Prolonged noise exposure-induced auditory threshold shifts in rats

Noise-induced hearing loss (NIHL) initially increases with exposure duration, but eventually reaches an asymptotic threshold shift (ATS) once the exposure duration exceeds 18-24h. Equations for predicting the ATS have been developed for several species, but not for rats, even though this species is extensively used in noise exposure research. To fill this void, we exposed rats to narrowband noise (NBN, 16-20kHz) for 5 weeks starting at 80dB SPL in the first week and then increasing the level by 6dB per week to a final level of 104dB SPL. Auditory brainstem responses (ABR) were recorded before, during, and following the exposure to determine the amount of hearing loss. The noise induced threshold shift to continuous long-term exposure, defined as compound threshold shift (CTS), within and above 16-20kHz increased with noise level at the rate of 1.82dB threshold shift per dB of noise level (NL) above a critical level (. C) of 77.2dB SPL i.e. CTS=. 1.82(NL-77.2). The normalized amplitude of the largest ABR peak measured at 100dB SPL decreased at the rate of 3.1% per dB of NL above the critical level of 76.9dB SPL, i.e., %ABR Reduction=. 3.1%(NL-76.9). ABR thresholds measured >30 days post-exposure only partially recovered resulting in a permanent threshold shift of 30-40dB along with severe hair cell loss in the basal, high-frequency region of the cochlea. In the rat, CTS increases with noise level with a slope similar to humans and chinchillas.