Intrinsic redox properties of N-methyl-D-aspartate receptor can determine the developmental expression of excitotoxicity in rat cortical neurons in vitro

The Redox Biology of Excitotoxic Processes: The NMDA Receptor, TOPA Quinone, and the Oxidative Liberation of Intracellular Zinc

This special issue of Frontiers in Neuroscience-Neurodegeneration celebrates the 50th anniversary of John Olney's seminal work introducing the concept of excitotoxicity as a mechanism for neuronal cell death. Since that time, fundamental research on the pathophysiological activation of glutamate receptors has played a central role in our understanding of excitotoxic cellular signaling pathways, leading to the discovery of many potential therapeutic targets in the treatment of acute or chronic/progressive neurodegenerative disorders. Importantly, excitotoxic signaling processes have been found repeatedly to be closely intertwined with oxidative cellular cascades. With this in mind, this review looks back at long-standing collaborative efforts by the authors linking cellular redox status and glutamate neurotoxicity, focusing first on the discovery of the redox modulatory site of the N-methyl-D-aspartate (NMDA) receptor, followed by the study of the oxidative conversion of 3,4-dihydroxyphenylalanine (DOPA) to the non-NMDA receptor agonist and neurotoxin 2,4,5-trihydroxyphenylalanine (TOPA) quinone. Finally, we summarize our work linking oxidative injury to the liberation of zinc from intracellular metal binding proteins, leading to the uncovering of a signaling mechanism connecting excitotoxicity with zinc-activated cell death-signaling cascades.

Dual action of L-Lactate on the activity of NR2B-containing NMDA receptors: from potentiation to neuroprotection

L-Lactate is a positive modulator of NMDAR-mediated signaling resulting in plasticity gene induction and memory consolidation. However, L-Lactate is also able to protect neurons against excito-toxic NMDAR activity, an indication of a mitigating action of L-Lactate on NMDA signaling. In this study, we provide experimental evidence that resolves this apparent paradox. Transient co-application of glutamate/glycine (1 μM/100 μM; 2 min) in primary cultures of mouse cortical neurons triggers a NMDA-dependent Ca²⁺ signal positively modulated by L-Lactate (10 mM) or DTT (1 mM) but decreased by Pyruvate (10 mM). This L-Lactate and DTT-induced potentiation is blocked by Ifenprodil (2 μM), a specific blocker of NMDARs containing NR2B sub-units. In contrast, co-application of glutamate/glycine (1 mM/100 μM; 2 min) elicits a NMDAR-dependent excitotoxic death in 49% of neurons. L-Lactate and Pyruvate significantly reduce this rate of cell death processes (respectively to 23% and 9%) while DTT has no effect (54% of neuronal death). This L-Lactate-induced neuroprotection is blocked by carbenoxolone and glibenclamide, respectively blockers of pannexins and K_ATP. In conclusion, our results show that L-Lactate is involved in two distinct and independent pathways defined as NMDAR-mediated potentiation pathway (or NADH pathway) and a neuroprotective pathway (or Pyruvate/ATP pathway), the prevalence of each one depending on the strength of the glutamatergic stimulus.

SNO-ing at the nociceptive synapse?

Nitric oxide is generally considered a pronociceptive retrograde transmitter that, by activation of soluble guanylyl cyclase-mediated cGMP production and activation of cGMP-dependent protein kinase, drives nociceptive hypersensitivity. The duality of its functions, however, is increasingly recognized. This review summarizes nitric-oxide-mediated direct S-nitrosylation of target proteins that may modify nociceptive signaling, including glutamate receptors and G-protein-coupled receptors, transient receptor potential channels, voltage-gated channels, proinflammatory enzymes, transcription factors, and redoxins. S-Nitrosylation events require close proximity of nitric oxide production and target proteins and a permissive redox state in the vicinity. Despite the diversity of potential targets and effects, three major schemes arise that may affect nociceptive signaling: 1) S-Nitrosylation-mediated changes of ion channel gating properties, 2) modulation of membrane fusion and fission, and thereby receptor and channel membrane insertion, and 3) modulation of ubiquitination, and thereby protein degradation or transcriptional activity. In addition, S-Nitrosylation may alter the production of nitric oxide itself.

Essential role of the redox-sensitive kinase p66shc in determining energetic and oxidative status and cell fate in neuronal preconditioning

Ischemic preconditioning is a phenomenon in which low-level stressful stimuli upregulate endogenous defensive programs, resulting in subsequent resistance to otherwise lethal injuries. We previously observed that signal transduction systems typically associated with neurodegeneration such as caspase activation are requisite events for the expression of tolerance and induction of HSP70. In this work, we sought to determine the extent and duration of oxidative and energetic dysfunction as well as the role of effector kinases on metabolic function in preconditioned cells. Using an in vitro neuronal culture model, we observed a robust increase in Raf and p66(Shc) activation within 1 h of preconditioning. Total ATP content decreased by 25% 3 h after preconditioning but returned to baseline by 24 h. Use of a free radical spin trap or p66(shc) inhibitor increased ATP content whereas a Raf inhibitor had no effect. Phosphorylated p66(shc) rapidly relocalized to the mitochondria and in the absence of activated p66(shc), autophagic processing increased. The constitutively expressed chaperone HSC70 relocalized to autophagosomes. Preconditioned cells experience significant total oxidative stress measured by F(2)-isoprostanes and neuronal stress evaluated by F(4)-neuroprostane measurement. Neuroprostane levels were enhanced in the presence of Shc inhibitors. Finally, we found that inhibiting either p66(shc) or Raf blocked neuroprotection afforded by preconditioning as well as upregulation of HSP70, suggesting both kinases are critical for preconditioning but function in fundamentally different ways. This is the first work to demonstrate the essential role of p66(shc) in mediating requisite mitochondrial and energetic compensation after preconditioning and suggests a mechanism by which protein and organelle damage mediated by ROS can increase HSP70.

Neurotoxic lipid peroxidation species formed by ischemic stroke increase injury

Stroke is the third leading cause of death in the United States, yet no neuroprotective agents for treatment are clinically available. There is a pressing need to understand the signaling molecules that mediate ischemic cell death and identify novel neuroprotective targets. Cyclopentenone isoprostanes (IsoPs), formed after free radical-mediated peroxidation of arachidonic acid, are used as markers of stress, but their bioactivity is poorly understood. We have recently shown that 15-A(2t)-IsoP is a potent neurotoxin in vitro and increases the free radical burden in neurons. In this work, we demonstrate that 15-A(2t)-IsoP is abundantly produced in stroke-infarcted human cortical tissue. Using primary neuronal cultures we found that minimally toxic exposure to 15-A(2t)-IsoP does not alter ATP content, but in combination with oxygen glucose deprivation resulted in a significant hyperpolarization of the mitochondrial membrane and dramatically increased neuronal cell death. In the presence of Ca(2+), 15-A(2t)-IsoP led to a rapid induction of the permeability transition pore and release of cytochrome c. Taken with our previous work, these data support a model in which ischemia causes generation of reactive oxygen species, calcium influx, lipid peroxidation, and 15-A(2t)-IsoP formation. These factors combine to enhance opening of the permeability transition pore leading to cell death subsequent to mitochondrial cytochrome c release. These data are the first documentation of significant 15-A(2t)-IsoP formation after acute ischemic stroke and suggest that the addition of 15-A(2t)-IsoP to in vitro models of ischemia may help to more fully recapitulate stroke injury.

Cyclopentenone isoprostanes are novel bioactive products of lipid oxidation which enhance neurodegeneration

Oxidative stress and subsequent lipid peroxidation are involved in the pathogenesis of numerous neurodegenerative conditions, including stroke. Cyclopentenone isoprostanes (IsoPs) are novel electrophilic lipid peroxidation products formed under conditions of oxidative stress via the isoprostane pathway. These cyclopentenone IsoPs are isomeric to highly bioactive cyclopentenone prostaglandins, yet it has not been determined if these products are biologically active or are formed in the brain. Here we demonstrate that the major cyclopentenone IsoP isomer 15-A2t-IsoP potently induces apoptosis in neuronal cultures at submicromolar concentrations. We present a model in which 15-A2t-IsoP induced neuronal apoptosis involves initial depletion of glutathione and enhanced production of reactive oxygen species, followed by 12-lipoxygenase activation and phosphorylation of extracellular signal-regulated kinase 1/2 and the redox sensitive adaptor protein p66shc, which results in caspase-3 cleavage. 15-A2t-IsoP application also dramatically potentiates oxidative glutamate toxicity at concentrations as low as 100 nm, demonstrating the functional importance of these molecules in neurodegeneration. Finally, we employ novel mass spectrometric methods to show that cyclopentenone IsoPs are formed abundantly in brain tissue under conditions of oxidative stress. Together these findings suggest that cyclopentenone IsoPs may contribute to neuronal death caused by oxidative insults, and that their activity should perhaps be addressed when designing neuroprotective therapies.

Mitochondrial nitric oxide mediates decreased vulnerability of hippocampal neurons from immature animals to NMDA

Mitochondrial membrane potential (DeltaPsim)-dependent Ca2+ uptake plays a central role in neurodegeneration after NMDA receptor activation. NMDA-induced DeltaPsim dissipation increases during postnatal development, coincident with increasing vulnerability to NMDA. NMDA receptor activation also produces nitric oxide (NO), which can inhibit mitochondrial respiration, dissipating DeltaPsim. Because DeltaPsim dissipation reduces mitochondrial Ca2+ uptake, we hypothesized that NO mediates the NMDA-induced DeltaPsim dissipation in immature neurons, underlying their decreased vulnerability to excitotoxicity. Using hippocampal neurons cultured from 5- and 19-d-old rats, we measured NMDA-induced changes in [Ca2+]cytosol, DeltaPsim, NO, and [Ca2+]mito. In postnatal day 5 (P5) neurons, NMDA mildly dissipated DeltaPsim in a NO synthase (NOS)-dependent manner and increased NO. The NMDA-induced NO increase was abolished with carbonyl cyanide 4-(trifluoromethoxy)phenyl-hydrazone and regulated by [Ca2+]mito. Mitochondrial Ca2+ uptake inhibition prevented the NO increase, whereas inhibition of mitochondrial Ca2+ extrusion increased it. Consistent with this mitochondrial regulation, NOS and cytochrome oxidase immunoreactivity demonstrated mitochondrial localization of NOS. Furthermore, NOS blockade increased mitochondrial Ca2+ uptake during NMDA. Finally, at physiologic O2 tensions (3% O2), NMDA had little effect on survival of P5 neurons, but NOS blockade during NMDA markedly worsened survival, demonstrating marked neuroprotection by mitochondrial NO. In P19 neurons, NMDA dissipated DeltaPsim in an NO-insensitive manner. NMDA-induced NO production was not regulated by DeltaPsim, and NOS immunoreactivity was cytosolic, without mitochondrial localization. NOS blockade also protected P19 neurons from NMDA. These data demonstrate that mitochondrial NOS mediates much of the decreased vulnerability to NMDA in immature hippocampal neurons and that cytosolic NOS contributes to NMDA toxicity in mature neurons.

Effect of NMDA on staurosporine-induced activation of caspase-3 and LDH release in mouse neocortical and hippocampal cells

To achieve a better understanding of developmentally regulated NMDA- and staurosporine-induced apoptotic processes, we investigated the concerted action of these agents on caspase-3 activity and LDH release in neocortical and hippocampal cell cultures at different stages in vitro (DIV). Hoechst 33342 and MAP-2 stainings were additionally employed to visualize apoptotic changes and cell damage. The vulnerability of neocortical cells to NMDA was more prominent at later culture stages, whereas hippocampal neurons were more susceptible to NMDA treatment at earlier stages. A persistent activation of caspase-3 by staurosporine was found at all experimental stages. Despite of certain differences in susceptibility to NMDA and staurosporine, both tissues responded to regulatory action of NMDA towards staurosporine-activated caspase-3 in a similar way. Combined treatment with NMDA and staurosporine resulted in a substantial increase in caspase-3 activity in neocortical and hippocampal neurons on 2 DIV. Additive effects were also observed in neocortical cultures on 12 DIV. In contrast, NMDA substantially inhibited staurosporine-induced caspase-3 activity on 7 DIV in neocortical and hippocampal cultures. Additionally, pro-apoptotic effects of 17beta-estradiol were attenuated by NMDA on 7 DIV. Changes in vulnerability to NMDA- and staurosporine-mediated activation of caspase-3 were not strictly related to LDH release. Our data revealed that NMDA can both enhance and inhibit the staurosporine-induced neuronal cell apoptosis. The pro-apoptotic effect of NMDA was exhibited at early and late culture stages, whereas the anti-apoptotic effect was transient occurring on 7 DIV only.

Sequential involvement of distinct glutamate receptors in domoic acid-induced neurotoxicity in rat mixed cortical cultures: effect of multiple dose/duration paradigms, chronological age, and repeated exposure

The increasing occurrence of poisoning accidents in marine animals caused by the amnesic shellfish toxin, domoic acid (DOM), necessitates a better understanding of the factors contributing to DOM neurotoxicity. Here we evaluated the contribution and temporal involvement of NMDA, non-NMDA- and metabotropic-type glutamate receptors (GluRs) in DOM-induced neuronal death using rat primary mixed cortical cultures. Co-application of antagonists for AMPA/kainate- (NBQX) and NMDA-type GluRs (D-AP5) but not for metabotropic GluRs reduced DOM toxicity induced by either of three EC50 dose/duration exposure paradigms. Maximal protection offered by D-AP5 and NBQX either extended or not to the 30- to 60-min period after DOM exposure, respectively. Antagonists were ineffective if applied with a 2-h delay, indicating the presence of a critical time window for neuronal protection after DOM exposure. Early effects correlated with neuronal swelling was seen as early as 10 min post-DOM, which has been linked to non-NMDAR-mediated depolarization and release of endogenous glutamate. That DOM toxicity is dictated by iGluRs is supported by the finding that increased efficacy and potency of DOM with in vitro neuronal maturation are positively correlated with elevated protein levels of iGluR subunits, including NR1, GluR1, GluR2/3, GluR5, and GluR6/7. We determined the time course of DOM excitotoxicity. At >10 microM maximal neuronal death occurs within 2 h, while doses < or = 10 microM continue to produce death during the subsequent 22-h washout period, indicating a quicker progression of the neuronal death cascade with high DOM concentrations. Accordingly, NBQX applied 30 min post-DOM afforded better protection against low dose/prolonged duration (3 microM/24 h) than against high dose/brief duration exposure (50 microM/10 min). Interestingly, prior exposure to subthreshold DOM dose-dependently aggravated toxicity produced by a subsequent exposure to DOM. These findings provide greater insight into the complex properties underlying DOM toxicity, including the sequential involvement of multiple GluRs, greater potency with increasing neuronal maturation and protein levels of iGluRs, varying efficacy depending on dose, duration, and prior history of DOM exposure.

Neonatal Toluene Exposure Alters Agonist and Antagonist Sensitivity and NR2B Subunit Expression of NMDA Receptors in Cultured Cerebellar Granule Neurons

Toluene has been reported to antagonize the function of N-methyl-D-aspartate (NMDA) receptors. In this study, the effects of neonatal toluene exposure on NMDA receptors in primarily cultured cerebellar granule neurons were examined. Sprague-Dawley rats were treated with toluene (0, 200, 500, and 1000 mg/kg, i.p.) from postnatal day (PN) 4 to PN 7. Under toluene-free conditions, Ca2+ signals of cultured neurons in response to glutamate and NMDA were measured for up to 14 days. The expression of NMDA receptor subunits (NR1, NR2A, and NR2B) at 5-14 days in vitro (DIV) were also determined. Neonatal toluene exposure dose-dependently reduced intracellular Ca2+ signals in response to glutamate/glycine and NMDA/glycine in cultured cerebellar granule neurons, and these effects were gradually decreased with time. Such toluene exposure did not influence the inhibition of Mg2+ or MK801 on NMDA-evoked responses, but it decreased the potency of ifenprodil (an NR2B preferring antagonist). The protein levels of NMDA receptor subunit NR2B were consistently reduced by toluene exposure at 5 DIV, but not at 14 DIV. These results demonstrate that neonatal toluene exposure induces long-term but reversible changes in the function and composition of NMDA receptors. Such changes during developmental stages may contribute to the cerebellar dysfunction observed in fetal solvent syndrome.

In, out, shake it all about: elevation of [Ca2+]i during acute cerebral ischaemia

Because of the extensive second messenger role played by calcium, free intracellular calcium levels are strictly regulated. Under normal physiological conditions, this is achieved through a combination of restricted calcium entry, efficient efflux and restricted intracellular mobility. Overall, the process of regulating free calcium is dependent on ATP derived from oxidative metabolism. Under conditions of cerebral ischaemia, ATP levels fall rapidly and calcium homeostasis becomes significantly disturbed resulting in the initiation of calcium-dependent neurodegenerative processes. In this review, the mechanisms underlying physiological calcium homeostasis and the links between calcium disregulation and neurodegeneration will be discussed.

Redox regulation of neuronal migration in a Down Syndrome model

Down Syndrome (DS), one of the major genetic causes of mental retardation, is characterized by disrupted corticogenesis produced, in part, by an abnormal layering of neurons in cortical laminas II and III. Because defects in the normal migration of neurons during corticogenesis can result in delayed cortical radial expansion and abnormalities in cortical layering, we have examined neuronal migration in murine trisomy 16 (Ts16), a mouse model for DS. Using an in vitro assay for chemotaxis, our data demonstrate that the number of acutely dissociated Ts16 cortical neurons migrating in response to glutamate or N-methyl-D-aspartate (NMDA), known chemotactic factors, was decreased compared to normal littermates, suggesting a defect in NMDA receptor- (NMDAR-) mediated events. Ts16 neurons did not lack NMDAR since expression of mRNA and protein for NMDAR subunits was observed in Ts16 cells. However, the number of cells that generated an observable current in response to NMDA was decreased compared to normal littermates. Similar to DS, Ts16 CNS demonstrated an inherent oxidative stress likely caused by the triplication of genes such as SOD1. To determine if the abnormal redox state was a factor in the failure of NMDAR-mediated migration in Ts16, we treated Ts16 neurons with either n-acetyl cysteine (NAC) or dithiothrietol (DTT), known antioxidants. The reduction in NMDAR-mediated migration observed in Ts16 neurons was returned to normal littermate values by NAC or DTT. Our data indicate that oxidative stress may play a key role in the abnormal glutamate-mediated responses during cortical development in the Ts16 mouse and may have an impact on neuronal migration at critical stages.

The neuroprotective agent ebselen modifies NMDA receptor function via the redox modulatory site

Ebselen is a seleno-organic compound currently in clinical trials for the treatment of ischemic stroke and subarachnoid hemorrhage. Its putative mode of action as a neuroprotectant is via cyclical reduction and oxidation reactions, in a manner akin to glutathione peroxidase. For this reason, we have investigated the effects of ebselen on the redox-sensitive NMDA receptor. We have found that ebselen readily reversed dithiothreitol (DTT) potentiation of NMDA-mediated currents in cultured neurons and in Chinese hamster ovary (CHO) cells expressing wild-type NMDA NR1/NR2B receptors. In contrast, ebselen was unable to modulate NMDA-induced currents in neurons previously exposed to the thiol oxidant 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), or in CHO cells expressing a mutant receptor lacking the NR1 redox modulatory site, suggesting that ebselen oxidizes the NMDA receptor via this site. In addition, ebselen was substantially less effective in modifying NMDA responses in neurons exposed to alkylating agent N-ethylmaleimide (NEM) following DTT treatment. Ebselen also reversed DTT block of carbachol-mediated currents in Cos-7 cells expressing the alpha(2)beta delta epsilon subunits of the acetylcholine receptor, an additional redox-sensitive ion channel. Ebselen was observed to significantly increase cell viability following a 30-min NMDA exposure in cultured neurons. In contrast, other more typical antioxidant compounds did not afford neuroprotection in a similar paradigm. We conclude that ebselen may be neuroprotective in part due to its actions as a modulator of the NMDA receptor redox modulatory site.

Reduction of functional N-methyl-D-aspartate receptors in neurons by RNase P-mediated cleavage of the NR1 mRNA

One approach to studying the functional role of individual NMDA receptor subunits involves the reduction in the abundance of the protein subunit in neurons. We have pursued a strategy to achieve this goal that involves the use of a small guide RNA which can lead to the destruction of the mRNA for a specific receptor subunit. We designed a small RNA molecule, termed 'external guide sequence' (EGS), which binds to the NR1 mRNA and directs the endonuclease RNase P to cleave the target message. This EGS has exquisite specificity and directed the RNase P-dependent cleavage at the targeted location within the NR1 mRNA. To improve the efficiency of this EGS, an in vitro evolution strategy was employed which led to a second generation EGS that was 10 times more potent than the parent molecule. We constructed an expression cassette by flanking the EGS with self-cleaving ribozymes and this permitted generation of the specified EGS RNA sequence from any promoter. Using a recombinant Herpes simplex virus (HSV), we expressed the EGS in neurons and showed the potency of the EGS to reduce NR1 protein within neurons. In an excitotoxicity assay, we showed that expression of the EGS in cortical neurons is neuroprotective. Our results demonstrate the utility of EGSs to reduce the expression of any gene (and potentially any splice variant) in neurons.

NMDA and glutamate evoke excitotoxicity at distinct cellular locations in rat cortical neurons in vitro

The development of cortical neurons in vivo and in vitro is accompanied by alterations in NMDA receptor subunit expression and concomitant modifications in the pharmacological profile of NMDA-activated ionic currents. For example, we observed that with decreasing NR2B/NR2A subunit expression ratio, the block of NMDA receptor-mediated whole-cell responses by the NR2B-selective antagonist haloperidol was also decreased. In mature cultures (>22 d in vitro), however, NMDA responses obtained from excised nucleated macropatches, which comprised a large portion of the soma, remained strongly antagonized by haloperidol. These results suggest that in more mature neurons NR1/NR2B receptors appear to be preferentially expressed in the cell body. As predicted from the whole-cell recording pharmacological profile, NMDA-induced toxicity was largely unaffected by haloperidol in mature cultures. However, haloperidol effectively blocked glutamate toxicity in the same cultures, suggesting that the neurotoxic actions of this amino acid were mostly due to the activation of somatic NMDA receptors. In experiments in which the potency of glutamate toxicity was increased by the transport inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid, the neuroprotective effects of haloperidol were significantly diminished. This was likely because of the fact that glutamate, now toxic at much lower concentrations, was able to reach and activate dendritic receptors under these conditions. These results strongly argue that exogenous glutamate and NMDA normally induce excitotoxicity at distinct cellular locations in mature mixed neuronal cultures and that NR1/NR2B receptors remain an important component in the expression of glutamate, but not NMDA-induced excitotoxicity.

Synaptic plasticity is impaired in rats with a low glutathione content

Long-term potentiation (LTP) is a sustained increase in the efficacy of synaptic transmission, based on functional changes involving pre- and postsynaptic mechanisms, and has been considered a cellular model for learning and memory. The sulphurated tripeptide glutathione acts as a powerful antioxidant agent within the nervous system. Recent in vitro studies suggest that the cellular redox status might influence the mechanisms involved in synaptic plasticity. It is not known, however, how glutathione depletion might affect LTP. In the present study, we evaluated the input-output relationships, LTP, and paired-pulse interactions in rats with low glutathione levels induced by systemic injection of diethylmaleate. Our results in anesthetized rats show that the basic synaptic transmission between the perforant pathway and the dentate gyrus granule cells was not affected by glutathione depletion. However, in the same synapses it was not possible to induce prolonged changes in synaptic efficacy (LTP). Paired-pulse facilitation was also absent in the treated animals, suggesting an impairment of short-term synaptic interactions. These findings indicate that low content of glutathione can impair short-term and long-term mechanisms of synaptic plasticity and stress the importance of the redox balance in the normal function of brain circuitry.

Reducing conditions significantly attenuate the neuroprotective efficacy of competitive, but not other NMDA receptor antagonists in vitro

Inappropriate activation of NMDA receptors during a period of cerebral ischaemia is a crucial event in the pathway leading to neuronal degeneration. However, significant research has failed to deliver a clinically active NMDA receptor antagonist, and competitive NMDA antagonists are ineffective in many experimental models of ischaemia. The NMDA receptor itself has a number of modulatory sites which may affect receptor function under ischaemic conditions. Using rat organotypic hippocampal slice cultures we have investigated whether the redox modulatory site affects the neuroprotective efficacy of NMDA receptor antagonists against excitotoxicity and experimental ischaemia (OGD). NMDA toxicity was significantly enhanced in cultures pretreated with a reducing agent. The noncompetitive antagonist MK-801 and a glycine-site blocker were equally neuroprotective in both normal and reduced conditions, but there was a significant rightward shift in the dose-response curves of the competitive antagonists APV and CPP and the uncompetitive antagonist memantine. OGD produced neuronal damage predominantly in the CA1 region, which was prevented by MK-801 and memantine, but not by APV or CPP. Inclusion of an oxidizing agent during the period of OGD had no effect alone, but significantly enhanced the neuroprotective potency of the competitive antagonists. These data clearly demonstrate that chemical reduction of the redox modulatory site of the NMDA receptor decreases the ability of competitive antagonists to block NMDA receptor-mediated neuronal damage, and that the reducing conditions which occur during simulated ischaemia are sufficient to produce a similar effect. This may have important implications for the design of future neuroprotective agents.

Cyanide interaction with redox modulatory sites enhances NMDA receptor responses

Activation of NMDA receptors plays an important role in cyanide neurotoxicity. Cyanide indirectly activates the receptor by inducing neuronal release of glutamate and also enhances receptor-mediated responses by a direct interaction with the receptor complex. This study investigated the mechanism in cerebellar granule cells by which cyanide enhances NMDA-induced Ca2+ influx. Cyanide (50 microM) increased the influx of Ca2+ over the NMDA concentration range of 0.5-500 microM. Experiments showed that cyanide does not interact with the receptor's glycine or PKC mediated phosphorylation regulatory sites. N-ethylmaleimide, a thiol alkylating agent which inactivates the redox regulatory sites of the receptor, blocked the enhancing effect of cyanide. Pretreatment of cells with 5,5-dithio-bis-2-nitrobenzoic acid (DTNB), a compound that oxidizes the receptor redox sites, had no effect on the response to cyanide. On the other hand, the nonpermeant reducing agents, dithiothreitol or cysteine, further increased the cyanide effect. These observations can be explained by cyanide interacting with redox sensitive disulfide groups that are not accessible to the non-permeant reducing agents. It is proposed that cyanide interacts with a redox site(s) located either on the intracellular receptor domain or in the transmembrane hydrophobic domain. Furthermore the enhancement by cyanide of the excitotoxic actions of NMDA involves receptor sites that are sensitive to oxidation/reduction and this interaction contributes to the neurotoxic action of cyanide.

Chronic brain oxidation in a glutathione peroxidase knockout mouse model results in increased resistance to induced epileptic seizures

Systemic administration of kainic acid (KA) to rodents results in limbic seizures and subsequent neurodegeneration similar to that observed in certain types of human epilepsy, and it is a commonly used animal model for this disease. Oxidative stress has been suggested to play a role in the neuronal injury associated with KA administration. Based on this observation, chronic treatment with antioxidants has been proposed as a possible protective therapy against neuronal damage associated with epileptic seizures. Here we demonstrate by histochemical, electrophysiological, and biochemical means that knockout mice with decreased activity of the protective antioxidant enzyme glutathione peroxidase, which display elevated basal brain oxidative stress levels, are resistant to KA-induced seizure activity and neurodegeneration. This appears to be a result of decreased NMDA receptor function due to oxidation of its NR1 subunit. This suggests that the chronic use of antioxidants as antiepileptic agents to modulate NMDA-dependent seizure-induced neurodegeneration may be detrimental rather than protective and calls into question their use as a therapeutic agent in the treatment of epilepsy.

Lack of interaction between nitric oxide and the redox modulatory site of the NMDA receptor

1. The inhibitory effects of nitric oxide (NO) on N-methyl-D-aspartate (NMDA) receptor function have been proposed to be mediated via the interaction of this gas with a redox-sensitive thiol moiety on the receptor. Here, we evaluated this suggested mechanism by examining the actions of various NO donors on native neuronal receptors as well as in wild-type and cysteine-mutated recombinant NMDA receptors expressed in Chinese hamster ovary (CHO) cells. 2. The NO donor N-ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydraxino)ethanamine (NOC-12; 100 microM) produced a rapid and readily reversible inhibition of whole-cell currents induced by NMDA (30 microM) in cultured cortical neurons. The inhibition was apparent at all holding potentials, though a more pronounced block was observed at negative voltages. The effects of NOC-12 disappeared when the donor was allowed to expire. A similar receptor block was observed with another NO-releasing agent, S-nitroso-N-acetylpenicillamine (SNAP; 1 mM). 3. The blocking effects of NO released by SNAP, 3-morpholinosydnonimine (SIN-1; 1 mM), and 3-[2-hydroxy-1-(1-methylethyl)-2-nitrosohydrazino]-1-propanamin e (NOC-5; 100 microM) on currents mediated by recombinant NRI/NR2B receptors were virtually indistinguishable from those observed on native receptors. Furthermore, mutating cysteines 744 and 798 of NR1, which constitute the principal redox modulatory site of the NR1/NR2B receptor configuration, did not affect the inhibition produced by NO. 4. The NR2A subunit may contribute its own redox-sensitive site. However, the effects of NO on NR1/NR2A receptors were very similar to those seen for all other receptor configurations evaluated. Hence, we conclude that NO does not exert its inhibition of NMDA-induced responses via a modification of any of the previously described redox-sensitive sites on the receptor.

Subunit-specific interactions of cyanide with the N-methyl-D-aspartate receptor

Cyanide can potentiate N-methyl-D-aspartate receptor-mediated physiological responses in neurons. Here we show that this phenomenon may be attributable to a subunit-specific chemical modification of the receptor directly by the toxin. N-Methyl-D-aspartate (30 microM)-induced whole cell responses in mature (22-29 days in vitro) rat cortical neurons were potentiated nearly 2-fold by a 3-5-min treatment with 2 mM potassium cyanide, as did a similar treatment with 4 mM dithiothreitol. A 1-min incubation with the thiol oxidant 5,5'-dithiobis(2-nitrobenzoic acid) (0.5 mM) readily reversed the potentiation induced by either cyanide or dithiothreitol. Cyanide did not increase further currents previously potentiated by dithiothreitol nor was it able to potentiate responses during brief co-application with the agonist. Transient expression studies in Chinese hamster ovary cells with wild-type and mutated recombinant N-methyl-D-aspartate subunits (NR) demonstrated that cyanide selectively potentiated NR1/NR2A receptors, presumably via the chemical reduction of NR2A. In contrast, currents mediated by NR1/NR2B receptors were somewhat diminished by the metabolic inhibitor. Some of the effects of cyanide on NR1/NR2B receptors may be mediated by the formation of a thiocyanate adduct with a cysteine residue located in NR1. Cyanide thus is able to distinguish chemically between two different N-methyl-D-aspartate receptor subtypes and produce diametrically opposing functional effects.

Dihydrokainate-sensitive neuronal glutamate transport is required for protection of rat cortical neurons in culture against synaptically released glutamate

Glutamate transport in nearly pure rat cortical neurons in culture (less than 0.2% astrocytes) is potently inhibited by dihydrokainate, l-serine-O-sulphate, but not by l-alpha-amino-adipate. This system allows for a test of the hypothesis that glutamate transport is important for protecting neurons against the toxicity of endogenous synaptically released glutamate. In support of this hypothesis, a 20-24 h exposure to 1 mm dihydrokainate reduced cell survival to only 14.8 +/- 9.8% in neuronal cultures (P < 0.001; n = 3), although it had no effect on neuronal survival in astrocyte-rich cultures (P > 0.05; n = 3). Dihydrokainate also significantly caused accumulation of glutamate in the extracellular medium of cortical neuronal cultures (6.6 +/- 4.9 micrometer, compared to 1.2 +/- 0.3 micrometer in control, n = 14, P < 0.01). The neurotoxicity of dihydrokainate was blocked by 10 micrometer MK-801, 10 micrometer tetrodotoxin, and an enzyme system that degrades extracellular glutamate. The latter two also abolished the accumulation of glutamate in the extracellular medium. Dihydrokainate (1 mm) inhibited the 45calcium uptake stimulated by 30 micrometer N-methyl-d-aspartate (NMDA), but not by higher concentrations consistent with a weak antagonist action of dihydrokainate at the NMDA receptor. Whole cell recordings showed that 1 mm dihydrokainate produced approximately 25% inhibition of 30 micrometer NMDA-induced current in cortical neurons. Dihydrokainate (1 mm) alone generated a small current (17% of the current produced by 30 micrometer NMDA) that was blocked by 30 micrometer 5,7-dichlorokynurenate and only weakly by 10 micrometer cyano-7-nitroquinoxaline-2,3-dione (CNQX). These results suggest that the toxicity of dihydrokainate in neuronal cultures is due to its ability to block glutamate transport in these cultures, and that dihydrokainate-sensitive neuronal glutamate transport may be important in protecting neurons against the toxicity of synaptically released glutamate.

Redox signaling and the emerging therapeutic potential of thiol antioxidants

Oxidation-reduction (redox) based regulation of signal transduction and gene expression is emerging as a fundamental regulatory mechanism in cell biology. Electron flow through side chain functional CH2-SH groups of conserved cysteinyl residues in proteins account for their redox-sensing properties. Because in most intracellular proteins thiol groups are strongly "buffered" against oxidation by the highly reduced environment inside the cell, only accessible protein thiol groups with high thiol-disulfide oxidation potentials are likely to be redox sensitive. The list of redox-sensitive signal transduction pathways is steadily growing, and current information suggests that manipulation of the cell redox state may prove to be an important strategy for the management of AIDS and some forms of cancer. The endogenous thioredoxin and glutathione systems are of central importance in redox signaling. Among the thiol agents tested for their efficacy to modulate cellular redox status, N-acetyl-L-cysteine (NAC) and alpha-lipoic acid hold promise for clinical use. A unique advantage of lipoate is that it is able to utilize cellular reducing equivalents, and thus it harnesses the metabolic power of the cell to continuously regenerate its reductive vicinal dithiol form. Because lipoate can be readily recycled in the cell, it has an advantage over N-acetyl-L-cysteine on a concentration:effect basis. Our current knowledge of redox regulated signal transduction has led to the unfolding of the remarkable therapeutic potential of cellular thiol modulating agents.

Effects of pyrroloquinoline quinone on glutamate-induced production of reactive oxygen species in neurons

Pyrroloquinoline quinone may act as a free radical scavenger and also as a modulator of the NMDA receptor associated redox modulatory site. Using the oxidation sensitive dye dihydroethidium, we examined the effects of pyrroloquinoline quinone on free radical production in cultured forebrain neurons following glutamate receptor activation. Both glutamate (100 microM) and hydrogen peroxide (30 mM) produced a rapid increase in dihydroethidium fluorescence indicating dye oxidation. Pyrroloquinoline quinone (5-200 microM) effectively inhibited dihydroethidium fluorescence induced by glutamate but not by hydrogen peroxide. Glutamate-induced dihydroethidium fluorescence was inhibited by the thiol oxidant 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB). Pyrroloquinoline quinone (50 microM) inhibited glutamate responses in control and in dithiothreitol treated neurons. However, pyrroloquinoline quinone did not further decrease the response to glutamate in DTNB treated neurons. These results suggest that pyrroloquinoline quinone inhibits the free radical-generating response to glutamate by oxidizing the NMDA receptor redox site and not by scavenging reactive oxygen species.