EFFECTS OF PINEALECTOMY ON RAT ESTROUS-CYCLE AND PITUITARY GONADOTROPIN RELEASE

In 4-day cyclic rats kept in a room with the lights on from 05.00 to 19.00 h, sham pinealectomy or pinealectomy on the morning of proestrus did not alter the length of the estrous cycle for 44 days or the time and magnitude of the rises in LH [luteinizing hormone], FSH [follicle stimulating hormone] and prolactin in the circulation in the afternoon on proestrous days 0, 20 or 44. On day 45, the light schedule was set forward 4 h to run from 09.00 to 23.00 h. The rats continued to have 7 additional consecutive 4-day estrous cycles. On day 27 after the resetting of the light schedule, the proestrous rises in serum LH, FSH and prolactin were delayed 4 h in all rats and a normal quota of eggs was ovulated that night. Other 4- and 5-day cyclic rats which had been made persistently estrous by anterior deafferentation of the medial basal hypothalamus (AC) underwent pinealectomy. These AC-pinealectomized rats were ovariectomized 60 days later, and histological examination of the ovaries revealed no evidence of recent ovulation. Five to six weeks after ovariectomy, sequential blood samples were withdrawn through indwelling atrial cannulas in the AC-pinealectomized-ovariectomized rats and in ovariectomized, pinealectomized-ovariectomized and AC-ovariectomized rats. Regular pulsatile rhythms in plasma LH were measured in all rats. S.c. injection of 50 .mu.g estradiol benzoate in oil lowered plasma LH levels in all 4 groups but caused an LH surge in the afternoon 2 days later only in the ovariectomized and pinealectomized-ovariectomized rats. The pineal gland in rats kept on a 14 h light:10 h darkness schedule does not play an active or permissive role in the timing or magnitude of LH, FSH or prolactin release at proestrus, the length of the estrous cycle, or LH release in ovariectomized rats.