Enkephalin-like immunoreactivity of olivocochlear nerve fibers in cochlea of guinea pig and cat (opioid/cholinergic neurons/organ of Corti/immunofluorescence/efferent synapses)

The distribution of enkephalin-like immunoreac- tivity in the cochlea of the guinea pig and cat was studied. Indirect immunofluorescence immunohistochemistry using antisera gen- erated against a methionine enkephalin-bovine thyroglobulin con- jugate was applied to surface preparations of the organ of Corti and cryostat sections of the whole of the cochlea. In the cochlear osseous spiral lamina, immunofluorescence was localized to un- myelinated fibers of the intraganglionic spiral bundle. In the organ of Corti, immunofluorescence was localized to a small number of fibers at inner hair cells, the inner spiral bundle, and tunnel spiral bundle, to tunnel crossing fibers at the level of the tunnel floor, to an occasional spiral outer fiber, and to the synaptic region of outer hair cells in the three rows of the basal turn of the cochlea. Less immunofluorescence was found in this region as one progressed towards the apex, with none seen at the apex. At the most apical region the inner spiral bundle became patchy and the tunnel spiral bundle developed arcades. There was no immunofluorescence found in spiral ganglion cells, in auditory nerve fibers, or in the hair cells of the organ of Corti. The findings were the same in cat as in guinea pig, the latter being studied in more detail. It was con- cluded that efferent, olivocochlear neurons of the cochlea, synaps- ing predominantly with primary auditory nerve fibers from the in- ner sensory cells or with the sensory cells, contain enkephalin-like immunoreactivity. Also, the findings indicate that endings of oli- vocochlear neurons that synapse predominantly with outer hair cells contain enkephalin-like immunoreactivity. It has previously been shown that olivocochlear neurons are likely to be cholinergic. Methionine enkephalin and leucine enkephalin are opioid pen- tapeptides originally isolated from the whole brain (1, 2). Im- munohistochemical mapping of enkephalin-like immunoreac- tivity in the central nervous system was done (3-5), and it was noted that in certain regions enkephalin-like immunoreactivity and opiate receptors had a similar distribution (3, 5). A much entertained working hypothesis is that enkephalins may act as neurotransmitters or modulate the action of neurotransmit- ters (6).