Development of the mechanism regulating the preovulatory surge of luteinizing hormone in sheep.

Development of the mechanism controlling cyclic LH secretion in the sheep was studied by examining the ability of estradiol to elicit LH surges in lambs at various ages. Silastic capsules containing estradiol were implanted sc for a 96-hour period at 3, 7, 12, 20, and 27 weeks of age. (First spontaneous ovulation occursss between 30 and 50 weeks). Although administration of estradiol failed to elicit a discharge of LH at 3 weeks of age, LH surges of progressively increasing magnitude (36 +/- 15, 73 +/- 28, 107 +/- 19 ng/ml) were elicited by estradiol as the lambs became older (7, 12, 20 weeks). By 27 weeks, the maximal serum LH level attained during the induced surge (123 +/- 29 ng/ml) was similar to that of the estrogen-induced LH surge in anestrous adults (179 +/- 23 ng/ml). Ovulation, however, did not occur in response to the induced LH surges. An additional experiment was performed to determine whether, as in the adult, progesterone can block the estradiol-induced LH discharge in the immature (12-week old) female. A sustained elevation of circulating progesterone to levels characteristic of the mid-luteal phase of the estrous cycle of the adult (3--4 ng/ml), beginning 24 h prior to insertion of the estradiol capsules, blocked the induced LH surge. These results demonstrate that, in immature female sheep, the LH surge mechanism is capable of functioning long before first ovulation occur and, further, suggest that timing of the initial preovulatory LH surge is limited by the ability of the ovary to produce the estradiol stimulus rather than by the ability of the hypothalamo-hypophyseal system to respond to the positive feedback action of estradiol. Additionally, the hypothalamo-hypophyseal mechanism whereby progesterone blocks the LH surge develops long before first ovulation.