Circadian surges of LH in the ovariectomised rat occur coincident with enhanced pituitary responsiveness to GnRH.

Experiments are described in which we have investigated the responsiveness to GnRH of pituitary glands obtained from ovariectomised rats in which LH surges were induced by steroid priming (a) by estrogen implants and (b) by estrogen-progesterone injections. These treatments generate LH surges late in the light phase. Stimulation with GnRH in vitro revealed a large increase in responsiveness coincident with the surge of LH. We suggest that this effect is mediated through a self-priming effect of GnRH and that this in vitro technique may be used to detect hypothalamic release of GnRH.