Modification by Exogenous Progesterone of Estrogen and Gonadotropin Secretion in the Cyclic Hamster*

Hamsters were injected sc with 25–500 μg progesterone (P) on day 3 of the estrous cycle (day 1, day of ovulation; day 4, proestrus). Normally on day 3, the corpora lutea are no longer functional and serum P is not detectable. After 25–100 μg P, the cycle was lengthened by 1 day, whereas after 250 or 500 μg P, the cycle was prolonged by 2 and 3 days, respectively. The follicles ultimately ovulating after 500 μg P were derived in part or in toto from a second wave of follicular maturation. However, the original antral follicles were the ovulating follicles after treatment with lower doses of P. After 100 or 500 μg P administered on day 3, the number of animals ovulating and the mean ovulation rate was reduced in hamsters injected with hCG during the next 24 h. By the afternoon of day 4, serum estradiol (E2) was djastically reduced in hamsters injected on the previous day with 250 or 500 μg P. However, their serum levels of FSH, LH, and PRL did not differ from control values. When 100μ500 μg P were injected on day 3, an abrupt fall in serum E2 occurred within 1 h; with 50 μg P, the fall in serum E2 did not occur until 3 h later. The drastic decline in serum E2 after 50 μg P was not associated at 1300 h with changes in serum levels of androstenedione or testosterone. For all doses of P, the decline in E2 was paralleled by temporally related decreases in serum LH. Concurrent injection of gonadotropins along with P, completely (hCG) or partially (LH) prevented the fall in serum E2. Serum FSH was unaffected by exogenous P. In fact, by the evening of day 4, serum FSH was elevated in the P-treated animals compared to controls. By 0900 h of day 4, serum E2 did not differ between hamsters treated with 50–100 jug P or peanut oil (80–100 pgLa Jolla, California 92037ml). However, with 500 μg P, serum E2 was still depressed to aproximately 25 pgLa Jolla, California 92037ml. These experiments demonstrate the rapidity with which changes in serum LH and E2 can be elicited in the diestrous hamster by relatively small amounts of exogenous P.