PRESYNAPTIC AND POSTSYNAPTIC M3-MUSCARINIC-RECEPTOR MESSENGER-RNAS INTHE RODENT PERIPHERAL AUDITORY-SYSTEM

The medial and lateral efferent innervations originate from distinct p arts of the superior olivary complex. Both use acetylcholine, respecti vely, to modulate the activity of outer hair cells (OHC), and spiral g anglion neurons (SGN) which are postsynaptic to the inner hair cells ( IHC). Besides predominantly activating nicotinic receptors, acetylchol ine recognizes muscarinic M3 receptors, whose the role(s) and cellular localization(s) are not yet firmly established. We used reverse trans cription and polymerase chain reaction to amplify the M3 receptor cDNA in the rat and guinea pig organ of Corti and spiral ganglion. Then, w e localized the M3 receptor mRNAs in cochleas and superior olivary com plex of both species. The M3 receptor cDNA was amplified from samples of brain, organ of Corti and spiral ganglion. Indeed, its correspondin g mRNA was localized in SGNs, OHCs and IHCs. However, in the apical tu rns, OHCs were often found unlabeled. In the superior olivary complex, M3 mRNAs were colocalized with choline acetyltransferase mRNAs in neu rons of the lateral superior olive and ventral nucleus of the trapezoi d body. These results suggest that the M3 receptor-induced inositol ph osphate formation described in previous studies [21] takes place in bo th postsynaptic (SGNs, OHCs) and presynaptic components of efferent co chlear synapses, and in cells that are not contacted by efferents in t he adult cochlea (IHCs).