Auditory degeneration after acoustic trauma in two genotypes of mice

Two strains of mice, CBA/Ca and C57BL/6J, were exposed to a steady noise (2-7 kHz) of 120 dB SPL for 5 min at 1, 3, 6, or 12 months of age. Threshold shifts were determined by recording auditory brainstem response 1 month after exposure and thereafter up to the age of 16 months (C57BL) or 23-27 months (CBA). With increasing age of exposure, susceptibility to acoustic trauma at middle frequencies (6.3-12.5 kHz) 1 month after exposure decreased in CBA mice but remained constant in C57BL mice. With increasing age after exposure, threshold shifts were retained at the middle frequencies in CBA mice exposed at 1 month of age and in C57BL mice of all exposed groups. The progress of the interaction between the previous noise damage and aging effects was generally the same for the two strains, first an additivity and then a blocking-like interaction. The rate of the progress in post-noise hearing did not exceed the spontaneous rate of aging. The differences between exposed and non-exposed groups decreased with advancing age. The results indicate that the interaction of noise trauma and aging effects depends on the susceptibility of the individual to acoustic trauma, affected frequencies, and the severity of noise-induced PTS. A previous noise damage did not potentiate the auditory degeneration either in CBA/Ca or in C57BL/6J mice.