Interactions of Estradiol and Progesterone on Pituitary Gonadotropin Secretion: Possible Sites and Mechanisms of Action

The effects of estradiol (E2) alone or with progesterone (P) on spontaneous or LHRH-induced releases of LH/FSH were examined 14-19 days after ovariectomy of rats. At 14 days, a Silastic E2 capsule was implanted s.c. (Day 0) and 72 h later (Day 3) 2 groups of rats received either sesame oil or progesterone (2 mg) s.c. Hourly blood samples were collected sequentially from 1200-1800 h on Day 3 and again at 0900 h and 1600 h on Days 4 and 5 (96 and 120 h post E2). Control groups included proestrous and untreated ovariectomized rats. Based on the data obtained we concluded that: a) Daily LH but not necessarily FSH surges occur on Days 2, 3, 4 and 5 but not on the day following E2 treatment (Day 1). b) The responsiveness of the pituitary gland to 250 ng LHRH is equal in both E2-treated (Day 3) and proestrous rats. However, the magnitude of the spontaneous LH surge in E2-treated rats (Day 3), was 60% less than in normal proestrous animals. Thus, E2 seems to reduce the amount of endogenous LHRH released on Day 3. c) Progesterone, given at 0900 h to E2 -treated rats (Day 3), resulted in afternoon LH and FSH surges that were earlier in their onset and of greater magnitude than those in E2-treated or normal proestrous rats. However, no plasma LH increases occurred in E2 P-treated rats on Days 4 and 5 unlike in rats receiving only E, d) If spontaneous LH surges were blocked with Nembutal on Day 3, the magnitude of the LH surge on Day 4 in E2 -treated rats was 2-fold greater than that observed on Day 4 in rats which spontaneously released LH on Day 3. E2 P-treated rats which P INTERACTIONS ON LH/FSU RELEASE 1181