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 an esthetized for bilateral ovariectomies (over) and recordings made 3 we eks later. During the week prior to recordings some ovariectomized gro ups received subcutaneous injections of 10, 100 or 500 mu g/kg Premari n in peanut oil, and other unoperated animals received vehicle injecti ons. Recordings from vertex/chin using needle electrodes and pure tone stimulus parameters were made under Rompun/Ketamine. The results usin g 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, 1 an, 1b, 11, 111, 111n, and 1V/V. Other posthoc comparisons at 40 kHz s timulation revealed differences between control and 100 mu g/kg Premar in groups for latencies of waves 1b, 1bn, 11 and 111. Latency reductio n appeared for waves 1b, 1bn, 11 and 111 for the 10 over group, but on ly at wave 11 for the 500 over group, compared to over-only animals. D ata from 8 kHz stimulation also demonstrated significant differences b etween 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. E xamination of the middle-latency responses revealed considerable varia bility of latencies and amplitudes among subjects with latencies of ov ariectomy animals significantly lengthened for the third component of the middle latency response at 40 kHz stimulation. These results sugge st that changes in neurophysiological processing of auditory informati on followed by ovariectomy are reversed by estrogen treatments which m ay alter activity in peripheral and central auditory structures. Modif ication of processing in the auditory system thus provides dependent m easures for study in the surgical model of menopause.