Expression and distribution of mu opioid receptors in the inner ear of the rat.

Opioid peptides have demonstrated modulatory effects on the vestibular afferent discharge and are putative vestibular efferent neuromodulators. The distribution of their receptors in the mammalian vestibular epithelia is not known. We used reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization, Western blots and immunohistochemistry to study the expression of mu opioid receptor (MOR) in the Scarpa's ganglia and cristae ampullares of rats. MOR transcript was only detected in the somata of the vestibular afferent neurons. MOR-like immunoreactivity was observed in the somata of vestibular afferents and in nerve terminals in the cristae ampullares epithelia both in the center and peripheral regions. Double labeling of cristae sections with the MOR1 antibody in combination with antibodies against calretinin (a marker for vestibular afferents terminating in calices) and peripherin (a marker for afferents terminating in boutons), respectively showed that MOR1 immunoreactivity was in calyx, dimorphic and bouton vestibular afferents. MOR immunoreactivity was not detected in vestibular efferent fibers identified with choline acetyltransferase immunohistochemistry. These results indicate that MOR may mediate effects of vestibular efferents on afferents.