Glutamate-nitric oxide-cGMP Research Articles

Lanthanum enhances glutamate-nitric oxide-3',5'-cyclic guanosine monophosphate pathway in the hippocampus of rats.

Lanthanum (La) appears to impair learning and memory and increase the toxicity of excitatory amino acids in the central nervous system. The mechanism underlying excitotoxicity induced by La is still unclear. The purpose of this study was to investigate the hippocampal impairment of La exposure and possible mechanism involving the glutamate-nitric oxide (NO)-3'-5'-cyclic guanosine monophosphate (cGMP) pathway. In this study, lactating rats were exposed to 0, 0.25, 0.50, and 1.0% lanthanum chloride (LaCl3) in drinking water, respectively. Their offsprings were exposed to LaCl3 by parental lactation and then administrated with 0, 0.25, 0.50, and 1.0% LaCl3 in drinking water for 1 month. The results showed that La exposure impaired the neuronal ultrastructure and significantly increased the glutamate level, intracellular calcium ion concentrations, and NR1 and NR2B expression in the hippocampi. La exposure significantly enhanced messenger RNA expression and activity levels of inducible NO synthase and increased NO and cGMP levels in the hippocampi in a dose-dependent manner. These results indicate that the mechanism underlying excitotoxicity induced by La is possibly due to alterations of the glutamate-NO-cGMP signaling pathway in the hippocampus. The study provides new findings that may help prevent and improve treatments for La-induced neurotoxicity.

Differential effects of chronic hyperammonemia on modulation of the glutamate–nitric oxide–cGMP pathway by metabotropic glutamate receptor 5 and low and high affinity AMPA receptors in cerebellum in vivo

Previous studies show that chronic hyperammonemia impairs learning ability of rats by impairing the glutamate–nitric oxide (NO)–cyclic guanosine mono-phosphate (cGMP) pathway in cerebellum. Three types of glutamate receptors cooperate in modulating the NO–cGMP pathway: metabotropic glutamate receptor 5 (mGluR5), (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartic acid (NMDA) receptors. The aim of this work was to assess whether hyperammonemia alters the modulation of this pathway by mGluR5 and AMPA receptors in cerebellum in vivo. The results support that in control rats: (1) low AMPA concentrations (0.1mM) activate nearly completely Ca2+-permeable (glutamate receptor subunit 2 (GluR2)-lacking) AMPA receptors and the NO–cGMP pathway; (2) higher AMPA concentrations (0.3mM) also activate Ca2+-impermeable (GluR2-containing) AMPA receptors, leading to activation of NMDA receptors and of NO–cGMP pathway. Moreover, the data support that chronic hyperammonemia: (1) reduces glutamate release and activation of the glutamate–NO–cGMP pathway by activation of mGluR5; (2) strongly reduces the direct activation by AMPA receptors of the NO–cGMP pathway, likely due to reduced entry of Ca2+ through GluR2-lacking, high affinity AMPA receptors; (3) strongly increases the indirect activation of the NO–cGMP pathway by high affinity AMPA receptors, likely due to increased entry of Na+ through GluR2-lacking AMPA receptors and NMDA receptors activation; (4) reduces the indirect activation of the NO–cGMP pathway by low affinity AMPA receptors, likely due to reduced activation of NMDA receptors.

Developmental exposure to polychlorinated biphenyls or methylmercury, but not to its combination, impairs the glutamate–nitric oxide–cyclic GMP pathway and learning in 3-month-old rats

Prenatal exposure to polychlorinated biphenyls (PCBs) or methylmercury (MeHg) contaminated food may affect brain development, leading to long-term alterations in cognitive function. Both types of contaminants, PCBs and MeHg, are often found together contaminating food, especially fish in some polluted areas. Exposure to combinations of neurotoxicants may exert different effects on the developing nervous system than exposure to individual contaminants. Developmental exposure (during pregnancy and lactation) to PCB126 or PCB153 impairs learning ability when the rats are 3 months old. Impairment of learning seems to be a consequence of impairment of the function of the glutamate–nitric oxide (NO)–cGMP pathway in brain in vivo. The aims of the present work were 1) to assess whether perinatal exposure to MeHg also affects the function of the glutamate–NO–cGMP pathway in brain in vivo analyzed by in vivo brain microdialysis and/or the ability to learn the Y maze task when the rats are 3 months old, and 2) to assess whether perinatal exposure to combinations of MeHg with PCB153 or PCB126 potentiates, decreases or does not modify the effects of the individual neurotoxicants. Perinatal exposure to PCB126, PCB153 or MeHg impaired the function of the glutamate–NO–cGMP pathway in cerebellum and learning ability. However, co-exposure to PCB126+MeHg or PCB153+MeHg inhibits the impairment of the pathway or learning ability. These results support that the function of this pathway modulates learning of the Y maze task. Moreover, they show that co-exposure to these PCBs and MeHg does not exacerbate, but reduces the effects on the ability to learn this task.

Alterations in soluble guanylate cyclase content and modulation by nitric oxide in liver disease

Hyperammonemia is the main responsible for the neurological alterations in hepatic encephalopathy in patients with liver failure. We studied the function of the glutamate–nitric oxide (NO)–cGMP pathway in brain in animal models of hyperammonemia and liver failure and in patients died with liver cirrhosis. Activation of glutamate receptors increases intracellular calcium that binds to calmodulin and activates neuronal nitric oxide synthase, increasing nitric oxide, which activates soluble guanylate cyclase (sGC), increasing cGMP. This glutamate–NO–cGMP pathway modulates cerebral processes such as circadian rhythms, the sleep-waking cycle, and some forms of learning and memory. These processes are impaired in patients with hepatic encephalopathy. Activation of sGC by NO is significantly increased in cerebral cortex and significantly reduced in cerebellum from cirrhotic patients died in hepatic coma. Portacaval anastomosis in rats, an animal model of liver failure, reproduces the effects of liver failure on modulation of sGC by NO both in cerebral cortex and cerebellum. In vivo brain microdialisis studies showed that sGC activation by NO is also reduced in vivo in cerebellum in hyperammonemic rats with or without liver failure. The content of α but not β subunits of sGC are increased both in frontal cortex and cerebellum from patients died due to liver disease and from rats with portacaval anastomosis. We assessed whether determination of activation of sGC by NO-generating agent SNAP in lymphocytes could serve as a peripheral marker for the impairment of sGC activation by NO in brain. Chronic hyperammonemia and liver failure also alter sGC activation by NO in lymphocytes from rats or patients. These findings show that the content and modulation by NO of sGC are strongly altered in brain of patients with liver disease. These alterations could be responsible for some of the neurological alterations in hepatic encephalopathy such as sleep disturbances and cognitive impairment.

A new NMDA-receptor blocker provides potential neuroprotection

Glutamate neurotoxicity is implicated in the pathogenesis of neurodegenerative diseases such as Huntington's, Alzheimer's and Parkinson's diseases, and also plays a role in brain damage caused by head trauma, stroke and brain-tumour progression.NMDA-receptor channels are very permeable to Ca2+. Glutamate-mediated Ca2+ influx into neurons sets up a glutamate–nitric oxide–cGMP pathway, which eventually results in cell death by necrosis or apoptosis. Therefore, drug development aimed at neuroprotection has targeted the NMDA receptor. Unfortunately, at present, NMDA-receptor antagonists have serious cognitive side effects that interfere with their therapeutic benefit. These antagonists have strong affinity, and bind not only to pathologically active receptors, but also to physiologically active receptor sites, thereby interfering with the normal processes of memory and learning. Non-competitive channel blockers bind preferentially to pathologically active receptors. A potential neuroprotectant non-competitive NMDA-channel blocker would need the attributes of low molecular weight, low receptor affinity and fast on–off kinetics.Planells-Cases et al. screened a restricted oligo-N-substituted glycine-based combinatorial library to find novel antagonists of the NMDA receptor [1xA novel n-methyl-d-aspartate receptor open channel blocker with in vivo neuroprotectant activity. Planells-Cases, R. et al. J. Pharmacol. Exp. Ther. 2002; 302: 163–173Crossref | PubMed | Scopus (46)See all References[1]. The rationale for this was the finding that arginine-rich hexapeptides blocked NMDA receptors with high efficacy and exhibited neuroprotectant activity in vitro. However, in vivo, the compounds were toxic at theraputic doses. The library identified trimers of N-alkylglycine with a 3,3-diphenylpropyl amino moeity that blocked NMDA-receptor channels. Structure–activity relationships demonstrated that one compound, N20C, selectively inhibited NMDA receptors with micromolar affinity, and had fast on–off kinetics and strong voltage dependence. It did not compete for binding with glycine or glutamate, but prevented high affinity MK801 binding. The authors concluded, therefore, that N20C is a non-competitive blocker that binds to the same non-competitive site as MK801, deep in the NMDA-receptor channel pore, thereby interfering with Ca2+ permeability. In vitro and in vivo experiments also demonstrated that N20C has a neuroprotectant effect on glutamate-induced neuronal cell death.The extent of neuroprotection provided by N20C was shown to be higher than that of memantine, and a better therapeutic profile was also suggested. The compound did not exhibit toxicity at high concentrations, and animals treated with N-alkylglycine did not display any conspicuous behavioural or motor deficits. Although the authors caution that a therapeutic index needs to be determined for this new open-channel blocker, this study represents an exciting step in the discovery of a neuroprotectant drug that has the potential to treat many neurodegenerative diseases.