OVARIAN CONTROL OF GONADOTROPIN HORMONE-RELEASING HORMONE PULSE-GENERATOR ACTIVITY IN THE RHESUS-MONKEY - DURATION OF THE ASSOCIATED HYPOTHALAMIC SIGNAL

The activity of the GnRH pulse generator in the rhesus monkey is assoc iated with abrupt increases in multiunit electrical activity (MUA) vol leys recorded from the mediobasal hypothalamus that precede each pulse of LH in the peripheral circulation. In long-term ovariectomized anim als the duration of these MUA volleys is 10-25 min and consists of a b rief initial 'overshoot' followed by a plateau phase that ends in a ra pid decline to baseline activity. In intact monkeys, however, the MUA volley lasts only 1-3 min, a duration equivalent to the overshoot in o variectomized animals. In addition, the maximal frequency of neuronal activity during each MUA volley is reduced in normal animals when comp ared to castrates. As shown in earlier studies, estradiol given to ova riectomized monkeys causes a reduction in the duration of MUA volleys to that characteristic of intact animals within 3-5 h. In contrast to this acute effect of estradiol, the increase in MUA volley duration fo llowing ovariectomy is a gradual phenomenon, 4-6 weeks being required to achieve the MUA volley duration observed in long-term ovariectomize d monkeys. A similar slow time course was observed for the increase in maximal neuronal frequency during each MUA volley. This protracted ef fect of ovariectomy on MUA volley duration and firing rate may be the consequence of hypothalamic remodelling but this consideration must be tempered by the observation that estradiol reverses these phenomena w ithin hours.