Estrogen-Induced Gonadotropin Release in Ovariectomized Rhesus Monkeys and Its Advancement by Progesterone1

The effects of estradiol-17β (E2) and progesterone (P), administered sc in silastic capsules, on the serum levels of LH and FSH were studied over four days in two groups of intact rhesus monkeys on days 3 to 9 of the intermenstrual period and in ovariectomized (ovx) animals. In the intact females (N = 5, Group 1), simultaneous LH and FSH surges occurred approximately 48 h after E2 treatment was begun (serum E2 levels of 200 to 400 pg/ml). With this same E2 treatment was accompanied by P (serum levels of 4 to 6 ng/ml, Group 2), LH release was blocked in 6 of 6 animals. In ovx animals (N = 9, Group 3), E2 alone produced an LH surge at 72 h (P>0.05 Group 1 vs 3), and a slight release of FSH with LH in 6 of the 9 animals. P administered simultaneously with E2 to these same ovx animals (Group 4) advanced (P<0.01 Group 3 vs 4) rather than blocked the LH surge. LH release occurred at 30 h and was again accompanied by a slight elevation of FSH in 4 of the 9 animals. P also failed to block the LH surge in 7 of 8 ovx animals pretreated for 11 days with 50 to 100 pg/ml of E2 (Group 5). Again the LH surge was advanced (P<0.01 Group 3 vs 5) to approximately 30 h and under these circumstances was accompanied by a distinct release of FSH. These data show that P advances the E2-induced release of LH in ovx rhesus monkeys. In intact females, however, it produces a different effect which cannot be replicated in ovx animals treated with tonic levels of 50 to 100 pg/ml E2 for two weeks.