Genotypic differences in behavioral, physiological and anatomical expressions of age-related hearing loss in the laboratory mouse.

The auditory nerve isoelectric thresholds, in response to tone pigs ranging from 5 to 20 kHz, are similar in the young C57BL/6 and CBA/J mice, although the latter genotype has somewhat more sensitive responses from 30 to 80 kHz. But their behavioral audiograms, obtained by the classical conditioning technique of Ehret, are very discrepant. Even though the behavioral audiogram of the CBA/J mouse can be predicted by measurements from its auditory nerve, the behavioral thresholds of the young C57BL/6 mouse are approximately 40 dB less sensitive than its electrophysiological measurements. Cochlear hair cell loss was not evident in young mice of either genotype. As the CBA/J approaches the end of its predicted life span, its auditory anatomy, physiology and behavior are not significantly altered; but those rare individuals, who exceed their predicted life span by 40% finally develop hearing loss of a mixed nature. By contrast, the C57BL/6 mouse shows a relatively rapid decline of hearing as it ages. By 200 days of age, its auditory nerve responses are 30 dB less sensitive at 5 kHz, and 55 dB less sensitive at 30 kHz, than similar measures taken at adolescence. Over this same age span, its behavioral sensitivity has only declined by 15 and 25 dB at these two frequencies. Hair cell counts correlated poorly with both behavioral and electrophysiological auditory measures in the C57BL/6 mouse.