Chapter 35: Postsynaptic actions of acetylcholine: the coupling of muscarinic receptor subtypes to neuronal ion channels

Abstract

In recent years, the application of molecular biological techniques has established that there are at least five subtypes of muscarinic receptor, each with a unique pattern of distribution. Radio-ligand binding displacement studies of antagonist pharmacology of the individual receptors expressed in appropriate host cells have shown that each receptor does have a discrete antagonist profile, but that there are no single antagonists with sufficient selectivity to be clearly diagnostic for any particular receptor type. This has hampered the elucidation of the roles of muscarinic receptor subtypes, particularly, in cells which may simultaneously express more than one subtype, as is probably the case for neurones. The use of cell lines transfected with a gene encoding a single muscarinic receptor subtype has extended the characterization of muscarinic receptor subtypes beyond the pharmacology of the recognition site and has generated an additional classification on the basis of the downstream cellular mechanisms to which each subtype preferentially couples. This approach can clearly provide indications of likely coupling mechanisms of subtypes in, for example, the CNS neurones. Initial biochemical experiments with cell lines, such as the neuroblastoma x glioma hybrid NG108-15 have established two major groupings of muscarinic receptor subtypes, with ml, m3, and m5 receptors coupling preferentially to stimulation of phospholipase C (PLC) through a Pertussis toxin-insensitive G-protein, with subsequent elevation of IP3, while m2 and m4 receptors couple through a Pertussis toxin-sensitive G-protein to preferentially inhibit adenylyl cyclase.This chapter describes the electrophysiological consequences of muscarinic receptor activation using chemically differentiated NG108-15 cells transfected with ml-m4 receptor genes. These cells are especially useful for electrophysiological studies, because they express a range of K + and Ca 2+ channels similar to those found in many neuronal cell types. However, NGl08-15 cells do natively express the m4 muscarinic receptor.