A nerve cell fires an AP when it is depolarised. Module 1. Lecture 2- Principles of Drug Action

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1 PCOL2012 Notes Module 1-Principles of Drug Action in the Nervous System... 2 Module 2-Drug Abuse, addiction and analgesia Module 3-Drug Treatment of Allergies and GI Disorders Module 4-Introduction of Drug Discovery and Development Sample Short Answer Questions

2 General Notes Depolarisation (excitation)=inside of cell becomes more positive than resting state (-70mV), usually happens through influx of Ca2+ and Na+ ions through channels Hyperpolarisation (inhibition)=inside of cell becomes more negative than resting membrane potential, usually happens through efflux of K= ions and influx of Clions A nerve cell fires an AP when it is depolarised Module 1 Lecture 2- Principles of Drug Action Pharmacology o The study of drugs and how they work o Pharmacodynamics=effects of a drug on the body o Pharmacokinetics=effect of body on drug How do drugs work? o Most bind to proteins, a few bind to nucleic acids o Four main drug targets Receptors Enzymes Carrier molecules/transporters Ion channels Drugs alter protein function by o Activation/agonism o Modulation o Inhibition/antagonism/block Receptors/Ligands Protein molecules that enable E/C molecules to alter intracellular events Agonists bind to (occupy) and activate the receptor to produce a response directly or indirectly (transduction) e.g. salbutamol Antagonists bind to/occupy but don t activate receptors e.g. atenolol Drugs work by binding to and activating receptors. Known as agonists e.g. salbutamol (ß receptor agonist, relaxes bronchial smooth muscle, opposes bronchoconstriction, which occurs in asthma). o Selective for ß2 (Gs) adrenoceptors on airway smooth muscle but excessive amounts causes activation of ß1 (Gs) adrenoceptors (on cardiac cells) which can cause increased heart rate Drugs work by binding to and antagonising receptors. Known as antagonists e.g atenolol (ß1 selective antagonist) o Activation of ß1 adrenoreceptors on cardiac cells cause increase in AC and camp causing increase in Ca2+ causing cardiac contraction ie increase HR and BP

3 o Antenolol antagonises the action of noradrenaline and adrenaline, decreases heart rate, reduces blood pressure) Ion Channels Allow ions (e.g. sodium, calcium, potassium) to cross membranes and are often voltage gated Different from receptor (ligand) operated channels above Blockers prevent ions from passing through e.g. lignocaine Modulators increase or decrease opening probability of channel Drugs work by blocking ion channels e.g. lignocaine (Na+ channel blocker: prevents action potentials and sensory sensation, and causes local anaesthesia) o It is blocking the ion channel directly so not a receptor In case of short answer-modulator example is DDT a common insecticide that opens sodium channels causing AP s to fire continuously and spontaneously leading to spasm and death Enzymes Metabolise substrates to products Enzyme inhibitors prevent metabolism/product formation e.g. aspirin

4 False substrates produce abnormal metabolite (subverts normal metabolic pathway) Can convert inactive drugs to active ones Drugs work by inhibiting enzymes e.g. aspirin (inhibits cyclooxygenase in platelets to prevent platelet clotting (ie thins blood). Inhibiting COX also inhibits prostaglandin production, which causes analgesia and reduces inflammation) example of prodrug is codeine which is converted in liver by CYP2D6 into morphine to work as an analgesic Transporters Transport substrates across membranes e.g. dopamine transporter DAT-pumps dopamine out of synaptic cleft back into cytosol or serotonin transporter-sert Inhibitors prevent transport e.g. fluoxetine False substrates can cause abnormal compounds to accumulate in cell Drugs work by inhibiting transporters: e.g. fluoxetine (inhibits serotonin/5ht uptake into nerve terminals in CNS, increases synaptic concentrations of 5HT: used to treat depression)

5 Receptor Proteins Receptors may be classified according to their: o effector linkages (transduction mechanism) o molecular structure Four receptor types: o Ligand gated ion channels (ionotropic) E.g. nicotinic ACh receptors at preganglionic fibres o G-protein coupled (metabotropic) e.g. muscarinic ACh receptors on smooth muscle cells and cardiac cells o Kinase linked receptors-involved with protein phosphorylation E.g. cytokine receptors which cytokines bind to o Nuclear receptors-occurs in nucleus, drugs need to be sufficiently lipophilic to enter nucleus E.g. oestrogen receptor which tamoxifen (antagonist) can bind to and prevent oestrogen binding to it to prevent cancer cell proliferation

6 The functions of G proteins Consists of three subunits (α,β,γ) anchored to the membrane Agonist binding to receptor causes coupling with α subunit and GTP/GDP exchange α subunit dissociates from complex and binds to a target (e.g. enzyme, ion channel) GTP is hydolysed resulting in GDP binding and re-association with β,γ complex

7 How Drugs Bind Reversible bind: typical of drug binding o Electrostatic attraction o Van der Waals bond o Hydrophobic interaction Irreversible binding: applies to some drugs o E.g. covalent bond Drug Receptor interaction Autonomic Terminology Term Autonomic Somatic Efferent Afferent Parasympathetic Sympathetic Enteric Ganglion Vascular Visceral Contraction Constriction Cholinergic ACh Receptor Adrenergic Adrenoceptor Definition Involuntary, unconscious automatic portion of the nervous system Voluntary, conscious portion of the nervous system Motor portion of the nervous system - from CNS to periphery Sensory portion of the nervous system - from periphery to CNS Part of ANS - originates in cranial nerves and sacral part of spinal cord Part of ANS - originates in thoracic and lumbar parts of spinal cord Part of ANS, semiautonomous, which specific control over GIT A mass of nerve cell bodies Relating to blood vessels Relating to internal organs Shortening Narrowing Term for nerve ending or synapse in which ACh is the main N/T A receptor that binds and is activated by ACh and related drugs Term for nerve ending or synapse in which NAd is the main N/T A receptor that binds and is activated by NAd and related drugs

8 NMJ (neuromuscular junction) NEJ (neuroeffector junction) Postsynaptic Presynaptic Autoreceptors Baroreceptor Reflex Refers to synapse between somatic nerve and skeletal muscle Refers to synapse between autonomic nerve and smooth muscle Receptor located on distal side of synapse, on effector cells Receptor located on nerve ending that modulates N/T release Presynaptic receptor that is activated by the N/T that it modulates Homeostatic mechanism that tries to maintain blood pressure L3-Theory of Experimental Design Lab 1 Intro o ACh mediated neurotransmission is fundamental to functioning of peripheral and central nervous system (could be a good intro to short answer) o Blockade can be lethal; gradual loss associated with cognitive, autonomic and neuromuscular dysfunction o AChE hydrolyses and inactivates Ach into choline and acetate ion thereby regulating the conc of ACh at the nerve synapse o At the synapse AChE is E/C but is attached to membrane as an insoluble TETRAMER (polymer comprising 4 monomer units) Effect of inhibiting AChE at cholinergic synapses ie excess ACh o At parasymp postganglionic synapses Basically activation of M2 and M3 Increased secretions (e.g. salivary, lacrimal, bronchial) Bronchoconstriction and increased gut motility Bradycardia (slow heart action) o At NMJ Muscle twitching Subsequent paralysis o In CNS Excitation leading to convulsions Depression leading to respiratory failure, unconsciousness and death Structure of AChE o Active site contains two subsites o 1. Peripheral Anionic Site - cation π interaction nothing to do with hydrolysis, just to bind receptor with substrate o 2. Catalytic site (esteratic site) catalytic triad is formed (glutamic acid, histidine and serine) and ultimately oxygen atom of serine is left with negative

9 charge which acts as nucleophile and attacks carbonyl group o an acetyl-enzyme intermediate is formed that is hydrolysed the enzyme can be acetylated, carbamylated or phosphorylated by cholinesterase inhibitors to slow or inhibit ACh breakdown AChE inhibitors o 3 main groups o 1. Short duration only important one is edrophnonium acts at NMJ It is a quaternary ammonium compound that binds to anionic site of enzyme only with ionic bond. Readily reversible as a result Main purposes are for diagnosis because improvement of muscle strength by ACh is characteristic of myasthenia gravis o 2. Medium duration Includes neostigmine and physostigmine Neostigmine acts at NMJ and is used for treatment of myasthenia gravis and reversal of competitive NMJ block Carbamates- posses basic groups that bind to anionic site. Transfer of cabamyl group to serine hydroxyl group of esteratic site occurs like with Ach, but the carmylated enzyme is hydrolysed much slower, taking mins rather than microseconds. The anticholinesterase drug is therefore hydrolysed but at a negligible rate compared to ACh and the slow recovery of the carbamylated enzyme means that the action of these drugs is medium lasting o 3. Irreversible phosphorylates serine hydroxyl group interact with only esteratic site and have no cationic group usually inactive phosphorylated enzyme is usually very stable. Enzyme can spontaneously hydrolyse and cause reactivation of cholinesterase however the process is very slow and often not clinically significant. Oximes can speed up this process another more common pathway is breaking of an alkyl like side chain on OP which leaves behind a negative charge on AChE that is resistant to nucleophilic attack meaning aged complex has formed that is completely irreversible recovery of enzymatic activity depends on synthesis of new enzyme molecules which can take weeks e.g. sarin (used as chemical war agent), acts at NMJ and postganglionic parasympathetic junction and malathion (prodrug) which is used as an insecticide and to kill head lice

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11 Myasthenia Gravis and Treatment Symptoms- muscle weakness (drooping eyelids) and fatigue Autoimmune disease resulting in severe loss (~70%) of nachrs from NMJ AChE inhibitors can be very useful in increasing AChs concs but if too many receptors are lost, not so effective An alternative approach is immunosuppression- limit antibody (that is destroying receptor) production Some AChEIs like tacrine have been used to increase cognitive function of people with Alzheimers but concerns with hepatoxicity and unwated cholinergic side effects so has been replaced. o Tacrine acts at CNS and is medium acting BuChE Less specific enzyme, can hydrolyse ACh well Soluble unlike AChE and is found in plasma Function still debated Metabolises suxamethonium (short acting neuromuscular junction blocker that is a muscle relaxant) which AChE doesn t Distribution Substrate specificity Function Acetylcholinesterase (AChE) Limited: neuromuscular junction* and neuronal synapse** Narrow: ACh and methacholine (to do with molecular confirmations of enzyme e.g. pocket to allow methyl group of methacholine to bind) Hydrolyses and terminates action of ACh at synapse: also new roles * includes neuroeffector ** autonomic ganglia and CNS Butyrylcholinesterase (BuChE) Widespread: liver, skin, brain, GI smooth muscle; soluble form in plasma Broad since BuChE has larger active site: BuCh, ACh, suxamethonium Physiological function unclear: keeps ACh levels low. Important role pharmacologically to metabolise some drugs (e.g. suxamethonium) Examples Of Uses Of Inhibitors Of AChE To treat dementia e.g. Alzheimer s disease (e.g. tacrine): limited effectiveness To reverse the action of neuromuscular blocking drugs (e.g. neostigmine) To test for (e.g. edrophonium i.v.) or treat myasthenia gravis (e.g. neostigmine) As pesticides, and in the treatment of head lice (e.g. malathion) As war gases (e.g. sarin)

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