Sex difference in the expression and regulation of nitric oxide synthase gene in the rat preoptic area.

Neuronal nitric oxide synthase (nNOS) mRNA-positive cells were visualized by non-isotopic in situ hybridization histochemistry in the organum vasculosum of the lamina terminalis (OVLT) and the preoptic area (POA) in gonadectomized juvenile female and male rats. In the rostral POA (rPOA) at the level of the anteroventral periventricular nucleus, nNOS mRNA-positive cells were distributed in an inverted V-shaped area over the third ventricle and were in close proximity to cell bodies of gonadotropin-releasing hormone (GnRH)-immunoreactive neurons. In the caudal POA (cPOA) at the level of the medial preoptic nucleus, no topological association existed between GnRH and nNOS. Throughout the rPOA, both the number and the area of nNOS mRNA positive cells were significantly larger in the gonadectomized females than in the gonadectomized males. Treatment with estradiol for 2 days, followed by progesterone in the next morning, which caused an increase in serum luteinizing hormone 6 h later, induced a significant reduction of the nNOS mRNA expression in the rPOA in the female but not in the male rat at the time of sacrifice. In the OVLT and the cPOA, ovarian steroids had no effect on nNOS mRNA expression of both sexes. The results indicate that nNOS mRNA expression in the rPOA is sexually dimorphic and regulated by ovarian steroids in a sex specific manner.