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 h ypothalamohypophysial-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 ma y 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 w ith, GnRH, although the latency in time between stimulus (coitus/estro gen) 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 pre ovulatory signals for GnRH surges are developed in both rabbits and pr imates.