Sexual differentiation of the surge mode of gonadotropin secretion: prenatal androgens abolish the gonadotropin-releasing hormone surge in the sheep.

In sheep, the surge mode of gonadotropin secretion is sexually differentiated, i.e. the LH surge is present in the female, but not in the male. The present study tested the hypothesis that sexual differentiation of the LH surge mechanism reflects a sex difference in the pattern of GnRH, and that prenatal androgens abolish the surge mode of GnRH secretion. We monitored the pattern of GnRH secretion in pituitary portal blood after acute treatment with estradiol in gonadectomized postpubertal males (n = 6), females (n = 6), and androgenized females (exposed prenatally to testosterone from day 30-90 in gestation, n = 7). Four capsules, each containing a 30-mm column of estradiol were implanted s.c. into each lamb to produce high physiologic concentrations of the hormone. Beginning 7 h later, portal and peripheral blood samples were collected hourly for 48 h for measurement of GnRH and LH, respectively. All females exhibited a GnRH surge beginning 13.0 +/- 0.4 h after estradiol treatment; this was accompanied by an LH surge. By contrast, only one male produced a small surge in GnRH (1.7 pg/min) with a latency of 32 h; a corresponding increase in LH occurred in this male. Likewise, among the androgenized females, only one exhibited GnRH and LH surges which began at about 22 h after estradiol treatment. Some of the androgenized females had sporadic increases in GnRH which were of lower amplitude than in the control females, and were unaccompanied by rises in LH. These findings provide the first direct evidence that the sex difference in the surge mode of LH secretion results from the sexual differentiation of the pattern of GnRH release. The study also suggests that androgens during prenatal development abolish the GnRH surge and subsequently, the generation of the LH surge.