Cochlear microphonics and action potentials mature and decline at different rates in the normal and pathologic mouse cochlea

A technique is described that can nontraumatically assess cochlear microphonic (CM) and cochlear action potential (AP) thresholds to pure tones in the intact mouse. This method was applied to the normal hearing CBA/J, and to the C57BL/6J, which has early-onset hearing loss. CM thresholds are most sensitive in both mice by 23 days of age, and the C57BL/6J CM responses degenerate rapidly thereafter, especially at the high-frequency end. The CMs of the CBA/J continue to change past 23 days, developing a threshold dip at 24 kHz, and becoming slightly less sensitive at 32 kHz. The AP of the C57BL/6J is also most sensitive at 23 days, and becomes gradually less sensitive to 32- and 64-kHz tones over the next 36 days. The CBA/J AP thresholds are not maximally sensitive until 41 days of age.