Loss of GABAB receptors in cochlear neurons: Threshold elevation suggests modulation of outer hair cell function by type II afferent fibers

Abstract

Despite pharmacological and immunohistochemical evidence for GABA as a neurotransmitter in the olivocochlear efferent bundle, a clear functional role of GABA in the inner ear has not emerged. To explore the role of metabotropic GABAB receptors, we characterized the cochlear phenotype of mice with targeted deletion of the GABAB1 subunit and determined its tissue localization using a mouse line expressing a GFP-tagged GABAB1 subunit under the endogenous promoter. Immunostaining revealed GABAB1 expression in both type I and type II ganglion cells and in their synaptic terminals under inner and outer hair cells, respectively. No GABAB1 expression was observed in hair cells. Mean cochlear thresholds, measured via both auditory brainstem responses and distortion product otoacoustic emissions (DPOAEs), were elevated by ∼10 dB in GABAB1-deficient mice, consistent with outer hair cell dysfunction. Olivocochlear efferent function, assessed via DPOAE suppression during efferent electrical stimulation, was unaffected by GABAB1 deletion. GABAB1-deficient mice showed increased resistance to permanent acoustic injury, with mean threshold shifts ∼25 dB smaller than wild-types after exposure to 8-16-kHz noise at 100 dB for 2 h. In contrast, there was no vulnerability difference to temporary acoustic injury following exposure to the same noise at 94 dB for 15 min. Our results suggest that GABAergic signaling in type II afferent neurons may be required for normal outer hair cell amplifier function at low sound levels and may also modulate outer hair cell responses to high-level sound. © 2008 Association for Research in Otolaryngology.