ACETYLCHOLINE AND ITS MUSCARINIC RECEPTOR TARGET
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 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 and are longer-acting congeners of ACh or natural alkaloids.
Cholinergic synapses are found at:
Autonomic effector sites innervated by postganglionic parasympathetic nerves (or, in the sweat glands, by postganglionic sympathetic nerves)
Sympathetic and parasympathetic ganglia and the adrenal medulla, innervated by preganglionic autonomic nerves
Motor end plates on skeletal muscle, innervated by somatic motor nerves
Certain synapses in the CNS, where ACh can have either pre- or postsynaptic actions
The actions of ACh and related drugs at autonomic effector sites are termed muscarinic, based on the observation that the alkaloid muscarine acts selectively at those sites and produces the same qualitative effects as ACh (see Table 8–1). Muscarinic receptors are present in autonomic ganglia and the adrenal medulla but primarily function to modulate the nicotinic actions of ACh at these sites (see Chapter 11). In the CNS, muscarinic receptors are widely distributed and mediate many important responses. The actions of ACh and its congeners at muscarinic receptors can be blocked competitively by atropine.
PROPERTIES AND SUBTYPES OF MUSCARINIC RECEPTORS
Muscarinic receptors comprise 5 distinct gene products, designated as M1 through M5 (see Table 8–3). Muscarinic receptors are GPCRs that in turn couple to various cellular effectors. Selectivity is not absolute but, in general, stimulation of M1, M3, and M5 receptors activates the Gq-PLC-IP3/DAG-Ca2+ pathway, resulting in a variety of Ca2+-mediated responses. By contrast, M2 and M4 muscarinic receptors couple to the pertussis toxin–sensitive G proteins, Gi and Go, to inhibit adenylyl cyclase and regulate specific ion channels.
The 5 muscarinic receptor subtypes are widely distributed in both the CNS and peripheral tissues; most cells express at least 2 subtypes (see Table 8–3). The M2 receptor is the predominant subtype in the cholinergic control of the heart, whereas the M3 receptor is the predominant subtype in the cholinergic control of smooth muscle, secretory glands, and the eye. The M1 receptor has an important role in the modulation of nicotinic cholinergic transmission in ganglia.
PHARMACOLOGICAL EFFECTS OF ACETYLCHOLINE