Age-related changes in the C57BL/6J mouse cochlea. I. Physiological findings.

The development and degeneration of cochlear output was studied in C57BL/6J mice from the time of hearing onset (12 days of age) through adulthood (50 days of age) using the eighth nerve compound action potential (N1). Mice were stimulated in free-field, using short rise-time tone bursts (2-35 kHz). Sensitivity to tones and threshold tuning curve sharpness (Q10) increased markedly between 12 and 20 days of age. Response latencies changed, in a complex way, to attain minimum values by 20 days of age. The form of intensity function was essentially identical in 12- and 16-day-old mice. In 30- and 40-day-old mice signs of both hearing development and degeneration were seen. For example, while N1 threshold intensities to low-frequency tones continued to decline, those to high frequencies began to increase. The results indicate that some of the previously observed age-related changes in central auditory function in C57BL/6J mice can be accounted for in terms of cochlear evolution. The observed changed in cochlear function are discussed in relation to outer-, middle-, and inner-ear modifications. A basis for the limits of the critical period for audiogenic seizure "priming' is hypothesized.