The major purpose of this study was to characterize the open-loop frequency of pulsatile GnRH release in the female rhesus monkey at an age (15-20 months) when the prepubertal restraint on the hypothalamic-pituitary axis is maximally imposed. Additionally, evidence for pulsatile GnRH release in agonadal males of comparable age was also sought. Episodic LH secretion from the pituitary was used as an indirect index of GnRH discharges. In order to maximize the sensitivity of this in situ bioassay, the responsiveness of the pituitary gonadotrophs was usually first heightened by an i.v. intermittent infusion of the synthetic peptide. Monkeys (five females, three males) were castrated between 9 and 14 months of age, implanted with indwelling venous catheters, fitted with nylon jackets and housed in specialized cages that permitted remote access to the venous circulation with minimal restraint and without interruption of the light-darkness cycle. In females, LH secretion was generally assessed at 20-day intervals during alternate nighttime (1900-0200 h) and daytime (0700-1400 h) windows. In males, LH was assessed less frequently and only at night. The mean frequency of pulsatile LH release in agonadal prepubertal females was 4 pulses/7 h during the night and 2 pulses/7 h during the day. These findings indicate that, prior to puberty in the female monkey, the GnRH pulse generator operates at a relatively slow frequency and is subjected to diurnal modulation. In males, evidence for robust pulsatile GnRH release was not observed. The striking difference in activity of the GnRH pulse generator in agonadal prepubertal male and female monkeys reinforces the view that the ontogeny of the hypothalamic drive to the pituitary-gonadal axis in higher primates, including man, is sexually differentiated.

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