Muscarinic Cholinergic Receptors

In the central nervous system (CNS), modulation of neuronal activity is brought about by the actions of endogenous “neurotransmitters” that specifically interact with receptor proteins located in the neuronal cell membrane. In the cholinergic system, acetylcholine is the primary neurotransmitter released in the CNS, and appears to alter neuronal excitability by affecting such processes as cGMP production and K⁺ flux (Heilbronn and Bartfai, 1978 ). In contrast to the periphery, in which the cholinergic receptor population is divided between nicotinic and muscarinic subclasses, the CNS-associated cholinergic receptor appears to be predominantly of the muscarinic type, although there is evidence that the spinal cord contains substantial amounts of nicotinic receptors (Heilbronn and Bartfai, 1978 ). Historically, the initial characterization of cholinergic actions was provided by A. S. V. Burgen. Although these early studies, using smooth muscle tissue (Burgen and Spero, 1968 ; Burgen et al., 1974 ), provided initial clues about the peripheral muscarinic receptors, since then there have been numerous investigations, especially following the advent of radioligand binding assays, from which have come many novel observations of physiological significance.