[The change of hypothalamic-pituitary-ovarian function during peripubertal period in female baboons (author's...

To clarify the relationship between the change in plasma levels of pituitary and ovarian hormones and the development of hypothalamic-pituitary-ovarian function in the course of sexual maturation, longitudinal studies were performed using seven peripubertal female baboons (Papio cynocephalus). Serial blood samples were collected every two weeks from 12 months before menarche to 8 months after menarche. Plasma levels of prolactin, LH, FSH and estradiol were determined by specific radioimmunoassay methods. Periodic injections of synthetic LHRH (100 microgram) + TRH (500 microgram) and conjugated estrogen (Premarin 20mg) at one month interval were also conducted throughout the period studied. Furthermore, daily blood samples were drawn from two baboons from the first day of menarche up to the subsequent onset of vaginal bleeding, and the fluctuations of the same kinds of plasma hormonal levels were analysed. The basal plasma levels of FSH showed the earliest significant increase seven months prior to menarche, while those of LH, prolactin and estradiol exhibited no apparent change. Statistically significant elevations of basal plasma LH, prolactin and estradiol levels were observed one month before menarche. Intravenous injections of LHRH which were given earlier than two months before menarche induced marked responses in FSH with limited changes in LH patterns. After this stage of puberty, the response of LH stimulated by LHRH administrations were augmented progressively, whereas those of FSH became impaired. The peak values of prolactin after TRH injections increased, obviously around the same period of sexual development. The LH positive feedback induced by Premarin was observed one month before menarche in four baboons whose LH responses were exaggerated significantly by LHRH, and the sex skin reached turgidity to a certain extent. The evaluation of the hormonal changes during the first menstrual cycles after menarche indicated that the follicular phases were prolonged due to delayed elevation of FSH during the early follicular phase, and the length of the luteal phases were shorter in comparison to those seen in the normal ovulatory cycles of adult baboons. The results from the present longitudinal studies on baboons revealed characteristic changes accompanied by the maturation of the hypothalamic-pituitary-ovarian function during the peripubertal period. The initial change was the increase of basal FSH levels, which was caused by the alteration of hypothalamic sensitivity to endogenous estrogen, and that FSH stimulated the ovary to secrete estrogen. The increased basal estradiol secretion influenced both hypothalamic and pituitary functions, namely, the dominant gonadotropin secretion by the pituitary shifted from FSH to LH, a positive feedback mechanism was established, and prolactin level secretions with enhanced response after TRH stimulations were increased. This sequence of events indicates that hypothalamic-pituitary-ovarian interaction occurs during maturation in the peripubertal period.