Biphasic Effect of Progesterone on the Release of Gonadotropin in Rats

The concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in serum following progesterone injection into spayed, estrogen-treated rats were measured by radioimmunoassay. Progesterone (2 mg), injected at noon 2, 3 or 4 days after the injection of a single dose of 20 μg estradiol benzoate, induced, a few hours later, a significant increase in serum LH. On the following 2 days after progesterone injection the LH levels were lower than those in the controls but on the 3rd day the serum LH rose again. The release of LH induced by progesterone was observed in the afternoon up to late in the evening but not in the morning. Estrogen priming is essential since no effect of progesterone was observed in spayed treated animals. Progesterone also exerted an inhibitory effect on the release of LH after the facilitatory period occurred. In fact, the release of LH which can be induced by the injection of either estrogen or progesterone was prevented by the administration of progesterone 1 day before. A rise in serum FSH following the injection of progesterone 3 days after a priming dose of estrogen was also observed. The present study provides evidence for a biphasic effect of progesterone on the release of LH. (Endocrinology89: 331, 1971)