Chapter 6: Neurotransmitter Systems
Glutamate-Gated Channels
3 glutamate receptor subtypes bear the names of their selective agonists: AMPA, NMDA, and kainate. Each of these is a glutamate-gated ion channel
Glutamate Receptor Subtypes
3 subtypes are *AMPA receptors, NMDA receptors*, and *kainate receptors*, each named for a different chemical agonist. The neurotransmitter glutamate activates all three receptor subtypes, but AMPA acts only at the AMPA receptor, NMDA acts only at the NMDA receptor, and so on
Opiates
A class of drugs, derived from the opium poppy, that are both medically important and commonly abused
Summary of In Situ Hybridization
A method for localiing specific mRNA transcripts for proteins
Summary of Immunocytochemistry
A method for viewing the location of specific molecules, including proteins, in section of brain tissue.
Immunocytochemistry
A method used to anatomically localize particular molecules to particular cells. Once the neurotransmitter candidate has been chemically purified, it is injected under the skin or into the bloodstream of an animal where it stimulates an immune reponse
Inhibition of Adenylyl Cyclase
The activation of a second type of NE receptor, called the α2 receptor, leads to the activation of Gi (the inhibitory G-protein). Gi suppressess the activity of adenylyl cyclase, and this effect can take precedence over the stimulatory system
Methods Used to Satisfy Criterion for Neurotransmitters
The 1st step in confirming the hypothesis that a molecule is a neurotransmitter is to show that the molecule is, in fact, localized in, and synthesized by, particular neurons. 2 of the most important techniques used today are immunocytochemistry and in situ hybridization
AMPA-Gated and NMDA-Gated Channels
The AMPA-gated and NMDA-gated channels mediate the bulk of fast excitatory synaptic transmission in the brain. AMPA receptors coexist with NMDA receptors at many synapses in the brain, so most glutamate-mediated excitatory postsynaptic potentials(EPSPs) have components contributed by both
5th Step in G-Protein Basic Mode of Operation
The Gα and Gβγ subunits come back together, allowing the cycle to begin again
4th Step in G-Protein Basic Mode of Operation
The Gα subunit is itself an enzyme that eventually breaks down GTP into GDP. Therefore, Gα eventually terminates its own activity by converting the bound GTP to GDP
Stimulation of Adenylyl Cyclase
The NE beta receptor is activated and in turn activates the stimulatory G-protein, Gs, which proceeds to stimulate the membrane-bound enzyme adenylyl cyclase. Adenylyl cyclase converts ATP to cAMP. The subsequent rise of cAMP in the cytosol activates a specific downstream enzyme called *protein kinase A (PKA)*
Divergence
The ability of one transmitter to activate more than one type of postsynaptic response
Reuptake of Catecholamine
The actions of catecholamines in the synaptic cleft are terminated by selective uptake of the neurotransmitters back into the axon terminal via Na^+-dependent transporters. Once inside the axon terminal, the catecholamines may be reloaded into synaptic vesicles for reuse, or they may be enzymatically destroyed by the action of *monoamine oxidase (MAO)*, an enzyme found on the outer membrane of mitochondria
3rd Step in G-Protein Basic Mode of Operation
The activated GTP-bound G-protein splits into two parts: the Gα subunit plus the GTP and the Gβγ complex. Both can then move on to influence various effector proteins
Serotonin
The amine neurotransmitter *serotonin*, also called 5-hydroxytroptamine and abbreviated *5-HT*, is derived from the amino acid tryptophan
Catecholamines
The amino acid tyrosine is the precursor for three different amine neurotransmitters that contain a chemical structure called a *catechol*. These neurotransmitters are collectively called *catecholamines*
Amino Acidergic Neuorons
The amino acids *glutamate (Glu)*, *glycine (Gly)*, and *gamma-amino-butyric acid (GABA)* serve as neurotransmitters at most CNS synapses. Of these, only GABA is unique to those neurons that use it as a neurotransmitter; the others are among the 20 amino acids that make up proteins
Opiods
The broader class of opiate-like chemicals, both natural and synthetic. Their effects include pain relief, euphoria, depressed breathing, and constipation
Catecholamine Neurotransmitters
The catecholamine neurotransmitters are *dopamine (DA)*, *norepinephrine (NE)*, and *epinephrine*, also called *adrenaline*
Conductance of Amino Acid-Gated Channels
The conductance of open channels helps determine the magnitude of their effects
Convention for Naming Neurotransmitters
The convention is to use the suffix -ergic when naming transmitters
Dale's Principle
The idea that a neuron has only one neurotransmitter. Many peptide-containing neurons violate Dale's principle because these cells usually release more than one neurotransmitter: an amino acid or amine and a peptide
Kinetics of Amino Acid-Gated Channels
The kinetics of the transmitter binding process and channel gating determine the duration of their effect
The Synthesis of Serotonin from Tryptophan
Tryptophan is converted first into an intermediary called 5-hydroxytrophan, 5-HTP, by the enzyme tryptophan hydroxylase. The 5-HT P is then converted to 5-HT, serotonin, by the enzyme 5-HTP decarboxylase
Differences Between NMDA and AMPA
1. NMDA-gated channels are permeable to Ca^2+... 2. Inward ionic current through NMDA-gated channels is voltage depedent
Microiontophoresis
A method used to assess the postsynaptic actions of a transmitter candidate. The tip of a glass pipette with a very fine tip is filled with an ionized solution and positioned next to the postsynaptic membrane of the neuron, and the transmitter candidate is ejected in very small amounts by passing electrical current through the pipette. The responses generated can be compared to those of generated by synaptic stimulation
Voltage Gated Dependency of NMDA Gated Channels
A normal negative resting membrane potentials, the channel becomes clogged by Mg^2+ ions, and this "magnesium block" prevents other ions from passing freely through. Mg^2+ pops out of the pore only when the membrane is depolarized, which usually follows activation of AMPA channels at the same and neighboring synapses
In Situ Hybridization
A useful method for confirming that a cell synthesized a particular protein or peptide. In order to see if the mRNA for a particular peptide is localized in a neuron, we chemically label the appropriate probe so it can be detected, apply it to a section of brain tissue, allow time for the probes to stick to any complementary mRNA strands, then wash away all the extra probes that have not stuck and search for labeled neurons
The Shortcut Pathway
A variety of neurotransmitters use the shortcut pathway, from receptor to G-protein to ion channel.
AMPA-Gated Channels
AMPA-gated channels are permeable to both Na^+ and K^+, and most of them are not permeable to Ca^2+. Activating them at normal, negative membrane potentials is to admit and excess of cation into the cell, causing a rapid and large depolarization. Thus, AMPA receptors at CNS synapses mediate excitatory transmission
Purinergic Receptors
ATP directly excites some neurons by gating cation channels. ATP binds to *purinergic receptors*, some of which are transmitter-gated ion channels.
Adenosine
Adenosine itself does not meet the standard definition of a neurotransmitter because it is not packaged in vesicles, but it does activate several adenosine-selective receptors
The Synthesis of Catecholamines from Tyrosine
All catecholaminergic neurons contain the enzyme *tyrosine hydroxylase(TH)*, which catalyzes the first step in the catecholamine synthesis, the conversion of tyrosine to a compound called *dopa*. Dopa is converted into the neurotransmitter DA by the enzyme dopa decarboxylase. The enzyme DBH converts DA to NE and finally, PNMT converts NE to epinephrine(adrenaline)
Amino Acid-Gated Channels
Amino acid-gated channels mediate most of the fast synaptic transmission in the CNS. Several properties of these channels distinguish them from one another and define their function within the brain. All these properties are a direct result of the molecular structure of the channels
Ligand
Any chemical compound that binds to a specific site on a receptor is called a *ligand* for that receptor. A ligand for a receptor can be an agonist, an antagonist, or the chemical neurotransmitter itself
Membrane-Delimited Pathway
Because all the action in the shortcut pathway occurs within the membrane, it is sometimes called the *membrane-delimited pathway*
Rate-Limiting Step in ACh Synthesis
Because the availability of choline limits how much ACh can be synthesized in the axon terminal, the transport of choline into the neuron is said to be the *rate-limiting step* in ACh synthesis
Benzodiazepines and Barbiturates Affect on GABAA Channel
Benzodiazepines and barbiturates are two classes of drugs that bid to their own distinct sites on the outside face fo the GABAA channel. By themselves, these drugs do very little to the channel. But when GABA is present, benzodiazepines increase the frequency of channel openings, while barbiturates increase the duration of channel openings. These action are selective for GABAA receptors and have no effect on glycine receptors
Catcholaminergic Neurons
Catecholaminergic neurons are found in regions of the nervous system involved in the regulation of movement, mood, attention, and visceral function
Synthesis of ACh
ChAT synthesizes ACh in the cytosol of the axon terminal by transferring an acetyl group from acetyl CoA to a choline, and then the neurotransmitter is concentrated in synaptic vesicles by the action of a vesicular ACh *transporter*
Uptake of Choline for Synthesis of ACh
Choline is taken up by the cholinergic axon terminals via a specific transporter that requires the cotransport of Na^+ to power the movement of choline
ACh Degredation
Cholinergic neurons manufacture the ACh degradative enzyme *acetylcholinesterase (AChE)*. AChE is secreted into the synaptic cleft and is associated with cholinergic axon terminal membranes. AChE degrades ACh into choline and acetic acid and much of the resulting choline is taken up by the cholinergic axon terminal via a choline transporter and reused for ACh synthesis
Retrograde Messengers
Communication in the direction from "post" to "pre" is called retrograde signaling; thus, endocannabinoids are *retrograde messengers*. Retrograde messengers serve as a kind of feedback system to regulate the conventional forms of synaptic transmission, which of course go from "pre" to "post"
Glutamate Receptors
Different drugs were also used to distinguish several subtypes of glutamate receptors, which mediate much of the synaptic excitation in the CNS.
1st Step in G-Protein Basic Mode of Operation
Each G-protein has three subunits, termed alpha, beta, and gamma. In the resting state, a guanosine diphosphate(GDP) molecule is bound to the Gα subunit, and the whole complex floats around on the inner surface of the membrane
Receptor Subtype
Each of the different receptors a neurotransmitter binds to. No two neurotransmitters bind to the same receptor; however, one neurotransmitter can bind to many different receptors
Result of Discharge of Stores of Ca^2+ by IP3
Elevations in cytosolic Ca^2+ can trigger widespread and long-lasting effects. One effect is the activation of the enzyme *calcium-calmodulin-dependent protein kinase (CaMK)*. CaMK is an enzyme implicated in the molecular mechanisms of memory
Ethanol's Affect on GABAA Receptors
Ethanol, the form of alcohol in beverages, strongly enhnaced the function of GABAA receptor. Ethanol has complex action that include effects on NMDA, glycine, nicotinic ACh, and serotonin receptors
Degradation of Neurotransmitter ATP
Following its release from synapses, ATP is degraded by extracellular enzymes, yielding adenosine.
Serotonin Reuptake
Following release from the axon terminal, serotonin (5-HT) is removed form the synaptic cleft by the action of a specific transporter. Numerous clinically useful antidepressant and antianxiety drugs, are selective inhibitors of serotonin reuptake. Once it is back in the cytosol of the serotonergic axon terminal, the transmitter is either reloaded into the synaptic vesicles or degraded by MAO
G-Proteins
G-proteins are a common link in most signaling pathways that start with a neurotransmitter receptor and end with effector proteins. G-protein is short for gaunosine triphosphate (GTP) binding proteins, which is actually a diverse family of about 20 types. Some types of G-proteins can be activated by many receptors
Second Messenger Cascades
G-proteins can exert their effects by directly activating certain enzymes which can trigger an elaborate series of biochemical reactions, a cascade that often ends in the activation of other "downstream" enzymes that alter neuronal function
G-Protein-Coupled Effector Systems
G-proteins exert their effects by binding to either to two types of proteins: G-protein-gated ion channels and G-protein-activated enzymes. Because the effects do not involve any other chemical intermediaries, the first route is sometimes called the *shortcut pathway*
GABA-Gated and Glycine-Gated Channels
GABA mediates most synaptic inhibition in the CNS, and glycine mediates most of the rest. Both the GABAA receptor and the glycine receptor gate a chloride channel. Inhibitory synapses are selective for anions(negative) instead of cations(positive)
Hybridization
If the sequence of nucleic acids in a strand of mRNA is known, it is possible to construct in the lab a complementary strand(a probe) that will stick to the mRNA molecule in a process called *hybridization*
2nd Step in G-Protein Basic Mode of Operation
If this GDP-bound G-protein bumps into the proper type of receptor and if that receptor has a transmitter molecule bound to it, then the G-protein releases its GDP and exchanges it for a GTP that it picks up from the cytosol
Protein Phospatase and Dephosphorylation
If transmitter-stimulated kinases were allowed to phosphorylate without some method of reversing the process, all proteins would quickly become saturated with phosphates, and further regulation would become impossible. Enzymes called *protein phosphatases* act rapidly to remove the phosphate groups
Requirements of NMDA Channels Passing Current
Inward ionic current through the NMDA channel is voltage dependent, in addition to being transmitter gated. Both glutamate and depolarization must coincide before the channel will pass current
Kainate Receptors
Kainate receptors exist throughout the brain, on both presynaptic and postsynaptic membranes, but their functions are not clearly understood
Protein Kinases and Phosphorylation
Key enzymes in many second messengers cascades are protein kinases(PKA, PKC, CaMK) which transfer phosphate to ATP floating in the cytosol to proteins, a reaction called *phosphorylation*.
Choline Acetyltransferase(ChAT)
Like nearly all presynaptic proteins, ChAT is manufactured in the soma and transported to the axon terminal. Only cholinergic neurons contain ChAT, so this enzyme is a good marker for cells that use ACh as a neurotransmitter
Basic Structure of G-Protein-Coupled Receptor
Most consist of a single polypeptide containing seven membrane-spanning alpha helices. Two the extracellular loops of the polypeptide form the transmitter binding sites. Structural variations here determine which neurotransmitters, agonists, and antagonists bind to receptor. Two of the intracellular loops can bind to and activate G-proteins. Structure variations here determine which G-proteins and, consequently, which effector systems are activated in response to transmitter binding
Convergence
Multiple neurotransmitters, each activating their own receptor type, can converge to influence the same effector system
Integrating Convergence and Divergence
Neurons integrate divergent and convergent signaling systems, resulting in a complex map of chemical effects
Cholinergic
Neurons that produce and release acetylcholine(ACh)
Noradrenergic
Neurons that use the amine neurotransmitter norepinephrine(NE)
Signal Amplification
One advantage of G-protein-coupled receptors over transmitter-gated channels is that the activation of one G-protein-coupled receptor can lead to the activation of many ion channels, called *signal amplification*
Gasotransmitters
One of the more exotic chemical messengers to be proposed for intercellular communication is a gaseous molecule, *nitric oxide (NO)*. Evidence for gasotransmitter functions is still sparse
Adenosine Triphosphate(ATP) as a Neurotransmitter
One of the most common small molecules other than amines and amino acids that function as neurotransmitters is ATP. ATP is often packaged in vesicles along with another classic transmitter and so act as co-transmitters
Neurosteroids
Other good candidates as natural modulators of GABAA receptors are the neurosteroids, which are natural metabolites of steriod hormones that are synthesized from cholesterol primarily in the gonads and adrenal glands, but also in glial cells of the brain. Some neurosteroids enhance inhibitory function while others suppress it, and they seem to do both by binding to their own sites on the GABAA receptor
Activating G-Proteins to Stimulate Phopholipase C (PLC)
PLC is an enzyme that floats in the membrane - like adenyl cyclase. PLC acts on a membrane phospholipid PIP2, splitting it to form two molecules that serve as second messengers: *diacylglycerol (DAG)* and *inositol-1,4,5-triphosphate (IP3)*. DAG, which is lipid-soluble, stays within the plane of the membrane where it activates a downstream enzyme, *protein kinase C (PKC)*. At the same time, the water-soluble IP3 diffuses away in the cytosol and binds to specific receptors on the smooth ER and other membrane-enclosed organelles in the cell. These receptors are IP3-gated calcium channels, so IP3 causes the organelles to discharge their stores of Ca^2+
Factors Affecting Serotonin Synthesis
Serotonin synthesis appears to be limited by the availability of tryptophan in the extracellular fluid bathing neurons. The source of brain tryptophan is the blood, and the source of the blood tryptophan is the diet(grains, meat, diary products, and chocolate are particularly rich in tryptophan)
Speed of Shortcut Pathway
Shortcut pathways are the fastest of the G-protein-coupled systems, having responses within 30-100 msec of neurotransmitter binding. This pathway is not as fast as a transmitter-gated channel, which uses not intermediary between receptor and channel, but is faster than the second messenger cascades
Endocannabinoids(Endogenous Cannabinoids)
Small lipid molecules that can be released from postynaptic neurons and act on presynaptic neurons
GABAergic Synapses
Synapses that use GABA
Glutamatergic Synapses
Synapses that use glutamate
Peptidergic
Synapses that use peptides
2nd Criteria for Considering a Molecule to be a Neurotransmitter
The molecule must be released by the presynaptic axon terminal upon stimulation
1st Criteria for Considering a Molecule to be a Neurotransmitter
The molecule must be synthesized and stored in the presynaptic neuron
3rd Criteria for Considering a Molecule to be a Neurotransmitter
The molecule, when experimentally applied, must produce a response in the postsynaptic cell that mimics the response produced by the release of neurotransmitter from the presynaptic neuron
Muscarinic Receptors Using Shortcut Pathway
The muscarinic receptors in the heart use this pathway. These ACh receptors are coupled by G-proteins to particular types of potassium channels, explaining why ACh slows the heart rate. In this case, the βγ subunits migrate laterally along the membrane until they bind to the right type of potassium channel and induce it to open
Acetylcholine(ACh)
The neurotransmitter at the neuromuscular junction and is therefore synthesized by all the motor neurons in the spinal cord and brain stem. ACh synthesis requires a specific enzyme, *choline acetyltransferase (ChAT)*
Pharmacology of Amino Acid-Gated Channels
The pharmacology of their binding sites describes which transmitters affect them and how drugs interact with them
The Synthesis of GABA from Glutamate
The precursor for GABA is glutamate, and the key synthesizing enzyme is glutamc acid decarboxylase(GAD). GABAergic neurons are distributed widely in the brain and are the major source of synaptic inhibition in the nervous system
Result of Benzodiazepines and Barbiturates When GABA is Present
The result in each case is more inhibitory Cl^- current, stronger inhibitory postsynaptic potentials(IPSPs), and the behavioral consequences of enhanced inhibition
Selectivity of Amino Acid-Gated Channels
The selectivity of the ion channels determines whether they produce excitation of inhibition and whether Ca^2+ enters the cell in significant amounts
Localization of Shortcut Pathway
The shortcut pathway is very localized compared with other effector systems. As the G-protein diffuses within the membrane, it apparently cannot move very far, so only channel nearby can be affected
Subdividing G-Proteins
The simplest scheme for subdividing the G-proteins is Gs, designating that the G-protein is stimulatory, and Gi, designating that the G-protein is inhibitory
The Basic Structure of Transmitter-Gated Channels
The structure of the nicotininc ACh receptor is a pentamer, an amalgam of five protein subunits arranged like the staves of a barrel to form a single pore through the membrane. 4 different types of polypeptides are used as subunits, designated alpha(α), beta(β), gamma(γ), and delta(δ). A channel is made from two alpha subunits, and one each of beta, gamma, and delta. There is one ACh binding site on each of the alpha subunits; the simultaneous binding of ACh to both sites it required for the channel to open
Reuptake of Amino Acid Neurotransmitters
The synaptic actions of the amino acid neurotransmitters are terminated by selective uptake into the presynaptic terminals and glia, once again via specific Na^+-dependent transporters. inside the terminal or glial cells, GABA is metabolized by the enzyme GABA transaminase
Ligand-Binding Method
The technique for studying receptors using radioactively or nonradioactively labeled ligands
NMDA-Gated Channels
These also cause excitation of a cell by admitting an excess of Na^+, but they differ from AMPA receptors in two very important ways
Naming Neurotransmitter Systems
These naming conventions are also used to identify the various neurotransmitter systems. For example, ACh and all the molecular machinery associated with it are collectively called the cholinergic system
Serotonergic Neurons
These neurons are relatively few in number, but appear to play an important role in brain systems that regulate mood, emotional behavior, and sleep
CB1 Receptors
These receptors are G-protein-coupled receptors, and their main effect is often to reduce the opening of presynaptic calcium channels. With its calcium channels inhibited, the ability of the presynaptic terminal to release its neurotransmitters(usually GABA[inhibitory] or glutamate[excitatory]) is impaired
1st Unusual Quality of Endocannabinoids
They are not packaged in vesicles like most other neurotransmitters; instead, they are manufactured rapidly and on demand
2nd Unusual Quality of Endocannabinoids
They are small and membrane permeable; once synthesized, they can diffuse rapidly across the membrane of their cell of origin to contact neighboring cells
3rd Unusual Quality of Endocannabinoids
They bind selectively to the CB1 type of cannabinoid receptor, which is mainly located on certain presynaptic terminals
Summary of Uses of Immunocytochemistry and In Situ Hybridization
Together, these methods enable us to see whether a neuron contains and synthesizes a transmitter candidate and molecules associated with that transmitter
G-Protein-Coupled Receptors
Transmission at these receptors involves three steps: (1) binding of the neurotransmitter to the receptor protein, (2) activation of G-proteins, and (3) activation of effector systems
Two ACh Receptor Subtypes
Two ACh receptor subtypes can be distinguished by the action of different drugs. In fact, the receptors were given the names of their agonists: *nicotinic ACh receptors* in skeletal muscle and *muscarinic ACh receptors* in the heart
Summary of Endocannabinoids
When a postsynaptic neuron is very active, it releases endocannabinoids, which suppress either the inhbitory or excitatory drive onto the neuron(depending on which presynaptic terminals have the CB1 receptors)
Autoradiography
When probes are radioactively labeled, hybridized probed can be detected by laying the the brain tissue on a sheet of special film that is sensitive to radioactive emissions. Radioactive cells are visible as clusters of small white dots. The technique for detecting and viewing viewing the distribution of radioactivity by using digital electronic imaging devices is called *autoradiography*
Co-Transmitters
When two or more transmitters are released from one nerve terminal
