Positive feedback by estrogen and progesterone on LH release in old and young rats.

Abstract

Plasma levels of luteinizing hormone (LH) and progesterone (P) were determined daily in samples obtained at 0800 1600 and 2400 hours from 6 beagle bitches during proestrus and estrus. Sexual behavior in response to males was also assessed. LH surges of 24-40 hours (4.4-11.1 ng/ml) were observed between 3 days before and 1 day after estrus. Estrous behavior was observed only after levels of P increased. The increase in P began either just before or at the same time as the initial LH surge. Ovariectomy was performed at approximately 4 8 20 38 44 and 50 hours after the LH peak when plasma levels of P were 1.3 2.8 4.2 3.3 4.4 and 6.9 ng/ml respectively. The respective ratios of ovulated to nonovulated mature follicles were 0:10 0:6 0:7 0:8 4:3 and 11:0. Sexual behavior was tested in ovariectomized bitches over a 5-week period during which estrogen was administered for 16 days beginning either 8 days before or after the initiation of 16 days of progesterone treatment. Full sexual receptivity occurred 8 days after superimposing estrogen treatment on progesterone and within 18 hours after superimposing progesterone treatment on estrogen. Removal of estrogen treatment did not affect sexual receptivity whereas it was completely abolished within 18 hours after cessation of progesterone treatment. The findings suggest that ovulation in the bitch occurs between 1 day before and 5 days after the onset of estrus and about 38-44 hours after the peak in LH and that the initial preovulatory increase in P is synergistic with the late-proestrous estrogen peak in promoting estrous behavior.