Neuronal death enhanced by N-methyl-D-aspartate antagonists

Compartmentalized NMDA receptor signalling to survival and death

The ability of Ca(2+) influx through the N-methyl d-aspartate subclass of glutamate receptor (NMDA receptor) to both kill neurons and to promote survival under different circumstances is well established. Here we discuss the signal pathways that mediate this dichotomous signalling, and the factors that influence whether an NMDA receptor-dependent Ca(2+) signal results in a net pro-survival or pro-death effect. The magnitude of NMDA receptor activation, be it intensity or duration, is of course very important in determining the nature of the response to an episode of NMDA receptor activity, with excitotoxic death pathways requiring higher levels than survival pathways. However, the NMDA receptor is not merely a conduit for Ca(2+) influx: the consequences of NMDA receptor activity can be influenced by signalling molecules that physically associate with the NMDA receptor or indeed the location (synaptic versus extrasynaptic) of the receptor. Furthermore, we discuss the possibility that the Ca(2+) effectors of survival and death are in different subcellular locations, and thus depend on the spatial characteristics of the Ca(2+) transient. A greater understanding of these issues may point to ways of selectively blocking pro-death signalling in neurological disorders such as stroke, where global NMDA receptor antagonists have proved ineffective.

Transient Receptor Potential Channels of the Melastatin Family and Ischemic Responses of Central Neurons

The excitotoxic theory of stroke, which implicated N-methyl-d-aspartate (NMDA) receptors as mediators of excessive Ca(2+) entry and neuronal death, generated a great deal of enthusiasm for the prospect of using NMDA receptor antagonists to prevent the associated brain injury. Unfortunately, these receptor antagonists failed to provide effective treatments for human stroke. In part, the failure is likely a consequence of having to administer these drugs within a very short therapeutic window after stroke and to the intolerable psychomimetic side effects associated with their use. However, new possibilities for therapeutic intervention are revealing themselves as our understanding of excitotoxicity evolves. We now recognize that ischemia and Ca(2+) toxicity in central neurons can be attributed to a variety of mechanisms recruited downstream of NMDA receptor activation. These include the activation of Ca(2+)-permeable transient receptor potential channels of the melastatin family. The more-delayed activation of these channels offers the tantalizing possibility that drugs targeting selected members of this family may possess a wider therapeutic window for preventing the debilitating consequences after stroke onset.

Gel-based hippocampal proteomic analysis 2 weeks following traumatic brain injury to immature rats using controlled cortical impact

Traumatic brain injury (TBI) to postnatal day 17 rats has been shown to produce acute changes in hippocampal global protein levels and spatial learning and memory deficits. The purpose of the present study was to analyze global hippocampal protein changes 2 weeks after a moderate ipsilateral controlled cortical impact in postnatal day 17 rats using 2-dimensional difference gel electrophoresis and mass spectrometry. Paired sham and ipsilateral injured hippocampal lysates were independently labeled with different fluorescent cyanine dyes and coseparated within the same immobilized pH gradient strips and slab gel based on isoelectric point and molecular mass. Significant changes in key proteins involved in glial and neuronal stress, oxidative metabolism, calcium uptake and neurotransmitter function were found 2 weeks after injury, and their potential roles in hippocampal plasticity and cognitive dysfunction were discussed.

Preconditioning doses of NMDA promote neuroprotection by enhancing neuronal excitability

Neuroprotection can be induced by low doses of NMDA, which activate both synaptic and extrasynaptic NMDA receptors. This is in apparent contradiction with our recent findings that extrasynaptic NMDA receptor signaling exerts a dominant inhibitory effect on prosurvival signaling from synaptic NMDA receptors. Here we report that exposure to low preconditioning doses of NMDA results in preferential activation of synaptic NMDA receptors because of a dramatic increase in action potential firing. Both acute and long-lasting phases of neuroprotection in the face of apoptotic or excitotoxic insults are dependent on this firing enhancement. Key mediators of synaptic NMDA receptor-dependent neuroprotection, phosphatidylinositol 3 kinase-Akt (PI3 kinase-Akt) signaling to Forkhead box subgroup O (FOXO) export and glycogen synthase kinase 3beta (GSK3beta) inhibition and cAMP response element-binding protein-dependent (CREB-dependent) activation of brain-derived neurotrophic factor (BDNF), can be induced only by low doses of NMDA via this action potential-dependent route. In contrast, NMDA doses on the other side of the toxicity threshold do not favor synaptic NMDA receptor activation because they strongly suppress firing rates below baseline. The classic bell-shaped curve depicting neuronal fate in response to NMDA dose can be viewed as the net effect of two antagonizing (synaptic vs extrasynaptic) curves: via increased firing the synaptic signaling dominates at low doses, whereas firing becomes suppressed and extrasynaptic signaling dominates as the toxicity threshold is crossed.

Interruption of functional recovery by the NMDA glutamate antagonist MK801 after compression of the sensorimotor cortex: implications for treatment of tumors or other mass-related brain injuries

Glutamate antagonists have recently been shown to limit tumor growth, providing potential new therapeutic targets and strategies against brain tumors. Here, we demonstrate that the glutamate NMDA receptor antagonist MK801, after a delay, adversely reverses functional recovery in rats with compressive mass lesions of the sensorimotor cortex. Our data suggest that the controlled focal cortical compression model may be a valuable pre-clinical tool to screen compounds for the treatment of brain tumors. It may be possible to use this model to develop interventions that maintain anti-cancer effects but with diminished harm to bystander tissue and brain plasticity.

Ca2+-Permeable Acid-sensing Ion Channels and Ischemic Brain Injury

Acidosis is a common feature of brain in acute neurological injury, particularly in ischemia where low pH has been assumed to play an important role in the pathological process. However, the cellular and molecular mechanisms underlying acidosis-induced injury remain unclear. Recent studies have demonstrated that activation of Ca(2+)-permeable acid-sensing ion channels (ASIC1a) is largely responsible for acidosis-mediated, glutamate receptor-independent, neuronal injury. In cultured mouse cortical neurons, lowering extracellular pH to the level commonly seen in ischemic brain activates amiloride-sensitive ASIC currents. In the majority of these neurons, ASICs are permeable to Ca(2+), and an activation of these channels induces increases in the concentration of intracellular Ca(2+) ([Ca(2+)](i)). Activation of ASICs with resultant [Ca(2+)](i) loading induces time-dependent neuronal injury occurring in the presence of the blockers for voltage-gated Ca(2+) channels and the glutamate receptors. This acid-induced injury is, however, inhibited by the blockers of ASICs, and by reducing [Ca(2+)](o). In focal ischemia, intracerebroventricular administration of ASIC1a blockers, or knockout of the ASIC1a gene protects brain from injury and does so more potently than glutamate antagonism. Furthermore, pharmacological blockade of ASICs has up to a 5 h therapeutic time window, far beyond that of glutamate antagonists. Thus, targeting the Ca(2+)-permeable acid-sensing ion channels may prove to be a novel neuroprotective strategy for stroke patients.

Nuclear Ca2+ and the cAMP response element-binding protein family mediate a late phase of activity-dependent neuroprotection

The mechanism by which physiological synaptic NMDA receptor activity promotes neuronal survival is not well understood. Here, we show that that an episode of synaptic activity can promote neuroprotection for a long time after that activity has ceased. This long-lasting or "late phase" of neuroprotection is dependent on nuclear calcium signaling and cAMP response element (CRE)-mediated gene expression. In contrast, neuroprotection evoked acutely by ongoing synaptic activity relies solely on the activation of the phosphatidylinositol 3-kinase/Akt pathway. This "acute phase" does not require nuclear calcium signaling and is independent of activation of the CRE-binding protein (CREB) family of transcription factors. Thus, activity-dependent neuroprotection comprises two mechanistically distinct phases that differ in their spatial requirements for calcium and in their reliance on the CREB family.

Glutamic acid reverses Pb2+-induced reductions of NMDA receptor subunits in vitro

The objective of this study is to determine the effects of Pb2+ on N-methyl-d-aspartate receptor (NMDAR) subunits--NR1C1, NR2A and NR2B in primary cultured neuronal cells. We hypothesize that L-glutamic acid (GA) reverses Pb2+-induced NMDAR damage. Neuronal cells were isolated from the fetus brain at 18-20th day of gestation of pregnant Sprague Dawley (SD) rats. All experiments were included three independent cell preparations (N=3). The neuronal cells were exposed to Pb2+ (10(-10), 10(-9), 10(-8) and 10(-7)M) for 24 h. Neurons were pretreated with NMDAR agonist--L-glutamic acid (GA) (200 microM) and antagonists dizocipine (MK-801, 50 nM) for 1h and then exposed to 10(-7)M of Pb2+ for 24 h. Finally, GA at 2, 0.2 and 0.02 mM was incubated with neurons prior to Pb2+ exposure. Aliquots of NR1, NR2A and NR2B proteins from cell homogenate were immunoprecipitated with protein A agarose and detected by Western blotting. The addition of GA unconventionally reversed the reductions of NMDAR by Pb at protein levels, whereas MK-801 exacerbated Pb2+-induced damage. The protection by GA against Pb2+-induced reduction of NMDAR was dose-dependent. These findings suggest that the administration of GA may be a potential approach to intervene the Pb2+-induced NMDAR alterations.

Memantine attenuates staurosporine-induced activation of caspase-3 and LDH release in mouse primary neuronal cultures

Developmental aspects of pro- and antiapoptotic action of some NMDA receptor antagonists in the central nervous system have been postulated. In order to further elucidate this problem, we investigated effect of memantine, an uncompetitive NMDA receptor antagonist and staurosporine alone and in combination on caspase-3 activity and lactate dehydrogenase (LDH) release in primary hippocampal, neocortical and striatal cell cultures on 7 and 12 days in vitro. The data showed that the vulnerability of neuronal cells to induction of caspase-3 activity by staurosporine was higher on 7 DIV than on 12 DIV, whereas staurosporine-mediated LDH release increased with days in vitro in striatal culture only. A specific inhibitor of caspase-3, AcDEVDCHO (60 microM), completely abolished the effect of staurosporine on this enzyme's activity, but only partially attenuated staurosporine-induced LDH release in hippocampal cells. Memantine alone (0.05-2.0 microM) did not induce any cytotoxic effect but attenuated the staurosporine-induced caspase-3 activity and LDH release in hippocampal cultured neurons on each investigated day in vitro. In striatal culture, memantine had a moderate inhibitory effect on staurosporine-evoked LDH release only on 7 DIV with no significant influence on caspase-3 activity. As for neocortical cultures, memantine partially inhibited staurosporine-induced neuronal injury only on 7 DIV. These data showed that the induction of caspase-3 activity by staurosporine was more profound in immature cells, however, the staurosporine neurotoxicity, as reflected by LDH release, only partially depended on caspase-3 activation and stage of cell development. Furthermore, memantine attenuated staurosporine-induced apoptosis more efficiently in hippocampal cultures than in neocortical and striatal ones, which points to tissue specificity of effects of this neuroprotectant.

3-Nitropropionic acid: a mitochondrial toxin to uncover physiopathological mechanisms underlying striatal degeneration in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a mutation in the gene encoding Huntingtin. The mechanisms underlying the preferential degeneration of the striatum, the most striking neuropathological change in HD, are unknown. Of those probably involved, mitochondrial defects might play an important role. The behavioural and anatomical similarities found between HD and models using the mitochondrial toxin 3-nitropropionic acid (3NP) in rats and primates support this hypothesis. Here, we discuss the recently identified mechanisms of 3NP-induced striatal degeneration. Two types of important factor have been identified. The first are the 'executioner' components that have direct roles in cell death, such as c-Jun N-terminal kinase and Ca2+-activated protease calpains. The second are 'environmental' factors, such as glutamate, dopamine and adenosine, which modulate the striatal degeneration induced by 3NP. Interestingly, these recent studies support the hypothesis that 3NP and mutated Huntingtin have certain mechanisms of toxicity in common, suggesting that the use of 3NP might give new insights into the pathogenesis of HD. Research on 3NP provides additional proof that the neurochemical environment of a given neurone can determine its preferential vulnerability in neurodegenerative diseases.

Inhibition of N-methyl-d-aspartate receptors increases paraoxon-induced apoptosis in cultured neurons

Organophosphorus (OP) compounds, used as insecticides and chemical warfare agents, are potent neurotoxins. We examined the neurotoxic effect of paraoxon (O,O-diethyl O-p-nitrophenyl phosphate), an organophosphate compound, and the role of NMDA receptors as a mechanism of action in cultured cerebellar granule cells. Paraoxon is neurotoxic to cultured rat cerebellar granule cells in a time- and concentration-dependent manner. Cerebellar granule cells are less sensitive to the neurotoxic effects of paraoxon on day in vitro (DIV) 4 than neurons treated on DIV 8. Surprisingly, the N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, enhances paraoxon-mediated neurotoxicity suggesting that NMDA receptors may play a protective role. Pretreatment with a subtoxic concentration of N-methyl-D-aspartate (NMDA) [100 microM] protects about 40% of the vulnerable neurons that would otherwise die from paraoxon-induced neurotoxicity. Moreover, addition of a neuroprotective concentration of NMDA 3 h after treatment with paraoxon provides the same level of protection. Because paraoxon-mediated neuronal cell death is time-dependent, we hypothesized that apoptosis may be involved. Paraoxon increases apoptosis about 10-fold compared to basal levels. The broad-spectrum caspase inhibitor (Boc-D-FMK) and the caspase-9-specific inhibitor (Z-LEHD-FMK) protect against paraoxon-mediated apoptosis, paraoxon-stimulated caspase-3 activity and neuronal cell death. MK-801 increases, whereas NMDA blocks paraoxon-induced apoptosis and paraoxon-stimulated caspase-3 activity. These results suggest that activation of NMDA receptors protect neurons against paraoxon-induced neurotoxicity by blocking apoptosis initiated by paraoxon.

Isoflurane-induced neuronal degeneration: an evaluation in organotypic hippocampal slice cultures

Prolonged exposure of postnatal day (PND) 7 rat pups to anesthetics, which act via N-methyl-D-aspartate antagonism and/or gamma-amino butyric acid enhancement, causes neurodegeneration and persistent behavioral deficits. We studied these findings in vitro and determined whether the age of rat pups used for study or duration of anesthetic exposure modulates resultant neurodegeneration. Organotypic hippocampal slices (OHSs) were prepared from rat pups on PNDs 4, 7, and 14 and cultured 7 or 14 days in vitro. The slices were exposed to 1.5% isoflurane or fresh gas for durations of 1, 3, or 5 h. Hippocampal CA1, CA3, and dentate gyrus neuronal survival was assessed 3 days later. Neuronal cell death was greatest in OHSs prepared from PND 7 rat pups (P < 0.001) and was most evident after 5 h exposure to isoflurane (P < 0.001). By eliminating variables such as hemodynamics, nutrition, oxygenation, and carbon dioxide elimination, this in vitro investigation supports both an age- and duration-dependent relationship between 1.5% isoflurane exposure and perinatal neuronal death.

The excitoprotective effect of N-methyl-D-aspartate receptors is mediated by a brain-derived neurotrophic factor autocrine loop in cultured hippocampal neurons

The neuroprotective effect and molecular mechanisms underlying preconditioning with N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons have not been described. Pre-incubation with subtoxic concentrations of the endogenous neurotransmitter glutamate protects vulnerable neurons against NMDA receptor-mediated excitotoxicity. As a result of physiological preconditioning, NMDA significantly antagonizes the neurotoxicity resulting from subsequent exposure to an excitotoxic concentration of glutamate. The protective effect of glutamate or NMDA is time- and concentration-dependent, suggesting that sufficient agonist and time are required to establish an intracellular neuroprotective state. In these cells, the TrkB ligand, brain-derived neurotrophic factor (BDNF) attenuates glutamate toxicity. Therefore, we tested the hypothesis that NMDA protects neurons via a BDNF-dependent mechanism. Exposure of hippocampal cultures to a neuroprotective concentration of NMDA (50 microM) evoked the release of BDNF within 2 min without attendant changes in BDNF protein or gene expression. The accumulated increase of BDNF in the medium is followed by an increase in the phosphorylation (activation) of TrkB receptors and a later increase in exon 4-specific BDNF mRNA. The neuroprotective effect of NMDA was attenuated by pre-incubation with a BDNF-blocking antibody and TrkB-IgG, a fusion protein known to inhibit the activity of extracellular BDNF, suggesting that BDNF plays a major role in NMDA-mediated survival. These results demonstrate that low level stimulation of NMDA receptors protect neurons against glutamate excitotoxicity via a BDNF autocrine loop in hippocampal neurons and suggest that activation of neurotrophin signaling pathways plays a key role in the neuroprotection of NMDA.

Neuronal damage after moderate hypoxia and erythropoietin

Both mild hypoxia and exogenous erythropoietin may protect the brain against subsequent severe hypoxia, and the conditioning effect of transient hypoxia is partly mediated by hypoxia-induced endogenous erythropoietin. We now observed in several experimental models that combining transient hypoxia and exogenous erythropoietin may cause neuronal damage. High-dose erythropoietin (40 IU/ml) profoundly impeded synaptic transmission of rat hippocampal slice cultures when used in conjunction with moderate hypoxia (10% O2 for two 8-h periods). Addition of erythropoietin increased viability of cultured rat embryonic cortical neurons at 21% O2 but decreased viability under hypoxic conditions (2% O2) in a dose-dependent fashion. Death of human neuronal precursor cells challenged by oxygen and glucose deprivation was increased by erythropoietin when cells were cultured under hypoxic but not under normoxic conditions. In neonatal rats exposed to moderate hypoxia plus erythropoietin, numbers of degenerating cerebral neurons were increased, as compared to controls or rats subjected to either hypoxia or erythropoietin alone. Thus, erythropoietin may aggravate rather than ameliorate neuronal damage when administered during transient hypoxia.

Peptide action in stroke therapy

Glutamate receptor antagonists, although effective in preventing in vitro excitotoxic death, also block the glutamatergic signalling that is essential for normal excitatory neurotransmission and neuronal survival. This has contributed to the failure of clinical trials employing glutamate receptor antagonists as stroke therapeutics. However, recent years have seen an increased understanding of the molecular organisation of glutamate receptors in the neuronal postsynaptic density. This and a dissection of their associated intracellular signalling cascades has allowed the identification of distinct pathways responsible for excitotoxicity. It has become possible to uncouple toxic signalling cascades from glutamate receptors by targeting the interactions of membrane receptors with downstream proteins. Toxic signalling can be effectively uncoupled from glutamate receptors using targeted, cell-permeable peptides to disrupt specific protein-protein interactions. This approach does not block essential excitatory neurotransmission, but attenuates neurotoxic signals specifically and reduces stroke damage. This novel approach to blocking excitotoxic signalling in cerebral ischaemia may constitute a practical approach to stroke therapy.

Valproate is neuroprotective against malonate toxicity in rat striatum: an association with augmentation of high-affinity glutamate uptake

The antiepileptic drug valproate (VPA) may be neuroprotective. We treated rats with VPA for 14 days (300 mg/kg twice daily) before intrastriatal injection of 1.5 micromol (1 M) of the succinate dehydrogenase inhibitor malonate. VPA-treated animals developed smaller lesions than control animals: 10 +/- 2 mm(3) versus 26 +/- 8 mm(3) (means +/- SD; P = 10(-4). Injection of NaCl that was equiosmolar with 1 M malonate caused lesions of only 1.2 +/- 0.4 mm(3) in control animals, whereas physiologic saline produced no lesion. VPA pretreatment reduced the malonate-induced extracellular accumulation of glutamate. This effect paralleled an increase in the striatal level of the glutamate transporter GLT, which augmented high-affinity glutamate uptake by 25%, as determined from the uptake of [(3)H] glutamate into striatal proteoliposomes. Malonate caused a 76% reduction in striatal adenosine triphosphate (ATP) content, but the glial, ATP-dependent formation of glutamine from radiolabeled glucose or glutamate was intact, indicating that glial ATP production supported uptake of glutamate. Striatal levels of HSP-70 and fos were reduced, and the levels of bcl-2 and phosphorylated extracellular signal-regulated kinase remained unaffected, but histone acetylation was increased by VPA treatment. The results suggest that augmentation of glutamate uptake may contribute importantly to VPA-mediated neuroprotection in striatum.

Symptomatic and Disease-Modifying Therapy for the Progressive Ataxias

BACKGROUND

The progressive ataxias are a diverse group of neurologic diseases that share features of degeneration of the cerebellum and its inflow/outflow pathways but differ in etiology, course, and associated noncerebellar system involvement. Some will have treatable causes, but for most, the pathophysiology is incompletely known.

REVIEW SUMMARY

Treatment strategies will include (1) definitive therapy when available, (2) symptomatic treatment and prevention of complications, and (3) rehabilitation and support resources. The physician will have to decide whether to introduce or approve the use of therapies based on as yet-unproven mechanisms or the use of complementary medicine approaches.

CONCLUSIONS

There are as yet no drugs that have been approved by the Food and Drug Administration for the treatment of the progressive ataxias and relatively few disease-modifying therapies, but symptomatic and rehabilitation interventions can greatly improve the quality of life of individuals with these disabling neurodegenerative disorders.

Taurine prevents the neurotoxicity of beta-amyloid and glutamate receptor agonists: activation of GABA receptors and possible implications for Alzheimer's disease and other neurological disorders

Alzheimer's disease (AD) and several other neurological disorders have been linked to the overactivation of glutamatergic transmission and excitotoxicity as a common pathway of neuronal injury. The beta-amyloid peptide (Abeta) is centrally related to the pathogenesis of AD, and previous reports have demonstrated that the blockade of glutamate receptors prevents Abeta-induced neuronal death. We show that taurine, a beta-amino acid found at high concentrations in the brain, protects chick retinal neurons in culture against the neurotoxicity of Abeta and glutamate receptor agonists. The protective effect of taurine is not mediated by interaction with glutamate receptors, as demonstrated by binding studies using radiolabeled glutamate receptor ligands. The neuroprotective action of taurine is blocked by picrotoxin, an antagonist of GABA(A) receptors. GABA and the GABA(A) receptor agonists phenobarbital and melatonin also protect neurons against Abeta-induced neurotoxicity. These results suggest that activation of GABA receptors decreases neuronal vulnerability to excitotoxic damage and that pharmacological manipulation of the excitatory and inhibitory neurotransmitter tonus may protect neurons against a variety of insults. GABAergic transmission may represent a promising target for the treatment of AD and other neurological disorders in which excitotoxicity plays a relevant role.

Inhibitory effects of angiotensin on NMDA-induced cytotoxicity in primary neuronal cultures

Primary cultures from the hypothalamus/thalamus/septum/midbrain (HTSM) region of 1-day-old mice were used to investigate the effects of angiotensin on NMDA excitotoxicity. Cell viability was determined following exposure to 1-10 mM glutamate or 0.01-10 mM NMDA. Cells exposed to 1 mM glutamate or 1 mM NMDA for 24 h showed a significant increase in cell death as determined by propidium iodide staining. HTSM cultures treated with 0.1 mM NMDA revealed both DNA laddering and positive staining for TUNEL, suggesting apoptosis as the primary mechanism for the cell death. We also determined whether angiotensin II (Ang II) modulated NMDA-induced cell death in HTSM-cultured neurons. Cells pre-treated with 10 nM Ang II showed a decrease in NMDA-induced cytotoxicity, TUNEL staining and DNA laddering. NMDA-induced cell death was also reduced when cells were pre-treated with the AT1 receptor antagonist losartan. In this study, we have shown that NMDA and glutamate induce cell death through the NMDA receptor, and that Ang II, acting primarily through the AT2 receptor, can attenuate this response.

Trialkylglycines: a new family of compounds with in vivo neuroprotective activity

Glutamate neurotoxicity is involved in the pathogenesis of neurodegenerative disorders such as Huntington's, Parkinson's and Alzheimer's diseases. It plays also a major role in the neuronal damage that occurs in brain ischemia and head trauma. Finding molecules that prevent or reverse glutamate neurotoxicity (excitotoxicity) is, therefore, of great interest. Strategies aimed at this end include the screening of libraries of compounds synthesized by combinatorial chemistry to find molecules that prevent neuronal death in vitro and in vivo. A library of trialkylglycines was screened to assess whether they prevent glutamate-induced neuronal death in primary cultures of cerebellar neurons. Two types of trialkylglycines have been found that significantly reduce the incidence of glutamate-induced neuronal death. The first type includes two compounds (referred to as 6-1-2 and 6-1-10) that efficiently prevent glutamate or NMDA-induced neuronal death. They also prevent excitotoxicity in vivo as assessed by using two animal models of excitotoxicity: acute intoxication with ammonia and a model of cerebral ischemia in rats. Trialkylglycines 6-1-2 and 6-1-10 prevent ammonia-induced (NMDA receptor-mediated) death of mice and neuronal degeneration in the model of cerebral ischemia. The trialkylglycines of the second type act as open channel blockers of the NMDA receptor. The first group of trialkylglycines does not block NMDA receptor channels and does not affect the glutamate-nitric oxide-cGMP pathway. Their molecular target has not yet been identified. These two types of trialkylglycines (especially those that do not affect NMDA receptor function) might represent effective drugs for the treatment of neurodegeneration. They are likely to be well tolerated and have fewer side effects than NMDA receptor antagonists.

Anatomical and functional cerebral variables associated with basal symptoms but not risperidone response in minimally treated schizophrenia

In schizophrenia, structural and functional cerebral variables show an unclear association with clinical features and their value as predictors of response to a typical antipsychotic agents has yet to be determined. The goal of this study was to investigate the relationships between clinical variables (baseline syndromes and response to risperidone) and anatomo-functional brain variables. We studied 19 minimally treated patients with schizophrenia of recent onset using magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (FDG-PET) under resting conditions. The following brain variables were studied: volume of the cerebrospinal fluid (CSF) and gray matter (GM) of the dorsolateral prefrontal cortex (DLPFC) and temporal lobe; hippocampal metabolic activity and volume; and metabolic activity of the DLPFC, temporal lobe, putamen and caudate. Anatomical volume measurements were corrected for age and intracranial size using regression parameters determined from a matched sample of control subjects. Using stepwise multiple regression, we assessed the relation between these brain measures and basal scores of symptom dimensions (positive, disorganization, negative and total), as well as their change in response to risperidone. We found that positive and disorganization symptoms improved with risperidone treatment and that hippocampal metabolism, DLPFC CSF volume, and temporal CSF volume predicted baseline symptoms. However, none of the brain measures predicted response to treatment. We conclude that there is evidence of a significant association between basal symptoms and DLPFC atrophy and limbic hyperactivity at rest in recent-onset schizophrenic patients.

Expression of NMDA receptor NR1, NR2A and NR2B subunit mRNAs during development of the human hippocampal formation

The N-methyl-d-aspartate receptor plays a critical role in the formation and maintenance of synapses during brain development. In the rodent, changes in subunit expression and assembly of the heteromeric receptor complex accompany these maturational processes. However, little is known about N-methyl-d-aspartate receptor subunit expression during human brain development. We used in situ hybridization to examine the distribution and relative abundance of NR1, NR2A and NR2B subunit messenger ribonucleic acids in the hippocampal formation and adjacent cortex of 34 human subjects at five stages of life (neonate, infant, adolescent, young adult and adult). At all ages, the three messenger ribonucleic acids were expressed in all subfields, predominantly by pyramidal neurons, granule cells and polymorphic hilar cells. However, their abundance varied across ontogeny. Levels of NR1 messenger ribonucleic acid in CA4, CA3 and CA2 subfields were significantly lower in the neonate than all other age groups. In the dentate gyrus, subiculum and parahippocampal gyrus, NR2B messenger ribonucleic acid levels were higher in the neonate than in older age groups. NR2A messenger ribonucleic acid levels remained constant, leading to an age-related increase in NR2A/2B transcript ratio. We conclude that N-methyl-d-aspartate receptor subunit messenger ribonucleic acids are differentially expressed during postnatal development of the human hippocampus, with a pattern similar but not identical to that seen in the rodent. Changes in subunit composition may thus contribute to maturational differences in human hippocampal N-methyl-d-aspartate receptor function, and to their role in the pathophysiology of schizophrenia and other neurodevelopmental disorders.

N-methyl-D-aspartate receptor blockade after status epilepticus protects against limbic brain damage but not against epilepsy in the kainate model of temporal lobe epilepsy

Most patients with temporal lobe epilepsy (TLE), the most common type of epilepsy, show pronounced loss of neurons in limbic brain regions, including the hippocampus. The massive neurodegeneration in the hippocampus is known as hippocampal sclerosis, and is considered one of the hallmarks of this type of difficult-to-treat epilepsy. There is a long and ongoing debate on whether this sclerosis is the result of an initial pathological event, such as a status epilepticus (S.E.), stroke or head trauma, which often precedes the development of TLE, or is caused by the spontaneous recurrent seizures (SRS) once epilepsy has developed. At present, pharmacological prevention of limbic sclerosis is not available. In a clinical situation, such prevention would only be possible if delayed cell death developing after an initial pathological event is involved. Assuming that sclerotic brain lesions provoke epileptogenesis and that delayed cell death is involved in these lesions, it should be possible to prevent both the lesions and the epilepsy by a prophylactic treatment after an initial insult such as an S.E. In order to test this hypothesis, we used a rat model of TLE in which limbic brain lesions and epilepsy with SRS develop after a kainate-induced S.E. A single low dose of the N-methyl-D-aspartate (NMDA) receptor blocker dizocilpine (MK-801) significantly reduced the damage in limbic regions, including the hippocampus and piriform cortex, and completely protected several rats from such damage when given after an S.E. of 90 min induced by kainate, strongly suggesting that delayed cell death is involved in the damage. This was substantiated by the use of molecular and immunohistochemical markers of delayed active ("programmed") cell death. However, the neuroprotection by dizocilpine did not prevent the development of SRS after the S.E., suggesting that structures not protected by dizocilpine may play a role in the genesis of SRS or that epileptogenesis is not the consequence of structural lesions in the limbic system. The only brain regions that exhibited neuronal damage in all rats with SRS were the hilus of the dentate gyrus and the mediodorsal thalamus, although treatment with dizocilpine reduced the severity of damage in the latter region. The data indicate that NMDA receptor blockade immediately after a prolonged S.E. is an effective means to reduce the damage produced by a sustained S.E. in several brain regions, including the hippocampus, but show that this partial neuroprotection of the limbic system does not prevent the development of epilepsy.

A dual role of adenosine A2A receptors in 3-nitropropionic acid-induced striatal lesions: implications for the neuroprotective potential of A2A antagonists

Reduction of A2A receptor expression is one of the earliest events occurring in both Huntington's disease (HD) patients and mice overexpressing the N-terminal part of mutated huntingtin. Interestingly, increased activity of A2A receptors has been found in striatal cells prone to degenerate in experimental models of this neurodegenerative disease. However, the role of A2A receptors in the pathogenesis of HD remains obscure. In the present study, using A2A-/- mice and pharmacological compounds in rat, we demonstrate that striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3NP) is regulated by A2A receptors. Our results show that the striatal outcome induced by 3NP depends on a balance between the deleterious activity of presynaptic A2A receptors and the protective activity of postsynaptic A2A receptors. Moreover, microdialysis data demonstrate that this balance is anatomically determined, because the A2A presynaptic control on striatal glutamate release is absent within the posterior striatum. Therefore, because blockade of A2A receptors has differential effects on striatal cell death in vivo depending on its ability to modulate presynaptic over postsynaptic receptor activity, therapeutic use of A2A antagonists in Huntington's as well as in other neurodegenerative diseases could exhibit undesirable biphasic neuroprotective-neurotoxic effects.

Reprogramming the host response in bacterial meningitis: how best to improve outcome?

Despite effective antibiotic therapy, bacterial meningitis is still associated with high morbidity and mortality in both children and adults. Animal studies have shown that the host inflammatory response induced by bacterial products in the subarachnoid space is associated with central nervous system injury. Thus, attenuation of inflammation early in the disease process might improve the outcome. The feasibility of such an approach is demonstrated by the reduction in neurologic sequelae achieved with adjuvant dexamethasone therapy. Increased understanding of the pathways of inflammation and neuronal damage has suggested rational new targets to modulate the host response in bacterial meningitis, but prediction of which agents would be optimal has been difficult. This review compares the future promise of benefit from the use of diverse adjuvant agents. It appears unlikely that inhibition of a single proinflammatory mediator will prove useful in clinical practice, but several avenues to reprogram a wider array of mediators simultaneously are encouraging. Particularly promising are efforts to adjust combinations of cytokines, to inhibit neuronal apoptosis and to enhance brain repair.

A dual role of adenosine A2A receptors in 3-nitropropionic acid-induced striatal lesions: implications for the neuroprotective potential of A2A antagonists

Reduction of A2A receptor expression is one of the earliest events occurring in both Huntington's disease (HD) patients and mice overexpressing the N-terminal part of mutated huntingtin. Interestingly, increased activity of A2A receptors has been found in striatal cells prone to degenerate in experimental models of this neurodegenerative disease. However, the role of A2A receptors in the pathogenesis of HD remains obscure. In the present study, using A2A-/- mice and pharmacological compounds in rat, we demonstrate that striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3NP) is regulated by A2A receptors. Our results show that the striatal outcome induced by 3NP depends on a balance between the deleterious activity of presynaptic A2A receptors and the protective activity of postsynaptic A2A receptors. Moreover, microdialysis data demonstrate that this balance is anatomically determined, because the A2A presynaptic control on striatal glutamate release is absent within the posterior striatum. Therefore, because blockade of A2A receptors has differential effects on striatal cell death in vivo depending on its ability to modulate presynaptic over postsynaptic receptor activity, therapeutic use of A2A antagonists in Huntington's as well as in other neurodegenerative diseases could exhibit undesirable biphasic neuroprotective-neurotoxic effects.

Blockade of ionotropic glutamate receptors produces neuronal apoptosis through the Bax-cytochrome C-caspase pathway: the causative role of Ca2+ deficiency

Blockade of ionotropic glutamate receptors induces neuronal cell apoptosis. We investigated if mitochondria-mediated death signals would contribute to neuronal apoptosis following administration of glutamate antagonists. The administration of MK-801 and CNQX (MK-801/CNQX), the selective antagonists of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors, produced widespread neuronal death in neonatal rat brain and cortical cell cultures. MK-801/CNQX-induced neuronal apoptosis was prevented by zVAD-fmk, a broad inhibitor of caspases, but insensitive to inhibitors of calpain or cathepsin D. Activation of caspase-3 was observed within 6-12 h and sustained over 36 h after exposure to MK-801/CNQX, which cleaved PHF-1 tau, the substrate for caspase-3. Activation of caspase-3 was blocked by high K+ and mimicked by BAPTA-AM, a selective Ca2+ chelator. Reducing extracellular Ca2+, but not Na+, activated caspase-3, suggesting an essential role of Ca2+ deficiency in MK-801/CNQX-induced activation of caspases. Cortical neurons treated with MK-801/CNQX triggered activation of caspase-9, release of cytochrome c from mitochondria, and translocation of Bax into mitochondria. The present study suggests that blockade of ionotropic glutamate receptors causes caspase-3-mediated neuronal apoptosis due to Ca2+ deficiency that is coupled to the sequential mitochondrial death pathway.

The glutamate AMPA receptor antagonist, YM872, attenuates cortical tissue loss, regional cerebral edema, and neurological motor deficits after experimental brain injury in rats

A massive increase in extracellular glutamate is thought to contribute to brain damage after traumatic brain injury. We examined the neuroprotective effect of the AMPA receptor antagonist YM872 in a rat head injury model using the fluid-percussion procedure. Male Sprague-Dawley rats were subjected to right lateral (parasagittal) fluid-percussion brain injury or sham injury. At 15 min postinjury, they received either YM872 (20 mg/kg/h, 20 mg/3 mL) or normal saline (vehicle) intravenously for 4 h. The administration of YM872 significantly improved the composite neuroscore at 1 and 2 weeks postinjury (p < 0.05), and markedly reduced the volume of tissue loss in the injured cortex (p < 0.05). It also significantly reduced cerebral edema in the ipsilateral parietal cortex at 48 h postinjury (p < 0.01). These results indicate that the posttraumatic administration of YM872 may be neuroprotective by ameliorating cortical tissue loss and regional cerebral edema, and suggest the importance of AMPA receptors in traumatic brain damage involving secondary injury processes.

The Yin and Yang of NMDA receptor signalling

Ca(2+) entry through the NMDA subtype of glutamate receptors has the power to determine whether neurons survive or die. Too much NMDA receptor activity is harmful to neurons - but so is too little. Is it a case of too much or too little Ca(2+) influx causing cell death or do other factors, such as receptor location or receptor-associated proteins, play a role? Understanding the mechanisms behind this dichotomous signalling is an important area of molecular neuroscience with direct clinical implications.

DNQX-induced toxicity in cultured rat hippocampal neurons: an apparent AMPA receptor-independent effect?

To evaluate the involvement of AMPA receptor activation in neuronal cell death and survival, rat hippocampal neurons in culture were treated with AMPA receptor antagonists. A 46 h treatment with 6,7-dinitroquinoxaline-2,3-dione (DNQX), added 2 h after cell plating, induces a dose-dependent neurotoxicity. Similar effects are also observed in more mature hippocampal neurons (treatment at 14 days in vitro). DNQX toxic effect is neuron-specific since cultured hippocampal glial cells are unaffected. Attempts to characterise the site of action of DNQX suggest that ionotropic glutamate receptors would not be implicated. Indeed, (i) other AMPA receptor antagonists are either ineffective or only moderately efficient in mimicking DNQX effects; (ii) AMPA alone or in the presence of cyclothiazide, as well as, other AMPA receptor agonists, do not reverse DNQX action; (iii) DNQX neurotoxicity is not likely to involve blockade of NMDA receptor glycine site, since this effect is neither mimicked by 7-chlorokynurenate nor reversed by D-serine. Thus, DNQX toxicity in cultured hippocampal neurons is apparently mediated through an ionotropic glutamate receptor-independent way.

Endoplasmic reticulum, Bcl-2 and Ca2+ handling in apoptosis

In the complex signalling interplay that allows extracellular signals to be decoded into activation of apoptotic cell death, Ca(2+) plays a significant role. This is supported not only by evidence linking alterations in Ca(2+) homeostasis to the triggering of apoptotic (and in some cases necrotic) cell death, but also by recent data indicating that a key anti-apoptotic protein, Bcl-2, has a direct effect on ER Ca(2+) handling. We will briefly summarise the first aspect, and describe in more detail these new data, demonstrating that (i) Bcl-2 reduces the state of filling of the ER Ca(2+) store and (ii) this Ca(2+) signalling alteration renders the cells less sensitive to apoptotic stimuli. Overall, these results suggest that calcium homeostasis may represent a pharmacological target in the fundamental pathological process of apoptosis.

Why did NMDA receptor antagonists fail clinical trials for stroke and traumatic brain injury?

Glutamate N-methyl-D-aspartate (NMDA) receptor antagonists (competitive receptor antagonists, ion channel blockers, and glycine antagonists)--such as selfotel, aptiganel, eliprodil, licostinel and gavestinel--failed to show efficacy in clinical trials of stroke or traumatic brain injury. This failure has been attributed to the deficient properties of the molecules that entered human trials and to inappropriate design of clinical studies. In this article we hypothesise that glutamate may be involved in the acute neurodestructive phase that occurs immediately after traumatic or ischaemic injury (excitotoxicity), but that, after this period, it assumes its normal physiological functions, which include promotion of neuronal survival. We propose that NMDA receptor antagonists failed stroke and traumatic brain injury trials in human beings because blockade of synaptic transmission mediated by NMDA receptors hinders neuronal survival.

Immune-related mechanisms participating in resistance and susceptibility to glutamate toxicity

Glutamate is an essential neurotransmitter in the CNS. However, at abnormally high concentrations it becomes cytotoxic. Recent studies in our laboratory showed that glutamate evokes T cell-mediated protective mechanisms. The aim of the present study was to examine the nature of the glutamate receptors and signalling pathways that participate in immune protection against glutamate toxicity. We show, using the mouse visual system, that glutamate-induced toxicity is strain dependent, not only with respect to the amount of neuronal loss it causes, but also in the pathways it activates. In strains that are genetically endowed with the ability to manifest a T cell-dependent neuroprotective response to glutamate insult, neuronal losses due to glutamate toxicity were relatively small, and treatment with NMDA-receptor antagonist worsened the outcome of exposure to glutamate. In contrast, in mice devoid of T cell-dependent endogenous protection, NMDA receptor antagonist reduced the glutamate-induced neuronal loss. In all strains, blockage of the AMPA/KA receptor was beneficial. Pharmacological (with alpha2-adrenoceptor agonist) or molecular intervention (using either mice overexpressing Bcl-2, or DAP-kinase knockout mice) protected retinal ganglion cells from glutamate toxicity but not from the toxicity of NMDA. The results suggest that glutamate-induced neuronal toxicity involves multiple glutamate receptors, the types and relative contributions of which, vary among strains. We suggest that a multifactorial protection, based on an immune mechanism independent of the specific pathway through which glutamate exerts its toxicity, is likely to be a safer, more comprehensive, and hence more effective strategy for neuroprotection. It might suggest that, because of individual differences, the pharmacological use of NMDA-antagonist for neuroprotective purposes might have an adverse effect, even if the affinity is low.

A novel N-methyl-D-aspartate receptor open channel blocker with in vivo neuroprotectant activity

Excitotoxicity has been implicated in the etiology of ischemic stroke, chronic neurodegenerative disorders, and very recently, in glioma growth. Thus, the development of novel neuroprotectant molecules that reduce excitotoxic brain damage is vigorously pursued. We have used an ionic current block-based cellular assay to screen a synthetic combinatorial library of trimers of N-alkylglycines on the N-methyl-D-aspartate (NMDA) receptor, a well known molecular target involved in excitotoxicity. We report the identification of a family of N-alkylglycines that selectively blocked the NMDA receptor. Notably, compound 3,3-diphenylpropyl-N-glycinamide (referred to as N20C) inhibited NMDA receptor channel activity with micromolar affinity, fast on-off blockade kinetics, and strong voltage dependence. Molecule N20C did not act as a competitive glutamate or glycine antagonist. In contrast, saturation of the blocker binding site with N20C prevented dizolcipine (MK-801) blockade of the NMDA receptor, implying that both drugs bind to the same receptor site. The N-alkylglycine efficiently prevented in vitro excitotoxic neurodegeneration of cerebellar and hippocampal neurons in culture. Attenuation of neuronal glutamate/NMDA-induced Ca(2+) overload and subsequent modulation of the glutamate-nitric oxide-cGMP pathway seems to underlie N20C neuroprotection. Noteworthy, this molecule exhibited significant in vivo neuroprotectant activity against an acute, severe, excitotoxic insult. Taken together, these findings indicate that N-alkylglycine N20C is a novel, low molecular weight, moderate-affinity NMDA receptor open channel blocker with in vitro and in vivo neuroprotective activity, which, in due turn, may become a tolerated drug for the treatment of neurodegenerative diseases and cancer.

Extrasynaptic NMDARs oppose synaptic NMDARs by triggering CREB shut-off and cell death pathways

Here we report that synaptic and extrasynaptic NMDA (N-methyl-D-aspartate) receptors have opposite effects on CREB (cAMP response element binding protein) function, gene regulation and neuron survival. Calcium entry through synaptic NMDA receptors induced CREB activity and brain-derived neurotrophic factor (BDNF) gene expression as strongly as did stimulation of L-type calcium channels. In contrast, calcium entry through extrasynaptic NMDA receptors, triggered by bath glutamate exposure or hypoxic/ischemic conditions, activated a general and dominant CREB shut-off pathway that blocked induction of BDNF expression. Synaptic NMDA receptors have anti-apoptotic activity, whereas stimulation of extrasynaptic NMDA receptors caused loss of mitochondrial membrane potential (an early marker for glutamate-induced neuronal damage) and cell death. Specific blockade of extrasynaptic NMDA receptors may effectively prevent neuron loss following stroke and other neuropathological conditions associated with glutamate toxicity.

Prevention of in vivo excitotoxicity by a family of trialkylglycines, a novel class of neuroprotectants

Excitotoxicity has been implicated in the etiology of ischemic stroke and chronic neurodegenerative disorders. Hence, the development of novel neuroprotectant molecules that ameliorate excitotoxic brain damage is vigorously pursued. We used a neuroprotection-based cellular assay to screen a synthetic combinatorial library of N-alkylglycine trimers. Two compounds (6-1-2 and 6-1-10) that efficiently prevented excitotoxic neurodegeneration in vitro and in vivo were identified. Both molecules protected primary cultures of cerebellar neurons against glutamate-induced neuronal death with an efficiency equivalent to N-methyl-D-aspartate (NMDA) receptor antagonists. These trialkylglycines did not block appreciably the NMDA receptor channel, or attenuated glutamate-induced increase of Ca(2+), or affect the glutamate-nitric oxide-cGMP pathway. Intraperitoneal injection of both peptoids in mice attenuated > or = 80% ammonia-induced, NMDA receptor-mediated animal death. Furthermore, these two molecules reduced by > or = 50% the neurodegeneration in striatum in a rat model of cerebral ischemia. Neuroprotection against ischemia was associated with decreased activation of caspase-3, reflecting prevention of apoptotic neuronal death. Collectively, the results reported indicate that these trialkylglycines are new neuroprotectant leads with important in vivo activity against excitotoxicity, and that they act on a novel, yet-unrecognized cellular target. These lead compounds may become tolerated drugs for the treatment of acute and chronic neurodegenerative diseases with fewer side effects than NMDA receptor antagonists.

Selective and biphasic effect of the membrane lipid peroxidation product 4-hydroxy-2,3-nonenal on N-methyl-D-aspartate channels

Increased oxyradical production and membrane lipid peroxidation occur in neurons under physiological conditions and in neurodegenerative disorders. Lipid peroxidation can alter synaptic plasticity and may increase the vulnerability of neurons to excitotoxicity, but the underlying mechanisms are unknown. We report that 4-hydroxy-2,3-nonenal (4HN), an aldehyde product of lipid peroxidation, exerts a biphasic effect on NMDA-induced current in cultured rat hippocampal neurons with current being increased during the first 2 h and decreased after 6 h. Similarly, 4HN causes an early increase and a delayed decrease in NMDA-induced elevation of intracellular Ca2+ levels. In contrast, 4HN affects neither the ion current nor the Ca2+ response to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). The initial enhancement of NMDA-induced current is associated with increased phosphorylation of the NR1 receptor subunit, whereas the delayed suppression of current is associated with cellular ATP depletion and mitochondrial membrane depolarization. Cell death induced by 4HN is attenuated by an NMDA receptor antagonist, but not by an AMPA receptor antagonist. A secreted form of amyloid precursor protein, previously shown to protect neurons against oxidative and excitotoxic insults, prevented each of the effects of 4HN including the early and late changes in NMDA current, delayed ATP depletion, and cell death. These findings show that the membrane lipid peroxidation product 4HN can modulate NMDA channel activity, suggesting a role for this aldehyde in physiological and pathophysiological responses of neurons to oxidative stress.