CHOLINERGIC TRANSMISSION 1 6/9/2018

Learning objectives At the end of the session students must be able to have: A clear understanding of the cholinergic transmission in the periphery and the ways in which drugs affect it. 2 6/9/2018

Learning objectives… Be able to describe:… the different types of Ach receptors and their functions, Synthesis and release of Ach Drugs that act on Ach receptors 3 6/9/2018

CHOLINERGIC TRANSMISSION Acetylcholine receptors are subdivided into: Nicotinic (nAChR) Muscarinic (mAChR) subtypes 4 6/9/2018

Nicotinic Receptors Directly coupled to cation channels Mediate fast excitatory synaptic transmission at the NMJ (neuromuscular junction), autonomic ganglia and at various sites in the CNS. Nicotinic receptors are subdivided into: Muscle – type NM ( muscle nAChR) Neuronal type NN (neuronal nAChR) Ganglionic type (ganglionic nAChR) 5 6/9/2018

6 6/9/2018

Nicotinic Receptors… Muscle nAChR and Neuronal nAChR differ in their molecular structure and pharmacology Muscle receptors are confined to the skeletal neuromuscular junction Neuronal receptors are widespread in the brain, and are heterogeneous with respect to their molecular composition and location Ganglionic receptors are responsible for transmission at sympathetic and parasympathetic ganglia 7 6/9/2018

Nicotinic Receptors… The nicotinic actions correspond to those of Ach acting on autonomic ganglia of the sympathetic and parasympathetic systems, the motor end place of voluntary muscle and the secretory cells of the adrenal medulla.

Muscarinic Receptors Muscarinic receptors (mAChRs) are G-protein coupled receptors, causing: Activation of phospholipase C (hence formation inositol triphosphate and diacylglycerol as second messengers) activation of K+ channels or inhibition of Ca+ channels. Inhibition of adenylate cyclase 9 6/9/2018

Muscarinic Receptors… Mediate Ach effects at postganglionic parasympathetic synapses (mainly heart, smooth muscle, glands) and contribute to ganglionic excitation. 10 6/9/2018

Exceptions: Ach causes generalized vasodilatation, even though most blood vessels have no parasympathetic innervations. Ach evokes secretion from sweat glands which are innervated by cholinergic fibres of the sympathetic nervous system. 11 6/9/2018

Muscarinic receptor-Subtypes Gene cloning has revealed five distinct types of muscarinic receptor : M1, M2, M3, M4, M5 but only four have been distinguished functionally and pharmacologically. Three of this (M1, M2, M3) are well characterized 12 6/9/2018

M1-receptors (‘neural’) found mainly on CNS, peripheral neurones and gastric parietal cells. Also in other glands salivary, lacrimal etc; cerebral cortex They mediate excitatory effects, for example the slow muscarinic excitation mediated by acetylcholine in sympathetic ganglia and central neurones. this excitation is produced by a decrease in K+ conductance, which causes membrane depolarisation. 13 6/9/2018

M1-receptors (‘neural’)… Deficiency of this kind of acetylcholine-mediated effect in the brain is possibly associated with dementia. Also involved in the increase of gastric acid secretion following vagal stimulation are selectively blocked by pirenzepine. 14 6/9/2018

M2-receptors (‘cardiac’) Occur in the heart, and also on the presynaptic terminals of periphery and central neurons. Exert inhibitory effects, mainly by increasing K+ conductance and inhibiting calcium channels causing decrease in cardiac rate and force of contraction (mainly of atria) and also mediate presynaptic inhibition They are selectively blocked by gallamine 15 6/9/2018

M3-receptors (‘glandular/smooth muscle’) Produce mainly excitatory effects i.e. stimulation of glandular secretions (salivary, bronchial, sweat etc.) and contraction of visceral smooth muscle Mediate relaxation (mainly vascular) of smooth muscle, which results from the release of nitric oxide from neighboring endothelial cells Are mainly blocked by darifenacin Selective agonist is cevimeline 16 6/9/2018

M1-; M2- and M3- receptors occur also in specific locations in the CNS 17 6/9/2018

M4 and M5 Receptors Are largely confined to the CNS Their functional role is not well characterized Read for more details 18 6/9/2018

All mAChR are activated by Ach and blocked by atropine The pharmacological classification of these receptor types relies on the limited selectivity of certain agonists and antagonists that can be distinguished between them. (Refer: Pharmacology by Rang & Dale 7th edition-Table 13.2 of chapter 13-”mAchR Subtypes) 19 6/9/2018

20 6/9/2018

PHYSIOLOGY OF CHOLINERGIC TRANSMISSION 21 6/9/2018

Synthesis and Release of Acetylcholine. Ach is synthesized within the nerve terminal from choline, which enters the neuron via carrier-mediated transport. 22 6/9/2018

ACh Synthesis… Requires acetylation of choline, utilizing acetyl CoA ( coenzyme A) as source of acetyl groups, which requires choline acetyl transferase (CAT), a cytosolic enzyme found only in cholinergic neurons. 23 6/9/2018

ACh Synthesis… Most of the ACh synthesized is packaged into synaptic vesicles at high concentration (about 100mmol/l) by carrier-mediated transport. Ach release occurs by Ca++ - mediated exocytosis. At the neuromuscular junction (NMJ), one presynaptic nerve impulse releases 100-500 vesicles. 24 6/9/2018

ACh Release: Requires calcium Triggers an interaction between several proteins associated with the vesicle [VAMPs = vesicle – associated membrane proteins, synaptobrevin synaptogmin] and nerve ending membrane (SNAPS, synaptosome- associated proteins, SNAPS25, syntazin and others). 25 6/9/2018

ACh Release… This interaction results in the fusion of the membranes of the vesicle with the nerve ending membranes, the opening of a pore to the extracellular space and the release of the stored transmitter. 26 6/9/2018

At the NMJ, Ach acts on nicotinic receptor to open cation channels, producing a rapid depolarisation (end plate potential), which normally initiates an action potential in the muscle fibre. Transmission at other “fast” cholinergic synapses (e.g. ganglionic) is similar. 27 6/9/2018

At “fast” cholinergic synapses, ACh is hydrolysed within about 1ms by acetylcholinesterase, so a presynaptic action potential produces only one post synaptic action potential. 28 6/9/2018

Transmission mediated by muscarinic receptors is much slower in its time course and synaptic structures are less clearly defined. In most cases Ach function as a modulator rather than as a direct transmitter. 29 6/9/2018

Main mechanisms of pharmacological block: inhibition of choline uptake, inhibition of Ach release, block of postsynaptic receptors or ion channels, persistent postsynaptic depolarisation 30 6/9/2018

Events and sites of drug action at a nicotinic cholinergic synapse Events and sites of drug action at a nicotinic cholinergic synapse. Acetylcholine (ACh) is shown acting postsynaptically on a nicotinic receptor controlling a cation channel (e.g. at the neuromuscular or ganglionic synapse), and also on a presynaptic nicotinic receptor that acts to facilitate ACh release during sustained synaptic activity. The nerve terminal also contains acetylcholinesterase (not shown); when this is inhibited, the amount of free ACh, and the rate of leakage of ACh via the choline carrier, is increased. Under normal conditions, this leakage of ACh is insignificant. At muscarinic cholinergic junctions (e.g. heart, smooth muscle and exocrine glands), both postsynaptic and presynaptic (inhibitory) receptors are of the muscarinic type. AcCoA, acetyl coenzyme A; AChE, acetylcholinesterase; CAT, choline acetyltransferase; CoA, coenzyme A. 31 6/9/2018

EFFECTS OF DRUGS ON CHOLINERGIC TRANSMISSION

They are subdivided according to their site of action: muscarinic agonists muscarinic antagonists ganglion-stimulating drugs ganglion-blocking drugs neuromuscular-blocking drugs anticholinesterases and other drugs that enhance cholinergic transmission 33 6/9/2018

34 6/9/2018

35 6/9/2018

36 6/9/2018

37 6/9/2018

38 6/9/2018