MECHANISMS GOVERNING ONSET OF OVARIAN CYCLICITY AT PUBERTY IN THE LAMB

The immature female sheep is capable of several aspects of adult reproductive endocrine function long before the onset of puberty. The prepubertal female can ovulate in response to an endogenous gonadotropin surge, which can be elicited by an endogenous estradiol rise which, in turn, can be produced by artificially increasing the rate of pulsatile LH discharge to an hourly frequency. Removal of the ovaries can produce an hourly frequency of LH pulses, but this frequency does not normally occur in the presence of the ovary prior to the beginning of puberty. This implies that the sequence of events leading to the first spontaneous gonadotropin surge is being restrained in the immature female by some mechanism inhibiting the onset of hourly LH pulses. Because the same levels of circulating estradiol which are capable of suppressing serum LH in the immature ovariectomized lamb are ineffective in this regard in the same females after the age of puberty, it is postulated that the increase in frequency of LH pulses to hourly at the onset of puberty is due to a decrease in the inhibitory feedback action of estradiol on tonic LH secretion. Thus, the anovulatory condition of the immature lamb can be considered as an active state of repression of reproductive function and puberty as a process of derepression. When viewed in this context, a striking similarity exists between the mechanism proposed for transition into adult-hood in the lamb and the postulated mechanism for transition into the breeding season in the adult. Because the onset of fertility in both the lamb at puberty and the adult at the beginning of the breeding season is associated with a pronounced decrease in response to estradiol negative feedback, it suggests either that an important component of the pubertal mechanism is yet operative during adulthood and regulates seasonal fertility, or that 'puberty' is an annual process in the adult female sheep.