Noise damage in the C57BL/CBA mouse cochlea.

The present study was designed to determine the response to noise of the auditory system of a genetically well-defined laboratory mouse in preparation for examining the effect of noise on mice with specific genetic mutations. The mice were C57BL/CBA F1 hybrids. Eight mice served as non-noise-exposed controls and 39 mice were exposed for 1-24 h to an octave band of noise with a center frequency of 2, 4 or 8 kHz and a sound pressure level of 100-120 dB. Auditory brainstem response thresholds were measured pre-exposure and several times post-exposure (i.e., 0-27 days) to determine the magnitude of the temporary threshold shift (TTS) and permanent threshold shift (PTS). After fixation by cardiac perfusion, the cochleas from each mouse were embedded in plastic, dissected into quarter turns of the cochlear duct and analyzed quantitatively. Immediately post-exposure, all mice had sizable TTSs at the tested frequencies (i.e., 3-50 kHz). At this time, two mice were killed. Thresholds of the other 37 mice recovered somewhat in the first 4 days post-exposure. One mouse fully recovered from its TTS; 10 mice were left with PTSs at all frequencies; 26 mice recovered at some frequencies but not others. Most mice with PTSs for 30-50 kHz had focal losses of inner and outer hair cells in the basal 20% of the organ of Corti, often with degeneration of adjacent myelinated nerve fibers in the osseous spiral lamina. On the other hand, mice with PTSs for the lower frequencies (i.e., 3-20 kHz) had stereocilia disarray without significant hair cell losses in the second and first turns. Considerable variability was found in the magnitude of hair cell losses in those mice that received identical noise exposures, despite their genetic homogeneity.