Abstract
Changes in the GnRH pulse generator activity were examined during the programmed treatment with ovarian steroids, which mimicked endocrine events in the luteal and follicular phases, by recording the hypothalamic multiple unit activity specifically associated with the pulsa-tile LH secretion (MUA volleys) in ovariectomized goats. The steroidal milieu of the luteal phase was simulated by subcutaneously implanting silastic capsules containing progesterone and estra-diol, and that of follicular phase by intravenously infusing estradiol at a gradually increasing infusion-rate after removal of the progesterone capsules. The administration of progesterone alone for 3 days had little influence on MUA volleys, but there was a significant reduction (p<0.05) in the MUA volley frequency following the additional administration of estradiol, which resulted in the low pulse frequency observed in the normal luteal phase. In the mimicked follicular phase the MUA volley frequency was increased from 10.0± 2.9 to 13.7±1.9 volleys/12 h after the proges-terone removal, but then decreased gradually in parallel with an increase in the plasma estradiol concentration. There was no increase in MUA volley frequency during the transition from the basal to the surge mode of LH secretion. These results suggest that the hypothalamic GnRH pulse generator activity is suppressed during the luteal phase by a synergetic action of progesterone and estradiol, and that the acceleration of the GnRH pulse generator activity does not take place during the preovulatory LH surge in the female goat.
