Nitric oxide (NO) synthase, the enzyme which converts arginine into citrulline plus NO, a highly active free radical, has been found in many neurons in the brain, including neurons in the hypothalamus. Our previous experiments showed that norepinephrine-induced prostaglandin E2 release from hypothalamic explants incubated in vitro is mediated by NO. Since the release of luteinizing hormone-releasing hormone (LHRH) is also driven by norepinephrine and prostaglandin E2, we hypothesized that NO might also control pulsatile release of LHRH in vivo, resulting in turn in pulsatile release of luteinizing hormone (LH). To ascertain the role of NO in control of pulsatile LH release in vivo, an inhibitor of NO synthase, NG_monomethyl-L-arginine (NMMA), was microinjected into the third cerebral ventricle (1 mg/5 pi) of conscious castrate male rats at time 0 and 60 min later; blood samples were taken every 10 min during this period. NMMA blocked pulsatile LH release within 20 min, and plasma LH concentration declined further without pulses after the ij'ection at 60 min. Pulsatile release of LH was not altered in diluent-injected controls. NMMA did not alter pulsatile release of follidestimulating hormone, which suggests that its release does not require NO. Incubation of medial basal hypothalami with norepinephrine (10 pM) ind.uced an increase in LHRH release that was inhibited by NMMA (300 PM). NMMA alone did not alter basal LHRH release, whereas it was augmented by sodium nitroprusside (100 FM), which releases NO spontaneously. This augmentation was prevented by hemoglobin (2 jug/ml), which binds the NO released by nitroprusside. Our previous experiments showed that norepinephrine-induced release of prostaglandin E2 is mediated by NO. Nitric oxidergic neurons were visualized in the median eminence adjacent to the LHRH terminals. The combined in vivo and in vitro results indicate that the pulsatile release of LHRH induced by norepinephrine is brought about by al-adrenergic activation of NO synthase.NO then induces prostaglandin E2 release that activates exocytosis of LHRH secretory granules into the portal vessels to induce pulsatile LH release.Nitric oxide (NO) released from vascular endothelium by cholinergic stimulation diffuses to the adjacent vascular smooth muscle and elicits relaxation (1-4). The mechanism by which this occurs begins with the release of acetylcholine from cholinergic terminals. It combines with muscarinic cholinergic receptors on the endothelial cells and increases intracellular Ca2+. The Ca2+ interacts with calmodulin to activate constitutive NO synthase, which then converts arginine into NO plus citrulline (1-4). Constitutive NO synthase occurs in the brain (5, 6); this enzyme has been purified and antibodies have been generated against it (7,8). NeuronsThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.containing the constitutive NO ...

Nitric oxide synthase-containing cells were visualized in the anterior pituitary gland by immunocytochemistry. Consequently, we began an evaluation of the possible role of NO in the control of anterior pituitary function.Prolactin is normally under inhibitory hypothalamic control, and in vitro the gland secretes large quantities of the hormone. When hemipituitaries were incubated for 30 min in the presence of sodium nitroprusside, a releaser of NO, prolactin release was inhibited. This suppression was completely blocked by the scavenger of NO, hemoglobin. Analogs of arginine, such as NG-monomethyl-L-arginine (NMMA, where NG is the terminal guanidino nitrogen) and nitroarginine methyl ester, inhibit NO synthase. Incubation of hemipituitaries with either of these compounds significantly increased prolactin release. Since in other tissues most of the actions of NO are mediated by activation of soluble guanylate cyclase with the formation of cyclic GMP, we evaluated the effects of cyclic GMP on prolactin release. Cyclic GMP (10 mM) produced an "40%o reduction in prolactin release. Prolactin release in vivo and in vitro can be stimulated by several peptides, which include vasoactive intestinal polypeptide and substance P. Consequently, we evaluated the possible role of NO in these stimulations by incubating the glands in the presence of either of these peptides alone or in combination with NMMA. In the case of vasoactive intestinal polypeptide, the significant stimulation of prolactin release was augmented by NMMA to give an additive effect. In the case of substance P, there was a smaller but significant release of prolactin that was not significantly augmented by NMMA. We conclude that NO has little effect on the stimulatory action of these two peptides on prolactin release. Dopamine (0.1 ,uM), an inhibitor of prolactin release, reduced prolactin release, and this inhibitory action was significantly blocked by either hemoglobin (20 jig/ml) or NMMA and was completely blocked by 1 mM nitroarginine methyl ester. Atrial natriuretic factor at 1 ,uM also reduced prolactin release, and its action was completely blocked by NMMA. In contrast to these results with prolactin, luteinizing hormone (LH) was measured in the same medium in which the effect of nitroprusside was tested on prolactin release, there was no effect of nitroprusside, hemoglobin, or the combination of nitroprusside and hemoglobin on luteinizing hormone release. Therefore, in contrast to its inhibitory action on prolactin release NO had no effect on luteinizing hormone release. Immunocytochemical studies by others have shown that NO synthase is present in the folliculostellate cells and also the gonadotrophs of the pituitary gland. We conclude that NO produced by either of these cell types may diffuse to the lactotropes, where it can inhibit prolactin release. NO appears to play little role in the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accorda...

Nitric oxide (NO) synthase (NOS), the enzyme that converts arginine into citrulline plus NO, the latter a highly active free radical, occurs in a large number of neurons in the brain, including certain neurons in the hypothalamus. Our previous experiments have shown that norepinephrine (NE)-induced prostaglandin E²(PGE²) release from medial basal hypothalamic explants (MBH) is mediated by NO. Because release of luteinizing hormone (LH)-releasing hormone (LHRH) is also driven by NE and PGE², we hypothesized that NO controls pulsatile release of LHRH in vivo, which in turn induces pulsatile LH release. Indeed, in vivo and in vitro experiments using an inhibitor of NOS (N^G-monomethyl-L-arginine; NMMA) demonstrated that pulsatile LH release is mediated by NO; LHRH release in vitro is also mediated by this free radical. Cytokines that are released from cells of the immune system during infection also inhibit LHRH release. We compared the action of one such cytokine, interleukin-1Α(IL-1Α), on LHRH release with that of substances which inhibit or induce NO release. Microinjection of IL-1Α(0.06 pmol in 2µl) into the third cerebral ventricle (3V) of conscious, castrated male rats had an action similar to that of 3V microinjection of NMMA (1 mg in 5µl): it blocked pulsatile LH, but not follicle-stimulating hormone (FSH) release. The only difference between the responses to NMMA and IL-1Αwas that the latency to onset was greater with IL-1Α. When both NMMA and IL-1Αwere microinjected together, there was an additive suppressive effect on LH release during the first hour after injection. As in the case of NMMA, there was no effect on pulsatile FSH release of IL-la injected alone or together with NMMA. The latter results suggest that hypothalamic control of FSH release is distinct from that of LH and does not involve NO. Previous in vitro experiments showed that IL-1Αalso has an action identical to that of NMMA; inhibition of NE-induced PGE²and LHRH release from MBH explants. Therefore, IL-1Αappears to act on its receptors on the NOergic neurons to prevent the release of NO, or on the LHRH terminals to inhibit the response to NO. To test the hypothesis that IL-1Αdirectly inhibits the response of LHRH terminals to NO, we incubated MBH explants with sodium nitroprusside (NP; 500µM)_twhich spontaneously releases NO. NP-induced LHRH release was inhibited by IL-1Α(10 pM).We conclude that IL-1Αdirectly suppresses the response of the LHRH terminals to NO.

Previous experiments in this and other laboratories have revealed that nitric oxids (NO) plays a role in controlling the release of corticotropin-releasing hormone (CRH) and luteinizing-hormone-releasing hormone (LHRH). Therefore, we have investigated its role in control of growth hormone (GH) release in conscious rats by microinjecting N^G-monomethyl-L-arginine (NMMA), an inhibitor of NO synthase (NOS), into the third ventricle (3V) of conscious, freely moving castrate male rats. An initial blood sample (0.3 ml) was drawn from an indwelling intra-atrial catheter just prior to injection of NMMA [1 mg in 5 µl of 0.9% NaCl (saline)] into the 3V. To maintain the inhibitory action on NOS, a second injection of NMMA was administered into the 3V 60 min after the first. Additional blood samples (0.3 ml) were removed at 10 min intervals for 120 min. Other animals received injections of the diluent at the same times and volumes as NMMA. Interleukin (IL)-1Α (0.06 pmol in 2 µl saline) was injected into the 3V immediately after the first injection of NMMA, whereas other animals received the NMMA diluent followed by IL-1Α. The effects of IL-1Α were almost identical to those of NMMA in that there was a dramatic lowering of plasma GH achieved primarily by a reduction in height of the GH pulses without a significant reduction in their number. When IL-1Α as well as NMMA were administered, results were similar to those with NMMA or IL-1Α alone except for a significant decrease (p < 0.025) in number of pulses and a small but significant increase in pulse height. The area under the plasma GH curve was highly significantly and similarly decreased in all treatment groups. Since NMMA decreased pulse height without altering frequency, this indicates that pulse height is controlled by NO, probably by stimulating release of GH-releasing hormone (GRH). Other experiments have shown that NO stimulates somatostatin release. Consequently, the elevation in pulse height in animals injected with IL-1Α plus NMMA above the level in the animals injected only with IL-1Α, may be due to suppression of somatostatin release in the IL-1Α-plus-NMMA-injected animals. Since the response to NMMA and IL-1Α was almost identical, we hypothesize that IL-1Α blocks the response of the GRH neuron to NO.