On the hypes and falls in neuroprotection: targeting the NMDA receptor

Neuroprotection against traumatic brain injury by a peptide derived from the collapsin response mediator protein 2 (CRMP2)

Neurological disabilities following traumatic brain injury (TBI) may be due to excitotoxic neuronal loss. The excitotoxic loss of neurons following TBI occurs largely due to hyperactivation of N-methyl-d-aspartate receptors (NMDARs), leading to toxic levels of intracellular Ca(2+). The axon guidance and outgrowth protein collapsin response mediator protein 2 (CRMP2) has been linked to NMDAR trafficking and may be involved in neuronal survival following excitotoxicity. Lentivirus-mediated CRMP2 knockdown or treatment with a CRMP2 peptide fused to HIV TAT protein (TAT-CBD3) blocked neuronal death following glutamate exposure probably via blunting toxicity from delayed calcium deregulation. Application of TAT-CBD3 attenuated postsynaptic NMDAR-mediated currents in cortical slices. In exploring modulation of NMDARs by TAT-CBD3, we found that TAT-CBD3 induced NR2B internalization in dendritic spines without altering somal NR2B surface expression. Furthermore, TAT-CBD3 reduced NMDA-mediated Ca(2+) influx and currents in cultured neurons. Systemic administration of TAT-CBD3 following a controlled cortical impact model of TBI decreased hippocampal neuronal death. These findings support TAT-CBD3 as a novel neuroprotective agent that may increase neuronal survival following injury by reducing surface expression of dendritic NR2B receptors.

On the promise of pharmacotherapies targeted at cognitive and neurodegenerative components of Down syndrome

Down syndrome (DS) is the phenotypic consequence of trisomy 21 and is the most common genetically defined cause of intellectual disability. The most complete, widely available, and well-studied animal model of DS is the Ts65Dn mouse. Recent preclinical successes in rescuing learning and memory deficits in Ts65Dn mice are legitimate causes for optimism that pharmacotherapies for cognitive deficits in DS might be within reach. This article provides a snapshot of potential pharmacotherapies for DS, with emphasis on our recent results showing that the N-methyl-D-aspartate receptor antagonist memantine can reverse learning and memory deficits in Ts65Dn mice. Because memantine has already been approved for the therapy of Alzheimer's dementia, we have been able to very rapidly translate these results into human research and are currently conducting a 16-week, randomized, double-blind, placebo-controlled evaluation of the efficacy, tolerability and safety of memantine hydrochloride on enhancing the cognitive abilities of young adults with DS. The design and current status of this clinical trial will be discussed, which will be followed by some speculation on the potential impact of this and future clinical trials in the field of DS.

Targeted over-expression of glutamate transporter 1 (GLT-1) reduces ischemic brain injury in a rat model of stroke

Following the onset of an ischemic brain injury, the excitatory neurotransmitter glutamate is released. The excitotoxic effects of glutamate are a major contributor to the pathogenesis of a stroke. The aim of this study was to examine if overexpression of a glutamate transporter (GLT-1) reduces ischemic brain injury in a rat model of stroke. We generated an adeno-associated viral (AAV) vector expressing the rat GLT-1 cDNA (AAV-GLT1). Functional expression of AAV-GLT1 was confirmed by increased glutamate clearance rate in non-stroke rat brain as measured by in vivo amperometry. AAV-GLT1 was injected into future cortical region of infarction 3 weeks prior to 60 min middle cerebral artery occlusion (MCAo). Tissue damage was assessed at one and two days after MCAo using TUNEL and TTC staining, respectively. Behavioral testing was performed at 2, 8 and 14 days post-stroke. Animals receiving AAV-GLT1, compared to AAV-GFP, showed significant decreases in the duration and magnitude of extracellular glutamate, measured by microdialysis, during the 60 minute MCAo. A significant reduction in brain infarction and DNA fragmentation was observed in the region of AAV-GLT1 injection. Animals that received AAV-GLT1 showed significant improvement in behavioral recovery following stroke compared to the AAV-GFP group. We demonstrate that focal overexpression of the glutamate transporter, GLT-1, significantly reduces ischemia-induced glutamate overflow, decreases cell death and improves behavioral recovery. These data further support the role of glutamate in the pathogenesis of ischemic damage in brain and demonstrate that targeted gene delivery to decrease the ischemia-induced glutamate overflow reduces the cellular and behavioral deficits caused by stroke.

Variation in the NMDA receptor 2B subunit gene GRIN2B is associated with differential language lateralization

Variations in the N-methyl-d-aspartate receptor 2B subunit gene (GRIN2B) have been associated with schizophrenia, a psychiatric disorder associated with reduced left-hemispheric language dominance. Here, we investigated, whether different polymorphisms in GRIN2B influence language lateralization and handedness in healthy individuals. In a cohort of 424 genetically unrelated participants we found significant association between the synonymous GRIN2B variation rs1806201 and language lateralization assessed using the dichotic listening task. Individuals carrying the heterozygous CT genotype exhibited more pronounced left-hemispheric language dominance as compared to both homozygous CC and TT individuals. Such an association was not identified for handedness. These findings suggest that variation in NMDA-receptors contributes to the interindividual variability of language lateralization.

Future Perspectives for Brain Pharmacotherapies: Implications of Drug Transport Processes at the Blood-brain Barrier

Years ago, ischemic stroke was regarded as a model disease for the development of neuroprotective therapies by the pharmacological industry. Results were disappointing. There are still no treatments available allowing the rescue of brain tissue once a stroke has occurred. Study failure is not only a problem in the stroke field. In other neurodegenerative conditions and in non-degenerative brain disorders, progress in drug development was also rather scarce until recently. An important factor in drug failure is the blood-brain barrier, which expresses active transporters that eliminate drugs from the brain. These transporters exhibit strong variations between different animals, which make it difficult to predict brain concentrations of drugs over species barriers. This paper claims that more detailed knowledge about: (1) the biology of blood-brain barrier transporters; (2) their regulation in brain disease, (3) the affinity of transporters to candidate drugs; and (4) the accumulation of drugs in brain tissue is needed for the overall success of clinical trials to be improved. An alternative strategy could be the use of disease-modifying treatments that do not have to enter the brain to exert their function. As such, restorative and anti-inflammatory strategies acting at the blood-brain interface might gain therapeutic potential in the future.

Variations in the GRIN2B gene are associated with risky decision-making

The dopaminergic system is known to modulate decision-making. As N-methyl-D-aspartate (NMDA) receptors strongly influence dopaminergic function, it is conceivable that the glutamatergic system is also involved in decision-making. We examined whether polymorphisms in the N-methyl-d-aspartate receptor 2B subunit gene (GRIN2B) influence decision-making using the Iowa Gambling Task (IGT). In total, 245 (n = 245, 127 female) healthy German students were included in the analysis. Two synonymous SNPs in exon 13, rs1806191 (H1178H) and rs1806201 (T888T) showed the strongest association with aspects of IGT performance. Females with a CC allele in rs1806201 made less use both of a win-stay strategy and demonstrated more exploratory behaviour during task execution. For rs1806191, we found a strong additive effect in usage of a win-stay strategy. This, partly sex-dependent, correlation of the win-stay/lose-shift behaviour with GRIN2B genotypes suggests that healthy individuals with certain GRIN2B variations respond differently to ambiguous conditions, possibly by altered perception of wins and losses. These findings underline the necessity to integrate the glutamatergic system when examining decision-making processes.

CaMKII in cerebral ischemia

Ischemic insults on neurons trigger excessive, pathological glutamate release that causes Ca²⁺ overload resulting in neuronal cell death (excitotoxicity). The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological excitatory glutamate signals underlying neuronal plasticity and learning. Glutamate stimuli trigger autophosphorylation of CaMKII at T286, a process that makes the kinase "autonomous" (partially active independent from Ca²⁺ stimulation) and that is required for forms of synaptic plasticity. Recent studies suggested autonomous CaMKII activity also as potential drug target for post-insult neuroprotection, both after glutamate insults in neuronal cultures and after focal cerebral ischemia in vivo. However, CaMKII and other members of the CaM kinase family have been implicated in regulation of both neuronal death and survival. Here, we discuss past findings and possible mechanisms of CaM kinase functions in excitotoxicity and cerebral ischemia, with a focus on CaMKII and its regulation.

Ginsenoside Rd attenuates redox imbalance and improves stroke outcome after focal cerebral ischemia in aged mice

We previously found that ginsenoside Rd (Rd), one of the main active ingredients in Panax ginseng, protects against ischemic brain damage induced by oxygen-glucose deprivation in vitro and middle cerebral artery occlusion (MCAO) in vivo. Considering stroke happens frequently in aged individuals, we herein sought to further define the protective effects of Rd in the aged mice. 16-18-month-old mice administered with Rd (0.1-200 mg/kg) or vehicle were subjected to transient MCAO. Rd at the doses of 10-50 mg/kg significantly reduced both cortical and striatal infarct volume. This protection was associated with an improvement in neurological function and was sustained for at least 2 weeks after the insult. Importantly, Rd was effective even when administered up to 4 h after recirculation. To evaluate the underlying mechanisms, oxidative DNA damage was identified by 8-hydroxy-deoxyguanosine immunostaining, oxidative protein damage was identified by the assessment of protein carbonyl, and lipid peroxidation was estimated by determining the malondialdehyde formation. Rd significantly suppressed the accumulations of DNA, protein and lipid peroxidation products at 24 h post-ischemia. Rd also protected mitochondria at 4 and 24 h after reperfusion as indicated by preserved respiratory chain complex activities and aconitase activity, lowered mitochondrial hydrogen peroxide production, and hyperpolarized mitochondrial membrane potential. Furthermore, Rd partly enhanced endogenous antioxidant activities following MCAO. Collectively, these findings demonstrated that Rd exerts neuroprotection against transient focal ischemia in the aged brain, which may be associated with the attenuation of redox imbalance.

Neuroprotective efficiency of NMDA receptor blockade in the striatum and CA3 hippocampus after various durations of cerebral ischemia in gerbils

The purpose of this study was to investigate neuroprotective efficiency of N-methyl D-aspartate (NMDA) receptor (NMDAR) blockade on the neuronal damage in the less studied and allegedly less affected CA3 hippocampus and striatum in the Mongolian gerbil model of global cerebral ischemia. The common carotid arteries of gerbils were occluded for 5, 10 or 15 minutes. Gerbils were given a low dose of non-competitive NMDA antagonist (MK-801, 3 mg/kg i.p.) or saline immediately after the occlusion in normothermic conditions. Neuronal damage was examined on 4th, 14th and 28th day after reperfusion. The effect of NMDAR blockade was followed in vivo by monitoring the neurological status of whole animals or at the cellular level by standard light- and confocal microscopy on brain slices. Increased duration of cerebral ischemia resulted in a progressive loss of striatal and CA3 hippocampal neurons. The most beneficial NMDAR blockade effect was observed when the neuronal damage was most severe - on the 28th day after 15-min ischemia. As judged by morphological and neurological data, the effect of ischemia is also apparent in the presumed less vulnerable regions (CA3 and striatum) which are functionally important in stroke plasticity. So, NMDAR blockade in normothermic conditions showed neuroprotective efficiency.

CGX-1007 prevents excitotoxic cell death via actions at multiple types of NMDA receptors

Glutamate induced excitotoxic injury through over-activation of N-methyl-D-aspartate receptors (NMDARs) plays a critical role in the development of many neurodegenerative diseases. The present study was undertaken to evaluate the role of CGX-1007 (Conantokin G) as a neuroprotective agent against NMDA-induced excitotoxicity. Conantokin G, a cone snail peptide isolated from Conus geographus is reported to selectively inhibit NR2B containing NMDARs with high specificity and is shown to have potent anticonvulsant and antinociceptive effects. CGX-1007 significantly reduced the excitotoxic cell death induced by NMDA in organotypic hippocampal brain slice cultures in a concentration-dependent manner. In contrast, ifenprodil, another NR2B specific antagonist failed to offer neuroprotection against NMDA-induced excitotoxicity. We further determined that the neuroprotection observed is likely due to the action of CGX-1007 at multiple NMDA receptor subtypes. In a series of electrophysiology experiments, CGX-1007 inhibited NMDA-gated currents in human embryonic kidney (HEK) 293 cells expressing NMDA receptors containing either NR1a/NR2B or NR1a/NR2A subunit combinations. CGX-1007 produced a weak inhibition at NR1a/NR2C receptors, whereas it had no effect on NR1a/NR2D receptors. Further, the inhibition of NMDA receptors by CGX-1007 was voltage-dependent with greater inhibition seen at hyperpolarized membrane potentials. The voltage-dependence of CGX-1007 activity was also observed in recordings of NMDA-gated currents evoked in native receptors expressed in cortical neurons in culture. Based on our results, we conclude that CGX-1007 is a potent neuroprotective agent that acts as an antagonist at both NR2A and NR2B containing receptors.

Synthesis of 4-(aminoalkyl) substituted 1,3-dioxanes as potent NMDA and σ receptor antagonists

Elongation of the distance between the oxygen heterocycle and the basic amino moiety or ring expansion of the oxygen heterocycle of the NMDA receptor antagonists dexoxadrol and etoxadrol led to compounds with promising NMDA receptor affinity. Herein the combination of both structural features, i.e. elongation of the O-heterocycle--amine distance with a 1,3-dioxane ring is envisaged. The synthesis of aminoethyl-1,3-dioxanes 13, 22, 23 and 29 was performed by transacetalization of various acetals with pentane-1,3,5-triol, activation of the remaining free OH moiety with tosyl chloride and subsequent nucleophilic substitution. The corresponding 3-aminopropyl derivatives 33-35 were prepared by substitution of the tosylates with KCN and LiAlH4 reduction. The highest NMDA receptor affinity was found for 1,3-dioxanes with a phenyl and an ethyl residue at the acetalic position (23) followed by diphenyl (22) and monophenyl derivatives (13). Generally the NMDA affinity of primary amines is higher than the NMDA affinity of secondary and tertiary amines. Altogether the primary amine 23a (Ki=24 nM) represents the most promising NMDA receptor antagonist of this series exceeding the NMDA affinity of the mono-homologues (2-aminoethyl)-1,3-dioxolanes (3,4) and (aminomethyl)-1,3-dioxanes (5,6). Whereas the primary amine 23a turned out to be selective against σ1 and σ2 receptors the benzylamine 13d was identified as potent (Ki=19 nM) and selective σ1 antagonist, which showed extraordinarily high antiallodynic activity in the capsaicin assay.

Saffron extract and trans-crocetin inhibit glutamatergic synaptic transmission in rat cortical brain slices

Saffron, the dried stigmata of Crocus sativus L., is used in traditional medicine for a wide range of indications including cramps, asthma, and depression. To investigate the influence of hydro-ethanolic saffron extract (CSE) and trans-crocetin on synaptic transmission, postsynaptic potentials (PSPs) were elicited by focal electrical stimulation and recorded using intracellular placed microelectrodes in pyramidal cells from rat cingulate cortex. CSE (10-200 μg/ml) inhibited evoked PSPs as well as the isolated NMDA and non-NMDA component of PSPs. Glutamate (500 μM) added into the organ bath induced membrane depolarization. CSE decreased glutamate-induced membrane depolarization. Additionally, CSE at 100 μg/ml decreased NMDA (20 μM) and kainate (1 μM)-induced depolarization, whereas AMPA (1 μM)-induced depolarization was not affected. Trans-crocetin (1-50 μM) showed inhibition of evoked PSPs and glutamate-induced membrane depolarization comparable to CSE. Trans-crocetin at 10 μM decreased NMDA (20 μM)-induced membrane depolarization, but did not inhibit the isolated non-NMDA component of PSPs. We conclude that trans-crocetin is involved in the antagonistic effect of CSE on NMDA but not on kainate receptors.

Knockout of Zn transporters Zip-1 and Zip-3 attenuates seizure-induced CA1 neurodegeneration

CA1 pyramidal neurons are the final integrators of information flow leaving the hippocampus, yet are singularly vulnerable to activity-dependent cell death. Zinc (Zn) entry into cells may add to this vulnerability. Here, we find that Slc39a1 and Slc39a3, members of the Zip (Zrt/Irt-like protein) plasmalemmal Zn transporter family, are predominantly expressed in the hippocampus. We examined Zip-1,3-deficient mice to investigate their role in neurodegeneration following intense synaptic activation. When isolated by blockade of NMDA receptors and voltage-gated calcium channels, the absence of both transporters slowed passive Zn uptake into CA1 neurons measured with intracellular fluorescent Zn dyes. In vivo CA1 cell damage following kainic acid exposure was greatly attenuated. Consistent with the hypothesis that Zn entry contributes to neurodegeneration, Znt-3-deficient mice lacking synaptic Zn also show less hippocampal cell damage following kainic acid injection. Zip transporters may provide selective therapeutic targets to protect these neurons from early Zn-induced neurodegeneration following injury.

Sepsis-associated encephalopathy and its differential diagnosis

Sepsis-associated encephalopathy (SAE) is defined as a diffuse cerebral dysfunction resulting from the systemic inflammatory response to an infection without direct infestation of the CNS. Although the pathophysiology of SAE is as yet unknown, some mechanisms have been suggested that involve BBB disruption as a consequence of proinflammatory mediators’ effects on endothelial cells. This leads to an increased passage of neurotoxic and proinflammatory mediators into the brain parenchyma, as well as an impairment of the movements of oxygen and metabolites through the BBB. Both neurons and glial cells are affected, resulting in neural functioning and neurotransmission impairment. The clinical translation of this process is an alteration of consciousness and awareness. SAE is a frequent condition in septic patients. Despite being considered reversible, SAE appears to be associated with long-term cognitive impairment. Detection and diagnosis can be challenging; it requires daily neurological assessment with the assistance of clinical scores. Use of biomarkers and neurophysiological testing is discussed. The aim of this article is to provide practical tools for detection of SAE, as well as an updated overview of its pathophysiology and therapeutic perspectives.

Changes in expression of NMDA-NR1 receptor subunits in the rostral ventromedial medulla modulate pain behaviors

NMDA receptors have an important role in pain facilitation in rostral ventromedial medulla (RVM) and the NR1 subunit is essential for its function. Studies suggest that the NMDA receptors in RVM are critical to modulate both cutaneous and muscle hypersensitivity induced by repeated intramuscular acid injections. We propose that increased expression of the NR1 subunit in the RVM is critical for the full development of hypersensitivity. To test this we used recombinant lentiviruses to over-express the NR1 subunit in the RVM and measured nociceptive sensitivity to cutaneous and muscle stimuli. We also downregulated the expression of NR1 in the RVM and measured the hyperalgesia produced by repeated-acid injections. Increasing the expression of NR1 in the RVM reduces cutaneous and muscle withdrawal threshold, and decreasing the expression of NR1 in the RVM increases the muscle withdrawal threshold and prevents the development of hyperalgesia in an animal model of muscle pain. These results suggest that the NR1 subunits in the RVM are critical for modulating NMDA receptor function, which in turn sets the 'tone' of the nervous system's response to noxious stimuli and tissue injury.

A novel family of negative and positive allosteric modulators of NMDA receptors

The N-methyl-D-aspartate (NMDA) receptor family regulates various central nervous system functions, such as synaptic plasticity. However, hypo- or hyperactivation of NMDA receptors is critically involved in many neurological and psychiatric conditions, such as pain, stroke, epilepsy, neurodegeneration, schizophrenia, and depression. Consequently, subtype-selective positive and negative modulators of NMDA receptor function have many potential therapeutic applications not addressed by currently available compounds. We have identified allosteric modulators with several novel patterns of NMDA receptor subtype selectivity that have a novel mechanism of action. In a series of carboxylated naphthalene and phenanthrene derivatives, compounds were identified that selectively potentiate responses at GluN1/GluN2A [e.g., 9-iodophenanthrene-3-carboxylic acid (UBP512)]; GluN1/GluN2A and GluN1/GluN2B [9-cyclopropylphenanthrene-3-carboxylic acid (UBP710)]; GluN1/GluN2D [3,5-dihydroxynaphthalene-2-carboxylic acid (UBP551)]; or GluN1/GluN2C and GluN1/GluN2D receptors [6-, 7-, 8-, and 9-nitro isomers of naphth[1,2-c][1,2,5]oxadiazole-5-sulfonic acid (NSC339614)] and have no effect or inhibit responses at the other NMDA receptors. Selective inhibition was also observed; UBP512 inhibits only GluN1/GluN2C and GluN1/GluN2D receptors, whereas 6-bromo-2-oxo-2H-chromene-3-carboxylic acid (UBP608) inhibits GluN1/GluN2A receptors with a 23-fold selectivity compared with GluN1/GluN2D receptors. The actions of these compounds were not competitive with the agonists L-glutamate or glycine and were not voltage-dependent. Whereas the N-terminal regulatory domain was not necessary for activity of either potentiators or inhibitors, segment 2 of the agonist ligand-binding domain was important for potentiating activity, whereas subtype-specific inhibitory activity was dependent upon segment 1. In terms of chemical structure, activity profile, and mechanism of action, these modulators represent a new class of pharmacological agents for the study of NMDA receptor subtype function and provide novel lead compounds for a variety of neurological disorders.

Functional heterogeneity of NMDA receptors in rat substantia nigra pars compacta and reticulata neurones

The nigra substantia nigra pars compacta (SNc) and substantia pars reticulata (SNr) form two major basal ganglia components with different functional roles. SNc dopaminergic (DA) neurones are vulnerable to cell death in Parkinson's disease, and NMDA receptor activation is a potential contributing mechanism. We have investigated the sensitivity of whole-cell and synaptic NMDA responses to intracellular ATP and GTP application in the SNc and SNr from rats on postnatal day (P) 7 and P28. Both NMDA current density (pA/pF) and desensitization to prolonged or repeated NMDA application were greater in the SNr than in the SNc. When ATP levels were not supplemented, responses to prolonged NMDA administration desensitized in P7 SNc DA neurones but not at P28. At P28, SNr neurones desensitized more than SNc neurones, with or without added ATP. Responses to brief NMDA applications and synaptic NMDA currents were not sensitive to inclusion of ATP in the pipette solution. To investigate these differences between the SNc and SNr, NR2 subunit-selective antagonists were tested. NMDA currents were inhibited by ifenprodil (10 microM) and UBP141 (4 microM), but not by Zn(2+) (100 nm), in both the SNr and SNc, suggesting that SNc and SNr neurones express similar receptor subunits; NR2B and NR2D, but not NR2A. The different NMDA response properties in the SNc and SNr may be caused by differences in receptor modulation and/or trafficking. The vulnerability of SNc DA neurones to cell death is not correlated with NMDA current density or receptor subtypes, but could in part be related to inadequate NMDA receptor desensitization.

Pathologically activated neuroprotection via uncompetitive blockade of N-methyl-D-aspartate receptors with fast off-rate by novel multifunctional dimer bis(propyl)-cognitin

Uncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists with fast off-rate (UFO) may represent promising drug candidates for various neurodegenerative disorders. In this study, we report that bis(propyl)-cognitin, a novel dimeric acetylcholinesterase inhibitor and gamma-aminobutyric acid subtype A receptor antagonist, is such an antagonist of NMDA receptors. In cultured rat hippocampal neurons, we demonstrated that bis(propyl)-cognitin voltage-dependently, selectively, and moderately inhibited NMDA-activated currents. The inhibitory effects of bis(propyl)-cognitin increased with the rise in NMDA and glycine concentrations. Kinetics analysis showed that the inhibition was of fast onset and offset with an off-rate time constant of 1.9 s. Molecular docking simulations showed moderate hydrophobic interaction between bis(propyl)-cognitin and the MK-801 binding region in the ion channel pore of the NMDA receptor. Bis(propyl)-cognitin was further found to compete with [(3)H]MK-801 with a K(i) value of 0.27 mum, and the mutation of NR1(N616R) significantly reduced its inhibitory potency. Under glutamate-mediated pathological conditions, bis(propyl)-cognitin, in contrast to bis(heptyl)-cognitin, prevented excitotoxicity with increasing effectiveness against escalating levels of glutamate and much more effectively protected against middle cerebral artery occlusion-induced brain damage than did memantine. More interestingly, under NMDA receptor-mediated physiological conditions, bis(propyl)-cognitin enhanced long-term potentiation in hippocampal slices, whereas MK-801 reduced and memantine did not alter this process. These results suggest that bis(propyl)-cognitin is a UFO antagonist of NMDA receptors with moderate affinity, which may provide a pathologically activated therapy for various neurodegenerative disorders associated with NMDA receptor dysregulation.

MK-801 effect on regional cerebral oxidative stress rate induced by different duration of global ischemia in gerbils

We investigated MK-801 effect on ischemia-induced oxidative stress-the most important factor that exacerbates brain damage by reperfusion. The common carotid arteries of gerbils were occluded for 5, 10, or 15 min. Immediately after the occlusion, MK-801 (3 mg/kg i.p.) or saline were given in normothermic conditions. The MK-801 effects were followed in vivo by monitoring the neurological status of animals and at the intracellular level by standard biochemical assays. We investigated nitric oxide levels, superoxide production, superoxide dismutase activity, index of lipid peroxidation (ILP), and reduced glutathione content in hippocampus, striatum, forebrain cortex, and cerebellum. The measurements took place at different times (1, 2, 4, 7, 14, and 28 days) after reperfusion. Increased duration of cerebral ischemia resulted in a progressive induction of oxidative stress. Our results revealed pattern of dynamic changes in each oxidative stress parameter level which corresponded with ischemia duration in all tested brain structures. Most sensitive oxidative stress parameters were ILP and superoxide production. Our study confirmed spatial distribution of ischemia-induced oxidative stress. Tested brain structures showed different sensitivity to each oxidative stress parameter. As judged by biochemical and neurological data, applied MK-801 showed neuroprotective efficiency by reduction of ischemia-induced oxidative stress in brain.

Effective post-insult neuroprotection by a novel Ca(2+)/ calmodulin-dependent protein kinase II (CaMKII) inhibitor

Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological glutamate signaling involved in higher brain functions. Here, we show CaMKII involvement in pathological glutamate signaling relevant in stroke. The novel inhibitor tatCN21 was neuroprotective even when added hours after glutamate insults. By contrast, the "traditional" inhibitor KN93 attenuated excitotoxicity only when present during the insult. Both inhibitors efficiently blocked Ca(2+)/CaM-stimulated CaMKII activity, CaMKII interaction with NR2B and aggregation of CaMKII holoenzymes. However, only tatCN21 but not KN93 blocked the Ca(2+)-independent "autonomous" activity generated by Thr-286 autophosphorylation, the hallmark feature of CaMKII regulation. Mutational analysis further validated autonomous CaMKII activity as the drug target crucial for post-insult neuroprotection. Overexpression of CaMKII wild type but not the autonomy-deficient T286A mutant significantly increased glutamate-induced neuronal death. Maybe most importantly, tatCN21 also significantly reduced infarct size in a mouse stroke model (middle cerebral arterial occlusion) when injected (1 mg/kg intravenously) 1 h after onset of arterial occlusion. Together, these data demonstrate that inhibition of autonomous CaMKII activity provides a promising therapeutic avenue for post-insult neuro-protection after stroke.

CXCL12 inhibits expression of the NMDA receptor's NR2B subunit through a histone deacetylase-dependent pathway contributing to neuronal survival

Homeostatic chemokines, such as CXCL12, can affect neuronal activity by the regulation of inhibitory and excitatory neurotransmission, but the mechanisms involved are still undefined. Our previous studies have shown that CXCL12 protects cortical neurons from excitotoxicity by promoting the function of the gene-repressor protein Rb, which is involved in the recruitment of chromatin modifiers (such as histone deacetylases (HDACs)) to gene promoters. In neurons, Rb controls activity-dependent genes essential to neuronal plasticity and survival, such as the N-methyl-D-aspartic acid (NMDA) receptor's subunit NR2B, the expression of which in the tetrameric ion channel largely affects calcium signaling by glutamate. In this study, we report that CXCL12 differentially modulates intracellular responses after stimulation of synaptic and extrasynaptic NMDA receptors, by a specific regulation of the NR2B gene that involves HDACs. Our results show that CXCL12 selectively inhibits NR2B expression in vitro and in vivo altering NMDA-induced calcium responses associated with neuronal death, while promoting prosurvival pathways that depend on stimulation of synaptic receptors. Along with previous studies, these findings underline the role of CXCL12/CXCR4 in the regulation of crucial components of glutamatergic transmission. These novel effects of CXCL12 may be involved in the physiological function of the chemokine in both developing and mature brains.

Kaitocephalin antagonism of glutamate receptors expressed in Xenopus oocytes

Kaitocephalin is the first discovered natural toxin with protective properties against excitotoxic-death of cultured neurons induced by N-methyl-d-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainic acid (kainate, KA) receptors. Nevertheless, the effects of kaitocephalin on the function of these receptors were unknown. In this work we report some pharmacological properties of synthetic (-)-kaitocephalin on rat brain glutamate receptors expressed in Xenopus laevis oocytes and, on the homomeric AMPA-type GluR3 and KA-type GluR6 receptors. Kaitocephalin was found to be a more potent antagonist of NMDA receptors (IC(50) = 75 +/- 9 nM) than of AMPA receptors from cerebral cortex (IC(50) = 242 +/- 37 nM) and from homomeric GluR3 subunits (IC(50) = 502 +/- 55 nM). Moreover, kaitocephalin is a weak antagonist of the KA-type receptor GluR6 (IC(50) ~ 100 muM) and of metabotropic (IC(50) > 100 muM) glutamate receptors expressed by rat brain mRNA.

Sepsis-associated encephalopathy and its differential diagnosis

Sepsis is often complicated by an acute and reversible deterioration of mental status, which is associated with increased mortality and is consistent with delirium but can also be revealed by a focal neurologic sign. Sepsis-associated encephalopathy is accompanied by abnormalities of electroencephalogram and somatosensory-evoked potentials, increased in biomarkers of brain injury (i.e., neuron-specific enolase, S-100 beta-protein) and, frequently, by neuroradiological abnormalities, notably leukoencephalopathy. Its mechanism is highly complex, resulting from both inflammatory and noninflammatory processes that affect all brain cells and induce blood-brain barrier breakdown, dysfunction of intracellular metabolism, brain cell death, and brain injuries. Its diagnosis relies essentially on neurologic examination that can lead one to perform specific neurologic tests. Electroencephalography is required in the presence of seizure; neuroimaging in the presence of seizure, focal neurologic signs or suspicion of cerebral infection; and both when encephalopathy remains unexplained. In practice, cerebrospinal fluid analysis should be performed if there is any doubt of meningitis. Hepatic, uremic, or respiratory encephalopathy, metabolic disturbances, drug overdose, withdrawal of sedatives or opioids, alcohol withdrawal delirium, and Wernicke's encephalopathy are the main differential diagnoses of sepsis-associated encephalopathy. Patient management is based mainly on controlling infection, organ system failure, and metabolic homeostasis, at the same time avoiding neurotoxic drugs.

Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

The neurodegenerative disease Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in early stages of the disease.

Neuroprotective effect of memantine combined with topiramate in hypoxic-ischemic brain injury

Glutamate receptor-mediated neurotoxicity is a major mechanism contributing to hypoxic-ischemic brain injury (HIBI). Memantine is a safe non-competitive NMDA receptor blocker characterized by its low affinity and fast unblocking kinetics. Topiramate is an AMPA/KA receptor blocker and use-dependent sodium channel blocker with several other neuroprotective actions and little neurotoxicity. We hypothesized that the coadministration of memantine and topiramate would be highly effective to attenuate HIBI in neonatal rats. Seven-day-old Sprague-Dawley rat pups were subjected to right common carotid artery ligation and hypoxia for 2 h, and then were randomly and blindly assigned to one of four groups: vehicle, memantine, topiramate and combination group. Brain injury was evaluated by gross damage and weight deficit of the right hemisphere at 22d after hypoxic-ischemia (HI) and by neurofunctional assessment (foot-fault test) at 21d post-HI. Acute neuronal injury was also evaluated by microscopic damage grading at 72 h post-HI. Results showed the combination of memantine and topiramate improved both pathological outcome and performance significantly. The drug-induced apoptotic neurodegeneration was assessed by TUNEL staining at 48 h post-HI and the result showed no elevated apoptosis in all observed areas. The result of the experiment indicates the combination therapy is safe and highly effective to reduce brain damage after HIBI.

Neuroprotection against neonatal hypoxia/ischemia-induced cerebral cell death by prevention of calpain-mediated mGluR1alpha truncation

Many cellular events are involved in ischemic neuronal death, and it has been difficult to identify those that play a critical role in the cascade triggered by lack of oxygen and glucose, although it has been widely recognized that overactivation of glutamate receptors represents one of the initiating factors. Different glutamate receptor antagonists, especially those for N-methyl-D-aspartate (NMDA) receptors, have achieved significant success in animal models of hypoxia/ischemia; however, these antagonists have failed in clinical trials. We previously reported that calpain-mediated truncation of metabotropic glutamate receptor 1alpha (mGluR1alpha) played a critical role in excitotoxicity, and that a TAT-mGluR1 peptide consisting of a peptide surrounding the calpain cleavage site of mGluR1alpha and the peptide transduction domain of the transactivating regulatory protein (TAT) of HIV was neuroprotective against excitotoxicity. In the present study we tested the effect of this peptide in in vitro and in vivo models of neonatal hypoxia/ischemia. TAT-mGluR1 peptide prevented oxygen/glucose deprivation- (OGD) and hypoxia/ischemia- (H/I) induced neuronal death in cultured hippocampal slices and neonatal rats, respectively. TAT-mGluR1 blocked H/I-induced mGluR1alpha degradation but had no effect on H/I-induced spectrin degradation, suggesting that neuroprotection was due to prevention of calpain-mediated mGluR1alpha truncation and not to calpain inhibition. Our results therefore suggest that mGluR1alpha truncation plays a critical role in neonatal hypoxia/ischemia and that blockade of this event may prevent the activation of many downstream cytotoxic cascades. Compared to glutamate receptor antagonists and general calpain inhibitors, TAT-mGluR1 may have limited side effects.

Antiapoptotic and neurotrophic effects of antidepressants: a review of clinical and experimental studies

Recent studies have strengthened the role of the abnormalities in neurotrophic pathways in the pathophysiology of depression. It has been shown that the depletion of growth factors, particularly brain-derived neurotrophic factor, may result in depression-like behavior in animals and may induce cellular changes that are reminiscent of those observed in depressed patients. Some authors even suggested that increased neuronal cell loss may contribute to the pathogenesis of depression. Hence, appreciable interest has been focused on the trophic and antiapoptotic effects of antidepressant drugs. In this paper, we put emphasis on the contribution of hippocampal atrophy, increased cell death and alterations in trophic factors to the pathogenesis of depression and their relationship to the potential of antidepressants to reverse these changes by modulating trophic factor cascades and preventing apoptosis. First, evidences for increased hippocampal atrophy and cell death in depression are discussed, followed by a review of selected studies of special interest that concern antiapoptotic action of antidepressant drugs. Next, depression-related neurotrophic abnormalities and their reversal by antidepressants are depicted. Finally, relationships among neurotrophins, antiapoptotic proteins and antioxidant enzymes in the pathology and treatment of depression are pointed out.

Increased cognitive functioning in symptomatic Huntington's disease as revealed by behavioral and event-related potential indices of auditory sensory memory and attention

Cognitive functions are thought to deteriorate globally in late stages of various neurodegenerative disorders. Here we describe that this general assumption is not justified and fails in Huntington's disease (HD). Presymptomatic gene mutation carriers (pHDs) and healthy controls performed worse compared with symptomatic HDs in an auditory signal detection task. During task performance, behavioral data and event-related potentials (ERPs) [i.e., MMN (mismatch negativity), P3a, and RON (reorienting negativity)] were recorded. Not only behavioral performance but also neurophysiological correlates of auditory sensory memory and attentional reorientation indicate enhanced performance occurring primal in late stages of a neurodegenerative disorder. Increased activity of the NMDA-receptor system, an assumed pathogenic mechanism in HD, might facilitate signal propagation at striatal level that enables more efficient task execution through a winner-take-all process. The results challenge the view that late stage neurodegeneration is necessarily related to a global decline in cognitive abilities in HD. In contrast, selectively enhanced cognitive functioning can emerge together with otherwise impaired cognitive functioning.

Bacopa monniera leaf extract ameliorates hypobaric hypoxia induced spatial memory impairment

Hypobaric hypoxia induced memory impairment has been attributed to several factors including increased oxidative stress, depleted mitochondrial bioenergetics, altered neurotransmission and apoptosis. This multifactorial response of the brain to hypobaric hypoxia limits the use of therapeutic agents that target individual pathways for ameliorating hypobaric hypoxia induced memory impairment. The present study aimed at exploring the therapeutic potential of a bacoside rich leaf extract of Bacopa monniera in improving the memory functions in hypobaric conditions. The learning ability was evaluated in male Sprague Dawley rats along with memory retrieval following exposure to hypobaric conditions simulating an altitude of 25,000 ft for different durations. The effect of bacoside administration on apoptosis, cytochrome c oxidase activity, ATP levels, and oxidative stress markers and on plasma corticosterone levels was investigated. Expression of NR1 subunit of N-methyl-d-aspartate receptors, neuronal cell adhesion molecules and was also studied along with CREB phosphorylation to elucidate the molecular mechanisms of bacoside action. Bacoside administration was seen to enhance learning ability in rats along with augmentation in memory retrieval and prevention of dendritic atrophy following hypoxic exposure. In addition, it decreased oxidative stress, plasma corticosterone levels and neuronal degeneration. Bacoside administration also increased cytochrome c oxidase activity along with a concomitant increase in ATP levels. Hence, administration of bacosides could be a useful therapeutic strategy in ameliorating hypobaric hypoxia induced cognitive dysfunctions and other related neurological disorders.

Blockade of NMDA receptor channels by 9-aminoacridine and its derivatives

9-Aminoacridine is known as "foot-in-the-door" NMDA receptor channel blocker because its binding prevents channel closure. Structural determinants of this mechanism of block were studied using a series of 9-aminoacridine derivatives. Experiments were performed on native NMDA receptors of hippocampal pyramidal neurons, isolated from rat brain slices. The use-dependence of block and kinetics of recovery from block were used to characterize mechanism of block produced by the compounds. Modifications, which preserve the flat structure of the tricyclic 9-aminoacridine moiety, affect blocking activity and kinetics but not the foot-in-the-door mechanism. On the contrary, disruption of the flat structure changes the mechanism of block to trapping. It is concluded that flat aromatic structure is one of the critical determinants of the action mechanism of 9-aminoacridine.

Expression of genes involved in excitatory neurotransmission in anoxic crucian carp (Carassius carassius) brain

The crucian carp, Carassius carassius, survives months without oxygen. During anoxia it needs to keep energy expenditure low, particularly in the brain, with its high rate of ATP use related to neuronal activity. This could be accomplished by reducing neuronal excitability through altered expression of genes involved in excitatory neurotransmission. Through cloning and the use of a recently developed real-time RT-PCR approach, with an external RNA control for normalization, we investigated the effect of 1 and 7 days of anoxia (12 degrees C) on the expression of 29 genes, including 8 3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits, 6 N-methyl-d-aspartate (NMDA) receptor subunits, 7 voltage-gated sodium and calcium channels, 4 glutamate transporters, and 4 genes involved in NMDA receptor-mediated neuroplasticity. The subunits of the majority of the gene families had expression profiles similar to those observed in the mammalian brain and showed remarkably stable expression during anoxia. This suggests that the genes may have similar functions in crucian carp and mammals, and that the excitatory abilities of the crucian carp brain are retained during anoxia. Although the data generally argue against profound neural depression ("channel arrest"), NMDA receptor subunit (NR) expression showed features that could mediate reduced neural excitability. Primarily, the NR2 subunit expression, which was dominated by NR2B and NR2D, resembled that seen in hypoxia-tolerant neonatal rats, and decreased anoxic expression of NR1, NR2C, and NR3A indicated reduced numbers of functional NMDA receptors. We also report the full-length sequence of crucian carp NR1 mRNA and a novel NR1 splice cassette introducing an N-glycosylation site into the extracellular S1S2 domain.

All quiet on the neuronal front: NMDA receptor inhibition by prion protein

The normal function of the prion protein (PrP)—the causative agent of mad cow or prion disease—has long remained out of reach. Deciphering PrP's function may help to unravel the complex chain of events triggered by PrP misfolding during prion disease. In this issue of the JCB, an exciting paper (Khosravani, H., Y. Zhang, S. Tsutsui, S. Hameed, C. Altier, J. Hamid, L. Chen, M. Villemaire, Z. Ali, F.R. Jirik, and G.W. Zamponi. 2008. J. Cell Biol. 181:551–565) connects diverse observations regarding PrP into a coherent framework whereby PrP dampens the activity of an N-methyl-d-aspartate (NMDA) receptor (NMDAR) subtype and reduces excitotoxic lesions. The findings of this study suggest that understanding the normal function of proteins associated with neurodegenerative disease may elucidate the molecular pathogenesis.