Release patterns of excitatory and inhibitory amino acids within the hypothalamic supraoptic nucleus in response to direct nitric oxide administration during forced swimming in rats

Different reactive species modulate the hypotensive effect triggered by angiotensins at CVLM of 2K1C hypertensive rats

Hypertension is associated with increased central activity of the renin-angiotensin system (RAS) and oxidative stress. Here, we evaluated whether reactive species and neurotransmitters could contribute to the hypotensive effect induced by angiotensin (Ang) II and Ang-(1-7) at the caudal ventrolateral medulla (CVLM) in renovascular hypertensive rats (2K1C). Therefore, we investigated the effect of Ang II, Ang-(1-7), and the Ang-(1-7) antagonist A-779 microinjected before and after CVLM microinjection of the nitric oxide (NO)-synthase inhibitor, (L-NAME), vitamin C (Vit C), bicuculline, or kynurenic acid in 2K1C and SHAM rats. Baseline values of the mean arterial pressure (MAP) in 2K1C rats were higher than in SHAM rats. CVLM microinjection of Ang II, Ang-(1-7), l-NAME, or bicuculline induced decreases in the MAP in SHAM and 2K1C rats. In addition, Vit C and A-779 produced decreases in the MAP only in 2K1C rats. Kynurenic acid increased the MAP in both SHAM and 2K1C rats. Only the Ang-(1-7) effect was increased by l-NAME and reduced by bicuculline in SHAM rats. L-NAME also reduced the A-779 effect in 2K1C rats. Only the Ang II effect was abolished by CVLM Vit C and enhanced by CVLM kynurenic acid in SHAM and 2K1C rats. Overall, the superoxide anion and glutamate participated in the hypotensive effect of Ang II, while NO and GABA participated in the hypotensive effect of Ang-(1-7) in CVLM. The higher hypotensive response of A-779 in the CVLM of 2K1C rats suggests that Ang-(1-7) contributes to renovascular hypertension.

Cell-specific retrograde signals mediate antiparallel effects of angiotensin II on osmoreceptor afferents to vasopressin and oxytocin neurons

Homeostatic control of extracellular fluid osmolality in rats requires a parallel excitation of vasopressin (VP) and oxytocin (OT) neurosecretory neurons by osmoreceptor afferents to regulate the amount of water and sodium in the urine under normal conditions. However, during decreased blood volume (hypovolemia), natriuresis is suppressed, whereas osmotically driven antidiuresis is enhanced to promote retention of isotonic fluid. Because Angiotensin II (Ang II) is released centrally to indicate hypovolemia, we hypothesized that Ang II can evoke a state-dependent switch in circuit function. Here, we show that Ang II, a neuropeptide released centrally during hypovolemia, suppresses osmoreceptor-mediated synaptic excitation of OT neurons while potentiating excitation of VP neurons. Ang II does this by inducing cell-autonomous release of nitric oxide by VP neurons and endocannabinoids by OT neurons to respectively enhance and reduce glutamate release by osmoreceptor afferents. These findings indicate that peptide modulators such as Ang II can regulate synaptic communication to achieve a state-dependent and target-specific modulation of circuit activity.

The nitric oxide donor SNAP-induced amino acid neurotransmitter release in cortical neurons. Effects of blockers of voltage-dependent sodium and calcium channels

BACKGROUND

The discovery that nitric oxide (NO) functions as a signalling molecule in the nervous system has radically changed the concept of neuronal communication. NO induces the release of amino acid neurotransmitters but the underlying mechanisms remain to be elucidated.

FINDINGS

The aim of this work was to study the effect of NO on amino acid neurotransmitter release (Asp, Glu, Gly and GABA) in cortical neurons as well as the mechanism underlying the release of these neurotransmitters. Cortical neurons were stimulated with SNAP, a NO donor, and the release of different amino acid neurotransmitters was measured by HPLC. The involvement of voltage dependent Na+ and Ca2+ channels as well as cGMP in its mechanism of action was evaluated.

CONCLUSIONS

Our results indicate that NO induces release of aspartate, glutamate, glycine and GABA in cortical neurons and that this release is inhibited by ODQ, an inhibitor of soluble guanylate cyclase. Thus, the NO effect on amino acid neurotransmission could be mediated by cGMP formation in cortical neurons. Our data also demonstrate that the Na+ and Ca2+ voltage- dependent calcium channels are involved in the NO effects on cortical neurons.

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

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.

Is major depressive disorder a metabolic encephalopathy?

Metabolic encephalopathy is an acute disturbance in cellular metabolism in the brain evoked by conditions of hypoxia, hypoglycaemia, oxidative stress and/or inflammation. It usually develops acutely or subacutely and is reversible if the systemic disorder is treated. If left untreated, however, metabolic encephalopathy may result in secondary structural damage to the brain. Most encephalopathies are present with neuropsychiatric symptoms, one in particular being depression. However, mood disorders are often co-morbid with cardiovascular, liver, kidney and endocrine disorders, while increasing evidence concurs that depression involves inflammatory and neurodegenerative processes. This would suggest that metabolic disturbances resembling encephalopathy may underscore the basic neuropathology of depression at a far deeper level than currently realized. Viewing depression as a form of encephalopathy, and exploiting knowledge gleaned from our understanding of the neurochemistry and treatment of metabolic encephalopathy, may assist in our understanding of the neurobiology of depression, but also in realizing new ideas in the pharmacotherapy of mood disorders.

Exogenous agmatine has neuroprotective effects against restraint-induced structural changes in the rat brain

Agmatine is an endogenous amine derived from decarboxylation of arginine catalysed by arginine decarboxylase. Agmatine is considered a novel neuromodulator and possesses neuroprotective properties in the central nervous system. The present study examined whether agmatine has neuroprotective effects against repeated restraint stress-induced morphological changes in rat medial prefrontal cortex and hippocampus. Sprague-Dawley rats were subjected to 6 h of restraint stress daily for 21 days. Immunohistochemical staining with beta-tubulin III showed that repeated restraint stress caused marked morphological alterations in the medial prefrontal cortex and hippocampus. Stress-induced alterations were prevented by simultaneous treatment with agmatine (50 mg/kg/day, i.p.). Interestingly, endogenous agmatine levels, as measured by high-performance liquid chromatography, in the prefrontal cortex and hippocampus as well as in the striatum and hypothalamus of repeated restraint rats were significantly reduced as compared with the controls. Reduced endogenous agmatine levels in repeated restraint animals were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. Moreover, administration of exogenous agmatine to restrained rats abolished increases of arginine decarboxylase protein levels. Taken together, these results demonstrate that exogenously administered agmatine has neuroprotective effects against repeated restraint-induced structural changes in the medial prefrontal cortex and hippocampus. These findings indicate that stress-induced reductions in endogenous agmatine levels in the rat brain may play a permissive role in neuronal pathology induced by repeated restraint stress.

Nitric oxide as inflammatory mediator in post-traumatic stress disorder (PTSD): evidence from an animal model

Post-traumatic stress disorder (PTSD) is a severe anxiety disorder that may develop after experiencing or witnessing a traumatic event. Recent clinical evidence has suggested the involvement of neurodegenerative pathology in the illness, particularly with brain imaging studies revealing a marked reduction in hippocampal volume. Of greater significance is that these anatomical changes appear to be positively correlated with the degree of cognitive deficit noted in these patients. Stress-induced increases in plasma cortisol have been implicated in this apparent atrophy. Although not definitive, clinical studies have observed a marked suppression of plasma cortisol in PTSD. The basis for hippocampal neurodegeneration and cognitive decline therefore remains unclear. Stress and glucocorticoids increase glutamate release, which is recognized as an important mediator of glucocorticoid-induced neurotoxicity. Recent preclinical studies have also noted that glutamate and nitric oxide (NO) play a causal role in anxiety-related behaviors. Because of the prominent role of NO in neuronal toxicity, cellular memory processes, and as a neuromodulator, nitrergic pathways may have an important role in stress-related hippocampal degenerative pathology and cognitive deficits seen in patients with PTSD. This paper reviews the preclinical evidence for involvement of the NO-pathway in PTSD, and emphasizes studies that have addressed these issues using time-dependent sensitization - a putative animal model of PTSD.

Stress-restress evokes sustained iNOS activity and altered GABA levels and NMDA receptors in rat hippocampus

RATIONALE

Stress-related glucocorticoid and glutamate release have been implicated in hippocampal atrophy evident in patients with post-traumatic stress disorder (PTSD). Glutamatergic mechanisms activate nitric oxide synthase (NOS), while gamma-amino-butyric acid (GABA) may inhibit both glutamatergic and nitrergic transmission. Animal studies support a role for NOS in stress.

OBJECTIVES

We have studied the role of NOS and glucocorticoids, as well as inhibitory and excitatory transmitters, in a putative animal model of PTSD that emphasizes repeated trauma.

METHODS

Hippocampal NOS activity, N-methyl-D-aspartate (NMDA) receptor binding characteristics and GABA levels were studied in Sprague-Dawley rats 21 days after exposure to a stress-restress paradigm, using radiometric analysis, radioligand studies and high-performance liquid chromatography (HPLC) analysis with electrochemical detection, respectively. The NOS isoform involved, and the role of stress-mediated corticosterone release in NOS activation, was verified with the administration of selective iNOS and nNOS inhibitors, aminoguanidine (50 mg/kg/day i.p.) and 7-nitroindazole (12.5 mg/kg/day i.p.), and the steroid synthesis inhibitor, ketoconazole (24 mg/kg/day i.p.), administered for 21 days prior to and during the stress procedure.

RESULTS

Stress evoked a sustained increase in NOS activity, but reduced NMDA receptor density and total GABA levels. Aminoguanidine or ketoconazole, but not 7-nitroindazole or saline, blocked stress-induced NOS activation.

CONCLUSIONS

Stress-restress-mediated glucocorticoid release activates iNOS, followed by a reactive downregulation of hippocampal NMDA receptors and dysregulation of inhibitory GABA pathways. The role of NO in neuronal toxicity, and its regulation by glutamate and GABA has important implications in stress-related hippocampal degeneration.

GABA selectively controls the secretory activity of oxytocin neurons in the rat supraoptic nucleus

Recently we reported that a single social defeat experience triggers the release of oxytocin (OXT) from somata and dendrites, but not axon terminals, of neurons of the hypothalamic-neurohypophysial system. To further investigate the regulatory mechanisms underlying this dissociated release, we exposed male Wistar rats to a 30-min social defeat and monitored release of the inhibitory amino acids gamma amino butyric acid (GABA) and taurine within the hypothalamic supraoptic nucleus (SON) using microdialysis. Social defeat caused a significant increase in the release of both GABA and taurine within the SON (up to 480%; P < 0.01 vs. prestress release). To reveal the physiological significance of centrally released GABA, the specific GABAA-receptor antagonist bicuculline (0.02 mm) was administered into the SON via retrodialysis. This approach caused a significant increase in the release of OXT both within the SON and into the blood under basal conditions and during stress (up to 300 and 200%, respectively; P < 0.05 vs. basal values), without affecting plasma vasopressin. Electrophysiological studies confirmed the selective action of bicuculline on the firing activity of OXT neurons in the SON. Taken together, our data demonstrate that GABA is released within the SON during emotional stress to act as a selective inhibitor of both central and peripheral OXT secretion.

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.

Neurobiology of antidepressant withdrawal: implications for the longitudinal outcome of depression

Inappropriate discontinuation of drug treatment and noncompliance are a leading cause of long-term morbidity during treatment of depression. Increasing evidence supports an association between depressive illness and disturbances in brain glutamate activity, nitric oxide synthesis, and gamma-amino butyric acid. Animal models also confirm that suppression of glutamate N-methyl-D-aspartate receptor activity or inhibition of the nitric oxide-cyclic guanosine monophosphate pathway, as well as increasing brain levels of gamma-amino butyric acid, may be key elements in antidepressant action. Imaging studies demonstrate, for the most part, decreased hippocampal volume in patients with depression, which may worsen with recurrent depressive episodes. Preclinical models link this potentially neurodegenerative pathology to continued stress-evoked synaptic remodeling, driven primarily by the release of glucocorticoids, glutamate, and nitric oxide. These stress-induced structural changes can be reversed by antidepressant treatment. In patients with depression, antidepressant withdrawal after chronic administration is associated with a stress response as well as functional and neurochemical changes. Preclinical data also show that antidepressant withdrawal evokes a behavioral stress response that is associated with increased hippocampal N-methyl-D-aspartate receptor density, with both responses dependent on N-methyl-D-aspartate receptor activation. Drawing from both clinical and preclinical studies, this article proposes a preliminary molecular perspective and hypothesis on the neuronal implications of adherence to and discontinuation of antidepressant medication.

Nitric oxide is not involved in the control of vasopressin release during acute forced swimming in rats

Neurons of the hypothalamo-neurohypophyseal system (HNS) are known to contain high amounts of neuronal nitric oxide (NO) synthase (nNOS). NO produced by those neurons is commonly supposed to be involved as modulator in the release of the two nonapeptides vasopressin (AVP) and oxytocin into the blood stream. Previous studies showed that forced swimming fails to increase the release of AVP into the blood stream while its secretion into the hypothalamus is triggered. We investigated here whether hypothalamically acting NO contributes to the control of the AVP release into blood under forced swimming conditions. Intracerebral microdialysis and in situ hybridization were employed to analyze the activity of the nitrergic system within the supraoptic nucleus (SON), the hypothalamic origin of the HNS. A 10-min forced swimming session failed to significantly alter the local NO release as indicated both by nitrite and, the main by-product of NO synthesis, citrulline levels in microdialysis samples collected from the SON. Microdialysis administration of NO directly into the SON increased the concentration of AVP in plasma samples collected during simultaneous forced swimming. In an additional experiment the effect of the defined stressor exposure on the concentration of mRNA coding for nNOS within the SON was investigated by in situ hybridization. Forced swimming increased the expression of nNOS mRNA at two and four hours after onset of the stressor compared to untreated controls. Taken together, our results imply that NO within the SON does not contribute to the regulation of the secretory activity of HNS neurons during acute forced swimming. Increased nNOS mRNA in the SON after forced swimming and the increase in AVP release in the presence of exogenous NO under forced swimming points to a possible role of NO in the regulation of the HNS under repeated stressor exposure.