AR-R15896AR blocks the early NMDA-induced loss of MAP2 in primary cortical cultures

Abstract

The low-affinity use-dependent N-methyl-D-aspartate (NMDA) receptor antagonist AR-R15896AR is neuroprotective in primary rat cortical cultures exposed to toxic concentrations of NMDA and reduces the magnitude of NMDA-triggered increases in [Ca²⁺],. Here we show using fluorescence staining and measurements of microtubule-associated protein-2 (MAP2) levels, that AR-R15896AR inhibits the NMDA- induced loss of MAP2 that occurs within 2 min following NMDA exposure. Understanding the multiple, Ca²⁺-triggered intracellular events that occur following NMDA receptor stimulation is important to the development of safe and effective neuroprotective agents. [Neurol Res 1999; 21: 524–528]     

AR-R15896AR, a low affinity, use-dependent, NMDA antagonist, is protective in a primate model of stroke

The low affinity, use-dependent, N-methyl- -aspartate (NMDA) antagonist, AR-R15896AR, is neuroprotective against transient focal cerebral ischemia in rats. We have examined the effect of AR-R15896AR, administered at a plasma level that is tolerated in acute stroke patients, on both functional and histopathologic measures in marmoset monkeys with permanent middle cerebral artery (MCA) occlusion. The M1 segment of the right MCA was permanently occluded (pMCAO) by bipolar coagulation in 11 marmosets. Five minutes later, the monkeys received either saline or AR-R15896 (4.5 mg/kg) intravenously, infused over 30 seconds. Also, osmotic minipumps were implanted subcutaneously to provide continuous drug or saline infusion for 48 hours. Drug-filled pumps released AR-R15896 at a rate of 1.1 mg/kg/h. The monkeys had been trained and tested preoperatively on a number of behavioral tasks and were retested 3 and 10 weeks after surgery. Three weeks after surgery, all the monkeys had a severe motor deficit, in that they were impaired at reaching with their contralesional arms, and perceptual neglect of contralesional space. AR-R15896AR-treated monkeys, however, had significantly less neglect than saline-treated monkeys, although the AR-R15896AR treatment had no effect on the motor deficit. By 10 weeks, the neglect had recovered, but not the motor deficit, and there were no differences between the 2 groups. Histopathologic analysis showed a reduction in the size of the infarction at several stereotaxic levels of the AR-R15896AR-treated monkeys. This study has shown that AR-R15896AR protected a select region of the brain against permanent focal cerebral ischemia in a primate species and attenuated the ensuing spatial neglect.     

Treatment of acute ischaemic stroke with the low-affinity,use-dependent NMDA antagonist AR-R15896AR. A safety and tolerability study

BACKGROUND AND PURPOSE: A low-affinity, use-dependent N-Methyl-D-Aspartate (NMDA) antagonist AR-R15896AR has neuroprotective properties in animal models of ischaemic stroke. The aim of the present study was to examine the safety and tolerability of a new and higher dosage regimen that would enable acute stroke patients to achieve and maintain neuroprotective plasma concentrations. METHODS: A randomised, multi-centre, double-blind, placebo-controlled, parallel group study was carried out at 19 centres in France, Germany and the Netherlands in patients with a clinical diagnosis of acute ischaemic stroke, and onset of symptoms within 12 hours before start of study drug administration. Two loading doses of 3.5 mg/kg of AR-R15896AR over 60 minutes, followed by a 2.5 mg/kg infusion over the next 120 minutes were given. Eight hours after the start of the loading dose infusion, the first maintenance dose (120 mg) was administered over 60 minutes. Eight further maintenance infusions were administered at intervals of 8 hours over a total treatment period of 3 days. Main variables were safety, tolerability and pharmacokinetics. Follow-up assessments also included the Barthel Index (BI) and the NIH Stroke Scale (NIHSS) at 4-7 days after the end of the last infusion and at 30 days after the onset of stroke. RESULTS: 103 patients with acute ischaemic stroke were randomised to either treatment with AR-R15896AR (70 patients) or placebo (33 patients). Mortality was not significantly different in the AR-R15896AR group compared with the placebo group (10 % vs. 6 %). Serious adverse events during treatment due to psychiatric conditions were associated with AR-R15896AR (3 vs. 0). Other side effects were more common in the group treated with AR-R15896AR: vomiting (29 % vs. 9 %), nausea (23 % vs. 12 %), fever (17 % vs. 12 %), agitation (7 % vs. 3 %), dizziness (7 % vs. 0 %), and hallucinations (6 % vs. 0 %). No significant difference between the two groups (with respect to the proportions of patients with favourable outcome) was detected in either the analysis of the BI or the NIHSS. Pharmacokinetic data showed that plasma concentrations of AR-R15896AR were in the expected neuroprotective range. CONCLUSION: In most of the patients with acute stroke receiving AR-R15896AR the intended high plasma levels were reached within a short time period. However, active treatment produced more side effects than placebo, thus indicating safety concerns and tolerability issues for use in high doses in an acute stroke population.     

Comparison of the neuroprotective effect of clomethiazole,AR-R15896AR and NXY-059 in a primate model of stroke using histological and behavioural measures

Three experimental neuroprotective agents (clomethiazole, AR-R15896AR and NXY-059) have recently been tested in a primate model of acute ischaemic stroke. As the experimental techniques used in all three studies were similar and the compounds were administered at clinically relevant doses, a comparative analysis of the functional benefits of these drug-treatments has now been performed. Furthermore a more detailed histological analysis of the neuroprotection afforded by the drugs has also been made. NXY-059 produced almost twice the degree of neuroprotection than that seen following clomethiazole or AR-R15896AR. Protection by NXY-059 was seen in measurements of damage to cortex and white matter. Clomethiazole and AR-R15896AR provided less protection of cortex and white matter than NXY-059. Conspicuously, AR-R15896AR was without effect in sub-cortical regions. NXY-059 was the only compound to produce a major, statistically significant improvement in the motor deficit induced by the stroke. All three drugs also reduced the degree of spatial neglect 3 weeks after pMCAO, and 10 weeks later only NXY-059 still provided significant additional functional benefit to the spontaneous improvement seen in stroked control animals not receiving treatment. The overview of the behavioural effects and these new histological findings suggest that NXY-059 was by far the most effective neuroprotective agent of the three examined.     

Efficacy of AR-R15896AR in the rat monofilament model of transient middle cerebral artery occlusion

The monofilament technique of transient middle cerebral artery occlusion (MCAO) was used in 3 separate studies to evaluate the efficacy of the low-affinity, use-dependent N-methyl-d-aspartate receptor antagonist, AR-R15896AR. First, a dose-response curve was attempted. Wister Kyoto rats received 2 hours of MCAO. Five minutes later, a 30-minute intravenous infusion of AR-R15896AR was given, followed by subcutaneous implantation of Alzet minipumps that were calibrated to maintain specified plasma levels (approximately 682, 1885, or 2682 ng/mL) of AR-R15896 (free base) for 1 week. The highest plasma level attained significantly decreased the percentage of damage to the subcortex, cortex, and total brain. Second, the high-dose, 1-week treatment regimen was repeated to determine if neuroprotection would extend to 8 weeks after MCAO. Indeed, in separate groups of animals, significant reduction in the percentage of damage, which was generally confined to the cortex and subcortex, was observed at 1, 2, 4, and 8 weeks. Third, verification was achieved in another laboratory. Lister Hooded rats received 60 minutes of transient MCAO. At 70 minutes, an acute dose of AR-R15896AR (20.3 mg/kg) was injected intraperitoneally and the rats were killed 23 hours later. This treatment group also exhibited significant reduction in the volume of infarction in the subcortex, cortex, and total brain. The outcome of these investigations supports the ongoing Phase II clinical trials in patients with acute stroke.     

Neuroprotective potential of ionotropic glutamate receptor antagonists

From the therapeutic point of view, the real challenge is not only to improve the symptoms, but to interfere with the pathomechanism of the disease. That is why a considerable interest has recently been devoted to developing glutamate receptors antagonists (mainly of the NMDA type) for acute and chronic neurodegeneration. Developing such a treatment that slows down the progression of the disease is extremely time and costs consuming. At present there is consensus that competitive NMDA receptor antagonists will not find therapeutic applications, in contrast to agents acting at the glycine_B site, or channel blockers. Recently, at least seven glycine_B antagonists (e.g. ACEA 1021, GV-150526, GV-196771A, ZD-9379, MRZ 2/576) and over 10 NMDA channel blockers (e.g. Remacemide, ARL-15896AR, HU-211, ADCI, CNS-5161, Neramexane-MRZ 2/579) have been under development, most of them as neuroprotective agents for acute (stroke, trauma) or chronic insult (e.g. Huntington’s or Alzheimer’s disease). Several substances selective for NR2B NMDA receptor subtypes such as: eliprodil, CP-101606 and Ro-25-6981 have been claimed to have a good neuroprotective profile. This presentation is an attempt to critically review preclinical and scarce clinical experience in the development of new NMDA receptor antagonists as neuroprotective agents according to the following scheme: rational, preclinical findings in animal models and finally clinical experience if available. The general impression is that NMDA receptor antagonists may find use in chronic type of neurodegeneration while AMPA antagonists seems to show better promise in acute insult.     

Lack of evidence for direct involvement of NMDA receptors or polyamines in blood–brain barrier injury after cerebral ischemia in rats

It is hypothesized that after various types of brain injury, blood–brain barrier (BBB) opening and vasogenic edema result from excessive neuronal release of glutamate and stimulation of capillary N-methyl-d-aspartate (NMDA) receptors linked to polyamine (putrescine) synthesis in endothelial cells. We produced cerebral ischemia in rats and measured BBB opening 6 h later as the increase in regional transfer constants (K_i) for blood to brain diffusion of []sucrose. Such BBB opening was not mitigated by drugs which block NMDA receptors (MK801 or AR-R 15896AR) or polyamine synthesis (difluoromethylornithine). These results question generality of the capillary NMDA receptor/polyamine hypothesis.     

Lippincott-Raven Publishers, Philadelphia © International League Against Epilepsy Remacemide Hydrochloride and ARL 15896AR Lack Abuse Potential: Additional Differences from Other Uncompetitive NMDA Antagonists

Summary: This study was designed to determine the possible abuse liability and phencyclidine-like effects of the low-affinity uncompetitive N-methyla-aspartate (NMDA) antagonists remacemide hydrochloride [(+min)-2–amino-N-(1 -methyl-1,2–diphenylethyl)-acetamine hydrochloride] and ARL 15896AR [(+)-a-phenyl-2–pyridine-ethanamine dihydrochloride]. For the abuse-liability studies, in rats trained to self-administer cocaine intravenously (0.1 mg/kg/injection), doses of remacemide HCI, ARL 15896AR, phencyclidine, and saline were made available, and the number of injections self-administered was recorded. In different sets of rats, we assessed the ability of these drugs to induce phencyclidine-like stereotyped behavior. Doses of the compounds were expressed as multiples of the 50% effective dose (ED,), as determined from the maximal electroshock (MES) test by using either oral or intravenous administration. None of the remacemide hydrochloride or ARL 15896AR doses was self-administered at a level higher than that of the saline vehicle, unlike cocaine and phencyclidine, which were self-administered at high and moderate levels, respectively. Unlike that with remacemide hydrochloride and ARL 15896AR, oral administration of the high-affinity uncompetitive NMDA receptor-antagonists phencyclidine, ARL 16247 [N-(3–ethylphenyl)-N-methyl-N'-naph-thylguanidine] and MK-801 engendered phencyclidine-like stereotypy at doses near their MES ED₅₀, values. These data confirm the unusual safety of remacemide hydrochloride and ARL 15896AR and demonstrate that they do not possess reinforcing properties. As such, they are unlikely to present a drug-abuse problem in human beings.     

Microtubule disruption, not calpain-dependent loss of MAP2, contributes to enduring NMDA-induced dendritic dysfunction in acute hippocampal slices

Brief exposure to excitotoxic agonists can result in substantial loss of the microtubule-associated protein MAP2 from neuronal dendrites, and accumulation in somata. A possible mechanism underling MAP2 loss is the activation of the calcium-dependent protease calpain by excessive dendritic Ca2+-loading. The present study examined mechanisms of MAP2 redistribution and loss of synaptic efficacy in the CA1 region of acutely prepared hippocampal slices. Brief NMDA exposure resulted in persistent and profound inhibition of postsynaptic potentials, and loss of MAP2 from dendritic compartments. When Ca2+ was removed during NMDA exposure, synaptic potentials recovered significantly during NMDA washout, and MAP2 loss was reduced. Calpain inhibition with MDL 28,170 (20 microM) did not prevent the loss of synaptic potentials, nor did it attenuate the initial aggregation of MAP2 into irregular dendritic swellings. However MDL 28,170 did reduce subsequent MAP2 loss from abnormal dendritic aggregates. Pre-exposure of slices to taxol (100 nM) effectively prevented microtubule depolymerization following NMDA exposure, as well as MAP2 disorganization and loss from apical dendrites. Slices treated with taxol also exhibited substantial recovery of synaptic potentials after transient NMDA stimulus. These results demonstrate a close correspondence between the maintained localization of MAP2 in apical dendrites and the recovery of postsynaptic potentials following transient NMDA exposure. In addition, it appears that rather than underlying the initial disruption of microtubule structure via MAP2 proteolysis, calpain activity instead may contribute to the degradation of irregularly aggregated MAP2 observed following microtubule depolymerization.     

Reduction of ischemic brain damage by nitrous oxide and xenon.

Neuronal death after ischemia-induced brain damage depends largely upon the activation of the N-methyl-D-aspartate (NMDA) excitatory glutamate receptor that is a target for many putative neuroprotective agents. Whereas the NMDA receptors mediate ischemic brain damage, blocking them is deleterious in humans. Here, the authors investigated whether nitrous oxide or xenon, which are gaseous anesthetics with a remarkably safe clinical profile that have been recently demonstrated as effective inhibitors of the NMDA receptor, may reduce the following: (1) ischemia-induced brain damage in vivo, when given after occlusion of the middle cerebral artery (MCAO), a condition needed to make these potentially neuroprotective agents therapeutically valuable; or (2) NMDA-induced Ca2+ influx in cortical cell cultures, a major critical event involved in excitotoxic neuronal death. The authors have shown that both nitrous oxide at 75 vol% and xenon at 50 vol% reduce ischemic neuronal death in the cortex by 70% and further decrease NMDA-induced Ca2+ influx by 30%. In addition, xenon at 50%, but not nitrous oxide at 75 vol%, further decreases ischemic brain damage in the striatum (a subcortical structure that is known to be resistant to neuroprotective interventions). However, at a higher concentration (75 vol%), xenon exhibits potentially neurotoxic effects. The mechanisms of the neuroprotective and potentially neurotoxic effects of nitrous oxide and xenon, as well as the possible therapeutic implications in humans, are discussed.     

Spermine is neuroprotective against anoxia and N-methyl-D-aspartate in hippocampal slices

Polyamines were implicated as either neurotoxic or neuroprotective in several models of stroke. Spermine augments the excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptor because this receptor is activated at micromolar spermine concentrations. However, at higher concentrations, spermine could be neuroprotective because it blocks the NMDA receptor and voltage-activated Ca(2+) channels. In this work, acute hippocampal slices were exposed to 1 mM spermine and either 10 min of anoxia or 0.5 mM NMDA. The percent recovery of population spikes was the measure of neuroprotection. One millimolar spermine was robustly neuroprotective; however, 0.1 mM spermine and 1 mM putrescine were not. The neuroprotective concentration of spermine was higher than the physiological concentration of free spermine. However, during an excitotoxic episode, extracellular Ca(2+) is decreased, enabling the inhibitory activity of lower spermine concentration. In addition, several noxious stimuli trigger the release of intracellular spermine and could raise local levels of spermine. Therefore, it is possible that spermine has a neuroprotective role in vivo.     

Nitric oxide applications prior and simultaneous to potentially excitotoxic NMDA-evoked calcium transients: cell death or survival

Nitric oxide (NO) is a molecule that plays a prominent role in neurotoxic as well as neuroprotective pathways. Here, we investigated the effects of NO on potentially excitotoxic glutamate-induced intracellular calcium ([Ca2+]i) dynamics. Our hypothesis was that pre- and coexposure to NO in conjunction with glutamate receptor stimulation modulates [Ca2+]i responses differentially. [Ca2+]i transients, assessed by the fluorescent cytosolic Ca2+ indicator dye fluo-4, were elicited in mouse striatal neurons by consecutive NMDA applications (200 microM for 100 s each). Subgroups of neuronal cultures were additionally exposed to a NO donor (S-nitroso-N-acetyl-d,l-penicillamine, SNAP, 50-500 microM), either by pre- (for 6 h prior to NMDA) or cotreatment (for 30 min during NMDA). Pretreatment with NO led to dramatically decreased NMDA-evoked [Ca2+]i rises in comparison to controls (NMDA alone). Annexin V/propidium iodide staining showed consistently that NO pretreatment is protective against NMDA-induced cell death. In contrast, NO/NMDA cotreatment caused a potentiation of [Ca2+]i rises, whereby the duration of [Ca2+]i transients following NMDA application was prolonged and remained at an increased plateau level. Simultaneous application of the mitochondrial permeability transition pore (mtPTP) blocker cyclosporin A (2 microM) during the NO/NMDA cotreatment prevented the deregulation of [Ca2+]i. The observed [Ca2+]i deregulation was accompanied by a decrease in the mitochondrial membrane potential as indicated by tetramethylrhodamine methylester (TMRM) fluorescence. These findings suggest that NO can act in a protective way due to preconditioning or can have a possibly detrimental impact in case of acute release. They provide a possible explanation for the ambivalence of NO in neurodegenerative processes where glutamate receptor stimulation and mitochondrial [Ca2+]i sequestration are causally involved.     

Pentamidine is an N-methyl-D-aspartate receptor antagonist and is neuroprotective in vitro.

Acquired immunodeficiency syndrome (AIDS) is frequently associated with dementia. The wide spectrum of neurological abnormalities associated with this dementia may involve a neurotoxin that activates the NMDA subtype of glutamate receptor in neurons. We have found that the antimicrobial agent pentamidine, which is prescribed for AIDS patients for the prophylaxis and treatment of Pneumocystis carinii pneumonia, is an effective NMDA receptor antagonist. Pentamidine inhibited 3H-dizocilpine binding to the NMDA receptor in rat brain membranes at a site separate from glutamate, glycine, and spermidine, with an affinity near 2 microM. Similar concentrations of pentamidine block NMDA-induced increases in intracellular Ca2+ and NMDA-induced currents in cultured forebrain and cortical neurons, apparently without use dependence or voltage dependence, suggesting that pentamidine may represent a novel chemical class of NMDA receptor antagonist. Finally, pentamidine protects neurons from the lethal effects of acute NMDA exposure in vitro. AS pentamidine may accumulate in the brain at relevant concentrations following repeated high-dose parenteral administration, these findings suggest that the drug may be neuroprotective in vivo.     

Interleukin-6 reduces NMDA-induced Ca2+ overload via prevention of Ca2+ release from intracellular store

A mechanism for neuroprotection of interleukin-6 (IL-6) via reduction of intracellular Ca2+ overload induced by N-methyl-D-aspartate (NMDA) was explored. Cerebellar granule neurons (CGNs) from postnatal 8-day infant rats were chronically exposed to IL-6 for 8 days. Confocal laser scanning microscope was used to measure dynamic changes of intracellular Ca2+ fluorescence intensity. NMDA triggered an acute and sustaining enhancement of intracellular Ca2+ fluorescence intensity in the cultured CGNs, whereas NMDA stimulation of the neurons that had been exposed to IL-6 reduced the intracellular Ca2+ fluorescence intensity relative to that of non-IL-6-pretreated neurons. Ethylene glycol bis(aminoethylether) tetraacetate (EGTA), a chelator of extracellular Ca2+, decreased the intracellular Ca2+ overload triggered by NMDA. The component of intracellular Ca2+ overload after EGTA treatment was prevented by IL-6 chronic exposure. Cotreatment with dantrolene sodium (DAN) and 2-aminoethoxydiphenylborate (2-APB), blockers of ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP3R), respectively, also decreased the intracellular Ca2+ overload triggered by NMDA. However, the component of intracellular Ca2+ overload after DAN and 2-APB treatment was only slightly but not significantly diminished by IL-6. These results suggest that IL-6 has a neuroprotective effect against NMDA-induced intracellular Ca2+ overload, and that the effect is implemented primarily via a suppression of Ca2+ release from the intracellular Ca2+ store.     

Adenosine A2A receptor blockade differentially influences excitotoxic mechanisms at pre- and postsynaptic sites in the rat striatum

Adenosine A(2A) receptor antagonists are being regarded as potential neuroprotective drugs, although the mechanisms underlying their effects need to be better studied. The aim of this work was to investigate further the mechanism of the neuroprotective action of A(2A) receptor antagonists in models of pre- and postsynaptic excitotoxicity. In microdialysis studies, the intrastriatal perfusion of the A(2A) receptor antagonist ZM 241385 (5 and 50 nM) significantly reduced, in an inversely dose-dependent way, the raise in glutamate outflow induced by 5 mM quinolinic acid (QA). In rat corticostriatal slices, ZM 241385 (30-100 nM) significantly reduced 4-aminopyridine (4-AP)-induced paired-pulse inhibition (PPI; an index of neurotransmitter release), whereas it worsened the depression of field potential amplitude elicited by N-methyl-D-aspartate (NMDA; 12.5 and 50 microM). The A(2A) antagonist SCH 58261 (30 nM) mimicked the effects of ZM 241385, whereas the A(2A) agonist CGS 21680 (100 nM) showed a protective influence toward 50 microM NMDA. In rat striatal neurons, 50 nM ZM 241385 did not affect the increase in [Ca(2+)](i) or the release of lactate dehydrogenase (LDH) induced by 100 and 300 microM NMDA, respectively. The ability of ZM 241385 to prevent QA-induced glutamate outflow and 4-AP-induced effects confirms that A(2A) receptor antagonists have inhibitory effects on neurotransmitter release, whereas the results obtained toward NMDA-induced effects suggest that A(2A) receptor blockade does not reduce, or even amplifies, excitotoxic mechanisms due to direct NMDA receptor stimulation. This indicates that the neuroprotective potential of A(2A) antagonists may be evident mainly in models of neurodegeneration in which presynaptic mechanisms play a major role.     

Synapses, NMDA receptor activity and neuronal Aβ production in Alzheimer's disease

A direct relationship has been established between synaptic activity and amyloid-β secretion. Dysregulation of neuronal calcium homeostasis was shown to increase production of amyloid-β, contributing to the initiation of Alzheimer's disease. Among the different routes of Ca(2+) entry, N-methyl-d-aspartate (NMDA) receptors, a subtype of ionotropic glutamate receptors, are especially involved in this process because of their ability to gate high levels of Ca(2+) influx. These receptors have been extensively studied for their crucial roles in synaptic plasticity that underlies learning and memory but also in neurotoxicity occurring during acute brain injuries and neurodegenerative diseases. For one decade, several studies provided evidence that NMDA receptor activation could have distinct consequences on neuronal fate, depending on their location. Synaptic NMDA receptor activation is neuroprotective, whereas extrasynaptic NMDA receptors trigger neuronal death and/or neurodegenerative processes. Recent data suggest that chronic activation of extrasynaptic NMDA receptors leads to a sustained neuronal amyloid-β release and could be involved in the pathogenesis of Alzheimer's disease. Thus, as for other neurological diseases, therapeutic targeting of extrasynaptic NMDA receptors could be a promising strategy. Following this concept, memantine, unlike other NMDA receptor antagonists was shown, to preferentially target the extrasynaptic NMDA receptor signaling pathways, while relatively sparing normal synaptic activity. This molecular mechanism could therefore explain why memantine is, to date, the only clinically approved NMDA receptor antagonist for the treatment of dementia.     

Brevetoxin augments NMDA receptor signaling in murine neocortical neurons

Brevetoxins (PbTx) are potent allosteric enhancers of voltage-gated sodium channel (VGSC) function and are associated with periodic "red tide" blooms. These neurotoxins produce neuronal injury and death in cerebellar granule cells (CGC) following acute exposure. In murine neocortical neurons, brevetoxin induces Ca(2+) influx that is mediated through both glutamatergic and non-glutamatergic pathways. Inasmuch as Src kinase is capable of upregulating the NMDA subtype of glutamate receptors, we determined whether Src kinase participated in PbTx-2-induced Ca(2+) influx. Inhibition of Src kinase blocked PbTx-2-induced Ca(2+) influx. PbTx-2 treatment moreover increased tyrosine phosphorylation of the NR2B subunit. A rise in intracellular [Na(+)] and phosphorylation of NMDA receptors by Src kinase is known to increase NMDA receptor activity. We therefore explored the influence of brevetoxin on NMDA receptor function. We found that PbTx-2 augments NMDA receptor-mediated Ca(2+) influx in both spontaneously oscillating mature neurons and in non-oscillatory immature neurons. PbTx-2 also enhanced the effect of bath-applied NMDA on extracellular signal-regulated kinase 2 (ERK2) activation. These results suggest that brevetoxin augments NMDA receptor signaling in neocortical neurons, and this upregulation may be mediated by coincidence of an elevation in intracellular [Na(+)] and Src kinase activation.     

Carnosine protects a primary cerebellar cell culture from acute NMDA toxicity

Activation of extrasynaptic NMDA receptors by high glutamate concentrations is one of the key pathogenic factors following a stroke. For this reason, the search for efficient neuroprotective agents that could reduce glutamate toxicity is a pressing need. Ca²⁺ overload in response to glutamate leads to activation of signaling cascades in the cell and the development of oxidative stress, which ultimately leads to apoptosis. Using a model system of acute excitotoxicity caused by 50 μM NMDA, which was used as a specific NMDA receptor activator, we demonstrated that during 2 hours of incubation the viability of the primary neuronal culture decreased by 30–50%. To demonstrate that the observed effect is associated not only with the Ca²⁺ influx into the cytoplasm through the activated NMDA receptors, we decreased the Ca²⁺ concentration in the medium. The lowered Ca2+ concentration, as well as its complete absence, did not affect NMDA toxicity. We tested carnosine, a naturally occurring dipeptide and promising antioxidant, as a neuroprotective agent. The addition of 2 mM carnosine prevented the decrease in cell viability caused by a 2-hour incubation with 50 μM NMDA, while it showed no effect on the viability of the cell culture in the control. Based on the results, we consider the further study of carnosine, its complexes, and analogues as neuroprotectors in cerebral ischemia promising.     

Nitric oxide modulates NMDA-induced increases in intracellular Ca2+ in cultured rat forebrain neurons.

We studied the effects of nitric oxide (NO) and the NO-releasing agents sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP) and isosorbide dinitrate (ISDN) on N-methyl-D-aspartate (NMDA)-induced increases in intracellular Ca2+ ([Ca2+]i), whole-cell patch-clamp currents and on glutamate-stimulated [3H]dizocilpine binding. NO and agents that release NO partially inhibit increases in [Ca2+]i at concentrations between 1 microM and 1 mM. These agents also decrease [Ca2+]i changes produced by kainate and potassium, but to a smaller extent. As the effects of NO are still present following alkylation of the redox modulatory site on the NMDA receptor this action of NO is probably not a consequence of oxidation of the redox site. In contrast to SNP, ISDN does not inhibit NMDA-induced whole cell patch-clamp currents suggesting that NO modulates [Ca2+]i via perturbation of a Ca2+ homeostatic process. Furthermore, SNP may have a direct action on the NMDA receptor complex in addition to the generation of NO. 8-Bromo-cGMP does not mimic the inhibitory effect of NO suggesting that this effect is not the result of NO stimulation of neuronal cGMP production. As the production of NO in neurons is dependent on increases in [Ca2+]i associated with NMDA receptor activation, these data suggest that NO-mediated decreases in [Ca2+]i may represent a novel feedback inhibitory mechanism for NO production in the brain.     

Repolarization of the plasma membrane shapes NMDA-induced cytosolic [Ca2+ ] transients.

After inactivation of NMDA receptors, restoration of basal cytosolic [Ca2+] ([Ca2+]c) is delayed. This may be caused by Ca2+ influx via reverse Na/Ca exchange or voltage-gated Ca2+ channels, and/or by Ca2+ efflux from internal stores. Monitoring of [Na+]c, [Ca2+]c, and plasma membrane potential in cultured cerebellar granule cells showed that repolarization of the plasma membrane and inactivation of voltage-gated Ca channels plays the most critical role in restoration of low [Ca2+]c following NMDA receptor inactivation. During NMDA receptor activation, however, an Na-dependent mechanism enhanced NMDA-induced elevation in [Ca2+]c. This mechanism did not involve Na,K-ATPase activation by Na+, because it operated even when Na,K-ATPase was inhibited.