Biphasic fluctuation of pituitary responsiveness to LH-RH during the rat estrous cycle. Role of endogenous LH-RH and ovarian steroids in its modulation.

The role of the endogenous LH-RH and gonadal steroids on the fluctuation of the pituitary responsiveness to an analog of LH-RH in rats was studied. A specific sheep serum against LH-RH and a superactive analog of LH-RH [D-Ala6,desGly-NH2(10)]LH-RH ethylamide, which does not cross-react with the LH-RH antiserum, were used. In control animals, the pituitary response to 400 ng of LH-RH varied during the estrous cycle, the greatest responses being on the days of proestrus (P) and estrus (E) and the lowest on diestrus I (DI) and diestrus II (DII). A similar pattern was observed in the same animals when 10 ng of [D-Ala6,desGly-NH2(10)]LH-RH ethylamide were used. In animals treated with anti-LH-RH serum, the LH response to the analog was greater than that in control rats on day I (DI) and day 2 (DII) and was smaller on day 3 (P) and on day 4 (E). The magnitude of the pituitary responsiveness to the analog in animals treated or not with the anti-LH-RH serum depended to some extent on the serum progesterone/estradiol ratio. The antiserum against LH-RH lowered the high serum levels of estradiol which are normally seen on P and E but not of progesterone. This indicates that a possible extraovarian source of progesterone exists, possibly from the adrenals. In conclusion, the cycling fluctuation of the plasma progesterone/estrogen ratio in the female rat could act at the pituitary level to modify LH-RH action.