Excitotoxicity and repair of cochlear synapses after noise-trauma induced hearing loss.

Guinea-pigs were exposed to a traumatic sound inducing up to 80 dB hearing loss. Beside the well described mechanical damage to outer hair cells, a total disruption of inner hair cell (IHC)-auditory nerve synapses was acutely observed within the traumatized area. To test the hypothesis that synaptic damage is due to an excessive release of glutamate by the IHCs, we examined the protective effect of the glutamate antagonist kynurenate on noise-induced hearing loss. The high degree of protection observed with kynurenate attests that dendritic damage is an important component in noise-induced hearing loss. Moreover, we demonstrate that a synaptic repair mechanism occurring within the first few days post-exposure is partly responsible for the recovery of temporary threshold shifts after an acoustic trauma.