Electrophysiological Recording of Hypothalamic GnRH Pulse Generator Activity in Castrated Male Goats

Abstract

Electrophysiological recording of the hypothalamic GnRH pulse generator activity was attempted in 3 long-term castrated male goats in which electrodes had been implanted in the medial basal hypothalamus (MBH). While multiple unit activity (MUA) signals from the electrode were continuously recorded, each animal received subcutaneous implants containing testosterone for 7 days (Experiment 1). They were similarly treated with estradiol alone, estradiol plus testosterone, and testosterone alone for 7 days each consecutively (Experiment 2). These steroidal treatments enabled to maintain the plasma levels of testosterone and estradiol within physiological ranges at 2-4 ng/ml and 3-5 pg/ml, respectively. Blood samples for the determination of pulsatile LH secretion were collected at 6-min intervals for 4 h on the last day of each treatment period. The short-lived sudden increase in MUA spikes (MUA volley) was detected being always preceding the occurrence of the LH pulse in the peripheral blood, and this synchrony of MUA volley with the LH pulse was constant in all the animals regardless of steroidal treatments. Testosterone treatment resulted in the increase of the MUA volley intervals from 27 min to 43 min (Experiment 1). Treatment with estradiol in Experiment 2 also resulted in prolonged MUA volley intervals similar to those of testosterone treatment and there was no additive or synergistic effect between these two steroids. The present study demonstrated that the electrophysiological manifestation of the hypothalamic GnRH pulse generator could be successfully monitored in male goats as reported in females, and it is suggested that estradiol may exert a negative feedback effect on the GnRH pulse generator activity in a similar manner to that of testosterone but at much lower doses in male goats.