Dose-dependent effect of nitric oxide synthase inhibition following transient forebrain ischemia in gerbils

Nitric oxide and the brain. Part 1: Mechanisms of regulation, transport and effects on the developing brain

Apart from its known actions as a pulmonary vasodilator, nitric oxide (NO) is a key signal mediator in the neonatal brain. Despite the extensive use of NO for pulmonary artery hypertension (PAH), its actions in the setting of brain hypoxia and ischemia, which co-exists with PAH in 20-30% of affected infants, are not well established. This review focuses on the mechanisms of actions of NO covering the basic, translational, and clinical evidence of its neuroprotective and neurotoxic properties. In this first part, we present the physiology of transport and delivery of NO to the brain and the regulation of cerebrovascular and systemic circulation by NO, as well the role of NO in the development of the immature brain. IMPACT: NO can be transferred from the site of production to the site of action rapidly and affects the central nervous system. Inhaled NO (iNO), a commonly used medication, can have significant effects on the neonatal brain. NO regulates the cerebrovascular and systemic circulation and plays a role in the development of the immature brain. This review describes the properties of NO under physiologic conditions and under stress. The impact of this review is that it describes the effects of NO, especially regarding the vulnerable neonatal brain, and helps understand the conditions that could contribute to neurotoxicity or neuroprotection.

Implication of MicroRNA503 in Brain Endothelial Cell Function and Ischemic Stroke

The role of miR-503 in brain endothelium and ischemic stroke (IS) remains unclear. We aimed to study the relationship between plasma miR-503 and the onset time, severity, subtypes, and von Willebrand Factor (vWF) level in IS patients and to investigate the roles and underlying mechanisms of miR-503 in middle cerebral artery occlusion (MCAO) mice and cultured cerebral vascular endothelial cells (ECs). In MCAO mice, the effects of plasma from acute severe IS patients (ASS) with or without miR-503 antagomir on brain and ECs damage were determined. In cultured human ECs, the effects of miR-503 overexpression or knockdown on the monolayer permeability, apoptosis, ROS, and NO generation were investigated. For mechanism study, the PI3K/Akt/eNOS pathway, cleaved caspase-3, and bcl-2 were analyzed. Results showed that plasma miR-503 was significantly increased in IS patients, especially in acute period and severe cases and subtypes of LAA and TACI, and was positively correlated with vWF. Logistic analysis indicated that miR-503 was an independent risk factor for IS, with the area under curve of 0.796 in ROC analysis. In MCAO mice, ASS pretreatment aggravated neurological injury, BBB damage, brain edema, CBF reduction, and decreased NO production while increased apoptosis and ROS generation in brain ECs, which were partly abolished by miR-503 antagomir. In cultured ECs, miR-503 overexpression and knockdown confirmed its effects on regulating monolayer permeability, cell apoptosis, NO, and ROS generation via PI3K/Akt/eNOS pathway or bcl-2 and cleaved caspase-3 proteins. These together indicate that miR-503 is a promising biomarker and novel therapeutic target for IS.

Dual action of NO synthases on blood flow and infarct volume consecutive to neonatal focal cerebral ischemia

Research into neonatal ischemic brain damage is impeded by the lack of a complete understanding of the initial hemodynamic mechanisms resulting in a lesion, particularly that of NO-mediated vascular mechanisms. In a neonatal stroke rat model, we recently show that collateral recruitment contributes to infarct size variability. Non-specific and selective NO synthase (NOS) inhibition was evaluated on cerebral blood-flow changes and outcome in a P7 rat model of arterial occlusion (left middle cerebral artery electrocoagulation with 50 min occlusion of both common carotid arteries). Blood-flow changes were measured by using ultrasound imaging with sequential Doppler recordings in both internal carotid arteries and basilar trunk. Cortical perfusion was measured by using laser Doppler flowmetry. We showed that global NOS inhibition significantly reduced collateral support and cortical perfusion (collateral failure), and worsened the ischemic injury in both gender. Conversely, endothelial NOS inhibition increased blood-flows and aggravated volume lesion in males, whereas in females blood-flows did not change and infarct lesion was significantly reduced. These changes were associated with decreased phosphorylation of neuronal NOS at Ser(847) in males and increased phosphorylation in females at 24h, respectively. Neuronal NOS inhibition also increased blood-flows in males but not in females, and did not significantly change infarct volumes compared to their respective PBS-treated controls. In conclusion, both nNOS and eNOS appear to play a key role in modulating arterial blood flow during ischemia mainly in male pups with subsequent modifications in infarct lesion.

Kainic Acid-induced Neuronal Death is Attenuated by Aminoguanidine but Aggravated by L-NAME in Mouse Hippocampus

Nitric oxide (NO) has both neuroprotective and neurotoxic effects depending on its concentration and the experimental model. We tested the effects of NG-nitro-L-arginine methyl ester (L-NAME), a nonselective nitric oxide synthase (NOS) inhibitor, and aminoguanidine, a selective inducible NOS (iNOS) inhibitor, on kainic acid (KA)-induced seizures and hippocampal CA3 neuronal death. L-NAME (50 mg/kg, i.p.) and/or aminoguanidine (200 mg/kg, i.p.) were administered 1 h prior to the intracerebroventricular (i.c.v.) injection of KA. Pretreatment with L-NAME significantly increased KA-induced CA3 neuronal death, iNOS expression, and activation of microglia. However, pretreatment with aminoguanidine significantly suppressed both the KA-induced and L-NAME-aggravated hippocampal CA3 neuronal death with concomitant decreases in iNOS expression and microglial activation. The protective effect of aminoguanidine was maintained for up to 2 weeks. Furthermore, iNOS knockout mice (iNOS(-/-)) were resistant to KA-induced neuronal death. The present study demonstrates that aminoguanidine attenuates KA-induced neuronal death, whereas L-NAME aggravates neuronal death, in the CA3 region of the hippocampus, suggesting that NOS isoforms play different roles in KA-induced excitotoxicity.

Cellular and molecular pathways of ischemic neuronal death

Three routes have been identified triggering neuronal death under physiological and pathological conditions. Excess activation of ionotropic glutamate receptors cause influx and accumulation of Ca2+ and Na+ that result in rapid swelling and subsequent neuronal death within a few hours. The second route is caused by oxidative stress due to accumulation of reactive oxygen and nitrogen species. Apoptosis or programmed cell death that often occurs during developmental process has been coined as additional route to pathological neuronal death in the mature nervous system. Evidence is being accumulated that excitotoxicity, oxidative stress, and apoptosis propagate through distinctive and mutually exclusive signal transduction pathway and contribute to neuronal loss following hypoxic-ischemic brain injury. Thus, the therapeutic intervention of hypoxic-ischemic neuronal injury should be aimed to prevent excitotoxicity, oxidative stress, and apoptosis in a concerted way.

Óxido nítrico: revisão

O óxido nítrico é um mediador gasoso responsável por uma variedade de fenômenos fisiológicos. A l-arginina é a precursora da síntese do óxido nítrico, na presença de óxido nítrico-sintase. Este artigo revê as funções das óxido nítrico-sintases e como o óxido nítrico atua na permeabilidade vascular e na síndrome de isquemia e reperfusão, assim como possíveis métodos para sua mensuração.

Cerebral blood flow, glucose metabolism and tunel-positive cells in the development of ischemia

Few in vivo studies were available about the relation between cerebral blood flow, glucose metabolism and the appearance of apoptotic cells in the development of cerebral infarct. To investigate this, we measured local cerebral blood flow (lCBF), local cerebral metabolic rate in glucose (lCMRglc), and histopathology in transient focal cerebral ischemia in the rat. A unilateral middle cerebral artery occlusion (MCAO) was induced for 2 h in Wistar-ST rats (n = 42). A histopathological study with hematoxylin-eosin staining and the TdT-mediated dUTP-biotin nick-end labeling (TUNEL) method was performed. lCBF was measured by means of the (14)C-iodoantipyrine autoradiography technique during MCAO (n = 6), and 1, 22 and 70 h after reperfusion. lCMRglc was also measured by autoradiography with (14)C-2-deoxyglucose in the animals 22 h after reperfusion. These parameters were assessed in each region of interest: the ischemic core, boundary zones (BZ-I and BZ-II) and remote area. The boundary zones were defined as the area based on TUNEL positivity (more than 5/field) at 22 h after reperfusion (BZ-I) and at 70 h after reperfusion (BZ-II). In the BZ-I, lCBF was decreased to 18% of the control during MCAO, and lCBF and lCMRglc showed 44 and 62% of the control, respectively, 22 h after reperfusion. In this area, TUNEL-positive cells increased at 22 h, then markedly decreased 70 h after reperfusion. In the BZ-II, lCBF decreased to 39% of the control during MCAO, then returned to about 90% of the control 22 h after reperfusion. lCMRglc was maintained near its normal range (82% of the control) 22 h after reperfusion. Histopathology of BZ-II was normal 22 h after reperfusion. The TUNEL positivity of neurons in our study was assumed to be a marker of apoptotic cells. Our data suggested that the apoptotic process plays an important role in the maturation of a cerebral infarct. Both lCBF and lCMRglc were maintained with only a mild reduction in the predisposing phase of apoptosis, suggesting that sufficient blood supply and glucose metabolism are required to promote the process of apoptosis.

Pathophysiology of perinatal brain damage

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favour of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralisation, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na(+)/K(+) pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channel, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarisation. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to pre-ischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the post-ischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of i.v. administration of magnesium or post-ischemic induction of cerebral hypothermia.

Roles of nitric oxide in brain hypoxia-ischemia

A large body of evidence has appeared over the last 6 years suggesting that nitric oxide biosynthesis is a key factor in the pathophysiological response of the brain to hypoxia-ischemia. Whilst studies on the influence of nitric oxide in this phenomenon initially offered conflicting conclusions, the use of better biochemical tools, such as selective inhibition of nitric oxide synthase (NOS) isoforms or transgenic animals, is progressively clarifying the precise role of nitric oxide in brain ischemia. Brain ischemia triggers a cascade of events, possibly mediated by excitatory amino acids, yielding the activation of the Ca2+-dependent NOS isoforms, i.e. neuronal NOS (nNOS) and endothelial NOS (eNOS). However, whereas the selective inhibition of nNOS is neuroprotective, selective inhibition of eNOS is neurotoxic. Furthermore, mainly in glial cells, delayed ischemia or reperfusion after an ischemic episode induces the expression of Ca2+-independent inducible NOS (iNOS), and its selective inhibition is neuroprotective. In conclusion, it appears that activation of nNOS or induction of iNOS mediates ischemic brain damage, possibly by mitochondrial dysfunction and energy depletion. However, there is a simultaneous compensatory response through eNOS activation within the endothelium of blood vessels, which mediates vasodilation and hence increases blood flow to the damaged brain area.

Nitric oxide production in the CA1 field of the gerbil hippocampus after transient forebrain ischemia : effects of 7-nitroindazole and NG-nitro-L-arginine methyl ester

BACKGROUND AND PURPOSE

The present study was designed to examine the time course of nitric oxide (NO) production and the source of NO in the CA1 field of the gerbil hippocampus after transient forebrain ischemia.

METHODS

The production of NO in the CA1 field of the hippocampus after transient ischemia was monitored consecutively by measuring total NO metabolites (NOx-, NO2- plus NO3-) with the use of brain microdialysis. 7-Nitroindazole (7-NI) and NG-nitro-L-arginine methyl ester were used to dissect the relative contributions of neuronal NO synthase and endothelial NO synthase to the NO production. The histological outcomes of 7-NI in 5- and 10-minute global ischemia were also evaluated.

RESULTS

The production of NO in the CA1 field of the hippocampus after ischemia was dependent on the severity of ischemia. Ischemia for 2 or 5 minutes did not induce a significant increase in NOx- levels in the CA1 field of the hippocampus after reperfusion, whereas the 10- and 15-minute ischemias produced significant and persistent increases in NOx- levels. 7-NI did not inhibit the basal NOx- levels and showed no effects on NOx- levels after 5 minutes of ischemia. However, it completely inhibited the increased NOx- levels after 10 or 15 minutes of ischemia. 7-NI provided minor neuroprotection in 5 minutes but not in 10 minutes of global ischemia.

CONCLUSIONS

The increased NO level in the CA1 field of the hippocampus after ischemia is produced mostly by neuronal NO synthase, whereas the basal NO level mainly originates from endothelial NO synthase. The observed neuroprotective effect of 7-NI in 5-minute global ischemia in gerbils may not be due to neuronal NO synthase inhibition by this drug.

Blockade of tetrahydrobiopterin synthesis protects neurons after transient forebrain ischemia in rat: a novel role for the cofactor

The generation of nitric oxide (NO) aggravates neuronal injury. (6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) is an essential cofactor in the synthesis of NO by nitric oxide synthase (NOS). We attempted to attenuate neuron degeneration by blocking the synthesis of the cofactor BH4 using N-acetyl-3-O-methyldopamine (NAMDA). In vitro data demonstrate that NAMDA inhibited GTP cyclohydrolase I, the rate-limiting enzyme for BH4 biosynthesis, and reduced nitrite accumulation, an oxidative metabolite of NO, without directly inhibiting NOS activity. Animals exposed to transient forebrain ischemia and treated with NAMDA demonstrated marked reductions in ischemia-induced BH4 levels, NADPH-diaphorase activity, and caspase-3 gene expression in the CA1 hippocampus. Moreover, delayed neuronal injury in the CA1 hippocampal region was significantly attenuated by NAMDA. For the first time, these data demonstrate that a cofactor, BH4, plays a significant role in the generation of ischemic neuronal death, and that blockade of BH4 biosynthesis may provide novel strategies for neuroprotection.

Chronic administration of adenosine A3 receptor agonist and cerebral ischemia: neuronal and glial effects

We have previously shown that chronic administration of the selective A3 receptor agonist N6-(3-iodobenzyl)-5'-N-methylcarboxoamidoadenosine (IB-MECA) leads to a significant improvement of postocclusive cerebral blood flow, and protects against neuronal damage and mortality induced by severe forebrain ischemia in gerbils. Using immunocytochemical methods we now show that chronic with IB-MECA results in a significant preservation of ischemia-sensitive microtubule associated protein 2 (MAP-2), enhancement of the expression of glial fibrillary acidic protein (GFAP), and a very intense depression of nitric oxide synthase in the brain of postischemic gerbils. These changes demonstrate that the cerebroprotective actions of chronically administered IB-MECA involve both neurons and glial cells, and indicate the possibility of distinct mechanisms that are affected in the course of chronic administration of the drug.

Nitric oxide synthase inhibitors L-NAME and 7-nitroindazole protect rat hippocampus against kainate-induced excitotoxicity

The role of nitric oxide in cerebral insults remains controversial. While numerous studies have used models of ischaemia and hypoxia, few have examined nitric oxide in the kainate model of excitotoxicity. Kainate (10 mg/kg) was administered to rats via the intraperitoneal (i.p.) route to induce submaximal damage to the CA1, CA2 and CA3a regions of the hippocampus after 7 days. Systemic injections of the nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), both at a dose of 5 mg/kg, reduced cell death in all three regions. As 7-NI selectively inhibits the neuronal form of NOS, this study suggests that nitric oxide produced from a neuronal and not epithelial source may contribute to neuronal damage in this model.

Modulation of extracellular glutamate concentration by nitric oxide synthase inhibitor in rat transient forebrain ischemia

The purpose of the present study was to clarify the effect of topical administration of a nitric oxide synthase inhibitor on extracellular glutamate concentration in transient forebrain ischemia. Two microdialysis probes were inserted into the bilateral striata of Wistar rats. NG-Nitro-L-arginine (L-NNA) with or without L-arginine was topically administered into the unilateral striatum through one of the microdialysis probes, while Ringer's solution was perfused into the contralateral striatum as the control, and 14 minutes of forebrain ischemia was applied. The extracellular glutamate concentration during ischemia and subsequent reperfusion was statistically significantly higher on the 100 microM L-NNA-perfused side than on the control side, but 1 mM L-NNA was ineffective. When 100 microM L-NNA was perfused together with 500 microM L-arginine, the glutamate concentration did not differ from that on the control side. Moreover, administration of 500 microM L-arginine significantly suppressed the glutamate elevation after reperfusion. The fact that the lower dose of L-NNA increased the accumulation of glutamate during ischemia and reperfusion without altering blood flow may indicate that nitric oxide affords protection against ischemia neuronal damage. However, since the higher dose of L-NNA did not affect the glutamate concentration, it appears that the effect of nitric oxide on extracellular glutamate concentration in forebrain ischemia differs, depending on the degree of the inhibition of NOS activity.

Neuroprotective effect of FK506, an immunosuppressant, on transient global ischemia in gerbil

Delayed neuronal death (DND) in CA1 region after transient global ischemia is a well-known phenomenon, but its mechanism has not been clarified. In order to examine the involvement of nitric oxide (NO) in DND, 7-nitro indazole (7NI), a selective neuronal NO synthase (nNOS) inhibitor, and FK506, an immunosuppressant which also inhibits nNOS, were administered intraperitoneally during and after transient global ischemia in gerbil. FK506 moderately ameliorated DND in a dose-dependent manner. However, 7NI showed only minor neuroprotective effects. These results show that DND is not mainly mediated by NO production via nNOS, and FK506 acts as a neuroprotective agent via unknown pathways other than nNOS inhibition.

Physiological and pathophysiological roles of nitric oxide in the central nervous system

Nitric oxide (NO) is produced by three distinct isoforms of nitric oxide synthases in the central nervous system. Here, the roles of nitric oxide in the central nervous system are reviewed under physiological and pathophysiological conditions. Under physiological conditions, NO plays a role in the regulation of cerebral blood flow and autoregulation, blood flow-metabolism coupling, neurotransmission, memory formation, modulation of neuroendocrine functions, and behavioral activity. Impairment of the NO-mediated cerebrovascular vasodilatation occurs during ischemia-reperfusion, diabetes, hypertension, subararchnoid hemorrhage, and various forms of shock. Enhancement of NO production in the brain occurs during stoke, seizures, and acute and chronic inflammatory and neurodegenerative disorders. The alterations of the expression of the various isoforms of nitric oxide synthases under the above conditions are discussed. Moreover, the molecular mechanisms of NO and peroxynitrite induced cellular injury are delineated. Finally, the current strategies available for selective pharmacological manipulation of individual nitric oxide synthase isoforms are discussed.