PHM 431 Exam 1

agonist

An agonist is any molecule that binds to a receptor and activates that receptor to cause a response.

What do pharmacologists do?

•Academic pharmacologist: research and teaching •Industrial pharmacologists: research (drug development) •Government pharmacologists: research, regulatory activities (Food and Drug Administration)

Specific ligand-gated cation channels G-protein-coupled receptors (GPCRs)

-7 transmembrane domains form an agonist binding pocket. -3rd intracellular loop binds G-protein and determines specificity of G-protein subtype interaction. Different receptors have a preferred G-protein. -C-terminal is a target for phosphorylation that regulates desensitization

Time course of intercellular signaling is determined by the type of receptor involved.

-A neuron releases a chemical messenger molecule (neurotransmitter) which diffuses across narrow space (synapse) to act on adjacent neurons. -time domain of response depends on signaling mechanisms activated: - ion channels (milliseconds) -enzymes (seconds - hours) -genes (days - years)

Absorption

-Absorption is the process by which drug molecules reach the plasma from the original site of administration. -All routes of drug administration, except for IV administration, require the drug to pass through cytoplasm and membranes of various cells to get to the plasma (circulation). -Absorption rates depend on: • Surface area of the absorption surface •Vascularization (the number of blood vessels draining the drug administration site) • Blood flow to the site of absorption (the number of blood vessels supplying the site) -There are several mechanisms of drug absorption but passive diffusion is the most common and important.

Action Potential Propagation

-Actions potentials propagate along the axon towards the nerve terminal (Figure 11). This occurs due to sequential movements of the depolarization and sequential opening and closing of sodium channels in the axon. -When the axon is depolarized, the sodium channels open causing a depolarization of nearby sodium channels. When these nearby sodium channels open, the action potential moves further down the axon toward the terminals. The sodium channels cycle from 1) closed, to 2) open following depolarization from the action potential, to 3) inactivated once the action potential has passed, and then 4) back to a closed state (Figure 12). -Some axons are wrapped by a myelin sheath, which is composed primarily of lipids (Figure 13). This wrapping increases the resistance of the axon and increases the speed of conduction down the axon because it prevents the loss of sodium ions from the axon. In myelinated axons, the action potential does not propagate as a wave but rather jumps from one node of ranvier to the next. This is called Saltatory Conduction (Figure 14). The nodes of ranvier are gaps in the myelination that contain a high number of sodium channels. The loss of myelination is a hallmark of some neurodegenerative diseases, such as multiple sclerosis.

Enteral Routes of Drug Administration (via the gastrointestinal tract): Oral

-Advantages: easy and convenient for most drugs (and for patients) - Disadvantages: 1) Time to onset of effect is slow and peak concentrations are reduced by the 1st pass effect. The First Pass Effect occurs because the blood supply draining the gastrointestinal tract flows through the liver. The liver has high levels of enzymes that metabolize (degrade) substances absorbed from the gastrointestinal tract. Morphine and heroin are victims of a large first pass effect and so cannot be administered orally (Figure 2). 2) Difficult to achieve very high or stable plasma (blood) concentrations (Figure 2). 3) Some drugs cannot be administered orally as they are poorly absorbed, or easily degraded, in the acidic environment of the stomach (pH<2) or the enzymes in the intestines

Membrane potential in cells

-All cells have a negative resting membrane potential (typically equal to -60mV). Membrane potential is due to the uneven distribution of ions across the membrane. The cell membrane is semi-permeable and does not readily allow the passage of large molecules across the membrane (such as proteins, lipids, other organic and inorganic anions). -A sodium/potassium ATPase pumps 3 sodium out and 2 potassium in. This is an energy dependent process that requires ATP hydrolysis. This creates a steep concentration gradient across the cell membrane and contributes to the membrane potential (Figure 6). -Potassium ions can leak through Potassium leak channels down their concentration gradient. Complete loss of potassium is prevented by electrostatic interaction with impermeable anions within the cell. This sets up the membrane potential (Figure 6). -Chloride ions are also pumped out of the cell. This sets up a steep chloride gradient from outside to inside. This will be important for GABA-mediated neurotransmission.

Receptor Antagonism

-Antagonists block the actions of agonists. There are several mechanisms that antagonists can use to block receptor activation. -Surmountable antagonism is possible n increasing agonist concentration rcomes antagonism -insurmountable antagonism occurs in increasing agonist concentration s not overcome antagonism

-Emax -Efficacy -Potency

-EC50 is the agonist concentration needed to produce one half of the maximum response. -Emax is the maximum effect. - efficacy is a measure of the maximum response an agonist can produce. The higher the max response the more efficacious a drug is. -Potency is a measure of the amount of agonist required to produce a given level of response. This is usually measured at the EC50 level. Drugs with lower EC50 values are more potent than drugs with high EC50 values

Excretion

-Excretion removes both the active and inactive drug. Urination is the major route of excretion. Kidney function is therefore critical to excretion of drug molecules. Excretion of drug molecules is slowed by any pathophysiological conditions which compromise renal function or alter urine formation. Also, drugs which affect renal function will alter (increase or decrease) excretion of other drugs. -Elimination: The rate of elimination of a drug from the body is the sum of the processes of metabolism and excretion.

first order kinetics

-For most drugs, elimination follows first order kinetics. The rate of elimination of drug is dependent on plasma concentration of the drug (Figure 13). The relationship between time and plasma concentration of the drug is nonlinear. -For any drug, a certain plasma volume can be completely cleared of that drug per unit time (mls/min). This elimination constant is independent of plasma drug concentration (g/ml) and is constant for a given drug assuming that metabolic and excretory mechanisms are not compromised (as with liver or kidney disease.)

Zero order kinetics

-For some drugs, elimination follows Zero order kinetics. The rate of elimination of the drug is constant and independent of plasma concentration of the drug. The relationship between time and plasma concentration of the drug is linear (Figure 14). -Alcohol is a drug metabolized by zero order kinetics. By taking an alcohol blood level the morning after drinking, it is possible to extrapolate back in time to establish blood alcohol levels.

Structure and GTPase activity of heterotrimeric G-proteins

-G-proteins consist of α, β and γ subunits. In the inactive state the α subunit binds GDP. -β and γ subunits are bound to each other, a lipid chain attached to γ subunit anchors β,γ complex to membrane; in the inactive (GDP bound) state the G-protein trimer is membrane bound. -agonist binding to the receptor causes a conformational shift in the 3rd intracellular loop which then binds the α subunit. Agonist binding increases the affinity of the receptor for the inactive (GDP bound) receptor. Receptor binding to the α subunit decreases affinity for GDP. GDP falls off and as there is a higher concentration of GTP then GDP, GTP binds causing the α subunit to dissociate from β,γ subunits. The α subunit interact with effector molecules. -the GTPase activity of the α-subunit is slow allowing the activated state to interact with its effectors. However when GTP is hydrolyzed to GDP the α subunit recombines with the β,γ subunits prior to a new cycle.

Specific ligand-gated cation channels Receptors for γ-amino butyric acid (GABA): GABAA receptor

-GABAA receptor is similar in structure to nAChR (heteropentameric receptor) but the GABAA receptor is a chloride (Cl- ) channel. GABAA receptors cause inhibitory responses. -Nearly every neuron in the brain is inhibited by GABA. GABA is the most important inhibitory neurotransmitter in the brain. -GABAA receptors formed from combinations of α with β and γ subunits. -GABAA receptors are highly modulated with multiple allosteric binding sites. GABAA receptor activity is modulated by alcohol, barbiturates, benzodiazepines and some steroids.

Specific ligand-gated cation channels Glutamate receptors

-Glutamate receptors are also ligand gated ion channels that are permeable to Na+ , K+ and Ca2+ . Similar function to nAChRs but structure is different. -Glutamate receptors are composed of 4 subunits with 3 transmembrane domains. There is also a "loop" between transmembrane domains 1 and 2, this may form the ion channel pore. -There are two classes of glutamate receptor: AMPA receptors and NMDA receptors (named for drugs which activate the receptor). -NMDA receptors are composed of NMDAR1 and NMDAR2 subunits. -NMDA receptors are targets for some drugs of abuse, PCP (phencyclidine for example). Ca2+ activates many intracellular signaling pathways including those that lead to changes in gene expression. NMDA receptors are very important in memory formation.

Phase 2 Metabolism

-In Phase 2 Metabolism, conjugation adds a chemical from the body to the drug molecule to synthesize a new and less reactive, more easily eliminated compound. Conjugation occurs after the Phase 1 metabolism. -The most common Phase 2 reactions are: •Acetylation: addition of an acetate • Glucuronidation: addition of a glucose molecule •Sulfonation: addition of a sulfate group -These reactions all make the drug less lipophilic and more hydrophilic, thus, the conjugated drug cannot cross membranes easily.

Factors Altering Biotransformation

-Induction: increased enzyme activity as a result of previous (same or different) drug exposure. Alcohol, induces, or stimulates production of enzymes that metabolize it and other drugs. This leads to "tolerance". Many other drugs also induce metabolic enzymes, which leads to drug-drug interactions in patients taking multiple drugs. -Inhibition: decreased enzyme activity as a result of previous drug exposure -Competition: co-administered drugs will lead to increased levels of both drugs due to drug-drug interactions on the metabolic pathways. This is often a problem in elderly patients who take many medications. -Natural, Herbal meds: herbal medications are also targets for drug metabolizing enzymes. These can interfere with drug metabolism increasing plasma concentrations of the drug.

Resting and active membrane potentials in neurons

-Neurons have similar mechanisms for setting up the resting membrane potential but neurons are excitable cells that transmit electrical signals. They also express voltage-gated sodium and potassium channels (Figure 7). These channels are proteins that form sodium and potassium permeable pores and they also have protein sequences that are sensitive to the membrane potential. -At -60mV, the sodium and potassium channels are closed, but when the potential across the membrane changes, the protein sequences undergo a conformational change, causing the channels to open (Figure 7). -The threshold for activation of sodium channels is -50mV. The threshold for activation of potassium channels is near 0mV. The sudden change in membrane potential caused by the opening of sodium and potassium channels is called an action potential (Figure 8). This is the principal mechanism of electrical signaling in the nervous system. The sudden opening of sodium channels and influx of sodium ions causes the upstroke of the action potential, when the membrane potential reaches 0mV, the potassium channels open and the sudden efflux of potassium ions causes the membrane potential to fall back to near -60mV. When this happens, the ion channels close and the sodium-potassium ATPase restores the resting distribution of ions. -Microelectrodes are used to study electrical behavior of neurons. Injections of small currents change the membrane potential in small increments. These are subthreshold (<- 50mV). There is no action potential and no output from the neurons. When larger currents are injected, the depolarization reaches -50mV, and sodium channels open causing an action potential to occur. This produces an output from the neuron and the action potential propagates down the axon (Figure 9). -Fast neurotransmitters like glutamate and acetylcholine produce membrane depolarization that mimics current injection. These fast synaptic excitatory responses can be subthreshold when a small amount of transmitter is released or suprathreshold when a large amount of transmitter is released. Large depolarizations elecit an action potential and output from the neuron (Figure 10).

Monoamine Oxidase

-Normally, MAO metabolizes the DA displaced from the vesicles. MAO inhibitors (MAOi) block degradation of DA. MAOi's (phenylzine, tranylcypromine) can potentiate the effects of amphetamine and related drugs. -Tyramine is contained in many foods such as beer, cheese, and red wine. Tyramine can have an amphetamine like effect. Thus, patients taking MAOi's need to be on special tyramine diets. -MAOi's are typically used to treat depression and panic attacks.

Pharmacodynamics

-Pharmacodynamics describes the interaction of a drug with its site of action. There are many ways a drug can interact with its site of action to produce an effect. These can be divided into two broad categories of interaction: 1) Receptor mediated - specific for drug-receptor interaction and involve high affinity and usually reversible binding. 2) Non-receptor mediated - usually low affinity (examples include antibiotics and laxatives).

What is Pharmacology and Toxicology?

-Pharmacology: the science of drugs and how they work to produce a therapeutic benefit. -Toxicology: the science of understanding the deleterious effects of drugs and other chemicals. -Pharmacologists develop drugs to treat disease or use drugs as tools to probe the function of living systems. -Pharmacy: the science of drug preparation and drug dispensing to patients.

What happens when the action potential reaches the nerve terminal?

-Pre-synaptic nerve terminals have high concentrations of neurotransmitter containing vesicles (Figure 15). Nerve terminals make connections with dendrites. The synaptic connection is highlighted by thick dark bands called active zones (Figure 15). These are probably high concentrations of proteins involved in neurotransmitter release, action, and recovery (ion channels, receptors, transporters, etc.) -There is a steep concentration gradient for calcium ions in the nerve terminal. Calcium concentrations are kept very low in the cytoplasm. Nerve terminals express voltage-gated calcium channels. These channels open during the depolarization caused by the action potential moving into the nerve terminal (Figure 16; 1 & 2). Calcium channels open quickly letting calcium into the nerve terminal, the concentration increases about 200 fold (Fig 16; 3). Calcium causes the vesicles to fuse with the presynaptic membrane and neurotransmitter is dumped in to the synaptic cleft (Fig 16; 4). Many specialized proteins are involved in calcium detection and vesicle fusion. -Calcium is quickly buffered back to resting levels by the mitochondria and by a sodium-calcium exchanger that pumps calcium back out of the nerve terminal (Fig 16; 5). -If enough neurotransmitter is released and binds to the post-synaptic receptors (Fig 16; 6) an action potential may be produced in the postsynaptic neuron to continue sending the message to the next neuron. This is the process of synaptic transmission from one neuron to the next.

Substance Use Disorder/Drug addiction is costly to society. Costs include:

-Productivity: lost wages, resources diverted, etc. -Health: costs for treatment of addiction, costs for treatment of drug abuse related diseases. -Other: criminal justice costs, social welfare costs.

Axonal transport

-Proteins are "shipped" to nerve terminals along axons and neurons (Figure 3). This is accomplished with microtubule proteins and motor molecules, such as kinesin, which move proteins and vesicles along the length of the axon (Figure 4). -Axonal transport is bi-directional. Anterograde (moving forward) transport occurs when proteins are transported from the cell body to nerve terminals. Retrograde transport (moving backward) occurs when molecules are transported back from the nerve terminal to the cell body. Retrograde transport functions to keep the nucleus informed about the status of the nerve terminal (Figure 4). -The same microtubules which participate in axonal transport also participate in cell division during mitosis. Microtubule poisons (vinblastine, taxol) are anticancer drugs. Taxol blocks the disassembly of microtubules and disrupts cell division and cell growth. In the figure to the right are a normal cell and a cell treated with Taxol (Figure 5).

Receptor structure and function

-Receptors are typically cell membrane proteins that are targets for intercellular (between cells) signaling molecules. -Each receptor protein is encoded by a single gene (one gene-one protein) -Most intercellular signaling molecules are polar, hydrophilic molecules that do not cross membranes easily. These intercellular signaling molecules include neurotransmitters (dopamine, acetylcholine, 5-hydroxytryptamine, for example), circulating hormones (insulin, epinenphrine, adrenocorticotrophin) -The exception is steroid hormones (estrogen, testosterone, for example) which are very lipophillic. Steroid receptors are intracellular receptors as steroids easily cross membranes.

Distribution

-Similar to absorption, the degree of ionization contributes to drug distribution: drugs exist in solution in ionized and non-ionized forms. Ionized drugs have a charge (+ or - ) and do not cross lipid membranes easily. -In peripheral tissue, gaps between cells allow ionized ions to diffuse from the circulation to the tissue (Figure 8). However, the blood brain barrier and placental barrier prevent uniform distribution of ionized drugs. Bloods vessels supplying the brain and placenta have specializations called tight junctions between endothelial cells and glial cells which prevent diffusion of ionized drugs into the brain (Figure 9). -Protein binding can also affect drug distribution into tissues from the blood. There are high levels of the protein albumin in the blood. Albumin can bind drugs forming albumin-drug complexes that get trapped in the blood and prevents movement of the drug into the tissue (Figure 10). -Drug-drug interactions can occur through competition for albumin binding. This can lead to higher concentrations of free drug for both compounds and thus can lead to higher levels of the drug in the tissue (Figure 11).

Law of Mass Action

-The Law of Mass Action describes the relationship between receptor concentration, agonist concentration, and agonist affinity for the receptor. It is used to predict receptor occupancy by the agonist. -[A] is the concentration of agonist [R] is the concentration of "free" receptor (not bound by agonist) -Kon is the rate constant for binding of A to R -Koff is the rate constant for unbinding of A from R -RT is the total receptor concentration (includes free and drug bound receptor) - The ratio Koff/Kon is a measure of affinity. If Koff is small (the rate of unbinding of the agonist is very slow) than the agonist has a high affinity for the receptor. -Koff/Kon is equal to the equilibrium dissociation constant, or Kd, which is a measure of affinity. -The law of mass action can help determine the number of receptors occupied by an agonist by using this final equation such that one can determine the number of bound receptors out of the total number of receptors by knowing the concentration of the agonist and the equilibrium dissociation constant.

Overview of Neuronal Structure

-The cell body contains the nucleus, mitochonidria, Golgi apparatus, ribosomes, and other subcellular organelles common to all cell types. These organelles are required for protein synthesis and other metabolic processes (Figure 1 & 2). -The dendrites are short processes that extend from the cell body. Dendrites express neurotransmitter receptors where they receive synaptic contacts from other neurons. These are the input structures (Figure 2). -The axon is a long process that carries electrical signals away from the cell body to other neurons. The terminals of axons contain neurotransmitter containing vesicles, other organelles (such as mitochondria), pre-synaptic receptors, and ion channels (Figure 2).

Metabolism

-The enzymes called cytochrome P450s act on drugs and other xenobiotics (foreign chemicals) to inactivate these compounds. These enzymes are located primarily in the liver but can also be in target tissues. -Sometimes the P450 enzymes increase the pharmacologic activity of a drug. Thus, an inactive molecule can be transformed into an active compound. Also, sometimes the P450 enzymes form a toxic metabolite, such as with acetaminophen or Tylenol. -In general, a drug undergoes 2 Phases of Metabolism.

Why develop new treatments?

-Treatment is less expensive than not treating or incarceration (1 yr methadone maintenance = $4,700 vs $18,400 for imprisonment)(US incarceration rates are higher than anywhere else in the developed world) -Every $1 invested in treatment yields up to $7 in reduced crime-related costs -Savings can exceed costs by 12:1 when health care costs are included -Reduced interpersonal conflicts -Improved workplace productivity (fewer drug-related accidents

Signaling via Gs - adenylate cyclase - protein kinase A (PKA)

-adenylate cyclase is a membrane bound enzyme activated by the αs subunit. -adenylate cyclase has two catalytic domains that bind ATP, cleave off two PO4 (phosphate) groups to make cyclic AMP (cAMP). -adenylate cyclase is dually regul and αi coupled receptors. αs ated by αs while αi inhibits adenylate cyclase. -Protein kinase A (PKA) is a heterodimer consisting of two regulatory subunits (R) and two catalytic subunits (C)(see figure below). In the resting state, R has high affinity for C inhibiting its catalytic activity. 4 cAMP molecules are required to activate PKA; cAMP binding to R decreases its affinity for C releasing it; unbound C has kinase activity and it phosphorylates a variety of substrates. -catalytic subunit of PKA phosphorylates (attaches PO4-- groups) other proteins including ion channels and transcription factors. This is a mechanism by which cell surface receptors coupled to Gs can alter gene expression. -PKA can alter gene expression through phosphorylation of the transcription factor cyclic AMP response element binding protein (CREB). CREB is constitutively bound to cyclic AMP response element (CRE) sequences on genes. In the unphosphorylated form, CREB is inactive. -Translocation of C (catalytic subunit of PKA) into the nucleus results in CREB phosphorylation Phosphorylated CREB binds an adaptor protein CREB binding protein (CBP) which now allows coupling to the transcription apparatus to initiate gene transcription. -This relates back to the first lecture about long term changes in gene expression caused by addictive drugs. Addictive drugs which act on neurons via an adenylate cyclase - PKA linked receptor can change gene expression which produces permanent changes in the structure or function of neurons!

Substance Use Disorder/drug addiction is multi-factorial

-both biological (genetics, developmental, early exposure, frequency of exposure) -environmental (socioeconomic, friends, family, coping skills)

G-protein coupled receptors have a specific mechanism for receptor down regulation.

-desensitization is temporary receptor inactivation (seconds to minutes). Downregulation is a decrease in receptor number lasting hours to days. -G-protein receptor kinases (GRKs) phosphorylate activated receptors (ligand-bound). GRKs phosphorylate amino acids on the intracellular end (Cterminal) of the receptor. GRKs are bound to β,γ subunits. -β-arrestin has a high affinity for phosphorylated receptors; β-arrestin inhibits receptor interaction with Gproteins. -β-arrestin promotes internalization to the endosome. -Receptors can be dephosphorylated and returned to the membrane. -receptor downregulation is associated with receptor degradation in the lysosome.

principles of drug addiction

-drugs are more likely to be addictive if they reliably produce euphoria or pleasant feelings. Drug intake is reinforcing as the subject wishes to take drug repeatedly. -drug dependence can lead to addiction but dependence does not equal addiction. Drug dependence is associated with drug withdrawal. -All drugs can produce dependence but not all drugs are addictive -Avoidance of withdrawal becomes part of the reinforcing profile of drug intake and potentiates addiction. -withdrawal symptoms contribute to the reinforcing properties of self-administration of drugs of abuse. Withdrawal signs are unpleasant and drug self-administration becomes important not only to achieve euphoria but to avoid withdrawal. -development of addiction and dependence (withdrawal risk) are associated with changes in gene expression. These changes can persist for months to years.

fast neurotransmitter (fNT)

-fast intercellular signaling mechanisms mediated by a fast neurotransmitter (fNT): rapid responses (depolarization, hyperpolarization) are due to a transient change in ion permeability. -receptors mediating these responses are ligand-gated ion channels that are permeable to Na+ , K+ and Ca2+ (excitatory) or Cl- (inhibitory), these receptors mediate responses in the millisecond time domain. -transmitter binding site and ion channel are the same macromolecule. -short term effects result from depolarization, longer term effects can result from increases in intracellular Ca2+

Addictive drugs

-legal (alcohol, cigarettes, and some prescription drugs) -illegal drugs (heroin, methamphetamine)

Specific ligand-gated cation channels 1. Nicotinic acetylcholine receptors (nAChRs):

-nAChRs are heteropentameric (5 subunits) ion channels composed of various subunit combinations. Each subunit has 4 membrane spanning domains. -Permeable to Na+ , K+ and Ca2+ -all receptors contain two α subunits which bind acetylcholine (ACh) -nAChR desensitization occurs without internalization. Receptor enters an inactive conformation. Receptor can not be activated from this state. -Desensitized receptors are different from inactive receptors in the resting state. Resting receptors can be activated when agonist is present. Desensitized receptors can not be activated.

Diffusion depends on

1) Ionization of drug molecules (Figure 6) Ionized (charged) molecules do not diffuse across the membrane while non-ionized (uncharged) molecules diffuse across the membrane. Charged molecules are hydrophilic (or water soluble), uncharged molecules are lipophilic (or fat soluble). 2) Concentration gradients (Figure 7) Non-ionized drugs move down their concentration gradient across cell membranes. There is a net movement of drug until equilibrium is reached. 3) Size Small molecules cross membranes more rapidly than large molecules

Mechanisms of pharmacodynamic tolerance (aka desensitization)

1) Receptor downregulation: repeated drug administration causes a decrease in the number of receptor or binding sites for the drug on the membrane surface of the target cell (Figure 10). Receptors can become internalized (trafficked to an intracellular compartment) where they either recycle back to the membrane or are degraded. If degraded, new receptors must be synthesized. 2) Effector molecule downregulation: repeated drug administration causes activation of intracellular signaling pathways. These respond to repeated stimulation by decreasing levels of effector molecules (Figure 11). 3) Cross tolerance - Two different drugs acting via the same receptor can produce tolerance to one another. Two different drugs acting at different receptors which converge on the same signaling pathway can produce cross tolerance (Figure 12).

Dopamine Neurons

A. Meso-limbic dopamine neurons These neurons contain nerve cell bodies in the ventral tegmental area in the midbrain region which project to the nucleus accumbens, frontal cortex, and amygdala. The nucleus accumbens also projects to the prefrontal cortex. Together these brain areas compose the limbic system. The amygdala is important for the regulation of mood, fear, and other emotions. Mesolimbic dopamine neurons are the targets of many drugs of abuse. These neurons are the primary neurons involved in reward and reward-seeking behaviors (Figure 1-1). B. Nigro-striatal dopamine neurons These neurons contain nerve cell bodies in the substantia nigra in the mid brain region which project to the striatum. The striatum is composed of the caudate and putamen and is located in the subcortical region of the forebrain. Nigro-striatal dopamine neurons are part of the extrapyramidal motor system. This system is involved in motor balance, gait, and muscle control and coordination. Parkinson's disease results from cell death of the nigro-striatal dopamine neurons (Figure 1-2). C. Tuberoinfundibular dopamine neurons These neurons have cell bodies in the hypothalamus which project to the pituitary gland where they regulate hormone secretion (prolactin). These neurons also project to the spinal cord (Figure 1-3).

American Society of Addiction Medicine (ASAM) definition

Addiction is a primary, chronic disease of brain reward, motivation, memory and related circuitry. Dysfunction in these circuits leads to characteristic biological, psychological, social and spiritual manifestations. This is reflected in an individual pathologically pursuing reward and/or relief by substance use and other behaviors.

Parenteral Routes of Administration (not via the GI tract) Intramuscular (i.m.)

Administration of drug directly into the muscle •Advantages 1) Slow release reduces toxicity 2) A depot or sustained release effect is possible increasing the duration of the effect (Figure 5). • Disadvantages: 1) Trained personnel are required to administer the drug. 2) Injection site will influence absorption 3) Absorption is sometimes erratic, especially for poorly soluble drugs. The solvent may be absorbed faster than the drug causing precipitation of the drug at the injection site.

DA storage

After synthesis, DA is stored in high concentrations in vesicles in the nerve terminal. DA is pumped into the vesicles by a vesicular monoamine transporter (VMAT) protein localized to the membrane of the DA vesicles.

Omega conotoxin

Calcium channels in nerve terminals are targets for toxins (omega conotoxin). Omega conotoxin is a blocker of N-type voltage gated calcium channels. Because the N-type calcium channels are involved in the sensation of pain, omega Conotoxin can be used to treat chronic pain disorders by intrathecal injection (Figure 17).

DA recovery

DA is recovered by the nerve that released it and DA is repackaged into the vesicle by the vesicular monoamine transporter (VMAT) which resides on the membrane of the vesicle. Any DA that escapes VMAT is scavenged by the mitochondria which contains the enzyme monoamine oxidase (MAO). MAO degrades DA to dihydroxyphenylacetate (DOPAC) an inactive metabolite (Figure 3).

Dopamine Transporter

DAT is a target for cocaine and amphetamine. Both drugs bind to DAT to block DA transport. This increases the DA concentration in the synaptic cleft leading to increased and prolonged activation of DA receptors (Figure 4).

Digitalis

Digitalis (common foxglove) is an inhibitor of the sodium/potassium ATPase. This is used to treat congestive heart failure. The inhibitor increases sodium concentrations thereby increasing the strength of cardiac contraction through a sodium calcium exchange mechanism in the heart.

DA Synthesis

Dopamine is synthesized from tyrosine, which is taken up from the blood supply. Tyrosine is hydroxylated by the enzyme tyrosine hydroxylase (TH). TH is the rate-limiting enzyme in dopamine synthesis. The product of TH activity is dihydroxyphenylalanine (DOPA). DOPA is decarboxylated by the enzyme DOPA decarboxylase to produce dopamine (DA). These reactions take place in the cytoplasm (Figure 2). L-DOPA is a drug used to treat Parkinson's disease. Parkinson's disease is caused by a depletion of DA neurons in the substantia nigra. L-DOPA increases DA synthesis for a while, but eventually enough neurons die off and L-DOPA no longer has a substantial effect.

Drug Tolerance

Drug Tolerance is the decrease in sensitivity to the effects of a drug after repeated drug administration (Figure 9).

Enteral Routes of Drug Administration (via the gastrointestinal tract): Rectal

Drugs administered this route are usually given by suppository or enema. Drugs administered this way include aspirin, theophylline, chlorpromazine, and some barbiturates. • Advantages: 1) Reduced first pass effect because veins draining the rectum lead directly to the general circulation thus bypassing the liver 2) Useful for patients unable to take drugs orally such as children • Disadvantages: 1) Erratic absorption - absorption is often incomplete 2) Not well accepted by most patients

Enteral Routes of Drug Administration (via the gastrointestinal tract): Sublingual

Drugs can be taken as tablets and held in the mouth or under the tongue. Nitroglycerin is administered this way for rapid relief of angina or chest pain. Nicotine containing chewing gum, such as Nicorette, used for cigarette smoking replacement therapy also used this mode of administration. • Advantages: 1) No first pass effect because the liver is bypassed. 2) Blood supply to the mouth is good and absorption is rapid. 3) Oral pH is neutral (~7.4) so the drugs remain stable. • Disadvantages: 1) Holding the dose in the mouth is inconvenient; any drug swallowed is subject to first pass metabolism. 2) Only small doses can be administered this way.

Parenteral Routes of Administration (not via the GI tract) Intravenous (i.v.)

Drugs may be given into a peripheral vein over 1-2 minutes or longer by infusion. Rapid injections are used to treat epileptic seizures, acute asthma, or cardiac arrhythmias. These are situations that require near instantaneous drug action. • Advantages: 1) Rapid onset of drug action (Figure 3) 2) Whole dose is delivered to the blood stream, no first pass effects -Disadvantages: 1) Can be difficult to find a vein 2) Because of the rapid response, toxicity can be a problem since all drugs can be toxic at high concentrations. Thus, keeping the level of the drugs within the therapeutic window becomes more difficult with i.v. administration (Figure 3). 3) IV administration requires trained personnel and needles must be kept sterile making this method expensive

Half life

For any drug a Half life (min) can be defined as the time required for the plasma concentration to decrease by 50%. Half life is constant for a given drug assuming metabolic and excretory mechanisms are not compromised.

Receptors with intrinsic guanylate cyclase activity.

Important in the heart and kidney and endocrine regulation of fluid volume. These are less important in the nervous system and we will not discuss these receptors any further.

Phase 1 Metabolism

In Phase 1 Metabolism, a drug can undergo: •Oxidation: the addition of an oxygen ion, hydroxylation produces a less lipophilic compound (Figure 12). • Reduction: addition of a hydrogen atom by conversion of nitro group (NO2) to amine (NH2). Replacement of hydroxyl (-OH) by a hydrogen atom. • Hydrolysis: enzyme catalyzed cleavage of compound with water to split the active molecule into two inactive compounds as shown below in the conversion of an ester to an acid and an alcohol. R1CO2R2 + H2O ⇌ R1CO2H + R2OH

drug craving in methamphetamine addicts

Increased neural activity in the prefrontal cortex is associated with drug craving in methamphetamine addicts. Brain activity measured as changes in blood flow (fMRI). Subjects on methylphenidate report craving and the craving score is correlated with activity in the prefrontal cortex.

Long-term risk for development of drug addiction is linked to

Long-term risk for development of drug addiction is linked to age at first exposure. Tracking adolescent drug use helps to predict future drug addiction liability.

DA release

Nerve-to-nerve connections are synapses (Greek "to clasp"). The space between the pre-synaptic nerve terminal and the post-synaptic neuron is the synaptic cleft. When DA is released from the terminal, it diffuses across the synaptic cleft where is activates postsynaptic DA receptors (G-protein coupled receptors). DA is cleared from the synaptic cleft by a high affinity dopamine transporter (DAT) localized to the pre-synaptic membrane (Figure 3).

Pharmacokinetics (ADME)

Pharmacokinetics is the process by which a drug is absorbed, distributed, metabolized, and excreted by the body (Figure 1). Pharmacokinetics describes how the action of the drug is time and space dependent. Pharmacokinetics describes the actions of the body on a drug, whereas pharmacodynamics describes the actions of a drug on the body.

When an agonist (drug that activates a receptor) binds to its receptor it activates a response but agonist binding also causes receptor internalization. This is a mechanism response for: A) Drug dependence B) Pharmacodynamic tolerance C) Pharmacokinetic tolerance D) Withdrawal response

b

BOTOX

Proteins involved in neurotransmitter release are also targets for toxins (botulinum toxin, BOTOX). Botulinum toxin blocks vesicle fusion and release thereby blocking neurotransmitter release and resulting in paralysis in the area exposed to the toxin. BOTOX has become a commonly used cosmetic to reduce wrinkles.

Sodium channels and potassium channels and toxins

Sodium channels and potassium channels are targets for toxins and local anesthetic drugs. Tetrodotoxin (TTX) is a potent neurotoxin that blocks voltage-gated sodium channels, thus blocking action potentials. TTX can be isolated from many animals including the pufferfish and blue ring octopus, shown below. Exposure to TTX results in paralysis and death due to blockade of action potentials in the heart. Scorpion toxins usually also block sodium channels. However, they are also poisonous scorpions that have potassium channel blocking toxins. Black Mamba snakes contain dendrotoxin, a potassium channel blocker that works at the Nodes of Ranvier to block potassium channels thereby prolonging the action potential, resulting in convulsions.

Rate of Elimination

The Rate of Elimination of the drug (g/min) is equal to the arithmetic product of the drug's plasma concentration and the drug's clearance. The rate of elimination is thus greater when the plasma concentration of the drug is high than when the plasma concentration of the drug is low.

Xenobiotic:

Xenobiotic: any substance not produced by your body

Movement of materials from the nerve terminal back to the nerve cell body is accomplished by: A) anterograde transport B) the endosome C) retrograde transport D) the Na+/K+ ATPase

c

Multiple sclerosis is an autoimmune diseases that damages myelin. Which of the following is a predicted pathophysiological effect of MS? A) Decreased speed of action potential propagation along axons B) Impaired axonal transport of materials from the cell body to the nerve terminal C) Disruption of proteins required for neurotransmitter release from nerve terminals D) Disruption of the blood brain barrier

a

Plasma protein binding of a drug is an example of: A) Chemical antagonism B) Receptor desensitization C) Physiological antagonism D) Irreversible antagonism

a

Which of the following is a disadvantage of the intravenous route of drug administration? A)High risk of exceeding the toxic drug level B)High level of protein binding C)Large first pass effect D)Slow onset of drug action

a

psychological disorder

a pattern of behaviors that negatively impact multiple life areas and create distress for the person experiencing these symptoms.

Competitive Irreversible Antagonism

agonist and antagonist compete for the same binding site but the antagonist binds irreversibly (Figure 7).

Competitive Reversible Antagonism

agonist and antagonist compete for the same receptor binding site in a reversible manner (Figure 5).

Non-competitive Antagonism

agonist and antagonist do not compete for the same binding site (allosteric binding site) can be both reversible and irreversible (Figure 6).

Drug:

any substance that brings about a change in biologic function through its chemical actions

Poison:

any substance that produces harmful effects on biological function through its chemical actions

Acetylation is an example of which of the following drug metabolism reactions? A)Cyt-P450 induction B)Phase 2 conjugation reaction. C)Increased lipophilicity D)Albumin conjugation

b

Functional magnetic resonance imaging (fMRI) is a tool for measure activity of specific brain areas. What is the functional basis for fMRI measurements? A. fMRI measures the electroencephalogram B. fMRI measures oxygenated blood flow to active brain areas C. fMRI measures the size of active brain regions D. fMRI directly measures brain electrical activity

b

Diagnostic and Statistical Manual of Mental Disorders (DSM-5) definition

diagnosis of drug addiction changed to: "Substance Use Disorder", requires a criterion of "drug craving."

Which of the following brain areas is not part of the mesolimbic dopamine system? A)nucleus accumbens B)hypothalamus C)prefrontal cortex D)ventral tegmental area

b

Ligand-gated ion channels:

binding site for agonist (neurotransmitter) and the ion channel are part of the same macromolecular complex. 1. mediate rapidly developing responses (onset is less than one millisecond) 2. mediate short duration responses 3. mediate rapidly desensitizing responses (tolerance).

Toxin:

biologically produced poison

Adolescent drug use is a risk factor for a lifelong drug problem. Why is this the case? A)Adolescents metabolize drugs very slowly, so higher blood levels of drugs are achieved and this increases the reward associated with drug use. B)The elimination rates of drugs are slower in adolescents allowing higher blood levels of drugs to be achieved. C)The prefrontal cortex (PFC) is a key decision making part of the brain, it is a target for addictive drugs and it is a late maturing part of the brain. D)All of the above

c

Which of the following best describes drug tolerance? A) The need to have the drug available after chronic use to maintain homeostasis B) Upleasant feelings experienced by the drug user when the drug is unavailable after chronic use C) Reduction in the drug response after repeated drug administration D) A feeling of euphoria after drug administration

c

Many factors contribute to drug addiction and continued drug use. Which of the following is not one of these factors? A)Drug withdrawal B)Drug tolerance C)Drug dependence D)1st order elimination kinetics

d

The equilibrium dissociation constant for a drug is a measure of drug: A) Efficacy B) Tolerance C) Dependence D) Affinity

d

Which of the following drug is least likely to be permeable to the blood brain barrier? A)Drug B: very hydrophobic (molecular weight = 240) B)Drug C: very lipophilic (molecular weight = 3,200) C)Drug D (very hydrophilic (molecular weight = 240) D)Drug A: very hydrophilic (molecular weight = 2,400)

d

Which of the following is a property of oral drug administration? A)Slow increase in blood levels of the drug. B)Higher safety level compared to other routes of administration C)Large first pass effect D)All of the above

d

medical problem

disorder attributed to a measurable change in the function and/or structure of cells, tissues or organs

Monitoring the future

is an ongoing study of drug use patterns among US teenagers. Study is run through the University of Michigan. Tracks drug use in a sample of US teenagers on a yearly basis

Physical dependence

is biological. Dependence is a state that develops as a result of tolerance (adaptation) to drug effects. Involves a resetting of mechanisms maintaining homeostasis

Innate Tolerance

is genetically determined sensitivity or insensitivity to a drugs effects the first time the drug is administered. Genetic polymorphisms can produce drug metabolizing enzymes with lower or higher activity. Leads to altered drug bioavailability. For example, a polymorphism in the gene encoding alcohol dehydrogenase (metabolizes ethanol) produces an enzyme with faster kinetics. Individuals with this polymorphism can tolerate higher alcohol intake because it is quickly metabolized.

G-protein coupled receptors:

largest class of cell surface receptors. Couple via a G-protein to activation or inhibition of intracellular (within a cell) signaling pathways.

Withdrawal syndromes

occur when drug administration stops and homeostasis is again disrupted. Signs and symptoms of withdrawal are generally opposite to the responses caused by drug administration.

Chemical Antagonism

protein binding (albumin) or chemical degradation (acid pH of the stomach) block the agonist from binding (Figure 4).

Tyrosine kinase receptors:

these are typically growth factor receptors, regulate growth and differentiation of cells. Insulin receptors are tyrosine kinase receptors.

Acquired Tolerance

tolerance that develops subsequent to drug administration (3 types) 1) Pharmacokinetic tolerance - changes in drug sensitivity due to changes in the distribution or metabolism of the drug. Alcohol can induce enzymes responsible for detoxification, barbiturates induce cytochrome P450 responsible for metabolism. 2) Pharmacodynamic tolerance - adaptive changes in the system in response to repeated drug administration. Receptor down-regulation, or down regulation of effector molecules activated by the receptor. 3) Learned tolerance - skills developed through experience with drugs. Common with alcohol abuse and can also occur with non-abused drugs.

Physiological Antagonism

typically seen with two agonists producing opposing responses (contraction vs. relaxation of vascular smooth muscle for example) (Figure 8).

Parenteral Routes of Administration (not via the GI tract) Subcutaneous (s.c.)

• Advantages: 1) Can be given by patient (ex. Insulin) 2) Absorption slow but usually complete (Figure 4) and can be improved by massage or heat 3) Slow release minimizes toxicity 4) Vasoconstrictors may be added to anesthetic agents to reduce their absorption, thereby prolonging their effects at the site of interest. •Disadvantages: 1) Can be painful; irritant drugs can damage local tissue 2) Can only be used with small doses (less than 2ml)

The mesolimbic dopamine system is the key brain circuit in drug addiction

•Drugs will be addictive if they produce euphoria. Euphoria leads to craving. •Most drugs of abuse act on mesolimbic dopamine system (reward system). •The faster the onset and more intense the euphoria, the higher the addiction liability •Mesolimbic dopamine reward system is a very late maturing brain pathway

Where did pharmacology come from?

•Interest in the effects of "natural products" as therapeutic agents (materia medica). •Pharmacology has a long history. Fermentation products (alcohol) were recognized very early for their " therapeutic" effects. "Therapeutic" effects of other natural products were also recognized by many ancient civilizations (opium, coca plants, tobacco). •Interest in plant derived toxins was also crucial to the development of pharmacology as a distinct science. •Claude Bernard (1850) was interested in the effects of curare on the neuromuscular system. Curare is obtained from plants (Strychnos toxifera) found in South America. Use as an "arrow poison" and it produces rapid paralysis. •Advances in chemistry particularly in Germany allowed purification of the active ingredients which allowed more detailed and careful studies of how these natural products interacted with the nervous system and with other tissues. •Chemistry advance also allowed synthesis of new drugs which might share some of the beneficial effects of natural products without side effects.