Neuropharmacology

What is neuropharmacology?

Study of drugs with actions at synapses. Because drugs act to change neurotransmission for some synapses in CNS, they change function of at least some neurons. Reveals actions of proteins and illustrates mechanisms of neurotransmission from behavioral perspective

What are agonistic effects in broad terms?

Any drug that promotes synthesis or release of neurotransmitter, blocks its degradation or reuptake, or mimics its actions at postsynaptic receptors

What are antagonistic effects in broad terms?

Any drug that blocks synthesis or release of neurotransmitter, enhances its degradation, or blocks its action at postsynaptic receptors

What is an agonist (strict definition)?

Drug that binds to neurotransmitter's receptor and produces same effect as neurotransmitter. Sometimes effect is larger because drug binds more efficiently or stays bound longer. In some cases drug fits not so well and fails to exert full effect (partial agonist)

What is an antagonist (strict definition)?

Binds somewhere in receptor complex and either interferes (partially or entirely) with neurotransmitter's binding to same receptor, or blocks effect agonist has when it binds. Can bind to same site as neurotransmitter or to allosteric site

What are the "three R's" where most drugs work?

Release, Reception, and Reuptake - the proteins that release neurotransmitter into cleft, receive it in pre- or post-synaptic membrane, and take it back up into presynaptic terminal

What is a key difference between ionotropic and metabotropic receptors?

Ionotropic receptors composed of 4-5 separate protein subunits forming pore. GPCRs are single proteins. Most GPCRs exert effects through G protein that indirectly opens/closes ion channel

Why are GPCRs powerful agents?

Can change opening of thousands of ion channels for many seconds or even minutes. Never an effect at only one ion channel. Can work at genome level to turn on/off groups of genes

What is a receptor?

Protein that folds to bind signaling agents (like neurotransmitter) and by binding that agent prompts change in cell activity. Ionotropic receptors change membrane potential; GPCRs lead to activation of intracellular effector proteins (usually enzymes)

Why is total number of receptors low?

Any receptor population exists at high concentrations but in very restricted spaces. Part of muscle fiber occupied by nicotinic receptors is <0.1% of surface area, so total number is low

What is saturation?

Maximum effect when drug has attached to all binding sites on receptor population. Occurs when drug is delivered in millimolar range. If drug requires concentration higher than few millimolar for saturating response, it's either poor drug with low affinity or absorbed by lipids/inert molecules

What is Effective Dose 50 (ED50)?

Dose of drug that produces 50% of that agonist's maximal effect. Best way to characterize strength of any agonist's effects. Used for in vivo studies (mg per kg body weight administered systemically)

What is Effective Concentration 50 (EC50)?

Concentration at which agonist produces half-maximal response. Used for in vitro studies when concentration of drug applied to isolated tissue or cells in culture can be accurately measured

What is a full agonist?

Agonist that produces full effect (100% signal) - binds to receptors as well or better than neurotransmitter and produces maximal effect equal to neurotransmitter

What is a partial agonist?

Agonist that binds not so well as neurotransmitter, so when it binds to receptors, produces maximal effect that is much reduced. Imperfect fit produces something less than full effect

What is tolerance?

With repeated drug application, receptors it binds and size of effect get smaller, so more drug must be added to produce effect. Produces rightward shift in dose-response curve. Changes in brain: removal of receptors from membrane (sequestration), degradation, long-term downregulation of receptor gene transcription

What is peripheral tolerance?

Peripheral tissues (particularly liver) can increase expression of genes for catabolic enzymes. Liver breaks down drug before it reaches brain. With long-term use, liver improves clearance through increased expression of catabolic enzymes

What is reverse tolerance?

Rare phenomenon when same degree of response to drug occurs at lower concentrations as it's taken repeatedly. Occurs with low concentration or low frequency administration. Develops for only some of many responses drug produces

Give example of reverse tolerance

Hyperactivity produced by amphetamine. Animal/person becomes hyperactive at lower drug concentrations when given daily, even though same individual displays tolerance (requires higher concentrations) to experience rewarding effects

What is potency?

Measured by ED50 or EC50 at which agonist produces half-maximal response. Independent of efficacy. Drugs can have matching efficacies but very different potencies (each has own EC50)

What is efficacy?

Maximal response produced by agonist at saturating concentration. Independent of potency. Drugs can have same potency (same EC50) but distinct efficacy (different maximal values). All agonists with lower maximal values are partial agonists

How can dose-response curves characterize receptors?

By testing same agonists in different tissues. Example: isoproterenol very effective for bronchodilation, epinephrine less so, norepinephrine even less. But for vasodilation, epinephrine and norepinephrine identical while isoproterenol shifted right. Reveals separate distinct receptors (β-adrenergic in bronchioles, α-adrenergic in arterioles)

What makes neurotransmitter bind well to receptor?

Match between transmitter and specific binding site on receptor molecule - the affinity of receptor (characterized by low Kd). Transmitter has shape that fits binding site and produces effect (open channel or turn on G protein)

What is the dissociation constant (Kd)?

Kd = k2/k1 = [L][R]/[LR]. Measure of how well ligand binds and stays bound to receptor. Low Kd is good. Kd is concentration of free ligand when half of receptor is bound. Lower Kd = higher affinity

What is a good Kd value for neurotransmitters?

Neurotransmitter worth its name works at least in millimolar range (Kd close to 1 micromole or 10^-6 M). Some neurotransmitters and many drugs work in nanomolar ranges (10^-9 M)

What are the three main varieties of antagonists?

1) Competitive (compete for agonist's binding site), 2) Noncompetitive (bind to allosteric sites, interfere with agonist binding), 3) Uncompetitive (bind to allosteric site on activated receptor, prevent effect without interfering with agonist binding)

What is a competitive antagonist?

Competes with binding of neurotransmitter/agonist at very same site on receptor protein. When high affinity or high concentration, can block all receptor sites. High concentration of agonist can out-compete antagonist and displace it, removing its effect

What is Ki (inhibition constant)?

Binding affinity of competitive antagonist. Measures antagonist's ability to displace agonist. Ki = I50/(1+[L]/Kd). Small Ki means antagonist has high affinity for ligand's binding site. When [L]=Kd, Ki = I50/2

What is I50 or IC50?

Concentration of antagonist that displaces half the agonist bound to its receptors. Used to calculate Ki

How does competitive antagonist affect dose-response curve?

Produces rightward shift. Agonist behaves as less potent (ED50 increased) but equally effective (maximal response at saturation remains same). Takes more agonist to get same job done because must displace antagonist first

What is an irreversible antagonist?

Recognizes same site as agonist but binds irreversibly. Usually forms covalent bond, or rarely has such high affinity no agonist can displace it. Reduces agonist's efficacy - maximal effect at saturating concentration reduced because permanently removes some proportion of receptors

Give example of irreversible antagonist

α-bungarotoxin (cobra toxin) - binds to acetylcholine binding site of nicotinic receptor and permanently blocks neurotransmitter's access. Powerful paralytic blocking neurotransmission between motor neurons and skeletal muscle

What is a noncompetitive antagonist?

Binds to allosteric sites on receptor protein (outside agonist binding sites). Binding alters receptor shape and interferes with agonist binding. Leaves neurotransmitter-binding site less capable of binding neurotransmitter molecules

How do you distinguish competitive from noncompetitive antagonist?

Simple observation: adding more agonist reverses effect of competitive antagonist but NOT noncompetitive antagonist. Noncompetitive antagonists cannot be displaced by agonist because they bind to different site

How does noncompetitive antagonist affect dose-response curve?

Lowers efficacy of agonist without changing potency (EC50 remains constant). Agonist cannot displace it; raising agonist concentration fails to drive response to maximal value. Effect akin to turning full agonist into partial agonist

What is an uncompetitive antagonist?

Binds to allosteric site on activated receptor. Does not interfere with agonist binding but interferes with receptor's ability to produce response. Changes outcome of receptor-ligand binding by interfering with final step or intermediate step

Give example of uncompetitive antagonist

Mg2+ at NMDA receptors. Binds at center of ion channel and prevents cations from passing even when glutamate bound to receptor. Occludes channel pore. Memantine is drug that works same way. Affinity of NMDA/glutamate same whether Mg2+ bound or not, but response blocked

What are nicotinic acetylcholine receptors?

Ionotropic receptors named for agonist nicotine (plant product). Non-specific cation channels. Antagonist is curare (competitive, plant compound)

What are muscarinic acetylcholine receptors?

GPCRs (metabotropic) named for agonist muscarine (plant product). Antagonist is atropine (competitive, from Belladonna plant, high affinity)

What does β-bungarotoxin do?

Snake toxin that greatly elevates acetylcholine release. Paradoxically works as indirect antagonist because it depletes synapses of acetylcholine, indirectly blocking cholinergic transmission

What does hemicholinium-3 do?

Blocks reuptake of choline. Since most choline used for ACh synthesis reaches cytoplasm of axon terminals by reuptake, blocking reuptake indirectly blocks cholinergic transmission

What is physostigmine (eserine)?

Reversible antagonist of acetylcholinesterase. Therapeutic value - prolongs availability of acetylcholine at nicotinic/muscarinic sites. Used to treat myasthenia gravis (autoimmune attack on nicotinic receptors at NMJ)

What are organophosphate compounds?

Used as insecticides and chemical weapons. Irreversibly antagonize acetylcholinesterase, inducing tetanic contraction of skeletal muscles and total parasympathetic outflow (heart rate and respiration plummet). Either effect is deadly. Example: sarin gas

What is botox (botulinum toxin)?

Cuts up SNARE complex and blocks vesicle fusion, preventing ACh release

What enzymes control monoamine concentration?

Two forms of monoamine oxidase (MAO-A and MAO-B) on surfaces of mitochondria and in extracellular fluid. Both break down dopamine, only MAO-A works on norepinephrine and 5-HT. COMT (catechol-O-methyltransferase) also contributes to breakdown of dopamine, norepinephrine, epinephrine

Why prescribe MAO-A inhibitors vs MAO-B inhibitors?

MAO-A inhibitors for depression (actions on NE and 5-HT). MAO-B inhibitors for Parkinson's Disease (actions on dopamine only)

What do cocaine and amphetamine do?

Stimulate release of dopamine and block its reuptake. Among most widely abused agents in Western societies. Prolonged use produces paranoia and disconnection from reality (classic schizophrenia symptoms)

What are neuroleptics (anti-psychotics)?

Drugs like haloperidol that work as antagonists of dopamine at two of five dopamine receptor subtypes (all GPCRs). Used to treat schizophrenia symptoms

What is apomorphine?

Broad spectrum agonist of dopamine receptors used in treatment of Parkinson's Disease

What does reserpine do?

Blocks Vesicular Monoamine Transporters that fill synaptic vesicles with any monoamine. Result is general depletion of these neurotransmitters

What are MAO inhibitors used for?

Treatment of clinical depression. Inhibit enzyme that breaks down 5-HT, NE, and DA, raising their levels

What are tricyclic antidepressants (TCAs)?

Block reuptake of all monoamines (5-HT, NE, DA). Used for depression but have anti-cholinergic properties complicating pharmacology

What are SSRIs?

Selective Serotonin Reuptake Inhibitors. Modern, very effective drugs for depression. Block reuptake of serotonin, minimal effect on dopamine and NE transporters. May work directly (increasing 5-HT in cleft) or indirectly (forcing downregulation in autoreceptors)

What are SNRIs?

Serotonin-Norepinephrine Reuptake Inhibitors. Deliberately target both SRTs (serotonin transporters) and NETs (norepinephrine transporters). Modern, very effective for depression

How many 5-HT receptors exist?

Fourteen types throughout body (blood platelets, GI tract cells, CNS and PNS neurons). Thirteen are GPCRs, one is ionotropic (5-HT3 - only ionotropic monoamine receptor)

What G proteins do different 5-HT GPCRs couple to?

Four coupled to Gs (stimulates AC to synthesize cAMP). Five coupled to Gi/o (inhibits AC). Four coupled to Gq (stimulates PLC to generate DAG and IP3). This diversity means neurons side-by-side can respond very differently to 5-HT

What is the 5-HT3 receptor?

Member of Cys-loop receptor family (like nicotinic AChRs, GABAA, glycine receptors). Pentamer - 5 subunits, each passes through membrane 4 times. Non-specific cation channel that opens to depolarize neurons. Only ionotropic monoamine receptor

How does 5-HT3 subunit composition affect function?

Only A subunits: channel allows Ca2+ to flow freely but very low conductance. A and B subunits: channel is Ca2+ impermeable but conductance >10-fold greater. Control of B subunit expression determines postsynaptic effect by controlling which cations enter and how much current

Where are 5-HT3 receptors highly expressed and why is this important?

Highest in dorsal medulla (dorsal motor nucleus and nucleus solitary tract) controlling emesis (vomiting). Presynaptic 5-HT3 receptor activation induces violent vomiting, particularly from radiation/chemotherapy. Selective 5-HT3 antagonists given to people undergoing these therapies

What is unusual about 5-HT3 receptor location?

Both presynaptic and postsynaptic. One of only few ionotropic receptor types inserted into membranes of presynaptic axon terminals. Presynaptic location serves to increase neurotransmitter release from any axon terminal expressing them

What are 5-HT1B receptors?

Typical release-regulating autoreceptors inserted into plasma membranes of axon terminals. Regulate release and synthesis of 5-HT by targeting plasma membrane ion channels (voltage-gated Ca2+ channels) and cytoplasmic enzymes (tryptophan hydroxylase)

What are 5-HT1A receptors?

Unusual autoreceptors inserted into dendrites and cell bodies of 5-HT neurons (not terminals). Coupled to Gi/o that indirectly gates K+ channel. Result is hyperpolarization whenever 5-HT concentration around cell body/dendrites becomes elevated

How does 5-HT1A work as negative feedback?

5-HT neurons release 5-HT from dendrites (Ca2+-mediated vesicle fusion, like terminal release). Dendritic release and binding to 5-HT1A on cell bodies/dendrites of neighboring neurons produces prolonged significant hyperpolarization. Spike rates drop, controlling "tone" of 5-HT response

Why is 5-HT1A control important?

Since 5-HT works principally through volume neurotransmission, control over spike rates of few thousand neurons in couple brainstem nuclei can fundamentally alter neuronal responses over much of CNS. 5-HT1A antagonists raise firing rates and drive increase in 5-HT release

What is dendritic release?

Release of neurotransmitter from dendrites, not just axon terminals. Ca2+-mediated event through fusion of synaptic vesicles with plasma membrane - looks and acts like release from terminals. Well established for DA from dendrites in substantia nigra and 5-HT

Why does acetylcholine termination differ from other neurotransmitters?

Acetylcholine is only major neurotransmitter whose actions terminated entirely by enzymatic degradation (by acetylcholinesterase). Other neurotransmitters terminated by reuptake, diffusion, and/or degradation

What is myasthenia gravis and how is it treated?

Disease from autoimmune attack on nicotinic receptors in neuromuscular junction. Symptoms relieved with physostigmine because it prolongs availability of acetylcholine at remaining nicotinic receptor sites

What is the only approved treatment for Alzheimer's symptoms?

Acetylcholinesterase inhibitors (like physostigmine) - one of only two approved treatments for symptoms

What happens with therapeutic range of dose-response curves?

Neurotransmitter or drug has threshold (concentration producing minimum effect) and ceiling (maximum effect when all receptors bound). Any further concentration increase yields no greater response. True receptors have saturation level, usually in millimolar range

How is dose-response curve typically plotted?

Semi-log curve: Y-axis is response in linear scale, X-axis is drug concentration in log scale. Allows better assessment of low dose effects and wide range of doses on same plot. Typically produces sigmoidal curve

Why does lipophilic drug require higher concentration?

Lipophilic drugs not only bind to specific receptor sites but also enter fatty tissues and are effectively removed from circulation. Fatty tissues not receptor sites (don't produce effect when drug present) - they just absorb drug

What curve shape results from linear dose-response plotting?

Hyperbolic curve when drug dose (X-axis) and drug response (Y-axis) plotted on linear scale

What is an inverse agonist?

Drug that binds to same site as agonist but has opposite effect. Requires some basal (constitutive) level of activity. Example: DMCM binds to benzodiazepine site on GABAA receptor and reduces GABA effect (inverse agonist to BDZ, negative allosteric modulator of GABAA)

What are benzodiazepines and GABAA receptors?

GABAA receptor is also benzodiazepine (BDZ) receptor. When BDZs bind, they make GABA effect bigger (larger hyperpolarization, agonistic effect). BDZ is agonist for BDZ receptor and positive allosteric modulator of GABAA receptor

What is flumazenil?

Antagonist to benzodiazepines - binds at BDZ binding site and blocks effect of BDZ, but baseline GABA effect remains unchanged

What is significance of volume neurotransmission for drug effects?

Because acetylcholine and monoamines work principally through volume neurotransmission, drugs acting on these neurotransmitters and receptors affect global functions such as mood, arousal, reward, and cognition

Why is 5-HT pharmacology most complex?

Has many receptor types including unusual autoreceptors on dendrites/cell bodies, and only ionotropic receptor for monoamine which works both postsynaptically and presynaptically

What determines whether drug binds to receptor?

Each step in neurotransmission requiring protein action is target of at least one drug. Drugs target proteins in synthesis, vesicular storage, release mechanism, pre/postsynaptic receptors, or reuptake transporters

Compare effects across monoamine drugs

Some drugs exert effects on more than one neurotransmitter. Both MAO inhibitors and tricyclic antidepressants raise levels of all monoamines (by separate mechanisms)