Sequential patterns of circulating luteinizing hormone and follicle-stimulating hormone in female sheep from early postnatal life through the first estrous cycles.

Patterns of circulating LH and FSH were examined during several 6-h periods in developing female sheep from 3 weeks of age through the first estrous cycles (30--40 weeks of age) and early pregnancy. With the onset of pulsatile LH secretion, beginning 11 weeks after birth, circulating LH increased to levels 2- to 5-fold greater than those observed in the adult. Thereafter, high and variable levels of circulating LH persisted through young adulthood when ovarian cyclicity including ovulation became manifest. Although prior to the initial estrus changes in frequency of LH release (number of LH discharges/6-h period) appeared to occur randomly from week to week, after first estrus, changes in frequency became predictable. During the early (day 1) and late (day 15) luteal phases of the first estrous cycles, when circulating progesterone was low (less than 0.6 ng/ml), the frequency of LH release was increased (5--7 discharges/6 h) while during the mid-luteal phase (day 7--12), when circulating progesterone was high (2--4 ng/ml), the frequency was diminished (0--2 discharges/6 h). Massive and sustained discharges of LH which resembled preovulatory surges were observed only shortly before first estrus. In contrast to the pulsatile release pattern of LH, concentrations of circulating FSH within the 6-h periods were relatively constant and within the range found in the adult. These findings lead to the following conclusions: a) changes in negative feedback control are not directly responsible for the onset of ovarian cyclicity in the sheep as indicated by the lack of differences in mean concentrations of circulating LH and FSH before and after first ovulation; b) the onset of the surge mode of gonadotropin secretion occurs only shortly before first ovulation; and c) progesterone may play an inhibitory role in regulating tonic LH secretion during the first estrous cycles.