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      Effects of Systemic Blockade of Nitric Oxide Synthases on Pulsatile LH, Prolactin, and GH Secretion in Adult Male Rats

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          Abstract

          Background: Nitric oxide (NO) has emerged as an important neurotransmitter involved in the control of the neuroendocrine function. NO acts at hypothalamic, pituitary, and gonadal levels. Previous data from our laboratory showed that blockade of NO generation, after systemic administration of a NO synthase inhibitor (Nω-nitro-arginine methyl ester, NAME), increased the luteinizing hormone (LH) secretion in intact and ovariectomized females, whereas a blockade of spontaneous and steroid-induced LH and prolactin surges after NO synthase inhibition has been also described. Methods and Results: Adult male rats were implanted with chronic intra-auricular cannulae and 5 days later sampled at 15-min intervals during 6 h (10.00–16.00 h). Administration of NAME (40 mg/kg at 08.00 and 13.00 h) stimulated significantly (p ≤ 0.01) the LH secretion, increasing LH pulse amplitude (0.58 ± 0.14 vs. 0.08 ± 0.01 ng/ml in controls), mean LH levels (0.64 ± 0.15 vs. 0.15 ± 0.03 ng/ml in controls), and area under curve (239 ± 56 vs. 57 ± 13 in controls). This effect was blocked by coadministration of sodium nitroprusside (SNP), a NO donor (0.5 mg/kg). The action of NAME was observed 3 h after administration, in contrast to the earlier response detected in female rats, and it appeared selective for LH, as prolactin and growth hormone secretion remained unchanged. Further analysis was carried out to determine whether the effect of NAME on the LH secretion was indirect and mediated by changes in testosterone release. To this end, adult male rats were decapitated 2 h after administration of NAME (40 mg/kg), SNP (0.5 mg/kg), or L-nitro-arginine methyl ester ( L-AME), a substrate for NOS (1 g/kg). The serum testosterone concentrations were unchanged after NAME administration, but inhibited by SNP and L-AME. Finally, the effect of NAME and SNP on in vitro testosterone secretion was analyzed. NAME (10 m M) did not affect basal testosterone production, but inhibited the human chorionic gonadotropin stimulated testosterone secretion. Conclusions: These data strongly suggest that the stimulatory effect of NAME on LH secretion is not due to an inhibition of testosterone release and is exerted at the hypothalamic-pituitary level.

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          Localization of nitric oxide synthase indicating a neural role for nitric oxide.

          Nitric oxide (NO), apparently identical to endothelium-derived relaxing factor in blood vessels, is also formed by cytotoxic macrophages, in adrenal gland and in brain tissue, where it mediates the stimulation by glutamate of cyclic GMP formation in the cerebellum. Stimulation of intestinal or anococcygeal nerves liberates NO, and the resultant muscle relaxation is blocked by arginine derivatives that inhibit NO synthesis. It is, however, unclear whether in brain or intestine, NO released following nerve stimulation is formed in neurons, glia, fibroblasts, muscle or blood cells, all of which occur in proximity to neurons and so could account for effects of nerve stimulation on cGMP and muscle tone. We have now localized NO synthase protein immunohistochemically in the rat using antisera to the purified enzyme. We demonstrate NO synthase in the brain to be exclusively associated with discrete neuronal populations. NO synthase is also concentrated in the neural innervation of the posterior pituitary, in autonomic nerve fibres in the retina, in cell bodies and nerve fibres in the myenteric plexus of the intestine, in adrenal medulla, and in vascular endothelial cells. These prominent neural localizations provide the first conclusive evidence for a strong association of NO with neurons.
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            Nitric Oxide (NO) Stimulates Gonadotropin Secretion in vitro through a Calcium-Dependent, cGMP-Independent Mechanism

            In the last few years, nitric oxide (NO) has emerged as an important intra- and intracellular messenger involved in the control of hypothalamic-pituitary function. The present experiments were undertaken in order to evaluate the pituitary component in the modulatory action of NO on gonadotropin secretion, as well as the second messenger pathway(s) involved. In a first step, we assessed LH and FSH secretion by hemipituitaries incubated in the presence of increasing concentrations of sodium nitroprusside (SNP), a potent NO donor, and cyclic guanosin monophosphate (cGMP), the second messenger for a wide range of NO actions. In addition, given that SNP induces the release of NO and cyanide ions, the response to SNP was tested in the presence of hemoglobin (an NO scavenger) or rhodanese + sodium thiosulfate (inactivators of cyanides) in order to ensure that the effects of SNP on gonadotropin secretion were mediated by the release of NO. SNP (10 –4 –10 –3   M ) stimulated gonadotropin secretion in our incubation system, whereas cGMP, at all doses tested, was ineffective. Similar results were obtained using dispersed pituitary cells. The stimulatory action of SNP is attributable to its ability to induce NO release since it was blocked by hemoglobin, but preserved after incubation with rhodanese + sodium thiosulfate. In further experiments, we aimed to identify the mechanism(s) underlying SNP-induced gonadotropin secretion. First, to evaluate the involvement of calcium (Ca 2+ ), the effects of SNP were analyzed in a calcium-free medium, after depletion of Ca 2+ stores by caffeine, in the presence of the Ca 2+ chelator ethylene glycol bis ( p -aminoethyl ether) N,N-tetra-acetic acid (EGTA), and after incubation with the Ca 2+ channel blockers verapamil and nifedipine. Second, to confirm that cGMP is not involved in the stimulatory action of SNP, the effects of the latter on gonadotropin secretion were tested in the presence of the antagonists of the guanylyl cyclases oxadiazoloquinoxaline and LY 83,583. Our results showed that the stimulatory action of SNP on gonadotropin release is blunted in Ca 2+ -free medium and after incubation with EGTA, verapamil, nifedipine, and caffeine. On the contrary, the effect of SNP remained unaltered after antagonization of guanylyl cyclases. We conclude that NO, acting at the pituitary level, stimulates gonadotropin secretion through a calcium-dependent, cGMP-independent mechanism.
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              Author and article information

              Journal
              HRE
              Horm Res Paediatr
              10.1159/issn.1663-2818
              Hormone Research in Paediatrics
              S. Karger AG
              1663-2818
              1663-2826
              2001
              2001
              05 December 2001
              : 55
              : 5
              : 229-235
              Affiliations
              Department of Physiology, Faculty of Medicine, University of Córdoba, Córdoba, Spain
              Article
              50001 Horm Res 2001;55:229–235
              10.1159/000050001
              11740144
              © 2001 S. Karger AG, Basel

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              Page count
              Figures: 4, Tables: 4, References: 26, Pages: 7
              Categories
              Original Paper

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