A muscarinic receptor agonist is an agent that activates the activity of the muscarinic acetylcholine receptor. The muscarinic receptor has different subtypes, labelled M1-M5, allowing for further differentiation.
Muscarinic acetylcholine receptors in the peripheral nervous system occur primarily on autonomic effector cells innervated by postganglionic parasympathetic nerves. Muscarinic receptors are also present in autonomic ganglia and on some cells (e.g., vascular endothelial cells) that, paradoxically, receive little or no cholinergic innervation. Within the central nervous system (CNS), the hippocampus, cortex, and thalamus have high densities of muscarinic receptors.
Acetylcholine (ACh), the naturally occurring neurotransmitter for these receptors, has virtually no systemic therapeutic applications because its actions are diffuse, and its hydrolysis, catalyzed by both acetylcholinesterase (AChE) and plasma butyrylcholinesterase, is rapid. Muscarinic agonists mimic the effects of ACh at these sites. These agonists typically are longer-acting congeners of ACh or natural alkaloids, some of which stimulate nicotinic as well as muscarinic receptors.