Cytosolic and membrane-bound cerebral nitric oxide synthase activity during hypoxia in cortical tissue of newborn piglets

Nitric oxide and the brain. Part 2: Effects following neonatal brain injury-friend or foe?

Nitric oxide (NO) has critical roles in a wide variety of key biologic functions and has intricate transport mechanisms for delivery to key distal tissues under normal conditions. However, NO also plays important roles during disease processes, such as hypoxia-ischemia, asphyxia, neuro-inflammation, and retinopathy of prematurity. The effects of exogenous NO on the developing neonatal brain remain controversial. Inhaled NO (iNO) can be neuroprotective or toxic depending on a variety of factors, including cellular redox state, underlying disease processes, duration of treatment, and dose. This review identifies key gaps in knowledge that should prompt further investigation into the possible role of iNO as a therapeutic agent after injury to the brain. IMPACT: NO is a key signal mediator in the neonatal brain with neuroprotective and neurotoxic properties. iNO, a commonly used medication, has significant effects on the neonatal brain. Dosing, duration, and timing of administration of iNO can affect the developing brain. This review article summarizes the roles of NO in association with various disease processes that impact neonates, such as brain hypoxia-ischemia, asphyxia, retinopathy of prematurity, and neuroinflammation. The impact of this review is that it clearly describes gaps in knowledge, and makes the case for further, targeted studies in each of the identified areas.

Flavone inhibits nitric oxide synthase (NOS) activity, nitric oxide production and protein S-nitrosylation in breast cancer cells

As the core structure of flavonoids, flavone has been proved to possess anticancer effects. Flavone's growth inhibitory functions are related to NO. NO is synthesized by nitric oxide synthase (NOS), and generally increased in a variety of cancer cells. NO regulates multiple cellular responses by S-nitrosylation. In this study, we explored flavone-induced regulations on nitric oxide (NO)-related cellular processes in breast cancer cells. Our results showed that, flavone suppresses breast cancer cell proliferation and induces apoptosis. Flavone restrains NO synthesis by does-dependent inhibiting NOS enzymatic activity. The decrease of NO generation was detected by fluorescence microscopy and flow cytometry. Flavone-induced inhibitory effect on NOS activity is dependent on intact cell structure. For the NO-induced protein modification, flavone treatment significantly down-regulated protein S-nitrosylation, which was detected by "Biotin-switch" method. The present study provides a novel, NO-related mechanism for the anticancer function of flavone.

Maternal Treatment with Propofol Attenuates Lipid Peroxidation after Transient Intrauterine Ischemia in the Neonatal Rat Brain

The purpose of this study was to investigate whether propofol has a neuroprotective effect on the fetal brain after intrauterine ischemia-reperfusion (I/R) injury in the rat fetus. Fetal brain ischemia was induced by clamping the utero-ovarian artery bilaterally for 30 min and reperfusion was achieved by removing the clamps for 2 h. A 40-mg/kg single dose of propofol was administered intraperitoneally 15 min before I/R injury. Lipid peroxidation in the brain tissue was determined as the concentration of thiobarbituric acid reactive substances (TBARS) for each fetal rat. Results showed that lipid peroxidation byproducts increased after I/R injury. Maternal treatment with propofol reduced TBARS compared to the I/R group. Propofol has been shown to have neuroprotective effects in intrauterine I/R-induced fetal brain damage in rats.

Postnatal changes in the nitric oxide system of the rat cerebral cortex after hypoxia during delivery

The impact of hypoxia in utero during delivery was correlated with the immunocytochemistry, expression and activity of the neuronal (nNOS) and inducible (iNOS) isoforms of the nitric oxide synthase enzyme as well as with the reactivity and expression of nitrotyrosine as a marker of protein nitration during early postnatal development of the cortex. The expression of nNOS in both normal and hypoxic animals increased during the first few postnatal days, reaching a peak at day P5, but a higher expression was consistently found in hypoxic brain. This expression decreased progressively from P7 to P20, but was more prominent in the hypoxic group. Immunoreactivity for iNOS was also higher in the cortex of the hypoxic rats and was more evident between days P0 and P5, decreasing dramatically between P10 and P20 in both groups of rats. Two nitrated proteins of 52 and 38 kDa, were also identified. Nitration of the 52-kDa protein was more intense in the hypoxic animals than in the controls, increasing from P0 to P7 and then decreasing progressively to P20. The 38-kDa nitrated protein was seen only from P10 to P20, and its expression was more intense in control than in the hypoxic group. These results suggest that the NO system may be involved in neuronal maturation and cortical plasticity over postnatal development. Overproduction of NO in the brain of hypoxic animals may constitute an effort to re-establish normal blood flow and may also trigger a cascade of free-radical reactions, leading to modifications in the cortical plasticity.

Nitric oxide synthesis inhibition during cerebral hypoxemia and reoxygenation with 100% oxygen in newborn pigs

The objective of our study was to evaluate the effects of N(sigma)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the nitric oxide (NO) pathway, on cerebral microcirculation during hypoxemia and reoxygenation with 100% oxygen in newborn pigs. Twenty-two pigs were randomized to hypoxemia [inspired fraction of oxygen (FIO(2)) 0.08; 20 min] and reoxygenation (FIO(2) 1.0; 60 min) or normoxia. The hypoxemic animals were further randomized to receive either an intravenous bolus injection of 5 mg/kg L-NAME (n = 8) or a corresponding volume of isotonic saline (n = 8) 30 min before the onset of hypoxemia. The normoxemic group (n = 6) received the same pretreatment with L-NAME. Cerebral hemodynamics were assessed by laser Doppler flowmetry and intracranial pressure monitoring. The cerebral NO concentration was continuously measured using an electrochemical sensor. Pretreatment with L-NAME resulted in a more severe systemic hypotension and reduced cerebral microcirculation during the period of hypoxemia compared with the saline/hypoxemia group. NO synthesis inhibition during reoxygenation with 100% oxygen, however, blunted the increase in NO concentration (p < 0.05) without reduction of cerebral blood flow and cerebral perfusion pressure. In conclusion, in this newborn pig model, pretreatment with a bolus infusion of L-NAME induced severe hypotension and reduced cerebral microcirculation during hypoxemia. However, it appears to have no significant adverse effect on cerebral hemodynamics during the period of reoxygenation with 100% oxygen. This deleterious effect during hypoxemia limits the use of L-NAME as a preventive drug but suggests beneficial effects during reoxygenation with 100% oxygen.

Hypoxia-induced lipid peroxidation in rat brain and protective effect of carnitine and phosphocreatine

The exposure to hypobaric hypoxia increased lipid peroxidation as indicated by thiobarbituric acid-reactive substances [TBARS] in rat brain. Plasma lactate/pyruvate ratio was used as a marker of hypoxia. We compared the protective effect of alpha-tocopherol with the effect of L-carnitine or phosphocreatine. Rats pretreated with alpha-tocopherol, L-carnitine, or phosphocreatine had lower TBARS levels after the exposure to hypobaric hypoxia. However, lactate/ pyruvate ratio was improved only in rats pretreated with L-carnitine or phosphocreatine. We conclude from our data that, contrary to alpha-tocopherol, protective effects of L-carnitine and phosphocreatine administrations are due to their regulation of metabolic reactions during hypobaric hypoxia rather than to their scavenger activity.

Effect of graded hypoxia on high-affinity Ca2+-ATPase activity in cortical neuronal nuclei of newborn piglets

Previous studies have shown that nuclear calcium signals control a variety of nuclear functions including gene transcription, DNA synthesis, DNA repair and nuclear envelope breakdown. The present study tested the hypothesis that the activity of the neuronal nuclear high affinity Ca2+-ATPase increases as a function of decreased energy metabolism in the cerebral cortex. Studies were performed in 11 ventilated newborn piglets, age 3-5 days, divided into normoxic (Nx, n = 4) and hypoxic (Hx, n = 7) groups. The animals were exposed to a single FiO2 in the range from 0.21 to 0.05 for one hr. Cerebral tissue hypoxia was confirmed biochemically by determining brain tissue ATP and phosphocreatine levels. Neuronal nuclei were isolated and the high-affinity Ca2+-ATPase activity determined. During graded hypoxia, cerebral tissue ATP decreased from 4.80 +/- 0.58 (normoxic) to 1.03 +/- 0.38 (ranging from 0.61-1.63) micromol/g brain (p < 0.05) and PCr decreased from 3.94 +/- 0.75 (normoxic) to 0.99 +/- 0.27 (ranging from 0.50 to 1.31) micromol/g brain (p < 0.05). The total high affinity Ca2+-ATPase activity in the hypoxic nuclei increased and ranged from 541 to 662 nmol/mg protein/hr, compared to activity in normoxic group of 327 to 446 nmol/mg protein/hr. During graded hypoxia, the level of nuclear high affinity Ca2+-ATPase activity correlated inversely with ATP (r = 0.91) and PCr levels (r = 0.82), with activity increasing as tissue high energy phosphates decreased. The results demonstrate that the decrease in cerebral energy metabolism during hypoxia is linearly correlated with an increase in activity of high affinity Ca2+-ATPase in cerebral cortical nuclei from immature brain. We propose that increased nuclear membrane high affinity Ca2+-ATPase activity, leading to increased nuclear Ca2+, will result in altered expression of apoptotic genes that could initiate programmed neuronal death.

Nitration as a mechanism of Na+, K+-ATPase modification during hypoxia in the cerebral cortex of the guinea pig fetus

Previous studies have shown that hypoxia induces nitric oxide synthase-mediated generation of nitric oxide free radicals leading to peroxynitrite production. The present study tests the hypothesis that hypoxia results in NO-mediated modification of Na+, K+-ATPase in the fetal brain. Studies were conducted in guinea pig fetuses of 58-days gestation. The mothers were exposed to FiO2 of 0.07% for 1 hour. Brain tissue hypoxia in the fetus was confirmed biochemically by decreased ATP and phosphocreatine levels. P2 membrane fractions were prepared from normoxic and hypoxic fetuses and divided into untreated and treated groups. The membranes were treated with 0.5 mM peroxynitrite at pH 7.6. The Na+, K+-ATPase activity was determined at 37 degrees C for five minutes in a medium containing 100 mM NaCl, 20 mM KCl, 6.0 mM MgCl2, 50 mM Tris HCl buffer pH 7.4, 3.0 mM ATP with or without 10 mM ouabain. Ouabain sensitive activity was referred to as Na+, K+-ATPase activity. Following peroxynitrite exposure, the activity of Na+, K+-ATPase in guinea pig brain was reduced by 36% in normoxic membranes and further 29% in hypoxic membranes. Enzyme kinetics was determined at varying concentrations of ATP (0.5 mM-2.0 mM). The results indicate that peroxynitrite treatment alters the affinity of the active site of Na+, K+-ATPase for ATP and decreases the Vmax by 35% in hypoxic membranes. When compared to untreated normoxic membranes Vmax decreases by 35.6% in treated normoxic membranes and further to 52% in treated hypoxic membranes. The data show that peroxynitrite treatment induces modification of Na+, K+-ATPase. The results demonstrate that peroxynitrite decreased activity of Na+, K+-ATPase enzyme by altering the active sites as well as the microenvironment of the enzyme. We propose that nitric oxide synthase-mediated formation of peroxynitrite during hypoxia is a potential mechanism of hypoxia-induced decrease in Na+, K+-ATPase activity.

Effects of chronic deep hypoxia on the expression of nitric oxide synthase in the rat brain

Experimental studies in extreme hypoxic conditions affecting the brain have been performed mainly in acute but not chronic models. Twenty rats were housed and exposed to decreasing concentrations of oxygen (from 21% to 7% over 130 days) and ten normal rats were used as control. Paraffin slices from representative sections containing cerebral cortex, cerebellum, striatum, hippocampus, thalamus and hypothalamus were incubated with antisera against nitric oxide synthase. Cortex and striatum showed small randomly distributed positive neurons with bipolar features, in greater numbers in the hypoxic group (p < 0.02). The granular layer of the cerebellum showed a strongly positive rim around some cell nuclei. Purkinje cells were immunopositive in hypoxic rats. Hipoccampal, thalamic and hypothalamic nuclei showed no quantitative differences in the number of positive neurons. The increased number of blood vessels and their dilation observed in some brain regions in hypoxic rats, mainly in ventral striatum, lead us to hypothesise that NOS may be overexpressed and act at these sites as vasomodulator and/or mediator of secondary cell injury affecting selective neuronal populations. We conclude that prolonged periods of adaptation to deep hypoxia reduces the effect of hypoxia on the upregulation of NOS in the brain tissue.

Inhibition of nitric oxide synthesis following severe hypoxia-ischemia restores autoregulation of cerebral blood flow in newborn lambs

Birth asphyxia impairs the autoregulatory ability of the cerebral blood flow. Inappropriate synthesis of vasodilatory nitric oxide may be important in this respect. We investigated if nitric oxide synthesis inhibition by N(omega)-nitro-L-arginine (NLA) could restore cerebral autoregulation after severe hypoxia-ischemia (HI). HI was induced in 15 newborn lambs. Cerebral blood flow (carotid artery blood flow [ml/min]: Qcar) and mean aortic blood pressure [mmHg]: MABP) were measured over a 30 min period before HI (pre-HI), 0-30 min after completion of HI (0-30 post-HI) and from 60 to 120 min post-HI (60-120 post-HI). Immediately after completion of HI, 5 lambs received a placebo (PLAC), 5 low dose NLA (10 mg/kg/iv: NLA-10) and 5 high dose NLA (40 mg/kg/iv: NLA-40). Pre-HI, all groups showed cerebral autoregulation with an upper limit of regulatory ability between 75 and 90 mm Hg. At 0-30 post-HI, all groups lacked autoregulatory ability of the cerebral vascular bed and showed an aortic blood pressure-passive Q(car). At 60-120 post-HI autoregulation was restored in NLA-10 and NLA-40-treated lambs (upper limit of autoregulation was shifted to higher MABP in NLA40-treated lambs), but not in placebo-treated lambs. At 60-120 post-HI MABP was higher in both NLA-groups than in PLAC group (83+/-15 [NLA-10] and 78+/-14 [NLA-40] vs. 65+/-9 mmHg [PLAC], P<0.05). We conclude that severe HI in newborn lambs induces impairment of the autoregulatory ability of the cerebral vascular bed. Even low-dose nitric oxide-synthesis inhibition started upon reperfusion restored autoregulation, suggesting a role for nitric oxide-induced vasodilation in the impairment of autoregulation of the cerebral blood flow after birth asphyxia.

Nitric oxide and nitric oxide synthase in the early phase of perinatal asphyxia of the rat

The role of nitric oxide, a compound involved in neurotransmission and regulation of cerebral blood flow, in cerebral ischemia is still not fully elucidated yet. Although well studied in adult systems of cerebral ischemia/hypoxia, information on nitric oxide in perinatal asphyxia is limited and, in particular, no direct evidence for its generation has been provided. We therefore decided to study nitric oxide generation in brain of asphyctic rat pups by biophysical and biochemical methods. We used a simple, non-invasive rat model resembling the clinical situation in perinatal asphyxia: rat pups delivered by Caesarean section were placed into a water bath at 37 degrees C still in patent membranes for various asphyctic periods (up to 20 min). Brain pH, cerebral blood flow, neuronal nitrix oxide synthase messenger RNA (by northern and dot blot analysis), immunoreactive protein (by western blot analysis) and nitric oxide synthase activity were determined; generation of nitric oxide was evaluated directly by electron paramagnetic resonance spectroscopy. Neuronal nitric oxide synthase messenger RNA activity and nitric oxide generation were unaffected, whereas neuronal nitric oxide synthase-immunoreactive protein of 150,000 mol. wt was decreased and of 136,000 mol. wt was increased with the length of the asphyctic period. This is the first report on direct evidence for the generation of nitric oxide in perinatal asphyxia and we demonstrate that nitric oxide production remains unaffected even by 20 min of asphyxia, at a time-point when cerebral blood flow was increased four-fold and severe acidosis was present. However, it was found that levels of immunoreactive neuronal nitric oxide synthase of 136,000 mol. wt were increased paralleling the length of asphyxia. Levels of the 150,000 mol. wt immunoreactive neuronal nitric oxide synthase protein decreased, suggesting a different regulation pattern. Thus, the present biochemical and biophysical results form the basis for further investigations on nitric oxide in perinatal asphyxia.

Oxidative damage in fetal rat brain induced by ischemia and subsequent reperfusion. Relation to arachidonic acid peroxidation

To determine whether ischemia followed by subsequent reperfusion can induce fetal cerebral oxidative damage, we created a model of fetal ischemia/reperfusion using rats at day 19 of pregnancy. Fetal ischemia was induced by unilateral occlusion of the utero-ovarian artery for 20 min. Reperfusion was achieved by releasing the occlusion and restoring the circulation for 30 min. The opposite uterine horn was used as control. We measured brain mitochondrial respiratory control index (RCI) and the concentration of thiobarbituric acid-reactive substances (TBARS) in each group. Arachidonic acid (AA) peroxidation induced by the incubation of brain microvessel fraction and AA was measured. AA peroxidation was also evaluated with and without aspirin, an inhibitor of cyclooxygenase and phenidone, which inhibits both of cyclooxygenase and lipoxygenase. The RCI significantly decreased by the occlusion with (p < 0.01) or without reperfusion (p < 0.05). The TBARS level significantly increased with occlusion plus reperfusion (p < 0.01). AA peroxidation was significantly greater in the occlusion and occlusion plus reperfusion groups than in the control groups (p < 0. 01). Aspirin did not affect peroxidation, while phenidone significantly inhibited it in a concentration-dependent manner (p < 0.001). Accordingly, ischemia followed by reperfusion is likely to induce fetal cerebral lipid peroxidation, which may inhibit mitochondrial respiratory activity. The phenidone-inhibited enzyme lipoxygenase may participate importantly in this peroxidation.

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.

Inhibitory effects of hypoxia and adenosine on N-methyl-D-aspartate-induced pial arteriolar dilation in piglets

Our previous studies have indicated that oxygen radicals, produced during reoxygenation following short-term arterial hypoxia, lead to sustained suppression of cerebral arteriolar responses to N-methyl-D-aspartate (NMDA). However, whether arteriolar dilator responses to NMDA are reduced during arterial hypoxia has never been examined. In this study, we determined whether hypoxia or hypoxia-related metabolites such as adenosine or nitric oxide (NO) will reduce NMDA-induced arteriolar dilation. We have also determined the location of NMDA receptor- and brain nitric oxide synthase (bNOS)-positive neurons in the cerebral cortex. In anesthetized piglets, pial arteriolar diameters were determined using intravital microscopy. Baseline arteriolar diameters were approximately 100 microns. Topical application of NMDA at concentrations of 10(-5), 5 x 10(-5) and 10(-4) M resulted in dose-dependent vasodilation (9 +/- 2, 18 +/- 2 and 29 +/- 2% above baseline, respectively, n = 21). Administration of theophylline (20 mg/kg, i.v.) had no effect on NMDA-dependent vasodilation, but it did block dilation to hypoxia (inhalation of 8.5% O2). In theophylline-treated animals, NMDA responses were completely abolished during hypoxia (28 +/- 2 vs. 2 +/- 1%, respectively to 10(-4) M, n = 7) while sodium nitroprusside (SNP, 10(-4) M) still dilated pial arterioles normally. NMDA-induced vasodilation was not modified after application and removal of adenosine (10(-4) M; n = 5) or SNP (10(-5) M; n = 4), or when SNP (10(-7) M) was coapplied with NMDA (n = 6). Conversely, coapplication of adenosine (10(-6) M) attenuated NMDA responses (31 +/- 5 vs. 20 +/- 3%, n = 7). We also found that NMDA receptor- and bNOS-containing neurons were located predominantly in layers II/III of the cortex. Proximity of these neurons to the cortical surface is consistent with diffusion of NO to pial arterioles as the mechanism of dilation to NMDA. We conclude that NMDA-induced cerebral arteriolar dilation is inhibited by hypoxia alone and by exogenous adenosine, but not by NO.

Cerebral ischemia/reperfusion increases endothelial nitric oxide synthase levels by an indomethacin-sensitive mechanism

In anesthetized piglets, endothelial and neuronal nitric oxide synthase (eNOS and nNOS, respectively) levels were investigated after global cerebral ischemia. Increased intracranial pressure was used to produce 5 or 10 minutes of global ischemia, which was verified visually by observing pial arteriolar blood flow and by a microsphere technique. At 4 to 6 hours of reperfusion, parietal cortex, hippocampus, and cerebellum were collected for immunohistochemical or immunoblot analysis. Immunohistochemical examination localized eNOS only to blood vessels and nNOS only to nonvascular cells, which were primarily neurons in all regions examined. Analysis of immunoblot data revealed significant increases in eNOS levels from 47 +/- 22 pixels/micrograms protein for time controls to 77 +/- 36 pixels/micrograms protein (75% increase) for ischemia in parietal cortex (n = 9 to 10) and 22 +/- 10 for control to 40 +/- 16 pixels/micrograms protein (40% increase) for ischemia in hippocampus (n = 7 to 8). Levels of eNOS in cerebellum also tended to be higher but were variable and not significant (n = 5 to 6). In contrast, changes in nNOS levels were not detected at 4 or 6 hours. The increase in eNOS levels detected on immunoblots also was apparent on tissue sections as an increase in intensity of staining. Cyclooxygenase-dependent mechanisms were investigated with respect to the ischemia-induced increase in eNOS levels. Pretreatment with the cyclooxygenase inhibitor indomethacin (5 mg/kg intravenously) abolished the ischemia-induced eNOS increase in parietal cortex and hippocampus (n = 7). Thus, we conclude that the eNOS response is rapid, specific to vessels, and involves an indomethacin-sensitive mechanism.

Function of cell membranes in cerebral cortical tissue of newborn piglets after hypoxia and inhibition of nitric oxide synthase

Hypoxia-induced brain cell membrane lipid peroxidation can be caused by free radicals that are produced during hypoxia. Recently, the production of nitric oxide (NO), a free radical, has been shown to be increased during cerebral hypoxia-ischemia. The present study tested the hypothesis that inhibition of NO synthase (NOS) reduced hypoxia-induced modifications of Na+,K+-ATPase activity, lipid peroxidation, and [3H]MK-801 binding to the N-methyl-D-aspartate (NMDA) receptor in cerebral cortical tissue of newborn piglets. Studies were performed in 26 newborn piglets. Cerebral NOS was inhibited by the i.v. administration of 25 or 50 mg/kg N(omega)-nitro-L-arginine (NNLA) over 30 min. Control animals received normal saline. Six groups of piglets were thus created (normoxia, no NNLA; normoxia + NNLA 25 mg/kg; normoxia + NNLA 50 mg/kg; hypoxia, no NNLA; hypoxia + NNLA 25 mg/kg; hypoxia + NNLA 50 mg/kg). One hour after the start of NNLA or saline infusion, hypoxia was induced by lowering the FiO2 to 0.07 in the three hypoxia groups, whereas in the three other groups normoxia was maintained. After 60 min of hypoxia, the brain was taken out and frozen. NOS activity, Na+,K+-ATPase activity, conjugated dienes, and [3H]MK-801 binding to the NMDA receptor of cerebral cortical tissue were determined. NOS activity was reduced to 34% of its baseline value with NNLA 25 mg/kg, and to 19-27% of its baseline value with NNLA 50 mg/kg, respectively. Administration of NNLA did neither significantly alter the hypoxia-induced production of conjugated dienes, indicating lipid peroxidation nor the decrease of Na+,K+-ATPase activity after hypoxia. [3H]MK-801 binding studies of the NMDA receptor, however, showed that NNLA preserved Bmax and Kd after hypoxia. We conclude that inhibition of NOS does not change the hypoxia-induced decrease of Na+,K+-ATPase activity and production of conjugated dienes in brain cell membranes. Inhibition of NOS preserved the binding of [3H]MK-801 to the NMDA receptor after hypoxia.

Interrelationships between astrocyte function, oxidative stress and antioxidant status within the central nervous system

Astrocytes have, until recently, been thought of as the passive supporting elements of the central nervous system. However, recent developments suggest that these cells actually play a crucial and vital role in the overall physiology of the brain. Astrocytes selectively express a host of cell membrane and nuclear receptors that are responsive to various neuroactive compounds. In addition, the cell membrane has a number of important transporters for these compounds. Direct evidence for the selective co-expression of neurotransmitters, transporters on both neurons and astrocytes, provides additional evidence for metabolic compartmentation within the central nervous system. Oxidative stress as defined by the excessive production of free radicals can alter dramatically the function of the cell. The free radical nitric oxide has attracted a considerable amount of attention recently, due to its role as a physiological second messenger but also because of its neurotoxic potential when produced in excess. We provide, therefore, an in-depth discussion on how this free radical and its metabolites affect the intra and intercellular physiology of the astrocyte(s) and surrounding neurons. Finally, we look at the ways in which astrocytes can counteract the production of free radicals in general by using their antioxidant pathways. The glutathione antioxidant system will be the focus of attention, since astrocytes have an enormous capacity for, and efficiency built into this particular system.