Loss of hair cells and threshold sensitivity during prolonged noise exposure in normotensive albino rats.

The relation between hearing loss and loss of hair cells after prolonged exposure to a simulated industrial noise environment was determined in normotensive albino rats. Hearing loss was assessed behaviorally by a conditioned suppression technique and electrophysiologically by auditory brainstem response to pulses of 1/3 octave filtered full cycle sine waves. The ears were analyzed in surface preparations and the hair cells counted. Sixty-two ears from 46 animals were analyzed after 1, 3, 7, and 15 months of exposure. A hearing loss of up to approximately 30 dB was consistently found with minimal loss of hair cells. Between 30 and 60 dB hearing loss there was a fair correspondence between loss of function and loss of hair cells. After 15 months exposure a relatively larger loss of hair cells was observed than expected from decrease of threshold sensitivity. The results indicate the existence of a systematic although not directly proportional relation between hair cell loss and loss of sensitivity during prolonged noise exposure. It is suggested that the correlation between hearing loss and hair cell loss is influenced, e.g., by duration and level of exposure, degree of hearing loss, and location along the basilar membrane.