PERIOVULATORY LHRH, LH AND FSH-SECRETION IN CYCLIC RATS TREATED WITH RU486 - EFFECTS OF EXOGENOUS LHRH AND LHRH ANTAGONIST ON LH AND FSH-SECRETION AT EARLY ESTRUS

The antiprogesterone RU486 injected on the morning of pro-oestrus blun ts the preovulatory secretion of LH and FSH and abolishes the secondar y secretion of FSH during oestrus without affecting ovulation in the r at. To ascertain whether the secretion of LHRH is involved in these ef fects, we studied the effects of RU486 (4 mg/0.2 mi oil), given s.c. a t 0800 h on pro-oestrus, on LHRH secretion into the pituitary stalk bl ood vessels and on peripheral plasma concentrations of LH and FSH at 1 800h on pro-oestrus and 0200h on oestrus. Furthermore, we determined t he effects of an s.c. injection of 1 mg of an LHRH antagonist (LHRH-A; ORG30276) at 2000 h on pro-oestrus and those of an i.p. injection of 100 ng LHRH (Peninsula 7201) at 0100 h on oestrus on serum concentrati ons of LH, FSH and oestradiol at 0200 h on oestrus in oil- and RU486-t reated rats. RU486 decreased LHRH secretion at 1800 h on prooestrus wh ile this was increased at 0200 h on oestrus. While the reduction of pr eovulatory LHRH secretion in RU486-treated rats coincided with a reduc tion in both LH and FSH surges during the evening of pro-oestrus, the increased LHRH secretion during the early hours of oestrus was only ac companied by an increased concentration of LH. An injection of LHRH st imulated, while that of LHRH-A inhibited serum concentrations of LH at 0200 h on oestrus in both oil- and RU486-treated rats. An injection o f LKRH-A had no effect on FSH concentration at 0200 h on oestrus in ei ther oil- or RU486-treated rats. On the contrary, exogenous LHRH incre ased FSH concentration at 0200 h on oestrus only in oil-treated rats. The results indicate that, in the rat, progesterone secretion during t he afternoon and evening of pro-oestrus enhances preovulatory LHRH and suppresses LHRH release during early oestrus into the pituitary stalk blood vessels on the afternoon of pro-oestrus and during early oestru s respectively. While the secretion of LH during early oestrus is blun ted by progesterone and entirely coupled to LHRH secretion, the second ary secretion of FSH during oestrus is not dependent on endogenous LHR H and at the same time is completely dependent on the actions (direct and/or indirect) of progesterone.