The Effects of Progesterone on Estrogen-Induced Luteinizing Hormone and Follicle-Stimulating Hormone Release in the Female Rhesus Monkey*

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Abstract
The effects of progesterone (P) in midcycle concentrations on the estradiol (E2) -induced gonadotropin release in the rhesus monkey were investigated by implanting Silastic capsules containing either crystalline E2 or P. All experiments were begun on day 3 or 4 of the menstrual cycle and finished 96 h later. In the control cycles E2 capsules (E2 increments to approximately 250 pg/ml) were implanted in all animals. In subsequent cycles E2 capsules were again implanted, but a P capsule was added (P increment to approximately 1.2 ng/ml) 0, 24, 32, and 46 h after the implantation of the E2 capsules (groups I, II, III, and IV, respectively). The time of maximum gonadotropin release in the E2 plus P cycles of all groups was advanced by approximately 12 h compared to their E2 control cycles (P 2 control means. When the time interval between the E2 and P implantation was increased, the magnitude of the mean Δmax of LH and FSH also increased. In group IV (P added 46 h after E2) the mean Δmax of the gonadotropins was 130% of the means in the E2 control cycles. These results point to a biphasic P effect with time on the E2-induced gonadotropin release. Similar effects were observed on the total amount of LH released: in group I the study cycles showed an average decrease of 30%, and in group IV total LH released was approximately the same as in the control cycles. Results for FSH were less marked. These data suggest that after adequate E2 priming (group IV), P accelerates the release of the gonadotropins from the pituitary. As a result, maximum levels are reached earlier, and the peaks are shorter and higher than in the E2 control cycles. These results suggest a physiological effect of the increasing P levels at midcycle on the timing, magnitude, and duration of gonadotropin release.