Little evidence for a chronotolerance effect for impulse noise exposure in the C57BL/6J mouse.

Noise-induced hearing loss affects a large number of adults and children worldwide, and continues to be a major public health problem. The cochlea is an organ that maintains delicate metabolic homeostasis and precise mechanical architecture. Disruption of either can cause temporary or permanent injury. Impulse noises, which are short-duration, high-level bursts of sound caused by explosions, such as gunfire, can injure the cochlea through combinations of mechanical and metabolic injury. Susceptibility to the metabolic component of noise injury may vary with the circadian rhythm, a phenomenon known as chronotolerance. Chronotolerance to noise injury has been demonstrated for a one-hour noise exposure at a fixed level, but chronotolerance for impulse noise-induced hearing loss has never been studied. Forty-four mice were exposed to 500 short-duration clicks at 137 dB peSPL at one of four hours after light onset: 2, 8, 14, or 20. Auditory brainstem response threshold shifts were measured at 3, 7, and 21 days after the exposure to measure hearing loss, and post mortem outer hair cell counts were used to confirm cochlear injury. The testing revealed no significant differences between the four exposure times for hearing threshold shifts, but did detect a small, but statistically significant, difference in outer hair cell loss, in which the loss was greatest for the mice exposed two hours after light offset. Therefore, a weak chronotolerance effect for impulse noise was detected, though the functional significance of the effect is low. Further investigation is required to more fully understand the relationship between circadian rhythm and hearing loss from different types of noise exposure.