Efflux of glutamate produced by short ischemia of varied severity in rat striatum

Spreading depolarizations mediate excitotoxicity in the development of acute cortical lesions

Spreading depolarizations (SD) are mass depolarizations of neurons and astrocytes that occur spontaneously in acute brain injury and mediate time-dependent lesion growth. Glutamate excitotoxicity has also been extensively studied as a mechanism of neuronal injury, although its relevance to in vivo pathology remains unclear. Here we hypothesized that excitotoxicity in acute lesion development occurs only as a consequence of SD. Using glutamate-sensitive microelectrodes, we found that SD induced by KCl in normal rat cortex elicits increases in extracellular glutamate (11.6±1.3μM) that are synchronous with the onset, sustainment, and resolution of the extracellular direct-current shift of SD. Inhibition of glutamate uptake with d,l-threo-β-benzyloxyaspartate (TBOA, 0.5 and 1mM) significantly prolonged the duration of the direct-current shift (148% and 426%, respectively) and the glutamate increase (167% and 374%, respectively) in a dose-dependent manner (P<0.05). These prolonged events produced significant cortical lesions as indicated by Fluoro-Jade staining (P<0.05), while no lesions were observed after SD in control conditions or after cortical injection of 1mM glutamate (extracellular increase: 243±50.8μM) or 0.5mM TBOA (glutamate increase: 8.5±1.6μM) without SD. We then used an embolic focal ischemia model to determine whether glutamate elevations occur independent of SD in the natural evolution of a cortical lesion. In both the ischemic core and penumbra, glutamate increased only in synchrony with anoxic terminal SD (6.1±1.1μM) and transient SDs (11.8±2.4μM), and not otherwise. Delayed terminal SDs were also observed in two animals at 98 and 150min after ischemic onset and induced similar glutamate elevations. Durations of SDs and glutamate increases were significantly correlated in both normal and ischemic animals (P<0.05). These data suggest that pathologically prolonged SDs are a required mechanism of acute cortical lesion development and that glutamate elevations and the mass electrochemical changes of SD and are merely different facets of the same pathophysiologic process.

Early and sharp nitric oxide production and anoxic depolarization in the rat hippocampus during transient forebrain ischemia

This study was designed to characterize nitric oxide (NO) production and anoxic depolarization in the rat hippocampus during transient forebrain ischemia using two NO synthase (NOS) inhibitors, L-N(5)-(1-iminoethyl)ornithine (L-NIO), a relatively selective endothelial NOS (eNOS) inhibitor, and 7-nitroindazole, a relatively selective neuronal NOS (nNOS) inhibitor, and an NO scavenger, [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] (carboxy-PTIO). We measured the mean arterial blood pressure, hippocampal blood flow, NO concentration and direct current potential before, during and after transient forebrain ischemia, which was induced by 4-vessel occlusion for 10 min. Saline, L-NIO (20 mg/kg), 7-nitroindazole (25 mg/kg), L-NIO (20 mg/kg)+7-nitroindazole (25 mg/kg) or carboxy-PTIO (1 mg/kg) was administered intraperitoneally 20 min before the onset of ischemia. We observed early and sharp NO production in the hippocampus during ischemia in the saline group. This NO increase during ischemia was significantly reduced by L-NIO (20 mg/kg)+7-nitroindazole (25 mg/kg) or carboxy-PTIO (1 mg/kg), but not L-NIO (20 mg/kg) or 7-nitroindazole (25 mg/kg). On the other hand, NO production after ischemia was significantly reduced by 7-nitroindazole (25 mg/kg), L-NIO (20 mg/kg)+7-nitroindazole (25 mg/kg) or carboxy-PTIO (1 mg/kg), but not L-NIO (20 mg/kg). The peak latency of NO production during ischemia always preceded the onset latency of anoxic depolarization in both the saline group and the carboxy-PTIO group. In the carboxy-PTIO group, the onset latency of anoxic depolarization was significantly longer than that in the saline group. Moreover, carboxy-PTIO significantly reduced the anoxic depolarization amplitude, compared with that of the saline group. These results suggest that both NOS-dependent and-independent NO formation contributes to early and sharp NO production during ischemia, and that this NO increase is, at least in part, related to the triggering of anoxic depolarization.

Naloxone lowers cerebrospinal fluid levels of excitatory amino acids after thoracoabdominal aortic surgery

OBJECTIVE

Although naloxone has been used to prevent ischemic spinal cord injury (SCI), its effect on excitatory amino acids (EAAs) has not been understood. We investigated the clinical significance of naloxone by measuring EAAs in the cerebrospinal fluid (CSF) in patients undergoing thoracoabdominal aortic surgery.

METHODS AND SUBJECTS

Twenty-seven patients (15 men and 12 women; mean age, 66 +/- 12 years) undergoing prosthetic replacement of the thoracoabdominal aorta (n = 19) or the descending thoracic aorta (n = 8) from April 1997 to June 2003 under distal perfusion and mild hypothermia were enrolled in this cohort study with historical controls. Their etiology was 7 dissections and 20 nondissections. In 16 patients (naloxone group), intravenous infusion of naloxone (1 microg/kg/h) was continued until the patients became alert. In the remaining 11 patients (control group) naloxone was not given. CSF drainage was used in all patients. CSF levels of EAAs, glutamate, aspartate, and glycine were measured at 6 points in time until 72 hours postoperatively, using a high-performance liquid chromatography method.

RESULTS

In 5 patients with SCI (2 patients in control group, 3 in naloxone group), CSF levels of glutamate and glycine continued to increase even at 72 hours postoperatively, and were significantly more elevated than those in patients without SCI ( P < .0001, glutamate; P = .0006, glycine). Postoperative maximum levels of CSF glutamate and glycine were also significantly higher in patients with postoperative SCI than in patients without SCI (glutamate: 215.3% +/- 158.6% vs 32.9% +/- 37.3% increase from baseline, P < .0001; glycine: 309.1% +/- 218.2% vs 89.2% +/- 103.1% increase from baseline, P = .0036). CSF levels of glutamate and aspartate in naloxone group were significantly lower than those in control group ( P = .0161, glutamate; P < .0001, aspartate). Postoperative maximum level of CSF aspartate was also significantly lower in the naloxone group than in the control group (8.3% +/- 75.5% vs 119.7% +/- 120.6% increase from baseline, P = .0077). In multivariate logistic regression analysis, postoperative maximum CSF glutamate >100% from baseline ( P < .001) and postoperative maximum level of CSF glycine ( P = .005)were identified as the independent risk factors for SCI. Both SCI ( P < .001) and postoperative maximum level of CSF glycine ( P = .005) were the independent predictors for postoperative maximum level of CSF glutamate >100% from baseline.

CONCLUSIONS

CSF levels of EAAs are elevated in patients with SCI. CSF glutamate is the strongest independent predictor of SCI. Naloxone is effective in lowering CSF levels of EAAs.

Expression of glutamate transporters and ionotropic glutamate receptors in GLAST knockout mice

In order to investigate the molecular mechanism underlying high seizure susceptibility of GLAST knockout mice, we carried out Western blotting for the expression of GLT-1, EAAC-1, and several kinds of glutamate receptors in the hippocampus and the cortex. Although no significant difference was observed between GLAST (+/+) and (-/-) mice in terms of expression of GLT-1 and EAAC-1 in the hippocampus, these proteins were over-expressed in the frontal cortex in GLAST (-/-) mice (GLT-1, about 210% increase; EAAC-1, about 180% increase). Expression of hippocampal Glu-R1 and Glu-R2 in GLAST (-/-) mice was remarkably increased (Glu-R1, about 140% increase; Glu-R2, about 160% increase), while Glu-R3 and NMDA receptors levels (NMDA-R1, 2A and 2B) were equal to those in control. Cortical levels of Glu-R1, -R2 and -R3 receptors in GLAST (-/-) mice were remarkably decreased (Glu-R1, about 60% decrease; Glu-R2, about 60% decrease; Glu-R3, about 70% decrease), while NMDA receptors were remarkably increased in comparison to those in GLAST (+/+) mice (N-R1, about 150% increase; N-R2A, about 150% increase; N-R2B, about 140% increase). These data suggest that the increased susceptibility to seizures in GLAST (-/-) mice might be derived from increased expression of Glu-R1 in the hippocampus coupled with decreased cortical expression of Glu-R2 and increased NMDA-R1 and -2A, -2B expression.

Blockade of NMDA-receptors or calcium-channels attenuates the ischaemia-evoked efflux of glutamate and phosphoethanolamine and depression of neuronal activity in rat organotypic hippocampal slice cultures

We have investigated the effects of various insults on extracellular glutamate and phosphoethanolamine levels as well as electrical activity alterations in the early period following these insults in organotypic hippocampal slice cultures. Cultures prepared from 7-day-old rats were maintained in vitro for 7-14 days and then metabolic inhibition was induced: cultures were briefly exposed to potassium cyanide to induce chemical anoxia, 2-deoxyglucose with glucose removal to produce hypoglycaemia, or a combination of both to simulate ischaemia. Chemical anoxia induced a small increase in glutamate and a reversible decrease in evoked field potentials and these were greatly potentiated following simulated ischaemia: high, biphasic glutamate efflux and irreversible field potential abolition as well as increase in phosphoethanolamine levels were observed. We have characterised the effects of treatments using NMDA-receptor antagonists and the L-type calcium channel blocker diltiazem. Anoxia-induced glutamate accumulation was prevented by MK-801 and diltiazem D-AP5. Following simulated ischaemia, diltiazem totally prevented glutamate and phosphoethanolamine accumulations, whereas MK-801 did not block the first phase of glutamate accumulation and D-AP5 prevented none. We demonstrated that glutamate and phosphoethanolamine ischaemic-evoked efflux as well as the recovery of electrical activity in organotypic hippocampal slice cultures are sensitive to both NMDA-receptor and calcium-channel blockade. This model thus represents a useful in vitro system for the study of ischaemic neurodegeneration paralleling results reported using in vivo models.

Value of (1)H-MRS using different echo times in neonates with cerebral hypoxia-ischemia

Previous studies have shown altered brain metabolism after cerebral hypoxia-ischemia, using magnetic resonance spectroscopy with echo times (TE) of 272 and 136 ms, based on peak-area or peak-height ratios. The present study examined the additional value of proton magnetic resonance spectroscopy with a short TE (31 ms) to predict a poor outcome in neonates with brain hypoxia-ischemia. Studies were performed in 21 full-term neonates with perinatal asphyxia in a 1.5 tesla magnetic field. Proton magnetic resonance spectroscopy was performed in a single volume of interest including the basal ganglia. TE of 272, 136 and 31 ms were used. After curve-fitting procedures, peak-areas as well as peak-height ratios of different brain metabolites were calculated, comparing patients with a poor versus a good outcome. Seven neonates out of 21 had a poor outcome. Neonates with a poor outcome showed a significantly lower N:-acetylaspartate/choline (NAA/Cho) and a significantly raised lactate/NAA (Lac/NAA) ratio using TE of 272 and 136 ms. Using a TE of 31 ms, no differences were found in glutamate/NAA (Glx/NAA), Glx/Cho, myo-inositol/NAA (mI/NAA), and mI/Cho ratios between neonates with a good and those with a poor outcome. Highest predictive values could be achieved for NAA/Cho with a TE of 136 ms. We conclude that low NAA/Cho and high Lac/NAA ratios predict a poor outcome in neonates with cerebral hypoxia-ischemia. TE of 272 and 136 ms have a better predictive value than a TE of 31 ms.

Microdialysis coupled to online enzymatic assays

In vivo sampling of interstitial fluid by using microdialysis fibers has become a standard and accepted procedure. This sampling method is generally coupled to offline analysis of consecutive dialysate samples by high-performance liquid chromatography or capillary electrophoresis, but this combination is not the best approach for some applications, especially those which require high temporal resolution and rapid data collection. The purpose of this review is to provide information on enzyme-based online assays, i.e., continuous analysis of the dialysate as it emerges from the outlet of the sampling device. We have focused on methods developed specifically for the analysis of solutions perfused at a very slow flow rate, i.e., a feature of microdialysis and ultrafiltration techniques. These methods include flow enzyme-fluorescence assays, flow enzyme-amperometric assays, and sequential enzyme-amperometric detection. Each type of assay is discussed in terms of principle, applications, advantages, and limitations. We also comment on implantable biosensors, an obvious next step forward for in vivo monitoring of molecules in neuroscience.

Evaluation of lipid peroxidation, cathepsin L and acid phosphatase activities in experimental brain ischemia-reperfusion

This investigation was conducted in rat brain tissues to elucidate the free radical induced cellular and subcellular membrane injuries in two different depth of global ischemia. Global moderate (penumbral) ischemia was performed on rat brains by bilateral vertebral arteries cauterization and temporary occlusion of the bilateral carotid arteries. Global severe ischemia was produced by a neck tourniquet in addition to four vessel occlusion. Somatosensory evoked potentials (SSEPs) were used as a feed back parameter to monitor electrophysiologically the ischemia. At the end of ischemic insult (0 min reperfusion) or various reperfusion periods (20, 60 and 240 min), all rats were decapitated and brains were frozen in liquid nitrogen. The brain tissues were prepared for the determination of cathepsin L (CL) and acid phosphatase (AP) activities in the supernatant (cytosolic) fraction (SF) and the fraction enriched with lysosomes (FEL). Further the level of thiobarbituric acid reactive substances (TBARS) of lipid peroxidation was assessed by the spectrophotometric methods. Severe ischemia-reperfusion was accompanied by a significant increase in TBARS levels and the SF/FEL ratio for CL and AP activities compared to the sham operated group and the concurrent reperfusion groups of moderate ischemia (p<0.05). There were no significant differences between the sham operated and moderate ischemia-reperfusion groups for the same parameters. Our data clearly demonstrate that; in rat brain although severe ischemia-reperfusion causes lipid peroxidation in cellular membranes and redistribution of lysosomal enzymes from lysosomes to cytoplasm due to lysosomal membrane injury, there are no changes in lysosomal membrane stability in moderate ischemia-reperfusion.

Glutamate release inhibitors: a critical assessment of their action mechanism

A number of important experimental data do not support the widespread hypothesis that Na(+)-channels block is cerebroprotective, essentially because it reduces presynaptic glutamate release: (i) the inhibition of exocytosis by these compounds is not specific to glutamate; (ii) aspartate efflux produced by various stimuli was also reduced, but aspartate cannot be released by exocytosis because it is not concentrated within presynaptic vesicles; and (iii) glutamate accumulated extracellularly during ischaemic or traumatic insult to the CNS is mainly of cytosolic origin. As an alternative, we propose that use-dependent Na(+)-channel blockers enhance the resistance of nerve cells to insults, primarily by decreasing their energy demand, and that reduced efflux of glutamate and other compounds is a consequence of attenuated cellular stress.

Neuroprotective strategies: voltage-gated Na+-channel down-modulation versus presynaptic glutamate release inhibition

Insufficient ATP production relative to cellular requirements is the key factor detrimental to neurons in neurological disorders associated with deficient oxygen/glucose supply or mitochondrial dysfunction. As a large part of the energy consumed by brain cells is used to maintain the Na+ gradient across the cellular membrane, reduction of energy demand by down-modulation of voltage-gated Na+-channels is a rational strategy for neuroprotection against these conditions. Preservation of the inward Na+ gradient is likely to be also beneficial as it is an essential driving force for vital ion exchanges and transport mechanisms (e.g. Ca2+-homeostasis and cell volume regulation). From these elements, I propose that use-dependent Na+-channel blockers increase the resilience of nerve cells to the primary insult and/or subsequent deleterious events, and that reduced efflux of glutamate and other compounds is only a consequence of cellular stress attenuation. The widespread hypothesis that down-modulation of Na+-channels is neuroprotective primarily through reduction of presynaptic glutamate release conflicts with strong experimental evidence.

A comparison of changes in nucleotide-protein interactions in the striatal, hippocampus and paramedian cortex after cerebral ischemia and reperfusion: correlations to regional vulnerability

[32P]Azido-purine analogs of ATP and GTP were used to detect changes in phosphorylation and nucleotide binding induced by ischemia and subsequent reperfusion in rat brain striatum, hippocampus and paramedian cortex (PM cortex) tissues. Major changes in phosphorylation were observed for a 130-kDa protein, tentatively identified as the Ca2+ transport ATPase, and calcium/calmodulin-dependent protein kinase II (CaM Kinase II) in all tissues. However, recovery of the phosphorylation of the 130-kDa protein occurred only in the PM cortex on reperfusion. A 200-300% increase in [32P]8N3ATP photoinsertions was observed in the striatum and hippocampus regions for a 43-kDa protein with an isoelectric point of 6.8. This protein was identified as glutamine synthetase (GS) and the increase in binding was found to be due to both increased copy number and activation by Mn2+. An increase in [32P]8N3GTP photoinsertion into a 55-kDa protein, identified as the beta-subunit of tubulin, was found only in the striatum and hippocampus. This indicates the depolymerization of microtubulin in these tissues. These changes correlate to the vulnerability of the striatum and hippocampus to ischemia-induced neuronal death.

Physiological effects and brain protection by hypothermia and cerebrolysin after moderate forebrain ischemia in rats

The "therapeutic window" of neuroprotective intervention due to hypoxic-ischemic brain injuries are initial disturbances of the neuronal function in regions of only moderate decrease of local cerebral blood flow (ICBF). Because of limited effects of single therapeutic principles therapeutic combinations should be tested. Neuroprotective effects of mild hypothermia and the nootropic drug Cerebrolysin (Cerebrolysin, EBEWE, Austria) on ICBF and development of brain edema were used. Four groups of adult Wistar rats (untreated and Cerebrolysin treated animals with 35 degrees C and 37 degrees C rectal temperature) were subjected to moderate forebrain ischemia by permanent bilateral carotid artery ligation for 6 h. The ICBF was measured continuously in the frontal and the occipital cortex by a 2-channel Laser Doppler flowmeter. The ECoG was derived from 4 ECoG leads above the frontal and occipital cortex and quantified by spectral analysis. Six hours after the onset of ischemia, the function of the blood-brain barrier to proteins was determined by staining with Evans Blue, the animals were sacrificed and the brain water content was estimated by gravimetry. Permanent bilateral carotid artery ligation led to an abrupt ICBF reduction to between 40-50% of baseline levels. Within a few minutes, however, the ICBF increased again to 50-80% of the baseline. The reduced spectral band power of the ECoG was correlated with the decreased ICBF values (p < 0.05) that indirectly indicated changes in the energy state of the neurons (p < 0.05). Changes in the ECoG appeared only with a delay of approximately 4 sec after the onset of ICBF reduction. Six hours after the onset of ischemia, a cytotoxic brain edema was shown in the frontoparietal cortex and hippocampus. Reducing the temperature by 2 degrees C diminished the decrease in ICBF between 10 min and 2 h after the onset of ischemia (p < 0.05). This effect was noted in the frontal but not in the occipital cortex. Furthermore, mild hypothermia prevented the loss of ECoG spectral power in the beta, alpha and theta bands (p < 0.05) as well as the development of cytotoxic brain edema. Cerebrolysin prevented the development of brain edema, too, both under normo- and hypothermic conditions. The ICBF was restored to higher levels in the occipital cortex in comparison both to the normothermic Cerebrolysin treated and hypothermic untreated rats (p < 0.05). This effect of Cerebrolysin was associated with only slight changes in ECoG, indicating that the neuronal activity state and the energy supply was obviously not decisively influenced. In conclusion, moderate ICBF reduction in rats to about 50-80% of baseline values was detectable in the ECoG by using spectral analysis. This reduction led to the development of cytotoxic brain edema in rats within 6 h. Thus, hypothermia prevents the development of cytotoxic brain edema. Cerebrolysin enhanced the effects of hypothermia on ICBF reduction and on the development of brain edema.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Mild hypothermia reduces penumbral glutamate levels in the rat permanent focal cerebral ischemia model

Although the cerebroprotective effects of hypothermia in focal models of ischemia are undisputed, the underlying mechanisms of this protection are still subject to much controversy. To analyze whether mild hypothermia attenuates glutamate levels in the penumbra surrounding permanent focal infarcts, extracellular glutamate concentration was analyzed bilaterally by microdialysis 20 minutes before to 120 minutes after a middle cerebral artery occlusion (MCAO) in rats. Normothermic animals (n = 11) had a baseline glutamate concentration of 1.14 +/- 0.40 mumol/ml (standard error of the mean) before the MCAO. Extracellular glutamate levels increased gradually after vessel occlusion to peak at 10.1 +/- 1.45 mumol/ml 80 minutes after the MCAO. This level gradually decreased to 5.72 +/- 1.67 mumol/ml by 120 minutes. Hypothermic animals (n = 11) had a baseline glutamate concentration of 1.73 +/- 0.83 mumol/ml before the MCAO. Extracellular glutamate levels increased after vessel occlusion but stabilized at 3.47 +/- 1.37 mumol/ml 30 minutes after the MCAO and remained stable until completion of the experiment. There were no significant differences in cortical blood flow between the normothermic and hypothermic groups at any time during the experiment. Infarct volumes, expressed as a percentage of the volume of the right (ipsilateral) hemisphere, were 19.8 +/- 2.16% in the normothermic group and 13.0 +/- 1.42% in the hypothermic group (P < 0.02). Although the normothermic penumbral glutamate levels began to increase immediately after the MCAO, they did not peak until 80 minutes after occlusion. In contrast, the normothermic core glutamate levels peaked within 30 minutes after the MCAO. Glutamate diffusion from the core region to the penumbra might account for this delay. Hypothermic cerebroprotection might involve a reduction in the pool of potentially diffusable glutamate in the core region but have little direct effect on glutamate release in the penumbra.

Differences in striatal extracellular amino acid concentrations between Wistar and Fischer 344 rats after middle cerebral artery occlusion

We hypothesized that the interstrain difference between Wistar and Fischer-344 (F344) rats in cerebral infarction volume after proximal middle cerebral artery (MCA) occlusion might be explained by differences in excitotoxicity between both rat strains. Using microdialysis we measured during a 5 h period after MCA occlusion the release of aspartate, glutamate and taurine in the cerebral cortex and the striatum. The volume of striatal infarction was comparable in Wistar and F344 rats. We found, however, in Wistar rats a significantly higher striatal efflux of aspartate and glutamate than in F344 rats, whereas the striatal taurine efflux was of a similar magnitude in the two strains. Because of the (variably) smaller volume of cortical infarction in Wistar rats (than that in F344), the location of the microdialysis probe-membrane with respect to the area of cortical infarction differed between Wistar rats. Hence, a reliable comparison between the quantitative amount of amino acids in the dialysate from the cortical probes of both rat strains could not be made. These results, demonstrating differences in striatal excitotoxicity between Wistar and F344 rats after MCA occlusion, are the first to show interstrain differences in striatal pathophysiology of focal ischemia between these normotensive rat strains. They do however not explain why MCA occlusion results in a significantly different volume of cortical infarction between Wistar and F344 rats. The F344 strain will probably show in a more sensitive way, as compared to Wistar rats, neuroprotective effects of agents that diminish excitotoxic damage during focal cerebral ischemia.

In vivo evidence of hydroxyl radical formation induced by elevation of extracellular glutamate after cerebral ischemia in the cortex of anesthetized rats

The in vivo interrelation between excitotoxicity and oxidative stress following cerebral ischemia in the cortex of anesthetized rats was investigated. Cerebral ischemia was induced by ligation of the bilateral common carotid arteries and the unilateral middle cerebral artery. Microdialysis perfusion with on-line high-performance liquid chromatography was used to monitor the hydroxyl radical levels. Extracellular hydroxyl radical levels were quantitated as the increased formation of 2.3 and 2.5 dihydroxybenzoic acid (DHBA), the hydroxylative products of salicylic acid contained in the microdialysis perfusion solutions. Elevated cortex extracellular glutamate content, resulting from the cerebral ischemia, caused an increase in the formation of hydroxyl radicals. Exogenous perfusion of authentic glutamate solutions through implanted microdialysis probes also resulted in increased hydroxyl radical formation in the cortex. The 2.3 and 2.5 DHBA levels remained elevated for an entire 80-min ischemic period. These results suggest that, after cerebral ischemia, increased oxidative stress did occur in anesthetized rats, and the oxidative stress may result from increased excitotoxicity.

Lowered brain glutathione by diethylmaleate decreased the glutamate release induced by cerebral ischemia in anesthetized rats

The effect of lowered brain glutathione content on the glutamate release following cerebral ischemia was investigated. Diethylmaleate (4 mmol/kg, i.p.), a commonly used chemical reagent for tissue glutathione depletion, significantly reduced the ischemia-induced glutamate release. The release of another excitatory amino acid aspartate was not affected by the diethylmaleate administration. These results suggested that part of the elevated glutamate content induced by ischemia might result from the cellular GSH.

Elevated extracellular glutamate levels increased the formation of hydroxyl radical in the striatum of anesthetized rat

Results from various in vitro experiments have indicated that excitotoxicity and oxidative stress are two interrelated major mechanisms in causing neuronal damage in brain disorders such as cerebral ischemia/reperfusion. Thus, we have conducted experiments to investigate whether in the striatum of anesthetized rats the elevated brain extracellular concentrations of glutamate could increase the formation of hydroxyl radical. Elevation of glutamate levels and trapping of hydroxyl radical were accomplished by perfusing Ringer solutions containing both glutamate and salicylic acid through microdialysis probes implanted in rat striatum. The formation of hydroxyl radical was quantitated as the increased amounts of 2,3 and 2,5 dihydroxybenzoic acid (DHBA) which were the hydroxylative products of salicylic acid. Eluted microdialysates were directly injected onto high performance liquid chromatography (HPLC) with electrochemical detector via an on-line automatic injector. This method was authenticated by in vitro studies employing Fenton-type hydroxyl radicals generation system. Our results indicated that elevated glutamate concentrations (15 mM, 1.5 mM, and 150 microM glutamate in perfusing solutions) would significantly increased both the concentrations of 2,3 and 2,5 DHBA. In conclusion, we have obtained direct evidence showing that the elevated glutamate concentrations in brain extracellular space would increase the formation of hydroxyl radical, and these results implied that oxidative stress occurring in brain disorders might be induced by excitotoxicity.

Extracellular glutamate: on-line monitoring using microdialysis coupled to enzyme-amperometric analysis

An enzyme-amperometric detector cell is described for flow analysis of glutamate in dialysate emerging from an implanted microdialysis probe. Its small size allows it to be placed within a few centimetres of the animal preparation, reducing the delay for data acquisition to around 2 min. The selectivity is provided by glutamate oxidase, immobilised with glutaraldehyde on surfaces adjacent to the 3-electrode system. A film of 1,2-diaminobenzene, electropolymerized on the platinum working electrode, eliminates interference from ascorbic acid and other endogenous electroactive compounds. The high sensitivity (< 0.5 mumol/l) and fast response time of the cell (90% of maximum response in 30 s) make it particularly suitable for investigating conditions that produce rapid changes in brain extracellular glutamate. This is illustrated by monitoring changes in extracellular glutamate subsequent to cardiac arrest, and K(+)-induced local depolarization.

HU-211, a nonpsychotropic cannabinoid, improves neurological signs and reduces brain damage after severe forebrain ischemia in rats

The purpose of the present study was to examine the dose-response relationship and the therapeutic time window for the synthetic nonpsychotropic cannabinoid (HU-211) as a neuroprotective agent in transient, severe forebrain ischemia in the rat. Adult Sprague-Dawley rats were subjected to 20 min common carotid artery occlusion (CCAo) 24 h after coagulation of both vertebral arteries. Thirty minutes after the onset of CCAo, rats received an iv injection of HU-211 2, 4, or 8 mg/kg in HPCD (n = 12, 18, and 11, respectively), or the appropriate vehicle (n = 20). Neurological signs were scored daily for 3 d following ischemia. A significant improvement (p < 0.01, Kruskal-Wallis nonparametric test, followed by Mann-Whitney U-test, p < 0.05) of neurological deficits by the 4 mg/kg dose of HU-211, was observed 24, 48, and 72 h after insult. On the third day post-CCAo, the rat brain was taken for histopathological evaluation of the CA-1 sector of the hippocampus. Counts of viable neurons in the hippocampal CA1 field showed significantly more live cells in the HU-211 (4 mg/kg) treated animals (P < 0.001, ANOVA followed by Duncan's post-hoc test, p < 0.05). The drug was equally effective when given 30 and 60 min after ischemia, but neuroprotection was no longer significant after 3 h. We suggest that HU-211 may be a potential treatment for postischemic brain damage in human beings.

Intraischemic hypothermia decreases the release of glutamate in the cores of permanent focal cerebral infarcts

The cerebroprotective effects of hypothermia in focal models of ischemia are well established, but little is known about the underlying mechanisms of this form of brain protection. Cortical cooling in global transient ischemic models suggests that hypothermia limits glutamate excitotoxicity by decreasing the release of glutamate during ischemia. Few studies have examined glutamate release in the more physiological model of permanent focal ischemia. In this study, we used a rat model of middle cerebral artery occlusion (MCAO) of permanent focal ischemia. Extracellular glutamate concentration was analyzed bilaterally by microdialysis for 30 minutes before MCAO to 120 minutes after MCAO. Normothermic animals (n = 13) had a baseline glutamate concentration of 9.23 +/- 2.5 mumol/ml (mean +/- standard error of the mean) before MCAO. Extracellular glutamate rose quickly after vessel occlusion and peaked at 33.95 +/- 6.3 mumol/ml 30 minutes after MCAO. By 60 minutes after MCAO, this level had decreased to 25.14 +/- 6.3 mumol/ml; glutamate levels decreased slightly to 21.35 +/- 6.8 mumol/ml by 120 minutes. Hypothermic animals (n = 11) had an initial extracellular glutamate concentration of 5.22 +/- 1.3 mumol/ml before MCAO. This value rose gradually to a maximum of 10.69 +/- 3.3 microns/ml at 50 minutes after MCAO and then returned to a baseline value of 2.58 +/- 1.2 mumol/ml by 120 minutes. Contralateral control glutamate dialysates in the normothermic and hypothermic groups remained near baseline throughout the experimental period. The mean percentages of right hemispheric volumes occupied by infarcts were 11.96 +/- 1.68% in the hypothermic group and 19.77 +/- 2.03% in the normothermic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Golgi electron microscopic study of the cerebral cortex after transient cerebral ischemia and reperfusion in the gerbil

Fine structures of defined neurons and their dendritic processes were studied in the cerebral cortex of gerbil brains by using Golgi electron microscopy during progressive cerebral ischemia for 10 and 20 min and after reperfusion for up to 72 h following transient ischemia for 20 min. The periphery of ascending dendrites of the vulnerable neurons in layers III and Vb became distended immediately after ischemia with swollen mitochondria and disintegrated microtubules, but the proximal portion of the same dendrites remained unchanged. After reperfusion for 6 h, distension of the dendroplasm of the impregnated dendrites in layer I receded, but the proximal portion of the same dendrites showed indentation caused by swollen astrocytic processes and derangement of microtubules inside. Polyribosomes in most neuronal perikarya were disaggregated, but severe neuronal damage was rarely found among those neuronal cell bodies impregnated by the Golgi method. Recovery with reaggregation of polyribosomes and realignment of microtubules was more clearly observed after reperfusion for 24 h and thereafter in impregnated neurons. These results indicated that impregnation during progressive ischemia occurred in many neurons with progressive structural damage but that impregnation during reperfusion occurred in a limited number of neurons with limited damage, allowing us to observe the recovery process, and that neuronal derangement in the dendrosomatic direction initially occurred both in the irreversibly damaged neurons and in the reversibly damaged ones. It is possible that disintegration of microtubules and the resulting disruption of dendritic transport may contribute to subsequent development of delayed neuronal death, if the recovery process does not take place promptly. Golgi electron microscopy is useful for ultrastructural investigation of defined neurons and their dendrites together and may be applicable for investigation of selected neuropathologic conditions.

Triggering and execution of neuronal death in brain ischaemia: two phases of glutamate release by different mechanisms

A reduced blood or oxygen supply to the brain leads to neuronal death caused by excessive activation of glutamate receptors. Recent evidence suggests that two distinct phases of glutamate release produce this death. During ischaemia or hypoxia, glutamate is released by reversed operation of glutamate uptake carriers. It activates N-methyl-D-aspartate (NMDA) receptors, increases the intracellular concentration of Ca2+, and triggers a long-lasting potentiation of NMDA-receptor-gated currents. After ischaemia, glutamate released by Ca(2+)-dependent exocytosis activates an excessive influx of Ca2+ largely through potentiated NMDA-receptor-channels, which leads to neuronal death. The therapeutic implications of such a scheme are discussed.

Evaluation of monoaminergic neurotransmitters in the rat striatum during varied global cerebral ischemia

Neurotransmitter release during cerebral ischemia has been extensively studied and is thought to play a key role in excitotoxic neuronal death. The changes in neurotransmitter release and its metabolism may reflect changes in cellular metabolism during ischemia. The purpose of this study is to assess alterations in extracellular dopamine and serotonin and their metabolites under varied degrees of ischemia in rat striatum to elucidate the pathophysiology of cerebral ischemia. Twenty rats were used to induce varied forebrain ischemia by means of bilateral common carotid artery occlusion along with hemorrhagic hypotension. Cerebral blood flow (CBF) in the striatum was measured every 40 minutes by methods of hydrogen clearance and maintained within certain ranges for 6 hours. Dopamine, serotonin, and their metabolites were measured every 20 minutes by in vivo microdialysis. Varying degrees of ischemia were obtained, ranging from 9.4 to 81.3% of control CBF. The animals were divided into three groups according to CBF levels measured 20 minutes after the induction of ischemia. In the mild ischemia group (n = 5), CBF ranged from 65 to 88% of baseline levels and resulted in only a slight increase of dopamine. In the moderate ischemia group (n = 10), CBF ranged from 21 to 48% of baseline levels and resulted in transient increases of dopamine (24-fold) and serotonin (5-fold). In the severe ischemia group (n = 5), CBF was below 14% of baseline levels and resulted in marked increases in dopamine (462-fold) and serotonin (225-fold). These alterations remained elevated for 3 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of neuronal responses to experimental ischemia in gerbil and rat hippocampal slices

The present study utilized in vitro gerbil and rat hippocampal slices to compare responses to experimental ischemia without species differences in the cerebrovasculature as a variable. Ischemic depolarization occurred faster in the gerbil (2.53 +/- 0.05 min) than in the rat (4.59 +/- 1.1 min). These results indicate that the gerbil's greater propensity to neuronal damage following short ischemic periods may be due to greater sensitivity of the gerbil brain itself.

Relevance of interstitial glutamate to selective vulnerability in focal cerebral ischemia

Glutamate concentrations in striatum and cortex were measured by means of in vivo cerebral microdialysis before and for 4 h after middle cerebral and ipsilateral common carotid artery occlusion in rats. The peak glutamate concentration reached 7.28 +/- 3.60 microM in dialysate from striatum and 5.64 +/- 2.24 microM in that from cortex. An index of exposure of each region to glutamate was calculated by integrating glutamate concentrations after occlusion. During ischemia the striatum was exposed to statistically higher cumulative concentrations of glutamate than the cortex (p < 0.01). The difference in vulnerability between striatum and cortex may arise from the additional time needed for the cortex to be exposed to cumulative threshold levels of glutamate.

Elevation of neuroactive substances in the cortex of cats during prolonged focal ischemia

Sustained accumulation of excitatory amino acids and other neuroactive substances may contribute to the delayed progression of infarction in focal ischemia. Following occlusion of the left middle cerebral artery (MCAO), extracellular amino acid and purine catabolite concentrations as well as local CBF were repeatedly monitored for up to 15 h in auditory (A) and somatosensory (SF) cortices of seven halothane-anesthetized cats using microdialysis/HPLC and hydrogen clearance. MCAO resulted in persistent reduction of local CBF, which was more severe in A (n = 6) than in SF (n = 6). Accordingly, higher transmitter amino acid and purine catabolite concentrations were found in A than in SF during ischemia. Aspartate, glutamate, and gamma-aminobutyrate (GABA) as well as hypoxanthine and inosine reached maximum levels 1-2 h after onset of ischemia (15-, 7-, 31-, 8-, and 14-fold increases, respectively). Maximum levels remained almost constant, with the exception of inosine, which decreased subsequently. Glycine seemed to increase with prolonged ischemia and reached maximum levels (10-fold) 15 h after occlusion. Adenosine peaked 30 min after occlusion (54-fold) and decreased thereafter to control levels within 1-2 h. One hour after occlusion, CBF thresholds for amino acid elevation were lower (glutamate and GABA approximately 20 ml 100 g-1 min-1 and glycine approximately 10 ml 100 g-1 min-1) than 6 and 15 h after occlusion (thresholds for all amino acids at approximately 30 ml 100 g-1 min-1). These results indicate that in prolonged ischemia, excitotoxicity is an important factor, particularly in border zones of ischemic foci.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in amino acid neurotransmitters and cerebral blood flow in the ischemic penumbral region following middle cerebral artery occlusion in the rat: correlation with histopathology

We simultaneously measured neurotransmitter amino acids by the microdialysis technique and cortical CBF by laser-Doppler flowmetry in the ischemic penumbral cortex of rats subjected to 2-h normothermic (36.5-37.5 degrees C) transient middle cerebral artery (MCA) clip-occlusion. Brains were perfusion-fixed 3 days later and infarct volume measured. CBF (% of preischemic values) fell to 32 +/- 2% (mean +/- SD) during ischemia and rose to 157 +/- 68% during recirculation. Extracellular glutamate levels increased from a baseline value of 7 +/- 3 microM to a peak value of 180 +/- 247 microM 20-30 min following onset of ischemia but subsequently returned to near baseline levels after 70 min of ischemia despite ongoing MCA occlusion. The threshold CBF for moderate glutamate release was 48%. Massive glutamate release was seen during the first 60 min of MCA occlusion in the two animals showing the largest infarcts and occurred at CBF values < or = 20% of control levels. Mean CBF during ischemia exhibited an inverse relationship with infarct volume, and the magnitude of glutamate release during ischemia was positively correlated with infarct volume. Extracellular gamma-aminobutyrate and glycine changes were similar to those of glutamate but showed no significant correlation with infarct volume. These results suggest that (a) accumulation of extracellular glutamate is an important determinant of injury in the setting of reversible MCA occlusion and (b) reuptake systems for neurotransmitter amino acids may be functional in the penumbra during transient focal ischemia.

Extracellular neuroactive amino acids in the rat striatum during ischaemia: comparison between penumbral conditions and ischaemia with sustained anoxic depolarisation

Changes in the extracellular levels of excitatory and inhibitory amino acid transmitters were studied in the rat striatum during penumbral ischaemia using intracerebral microdialysis. Effects of penumbral forebrain ischaemia were compared with those of ischaemia with sustained anoxic depolarisation and K+ (100 mM). Comparisons were also made between different groups of animals at 2 and 24 h after dialysis probe implantation. The K+ stimulus did not provoke any release of excitatory amino acids in the 24-h group, probably reflecting a decrease of functional synapses adjacent to the probe. During 30 min of penumbral ischaemia, excitatory amino acids did not reach critical concentrations in the extracellular fluid, and increases in levels of inhibitory/modulatory amino acids were similar. On the other hand, severe transient ischaemia resulted in massive synchronous release of many neuroactive excitatory and inhibitory compounds, in both the 2- and 24-h groups. These and other data suggest that changes during severe ischaemia may arise from both neurotransmitter and metabolic pools. It is concluded that ischaemic damage in the penumbra may not be related to extracellular neuroactive amino acid changes generated within this region.

Cardiovascular responses to global cerebral ischemia: role of excitatory amino acids in the ventrolateral medullary pressor area

The object of this study was to investigate the role of the ventrolateral medullary pressor area in mediating the cardiovascular responses to experimentally induced global cerebral ischemia, and to test if excitatory amino acids or acetylcholine are the transmitters released in this brain region during these responses. The cerebral ischemic response was elicited in pentobarbital-anesthetized, artificially ventilated male Wistar rats by bilateral ligation of vertebral arteries followed by temporary clamping of the common carotid arteries. The pressor area was identified by microinjections of L-glutamate. Inhibition of neurons in this area by microinjections of muscimol, a gamma-aminobutyric acid receptor agonist, abolished the ischemic response, which demonstrated that this area is important in mediating these responses. Microinjections of a broad-spectrum excitatory amino acid receptor blocker (kynurenate), of specific antagonists for N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors (injected alone or in combination), and of atropine failed to block the ischemic responses. These results indicate that: 1) the ventrolateral medullary pressor area mediates pressor responses to cerebral ischemia, and 2) excitatory amino acids or acetylcholine in this area do not mediate the cardiovascular responses to cerebral ischemia.

Extracellular dopamine and serotonin in the rat striatum during transient ischaemia of different severities: a microdialysis study

Generalised neurotransmitter overflow into the extracellular space on cerebral ischaemia has been widely reported and implicated in events leading to subsequent neuronal death. As little is known about the effect of depth of ischaemia on these changes, we have subjected anaesthetised rats to a sequence of four challenges [high K+ stimulus, moderate (penumbral) ischaemia, severe ischaemia, cardiac arrest] and have concurrently monitored both electrophysiological parameters and changes in extracellular dopamine, serotonin, and their metabolites in the striatum. Of particular relevance to human stroke therapy was penumbral ischaemia, where ionic homeostasis was maintained even though electrical function was lost. All challenges increased extracellular monoamines, although levels were significantly greater when ischaemia was severe enough to produce sustained anoxic depolarisation. Baseline levels were rapidly restored during recovery phases. Acidic monoamine metabolites decreased significantly during each insult, returning to basal levels during reperfusion after moderate ischaemia, and to significantly higher levels after severe ischaemia. Results indicate that sustained anoxic depolarisation may be a critical factor in determining outcome after ischaemia, being associated with significantly greater release of monoamines, and impairment of electrical function recovery.