Abstract
Nitric oxide (NO), formed by conversion of arginine to citrulline and NO by NO synthase, mediates relaxation of vascular smooth muscle. NO synthase has been demonstrated by immunocytochemical methods in neurons in various parts of the central nervous system including the hypothalamus. The latter finding suggested to us that NO might play a role in controlling the release of hypothalamic peptides. We have previously shown that norepinephrine mediates the release of luteinizing hormone-releasing hormone (LHRH) from LHRH terminals in the median eminence into the hypophyseal portal veins, which transport LHRH to the anterior pituitary gland to trigger release of luteinizing hormone from gonadotrophs. LHRH release from these terminals requires increased release of prostaglandin E2 (PGE2). PGE2 activates adenylate cyclase to produce cAMP, and then cAMP induces the exocytosis of LHRH secretory granules. In view of the evidence above and because of the developing evidence for the importance of NO in the central nervous system, it occurred to us that NO might be involved in this process. Consequently, we evaluated the role of NO in the release of PGE2 from medial basal hypothalamic fragments. As previously reported, norepinephrine (10 microM) increased PGE2 release from the hypothalamic fragments. The inhibitor of NO synthase NG-monomethyl-L-arginine (NMMA, 300 microM) blocked the stimulation of PGE2 release induced by norepinephrine but had no effect on the basal release of PGE2. Sodium nitroprusside (100 microM), which liberates NO, also elevated PGE2 release from the hypothalamic fragments. This elevation was not affected by NMMA, presumably because NMMA blocks enzymatic generation of NO but does not alter NO liberated by nitroprusside. When the NO liberated by nitroprusside was inactivated by hemoglobin (2 micrograms/ml), the effect of nitroprusside on PGE2 release was completely inhibited. Neither NMMA nor hemoglobin altered the basal release of PGE2, which indicates that NO is not responsible for basal PGE2 release. Addition of L-arginine (10 microM to 1 mM), the substrate for NO synthase, had no effect on basal PGE2 production. These results indicate that NO synthase is not activated in unstimulated hypothalamic fragments in vitro. The results suggest that norepinephrine activates NO synthase leading to the production of NO, which subsequently activates cyclooxygenase and results in the production of PGE2. PGE2 then activates adenylate cyclase leading to generation of increased cAMP, which induces exocytosis of secretory granules of LHRH and other neuropeptides released by PGE2. The indication that NO is essential to norepinephrine-induced release of PGE2 from hypothalamic fragments provides insight into the mechanism of LHRH release and the results open the possibility that the importance of NO to neuronal functions may be widespread in the nervous system.