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

The hypothalamic component of the reproductive axis in vertebrates is compr ised of a pulse generator that stimulates the release of GnRH. Several line s 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 a gonadal, prepubertal male monkey, we observed a daytime profile of LH secre tion, which suggests an apulsatile mode of GnRH release, The first purpose of this study was to describe this observation of apulsatile LH release dur ing the peripubertal transition. Furthermore, we have explored the dependen ce of this form of LH secretion on GnRH release. Five male rhesus monkeys ( Macaca mulatta) were castrated prepubertally and were treated with an inter mittent infusion of GnRH to prematurely sensitize the juvenile pituitary to endogenous GnRH release. Alternate daytime (1100-1800 h) and nighttime (19 00-0200 h) assessments of LH release were performed at 10-day intervals thr oughout the peripubertal transition with samples taken every 12 min. In a s econd experiment, four agonadal males which demonstrated an apulsatile prof ile of LH release were maintained on an infusion of physiological saline an d were treated with the GnRH antagonist Nal-Glu (i.m., 500 mu g/kg). Circul ating levels of LH were determined 22 h after antagonist treatment. In peri pubertal animals, circulating levels of LH were similar between morning and evening assessments. However, pulse frequency was significantly lower duri ng the daytime. GnRH antagonist reduced LH levels by 72% and a similar redu ction in response to an exogenous GnRH test stimulus occurred. These findin gs suggest an apulsatile mode of GnRH release.