Temporospatial expression and cellular localization of VGLUT3 in the rat cochlea

Vesicular glutamate transporter 3 (VGLUT3) plays an important role in hearing, and VGLUT3 knockout mice are deaf. However, the mechanisms whereby VGLUT3 exerts its effects in the cochlea are not well established. Elucidating the developmental and aging dynamics of VGLUT3 localization and expression in the cochlea would aid a functional understanding of auditory glutamatergic transmission. In this study, we characterized the expression of VGLUT3 in rat auditory epithelium and spiral ganglion cells (SGCs) during postnatal development and with age using immunohistochemistry and quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). We observed VGLUT3 expression in cochlea inner hair cells (IHCs) and SGCs as early as postnatally, and its expression increased gradually with maturity. However, the expression of VGLUT3 in the IHCs decreased in the aging rats compared to postnatal day 60 (P60), and VGLUT3 was still expressed in the remaining SGCs in the aging rats. VGLUT3-immunopositive punctate structures were only found in the cytoplasm of SGCs, not IHCs, at P7, or later. Variation in VGLUT3 mRNA expression evaluated by quantitative real time RT-PCR was consistent with the immunohistochemical data except for no statistical difference between P60 and aging rats in the whole cochlea level. Our data suggest that VGLUT3 likely has developmental and physiological roles in the rat cochlea during postnatal development as well as later in life. VGLUT3 may have additional unknown roles in SGCs besides glutamate transport.