Susceptibility of DPOAEs to sound overexposure in inbred mice with AHL.

The notion that three inbred strains of mice, i.e., C57BL/6J (C57), BALB/cByJ (BALB), and WB/ReJ (WB), which exhibit differential rates of age-related hearing loss (AHL), may also exhibit differential susceptibility to noise-induced hearing loss was tested by comparing the effects of sound overexposure on these strains. The aftereffects of noise overstimulation on the distortion-product otoacoustic emissions (DPOAEs) of these three strains were compared and contrasted to those for the CBA/CaJ (CBA) strain, which does not show changes in hearing threshold sensitivity up to 15 months of age. Two cohorts of mice, one at 2.5 and the other at 6 months of age, were first exposed to a tonal overstimulation paradigm, were allowed to recover, and then were later re-exposed to an octave band noise (OBN), at 3 or 7 months of age, respectively. The two sound exposure episodes were designed to produce either a temporary (tonal exposure) or permanent (OBN exposure) reduction in the levels of the 2f1 - f2 DPOAE in the WB strain, which exhibited the fastest rate of AHL. Although the tonal paradigm resulted in a temporary decrease in DPOAE levels for all strains at both ages, the 2.5-month BALBs showed the greatest susceptibility to this overexposure, while the 2.5-month WBs exhibited the least effects on DPOAEs. At the older age of 6 months, tonal overexposure produced essentially the same reduction in DPOAE levels for all four strains. In addition, there were no differences noted between CBAs and C57s, at either of the two ages. The OBN paradigm resulted in a permanent decrease in DPOAE levels in all the strains exhibiting early AHL, i.e., the C57, BALB, and WB mice, for frequencies about one-half to an octave higher than the exposure frequency, regardless of age. In contrast, the CBA strain was not significantly affected by the OBN overexposure.