Characterization of glutamate, aspartate, and GABA release from ischemic rat cerebral cortex

Enhancement of the releases of GABA and glycine during ischemia in rat spinal dorsal horn

The present study examined a change in spontaneous inhibitory postsynaptic currents (sIPSCs) following ischemia in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. At about 10 min after superfusion of an oxygen- and glucose-free medium, sIPSCs were remarkably increased in amplitude and frequency when compared with those in the control. In a phase of the increase in sIPSC activities, GABAergic and glycinergic sIPSCs, which were observed in the presence of strychnine and bicuculline, respectively, with TTX, were increased greatly in frequency with a minimal change in their amplitudes. It is concluded that the in vitro ischemia increases the spontaneous quantal releases of GABA and glycine to SG neurons from nerve terminals; a part of this enhancement is possibly due to an increase in spontaneous activities of inhibitory interneurons. GABA released thus might serve to inhibit the release of l-glutamate from nerve terminals.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Neurotoxic and neuroprotective effects of the glutamate transporter inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) during physiological and ischemia-like conditions

Maintenance of low extracellular glutamate ([Glu](O)) preventing excitotoxic cell death requires fast removal of glutamate from the synaptic cleft. This clearance is mainly provided by high affinity sodium-dependent glutamate transporters. These transporters can, however, also be reversed and release glutamate to the extracellular space in situations with energy failure. In this study the cellular localisation of the glutamate transporters GLAST and GLT-1 in organotypic hippocampal slice cultures was studied by immunofluorescence confocal microscopy, under normal culture conditions, and after a simulated ischemic insult, achieved by oxygen and glucose deprivation (OGD). In accordance with in vivo findings, GLAST and GLT-1 were primarily expressed by astrocytes under normal culture conditions, but after OGD some damaged neurons also expressed GLAST and GLT-1. The potential damaging effect of inhibition of the glutamate transporters by DL-threo-beta-benzyloxyaspartate (DL-TBOA) was studied using cellular uptake of propidium iodide (PI) as a quantitative marker for the cell death. Addition of DL-TBOA for 48 h was found to induce significant cell death in all hippocampal regions, with EC(50) values ranging from 38 to 48 microM for the different hippocampal subregions. The cell death was prevented by addition of the glutamate receptor antagonists NBQX and MK-801, together with an otherwise saturating concentration of DL-TBOA (100 microM). Finally, the effect of inhibition of glutamate release, via reverse operating transporters during OGD, was investigated. Addition of a sub-toxic (10 microM) dose of DL-TBOA during OGD, but not during the subsequent 48 h recovery period, significantly reduced the OGD-induced PI uptake. It is concluded: (1) that the cellular expression of the glutamate transporters GLAST and GLT-1 in hippocampal slice cultures in general corresponds to the expression in vivo, (2) that inhibition of the glutamate transporters induces cell death in the slice cultures, and (3) that partial inhibition during simulation of ischemia by OGD protects against the induced PI uptake, most likely by blocking the reverse operating transporters otherwise triggered by the energy failure.

Characterization of modes of release of amino acids in the ischemic/reperfused rat cerebral cortex

Brain extracellular levels of glutamate, aspartate, GABA and glycine increase rapidly following the onset of ischemia, remain at an elevated level during the ischemia, and then decline over 20-30 min following reperfusion. The elevated levels of the excitotoxic amino acids, glutamate and aspartate, are thought to contribute to ischemia-evoked neuronal injury and death. Calcium-evoked exocytotic release appears to account for the initial (1-2 min) efflux of neurotransmitter-type amino acids following the onset of ischemia, with non-vesicular release responsible for much of the subsequent efflux of these and other amino acids, including taurine and phosphoethanolamine. Extracellular Ca(2+)-independent release is mediated, in part by Na(+)-dependent amino acid transporters in the plasma membrane operating in a reversed mode, and by the opening of swelling-induced chloride channels, which allow the passage of amino acids down their concentration gradients. Experiments on cultured neurons and astrocytes have suggested that it is the astrocytes which make the primary contribution to this amino acid efflux. Inhibition of phospholipase A(2) attenuates ischemia-evoked release of both amino and free fatty acids from the rat cerebral cortex indicating that this group of enzymes is involved in amino acid efflux, and also accounting for the consistent ischemia-evoked release of phosphoethanolamine. It is, therefore, possible that disruption of membrane integrity by phospholipases plays a role in amino acid release. Recovery of amino acid levels to preischemic levels requires their uptake by high affinity Na(+)-dependent transporters, operating in their normal mode, following restoration of energy metabolism, cell resting potentials and ionic gradients.

Hippocampal injury and neurobehavioral deficits are improved by PD 81,723 following hyperglycemic cerebral ischemia

We investigated the effects of PD 81,723, an allosteric enhancer for the adenosine A(1) receptor subtype, on hippocampal injury and Morris water maze (MWM) performance following hyperglycemic cerebral ischemia and reperfusion (4-VO, 10 min) in the rat. PD 81,723 (3 or 10 mg/kg) or the equivalent volume of saline was administered intraperitoneally 30 min prior to ischemia. Moderate hyperglycemia was achieved by administration of D-glucose (3g/kg, i.p.) 15 min prior to induction of ischemia. Morris water maze trials were performed on the 6th, 7th, and 8th days after the ischemic insult. The rat brains were sectioned (8 microm), stained with cresyl violet/acid fuchsin, and evaluated for hippocampal ischemic injury by an experimenter blinded to the treatment conditions. At the higher dose, PD 81,723 (10 mg/kg) had no effect on hippocampal injury or MWM performance following hyperglycemic ischemia compared to corresponding saline-treated animals. In contrast, a lower dose of PD 81,723 (3 mg/kg) resulted in significant (P < 0.05, n = 8) reduction of hippocampal injury following hyperglycemic ischemia. Furthermore, corresponding Morris water maze performance (latency, learning index, and cumulative distance swum) was significantly improved by PD 81,723 (P < 0.05, n = 8). The results of the present study suggest that, in the presence of PD 81,723, an A(1) allosteric enhancer, endogenously produced adenosine is sufficient to exert significant neuroprotection during hyperglycemic ischemia. Moreover, the present study provides further evidence for a neuromodulatory role of adenosine during hyperglycemic ischemia.

Purification of a neuroprotective component of Parawixia bistriata spider venom that enhances glutamate uptake

(1) In this study, we examined the effects of crude venom from the spider Parawixia bistriata on glutamate and GABA uptake into synaptosomes prepared from rat cerebral cortex. Addition of venom to cortical synaptosomes stimulated glutamate uptake and inhibited GABA uptake in a concentration-dependent manner. (2) The venom was fractionated using reverse-phase high-performance liquid chromatography on a preparative column. The fraction that retained glutamate uptake-stimulating activity was further purified on a reverse-phase analytical column followed by ion-exchange chromatography. (3) The active fraction, referred to as PbTx1.2.3, stimulated glutamate uptake in synaptosomes without changing the K(M) value, and did not affect GABA uptake. Additional experiments showed that the enhancement of glutamate uptake by PbTx1.2.3 occurs when ionotropic glutamate receptors or voltage-gated sodium and calcium channels are completely inhibited or when GABA receptors and potassium channels are activated, indicating that the compound may have a direct action on the transporters. (4) In an experimental model for glaucoma in which rat retinas are subjected to ischemia followed by reperfusion, PbTx1.2.3 protected neurons from excitotoxic death in both outer and inner nuclear layers, and ganglion cell layers. (5) This active spider venom component may serve as a basis for designing therapeutic drugs that increase glutamate clearance and limit neurodegeneration.

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

Simulated ischemic conditions (hypoxia-hypoglycaemia) in vitro enhanced glutamate efflux from rat cerebrocortical prisms. Here we characterised efflux mechanisms using pharmacological tools. The Na(+) channel blocker TTX (1 microM) did not affect ischemia-induced efflux, while sipatrigine (100 microM), a Na(+)/Ca(2+) channel blocker and omega-conotoxin MVIIC (2 microM), an N/P/Q type Ca(2+) channel blocker, inhibited efflux by fractions of 0.53 and 0.46, respectively (1.00 corresponding to total inhibition). Omission of extracellular Ca(2+) and addition of EGTA (2 mM) inhibited ischemia-induced efflux only during the first 25 min of incubation. A similar result was observed on omission of extracellular Ca(2+) together with addition of La(3+) (10 microM) and Mg(2+) (6 mM). TTX, sipatrigine and La(3+)/Mg(2+) all inhibited control efflux. Ischemia-induced efflux was sensitive to the volume activated anion channel inhibitor NPPB (100 microM) only after the first 25 min of incubation, with the maximal fraction inhibited being 0.54. The glutamate transporter inhibitor D,L-TBOA reduced ischemia-induced efflux throughout a 45-min incubation period, and enhanced efflux from control tissue. D,L-TBOA inhibited efflux at 30 min by a maximum fraction of 0.49, at 50 microM. These data indicate that the early phase of ischemia-induced glutamate efflux is in part Ca(2+) dependent, while the later phase involves volume activated anion currents and both phases involve excitatory amino acid transporters.

Preconditioned resistance to oxygen-glucose deprivation-induced cortical neuronal death: alterations in vesicular GABA and glutamate release

Central neurons exposed to several types of sublethal stress, including ischemia, acquire resistance to injury induced by subsequent ischemic insults, a phenomenon called ischemic preconditioning. We modeled this phenomenon in vitro, utilizing exposure to 45 mM KCl to reduce the vulnerability of cultured murine cortical neurons to subsequent oxygen-glucose deprivation. Twenty-four hours after preconditioning, cultures exhibited enhanced depolarization-induced, tetanus toxin-sensitive GABA release and a modest decrease in glutamate release. Total cellular GABA levels were unaltered. Inhibition of GABA degradation with the GABA transaminase inhibitor (+/-)-gamma-vinyl GABA, or addition of low levels of GABA, muscimol, or chlormethiazole to the bathing medium, mimicked the neuroprotective effect of preconditioning against oxygen-glucose deprivation-induced death. However, neuronal death was enhanced by higher levels of these manipulations, as well as by prior selective destruction of GABAergic neurons by kainate. Finally, selective blockade of GABA(A) receptors during oxygen-glucose deprivation or removal of GABAergic neurons eliminated the neuroprotective effects of prior preconditioning. Taken together, these data predict that presynaptic alterations, specifically enhanced GABA release together with reduced glutamate release, may be important mediators of ischemic preconditioning, but suggest caution in regard to interventions aimed at increasing GABA(A) receptor activation.

GABA-mediated inhibition of glutamate release during ischemia in substantia gelatinosa of the adult rat

An ischemia-induced change in glutamatergic transmission was investigated in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique; the ischemia was simulated by superfusing an oxygen- and glucose-free medium (ISM). Following ISM superfusion, 21 of 37 SG neurons tested produced an outward current (23 +/- 4 pA at a holding potential of -70 mV), which was followed by a slow and subsequent rapid inward current; the remaining neurons had only inward currents. During such a change in holding currents, spontaneous excitatory postsynaptic currents (EPSCs) were remarkably decreased in a frequency with time (half-decay time of the frequency: about 65 s). The frequency of spontaneous EPSCs was reduced to 28 +/- 13% (n = 37) of the control level during the generation of the slow inward current (about 4 min after the beginning of ISM superfusion) without a change in the amplitude of spontaneous EPSCs. When ISM was superfused together with either bicuculline (10 microM) or CGP35348 (20 microM; GABA(A) and GABA(B) receptor antagonists, respectively), spontaneous EPSC frequency reduced by ISM recovered to the control level and then the frequency markedly increased [by 325 +/- 120% (n = 22) and 326 +/- 91% (n = 17), respectively, 4 min after ISM superfusion]; this alteration in the frequency was not accompanied by a change in spontaneous EPSC amplitude. Superfusing TTX (1 microM)-containing ISM resulted in a similar recovery of spontaneous EPSC frequency and following increase (by 328 +/- 26%, n = 12) in the frequency; strychnine (1 microM) did not affect ISM-induced changes in spontaneous EPSC frequency (n = 5). It is concluded that the ischemic simulation inhibits excitatory transmission to SG neurons, whose action is in part mediated by the activation of presynaptic GABA(A) and GABA(B) receptors, probably due to GABA released from interneurons as a result of an ischemia-induced increase in neuronal activities. This action might protect SG neurons from an excessive excitation mediated by L-glutamate during ischemia.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

Different effects of eNOS and nNOS inhibition on transient forebrain ischemia

To clarify the functions of nitric oxide (NO) induced by either neuronal NO synthase (nNOS) or endothelial NO synthase (eNOS) after transient cerebral ischemia, we investigated the effects of L-N(5)-(1-iminoethyl)ornithine (L-NIO), a relatively selective eNOS inhibitor, and 7-nitroindazole (7-NI), a relatively selective nNOS inhibitor, on hippocampal dysfunction caused by cerebral ischemia. We measured mean arterial blood pressure (MABP), hippocampal blood flow, direct current (DC) potential, CA1 population spike (PS) and extracellular concentrations of glutamate from rat hippocampus after transient forebrain ischemia, which was induced by four-vessel occlusion for 10 min. L-NIO (20 mg/kg) and 7-NI (25 mg/kg) were administered intraperitoneally 20 min before ischemia. L-NIO, but not 7-NI, increased MABP before, during and after ischemia, compared with the vehicle group. 7-NI, but not L-NIO, reduced the amplitude of anoxic depolarization induced by ischemia. 7-NI recovered the PS amplitude in part 60 min after ischemia. 7-NI, but not L-NIO, reduced the ischemia-induced levels of glutamate. These results indicate that nNOS inhibition with 7-NI improves, at least in part, hippocampal dysfunction after ischemia, while eNOS inhibition with L-NIO worsens it.

Ischemia-induced taurine release is modified by nitric oxide-generating compounds in slices from the developing and adult mouse hippocampus

The novel neurotransmitter/neuromodulator nitric oxide (NO), which is linked to the activation of the N-methyl-D-aspartate class of glutamate receptors, has been shown to modify transmitter release in brain tissue. Release of the inhibitory amino acid taurine is also markedly enhanced by N-methyl-D-aspartate and NO-producing agents under normal conditions in the mouse hippocampus. The release of preloaded [3H]taurine from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice was characterized under ischemic conditions in the presence of different NO-generating compounds, hydroxylamine, sodium nitroprusside, and S-nitroso-N-acetylpenicillamine (SNAP), using a superfusion system. The ischemia-induced taurine release at both ages was markedly enhanced by 1.0 mM nitroprusside and 1.0 mM SNAP, whereas 5.0 mM hydroxylamine was effective only in adults. The nitroprusside- and SNAP-induced releases were reduced by the inhibitors of NO synthase (nitroarginine and 7-nitroindazole) and NO-sensitive soluble guanylyl cyclase [1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one], suggesting involvement of the NO/cGMP pathway. The release in ischemia in the absence of Na+ was modified by NO compounds only in adults; the 0.1 mM N-methyl-D-aspartate stimulated taurine release at both ages. The enhanced release of taurine associated with NO production could be beneficial to brain tissue under cell-damaging conditions and corroborates the neuroprotective role of this amino acid, particularly in the immature brain.

Mechanisms of glutamate release in the rat spinal cord slices during metabolic inhibition

Glutamate toxicity is a viable hypothesis to explain the expanding tissue degeneration occurring after traumatic or ischemic spinal cord injury. One important component in this process is the acute, excessive release of glutamate. In the current communication, the glycolytic inhibitor iodoacetate was used to induce metabolic inhibition in spinal cord slices and thereby provide an in vitro model to study the mechanisms of pathological glutamate release in the spinal cord. The evoked glutamate release was not Ca2+-dependent. Exclusion of NaCl reduced the evoked release of endogenous glutamate by 56%, while excluding Na+ increased release. Glutamate release was also reduced by the PLA2 inhibitors indomethacin (40%), arachidonyltrifluoromethyl ketone (45%) and 4-bromophenacyl bromide (36%). Blocking reverse glutamate transport by preincubation with 1 mM dihydrokainic acid reduced evoked release by 41%. However, when the dihydrokainic acid and arachidonyltrifluoromethyl ketone treatments were combined, no additive effect of the two substances was seen. These findings suggest that glutamate is released by three mechanisms from the energy compromised spinal cord: (1) in response to cellular swelling, most likely by the regulatory volume decrease, (2) by PLA2-mediated breakdown of the cell membrane and diffusion of glutamate down its concentration gradient, and (3) through reversal of the glutamate transporter.

Knocking out the glial glutamate transporter GLT-1 reduces glutamate uptake but does not affect hippocampal glutamate dynamics in early simulated ischaemia

Glutamate release in ischaemia triggers neuronal death. The major glial glutamate transporter, GLT-1, might protect against glutamate-evoked death by removing extracellular glutamate, or contribute to death by reversing and releasing glutamate. Previous studies of the role of GLT-1 in ischaemia have often used the GLT-1 blocker dihydrokainate at concentrations that affect transporters other than GLT-1 and which affect kainate, N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. In hippocampal slices from postnatal day 14 mice lacking GLT-1, the current response of area CA1 pyramidal cells to superfused AMPA and NMDA (which are not taken up) was unaffected, whereas the response to 100 microm glutamate was more than doubled relative to that in wild-type littermates, a finding consistent with a decrease in glutamate uptake. In response to a few minutes of simulated ischaemia, pyramidal cells in wild-type mice showed a large and sudden inward glutamate-evoked current [the anoxic depolarization (AD) current], which declined to a less inward plateau. In mice lacking GLT-1, the time to the occurrence of the AD current, its amplitude, the size of the subsequent plateau current and the block of the plateau current by glutamate receptor blockers were all indistinguishable from those in wild-type mice. We conclude that GLT-1 does not contribute significantly to glutamate release or glutamate removal from the extracellular space in early simulated ischaemia. These data are consistent with glutamate release being by reversal of neuronal transporters, and with uptake into glia being compromised by the ischaemia-evoked fall in the level of ATP needed to convert glutamate into glutamine.

Neuronal and inducible nitric oxide synthase expression and protein nitration in rat cerebellum after oxygen and glucose deprivation

A perfusion model of global cerebral ischemia was used for the immunohistochemical study of changes in the glutamate-nitric oxide (NO) system in the rat cerebellum and cerebellar nuclei during a 0-14 h reperfusion period after 30 min of oxygen and glucose deprivation, with and without administration of 1.5 mM N(omega)-nitro-L-arginine methyl ester (L-NAME). While immunostaining for N-methyl-D-aspartate receptor subunit 1 (NMDAR1) showed no marked changes during the reperfusion period, neuronal NO synthase (nNOS) immunostaining increased in stellate and basket cells, granule cells and neurons of the cerebellar nuclei. However, global cerebellar nNOS concentrations determined by Western blotting remained largely unchanged in comparison with actin expression. Inducible NOS (iNOS) immunostaining appeared in Purkinje cells and neurons of the cerebellar nuclei after 2-4 h of reperfusion and intensified during the 6-14 h period. This was reflected by an increase in global cerebellar iNOS expression determined by Western blotting. Immunostaining for protein nitrotyrosine was seen in Purkinje cells, stellate and basket cells, neurons of the cerebellar nuclei and glial cells in controls, and showed a progressive translocation in Purkinje cells and neurons of the cerebellar nuclei from an initial perinuclear or nuclear location towards the periphery. At the end of the reperfusion period the Purkinje cell apical dendrites were notably retracted and tortuous. Prior and concurrent L-NAME administration eliminated nitrotyrosine immunostaining in controls and blocked or reduced most of the postischemic changes observed. The results suggest that while nNOS expression may be modified in certain cells, iNOS is induced after a 2-4 h period, and that changes in protein nitration may be associated with changes in cell morphology.

Comparison of effects of DL-threo-beta-benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [3h]D-aspartate in astrocytes and glutamatergic neurons

Uptake and release processes in cerebellar astrocytes and granule neurons (glutamatergic) for glutamate were investigated by the use of [3H]D-aspartate, a non-metabolizable glutamate analog. The effects of DL-threo-beta-benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [3H]D-aspartate were studied. Both compounds inhibited potently uptake of [3H]D-aspartate in neurons and astrocytes (IC50 values 10-100 microM), DL-TBOA being slightly more potent than t-2,4-PDC. Release of preloaded [3H]D-aspartate from neurons or astrocytes could be stimulated by addition of excess t-2,4-PDC whereas addition of DL-TBOA had no effect on [3H]D-aspartate efflux. Moreover, DL-TBOA inhibited significantly the depolarization-induced (55 mM KCI) release of preloaded [3H]D-aspartate in the neurons. The results reflect the fact that DL-TBOA is not transported by the glutamate carriers while t-2,4-PDC is a substrate which may heteroexchange with [3H]D-aspartate. It is suggested that DL-TBOA may be used to selectively inhibit depolarization coupled glutamate release mediated by reversal of the carriers.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

The effect of streptozotocin-induced diabetes on the release of excitotoxic and other amino acids from the ischemic rat cerebral cortex

OBJECTIVE

Hyperglycemic stroke results in increased neuronal damage, the exact mechanism of which is unknown. Lactic acidosis has been implicated; however, increases in the excitotoxic amino acid glutamate, which correlate with increased neuronal damage, may be the cause for the increased damage seen in hyperglycemic stroke.

METHODS

Ten Sprague-Dawley rats were treated with streptozotocin (STZ; 50 mg/kg), and 12 normoglycemic rats were used as controls. Using a four-vessel occlusion model, global ischemia was assessed at 5 to 7 days after treatment in five animals (acute STZ group) or at 4 to 6 weeks after treatment in five animals (chronic STZ group). The cortical cup model was used to collect superfusates under basal, ischemic, and reperfusion conditions and analyzed for nine different amino acids using high-performance liquid chromatography.

RESULTS

Plasma glucose levels were significantly higher in the acute and chronic STZ groups as compared with the control group. Plasma lactate levels were higher in the acute STZ group as compared with the control or chronic STZ groups. Extracellular cortical glutamate levels were significantly reduced during reperfusion in the acute STZ group and during ischemia/reperfusion in the chronic STZ group as compared with the controls. Levels of extracellular gamma-aminobutyric acid were significantly reduced in the acute and chronic STZ groups as compared with the controls.

CONCLUSION

A chronic state of hyperglycemia results in reduction in extracellular brain glutamate levels during ischemia/reperfusion and therefore does not appear to be responsible for the increased neuronal damage seen in diabetic stroke. Chronic hyperglycemia also causes decreased extracellular gamma-aminobutyric acid levels, which, because of the loss of the inhibitory effects of this neurotransmitter, could contribute to the increased damage observed in hyperglycemic stroke.

Characterization of endogenous amino acid efflux from hippocampal slices during chemically-induced ischemia

Using sodium (NaN3)-induced anoxia plus aglycaemia as a model of chemically-induced ischemia, we have characterized the endogenous release of excitatory and inhibitory amino acids from superfused hippocampal slices. Chemical ischemia produced an azide (1-30 mM) dose-dependent increase in the efflux of glutamate, aspartate and GABA. These increases were attenuated to varying degrees by removal of Ca2+, or the addition of the voltage dependent Na+-channel blocker tetrodotoxin (TTX), the selective Ca2+ channel blockers conotoxin MVIIA, MVIIC, and nifedipine, the NMDA antagonist MK801, the AMPA antagonist GYKI-52466. Similarly, addition of the GLT-1 glutamate transport inhibitor dihydrokainate (DHK) and the anti-estrogen/anion channel blocker tamoxifen also attenuated the efflux of glutamate and GABA. It would therefore appear that the increases in amino acid efflux induced by chemical ischemia originates from both the neuronal pool, via conventional exocytotic release, and glial sources via reversal of the GLT-1 transporter and anion channel regulated cell swelling.

Studies on the effects of lactate transport inhibition, pyruvate, glucose and glutamine on amino acid, lactate and glucose release from the ischemic rat cerebral cortex

A rat four vessel occlusion model was utilized to examine the effects of ischemia/reperfusion on cortical window superfusate levels of amino acids, glucose, and lactate. Superfusate aspartate, glutamate, phosphoethanolamine, taurine, and GABA were significantly elevated by cerebral ischemia, then declined during reperfusion. Other amino acids were affected to a lesser degree. Superfusate lactate rose slightly during the initial ischemic period, declined during continued cerebral ischemia and then was greatly elevated during reperfusion. Superfusate glucose levels declined to near zero levels during ischemia and then rebounded beyond basal levels during the reperfusion period. Inhibition of neuronal lactate uptake with alpha-cyano-4-hydroxycinnamate dramatically elevated superfusate lactate levels, enhanced the ischemia/reperfusion evoked release of aspartate but reduced glutamine levels. Topical application of an alternative metabolic fuel, glutamine, had a dose dependent effect. Glutamine (1 mM) elevated basal superfusate glucose levels, diminished the decline in glucose during ischemia, and accelerated its recovery during reperfusion. Lactate levels were elevated during ischemia and reperfusion. These effects were not evident at 5 mM glutamine. At both concentrations, glutamine significantly elevated the superfusate levels of glutamate. Topical application of sodium pyruvate (20 mM) significantly attenuated the decline in superfusate glucose during ischemia and enhanced the levels of both glucose and lactate during reperfusion. However, it had little effect on the ischemia-evoked accumulation of amino acids. Topical application of glucose (450 mg/dL) significantly elevated basal superfusate levels of lactate, which continued to be elevated during both ischemia and reperfusion. The ischemia-evoked accumulations of aspartate, glutamate, taurine and GABA were all significantly depressed by glucose, while phosphoethanolamine levels were elevated. These results support the role of lactate in neuronal metabolism during ischemia/reperfusion. Both glucose and glutamine were also used as energy substrates. In contrast, sodium pyruvate does not appear to be as effectively utilized by the ischemic/reperfused rat brain since it did not reduce ischemia-evoked amino acid efflux.

Characterization of transient focal ischemia-induced increases in extracellular glutamate and aspartate in spontaneously hypertensive rats

Using middle cerebral artery occlusion (MCAO) and in vivo microdialysis, we have evaluated the changes in extracellular concentrations of the excitatory amino acids (EAA) glutamate and aspartate during varying periods of MCAO (0, 30, 60 min) in the striatum of spontaneously hypertensive rats (SHR). A positive correlation between occlusion time-dependent elevations in EAAs and the resulting ischemic injury was observed. This is the first demonstration of the temporal profile of EAA efflux during transient focal ischemia in SHRs. Possible sources and mechanisms of ischemia-induced EAA efflux were examined during 60 min of MCAO. Removal of Ca(2+) from the microdialysis infusion media significantly attenuated ischemia-induced increases in both glutamate (from ischemic peak of 4892 +/- 1298 to 1144 +/- 666% of preischemic values) and aspartate (from 2703 +/- 682 to 2090 +/- 599% of preischemic values). Similarly, infusion of the voltage dependent Na(+) channel blocker tetrodotoxin (TTX; 10 microM) significantly attenuated MCAO-induced increases in glutamate (to 1313 +/- 648%) and aspartate (to 359 +/- 114%). Infusion of the GLT-1 selective nontransportable inhibitor, dihydrokainate (DHK; 1 mM) also significantly attenuated the ischemia-induced increases in both EAAs (1285 +/- 508 and 1366 +/- 741% of the preischemic levels, respectively). These results indicate that during transient focal ischemia the increase in extracellular EAAs originates from both the neuronal pool, via conventional exocytotic release, and glial sources via the reversal of the GLT-1 transporter.

Inhibition of Na(+)/H(+) exchange by 5-(N-ethyl-N-isopropyl)-amiloride reduces free fatty acid efflux from the ischemic reperfused rat cerebral cortex

Brain tissue acidosis is considered to be a contributor to ischemic brain injury. The deleterious effects of marked acidosis may be associated with reperfusion and an excessive entry of Na(+) into cerebral neurons and glia as intracellular pH is restored by Na(+)/H(+) exchange. Normalization of pH, with activation of many calcium-dependent and other phospholipases and proteases with pH optima in the neutral or alkaline range, could account for the pronounced elevation in extracellular levels of free fatty acids which occurs during reperfusion following cerebral ischemia. In the present investigation we evaluated the effects of inhibition of Na(+)/H(+) exchange with N-(N-ethyl-N-isopropyl)-amiloride (EIPA; 25 microM) applied topically onto the rat cerebral cortex prior to and during ischemia. Free fatty acid levels in cortical superfusates, withdrawn at 10-min intervals from bilateral cortical windows, were analyzed by high pressure liquid chromatography. EIPA application effectively inhibited the increases in arachidonic and linoleic acid release observed in the control rats during reperfusion, and non-significantly depressed that of palmitic and oleic acids. Superfusate levels of glucose, which decline to near zero levels during ischemia and then rebound during reperfusion, were not affected by EIPA administration. Lactate levels in cortical superfusates from EIPA-treated animals rose more rapidly during reperfusion than did those in the control rats and then significantly declined towards basal levels. The data indicate that inhibition of Na(+)/H(+) exchange prevented the activation of phospholipases that usually occurs during reperfusion following a cerebral ischemic episode. These results are the first demonstration of such an effect and may provide an explanation for the cerebroprotective effects that have been observed in stroked animals following administration of Na(+)/H(+) exchange inhibitors.

The effect of intravenous insulin on accumulation of excitotoxic and other amino acids in the ischemic rat cerebral cortex

Insulin has been reported to be neuroprotective during cerebral ischemia/reperfusion. However, it may also increase the sensitivity of cultured cortical neurons to glutamate toxicity. The experiments described here utilized a rat four-vessel occlusion model with cerebral cortical windows to determine the effects of intravenous insulin, alone (I) or combined with glucose (IG) to maintain physiologic blood glucose levels, on the extracellular accumulation of amino acids in superfusates of the cerebral cortex. Aspartate, phosphoethanolamine, taurine and gamma-aminobutyric acid were increased in the I and IG groups and glutamate was increased in the IG group compared to controls during ischemia/reperfusion. Insulin treatment attenuated the rebound in cortical superfusate glucose levels in both groups of animals during reperfusion. The increases in amino acid release during reperfusion may be due to a lack of glycolytically derived energy available for amino acid uptake systems and ionic pumps.

Transporter reversal as a mechanism of glutamate release from the ischemic rat cerebral cortex: studies with DL-threo-beta-benzyloxyaspartate

Elevated levels of the excitotoxic amino acids, glutamate and aspartate, have been implicated in the pathogenesis of neuronal injury and death induced by cerebral ischemia. This study evaluated the contribution of reversed high-affinity, Na(+)-dependent, glutamate transport to the ischemia-evoked release of glutamate and aspartate using DL-threo-beta-benzyloxyaspartate (DL-TBOA), a newly developed competitive, non-transported blocker of the EAAT 1-3 transporters. Changes in the extracellular levels of these and other amino acids, and of glucose and lactate in cerebral cortical superfusates during four-vessel occlusion-elicited global cerebral ischemia were examined using a cortical window technique. Basal and ischemia-evoked amino acid, glucose and lactate efflux were compared in control versus DL-TBOA (100 microM; applied topically for 35 min prior to ischemia) animals. Twenty minutes of ischemia caused large increases in aspartate, glutamate, GABA and taurine effluxes into cortical superfusates, with non-significant effects on the efflux of glycine, glutamine, alanine and serine. Application of DL-TBOA caused a 2-fold increase in basal, preischemic, extracellular glutamate levels, but did not affect those of the other compounds. In the presence of DL-TBOA, ischemia-evoked release of aspartate, glutamate, taurine and glutamine was significantly reduced; that of the other amino acids was not affected. The ischemia-evoked declines in glucose were significantly attenuated, and lactate release was enhanced above that in control animals. The amino acid data are interpreted as indicating that aspartate and glutamate releases were reduced as a consequence of DL-TBOA inhibition of reversed transport by high-affinity, Na-dependent carriers, predominantly involving the glial EAAT 2 transporter. The reduction in ischemia-evoked taurine release is interpreted as being due to a decrease in cell swelling prior to and during the initial phase of ischemia due to reduced entry of the Na(+), and other ions, associated with a decreased glutamate uptake. Glucose-sparing and availability for lactate formation would also result from a reduced glutamate/Na(+) uptake. These results indicate that reversed transport, primarily from glial cells by the EAAT 2 carrier, is responsible for a substantial (42 and 56%) portion of the ischemia-evoked increase in extracellular glutamate and aspartate levels, respectively. As a potent, competitive, non-transported blocker of high-affinity, Na(+)-dependent, glutamate transporters, DL-TBOA promises to be a valuable new compound for the study of glutamatergic mechanisms.

Melittin enhances amino acid and free fatty acid release from the in vivo cerebral cortex

The effects of the phospholipase activator melittin on amino acid and free fatty acid release from the rat cerebral cortex were monitored and compared with those of a secretory PLA(2), using a cortical cup technique with topical application of these agents in artificial cerebrospinal fluid. Melittin (10 microg/ml; 3.5 microM) elicited a rapid increase in the levels of superfusate amino acids; aspartate, glutamate, GABA, glycine, taurine, glutamine, phosphoethanolamine, alanine, serine and the free fatty acids arachidonic, linoleic, palmitic and oleic acid. PLA(2) (25 microg/ml) also enhanced amino acid efflux but its effects were significantly slower to develop than those of melittin. The results confirm previous indications of an ability of phospholipases to augment extracellular levels of several amino acids, including the excitotoxins glutamate and aspartate, and further implicate phospholipase activation as a significant contributor to cerebral ischemic injury. Melittin has the potential to be a useful tool with which to evaluate the role of phospholipases in ischemia injury.

Failure of kynurenic acid to inhibit amino acid release from the ischemic rat cerebral cortex

The ability of a broad spectrum glutamatergic receptor antagonist, kynurenic acid (1 and 5 mM) to attenuate release of excitant and other amino acids from the ischemic cerebral cortex was examined in a four vessel occlusion rat model. Kynurenic acid, administered topically onto the cortex in artificial cerebrospinal fluid using bilateral cortical cups, failed to attenuate ischemia-evoked release of aspartate, glutamate, phosphoethanolamine, taurine and at 1 (but not 5) mM it depressed GABA release. There was no effect on basal, pre-ischemic, release. This result suggests that ischemia-evoked amino acid release is not a significant consequence of the activation of ionotropic receptors by synaptically released glutamate.

Block of rapid depolarization induced by in vitro energy depletion of rat dorsal vagal motoneurones

1. The ionic mechanisms contributing to the rapid depolarization (RD) induced by in vitro ischaemia have been studied in dorsal vagal motoneurones (DVMs) of brainstem slices. Compared with CA1 hippocampal neurones, RD of DVMs was slower, generally occurred from a more depolarized membrane potential and was accompanied by smaller increases in [K+]o. 2. RD was not induced by elevation of [K+]o to values measured around DVMs during in vitro ischaemia or by a combination of raised [K+]o and 2-5 microM ouabain. 3. Neither TTX (5-10 microM) nor TTX combined with bepridil (10-30 microM), a Na+-Ca2+ exchange inhibitor, slowed RD. Block of voltage-dependent Ca2+ channels with Cd2+ (0.2 mM) and Ni2+ (0.3 mM) led to an earlier onset of RD, possibly because [K+]o was higher than that measured during in vitro ischaemia in the absence of divalent ions. 4. When [Na+]o was reduced to 11.25-25 mM, RD did not occur, although a slow depolarization was observed. RD was slowed (i) by 10 mM Mg2+ and 0.5 mM Ca2+, (ii) by a combination of TTX (1.5-5 microM), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D-2-amino-5-phosphonovalerate (AP5, 50 microM) and (iii) by TTX (1.5-5 microM) and AP5 (50 microM). 5. Ni2+ at concentrations of 0.6 or 1.33 mM blocked RD whereas 0.6 mM Cd2+ did not. A combination of Cd2+ (0.2 mM), Ni2+ (0.3 mM), AP5 (50 microM) and bepridil (10 microM) was largely able to mimic the effects of high concentrations of Ni2+. 6. It is concluded that RD is due to Na+ entry, predominantly through N-methyl-D-aspartate receptor ionophores, and to Ca2+ entry through voltage-dependent Ca2+ channels. These results are consistent with known changes in the concentrations of extracellular ions when ischaemia-induced rapid depolarization occurs.

Mechanisms of D-aspartate release under ischemic conditions in mouse hippocampal slices

The release of preloaded D-[3H]aspartate, an unmetabolizable analogue of L-glutamate, was studied in superfused hippocampal slices from 7-day-old and 3-month-old (adult) mice under various cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and the presence of free radicals and metabolic poisons. The release was generally markedly enhanced in most of the above conditions, the responses being greater in adults than in developing mice. The presence of dinitrophenol had the most pronounced effect at both ages, followed by NaCN- and free-radical-containing media and ischemia. Hypoxia did not affect release in the immature hippocampus. Under most conditions K+ stimulation (50 mM) was still able markedly to enhance D-aspartate release. This potentiation under cell-damaging conditions in both adult and developing hippocampus signifies that increased L-glutamate release contributes to excitotoxicity and subsequent cell death. The mechanisms of ischemia-induced release of D-aspartate were analyzed in the adult hippocampus using ion channel inhibitors and modified superfusion media. The induced release proved to be partly Ca(2+)-dependent and partly Ca(2+)-independent. The results obtained with Na+ omission and homo- and heteroexchange with D-aspartate and L-glutamate demonstrated that a part of the release in normoxia and ischemia is mediated by the reversal of Na(+)-dependent glutamate transporters. The Na+ channel blockers amiloride and riluzole reduced the ischemia-induced release, also indicating the involvement of Na+ channels. In addition to this, the enhanced release of D-aspartate may comprise a swelling-induced component through chloride channels.

Effects of hyperosmolarity and ion substitutions on amino acid efflux from the ischemic rat cerebral cortex

The contributions of sodium and chloride ions and of osmotic stresses to the ischemia-evoked efflux of excitotoxic and other amino acids were explored using a rat four vessel occlusion model. Replacement of Na+ with choline or N-methyl-D-glucamine (NMDG) and of Cl- with sulfate or gluconate was used to evaluate the contribution that these ions make to amino acid efflux. The contribution of ischemia-evoked swelling to amino acid release was studied by applying mannitol or sucrose to minimize the cell volume increases and the compensatory regulatory volume decrease evoked efflux of amino acids. Aliquots of artificial cerebrospinal fluid (aCSF), appropriately adjusted for ion replacement or 150 mM mannitol or sucrose, were pipetted into cortical cups 35 min prior to ischemia and perfusate samples were obtained prior to, during and following ischemia (20 min) and reperfusion (40 min). Replacement of Na+ by NMDG depressed basal (normoxic) efflux of most amino acids, with choline substitution having little effect. During ischemia NMDG substitution increased glutamate and GABA efflux and choline enhanced the release of aspartate, glutamate, GABA and taurine. A reduction in extracellular Na+ would facilitate reversal of Na+-dependent transporters with extrusion of amino acids. Another possible explanation for the elevated release is that the absence of Na+ would inhibit the Ca2+/Na+ counter transport system, with a deleterious accumulation of intracellular Ca2+. Chloride replacement with sulfate or gluconate enhanced the efflux of aspartate, glutamate, GABA and taurine during ischemia. Removal of Cl- would depolarize cells, and block the Cl--dependent action of inhibitory amino acid transmitters, with both actions enhancing the ischemic injury and, consequently, amino acid release. Exposure to hyperosmotic mannitol (150 mM) aCSF enhanced ischemia-evoked release of some amino acids (taurine, GABA) and decreased that of aspartate and phosphoethanolamine. Sucrose aCSF enhanced the ischemia-evoked release of most amino acids. A potential explanation for these observations is that both agents may be able to rapidly penetrate the plasma membranes of ischemic neurons, actually contributing to the release of other osmolytes. The unanticipated nature of many of the observations made during these manipulations of the aCSF serves to accentuate the complex nature of the mechanisms responsible for the ischemia-evoked amino acid efflux into the interstitial spaces.

Inhibition of ischemia-induced glutamate release in rat striatum by dihydrokinate and an anion channel blocker

BACKGROUND AND PURPOSE

Increased activation of excitatory amino acid (EAA) receptors is considered a major cause of neuronal damage. Possible sources and mechanisms of ischemia-induced EAA release were investigated pharmacologically with microdialysis probes placed bilaterally in rat striatum.

METHODS

Forebrain ischemia was induced by bilateral carotid artery occlusion and controlled hypotension in halothane-anesthetized rats. During 30 minutes of ischemia, microdialysate concentrations of glutamate and aspartate were measured in the presence of a nontransportable blocker of the astrocytic glutamate transporter GLT-1, dihydrokinate (DHK), or an anion channel blocker, 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS), administered separately or together through the dialysis probe.

RESULTS

In control striata during ischemia, glutamate and aspartate concentrations increased 44+/-13 (mean+/-SEM) times and 19+/-5 times baseline, respectively, and returned to baseline values on reperfusion. DHK (1 mmol/L in perfusate; n=8) significantly attenuated EAA increases compared with control (glutamate peak, 9. 6+/-1.7 versus control, 15.4+/-2.6 pmol/ microL). EAA levels were similarly decreased by 10 mmol/L DHK. DNDS (1 mmol/L; n=5) also suppressed EAA peak increases (glutamate peak, 5.8+/-1.1 versus control, 10.1+/-0.7 pmol/ microL). At a higher concentration, DNDS (10 mmol/L; n=7) further reduced glutamate and aspartate release and also inhibited ischemia-induced taurine release. Together, 1 mmol/L DHK and 10 mmol/L DNDS (n=5) inhibited 83% of EAA release (glutamate peak, 2.7+/-0.7 versus control, 10.9+/-1.2 pmol/ microL).

CONCLUSIONS

These findings support the hypothesis that both cell swelling-induced release of EAAs and reversal of the astrocytic glutamate transporter are contributors to the ischemia-induced increases of extracellular EAAs in the striatum as measured by microdialysis.

Neurochemical and morphological responses to acutely and chronically implanted brain microdialysis probes

The purpose of this study was to compare, in rats, brain microdialysis results obtained using microdialysis probes implanted acutely for 2 h versus probes implanted chronically for 24 h in the caudate. Specific comparisons included: (1) dialysate purine and amino acid profiles during cerebral ischemia; (2) diffusional characteristics of the microdialysis probe; and (3) tissue morphology surrounding the probe. During ischemia, the increase in dialysate levels of adenosine, inosine, and hypoxanthine was less pronounced from probes implanted chronically, while dialysate xanthine levels increased to a greater extent. An increase in dialysate amino acid neurotransmitters during cerebral ischemia was observed in the acutely implanted probes within 10 min of the onset of cerebral ischemia; in the chronically implanted probes this increase did not occur until after 50 min of severe ischemia. Both in vitro and in vivo tests revealed a diffusional barrier in chronically implanted probes. Moreover, the tissue surrounding chronically implanted probes exhibited a high degree of inflammation, and fibrin deposits were substantial. In addition, uric acid levels (an indicator of tissue injury) sampled from chronically implanted probes were 7-fold greater than levels sampled from acutely implanted probes. These data raise concerns about the use of chronically implanted microdialysis probes for the measurement of purine and amino acid profiles during cerebral ischemia.

Mechanisms of amino acid release from the isolated anoxic/reperfused rat heart

Loss of amino acids into the coronary artery perfusate, which is exacerbated during anoxic stress, may have important implications for the ability of hearts subjected to ischemia or anoxia to recover function during reoxygenation. This work investigates the mechanisms underlying the amino acid efflux. Rat Langendorff heart preparations were used to study amino acid loss into coronary artery perfusates during anoxia or anoxia/reoxygenation sequences. Coronary flow rates, heart rates and intra-aortic pressures were recorded. Changes in myocardial amino acid concentrations were equated with amino acid levels in collected anoxic perfusate. With the exception of taurine, the differences in amino acid levels between normoxic and anoxic hearts were smaller than the amounts lost into the coronary perfusates, indicating ongoing replenishment of most amino acids during the anoxic episode. Fifteen-minute periods of exposure to low oxygen levels (P02 18-20 mmHg) resulted in large percentage increases in perfusate amino acid levels which returned slowly towards control levels upon reoxygenation. Anion channel blockers, anthracene-9-carboxylic acid, furosemide, and 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid (SITS), depressed anoxia-elicited increases in amino acid release. Phospholipase inhibition with quinacrine, 4-bromophenacyl bromide and 7,7-dimethyl-eicosadenoic acid (DEDA) depressed the anoxia-evoked release of amino acids. Combined applications of SITS and DEDA exhibited additive effects, virtually abolishing anoxia-evoked release of all the amino acids. The protein kinase C inhibitor, chelerythrine chloride, and the protein tyrosine kinase inhibitors, genistein and lavendustin A, inhibited anoxia-evoked amino acid release. Polyunsaturated fatty acids, arachidonic and linoleic, reduced anoxia-evoked amino acid release whereas monosaturated (oleic) and saturated (stearic) acids were ineffective. The glutamate transport inhibitor, dihydrokainate, depressed anoxia-evoked glutamate and aspartate release. These results suggest that at least three possible mechanisms for the anoxia-evoked amino acid efflux including (a) diffusional release through volume activated anion channels, (b) leakage across myocyte plasma membranes as a consequence of phospholipase activation and (c) reversal of Na+ dependent high-affinity transporters.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

Na+ channel block prevents the ischemia-induced release of norepinephrine from spinal cord slices

The principal finding of the present study with rat spinal cord slices was the novel demonstration of the [Ca2+]o-independent effect of ischemia on norepinephrine release and its antagonism by tetrodotoxin and low temperature (10 degrees C). Our finding that tetrodotoxin antagonized the effects of glucose deprivation on norepinephrine release in a [Ca2+]o-independent way suggests that Na+ channel block alone, i.e., the prevention of Na+ accumulation, may account for the protective action. Low temperature completely prevented the effect of ischemia on norepinephrine release but did not change the release associated with axonal activity. This finding is in good agreement with the observation that small changes in brain temperature critically determine the extent of neuronal injury from ischemia and suggests that both [Ca2+]o-independent release and cell injury are associated with the norepinephrine membrane carrier. It is suggested, therefore, that drugs able to attenuate the increase in [Na+]i during ischemia may be useful agents to protect against ischemic damage if given before the insult.

Tamoxifen, a chloride channel blocker, reduces glutamate and aspartate release from the ischemic cerebral cortex

The effects of the anti-estrogen, anion channel blocker, tamoxifen on amino acid release from the ischemic rat cerebral cortex was investigated using a cortical cup technique. Tamoxifen (20 microM in artificial cerebrospinal fluid), applied topically, inhibited the ischemia-evoked efflux of aspartate, glutamate, taurine and phosphoethanolamine. Reductions in the ischemia-evoked levels of these amino acids suggest that tamoxifen may attenuate chloride-related osmotic cell swelling and the associated regulatory volume decrease (RVD) release of amino acids.

Amino acid release during volume regulation by cardiac cells: cellular mechanisms

Mechanisms of amino acid efflux during volume regulation in hypoosmotically treated isolated rat hearts were studied by collecting the coronary artery perfusate and analysis by high pressure liquid chromatography. Hypoosmotic stress resulted in marked percentage increases in perfusate taurine, aspartate and glutamate levels, smaller increases in phosphoethanolamine, glycine and alanine and non-significant increases in serine and glutamine. Amino acid levels declined during reperfusion with isosmotic perfusate. The anion channel blocker 4-acetamido-4-isothiocyanostilbene-2:2'-disulfonic acid (SITS, 500 microM) significantly reduced hypoosmotic release of taurine, aspartate, glutamate and glycine. Furosemide reduced hypoosmotically-evoked releases of taurine, glycine, alanine and phosphoethanolamine. The polyunsaturated amino acids, arachidonic and linoleic also reduced amino acid efflux. Phospholipase A2 inhibition with 7,7-dimethyleicosadienoic acid (DEDA, 2 microM) reduced osmotically-evoked releases of taurine, aspartate and glutamate. 4-Bromophenacyl bromide (1 microM) inhibited osmotically-evoked release of glutamate and glycine. Combined applications of SITS + DEDA markedly reduced osmotically evoked release of all eight amino acids. Glutamate and aspartate effluxes were not inhibited by the glutamate transport inhibitor dihydrokainic acid (1 mM). These results indicate that the hypoosmotic stress, by inducing cell swelling, can initiate an amino acid efflux as part of a regulatory volume decrease. An opening of anion-permeant channels and phospholipase activation appear to be involved in the regulatory volume decrease phenomenon.

Sodium channel blockade unmasks two temporally distinct mechanisms of striatal dopamine release during hypoxia/hypoglycaemia in vitro

Massive striatal dopamine release during cerebral ischaemia has been implicated in the resulting neuronal damage. Sodium influx is an early event in the biochemical cascade during ischaemia and blockade of sodium channels may increase resistance to ischaemia by reducing energy demand involved in compensation for sodium and potassium fluxes. In this study, we have determined the effects of opening and blockade of voltage-gated sodium channels on hypoxia/hypoglycaemia-induced dopamine release. Slices of rat caudate nucleus were maintained in a slice chamber superfused by an oxygenated artificial cerebrospinal fluid containing 4 mM glucose. Ischaemia (hypoxia/hypoglycaemia) was mimicked by a switch to a deoxygenated artificial cerebrospinal fluid containing 2 mM glucose and dopamine release was measured using fast cyclic voltammetry. In drug-free (control) slices, there was a 2-3 min delay after the onset of hypoxia/hypoglycaemia followed by a rapid dopamine release event which was associated with anoxic depolarization. In slices treated with the Na+ channel opener, veratridine (1 microM), the time to onset of dopamine release was shortened (101 +/- 20 s, compared with 171 +/- 8 s in controls, P < 0.05). Conversely, phenytoin (100 microM), lignocaine (200 microM) and the highly selective sodium channel blocker, tetrodotoxin (1 microM) markedly delayed and slowed dopamine release vs paired controls. In the majority of cases, dopamine release was biphasic after sodium channel blockade: a slow phase preceded a more rapid dopamine release event. The latter was associated with anoxic depolarization. Neither the fast nor the slow release events were affected by pretreatment with the selective dopamine uptake blocker GBR 12935 (0.2 microM), suggesting that uptake carrier reversal did not contribute to these events. In conclusion, sodium channel antagonism delays and slows hypoxia/hypoglycaemia-induced dopamine release in vitro. Furthermore, sodium channel blockade delays anoxic depolarization and its associated neurotransmitter release, revealing an earlier dopamine release event that does not result from reversal of the uptake carrier.

Glutamate measured by 6-s resolution brain microdialysis: capillary electrophoretic and laser-induced fluorescence detection application

In the present experiment the combination of brain microdialysis and CZE-LIFD permitted the measurement of glutamate in 100 nl microdialysis samples collected every 5 or 6 s. Samples were collected every 6 s, in rats anesthetized with two different anesthetic agents (ketamine and sodium thiopental). A microdialysis probe was inserted in the cortex of an anesthetized rat in the territory irrigated by the middle cerebral artery. The artery was clamped for 30 s and then released. The samples were derivatized with fluorescein isothiocyanate I (FITC) by means of a continuous-flow reactor, collected and injected into a home-made CZE-LIFD instrument. Glutamate decreased immediately after clamping the artery in ketamine anesthetized rats and increased 1 min after the onset of the ischemia in sodium thiopental anesthetized rats. In another experiment a 60 mM KCl solution was injected through a microdialysis probe inserted in the hippocampus of an anesthetized rat. In the first 5 s after the KCl solution reached the tissue, glutamate increased but gamma-aminobutytic acid and glutamine did not. The experiments show that time resolution of brain microdialysis can be reduced to a few seconds if the analytical technique is the proper one.

Adenosine A1 receptor activation preferentially protects cultured cerebellar neurons versus astrocytes against hypoxia-induced death

Administration of adenosine A1 receptor agonists in vivo is neuroprotective in various stroke models. Experiments using either mixed cultures of neurons and astrocytes or brain slices, in which several cell types are present, have demonstrated that activation of A1 receptors also id protective against hypoxia and/or hypoglycemia in vitro. In this study, we have examined the effect of the A1 agonist cyclopentyladenosine (CPA) on cellular damage, measured by efflux of lactate dehydrogenase (LDH), in highly enriched primary cultures of either neurons of astrocytes exposed to different metabolic insults. CPA reduced neuronal LDH release induced by a combination of hypoxia and substrate deprivation ("simulated ischemia"; IC50 = 28 nM) of by hypoxia alone (IC50 = 170 nM). In contrast, CPA had no effect on neuronal damage induced by substrate deprivation alone, not did it affect ischemic death to astrocytes. The neuroprotective effect of CPA during simulated ischemia and hypoxia were reversed by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). These data demonstrate that activation of an adenosine A1 receptor on neurons, but not astrocytes, is protective against cellular damage of death induced specifically by hypoxia as opposed to other metabolic insults such as hypoglycemia.

Inhibition by anion channel blockers of ischemia-evoked release of excitotoxic and other amino acids from rat cerebral cortex

Neuronal and glial cell swelling occurs rapidly in ischemia as part of the cytotoxic response. Astrocytic swelling is known to result in large extracellular increases in certain amino acids, including glutamate, aspartate and taurine, as part of the regulatory volume decrease (RVD) response inherent to these and other cells. RVD in astrocytic cultures is inhibited by anion channel blockers. In this study, we compared the effects of three anion channel blockers on the ischemia/reperfusion-evoked release of amino acids from the in vivo rat cerebral cortex. Twenty minutes of four vessel cerebral ischemia caused significant increases in cortical superfusate levels of aspartate, glutamate, GABA, taurine and phosphoethanolamine. During reperfusion there were delayed increases in the level of glycine, alanine and serine. Glutamine levels were not affected. Cl- channel blockers, 4-acetamido-4'-isothiocyanostrilbene-2,2'-disulfonic acid (SITS, 2 mM), 5-nitro-2-(3-phenyl-propylamino)benzoic acid (NPPB, 350 microM) and dipyridamole (200 microM) depressed basal releases of glutamate and taurine and the ischemia/reperfusion-evoked releases of aspartate, glutamate, taurine and phosphoethanolamine. The results suggest that diffusion of amino acids through an anion channel may be partially responsible for the elevated extracellular levels of excitotoxic and other amino acids that occur during cerebral ischemia/reperfusion.

Nitric oxide modulates synaptic glutamate release during anoxia

The ability of nitric oxide to enhance vesicular glutamate release during anoxia was examined in the present study. Whole-cell patch clamp recordings were obtained from CA1 pyramidal neurons in rat hippocampal slices perfused in media containing tetrodotoxin. These cells exhibit spontaneous inward currents previously identified as glutamatergic miniature excitatory postsynaptic currents (mEPSCs). The frequency of these mEPSCs increases during exposure to anoxia. The anoxia-induced increase in frequency is reduced when experiments are performed in the presence of the competitive nitric oxide (NO)-synthase inhibitors N(G)-nitro-L-arginine methyl ester and N(G)-nitro-L-arginine, as well as reduced hemoglobin. Arginine reversed the suppression by the NO-synthase inhibitors. The N-methyl-D-aspartate (NMDA) receptor antagonists 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid and MK-801 also suppressed the anoxia-induced increase in mEPSC frequency. These data indicate that NMDA receptor-activated NO production may enhance vesicular synaptic glutamate release, which would in turn contribute to excitotoxicity during hypometabolic states.

Free radicals and the ischemia-evoked extracellular accumulation of amino acids in rat cerebral cortex

The effects of free radical generating systems on basal and ischemia/reperfusion-evoked release of amino acids into cortical superfusates was examined in the rat using the cortical cup technique. Xanthine oxidase plus xanthine significantly enhanced GABA levels 358 fold over controls during 20 min of four vessel occlusion. Glutamate and phosphoethanolamine release following reperfusion were also elevated. Prostaglandin synthase plus arachidonic acid significantly enhanced the ischemia-evoked release of all amino acids (aspartate 360 fold; glutamate 433 fold; glycine 6 fold; GABA 689 fold; phosphoethanolamine 69 fold) and increased the pre-ischemic levels of glutamate, glycine and phosphoethanolamine. Administration of H2O2 plus ferrous sulfate significantly elevated both pre-ischemic amino acid release and ischemia-evoked release. A role for free radical generating systems in the development of ischemic injury is supported by the ability of superoxide dismutase plus catalase to reduce ischemia-evoked amino acid efflux into cortical superfusates. Thus, the species of free radical produced, as well as the amount generated, may after the pattern of amino acid release under both ischemic and non-ischemic conditions.

Release of the excitotoxic amino acids, glutamate and aspartate, from the isolated ischemic/anoxic rat heart

An isolated rat Langendorff heart preparation has been developed as a model in which to study the release of glutamate, aspartate and other amino acids during ischemia, anoxia and hypoglycemia. 15 min periods of ischemia resulted in large increases in perfusate levels of glutamate, aspartate, glycine, phosphoethanolamine, serine, alanine, taurine and glutamine. Amino acid levels returned towards pre-ischemic levels in subsequent perfusate collections. Anoxia (15 min duration) increased perfusate levels of most of the measured amino acids, with glutamate and aspartate being particularly affected. In contrast to ischemia, glutamate and aspartate levels declined slowly following reoxygenation. Hypoglycemia (15 min) resulted in small but significantly elevated levels of glutamate and glycine in heart perfusates. As the effects of ischemia or anoxia on glutamate and aspartate release from the heart appear to be comparable to those observed in the brain, it is proposed that the heart preparation may be a suitable model in which to study the ischemia-evoked release of these amino acids in the absence of complications arising from their depolarizing and excitotoxic actions on central neurons.

Mechanism of glutamate release from rat hippocampal slices during in vitro ischemia

There was a large release of endogenous glutamate and of pre-accumulated [3H]-D-aspartate from rat hippocampal slices during deprivation of oxygen and glucose (in vitro ischemia). The role of Na(+)-dependent glutamate transporters in this process was investigated. The release of both glutamate and [3H]-D-aspartate was largely blocked by two competitive substrate analogues of the Na(+)-dependent glutamate transporters (L-trans-pyrrolidine-2,4-dicarboxylate and D,L-threo-B-hydroxyaspartate) if the substrate analogues were intracellularly loaded prior to the ischemia. The pre-loaded analogue, D,L-threo-B-hydroxyaspartate, did not block exocytotic release of glutamate, induced by high-potassium. Dihydrokainate, an inhibitor of a subset of the Na(+)-dependent transporters, did not inhibit ischemia-induced release of glutamate or [3H]-D-aspartate. However, it did block release induced by veratridine, which was also blocked by the pre-loaded substrate analogues. Dihydrokainate could still inhibit veratridine-induced release during ischemia, showing that conditions during ischemia did not reduce its efficacy. It is concluded that release of glutamate during ischemia is largely via reversal of the Na(+)-dependent glutamate transport system. The differential effects of dihydrokainate and the competitive substrate analogues on ischemia-induced release indicate that this release occurs via a subset of the glutamate transporters that are present in the hippocampus.

Ischemic disruption of glutamate homeostasis in brain: quantitative immunocytochemical analyses

More than 10 years ago, it was shown by microdialysis that the excitatory transmitter glutamate accumulates in the interstitial space of brain subjected to ischemic insult. This was one of the key observations leading to the formulation of the "glutamate hypothesis' of ischemic cell death. It is now assumed that even a transient glutamate overflow may set in motion a number of events that ultimately cause cell loss in vulnerable neuronal populations. The aim of the present review is to discuss the intracellular changes that underlie the dysregulation of extracellular glutamate during and after ischemia, with emphasis on data obtained by postembedding, electron microscopic immunogold cytochemistry. While the time resolution of this approach is necessarily limited, it can reveal, quantitatively and at a high level of spatial resolution, how the intracellular pools of glutamate and metabolically related amino acids are perturbed during and after an ischemic insult. Moreover, this can be done in animals whose extracellular amino acid levels are monitored by microdialysis, allowing a direct correlation of extra- and intracellular changes. Immunogold analyses of brains subjected to ischemia have identified dendrites and neuronal somata as likely sources of glutamate efflux, probably mediated by reversal of glutamate uptake. The vesicular glutamate pool has been found to be largely unchanged after 20 min of ischemia. Ischemia causes an increased glutamate content and an increased glutamate/glutamine ratio in glial cells, as revealed by double immunogold labelling. This argues against the idea that glial cells contribute to the extracellular overflow of glutamate in the ischemic brain.