Muscarinic acetylcholine receptor binding in the guinea pig cochlea

Because of the possibility that olivocochlear bundles may use acetylcholine as an inhibitory transmitter, we examined the cochlea of the guinea pig for cholinergic receptor binding sites. Binding of the potent muscarinic antagonist [3H]quinuclidinyl benzilate (3H-QNB) showed that the cochlea has sites with the pharmacological specificity of muscarinic cholinergic receptors. Specific 3H-QNB binding was saturated at 0.8 nM and was half-saturated at 0.03 nM concentrations. Total 3H-QNB binding was reduced 70% in the presence of saturating doses of acetylcholine, oxotremorine and atropine, and half-maximal competition occurred at doses comparable to those at other muscarinic sites in the central and peripheral nervous systems. The nicotinic specific antagonist d-tubocurarine did not block 3H-QNB binding at concentrations known to be effective in electrophysiological experiments, indicating the sites measured here were not the mixed muscarinic-nicotinic receptor type postulated for some systems. Localization of binding sites, as done by hand microdissection methods, showed that basal cochlear turns had five times the number of sites as apical turns. Most of the sites were evenly divided between the bony modiolus and the auditory nerve, although some sites were measured in sensory tissue. These results support the idea that cholinergic communication occurs in the cochlea, but they are also consistent with the hypothesis that other receptor mechanisms may be involved in olivocochlear inhibition.