Developmental regulation of glycine receptors at efferent synapses of the murine cochlea.

Efferent olivocochlear feedback innervation modulates the stream of auditory information from cochlea to brainstem by regulating auditory nerve activity and controlling the contribution of cochlear outer hair cells to basilar membrane motion. In our previous work, we gave a first description of glycine receptors (GlyRs) in the rat cochlea indicating a possible localization at efferent cochlear synapses. Here, we analyze the developmental regulation of GlyR transcripts and protein within the developing murine organ of Corti (postnatal days P0-P21). Using quantitative RT-PCR, GlyRα1 and α2 were identified as the predominant GlyRα subunit transcripts before the onset of hearing (<P12), whereas GlyRα3 prevailed afterwards. Compared to GlyRα subunits, high levels of GlyRβ mRNA were detected from P0-P21. Nested RT-PCR of isolated hair cells revealed a translocation of GlyRα and β transcripts from inner to outer hair cells paralleling the shift of efferent cochlear innervation from inner to outer hair cells around the onset of hearing. This observation was verified on the protein level by immunostaining of GlyRα protein on cochlear cryosections. Finally, postsynaptic clusters of α3-containing GlyRs were located to efferent synapses of the medial and lateral olivocochlear bundle in the murine organ of Corti beyond the onset of hearing. In summary, the distinct developmental regulation of GlyRs in the murine cochlea advocates a contribution of these chloride channels to efferent olivocochlear innervation.