Acute Effects of Exogenous Progesterone on Follicular Steroidogenesis in the Cyclic Rat*

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Abstract
The effects of exogenous progesterone (P) on follicular steroidogenesis and serum gonadotropins in the cyclic rat were determined using in vivo and in vitro approaches. A single sc injection of 800 μg progesterone at 0900 h on diestrus II (day 0) consistently postponed ovulation by 24 h in 4-day cyclic rats, with no change in the ovulation rate. The preovulatory surges in serum LH and FSH were delayed by 1 day and occurred on day 2 in the P-treated animals. After the injection of 800 μg P at 0900 h on diestrus II, peak serum levels of P were reached 3 h later and thereafter decreased to basal levels by 1500 h on day 1. Within 1 h after injecting P, serum LH and FSH decreased to values below control levels; serum LH did not recover during the next 48 h, whereas serum FSH returned to basal levels within 6 h. An abrupt fall in serum 17β-estradiol occurred within 1 h after the administration of P. This drastic decline in serum estradiol was paralleled by temporally related decreases in serum LH and FSH but not by changes in serum testosterone. By 0900 h on day 1 (24 h after P injection), serum estradiol returned to values within the range observed on diestrus II, then gradually increased to proestrous values by day 2. Within 3 h after the administration of P, the initial content and the in vitro production rate of P, testosterone, and 17β-estradiol by antral follicles decreased in the P-treated animals compared to that in follicles of control animals, whereas the follicular content and production rate of 20α-dihydroprogesterone were unaffected. Thus, in the cyclic rat, the inhibitory effects of exogenous P on follicular steroidogenesis are mediated in this dose range through the hypothalamic-pituitary axis, especially via lowering serum LH.