AUDITORY BRAIN-STEM RESPONSES AFTER OVARIECTOMY AND ESTROGEN REPLACEMENT IN RAT

Previous work has suggested possible influences of ovarian hormones on evoked potentials in the auditory system. The aim of this project was to study the effects of ovariectomy and subsequent administration of estrogen replacement on the auditory brainstem response and the middle latency response. Groups of 90 day-old Long-Evans hooded rats were anesthetized for bilateral ovariectomies (over) and recordings made 3 weeks later. During the week prior to recordings some ovariectomized groups received subcutaneous injections of 10, 100 or 500 mu g/kg Premarin in peanut oil, and other unoperated animals received vehicle injections. Recordings from vertex/chin using needle electrodes and pure tone stimulus parameters were made under Rompun/Ketamine. The results using 40 kHz tone stimuli showed that mean latencies for over animals were longer than animals in the 100 mu g/kg Premarin group for waves 1a, 1an, 1b, 11, 111, 111n, and 1V/V. Other posthoc comparisons at 40 kHz stimulation revealed differences between control and 100 mu g/kg Premarin groups for latencies of waves 1b, 1bn, 11 and 111. Latency reduction appeared for waves 1b, 1bn, 11 and 111 for the 10 over group, but only at wave 11 for the 500 over group, compared to over-only animals. Data from 8 kHz stimulation also demonstrated significant differences between the over and over 100 groups at waves 1bn and Vn. Observations of interpeak latency differences, especially between waves 1a and 11, suggested central as well as cochlear involvement in hormone action. Examination of the middle-latency responses revealed considerable variability of latencies and amplitudes among subjects with latencies of ovariectomy animals significantly lengthened for the third component of the middle latency response at 40 kHz stimulation. These results suggest that changes in neurophysiological processing of auditory information followed by ovariectomy are reversed by estrogen treatments which may alter activity in peripheral and central auditory structures. Modification of processing in the auditory system thus provides dependent measures for study in the surgical model of menopause.