Ferchmin 2014 Introduction to Neuropharmacology Pharmacology is the science that study the interaction of drugs with biological systems either in vivo, ex vivo or in vitro. Pharmacology comes from pharmakon, poison or drug in Greek. In Spanish there is a handy word farmaco that is lacking in English. Drug is defined as any natural or synthetic molecule which causes an effect on biological systems. If substances have medicinal properties, they are considered medicines or pharmaceuticals. Pharmacology includes the study of synthesis and drug design, mechanism of action, signal transduction, drug interactions, toxicology, therapy, and medical applications. The three branches of pharmacology are pharmacodynamics, pharmacokinetics and toxicology. Pharmacodynamics, broadly defined, studies the effects of drugs on biological systems by interactions with receptors. Pharmacokinetics studies the absorption, distribution, metabolism, and excretion of chemicals from the biological systems. Toxicology studies the toxic effects of drugs. It must be kept in mind that all chemicals are toxic and even lethal at high dose. Neuropharmacology is pharmacology applied to the nervous system We will summarily address the following subjects Definition of basic concepts: agonists, antagonists, and positive modulators; efficacy and potency (EC50, IC50). An overview of the main neurotransmitter systems in the brain: glutamate, GABA, acetylcholine, etc. There are four main protein targets with which drugs can interact: enzymes (e.g. sarin that inhibits acetyl cholinesterase), membrane carriers, ion channels (e.g. nimodipine and voltage-gated Ca2+ channels) and receptors. Receptors interact with agonists, antagonists, partial agonists and inverse agonists. The binding of drugs with “inert” proteins can also have an important effect. Definitions KD: The equilibrium dissociation constant represents the concentration of ligand occupying half of the maximum receptor population. KD is a measure of affinity. Interestingly, the smaller the KD the higher the affinity. Potency of a drug is the concentration which elicits a given response often relative to other drug. More precisely, potency is the concentration that produces 50% of the maximum response. It can also be defined as effective dose 50% or ED50. Potency is equivalent to the Km of enzymes. Efficacy is the maximum response produced by a particular agonist. Efficacy is equivalent to Vmax in enzymes. Clearly, the concepts used in pharmacology and presented here are borrowed from the Michaelis–Menten enzyme kinetics model. There are several manners to do DRC. Often a cumulative DRC is the most practical Copyright © 2004 Allyn and Bacon “Review” of receptors No formal system for the classification of receptors exists. A receptor is usually defined as a protein that binds a relatively specific molecule called the ligand. The binding of the ligand to its receptor has a functional effect. The two main types of receptors could be defined as ionotropic and metabotropic. The ionotropic receptors have intrinsic channels that allow currents of either cations or anions. In general, the cation selective receptors are depolarizing while the anion selective ones are hyperpolarizing. Receptors with cationic channels conduct Na+, K+ or Ca2+ ions. Receptors with anionic channels conduct Cl- and to a lesser degree HCO3-. Some ionotropic receptors are both, ligand and voltage gated. Directly or indirectly all ionotropic receptors are modulated by the voltage across the membrane. Metabotropic receptors, after binding the ligand cause a metabolic change in the cell that often, but not always, mediate indirectly modulation of ion channels usually through second messenger systems. Metabotropic receptors are slower than ionotropic receptors because of the nature of the mechanism that mediates the physiological effect. Therefore, the skeletal voluntary muscles have nicotinic receptors while the smooth muscles have muscarinic receptors. The prototypic metabotropic receptor is the muscarinic acetylcholine receptor and the prototypic ionotropic receptor is the nicotinic acetylcholine receptor. In the brain the main excitatory (depolarizing) transmitter is glutamate. Glutamate binds to several metabotropic and several ionotropic receptors. The main ionotropic receptor is the AMPA receptor. AMPA is an abbreviation of (±)alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, obviously this is the last complete name that is shown here. You will have to put up with abbreviation only. The other glutamate ionotropic receptor is the kainate receptor. AMPA is a synthetic compound and kainate is an antihelminthic Japanese folk remedy. They are tools to differentiate subtypes of receptors, they are specific for a specific subtype of glutamate receptor. Neither AMPA nor kainate are endogenous!!!! They are just tools. Both AMPA and kainate have several subtypes that differ in ion selectivity and other parameters. Some subtypes of AMPA and kainate are more permeant to Ca2+ than others. The NMDA type of glutamate receptor is a ligand and voltage gated receptor. Its natural (endogenous) ligands are: spermine, glycine and glutamate. Glutamate is the actual neurotransmitter. The importance of spermine and glycine varies with the subtype of the NMDA receptor. The NMDA receptor is normally blocked with the Mg2+ ion. Only a considerable degree of depolarization removes the inhibition due to Mg2+ and allows the NMDA receptor to conduct Na+, K+ and Ca2+. NMDA is a synthetic molecule that specifically binds the glutamate receptor of NMDA type. NMDA stands for N-methyl-D-aspartate. Try to draw this formula. The most common amino acid neurotransmitters are: Glutamate; Gamma-aminobutyric acid (GABA) and Glycine. NMDA receptor: A specialized ionotropic glutamate receptor that controls a calcium channel that is normally blocked by Mg2+ ions. It has several other binding sites for ≠ ligands. AMPA receptor is an ionotropic glutamate receptor that controls a sodium channel. It is stimulated by AMPA and blocked by CNQX. Both compounds are synthetic. AMPA receptor is the most common glutamate receptor in the CNS. Glutamatergic transmision What is excitotoxicity? Epilepsy and excitotoxicity can be stopped or ameliorated by hyperpolarizing currents mediated by inhibitory transmitters. GABA (γ-aminobutyric acid) and glycine are two main hyperpolarizing and therefore inhibitory neurotransmitters. Inhibition of GABA transmission induces seizures. GABA is synthesized from Glutamate by the enzyme Glutamic Acid Decarboxylase (GAD) which is a marker of GABAergic neurons. Strychnine, a glycinergic antagonist, also causes seizures and for that purpose was used to control rodents, dogs and others. GABAergic transmission muscimol There are two main type of GABA receptors: GABAA (ionotropic) and GABAB (metabotropic) GABAA receptor consists of a channel for Cl-. Drugs that interact with GABA are: Muscimol (agonist); Bicuculline (antagonist); Pentylenetetrazol (antagonist) Benzodiazepines*; Barbiturates* and Ethanol are positive modulators. They activate the receptor together with GABA. There is no effect in absence of GABA GABAB receptors (GABAB R) are metabotropic transmembrane receptors for GABA that are linked via G-proteins to potassium channels. Baclofen is an agonist for GABAB receptors and it is used as a muscle relaxant. Glycine Glycine is an amino acid and a hyperpolarizing neurotransmitter in the spinal cord and some other areas of the CNS where it binds to strychnine sensitive sites (strychnine binding sites). This glycine-binding site at the glycine receptor is different from the glycine-binding site at the NMDA receptor. Strychnine: A direct antagonist for the glycine receptor is a poison because it causes fatal convulsions. Acetylcholine (Ach) Acetylcholine: Otto Loewi, 1921. Was the first neurotransmitter discovered. The cholinergic pharmacology is well studied and has many practical applications. Botulinum toxin: Prevents the release of ACh from nerve terminals. Black widow spider venom: A poison produced by the black widow spider that triggers the release of acetylcholine. There are many inhibitors of acetylcholine esterase including warfare never agents and insecticides that cause a cholinergic a crisis by overstimulation Hemicholinium: A drug that inhibits the uptake of choline. Vesamicol inhibits the reloading of vesicles with acetylcholine. The two cholinergic receptor families are the nicotinic and the muscarinic receptors Nicotinic acetylcholine receptors A model for AChR 3-D structure indicating possible locations for the binding-sites for non-competitive inhibitors and positive modulators. Ovals indicate the binding location of channel blockers (black), competitive ligands (blue), agonist sites (red) and positive modulators (grey oblique). Adapted from Hansen and Taylor, 2007. There are several nicotinic receptor subtypes. They are all pentamers and share a common design but have different subunits. 7 7 Cationic pore 7 7 7 Homomeric α7 neuronal nAChR 2 4 Cationic pore 2 4 2 Heteromeric α4β2 neuronal nAChR 1 Cationic pore 1 The adult muscle receptor These receptors are activated and desensitized by nicotine to a different degree in a subtype and agonist concentration dependent manner. Neuronal type nAChR present in physiologically significant concentration in hippocampal area CA1 α7 Muscle type nAChR α7 α7 α7 α7 α7 Inhibited by α7-bungarotoxin, methyllycaconitine and 4R, the tobacco cembranoid α4β2 α4 β2 β2 α4 β2 Inhibited by dihydro-β-erythroidine α7 nAChR Homomeric More permeable to calcium than other nAChRs Functions: stabilizing synapse formation during development; learning and memory; neuronal survival, etc. Muscarinic receptors were originally defined by their activation by muscarine derived from the mushroom Amanita muscaria that was used to kill flies. In man it causes convulsions and death. The muscarinic receptors are part of the family of G-protein-coupled. Muscarinic receptors are involved in many physiological functions like heart rate, contraction of smooth muscles and control of neuronal excitability. There are five types of muscarinic M1-M5. M1, M3 and M5 subtypes cause the activation of phospholipase C, generating two secondary messengers (IP3 and DAG) eventually leading to an intracellular increase of calcium, while M2 and M4 inhibit adenylate cyclase, thereby decreasing the production of the second messenger cAMP. The M1 receptors increase neuronal excitability and for this reason are involved in cognitive enhancement. However excessive stimulation of M1 like in case of warfare nerve agent poisoning it causes central seizures and fatal brain damage. Copyright © 2004 Allyn and Bacon Warning: In this introductory lecture the exciting fields of catecholamines, serotonin and other neurotransmitters were not discussed. You will learn this topics later if you will need them. With reference to the next slide. Agonist is the compound that causes the effect of the natural ligand. Antagonist is the compound that blocks the effect of the natural ligand. We assume that agonist and antagonist bind to the receptors to produce their effect. However, old fashion, not very molecular pharmacologist often call agonists or antagonist drugs that affect synthesis or break down of the transmitter. Copyright © 2004 Allyn and Bacon Copyright © 2004 Allyn and Bacon The non-official first law of pharmacology states that: Some drugs are selective but none is specific. The forbidden first law of clinical pharmacology states that: The probability of a clinician to prescribe a drug is inversely proportional to the $@!#^&*(^%! of the representative of the pharmaceutical company and directly proportional to other advertising gimmicks of the pharmaceutical company