Strength and Duration Characteristics of the Facilitory and Inhibitory Effects of Progesterone on the Estrogen-Induced Gonadotropin Surge in the Female Rhesus Monkey*

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Abstract
The dose-dependent effects of progesterone (P) on estradiol (E2)-induced gonadotropin release were investigated in the early follicular phase of female rhesus monkeys by sc implanting Silastic capsules containing either crystalline E2 or P. In group 1, E2 alone (250 pg/ml) and E2 in combination with P (1.5, 3, and 5 ng/ml) were administered in four consecutive experiments. The lowest P dosage (1.5 ng/ml) advanced the time of the maximal gonadotropin release, but the magnitude seemed to be decreased compared to that in the experiment with E2 alone. The higher P dosages (3 and 5 ng/ml) blocked the previously observed E2-induced gonadotropin surges. In group 2, E2 alone (250 pg/ml) and E2 in combination with P (1.5, 5, and 20 ng/ml), added 46 h after E2, were administered in four consecutive experiments. In all experiments, P advanced and increased the gonadotropin peaks. The FSH surges could be increased even further in the experiments with the two highest P dosages (5 and 20 ng/ml). After this period of facilitated gonadotropin secretion, we observed lower gonadotropin levels than those in the control experiment with E2 alone. This effect became more apparent by increasing the P dosage. These results point to a biphasic effect of P across time and dosage on the release of gonadotropins.

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