Noise-induced aspartate and glutamate efflux in the guinea pig cochlea andhearing loss

Aspartate and glutamate were monitored in the scala tympani of the guinea p ig cochlea using in vivo microdialysis before and during noise exposure; Mo derate level broad band noise [105 dB sound pressure level (SPL), 30 min] n either altered the levels of aspartate or glutamate, nor auditory brainstem response (ABR) thresholds. High level noise exposure (135 dB SPL, 30 min) caused a large increase in aspartate (330%), a smaller increase in glutamat e (150%), and a permanent ABR threshold shift of 60-75 dB between 2.0 and 1 2.5 kHz. Morphological analysis of the cochlea revealed a collapse of suppo rting structures, swelling of the afferent dendrites under the inner hair c ells, and outer hair cell loss. Pretreatment with the NMDA antagonist, MK 8 01 (1 mg/kg body weight, i.p.) 1 h before noise exposure protected the affe rent dendrites from swelling but did not protect the collapse of supporting structures, outer hair cell loss, or auditory thresholds. In conclusion, t he noise-induced increase in aspartate and glutamate release in the cochlea and the protective effect of NMDA antagonism suggest that these two neurot ransmitters are involved in noise-induced hearing loss.