Coital and estrogen signals: a contrast in the preovulatory neuroendocrine networks of rabbits and rhesus monkeys.

Certain brain peptides and catecholamines function in activating the hypothalamohypophysial-ovarian axis in both rabbits and rhesus monkeys. The natural stimulus for a surge release of GnRH is coitus in rabbits, whereas the initial excitatory signal is ovarian steroids in monkeys. Despite this contrast in initial signals, specific neurochemicals may serve as common stimuli for GnRH secretion in both species. Evidence is presented that one such substance is norepinephrine (NE), which is released from the mediobasal hypothalamus before, or simultaneously with, GnRH, although the latency in time between stimulus (coitus/estrogen) and response (GnRH/LH release) is very different. Moreover, both stimuli activate NE gene expression in cells located in the brainstem. We suggest that the brainstem is an extra-hypothalamic site where preovulatory signals for GnRH surges are developed in both rabbits and primates.