Influence of carbon monoxide, and its interaction with nitric oxide, on the adrenocorticotropin hormone response of the normal rat to a physico-emotional stress

The role of brain gaseous neurotransmitters in anxiety

Although anxiety is perhaps one of the most significant current medical and social problems, the neurochemical mechanistic background of this common condition remains to be fully understood. Multifunctional regulatory gasotransmitters are novel, atypical inorganic factors of the brain that are involved in the mechanisms of anxiety responses. Nitric oxide (NO) signaling shows ambiguous action in animal models of anxiety, while NO donors exert anxiogenic or anxiolytic effect depending on their chemical structure, dose, treatment schedule and gas release rapidity. The majority of NO synthase inhibitors act as a relatively potent axiolytic agents, while hydrogen sulfide (H₂S) and carbon monoxide (CO) delivered experimentally in the form of "slow" or "fast" releasing donors have recently been considered as anxiolytic neurotransmitters. In this comprehensive review we critically summarize the literature regarding the intriguing roles of NO, H₂S and CO in the neuromolecular mechanisms of anxiety in the context of their putative, yet promising therapeutic application. A possible mechanism of gasotransmitter action at the level of anxiety-related synaptic transmission is also presented. Brain gasesous neuromediators urgently require further wide ranging studies to clarify their potential value for the current neuropharmacology of anxiety disorders.

CB1 receptor activation in the rat paraventricular nucleus induces bi-directional cardiovascular effects via modification of glutamatergic and GABAergic neurotransmission

We have shown previously that the cannabinoid receptor agonist CP55940 microinjected into the paraventricular nucleus of the hypothalamus (PVN) of urethane-anaesthetized rats induces depressor and pressor cardiovascular effects in the absence and presence of the CB₁ antagonist AM251, respectively. The aim of our study was to examine whether the hypotension and/or hypertension induced by CP55940 given into the PVN results from its influence on glutamatergic and GABAergic neurotransmission. CP55940 was microinjected into the PVN of urethane-anaesthetized rats twice (S₁ and S₂, 20 min apart). Antagonists of the following receptors, NMDA (MK801), β₂-adrenergic (ICI118551), thromboxane A₂–TP (SQ29548), angiotensin II–AT₁ (losartan) or GABA_A (bicuculline), or the NO synthase inhibitor L-NAME were administered intravenously 5 min before S₂ alone or together with AM251. The CP55940-induced hypotension was reversed into a pressor response by AM251, bicuculline and L-NAME, but not by the other antagonists. The CP55940-induced pressor effect examined in the presence of AM251 was completely reversed by losartan, reduced by about 50–60 % by MK801, ICI118551 and SQ29548, prevented by bilateral adrenalectomy but not modified by bicuculline and L-NAME. Parallel, but smaller, changes in heart rate accompanied the changes in blood pressure. The bi-directional CB₁ receptor-mediated cardiovascular effects of cannabinoids microinjected into the PVN of anaesthetized rats depend on stimulatory glutamatergic and inhibitory GABAergic inputs to the sympathetic tone; the glutamatergic input is related to AT₁, TP and β₂-adrenergic receptors and catecholamine release from the adrenal medulla whereas the GABAergic input is reinforced by NO.

Role of hypothalamic corticotropin-releasing factor in mediating alcohol-induced activation of the rat hypothalamic-pituitary-adrenal axis

Alcohol stimulates the hypothalamic-pituitary-adrenal (HPA) axis through brain-based mechanisms in which endogenous corticotropin-releasing factor (CRF) plays a major role. This review first discusses the evidence for this role, as well as the possible importance of intermediates such as vasopressin, nitric oxide and catecholamines. We then illustrate the long-term influence exerted by alcohol on the HPA axis, such as the ability of a first exposure to this drug during adolescence, to permanently blunt neuroendocrine responses to subsequent exposure of the drug. In view of the role played by CRF in addiction, it is likely that a better understanding of the mechanisms through which this drug stimulates the HPA axis may lead to the development of new therapies used in the treatment of alcohol abuse, including clinically relevant CRF antagonists.

Role of heme oxygenase in modulating endothelial function in mesenteric small resistance arteries of spontaneously hypertensive rats

It has been proposed that endothelial dysfunction is due to the excessive degradation of nitric oxide (NO) by oxidative stress. The enzyme heme-oxygenase (HO) seems to exert a protective effect on oxidative stress in the vasculature, both in animal models and in humans. The objective of this study is to evaluate the effects of inhibition or activation of HO on endothelial function in mesenteric small resistance arteries of spontaneously hypertensive rats (SHR). Six SHR were treated with cobalt protoporphyrin IX 50 mg/Kg (CoPP), an activator of HO; six SHR with stannous mesoporphyrin 30 mg/Kg (SnMP), an inhibitor of HO, and six SHR with saline. As controls, six Wistar-Kyoto rats (WKY) were treated with CoPP, six WKY with SnMP, and six WKY with saline. Drugs were injected in the peritoneum once a week for 2 weeks. Systolic blood pressure (SBP) was measured (tail cuff method) before and after treatment. Mesenteric small resistance arteries were mounted on a micromyograph. Endothelial function was evaluated as a cumulative concentration-response curve to acetylcholine (ACH), before and after preincubation with N(G)-methyl-L-arginine (L-NMMA, inhibitor of NO synthase), and to bradykinin (BK). In SHR treatment with CoPP, improved ACH-and BK-induced vasodilatation (ANOVA p < 0.001) and this improvement was abolished by L-NMMA (ANOVA p < 0.001). SnMP was devoid of effects on endothelial function. In WKY, both activation and inhibition of HO did not substantially affect endothelium-mediated vasodilatation. The stimulation of HO seems to induce an improvement of endothelial dysfunction in SHR by possibly reducing oxidative stress and increasing NO availability.

Roles of nitric oxide, carbon monoxide, and hydrogen sulfide in the regulation of the hypothalamic-pituitary-adrenal axis

The importance of stress in modifying human behavior and lifestyle is no longer a matter of debate. Although mild stress enhances the immune response and prevents infections, prolonged stress seems to play pathogenic roles in depression and neurodegenerative disorders. The body has developed an adaptive stress response consisting of cardiovascular, metabolic, and psychological changes, which act in concert to eliminate stressors. One of the major components of this response is the hypothalamic-pituitary-adrenal axis, also known as the stress axis. Over the last 30 years, many studies have documented the integrated stress-axis regulation by neurotransmitters. They have also demonstrated that gaseous neuromodulators, such as NO, CO, and H(2)S, regulate the hypothalamic release of neuropeptides. The specific effects (stimulatory vs. inhibitory) of these gases on the stress axis varies, depending on the type of stress (neurogenic or immuno-inflammatory), its intensity (low or high), and the species studied (rodents or humans). This review examines the complex roles of NO, CO, and H(2)S in modulation of stress-axis activity, with particular emphasis on the regulatory effects they exert at the hypothalamic level.

Mediation of the behavioral, endocrine and thermoregulatory actions of ghrelin

The action of ghrelin on telemetrically recorded motor activity and the transmission of the effects of this neuropeptide on spontaneous and exploratory motor activity and some related endocrine and homeostatic parameters were investigated. Different doses (0.5-5 microg) of ghrelin administered intracerebroventricularly caused significant increases in both square crossing and rearing activity in the "open-field" apparatus, while only the dose of 5 microg evoked a significant increase in the spontaneous locomotor activity recorded by telemetry. Ghrelin also induced significant increases in corticosterone release and core temperature. To determine the transmission of these neuroendocrine actions, the rats were pretreated with different antagonists, such as a corticotropin-releasing hormone (CRH) antagonist (alpha-helical CRH(9-41)), the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME), haloperidol, cyproheptadine or the cyclooxygenase inhibitor noraminophenazone (NAP). The open-field and biotelemetric observations revealed that the motor responses were diminished by pretreatment with the CRH antagonist and haloperidol. In the case of HPA (hypothalamic pituitary adrenal) activation, only cyproheptadine pretreatment proved effective; haloperidol and L-NAME did not modify the corticosterone response. NAP had only a transient, while cyproheptadine elicited a more permanent impact on the hyperthermic response evoked by ghrelin; the other antagonists proved to be ineffective. The present data suggest that both CRH release and dopaminergic transmission may be involved in the ghrelin-evoked behavioral responses. On the other hand, ghrelin appears to have an impact on the HPA response via a serotonergic pathway and on the hyperthermic response via a cyclooxygenase and a serotonergic pathway.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Heme oxygenase and its products in the nervous system

Heme oxygenase (HO) cleaves the tetrapyrrolic ring of cellular heme moieties liberating carbon monoxide (CO) and equimolar amounts of free iron and biliverdin (BV). BV is in turn converted into bilirubin (BR) by the cytosolic enzyme BV reductase. Three HO isoforms have been described to date: HO-1, HO-2, and HO-3. All these isoforms are present in nervous tissue with different localizations and regulation. CO, the gaseous product of HO, exerts its biological effects through the activation of soluble guanylyl cyclase, but alternative signaling pathways, such as the activation of cyclooxygenase, have also been reported in the brain. In vitro and in vivo studies showed that CO, at the hypothalamic level, plays a key role in the modulation of stress response because it inhibits the release of antiinflammatory neuropeptides, such as corticotropin-releasing hormone and arginine vasopressin, and increases body temperature in rodents exposed to psychological stressors (stress fever). In the last few years, a new role of BR as an endogenously produced antioxidant has emerged, and several reports have shown that BR contributes to prevent cell damage mediated by reactive oxygen species, as well as nitric oxide and its congeners.

The role of carbon monoxide and nitric oxide in hyperosmolality-induced atrial natriuretic peptide release by hypothalamus in vitro

We evaluated the participation of the nitrergic and carbon monoxide (CO) systems in the atrial natriuretic peptide (ANP) release induced by osmotic stimulation of the rat anterior and medial basal hypothalamus (BH) fragments in vitro. The increase in the medium osmolality (NaCl, 340 mOsm/kg H2O) induced an elevated ANP release, which was associated with a decrease in nitric oxide synthase (NOS) activity (p<0.001), nitric oxide (NO) production and nitrate (p<0.001) release into the medium. The NO donors sodium nitroprusside (SNP, 300 microM), S-nitroso-N-acetylpenicillamine (SNAP, 300 microM) and 3-morpholinylsydnoneimine chloride (SIN-1, 300 microM) promoted a significant decrease in ANP release in response to hyperosmolality (p<0.001). ANP release observed in the present study did not result from injury to the BH caused by the increase in medium osmolality nor a toxic effect of the NO donors as demonstrated by the ANP release after incubation with KCl (56 mM). Furthermore, hyperosmolality or NO donors did not increase the LDH content in the medium. The hyperosmotic-induced ANP release and reduction of NOS activity were prevented by the heme oxygenase inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG). In conclusion, these results suggest that NO, the production of which is dependent on CO, modulates the osmolality-induced ANP release by BH fragments.

Nitric oxide is involved in lithium-induced immediate early gene expressions in the adrenal medulla

This study was conducted to determine if nitric oxide (NO) is involved in lithium-induced expression of c-Fos and inducible cAMP early repressor (ICER) in the adrenal gland. Rats received an intraperitoneal injection of isotonic lithium (76 mg/kg) with either an intracerebroventricle (i.c.v., 250 microg) or intraperitoneal (i.p., 30 mg/kg) N(omega)-nitro-L-arginine methyl ester (L-NAME) pretreatment. The adrenal expression of c-Fos and ICER was examined by in situ hybridization 1 h after the lithium injection. The cortical c-Fos/ICER expression induced by lithium was not modulated by L-NAME pretreatment. However, lithium-induced medullary expression of c-Fos was attenuated by central L-NAME, and ICER by systemic L-NAME. These results suggest that nitric oxide is, at least partly, involved in lithium-induced c-Fos/ICER expression in the adrenal medulla, and that central nitric oxide may play a different role from peripheral nitric oxide in lithium-induced activation of adrenal medulla.

Colocalization of urocortin and neuronal nitric oxide synthase in the hypothalamus and Edinger-Westphal nucleus of the rat

Different lines of studies suggest that both the corticotropin-releasing hormone-related peptide Urocortin I (Ucn) and the neuromodulator nitric oxide (NO) are involved in the regulation of the complex mechanisms controlling feeding and anxiety-related behaviors. The aim of the present study was to investigate the possible interaction between Ucn and NO in the hypothalamic paraventricular nucleus (PVN), an area known to be involved in the modulation of these particular behaviors. Therefore, we mapped local mRNA and peptide/protein presence of both Ucn and the NO producing neuronal NO synthase (nNOS). This investigation was extended to include the hypothalamic supraoptic nucleus (SON) and the Edinger-Westphal nucleus area (EW), the latter being one of the major cellular Ucn-expressing sites. Furthermore, we compared the two predominantly used laboratory rat strains, Wistar and Sprague-Dawley. Ucn mRNA and immunoreactivity were detected in the SON and in the EW. A significant difference between Wistar and Sprague-Dawley rats was found in mRNA levels in the EW. nNOS was detected in all brain areas analyzed, showing a significantly lower immunoreactivity in the PVN and EW of Sprague-Dawley versus Wistar rats. Contrary to some previous reports, no Ucn mRNA and only a very low immunoreactivity were detectable in the PVN of either rat strain. Interestingly, double-labeling immunofluorescence revealed that in the SON approximately 75% of all cells immunoreactive for Ucn were colocalized with nNOS, whereas in the EW only approximately 2% of the Ucn neurons were found to contain nNOS. These findings suggest an interaction between Ucn and NO signaling within the SON, rather than the PVN, that may modulate the regulation of feeding, reproduction, and anxiety-related behaviors.

The involvement of dopamine and nitric oxide in the endocrine and behavioural action of endomorphin-1

Previous publications have demonstrated a prominent central and corticotropin releasing hormone-mediated action of the endomorphins (EMs) on both open-field behaviour and the hypothalamo-pituitary-adrenal (HPA) axis. In the present experiments, the direct action of endomorphin-1 (EM1) on pituitary adrenocorticotropic hormone (ACTH) release, adrenal corticosterone secretion and the roles of nitric oxide (NO) and dopamine (DA) in the HPA and behavioural responses elicited by EM1 were investigated in mice. In vitro perifusion studies indicated that the action of EM1 on the HPA system appears to be confined to the hypothalamus, as EM1 did not influence the corticosterone secretion from adrenal slices and moderately attenuated the ACTH release from anterior pituitary slices. In in vivo experiments, NG-nitro-L-arginine (L-NNArg) pretreatment brought about a profound inhibition of both the endocrine and the behavioural responses. On the other hand, haloperidol completely abolished the increases in square crossing and rearing, without affecting corticosterone release. The direct action of EM1 on striatal DA release was therefore also investigated in an in vitro superfusion system. Although EM1 did not influence the basal release of tritiated DA, it significantly enhanced the transmitter release evoked by electric impulses and pretreatment with L-NNArg resulted in a considerable inhibition of the release elicited by EM1. In conclusion, our endocrine studies suggest an important role of NO in the mediation of the EM1-evoked corticosterone secretion. They also indicate that EM1 activates the HPA axis at a hypothalamic level and dopamine is not involved in this process. In contrast, the behavioural experiments reflect that the locomotor activation induced by EM1 is mediated by NO and dopamine, and the superfusion studies demonstrate that NO transmits the dopamine release enhancing effect of EM1.

Role of the brain heme oxygenase-carbon monoxide pathway in stress fever in rats

This study was aimed at testing the hypothesis that the brain heme oxygenase (HO)-carbon monoxide (CO) pathway plays a role in stress fever. To this end, the effect of the HO inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG), on restraint-induced fever was tested. Intracerebroventricular ZnDPBG (200 nmol) did not affect the body core temperature of unrestrained rats, but markedly attenuated restraint-induced fever. However, at the same dose, intraperitoneal ZnDPBG did not affect the febrile response to restraint. Taken together, these results indicate that the brain HO-CO pathway plays a major role in the genesis of stress fever in rats.

Role of nitric oxide in regulating the rat hypothalamic-pituitary-adrenal axis response to endotoxemia

This work examines the role of nitric oxide (NO) in the periphery (i.e., on the pituitary) and the brain (particularly on corticotropin-releasing factor [CRF] and vasopressin [VP] neurons in the paraventricular nucleus [PVN] of the hypothalamus) as a modulator of the ACTH response to lipopolysaccharide. We previously showed that NO restricted the pituitary response to VP while it facilitated the synthesis of PVN CRF and VP. In our experience, only relatively high doses of lipopolysaccharide (>50 microg/kg, injected intravenously [i.v.]) cause detectable increases in PVN neuronal activation. Our hypothesis, therefore, was that pituitary NO-VP interactions would predominate in rats injected with a low dose of lipopolysaccharide (0.5 microg/kg, i.v.) while the stimulatory influence of the gas on PVN neuronal activity would play an important role following i.v. injection of a large dose of lipopolysaccharide (50 microg/kg, i.v.). We observed that the ability of 0.5 microg/kg lipopolysaccharide to release ACTH was significantly enhanced by the subcutaneous (s.c.), but not the intracerebroventricular (i.c.v.) injection of L-NAME, an arginine derivative that blocks NO synthesis. The effect of s.c. L-NAME was reversed by immunoneutralization of endogenous VP, which indicated that in this model, the ability of lipopolysaccharide to release ACTH depended, at least in part, on the influence exerted by NO on the pituitary response to VP. In rats injected with the high lipopolysaccharide dose, the s.c. injection of L-NAME decreased plasma ACTH levels compared to those in rats pretreated with the vehicle. The effect of s.c. L-NAME was not significantly altered by VP antibodies. These results indicate that in this model, the primary influence of NO was exerted in the PVN and/or its afferents and that it did not depend on a peripheral, VP-mediated effect of the gas. On the one hand, these data are at odds with our finding that the i.c.v. injection of L-NAME only marginally altered the ACTH response to the large dose of lipopolysaccharide. As i.c.v. injected L-NAME should have primarily decreased hypothalamic, but not pituitary NOS, its only modest influence on ACTH release may have been due to a balance between stimulating and inhibiting effects of NO within the brain. As high doses of lipopolysaccharide increase brain levels of prostaglandin, monoamine, and proinflammatory cytokines, it will be important to investigate the influence exerted by NO on these secretagogues and on their interactions with PVN CRF and VP neurons, which may help us resolve the issues raised by our results. Collectively, these data support our hypothesis that the mechanisms mediating the ACTH response to a low lipopolysaccharide concentration involve the inhibitory VP-mediated influence of NO on pituitary activity. By contrast, the stimulatory effect of high doses of lipopolysaccharide on ACTH release depends, at least in part, on the ability of NO to upregulate PVN neuronal activity.

Comparison between the influence of the intravenous and intracerebroventricular injection of a nitric oxide donor on adrenocorticotropic hormone release and hypothalamic neuronal activity

We investigated the ability of the nitric oxide (NO) donor 3-morpholino-sydnonimine (SIN-1) to release adrenocorticotropic hormone (ACTH) and up-regulate hypothalamic neurones following its intravenous (i.v.) injection. i.v. SIN-1 (0.2-1.8 mg/kg) produced dose-related increases in plasma ACTH levels which were blocked by prior neutralization of endogenous corticotropin-releasing factor (CRF) but not by vasopressin antibodies. In contrast, the intracerebroventricular (i.c.v.) injection of 50-microg SIN-1 released significantly larger amounts of ACTH, a response blunted by either CRF or vasopressin antibodies. While i.c.v. SIN-1 markedly up-regulated transcripts of the immediate early gene NGFI-B in the paraventricular nucleus (PVN) of the hypothalamus, no such response was observed following the i.v. injection of up to 2.0 mg/kg SIN-1. Finally, we found no evidence that the influence of the peripheral administration of SIN-1 on ACTH secretion is mediated by altered pituitary responsiveness to CRF or vasopressin. The fact that NO has a profound hypotensive influence in the periphery suggests that it may have released ACTH through this mechanism, although the absence of PVN neuronal response in regions that are activated by decreased blood pressure casts some doubt on this hypothesis. As the systemic injection of arginine derivatives that block NOS activity potently augment the ACTH response to circulating pro-inflammatory cytokines or vasopressin, the present findings indicate that the mechanisms responsible for this phenomenon are distinct from those responsible for ACTH released by i.v. SIN-1.

Cross talk between carbon monoxide and nitric oxide

Carbon monoxide and nitric oxide are two endogenously produced gases that can act as second messenger molecules. Heme oxygenase and nitric oxide synthase are the enzyme systems responsible for generating carbon monoxide and nitric oxide, respectively. Both carbon monoxide and nitric oxide share similar properties, such as the ability to activate soluble guanylate cyclase to increase cyclic GMP. It is becoming increasingly clear that these two gases do not always work independently, but rather can modulate each other's activity. Although much is known about the heme oxygenase/carbon monoxide and nitric oxide synthase/nitric oxide pathways, how these two important systems interact is less well understood. This review attempts to define the current known relationship between carbon monoxide and nitric oxide as it relates to their production and physiological function.

Microinfusion of a nitric oxide donor in discrete brain regions activates the hypothalamic-pituitary-adrenal axis

We previously showed that the intracerebroventricular injection of the nitric oxide (NO) donor 3-morpholino-sydnonimine (SIN-1) released adrenocorticotropic hormone (ACTH) and upregulated transcripts for corticotropin-releasing factor (CRF) and vasopressin in the paraventricular nucleus (PVN) of the rat hypothalamus. In the present work, we microinfused SIN-1 into the PVN itself, the amygdala, the hippocampus or the frontal cortex to identify the brain regions that modulate the influence of NO on the hypothalamic-pituitary-adrenal (HPA) axis. Microinfusion into the PVN, which contains most of the CRF and vasopressin neurones that control HPA axis activity, significantly released ACTH. Microinfusion into the amygdala or the hippocampus, areas which also regulate HPA axis activity, similarly increased plasma ACTH levels. However, these responses were smaller and showed a delayed onset, compared to that observed following PVN treatment. In contrast, microinfusion of SIN-1 into the frontal cortex, which is not believed to exert a major direct influence on the HPA axis, was without effect. The observation that compared to microinfusion into the PVN, peak ACTH levels were both smaller and delayed when SIN-1 was microinfused into the amygdala or the hippocampus, and that SIN-1 only increased NO levels when injected into the PVN, suggests that the NO donor injected outside the PVN activates this nucleus by targeting pathways that connect it to these other regions rather than by leakage. Collectively, our results provide important clues regarding the putative role of these regions in modulating the influence of NO on the HPA axis.

Gaseous neuromodulators in the control of neuroendocrine stress axis

The gaseous neuromodulator carbon monoxide has been shown to reduce the stimulated release of stress neuropeptides, such as vasopressin and oxytocin, from the rat hypothalamus in vitro, while evidence concerning corticotropin-releasing hormone is controversial. In vivo studies have been conducted in the rat, inhibiting heme oxygenase activity--and hence carbon monoxide biosynthesis--in the central nervous system by means of specific heme oxygenase blockers; these studies showed that basal heme oxygenase activity tends to oppose exaggerated increases in vasopressin secretion following immune-inflammatory challenges, whereas it favors the normal rise in circulating ACTH which follows footshock. Another gas normally produced in mammalian brains under basal conditions, hydrogen sulfide, also appears to play a role in the control of the hypothalamo-pituitary-adrenal axis. Indeed, increases in hydrogen sulfide levels within the hypothalamus, either obtained with hydrogen sulfide-enriched media or by the addition of the hydrogen sulfide precursor S-adenosyl-methionine, are associated with the inhibition of the stimulated release of corticotropin-releasing hormone from rat hypothalamic explants. Parellel in vivo experiments in the rat under resting conditions and after stress-induced adrenocortical activation show that S-adenosyl-methionine significantly reduces the rise in serum corticosterone levels caused by 1-h exposure to cold. These results demonstrate the pathophysiological importance of both carbon monoxide and hydrogen sulfide in the regulation of neuroendocrine function.

Relative importance of nitric oxide and carbon monoxide in regulating the ACTH response to immune and non-immune signals

The present work investigated the effect of nitric oxide (NO) or carbon monoxide (CO) in the ACTH response to an immune signal [the intravenous injection of interleukin-1 β (IL-1β)] or to a neurogenic stressor (mild intermittent inescapable foot shocks). The arginine derivative N(ω)-L-arginine methylester (L-NAME), which non-specifically inhibits NO formation induced by all constitutive forms of NO synthase (NOS), significantly augmented the effect of IL-1P,but blockade of CO formation with metalloporphyrins was without effect. On the other hand, L-NAME blunted the effect of shocks on the early phase of ACTH release, while we had reported earlier that metalloporphyrins exerted a similar effect. This effect was mimicked by blockade of neuronal (n) NOS by N(ω)-Propyl-L-arginine (PA), although the resulting decrease in hormone levels was less than that induced by L-NAME. These results indicate that endogenous NO, but not CO, interferes with ACTH released by a peripheral immune signal. In contrast, NO formed by nNOS enhances the ability of shocks to induce ACTH secretion.

The roles of carbon monoxide and nitric oxide in the control of the neuroendocrine stress response: complementary or redundant

There is widespread evidence in favour of nitric oxide (NO) acting as a gaseous neurotransmitter in the central nervous system, diffusing from its cells of origin and affecting surrounding neuronal tissue in evanescent three-dimensional waves. This is also true of the hypothalamus, where amongst other activities NO inhibits stimulation of corticotrophin-releasing hormone (CRH) and vasopressin release by inflammatory stressors, effects thought to be mediated by binding with soluble guanylate cyclase (sGC). Carbon monoxide is being increasingly recognised as another gaseous neuromodulator, but with principal effects on other hemoproteins such as cyclo-oxygenase, and a distinctly different profile of localisation.NO is predominantly a pro-inflammatory agent in the periphery while CO is often anti-inflammatory. In the hypothalamus, the actions of CO are also distinct from those of NO,with marked antagonistic effects on the inflammatory release of vasopressin, both in vitro and in vivo, but with little involvement in the regulation of CRH. Thus, it would appear that these apparently similar gases exert quite distinct and separate effects, although they cause broadly similar overall changes in the secretion of neuroendocrine stress hormones. We conclude that these two gases may play significant but different roles in the control of the neuroendocrine stress response, but one common feature may be attenuation of inflammation-induced release of stress hormones.

Increased activity of the hypothalamic-pituitary-adrenal axis of rats exposed to alcohol in utero: role of altered pituitary and hypothalamic function

Prenatal exposure to ethanol (E) enhances the offspring's ACTH and corticosterone responses to stressors. Here, we determined the role of increased pituitary responsiveness and/or PVN neuronal activity in this phenomenon. Pregnant rats were exposed to E vapors during days 7-18 of gestation, and we compared the responses of their 55- to 60-day-old offspring (E rats) to those of control (C) dams. PVN mRNA levels of the immediate early genes (IEGs) c-fos and NGFI-B, which were low under basal conditions in all groups, showed a peak response 15 min after shocks and 45 min after LPS treatment. These responses were significantly enhanced in E, compared to C offspring of both genders. CRF, but not VP hnRNA levels were also significantly higher in the PVN of shocked E offspring. Resting median eminence content of CRF and VP, and pituitary responsiveness to CRF, were unchanged, while responsiveness to VP was marginally increased in females. These results indicate that prenatal alcohol selectively augments the neuronal activity of hypothalamic CRF perikarya.

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.

Differential profiles of nitric oxide and norepinephrine releases in the paraventricular nucleus region in response to mild footshock in rats

The purpose of this study was to determine whether the application of mild intermittent footshock stress can cause changes in the nitric oxide (NO) and norepinephrine (NE) releases in the hypothalamic paraventricular nucleus (PVN) region and medial prefrontal cortex (mPFC). Extracellular levels of NO metabolites and NE in the PVN region and mPFC were determined using an in vivo brain microdialysis technique in conscious rats. In the PVN region, we demonstrated that perfusion of N-methyl-D-aspartate through a microdialysis probe resulted in a dose-dependent increase in NO metabolite levels, whereas intraperitoneal administration of N(G)-nitro-L-arginine methyl ester produced a dose-dependent reduction in the levels of NO metabolites. The levels of NO metabolites in the PVN region increased after intraperitoneal administration of interleukin-1beta in a dose-dependent manner, as we previously reported. This increase in NO metabolite levels was abolished 60 min after systemic administration of N(G)-nitro-L-arginine methyl ester compared to the vehicle-treated control group. Twenty minutes of intermittent footshock induced NE release but did not induce NO release in the PVN region. On the contrary, in the mPFC, 20 min of intermittent footshock induced both NO and NE releases. The present results reveal different patterns and time courses in NO and NE releases between the PVN region and the mPFC in response to mild intermittent footshock stress. These findings are likely to have helpful suggestions for our understanding of the hypothalamic-pituitary-adrenal axis and the limbic forebrain system response to different kinds of stress.

Endotoxin stimulates nitric oxide production in the paraventricular nucleus of the hypothalamus through nitric oxide synthase I: correlation with hypothalamic-pituitary-adrenal axis activation

Nitric oxide (NO) is an unstable gas that is produced in brain tissues involved in the control of the activity of the hypothalamus-pituitary-adrenal (HPA) axis. Transcripts for constitutive neuronal NO synthase (NOS I), one of the enzymes responsible for NO formation in the brain, is up-regulated by systemic endotoxin [lipopolysaccharide (LPS)] injection. However, this change is delayed compared with LPS induced-ACTH release, which makes it difficult to determine whether it is functionally important for the hormonal response. To obtain a more resolutive time course of the NO response, we first measured NO in microdialysates of the paraventricular (PVN) nucleus of the hypothalamus. The iv injection of 100 microg/kg LPS induced a rapid and short-lived increase in concentrations of this gas, which corresponded to the initiation of the ACTH response. LPS-induced Ca2+-dependent NOS activity in the PVN as well as the number of PVN cells expressing citrulline (a compound produced stoichiometrically with NO) also increased significantly over a time course that corresponded to ACTH and corticosterone release. Finally, blockade of NO production with the arginine derivative Nomega-nitro-L-argininemethylester (L-NAME; 50 mg/kg, sc), which attenuated the ACTH response to LPS, virtually abolished basal NOS activity in the PVN, as well as anterior and neurointermediate lobes of the pituitary, and prevented the appearance of citrulline in the PVN of rats injected with LPS. Collectively, these results show that LPS-induced activation of the HPA axis correlates with the activation of the PVN NOergic system, and supports a stimulatory role for NO in the modulation of the HPA axis in response to immune challenges.

Gender, sex steroids, corticotropin-releasing factor, nitric oxide, and the HPA response to stress

We used two stresses--exposure to mild electrofoot shocks (a neurogenic stress) and acute alcohol injection (a systemic stress)--to investigate the influence of gender and circulating sex steroids on ACTH and corticosterone released by adult rats. Both types of stresses significantly increased plasma levels of these hormones. Following exposure to shocks, intact females secreted significantly more ACTH than intact males, a difference that was abolished by ovariectomy. Gender differences in corticosterone responses were sometimes, but not always, present. In contrast, in this series of experiments males released more ACTH when acutely injected with alcohol, while there was no obvious effect of sex on corticosterone secretion. Corticotropin-releasing factor (CRF) antagonists were more effective at reducing ACTH compared to corticosterone levels. Finally, pituitary response to CRF, but much less so to vasopressin (VP), was larger in intact females compared to intact males. Blockade of endogenous nitric oxide formation slightly enhanced the effect of CRF in males, but not in females, and while it produced the expected enhancement of VP-induced ACTH release, this effect was more pronounced in females. Collectively, these results provide evidence for an influence of circulating sex steroids on pituitary and adrenal activity under some, but not all circumstances.

Inhibition of heme oxygenase in the central nervous system potentiates endotoxin-induced vasopressin release in the rat

Previous in vitro studies have shown that increases in endogenous carbon monoxide (CO) generation via activation of the enzyme heme oxygenase (HO) within the rat hypothalamus are associated with the reduced release of the neuropeptides, vasopressin (AVP) and oxytocin, while evidence concerning corticotrophin-releasing hormone (CRH) is controversial. The present study investigated whether there is also a functional relationship between the HO-CO pathway and AVP and corticosterone (Cort) in vivo. Male Wistar rats were challenged with bacterial lipopolysaccharide (LPS) at doses producing significant activation of the hypothalamo-pituitary-adrenal (HPA) axis. LPS was given alone or after pretreatment with the HO inhibitor Sn-protoporphyrin-9 (SnPP9). The latter was injected either intraperitoneally (i.p.) or by intracerebroventricular (i.c.v.) route. SnPP9 given i.p. failed to modify either basal or LPS-stimulated levels of AVP and Cort. On the contrary, i.c.v. SnPP9 strongly potentiated LPS-induced AVP release and significantly enhanced basal serum Cort levels, although it failed to potentiate stimulation by LPS. The LPS + i.c.v. SnPP9 also significantly reduced the hypothalamic stores of AVP compared to controls, correlating with increased circulating levels of AVP. Taken collectively, these data are in concordance with previous in vitro observations showing that the HO-CO pathway acts centrally to attenuate endotoxin-stimulated AVP release, while having less effects on the pituitary-adrenal axis.

Nitric oxide stimulates ACTH secretion and the transcription of the genes encoding for NGFI-B, corticotropin-releasing factor, corticotropin-releasing factor receptor type 1, and vasopressin in the hypothalamus of the intact rat

We investigated the effect of the intracerebroventricular injection of the nitric oxide (NO) donor 3-morpholino-sydnonimine (SIN-1) on the release of adrenocorticotropin hormone (ACTH) and the neuronal response of hypothalamic neurons responsible for this release. Rats that were administered SIN-1 showed significant elevations in plasma ACTH levels, a response that was virtually abolished by antibodies against corticotropin-releasing factor (CRF) and significantly blunted by vasopressin (VP) antiserum. SIN-1 also upregulated heteronuclear (hn) transcripts for CRF and VP and messenger RNA (mRNA) levels for the immediate early gene NGFI-B and for CRF receptor type 1 (CRF-R(1)) in the parvocellular portion of the paraventricular nucleus (PVN) of the hypothalamus. Blockade of prostaglandin synthesis with ibuprofen did not alter the ACTH or the PVN response to SIN-1. The central nucleus of the amygdala and the supraoptic nucleus, regions that are involved in autonomic adjustments to altered cardiovascular activity, also responded to SIN-1 with elevated NGFI-B mRNA levels. However, the only change in mean arterial blood pressure caused by this NO donor was a transient and modest increase. To our knowledge, this is the first demonstration that in the intact rat NO stimulates the activity of PVN neurons that control the hypothalamic-pituitary-adrenal axis. It must be noted, however, that our results do not allow us to determine whether this effect was direct or mediated through PVN afferents. This study should help resolve the controversy generated by the use of isolated brain tissues to investigate the net effect of NO on hypothalamic peptide production.