Serotonergic innervation of the organ of Corti

The olivocochlear efferent system of the mammalian cochlea, which is divide d into two lateral and medial bundles, contains numerous neuroactive substa nces (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). The se have been located at the brainstem in neurons belonging to the lateral s uperior olive (lateral efferent system) or in neurons of the periolivary re gion around the medial superior olive and the trapezoid body (medial effere nt system). All of these substances were found in weil-characterized projec tions corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respecti vely. All could be involved in the modulation of the auditory process, as i s suggested by the cochlear turnover increases observed in some of them (i. e. enkephalins or dopamine) induced by sound stimulation. Recently, the pre sence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly parti cular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a p reviously unknown cochlear innervation.