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At the end of this session, the student should be able to: • Describe cholinergic transmission • Enumerate types of cholinergic receptors, clinically relevant sites where present & response on stimulation PH 1.14 • Enumerate choline esters & alkaloids & mention clinical uses of each Parasympathomimetics with basis • Enumerate anticholinesterases, describe their important uses with Cholinergic drugs preferred agent & basis of use • Describe clinically relevant differences between Physostigmine & Neostigmine • Explain why Physostigmine is preferred for t/t of glaucoma & Belladona poisoning • Explain why Edrophonium is used for diagnostic purpose but not 1 preferred for therapeutic purpose 2 Parasympathetic Nervous System (Craniosacral Outflow) • Explain why anticholinesterases are not used for reversing the action SA & AV Node Bronchi/Bronchial of succinylcholine Circular Muscle of Iris Glands Ciliary Muscle • Explain t/t of early mushroom poisoning & acute organophosphate Stomach poisoning • Write a brief note on: Small Intestine Lacrimal Gland a) Edrophonium Bile Ducts b) Pralidoxime Gallbladder c) Cholinergic crisis Submaxillary & Kidney Sublingual d) Myasthenic crisis Glands Large Intestine Bladder Parotid Gland Genitalia 3 4 Sympathetic Nervous System (Thoracolumbar Outflow) Radial Muscle of Iris Ciliary Muscle Cholinergic and Adrenergic System Sublingual,Submaxillary & Parotid Gland • Accordingly: Pilomotor Muscles SA & AV Nodes Sweat Glands His-Purkinje System – Cholinergic Drugs, i.e. they act by releasing Myocardium acetylcholine Bronchi/Bronchial Glands • But also utilize nitric oxide (NO) or peptides for transmission Stomach Blood Vessels – Noradrenergic (commonly called "adrenergic") Drugs - Kidneys act by releasing norepinephrine (NA) Paravertebral Ganglia Intestines Bladder/ Genitalia Prevertebral Ganglia 5 6 1 . Sites of Cholinergic Transmission Schematic diagram comparing some anatomic 1. All preganglionic sites (Both Parasympathetic and sympathetic) and neurotransmitter features of autonomic and 2. All Postganglionic Parasympathetic sites and sympathetic to somatic motor nerves. Only the primary transmitter substances are shown. Smooth muscle of vessels sweat gland and blood vessels residing in skeletal muscle Parasympathetic ganglia are not shown 3. Skeletal Muscles because most are in or near the wall of the organ innervated. Cholinergic nerves are shown 4. CNS: Cortex, basal ganglia, spinal cord and others in blue; noradrenergic in red; and dopaminergic in green. Note that some sympathetic postganglionic fibers release acetylcholine or Parasympathetic Stimulation – Acetylcholine (Ach) release at dopamine rather than norepinephrine. The neuroeffector junction - biological effects adrenal medulla, a modified sympathetic ganglion, receives sympathetic preganglionic Sympathetic stimulation – Noradrenaline (NA) at neuroeffector fibers and releases epinephrine and junction - biological effects norepinephrine into the blood. ACh, acetylcholine; D, dopamine; Epi, epinephrine; M, muscarinic receptors; N, nicotinic receptors; NE, norepinephrine. 7 8 Cholinergic Transmission: • Cholinergic neurons contain large numbers of small membrane-bound vesicles concentrated near the synaptic portion of the cell membrane • ACh is synthesized in the cytoplasm from acetyl-CoA and choline by enzyme, Choline acetyltransferase (ChAT) • Acetyl-CoA is synthesized in mitochondria present in large numbers in the nerve ending • Choline is transported from the extracellular fluid into the neuron terminal by a sodium-dependent membrane carrier (carrier A). Hemicholinium ❖The action of the choline transporter is the rate-limiting step in ACh synthesis 9 10 Cholinergic Transmission: Cholinergic Transmission: • Synthesized, ACh is transported from the • After release - ACh molecules may bind cytoplasm into the vesicles by an antiporter that to and activate cholinoceptors removes protons (carrier B). Vesamicol • Eventually (and usually very rapidly), all • Release is dependent on extracellular Ca2+ of the ACh released will diffuse within and occurs when an AP reaches the terminal range of an acetylcholinesterase and triggers sufficient influx of Ca2+ ions (AChE) molecule • The increased Ca2+ concentration • AChE splits ACh into choline and "destabilizes" the storage vesicles by acetate, interacting with special proteins associated with the vesicular membrane (VAMPs) • Most cholinergic synapses are richly supplied with AChE; the half-life of ACh •Fusion of the vesicular membranes with the in the synapse is therefore very short. terminal membrane results in exocytotic expulsion of ACh into the synaptic cleft • Another cholinesterase with a lower specificity for ACh, • The ACh vesicle release process is blocked butyrylcholinesterase [pseudo by botulinum toxin through the enzymatic cholinesterase] removal of two amino acids from one or more of the fusion proteins. Black widow spider?? 11 12 2 . Differences between 2 AChEs Cholinergic receptors - 2 types True AChE Pseudo AChE • Muscarinic (M) and Nicotinic (N): Distribution All cholinergic sites, Plasma, liver, Intestine RBCs, grey matter and white matter Action on: Acetycholine Very Fast Slow Methacholine Slower Not hydrolyzed Inhibition More sensitive to More sensitive to Physostigmine Organophosphates Nicotinic (N) – Muscarinic ligand gated Function Termination of Ach Hydrolysis of Ingested (M) - GPCR action Esters 13 14 15 16 Acetylcholine (cholinergic receptors) Muscarinic Receptors - Subtypes – Muscarinic Receptors Amanita muscaria • Pharmacologically - M1, M2, M3, M4 and M5 • M4 and M5 are present in certain areas of Brain and regulate 1. Selectively stimulated by Muscarine and blocked by Atropine – other neurotransmitters all are G-protein coupled receptors • M1, M3 and M5 fall in one class, while M2 and M4 in another 2. Primarily located in heart, eye, smooth muscles and glands of class GIT • Most organs usually have more than one subtype but one 3. Blood vessels: All blood vessels have muscarinic receptors although no cholinergic innervation subtype predominates in a tissue 17 18 3 . Muscarinic Receptor Subtypes M1 M2 M3 Location Autonomic ganglia, Heart and CNS SMs of Viscera, Eye, Gastric glands and CNS exocrine glands and endothelium Functions EPSP Less impulse Visceral SM Histamine release & generation, less contraction, acid secretion velocity of Constriction of CNS - learning and conduction, pupil, motor functions decreased contraction of contractility, less Ciliary muscle Ach release vasodilatation Agonist Oxotremorine Methacholine Bethanechol Antagonist Pirenzepine Methoctramine & Darifenacin 19 Triptramine 20 Nicotinic (N) Receptors Sites of Cholinergic transmission and types of Receptors Site Types Selective Selective ➢ Nicotinic receptors: nicotinic actions of ACh are agonist antagonist those that can be reproduced by the injection of All Postganglionic Nicotine (Nicotiana tabacum) Parasympathetic Muscarinic Muscarine Atropine Postganglionic ➢ Can be blocked by tubocurarine and sympathetic to hexamethonium sweat gland & BV • ligand-gated ion channels Ganglia (Both Para and NN DMPP Hexamethonium sympathetic and also – activation results in a rapid increase in cellular Adrenal Medulla permeability to Na+ and Ca++ resulting - depolarization and initiation of action potential Skeletal Muscle NM PTMA Curare CNS Muscarinic Muscarine Atropine Oxotremorine 21 22 Ganglia Concept - summary Cholinergic Drugs or Cholinomimetic or Parasympathomimetics Drugs producing actions similar to Ach – by interacting with Cholinergic receptors or by increasing availability of Ach at these sites. 23 24 4 . Classification - Direct-acting (receptor Cholinergic Drugs – Indirect acting agonists ) • Reversible anticholinesterases: • Choline Esters – Natural: Physostigmine – Natural: Acetylcholine – Synthetic: neostigmine, pyridostigmine, rivastigmine, donepezil, – Synthetic: Methacholine, Carbachol and gallantamine, edrophonium, ambenonium, demecarium Bethanechol • Irreversible anticholinesterases: – Organophosphorous Compounds (OPC) – Diisopropyl • Alkaloids: Pilocarpine, Muscarine, Arecoline fluorophosphate (DFP), Ecothiophate, Parathion, malathion, diazinon (insecticides and pesticides) – Synthetic: Oxotremorine – Tabun, sarin, soman (nerve gases in war) – Carbamate Esters: Carbaryl and Propoxur (Baygon) 25 26 Question… Ach actions - Muscarinic 1. Heart: M2 – Hyperpolarization of SA node, reduction in impulse generation • What side effects might you expect to and Bradycardia see in a patient taking a cholinergic drug? – RP in SAN and PF increased but atrial muscles fibers abbreviated – Slowing of AV conduction and His-purkinje fibres – partial or complete block • Hint… Cholinergic = “Colon-Urgent” – Decrease in ventricular contractility 2. Blood Vessels: M3 – Cholinergic innervations is limited – skin of face and neck – But, M3 present in all type of blood vessel – Vasodilatation by Nitric oxide (NO) release – Penile erection 27 28 Muscarinic action – contd. Ach actions - Nicotinic 3. Smooth Muscles: M3 1. Autonomic ganglia: – Abdominal cramps, diarrhoea – due to increased peristalsis and – Both Sympathetic and parasympathetic ganglia are stimulated relaxed sphincters – After atropine injection Ach causes tachycardia and rise in BP – Voiding of Bladder – Bronchial SM contraction – dyspnoea, attack of asthma etc. 2. Skeletal muscle – IV injection – no effect 4. Glands: M3 – Application causes contraction of skeletal muscle – Increased secretions: sweating, salivation, lacrimation, tracheobronchial