A study on the mechanism by which sodium nitroprusside, a nitric oxide donor, applied to the anteroventral third ventricular region provokes facilitation of vasopressin secretion in conscious rats

An Essential role for DeltaFosB in the median preoptic nucleus in the sustained hypertensive effects of chronic intermittent hypoxia

One of the main clinical features of obstructive sleep apnea is sustained hypertension and elevated sympathetic activity during waking hours. Chronic intermittent hypoxia (CIH), animal model of the hypoxemia associated with obstructive sleep apnea, produces a similar sustained increase in blood pressure. This study determined the role of ΔFosB in the median preoptic nucleus (MnPO) in the sustained increase in mean arterial pressure associated with CIH. Rats were injected in the MnPO with viral vectors that expressed green fluorescent protein alone or green fluorescent protein plus a dominant-negative construct that inhibits the transcriptional effects of ΔFosB. In green fluorescent protein-injected rats and uninjected controls, 7-day exposure to CIH increased mean arterial pressure by 7 to 10 mm Hg during both intermittent hypoxia exposure and normoxia. Dominant-negative inhibition of MnPO ΔFosB did not affect changes in mean arterial pressure during intermittent hypoxia exposure but significantly reduced the sustained component of the blood pressure response to CIH during the normoxic dark phase. Inhibition of MnPO ΔFosB reduced the FosB/ΔFosB staining in the paraventricular nucleus and rostral ventrolateral medulla but not the nucleus of the solitary tract. PCR array analysis identified 6 activator protein 1-regulated genes expressed in the MnPO that were increased by CIH exposure, ace, ace2, nos1, nos3, prdx2, and map3k3. Dominant-negative inhibition of ΔFosB in the MnPO blocked increased expression of each of these genes in rats exposed to CIH except for Prdx2. ΔFosB may mediate transcriptional activity in MnPO necessary for sustained CIH hypertension, suggesting that neural adaptations may contribute to diurnal hypertension in obstructive sleep apnea.

The dynamic distribution of fluoro-gold and its interrelation with neural nitric oxide synthase following intracerebroventricular injection into rat brain

We mapped the dynamic distribution of fluoro-gold (FG) within rat brain following intracerebroventricular (icv) injection into the lateral ventricle and observed its interrelation with neural nitric oxide synthase (nNOS) using FG fluorescent microphotography combined with nNOS immunohistochemistry. We also detected the amount of icv administered FG entering the peripheral circulation using a fluorescence microplate assay. The degree of periventricular penetration of FG was significantly increased over time. At 2 min after icv injection, FG primarily labeled the choroid plexus in the lateral and third ventricles, with limited penetration into the ependyma and the subependyma of the same ventricles. Some FG/nNOS-double labeled cerebrospinal fluid-contacting neurons were observed in these ventricles as well. At 15 and 30 min, FG penetrated mainly into forebrain ventricular organs and parenchymal structures. Many FG/nNOS double labeled neurons were found at each of these sites. In addition, at 30 min intense FG labeling was found in the hypophysis, while limited periventricular penetration of FG was detected in the hindbrain circumventricular areas. In the peripheral circulation, a low concentration of FG was detected 2 min after icv injection. The concentration increased slowly, peaked at 20 min, then gradually decreased until the end of the experiment at 30 min. These findings indicate that dynamic penetration of icv administrated agents into the periventricular tissues and peripheral circulation should be considered when designing icv experiments.

Neural nitric oxide gene inactivation affects the release profile of oxytocin into the blood in response to forced swimming

This study was undertaken to examine the importance of nitric oxide (NO) generated by the neural isoform of the nitric oxide synthase (nNOS) on the activity of the hypothalamic neurohypophyseal system in neural nitric oxide synthase knock-out (KO) and wild-type (WT) mice under basal conditions and in response to forced swimming. The intensity of the hybridisation signal for vasopressin (AVP) in the hypothalamic supraoptic nucleus (SON) was significantly higher in KO mice when compared with WT, whereas oxytocin (OXT) basal mRNA levels were similar in both groups. Although the basal peripheral release of AVP and OXT was equivalent in both genotypes, we observed in KO mice a significant drop of AVP and OXT plasma values 15 min after stressor onset and a robust increase in the OXT plasma concentration at 60 min. These findings suggest that in the male mouse, NO inhibits AVP gene transcription in magnocellular neurones of the SON and collaborates in maintaining constant AVP and OXT plasma levels following acute stressor exposure, exerting a bimodal regulatory action on OXT secretion. We conclude that NO is involved in the regulation of magnocellular neurones of the SON, and it is preferentially implicated in the attenuation of the peripheral release of OXT induced by acute stressor exposure.

Noradrenergic regulation in mouse supraoptic nucleus involves a nitric oxide pathway only to regulate arginine-vasopressin expression and not oxytocin expression

Noradrenalin (NA) regulates the expression of arginine-vasopressin (AVP) and oxytocin (OT) by magnocellular neurons in the supraoptic nucleus (SON) of the hypothamalus. Nitric oxide (NO) may be one of the factors involved in the NA signaling pathway regulating AVP and OT expression. To test this possibility, we used an ex vivo experimental model of mouse hypothalamus slices. Increases in AVP and OT levels in the SON were detected by immunohistochemistry and immunoenzyme assays after 1 hr and 4 hr incubations with NA (10(-4) M). There was also an increase in the expression and activity of neuronal NOS and inducible NOS in the SON as assessed by immunohistochemical and histoenzymological analysis of NADPH-diaphorase, whereas endothelial NOS was undetectable. To specify the role of NO, the slices were treated with NA and L-arginine methyl ester (L-NAME, an NOS inhibitor; 3 microM). This treatment for 1 hr abolished the NA-induced increase in AVP. Treatment with sodium nitroprusside (SNP, an NO donor; 0.1 mM) increased AVP levels, confirming that NO regulates AVP expression. Addition of 1 mM EGTA during the incubation with NA reduced the AVP increase by half, indicating that both nNOS and iNOS activities are involved in the regulation. A 1-hr treatment with L-NAME did not prevent the increase in OT induced by NA; similarly, treatment with SNP had no effect. These findings show that NO is involved in the regulation of AVP expression by NA and that NA control of OT expression is independent of NO.

Neuronal nitric oxide synthase inhibition differentially affects oxytocin and vasopressin secretion in salt loaded rats

Nitric oxide, an endogenous gas produced by nitric oxide synthase (NOS), has been described as a neuromodulator of hormone secretion, including the neurohypophysial peptides oxytocin (OT) and vasopressin (AVP), hormones involved in the sodium and water homeostasis. The presence of NOS in the hypothalamic nuclei as well as in the circumventricular organs suggests a nitrergic regulation of OT and AVP secretion. Thus, the aim of this study was to evaluate the effect of 7-nitroindazole (7-NI), a selective inhibitor of neuronal NOS, in the plasma OT and AVP levels in rats submitted to a short and long-term salt loading. We also evaluated the NOS activity in the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. Our data showed an increase of plasma OT and AVP levels in both short and long-term salt loading. The augment of plasma OT and AVP levels was accompanied by an increase of NOS activity in the SON and PVN. The injection of 7-NI potentiated the increase of plasma OT induced by salt loading, but inhibited the increase of plasma AVP in the same experimental conditions. These results indicate that, under short and prolonged osmotic stimulation, nitric oxide may differentially control the neurohypophysial secretion.

Nitric oxide modulation of the hypothalamo-neurohypophyseal system

Nitric oxide (NO), a free radical gas produced endogenously from the amino acid L-arginine by NO synthase (NOS), has important functions in modulating vasopressin and oxytocin secretion from the hypothalamo-neurohypophyseal system. NO production is stimulated during increased functional activity of magnocellular neurons, in parallel with plastic changes of the supraoptic nucleus (SON) and paraventricular nucleus. Electrophysiological data recorded from the SON of hypothalamic slices indicate that NO inhibits firing of phasic and non-phasic neurons, while L-NAME, an NOS inhibitor, increases their activity. Results from measurement of neurohypophyseal hormones are more variable. Overall, however, it appears that NO, tonically produced in the forebrain, inhibits vasopressin and oxytocin secretion during normovolemic, isosmotic conditions. During osmotic stimulation, dehydration, hypovolemia and hemorrhage, as well as high plasma levels of angiotensin II, NO inhibition of vasopressin neurons is removed, while that of oxytocin neurons is enhanced. This produces a preferential release of vasopressin over oxytocin important for correction of fluid imbalance. During late pregnancy and throughout lactation, fluid homeostasis is altered and expression of NOS in the SON is down- and up-regulated, respectively, in parallel with plastic changes of the magnocellular system. NO inhibition of magnocellular neurons involves GABA and prostaglandin synthesis and the signal-transduction mechanism is independent of the cGMP-pathway. Plasma hormone levels are unaffected by i.c.v. 1H-[1, 2, 4]oxadiazolo-[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor) or 8-Br-cGMP administered to conscious rats. Moreover, cGMP does not increase in homogenates of the neural lobe and in microdialysates of the SON when NO synthesis is enhanced during osmotic stimulation. Among alternative signal-transduction pathways, nitrosylation of target proteins affecting activity of ion channels is considered.