A striking sex difference in the gonadotropin response to gonadectomy during infantile development in the rhesus monkey (Macaca mulatta).

The present study was conducted to examine further the view that the early postnatal ontogeny of the neuroendocrine control system that regulates ovarian function in the rhesus monkey differs from that which governs testicular activity in this species. To this end, time courses of gonadotropin secretion were determined from birth until 40 weeks of age in neonatally ovariectomized (n = 6) and gonad-intact (n = 4) female rhesus monkeys by monitoring circulating LH and FSH concentrations in blood samples collected at weekly intervals. The developmental patterns of FSH and LH secretion in agonadal infantile females were then compared to those in a group of neonatally orchidectomized animals (n = 4). In addition, moment to moment changes in circulating LH concentrations were determined in agonadal male (n = 2) and female infants (n = 6) at approximately 1 month of age by collecting sequential blood samples every 10-20 min for 5-8 h. Ovariectomy at 1 week of age elicited a truncated and abbreviated rise in circulating LH concentrations, which contrasted to a postcastration LH hypersecretion of substantial magnitude and duration in males. The initial FSH response to neonatal ovariectomy and orchidectomy were similar, with both males and females exhibiting a striking and progressive rise in circulating levels of this gonadotropin during the immediate weeks following castration. Thereafter, however, a dramatic sex difference in the time course of open loop FSH secretion emerged; in males, plasma FSH concentrations declined to undetectable levels by 24 weeks of age, while in females, plasma concentrations of this gonadotropin were sustained at elevated levels for the duration of the 40-week period of observation. As a result of the foregoing sex difference in the early postnatal ontogeny of open loop gonadotropin secretion, circulating FSH to LH ratios in ovariectomized infantile female monkeys (2.3:8.1) were consistently greater than those in agonadal males (0.5:3.8). Interestingly, the elevated FSH to LH ratios in castrated females were associated with a relatively slow LH pulse frequency of approximately one pulse every 3-4 h. The expected circhoral frequency of pulsatile LH release was observed in agonadal males of comparable age. The persistence of sex differences in gonadotropin secretion in infantile rhesus monkeys several weeks after removal of the gonads suggests that the origin of these differences must reside at the hypothalamic-pituitary level rather than at the level of the gonad.(ABSTRACT TRUNCATED AT 400 WORDS)