PHAR2011 Exam 1

muscarinic receptor agonists

~Ach, the endogenous muscarinic and nicotinic receptor agonist, is virtually of no therapeutic use because: (1) it is not selective for receptor sub-types (2) it is very quickly hydrolysed by acetylcholine esterase (AChE) ~(1) ACh- muscarinic and nicotinic receptor, hydrolyzed by AChE (2) carbachol- muscarinic and nicotinic receptor, not hydrolyzed (3) methacholine- muscarinic and slightly nicotinic, hydrolyzed by AChE (4) bethanecol- muscarinic only, not hydrolyzed (5) muscarine- muscarinic only, not hydrolyzed (6) pilocarpine- muscarinic only, not hydrolyzed ~Muscarinic receptor agonists are mainly used in glaucoma [Stimulate muscurinic receptors on ciliary muscle, sphincter pupillae to cause contraction] [In open angle glaucoma-increase trabecular outflow by contracting the longitudinal part of ciliary muscle] [In angle closure-contraction of the sphincter pupillae causes miosis, pulls iris away from trabecular meshwork and opens the angle] ~carbachol- Pharmacology: stimulates muscarinic receptors on ciliary and sphincter pupillae [Uses: 1. Topical ocular administration is used for open-angle glaucoma. 2. Intraocular administration is used to produce miosis during cataract surgery] [Side effect: Rare when topical uses. Ciliary spasms with resultant temporary decrease of visual acuity may occur] [Contraindications: Miotics are contraindicated where constriction is undesirable such as in acute iritis.] ~pilocarpine/bethanechol- Pharmacology: selective for muscarinic receptors, stimulates smooth muscle contraction and gland secretion [Pilocarpine is used in Chronic open-angle glaucoma and acute angleclosure glaucoma (eye drop,1%- 4%), Dry mouth, for diagnosis test of cystic fibrosis (CF)] [Bethanechol is used to treat urinary retention (difficulty urinating), resulting from general anaesthetics, delivering a baby, diabetic neuropathy] ~Common side effects of muscarinic agonists: excessive sweating, excessive salivation, bronchospasm, bradycardia, hypotension, diarrhea, ciliary spasms and miosis [Contraindications: asthma, peptic ulcer, coronary insufficiency, intestinal obstruction, diarrhea, overactive bladder]

Non-depolarising blocking agents

~Curare (tubocurarine): original blocker. It was isolated from South American plants and used as a poison on arrows ~Vecuronium (pure NM blocker) is a synthetic drug which has fast onset and shorter duration of action with few side effects and as such is widely used as adjunct to general anaesthesia for all surgical procedures [Used during surgery to relax muscles, Increase safety of anaesthetics, Do not cross BBB]

Uptake 1 / Noradrenaline Transporter (NET) Inhibitors

~Desipramine (Antidepressant)- In depression levels of noradrenaline (and serotonin) are low. Aim to increase the levels to help relieve the symptoms [Mechanism of Action- Block the uptake of noradrenaline into the nerve terminal, Increasing the duration of action of the released noradrenaline] [In depression, helps to restore noradrenaline levels to normal] [Adverse Effects- Postural hypotension (α1‐AR), sedation (Histamine H1‐R), dry mouth, blurred vision, constipation (Muscarinic‐R)] [Contraindications- Monoamine oxidase inhibitors] ~Cocaine- Mechanism of Action: Block the uptake of noradrenaline into the nerve terminal, Increasing the duration of action of the released noradrenaline [Indications- Local anaesthetic/drug of abuse] [Adverse Effects & Toxicity- Hypertension, excitement, convulsions, dependence]

addiction (2)

~Ethanol has multiple actions to inhibit neuronal activity- potentiation of inhibitory neurotransmission at GABA and glycinergic synapses, decreased activity of voltage dependent Ca2+ channels also reduces neuronal excitability, inhibition of glutamate mediated excitatory neurotransmission ~nicotine- nicotine rapidly absorbed and distributed into brain where it strongly binds to and activates central nicotinic acetylcholine receptors which generally excites neurons; rapidly metabolized although accumulates in heavy smokers] [some peripheral actions in the autonomic nervous system] [treatment: abstinence and replacement therapy, verenicline, bupropione.] [subtle behavioural but severe withdrawal and clear adverse health effects] ~Marijuana- Specific receptors, endogenous ligands (e.g., anandamide) and transport systems. [rapidly absorbed; slow removal (highly lipophilic).] [possible medical uses as anti-emetic and appetite stimulator; antihypertensive; analgesic and for movement disorders] ~Cannabinoid receptors- 7-TM domain, G-protein coupled receptors; CB1 and CB2 [(1) CB1: brain; mediates euphoric and most therapeutic effects; decreases neuronal activity (↓cAMP, ↓Ca2+ channels, ↑ K+ channels) (2) CB2: 50% homology; ↓ cAMP; immunosuppressant effects] ~hallucinogens- Mostly interact with serotonin neurotransmission (5-HT2 receptor activation) [LSD and related substances: psilocibin (indoleamine) and mescaline (phenylethylamine)] [naturally occurring (mushrooms, cactus) and synthetic compounds: MDMA (ecstasy)] [others, possibly via NMDA glutamate receptors: PCP (angel dust); ketamine (anaesthetic)]

nicotinic receptors antagonists

~Hexamethonium- an antagonist at ganglionic nicotinic receptors, was the first clinically useful antihypertensive agent (obsolete). Useful pharmacological tool. ~Trimethaphan is the only ganglionic blocking agent still used clinically. This short acting agent is used in hypertensive emergencies.

Drugs affecting Serotonin neurotransmission

~MDMA (3,4‐methylenedioxy methamphetamine ‐ Ecstasy)- Mechanisms: Indirect serotonergic agonist, increasing the amount of serotonin released into the synapse [High affinity substrate for serotinin transporter (SERT), 10x less affinity for dopamine and noradrenaline transporters] [Also can act directly as an agonist at 5HT2, 1B receptors [Toxicity: Can selectively kill serotonin neurons] [Indication- Drug of abuse (not used clinically), Altered perception, mood elevation] [Adverse Effects- Tachycardia, hyperthermia, panic, toxicity, increased secretion of antidiuretic hormone→thirst→over‐ hydration→hyponatraemia, rebound dysphoria] [Contraindications- Some antidepressants and drugs increasing serotonin levels] ~Selective Serotonin Reuptake inhibitors (SSRIs)- Fluoxetine (Antidepressant) [In depression levels of serotonin are low. Aim to increase the levels to help relieve the symptoms] [Mechanism of Action of Fluoxetine- Block the uptake of serotonin into the nerve terminal, Increasing the duration of action of the released serotonin] [Helps to restore serotonin levels to normal] [Indication- Depression, Anxiety disorders] [Adverse Effects- Nausea, insomnia, agitation, weight change, loss of libido, many drug interactions (due to inhibiting drug metabolism)] [Contraindications- Other antidepressants, St John's wort]

depolarization block

~Main mechanisms of pharmacological block: inhibition of choline uptake, inhibition of ACh release, block of postsynaptic receptors or ion channels, persistent postsynaptic depolarisation. ~when the excitatory nAChRs are persistently activated, and it results from a decrease in the electrical excitability of the postsynaptic cell ~The main reason for the loss of electrical excitability during a period of maintained depolarisation is that the voltage-sensitive sodium channels (see Ch. 4 ) become inactivated (i.e. refractory) and no longer able to open in response to a brief depolarising stimulus

β1-Adrenergic Selective Agonists: Dobutamine

~Mechanism of Action- (1) Increases the strength (inotropic) of heart contraction (β1), has more prominent inotropic than chronotropic effects. (−) isomer of dobutamine is a agonist at α1 adrenoceptors increasing blood pressure, (+)-dobutamine is a α1 adrenoceptors antagonist decreasing blood pressure. ~Indications- Cardiac failure, Heart block, Cardiac arrest ~Adverse Effects- Increased heart rate, increased or decreased blood pressure ~Route of Administration- I.V. or subcutaneous (subcut) ~Pharmacokinetic parameters: ½ life ~ 2 min (iv) ~2h (subcut)

addiction

~Mesolimbic dopaminergic pathway [reward pathway]- all drugs of addiction activate this DA pathway via different mechanisms, other pleasurable stimuli also activate pathway to some extent (especially if craving, eg hungry rats shown food) [other neurotransmitter pathways involved, dopamine theory does not fully explain craving & addiction] ~Stimulants: Cocaine, amphetamines, ecstasy, caffeine [Act on aminergic neurotransmission (adrenalin, noradrenaline, dopamine, serotonin)] [Euphoria, increased energy (locomotor stimulation) and alertness; sympathomimetic, stereotypical behaviour, appetite suppression] [Caffeine: inhibits phosphodiesterase, increases Adr. & Nor., many other actions (adenosine receptor antagonist)] [Cocaine: blocks DA uptake, also increases Nor & 5-HT, blocks voltage dependent Na+ channels (local anaesthetic)] [Amphetamine: increases DA release, blocks DA uptake, also increases Nor & 5-HT] [Ecstasy: increases serotonin, DA and Nor] ~Opioids cause analgesia by decreasing neuronal excitability and neurotransmission [Opioid agonists (morphine, codeine, heroin), Endogenous opioids (endorphins, enkephalins, dynorphins), Opioid antagonists (naloxone, naltrexone)] ~Heroin- Diacetyl morphine; metabolized to morphine; further metabolized in liver and excreted in urine. [Effects: intense rush followed by dreamy state, side effects (nausea, constipation) overdose & addiction] [Effective dose: about 10 mg in inexperienced users, marked tolerance] [Withdrawal symptoms (craving, restlessness, fever, dysphoria, insomnia); begin within about 10 hours and last, initially for 10- 12 days and may persist indefinitely.] [Treatment: antagonists (naloxone; naltrexone), partial agonists (buprenorphine), agonists (methadone)]

drugs acting on noradrenergic transmission

~Mixed (α- and β-) adrenoceptor agonists- (1) adrenaline- Asthma (emergency treatment), anaphylactic shock, cardiac arrest [Added to local anaesthetic solutions] [Main hormone of adrenal medulla] (2) noradrenaline- Sometimes used for hypotension in intensive care [Transmitter at postganglionic sympathetic neurons, and in CNS] [unwanted effects- Hypertension, vasoconstriction, tachycardia (or reflex bradycardia), ventricular dysrhythmias] (3) Isoprenaline- non selective b agonist (treats asthma) (4) Dobutamine- Cardiogenic shock (b1 agonist non selective) (5) Salbutamol (b2 agonist)- Asthma, premature labour [unwanted effects- Tachycardia, dysrhythmias, tremor, peripheral vasodilatation] [Salmeterol and Terbutaline are the same] (6) Mirabegron (b3 agonist)- Symptoms of overactive bladder [unwanted effects- Tachycardia] (7) Phenylephrine (a1 agonist)- Nasal decongestion [unwanted effects- Hypertension, reflex bradycardia] (8) Clonidine (a2 partial agonist)- Hypertension, migraine [unwanted effects- Drowsiness, orthostatic hypotension, oedema and weight gain, rebound hypertension] ~a antagonists- (1) Phenoxybenzamine (non selective irreversible a antagonist)- Phaeochromocytoma [unwanted effects- Postural hypotension, tachycardia, nasal congestion, impotence] ~b antagonists- (1) Propranolol (non selective b antagonist)- Angina, hypertension, cardiac dysrhythmias, anxiety, tremor, glaucoma [unwanted effects- Bronchoconstriction, cardiac failure, cold extremities, fatigue and depression, hypoglycaemia] (2) Metoprolol (b1 antagonist)- Angina, hypertension, dysrhythmias (3) Nebivolol (b1 antagonist)- Hypertension ~Mixed (α-/β-) antagonists- (1) Labetalol- hypertension in pregnancy (2) Carvedilol- heart failure

Monoamine Oxidase Inhibitors

~Moclobemide- Mechanism of Action: Inhibits Monoamine oxidase (MAO) leading to decreased metabolism of noradrenaline, dopamine and serotonin. Synaptic concentrations of serotonin, noradrenaline and dopamine are increased [Indication- Major depression] [Adverse Effects- weight gain, CNS stimulation, insomnia and "cheese effect" (hypertensive crisis when foods containing tyramine are eaten (liver, red wine, cheese & vegemite))] [Contraindications- Other antidepressants, drugs affecting catecholamines (noradrenaline & dopamine) and serotinin]

muscarinic agonists

~Muscarinic agonists, as a group, are often referred to as parasympathomimetic , because the main effects that they produce in the whole animal resemble those of parasympathetic stimulation ~They are agonists at both mAChRs and nAChRs, but act more potently on mAChRs ~Bethanechol , pilocarpine and cevimeline are the only ones used clinically ~The key features of the ACh molecule that are important for its activity are the quaternary ammonium group, which bears a positive charge, and the ester group, which bears a partial negative charge and is susceptible to rapid hydrolysis by cholinesterase [variants of the choline ester structure ( Table 13.3 ) have the effect of reducing the susceptibility of the compound to hydrolysis by cholinesterase, and altering the relative activity on mAChRs and nAChRs.] ~cardiovascular effects- These include cardiac slowing and a decrease in cardiac output due both to the reduced heart rate and to a decreased force of contraction of the atria (the ventricles have only a sparse parasympathetic innervation and a low sensitivity to muscarinic agonists). Generalised vasodilatation also occurs (mediated by nitric oxide, NO ) and, combined with the reduced cardiac output, produces a sharp fall in arterial pressure ~smooth muscle effects- Smooth muscle generally contracts in direct response to muscarinic agonists, in contrast to the indirect effect via NO on vascular smooth muscle. Peristaltic activity of the gastrointestinal tract is increased, which can cause colicky pain, and the bladder and bronchial smooth muscle also contract. ~effects on eye- Activation of the constrictor pupillae muscle by muscarinic agonists in these circumstances lowers the intraocular pressure and treats glaucoma ~In addition to these peripheral effects, muscarinic agonists that are able to penetrate the blood-brain barrier produce marked central effects due to activation mainly of M 1 receptors in the brain. These include tremor, hypothermia and increased locomotor activity, as well as improved cognition ~Important compounds include acetylcholine , carbachol , methacholine , muscarine and pilocarpine . They vary in muscarinic/nicotinic selectivity, and in susceptibility to cholinesterase ~Main effects are bradycardia and vasodilatation (endothelium-dependent), leading to fall in blood pressure; contraction of visceral smooth muscle (gut, bladder, bronchi, etc.); exocrine secretions, pupillary constriction and ciliary muscle contraction, leading to decrease of intraocular pressure

nicotinic receptors

~Nicotinic receptors are a pentameric (5) proteins, Made up of combinations of alpha, beta, gamma, delta or epsilon subunits [Neuronal-types (NN) are formed with alpha or alpha and beta subunits] [Muscle-types (NM) are formed with alpha, beta, gamma, delta or epsilon] ~Nicotinic receptors are located: 1. At the ganglia of the autonomic nervous system 2. At the neuromuscular junction 3. At adrenal medulla 4. In the CNS [neuronal (Nn) are Located on both the sympathetic and parasympathetic autonomic ganglia, adrenal medulla and CNS] [neuromuscular (Nm) are located at postsynaptic skeletal muscle] ~neuromuscular receptors- (1) Nerve terminal: Action potential signals, ACh release (2) Motor end plate: Two ACh molecules bind to alpha subunit of N receptor, Opens cation channel, Na+ influx, Membrane depolarized, Muscle fiber contraction ~Neuromuscular blocking agents (Effects mostly due to motor paralysis)- (1) Non-depolarising agents - act to block nicotinic receptors, Competitive antagonists, increased ACh can reverse effect, tetanic fade- blockage of presynaptic autoregulation gradually reduces ACh release, Can block pre and post synaptic nicotinic receptors, The majority of clinically used neuromuscular blocking agents are non-depolarising (2) Depolarising blocking agents- Agonists at nicotinic receptors, Causes muscle twitching before paralysis, Maintains muscle depolarisation, unaffected by increased ACh

non-vesicular release mechanisms

~Nitric oxide (see Ch. 20 ) and arachidonic acid metabolites (e.g. prostaglandins; Ch. 17 ) are two important examples of mediators that are released from the cytosol by diffusion across the membrane or by carrier-mediated extrusion, rather than by exocytosis. The mediators are not stored but escape from the cell as soon as they are synthesised ~Acetylcholine, noradrenaline (norepinephrine) and other mediators can leak out of nerve endings from the cytosolic compartment, independently of vesicle fusion, by utilising carriers in the plasma membrane

b-adrenoreceptor antagonists

~Non-selective between β 1 and β 2 adrenoceptors: propranolol , alprenolol , oxprenolol . ~β 1 -selective: atenolol , nebivolol . ~Alprenolol and oxprenolol have partial agonist activity. ~Important hazards are bronchoconstriction, and bradycardia and cardiac failure (possibly less with partial agonists). ~Side effects include cold extremities, insomnia, depression, fatigue. ~Some show rapid first-pass metabolism, hence poor bioavailability. ~Some drugs (e.g. labetalol , carvedilol ) block both α and β adrenoceptors. ~clinical uses- (1) Cardiovascular: angina pectoris, myocardial infarction and following infarction, prevention of recurrent dysrhythmias (especially if triggered by sympathetic activation), heart failure (in well-compensated patients), hypertension (no longer first choice) (2) Other uses: glaucoma (e.g. timolol eye drops), thyrotoxicosis, as adjunct to definitive treatment (e.g. preoperatively), anxiety to control somatic symptoms (e.g. palpitations, tremor), migraine prophylaxis, benign essential tremor (a familial disorder)

The phospholipase C/inositol phosphate system

~PIP 2 is the substrate for a membrane-bound enzyme, phospholipase Cβ (PLCβ), which splits it into diacylglycerol (DAG) and inositol (1,4,5) trisphosphate (IP 3), both of which function as second messengers [The activation of PLCβ by various agonists is mediated through a G protein (Gq)] [After cleavage of PIP 2 , the status quo is restored, DAG being phosphorylated to form phosphatidic acid (PA), while the IP 3 is dephosphorylated and then recoupled with PA to form PIP 2 once again] [DAG activates PKC (which controls many cellular functions by phosphorylating a variety of proteins) and IP3 increases intracellular Ca2+] [increased free Ca 2+ initiates many events, including contraction, secretion, enzyme activation and membrane hyperpolarisation] ~Receptor-linked G proteins also control: Ion channels by opening potassium channels, resulting in membrane hyperpolarisation or inhibiting calcium channels, thus reducing neurotransmitter release

presynaptic modulation

~The presynaptic terminals that synthesise and release transmitter in response to electrical activity in the nerve fibre are often themselves sensitive to transmitter substances and to other substances that may be produced locally in tissues (shows the inhibitory effect of adrenaline on the release of acetylcholine) (The release of noradrenaline from nearby sympathetic nerve terminals can also inhibit release of acetylcholine) ~These are examples of heterotropic interactions , where one neurotransmitter affects the release of another. Homotropic interactions also occur, where the transmitter, by binding to presynaptic autoreceptors, affects the nerve terminals from which it is being released. This type of autoinhibitory feedback acts powerfully at noradrenergic nerve terminals

transmitters in the autonomic nervous system

~The two main neurotransmitters that operate in the autonomic system are acetylcholine and noradrenaline ~All autonomic nerve fibres leaving the central nervous system release acetylcholine, which acts on nicotinic receptors ~All postganglionic parasympathetic fibres release acetylcholine, which acts on muscarinic receptors ~All postganglionic sympathetic fibres (with one important exception) release noradrenaline, which may act on either α or β adrenoceptors (see Ch. 14 ). The exception is the sympathetic innervation of sweat glands, where transmission is due to acetylcholine acting on muscarinic receptors ~Transmitters other than noradrenaline and acetylcholine (NANC transmitters) are also abundant in the autonomic nervous system. The main ones are nitric oxide and vasoactive intestinal peptide (parasympathetic), ATP and neuropeptide Y (sympathetic). Others, such as 5-hydroxytryptamine, GABA and dopamine, also play a role

addiction (3)

~heroin- inhibitory neurotransmitters inhibits dopamine release, natural opiates cause dopamine to be released, heroin mimics natural opiates do dopamine floods synapse, natural opiates are painkillers ~ecstasy- serotonin transporters remove serotonin from synaptic cleft, ecstasy mimics serotonin and is taken up, alters transporters and reverses so transporters move serontonin out of cell, escess serotonin is trapped in cleft and binds again and again to postsynaptic receptors, indirectly releases dopamine (stimulated by excess serotonin) ~marijuana- before inhibitory neurotransmitters inhibit dopamine release, native cannabonoid (anandamide) releases dopamine, THC mimics anadamide, anandamide is broken down very quickly while THC isnt ~meth- dopamine transporters remove dopamine from cleft, meth mimics dopamine and is brought into cell by transporters, meth enters vesicles forcing dopamine out, excess dopamine in cell causes transporters to pump it out, binds to postsynaptic receptors ~alcohol- inhibitory GABA neurotransmitters, glutamate is excitatory neurotransmitter, alcohol interacts w/ GABA receptors to make them even more inhibitory and binds to glutamate receptors preventing glutamate from exciting cell ~cocaine- dopamine transporters remove dopamine from cleft, cocaine blocks transporters leaving dopamine blocked in cleft, binds to postsynaptic receptors ~LSD- chemically resembles serotonin and binds to serotonin receptors, sometimes has inhibitory and sometimes has excitatory effects which is why it has such complex sensory effects

adrenergic transmission

~nerve fibers that release noradrenaline at a synapse when a nerve impulse passes ~At sympathetic synapses: NE (NA) is the neurotransmitter released from most sympathetic postganglionic neurons [Once released, it stimulates alpha1, and β adrenoceptors on the effectors and alpha2 receptors on presynaptic terminals] [ACh is the neurotransmitter released by sympathetic postganglionic nerves innervating the sweat glands, where it stimulates muscarinic receptors] ~Cholinergic receptors- Respond to ACh, Nicotinic N [NM (neuromuscular junction), NN (autonomic ganglia, adrenal medulla & CNS)], Muscarinic M [M1 (CNS, glands), M2 (heart & smooth muscle), M3 (smooth muscle & glands), M4 (nerve cells), M5 (?)] ~Adrenoceptors- Respond to catecholamines, alpha-adrenoreceptors [alpha1 (smooth muscle), alpha2 (presynaptic nerves)], β-adrenoreceptors [β1 (heart), β2 (smooth muscle), β3 (fat tissue, bladder)] ~Muscarinic receptors and adrenergic receptors are G-protein coupled receptors, and nicotinic receptors are ion channels

eye prac

~pilocarpine is muscarinic agoinst, tropicamide is muscarinic antagonist, phenylephrine is alpha 1 adrenergic agonist ~parasympathetic to ciliary muscle and sphincter pupillae [sympathetic to dilator pupillae] ~Sphincter smooth muscle - parasympathetic innervation causes pupil constriction ~Dilator smooth muscle - sympathetic innervation causes dilation of the pupil ~Focus on Distant Objects- Ciliary muscle relaxes suspensory ligaments under tension, lens is flattened ~Focus on Near Objects- Ciliary muscle contracts- muscarinic- reduces tension on suspensory ligaments, lens becomes rounded ~ACh agonists contract ciliary muscle and increase aqueous humour outflow which result in a reduced intraocular pressure ~sympathetic relaxes ciliary muscle (beta 2 receptors) and contracts dilator smooth muscle (alpha 1 receptors) ~parasympathetic contracts ciliary muscle (M3) and contracts sphincter smooth muscle (M3) ~pupil size Conclusion: Pilocarpine significantly constricted the pupil (a miotic), whereas phyenylephrine and tropicamide highly significantly dilated the pupil (myriatics). Tropicamide was more effective than phenylephrine. ~near point of vision Conclusion: Tropicamide significantly reduced the accommodation of the eye due to the paralysis of the ciliary muscle (near vision affected). The effect of pilocarpine and phyenylephrine was minimal. ~visual acuity Conclusion: Pilocarpine significantly reduced visual acuity (far vision affected) due to the contraction of the ciliary muscle. The effect of phyenylephrine and tropicamide was minimal. ~The pupil has two muscles: (a) The sphincter pupillae which is innervated by the 3rd cranial nerve. The nerve fibres come from the ciliary ganglion and are postganglionic. They are part of the parasympathetic nervous system. (b) The radial muscle fibres of the dilator pupillae innervated by fibres of the cervical sympathetic system arising from the superior cervical ganglion. ~Sympathomimetic drugs cause blanching of the conjunctival vessels and dilation of the pupil. They have little effect on accommodation. ~Parasympathomimetic drugs cause constriction of the pupil, spasm of the ciliary muscle, difficulty in accommodation for far vision and a fall in intraocular pressure. ~Parasympatholytic drugs cause dilation of the pupil, paralysis of the ciliary muscle so that the eye cannot accommodate for near vision and in rare instances a rise in intraocular pressure.

biased agonism

~receptors are not actually restricted to two distinct states but have much greater conformational flexibility, so that there is more than one inactive and active conformation ~Receptors that couple to second messenger systems can couple to more than one intracellular effector pathway, giving rise to two or more simultaneous responses [different agonists can exhibit bias for the generation of one response over another even although they are acting through the same receptor probably because they stabilise different conformational states of the receptor]

drug target- exocytosis (Botulinum toxin (BOTOX®))

~A neurotoxin produced by Clostridium botulinum, a bacterium that causes food poisoning (botulism, lethal) [A potent neuroparalytic agent] [It inhibits Ca2+-dependent ACh release] ~normal mode of action- (1). Nerve transmission involves SNARE proteins (2) &(3). SNAREs interacting with vesicles facilitates vesicle docking and fusion with presynaptic membrane. (4). ACh is released into the synaptic cleft. (5). ACh binds to its receptors and triggers cellular function ~BOTOX mode of action- (1). Botulinum toxins enter the cell by endocytosis (2). Release of the light chain (3). The toxin breaks SNARE proteins, which results in an inability to form SNARE complexes, thus vesicle and synaptic membranes cannot fuse ~treatment of (1) Dystonia, such as strabismus (crossed eyes, lazy eye) (2) Blepharospasm (excessive blinking) (3) Cervical dystonia (head, neck muscle spasm) (4) Stuttering (5) Hyperhidrosis (excessive sweating) (6) Achalasia (failure of the lower oesophageal sphincter to relax) (7) Chronic migraine ~ Currently used in some patients with overactive bladder (incontinence) by local injection (approved by FDA in 2013). Effectively lasting 6 to 12 months and dramatically improving patients' quality of life ~Other clinical uses of Botox (not approved by FDA)- (1) Movement disorders associated with stroke, multiple sclerosis, Parkinson's disease, or cerebral palsy (2) Diabetic neuropathy (3) Wound healing (4) Excessive salivation (5) Cosmetic uses (to treat facial wrinkles)

Acetylcholine Synthesis and Release

~ACh is synthesised within the nerve terminal from choline, which is taken up into the nerve terminal by a specific transporter ~the concentration of choline in the blood and body fluids is normally about 10 µmol/l, but in the immediate vicinity of cholinergic nerve terminals it increases, probably to about 1 mmol/l, when the released ACh is hydrolysed, and more than 50% of this choline is normally recaptured by the nerve terminals ~free choline within the nerve terminal is acetylated by a cytosolic enzyme, choline acetyltransferase (CAT), which transfers the acetyl group from acetyl coenzyme A. The rate-limiting process in ACh synthesis appears to be choline transport, which is determined by the extracellular choline concentration and hence is linked to the rate at which ACh is being released ~Cholinesterase is present in the presynaptic nerve terminals, and ACh is continually being hydrolysed and resynthesised ~Cholinergic vesicles accumulate ACh actively, by means of a specific transporter belonging to the family of amine transporters. Accumulation of ACh is coupled to the large electrochemical gradient for protons that exists between acidic intracellular organelles and the cytosol; it is blocked selectively by the experimental drug vesamicol ~ollowing its release, ACh diffuses across the synaptic cleft to combine with receptors on the postsynaptic cell. Some of it succumbs on the way to hydrolysis by acetylcholinesterase (AChE), an enzyme that is bound to the basement membrane that lies between the pre- and postsynaptic membranes. At fast cholinergic synapses (e.g. the neuromuscular and ganglionic synapses), but not at slow ones (smooth muscle, gland cells, heart, etc.), the released ACh is hydrolysed very rapidly (within 1 ms), so that it acts only very briefly. ~Approximately two million ACh molecules combine with receptors, of which there are about 30 million on each muscle fibre, the rest being hydrolysed without reaching a receptor. The ACh molecules remain bound to receptors for, on average, about 2 ms, and are quickly hydrolysed after dissociating, so that they cannot combine with a second receptor. The result is that transmitter action is very rapid and very brief, which is important for a synapse that has to initiate speedy muscular responses, and that may have to transmit signals faithfully at high frequency ~Acetylcholine release is regulated by mediators, including ACh itself, acting on presynaptic receptors- inhibitory M 2 receptors participate in autoinhibition of ACh release; other mediators, such as noradrenaline, also inhibit the release of ACh

Electrical Events in Transmission at Fast Cholinergic Synapses

~Acetylcholine, acting on the postsynaptic membrane of a nicotinic (neuromuscular or ganglionic) synapse, causes a large increase in its permeability to cations, particularly to Na + and K + , and to a lesser extent Ca 2+ . The resulting inflow of Na + depolarises the postsynaptic membrane. This transmitter-mediated depolarisation is called an endplate potential ( epp ) in a skeletal muscle fibre, or a fast excitatory postsynaptic potential ( fast epsp ) at the ganglionic synapse ~In a muscle fibre, the localised epp spreads to adjacent, electrically excitable parts of the muscle fibre; if its amplitude reaches the threshold for excitation, an action potential is initiated, which propagates to the rest of the fibre and evokes a contraction ~at neuromuscular junction- the synapse ensures faithful 1 : 1 transmission despite the impedance mismatch between the fine nerve fibre and the much larger muscle fibre. The amplitude of the epp is normally more than enough to initiate an action potential - indeed, transmission still occurs when the epp is reduced by 70-80%, showing a large margin of safety so that fluctuations in transmitter release (e.g. during repetitive stimulation) do not affect transmission ~Transmission at the ganglionic synapse is more complex than at the neuromuscular junction. Although the primary event at both is the epp or fast epsp produced by ACh acting on nAChRs, this is followed in the ganglion by a succession of much slower postsynaptic responses: (1) A slow inhibitory (hyperpolarising) postsynaptic potential (slow ipsp) , lasting 2-5 s. This mainly reflects a muscarinic (M 2 )-receptor-mediated increase in K + conductance (2) A slow epsp , which lasts for about 10 s. This is produced by ACh acting on M 1 receptors, which close K + channels (3) A late slow epsp , lasting for 1-2 min. This is thought to be mediated by a peptide co-transmitter, which may be substance P in some ganglia, and a gonadotrophin-releasing hormone-like peptide in others (see Ch. 12 ). Like the slow epsp, it is produced by a decrease in K + conductance.

drug target- acetylcholinesterase

~Acetylcholinesterase (AChE)- Terminates the actions of acetylcholine. [Acetylcholine > Choline + Acetate] [AChE keeps ACh below detection levels] [AChE is found in the cholinergic synapses- dendrites, axons, neuromuscular junctions, smooth muscle cells and other parasympathetic target tissues] ~AChE inhibitors- Enhance the actions of ACh and have actions similar to ACh agonists [Autonomic actions: enhancement of ACh activity at parasympathetic synapses (bradycardia, increased saliva secretion, increased gut contractility)] [Neuromuscular junction: repeated firing of the muscle fiber leading to twitching and increased muscle contraction] [AChE inhibitors that cross BBB (eg physostigmine) can cause profound CNS effects] ~Medium duration - Physostigmine (calabar beans) and neostigmine - have an amine group and a ester group (similar to ACh) and bind to the active site of AChE where they are slowly hydrolysed. ~Irreversible anticholinesterases - organophosphate compounds - bind covalently to the AChE active site. Developed as nerve gases (sarin) & insecticides (parathion) ~neostigmine- Medium duration, Used after surgery to reverse neuromuscular junction blockage ~physostigmine/ecothiopate- Long to medium duration, Can be used as eye drops to treat glaucoma (uncommon) [Physostigmine passes BBB: antidote for CNS effects of atropine and other anticholinergic overdoses. ~neostigmine, pyridostigmine- Medium duration, Treatment of urinary retention; myasthenia gravis (autoimmune disease with loss of nicotinic receptors at the NMJ leading to muscle weakness) ~Myasthenia gravis- MG is a complex autoimmune disorder in which antibodies destroy neuromuscular connections [Symptoms may vary: Eyelid drooping, Facial paralysis, Hoarseness / changing voice, Drooping head, Difficulty talking, Difficulty breathing, Fatigue]

calcium extrusion mechanisms

~Active transport of Ca 2+ outwards across the plasma membrane, and inwards across the membranes of the ER or SR, depends on the activity of distinct Ca 2+ -dependent ATPases, 2 similar to the Na + /K + -dependent ATPase that pumps Na + out of the cell in exchange for K + ~Calcium is also extruded from cells in exchange for Na + , by Na + -Ca 2+ exchange [The exchanger transfers three Na + ions for one Ca 2+ , and therefore produces a net depolarising current when it is extruding Ca 2+ . The energy for Ca 2+ extrusion comes from the electrochemical gradient for Na + , not directly from ATP hydrolysis. This means that a reduction in the Na + concentration gradient resulting from Na + entry will reduce Ca 2+ extrusion by the exchanger, causing a secondary rise in [Ca 2+ ] i ]

protein targets for drug binding (2)

~Although binding can be measured directly, it is usually a biological response (such as a rise in blood pressure) that we are interested in and this is often plotted as a concentration - effect curve ( in vitro ) or dose - response curve ( in vivo) [ This allows us to estimate the maximal response that the drug can produce ( E max ), and the concentration or dose needed to produce a 50% maximal response (EC 50 or ED 50 )] [The E max , EC 50 and slope parameters are useful for comparing different drugs that produce qualitatively similar effects] [log concentration is used to make the curve linear] ~concentration-effect curves cannot be used to measure the affinity of agonist drugs for their receptors, because the response produced is not, as a rule, directly proportional to receptor occupancy. This often arises because the maximum response of a tissue may be produced by agonists when they are bound to less than 100% of the receptors. Under these circumstances the tissue is said to possess spare receptors (Economy of hormone or transmitter secretion is thus achieved at the expense of providing more receptors) ~Full agonists bind to receptors and very efficiently give a response, Partial agonists are less 'efficacious' - 1. Never achieve maximum effect. 2. Also act as an antagonist. [The dose response curve for a partial agonist therefore does not reach the maximal response obtained for a full agonist (looks like combination of dose response curve for agonist and antagonist)] ~Inverse Agonists- Some receptors are constitutively active, even in the absence of any agonist (An Inverse agonist restores the receptor to its inactive state) [Receptors exist in equilibrium between active and inactive forms, The presence of an agonist will increase the proportion of active receptors, The presence of an inverse agonist will increase the proportion of inactive receptors., Mechanism of action of inverse agonists is thought to involve the destabilization of Gprotein receptor coupling.] ~allosteric modulators- These compounds bind to a separate site on the receptor from agonists called an allosteric site. (Occupation of this site can either increase or decrease the response to an endogenous agonists, depending on whether it is positive or negative modulation) ~Therapeutic ratio = LD50/ED50 [Toxic Ratio = TD50/ED50] ~The number of receptors in a cell is not static but dynamic, There is a high turnover of receptors as they are continuously removed or replaced, Repeated drug treatment may up-regulate or down regulate the receptor numbers (down regulation=tolerance, up regulation=sensitization) ~causes of desensitization- (1) Change in receptors (phosphorylation) (2) Down regulation of receptors (internalization / reduced expression) (3) Depletion of mediators (4) Increased metabolic breakdown

termination of transmitter action

~At cholinergic synapses ( Ch. 13 ), the released acetylcholine is inactivated very rapidly in the synaptic cleft by acetylcholinesterase ~In most other cases (see Fig. 12.8 ), transmitter action is terminated by active reuptake into the presynaptic nerve, or into supporting cells such as glia. Such reuptake depends on transporter proteins (see Ch. 4 ), each being specific for a particular transmitter ~Membrane transporters usually act as co-transporters of Na + , Cl − and transmitter molecules, and it is the inwardly directed 'downhill' gradient for Na + that provides the energy for the inward 'uphill' movement of the transmitter

drugs acting on adrenoreceptors

~Broadly speaking, β-adrenoceptor agonists are useful as smooth muscle relaxants (especially in the airways), while β-adrenoceptor antagonists (often called β blockers) are used mainly for their cardiodepressant effects. α-Adrenoceptor antagonists are used mainly for their vasodilator effects in cardiovascular indications and also for the treatment of prostatic hyperplasia. Adrenaline, with its mixture of cardiac stimulant, vasodilator and vasoconstrictor actions is uniquely important in cardiac arrest ( Ch. 21 ). Selective α-adrenoceptor agonists have relatively few clinical uses. ~summary of adrenoreceptor agonists- (1) Noradrenaline and adrenaline show relatively little receptor selectivity (2) Selective α 1 agonists include phenylephrine and oxymetazoline (3) Selective α 2 agonists include clonidine and α-methylnoradrenaline [They cause a fall in blood pressure, partly by inhibition of noradrenaline release and partly by a central action] (4) Selective β 1 agonists include dobutamine [Increased cardiac contractility may be useful clinically, but all β 1 agonists can cause cardiac dysrhythmias] (5) Selective β 2 agonists include salbutamol , terbutaline and salmeterol ; used mainly for their bronchodilator action in asthma (6) A selective β 3 agonist, mirabegron , is used to treat overactive bladder; β 3 agonists promote lipolysis and have potential in the treatment of obesity.

clinical uses of adrenoreceptor agonists

~Cardiovascular system: (1) cardiac arrest: adrenaline (2) cardiogenic shock: dobutamine (β 1 agonist). ~Anaphylaxis (acute hypersensitivity): adrenaline . ~Respiratory system: (1) asthma: selective β 2 -receptor agonists ( salbutamol , terbutaline , salmeterol , formoterol ) (2) nasal decongestion: drops containing xylometazoline or ephedrine for short-term use. ~Miscellaneous indications: (1) adrenaline : with local anaesthetics to prolong their action (2) premature labour ( salbutamol) (3) α 2 agonists (e.g. clonidine ): to lower blood pressure and intraocular pressure; as an adjunct during drug withdrawal in addicts; to reduce menopausal flushing, especially when oestrogen is contraindicated as in patients with breast cancer; and to reduce frequency of migraine attacks (4) A β 3 agonist, mirabegron : to treat urgency, increased micturition frequency and incontinence (overactive bladder symptoms).

Ion Channels as Drug Targets

~Channels are generally either cation selective or anion selective. The main cation-selective channels are selective for Na + , Ca 2+ or K + , or non-selective and permeable to all three. Anion channels are mainly permeable to Cl − ~(1) voltage gated channels- Commonly, the channel opening (activation) induced by membrane depolarisation is short lasting, even if the depolarisation is maintained. This is because, with some channels, the initial activation of the channels is followed by a slower process of inactivation (2) ligand gated channels - activated by binding of a chemical ligand to a site on the channel molecule [Some ligand-gated channels in the plasma membrane respond to intracellular rather than extracellular signals, the most important being the following] (3) calcium release channels (4) store operated calcium channels- When the intracellular Ca 2+ stores are depleted, 'store-operated' channels (SOCs) in the plasma membrane open to allow Ca 2+ entry ~Voltage-gated channels generally include one transmembrane helix that contains an abundance of basic (i.e. positively charged) amino acids. When the membrane is depolarised, so that the interior of the cell becomes less negative, this region - the voltage sensor - moves slightly towards the outer surface of the membrane, which has the effect of opening the channel (see Bezanilla, 2008 ). Many voltage-activated channels also show inactivation , which happens when an intracellular appendage of the channel protein moves to plug the channel from the inside

postsynaptic modulation

~Chemical mediators often act on postsynaptic structures, including neurons, smooth muscle cells, cardiac muscle cells, etc., in such a way that their excitability or spontaneous firing pattern is altered. In many cases, as with presynaptic modulation, this is caused by changes in calcium and/or potassium channel function mediated by a second messenger. ~neuromodulation , because the mediator acts to increase or decrease the efficacy of synaptic transmission without participating directly as a transmitter [Many neuropeptides, for example, affect membrane ion channels in such a way as to increase or decrease excitability and thus control the firing pattern of the cell. Neuromodulation 1 is loosely defined but, in general, involves slower processes (taking seconds to days) than neurotransmission (which occurs in milliseconds), and operates through cascades of intracellular messengers ( Ch. 3 ) rather than directly on ligand-gated ion channels.]

release of chemical mediators/exocytosis

~Chemical mediators that are released from cells fall into two main groups (1) Mediators that are preformed and packaged in storage vesicles - sometimes called storage granules - from which they are released by exocytosis (neurotransmitters and hormones) (2) Mediators that are produced on demand and are released by diffusion or by membrane carriers which are released from the postsynaptic cell to act retrogradely on nerve terminals. ~Calcium ions play a key role in both cases, because a rise in [Ca 2+ ] i initiates exocytosis and is also the main activator of the enzymes responsible for the synthesis of diffusible mediators ~Exocytosis, occurring in response to an increase of [Ca 2+ ] i , is the principal mechanism of transmitter release [involves fusion between the membrane of synaptic vesicles and the inner surface of the plasma membrane. The vesicles are preloaded with stored transmitter, and release occurs in discrete packets, or quanta, each representing the contents of a single vesicle] ~In nerve terminals specialised for fast synaptic transmission, Ca 2+ enters through voltage-gated calcium channels and the synaptic vesicles are 'docked' at active zones - specialised regions of the presynaptic membrane from which exocytosis occurs, situated close to the relevant calcium channels and opposite receptor-rich zones of the postsynaptic membrane [where speed is less critical, Ca 2+ may come from intracellular stores] ~It is common for secretory cells, including neurons, to release more than one mediator (for example, a 'fast' transmitter such as glutamate and a 'slow' transmitter such as a neuropeptide) from different vesicle pools. The fast transmitter vesicles are located close to active zones, while the slow transmitter vesicles are further away. Release of the fast transmitter, because of the tight spatial organisation, occurs as soon as the neighbouring calcium channels open, before the Ca 2+ has a chance to diffuse throughout the terminal, whereas release of the slow transmitter requires the Ca 2+ to diffuse more widely. As a result, release of fast transmitters occurs impulse by impulse, even at low stimulation frequencies, whereas release of slow transmitters builds up only at higher stimulation frequencies. The release rates of the two therefore depend critically on the frequency and patterning of firing of the presynaptic neuron ~Calcium causes exocytosis by binding to the vesicle-bound protein synaptotagmin , and this favours association between a second vesicle-bound protein, synaptobrevin , and a related protein, synaptotaxin , on the inner surface of the plasma membrane. This association brings the vesicle membrane into close apposition with the plasma membrane, causing membrane fusion. This group of proteins, known collectively as SNAREs, plays a key role in exocytosis ~Having undergone exocytosis, the empty vesicle 6 is recaptured by endocytosis and returns to the interior of the terminal, where it fuses with the larger endosomal membrane. The endosome buds off new vesicles, which take up transmitter from the cytosol by means of specific transport proteins and are again docked on the presynaptic membrane

cholinergic transmission

~Cholinergic: a neuron or axon that is capable of releasing the neurotransmitter, acetylcholine (ACh), when a nerve impulse passes [The cholinergic system includes: (1) The entire parasympathetic nervous system (2) The preganglionic neurons of the sympathetic nervous system (3) The postganglionic sympathetic neurons innervating sweat glands (4) Somatic motor neurons (5) The CNS] ~At ganglia- ACh is the neurotransmitter at all autonomic ganglia, Released by preganglionic nerve endings, ACh stimulates nicotinic (N) receptors on the postganglionic neurons, and adrenal medulla ~At parasympathetic synapses- ACh is also the neurotransmitter at all parasympathetic nerve endings, Following its release ACh stimulates muscarinic (M) receptors on the innervated tissues (effectors) ~At neuromuscular junctions- ACh is also the neurotransmitter of somatic nerves, Following its release ACh stimulates nicotinic (N) receptors on the motor end plates of skeletal muscles to cause muscle contraction

drugs that inhibit cholinesterase

~Cholinesterase inhibitors affect peripheral as well as central cholinergic synapses ~enhancement of ACh activity at parasympathetic postganglionic synapses (i.e. increased secretions from salivary, lacrimal, bronchial and gastrointestinal glands; increased peristaltic activity; bronchoconstriction; bradycardia and hypotension; pupillary constriction; fixation of accommodation for near vision; fall in intraocular pressure) ~The twitch tension of a muscle stimulated via its motor nerve is increased by anticholinesterases, owing to repetitive firing in the muscle fibre associated with prolongation of the epp. Normally, the ACh is hydrolysed so quickly that each stimulus initiates only one action potential in the muscle fibre, but when AChE is inhibited this is converted to a short train of action potentials in the muscle fibre, and hence greater tension ~In large doses, such as can occur in poisoning, anticholinesterases initially cause twitching of muscles. This is because spontaneous ACh release can give rise to epps that reach the firing threshold. Later, paralysis may occur due to depolarisation block, which is associated with the build-up of ACh in the plasma and tissue fluids

Type 4: Nuclear Receptors (2)

~Class I consists largely of endocrine steroid receptors, including the glucocorticoid and mineralocorticoid receptors (GR and MR), as well as the oestrogen, progesterone and androgen receptors (ER, PR and AR, respectively). The hormones (e.g. glucocorticoids) recognised by these receptors generally act in a negative feedback fashion to control biological events (see Ch. 33 for more details). In the absence of their ligand, these NRs are predominantly located in the cytoplasm, complexed with heat shock and other proteins and possibly reversibly attached to the cytoskeleton or other structures. Following diffusion (or possibly transportation) into the cell from the blood, ligands bind their NR partner with high affinity. These liganded receptors generally form homodimers and translocate to the nucleus, where they can transactivate or transrepress genes by binding to 'positive' or 'negative' HREs. Once bound, the NR recruits other proteins to form complexes that promote transcription of multiple genes ~Class II NRs are generally bound to co-repressor proteins. These dissociate when the ligand binds and allows recruitment of co-activator proteins and hence changes in gene transcription. They tend to mediate positive feedback effects (e.g. occupation of the receptor amplifies rather than inhibits a particular biological event). ~(1) Class I NRs are present in the cytoplasm, form homodimers in the presence of their partner, and migrate to the nucleus. Their ligands are mainly endocrine in nature (e.g. steroid hormones) (2) Class II NRs are generally constitutively present in the nucleus and form heterodimers with the retinoid X receptor. Their ligands are usually lipids (e.g. the fatty acids).

drugs that act presynaptically

~Drugs That Inhibit Acetylcholine Synthesis- The rate-limiting process appears to be the transport of choline into the nerve terminal. Hemicholinium blocks this transport and thereby inhibits ACh synthesis ~Drugs That Inhibit Acetylcholine Release- Agents that inhibit Ca 2+ entry include Mg 2+ and various aminoglycoside antibiotics (Acetylcholine release by a nerve impulse involves the entry of Ca 2+ into the nerve terminal)

drugs acting on noradrenergic transmission (2)

~Drugs affecting NA synthesis- (1) α-Methyl-p-tyrosine- Inhibits tyrosine hydroxylase (2) Carbidopa- inhibits dopa decarboxylase (3) Methyldopa- False transmitter precursor (4) L-DOPS - Converted to NA by dopa decarboxylase, thus increasing NA synthesis and release ~Drugs that release NA (indirectly acting sympathomimetic amines)- (1) Tyramine- NA release (2) amphetamine- NA release, MAO inhibitor, NET inhibitor, CNS stimulant (3) ephidrine- NA release, β agonist, weak CNS stimulant action ~Drugs that inhibit NA release- (1) Reserpine- Depletes NA stores by inhibiting VMAT (2) guanethidine- Inhibits NA release Also causes NA depletion and can damage NA neurons irreversibly ~Drugs affecting NA uptake- (1) imipramine- Blocks neuronal transporter (NET) [Also has atropine-like action] (2) cocaine- Local anaesthetic; blocks NET CNS stimulant

neuromuscular blocking drugs

~Drugs can block neuromuscular transmission either by acting presynaptically to inhibit ACh synthesis or release, or by acting postsynaptically either (a) by blocking ACh receptors (or in some cases the ion channel) or (b) by activating ACh receptors and thus causing persistent depolarisation of the motor endplate ~Non-depolarising blocking agents act as competitive antagonists (see Ch. 2 ) at the ACh receptors of the endplate [The amount of ACh released by a nerve impulse normally exceeds by several-fold what is needed to elicit an action potential in the muscle fibre. It is therefore necessary to block 70-80% of the receptor sites before transmission actually fails] ~Substances that block choline uptake: for example hemicholinium (not used clinically). ~Substances that block acetylcholine release: aminoglycoside antibiotics , botulinum toxin . ~Drugs used to cause paralysis during anaesthesia are as follows: (1) Depolarising neuromuscular-blocking agents: suxamethonium , short acting and used during induction of anaesthesia and intubation of the airway. (2) Non-depolarising neuromuscular-blocking agents: tubocurarine , pancuronium , atracurium , vecuronium , mivacuronium . These act as competitive antagonists at nicotinic acetylcholine receptors and differ mainly in duration of action; used to maintain neuromuscular relaxation throughout an operation which may be of several hours duration or when unconscious in an intensive care unit. ~Non-depolarising block is reversible by anticholinesterase drugs, depolarising block is not

a-adrenoreceptor antagonists

~Drugs that block α 1 and α 2 adrenoceptors (e.g. phenoxybenzamine and phentolamine ) were once used to produce vasodilatation in the treatment of peripheral vascular disease, but this use is now largely obsolete. ~Selective α 1 antagonists (e.g. prazosin , doxazosin , terazosin ) are used in treating hypertension and for benign prostatic hypertrophy. Postural hypotension, stress incontinence and impotence are unwanted effects. ~Tamsulosin is α 1A selective and acts mainly on the urogenital tract. It is used to treat benign prostatic hypertrophy and may cause less postural hypotension than other α 1 agonists. ~Yohimbine is a selective α 2 antagonist. It is not used clinically. ~clinical uses- (1) Severe hypertension: α 1 -selective antagonists (e.g. doxazosin ) in combination with other drugs (2) Benign prostatic hypertrophy (e.g. tamsulosin , a selective α 1A -receptor antagonist) (3) Phaeochromocytoma: phenoxybenzamine (irreversible antagonist) in preparation for surgery.

drugs that enhance cholinergic transmission

~Drugs that enhance cholinergic transmission act either by inhibiting cholinesterase (the main group) or by increasing ACh release ~There are two distinct types of cholinesterase, namely acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE, sometimes called pseudocholinesterase), closely related in molecular structure but differing in their distribution, substrate specificity and functions ~The bound AChE at cholinergic synapses serves to hydrolyse the released transmitter and terminate its action rapidly. Soluble AChE is also present in cholinergic nerve terminals, where it has a role in regulating the free ACh concentration ~Butyrylcholinesterase (BuChE) has a widespread distribution, being found in tissues such as liver, skin, brain and gastrointestinal smooth muscle, as well as in soluble form in the plasma. It is not particularly associated with cholinergic synapses, and its physiological function is unclear. It has a broader substrate specificity than AChE. It hydrolyses the synthetic substrate butyrylcholine more rapidly than ACh, as well as other esters, such as procaine , suxamethonium and propanidid

drugs acting on noradrenergic nerve terminals

~Drugs that inhibit noradrenaline synthesis include: (1) α-Methyltyrosine : blocks tyrosine hydroxylase; not used clinically (2) carbidopa : blocks dopa decarboxylase and is used in treatment of parkinsonism; not much effect on noradrenaline synthesis. ~Methyldopa gives rise to false transmitter (methylnoradrenaline), which is a potent α 2 agonist, thus causing powerful presynaptic inhibitory feedback (also central actions). Its use as an antihypertensive agent is now limited mainly to during pregnancy. ~Reserpine blocks noradrenaline accumulation in vesicles by VMAT, thus depleting noradrenaline stores and blocking transmission. Effective in hypertension but may cause severe depression. Clinically obsolete. ~Noradrenergic neuron-blocking drugs (e.g. guanethidine , bethanidine ) are selectively concentrated in terminals and in vesicles (by NET and VMAT respectively), and block transmitter release, partly by local anaesthetic action. Effective in hypertension but cause severe side effects (postural hypotension, diarrhoea, nasal congestion, etc.), so now little used. ~6-Hydroxydopamine is selectively neurotoxic for noradrenergic neurons, because it is taken up and converted to a toxic metabolite. Used experimentally to eliminate noradrenergic neurons, not used clinically. ~Indirectly acting sympathomimetic amines (e.g. amphetamine , ephedrine , tyramine ) are accumulated by NET and displace noradrenaline from vesicles, allowing it to escape. Effect is much enhanced by monoamine oxidase (MAO) inhibition, which can lead to severe hypertension following ingestion of tyramine-rich foods by patients treated with MAO inhibitors. ~Indirectly acting sympathomimetic agents are central nervous system stimulants. Methylphenidate and atomoxetine are used to treat attention deficit-hyperactivity disorder. ~Drugs that inhibit NET include cocaine and tricyclic antidepressant drugs. Sympathetic effects are enhanced by such drugs.

protein targets for drug binding (1)

~Four main kinds of regulatory protein are commonly involved as primary drug targets- receptors, enzymes, carrier molecules (transporters), ion channels [Specificity is reciprocal: individual classes of drug bind only to certain targets, and individual targets recognise only certain classes of drug] [No drugs are completely specific in their actions. In many cases, increasing the dose of a drug will cause it to affect targets other than the principal one, and this can lead to side effects] ~agonists , which 'activate' the receptors, and antagonists , which combine at the same site without causing activation, and block the effect of agonists on that receptor ~Binding and activation represent two distinct steps in the generation of the receptor-mediated response by an agonist [If a drug binds to the receptor without causing activation and thereby prevents the agonist from binding, it is termed a receptor antagonist] [The tendency of a drug to bind to the receptors is governed by its affinity , whereas the tendency for it, once bound, to activate the receptor is denoted by its efficacy] ~Drugs with intermediate levels of efficacy, such that even when 100% of the receptors are occupied the tissue response is submaximal, are known as partial agonists , to distinguish them from full agonists , the efficacy of which is sufficient that they can elicit a maximal tissue response ~binding curve defines the relationship between concentration and the amount of drug bound (B) allowing the affinity of the drug for the receptors to be estimated, as well as the binding capacity ( B max ), representing the density of receptors in the tissue ~1. Binding of drug to receptor - affinity 2. Response to binding - efficacy (intrinsic activity). ~Effect = Maximal Effect x [D]/(KD + [D]), Receptors occupied = [D]/(KD + [D]), Receptors occupied = ([D]/KD)/ ([D]/KD+ 1) where [D]=concentration of drug and KD=dissociation constant=EC50 ~Competitive Antagonism- Agonists and antagonists compete for the same receptor sites, Maximal effect unchanged (ie antagonism is surmountable), Parallel shift to the right [Agonist and antagonist compete at the high affinity receptor sites - the agonist can activate the receptor but the antagonist can not. The dose-response curve of the agonist is shifted to the right. A high enough dose of agonist can overcome the antagonist.] ~Non-competitive Antagonism- Irreversible antagonists can act on the receptor itself, binding irreversible, Cause a change in the receptor so that the agonist can no longer bind, A maximum effect is no longer produced. [Non-competitive antagonists bind covalently to the receptor (do not wash out). This is the equivalent of removing a number of receptors from the system. Therefore a max effect is no longer produced (except in the case of spare receptors)] ~Because of the existence of spare receptors the ED50 for an agonist may not be equal to its KD and the presence of spare receptors shifts the dose-response curve to the left of the KD ~Physiological Antagonism- Occurs when 2 agonists act on different receptors to produce opposite effects, The drugs have different mechanisms of action (Eg bronchoconstriction due to histamine can be treated with adrenaline which acts as a vasodilator)

G proteins

~Guanine nucleotides bind to the α subunit, which has enzymic (GTPase) activity, catalysing the conversion of GTP to GDP. The β and γ subunits remain together as a βγ complex. The 'γ' subunit is anchored to the membrane through a fatty acid chain, coupled to the G protein through a reaction known as prenylation ~In the 'resting' state the G protein exists as an αβγ trimer, which may or may not be precoupled to the receptor, with GDP occupying the site on the α subunit. When a GPCR is activated by an agonist molecule, a conformational change occurs (This agonist-induced interaction of αβγ with the receptor causes the bound GDP to dissociate and to be replaced with GTP (GDP-GTP exchange), which in turn causes dissociation of the G protein trimer, releasing α-GTP and βγ subunits) [these are the 'active' forms of the G protein, which diffuse in the membrane and can associate with various enzymes and ion channels, causing activation of the target] ~Association of α or βγ subunits with target enzymes or channels can cause either activation or inhibition, depending on which G protein is involved [G protein activation results in amplification, because a single agonist-receptor complex can activate several G protein molecules in turn, and each of these can remain associated with the effector enzyme for long enough to produce many molecules of product] ~The product (see below) is often a 'second messenger', and further amplification occurs before the final cellular response is produced. Signalling is terminated when the hydrolysis of GTP to GDP occurs through the GTPase activity of the α subunit. The resulting α-GDP then dissociates from the effector, and reunites with βγ, completing the cycle

G proteins (2)

~Guanine nucleotides bind to the α subunit, which has enzymic (GTPase) activity, catalysing the conversion of GTP to GDP. The β and γ subunits remain together as a βγ complex. The 'γ' subunit is anchored to the membrane through a fatty acid chain, coupled to the G protein through a reaction known as prenylation ~In the 'resting' state, the G protein exists as an αβγ trimer, which may or may not be precoupled to the receptor, with GDP occupying the site on the α subunit. When a GPCR is activated by an agonist molecule, a conformational change occurs, involving the cytoplasmic domain of the receptor, inducing a high affinity interaction of αβγ and the receptor [causing the bound GDP to dissociate and to be replaced with GTP (GDP-GTP exchange), which in turn causes dissociation of the G protein trimer, releasing α-GTP and βγ subunits; these are the 'active' forms of the G protein, which diffuse in the membrane and can associate with various enzymes and ion channels, causing activation of the target] ~Association of α or βγ subunits with target enzymes or channels can cause either activation or inhibition, depending on which G protein is involved ~G protein activation results in amplification, because a single agonist-receptor complex can activate several G protein molecules in turn, and each of these can remain associated with the effector enzyme for long enough to produce many molecules of product ~Signalling is terminated when the hydrolysis of GTP to GDP occurs through the GTPase activity of the α subunit. The resulting α-GDP then dissociates from the effector, and reunites with βγ, completing the cycle

GPCR desensitisation

~Homologous desensitisation is restricted to the receptors activated by the desensitising agonist, while heterologous desensitisation affects other GPCRs in addition ~(1) phosphorylated by specific membrane-bound GPCR kinases (GRKs) (2) The phosphorylated receptor serves as a binding site for arrestins, intracellular proteins that block the interaction between the receptor and the G proteins producing a selective homologous desensitisation (3) Arrestin binding also targets the receptor for endocytosis (4) The internalised receptor can then either be dephosphorylated and reinserted into the plasma membrane (resensitisation) or trafficked to lysosomes for degradation (inactivation) ~G,alpha,i inhibits cAMP production, G,alpha,s stimulates cAMP production, G,alpha,q increases DAG and IP3 production, G,alpha,12/13 activates Rho

G proteins (3)

~In addition, there is a family of about 20 cellular proteins, regulators of G protein signalling (RGS) proteins that possess a conserved sequence that binds specifically to α subunits to increase greatly their GTPase activity, so hastening the hydrolysis of GTP and inactivating the complex. RGS proteins thus exert an inhibitory effect on G protein signalling, a mechanism that is thought to have a regulatory function in many situations ~targets for G proteins- (1) adenylyl cyclase , the enzyme responsible for cAMP formation (2) phospholipase C , the enzyme responsible for inositol phosphate and diacylglycerol (DAG) formation (3) ion channels , particularly calcium and potassium channels (4) Rho A/Rho kinase , a system that regulates the activity of many signalling pathways controlling cell growth and proliferation, smooth muscle contraction, etc. (5) mitogen-activated protein kinase (MAP kinase), a system that controls many cell functions, including cell division.

Protein Phosphorylation and Kinase Cascade Mechanisms

~In many cases, ligand binding to the receptor leads to dimerisation. The association of the two intracellular kinase domains allows a mutual autophosphorylation of intracellular tyrosine residues to occur. The phosphorylated tyrosine residues then serve as high-affinity docking sites for other intracellular proteins that form the next stage in the signal transduction cascade. One important group of such proteins is known as the SH2 domain proteins. These possess a highly conserved sequence of about 100 amino acids, forming a recognition site for the phosphotyrosine residues of the receptor. Individual SH2 domain proteins, of which many are now known, bind selectively to particular receptors, so the pattern of events triggered by particular growth factors is highly specific. [The end result is to activate or inhibit, by phosphorylation, a variety of transcription factors that migrate to the nucleus and suppress or induce the expression of particular genes] ~The membrane-bound form of guanylyl cyclase , the enzyme responsible for generating the second messenger cGMP in response to the binding of natriuretic peptides, resembles the receptor tyrosine kinase family and is activated in a similar way by dimerisation when the agonist is bound.

epithelial ion transport

~In the case of Na + transport, secretion occurs because Na + enters the cell passively at one end and is pumped out actively at the other, with water following passively. Critical to this mechanism is a class of highly regulated epithelial sodium channels (ENaCs) that allow Na + entry [They are regulated mainly by aldosterone, a hormone produced by the adrenal cortex that enhances Na + reabsorption by the kidney- Aldosterone, like other steroid hormones, exerts its effects by regulating gene expression (see Ch. 3 ), and causes an increase in ENaC expression, thereby increasing the rate of Na + and fluid transport] ~Both Na + and Cl − transport are regulated by intracellular messengers, notably by Ca 2+ and cAMP, the latter exerting its effects by activating protein kinases and thereby causing phosphorylation of channels and transporters

co-transmission

~It is the rule rather than the exception that neurons release more than one transmitter or modulator, each of which interacts with specific receptors and produces effects, often both pre- and postsynaptically ~benefits- (1) One constituent of the cocktail (e.g. a peptide) may be removed or inactivated more slowly than the other (e.g. a monoamine), and therefore reach targets further from the site of release and produce longer-lasting effects (2) The balance of the transmitters released may vary under different conditions. At sympathetic nerve terminals, for example, where noradrenaline and NPY are stored in separate vesicles, NPY is preferentially released at high stimulation frequencies, so that differential release of one or other mediator may result from varying impulse patterns. Differential effects of presynaptic modulators are also possible; for example, activation of β adrenoceptors inhibits ATP release while enhancing noradrenaline release from sympathetic nerve terminals ~As well as functioning directly as neurotransmitters, chemical mediators may regulate presynaptic transmitter release and neuronal excitability [Both are examples of neuromodulation and generally involve second messenger regulation of membrane ion channels] ~Inhibitory presynaptic autoreceptors occur on noradrenergic and cholinergic neurons, causing each transmitter to inhibit its own release ( autoinhibitory feedback )

Type 2: G Protein-Coupled Receptors

~Many neurotransmitters, apart from peptides, can interact with both GPCRs and ligand-gated channels, allowing the same molecule to produce fast (through ligand-gated ion channels) and relatively slow (through GPCRs) effects ~Individual peptide hormones, on the other hand, generally act either on GPCRs or on kinase-linked receptors, but rarely on both, and a similar choosiness applies to the many ligands that act on nuclear receptors ~Protease-activated receptors- Many proteases, such as thrombin activate PARs by snipping off the end of the extracellular N-terminal tail of the receptor to expose five or six N-terminal residues that bind to receptor domains in the extracellular loops, functioning as a 'tethered agonist' [A PAR molecule can be activated only once, because the cleavage cannot be reversed, so continuous resynthesis of receptor protein is necessary. Inactivation occurs by a further proteolytic cleavage that frees the tethered ligand, or by desensitisation, involving phosphorylation, after which the receptor is internalised and degraded, to be replaced by newly synthesised protein] ~(1) Structures comprise seven membrane-spanning α-helices, often linked as dimeric structures (2) The third intracellular loop interacts with the G protein (3) The G protein is a membrane protein comprising three subunits (α, β, γ), the α subunit possessing GTPase activity (4) When the trimer binds to an agonist-occupied receptor, the α subunit binds GTP, dissociates and is then free to activate an effector (e.g. a membrane enzyme). In some cases, the βγ subunit is the activator species (5) Activation of the effector is terminated when the bound GTP molecule is hydrolysed, which allows the α subunit to recombine with βγ

α2-Adrenergic Agonists: Clonidine

~Mechanism of Action- Activation of presynaptic α2-AR inhibits the release of noradrenaline, leading to decreased blood pressure and heart rate and decreased aqueous humor production. Smaller doses of clonidine may be used for migraine prophylaxis and the alleviation of symptoms in menopausal flushing. The mechanism of action appears to be modification of the response of blood vessels to vasoconstrictor and vasodilator stimuli including noradrenaline and angiotensin. ~Indications- Hypertension, glaucoma, migraine, menopausal flushing, alcohol withdrawal ~Contraindications- Severe bradyarrhythmia resulting from either sick sinus syndrome or AV block ~Adverse Effects- Dizziness, sedation, orthostatic hypotension, dry mouth, depression, sleep disorders, GI upset, headache, erectile dysfunction, fatigue

α1A-Adrenergic Selective Antagonist: Tamsulosin

~Mechanism of Action- Inhibits smooth muscle contraction. The binding of tamsulosin to α1A-adrenoceptors in the prostate results in relaxation of prostate smooth muscle followed by improvements in urodynamics. ~Indications- Relief of lower urinary tract symptoms (LUTS) associated with benign prostatic hyperplasia (BPH). ~Contraindications- Cataract surgery, A history of orthostatic hypotension ~Adverse Effects- Abnormal ejaculation (failure of ejaculation or retrograde ejaculation)(D3-R), amblyopia (poor pupil dilation) (α1-AR), dizziness, floppy iris syndrome (5HT1A-R), hypotension (with overdose) (α1B-AR), priapism (very rare) (α1-AR) ~Tamsulosin's side effects of abnormal ejaculation and floppy iris syndrome are due to its off-target affinity for the dopmaine D3 receptor and the serotonin 5HT1A receptor

α1-Adrenergic Antagonists: Prazosin

~Mechanism of Action- Inhibits vasoconstriction causing a decrease in total peripheral resistance, inhibits smooth muscle contraction ~Indications- Hypertension, Benign prostatic hyperplasia, Raynaud's phenomenon and Raynaud's disease ~Contraindications- Hypotension ~Adverse Effects- Postural Hypotension (α1), blurred vision (α1), nasal congestion (α1), priapism (a painful penile erection that sustains for hours) (α1), tachycardia, palpitations.

β2-Adrenergic Selective Agonists: Salbutamol & Terbutaline

~Mechanism of Action- Stimulation of β2-AR causes relaxation of the smooth muscle of the bronchi, uterus and blood vessels, it decreases the duration of skeletal muscle contraction and increases glycolysis and glycogenolysis ~Indications- Treatment of bronchospasm in asthma or chronic obstructive pulmonary disease (COPD), pre-term labour ~Contraindications- hypertension or with heart disease, especially in patients with tachyarrhythmias, coronary artery disease, or congestive cardiac failure. ~Adverse Effects- Inhalation: tachycardia (β1), headache, flushing and skeletal muscle tremor (β2), iv/oral: tachycardia, palpitations, peripheral vasodilatation with associated hypotension, flushing and headache, disturbance of carbohydrate metabolism and ketosis (particularly in diabetic patients), hand tremors. obstetric use: as above and increased fetal heart rate ~Route of Administration- Inhalation (limits systemic effects), oral, iv ~Pharmacokinetic parameters Terbutaline is not a substrate for COMT ∴ Decreased MAO metabolism

β3-Adrenergic Selective Agonist: Mirabegron

~Mechanism of Action- Stimulation of β3-AR causes relaxation of bladder smooth muscle, decreasing voiding frequency in patients with over active bladder. ~Indications- Symptomatic treatment of urgency, increased micturition frequency and/or urgency incontinence in patients with overactive bladder (OAB) syndrome ~Precautions- Severe uncontrolled hypertension. Patients taking drugs metabolised by CYP2D6. ~Adverse Effects- Hypertension, tachycardia, nasopharyngitis, urinary tract infection and headache ~Route of Administration- oral

α1-Adrenergic Agonist: Phenylephrine

~Mechanism of Action- Vasoconstriction resulting in redistribution of local blood flow, which reduces oedema of the nasal mucosa, thus improving ventilation, drainage and nasal stuffiness. ~Indications- Nasal decongestion, treatment of uveitis, mydriasis : to facilitate opthalmoscopy & cataract surgery ~Contraindication- Cardiac conditions, antidepressants (monoamine oxidase inhibitors) ~Adverse Effects- Hypertension, tachycardia, arrhythmias, central nervous system stimulation (anxiety, dizziness, nervousness, irritability, excitability, insomnia, restlessness, trembling, headache, hyperactivity). Children and the elderly are more likely to experience adverse effects than other age groups. ~Absorption and metabolism- When taken orally it is metabolised by monoamine oxidase

β1-Adrenergic Selective Antagonists: Metoprolol

~Mechanism of Action- decrease in heart rate > reduction in cardiac output > oxygenation of the myocardium is improved, suppression of renin release from the kidneys > decreased blood pressure. ~Indications- Oral: Hypertension, angina pectoris, myocardial infarction, migraine prophylaxis. Intravenous: Disturbances of cardiac rhythm, in particular supraventricular tachyarrhythmias. ~Contraindications- Congestive heart failure, hypotension ~Adverse Effects- Cardiac failure, bradycardia, cold extremities, hypoglycaemia (at high dose) & bronchospasm (at high dose) dizziness, headache, dyspnoea on exertion β1-AR selectivity reduces pulmonary and glucose metabolism side effects therefore safer for patients with asthma and diabetes

Ganglion Stimulants

~Most nAChR agonists act on either neuronal (ganglionic and CNS) nACh receptors or on striated muscle (motor endplate) receptors but not, apart from nicotine and ACh, on both ~Both sympathetic and parasympathetic ganglia are stimulated, so effects are complex, including tachycardia and increase of blood pressure; variable effects on gastrointestinal motility and secretions; increased bronchial, salivary and sweat secretions. Additional effects result from stimulation of other neuronal structures, including sensory and noradrenergic nerve terminals ~Ganglion-Blocking Drugs- Ganglion block can occur by several mechanisms: (1) By interference with ACh release, as at the neuromuscular junction (2) By prolonged depolarisation. Nicotine can block ganglia, after initial stimulation, in this way, as can ACh itself if cholinesterase is inhibited so that it can exert a continuing action on the postsynaptic membrane (3) By interference with the postsynaptic action of ACh. The few ganglion-blocking drugs of practical importance act by blocking neuronal nAChRs or the associated ion channels. ~In practice, the main effect is a marked fall in arterial blood pressure resulting mainly from block of sympathetic ganglia, which causes arteriolar vasodilatation, and the block of cardiovascular reflexes. In particular, the venoconstriction, which occurs normally when a subject stands up and which is necessary to prevent the central venous pressure (and hence cardiac output) from falling sharply, is reduced. Standing thus causes a sudden fall in arterial pressure ( postural hypotension ) that can cause fainting. Similarly, the vasodilatation of skeletal muscle during exercise is normally accompanied by vasoconstriction elsewhere (e.g. splanchnic area) produced by sympathetic activity. If this adjustment is prevented, the overall peripheral resistance falls and the blood pressure also falls ( postexercise hypotension ).

Regulation of Intracellular Calcium

~Most of the Ca 2+ in a resting cell is sequestered in organelles, particularly the endoplasmic or sarcoplasmic reticulum (ER or SR) and the mitochondria, and the free [Ca 2+ ]i is kept to a low level, about 100 nmol/l [The Ca 2+ concentration in extracellular fluid, [Ca 2+ ]o , is about 2.4 mmol/l, so there is a large concentration gradient favouring Ca 2+ entry] [[Ca 2+ ]i is kept low (a) by the operation of active transport mechanisms that eject cytosolic Ca 2+ through the plasma membrane and pump it into the ER, and (b) by the normally low Ca 2+ permeability of the plasma and ER membranes] ~Regulation of [Ca 2+ ]i involves three main mechanisms: (1) control of Ca 2+ entry (2) control of Ca 2+ extrusion (3) exchange of Ca 2+ between the cytosol and the intracellular stores

Muscarinic and Nicotinic Actions of Acetylcholine

~Muscarinic actions closely resemble the effects of parasympathetic stimulation, as shown in Table 12.1 . After the muscarinic effects have been blocked by atropine , larger doses of ACh produce nicotine-like effects, which include: stimulation of all autonomic ganglia, stimulation of voluntary muscle, secretion of adrenaline from the adrenal medulla. ~Small and medium doses of ACh produce a transient fall in blood pressure due to arteriolar vasodilatation and slowing of the heart - muscarinic effects that are abolished by atropine (muscarinic antagonist). A large dose of ACh given after atropine produces nicotinic effects: an initial rise in blood pressure due to a stimulation of sympathetic ganglia and consequent vasoconstriction, and a secondary rise resulting from secretion of adrenaline ~The muscarinic actions correspond to those of ACh released at postganglionic parasympathetic nerve endings and The nicotinic actions correspond to those of ACh acting on autonomic ganglia of the sympathetic and parasympathetic systems, the motor endplate of voluntary muscle and the secretory cells of the adrenal medulla

muscarinic antagonists

~Muscarinic receptor antagonists ( parasympatholytic drugs ; Table 13.5 ) are competitive antagonists whose chemical structures usually contain ester and basic groups in the same relationship as ACh, but they have a bulky aromatic group in place of the acetyl group ~All the muscarinic antagonists produce basically similar peripheral effects, although some show a degree of selectivity, for example for the heart or bladder, reflecting heterogeneity among mAChRs ~main effects are (1) inhibition of secretions (2) causes tachycardia through block of cardiac mAChRs (3) The pupil is dilated ( mydriasis ) by atropine administration, and becomes unresponsive to light. Relaxation of the ciliary muscle causes paralysis of accommodation ( cycloplegia ), so that near vision is impaired. Intraocular pressure may rise; although this is unimportant in normal individuals, it can be dangerous in patients suffering from narrow-angle glaucoma (4) Gastrointestinal motility is inhibited by atropine (5) Bronchial, biliary and urinary tract smooth muscle are all relaxed by atropine (6) Atropine produces mainly excitatory effects on the CNS. At low doses, this causes mild restlessness; higher doses cause agitation and disorientation Most important compounds are atropine , hyoscine , ipratropium and pirenzepine ~Main effects are inhibition of secretions; tachycardia, pupillary dilatation and paralysis of accommodation; relaxation of smooth muscle (gut, bronchi, biliary tract, bladder); inhibition of gastric acid secretion (especially pirenzepine ); central nervous system effects (mainly excitatory with atropine ; depressant, including amnesia, with hyoscine ), including antiemetic effect and antiparkinsonian effect

Summary - cholinergic drugs

~Parasympathomimetics: drugs act by MIMICKING ACh- (1) Muscarinic receptor agonists: Pilocarpine (treating glaucoma, topical application), Bethanechol (postoperative urinary retention) (2) Nicotinic receptor agonists- Nicotine (an aid for the relief of nicotine withdrawal symptoms) (3) Acetylcholinesterase inhibitors- Physostigmine (glaucoma)- Neostigmine, pyridostigmine (myasthenia gravis) ~Anticholinergics: drugs serve to REDUCE the effects mediated by ACh- (1) Antimuscarinic agents (widely used, incl OAB): atropine, hyoscine, tropicamide, tolterodine, darifenacin (2) Ganglionic blockers- Hexamethonium. The first antihypertensive drug (profound side effects, obsolete) (3) Botulinum toxin: a neurotoxin. It blocks exocytosis of ACh (dystonia, cosmetic surgery, OAB). (4) NMJ blocking drugs: depolarising (succinylcholine) and nondepolarising (tubocurarine) agents. Used as adjunct to general anaesthesia

clinical uses of muscarinic receptor agents

~Pilocarpine, carbachol- agonism- M3- glaucoma; dry mouth ~Bethanechol- agonism- M3- urinary retention ~Atropine- non-selective antagonism- M2, M3- resuscitation (bradycardia); anticholinesterase poisoning; diarrhoea, antispasmodic; in anaesthesia ~Tropicamide- non-selective antagonism- M3- produce mydriasis and cycloplegia in ophthalmologic diagnostics ~Tolterodine, Oxybutynin, Darifenacin- non-selective antagonism- selective M3- overactive bladder ~Pirenzepine- decreases gastric acid secretion- M1- peptic ulcer ~Ipratropium- non-selective antagonism- M3- asthma, bronchitis ~Hyoscine- CNS depression- M3, M5?- motion sickness ~Benzatropine- Cholinergic excess and dopaminergic deficiency- M1, M2- Parkinson's disease

Depolarising blocking agents

~Succinylcholine- A depolarizing neuromuscular blocker. It imitates the action of ACh at NMJs [Ultra short action due to hydrolysis by plasma cholinesterase (butyrylcholinesterase), which has slower action compared to acetylcholinesterase)] [Uses: As a skeletal muscle relaxant during emergency intubation procedures] [Problem: can cause apnea, bradycardia, hyperkalemia]

binding when more than one drug is present

~Suppose that two drugs, A and B, which bind to the same receptor with equilibrium dissociation constants K A and K B , respectively, are present at concentrations x A and x B . If the two drugs compete (i.e. the receptor can accommodate only one at a time), then, by application of the same reasoning as for the one-drug situation described above, the occupancy by drug A is given by pA=(xA/KA)/[(xA/KA)+(xB/KB)+1] [adding drug B, as expected, reduces the occupancy by drug A] [extent of the rightward shift, on a logarithmic scale, represents the ratio ( r A , given by x A ′/ x A where x A ′ is the increased concentration of A) by which the concentration of A must be increased to overcome the competition by B] [rA=(xB/KB)+1 Thus r A depends only on the concentration and equilibrium dissociation constant of the competing drug B, not on the concentration or equilibrium dissociation constant of A] ~For competitive antagonism, r shows the following characteristics: (1) It depends only on the concentration and equilibrium dissociation constant of the antagonist, and not on the size of response that is chosen as a reference point for the measurements (so long as it is submaximal) (2) It does not depend on the equilibrium dissociation constant of the agonist (3) It increases linearly with x B , and the slope of a plot of ( r A −1) against x B is equal to 1/ K B ; this relationship, being independent of the characteristics of the agonist, should be the same for an antagonist against all agonists that act on the same population of receptors

the autonomic nervous system/basic anatomy and physiology (1)

~The autonomic nervous system conveys all the outputs from the central nervous system to the rest of the body, except for the motor innervation of skeletal muscle ~The autonomic nervous system is largely outside the influence of voluntary control. The main processes that it regulates, to a greater or lesser extent, are: (1) contraction and relaxation of vascular and visceral smooth muscle (2) all exocrine and certain endocrine secretions (3) the heartbeat (4) energy metabolism, particularly in liver and skeletal muscle. ~the autonomic efferent pathway consists of two neurons arranged in series, whereas in the somatic motor system a single motor neuron connects the central nervous system to the skeletal muscle fibre [The two neurons in the autonomic pathway are known, respectively, as preganglionic and postganglionic] [In the sympathetic nervous system, the intervening synapses lie in autonomic ganglia, which are outside the central nervous system, and contain the nerve endings of preganglionic fibres and the cell bodies of postganglionic neurons. In parasympathetic pathways, the postganglionic cells are mainly found in the target organs] ~The cell bodies of the sympathetic preganglionic neurons lie in the lateral horn of the grey matter of the thoracic and lumbar segments of the spinal cord, and the fibres leave the spinal cord in the spinal nerves as the thoracolumbar sympathetic outflow . The preganglionic fibres synapse in the paravertebral chains of sympathetic ganglia, lying on either side of the spinal column. These ganglia contain the cell bodies of the postganglionic sympathetic neurons, the axons of which rejoin the spinal nerve. Many of the postganglionic sympathetic fibres reach their peripheral destinations via the branches of the spinal nerves

cholinergic transmission (2)

~The cholinergic system includes: The entire parasympathetic nervous system, The preganglionic neurons of the sympathetic nervous system, The postganglionic sympathetic neurons innervating sweat glands, Somatic motor neurons, The CNS ~At the autonomic level: the cholinergic controls smooth muscle activity, exocrine (and some endocrine) secretions, heart rate and cardiac output, certain metabolic processes [At somatic motor neurons, ACh trigger the contraction of skeletal muscle fibres] [In CNS, the cholinergic regulates plasticity, arousal and reward.] ~drugs that target cholinergic transmission- 1. Inhibition of Re-uptake: hemicholinium 2.Blockage of vasicular ACh transport (VAT): vesamicol 3.Inhibition of exocytosis of ACh containing vesicles: Botulinum toxin and calcium channel blockers 4.Receptor agonists/antagonists 5.Acetylcholinesterase inhibitors ~Drugs act by STIMULATING or MIMICKING ACh- (1) Muscarinic receptor agonists- Pilocarpine (treating glaucoma, topical application), Bethanechol (postoperative urinary retention) (2) Nicotinic receptor agonists- Nicotine (an aid for the relief of nicotine withdrawal symptoms) (3) Acetylcholinesterase inhibitors- Physostigmine (glaucoma), Neostigmine, pyridostigmine (myasthenia gravis) ~Drugs serve to REDUCE the effects mediated by ACh- (1) Antimuscarinic agents (widely used): atropine (2) Botulinum toxin: blocks exocytosis of ACh [Clinical uses: dystonia, cosmetic surgery, urinary incontinence] (3) Neuromuscular blocking drugs: depolarising and nondepolarising agents. Used as adjunct to general anaesthesia (4) Ganglionic blockers: hexamethonium ~receptors- (1) muscarinic [GPCRs, respond to ACh via IP3 and cAMP, located in periphreal tissues and CNS, M1 (CNS, glands), M2 (heart and SM), M3 (SM and glands), M4 (nerve cells), M5 (?)] [M1, M3 and M5 are coupled to activation of the Gq/11-phosphatidyl inositol pathway to increase Ca++ influx while M2 and M4 are linked to Gi and adenylyl cyclase inhibition to reduce cAMP formation] [heart-bradycardia (M2), blood vessels- vasodilation (M3), lungs- constriction (M3), gastro- contraction (M3), bladder- contraction (M3), glands- increase secretion (M3), stomach- increase HCl secretion (M1)] (2) nicotinic [ligand gated ion channels, 3 classes are muscle/ganglionic/CNS types, respond to ACh by increasing cation permeability and mediating fast excitatory synaptic transmission, located in ANS ganglia/CNS/neuromuscular junctions of somatic nervous system] [receptors are Nm (neuromuscular) and Nn (autonomic ganglia, medulla, CNS)]

muscarinic receptor antagonists

~The common effects after M receptor blockage: (1) CNS: anti-tremor and used against motion sickness (2) visual system: pupil dilation and poor accommodation (muscle action) [decreases secretions:- tears, saliva, gastric acid, sweat] [CVS: tachycardia, little effect on BP] [bladder: - reduced micturition] [GIT: - relaxation] ~atropine- (1) Resuscitation: Injection of atropine is used in the treatment of bradycardia in cardiac arrest after myocardial infarction (2) Premedication in anaesthesia: inhibition of bronchial and salivary secretions, and reflex bronchoconstriction (3) Antidote for organophosphate poisoning and AChE inhibitor overdose: Some insecticides and nerve gases destroy acetylcholinesterase, so the action of ACh becomes prolonged - Cholinergic crisis (4) Anti-spasmodic and anti-diarrhoea: GIT inhibition (5) Mydriatic to facilitate eye examination (but tropicamide are generally preferred due to its short duration of effect (4 - 8 hours). ~Other clinical uses of antimuscarinics- Overactive bladder (urge incontinence) (e.g. oxybutynin, tolterodine, darifenacin), Motion sickness (hyoscine, TravelCam), Parkinson's (benztropine), Asthma (ipratropium, a bronchodilator), Peptic ulcer (pirenzepine, M1 selective, is used to reduce gastric acid secretion) ~Side effects of muscarinic antagonists- Gut motility is reduced leading to constipation, Retention of urine, Blurred vision/photophobia, Exocrine gland secretions are inhibited: decreased saliva secretion leading to dry mouth decreased sweating leading to hyperthermia [Most clinically useful agents do not to cross the blood brain barrier (BBB) but high doses of antagonists that do cross BBB can cause CNS excitation (agitation, disorientation, hyperactivity)] [Toxic doses lead to depression, circulatory and respiratory failure (treated with physostigmine, the AChE inhibitor).]

muscle contraction (2)

~The contractile machinery of smooth muscle is activated when the myosin light chain undergoes phosphorylation, causing it to become detached from the actin filaments. This phosphorylation is catalysed by a kinase, myosin light-chain kinase (MLCK), which is activated when it binds to Ca 2+ -calmodulin (see p. 61 , Fig. 4.9 ). A second enzyme, myosin phosphatase , reverses the phosphorylation and causes relaxation ~In smooth muscle, contraction can occur without action potentials, for example when agonists at G protein-coupled receptors lead to IP 3 formation

Quantitative Aspects of Drug-Receptor Interactions

~The first step in drug action on specific receptors is the formation of a reversible drug-receptor complex ~Suppose that a piece of tissue, such as heart muscle or smooth muscle, contains a total number of receptors, N tot , for an agonist such as adrenaline. When the tissue is exposed to adrenaline at concentration x A and allowed to come to equilibrium, a certain number, N A , of the receptors will become occupied, and the number of vacant receptors will be reduced to N tot − N A [Normally, the number of adrenaline molecules applied to the tissue in solution greatly exceeds N tot , so that the binding reaction does not appreciably reduce x A] [The magnitude of the response produced by the adrenaline will be related (even if we do not know exactly how) to the number of receptors occupied, so it is useful to consider what quantitative relationship is predicted between N A and x A] ~A (drug Xa) + R (free receptor Ntot - Na) ⇌ AR (complex Na) [The Law of Mass Action (which states that the rate of a chemical reaction is proportional to the product of the concentrations of reactants) can be applied to this reaction] ~Rate of forward reaction = k+1*xA*(Ntot−NA) and Rate of backward reaction = k−1*NA [at equilibrium the 2 rates are equal k+1xA(Ntot−NA)=k−1NA] ~ equilibrium dissociation constant ( K A )= KA=k−1/k+1=xA(Ntot−NA)/NA [equilibrium dissociation constant , K A , is a characteristic of the drug and of the receptor; it has the dimensions of concentration and is numerically equal to the concentration of drug required to occupy 50% of the sites at equilibrium] [The higher the affinity of the drug for the receptors, the lower will be the value of K A] ~The proportion of receptors occupied, or occupancy ( p A ), is N A / N tot , which is independent of N tot [pA=xA/(xA+k−1/k+1=xA/xA+KA] [Thus if the equilibrium dissociation constant of a drug is known we can calculate the proportion of receptors it will occupy at any concentration- pA=(xA/KA)/(xA/KA)+1] ~In this case, the relationship between the amount bound ( B ) and ligand concentration ( x A ) should be: B=Bmax*xA/(xA+KA) where B max is the total number of binding sites in the preparation [to display the results in linear form it may be rearranged into B/xA=Bmax/(KA−B/KA)] [A plot of B / x A against B (known as a Scatchard plot ) gives a straight line from which both B max and K A can be estimated]

pharmacology of ion channels

~The gating and permeation of both voltage-gated and ligand-gated ion channels is modulated by many factors (1) Ligands that bind directly to various sites on the channel protein (by blocking the channel or by affecting the gating process, thereby either facilitating or inhibiting the opening of the channel) (2) Mediators and drugs that act indirectly, mainly by activation of GPCRs [by affecting the state of phosphorylation of individual amino acids located on the intracellular region of the channel protein- this modulation involves the production of second messengers that activate protein kinases] [The opening of the channel may be facilitated or inhibited, depending on which residues are phosphorylated] (3) Intracellular signals, particularly Ca 2+ and nucleotides such as ATP and GTP [Many ion channels possess binding sites for these intracellular mediators] ~Short-term regulation of receptor function generally occurs through desensitisation , as discussed above. Long-term regulation occurs through an increase or decrease of receptor expression

calcium release mechanisms

~The inositol trisphosphate receptor (IP3R) is activated by inositol trisphosphate (IP3), a second messenger produced by the action of many ligands on G protein-coupled receptors. IP3R is a ligand-gated ion channel, although its molecular structure differs from that of ligand-gated channels in the plasma membrane. This is the main mechanism by which activation of G protein-coupled receptors causes an increase in [Ca 2+ ] i . ~Ryanodine receptors (RyR)- In skeletal muscle RyRs on the SR are physically coupled to dihydropyridine receptors on the T-tubules; this coupling results in Ca 2+ release following the action potential in the muscle fibre. In other muscle types RyRs respond to Ca 2+ that enters the cell through membrane calcium channels by a mechanism known as calcium-induced calcium release (CICR) ~other second messengers- cADPR acts by increasing the sensitivity of RyRs to Ca 2+ , thus increasing the 'gain' of the CICR effect. NAADP releases Ca 2+ from lysosomes by activating two-pore domain calcium channels ~Under normal conditions, mitochondria accumulate Ca 2+ passively as a result of the intramitochondrial potential, which is strongly negative with respect to the cytosol. This negativity is maintained by active extrusion of protons, and is lost - thus releasing Ca 2+ into the cytosol - if the cell runs short of ATP, for example under conditions of hypoxia ~Calmodulin is a dimer, with four Ca 2+ binding sites. When all are occupied, it undergoes a conformational change, exposing a 'sticky' hydrophobic domain that lures many proteins into association, thereby affecting their functional properties

the autonomic nervous system/basic anatomy and physiology (2)

~The parasympathetic nerves emerge from two separate regions of the central nervous system. The cranial outflow consists of preganglionic fibres in certain cranial nerves, namely the oculomotor nerve (carrying parasympathetic fibres destined for the eye), the facial and glossopharyngeal nerves (carrying fibres to the salivary glands and the nasopharynx), and the vagus nerve (carrying fibres to the thoracic and abdominal viscera). The ganglia lie scattered in close relation to the target organs; the postganglionic neurons are very short compared with those of the sympathetic system. Parasympathetic fibres destined for the pelvic and abdominal viscera emerge as the sacral outflow from the spinal cord in a bundle of nerves known as the nervi erigentes ~In some places (e.g. in the visceral smooth muscle of the gut and bladder, and in the heart), the sympathetic and the parasympathetic systems produce opposite effects, but there are others where only one division of the autonomic system operates. The sweat glands and most blood vessels , for example, have only a sympathetic innervation, whereas the ciliary muscle of the eye has only a parasympathetic innervation ~Sympathetic activity increases in stress ('fight or flight' response), whereas parasympathetic activity predominates during satiation and repose. Both systems exert a continuous physiological control of specific organs under normal conditions, when the body is at neither extreme

two state receptor model

~The receptor is shown in two conformational states, 'resting' (R) and 'activated' (R*), which exist in equilibrium [Normally, when no ligand is present, the equilibrium lies far to the left, and few receptors are found in the R* state] [For constitutively active receptors, an appreciable proportion of receptors adopt the R* conformation in the absence of any ligand] [Agonists have higher affinity for R* than for R, so shift the equilibrium towards R*] [The greater the relative affinity for R* with respect to R, the greater the efficacy of the agonist] [An inverse agonist has higher affinity for R than for R* and so shifts the equilibrium to the left] [A 'neutral' antagonist has equal affinity for R and R* so does not by itself affect the conformational equilibrium but reduces by competition the binding of other ligands] ~If its preference for R* is very large, nearly all the occupied receptors will adopt the R* conformation and the drug will be a full agonist (positive efficacy); if it shows only a modest degree of selectivity for R* (say 5-10-fold), a smaller proportion of occupied receptors will adopt the R* conformation and it will be a partial agonist; if it shows no preference, the prevailing R : R* equilibrium will not be disturbed and the drug will be a neutral antagonist (zero efficacy), whereas if it shows selectivity for R it will shift the equilibrium towards R and be an inverse agonist (negative efficacy) [We can therefore think of efficacy as a property determined by the relative affinity of a ligand for R and R*]

Calcium Entry Mechanisms

~There are four main routes by which Ca 2+ enters cells across the plasma membrane (1) voltage-gated calcium channels (2) ligand-gated calcium channels (3) store-operated calcium channels (SOCs) (4) Na + -Ca 2+ exchange (which works both ways so can also put calcium out) ~voltage gated calcium channels- vertebrate cells also possess voltage-activated calcium channels capable of allowing substantial amounts of Ca 2+ to enter the cell when the membrane is depolarised. These voltage-gated channels are highly selective for Ca 2+ and do not conduct Na + or K + ; they are ubiquitous in excitable cells and cause Ca 2+ to enter the cell whenever the membrane is depolarised, for example by a conducted action potential ~ligand gated channels- Most ligand-gated cation channels that are activated by excitatory neurotransmitters are relatively non-selective, and conduct Ca 2+ ions as well as other cations ~store operated calcium channels- SOCs are very low-conductance channels that occur in the plasma membrane and open to allow entry when the ER stores are depleted, but are not sensitive to cytosolic [Ca 2+ ] i [Like the ER and SR channels, these channels can serve to amplify the rise in [Ca 2+ ] i resulting from Ca 2+ release from the stores]

Type 3: Kinase-Linked and Related Receptors

~They are activated by a wide variety of protein mediators, including growth factors and cytokines, and hormones such as insulin and leptin, whose effects are exerted mainly at the level of gene transcription. Most of these receptors are large proteins consisting of a single chain of up to 1000 residues, with a single membrane-spanning helical region, linking a large extracellular ligand-binding domain to an intracellular domain of variable size and function ~Receptor tyrosine kinases (RTKs)- These receptors have a tyrosine kinase moiety in the intracellular region/They include receptors for many growth factors [(1) growth factor binds (2) conformation change dimerization (3) tyrosine autophosphorylation (4) phosphorylation of protein Grb2 (5) activation of RAS (GDP/GTP exchange) (6) activation of kinase cascade (Raf > Mek > MAP kinase > various transcription factors > gene transcription)] ~Receptor serine/threonine kinases . This smaller class is similar in structure to RTKs but they phosphorylate serine and/or threonine residues rather than tyrosine. The main example is the receptor for transforming growth factor (TGF) ~Cytokine receptors- These receptors lack intrinsic enzyme activity/When occupied they activate various tyrosine kinases [(1) cytokine binds (2) dimerization conformation change and activation of JAK (tyrosine kinase) (3) phosphorylation of receptor and Jak (4) binding and phosphorylation of protein Stat (5) dimerization of Stat (6) gene transcription] ~Signal transduction generally involves dimerisation of receptors, followed by autophosphorylation of tyrosine residues. The phosphotyrosine residues act as acceptors for the SH2 domains of a variety of intracellular proteins, thereby allowing control of many cell functions [They are involved mainly in events controlling cell growth and differentiation, and act indirectly by regulating gene transcription] [Two important pathways are (1) the Ras/Raf/mitogen-activated protein (MAP) kinase pathway, which is important in cell division, growth and differentiation (2) the Jak/Stat pathway activated by many cytokines, which controls the synthesis and release of many inflammatory mediators] ~There are four main types of catalytic receptors- (1) Receptor Tyrosine Kinase (RTK): Contain intrinsic tyrosine kinase activity (EGF, VEGF, Insulin) (2) Receptor Serine/Threonine Kinase: Contain intrinsic serine/threonine kinase activity (TGF-β) (3) Tyrosine-Kinase Associated Receptors: Receptors that associate with proteins that have tyrosine kinase activity (Cytokine Receptors) (4) Receptor Guanylyl Cyclases: Contain intrinsic cyclase activity (ANP) ~Catalytic (kinase-linked) receptors dimerise when bound to ligand to form the functional receptor

receptor proteins

~Type 1: ligand-gated ion channels (also known as ionotropic receptors)- receptors on which fast neurotransmitters act [cause hyperpolarization/depolarization of cell which causes cellular affects- acts in milliseconds] ~Type 2: G protein-coupled receptors (GPCRs). These are also known as metabotropic receptors or 7-transmembrane (7-TM or heptahelical) receptors . They are membrane receptors that are coupled to intracellular effector systems primarily via a G protein [receptors for hormones or slow transmitters] [G proteins cause second messengers to increase/decrease Ca2+ or protein phosphorylation, etc which causes cellular affects] [time scale of seconds] ~Type 3: kinase-linked and related receptors- They comprise an extracellular ligand-binding domain linked to an intracellular domain by a single transmembrane helix. In many cases, the intracellular domain is enzymic in nature (with protein kinase or guanylyl cyclase activity) [protein phosphorylation causes gene transcription which causes protein synthesis which causes cellular affects] [timescale of hours] ~Type 4: nuclear receptors. These are receptors that regulate gene transcription [Receptors of this type also recognise many foreign molecules, inducing the expression of enzymes that metabolise them] [bound receptors move into the nucleus and bind to DNA changing gene transcription] [time scale of hours] ~ Much of the sequence variation that accounts for receptor diversity arises at the genomic level, i.e. different genes give rise to distinct receptor subtypes. Additional variation arises from alternative mRNA splicing, which means that a single gene can give rise to more than one receptor isoform

excitation

~Under resting conditions, all cells maintain a negative internal potential between about −30 mV and −80 mV, depending on the cell type. This arises because (a) the membrane is relatively impermeable to Na + , and (b) Na + ions are actively extruded from the cell in exchange for K + ions by an energy-dependent transporter, the Na + pump (or Na + -K + -ATPase). The result is that the intracellular K + concentration, [K + ] i , is higher, and [Na + ] i is lower, than the respective extracellular concentrations ~the action potential is generated by the interplay of two processes: 1. a rapid, transient increase in Na + permeability that occurs when the membrane is depolarised beyond about −50 mV 2. a slower, sustained increase in K + permeability. [Because of the inequality of Na + and K + concentrations on the two sides of the membrane, an increase in Na + permeability causes an inward (depolarising) current of Na + ions, whereas an increase in K + permeability causes an outward (repolarising) current] [he first event is a small depolarisation of the membrane, produced either by transmitter action or by the approach of an action potential passing along the axon. This opens sodium channels, allowing an inward current of Na + ions to flow, which depolarises the membrane still further. The process is thus a regenerative one, and the increase in Na + permeability is enough to bring the membrane potential close to E Na . The increased Na + conductance is transient, because the channels inactivate rapidly and the membrane returns to its resting state] [repolarisation is assisted by the opening of voltage-dependent K + channels. These function in much the same way as sodium channels, but their activation kinetics are about 10 times slower and they do not inactivate appreciably. This means that the potassium channels open later than the sodium channels, and contribute to the rapid termination of the action potential]

Type 4: Nuclear Receptors (1)

~Unlike the other receptors described in this chapter, the NRs can directly interact with DNA. For this reason we should really regard them as ligand-activated transcription factors that transduce signals by modifying gene transcription. Another unique property is that NRs are not embedded in membranes like GPCRs or ion channels, but are present in the soluble phase of the cell. Some, such as the steroid receptors, become mobile in the presence of their ligand and can translocate from the cytoplasm to the nucleus, while others, such as the RXR, probably dwell mainly within the nuclear compartment. ~structure of nuclear receptors- The heterogenous N-terminal domain harbours the AF1 (activation function 1) site. This binds cell-specific transcription factors that modify the properties of the receptor. The highly conserved core domain comprises two 'zinc fingers'; cysteine- (or cystine-/histidine-) rich loops in the amino acid chain that are held in a particular conformation by zinc ions and which are responsible for DNA recognition and binding. The flexible hinge region in the molecule allows the receptor to dimerise with other NRs, and the C-terminal domain, which contains the ligand-binding module, is specific to each class of receptor ~Once in the nucleus, the ligand-bound receptor recruits large complexes of other proteins including co-activators or co-repressors to modify gene expression through its AF1 and AF2 domains

partial agonists

~ability of a drug molecule to activate the receptor is actually a graded, rather than an all-or-nothing, property ~Some compounds (known as full agonists ) can produce a maximal response (the largest response that the tissue is capable of giving), whereas others ( partial agonists ) can produce only a submaximal response ~The lower the efficacy of the drug the lower the maximum response and slope of the log concentration-response curve [efficacy describes the 'strength' of the agonist-receptor complex in evoking a response of the tissue] [efficacy describes the tendency of the drug-receptor complex to adopt the active (AR*), rather than the resting (AR), state] [A drug with zero efficacy ( e = 0) has no tendency to cause receptor activation, and causes no tissue response] [A full agonist is a drug whose efficacy is sufficient that it produces a maximal response when less than 100% of receptors are occupied] [A partial agonist has lower efficacy, such that 100% occupancy elicits only a submaximal response] ~intrinsic efficacy of the tissue- depending on tissue characteristics, a given drug may appear as a full agonist in one tissue but a partial agonist in another, and drugs may differ in their relative agonist potencies in different tissues, though the receptor is the same

channel function

~action potentials are initiated by membrane currents that cause depolarisation of the cell. These currents may be produced by synaptic activity, by an action potential approaching from another part of the cell, by a sensory stimulus or by spontaneous pacemaker activity. The tendency of such currents to initiate an action potential is governed by the excitability of the cell, which depends mainly on the state of (a) the voltage-gated sodium and/or calcium channels, and (b) the potassium channels of the resting membrane ~Anything that increases the number of available sodium or calcium channels, or reduces their activation threshold, will tend to increase excitability, whereas increasing the resting K + conductance reduces it ~Voltage-gated channels can exist in three functional states ( Fig. 4.8 ): resting (the closed state that prevails at the normal resting potential), activated (the open state favoured by brief depolarisation) and inactivated (the blocked state resulting from a trap door-like occlusion of the open channel) [After the action potential has passed, many sodium channels are in the inactivated state; after the membrane potential returns to its resting value, the inactivated channels take time to revert to the resting state and thus become available for activation once more. In the meantime, the membrane is temporarily refractory . Each action potential causes the channels to cycle through these states. The duration of the refractory period determines the maximum frequency at which action potentials can occur] ~Of particular importance are drugs that bind most strongly to the inactivated state of the channel and thus favour the adoption of this state, thus prolonging the refractory period and reducing the maximum frequency at which action potentials can be generated. This type of block is called use dependent , because the binding of such drugs increases as a function of the rate of action potential discharge, which governs the rate at which inactivated - and therefore drug-sensitive - channels are generated ~voltage dependance- Most sodium channel-blocking drugs are cationic at physiological pH and are therefore affected by the voltage gradient across the cell membrane. They block the channel from the inside, so that their blocking action is favoured by depolarisation

adrenergic receptor (lecture notes)

~adrenaline vs noradrenaline potency- (1) α1 noradrenaline ≥ adrenaline (2) α2 noradrenaline ≥ adrenaline (3) β1 noradrenaline = adrenaline (4) β2 adrenaline > noradrenaline (5) β3 noradrenaline ≥ adrenaline ~effects- (1) heart- b1 increases heart rate, velocity and contractility (2) blood vessels- a1 increases constriction, b2 dilation (3) kidneys- b1 increases renin (4) skeletal muscle- a1 constriction, b2 dilation (5) lungs- a1 bronchoconstriction, b2 bronchodilation (6) eyes- a1 constriction of radial muscle, b2 ciliary muscle relaxation (7) liver- a1/b2 increases gluconeogenesis and glycogenolysis (8) pancreas- a2 decreases insulin and b2 increases insulin (9) fat- b3 lipolysis/thermogenesis (10) GI tract- a1 sphincter contraction, b2 decreased tone and motility (11) prostate- a1 prostate contraction (12) bladder- a1 sphincter contraction, b3 detrusor relaxation ~Isoprenaline β agonist, Dobutamine β1 agonist, Salbutamol&Terbutaline β2 agonist, Mirabegron β3 agonist ~Phenylephrine α1 agonist, Clonidine α2 agonist, Propranolol β antagonist, Metoprolol β1 antagonist, Prazosin α1 antagonist, Tamsulosin α1A antagonist

The adenylyl cyclase/cAMP system

~cAMP is a nucleotide synthesised within the cell from ATP by the action of a membrane-bound enzyme, adenylyl cyclase. It is produced continuously and inactivated by hydrolysis to 5′-AMP by the action of a family of enzymes known as phosphodiesterases [Many different drugs, hormones and neurotransmitters act on GPCRs and increase or decrease the catalytic activity of adenylyl cyclase, thus raising or lowering the concentration of cAMP within the cell] ~increased cAMP increases the amount of PKA which activates lipase (increased lipolysis), inactivates glycogen synthase (reduced glycogen synthesis), activates phosphorylase kinase which turns glycogen into glucose ~Other examples of regulation by cAMP-dependent protein kinases include the increased activity of voltage-gated calcium channels in heart muscle cells- Phosphorylation (from increased amounts of PKA) of these channels increases the amount of Ca 2+ entering the cell during the action potential, and thus increases the force of contraction of the heart ~In smooth muscle, cAMP-dependent protein kinase phosphorylates (thereby inactivating) another enzyme, myosin light-chain kinase , which is required for contraction. This accounts for the smooth muscle relaxation produced by many drugs that increase cAMP production in smooth muscle ~receptors linked to Gi inhibit cAMP and receptors linked to Gs activate cAMP

Type 1: Ligand-Gated Ion Channels

~consists of a pentameric assembly of different subunits, of which there are four types, termed α, β, γ and δ, each of molecular weight ( M r ) 40-58 kDa [each contains four membrane-spanning α-helices, inserted into the membrane] ~The pentameric structure (α 2 , β, γ, δ) possesses two acetylcholine binding sites, each lying at the interface between one of the two α subunits and its neighbour. Both must bind acetylcholine molecules in order for the receptor to be activated ~When acetylcholine molecules bind, a conformation change occurs in the extracellular part of the receptor, which twists the α subunits, causing the kinked M 2 segments to swivel out of the way, thus opening the channel ~The use of site-directed mutagenesis, which enables short regions, or single residues, of the amino acid sequence to be altered, has shown that a mutation of a critical residue in the M 2 helix changes the channel from being cation selective (hence excitatory in the context of synaptic function) to being anion selective (typical of receptors for inhibitory transmitters such as GABA and glycine). Other mutations affect properties such as gating and desensitisation of ligand-gated channels ~Binding of one agonist molecule to one site increases the affinity of binding at the other site (positive co-operativity) and both sites need to be bound for the receptor to be activated and the channel to open. ~Receptors of this type control the fastest synaptic events in the nervous system, in which a neurotransmitter acts on the postsynaptic membrane of a nerve or muscle cell and transiently increases its permeability to particular ions [cause an increase in Na + and K + permeability and in some instances Ca 2+ permeability. At negative membrane potentials this results in a net inward current carried mainly by Na + , which depolarises the cell and increases the probability that it will generate an action potential]

pharmacology intro

~drug- chemical substance of known structure, other than a nutrient or an essential dietary ingredient which, when administered to a living organism, produces a biological effect [may be synthetic chemicals, chemicals obtained from plants or animals, or products of genetic engineering] ~medicine is a chemical preparation, which usually, but not necessarily, contains one or more drugs, administered with the intention of producing a therapeutic effect [Medicines usually contain other substances (excipients, stabilisers, solvents, etc.) besides the active drug, to make them more convenient to use] ~drug molecules must exert some chemical influence on one or more cell constituents in order to produce a pharmacological response [drug molecules must get so close to these constituent cellular molecules that the two interact chemically in such a way that the function of the latter is altered] [non-uniform distribution of the drug molecule within the body or tissue, which is the same as saying that drug molecules must be 'bound' to particular constituents of cells and tissues in order to produce an effect] ~pharmacodynamics- what the drug does to the body (The study of the molecular, biochemical, and physiological effects of drugs on cellular systems and their mechanisms of action) [pharmacokinetics- what the body does to the drug (The study of the absorption, distribution, and excretion of drugs)] ~Drugs with activity at high concentrations -little structural specificity. Cause physical change (general anaesthetics). [Drugs acting at low concentrations - structural specificity. Act by chemical rather than physical interaction (Isoprenaline).] ~Agonists mimic endogenous ligands. (They bind to a receptor and cause a secondary effect.) [An antagonist binds to a receptor and prevents the action of an agonist. (Most antagonists are competitive and reversible.)] ~Drug - receptor interaction: (1) Assumed that the effect of a drug is proportional to the fraction of receptors occupied. (2) Assumed that the maximal effects occurs when all receptors are occupied. [These assumptions are not always true.]

Effects of Drugs on Cholinergic Transmission

~drugs can influence cholinergic transmission either by acting on postsynaptic ACh receptors as agonists or antagonists, or by affecting the release or destruction of endogenous ACh. ~(1) muscarinic agonists (2) muscarinic antagonists (3) ganglion-stimulating drugs (4) ganglion-blocking drugs (5) neuromuscular-blocking drugs (6) anticholinesterases and other drugs that enhance cholinergic transmission.

competitive antagonism

~in the presence of a competitive antagonist, the agonist occupancy (i.e. proportion of receptors to which the agonist is bound) at a given agonist concentration is reduced, because the receptor can accommodate only one molecule at a time. However, because the two are in competition, raising the agonist concentration can restore the agonist occupancy (and hence the tissue response). The antagonism is therefore said to be surmountable , in contrast to other types of antagonism (see below) where increasing the agonist concentration fails to overcome the blocking effect ~predicts that in the presence of a fixed concentration of the antagonist, the log concentration-effect curve for the agonist will be shifted to the right, without any change in slope or maximum - the hallmark of competitive antagonism [shift is expressed as a dose ratio , r (the ratio by which the agonist concentration has to be increased in the presence of the antagonist in order to restore a given level of response). Theory predicts that the dose ratio increases linearly with the concentration of the antagonist] ~Irreversible competitive (or non-equilibrium ) antagonism occurs when the antagonist binds to the same site on the receptor as the agonist but dissociates very slowly, or not at all, from the receptors, with the result that no change in the antagonist occupancy takes place when the agonist is applied (eversible competitive antagonism described above reflect the fact that agonist and competitive antagonist molecules do not stay bound to the receptor but bind and rebind continuously) [Irreversible competitive antagonism occurs with drugs that possess reactive groups that form covalent bonds with the receptor]

autonomic pharmacology

~nervous system- central nervous system (brain and spinal cord) and peripheral nervous system [splits into somatic nervous system (innervates skeletal muscle) and autonomic nervous system (innervates visceral organs)] [autonomic is split into sympathetic and parasympathetic] ~somatic only has one neuron that connects spinal cord to muscle while autonomic has 2 neurons (postganglionic and preganglionic) ~The general principles of ANS- (1) Involuntary control - self governing (2) Dual innervations - organs receive impulses from both sympathetic and parasympathetic motor neurons (3) Antagonistic actions - excitatory v.s. inhibitory, leading to a physiological balance - homeostasis ~sympathetic- short preganglionic fibers, long postganglionic fibers, ACh transmitter at ganglia, norepinephrine (NE) or noradrenaline(NA) at effectors, nicotinic receptors at ganglia and alpha/beta andregenic at effectors ~parasympathetic- long preganglionic fibers, short postganglionic fibers, ACh transmitter, nicotinic receptors at ganglia, muscarinic receptors at effectors ~parasympathetic response- decreased heart rate, decreased cardiac output, no effect on blood vessels, constricts pupils, bronchiole constriction, increased gastric acid secretion, contracts smooth muscle [sympathetic response- increased heart rate, increased cardiac output, constriction of blood vessels in visceral organs and skin, dilation of blood vessels in muscle, dilates pupils, dilation of bronchioles, decreased urination, relaxes smooth muscle] ~Most internal organs are under antagonistic control, in which one autonomic branch is inhibitory and the other excitatory [Exceptions: sweat glands and the smooth muscle in most blood vessels. These are innervated only by the sympathetic branch and rely on tonic (up and down) control.]

nicotinic receptors agonists

~nicotine- Clinical use of nicotine is to treat nicotine dependence in order to quit smoking [Pharmacologic effects (1) Stimulation followed by depression at nicotinic receptors (2) Strong psychologic, physical dependence] [Delayed adverse effects of tobacco use (1) Lung disease: cancer, chronic obstructive pulmonary disease, chronic cough (2) Heart disease: atherosclerosis] ~Varenicline- a partial agonist at alpha4beta2 neuronal nicotinic receptors. It has two ways of action: (1) Agonist action: it selectively stimulates the receptors to alleviate symptoms of craving and withdrawal (2) Antagonist action: It also results in blockade of the rewarding and reinforcing effects of smoking by preventing nicotine binding to the receptors

acetylcholine receptors

~nicotinic receptors- the 3 types are Muscle receptors are confined to the skeletal neuromuscular junction; ganglionic receptors are responsible for transmission at sympathetic and parasympathetic ganglia; and CNS-type receptors are widespread in the brain ~All nAChRs are pentameric structures that function as ligand-gated ion channels [The two binding sites for ACh (both of which need to be occupied to cause the channel to open) reside at the interface between the extracellular domain of each of the α subunits and its neighbour] ~muscarinic receptors- Muscarinic receptors (mAChRs) are typical G protein-coupled receptors (see Ch. 3 ), and five molecular subtypes (M 1 -M 5 ) are known. The odd-numbered members of the group (M 1 , M 3 , M 5 ) couple with G q to activate the inositol phosphate pathway ( Ch. 3 ), while the even-numbered receptors (M 2 , M 4 ) open potassium (K ATP ) channels causing membrane hyperpolarisation as well as acting through G i to inhibit adenylyl cyclase and thus reduce intracellular cAMP. Both groups activate the MAP kinase pathway ~M 1 receptors ('neural') are found mainly on CNS and peripheral neurons and on gastric parietal cells. They mediate excitatory effects [This excitation is produced by a decrease in K + conductance, which causes membrane depolarisation] ~M 2 receptors ( 'cardiac' ) occur in the heart, and also on the presynaptic terminals of peripheral and central neurons. They exert inhibitory effects, mainly by increasing K + conductance and by inhibiting calcium channels [M 2 -receptor activation is responsible for cholinergic inhibition of the heart, as well as presynaptic inhibition in the CNS and periphery ( Ch. 12 ). They are also co-expressed with M 3 receptors in visceral smooth muscle, and contribute to the smooth-muscle-stimulating effect of muscarinic agonists in several organs] ~M 3 receptors ( 'glandular/smooth muscle' ) produce mainly excitatory effects, i.e. stimulation of glandular secretions (salivary, bronchial, sweat, etc.) and contraction of visceral smooth muscle. M 3 receptors also mediate relaxation of smooth muscle (mainly vascular), which results from the release of nitric oxide from neighbouring endothelial cells

Catecholamines

~noradrenaline ( norepinephrine ), a transmitter released by sympathetic nerve terminals ~adrenaline ( epinephrine ), a hormone secreted by the adrenal medulla ~dopamine , the metabolic precursor of noradrenaline and adrenaline, also a transmitter/neuromodulator in the central nervous system ~isoprenaline ( isoproterenol ), a synthetic derivative of noradrenaline, not present in the body.

noradrenergic transmission

~noradrenaline, by acting on presynaptic β 2 receptors, can regulate its own release, and also that of co-released ATP (see Ch. 12 ). This is believed to occur physiologically, so that released noradrenaline exerts a local inhibitory effect on the terminals from which it came - the so-called autoinhibitory feedback mechanism ~The action of released noradrenaline is terminated mainly by reuptake of the transmitter into noradrenergic nerve terminals. Some is also sequestered by other cells in the vicinity. Circulating adrenaline and noradrenaline are degraded enzymically, but much more slowly than acetylcholine ~About 75% of the noradrenaline released by sympathetic neurons is recaptured and repackaged into vesicles. This serves to cut short the action of the released noradrenaline, as well as recycling it. The remaining 25% is captured by non-neuronal cells in the vicinity, limiting its local spread. These two uptake mechanisms depend on distinct transporter molecules. Neuronal uptake is performed by the plasma membrane noradrenaline transporter (generally known as NET, the norepinephrine transporter ) ~catecholamines are metabolised mainly by two intracellular enzymes: monoamine oxidase (MAO) and catechol- O -methyl transferase (COMT) ~Transmitter synthesis involves the following: (1) L-tyrosine is converted to dihydroxyphenylalanine (dopa) by tyrosine hydroxylase (rate-limiting step) (Tyrosine hydroxylase occurs only in catecholaminergic neurons) (2) Dopa is converted to dopamine by dopa decarboxylase (3) Dopamine is converted to noradrenaline by dopamine β-hydroxylase (DBH), located in synaptic vesicles (4) In the adrenal medulla, noradrenaline is converted to adrenaline by phenylethanolamine N -methyltransferase. ~Transmitter storage: noradrenaline is stored at high concentration in synaptic vesicles, together with ATP, chromogranin and DBH, all of which are released by exocytosis. Transport of noradrenaline into vesicles occurs by a reserpine-sensitive transporter (VMAT). Noradrenaline content of cytosol is normally low due to monoamine oxidase in nerve terminals. ~Transmitter release occurs normally by Ca 2+ -mediated exocytosis from varicosities on the terminal network. Non-exocytotic release occurs in response to indirectly acting sympathomimetic drugs (e.g. amphetamine ), which displace noradrenaline from vesicles. Noradrenaline escapes via the NET transporter (reverse transport) ~Co-transmission occurs at many noradrenergic nerve terminals, ATP and neuropeptide Y being frequently co-released with NA. ATP mediates the early phase of smooth muscle contraction in response to sympathetic nerve activity

allosteric modulation

~receptor proteins possess many other ( allosteric ) binding sites through which drugs can influence receptor function in various ways, by increasing or decreasing the affinity of agonists for the agonist binding site, by modifying efficacy or by producing a response themselves ~Allosteric drugs bind at a separate site on the receptor to 'traditional' agonists (now often referred to as 'orthosteric' agonists). They can modify the activity of the receptor by (i) altering agonist affinity, (ii) altering agonist efficacy or (iii) directly evoking a response themselves ~other forms of antagonism- (1) chemical antagonism [situation where the two substances combine in solution; as a result, the effect of the active drug is lost] (2) pharmacokinetic antagonism [situation in which the 'antagonist' effectively reduces the concentration of the active drug at its site of action] (3) block of receptor-response linkage [situation where the antagonist blocks at some point downstream from the agonist binding site on the receptor, and interrupts the chain of events that leads to the production of a response by the agonist] (4) physiological antagonsim [interaction of two drugs whose opposing actions in the body tend to cancel each other]

targets for drug action

~receptors- for agonist it binds and can cause direct (ion channel opening/closing) or transduction mechanisms (enzyme activation/inhibition, ion channel modulation, DNA transcription) [antagonist binds and no effects/endogenous mediators blocked] ~ion channels- blockers block ion permeation and mediators increase/decrease opening probability [drugs can affect ion channel function in several ways: (1) By binding to the channel protein itself, either to the ligand-binding ( orthosteric ) site of ligand-gated channels, or to other ( allosteric ) sites which plugs the channel physically (2) By an indirect interaction, involving a G protein and other intermediaries (3) By altering the level of expression of ion channels on the cell surface] ~enzymes- inhibitors bind to enzyme and normal action is inhibited, false substrate binds and abnormal metabolite is produced, prodrug binds and an active drug is produced ~transporters- normal transport, inhibitors block transport, false substrate transported causes abnormal compound to accumulate

biogenic amines

~serotonin (obsession, compulsion, aggression, memory, appetite, sex, irritability, anxiety, mood, cognitive function) , histamine (sleep, appetite, sex, memory), dopamine (reward, motivation, mood, attention, cognitive function, sex, appetite, aggression), noradrenaline (Alertness, Concentration, Energy, cognitive function, attention, anxiety, impulse, irritability), adrenaline ~Many central nervous system (CNS) acting drugs affect the synthesis, degradation or re‐uptake of one of more of the biogenic amines ~Neurotransmitters- Noradrenaline, dopamine, serotonin [Peripheral effects (blood pressure, platelets, gastrointestinal tract, kidney etc .....)] [Central nervous system (CNS) effects - brain and spinal cord] [(1) Noradrenaline - fear, stress, mood, memory, attention (2) Dopamine - movement, addiction, vomiting, psychosis (3) Serotonin - sleep, mood, migraine, hallucinations] ~Each stage of neurotransmission is a potential site of drug action- 1. Action potential 2. Calcium channel opens 3. Release 4. Activation of receptors 5. Postsynaptic cell signalling 6. Degradation 7. Reuptake ~Noradrenaline neurotransmission- (1) Stimulants (amphetamine, cocaine) (2) Antidepressants (desiprimine, moclobemide) ~Serotonin neurotransmission- (1) Stimulants (MDMA/Ecstasy) (2) Antidepressants (fluoxetine) ~Indirectly Acting Sympathomimetic Amines- Amphetamine, Ephedrine, Tyramine [Mechanism of Action- Increased release of noradrenaline from nerve terminals leading to increased activation of adrenergic receptors. Leading to hypertension, hyperthermia, tachycardia, mydriasis, diaphoresis, central nervous system excitation] [Absorption and metabolism- Tyramine normally destroyed by MAO in the gut] [Indications- CNS stimulant in narcolepsy, ADHD, appetite suppressant, nasal decongestion (ephedrine)] [Adverse Effects & Toxicity- Tachycardia, insomnia, acute psychosis with overdose, dependence, anorexia, pulmonary hypertension] [act on VMAT, NET or postsynaptic receptors]

muscle contraction

~skeletal muscle- Skeletal muscle possesses an array of transverse T-tubules extending into the cell from the plasma membrane. The action potential of the plasma membrane depends on voltage-gated sodium channels and propagates rapidly from its site of origin, the motor endplate to the rest of the fibre [The T-tubule membrane contains voltage-gated calcium channels termed dihydropyridine receptors (DHPRs), that respond to membrane depolarisation conducted passively along the T-tubule when the plasma membrane is invaded by an action potential. DHPRs are located extremely close to ryanodine receptors in the adjacent SR membrane, and activation of these RyRs causes release of Ca 2+ from the SR] [depolarisation rapidly activates the RyRs, releasing a short puff of Ca 2+ from the SR into the sarcoplasm. The Ca 2+ binds to troponin, a protein that normally blocks the interaction between actin and myosin. When Ca 2+ binds, troponin moves out of the way and allows the contractile machinery to operate. Ca 2+ release is rapid and brief, and the muscle responds with a short-lasting 'twitch' response.] ~cardiac muscle- The cardiac action potential varies in its configuration in different parts of the heart, but commonly shows a 'plateau' lasting several hundred milliseconds following the initial rapid depolarisation. T-tubules in cardiac muscle contain L-type calcium channels, which open during this plateau and allow Ca 2+ to enter. This Ca 2+ entry acts on RyRs (a different molecular type from those of skeletal muscle) to release Ca 2+ from the SR. With minor differences, the subsequent mechanism by which Ca 2+ activates the contractile machinery is the same as in skeletal muscle ~smooth muscle- The action potential of smooth muscle is generally a rather lazy and vague affair compared with the more military behaviour of skeletal and cardiac muscle, and it propagates through the tissue much more slowly and uncertainly. The action potential is, in most cases, generated by L-type calcium channels rather than by voltage-gated sodium channels, and this is one important route of Ca 2+ entry [Smooth muscle cells also store Ca 2+ in the ER, from which it can be released when the IP 3 R is activated. IP 3 is generated by activation of many types of G protein-coupled receptor. Thus, in contrast to skeletal and cardiac muscle, Ca 2+ release and contraction can occur in smooth muscle when such receptors are activated without necessarily involving depolarisation and Ca 2+ entry through the plasma membrane]

adrenoreceptors

~smooth muscle- (1) blood vessels- alpha 1 constricts, alpha 2 constricts or dilates, b2 dilates (2) bronchi- a1 constricts, b2 dilates (3) gastro- a1 relaxes, b2 relaxes, a2 relaxes (4) uterus- a1 contract, b2 relax (5) bladder- a1 contract, b2/b3 relax (6) iris- a1 contract, b2 relax ~heart- (1) rate- b1/b2 increase (2) force of contraction - b1/b2 increase ~other tissues/cells- (1) skeletal muscle- b2 causes increased muscle mass and speed of contraction/glycogenolysis, b3 causes thermogenesis (2) liver- a1 and b2 cause glycogenolysis (3) fat- b3 causes lipolysis (4) pancreatic- a2 decrease insulin secretion ~nerve terminals- (1) adrenergic- a2 decrease release and b2 increase release (2) cholinergic- a2 decrease release ~ α 1 receptors are particularly important in the cardiovascular system and lower urinary tract, while α 2 receptors are predominantly neuronal, acting to inhibit transmitter release both in the brain and at autonomic nerve terminals in the periphery. The distinct functions of the different subclasses of α 1 and α 2 adrenoceptors remain for the most part unclear; they are frequently co-expressed in the same tissues, and may form heterodimers, making pharmacological analysis difficult. ~ α 1 receptors- Phospholipase C activation (↑ Inositol trisphosphate ↑ Diacylglycerol ↑ Ca 2+) [potency- NA > A >> ISO] [agonists- Phenylephrine Methoxamine] [antagonists- Prazosin, Doxazocin] ~α 2 receptors- inhibit adenyl cyclase (↓ cAMP , ↓ Calcium channels , ↑ Potassium channels) [potency- A > NA >> ISO] [agonists- Clonidine] [antagonists- Yohimbine, Idazoxan] ~β 1 receptors- ↑ cAMP [potency- ISO > NA > A] [agonists- Dobutamine, Xamoterol] [antagonists- Atenolol , Metoprolol] ~β 2- ↑ cAMP [potency- ISO > A > NA] [agonists- Salbutamol , Terbutaline ,Salmeterol ,Formoterol , Clenbuterol] [antagonists- Butoxamine] ~β 3- ↑ cAMP [potency- ISO > NA = A] [agonists- Mirabegron] ~The main effects of receptor activation are as follows: (1) α 1 receptors: vasoconstriction, relaxation of gastrointestinal smooth muscle, salivary secretion and hepatic glycogenolysis (2) α 2 receptors: inhibition of: transmitter release (including noradrenaline and acetylcholine release from autonomic nerves); platelet aggregation; vascular smooth muscle contraction; insulin release (3) β 1 receptors: increased cardiac rate and force (4) β 2 receptors: bronchodilatation; vasodilatation; relaxation of visceral smooth muscle; hepatic glycogenolysis; muscle tremor (5) β 3 receptors: lipolysis and thermogenesis; bladder detrusor muscle relaxation.

denervation supersensitivity

~that if a nerve is cut and its terminals allowed to degenerate, the structure supplied by it becomes supersensitive to the transmitter substance released by the terminals ~mechanisms of this phenomena- (1) Proliferation of receptors. This is particularly marked in skeletal muscle, in which the number of acetylcholine receptors increases 20-fold or more after denervation; the receptors, normally localised to the endplate region of the fibres ( Ch. 13 ), spread over the whole surface. Elsewhere, increases in receptor number are much smaller, or absent altogether. (2) Loss of mechanisms for transmitter removal. At noradrenergic synapses, the loss of neuronal uptake of noradrenaline (see Ch. 14 ) contributes substantially to denervation supersensitivity. At cholinergic synapses, a partial loss of cholinesterase occurs (see Ch. 13 ) (3) Increased postjunctional responsiveness. Smooth muscle cells become partly depolarised and hyperexcitable after denervation (due in part to reduced Na + -K + -ATPase activity; see Ch. 4 ) and this phenomenon contributes appreciably to their supersensitivity. Increased Ca 2+ signalling, resulting in enhanced excitation-contraction coupling, may also occur.

Dale's principle of chemical transmission

~that there are situations where different transmitters are released from different terminals of the same neuron. Further, most neurons release more than one transmitter (see co-transmission, p. 149 ) and may change their transmitter repertoire, for example during development or in response to injury

overactive bladder

~the bladder micturition reflex- Normal: Detrusor muscle contracting when bladder is full, Overactive bladder: Detrusor muscle contracting before bladder is full [M3 receptors are located on detrusor and contract detrusor muscle to urinate] ~Symptoms include: (1) Frequency voiding more than 8 times per day (2) Urgency a sudden/ unavoidable desire to void (3) Nocturia waking at night to void (4) Urge incontinence incontinence with strong urgency [Main treatment is with antimuscarinics (oxybutynin and tolterodine), as these drug inhibit micturition] ~treatments- (1) Tolterodine- a competitive non-selective M receptor antagonist [both tolterodine and its motabolite 5-hydroxymethyl toterodine are active] [p.o. 1 or 2 mg twice daily] [half life 2-6 hours] (2) Oxybutynin- a competitive non-selective M receptor antagonist [p.o. 5 mg 2-3 time daily] [half life 2 hours] (3) Darifenacin- a selective M3 receptor antagonist (9- to 59-fold more selective for M3 than M1, M2, M4 and M5) [p.o. 7.5 mg or 15 mg, once daily] [half-life for chronic dosing is >12 h] ~mechanism of action- Tolterodine, oxybutynin and darifenacin block the M3 receptor on the bladder detrusor muscle, resulting in (1) increased bladder (vesical) capacity (2) diminished the frequency of contractions of the detrusor muscle (3) delayed the initial desire to void [They thus decrease urgency and the frequency of both incontinent episodes and voluntary urination] ~side effects- (1) Gut motility is reduced leading to constipation (2) Retention of urine (3) Blurred vision, photophobia (4) Exocrine gland secretions are inhibited: decreased saliva secretion leading to dry mouth, decreased sweating leading to hyperthermia [Most clinically useful agents do not to cross the blood brain barrier (BBB) but high doses of antagonists that do cross BBB can cause CNS excitation (agitation, disorientation, hyperactivity)] [Toxic doses lead to depression, circulatory and respiratory failure (treated with physostigmine, the AChE inhibitor)]

Desensitisation and Tolerance

~tolerance- the effect of a drug gradually diminishes when it is given continuously or repeatedly [Many different mechanisms can give rise to these phenomena- change in receptors, translocation of receptors, exhaustion of mediators, increased metabolic degradation of the drug, physiological adaptation, active extrusion of drug from cells] [the desensitised state is caused by a conformational change in the receptor, resulting in tight binding of the agonist molecule without the opening of the ionic channel- Phosphorylation of intracellular regions of the receptor protein is a second, slower mechanism by which ion channels become desensitised] ~Most G protein-coupled receptors also show desensitisation [Phosphorylation of the receptor interferes with its ability to activate second messenger cascades, although it can still bind the agonist molecule] ~translocation of receptors- Prolonged exposure to agonists often results in a gradual decrease in the number of receptors expressed on the cell surface, as a result of internalisation of the receptors [internalised receptors are taken into the cell by endocytosis of patches of the membrane, a process that normally depends on receptor phosphorylation and the subsequent binding of arrestin proteins to the phosphorylated receptor]

Constitutive Receptor Activation and Inverse Agonists

~where an appreciable level of activation ( constitutive activation ) may exist even when no ligand is present [receptor mutations occur - either spontaneously, in some disease states or experimentally created - that result in appreciable constitutive activation] [Resting activity may be too low to have any effect under normal conditions but become evident if receptors are overexpressed- say, 1% of receptors are active in the absence of any agonist, in a normal cell expressing perhaps 10 000 receptors, only 100 will be active. Increasing the expression level 10-fold will result in 1000 active receptors, producing a significant effect] [Under these conditions, it may be possible for a ligand to reduce the level of constitutive activation; such drugs are known as inverse agonists] ~Inverse agonists can be regarded as drugs with negative efficacy, to distinguish them from agonists (positive efficacy) and neutral antagonists (zero efficacy) [Neutral antagonists, by binding to the agonist binding site, will antagonise both agonists and inverse agonists] ~The degree of receptor activation (vertical scale) increases in the presence of an agonist and decreases in the presence of an inverse agonist- Addition of a competitive antagonist shifts both curves to the right

The Effects of Adrenergic Stimulation in the Cardiovascular System

~β-blockers do not effect the blood pressure of normal subjects ~β-blockers do effect the blood pressure of hypertensive patients by reducing renin levels ~β-blockers reduce maximal exercise tolerance in normal subjects ~β-blockers inhibit the increase in heart rate and therefore mask the signs of hypoglycemia ~β-receptor antagonists increase the likelihood of exercise-induced hypoglycemia in diabetic patients ~Signs of Hypoglycemia- (1) Shakiness, anxiety, nervousness, tremors (2) Palpitations, tachycardia (increased heart rate greater than 100 bpm) (3) Sweating, feeling of warmth (4) Pallor, coldness, clamminess (5) Dilated pupils (6) Feeling of numbness "pins and needles" in the fingers (7) Hunger (8) Nausea, vomiting, abdominal discomfort (9) Headache

β-Adrenergic Agonists: Isoprenaline

~β-non-selective ~Mechanism of Action- (1) Increases the strength (inotropic) and rate (chronotropic) of heart contraction (β1) (2) Relaxes the smooth muscle of bronchi, skeletal muscle vasculature and gastrointestinal tract (β2) ~Indications- Heart block, cardiac arrest, bronchospasm (e.g. asthma) ~Adverse Effects- palpitations, increased heart rate (tachycardia) (β1), headache and flushing (β2), mild tremors (β2), hypotension (β2). ~Route of Administration- Intra-venous (IV) ~Pharmacokinetic parameters: ½ life ~ 2 min, metabolised by COMT, poor substrate for MAO

β-Adrenergic Antagonists: Propranolol

~β-non-selective ~Mechanism of Action- (1) β1: decrease in heart rate > reduction in cardiac output > oxygenation of the myocardium is improved, suppression of renin release from the kidneys > decreased blood pressure (2 )β2: decreased insulin levels, decreases serum free fatty acid concentrations and increased triglyceride levels, reduce intraocular pressure by reducing aqueous humor production. ~Indications- Glaucoma, Hypertension, Angina, cardiac dysrhythmias, migraine, anxiety tremor. ~Contraindications- Congestive heart failure, hypotension, patients prone to hypoglycaemia (diabetics), asthma. ~Adverse Effects- Cardiac failure (β1), bradycardia (β1), bronchospasm in susceptible patients (asthmatics) (β2), hypoglycaemia (β2) in susceptible patients (neonates, infants, children, elderly patients, patients on haemodialysis, patients on concomitant anti-diabetic therapy), cold extremities, fatigue.