Pituitary gonadotropin inhibition by a highly active analog of luteinizing hormone-releasing hormone

Chronic treatment with highly active analogs of luteinizing hormone (LH)-releasing hormone (LH-RH) induces "paradoxical" antifertility effects. Intact male rats, a reduction in testosterone production is observed after the administration of [D-Ser(But)6]-LH-RH(1-9)ethylamide (burserelin, Hoe 766), 50 ng/day subcutaneously for 4 weeks. In castrated male rats treated with the same dose of analog, plasma LH levels were reduced from days 14 to 28 and plasma follicle-stimulating hormone (FSH) levels were reduced from days 21 to 28 of treatment. Pituitary LH and FSH concentrations were also decreased. The plasma prolactin level was reduced at 14 days of treatment. The hypothalamic LH-RH content remained unchanged and the adrenal corticosterone content was lowered. These findings indicate a direct inhibitory effect of the analog on gonadotropin secretion in the absence of the gonads, and may explain some paradoxical antifertility effects observed with high doses of LH-RH analogs which exceed the physiologic dose range.