Biphasic Fluctuation of Pituitary Responsiveness to LH-RH during the Rat Estrous Cycle

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Abstract
The role of the endogenous LH-RH [luteinizing hormone-releasing hormone, luliberin] 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-NH210]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-NH210]LH-RH ethylamide were used. In animals treated with anti-LH-RH serum, the lutropin response to the analog was greater than that in control rats on DI and 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. The cycling fluctuation of the plasma progesterone/estrogen ratio in the female rat could act at the pituitary level to modify LH-RH action.