Cholinergic System

The important structural features of the neurotransmitter acetylcholine are the strongly positive quaternary amine in the choline part of the molecule and the partial negative charged ester component. Choline receptor antagonists have either a tertiary or a quaternary amine (or both). Acetylcholine receptors are classified as either muscarinic or nicotinic. Nicotinic receptors are widespread in the body and are found in both sympathetic and parasympathic nervous systems. Drug actions at the nicotinic receptors of the neuromuscular junction, which is not part of the ANS, are covered on pages 644-651.

Muscarinic Receptors

The muscarinic receptors are G-protein coupled receptors. Five subtypes have been identified (Mi_5) but the most important ones are M1, M2 and M3, which are all antagonized by atropine. M1 receptors are found in the central nervous system (CNS), autonomic ganglia and gastric parietal cells, M2 receptors are found in the heart and at pre synaptic sites, and M3 receptors are found in smooth muscle, vascular endothelium (causing vasodilatation) and in exocrine glands.

Muscarinic Agonists

• Examples—carbachol, pilocarpine

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