Extracellular accumulation of glutamate in the hippocampus induced by ischemia is not calcium dependent--in vitro and in vivo evidence

New insights in ferroptosis: Potential therapeutic targets for the treatment of ischemic stroke

Stroke is a common disease in clinical practice, which seriously endangers people's physical and mental health. The neurovascular unit (NVU) plays a key role in the occurrence and development of ischemic stroke. Different from other classical types of cell death such as apoptosis, necrosis, autophagy, and pyroptosis, ferroptosis is an iron-dependent lipid peroxidation-driven new form of cell death. Interestingly, the function of NVU and stroke development can be regulated by activating or inhibiting ferroptosis. This review systematically describes the NVU in ischemic stroke, provides a comprehensive overview of the regulatory mechanisms and key regulators of ferroptosis, and uncovers the role of ferroptosis in the NVU and the progression of ischemic stroke. We further discuss the latest progress in the intervention of ferroptosis as a therapeutic target for ischemic stroke and summarize the research progress and regulatory mechanism of ferroptosis inhibitors on stroke. In conclusion, ferroptosis, as a new form of cell death, plays a key role in ischemic stroke and is expected to become a new therapeutic target for this disease.

Effects of pharmacological inhibition of AMP-activated protein kinase on GLUT3 expression and the development of ischemic tolerance in astrocytes

Ischemic tolerance resulting from preconditioning ischemia is a neuroprotective mechanism. In cultured astrocytes, its development depends on regulation of the expression of glucose transporter 3 (GLUT3) by the stress sensor/effector AMP-activated protein kinase (AMPK). Here we demonstrate that GLUT3 is upregulated during preconditioning and then downregulated during recovery. We also found that, although AMPK inhibition during preconditioning initially suppressed the upregulation of GLUT3 as shown previously, this was followed by a period of GLUT3 upregulation, enhanced glycogen accumulation, and enhanced tolerance to a subsequent ischemic challenge. These results reveal that AMPK has a complex influence on ischemic tolerance.

Ischemia induces release of endogenous amino acids from the cerebral cortex and cerebellum of developing and adult mice

Ischemia enhanced release of endogenous neuroactive amino acids from cerebellar and cerebral cortical slices. More glutamate was released in adult than developing mice. Taurine release enhanced by K⁺ stimulation and ischemia was more than one magnitude greater than that of GABA or glutamate in the developing cerebral cortex and cerebellum, while in adults the releases were almost comparable. Aspartate release was prominently enhanced by both ischemia and K⁺ stimulation in the adult cerebral cortex. In the cerebellum K⁺ stimulation and ischemia evoked almost 10-fold greater GABA release in 3-month olds than in 7-day olds. The release of taurine increased severalfold in the cerebellum of 7-day-old mice in high-K⁺ media, whereas the K⁺-evoked effect was rather small in adults. In 3-month-old mice no effects of K⁺ stimulation or ischemia were seen in the release of aspartate, glycine, glutamine, alanine, serine, or threonine. The releases from the cerebral cortex and cerebellum were markedly different and also differed between developing and adult mice. In developing mice only the release of inhibitory taurine may be large enough to counteract the harmful effects of excitatory amino acids in ischemia in both cerebral cortex and cerebellum, in particular since at that age the release of glutamate and aspartate cannot be described as massive.

Phenolic alkaloids from Menispermum dauricum rhizome protect against brain ischemia injury via regulation of GLT-1, EAAC1 and ROS generation

Menispermum dauricum rhizome has been widely used in China to treat various cardiovascular and thrombosis disorders. Some studies have reported that the phenolic alkaloids of Menispermum dauricum rhizome (PAM) have protective effects against brain ischemia injury, but the mechanism of this action remains to be clarified. In the present study, we investigated the possible mechanisms of action of PAM on experimental brain ischemia injury. Oxygen and glucose deprivation (OGD) in rat primary cortical cultures and middle cerebral artery occlusion in rats were used to mimic ischemia-reperfusion injury, respectively. The results suggested that PAM protected rat primary cortical cultures against OGD-reoxygenation induced cytotoxicity. PAM decreased extracellular glutamate content and markedly prevented the effects induced by OGD on protein level of GLT-1 and EAAC1 glutamate transporters. In addition, it reduced intracellular ROS generation. In vivo, PAM significantly reduced cerebral infarct area and ameliorated neurological functional deficits at different time points. Our findings revealed that the possible mechanism of action of PAM protected against brain ischemia injury involves regulation of GLT-1, EAAC1 and ROS generation.

Cordycepin protects against cerebral ischemia/reperfusion injury in vivo and in vitro

Cordycepin, (3'-deoxyadenosine), a bioactive compound of Cordyceps militaris, has been shown to exhibit many pharmacological actions, such as anti-inflammatory, antioxidative and anticancer activities. Little is known about the neuroprotective action of cordycepin as well as its molecular mechanisms. In this study, cordycepin was investigated for its neuroprotective potential in mice with ischemia following 15 min of the bilateral common carotid artery occlusion and 4h of reperfusion. The effect of cordycepin was also studied in mice brain slices treated with oxygen-glucose deprivation (OGD) injury. Our results showed that cordycepin was able to prevent postischemic neuronal degeneration and brain slice injury. Excitatory amino acids such as glutamate and aspartate in brain homogenized supernatant, which were increased in ischemia/reperfusion group, were detected by high performance liquid chromatography (HPLC). The results showed that cordycepin was able to decrease the extracellular level of glutamate and aspartate significantly. Moreover, cordycepin was able to increase the activity of superoxide dismutase (SOD) and decrease the level of malondialdehyde (MDA), ameliorating the extent of oxidation. Furthermore, matrix metalloproteinase-3(MMP-3), a key enzyme involved in inflammatory reactions, was markedly increased after ischemia reperfusion, whereas cordycepin was able to inhibit its expression obviously. In conclusion, our in vivo and in vitro study showed that cordycepin was able to exert a potent neuroprotective function after cerebral ischemia/reperfusion.

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the 'what' of brain function, the 'how' of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the 'how' that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic-extrasynaptic receptor trafficking, neuron-glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.

The Effect of Cyclohexyladenosine on the Penischemic Increases of Hippocampal Glutamate and Glycine in the Rabbit

We investigated the ability of N6-cyclohexyladenosine (CHA), a potent and selective agonist of the adenosine A1 receptor, to attenuate elevations of levels of extracellular hippocampal glutamate and glycine that result from episodes of transient global cerebral ischemia (TGCI). A total of 30 New Zealand white rabbits were randomly assigned to receive 0 (n = 5), 0.1 (n = 8), 1.0 (n = 6), 10 (n = 6), or 100 (n = 5) microM CHA. The drug was dissolved in artificial CSF (vehicle) and administered via a microdialysis probe placed stereotactically into the dorsal hippocampus. A second microdialysis probe placed into the contralateral hippocampus of each animal was perfused with vehicle alone. Ten minutes of TGCI was induced by neck tourniquet inflation and deliberate hypotension from 0 to 10 min. Microdialysis samples were collected as follows: every 20 min preischemia (at -80, -60, -40, -20, and 0 min); every 5 min during ischemia and in the immediate reperfusion period (at 5, 10, 15, and 20 min); and every 20 min for the remainder of the reperfusion period (at 40, 60, and 80 min). Samples were then analyzed for their concentration of glutamate and glycine by HPLC. Following 10 min of ischemia, glutamate levels increased to a peak of 3.28 +/- 0.55 times baseline and returned to preischemic levels by 40 min, i.e., during reperfusion. Glycine concentrations increased to 5.41 +/- 0.91 times over baseline and remained elevated for the duration of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of glutamate receptors of the NMDA type in the modulation of acetylcholine and glutamate overflow from the guinea pig ileum during in vitro hypoxia and hypoglycaemia

The involvement of NMDA glutamate receptors in the effects of glucose/oxygen deprivation (in vitro ischaemia) on spontaneous endogenous acetylcholine and glutamate overflow from the guinea pig ileum was studied. Neurotransmitter overflow was measured by HPLC. Deprivation of glucose in the medium slightly reduced acetylcholine overflow, and did not significantly influence glutamate overflow. During oxygen deprivation and glucose/oxygen deprivation, acetylcholine overflow augmented with a biphasic modality: an early peak was followed by a long lasting increase, whereas glutamate overflow increased with a rapid and sustained modality. The effects of glucose/oxygen deprivation on both acetylcholine and glutamate overflow were abolished after reperfusion with normal oxygenated medium. Acetylcholine and glutamate overflow induced by glucose/oxygen deprivation were significantly reduced in the absence of external Ca(2+) as well as by the addition of the mitochondrial Na(+)-Ca(2+) exchanger blocker, CGP 37157, and of the endoplasmic reticulum Ca(2+)/ATPase blocker, thapsigargin. +/-AP5, an NMDA receptor antagonist, and 5,7-diCl-kynurenic acid, an antagonist of the glycine site associated to NMDA receptor, markedly depressed glucose/oxygen deprivation-induced acetylcholine and glutamate overflow as well. Our results suggest that in vitro simulated ischaemia evokes acetylcholine and glutamate overflow from the guinea pig ileum, which is partly linked to an increase in intracellular Ca(2+) concentration dependent on both Ca(2+) influx from the extracellular space and Ca(2+) mobilization from the endoplasmic reticulum and mitochondrial stores. During glucose/oxygen deprivation, ionotropic glutamate receptors of the NMDA type exert both a positive feedback modulation of glutamate output and contribute to increased acetylcholine overflow.

Therapeutic time window for the neuroprotective action of MK-801 after decapitation ischemia: hippocampal slice data

Neuroprotective action of MK-801 administrated pre- and postischemically, in vivo or in vitro, respectively, was studied on hippocampal slices using decapitation ischemia model. Recovery of orthodromic population spikes in CA1 region was measured during postischemic incubation of the slices with oxygenated artificial cerebrospinal fluid (ACSF). The ability of postischemically applied MK-801 to restore the electrical activity dramatically depended on the timing of its application during the reoxygenation period. When applied in vitro, together with the start of reoxygenation, MK-801 was as effective as in the case of in vivo administration before the ischemia. The delay in the in vitro administration for only a few minutes led to a dramatic decrease in the drug effectiveness. When applied in 30 min after the start of reoxygenation, MK-801 was totally ineffective. The dose/response relationship between MK-801 concentration and the amplitude of recovered orthodromic population spikes of hippocampal pyramidal neurons is logarithmic. The ED(50) value for the action of "postischemic" MK-801 is ca. 10(-5) M. Preischemic in vivo application of the drug [intraperitoneal (i.p.) injection 15 min prior to decapitation] results in ED(50) ca. 0,2 mg/kg. The slope of both dose/concentration-response curves is similar. The time course of population spike recovery after 90-min ischemia is identical for pre- and postischemic action of MK-801 (estimated for ED(50) in both cases). These data allow to suggest that "preischemic" MK-801 is predominantly active as a neuroprotector only after ischemia, within a short therapeutic window at the start of the reoxygenation period.

α-Sialylcholesterol enhances the depolarization-induced release of acetylcholine and glutamate in rat hippocampus: in vivo microdialysis study

The effects of alpha-sialylcholesterol (alpha-SC), a synthetic ganglioside analogue, on synaptic neurotransmission were studied using in vivo microdialysis technique. Application of alpha-SC through a microdialysis probe enhanced high potassium-evoked release of acetylcholine and glutamate in the hippocampal CA3 region of Wistar rats. The experiments using synaptosomes and FM1-43, a fluorescent styryl dye used for studies of neurotransmitter release mechanisms, showed that alpha-SC increased depolarization-induced loss of dye but it did not evoke the dye loss at resting condition. These results indicate that alpha-SC promotes a depolarization-induced exocytotic neurotransmitter release in the brain under in vivo conditions. Application of alpha-SC increased the level of glutamate but not that of acetylcholine, suggesting that alpha-SC affects spontaneous glutamate release and/or transport system at the brain region.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

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.

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.

Glutamate release by an Na+ load and oxidative stress in nerve terminals: relevance to ischemia/reperfusion

Previously we have reported that oxidative stress induced by hydrogen peroxide exacerbates the effect of an Na+ load in isolated nerve terminals, with a consequence of an ATP depletion, [Ca2+]i and [Na+]i deregulation, and collapse of mitochondrial membrane potential. In the present study, the release of glutamate in response to a combined effect of an [Na+] load and oxidative stress was measured in isolated nerve terminals over an incubation for 15 min. Exposure to hydrogen peroxide (100 micro m) had no effect on the release of glutamate, but significantly enhanced the Ca2+-independent glutamate release induced by a small [Na+] load achieved with 10 micro m veratridine. The effect of a larger Na+ load induced by 40 micro m veratridine was not further increased by hydrogen peroxide; in contrast the external Ca2+-dependent glutamate release was completely eliminated by the oxidant under this condition. The effects of oxidative stress superimposed on a Na+ load are consistent with at least two factors: (i) a relatively modest Na+ load induced by veratridine is augmented by H2O2 giving rise to an increased Ca2+-independent release of glutamate (ii) oxidative stress in combination with a larger Na+ load causes severe ATP depletion limiting the Ca2+-dependent vesicular glutamate release. Given the concurrent presence of an Na+ load and oxidative stress in ischemia/reperfusion these results indicate that the extent of the Na+ load developing during the ischemic period could determine the release of glutamate induced by an oxidative stress during reperfusion.

Trimetazidine as a potential neuroprotectant in transient global ischemia in gerbils: a behavioral and histological study

The effect of Trimetazidine (TMZ) as a potential neuroprotectant against stroke was studied in the gerbil model of transient forebrain global ischemia. Animals were subjected to a 5-min period of ischemia and assessed 4 and 21 days later. Gerbils were divided into two groups: in group one, gerbils were treated with TMZ at a dose of 25 mg/kg given by intraperitoneal injection prior to ischemia. In group two, gerbils were treated with TMZ at a dose of 25 mg/kg given intraperitoneally after ischemia. Saline-injected gerbils served as controls. Histological evaluation of neuronal damage was carried out using the silver staining technique in gerbils 4 and 21 days after the start of the experimental protocol. Behavioral functions were assessed in gerbils from the 14th to the 21st day after the start of the experimental protocol using the Morris water maze test. Results obtained from this study showed no significant difference between saline treated TMZ-treated gerbils when TMZ was administered after ischemia. When TMZ was administered prior to ischemia, there was a reduction in neuronal damage although it did not reach statistical significance and a statistically significant improvement in behavior. We conclude that TMZ shows signs of promise as a neuroprotective agent, and further studies should look at pre-treatment with different doses and different times.

Analysis of high intracellular [Na+]-induced release of [3H]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellular sodium-induced transmitter release in the CNS through the characterisation of the veratridine-evoked (40 microM) noradrenaline release from rat hippocampal slices. The response to veratridine was completely inhibited by tetrodotoxin (1 microM), indicating that the effect is due to the activation of sodium channels. Omission of Ca2+ from the superfusion fluid inhibited the veratridine-evoked release by 72%, showing that the majority of release results from external Ca2+-dependent exocytosis. The residual Ca2+-independent release was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 microM) suggesting that intracellular Ca2+ stores are not involved in this component of veratridine effect. The noradrenaline uptake blockers, desipramine (10 microM) and nisoxetine (10 microM), inhibited the external Ca2+-independent release by 50 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to uptake blockers, lowering the temperature, another possibility to inhibit transporter function, completely inhibited the effect of veratridine in the absence of Ca2+. Further experiments revealed that low temperature (20 and 12 degrees C) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+]i) in rat cortical synaptosomes (68 and 78% inhibition, respectively). The clinical relevance of our data is that during ischemia a massive release of transmitters occurs mainly due to the elevation of [Na+]i, which contributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of ischemia because it has a dual action on this process. Firstly, it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly, it inhibits the sodium influx and therefore prevents the unwanted elevation of [Na+]i. Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions.

Excessive release of [3H] noradrenaline by veratridine and ischemia in spinal cord

In this study, the properties of ischemic condition-induced and veratridine-evoked [3H]noradrenaline ([3H]NA) release from rat spinal cord slices were compared. It was expected that ischemia mimicked by oxygen and glucose deprivation results in the impairment of Na+/K+ -ATPase with a consequent elevation of the intracellular Na+ -level which reverses the NA carrier and promotes excessive NA release, and veratridine, by the activation of Na+ channels, releases NA both carrier-mediated and Ca2+ -dependent, i.e. vesicular manner. In our experiments, veratridine (1-100 microM) dose-dependently increased the resting [3H]NA release, and its effect was only partially blocked by low temperature or the lack of external calcium, whereas the sodium channel inhibitor tetrodotoxin (TTX, 1 microM) completely prevented it, indicating that veratridine induces NA release via axonal depolarization and reversing the transporters by eliciting Na+ -influx. In contrast to TTX, the local anesthetic lidocaine (100 microM) only partially blocked the veratridine-induced [3H]NA release due to its inhibitory action on K+ channels. The ischemia-induced [3H]NA release was abolished at 12 degrees C, a temperature known to block only the transporter-mediated release of transmitters. However, lidocaine was also partially effective to reverse the action of ischemia on the NA release, indicating that lidocaine is not a useful compound in the treatment of spinal cord-injured patients against the excessive excytotoxic NA release.

Adenosine induces initial hypoxic-ischemic depression of synaptic transmission in the rat hippocampus in vivo

The present study was designed to investigate the role of adenosine in the hypoxic depression of synaptic transmission in rat hippocampus. An in vivo model of hypoxic synaptic depression was developed in which the common carotid artery was occluded on one side in the urethane-anesthetized rat. Inspired oxygen levels were controlled through a tracheal cannula. Rats were placed in a stereotaxic apparatus for stimulation and recording of bilateral hippocampal field excitatory postsynaptic potentials. The percent inspired oxygen could be reduced to levels that produced a reversible and repeatable depression of evoked synaptic transmission restricted to the hippocampus ipsilateral to the occlusion. Further reduction in the level of inspired oxygen depressed synaptic transmission recorded from both hippocampi. The adenosine nonselective antagonist caffeine and the A(1) selective antagonist 8-cyclopentyltheophylline prevented the initial depression in synaptic transmission. We conclude that the initial depression of synaptic transmission observed in the rat hippocampus in vivo is due to endogenous adenosine acting at neuronal adenosine A(1) receptors.

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.

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.

Effects of adenosine agonists and an antagonist on excitatory transmitter release from the ischemic rabbit hippocampus

The purpose of this study was to determine the effects of adenosine agonists and an antagonist on ischemia-induced extracellular glutamate concentrations in an animal model of transient cerebral ischemia using in vivo cerebral microdialysis. Fifty New Zealand white rabbits were randomly assigned to one of five groups (normothermia, hypothermia, cyclopentyladenosine (CPA), theophylline, or propentofylline). Microdialysis probes were stereotactically placed in the dorsal hippocampus. Twenty minutes before the onset of ischemia, either 1 mg/kg CPA, 5 mg/kg propentofylline, or 20 mg/kg theophylline were administered intravenously. Esophageal temperature was maintained at 38 degrees C, except in the hypothermic animals, which were cooled to 30 degrees C throughout the entire experiment. Two 12-min periods of cerebral ischemia, separated by a 105-min interval of reperfusion, were produced by inflating a neck tourniquet. High-performance liquid chromatography was used to determine the glutamate concentration in the microdialysate. There were no significant increases in glutamate concentrations during the first ischemic period in any of the five groups. During the second ischemic episode, glutamate concentrations in the normothermic group peaked at levels approximately three times higher than the initial values. A similar pattern of changes in glutamate concentrations was observed in the CPA, propentofylline, and theophylline groups. In the hypothermic group, the concentrations of glutamate remained at baseline levels during the entire experiment. Contrary to expectations, neither the adenosine agonists (CPA, propentofylline) nor the antagonist (theophylline) had any effect on extracellular glutamate concentrations in the peri-ischemic period. Although adenosine and its analogs may be cerebroprotective agents, their mechanism of action is not fully understood. The data derived from this study indicates that the acute administration of such agents had no effect on ischemia-induced glutamate release within the hippocampus under these experimental conditions. Based on these results, further work is needed to compare in vivo versus in vitro experimental results in acute and long-term treatment studies with adenosine receptor agonists and antagonists.

Direct measurement of adenosine release during hypoxia in the CA1 region of the rat hippocampal slice

We have used an enzyme-based, twin-barrelled sensor to measure adenosine release during hypoxia in the CA1 region of rat hippocampal slices in conjunction with simultaneous extracellular field recordings of excitatory synaptic transmission. When loaded with a combination of adenosine deaminase, nucleoside phosphorylase and xanthine oxidase, the sensor responded linearly to exogenous adenosine over the concentration range 10 nM to 20 microM. Without enzymes, the sensor when placed on the surface of hippocampal slices recorded a very small net signal during hypoxia of 40 +/- 43 pA (mean +/- s.e.m.; n = 7). Only when one barrel was loaded with the complete sequence of enzymes and the other with the last two in the cascade did the sensor record a large net difference signal during hypoxia (1226 +/- 423 pA; n = 7). This signal increased progressively during the hypoxic episode, scaled with the hypoxic depression of the simultaneously recorded field excitatory postsynaptic potential and was greatly reduced (67 +/- 6.5 %; n = 9) by coformycin (0.5-2 microM), a selective inhibitor of adenosine deaminase, the first enzyme in the enzymic cascade within the sensor. For 5 min hypoxic episodes, the sensor recorded a peak concentration of adenosine of 5.6 +/- 1.2 microM (n = 16) with an IC(50) for the depression of transmission of approximately 3 microM. In slices pre-incubated for 3-6 h in nominally Ca(2+)-free artificial cerebrospinal fluid, 5 min of hypoxia resulted in an approximately 9-fold greater release of adenosine (48.9 +/- 17.7 microM; n = 6). High extracellular Ca(2+) (4 mM) both reduced the adenosine signal recorded by the sensor during hypoxia (3.5 +/- 0.6 microM; n = 4) and delayed the hypoxic depression of excitatory synaptic transmission.

Ischemia but not anoxia evokes vesicular and Ca(2+)-independent glutamate release in the dorsal vagal complex in vitro

Whole cell recordings of fura-2 dialyzed vagal neurons of brain stem slices were used to monitor interstitial glutamate accumulation within the dorsal vagal complex. Anoxia produced a sustained outward current (60 pA) and a moderate [Ca(2+)](i) rise (40 nM). These responses were neither mimicked by [1S,3R]-1-aminocyclo-pentane-1, 3-dicarboxylic acid nor affected by Ca(2+)-free solution, 6-cyano-7-nitroquino-xaline-2,3-dione (CNQX), 2-amino-5-phosphonovalerate (APV), or tetrodotoxin. Anoxia or cyanide in glucose-free saline (in vitro ischemia) as well as ouabain or iodoacetate elicited an initial anoxia-like [Ca(2+)](i) increase that turned after several minutes into a prominent Ca(2+) transient (0.9 microM) and inward current (-1.8 nA). APV plus CNQX (plus methoxyverapamil) inhibited this inward current as well as accompanying spontaneous synaptic activity, and reduced the secondary [Ca(2+)](i) rise to values similar to those during anoxia. Each of the latter drugs delayed onset of both ischemic current and prominent [Ca(2+)](i) rise by several minutes and attenuated their magnitudes by up to 40%. Ca(2+)-free solution induced a twofold delay of the ischemic inward current and suppressed the prominent Ca(2+) increase but not the initial moderate [Ca(2+)](i) rise. Cyclopiazonic acid or arachidonic acid in Ca(2+)-free saline delayed further the ischemic current, whereas neither inhibitors of glutamate uptake (dihydrokainate, D,L-threo-beta-hydroxyaspartate, L-transpyrrolidone-2,4-dicarboxylate) nor the Cl(-) channel blocker 5-nitro-2-(3-phenylpropyl-amino) benzoic acid had any effect. In summary, the response to metabolic arrest is due to activation of ionotropic glutamate receptors causing Ca(2+) entry via N-methyl-D-aspartate receptors and voltage-activated Ca(2+) channels. An early Ca(2+)-dependent exocytotic phase of ischemic glutamate release is followed by nonvesicular release, not mediated by reversed glutamate uptake or Cl(-) channels. The results also show that glycolysis prevents glutamate release during anoxia.

Release of monoamines and nitric oxide is involved in the modulation of hyperpolarization-activated inward current during acute thalamic hypoxia

Using slices of the dorsal lateral geniculate nucleus, it has been shown that, in the presence of excitatory and inhibitory amino acid antagonists, brief periods of hypoxia (3-4 min of 95% N(2)/5% CO(2)) induce in thalamocortical neurons an increase in instantaneous input conductance (G(N)) accompanied by an inward shift in baseline holding current (I(BH)). These effects have been suggested to be mediated, at least in part, by a positive shift in the voltage-dependence of the hyperpolarization-activated, mixed Na(+)/K(+) current (I(h)) and a change in its activation kinetics which transforms it into an almost instantaneously activated current. In this study, using the whole-cell patch-clamp technique, the contribution of an increased Ca(2+)-dependent transmitter release to the hypoxic response of thalamocortical neurons was further investigated using (i) blockers of calcineurin, a Ca(2+)/calmodulin-activated phosphatase that selectively regulates Ca(2+)-dependent release, and (ii) antagonists of neurotransmitters that are known to modulate I(h). Thalamocortical neurons (n = 23) recorded with electrodes filled with calcineurin autoinhibitory fragment (30-250 microM), a membrane impermeable blocker of calcinuerin, showed no difference either in resting, or in the hypoxia-induced changes in, G(N), I(BH) and I(h), when compared to thalamocortical cells patched with electrodes that did not contain calcineurin autoinhibitory fragment. In contrast, in 18 of these neurons recorded with calcineurin autoinhibitory fragment-filled electrodes, bath application either of cyclosporin-A (20 microM) or tacrolimus (50-100 microM), two membrane permeable blockers of calcineurin, abolished the effects of hypoxia on G(N), I(BH), and I(h). Separate application of noradrenaline, serotonin, histamine and nitric oxide antagonists produced only a small depression of the hypoxic response, while concomitant bath application of these antagonists decreased the hypoxia-induced changes in G(N) and I(BH) by 55 and 42%, respectively (n = 12). Concomitant bath application of 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo-guanosine-3'5'-cyclicmonophosphate (both 1mM), which are known to mediate the action of these transmitters on I(h), increased G(N) (40%), decreased I(h) time-constant of activation (30%) and significantly occluded (50%) the hypoxia-induced effect on G(N) and I(BH). Thalamocortical neurons (n = 6) patched with electrodes filled with 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo-guanosine-3'5'-cyclicmonophosphate (both 1 mM) showed a larger G(N) than the one recorded with the standard internal solution, and a significant depression of the hypoxia-induced changes in G(N) and I(BH). These results indicate that during acute thalamic hypoxia an increased release of noradrenaline, serotonin, histamine and nitric oxide is responsible for transforming I(h) into an instantaneously activating current via cyclic AMP- and cyclic GMP-mediated 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.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

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.

Reverse transport of glutamate during depolarization in immature hippocampal slices

We studied the source of extracellular glutamate released by hippocampal slices obtained from P14 or adult rats, during 50 mM K+ depolarization by using two potent inhibitors of Na+-dependent glutamate transport: l-trans-pyrrolidine-2,4-dicarboxylate (PDC), which is a relatively non-selective inhibitor of various glutamate transporter subtypes and dihydrokainic acid (DHK), a specific inhibitor of the glial transporter, GLT-1. Most depolarization-induced glutamate release was Ca2+-dependent in adults, while in P14 slices most glutamate release was Ca2+-independent. PDC decreased depolarization-induced glutamate release in P14 slices but not in adults. DHK increased glutamate release in adults but not in P14 slices. These data suggest that most depolarization-induced glutamate release in immature hippocampal slices is due to reversal of transport through a PDC-sensitive Na+-dependent glutamate transporter, presumably acting on presynaptic or cytoplasmic neuronal pools, and is not due to exocytosis from vesicular pools.

Excessive release of [3H]noradrenaline and glutamate in response to simulation of ischemic conditions in rat spinal cord slice preparation: effect of NMDA and AMPA receptor antagonists

In the present study we investigated the effects of NMDA and non-NMDA glutamate receptor antagonists on the ischemia-evoked release of [3H]noradrenaline from rat spinal cord slices. An in vitro ischemia model (oxygen and glucose deprivation) was used to simulate the ischemic conditions known to cause neuronal injury. Spinal cord slices were loaded with [3H]noradrenaline and superfused with Krebs solution in a micro-organ bath. Both axonal stimulation and ischemia increased the release of [3H]noradrenaline, but the release in response to glucose and oxygen deprivation was [Ca2+]o independent. Dizocilpine (MK-801), an NMDA receptor antagonist, suppressed the release of [3H]noradrenaline produced by ischemia, while it enhanced the release of [3H]noradrenaline evoked by electrical field stimulation. In contrast, LY300168 (GYKI-53655) [(+/-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylen e-dioxy-5H-2.3-benzodiazepine] and its (-)isomer LY303070 (GYKI-53784) [(-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylene- dioxy-5H-2.3-benzodiazepine] AMPA receptor antagonists, had no effect on the release of [3H]noradrenaline evoked by either electrical stimulation or ischemia. Desipramine, a noradrenaline uptake inhibitor, potentiated the release of [3H]noradrenaline evoked by ischemia, while in the absence of [Ca2+]o but under conditions when [3H]noradrenaline release was further increased, it reduced the release. Dizocilpine also decreased glutamate and aspartate release, measured by high performance liquid chromatography, during ischemia. It is concluded that glutamate release and NMDA receptors, but not AMPA receptors, are involved in the acute effect of oxygen and glucose deprivation on the excessive release of noradrenaline and that this release is not related to physiological axonal conduction.

Net efflux of cysteine, glutathione and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glutamyl transpeptidase

Extracellular metabolism of the protective substance glutathione (gamma-glutamyl-cysteinyl-glycine) may generate cysteine, glycine, several gamma-glutamyl-containing dipeptides and possibly free glutamate, all of which could participate in neurotoxicity. In the present study, we have examined how blockage of gamma-glutamyl transpeptidase, the key enzyme in glutathione degradation, influences the extracellular concentrations of glutathione, cysteine and related metabolites during anoxia/aglycemia of rat hippocampal slices. The net efflux, i.e., the increase in extracellular concentration due to changes in release and/or uptake, of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate, gamma-glutamyl-glutamine, glutathione, gamma-glutamyl-cysteine and glutamate increased as a result of anoxia/aglycemia. These increases in net efflux of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate and gamma-glutamyl-glutamine were reduced or blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase. In contrast, acivicin caused an increase in both basal and anoxia/aglycemia-induced net efflux of glutathione whereas the basal and anoxia/aglycemia-induced efflux of glutamate was unchanged by acivicin treatment. The effect of acivicin on the efflux of gamma-glutamyl-cysteine was similar to that of glutathione although less pronounced. Addition of beta-mercaptoethanol to the incubation medium during and after 30 min of anoxia/aglycemia decreased the net efflux of cysteine sulfinate specifically, indicating that the increase in cysteine sulfinate during anoxia/aglycemia may be partly derived from the spontaneous oxidation of cysteine. The results suggest that gamma-glutamyl transpeptidase may be involved in the regulation of the extracellular concentrations of cysteine, several gamma-glutamyl-containing dipeptides and glutathione but not glutamate during ischemia.

Mechanisms of ischemia-induced taurine release in mouse hippocampal slices

Taurine release in the hippocampus is markedly potentiated in various cell-damaging conditions, including ischemia and excitotoxic damage produced by glutamate. The increase in the levels of taurine may provide an important protective mechanism against excitotoxicity. The mechanisms of the enhanced release were now studied in mouse hippocampal slices using a superfusion system. The basal release of [3H]taurine was significantly increased in Na+-deficient media in normal conditions, whereas the ischemia-evoked release was decreased, indicating the participation of Na+-dependent transport processes. The involvement of taurine transport carriers in the release was confirmed with the structural analogs, hypotaurine and beta-alanine. These amino acids potentiated the release by trans-stimulation in normoxia. In Na+-free conditions, this heteroexchange was not discernible, the carriers not being functional without Na+. In ischemia, the marked potentiation of taurine release by hypotaurine and beta-alanine further indicates that the Na+-requiring transporters also operate in ischemia. The effects of membrane disruption on taurine release due to activation of phospholipases were estimated using phospholipase and protein kinase inhibitors, which had no marked effects on hippocampal taurine release. The chloride channel blockers, 4-acetamido-4'-isothiocyanostilbene-2, 2'-disulphonate (SITS) and diisothiocyanostilbene-2,2'-disulphonate (DIDS), reduced the ischemia-induced release, suggesting that taurine diffusion through an anion channel is partially responsible for the enhanced release in ischemia.

Modulation of ischemia-evoked release of excitatory and inhibitory amino acids by adenosine A1 receptor agonist

Adenosine has been reported to have beneficial effects against ischemic brain damage, although the mechanisms are not fully clarified. To examine the role of adenosine on the ischemia-evoked release of neurotransmitters, we applied a highly selective agonist for adenosine A1 receptor, 2-chloro-N6-cyclopentyladenosine (CCPA), into the ischemic brain using in vivo brain dialysis, which directly delivered the agonist to the local brain area. Concentrations of extracellular amino acids (glutamate, aspartate, gamma-aminobutyric acid (GABA) and taurine) and regional blood flow in the striatum of spontaneously hypertensive rats (SHRs) were monitored during cerebral ischemia elicited by bilateral carotid artery occlusion for 40 min and recirculation. Striatal blood flow and basal levels of amino acids were not affected by direct perfusion of CCPA (10 microM or 100 microM). During ischemia, concentrations of glutamate, aspartate, GABA and taurine increased up to 37-, 30-, 96- and 31-fold, respectively, when vehicle alone was administered. Administration of CCPA did not affect the changes in regional blood flow during ischemia and reperfusion. Perfusion of CCPA (100 microM), however, significantly attenuated the ischemia-evoked release of aspartate (by 70%) and glutamate (by 73%). The ischemia-induced increase of GABA tended to be decreased by CCPA, although it was not statistically significant. In contrast, both low and high concentrations of CCPA had little effect on the release of taurine during ischemia. These results suggest that stimulation of adenosine A1 receptors selectively attenuated the ischemia-evoked release of excitatory amino acids, but not of inhibitory amino acids without affecting blood flow. This modulation of the release of amino acids by adenosine A1 receptor agonists may play a protective role against ischemic neuronal damage.

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.

Peptide N- and P/Q-type Ca2+ blockers inhibit stimulant-induced hyperactivity in mice

omega-Conotoxin GVIA and omega-agatoxin IVA are specific peptide blockers of N- and P/Q-type calcium channel, respectively. Effects of their intracerebroventricular injection (1-3 pmol/mouse) on psychostimulant-induced hyperactivity were investigated in mice. omega-Conotoxin GVIA antagonized methylphenidate-, methamphetamine- and phencyclidine-induced hyperactivity in a dose-dependent manner. omega-Agatoxin IVA blocked methylphenidate-induced but not methamphetamine- or phencyclidine-induced hyperactivity. Neither peptides showed any effect on apomorphine-induced hyperactivity or spontaneous activity, suggesting that the inhibitory effects on psychostimulant-induced hyperactivity are not due to dopamine receptor blockage or nonspecific behavioral depression. Antagonism of calcium channels, particularly N-type, may ameliorate activation of the dopaminergic system induced by increased dopamine release.

Interaction between tetanus long-term potentiation and hypoxia-induced potentiation in the rat hippocampus

Interaction between tetanus long-term potentiation and hypoxia-induced potentiation in the rat hippocampus. J. Neurophysiol. 78: 2475-2482, 1997. The interaction between tetanus-induced long-term potentiation (LTP) and hypoxia-induced potentiation was investigated by performing extracellular recordings in the CA1 region of rat hippocampus using a two-pathway design. Hippocampal slices were placed in an interface chamber containing artificial cerebrospinal fluid (ACSF) solution with high magnesium concentration. Hypoxia was induced by replacing the 5% CO2-95% O2 gas mixture with 5% CO2-95% N2 for 2 min. Tetanus-LTP was induced with 1-s, 100-Hz current pulses. Significant hypoxia-induced potentiation of the slope of the dendritic excitatory postsynaptic potential (EPSP) was found in ACSF containing 2 mM of magnesium 2, 27 +/- 10% (mean +/- SE; n = 16; P < 0.01) with no change in the mean amplitude of the presynaptic volley. All experiments in which a stable control baseline was obtained were used for data analysis. The data show that short episodes (2 min) of hypoxia can induce LTP of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated synaptic transmission. The present study demonstrated that after tetanus-LTP, 33 +/- 3% (n = 10; P < 0.01), hypoxia further potentiated the field EPSP slopes by a mean value of 16 +/- 5% (n = 10; P < 0.05). Moreover, using a two-pathway design, we showed that hypoxia produced similar potentiation in both the control [19 +/- 5%; n = 10; P < 0.01) and tetanus-induced LTP pathway, and the total potentiation produced by a combination of tetanus then hypoxia, 63 +/- 13% (n = 10; p < 0.01), was significantly larger (P < 0.01) than hypoxia alone. These data suggest that hypoxia-induced potentiation is additive with tetanus-LTP. Occlusion experiments were performed to verify whether the mechanisms responsible for hypoxia-induced potentiation are independent of preexisting synaptic levels induced by high-frequency stimulation. Hypoxia produced significant potentiation (23 +/- 7%; n = 7; P < 0.05) after successful occlusion of the LTP pathway. Therefore, because the magnitude of hypoxia-induced potentiation is both independent of preexisting synaptic levels and also additive, synaptic specificity associated with LTP is preserved. The magnitude of tetanus-LTP induced 20 min after hypoxia (15 +/- 4%; n = 10) was significantly smaller (P < 0.01) relative to LTP after normoxic conditions (33 +/- 3%; n = 10). Additionally, hypoxia blocked the transient, robust potentiation occurring during the early phase of LTP induction. This study suggests that although hypoxia modifies neuronal processing by general excitation, synaptic specificity associated with tetanus-LTP still is preserved. However, hypoxia can disrupt neuronal processing by inhibiting new modification of synaptic transmission.

Zonisamide as a neuroprotective agent in an adult gerbil model of global forebrain ischemia: a histological, in vivo microdialysis and behavioral study

Brief periods of global cerebral ischemia are known to produce characteristic patterns of neuronal injury both in human studies and in experimental animal models. Ischemic damage to vulnerable areas such as the CA1 sector of the hippocampus is thought to result from excitotoxic amino acid neurotransmission. The objective of this study was to determine the ability of a novel sodium channel blocking compound, zonisamide, to reduce neuronal damage by preventing the ischemia-associated accumulation of extracellular glutamate. Using a gerbil model, animals were subjected to 5 min ischemic insults. Both pre- and post-ischemic drug administration (zonisamide 150 mg/kg) were studied. Histological brain sections were prepared using a silver stain at 7 and 28 days post ischemia. The animals sacrificed at 28 days also underwent behavioral testing using a modified Morris water maze. In vivo microdialysis was performed on a separate group of animals in order to determine the patterns of ischemia-induced glutamate accumulation in the CA1 sector of the hippocampus. Pyramidal cell damage scores in the CA1 region of the hippocampus were significantly reduced in animals pre-treated with zonisamide compared to saline-treated controls, both at 7 days (drug pre-treated: 0.812 +/- 0.28, n = 8; controls: 1.625 +/- 0.24, n = 8; *P < 0.05) and 28 (drug pre-treated: 0.833 +/- 0.22, n = 12; controls: 1.955 +/- 0.26, n = 11; **P < 0.01) days post ischemia. However, animals receiving zonisamide post-treatment did not display significant differences from controls. Behavioral studies also showed significant preservation of function in drug-treated animals. Microdialysis studies confirmed a reduction in glutamate release in drug-treated animals compared to saline-treated controls. Our data suggest that zonisamide is effective in reducing neuronal damage by a mechanism involving decreased ischemia-induced extracellular glutamate accumulation and interruption of excitotoxic pathways.

Neural activity and intracellular Ca2+ mobilization in the CA1 area of hippocampal slices from immature and mature rats during ischemia or glucose deprivation

To investigate the correlation between neural activity and intracellular Ca2+ ([Ca2+]i) mobilization in immature and adult brain during ischemia (hypoxia and glucose deprivation) and deprivation of glucose, hippocampal slices were prepared from 7-, 10-day-old and adult rats. Population spikes (PS) and antidromic responses (AR) were recorded in the pyramidal cell layer of the CA1 area as an index of neural function. [Ca2+]i mobilization of the stratum radiatum in the CA1 area was measured using the fluorescent dye fura-2 AM. The rise in [Ca2+]i occurred earlier in the adult animal and the decay times for the orthodromic PS and antidromic responses were shorter in the adult during ischemia. The field potentials and antidromic responses decreased substantially prior to the elevation of [Ca2+]i in both developing and adult brains. Furthermore, ATP levels decreased substantially before the elevation of [Ca2+]i during ischemia. These results suggest that neural activity and intracellular Ca2+ homeostasis in the immature rats brain are more resistant to energy failure than adult rats and that neuronal activity in the developing and adult brain is impaired initially by energy depletion during ischemia. In the immature animal, during glucose deprivation, the antidromic responses were slowly decayed or even failed to extinguish and [Ca2+]i levels were maintained for a longer period or even failed to rise in spite of the rapid loss of PS. Furthermore, ATP levels were well preserved at the time of PS loss. These results agree well with our previous reports showing that glucose plays an important role in the preservation of synaptic transmission in addition to its major function as an energy substrate.

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.

L-alpha-aminoadipate reduces glutamate release from brain tissue exposed to combined oxygen and glucose deprivation

The effect of the glutamate analogue L-alpha-aminoadipate (alpha AA) on the release of glutamate and gamma-aminobutyric acid (GABA) from rat hippocampal slices was investigated in vitro. Oxygen/glucose deprivation caused a large release of glutamate and GABA. alpha AA added during energy deprivation reduced the glutamate release in a dose-dependent manner (56% reduction at 5 mM), whereas GABA release was unchanged. We speculate that ischemic glutamate release from the brain is mediated by a low affinity transport mechanism that is blocked by alpha AA.

Immediate and delayed effects of in vitro ischemia on glutamate efflux from guinea-pig cerebral cortex slices

Immediate and delayed effects of glucose deprivation, oxygen deprivation (hypoxia) and both oxygen and glucose deprivation (in vitro ischemia) on glutamate efflux from guinea pig cerebral cortex slices were studied. Immediate effects were evaluated by measuring changes of glutamate efflux during the metabolic insults. Delayed effects were evaluated by measuring the response of the tissue to a 50 mM KCI pulse applied 60 min after the metabolic insults. Deprivation of glucose in the medium did not induce either immediate or delayed effects, while hypoxic condition produced an immediate slight stimulation of glutamate efflux without any delayed effect. Conversely, in vitro ischemia produced both immediate and delayed effects on glutamate efflux. During in vitro ischemia glutamate efflux dramatically increased in a calcium-independent and tetrodotoxin-sensitive manner; this effect was potentiated by a low sodium containing medium. The blockade of the sodium/potassium ATPase exchanger by ouabain caused a glutamate outflow similar to that induced by in vitro ischemia. On the whole, these data demonstrate the central role played by the sodium electrochemical gradient and by the membrane glutamate uptake system in the glutamate overflow induced by in vitro ischemia. Moreover, in slices previously exposed to both oxygen and glucose deprivation the effect of KCI on glutamate efflux was potentiated. This in vitro ischemia-induced delayed potentiation of neurotransmitter efflux, until now unreported in the literature, was found to be selectively restricted to glutamatergic structures and to be mainly due to an enhancement of the exocytotic component of glutamate release.

Release of [3H]-D-aspartate from primary astrocyte cultures in response to raised external potassium

There are significant Ca2+-independent increases in extracellular glutamate and aspartate during various CNS insults such as ischemia and anoxia. However, the cellular sources of such presumed nonvesicular excitatory amino acid (EAA) release have not been established. To further explore potential mechanisms and sites for EAA release, we studied the release of preloaded [3H]-D-aspartate from primary cultured astrocytes prepared from the cerebral cortices of rat pups. Two phases of release were seen in response to raised KCl. The first phase was small and transient, and the second phase was slower and increased progressively. The initial phase of [3H]-D-aspartate release was greatly enhanced by ouabain pretreatment and was inhibited when astrocytes were preexposed to the EAA transport inhibitor threo-hydroxy beta-aspartic acid (THBA). Neither of these manipulations affected the second release component. The second phase of release was inhibited by an anion channel blocker, L-644,711, which is known to inhibit hypotonic swelling-induced release of EAA. Ouabain also resulted in the first phase of release occurring at lower [K+]o. Omission of Ca2+ had no effect on either phase of [3H]-D-aspartate release. These results support the hypothesis that the first component of release in cultured astrocytes is a reversal of the glutamate transporter, and the second component is a result of high KCl-induced swelling. Because marked increases in [K+]o are well established in CNS pathologies such as ischemia, such release may represent a significant source for the increased extracellular EAAs seen in such conditions.

Efflux of gamma-aminobutyric acid caused by changes in ion concentrations and cell swelling simulating the effect of cerebral ischaemia

The relationships among ischaemic GABA efflux from brain tissue and extracellular and intracellular concentrations of sodium, chloride and potassium ions were investigated by means of 1) transverse hippocampal slices from rat and 2) functional expression of a high affinity GABA transporter in Xenopus oocytes. Brain slices were incubated for 20 min in medium where extracellular sodium and chloride were substituted with impermeant ions. Isethionate (Iseth) substitution for chloride generated a 7-fold increase in GABA efflux. Choline (Chol) but not N-methyl-D-glucamine (NMDG) substitution for sodium likewise increased GABA efflux. Reducing the osmolarity of the medium by decreasing both sodium and chloride concentrations (Hyp) increased GABA efflux 3-fold. This release was blocked by mannitol (Man). Blocking sodium channels with 1 microM of tetrodotoxin (TTX) also increased the release 3-fold. Energy deprivation (ED) increased the GABA release 50-fold. ED/Iseth left the release unchanged, ED/Chol increased the GABA efflux by 23%, whereas ED/NMDG reduced the release by 41%. Adding mannitol did not block the ED-evoked release, whereas TTX reduced it by 52%. Release of preloaded [3H]-GABA from oocytes expressing the GAT-1 GABA transporter was then examined. Depolarisation by current injection or 100 mM extracellular K+ did not increase GABA release. Sodium chloride injection, however, caused membrane depolarisation and a 100-fold increased GABA efflux from the oocytes. This release was blocked when the osmolarity was increased extracellularly by adding mannitol. These results show that 1) TTX releases GABA from brain tissue but blocks release during ED, 2) the high affinity GABA carrier must be altered in order to reverse, 3) ischaemic GABA release is sodium independent, and is modulated by large cations, 4) mannitol blocks the reversal of high affinity carriers in oocytes, but the release from brain slices during ED is unaffected. Taken together, the results suggest that ischaemic release of GABA from brain tissue does not occur by means of reversed high affinity carriers alone, but rather that it is controlled by more complex mechanisms.

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.