Dose-response hearing loss for white noise in the Sprague-Dawley rat

Abstract

The effect of noise exposure on the inner ear is well documented. However, the sensitivity to noise-induced damage varies with animal species. The purpose of this investigation was to generate a hearing loss dose-response curve for a 20-day white noise exposure in the Sprague-Dawley rat. Eight male rats were exposed to 110, 100, 95 or 85 dB sound pressure level (SPL) for 6 hr/day, 5 days/week for 4 weeks in a sound-attenuated chamber fitted with a bank of overhead speakers. Controls were placed in an identical chamber without speakers. Four weeks after the exposure period ended, brainstem auditory evoked response (BAER) thresholds were recorded in all rats at 32, 16, 8, 4, 2, 1, and 0.5 kHz. Rats were then killed and cochlear tissues were processed for surface preparation. Hair cells were counted. Outer hair cell loss in the organ of Corti was observed in rats exposed to 95, 100, and 110 dB. Summary cytocochleograms were prepared for each rat by graphing the percentage of all hair cells remaining vs the percentage of distance along the basilar membrane as measured from the apex. The summary cytocochleograms were averaged and the area above this curve, corresponding to hair cell loss, was calculated. BAER threshold elevations (dB) were converted into percentage loss overall hearing function. Noise exposure level (dB) and (1) percentage area above the group average summary cytocochleogram and (2) percentage hearing loss were plotted as dose-response curves. Log-probit analysis was used to calculate ED50 noise exposure levels of 117 and 104 dB (SPL), (1), and (2), respectively. No observable effect level, lowest observable adverse effect level, and frank effect levels are identified.