Mechanism of the first spontaneous gonadotrophin surge and that induced by pregnant mare serum and effects of neonatal androgen in rats.

The mechanism of the first (pubertal) preovulatory gonadotrophin surge was investigated in Wistar rats by measuring (1) LH releasing factor (LH-RF) in pituitary stalk plasma and in extracts of hypothalamic and preoptic tissue, (2) LH and FSH in peripheral plasma and extracts of anterior pituitary tissue and (3) the LH and FSH response to LH-RF. The mechanism of the first surge appeared broadly to resemble that in the adult. That is, the gonadotrophin surge which occurs on the afternoon of the day before vaginal opening was found to coincide with a surge of immunoreactive LH-RF in pituitary stalk plasma and a significant increase in pituitary responsiveness to LH-RF. The magnitude of the change in pituitary responsiveness was, however, less at puberty than in the adult while the peak of the LH-RF surge was higher at puberty. The surges of LH-RF in stalk plasma and gonadotrophin in peripheral plasma corresponded relatively precisely with a fall in the preoptic and hypothalamic content of LH-RF and in the pituitary content of LH and FSH, suggesting that, in contrast with the adult, the synthesis of LH-RF and gonadotrophin at puberty cannot keep up with their release. A significant increase in stalk plasma LH-RF concentration occurring concomitantly with a surge of LH could be induced on the afternoon of Day 32 of life by administering pregnant mare serum (PMS) ON Day 30 (about 10 days before vaginal opening). This, together with other evidence, suggests that the timing of the first gonadotrophin surge depends upon the capacity of the ovary to secrete oestradiol in the form of a surge. The fact that no significant increase in the pituitary responsiveness to LH-RF occurred in PMS-treated rats could account for the fact that the height of the gonadotrophin surge in these animals is only a third that of the spontaneous surge. No significant change in peripheral plasma LH or stalk plasma LH-RF concentrations was found around the time of vaginal opening or cornification in female rats given 1.25 mg testosterone propionate on Day 4 (androgenized female rats). Studies on the effect of ovariectomy and/or adrenalectomy suggested that the ovary and adrenal are involved in the timing of vaginal opening in normal but not in androgenized female animals.