Functional role of GABAergic innervation of the cochlea:: Phenotypic analysis of mice lacking GABAA receptor subunits α1, α2, α5, α6, α2, α3, or δ

The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABA(A) receptor subunit (alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta). Four of the lines (alpha 1, alpha 2, alpha 6, and delta) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (alpha 5, beta 2, and beta 3) showed threshold elevations indicative of outer hair cell dysfunction. alpha 5 and beta 2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in alpha 5 (> 6 weeks) than beta 2 (< 6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (beta 2 and beta 3) showed signs of neuropathy, either decreased density of afferent innervation (beta 3) or decreased neural responses without concomitant attenuation of hair cell responses (beta 2). One of the lines (beta 2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.