Temporary anestrus induced by injection of luteinizing hormone-releasing hormone in hamsters

The photoperiod-induced anovulatory state of the Syrian hamster (Mesocricetus auratus) is characterized by a daily rhythm of circulating LH and FSH with a surge in blood levels occurring each afternoon. The association between anestrus and the presence of daily gonadotropin surges suggests that initiation of the characteristic daily rhythm of LH and/or FSH after transfer to short days results in blockade of ovulation. We tested whether daily injection of LHRH affects the 4-day rhythm of vaginal estrous cycles, induces daily surges of LH and FSH, blocks ovulation, and induces regression of the ovaries. LHRH (1 μg/animal) was injected (s.c.) at 1630 h each day into females with regular 4-day estrous cycles. Injections began on the afternoon of estrus and continued for 12 days (experiment 1) or 8 days (experiments 2 and 3). In saline-injected controls, vaginal discharges occurred regularly at 4-day intervals during the injection period in all experiments. In contrast, vaginal estrus was not observed during the period of LHRH injection in any animals. Ova were found in oviducts of all saline- injected controls (8.9 ± 0.8; n = 8) on the morning of estrus (after 8 injections), but no ova were found in oviducts of LHRH-injected females. Levels of plasma LH but not FSH increased after injection of LHRH each day, whereas the 4-day estrous cycle rhythm of gonadotropins was not disturbed in saline-injected controls. Vaginal estrous cycles and a 4-day rhythm of LH release resumed within 7 days after LHRH injections were stopped. These results suggest that daily surges of LH are an essential component of the anovulatory condition and that a daily rhythm of LH is sufficient to induce the anestrous state. They also suggest that the time delay associated with photoperiod-induced anestrus is due to a mechanism functioning at or above the generation of the LHRH signal to the pituitary.