Estradiol coupling to endothelial nitric oxide stimulates gonadotropin-releasing hormone release from rat median eminence via a membrane receptor

The median eminence (ME). which is the common termination field for adenohypophysiotropic systems, has been shown to produce nitric oxide (NO), a signaling molecule involved in neuroendocrine secretion. Using an ex vivo technique, 17 beta-estradiol exposure to ME fragments, including vascular tissues, stimulated NO release within seconds in a concentration-dependent manner, whereas 17 alpha-estradiol or testosterone had no effect. 17 beta-Estradiol conjugated to BSA (E-2-BSA) also stimulated NO release, suggesting mediation by a membrane surface receptor. Tamoxifen, an estrogen receptor inhibitor, antagonized the action of both 17 beta-estradiol and E-2-BSA. Furthermore, estradiol-stimulated NO stimulates GnRH release. This was demonstrated by hemoglobin (a NO scavenger), N-omega-nitro-L-arginine methyl ester, and L-N-5-(1-iminocthyl)ornithine (nitric oxide synthase inhibitors) inhibition of estradiol stimulated NO and GnRH release. In this regard, L-N-5-(1-iminoethyl)ornithine, specific for endotheliol constitutive nitric oxide synthase, was significantly more potent, suggesting that the estradiol-stimulated NO release arose from vascular endothelial cells. Additionally, the NO-stimulated GnRH release occurs via guanylyl cyclase activation in GnRH nerve terminals, as ODQ, a potent and selective inhibitor of NO-sensitive guanylyl cyclase, abolished the estradiol-stimulated GnRH release. The results suggest that at physiological concentrations, 17 beta-estradiol may have immediate actions on ME endothelial cells via nongenomic signaling pathways leading to NO-stimulated GnRH release.