The concentration of immunoreactive luteinizing hormone releasing factor (LH-RF) was measured in pituitary stalk blood collected in the afternoon of expected pro-oestrus from rats ovariectomized and given oil, oestradiol benzoate (OB) or progesterone on the morning of dioestrus. Compared with the values in intact rats, LH-RF concentrations were reduced in ovariectomized animals treated with oil or progesterone but not with OB. Administration of progesterone at 12.30 h of expected pro-oestrus reduced the LH-RF concentrations in OB-treated rats; 5α-pregnan-3β-ol-20-one was not as effective as progesterone. The apparent inhibitory effect of progesterone, which was dose-dependent, was not found in long-term adrenalectomized rats, suggesting that it may be due to the production of an inhibitory substance by the adrenal consequent upon steroid administration. These and other data show that oestradiol triggers the spontaneous LH surge by stimulating the release of LH-RF as well as by enhancing pituitary responsiveness. A different mechanism may underlie the reflex surge of LH in rats exposed to constant light for, in these animals, progesterone was found to stimulate LH-RF release.
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