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(Stroke. 1996;27:913-918.)
© 1996 American Heart Association, Inc.

Articles

Effects of Hypothermia on the Rate of Excitatory Amino Acid Release After Ischemic Depolarization

Ken Nakashima, MD Michael M. Todd, MD

From the Neuroanesthesia Research Laboratory, Department of Anesthesia, University of Iowa College of Medicine (Iowa City). K.N. was a research fellow from the Critical Care Medical Center, Yamaguchi University, Ube, Yamaguchi, Japan.

Correspondence to Michael Todd, MD, Department of Anesthesia, 6JCP, University of Iowa Hospitals and Clinics, Iowa City, IA 52242. E-mail mtodd@blue.weeg.uiowa.edu.

	Abstract

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Background and Purpose Hypothermia slows the increase in extracellular excitatory amino acid (EAA) concentrations during temporary cerebral ischemia. However, it is unclear whether hypothermia slows the rate of EAA release or just delays the time until the first sharp increase (which occurs coincident with terminal depolarization).

Methods Pericranial temperatures were adjusted to 38°C, 34°C, 31°C, or 25°C in halothane-anesthetized rats. The cortical DC voltage was recorded from a glass microelectrode while the cortical concentrations of glutamate, aspartate, glycine, and -aminobutyric acid (GABA) were measured by microdialysis. A cardiac arrest was induced with intravenous KCl, and the times until electroencephalograph isoelectricity and terminal depolarization were recorded. Dialysate concentrations of the four compounds were measured at 10, 20, and 30 minutes after depolarization.

Results The times to isoelectricity and depolarization varied inversely with temperature; depolarization time increased from 70±9 seconds at 38°C (mean±SD) to 294±34 seconds at 25°C. The dialysate concentrations of all four compounds increased during ischemia, and the rate of increase was inhibited by cooling. After 30 minutes of ischemia, glutamate concentration in 38°C animals was 58.4±31.8 µmol/L; this decreased to 15.9±8.4 µmol/L at 25°C. The magnitude of the effects of temperature on amino acid release differed with the compound measured. For glutamate, the calculated Q10 was 3.63. Corresponding values for aspartate and glycine were 3.68 and 1.95, respectively. By contrast, Q10 for GABA release was 6.31, indicating greater sensitivity to cooling.

Conclusions These results suggest that effects of hypothermia on EAA concentrations during cerebral ischemia may be the result of both a delay until initial EAA release as well as a direct effect of temperature on the rate of amino acid release. The observed temperature effects are more consistent with carrier-mediated processes controlling EAA release.

Key Words: cerebral ischemia, global • excitatory amino acids • glutamates • hypothermia • rats

	Introduction

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Hypothermia is the most effective means known for protecting the brain against ischemic injury; recent work indicates that decreases of even 2°C to 5°C in brain temperature may be beneficial.^{1 2 3} However, the mechanism of hypothermic protection remains uncertain. Whereas profound hypothermia markedly reduces cerebral metabolic demand and delays the onset of energy failure, only minor changes in cerebral metabolic rate occur during mild hypothermia.⁴ This makes simple metabolic depression an unlikely or incomplete explanation for its efficacy.

In recent years, several groups have proposed that the protective effects of hypothermia are related to inhibition of the release of EAAs. These compounds, including glutamate and aspartate, are released into the extracellular space at or near the moment of anoxic membrane depolarization, and the resultant high concentrations play an important role in delayed (and perhaps immediate) neuronal death. Microdialysis studies by several groups have shown that extracellular concentrations of glutamate during temporary global and focal ischemia are reduced by mild hypothermia.^{5 6 7 8}

Although these results provide one explanation for the efficacy of hypothermia, several questions are unanswered. Most laboratory models involve only 5 to 15 minutes of cerebral ischemia, and microdialysis provides only very broad "average" values for EAA concentrations during that period, since 5- to 10-minute sampling periods are typically used. On the basis of these considerations, there are two distinctly different ways that measured extracellular glutamate concentrations could be reduced during temporary ischemia. First, hypothermia may simply delay glutamate release, perhaps by slowing energy depletion and increasing the time until anoxic depolarization. If the ischemic interval is fixed, and if EAA release ends with reperfusion, then reducing the total time during which glutamate is escaping into the extracellular space (eg, from 9 to 6 minutes) would reduce the total amount of compound collected during a 10-minute dialysis interval. Alternatively, hypothermia may directly reduce the rate (in micromoles per liter per minute) at which glutamate is released. Obviously, both of these changes may occur at the same time, eg, a delay in the time to first release and a subsequent reduction in the rate of release.

The following experiments were designed to directly examine the influence of temperature on the rate of postischemic glutamate release. This was done by beginning the dialysate collection only after recording a depolarization event from the cortex.

	Materials and Methods

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All aspects of this study were approved by the University of Iowa Animal Care and Use Committee. Male Sprague-Dawley rats weighing 325 to 350 g were used. Animals were fasted overnight, with free access to water until they were anesthetized. Anesthesia was induced with 4% halothane in O₂ in a closed plastic box. After tissue infiltration with 1% lidocaine, a tracheotomy was performed and mechanical ventilation started with an inspired gas mixture of 1.5% halothane in O₂, using a tidal volume of 2.5 to 3.5 mL and a rate of 32 to 45 breaths per minute. A femoral artery and vein were cannulated, again after tissue infiltration with 1% lidocaine, and were used for blood pressure monitoring and arterial blood sampling and for the infusion of fluids and drugs. Muscle relaxation was achieved with 0.2 mg pancuronium IV given as needed. The animal was turned prone, and the head was fixed in a stereotaxic frame (David Kopf Instruments). The scalp was infiltrated with 1% lidocaine and reflected laterally to expose the calvarium. Two small (2x2 mm) left-sided craniectomies were drilled, one frontal (centered 2 mm anterior to bregma and 2 mm left of the midline) and the other parietal (located 7 mm caudal to the bregma and 5 mm lateral). All drilling was done under an operating microscope, using a high-speed electrical drill. The drilling sites were irrigated with cool saline to avoid thermal trauma.

When surgery was complete, the inspired halothane concentration was reduced to 0.8% (as verified with a Datex Anesthetic Agent Monitor 222, Datex Instrumentarium Corp), combined with 50% N₂O in O₂. A continuous infusion of lactated Ringer's solution was started at the rate of 1 mL/h, and supplemental doses of 6% hetastarch were given to maintain MAP >80 mm Hg if necessary. Platinum needle electrodes were inserted into the temporalis muscles bilaterally to permit the recording of a single biparietal EEG. Temperature was recorded in two locations: a needle thermistor (YSI model 524, Yellow Springs Instrument Co Inc) was placed into the pericranial tissue adjacent to the craniotomy, and a YSI model 401 probe was inserted rectally.

After these preparations, a small slit was made in the dura of the left frontal cranial window, and a saline-filled glass micropipette with tip diameter of 5 µm was inserted 0.5 mm into the cortical surface with a micromanipulator. Care was taken to avoid damage to cortical vessels. A Ag/AgCl wire in the barrel served as the electrical contact, and a Ag/AgCl rod was inserted into the dorsal neck muscles as the reference. The DC potential between these electrodes was measured with a Grass 7P122 amplifier (Grass Instrument Co) equipped with a Grass H1P5 high-impedance input probe. Then, through the small slit in the dura of the parietal burr hole, a microdialysis probe (4-mm membrane, 0.24-mm OD; CMA/11, CMA) was inserted 4 mm into the left parietal cortex. The probe was perfused using a microinjection pump (EP-60, Eicom) at a constant flow rate of 2 µL/min. The perfusion medium consisted of Ringer's solution with the following ion content: (mmol/L) 147 Na⁺, 4 K⁺, 2.3 Ca²⁺, 155.6 Cl^-; pH 6.0, 285 mOsm/kg H₂O.

Levels of hematocrit, pH, PaCO₂, and PaO₂ were determined intermittently, and ventilation was adjusted if necessary to maintain PaO₂ >100 mm Hg and PaCO₂ between 35 and 42 mm Hg. All blood gas values were measured at 37°C and reported without temperature correction (alpha-stat management).

When preparation was complete, pericranial temperature was adjusted to and maintained at target values of 38°C, 34°C, 31°C, or 25°C with a warming blanket in normothermic animals or, in hypothermic rats, an ice water–perfused water jacket placed around the animal's body. Heparin (50 U IV) was administered to all rats. No measurements were made until at least 90 minutes after insertion of the dialysis cannula and 60 minutes after reaching the target temperature. At this point, a single 10-minute (20 µL) sample of dialysate was collected. Each animal was then killed with 0.5 mL saturated KCl given intravenously. The subsequent times (in seconds) to the appearance of an isoelectric EEG and to terminal ischemic depolarization were recorded. A 20-µL dialysate sample was then collected 11, 21, and 31 minutes after appearance of depolarization. The 1-minute delay was introduced to minimize the amount of predepolarization dialysate that would be present in the first sample (ie, to account for "dead space" in the dialysis tubing). Pericranial temperatures were maintained at the prearrest values throughout the ischemic period. Animals were not resuscitated.

Dialysate concentrations of glutamate, aspartate, glycine, and GABA were determined using high-performance liquid chromatography (EP-10, Eicom) with an ODC-C18 column (particle diameter of 5 µm; column of 6 mm ID and 150-mm length; Eicompack MA-5ODS, Eicom) and an electrochemical detector (ECD-100, Eicom). Samples were derivitized with o-phthalaldehyde. The mobile phase consisted of 80% 0.1 mol/L phosphate buffer/20% methanol and was run at the rate of 1.0 mL/min. Amino acid concentrations were determined using calibration curves derived from external standards.

To examine the effects of temperature on dialysis probe recovery rates, probes were placed into solutions containing both glutamate and aspartate and maintained at either 38°C or 25°C. This was done before the probes were inserted into the animal and again after removal. However, all in vivo data are reported as the directly measured concentrations in the dialysate, with no attempt made to calculate actual extracellular values.

Data Analysis
All physiological variables were compared among groups using a factorial ANOVA. Changes in dialysate concentrations versus time and versus temperature were examined using two-factor ANOVA, with temperature as the between-group variable and with time treated as a repeated measure. Because of the enormous possible number of post hoc tests that could be performed, we restricted ourselves to answering one question: at what postdepolarization interval (10, 20, or 30 minutes) did the dialysate concentration of each compound become significantly greater than before ischemia? This was done using a post hoc Fisher's least-significant difference test, with significance accepted only for P.02.

To compare the impact of temperature on the rate of release of the different measured compounds, we calculated Q10 values for each amino acid. The Q10 (no units) is defined in hypothermia research as the ratio of two values measured 10°C apart. For example, if the rate of a reaction is 20 µmol·L^-1·min^-1 at 38°C and 10 µmol·L^-1·min^-1 at 28°C, Q10 would be 20 divided by 10, or 2.0. When measurements are not made at precise 10°C intervals, the equation below is used.

To make this calculation, it was first necessary to estimate the "rate of release" (in micromoles per liter per minute) for the different EAAs at different temperatures. We first calculated the rate of release for each amino acid in each animal by plotting the dialysate concentration versus collection time, taking the prearrest (baseline) interval as time zero. This approach assumes no change in dialysate concentration before depolarization (since all collection intervals were timed from depolarization, not from the initial arrest). Also, since the concentration of compound measured in a 10-minute collection is the "average" of the concentrations during that interval, we plotted the regression using "adjusted times," ie, 5, 15, and 25 minutes (the midpoints of each collection interval). Next, rather than calculating the rate by simply subtracting the baseline value from the last sample and dividing by time, we used a least-squares method to fit a straight line to the four data points in each animal. The slope of this line was taken as the release rate for that animal. Mean±SD values for each temperature group (and for each amino acid) were then calculated. Using the average values at 38°C and 25°C, we calculated Q10 as the antilog of the following value: ([log(Rate of Release at 38°C/Rate of Release at 25°C)]/Temperature[=13°C])x10.

This approach assumes that the extracellular/dialysate concentrations of amino acid rise in a linear fashion. Inspection of the data indicates that this was true for the pooled data and individually in most animals. However, in a few cases, the data appeared to be better fitted by a nonlinear least-squares method. Unfortunately, expressing the slope of a curved line is difficult, hence we chose the linear approach for simplicity.

A similar Q10 was calculated for the effect of temperature on the times to isoelectricity and terminal depolarization.

	Results

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Prearrest values for temperature (rectal and pericranial), MAP, arterial blood gases, hematocrit, and blood glucose are shown in the Table. In addition, the Table shows the times to EEG isoelectricity and terminal depolarization in the four groups. As expected, there was a progressive increase in these times with decreasing temperature. The calculated Q10 for the time to isoelectricity was 1.88. Temperature had a greater effect on the latency to depolarization (Q10 of 3.17).

View this table: [in this window] [in a new window]   Table 1. Physiological Parameters Versus Temperature

In vitro recovery rates for the probes changed depending on use. Glutamate recovery rate of unused probes was 18.6±7.9% at 38°C, decreasing to 12.0±9.6% after removal from the animal (mean±SD). In vitro recovery was not significantly influenced by temperature (all probes tested at both 38°C and 25°C). Baseline (preischemic) in vivo dialysate concentrations of glutamate, aspartate, and glycine also did not differ among the four different temperature groups. Combined baseline values for these three amino acids were 5.82±4.92, 0.75±1.62, and 2.64±2.97 µmol/L, respectively. GABA was undetectable under preischemic conditions, regardless of temperature.

Postdepolarization changes in the dialysate concentrations of the four measured compounds are shown in the Figure. Regardless of temperature, ischemia and depolarization were followed by a progressive increase in the concentrations of all compounds. The magnitude of this increase was progressively attenuated by hypothermia. For example, 30 minutes after depolarization, dialysate concentrations of glutamate in normothermic animals had increased to 58.4±31.8 µmol/L. By contrast, concentrations of only 15.9±8.4 µmol/L were measured in animals at 25°C. Expressed differently, the calculated rate of glutamate release decreased from 2.10±1.21 µmol·L^-1·min^-1 at 38°C to 0.42±0.29 µmol·L^-1·min^-1 at 25°C.

View larger version (39K): [in this window] [in a new window]   Figure 1. Dialysate concentrations for glutamate, aspartate, glycine, and GABA at the four experimental temperatures. All concentrations are reported as those directly measured in the dialysate and are given as mean±SD. Probes were perfused continuously (2 µL/min), and 10-minute aliquots (20 µL) were used to determine amino acid concentrations. These aliquots were collected either before circulatory arrest (baseline) or 10, 20, and 30 minutes after the onset of terminal depolarization as assessed with a microelectrode in the ipsilateral cortex. There was a statistically significant increase in the concentrations of all compounds in all groups over time, and the rate of rise differed significantly with temperature (two-way ANOVA with time as a repeated measure). In addition, a post hoc Fisher's least-significant difference test was performed to define the point in time (designated with *) at which the concentration of the measured EAA first statistically exceeded baseline (at the P<.02 level). For example, a significant increase in glutamate (top left) was first seen at 10 minutes in 38°C animals, at 20 minutes in both the 34°C and 31°C animals, and only at 30 minutes in 25°C rats. Conc indicates concentration; t, temperature.

The effect of temperature on rates of release is best summarized by calculating Q10 values. For the four transmitters, Q10 values were glutamate 3.63, aspartate 3.68, glycine 1.95, and GABA 8.23. (Since GABA was undetectable in preischemic samples, there are difficulties in calculating Q10. If the rate of rise for GABA is calculated using only the 10-, 20-, and 30-minute data points, a Q10 of 6.10 was found.)

	Discussion

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Although mechanistic details are still being debated, glutamate and other excitatory neurotransmitters have clearly been shown to play a role in the pathophysiology of cerebral ischemic injury.^{9 10 11} High glutamate concentrations are neurotoxic in vivo,¹² and extracellular glutamate concentrations increase markedly during ischemia.^{5 9 13 14 15} Blockade of both N-methyl-D-aspartate and -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors reduces neurological injury at least in some models.¹⁶ In addition, chemical inhibition of release appears to be protective.^{17 18} It is hence reasonable to hypothesize that other interventions that limit glutamate release might be of therapeutic value.

The protective efficacy of profound hypothermia has been known for decades. The impact of mild hypothermia on postischemic energy failure was reported in 1981.¹⁹ In 1987, Busto et al¹ clearly demonstrated that mild hypothermia could also have major effects on histopathologic outcome after forebrain ischemia, a finding confirmed by Minamisawa et al in 1990^{2 3} and by many others since. Unlike deep hypothermia, protection produced by these small temperature reductions cannot be readily explained on the traditional basis of "metabolic suppression," and a concerted effort has been made to find other explanations. One possible mechanism is a reduction in the extracellular concentrations of EAAs measured during temporary ischemia. In 1989, Busto et al⁵ subjected rats to 20 minutes of global ischemia at head temperatures of 36°C, 33°C, and 30°C. Glutamate concentrations (as determined by microdialysis) increased almost fivefold in the normothermic animals (with the peak occurring in the sample collected at the end of the ischemic interval). By contrast, glutamate concentrations were unchanged in hypothermic animals (or at most doubled in the 33°C animals). Similar findings have been obtained by several groups, working in various species, using either global or focal ischemic insults.^{6 7 8 20 21 22 23}

What remains unclear is the mechanism by which small temperature changes prevent these increases. Extracellular EAA concentrations must rise as the result of increased release and/or decreased reuptake. Since it is difficult to imagine a process by which hypothermia would stimulate EAA reuptake, it is reasonable to conclude that the effects of cooling are due to changes in EAA release. Two different processes could be operating (and these are not mutually exclusive). First, cold could simply delay the time until the start of postischemic release. If a temporary insult is studied (eg, 10 minutes of forebrain ischemia followed by reperfusion), a reduction in glutamate concentrations might be seen if the onset of release was simply delayed for several minutes. It is known that a marked increase in extracellular EAA concentrations begins coincident with the appearance of ischemic/anoxic cortical depolarization.^{24 25 26 27} It has also been shown that the time delay until anoxic depolarization varies directly with temperature.^{4 28} In the present study, reducing cranial temperature from 38°C to 31°C increased depolarization times from 70±9 to 169±23 seconds. If this had been a 10-minute ischemic event, the available time for glutamate release (assuming that release started coincident with depolarization) would have been reduced from 8.8 to 7.2 minutes. This should reduce the total amount of glutamate released during the ischemic interval by almost 20%. At lower temperatures, the reduction would be even greater (eg, at 25°C, the depolarization delay is almost 5 minutes). In addition, since equilibration time between the extracellular space and the dialysis fluid is not immediate, even greater reductions in measured glutamate might be recorded. Nevertheless, the possible magnitude of the observed depolarization delay does not seem sufficient to explain the dramatic reductions in recovered glutamate seen by various authors.

Because dialysate collections did not start until after a DC shift was observed, our data were not altered by depolarization delays. The results clearly show that hypothermia slows the rate at which extracellular concentrations of all measured compounds increase. For example, in normothermic rats, the dialysate concentration of glutamate increased at the rate of 2.1 µmol·L^-1·min^-1, a value similar to that seen by others. When temperature was reduced to 25°C, this decreased to 0.4 µmol·L^-1·min^-1. This temperature effect can be summarized by the Q10, ie, the ratio between the rates of increase at two temperatures 10°C apart (eg, 38°C and 28°C). For glutamate, Q10 was calculated as 3.63. Generally similar values were seen for aspartate (3.68). By contrast, Q10 for glycine was somewhat lower (1.95), whereas that for GABA was much greater (6 to 9). This indirectly suggests that several release mechanisms may be involved for glutamate/aspartate, glycine, and GABA, each with different temperature sensitivities.

There are several means by which hypothermia might slow EAA release. Normal Ca²⁺-dependent vesicular release is unlikely to play an important role because this is inhibited by energy failure; tissue depolarization and ischemic Ca²⁺ entry do not begin until tissue ATP concentrations are between 15% to 25% of normal.^{29 30} Although hypothermia delays the time until depolarization, ATP concentrations at this point are not different from those seen during normothermia.³⁰ Therefore, our dialysate collections did not begin until energy failure was nearly complete. Many have suggested that EAA release during severe ischemia is the result of a reversal of the normal Na⁺-linked glutamate/aspartate reuptake carrier.^{31 32} Hypothermia might slow this process by directly influencing carrier kinetics, or it might slow the increase in cytoplasmic Na⁺ or the increase in extracellular K⁺ (which will also inhibit inward transport activity). At present, insufficient information is available to sort out these possibilities. Nevertheless, our data indirectly support some form of carrier-mediated process controlling glutamate release during ischemia. Specifically, temperature changes such as those seen here (eg, 12°C) have little effect on simple diffusion (ie, Q10 is close to 1.0), and it is unlikely that our observations could be the result of glutamate passively "leaking" out of nerve terminals or astrocytes.

A number of methodological comments are needed. Instead of the temporary and incomplete (albeit severe) ischemic models used by many, we chose only to work with animals subjected to complete global ischemia (cardiac arrest). This was done to ensure that all dialysis probes were implanted in tissues with identical ischemic conditions (ie, cerebral blood flow of 0). We also used acutely implanted cortical probes. The cortical location was chosen to ensure that we would be able to record DC potentials and sample extracellular fluid from approximately similar tissue compartments. The acute nature of the preparation was for convenience. Although this will result in some tissue injury and immediate glutamate release (as evidenced by the modestly elevated baseline concentrations of glutamate in the dialysate), we do not believe that it has any major impact on the time-related changes seen with complete ischemia. In fact, the changes seen in normothermic animals are similar to those reported by others. We also did not demonstrate any important effect of temperature on the recovery rates for glutamate and aspartate in vivo, although we did not examine temperature effects on glycine or GABA recovery. This is also in keeping with the limited effects of mild temperature changes on simple diffusion.

In summary, our results demonstrate that hypothermia can influence postischemic extracellular glutamate concentrations in two ways, first by delaying the time until depolarization and then by directly slowing the rate of postdepolarization increase. The magnitude of this temperature effect is compatible with a carrier-mediated process, and the differing Q10s for glutamate/aspartate, glycine, and GABA suggest that different release/reuptake mechanisms may be involved.

	Selected Abbreviations and Acronyms

 

EAA = excitatory amino acid EEG = electroencephalogram GABA = -aminobutyric acid MAP = mean arterial pressure

	Acknowledgments

 
We would like to thank Dr Tsuyoshi Maekawa (Chairman, Department of Critical Care and Emergency Medicine, Yamaguchi University) and Dr John Tinker (Head, Department of Anesthesia, University of Iowa) for their support of this work. In addition, we would like to thank EICOM Inc (Kyoto, Japan) for their loan of the high-performance liquid chromatography equipment and the microdialysis perfusion pumps.

Received October 2, 1995; revision received January 30, 1996; accepted February 9, 1996.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

1. Busto R, Dietrich WD, Globus MYT, Valdes I, Scheinberg P, Ginsberg MD. Small differences in intraischemic brain temperature critically determine the extent of ischemic neuronal injury. J Cereb Blood Flow Metab. 1987;7:729-738. [Medline] [Order article via Infotrieve]
2. Minamisawa H, Smith ML, Siesjö BK. The effect of mild hyperthermia and hypothermia on brain damage following 5, 10, and 15 minutes of forebrain ischemia. Ann Neurol. 1990;28:26-33. [Medline] [Order article via Infotrieve]
3. Minamisawa H, Nordstrom CH, Smith ML, Siesjö BK. The influence of mild body and brain hypothermia on ischemic brain damage. J Cereb Blood Flow Metab. 1990;10:365-374. [Medline] [Order article via Infotrieve]
4. Nakashima K, Todd MM, Warner DS. The relation between cerebral metabolic rate and ischemic depolarization: a comparison of the effects of hypothermia, pentobarbital and isoflurane. Anesthesiology. 1995;82:1199-1208. [Medline] [Order article via Infotrieve]
5. Busto R, Globus MYT, Dietrich WD, Martinez E, Valdes I, Ginsberg MD. Effect of mild hypothermia on ischemia-induced release of neurotransmitters and free fatty acids in rat brain. Stroke. 1989;20:904-910. [Abstract/Free Full Text]
6. Lo EH, Steinberg GK, Panahian N, Maidment NT, Newcomb R. Profiles of extracellular amino acid changes in focal cerebral ischaemia: effects of mild hypothermia. Neurol Res. 1993;15:281-287. [Medline] [Order article via Infotrieve]
7. Matsumoto M, Scheller MS, Zornow MH, Strnat MAP. Effects of S-emopamil, nimodipine, and mild hypothermia on hippocampal glutamate concentrations after repeated cerebral ischemia in rabbits. Stroke. 1993;24:1228-1234. [Abstract/Free Full Text]
8. Illievich UM, Zornow MH, Choi KT, Scheller MS, Strnat MAP. Effects of hypothermic metabolic suppression on hippocampal glutamate concentrations after transient global cerebral ischemia. Anesth Analg. 1994;78:905-911. [Abstract/Free Full Text]
9. Benveniste H. The excitotoxin hypothesis in relation to cerebral ischemia. Cerebrovasc Brain Metab Rev. 1991;3:213-245. [Medline] [Order article via Infotrieve]
10. Siesjö BK. Pathophysiology and treatment of focal cerebral ischemia, I: pathophysiology. J Neurosurg. 1992;77:169-184. [Medline] [Order article via Infotrieve]
11. Greenamyre JT, Porter RH. Anatomy and physiology of glutamate in the CNS. Neurology. 1994;44:S7-S13. [Medline] [Order article via Infotrieve]
12. Choi DW, Koh J, Peters S. Pharmacology of glutamate neurotoxicity in cortical cell culture: attenuation by NMDA antagonists. J Neurosci. 1988;8:185-196. [Abstract]
13. Benveniste H, Drejer J, Schousboe A, Diemer NH. Elevation of the extracellular concentrations of glutamate and aspartate in rat hippocampus during transient cerebral ischemia monitored by intracerebral microdialysis. J Neurochem. 1984;43:1369-1374. [Medline] [Order article via Infotrieve]
14. Hagberg H, Lehmann A, Sandberg M, Nystrom B, Jacobson I, Hamberger A. Ischemic-induced shift of inhibitory and excitatory amino acids from intra- to extracellular compartments. J Cereb Blood Flow Metab. 1985;5:413-419. [Medline] [Order article via Infotrieve]
15. Baker AJ, Zornow MH, Scheller MS, Yaksh TL, Skilling SR, Smullin DH, Larson AA, Kuczenski R. Changes in extracellular concentrations of glutamate, aspartate, glycine, dopamine, serotonin, and dopamine metabolites after transient global ischemia in the rabbit brain. J Neurochem. 1991;57:1370-1379. [Medline] [Order article via Infotrieve]
16. Siesjö BK. Pathophysiology and treatment of focal cerebral ischemia, II: mechanism of damage and treatment. J Neurosurg. 1992;77:337-354. [Medline] [Order article via Infotrieve]
17. Meldrum BS, Swan JH, Leach MJ, Millan MH, Gwinn R, Kadota K, Graham SH, Chen J, Simon RP. Reduction of glutamate release and protection against ischemic brain damage by BW 1003C87. Brain Res. 1992;593:1-6. [Medline] [Order article via Infotrieve]
18. Graham SH, Chen J, Sharp FR, Simon RP. Limiting ischemic injury by inhibition of excitatory amino acid release. J Cereb Blood Flow Metab. 1993;13:88-97. [Medline] [Order article via Infotrieve]
19. Berntman L, Welsh FA, Harp JR. Cerebral protective effect of low-grade hypothermia. Anesthesiology. 1981;55:495-498. [Medline] [Order article via Infotrieve]
20. Baker AJ, Zornow MH, Grafe MR, Scheller MS, Skilling SR, Smullin DH, Larson AA. Hypothermia prevents ischemia-induced increases in hippocampal glycine concentrations in rabbits. Stroke. 1991;22:666-673. [Abstract/Free Full Text]
21. Mitani A, Kataoka K. Critical levels of extracellular glutamate mediating gerbil hippocampal delayed neuronal death during hypothermia: brain microdialysis study. Neuroscience. 1991;42:661-670. [Medline] [Order article via Infotrieve]
22. Patel PM, Drummond JC, Cole DJ, Yaksh TL. Differential temperature sensitivity of ischemia-induced glutamate release and eicosanoid production in rats. Brain Res. 1994;650:205-211. [Medline] [Order article via Infotrieve]
23. Illievich UM, Zornow MH, Choi KT, Strnat MAP, Scheller MS. Effects of hypothermia or anesthetics on hippocampal glutamate and glycine concentrations after repeated transient global cerebral ischemia. Anesthesiology. 1994;80:177-186. [Medline] [Order article via Infotrieve]
24. Ueda Y, Obrenovitch TP, Lok SY, Sarna GS, Symon L. Efflux of glutamate produced by short ischemia of varied severity in rat striatum. Stroke. 1992;23:253-259. [Abstract/Free Full Text]
25. Ueda Y, Obrenovitch TP, Lok SY, Sarna GS, Symon L. Changes in extracellular glutamate concentration produced in the rat striatum by repeated ischemia. Stroke. 1992;23:1125-1131. [Abstract/Free Full Text]
26. Fabricius M, Jensen LH, Lauritzen M. Microdialysis of interstitial amino acids during spreading depression and anoxic depolarization in rat neocortex. Brain Res. 1993;612:61-69. [Medline] [Order article via Infotrieve]
27. Perez-Pinzon MA, Nilsson GE, Lutz PL. Relationship between ion gradients and neurotransmitter release in the newborn rat striatum during anoxia. Brain Res. 1993;602:228-233. [Medline] [Order article via Infotrieve]
28. Astrup J, Rehncrona S, Siesjö BK. The increase in extracellular potassium concentration in the ischemic brain in relation to the preischemic functional activity and cerebral metabolic rate. Brain Res. 1980;199:161-174. [Medline] [Order article via Infotrieve]
29. Ekholm A, Katsura K, Siesjö BK. Coupling of energy failure and dissipative K+ flux during ischemia: role of preischemic plasma glucose concentration. J Cereb Blood Flow Metab. 1993;13:193-200. [Medline] [Order article via Infotrieve]
30. Verhaegen M, Iaizzo P, Todd M. A comparison of the effects of hypothermia, pentobarbital and isoflurane on cerebral energy stores at the time of ischemic depolarization. Anesthesiology. 1995;82:1208-1215.
31. Nicholls D, Attwell D. The release and uptake of excitatory amino acids. Trends Pharmacol Sci. 1990;11:462-468. [Medline] [Order article via Infotrieve]
32. Szatkowski M, Attwell D. Triggering and execution of neuronal death in brain ischaemia: two phases of glutamate release by different mechanisms. Trends Neurosci. 1994;17:359-365. [Medline] [Order article via Infotrieve]

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