Regulation of Gonadotropin Release by Luteinizing Hormone-Releasing Hormone and Estrogen in Chimpanzees*

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Abstract
Plasma LH and FSH levels were determined by RIA before and after an iv bolus of LHRH in adult female chimpanzees. The pre-LHRH concentration of 17β-estradiol (E2) was determined by RIA. Menstruating and postpartum chimpanzees (mean E2, 29.8 and 45.0 pg/ml, respectively) showed a monophasic LH response characterized by a 2- to 3-fold increase in LH levels within 15 min of LHRH injection, followed by a slow decline. In animals which had elevated estrogen levels, namely midfollicular animals (mean E2, 73.6 pg/ml), menstruating animals pretreated with 5 /μg estradiol benzoate for 2 days (mean E2, 283.2 pg/ml), and postpartum chimpanzees administered 50 μg mestranol/day for 7 days, LH showed a reduced or similar initial rise, followed by a plateau or slight decline during the first 60 min after LHRH but, thereafter, a progressive secondary increase that continued throughout the second 60 min. This secondary augmentation was greatly exaggerated when a long acting LHRH analog (desGly10-[D-Trp6, Pro9, NEt]LHRH) was administered. FSH responses were inhibited during the first 2 h when estrogen levels were elevated, except in animals administered the long acting analog which showed a secondary augmentation of FSH after 3h. These results show that the relative gonadotropin increment after LHRH administration in the chimpanzee is similar to the human and that estrogen can differentially modulate the LH and FSH responses to LHRH, partly by preferential augmentation of LH release, as seen in this study 1 h after LHRH challenge.