Serotonergic innervation of the organ of corti of the cat cochlea

The presence and distribution of serotonin-containing fibers within the adult cat cochlea is reported here for the first time. The 5-HT-like immunoreactive fibers were mainly found in the middle coils of the cat cochlea, following a peripheral distribution similar to that of the olivocochlear lateral efferent system (OLES). Thus, 5-HT-like immunoreactive varicose fibers were found within the intraganglionic spiral bundle, the inner and the tunnel bundles, although, some variation with the distribution of other neuroactive substances of the OLES were observed. In particular, some isolated fibers from the tunnel bundle branched towards the first row of the outer hair cells. Cochlear serotonergic fibers might be involved in the auditory processing within the cochlea as efferent modulating fibers.