Regulatory role of nitric oxide over extracellular taurine in the hippocampus of freely moving rats

Metabolomic Endotype of Asthma

Metabolomics, the quantification of small biochemicals in plasma and tissues, can provide insight into complex biochemical processes and enable the identification of biomarkers that may serve as therapeutic targets. We hypothesized that the plasma metabolome of asthma would reveal metabolic consequences of the specific immune and inflammatory responses unique to endotypes of asthma. The plasma metabolomic profiles of 20 asthmatic subjects and 10 healthy controls were examined using an untargeted global and focused metabolomic analysis. Individuals were classified based on clinical definitions of asthma severity or by levels of fraction of exhaled NO (FENO), a biomarker of airway inflammation. Of the 293 biochemicals identified in the plasma, 25 were significantly different among asthma and healthy controls (p < 0.05). Plasma levels of taurine, lathosterol, bile acids (taurocholate and glycodeoxycholate), nicotinamide, and adenosine-5-phosphate were significantly higher in asthmatics compared with healthy controls. Severe asthmatics had biochemical changes related to steroid and amino acid/protein metabolism. Asthmatics with high FENO, compared with those with low FENO, had higher levels of plasma branched-chain amino acids and bile acids. Asthmatics have a unique plasma metabolome that distinguishes them from healthy controls and points to activation of inflammatory and immune pathways. The severe asthmatic and high FENO asthmatic have unique endotypes that suggest changes in NO-associated taurine transport and bile acid metabolism.

Evidence for involvement of nitric oxide and GABA(B) receptors in MK-801- stimulated release of glutamate in rat prefrontal cortex

Systemic administration of NMDA receptor antagonists elevates extracellular glutamate within prefrontal cortex. The cognitive and behavioral effects of NMDA receptor blockade have direct relevance to symptoms of schizophrenia, and recent studies demonstrate an important role for nitric oxide and GABA(B) receptors in mediating the effects of NMDA receptor blockade on these behaviors. We sought to extend those observations by directly measuring the effects of nitric oxide and GABA(B) receptor mechanisms on MK-801-induced glutamate release in the prefrontal cortex. Systemic MK-801 injection (0.3 mg/kg) to male Sprague-Dawley rats significantly increased extracellular glutamate levels in prefrontal cortex, as determined by microdialysis. This effect was blocked by pre-treatment with the nitric oxide synthase inhibitor L-NAME (60 mg/kg). Reverse dialysis of the nitric oxide donor SNAP (0.5-5 mM) directly into prefrontal cortex mimicked the effect of systemic MK-801, dose-dependently elevating cortical extracellular glutamate. The effect of MK-801 was also blocked by systemic treatment with the GABA(B) receptor agonist baclofen (5 mg/kg). In combination, these data suggest increased nitric oxide formation is necessary for NMDA antagonist-induced elevations of extracellular glutamate in the prefrontal cortex. Additionally, the data suggest GABA(B) receptor activation can modulate the NMDA antagonist-induced increase in cortical glutamate release.

Repeated exposure of adult rats to Aroclor 1254 induces neuronal injury and impairs the neurochemical manifestations of the NMDA receptor-mediated intracellular signaling in the hippocampus

Aroclor 1254 is a mixture of polychlorinated biphenyls (PCBs), a class of environmental toxins which cause a wide spectrum of neurotoxic effects. Learning and memory deficits are the profound effects of PCBs which may be related to hippocampal dysfunction. To get insight into the underlying neurochemical mechanisms, we employed the microdialysis technique to investigate the effect of repeated exposure of adult male Wistar rats to Aroclor 1254 (10mg/kg b.w., daily, ig., for 14days), on the neurochemical parameters of NMDA receptor-mediated glutamatergic signaling in the hippocampus in vivo assessed using the microdialysis technique. The results demonstrated that exposure to Aroclor 1254, which was associated with substantial neuronal damage and loss in the hippocampus, markedly decreased the NMDA-induced extracellular accumulation of newly loaded (45)CaCl(2), cGMP and glutamate, and reduced the basal content of the NO precursor, arginine, indicating inhibition of the NMDA/NO/cGMP pathway. Aroclor 1254 exposure also decreased the basal microdialysate content of glutamate and glutamine, which may cause inadequate supply of the neurotransmitter glutamate, while the level of two other neuroactive amino acids, aspartate or taurine was not affected by the exposure. The results underscore neuronal lesion and inhibition of NMDA receptor-mediated glutamatergic signaling in hippocampus as a potential major contributor to the cognitive deficits associated with exposure to PCB.

Nitric oxide is involved in taurine release in the mouse brain stem under normal and ischemic conditions

Nitric oxide (NO) has been shown to regulate neurotransmitter release in the brain; both inhibitory and excitatory effects have been seen. Taurine is essential for the development and survival of neural cells and protects them under cell-damaging conditions. In the brain stem, it regulates many vital functions such as cardiovascular control and arterial blood pressure. Now we studied the effects of the NO-generating compounds hydroxylamine (HA), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) on the release of preloaded [(3)H]taurine under normal and ischemic conditions in slices prepared from the mouse brain stem from developing (7-day-old) to young adult (3-month-old) mice. In general, the effects of NO on the release were somewhat complex and difficult to explain, as expected from the multifunctional role of NO in the central nervous system. The basal initial release under normal conditions was enhanced by the NO donors 5 mM HA and 1.0 mM SNAP at both ages, but SNP was inhibitory in developing mice. The release was markedly enhanced by K(+) stimulation. The effects of HA, SNAP and SNP on the basal release were not antagonized by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA, 1.0 mM), demonstrating that mechanisms other than NO synthesis are involved. Taurine release in developing mice in the presence of SNP was reduced by the inhibitor of soluble guanylate cyclase, 1H-(1,2,3)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), indicating the possible involvement of cGMP. In normoxia, N-methyl-D-aspartate (NMDA, 1.0 mM) enhanced the SNAP- and HA-evoked taurine release in developing mice and the HA-evoked release in adults. In ischemia, both K(+) stimulation and NMDA potentiated the NO-induced release, particularly in the immature mice, probably without the involvement of the NO synthase or cGMP. The substantial release of taurine in the developing brain stem evoked by NO donors together with NMDA might represent signs of important mechanisms against excitotoxicity which protect the brain stem under cell-damaging conditions.

Unexpected effects of nitric oxide synthase inhibitors on extracellular nitrite levels in the hippocampus in vivo

The aim of this study was to determine whether extracellular nitric oxide levels in the hippocampus of freely moving animals were reduced by the administration of nitric oxide synthase (NOS) inhibitors via a microdialysis probe. Our results show that extracellular nitrite levels were increased following the infusion of N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), in the case of the latter, the response was biphasic. In contrast, infusion of both inhibitors together resulted in a substantial reduction in nitrite when compared to control. More predictably, the infusion of NMDA elicited an increase in extracellular nitrite levels. This effect was biphasic, the second phase occurring some 3 h after the drug infusion period had ended. When NMDA was infused in the presence of L-NAME, no agonist-induced increase in nitrite production was recorded, in fact nitrite levels were found to decline to below control values. There was no immediate increase in nitrite levels when NMDA was infused in the presence of 7-NI, although this may have been partially obscured by the biphasic effect of the inhibitor. It did appear, however, that the second phase of the NMDA-induced response was attenuated by 7-NI. No NMDA-evoked increase in nitrite was evident when the agonist was infused in the presence of a combination of both inhibitors. We have no clear explanation for the data presented here but suggest that up-regulated activity of particular NOS isoforms might compensate for the inhibition of the other by a mechanism yet to be elucidated. In addition, we propose that caution be exercised when interpreting results from in vivo microdialysis studies where NOS inhibitors are administered directly into the brain via a probe.

Release of arginine, glutamate and glutamine in the hippocampus of freely moving rats: Involvement of nitric oxide

Using in vivo microdialysis, we have monitored the release of three amino acids (arginine, glutamate and glutamine) in the hippocampus of freely moving rats in response to various drugs. In response to N-methyl-d-aspartate (NMDA) infusion, extracellular glutamate was increased, glutamine was decreased and arginine remained unchanged. By contrast, alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) elicited an increase in arginine release but had no effect on either glutamate or glutamine. When S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, was infused into the hippocampus, an increase in glutamate, a decrease in glutamine and no change in arginine were recorded. The effect of SNAP on extracellular glutamine levels was reversed by prior infusion of the guanylate cyclase inhibitor oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), however its effect on glutamate release was unchanged. Interestingly, SNAP was found to promote the release of arginine in the presence of ODQ. We also assessed the effect of two nitric oxide synthase inhibitors, N-nitro-l-arginine methylester (l-NAME) and 7-nitroindazole (7-NI), on the release of these amino acids. l-NAME was found to increase arginine and glutamate levels but decrease those of glutamine. In contrast, 7-NI reduced the release of all three amino acids. The results presented here confirm some but not all of the findings previously obtained using in vitro preparations. In addition, they suggest that complex relationships exist between the release of these amino acids, and that endogenous NO plays an important role in regulating their release.