A potential apulsatile mode of GnRH release in the male rhesus monkey (Macaca mulatta)

The hypothalamic component of the reproductive axis in vertebrates is comprised of a pulse generator that stimulates the release of GnRH. Several lines of evidence are in agreement that the activity of this pulse generator is intermittent and results in the pulsatile pattern of GnRH and LH release. During a recent investigation of the re-initiation of LH secretion in the agonadal, prepubertal male monkey, we observed a daytime profile of LH secretion, which suggests an apulsatile mode of GnRH release. The first purpose of this study was to describe this observation of apulsatile LH release during the peripubertal transition. Furthermore, we have explored the dependence of this form of LH secretion on GnRH release. Five male rhesus monkeys (Macaca mulatta) were castrated prepubertally and were treated with an intermittent infusion of GnRH to prematurely sensitize the juvenile pituitary to endogenous GnRH release. Alternate daytime (1100-1800 h) and nighttime (1900-0200 h) assessments of LH release were performed at 10-day intervals throughout the peripubertal transition with samples taken every 12 min. In a second experiment, four agonadal males which demonstrated an apulsatile profile of LH release were maintained on an infusion of physiological saline and were treated with the GnRH antagonist Nal-Glu (i.m., 500 microgram/kg). Circulating levels of LH were determined 22 h after antagonist treatment. In peripubertal animals, circulating levels of LH were similar between morning and evening assessments. However, pulse frequency was significantly lower during the daytime. GnRH antagonist reduced LH levels by 72% and a similar reduction in response to an exogenous GnRH test stimulus occurred. These findings suggest an apulsatile mode of GnRH release.