Human chorionic gonadotropin blocks the estrogen-induced luteinizing hormone release in long-term castrated rhesus...

We studied the effects of human chorionic gonadotropin (hCG) administration on the biphasic response of luteinizing hormone (LH) to estrogen challenge in long-term castrated, female rhesus monkeys. Estradiol benzoate (EB), 400 micrograms, was administered to all animals at time zero; hCG, 2000 IU, was administered either concomitantly or 12 hours after EB. Blood samples were obtained every 6 hours for 58 hours, and then at 72 and 82 hours. Control animals received EB alone. Serum LH and beta hCG were determined by radioimmunoassay. Control animals showed a typical biphasic response to EB: an initial decrease in LH levels, 50% to 70% of baseline, from +6 hours until +30 hours; then a large increase, 150% to 190% of baseline, at +48 hours. Animals treated with hCG at time zero showed an initial decrease in LH, 40% to 70% of baseline, but LH returned to basal levels at +42 hours. Animals treated with hCG 12 hours after EB showed an initial decrease in LH, 45% to 55% of baseline, but LH returned to basal levels at +36 hours. These results suggest that an LH ultrashort-loop negative feedback may exist in the nonhuman primate.