Evidence for an inhibitory effect of nitric oxides on neuropeptide secretion from isolated neural lobe of the rat pituitary gland

The modulatory effect of nitric oxide in pro- and anti-convulsive effects of vasopressin in PTZ-induced seizures threshold in mice

Vasopressin neuropeptides play an important role in the several cognitive, social, and neuroendocrine functions. Also, several studies report the involvement of nitrergic system in the vasopressin functions in central nervous system. This study investigates the effect of Arginine-Vasopressin (AVP) in pentylenetetrazol (PTZ)-induced seizures threshold and the probable role of nitric oxide (NO). AVP is administered intraperitoneally (0.01-20μg/kg, i.p.) 30min before induction of seizures. Administration of AVP (0.1μg/kg) significantly lowered the PTZ-induced seizures threshold. But, administration of AVP (10 and 20μg/kg) increased the seizures threshold, significantly. Pretreatment of SR 49059 (V1a receptor antagonist, 2mg/kg, i.p.) just reversed the pro-convulsant effect of AVP. Meanwhile, SSR 149415 (V1b receptor antagonist, 10mg/kg, i.p.) pretreatment reversed both pro-and anti-convulsant effects of AVP. The nitric oxide precursor, L-arginine (60mg/kg, i.p.) increased pro-convulsant effect of AVP, but did not change anticonvulsant activity. The nitric oxide synthase (NOS) inhibitor L-NAME (10mg/kg, i.p.) reversed both pro- and anti-convulsant effect of AVP. Selective inducible NOS inhibitor, aminoguanidine (100mg/kg, i.p.) just reversed the anti-convulsant effects of AVP. The results of the present study showed nitric oxide system may contribute to the biphasic effects of AVP on PTZ-induced seizures. V1a receptor may modulate only the proconvulsive effect. While, V1b receptors can mediate both the pro- and anti-convulsive effect of AVP.

Nitric oxide rectifies acid-base disturbance and modifies thyroid hormone activity during net confinement of air-breathing fish (Anabas testudineus Bloch)

Nitric oxide (NO), a short-lived freely diffusible radical gas that acts as an important biological signal, regulates an impressive spectrum of physiological functions in vertebrates including fishes. The action of NO, however, on thyroid hormone status and its role in the integration of acid-base, osmotic and metabolic balances during stress are not yet delineated in fish. Sodium nitroprusside (SNP), a NO donor, was employed in the present study to investigate the role of NO in the stressed air-breathing fish Anabas testudineus. Short-term SNP treatment (1 mM; 30 min) interacted negatively with thyroid axis, as evident in the fall of plasma thyroxine in both stressed and non-stressed fish. In contrast, the cortisol responsiveness to NO was negligible. SNP challenge produced systemic alkalosis, hypocapnia and hyperglycemia in non-stressed fish. Remarkable acid-base compensation was found in fish kept for 60 min net confinement where a rise in blood pH and HCO(3) content occurred with a reduction in PCO(2) content. SNP challenge in these fish, on the contrary, produced a rise in oxygen load together with hypocapnia but without an effect on HCO(3) content, indicating a modulator role of NO in respiratory gas transport during stress response. SNP treatment reduced Na(+), K(+) ATPase activity in the gill, intestine and liver of both stressed and non-stressed fish, and this suggests that stress state has little effect on the NO-driven osmotic competence of these organs. On the other hand, a modulatory effect of NO was found in the kidney which showed a differential response to SNP, emphasizing a key role of NO in kidney ion transport and its sensitivity to stressful condition. H(+)-ATPase activity, an index of H(+) secretion, downregulated in all the organs of both non-stressed and stressed fish except in the gill of non-stressed fish and this supports a role for NO in promoting alkalosis. The data indicate that, (1) NO interacts antagonistically with T(4), (2) modifies respiratory gas transport and (3) integrates acid-base and osmotic actions during stress response in air-breathing fish. Collectively, this first evidence in fish indicate that NO can promote compensatory physiologic modification and that can reduce the magnitude of stress-induced acid-base and osmotic disturbance and that suggests a role for NO in the ease and ease response of this fish.

Anteroventral third ventricle (AV3V) lesion affects hypothalamic neuronal nitric oxide synthase (nNOS) expression following water deprivation

Neuronal nitric oxide synthase (nNOS) has been reported to be up-regulated in the hypothalamic supraoptic nucleus (SON) during dehydration which in turn could increase nitric oxide (NO) production and consequently affect arginine vasopressin (AVP) secretion. The anteroventral third ventricle (AV3V) region has strong afferent connections with the SON. Herein we describe our analysis of the effects of an AV3V lesion on AVP secretion, and c-fos and nNOS expression in the SON following dehydration. Male Wistar rats had their AV3V region electrolytically lesioned or were sham operated. After 21 days they were submitted to dehydration or left as controls (euhydrated). Two days later, one group was anaesthetized, perfused and the brains were processed for Fos protein and nNOS immunohistochemistry (IHC). Another group was decapitated, the blood collected for hematocrit, osmolality, serum sodium and AVP plasma level analysis. The brains were removed for measurement of neurohypophyseal AVP content, and the SON was punched out and processed for nNOS detection by western blotting. The AV3V lesion reduced AVP plasma levels and c-fos expression in the SON following dehydration (P<0.05). Western blotting revealed an up-regulation of nNOS in the SON of control animals following dehydration, whereas such up-regulation was not observed in AV3V-lesioned rats (P<0.05). We conclude that the AV3V region plays a role in regulating the expression of nNOS in the SON of rats submitted to dehydration, and thus may affect the local nitric oxide production and the secretion of vasopressin.

Carbon monoxide and nitric oxide modulate hyperosmolality-induced oxytocin secretion by the hypothalamus in vitro

OT (oxytocin) is secreted from the posterior pituitary gland, and its secretion has been shown to be modulated by NO (nitric oxide). In rats, OT secretion is also stimulated by hyperosmolarity of the extracellular fluid. Furthermore, NOS (nitric oxide synthase) is located in hypothalamic areas involved in fluid balance control. In the present study, we evaluated the role of the NOS/NO and HO (haem oxygenase)/CO (carbon monoxide) systems in the osmotic regulation of OT release from rat hypothalamus in vitro. We conducted experiments on hypothalamic fragments to determine the following: (i) whether NO donors and NOS inhibitors modulate OT release and (ii) whether the changes in OT response occur concurrently with changes in NOS or HO activity in the hypothalamus. Hyperosmotic stimulation induced a significant increase in OT release that was associated with a reduction in nitrite production. Osmotic stimulation of OT release was inhibited by NO donors. NOS inhibitors did not affect either basal or osmotically stimulated OT release. Blockade of HO inhibited both basal and osmotically stimulated OT release, and induced a marked increase in NOS activity. These results indicate the involvement of CO in the regulation of NOS activity. The present data demonstrate that hypothalamic OT release induced by osmotic stimuli is modulated, at least in part, by interactions between NO and CO.

Carbon monoxide and nitric oxide modulate hyperosmolality-induced oxytocin secretion by the hypothalamus in vitro

OT (oxytocin) is secreted from the posterior pituitary gland, and its secretion has been shown to be modulated by NO (nitric oxide). In rats, OT secretion is also stimulated by hyperosmolarity of the extracellular fluid. Furthermore, NOS (nitric oxide synthase) is located in hypothalamic areas involved in fluid balance control. In the present study, we evaluated the role of the NOS/NO and HO (haem oxygenase)/CO (carbon monoxide) systems in the osmotic regulation of OT release from rat hypothalamus in vitro. We conducted experiments on hypothalamic fragments to determine the following: (i) whether NO donors and NOS inhibitors modulate OT release and (ii) whether the changes in OT response occur concurrently with changes in NOS or HO activity in the hypothalamus. Hyperosmotic stimulation induced a significant increase in OT release that was associated with a reduction in nitrite production. Osmotic stimulation of OT release was inhibited by NO donors. NOS inhibitors did not affect either basal or osmotically stimulated OT release. Blockade of HO inhibited both basal and osmotically stimulated OT release, and induced a marked increase in NOS activity. These results indicate the involvement of CO in the regulation of NOS activity. The present data demonstrate that hypothalamic OT release induced by osmotic stimuli is modulated, at least in part, by interactions between NO and CO.

Central nitric oxide blocks vasopressin, oxytocin and atrial natriuretic peptide release and antidiuretic and natriuretic responses induced by central angiotensin II in conscious rats

The presence of nitric oxide synthase (NOS), the enzyme that catalyses the formation of nitric oxide (NO), in the circumventricular organs and magnocellular neurones suggests an important role of NO in the modulation of vasopressin (AVP) and oxytocin (OT) release. Intracerebroventricular (I.C.V.) injection of angiotensin II (Ang II) stimulates the release of AVP, OT and atrial natriuretic peptide (ANP), with the resultant antidiuretic and natriuretic effects. This study investigated the interaction between nitrergic and angiotensinergic pathways on the release of AVP, OT and ANP and on urinary volume and sodium excretion in water-loaded rats. Unanaesthetized, freely moving, male Wistar rats received two water loads followed by an injection into the lateral ventricle of an inhibitor of NOS (L-NAME), a NO donor [3-morpholinylsydnoneimine chloride (SIN-1) or S-nitroso-N-acetyl penicillamine (SNAP)] or vehicle (isotonic saline) and, 20 min after, they received a second I.C.V. injection of Ang II or vehicle. Injections of L-NAME or Ang II produced an increase in plasma levels of AVP, OT and ANP, a reduction in urinary volume and an increase in sodium excretion. Pretreatment with L-NAME enhanced the Ang II-induced increase in AVP, OT and ANP release, as well as the antidiuresis and natriuresis. Injection of SIN-1 or SNAP did not modify hormonal plasma levels and urinary parameters. In contrast SNAP blocked the AVP, OT and ANP release, as well as antidiuretic and natriuretic responses induced by ANG-II. Thus, the central nitrergic system can act to inhibit AVP, OT and ANP secretion and the antidiuretic and natriuretic effects in response to Ang II.

Regulation of interleukin-6 secretion in murine pituicytes

Pituicytes, the astrocytic glial cells of the neural lobe, are known to secrete interleukin-6 and nitric oxide upon stimulation with various inflammatory mediators, i.e. interleukin-1beta. Nitric oxide is described to modulate the secretion of interleukin-6 in various cell types. The aim of the present study was to investigate the effect of nitric oxide on interleukin-1beta induced interleukin-6 secretion. Furthermore the effect of interferon-gamma on interleukin-6 and nitric oxide release was investigated. Cultures of pituicytes were prepared of neural lobes from male mice. The effect of interleukin-1beta and interferon-gamma on interleukin-6 and nitric oxide secretion was investigated in pituicytes cultured for 14 days. The secretion of interleukin-6 and nitric oxide was determined after 24 h of stimulation. Pituicytes secrete interleukin-6 upon stimulation with interleukin-1beta dose dependently but did not induce any detectable nitric oxide release. Co-stimulation with interferon-gamma and interleukin-1beta induced a significant nitric oxide release. In addition interferon-gamma inhibits interleukin-1beta induced interleukin-6 secretion dose dependently. The observed effect of interferon-gamma on interleukin-6 secretion was not affected by the specific inducible nitric oxide synthase inhibitor 1400W (N-(3-[aminomethyl]benzyl)acetamidine). Furthermore interferon-gamma dose dependently inhibits unstimulated interleukin-6 secretion. Use of the nitric oxide releaser DETA/NO (2,2'-(hydroxynitrosohydrazono)bis-ethanimine) demonstrated that nitric oxide does not inhibit interleukin-1beta induced interleukin-6 secretion. These results demonstrated that nitric oxide has no influence on interleukin-1beta induced interleukin-6 secretion in cultured pituicytes. However the results are showing that interferon-gamma has an inhibitory effect on interleukin-6 secretion.

Modulation of Ca(v)1 and Ca(v)2.2 channels induced by nitric oxide via cGMP-dependent protein kinase

The unconventional gaseous transmitter nitric oxide (NO) markedly influences most of mechanisms involved in the regulation of intracellular Ca2+ homeostasis. In excitable cells, Ca2+ signaling mainly depends on the activity of voltage-gated Ca2+ channels (VGCCs). In the present paper, we will review data from our laboratory and others characterizing NO-induced modulation of Ca(v)1 (L-type) and Ca(v)2.2 (N-type) channels. In particular, we will explore experimental evidence indicating that NO's inhibition of channel gating is produced via cGMP-dependent protein kinase and examine some of the numerous cell functions that are potentially influenced by the action of NO on Ca2+ channels.

NO inhibition of the magnocellular neuroendocrine system in rats is independent of cGMP signaling pathway

Our objective was to test the hypothesis that the cGMP signal-transduction mechanism mediates nitric oxide's (NO) modulation of oxytocin (OT) and vasopressin (VP) secretion from the hypothalamo-neurohypophysial system. Three studies were conducted in adult male Sprague-Dawley rats: (1a) Euhydrated rats received an intracerebroventricular (icv) infusion (1 microl/min for 30 min) of artificial cerebrospinal fluid (aCSF), vehicle (2.6% dimethyl sulfoxide [DMSO]) or 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (0.05 microg/microl), an inhibitor of soluble guanylyl cyclase (sGC). ODQ did not affect basal levels of plasma VP or OT; (1b) Rats dehydrated for 24 h received aCSF or 8-Br-cGMP (icv), a membrane-permeable analog of cGMP, and plasma hormones were measured 2 min later. 8-Br-cGMP did not significantly change VP or OT levels; (2) Rats ingested water or 2% NaCl for 4 days, and NO synthase (NOS) and sGC activities were measured in posterior pituitaries, the anatomical site of hormone secretion. Salt loading enhanced (P < 0.001) production of [(14)C]citrulline, the coproduct of NO synthesis, without altering cGMP; (3) One SON was microdialyzed with [(14)C]arginine and NOS and sGC activities were quantified in microdialysates during intravenous (iv) infusion of isotonic or hypertonic saline in awake and anesthetized rats. In awake rats, [(14)C]citrulline recovery, but not cGMP, increased (P < 0.05) during intravenous infusion of both isotonic and hypertonic solutions, and after insertion of microdialysis probe itself. In anesthetized rats, however, where basal NOS activity is low, intravenous infusion of hypertonic, but not isotonic solution, increased [(14)C]citrulline recovery without affecting cGMP. Thus, in the forebrain, neither NO produced basally nor during osmotic stimulation depends on cGMP to modulate plasma vasopressin and oxytocin 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.

Inducible nitric oxide synthase is responsible for nitric oxide release from murine pituicytes

This study investigated whether pituicytes were able to produce and release nitric oxide (NO), and which type of nitric oxide synthase (NOS) would be responsible for this phenomenon. Lipopolysaccharide (LPS) 1 micro g/ml was used as inflammatory mediator. Because pituicytes are known to secrete interleukin (IL)-6 upon stimulation with LPS, this parameter was also investigated. Cultured pituicytes, from 4-week-old male mice, were stimulated with LPS for 6 h or 24 h. At 24 h, there was a significant increase in accumulated nitrite indicating NO formation. In contrast, IL-6 release was already significantly higher 6 h after stimulation and further increased at 24 h. The correlation between accumulated nitrite and secreted IL-6 was 0.84 after 24 h of incubation with LPS. The expression of inducible NOS (iNOS) mRNA in the pituicytes was significantly higher than the control level after 6 h and 24 h of exposure to LPS, with levels at 6 h being significantly higher than those at 24 h. There was no detected expression of endothelial NOS or neuronal NOS mRNA. Cultured pituicytes were also subjected to immunocytochemistry for iNOS protein at 6, 12, and 24 h after stimulation with LPS. Most cells were positive for iNOS, but there were no observable differences with the time points that we used. Collectively, these results show that pituicytes are able to produce NO, and that the inducible form of NOS is responsible for this production. Furthermore, there is a weak correlation between NO and IL-6 released from pituicytes after 24 h of stimulation with LPS.

Nitrergic modulation of vasopressin, oxytocin and atrial natriuretic peptide secretion in response to sodium intake and hypertonic blood volume expansion

The central nervous system plays an important role in the control of renal sodium excretion. We present here a brief review of physiologic regulation of hydromineral balance and discuss recent results from our laboratory that focus on the participation of nitrergic, vasopressinergic, and oxytocinergic systems in the regulation of water and sodium excretion under different salt intake and hypertonic blood volume expansion (BVE) conditions. High sodium intake induced a significant increase in nitric oxide synthase (NOS) activity in the medial basal hypothalamus and neural lobe, while a low sodium diet decreased NOS activity in the neural lobe, suggesting that central NOS is involved in the control of sodium balance. An increase in plasma concentrations in vasopressin (AVP), oxytocin (OT), atrial natriuretic peptide (ANP), and nitrate after hypertonic BVE was also demonstrated. The central inhibition of NOS by L-NAME caused a decrease in plasma AVP and no change in plasma OT or ANP levels after BVE. These data indicate that the increase in AVP release after hypertonic BVE depends on nitric oxide production. In contrast, the pattern of OT secretion was similar to that of ANP secretion, supporting the view that OT is a neuromodulator of ANP secretion during hypertonic BVE. Thus, neurohypophyseal hormones and ANP are secreted under hypertonic BVE in order to correct the changes induced in blood volume and osmolality, and the secretion of AVP in this particular situation depends on NOS activity.

Nitric oxide and neuromodulation in the caudal neurosecretory system of teleosts

Although evidence exists that nitric oxide (NO) mediates neuroendocrine secretion in mammals, the involvement of NO in the neuroendocrine regulation of non-mammalian vertebrates has yet to be investigated in detail. The present review conveys several recent data, suggesting that NO plays a modulatory role in the caudal neurosecretory system (CNSS) of teleosts. The presence and distribution of neuronal NO synthase (nNOS) was demonstrated in the CNSS of the Nile tilapia Oreochromis niloticus by means of NADPHd histochemistry, NOS immunohistochemistry, NOS immunogold electron microscopy, the citrulline assay for NOS activity and Western blot analysis. NO production by the caudal spinal cord homogenates was also evaluated by the oxyhemoglobin assay. On the whole, these findings indicate that caudal neurosecretory cells express NOS enzymes and presumably produce NO as a cotransmitter. Moreover, the comparison of the nNOS distribution with that of urotensins I and II (UI and UII) suggests that neurosecretory Dahlgren cells belong to two different functional subpopulations: a population of UI/UII secreting nitrergic neurons and a population of non-nitrergic neurons, which principally secrete UII. These results implicate NO as a putative modulator of the release of urotensins from the neurosecretory axon terminals. Therefore, like in mammals, NO appears to influence neuroendocrine secretion in teleosts.

Activation of hypothalamic neuronal nitric oxide synthase in lithium-induced diabetes insipidus rats

The expression of the neuronal nitric oxide synthase (nNOS) gene in the paraventricular (PVN) and supraoptic nuclei (SON) in rats with lithium (Li)-induced polyuria was examined by using in situ hybridization histochemistry. The state of the thyroid axis in these rats was also examined by in situ hybridization histochemistry for thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) mRNAs and radioimmunoassay for circulating thyroid hormones. Adult male Wistar rats consuming a diet that contained LiCl (60 mmol/kg) for 4 weeks developed remarkable polyuria. The urine in the Li-treated rats was hypotonic and had a large volume and low ionic concentration. The nNOS mRNA in the PVN and SON was significantly increased in the Li-treated rats in comparison with that in control. The increased levels of the nNOS mRNA in the PVN and SON were confirmed by NADPH-diaphorase histochemical staining. There were no differences of TRH mRNA in the PVN, TSH mRNA in the anterior pituitary and plasma concentrations of free T3 and free T4 between Li-treated rats and control rats. These results suggest that Li-induced diabetes insipidus may activate nNOS in the PVN and SON without change of the thyroid axis.

Neurokinin A inhibits oxytocin and GABA release from the posterior pituitary by stimulating nitric oxide synthase

Neurokinin A (NKA) is a tachykinin that participates in the control of neuroendocrine functions. The posterior pituitary lobe (PP) contains abundant nitric oxide synthase (NOS), suggesting that nitric oxide (NO) may play a role in controlling the release of neuropeptides and neurotransmitters. In the present project, we investigated the in vitro effect of NKA on oxytocin release from hypothalamic explants and PP of male rats and the possible involvement of NO in the action of NKA. Since NKA inhibits gamma-aminobutyric acid (GABA) release from PP, we also examined the role of NO in the effect of NKA on basal and K(+)-evoked GABA release. NKA (10(-7)-10(-5) M) significantly decreased oxytocin release from PP, whereas it did not affect its release from hypothalamic explants. The inhibitory effect of NKA on oxytocin release from PP was completely blocked by the NOS inhibitors N(G)-monomethyl-L-arginine (L-NMMA, 0.5 mM) or N(G)-nitro-L-arginine-methyl-ester (L-NAME, 1 mM). Sodium nitroprusside (0.5 mM), an NO releaser, had no effect on basal GABA release but significantly decreased K(+)-evoked GABA release. L-NMMA (0.3 mM) and L-NAME (0.5 mM) increased K(+)-evoked GABA release, indicating that NO plays an inhibitory role in GABA release from PP. The inhibition in both basal and K(+)-evoked GABA release induced by NKA (10(-7) M) was reduced by L-NAME (1 mM). Also, NKA (10(-7) M) increased NO synthesis as measured by [(14)C] citrulline production. Considered all together, our data indicate that NO may mediate the inhibitory effect of NKA on the release of both oxytocin and GABA from PP.

Colocalization of neuronal NO synthase with urotensins I and II in the caudal neurosecretory neurons and the urophysis of the teleost oreochromis niloticus: a gold immunoelectron microscopic study

The intracellular distribution of neuronal nitric oxide synthase (nNOS) was studied in the caudal neurosecretory system of a teleost, Oreochromis niloticus (Cichlids), by means of post-embedding immunogold labeling with a polyclonal antibody directed against nNOS of human origin. Ultrastructural examination demonstrated that neuronal NOS-like molecules are distributed within the Dahlgren cell perikarya, the neurosecretory axons, and the urophysial axon terminals. In the neurosecretory somata, gold particles for nNOS were mainly cytosolic, whereas in the neurosecretory axons and axon terminals they were associated with the membrane and/or the dense core of neurosecretory granules. Double immunogold labelings for nNOS/urotensin I (UI) and nNOS/urotensin II (UII) demonstrated that nNOS-like molecules are colocalized with UI and/or UII in the neurosecretory granules contained within the urophysial terminals. The present findings suggest that both a soluble cytosolic and a particulate neuronal NOS are expressed in the caudal neurosecretory neurons. They confirm previous biochemical data on the same species.

Nitric oxide and carbon monoxide have a stimulatory role in the hypothalamic-pituitary-adrenal response to physico-emotional stressors in rats

We tested the hypothesis that nitric oxide and carbon monoxide, which are produced in the brain by nitric oxide synthase (NOS) and heme oxygenase (HO), modulate the hypothalamic-pituitary-adrenal response to physico-emotional stressors by acting at the hypothalamus. Accordingly, we determined 1) whether the intracerebroventricular (icv) injection of NOS or HO inhibitors at doses that were confined to the brain attenuated electroshock-induced ACTH release; and 2) whether the decreases in this ACTH response were concurrent with decreases in NOS or HO activity levels at the hypothalamus. Icv injection of the NOS inhibitor Nomega-nitro-L-arginine-methylester (L-NAME; 50 microg) or the HO inhibitor tin protoporphyrin (SnPP; 20-25 microg) significantly blunted the plasma ACTH response to a 45-min session of intermittent electroshocks. Importantly, in these same animals there were concurrent decreases in hypothalamic NOS or HO activities, respectively. There were little or no effects of these inhibitors on anterior pituitary NOS or HO activities, indicating that there was only minimal leakage of the drug from the brain after icv administration. The specificity of action of these inhibitors was confirmed by the fact that SnPP did not affect NOS activity, and L-NAME did not affect HO activity. Finally, L-NAME produced no effect, whereas SnPP produced only transient increases in blood pressure, suggesting that these inhibitors do not affect activity indirectly through alterations in blood pressure. These data support the hypothesis that in the whole animal, both NO and CO exert a stimulatory influence on the acute ACTH response to physico-emotional stressors, and that the hypothalamus is the critical site of their actions.

Centrally produced nitric oxide and the regulation of body fluid and blood pressure homeostases

1. Nitric oxide (NO) tonically inhibits the basal release of vasopressin and oxytocin into plasma. 2. Nitric oxide inhibition on vasopressin secretion is removed, while that on oxytocin is enhanced, during water deprivation, hypovolaemia, moderate osmotic stimulation and angiotensin (Ang)II. This results in a preferential release of vasopressin over oxytocin that promotes conservation of water. 3. Nitric oxide facilitates drinking behaviour stimulated by water deprivation, osmotic stimulation, haemorrhage and AngII. Together with the hormonal response, NO produces a positive water balance during reductions in intracellular and intravascular volumes. 4. Nitric oxide produced within the central nervous system maintains resting arterial blood pressure partially by attenuating the pressor actions of AngII and prostaglandins. 5. Central production of NO is enhanced during osmotic stimulation to counterbalance the salt-induced pressor response. 6. Paradoxically, central production of NO is also enhanced during haemorrhage, presumably to maintain peripheral vasodilation and blood flow to vital organs.

Vasopressin does not effect hypertension caused by long-term nitric oxide inhibition

Nitric oxide attenuates both vasopressin-induced vasoconstriction and vasopressin release. We tested whether hypertension and renal dysfunction elicited by chronic inhibition of nitric oxide (NO) synthesis using N(G)-nitro-L-arginine (L-NNA) could be mediated in part by vasopressin V(1A) receptors. Male rats were treated orally for 6 weeks with L-NNA (15 mg/kg per day), a nonpeptide V(1A) receptor antagonist (2S)-1-[(2R,3S)-5-chloro-3-(2-chlorophenyl)-1-(3, 4-dimethoxybenzene-sulfonyl)-3-hydroxy-2, 3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2-carboxamide (SR 49059, 30 mg/kg per day), or a combination of SR 49059 and L-NNA (same doses), or they received no treatment. Both drugs were added to the food. Measurements were performed in conscious rats (urine collection in metabolic cages, tail-cuff arterial pressure) and at the end of the study in anesthetized rats (clearance measurements). L-NNA produced sustained hypertension, decreased glomerular filtration rate, and increased renal vascular resistance, plasma renin activity, and urinary albumin excretion. SR 49059 had no effect per se on these parameters and also did not attenuate the hypertension and renal dysfunction induced by L-NNA. Surprisingly, SR 49059 potentiated L-NNA-induced hypertension at the end of the 6-week treatment. However, the blood pressure response and the renal and mesenteric vasoconstriction elicited by exogenous vasopressin were attenuated in rats treated with SR 49059. L-NNA did not change plasma vasopressin concentration or 24-hour urinary vasopressin excretion. Our findings suggest that activation of vasopressin V(1A) receptors does not contribute to the hypertension and renal dysfunction induced by chronic NO synthesis inhibition. They also document unchanged plasma vasopressin concentration in NO-deficient hypertension.

Direct actions of nitric oxide on rat neurohypophysial K+ channels

1. Nitric oxide (NO) has been shown to modulate neuropeptide secretion from the posterior pituitary. Here we show that NO activates large-conductance Ca2+-activated K+ (BK) channels in posterior pituitary nerve terminals. 2. NO, generated either by the photolysis of caged-NO or with chemical donors, irreversibly enhanced the component of whole-terminal K+ current due to BK channels and increased the activity of BK channels in excised patches. NO also inhibited the transient A-current. The time courses of these effects on K+ current were very different; activation of BK channels developed slowly over several minutes whereas inhibition of A-current immediately followed NO uncaging. 3. Activation of BK channels by NO occurred in the presence of guanylyl cyclase inhibitors and after removal of ATP or GTP from the pipette solution, suggesting a cGMP-independent signalling pathway. 4. The sulfhydryl alkylating agent N-ethyl maleimide (NEM) increased BK channel activity. Pretreatment with NEM occluded NO activation. 5. NO activation of BK channels occurred independently of voltage and cytoplasmic Ca2+ concentration. In addition, NO removed the strict Ca2+ requirement for channel activation, rendering channels highly active even at nanomolar Ca2+ levels. 6. These results suggest that NO, or a reactive nitrogen byproduct, chemically modifies nerve terminal BK channels or a closely associated protein and thereby produces an increase in channel activity. Such activation is likely to inhibit impulse activity in posterior pituitary nerve terminals and this may explain the inhibitory action of NO on secretion.

Central actions of nitric oxide in regulation of autonomic functions

The identification of nitric oxide (NO) as a gaseous, nonconventional neurotransmitter in the central nervous system has led to an explosion of studies aimed at learning about the roles of NO, not only at a cellular level, but also in regulating the activity of specific physiological systems that are coordinated by the brain. In the 1980s, publications began to appear which pointed to a role for NO in regulating peripheral autonomic function. In the 1990s, it became apparent that NO also acts centrally to affect autonomic responses. In this review, I will discuss the state of the current knowledge about the central role of NO in physiological functions which are related specifically to the control of sympathetic output. Studies which do not differentiate a central from a peripheral role for NO in these functions have not been included. After a brief discussion about the cellular events in which NO is involved, the distribution of NO-producing neurons in central autonomic areas of the brain will be presented. The more general actions of central NO in regulating sympathetic activity, as assessed with i.c.v. injections of pharmacological agents, will be followed by more specific sites of action achieved with microinjections into discrete brain areas. The review will be concluded with discussions about central NO in two physiological states of sympathetic imbalance, hypertension and stress.

Oxidative stress and a murine superoxide dismutase-1 mutation promoting amyotrophic lateral sclerosis alter neurosecretion in the hypothalamo-neurohypophyseal axis

In this study, we examined the effects of oxidative stress on a nitric oxide (NO)-regulated neuroendocrine function, the release of arginine vasopressin (AVP) by the hypothalamo-neurohypophyseal axis. Treatment of mouse-isolated hypothalami and neurointermediate lobes (NIL) with H2O2 increased AVP release. This effect was inhibited by copper-zinc superoxide dismutase-1 (SOD1) analogs. By measuring cGMP accumulation as an indicator of biologically active NO, we found that H2O2 treatment decreased cGMP formation in both hypothalami and NIL. We have previously shown that NO inhibits AVP release by a cGMP-independent mechanism. Given that H2O2 stimulated AVP release, while it reduced cGMP production, our findings strongly suggest that oxidative damage affects neurosecretion by reducing NO availability. To test whether such a mechanism may operate under pathological conditions with pronounced oxidative stress, we compared neurosecretion in wild-type and transgenic mice carrying a mutated form of SOD1 associated with human familial amyotrophic lateral sclerosis. Reminiscent of the data obtained from H2O2-treated tissues, hypothalami and NIL from SOD1 mutants displayed decreased cGMP accumulation and increased AVP release, compared with tissues from wild-type littermates. Since neuronal NO synthase expression was not modified, we conclude that the perturbed free radical metabolism associated with the SOD1 mutation is likely to trap NO, and thereby alter neurosecretion, a mechanism that can be exacerbated in specific physiopathological conditions.

Immune stress activates putative nitric oxide-producing neurons in rat brain: cumulative effects with restraint

Immune and restraint stresses induce changes in the hypothalamo-pituitary-adrenal axis activity and autonomic function. In the hypothalamus, the paraventricular nucleus (PVN) plays an integral role, and nitric oxide (NO) is hypothesized to participate in this process. We used 1) intravenous injections of lipopolysaccharide (LPS, 125 microg/kg) to identify activated (Fos-positive) putative NO-producing neurons, 2) retrograde tracing to determine if autonomic medullary regions signal the PVN to mediate this activation, and 3) intravenous LPS injections plus restraint stress to determine if responses to restraint are altered by the presence of immune stress. At 2 hours after LPS injections, approximately 15% of putative NO-producing neurons were activated in the nucleus of the tractus solitarius (NTS) and ventrolateral medulla (VLM); about half of the putative NO neurons in the PVN were activated. In LPS + restraint rats, the percentage of activated putative NO neurons in the PVN was not significantly different from LPS-treated rats, but the numbers of putative NO neurons and activated NO neurons per section increased significantly. Retrogradely labeled neurons were found mostly in the middle NTS and VLM, and about 75% were activated. No neurons in the NTS or VLM were triple labeled. The results show that putative NO-producing neurons in the PVN, NTS, and VLM are activated by circulating LPS. However, the LPS-induced signaling to the PVN likely occurs through pathways other than the NO network of neurons in NTS or VLM. Finally, superimposition of restraint stress onto animals already exposed to immune stress stimulates the NO system in the PVN to a greater extent than either stress alone.

Hypovolemia upregulates the expression of neuronal nitric oxide synthase gene in the paraventricular and supraoptic nuclei of rats

We have examined the effects of isotonic hypovolemia on the expression of the neuronal nitric oxide synthase (nNOS) gene in the paraventricular (PVN) and supraoptic nuclei (SON) of the rat, using in situ hybridization histochemistry with a 35S-labelled oligodeoxynucleotide probe complementary to nNOS mRNA. Intraperitoneal (i.p.) administration of polyethylene glycol (PEG) (MW 4000, 20 ml/kg body weight) dissolved in 0.9% saline (20% w/v) induced isotonic hypovolemia. The expression of the nNOS gene in the PVN and SON 6 h after i.p. administration of PEG was increased significantly in comparison with controls. The dual staining for NADPH diaphorase activity and Fos-like immunoreactivity (Fos-LI) showed that at 3 and 6 h after i.p. administration of PEG, a subpopulation of NADPH diaphorase-positive cells in the PVN and SON exhibited nuclear Fos-LI. These results suggest that NO in the PVN and SON may be involved in the neuroendocrine and autonomic responses to non-osmotic hypovolemia.

Modulatory effect of L-NAME, a specific nitric oxide synthase (NOS) inhibitor, on stress-induced changes in plasma adrenocorticotropic hormone (ACTH) and corticosterone levels in rats: physiological significance of stress-induced NOS activation in hypothalamic-pituitary-adrenal axis

We investigated whether NG-nitro-L-arginine methyl ester (L-NAME), a specific inhibitor of nitric oxide synthase (NOS), can modify the stress-induced adrenocorticotropic hormone (ACTH) and corticosterone responses, because we found that immobilization-induced stress increases NOS mRNA and protein levels and enzyme activity in the adrenal cortex. The physiological significance of these phenomena, however, remains unknown. Plasma ACTH and corticosterone levels were determined by radioimmunoassay (RIA) of systemic blood samples and NOS enzyme activity was measured as the rate of [3H]arginine conversion to [3H]citrulline in the presence of tissue homogenate of adrenal cortex separated from the adrenal gland. The NOS enzyme activity in the adrenal cortex of rats pre-injected with saline at 2 h after the 2-h immobilization was significantly higher (P < 0.01) than that in the non-stressed controls. Pre-injection of L-NAME (100 mg/kg, s.c.) almost completely abolished the activity. This dose of L-NAME maintained a significantly elevated plasma corticosterone level (P < 0.05, compared with basal level) even 2 h after the 2-h stress, whereas the plasma corticosterone level in rats pre-injected with saline returned to the basal level at the same time point. Plasma ACTH level in L-NAME-pre-treated rats was higher than that in those pre-treated with saline 2 h after the stress, but the difference was not significant. This dose of L-NAME did not influence plasma ACTH or corticosterone levels under resting conditions without stress. These findings suggest that the stress-induced increase in NO synthesis in the adrenal cortex can modify the stress-induced corticosterone response to facilitate the recovery from the elevated corticosterone secretion by stress in the adrenal cortex to the resting basal level.

The involvement of nitric oxide in the secretion of beta-endorphin from the pituitary intermediate lobe of the rat

Nitric oxide (NO) generated by the enzyme nitric oxide synthase (NOS) has been implicated in the regulation of a variety of endocrine functions. A number of biochemical and anatomical studies have demonstrated the presence of neuronal NOS (nNOS) in the neuroendocrine axis and have shown significant effects of NO on the release of hypothalamic and pituitary hormones. Using a C-terminal directed peptide antibody that is specific for nNOS we have found a predominance of nNOS in the neural lobe of the pituitary and in a single layer of epithelial cells, possibly a remnant of Rathke's pouch that form a border between the intermediate lobe and the anterior lobe. Furthermore, we have examined the effect of sodium nitroprusside (SNP), a donor of NO on the secretion of beta-endorphin (beta-EP) from the isolated neuro-intermediate lobe (NIL) of the rat and cultured rat melanotrophs. It was shown that in explant cultures of intact neuro-intermediate lobes, SNP (100 microM) was able to cause an inhibition of beta-EP secretion. In the presence of sulpiride (10 microM), a dopamine D2-receptor antagonist, there was a partial reversal of the SNP effect. On the other hand SNP did not affect beta-EP secretion in primary cultures of melanotrophs that no longer possessed any innervation. Taken together these data suggest that NO has an indirect inhibitory effect on the secretion of beta-EP by the intermediate lobe via the release of dopamine.

Subcellular localization of neuronal nitric oxide synthase in the brain of a teleost; an immunoelectron and confocal microscopical study

The subcellular localization of neuronal nitric oxide (NO) synthase (NOS)-immunoreactive (NOSir) elements in the brain of the Atlantic salmon was investigated by means of electron microscopy and confocal laser scanning microscopy. NOSir structures are present only in neuronal elements. In neuronal processes, strong NOS immunoreactivity was mainly localized within synaptic vesicles or seen as a dense accumulation associated with the plasma membrane of dendrites and at terminal formations. NOSir precipitate was also associated with microtubuli and mitochondrial outer membranes. The highest accumulation of NOS immunoreactivity was found in dendrites located in close apposition to immunonegative myelinated or unmyelinated neural processes. Several NOSir and unmyelinated immunonegative profiles formed synaptic specializations. Immunonegative neurons in contact with NOSir processes always contained round clear synaptic vesicles. In neuronal somata, strong NOS immunoreactivity was localized in the cristae of some large mitochondria, whereas vacuoles and the endoplasmic reticulum showed a relatively weak staining. Confocal microscopic analysis of NOS immunofluorescence showed a corresponding subcellular localization of NOS in different brain regions, but also indicated the presence of NOS axosomatic terminals. Our data show that specific neurons contain a neuronal NOS-like molecule which to a high degree is stored in vesicles and is accumulated at various sites along the neuronal processes or at specific synaptic terminal formations. Thus, NO may be formed and exert its action at various sites along the processes of NOS-synthesizing neurons. The present study provides evidence at the ultrastructural level that NO may play a messenger role in neural circuits involved in visual and hypophysiotrophic brain functions.

Nitric oxide inhibits neuronal activity in the supraoptic nucleus of the rat hypothalamic slices

The presence of abundant nitric oxide synthase (NOS) in magnocellular neurons of the rat hypothalamus suggests that nitric oxide (NO) may be involved in controlling the release of oxytocin and vasopressin. To test this possibility, we examined the effect of NO-related drugs on extracellular discharges of 124 supraoptic nucleus (SON) neurons from slices of rat hypothalamus in vitro. Twenty-three (43%) of 53 neurons were inhibited by sodium nitroprusside (SNP), a spontaneous releaser of NO, at 1-3 mM. This inhibition was prevented by preincubation of the slices with 1 microM hemoglobin, an inactivator of NO (n = 14), whereas hemoglobin alone enhanced neuronal activity in seven (35%) of 20 neurons. L-Arginine (1 mM), a precursor of NO, inhibited neuronal activity in five (36%) of 14 neurons, while D-arginine (1 mM), the inactive counterpart of L-arginine, was ineffective (n = 12). N-omega-nitro-L-arginine methyl ester (L-NAME, 10 microM), an inhibitor of NOS, also enhanced neuronal activity in five (29%) of 17 neurons, while N-omega-nitro-D-arginine methyl ester (DNAME, 10 microM), the inactive enantiomer of L-NAME, was without effect (n = 11). Together, our data show that NO exerts predominantly an inhibitory effect on SON neurons and may serve as a negative feedback loop in controlling release of oxytocin and vasopressin.

Nitric oxide stimulates both the basal and reflex release of vasopressin in anesthetized rats

In view of the fact of the high concentration of neuronal nitric oxide synthase (NOS) in the supraoptic and paraventricular nuclei as well as the posterior pituitary, it has been supposed that nitric oxide (NO) may be involved in the regulation of hormone secretion from these sites. In the present study, L-arginine, the precursor of NO, and N(omega)-nitro-L-arginine methyl ester (L-NAME), the NOS inhibitor, were intracerebroventricularly (i.c.v.) administered into the pentobarbital anesthetized rats. The results showed that plasma arginine vasopressin (AVP) concentration increased after the injection of L-arginine (1.0 mg in microliters, i.c.v.) and decreased after the L-NAME (270 micrograms in 5 microliters, i.c.v.). The elevated plasma AVP level in response to short-term hypotension induced by intravenous infusion of sodium nitroprusside was also significantly reduced by i.c.v. administration of L-NAME. The results indicate that NO acts as a stimulating factor to both the basal and reflex release of AVP, opposing the view that NO plays an inhibitory role in the regulation of AVP secretion.

Immunohistochemical mapping of nitric oxide synthase in the rat hypothalamus and colocalization with neuropeptides

The localization and distribution of nitric oxide synthase in the hypothalamus have been studied with an immunohistochemical technique using antibodies to neuronal rat nitric oxide synthase. Subsequent double-labeling experiments examined the colocalization patterns of nitric oxide synthase and several peptides. Our results demonstrate a widespread occurrence of nitric oxide synthase-immunoreactive nerve cell bodies and processes throughout the hypothalamus, especially in various parts of the preoptic region, in the supraoptic and paraventricular nuclei, the lateral hypothalamic area, the ventromedial and dorsomedial nuclei, the arcuate nucleus and various parts of the mammillary region. Double labeling experiments showed that nitric oxide synthase-like immunoreactivity coexists with substance P-like immunoreactivity in the medial preoptic area, with oxytocin-, cholecystokinin-and galanin message-associated peptide-like immunoreactivity in the supraoptic nucleus, with enkephalin, oxytocin- and corticotropin releasing factor-like immunoreactivity in the paraventricular nucleus and with enkephalin-like immunoreactivity in the arcuate nucleus. Furthermore, in the ventromedial nucleus, nitric oxide synthase-like immunoreactivity coexisted with enkephalin-, substance P-, and somatostatin-like immunoreactivity, and in the dorsomedial nucleus with enkephalin-, galanin message-associated peptide-and substance P-like immunoreactivity. In the mammillary region nitric oxide synthase-like immunoreactivity coexisted with enkephalin-, cholecystokinin-, and substance P-like immunoreactivity. Among these neuropeptides, enkephalin and substance P were most frequently found in nitric oxide synthase-immunoreactive neurons. We conclude that nitric oxide synthase-immunoreactive neurons contain neuropeptides in various parts of the hypothalamus, and that nitric oxide in the hypothalamus may be involved in a variety of neuroendocrine and autonomic functions.

Evidence for a role of nitric oxide in the corticotropin-releasing factor release induced by interleukin-1 beta

Interleukin-1 beta stimulates corticotropin-releasing factor (CRF) secretion from the hypothalamus involving the activation of prostaglandins. This study investigated the possibility that nitric oxide (NO) acts as a mediator of interleukin-1-induced CRF release. An in vitro rat hypothalami continuous perifusion system was used. Pre- and co-incubation of hypothalami with either the NO synthase inhibitor, NG-nitro-L-arginine (1 mM), or the NO scavenger, hemoglobin (10 microM), induced a marked reduction in the effect of interleukin-1 (3 pM) on CRF secretion. The effect of NG-nitro-L-arginine was prevented by pre-exposure of hypothalami to L-arginine (1 mM). We also studied whether the involvement of NO in this interleukin-1 effect could involve a prostaglandin action. The concurrent treatment with NG-nitro-L-arginine and indomethacin (14 microM)--an inhibitor of prostaglandin production--reduced interleukin-1-induced CRF release to the same level as NG-nitro-L-arginine alone, suggesting that prostaglandins might interact with NO on this interleukin-1 effect. These results suggest that NO plays a role in the in vitro stimulatory action of interleukin-1 on hypothalamic CRF secretion.