Effect of RU486 injected on proestrous morning on LHRH, LH and 17 beta-estradiol secretion during the estrous cycle in rat

The antiprogesterone RU486 injected on proestrus to cyclic rats advances th e preovulatory surge of LH, resulting in a 3-day estrous cycle. To ascertai n whether proestrous progesterone secretion regulates ovulation by synchron izing the functions of the ovary, the pituitary and the hypothalamus, in th is study the effects of RU486 (4 mg/0.2 mi oil/s.c.) at 09:00 h on proestru s (day 1), on LHRH secretion into the pituitary stalk blood vessels and on peripheral plasma concentrations of LH and 17 beta-estradiol at 10:00 h and at 18:00 h on each day of the estrous cycle, have been investigated. Contr ol rats present the expected surges of LHRH and LH at 18:00 h on day 1 and a second increase in LHRH but no LH secretion was found in the afternoon of day 2. RU486 decreases both LHRH and LH surges in the afternoon of day 1, while it increases plasma concentrations of LHRH and LH in the morning of d ay 2. During the rest of the estrous cycle RU486-injected rats show high ba sal secretion of LH in comparison to controls, and at 18:00 h of day 4 adva nced preovulatory surges of both LHRH and LH are present in RU486-injected rats. In relation to 17 beta-estradiol concentrations, RU486 injection incr eases those on day 3 and induces an advanced preovulatory surge of 17 beta- estradiol in the morning of day 4. These results show that, in the l-day cy clic rat, the 24 hours-shortening of the estrous cycle induced by the antip rogesterone RU486 is due to the enhanced secretion of estrogen during the d ays of metestrus and diestrus, which stimulates the release of a surge of L HRH in the afternoon of diestrus and this, in turn, induces the advanced pr eovulatory surge of LH.