Microdialysis of interstitial amino acids during spreading depression and anoxic depolarization in rat neocortex

Intracerebral microdialysis markedly inhibits the propagation of cortical spreading depression

It is accepted that the ionic composition of the medium perfused through a microdialysis probe should match that of the extracellular fluid (ECF) under physiological conditions. In contrast, the possibility that control artificial cerebrospinal fluid may influence the experimental or pathological conditions under study, by buffering changes in the ECF composition, has been neglected. Spreading depression (SD) is a propagating transient suppression of electrical activity due to cellular depolarization which may contribute to neuronal damage in focal ischaemia, and underlie the migraine aura Here we report that microdialysis markedly inhibits SD propagation, by buffering the sudden increase in extracellular K+ associated with this event. This effect is independent of the microdialysis flow rate and does not result from tissue injury following probe implantation. This finding clearly illustrates that microdialysis can influence the pathological conditions under investigation.

Origins of glutamate release in ischaemia

Using microdialysis coupled to on-line detection of glutamate, and recording electrical activity and field potential at the same tissue site, We have shown that the increase in extracellular glutamate under global penumbral conditions in minor. However, in the border of the ischaemic core, recurrent spreading depression is presumably associated with transient vesicular release of glutamate (exocytosis). With ischaemic insults severe enough to provoke anoxic depolarization, such as in the ischaemic core, exocytosis only occurred for a few minutes because it requires ATP hydrolysis, and the magnitude of this release was minor in comparison with that of the total glutamate efflux. Subsequent experiments with a selective inhibitor of high-affinity glutamate transporters suggested that reversal of glutamate uptake may not be a major contributor to the sustained release of glutamate in this condition. These results, and other consideration, do not favour the view that presynaptic glutamate release and reversed glutamate uptake are suitable targets for neuroprotection in ischaemia. Acting postsynaptically to inhibit recurrent spreading depression (NMDA-receptor antagonists) or to modulate long-lasting enhancement of synaptic efficiency ('anoxia-induced long-term potentiation' appear to be more rational strategies.

Gap junctions are required for the propagation of spreading depression

Spreading depression (SD) is a slowly propagating depression of cerebral neuronal activity and transmembrane ionic gradients, that arises in response to a variety of noxious stimuli. SD bears a strong resemblance to gap junction-mediated calcium waves among cultured astrocytes. Here, we show that gap junction-mediated intercellular diffusion is necessary for the generation of SD. Waves of SD in the isolated chicken retina were blocked by five different inhibitors of gap junctional coupling, which was assessed by the intercellular transit of Lucifer Yellow (LY). Each of these gap junction blockers inhibited both the migration of SD and the diffusion of LY in a dose-dependent manner. In contrast, glutamate-evoked calcium influx into retinal cells was not affected by these compounds. The results indicate that intercellular coupling through gap junctions is required for SD. Gap junction-mediated communication might therefore constitute an important mechanism in both normative and pathological brain function.

A comparison of the effects of a sodium channel blocker and an NMDA antagonist upon extracellular glutamate in rat focal cerebral ischemia

Agents such as 619C89 decrease extracellular glutamate concentrations by a primary action at voltage sensitive sodium channels, but NMDA antagonists also have been shown to decrease extracellular glutamate concentration after ischemia. To address the question as to whether 619C89's effect upon extracellular glutamate concentrations is any different than the effect of the NMDA antagonist dextrorphan, 24 rats were given either optimally neuroprotective doses of these drugs or saline prior to middle cerebral artery occlusion. In caudate, the 619C89-treated, but not dextrorphan-treated rats had less microdialysate glutamate than ischemic controls. In cortex, both 619C89- and dextrorphan-treated groups had significantly decreased glutamate compared with ischemic controls. These results support a specific effect of 619C89 upon glutamate release in caudate but not cortex.

Halothane, but not alpha-chloralose, blocks potassium-evoked cortical spreading depression in cats

The effects of two anesthetics, halothane and alpha-chloralose, on induction of spreading depression and on extracellular glutamate elevation after intracortical potassium administration were investigated in artificially ventilated (30% oxygen/70% nitrous oxide) cats. High potassium concentrations were achieved using either direct KCl injections (7 microliters, 150 mM via a micropipette) or microdialysis by supplementing 100, 300 or 500 mM KCl, respectively, for 10 min to the perfusion solution (Ringer's). Changes of the cortical DC potential were recorded adjacent (1-2 mm: electrode DC1) and distant (6-7 mm: electrode DC2) to the injection site. Either under halothane (0.75% in the respiratory gas mixture) or under alpha-chloralose (60 mg/kg i.v.) anesthesia, prolonged negative shifts of the DC potential reflecting the elevated potassium levels after KCl injection were measured near the injection site (electrode DC1). In contrast, spreading depressions (transient short DC deflections) were almost exclusively observed under alpha-chloralose. Spreading depressions recorded with electrode DC1 were superimposed on the prolonged negative DC shifts and they propagated frequently to the more distant site (DC2). Upon KCl administration, dose dependent elevations of extracellular glutamate were measured. These elevations were not significantly altered by the type of anesthesia. Our results suggest that in cats, spreading depression induction is affected by anesthesia, i.e., spreading depression induction is inhibited by halothane as compared to alpha-chloralose. Furthermore, factors other than glutamate or high potassium seem to contribute to spreading depression induction.

Effects of Ca2+ channel blockers on cortical hypoperfusion and expression of c-Fos-like immunoreactivity after cortical spreading depression in rats

1. We examined the effects of two Ca2+ channel blockers, lomerizine (KB-2796) and flunarizine, on the cortical hypoperfusion (measured by hydrogen clearance and laser Doppler flowmetry methods) and cortical c-Fos-like immunoreactivity that follow KCl-induced cortical spreading depression in anaesthetized rats. Cortical spreading depression was induced by application of 1 M KCl for 30 s to the cortical surface, 3.0 mm posterior to the area of cerebral blood flow measurement. 2. In control rats, KB-2796 (0.3 and 1 mg kg-1, i.v.) dose-dependently increased cerebral blood flow significantly at 30 min and 15 min, respectively, after its administration. Flunarizine (1 mg kg-1, i.v.) significantly increased cerebral blood flow 15 min after its administration. In contrast, dimetotiazine (3 mg kg-1, i.v.), a 5-HT2 and histamine H1 antagonist, failed to affect cerebral blood flow significantly. 3. After KCl application to the cortex, cerebral blood flow monitored by the laser Doppler flowmetry method increased transiently, for a few minutes, then fell and remained approximately 20 to 30% below control for at least 60 min. Cerebral blood flow monitored by the hydrogen clearance method was also approximately 20 to 30% below baseline for at least 60 min after KCl application. KB-2796 (0.3 and 1 mg kg-1, i.v.) and flunarizine (1 and 3 mg kg-1, i.v.) administered 5 min before KCl application inhibited the cortical hypoperfusion that followed KCl application, but dimetotiazine (1 and 3 mg kg-1, i.v.) did not. 4. An indicator of neuronal activation, c-Fos-like immunoreactivity, was detected in the ipsilateral, but not in the contralateral frontoparietal cortex 2 h after KCl application. No c-Fos-like immunoreactivity was seen on either side of the brain in the hippocampus, thalamus, striatum or cerebellum. 5. KB-2796 (1 mg kg-1, i.v.) and flunarizine (3 mg kg-1, i.v.), but not dimetotiazine (3 mg kg-1, i.v.), significantly attenuated the expression of c-Fos-like immunoreactivity in the ipsilateral frontoparietal cortex. 6. These findings suggest that the inhibitory effects of KB-2796 and flunarizine on the cortical hypoperfusion and expression of c-Fos-like immunoreactivity induced by spreading depression are mediated via the effects of Ca(2+)-entry blockade, which may include an increase in cerebral blood flow and the prevention of excessive Ca2+ influx into brain cells. KB-2796 and flunarizine may prove useful as inhibitors of cortical spreading depression in migraine.

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.

A comparison between the stimulated and paroxysmal release of endogenous amino acids from rat cerebellar, striatal and hippocampal slices: a manifestation of spreading depression?

Spreading depression, which can be evoked by a variety of stimuli both in vitro and in vivo, is associated with profound changes in extracellular ion concentrations and enhanced release of neurotransmitter amino acids. We have observed a transient spontaneous release of amino acids in slice preparations obtained from rat cerebellum, striatum and hippocampus; this phenomenon has similar properties to stimulus-evoked spreading depression. Aspartate, glutamate, glutamine, serine, glycine and gamma-aminobutyric acid (GABA) release were potentiated during these episodes in all three brain regions, with a variable effect upon taurine release. When compared to glutamate release, a consistently high release of aspartate, glycine and serine was observed. Amino acid release, evoked by whole slice depolarization using veratridine (10-25 microM) or elevated potassium (35-60 mM) consistently enhanced glutamate release, and to a lesser extent aspartate release, but had negligible effect upon the other amino acids. Thus, the release profiles for spontaneous and depolarization-evoked release are markedly different. We suggest that the spontaneous release observed in brain slices represents a spreading depression-like phenomenon; the putative roles of the amino acids are discussed.

Intracerebral human microdialysis. In vivo study of an acute focal ischemic model of the human brain

BACKGROUND AND PURPOSE

In vivo microdialysis was introduced in 1982 as a technique to study cerebral neurochemistry in awake, freely moving animals. In small animals, bilateral carotid occlusion produces a 7- to 10-fold increase in extracellular glutamate concentrations. This rapidly falls with reperfusion. Increase in extracellular glutamate is currently believed to be a major factor in initiating neuronal injury. Glutamate antagonists are currently undergoing clinical trials in acute stroke. Human data on the extracellular levels of glutamate and other amino acids in the normal or ischemic brain are limited. In this communication we wish to report the extracellular concentrations of glutamate, serine, glutamine, glycine, taurine, alanine, and gamma-aminobutyric acid, as monitored by in vivo microdialysis, in the simulated ischemic model of the temporal lobe of the human brain.

METHODS

Intracerebral microdialysis was carried out in five patients who underwent resection of the temporal lobe for intractable epilepsy. Surgical excision leads to an acute (from partial to total, ie, from incomplete to complete) ischemic state of the resected brain. This was our model to study the changes in human extracellular fluid during acute focal ischemic conditions.

RESULTS

Extracellular glutamate concentrations were 15 to 30 mumol/L in the preischemic samples. This increased to 380.69 +/- 42.14 mumol/L with partial (incomplete) ischemia and reached a peak of 1781.67 +/- 292.34 mumol/L (> 100-fold) with total isolation of the temporal pole (complete ischemia). The levels fell to 394.52 +/- 72.93 mumol/L 20 minutes after resection. Similar trends were observed with the onset of ischemia in the dialysate levels of serine, glutamine, glycine, alanine, taurine, and gamma-aminobutyric acid.

CONCLUSIONS

Our results show that there is a significant increase in extracellular glutamate and other neurotransmitters with ischemia in the temporal lobe model of the human brain. This increase is of a higher magnitude than that in small animals.

Ca(2+)- and Cl(-)-dependent, NMDA receptor-mediated neuronal death induced by depolarization in rat hippocampal organotypic cultures

The neurotoxicity induced by depolarization with high-K+ was investigated in rat hippocampal organotypic slice cultures. The exposure of cultures to 90 mM K+ solution for 30 min caused a severe neuronal injury in CA1 region while less damage was observed in CA3 and dentate gyrus over the following day. This neurotoxicity was prevented in a concentration dependent manner by NMDA antagonist MK-801 or CPP. Non-NMDA antagonist, DNQX, had no protective effect. Omission of Ca2+ from the exposure solution prevented the neurotoxicity. Voltage-dependent Ca2+ channel blockers, nifedipine and flunarizine, failed to prevent the neurotoxicity. These results suggest that the Ca2+ influx through the NMDA receptor is predominantly involved in this neurotoxicity. Apparent tissue swelling was observed immediately after the depolarization. This swelling was completely inhibited by omission of Cl- from the exposure solution, accompanied with complete protection against neurotoxicity. This suggests that Cl(-)-dependent tissue swelling also largely contributes to the neurotoxicity. Depolarization with application of MK-801 (10 microM) or omission of Ca2+ from the solution still caused apparent swelling, despite these treatment protected neuronal death. We hypothesize that Cl(-)-dependent tissue swelling may be involved in the release of the excitatory amino acid, which activates the NMDA receptor.

Labeling of metabolic pools by [6-14C]glucose during K(+)-induced stimulation of glucose utilization in rat brain

[6-14C]Glucose is the tracer sometimes recommended to assay cerebral glucose utilization (CMRglc) during transient or brief functional activations, but when used to study visual stimulation and seizures in other laboratories, it underestimated CMRglc. The metabolic fate of [6-14C]glucose during functional activation of cerebral metabolism is not known, and increased labeling of diffusible metabolites might explain underestimation of CMRglc and also reveal trafficking of metabolites. In the current studies cerebral cortex in conscious rats was unilaterally activated metabolically by KCl application, and CMRglc was determined in activated and contralateral control cortex with [6-14C]glucose or 2-[14C]deoxy-glucose ([14C]DG) over a 5- to 7-min interval. Local 14C concentrations were determined by quantitative autoradiography. Labeled precursor and products were measured bilaterally in paired cortical samples from funnel-frozen brains. Left-right differences in 14C contents were small with [6-14C]glucose but strikingly obvious in [14C]DG autoradiographs. CMRglc determined with [6-14C]glucose was slightly increased in activated cortex but 40-80% below values obtained with [14C]DG. [14C]Lactate was a major metabolite of [6-14C]glucose in activated but not control cortex and increased proportionately with unlabeled lactate. These results demonstrate significant loss of labeled products of [6-14C]glucose from metabolically activated brain tissue and indicate that [14C]DG is the preferred tracer even during brief functional activations of brain.

Hypoxic-ischemic injury in the neonatal rat brain: effects of pre- and post-treatment with the glutamate release inhibitor BW1003C87

In a model of perinatal hypoxia-ischemia (HI) we examined the neuroprotective efficacy of pre- and post-treatment with the glutamate release inhibitor BW1003C87 [5-(2,3,5-trichlorophenyl)-2,4-diamino-pyrimidine). Ipsilateral brain damage developed in 99% of rat pups subjected to HI (unilateral common carotid artery ligation and 100 min of 7.7% oxygen exposure) with a 26 +/- 16% (mean +/- S.D.) weight deficit of the damaged hemisphere 2 weeks after the insult. Pre-treatment with BW1003C87 (10 mg/kg intraperitoneally) reduced the brain damage by 46% (P < 0.05). A higher dose (20 mg/kg) of pre-treatment was not tolerated. Administration of BW1003C87 did not affect the rectal temperature of the rats. Post-treatment with BW1003C87 (10-30 mg/kg) offered no neuroprotection in this model. In conclusion, there was a neuroprotective effect from pre- but not post-treatment with BW1003C87 in this model, supporting the concept that intra-ischemic excitatory amino acid release is important for development of brain damage. The lack of post-treatment effect indicates that BW1003C87 did not attenuate deleterious EAA cycling during reflow in the neonatal brain.

Endogenous adenosine exerts inhibitory effects upon the development of spreading depression and glutamate release induced by microdialysis with high K+ in rat hippocampus

Spreading depression (SD) is known to be involved in the N-methyl-D-aspartate receptor-mediated neuronal damage. In urethane-anesthetized rats, we examined the release of adenosine and glutamate during SD induced by microdialysis of high K+ perfusate through the hippocampal CA1 area. The effects of endogenous adenosine upon SD were studied by applying an adenosine antagonist, theophylline (1 mM) and by a simultaneous application of adenosine uptake blockers, dipyridamole (DPR) (100 microM) and nitrobenzylthioinosine (NBI) (50 microM). The dialysates were sampled every 5 or 10 min and analyzed by HPLC. SD was identified by flattening of background EEg and disappearance of population spikes recorded from the pyramidal cell layer of CA1 area by a glass microelectrode. Adenosine and glutamate release was enhanced significantly in association with the occurrence of SD. Theophylline increased the release of glutamate and the incidence of SD and decreased the latency of the SD occurrence. DPR+NBI decreased the release of glutamate and the occurrence of SD, but increased extracellular adenosine concentration. The effects of DPR+NBI were blocked by application of a selective antagonist of adenosine A1 receptor, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microM). These findings suggest that endogenous adenosine exerts inhibitory influences upon the development of SD and the glutamate release through the A1 receptor in rat hippocampus.

Prevention of periinfarct direct current shifts with glutamate antagonist NBQX following occlusion of the middle cerebral artery in the rat

The effect of the glutamate (AMPA subtype) receptor antagonist NBQX on periinfarct direct current (DC) shifts and cortical ATP depletion volume was examined in rats subjected to 3 h of occlusion of the middle cerebral artery (MCA). MCA occlusion produced an immediate DC shift in the periphery of the ischemic territory. Vehicle-treated (untreated) animals developed one to five additional DC shifts (median, 2) during the 3-h occlusion time. NBQX treatment (2 x 30 mg/kg i.v. immediately after MCA occlusion and 1 h later) significantly reduced the number of DC deflections (median, 0; range, 0-2; p < 0.05) without changing blood flow in the border zone of the infarct (untreated, 50.6 +/- 10.6%; NBQX-treated: 51.9 +/- 7.7% of control; mean +/- SD). NBQX treatment significantly decreased the cortical volume of ATP depletion (untreated, 75.3 +/- 11.4 mm3; NBQX-treated, 47.9 +/- 10.1 mm3; p < 0.05). Moreover, a significant linear relationship between the number of periinfarct DC shifts and the volume of cortical ATP depletion was obtained (y = 38.3 + 9.4x; r = 0.866; p < 0.001). The reduction of brain infarct volume by NBQX treatment is explained by the suppression of DC shifts and the decrease of metabolic workload in hemodynamically compromised cortex.

Rapid postmortem changes in the cellular localisation of amino acid transmitters in the retina as assessed by immunocytochemistry

We have assessed by means of immunocytochemistry, the cellular distributions of the amino acid transmitters GABA, glycine and glutamate, and the free-radical scavenger taurine, in the retinae of adult rabbits at various times after death. Within 10 min of death, horizontal cells began to display immunoreactivity for GABA, whilst displaced amacrine cells began to display immunoreactivity for glycine. By 40 min postmortem, GABA was present in glial cells. Glutamate, which is not normally detectable in retinal glia, was detected in such glia by 20 min postmortem. By contrast immunocytochemically detectable glycine did not accumulate in glia. There was a gradual diminution of immunoreactivity for taurine in glial cells and photoreceptors. By 2 h postmortem, most immunoreactivity had disappeared from the retina. We conclude that amino acid transmitters show rapid changes in their distributions immediately after death, which may be related to changes in the patterns of transmitter release and uptake, and changes in degradation mechanisms. The rapid changes in cellular localisation of amino acid immunoreactivity illustrated in this study, indicate that the fixation of nervous tissues must be performed rapidly. Moreover, the massive loss of immunoreactivity by 2 h postmortem suggests that any assays for content of these transmitters at this, and subsequent time-points, will bear little resemblance to the values obtained at the time of death.

The early events of oxygen and glucose deprivation: setting the scene for neuronal death?

It is generally thought that neuronal death caused by a reduction in oxygen or glucose supply, or both, occurs as a result of massive increases in the extracellular concentrations of excitatory amino acid neurotransmitters, particularly glutamate. A pertinent question is what happens before this increase, because measures which prevent extracellular accumulation of glutamate could have potential for clinical use in, for example, management of acute stroke. This article will review the major pathophysiological responses which occur up until the time of accumulation of glutamate. Withdrawal of energy substrate quickly leads to modest changes in membrane potential and intracellular and extracellular ion concentrations. Depression of action-potential-dependent synaptic transmission occurs a little later and might, in part, reflect actions of adenosine. Increases in the extracellular concentration of excitatory amino acids to neurotoxic levels take place only as membrane potential falls rapidly towards 0mV, coincident with massive changes in ion gradients.

Involvement of calcitonin gene-related peptide (CGRP) and nitric oxide (NO) in the pial artery dilatation elicited by cortical spreading depression

The aim of the present study was to examine whether the initial transient arterial dilatation during cortical spreading depression (CSD) was mediated by the release of calcitonin gene-related peptide (CGRP) and/or nitric oxide (NO). This question is of interest as the initial phase of CSD appears to be a model of events occurring during functional hyperemia and during the first period of classic migraine. Using an open cranial window technique, pial arterial diameter in the parietal cortex of cats was recorded with an image splitting method. Employing micropuncture technique, perivascularly applied CGRP8-37 did not alter the resting diameter of pial arteries but antagonized concentration dependently (5 x 10(-9)-10(-6) M) the dilatation (35%) due to 5 x 10(-8) M CGRP. NG-Nitro-L-Arginine (NOLAG, 10(-4) M) also had no effect on resting diameter of pial arteries, indicating that their resting tone is neither mediated by a continuous release of CGRP nor of NO. CSD was triggered by a remote intracortical injection of KCl (150 mM) and recorded by a microelectrode placed adjacent to the artery under investigation. CSD elicited a transient negative DC shift which was accompanied by a peak dilatation of 44 +/- 5.2% (S.E.M.). This dilatation was reduced by approximately 50% during topical application of 10(-7) M CGRP8-37 and 10(-4) M NOLAG each. A 75% inhibition of the CSD-induced dilatation was found during simultaneous application of both compounds. These data indicate that the initial dilatation during CSD is mediated, at least in part, by a release of CGRP and NO.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate-mediated injury in focal cerebral ischemia: the excitotoxin hypothesis revised

Neuronal injury following focal cerebral ischemia is widely attributed to the excitotoxic effects of glutamate. However, critical analysis of published data on glutamate toxicity in vitro and the comparison of these data with in vivo release of glutamate and the therapeutic effect of glutamate antagonists raises doubts about a neurotoxic mechanism. An alternative explanation for glutamate-mediated injury is hypoxia due to peri-infarct spreading depression-like depolarizations. These depolarizations are triggered in the core of the ischemic infarct and spread at irregular intervals into the peri-infarct surrounding. In ischemically uncompromised tissue, the metabolic workload associated with spreading depression is coupled to an increase in blood flow and oxygen supply, assuring maintenance of oxidative respiration. In the penumbra region of focal ischemia, the hemodynamic constraints of collateral blood circulation prevail the adequate adjustment of oxygen delivery, leading to transient episodes of relative tissue hypoxia. The hypoxic episodes cause a suppression of protein synthesis, a gradual deterioration of energy metabolism and a progression of irreversibly damaged tissue into the penumbra zone. The generation of peri-infarct spreading depressions and the associated metabolic workload can be suppressed by NMDA and non-NMDA antagonists. As a result, the penumbral inhibition of protein synthesis and the progressing energy failure is also prevented, and the volume of ischemic infarct decreases. Interventions to improve ischemic resistance should therefore aim at improving the oxygen supply or reducing the metabolic workload in the penumbra region.