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 blunts the preovulatory secretion of LH and FSH and abolishes the secondary secretion of FSH during oestrus without affecting ovulation in the rat. To ascertain whether the secretion of LHRH is involved in these effects, we studied the effects of RU486 (4 mg/0.2 mi oil), given s.c. at 0800 h on pro-oestrus, on LHRH secretion into the pituitary stalk blood vessels and on peripheral plasma concentrations of LH and FSH at 1800h on pro-oestrus and 0200h on oestrus. Furthermore, we determined the 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 concentrations of LH, FSH and oestradiol at 0200 h on oestrus in oil- and RU486-treated rats. RU486 decreased LHRH secretion at 1800 h on prooestrus while this was increased at 0200 h on oestrus. While the reduction of preovulatory LHRH secretion in RU486-treated rats coincided with a reduction in both LH and FSH surges during the evening of pro-oestrus, the increased LHRH secretion during the early hours of oestrus was only accompanied by an increased concentration of LH. An injection of LHRH stimulated, while that of LHRH-A inhibited serum concentrations of LH at 0200 h on oestrus in both oil- and RU486-treated rats. An injection of LKRH-A had no effect on FSH concentration at 0200 h on oestrus in either oil- or RU486-treated rats. On the contrary, exogenous LHRH increased FSH concentration at 0200 h on oestrus only in oil-treated rats. The results indicate that, in the rat, progesterone secretion during the 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 oestrus respectively. While the secretion of LH during early oestrus is blunted by progesterone and entirely coupled to LHRH secretion, the secondary secretion of FSH during oestrus is not dependent on endogenous LHRH and at the same time is completely dependent on the actions (direct and/or indirect) of progesterone.