Pharmacology of ischemia-induced glutamate efflux from rat cerebral cortex in vitro

Fischer's oligopeptide ratio in ischemic hypoxia: prophylactic amendment of sophoretin and melatonin supplementation

Aim: The fundamental pathophysiology of ischemic-hypoxia is oxygen depletion. Fischer's ratio is essential for monitoring hypoxia intensity. Methods: the current study highlighted the prophylactic role of sophoretin (QRC) and/or melatonin (MLN) versus sodium nitrite (SN) brain hypoxia. Results: Prophylactic treatment with sophoretin and MLN, was preceded with hypoxia-induction via sodium nitrite (60 mg/kg, S.C.). SN decreased hemoglobin (Hb), elevated HIF-α, HSP-70, IL-6 and TNF-α. Sophoretin and/or MLN restored the ameliorated inflammatory biomarkers, modulated norepinephrine, dopamine, serotonin and gamma-aminobutyric acid (GABA). Similarly, single-cell gel electrophoresis (SCGE or COMET) DNA damage assay confirmed this finding. Conclusion: Treatment via sophoretin and MLN was the most effective therapy for improving sodium nitrite-induced brain injury.

Gyroxin, a toxin from Crotalus durissus terrificus snake venom, induces a calcium dependent increase in glutamate release in mice brain cortical synaptosomes

Gyroxin is a thrombin-like toxin obtained from the venom of the South American rattlesnake, Crotalus durissus terrificus. Literature has reported "gyroxin syndrome" characterized, in mice, as series of aberrant motor behavior, known as barrel rotation, mainly after intraperitoneal administration. Despites several studies, a physiological mechanism of "gyroxin syndrome" are still not completely understood. In this context, alterations on the central nervous system (CNS), especially causing neurotoxic events, are pointed out as likely candidates. Then, we decided to investigate whether gyroxin induces alterations in glutamate release, one of the most important neurotransmitter involved in neurotoxicity. For that, we performed all experiments, in vitro, using a model of mice brain cortical synaptosomes. Notably, our results indicate that the administration of gyroxin on purified presynaptic brain cortical terminals resulted in an extracellular Ca²⁺- dependent raise in glutamate release. Indeed, our results also showed that gyroxin increases intrasynaptosomal calcium (Ca²⁺) levels through acting on voltage gated calcium channels (VGCC), specifically N and P/Q subtypes. Moreover, our data show that gyroxin increases exocytosis rate. Interestingly, these data suggest that gyroxin might induce neurotoxicity by increasing glutamate levels. However, future investigations are needed in order to elucidate the nature of the following events.

Carnosine and L-arginine attenuate the downregulation of brain monoamines and gamma aminobutyric acid; reverse apoptosis and upregulate the expression of angiogenic factors in a model of hemic hypoxia in rats

PURPOSE

The purpose of the present study was to investigate the preventive effect of L-arginine (ARG) and carnosine (CAR) on hypoxia-induced neurotoxicity in rats. The impact on neuro-inflammation, apoptosis, angiogenesis, and the brain levels of monoamines and GABA were investigated.

METHODS

Rats were divided into the following: normal control, hypoxia model induced by sodium nitrite (75 mg/kg s.c), and hypoxic rats pre-treated with CAR (250 mg/kg), ARG (200 mg/kg), and their combination.

RESULTS

Data revealed that hypoxia induced significant elevation of hypoxia inducible factor-1α (HIF-1α), vascular endothelial growth factor (VEGF), and its receptor reflecting the stimulation of angiogenesis. Hypoxia also resulted in increased inflammatory mediators-including nuclear factor kappa B (NF-κB), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6). In addition, hypoxia initiates cerebral apoptosis as revealed by increased caspase-3 and BAX with reduced Bcl-2. These changes were associated with reduced brain levels of GABA and monoamines including noradrenaline (NADR), dopamine (DOP), and serotonin (SER). Pre-treatment with ARG and/or CAR significantly mitigated the neural changes induced by hypoxia and attenuated the elevated levels of NF-κB, TNF-α, IL-6, caspase-3, and BAX, while ameliorated the reduced levels of Bcl-2, NADR, DOP, SER, and GABA, with the best improvement observed with the combination. Further elevation of the angiogenic markers was observed indicating their role in boosting oxygen delivery to brain.

CONCLUSION

CAR, ARG, and, importantly, their combination could effectively protect against hypoxia-induced neurotoxicity, via their angiogenic, anti-inflammatory, and anti-apoptotic properties in addition to reversing the effect on GABA and monoamines.

Glutamate as a potential "survival factor" in an in vitro model of neuronal hypoxia/reoxygenation injury: leading role of the Na+/Ca2+ exchanger

In brain ischemia, reduction in oxygen and substrates affects mitochondrial respiratory chain and aerobic metabolism, culminating in ATP production impairment, ionic imbalance, and cell death. The restoration of blood flow and reoxygenation are frequently associated with exacerbation of tissue injury, giving rise to ischemia/reperfusion (I/R) injury. In this setting, the imbalance of brain bioenergetics induces important metabolic adaptations, including utilization of alternative energy sources, such as glutamate. Although glutamate has long been considered as a neurotoxin, it can also be used as intermediary metabolite for ATP synthesis, and both the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺-dependent excitatory amino-acid transporters (EAATs) are essential in this pathway. Here we analyzed the role of NCX in the potential of glutamate to improve metabolism and survival of neuronal cells subjected to hypoxia/reoxygenation (H/R). In SH-SY5Y neuroblastoma cells differentiated into a neuron-like state, H/R produced a significant cell damage, a decrease in ATP cellular content, and intracellular Ca²⁺ alterations. Exposure to glutamate at the onset of the reoxygenation phase attenuated H/R-induced cell damage and evoked a significant raise in intracellular ATP levels. Furthermore, we found that in H/R cells NCX reverse-mode activity was reduced, and that glutamate limited such reduction. All the effects induced by glutamate supplementation were lost when cells were transfected with small interfering RNA against NCX1 and EAAT3, suggesting the need of a specific functional interplay between these proteins for glutamate-induced protection. Collectively, our results revealed the potential beneficial effect of glutamate in an in vitro model of H/R injury and focused on the essential role exerted by NCX1. Although preliminary, these findings could be a starting point to further investigate in in vivo systems such protective effect in ischemic settings, shedding a new light on the classical view of glutamate as detrimental factor.

Protection by taurine of rat brain cortical slices against oxygen glucose deprivation- and reoxygenation-induced damage

Taurine neuroinhibitory features have suggested its potential for neuroprotection. The aim of the present study was to assess whether it prevents or counteracts brain ischemia and reperfusion-induced cell injury. Rat brain cortical slices were subjected to oxygen/glucose deprivation and reperfusion. Tissue damage was assessed by measuring the release of glutamate and lactate dehydrogenase (LDH) during reperfusion and by determining final tissue water gain, taken as an index of cell swelling. When added during the reperfusion period taurine did not significantly affect oxygen/glucose deprivation-induced LDH and glutamate release, while it antagonised tissue water gain in a concentration-dependent manner (IC(50)=46.5 microM). The latter effect was antagonised by 50% when a taurine transport inhibitor, 2-(guanidino)ethanesulphonic acid (GES), or a GABA(A) receptor antagonist, bicuculline, was added together with taurine, while it was completely abolished when both GES and bicuculline or the volume-sensitive outwardly rectifying (VSOR) Cl(-) channel blocker, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), was used. On the contrary, when present throughout the entire experiment, taurine significantly reduced oxygen/glucose deprivation-induced LDH and glutamate release with a maximal effect (45% reduction) between 5 and 20 mM. Taurine antagonised also tissue water gain according to a "U-shaped" concentration-response curve, which was significant within the range of 0.01-1.0 mM concentration. This effect was partially counteracted by GES as well as by bicuculline and fully reverted by NPPB. In conclusion, since brain edema is a major contributing factor to morbidity and mortality in stroke, the present findings give the rational basis for assessing taurine efficacy in reducing brain edema in vivo.

Inability of volatile anesthetics to inhibit oxygen-glucose deprivation-induced glutamate release via glutamate transporters and anion channels in rat corticostriatal slices

Ischemia-induced extracellular glutamate accumulation and the subsequent excitotoxicity contribute significantly to ischemic brain injury. Volatile anesthetics have been shown to reduce ischemic brain injury. Here, we showed that oxygen-glucose deprivation (OGD, to simulate ischemia in vitro) increased extracellular glutamate accumulation in the corticostriatal slices of adult rats. This increased accumulation was reduced by dihydrokinate, a glutamate transporter type 2 inhibitor, and 4,4'-dinitrostilbene-2,2'-disulfonic acid, a blocker for volume-activated anion channels. The volatile anesthetics isoflurane, sevoflurane and desflurane at clinically relevant concentrations did not affect the OGD-induced extracellular glutamate accumulation from brain slices of adult rats. Isoflurane also did not change the OGD-induced extracellular glutamate accumulation from brain slices of newborn/young rats. These results suggest that the OGD-induced glutamate accumulation involves reversed transport of glutamate via glutamate transporters and volume-activated anion channels. Volatile anesthetics may not inhibit this extracellular glutamate accumulation.

Magnesium sulphate reduces cell volume in physiological conditions but not in the cytotoxic oedema during global brain ischemia

PRIMARY OBJECTIVE

To establish the effect of MgSO4 on brain cellular volume during physiological and global ischemia using impedanciometric method.

RESEARCH DESIGN

Impedanciometric measure in the brain before and during global brain ischemia, with or without intravenous infusion of MgSO4.

METHODS AND PROCEDURES

Male Sprague-Dawley rats were anaesthetized (thiobarbital, 60 mg kg-1 i.p.). Ringer solution (n=9) or MgSO4 (n=8; 1 mmol Kg-1) where i.v. isovolumetrically administered. Sub-cortical impedance was recorded before and after the infusion of ringer or MgSO4 and during global cerebral ischemia induced by a cardiopulmonary arrest.

MAIN OUTCOMES AND RESULTS

In non-ischemic conditions, MgSO4 infusion induced higher voltage values than those of ringer infusion (Wilcoxon, Z=2.49; p=0.01). During global cerebral ischemia, the MgSO4 infused animals showed a fast drop of voltages (82%) in the first 5 minutes, 4-fold the values of ringer infused animals (p<0.0001). In the following 15 minutes no differences were found between ringer and MgSO4 infused animals.

CONCLUSIONS

The results suggest that MgSO4 significantly reduced brain cell volume in physiological conditions but not in global brain ischemia.

Enhanced activity of GABA receptors inhibits glutamate release induced by focal cerebral ischemia in rat striatum

Cerebral ischemia causes an excess release of glutamate, which can injure neurons. The striatum is one of the important regions vulnerable to hypoxia and ischemia. Using push-pull perfusion technique, we investigated the regulatory role of gamma-aminobutyric acid (GABA) and its receptors in modifying the amount of glutamate in rat striatum with ischemia. Perfusion with exogenous GABA (1 mM) inhibited cerebral ischemia-induced glutamate release by as much as 47%. We further characterized relative roles of subtype receptors of GABA on glutamate release by using pharmacological tools. While baclofen (500 microM), a GABA(B) receptor agonist, suppressed ischemia-induced glutamate release by 52%, GABA(B) receptor antagonist saclofen (500 microM) failed to produce a significant increase of glutamate release. The GABA(A) receptor agonist muscimol (500 microM) also reduced by 38% the release of glutamate induced by cerebral ischemia but the GABA(A) receptor antagonist bicuculline (500 microM) had very little effect. The present study demonstrates that the excessive release of glutamate or the overly activated glutamate receptor, triggered by cerebral ischemia, can be down-regulated by exogenous GABA or by increased activity of GABA receptors, especially the presynaptic GABA(B) receptors, which might be one of the important mechanisms to protect against striatum neuronal damage from over stimulation by excessive glutamate during ischemia.

Changes in the spontaneous calcium oscillations for the development of the preconditioning-induced ischemic tolerance in neuron/astrocyte co-culture

Spontaneous Ca(2+) oscillations are believed to contribute to the regulation of gene expression. Here we investigated whether and how the dynamics of Ca(2+) oscillations changed after sublethal preconditioning (PC) for PC-induced ischemic tolerance in neuron/astrocyte co-cultures. The frequency of spontaneous Ca(2+) oscillations significantly decreased between 4 and 8 h after the end of PC in both neurons and astrocytes. Treatment with 2-APB, an inhibitor of IP3 receptors, decreased the oscillatory frequency, induced ischemic tolerance and a down-regulation of glutamate transporter GLT-1 contributing to the increase in the extracellular glutamate during ischemia. The expression of GLT-1 is known to be up-regulated by PACAP. Treatment with PACAP38 increased the oscillatory frequency, and antagonized both the PC-induced down-regulation of GLT-1 and ischemic tolerance. These results suggested that the PC suppressed the spontaneous Ca(2+) oscillations regulating the gene expressions of various proteins, especially of astrocytic GLT-1, for the development of the PC-induced ischemic tolerance.

The anion channel blocker, 4,4′-dinitrostilbene-2,2′-disulfonic acid prevents neuronal death and excitatory amino acid release during glycolysis inhibition in the hippocampus in vivo

Neuronal death associated with cerebral ischemia and hypoglycemia is related to increased release of excitatory amino acids (EAA) and energy failure. The intrahippocampal administration of the glycolysis inhibitor, iodoacetate (IOA), induces the accumulation of EAA and neuronal death. We have investigated by microdialysis the role of exocytosis, glutamate transporters and volume-sensitive organic anion channel (VSOAC) on IOA-induced EAA release. Results show that the early component of EAA release is inhibited by riluzole, a voltage-dependent sodium channel blocker, and by the VSOAC blocker, tamoxifen, while the early and late components are blocked by the glutamate transport inhibitors, L-trans-pyrrolidine 2,4-dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA); and by the VSOAC blocker 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Riluzole, DL-TBOA and tamoxifen did not prevent IOA-induced neuronal death, while PDC and DNDS did. The VSOAC blockers 5-nitro-2-(3-phenylpropyl-amino) benzoic acid (NPPB) and phloretin had no effect either on EAA efflux or neuronal damage. Results suggest that acute inhibition of glycolytic metabolism promotes the accumulation of EAA by exocytosis, impairment or reverse action of glutamate transporters and activation of a DNDS-sensitive mechanism. The latest is substantially involved in the triggering of neuronal death. To our knowledge, this is the first study to show protection of neuronal death by DNDS in an in vivo model of neuronal damage, associated with deficient energy metabolism and EAA release, two conditions involved in some pathological states such as ischemia and hypoglycemia.

Glutamate receptor-mediated inhibition of L-glutamate efflux from cerebral cortex in vitro

We tested whether glutamate receptor ligands affect oxygen-glucose deprivation-evoked L-glutamate efflux from adult rat cerebrocortical prisms. The uncompetitive NMDA antagonist AR-R15896AR inhibited efflux (IC50 34 microM, 87% maximal inhibition). AMPA/kainate receptor blockade (NBQX, 100 microM) or Group II metabotropic glutamate receptor activation (DCG-IV, 10 microM) inhibited efflux (41%, 67% respectively) but Group I mGluR blockade (CPCCOEt/MPEP, 10 microM) was without effect. These data support a modulatory effect of glutamate receptors on L-glutamate efflux.

Concentrations of extracellular free zinc (pZn)e in the central nervous system during simple anesthetization, ischemia and reperfusion

"Free Zn2+" (rapidly exchangeable Zn2+) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn2+ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn2+ in the brain nor the presumed co-release of free Zn2+ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn2+ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 microM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn2+ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn2+ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn2+ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.

Acute effects of 17beta-estradiol on the extracellular concentration of excitatory amino acids and energy metabolites during transient cerebral ischemia in male rats

Elevation of extracellular levels of amino acids has been implicated in the pathogenesis of stroke. The failure of brain energy metabolism due to the lack of oxygen and glucose contributes also to cell loss. Estrogen has been shown to protect brain cells against ischemia by a still unclear mechanism. We used intracerebral microdialysis to monitor the effects of acute 17beta-estradiol treatment on the release of glutamate and aspartate and on the levels of the energy metabolites glucose and lactate. In male rats subjected to 90 min of transient middle cerebral artery occlusion followed by 24-h reperfusion, acute treatment with 17beta-estradiol (0.8 mg/kg, i.v.) at the time of occlusion reduced the ischemic infarct by about 50%. In these treated rats, the ischemia-induced increases of extracellular levels of glutamate and aspartate were significantly and rapidly reduced. The reduction of glucose level during occlusion was not affected by 17beta-estradiol treatment; however, the increase of extracellular lactate was reduced during occlusion and reperfusion, probably due to the reduced glutamate-driven astrocytic glycolysis. These data suggest that acute treatment with 17beta-estradiol at the onset of occlusion significantly reduces the ischemia-induced excitotoxicity in the cortex, a mechanism that may participate in the neuroprotective effect on cellular survival.

Effects of chemical ischemia in cerebral cortex slices. Focus on nitric oxide

Superfused rat cerebral cortex slices were submitted to a continuous electrical (5 Hz) stimulation and treated with sodium azide (1-10 mM) in the presence of 2 mM 2-deoxyglucose ("chemical ischemia"). Presynaptic cholinergic activity, evaluated as acetylcholine release, was inhibited depending on the sodium azide concentrations and on the length of application (5-30 min). Following a 5-min treatment with 10 mM sodium azide, acetylcholine release was reduced to 45+/-2.3%; simultaneously, there was a 15- and 10-fold increase in glutamate and nitric oxide effluxes, respectively. After restoring normal superfusion conditions, acetylcholine release recovered to 70+/-3.1% of the controls; the N-methyl-D-aspartate receptor antagonist MK-801 (10 microM) as well as the nitric oxide scavengers, haemoglobin (20 microM) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (150 microM), improved the recovery in presynaptic activity, showing that both glutamate and nitric oxide play detrimental roles in chemical ischemia. On the other hand, the post-ischemic recovery was worsened by the guanylylcyclase inhibitor 1H-[l,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (10 microM), suggesting that the activation of such a pathway plays a neuroprotective role and that the nitric oxide-induced harmful effects depend on different mechanisms. Chemical ischemia-evoked nitric oxide efflux partly derived from its calcium-dependent endogenous synthesis, since both the intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (1 mM), and the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (100 microM), substantially prevented sodium azide effects. Nitric oxide efflux was only weakly reduced by MK-801 and was not modified by either the L-type calcium channel blocker, nifedipine (10 microM) or the N-type calcium channel blocker omega-conotoxin (0.5 microM), thus suggesting a prevailing intracellular calcium-dependence of nitric oxide production, although a partial extracellular calcium source cannot be ruled out. These findings show that sodium azide plus 2-deoxyglucose treatment is a useful protocol to induce brain ischemia in vitro and underline the involvement of nitric oxide in the complex events following the ischemic insult.

Protein S100B release from rat brain slices during and after ischemia: comparison with lactate dehydrogenase leakage

One hour of ischemia significantly increased protein S100B release from rat brain slices without altering lactate dehydrogenase leakage. Reoxygenation of the ischemic slices, however, increased the levels of these biochemical markers in the medium. Although removal of extracellular Ca(+2) ions from the medium did not alter the basal lactate dehydrogenase leakage from cortical slices, an excessive increase in basal protein S100B release was seen under this condition. Ischemia and/or reoxygenation induced enhancements in these markers were attenuated by removal of Ca(+2) ions from the medium. Ischemia significantly increased glutamate release, but neither ischemia nor reoxygenation induced rises in protein S100B and lactate dehydrogenase levels were altered by glutamate receptor antagonists. Rising the glutamate levels in the medium by each ouabain or exogenous glutamate, moreover, failed in exerting an ischemia like effect on protein S100B and LDH outputs. In contrast, exogenous glutamate added into the medium protected the slices against reoxygenation induced increments in protein S100B and lactate dehydrogenase levels. These results indicate that protein S100B has a greater sensitivity against ischemia than lactate dehydrogenase in in vitro brain slice preparations. Since neither exogenous glutamate nor enhancements of the extracellular glutamate levels by ouabain had an ischemia like effect, and since glutamate receptor antagonists were also unsuccessful, it seems unlikely that ischemia-induced increase in glutamate release is directly involved in protein S100B release or lactate dehydrogenase leakage determined in the present study.

Strain variability, injury distribution, and seizure onset in a mouse model of stroke in the immature brain

Neonatal stroke is an important cause of neurologic morbidity and cerebral palsy. Recently, we have determined that in postnatal day 12 CD1 mice unilateral carotid ligation alone results in seizures and brain injury. We have shown that, in this model, seizure scores correlate with brain injury scores. We have applied this model to another strain of mice to assess strain-related differences in vulnerability to seizures and brain injury after unilateral carotid ligation. Under isoflurane anesthesia, unilateral right-sided carotid ligation was performed in postnatal day 12 C3HeB/FeJ mice followed by a 4-hour period of observation in a 35 degrees C incubator. Seizure scores and brain jury scores were assigned and compared to scores in mice receiving sham surgery. Timing of seizure onset and regional distribution of brain injury were compared in the CD1 and C3HeB/FeJ mice. Unilateral carotid ligation in postnatal day 12 C3HeB/FeJ mice resulted in seizure behavior and brain injury in some animals, with similar time to seizure onset and regional injury distribution, but affected a significantly smaller percentage of C3HeB/FeJ pups than that observed in postnatal day 12 CD1 mice, indicating strain-related vulnerability in this model.

Gamma-amino butyric acid type B receptors stimulate neutrophil chemotaxis during ischemia-reperfusion

Serine/threonine kinase Akt, or protein kinase B, has been shown to regulate a number of neutrophil functions. We sought to identify Akt binding proteins in neutrophils to provide further insights into understanding the mechanism by which Akt regulates various neutrophil functions. Proteomic and immunoprecipitation studies identified gamma-amino butyric acid (GABA) type B receptor 2 (GABA(B)R2) as an Akt binding protein in human neutrophils. Neutrophil lysates subjected to Akt immunoprecipitation followed by immunoblotting with anti-GABA(B)R2 demonstrated Akt association with the intact GABA(B)R. Similar results were obtained when reciprocal immunoprecipitations were performed with anti-GABA(B)R2 Ab. Additionally, GABA(B)R2 and Akt colocalization was demonstrated by confocal microscopy. A GABA(B)R agonist, baclofen, activated Akt and stimulated neutrophil-directed migration in a PI3K-dependent manner, whereas CGP52432, a GABA(B)R antagonist blocked such effects. Baclofen, stimulated neutrophil chemotaxis and tubulin reorganization in a PI3K-dependent manner. Additionally, a GABA(B)R agonist failed to stimulate neutrophil superoxide burst. We are unaware of the association of GABA(B)R with Akt in any cell type. The present study shows for the first time that a brain-specific receptor, GABA(B)R2 is present in human neutrophils and that it is functionally associated with Akt. Intraventricular baclofen pretreatment in rats subjected to a stroke model showed increased migration of neutrophils to the ischemic lesion. Thus, the GABA(B)R is functionally expressed in neutrophils, and acts as a chemoattractant receptor via an Akt-dependent pathway. The GABA(B)R potentially plays a significant role in the inflammatory response and neutrophil-dependent ischemia-reperfusion injury such as stroke.

Reversed operation of glutamate transporter GLT-1 is crucial to the development of preconditioning-induced ischemic tolerance of neurons in neuron/astrocyte co-cultures

Sublethal ischemia leads to increased tolerance against subsequent prolonged cerebral ischemia in vivo. In the present study, we investigated the roles of the astrocytic glutamate (Glu) transporter GLT-1 in preconditioning (PC)-induced neuronal ischemic tolerance in cortical neuron/astrocyte co-cultures. Ischemia in vitro was simulated by subjecting cultures to both oxygen and glucose deprivation (OGD). A sublethal OGD (PC) increased the survival rate of neurons significantly when cultures were exposed to a lethal OGD 24 h later. The extracellular concentration of Glu increased significantly during PC, and treatment with an inhibitor of N-methyl-D-actetate (NMDA) receptors significantly reversed the PC-induced ischemic tolerance of neurons, suggesting that the increase in extracellular concentration of Glu during PC was critical to the development of PC-induced neuronal ischemic tolerance via the activation of NMDA receptors. Treatment with a GLT-1 blocker during PC suppressed this increase in Glu significantly, and antagonized the PC-induced neuronal ischemic tolerance. This study suggested that the reversed operation of GLT-1 was crucial to the development of neuronal ischemic tolerance.

Analysis of the brain bioavailability of peripherally administered magnesium sulfate: A study in humans with acute brain injury undergoing prolonged induced hypermagnesemia*

OBJECTIVE

Based on preclinical investigations, magnesium sulfate (MgSO4) has gained interest as a neuroprotective agent. However, the ability of peripherally administered MgSO4 to penetrate the blood-brain barrier is limited in normal brain. The current study measured the passage of intravenously administered Mg into cerebrospinal fluid in patients with brain injury requiring ventricular drainage.

DESIGN

A prospective evaluation of the cerebrospinal fluid total and ionized magnesium concentration, [Mg], during sustained hypermagnesemia was performed.

SETTING

Neurosciences intensive care unit at a major teaching institution.

PATIENTS

Thirty patients with acute brain injury secondary to subarachnoid hemorrhage, traumatic brain injury, primary intracerebral hemorrhage, subdural hematoma, brain tumor, central nervous system infection, or ischemic stroke were studied.

INTERVENTIONS

Patients underwent 24 hrs of induced hypermagnesemia during which total and ionized cerebrospinal fluid [Mg] was measured. Serum [Mg] was adjusted to 2.1-2.5 mmol/L. Cerebrospinal fluid [Mg] was measured at baseline, at 12 and 24 hrs after onset of infusion, and at 12 hrs following infusion termination.

MEASUREMENTS AND MAIN RESULTS

At baseline, total (1.25 +/- 0.14 mmol/L) and ionized (0.80 +/- 0.10 mmol/L) cerebrospinal fluid [Mg] was greater than serum total (0.92 +/- 0.18 mmol/L) and ionized (0.63 +/- 0.07 mmol/L) [Mg] (p < .05). Total (1.43 +/- 0.13 mmol/L) and ionized (0.89 +/- 0.12 mmol/L) cerebrospinal fluid [Mg] was maximally increased by 15% and 11% relative to baseline, respectively, during induced hypermagnesemia (p < .05).

CONCLUSIONS

Hypermagnesemia produced only marginal increases in total and ionized cerebrospinal fluid [Mg]. Regulation of cerebrospinal fluid [Mg] is largely maintained following acute brain injury and limits the brain bioavailability of MgSO4.

Retinal glutamate transporter activity persists under simulated ischemic conditions

Elevated extracellular concentrations of the neurotransmitter glutamate are neurotoxic and directly contribute to CNS damage as a result of ischemic pathologies. However, the main contributors to this uncontrolled rise in glutamate are still unconfirmed. It has been reported that the reversal of high-affinity glutamate transporters is a significant contributing factor. Conversely, it has also been observed that these transporters continue to take up glutamate, albeit at a reduced saturation concentration, under ischemic conditions. We sought to determine whether glutamate transporters continue to remove glutamate from the extracellular space under ischemic conditions by pharmacologically modulating the activity of high-affinity retinal glutamate transporters during simulated ischemia in vitro. Retinal glutamate transporter activity was significantly reduced under these ischemic conditions. The suppression of retinal glutamate transporter activity, with the protein kinase C inhibitor chelerythrine, significantly reduced ischemic glutamate uptake and enhanced retinal neurodegeneration. These findings imply a limited but protective role for retinal glutamate transporters under certain ischemic conditions, suggesting that pharmacological enhancement of high-affinity glutamate transporter activity may reduce tissue damage and loss of function resulting from toxic extracellular glutamate concentrations.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.