Pure tone overstimulation changes the micromechanical properties of the inner hair cell stereocilia

The effect of permanent noise-induced hearing loss on the auditory brainstem response (ABR) and the micromechanical properties of cochlear hair cell stereocilia in guinea pigs was investigated. The threshold of movement of the stereocilia was measured by applying force from a fluid filled pipette. After exposure to a 1.0 kHz pure tone signal at 105 dB(A) for 72 h the threshold of the ABR was broadly elevated by approximately 50 dB. Inner hair cell stereocilia showed a decrease in threshold while the outer hair cell stereocilia bundles remained unaltered. This effect was localized to the 13-15 mm distance from the stapes corresponding to the region of maximal stimulation. The effect was recorded within 1 h of exposure and remained constant with exposures up to 7 days. Following a one month recovery period from sound exposure, normal threshold values of stereocilia movement were observed, indicating recovery. At this time, swelling of the afferent dendrites beneath the inner hair cells was observed throughout the cochlea together with approximately 30% scattered loss of outer hair cells in the 13 to 15 mm region. The ABR showed some recovery (approximately 20 dB), yet a threshold shift remained.