Compound action potential input/output functions in young and quiet-aged gerbils.

Auditory-nerve compound action potentials (CAP) and cochlear microphonic (CM) potentials were measured with round window electrodes in two sets of quiet-reared gerbils: young (N = 9 ears, 4-7 months) and aged (N = 11 ears, 35-37 months). CAP thresholds, measured at probe frequencies from 0.5 to 25.6 kHz, are plotted as audibility curves. Input/output (I/O) functions were derived from CAP and CM amplitude measurements at six frequencies. When compared to young controls, CAP audibility curves from aged animals all show some degree of threshold shift, ranging from minimal to severe, as well as increased variability. Our data suggest that some of the variability in the aged-animal audibility curves can be attributed to variations in individual genetic factors. Maximum CAP amplitudes for the aged animals average significantly less than those of the young controls at all frequencies tested. Young control I/O functions are generally steeper than those of the aged gerbils. Differences in the CM amplitudes of the young and aged gerbils are not as clear cut as the differences in the CAP. Possible mechanisms explaining the decrease in amplitudes and slopes of the CAP I/O functions in aged animals include changes in numbers or thresholds of primary ganglion cells, or a decrease in synchrony in discharges of auditory-nerve fibers.