Temporal evolution of regional energy metabolism following focal cerebral ischemia in the rat

Metabolomic and Imaging Mass Spectrometric Assays of Labile Brain Metabolites: Critical Importance of Brain Harvest Procedures

Metabolomic technologies including imaging mass spectrometry (IMS; also called mass spectrometry imaging, MSI, or matrix-assisted laser desorption/ionization-mass spectrometry imaging, MALDI MSI) are important methods to evaluate levels of many compounds in brain with high spatial resolution, characterize metabolic phenotypes of brain disorders, and identify disease biomarkers. ATP is central to brain energetics, and reports of its heterogeneous distribution in brain and regional differences in ATP/ADP ratios reported in IMS studies conflict with earlier studies. These discordant data were, therefore, analyzed and compared with biochemical literature that used rigorous methods to preserve labile metabolites. Unequal, very low regional ATP levels and low ATP/ADP ratios are explained by rapid metabolism during postmortem ischemia. A critical aspect of any analysis of brain components is their stability during and after tissue harvest so measured concentrations closely approximate their physiological levels in vivo. Unfortunately, the requirement for inactivation of brain enzymes by freezing or heating is not widely recognized outside the neurochemistry discipline, and procedures that do not prevent postmortem autolysis, including decapitation, brain removal/dissection, and 'snap freezing' are commonly used. Strong emphasis is placed on use of supplementary approaches to calibrate metabolite abundance in units of concentration in IMS studies and comparison of IMS results with biochemical data obtained by different methods to help identify potential artifacts.

Spreading depolarization remarkably exacerbates ischemia-induced tissue acidosis in the young and aged rat brain

Spreading depolarizations (SDs) occur spontaneously in the cerebral cortex of subarachnoid hemorrhage, stroke or traumatic brain injury patients. Accumulating evidence prove that SDs exacerbate focal ischemic injury by converting zones of the viable but non-functional ischemic penumbra to the core region beyond rescue. Yet the SD-related mechanisms to mediate neurodegeneration remain poorly understood. Here we show in the cerebral cortex of isoflurane-anesthetized, young and old laboratory rats, that SDs propagating under ischemic penumbra-like conditions decrease intra and- extracellular tissue pH transiently to levels, which have been recognized to cause tissue damage. Further, tissue pH after the passage of each spontaneous SD event remains acidic for over 10 minutes. Finally, the recovery from SD-related tissue acidosis is hampered further by age. We propose that accumulating acid load is an effective mechanism for SD to cause delayed cell death in the ischemic nervous tissue, particularly in the aged brain.

The application of permanent middle cerebral artery ligation in the mouse

Focal cerebral ischemia is among the most common type of stroke seen in patients. Due to the clinical significance there has been a prolonged effort to develop suitable animal models to study the events that unfold during ischemic insult. These techniques include transient or permanent, focal or global ischemia models using many different animal models, with the most common being rodents. The permanent MCA ligation method which is also referred as pMCAo in the literature is used extensively as a focal ischemia model in rodents. This method was originally described for rats by Tamura et al. in 1981. In this protocol a craniotomy was used to access the MCA and the proximal regions were occluded by electrocoagulation. The infarcts involve mostly cortical and sometimes striatal regions depending on the location of the occlusion. This technique is now well established and used in many laboratories. Early use of this technique led to the definition and description of "infarct core" and "penumbra", and it is often used to evaluate potential neuroprotective compounds. Although the initial studies were performed in rats, permanent MCA ligation has been used successfully in mice with slight modifications. This model yields reproducible infarcts and increased post-survival rates. Approximately 80% of the ischemic strokes in humans happen in the MCA area and thus this is a very relevant model for stroke studies. Currently, there is a paucity of effective treatments available to stroke patients, and thus there is a need for good models to test potential pharmacological compounds and evaluate physiological outcomes. This method can also be used for studying intracellular hypoxia response mechanisms in vivo. Here, we present the MCA ligation surgery in a C57/BL6 mouse. We describe the pre-surgical preparation, MCA ligation surgery and 2,3,5 Triphenyltetrazolium chloride (TTC) staining for quantification of infarct volumes.

Rodent models of focal cerebral ischemia

This unit presents models that are both used to study ischemic mechanisms and to test for neuroprotective agents or agents that enhance recovery from stroke. The Tamura model is one of the best characterized focal ischemia models in which the middle cerebral artery is occluded by electrocoagulation. Also described is the intraluminal monofilament model, the spontaneously hypertensive rat (SHR), and the newer endothelin-1 model. The rationale behind the use of animal models, the various types of models and advantage and disadvantages of each model are presented.

Compromised metabolic recovery following spontaneous spreading depression in the penumbra

Spreading depression (SD) has been demonstrated following focal ischemia, and the additional workload imposed by SD on a tissue already compromised by a marked reduction in blood flow may contribute to the evolution of irreversible damage in the ischemic penumbra. SD was elicited in one group of rats by injecting KCl directly into a frontal craniectomy and the wave of depolarization was recorded in two craniectomies 3 and 6 mm posterior to the first one. In a second group, the middle cerebral artery was occluded using the monofilament technique and a recording electrode was placed 5 mm lateral to the midline and 0.2 mm posterior to bregma. To determine the metabolic response in the penumbral region of the cortex ipsilateral to the occlusion, brains from both groups were frozen in situ when the deflection of the SD was maximal. The spatial metabolic response of SD in the ischemic cortex was compared to that in the non-ischemic cortex. Coronal sections of the brains were lyophilized, pieces of the dorsolateral cortex were dissected and weighed, and analyzed for ATP, P-creatine, inorganic phosphate (Pi), glucose, glycogen and lactate at varying distances anterior and posterior to the recording electrode. ATP and P-creatine levels were significantly decreased at the wavefront in both groups and the levels recovered after passage of the wavefront in the normal brain, but not in the ischemic brain. Glucose and glycogen levels were significantly decreased and lactate levels significantly increased in the tissue after the passage of the wavefront. While the changes in the glucose-related metabolites persisted during recovery even in anterior portions of the cortex in both groups in the aftermath of the SD, the magnitude of the changes was greater in the penumbra than in the normal cortex. SD appears to impose an equivalent increase in energy demands in control and ischemic brain, but the ability of the penumbra to recover from the insult is compromised. Thus, increasing the energy imbalance in the penumbra after multiple SDs may hasten the deterioration of the energy status of the tissue and eventually contribute to terminal depolarization and cell death, particularly in the penumbra.

Type I glutaric aciduria, part 2: a model of acute striatal necrosis

Type I glutaric aciduria (GA1) is an inborn error of organic acid metabolism that is associated with acute neurological crises, typically precipitated by an infectious illness. The neurological crisis coincides with swelling, metabolic depression, and necrosis of basal ganglia gray matter, especially the putamina and can be visualized as focal, stroke-like, signal hyperintensity on MRI. Here we focus on the stroke-like nature of striatal necrosis and its similarity to brain injury that occurs in infants after hypoxia-ischemia or systemic intoxication with 3-nitropropionic acid (NPA). These conditions share several features including abrupt onset, preferential effect in the striatum and age-specific susceptibility. The pathophysiology of the conditions is reviewed and a model proposed herein. We encourage investigators to test this model in an appropriate experimental system.

Increased susceptibility of striatal mitochondria to calcium-induced permeability transition

Mitochondria were simultaneously isolated from striatum and cortex of adult rats and compared in functional assays for their sensitivity to calcium activation of the permeability transition. Striatal mitochondria showed an increased dose-dependent sensitivity to Ca2+ compared with cortical mitochondria, as measured by mitochondrial depolarization, swelling, Ca2+ uptake, reactive oxygen species production, and respiration. Ratios of ATP to ADP were lower in striatal mitochondria exposed to calcium despite equal amounts of ADP and ATP under respiring and nonrespiring conditions. The Ca2+-induced changes were inhibited by cyclosporin A or ADP. These responses are consistent with Ca2+ activation of both low and high permeability pathways constituting the mitochondrial permeability transition. In addition to the striatal supersensitivity to induction of the permeability transition, cyclosporin A inhibition was less potent in striatal mitochondria. Immunoblots indicated that striatal mitochondria contained more cyclophilin D than cortical mitochondria. Thus striatal mitochondria may be selectively vulnerable to the permeability transition. Subsequent mitochondrial dysfunction could contribute to the initial toxicity of striatal neurons in Huntington's disease.

Uncoupling protein-2 prevents neuronal death and diminishes brain dysfunction after stroke and brain trauma

Whereas uncoupling protein 1 (UCP-1) is clearly involved in thermogenesis, the role of UCP-2 is less clear. Using hybridization, cloning techniques and cDNA array analysis to identify inducible neuroprotective genes, we found that neuronal survival correlates with increased expression of Ucp2. In mice overexpressing human UCP-2, brain damage was diminished after experimental stroke and traumatic brain injury, and neurological recovery was enhanced. In cultured cortical neurons, UCP-2 reduced cell death and inhibited caspase-3 activation induced by oxygen and glucose deprivation. Mild mitochondrial uncoupling by 2,4-dinitrophenol (DNP) reduced neuronal death, and UCP-2 activity was enhanced by palmitic acid in isolated mitochondria. Also in isolated mitochondria, UCP-2 shifted the release of reactive oxygen species from the mitochondrial matrix to the extramitochondrial space. We propose that UCP-2 is an inducible protein that is neuroprotective by activating cellular redox signaling or by inducing mild mitochondrial uncoupling that prevents the release of apoptogenic proteins.

Evaluation of MCAO stroke models in normotensive rats: standardized neocortical infarction by the 3VO technique

The temporary three-vessel occlusion (3VO) technique with a surgical approach for middle cerebral artery (MCA) produces consistent cerebral infarction in the neocortex in normotensive rats. The intraluminal thread-occlusion technique with an endovascular approach targeting the MCA occlusion (MCAO) is more widely used since it does not require complicated intracranial procedures. The aim of this study was to review the methods/models for MCAO stroke in normotensive rats and to evaluate a 3VO stroke model that provides consistent degrees and variance of cortical stroke injury for additional discussion. First, we analyzed a model with modified temporary 3VO technique requiring less complicated procedures than the temporary 3VO model, i.e., temporary occlusion of the bilateral common carotid arteries (CCAs) superimposed on a permanent occlusion of the MCA, in Sprague-Dawley rats or C57BL/6J mice. In the microvascular tissue (cerebral) perfusion study, significant reductions in regional cerebral perfusion during the 3VO accompanied a rapid return to baseline after release of the CCAs, showing that the technique induces temporary focal ischemia. The average sizes and variances of the neocortical infarction in this model, together with those in the other normotensive rat models caused by the 3VO technique in the literature, indicated a standard size and variance of infarcted lesion in the control groups relative to the specific ischemic period. However, stroke injuries in the neocortex induced by the thread occlusion technique showed greater variability with less consistent lesion sizes. Inclusion/exclusion criteria to avoid inappropriate cases with too mild (no/faint infarction) or too great (huge/fatal infarction) severity in the ischemic injury may differ between laboratories in the thread occlusion model.

Direct visualization of trapped erythrocytes in rat brain after focal ischemia and reperfusion

Partial microcirculatory stasis after cerebral ischemia and reperfusion is a potential factor in delayed cell death. Sometimes described as the "no-reflow" phenomenon, limitations in current detection techniques have left the extent and spatial distribution of the phenomenon undetermined, which has led to some doubt as to its actual existence. The authors describe a new method, based on erythrocyte autofluorescence, that allows the erythrocytes trapped in the microvasculature, and thus blocking recirculation, to be directly visualized. Using this method, the authors have examined the spatial and temporal characteristics of this phenomenon in the rat intraluminal model of cerebral ischemia and reperfusion. Up to 15% of the capillaries in the ischemic penumbra remained occluded at least 2 hours after reperfusion. The amount of capillary bed showing trapped erythrocytes was more severe in the ischemic penumbra region than in the ischemic core. These results indicate that the no-reflow phenomenon may contribute to the developing damage in ischemic penumbra region, leading to additional injury after reperfusion.

1H magnetic resonance spectroscopic imaging of permanent focal cerebral ischemia in rat: longitudinal metabolic changes in ischemic core and rim

The purpose of this study was to determine whether regional differences in metabolites can be seen chronologically in permanent focal cerebral ischemia using 1H magnetic resonance spectroscopic imaging (MRSI), and whether these changes reflect pathological outcome. Regional variation in metabolites after permanent focal ischemia were investigated longitudinally in rats using 1H MRSI for a total of 7 days and then compared to histopathological findings. Four hours after the induction of ischemia, N-acetyl-L-aspartate (NAA) levels in the lateral caudo-putamen and the somatosensory cortex, core ischemic regions, decreased 22 and 40%, respectively. This reduction in NAA was coupled with a marked rise in lactate. In the medial caudo-putamen, the ischemic rim, however, NAA was preserved in spite of a marked increase in lactate. By 24 h post ischemia, the levels of NAA in medial caudo-putamen (ischemic rim in caudate) also decreased significantly. However NAA in cingulated cortex (ischemic rim in cortex) decreased more gradually between 24 and 48 h. This regional difference can reflect the severity of metabolic derangement in the acute stage. After 96 h following ischemia, the levels of all metabolites detected by 1H MRSI had decreased and the levels of NAA decline reflected the severity of histopathological damage. In conclusion, the regional metabolic differences could be assessed by 1H MRSI chronologically, and the depth of NAA decline reflected histopathological changes in the chronic stage.

Hyperbaric oxygen reduces infarct volume in rats by increasing oxygen supply to the ischemic periphery

OBJECTIVE

Hyperbaric oxygen (HBO) increases oxygen supply to anoxic areas. To examine the therapeutic effect of HBO on ischemic stroke, we measured infarct volume as well as cerebral blood flow (CBF), oxygen supply, and lipid peroxidation in the ischemic periphery.

DESIGN

Prospective experimental study in rats.

SETTING

Experimental laboratory in a university teaching hospital.

SUBJECTS

Thirty-eight adult rats.

INTERVENTION

The rats were anesthetized (1% halothane) and intubated. Focal ischemia was induced by ligating the right middle cerebral and right common carotid arteries. Nineteen animals were exposed to 2 hrs of HBO (100% oxygen, 3 atmospheres absolute), initiated 10 mins after the onset of ischemia. The remaining animals were kept at ambient pressure and used as controls.

MEASUREMENTS AND MAIN RESULTS

At the initiation of ischemia, CBF measured by a laser-Doppler flow probe placed in the ischemic periphery was reduced to 47%+/-11% and 51%+/-15% of normal levels in animals exposed or not to HBO, respectively. These altered values were not affected further by administration of HBO and remained stable throughout a 2-hr observation period. Arterial oxygen pressure and content were significantly increased to 1571+/-130 torr (209.41+/-17.32 kPa; p < .0001) and 1.03+/-0.04 mmol/dL (p < 0.0001), respectively, in HBO-treated animals compared with nontreated animals (139+/-14 torr [18.53+/-1.87 kPa] and 0.86+/-0.04 mmol/dL, respectively). The calculated increase in the oxygen supply to the ischemic periphery was 20%. The infarct volume of HBO-treated animals measured 24 hrs after the onset of focal cerebral ischemia was significantly reduced by 18% (HBO-treated, 132+/-13 mm3 vs. nontreated, 161+/-29 mm3; p = .02). Lipid peroxidation was unchanged after 120 mins of HBO administration in the cerebral cortex where the laser-Doppler flow probe was placed.

CONCLUSIONS

HBO at 3 atmospheres absolute reduced infarct volume by increasing oxygen supply to the ischemic periphery without aggravating lipid peroxidation, suggesting that HBO can be useful in treating stroke victims.

Comparative effect of flunarizine in two animal models of stroke

1. A common 'stroke model', the rat middle cerebral artery occlusion preparation, was used to compare photocoagulation with mechanical occlusion. 2. Areas of cerebral infarction and of vascular leakage were compared using these two methods. The general pattern of damage was similar, with central infarcted areas surrounded by larger vascular leakage areas. 3. The two models differed in the extent of their vascular leakage areas and in their response to the cerebroprotective agent flunarizine. 4. We conclude that the two models are not equivalent, probably because they do not damage the vascular endothelium to the same degree.

Focal cerebral ischaemia induces a decrease in activity and a shift in ouabain affinity of Na+, K+-ATPase isoforms without modifications in mRNA and protein expression

In a mouse model of focal cerebral ischaemia, we observed after 1 h of ischaemia, that the total Na+, K+-ATPase activity was decreased by 39.4%, and then did not vary significantly up to 6 h post-occlusion. In the sham group, the dose-response curves for ouabain disclosed three inhibitory sites of low (LA), high (HA) and very high (VHA) affinity. In ischaemic animals, we detected the presence of only two inhibitory sites for ouabain. After 1 h of permanent occlusion, the first site exhibited a low affinity while the second site presented an affinity intermediate between those of HA and VHA sites, which evolved after 3 h and 6 h of occlusion towards that of the VHA site. The presence of only two ouabain sites for Na+, K+-ATPase after ischaemia could result from a change in ouabain affinity of both HA and VHA sites (alpha2 and alpha3 isoforms, respectively) to form a unique component. Irrespective of the duration of ischaemia, the smaller activity of this second site accounted entirely for the loss in total activity. Surprisingly, no modifications in protein and mRNA expression of any alpha or beta isoforms of the enzyme were observed, thus suggesting that ischaemia could induce intrinsic modifications of the Na+, K+-ATPase.

Neuroprotective effect of YM-39558 in focal cerebral ischemia in cats

We studied the effect of YM-39558, orotic acid ethylester, in a focal cerebral ischemia model in anesthetized cats. YM-39558 has good permeability across the blood brain barrier, and in the brain is hydrolyzed to orotic acid, the main active substance. Cats were subjected to permanent occlusion of the middle cerebral artery (MCA) for 6 h, then killed and examined histologically. Treatment with YM-39558 (intravenous infusion of 11.8 mg (10 mg as orotic acid)/6 ml per kg per h) starting 15 min after MCA occlusion markedly reduced the volume of ischemic damage (from 2450 +/- 82 mm3 of the cerebral hemisphere in the saline-treated cats to 1644 +/- 123 mm3 in the YM-39558-treated cats, P < 0.01). In contrast, YM-39558 (2.26 and 1.18 mg/0.8 ml per kg per h) showed no significant protective effect on ischemic damage. No significant differences were observed between saline- and YM-39558-treated cats concerning physiological variables including brain temperature. This evidence for the neuroprotective efficacy of YM-39558 in gyrencephalic species suggests its therapeutic potential in the treatment of stroke in humans.

Progressive impairment of brain oxidative metabolism reversed by reperfusion following middle cerebral artery occlusion in anaesthetized baboons

A better understanding of the temporospatial evolution of ischaemic brain tissue towards necrosis would be of crucial value to establish and validate therapeutic strategies for stroke in man. By means of sequential positron emission tomographic (PET) studies performed through the acute to the chronic stages of infarction, we addressed the question of the effect of 6 h temporary occlusion of the middle cerebral artery (MCAO) on the evolution of the volume of severely hypometabolic tissue in anaesthetized baboons and compared it to that reported previously in permanently occluded baboons. Thirteen anaesthetized baboons underwent serial PET (15O steady-state technique) examinations before and 1, 4, 7, 24-48 h and 15-62 days following transorbital MCAO. Reperfusion, at 6 h post-occlusion, was assessed by Doppler sonography and cerebral blood flow (CBF) values after clip removal. In each baboon, the infarct volume was calculated by standard histological procedures 20-91 days after MCAO. The severely hypometabolic tissue volume, as defined by a threshold of oxidative metabolism, showed a progressive increase for up to 24-48 h in a not dissimilar manner to that found in baboons with permanent occlusion. However, these hypometabolic volumes regressed in the chronic stage (p < 0.05). Permanent and temporary occluded baboons, when taken together, showed a highly significant relationship between histological and chronic hypometabolic volumes (r = 0.84; p < 0.001). Moreover, the final hypometabolic volume where cerebral metabolic rate of oxygen (CMRO2) was below 40% of contralateral metabolism corresponded well to that of histological infarction volume. We conclude that, in anaesthetized baboons, restoration of blood flow will reverse (even if not immediately) the progressive derangement of metabolism after MCAO and markedly limit the final volume of consolidated infarction.)

Characterizing the target of acute stroke therapy

BACKGROUND

The development of effective therapies for acute ischemic stroke presumes the existence of potentially salvageable ischemic tissue when therapy is initiated because it is widely assumed that the effectiveness of most acute stroke therapies under development is related to reducing ultimate infarct size to promote functional improvement. Such salvageable ischemic tissue was previously labeled the ischemic penumbra and must be distinguished from irreversible injury.

SUMMARY OF REVIEW

Pathological identification of irreversibility (infarction) appears to lag behind the actual development of this condition, and reversible injury after focal ischemia should be differentiated from infarction. Imaging and biochemical markers apparently can provide clues for distinguishing potentially salvageable from irreversibly injured ischemic tissue in experimental and clinical stroke. Recent positron emission tomography and MRI studies suggest that these clinically available imaging technologies will be useful for determining the presence of ischemic penumbra in individual stroke patients. The progression from potentially reversible to irreversible injury after focal brain ischemia has many potential mechanisms that may be synergistic and vary among individuals.

CONCLUSIONS

Delineating and prioritizing these mechanisms provides the opportunity to develop multiple potential acute stroke therapies that ultimately will be used in combination, perhaps directed by imaging technology.

Effects of LY231617 and angiotensin IV on ischemia-induced deficits in circular water maze and passive avoidance performance in rats

The antioxidant LY231617 has previously been shown to offer significant protection against postischemic cell death in the hippocampus and corpus striatum of rats. The present results extend this observation by demonstrating a concomitant protection against the spatial memory deficits that accompany damage to the hippocampus, as measured by the circular water maze task. These animals were further tested for changes in associative memory by employing a passive avoidance conditioning task. No deficits in passive avoidance conditioning were measured among the 4-vessel occlusion animals treated with LY231617 or vehicle. However, the intracerebroventricular injection of angiotensin IV (Ang IV) immediately prior to foot-shock conditioning improved retention of the conditioned response during the subsequent 2-day period. These results suggest that LY231617 can offer considerable protection against global ischemia-induced cell death in the hippocampus with resulting preservation of spatial memory abilities. In addition, untreated animals that suffered cell losses in the hippocampus remained capable of responding to the facilitory effect of centrally administered Ang IV on a non-spatial memory task. The hypothesized mechanisms of the protection characteristics of LY231617, and the nootropic effect of Ang IV, are discussed.

Changes in levels of monoamines and their metabolites in incompletely ischemic brains of spontaneously hypertensive rats

In order to investigate changes in levels of monoamines and their related substances together with those of other neurotransmitters (acetylcholine and GABA), choline and substances related to energy metabolism (ATP, lactate and glucose) accompanying incomplete cerebral ischemia, a bilateral common carotid artery occlusion model of spontaneously hypertensive rats (SHR) was utilized. Animals were subjected to 1 or 2 h ischemia. Then the concentrations of substances were measured in the cerebral cortex, hippocampus and striatum and compared with control values. Due to the incomplete ischemia, ATP showed a moderate decrease, while lactate and choline increased remarkably, and GABA underwent a moderate increase. With regard to monoamines, both noradrenaline and serotonin levels were reduced in the cerebral cortex and hippocampus, whereas dopamine levels increased in the hippocampus. All monoamine metabolites, i.e. metabolites by monoamine oxidase (MAO), metabolites by catechol-O-methyltransferase (COMT), and metabolites by both MAO and COMT, underwent increases. The 3-methoxytyramine level in particular showed marked increases. Furthermore levels of precursor amino acids as well as 5-hydroxytryptophan rose. Acetylcholine decreased moderately only in the cerebral cortex. Among these changes, sustained increases in all the monoamine metabolites were characteristic of changes in the incompletely ischemic brain, suggesting that both COMT and MAO retain their activities in the incompletely ischemic brain.

L-arginine ameliorates cerebral blood flow and metabolism and decreases infarct volume in rats with cerebral ischemia

Effects of L-arginine, 300 mg/kg, i.p., on the regional cerebral blood flow (rCBF), brain metabolism, and infarct volume were examined in spontaneously hypertensive rats subjected to occlusion of both left middle cerebral artery and left common carotid artery. Rats treated with L-arginine had higher rCBF, determined by hydrogen clearance method, in the ischemic core (7 +/- 1 ml/100 g/min, mean +/- S.E.M.) and penumbral regions (16 +/- 2) than did rats treated with saline (5 +/- 0 and 7 +/- 1, respectively). Simultaneously, L-arginine attenuated metabolic derangement in the ischemic tissue at 60 min, i.e. well maintained adenosine triphosphate (ATP) in ischemic region (1.29 +/- 0.07 mmol/kg in L-arginine group vs. 1.05 +/- 0.06 in saline group), and also close to normal levels in ATP (2.61 +/- 0.02 mmol/kg vs. 2.45 +/- 0.05), glucose (2.29 +/- 0.12 mmol/kg vs. 1.80 +/- 0.17) and lactate (1.63 +/- 0.10 mmol/kg vs. 2.24 +/- 0.21) in periischemic region. In another experiment, the effects of L-arginine on rCBF in the subcortical regions and on infarct volume were evaluated. L-arginine, compared with saline, increased rCBF by 8 ml/100 g/min in the ischemic side and reduced infarct volume by 29% at 24 h of ischemia. These findings support that L-arginine may be potentially useful for the treatment of acute cerebral ischemia.

Regulation of FFA by the acyltransferase pathway in focal cerebral ischemia-reperfusion

Cerebral insult is associated with a rapid increase in free fatty acids (FFA) and arachidonic acid release has been linked to the increase in eicosanoid biosynthesis. In transient focal cerebral ischemia induced by middle cerebral artery (MCA) occlusion, there is an inverse relationship between the increase in FFA and the decrease in ATP, both during the ischemia period and at later time periods after reperfusion. In this study, the focal cerebral ischemia model was used to examine incorporation of [14C]arachidonic acid into the glycerolipids in rat MCA cortex at different reperfusion times after a 60 min ischemia. The label was injected intracerebrally into left and right MCA cortex 1 hr prior to decapitation. Labeled arachidonic acid was incorporated into phosphatidylcholine, phosphatidylethanolamine and neutral glycerides. With increasing time (4-16 hr) after a 60 min ischemia, an inhibition of labeled arachidonate uptake could be found in the right ischemic MCA cortex, whereas the distribution of radioactivity among the major phospholipids was not altered. When compared to labeled PC, there was a 3-4 fold increase in incorporation of label into phosphatidic acid and triacylglycerols (TG) in the right MCA cortex, suggesting of an increase in de novo biosynthesis of TG. In an in vitro assay system, synaptosomal membranes isolated from MCA cortex 8 and 16 hr after a 60 min ischemia showed a significant decrease in arachidonoyl transfer to lysophospholipids, due mainly to a decrease in lysophospholipid:acylCoA acyltransferase activity. Assay of phospholipase A2 activity with both syaptosomes and cytosol, however, did not show differences between left and right MCA cortex or with time after reperfusion. These results suggest that besides ATP availability, the decrease in acyltransferase activity may also contribute to the increase in FFA in cerebral ischemia-reperfusion.

Inositol trisphosphate, polyphosphoinositide turnover, and high-energy metabolites in focal cerebral ischemia and reperfusion

BACKGROUND AND PURPOSE

Although the signaling pathway involving polyphosphoinositide (poly-PI) hydrolysis and release of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is an important mechanism for regulation of neuronal calcium homeostasis, the effect of cerebral ischemia-reperfusion on this calcium signaling pathway is not well understood. Because activity of this pathway is dependent on availability of ATP, this study is aimed at examining the poly-PI signaling pathway and high-energy metabolites in a rat stroke model.

METHODS

Focal cerebral ischemia in rats was induced by temporary occlusion of the right middle cerebral artery and both common carotid arteries. Levels of Ins(1,4,5)P3 were determined by use of the radioreceptor binding assay. Poly-PI turnover in rat cortex was assessed with an in vivo protocol involving intracerebral injection of [3H] inositol and systemic administration of lithium. High-energy metabolites (ATP, ADP, and AMP) were analyzed by high-performance liquid chromatography.

RESULTS

Ischemia induced an increase in poly-PI turnover in the right middle cerebral artery cortex, but reperfusion led to a decline in this signaling activity. However, Ins(1,4,5)P3 levels decreased during ischemia, and these levels were not restored if ischemic insults were longer than 30 minutes. ATP levels decreased to 26% of control during ischemia and recovered to 80% of control during the initial 4 hours of reperfusion; these changes were followed by a second phase of decline.

CONCLUSIONS

Results show an important relationship between ischemia-induced depletion of high-energy metabolites and poly-PI signaling activity. However, the uncoupling between Ins(1,4,5)P3 and ATP during reperfusion after severe ischemia suggests that metabolism of Ins(1,4,5)P3 is more stringently regulated than ATP.

Stability of thrombosis induced by electrocoagulation of rat middle cerebral artery

BACKGROUND AND PURPOSE

Although it is often assumed in experimental stroke studies that cautery-induced occlusion is permanent, surgeons commonly expect cauterized vessels to recanalize spontaneously. We used the rat middle cerebral artery to determine if electrocoagulation would produce a permanent occlusion in this preparation.

METHODS AND RESULTS

A standard bipolar coagulator, calibrated to determine actual power output, was adjusted to induce platelet aggregation in the middle cerebral artery of anesthetized Sprague-Dawley rats without inducing bleeding through the arterial wall. A reliable temporary thrombosis was induced by a Malis Bipolar Coagulator set to deliver 10 bursts of 1.5 seconds each at a rate of 24 min-1 and a power setting of 3 W. This thrombus was responsive to the antithrombotic agent flunarizine. An apparently permanent occlusion was produced by 30 bursts at 3 W followed by 20 bursts at 5 W. To our surprise, seven of seven such occlusions recanalized spontaneously within 4 hours.

CONCLUSIONS

The electrocoagulation process commonly used in experimental stroke studies may produce only a temporary occlusion of the rat middle cerebral artery.

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.

Temporary vessel occlusion in spontaneously hypertensive and normotensive rats. Effect of single and multiple episodes on tissue metabolism and volume of infarction

Temporary occlusion of an intracranial artery is frequently necessary in the surgical management of intracranial aneurysms, arteriovenous malformations, and tumors. While the risks of vessel damage associated with clip application have been lessened by improved design, the threat of ischemic damage remains. It is unclear whether multiple, brief periods of clip application are more or less safe than a single period of occlusion, and whether the underlying cerebrovascular status influences the outcome from either method. The effect of each of these paradigms (single: 1-hour occlusion; multiple: three 20-minute episodes separated by 10 minutes of reperfusion) on histopathological outcome was assessed in a middle cerebral artery (MCA) occlusion model using both normotensive and spontaneously hypertensive rats. The mean volume of infarction (+/- standard error of the mean) was not different between the single-ischemic (49.4 +/- 17.3 cu mm) and the multiple-ischemic (42.9 +/- 12.9 cu mm) episode groups of normotensive rats, whereas in the spontaneously hypertensive rats a significant difference existed between the volume of infarction for the single-occlusion group (126.7 +/- 18.7 cu mm) and the multiple-occlusion group (162.4 +/- 15.5 cu mm) (p < 0.05). The metabolic data obtained from spontaneously hypertensive animals did not provide an explanation for the larger infarction in that there were no significant differences between the single- and multiple-occlusion groups with respect to tissue glucose, adenosine triphosphate, or lactate levels. The results suggest that intermittent reperfusion may have different effects depending not only on the degree and duration of ischemia and reperfusion, but also on the underlying cerebrovascular status.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Ischemic tolerance and extracellular amino acid concentrations in gerbil hippocampus measured by intracerebral microdialysis

Preconditioning of the brain with sublethal ischemia induces tolerance to subsequent longer periods of ischemia. To elucidate the role of excitatory and inhibitory amino acids in the induction of ischemic tolerance, we measured the extracellular concentrations of the amino acids in the gerbil hippocampus with intracerebral microdialysis. Mongolian gerbils were subjected to 3 min of forebrain ischemia 4 days after preconditioning with 2 min of ischemia or sham operation. Microdialysis probes were implanted into the hippocampus before the second ischemia and the amino acid concentrations in the dialysates were measured with HPLC. During and immediately after 3 min of ischemia without preconditioning, the concentrations of glutamate, glycine, gamma-aminobutyric acid, and taurine, but not glutamine, increased significantly. The increased amino acid levels rapidly returned to baseline after reperfusion. Preconditioning of the brain did not alter the amount of any amino acid released during and after the second ischemia. The excitotoxic index also unchanged in the preconditioned hippocampus. Thus, the results clearly show that ischemic tolerance is not induced through the alteration of the amounts of excitatory and inhibitory amino acids released during subsequent ischemia.

Effects of repeated cerebral ischemia on extracellular amino acid concentrations measured with intracerebral microdialysis in the gerbil hippocampus

BACKGROUND AND PURPOSE

To clarify the role of elevated extracellular amino acid concentrations during ischemia on the cumulative neuronal damage after repeated cerebral ischemic insults, using a microdialysis technique we measured concentrations of the amino acids glutamate, glutamine, glycine, taurine, and gamma-aminobutyric acid in the gerbil hippocampus over three 2-minute forebrain ischemic insults induced at 1-hour intervals.

METHODS

Under light anesthesia, the bilateral common carotid arteries were occluded with aneurysm clips at 1-hour intervals. Samples were collected by microdialysis at 10-minute intervals, and the amino acid concentrations were determined using a high-performance liquid chromatography system.

RESULTS

During and immediately after the first ischemic insult, concentrations of glutamate, glycine, and taurine, but not glutamine, increased significantly. Glutamate and taurine concentrations rose again during the second and third ischemic insults, but the increases were smaller than those during the first insult. By contrast, glutamine concentrations increased slightly but significantly during the second and third ischemic insults. The extracellular concentration of gamma-aminobutyric acid before the ischemic insults was below the level of detectability but increased markedly during each ischemic insult, with similar declines in the amounts released during later insults. Concentrations of all amino acids returned to baseline after 10 minutes of reperfusion and remained at baseline until the subsequent ischemic insult was induced.

CONCLUSIONS

It is well established that glutamate released during ischemia plays a crucial role in ischemia-induced neuronal death. However, the present results indicate that cumulative neuronal damage following sublethal ischemic insults is not caused by an exaggerated release of excitatory amino acids during subsequent ischemic insults but strongly suggest that increased intracellular reactions leading to cell death play a major role.

Protective effects of serotonin reuptake inhibitors, citalopram and clomipramine, against hippocampal CA1 neuronal damage following transient ischemia in the gerbil

To clarify the role of serotonin in cerebral ischemia, we examined the effects of selective serotonin reuptake inhibitors, citalopram and clomipramine, on ischemic neuronal damage in the gerbil. Pretreatment with citalopram (40 mg/kg i.p.) and clomipramine (20 mg/kg i.p.) protected against neuronal destruction of hippocampal CA1 pyramidal cells following 5 min of forebrain ischemia. Furthermore, microdialysis assays showed that a striking increase in extracellular excitatory amino acid levels during ischemia was significantly inhibited by pretreatment with citalopram and clomipramine. However, citalopram (40 mg/kg i.p.) did not alter the extracellular amino acid concentrations in normal gerbils. Thus, serotonin reuptake inhibitors have a protective effect against ischemic neuronal damage. Furthermore, the present result suggests that the protective effect is mediated through prevention of the accumulation of extracellular excitatory amino acids during and after ischemia.

Effects of pretreatment with sublethal ischemia on the extracellular glutamate concentrations during secondary ischemia in the gerbil hippocampus evaluated with intracerebral microdialysis

To elucidate the role of glutamate in the pathogenesis of altered neuronal vulnerability following sublethal ischemia, we measured the extracellular concentrations of glutamate in the gerbil hippocampus using a microdialysis technique. During and immediately after 3-min forebrain ischemia, the extracellular glutamate level showed a striking increase. However, pretreatment with 2-min ischemia 1 h or 4 days before secondary 3-min ischemia did not alter the amount of glutamate released. The result indicates that cumulative damage and tolerance, which take place after pretreatment with sublethal ischemia, are not induced through modification of the amount of glutamate released during secondary ischemia.

Hyperglycemic versus normoglycemic stroke: topography of brain metabolites, intracellular pH, and infarct size

Hyperglycemia aggravates brain pathologic outcome following middle cerebral artery (MCA) occlusion in cats. We presently determined if hyperglycemia during occlusion leads to high lactic acid accumulations in the ischemic MCA territory. We measured brain metabolite concentrations in 14 MCA territory sites at 0.5 and 4 h following occlusion in hyper- (20 mM) and normoglycemic (5 mM) cats and correlated these results with previous brain pathologic findings. Hyper- versus normoglycemia during MCA occlusion resulted in significantly higher lactate concentrations in the ischemic territory and more numerous loci with lactates greater than 17 mumol/g. At 0.5 h of occlusion, ATP levels were lower in normoglycemic cats, while at 4 h, ATP was similarly reduced (40%) in both glycemia groups. At 4 h, PCr was more reduced in hyperglycemics secondary to a greater brain tissue acidosis. Carbohydrate substrates at 0.5 h were more markedly depleted in normoglycemics, likely limiting lactate accumulation (34.3% versus only 5.0% of sites in hyperglycemics with glucose less than 0.5 mumol/g). Although lactate was markedly elevated at both 0.5 and 4 h in hyperglycemic ischemic territories, clip release at 4 versus 0.5 h yields a significantly poorer brain pathologic outcome. Correspondingly, intracellular pH, calculated from the creatine kinase equilibrium, was more markedly depressed at 4 than at 0.5 h of occlusion, demonstrating a time-dependent dissociation between tissue lactate and hydrogen ion accumulations. The present findings show that following MCA occlusion (a) hyperglycemia increases the magnitude and topographic extent of marked tissue lactic acidosis, (b) infarct size following 0.5 h of clip release correlates more closely with tissue acidosis than with lactate concentrations, (c) ischemic tissue ATP concentrations correlate poorly with infarct size, (d) normoglycemia limits lactate accumulation during focal ischemia because tissue glucose is depleted, and (e) early during ischemia, tissue buffering or antiport mechanisms may prevent marked increases in intracellular hydrogen ion activity.

Focal and perifocal changes in tissue energy state during middle cerebral artery occlusion in normo- and hyperglycemic rats

The objective of the present study was to assess changes in cellular energy metabolism in focal and perifocal areas of a stroke lesion and to explore how these changes are modulated by preischemic hyperglycemia. A model for reversible occlusion of the middle cerebral artery (MCA) in rats was used to study changes in energy metabolism. Following MCA occlusion for 5, 15, or 30 min in normoglycemic rats, the tissue was frozen in situ, and samples from the lateral caudoputamen and from two neocortical areas were collected for metabolite analyses, together with a control sample from the contralateral, nonischemic hemisphere. Two other groups, subjected to 30 min of MCA occlusion, were made hyperglycemic by acute glucose infusion or by prior injection of streptozotocin. Enzymatic techniques were used for measurements of phosphocreatine, creatine, ATP, ADP, AMP, glycogen, glucose, pyruvate, and lactate. The neocortex of the contralateral, nonischemic hemisphere had labile metabolites that were similar to those measured in control animals. Ipsilateral neocortex bordering the focus, and thus constituting the "penumbra," showed mild to moderate ischemic changes. In the "focus" (lateral caudoputamen plus the overlying neocortex), deterioration of energy state was rapid and relatively extensive (ATP content 20-40% of control). After 5 min of occlusion, no further deterioration of metabolic parameters was observed. Substrate levels were markedly reduced, and lactate content rose to approximately 10 mM kg-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemic thresholds of cerebral protein synthesis and energy state following middle cerebral artery occlusion in rat

The ischemic threshold of protein synthesis and energy state was determined 1, 6, and 12 h after middle cerebral artery (MCA) occlusion in rats. Local blood flow and amino acid incorporation were measured by double tracer autoradiography, and local ATP content by substrate-induced bioluminescence. The various images were evaluated at the striatal level in cerebral cortex by scanning with a microdensitometer with 75 microns resolution. Each 75 x 75 microns digitized image pixel was then converted into the appropriate units of either protein synthesis, ATP content, or blood flow. The ischemic threshold was defined as the flow rate at which 50% of pixels exhibited complete metabolic suppression. One hour after MCA occlusion, the threshold of protein synthesis was 55.3 +/- 12.0 ml 100 g-1 min-1 and that of energy failure was 18.5 +/- 9.8 ml 100 g-1 min-1. After 6 and 12 h of MCA occlusion, the threshold of protein synthesis did not change (52.0 +/- 9.6 and 56.0 +/- 6.5 ml 100 g-1 min-1, respectively) but the threshold of energy failure increased significantly at 12 h following MCA occlusion to 31.9 +/- 9.7 ml 100 g-1 min-1 (p less than 0.05 compared to 1 h ATP threshold value; all values are mean +/- SD). In focal cerebral ischemia, therefore, the threshold of energy failure gradually approached that of protein synthesis. Our results suggest that with increasing duration of ischemia, survival of brain tissue is determined by the high threshold of persisting inhibition of protein synthesis and not by the much lower one of acute energy failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Transhemispheric diaschisis. A review and comment

We review here the literature in both animal models and humans concerning electrical activity, blood flow, and metabolism in the hemisphere contralateral to unilateral cerebral ischemia. We analyze the data by periods based on the time from initial injury to emphasize the time course of transhemispheric diaschisis. Contralateral electrical activity, such as evoked potential amplitude, is increased in the late stages after unilateral infarction, with the data from the more acute periods being inconclusive. Contralateral blood flow changes probably depend on the magnitude of the ischemic injury, with a larger insult resulting in a decrease not seen with smaller insults. Some studies have shown a decrease in contralateral blood flow over the first week followed by a gradual return toward baseline. Most measures of contralateral metabolism show a time course similar to blood flow, that is, a decrease followed by gradual recovery. The effects of corpus callosum section on transhemispheric diaschisis are not yet established. We provide examples to show that under certain conditions, diaschisis may represent a loss of remote inhibition rather than a loss of remote facilitation, as von Monakow originally suggested. By following the contralateral changes over time, particularly during the first minutes and hours of ischemia, insight will be gained into the brain's responses remote from the focus of ischemic injury. These responses should bear a relation to the brain's defense mechanisms ipsilaterally to the region of ischemia.

NADH fluorescence and regional energy metabolites during focal ischemia and reperfusion of rat brain

Transient focal ischemia was produced in rat brain using simultaneous, reversible occlusion of the middle cerebral artery (MCA) and both carotid arteries. NADH tissue fluorescence and regional levels of ATP and lactate were measured after occlusion for 1 or 2.5 h and after reperfusion for 1 or 24 h following a 2.5-h insult. Occlusion for 1 or 2.5 h caused a marked but microheterogenous increase in NADH fluorescence, which was restricted to the MCA territory of the ipsilateral cortex. In this ischemic core, tissue levels of ATP were nearly depleted, while lactate accumulated to 10-13 mmol/kg. Metabolic alterations were less pronounced in regions adjacent to the ischemic core; however, one border region experienced a progressive increase in lactate between 1 and 2.5 h. NADH fluorescence and metabolite levels were not significantly altered in subcortical structures. In animals reperfused after a 2.5-h insult, NADH fluorescence diminished in the ischemic core to abnormally low levels, ATP was restored only to 37-50% of control, and lactate remained elevated. By 24 h, histologic infarction was evident in the regions with metabolic impairment. These results indicate that focal depletion of energy metabolites for 2.5 h caused irreversible impairment of energy metabolism and focal infarction even though lactate accumulation was moderate.

Experimental and clinical main forms of hypoxic-ischaemic brain damage and their monitoring

A short review of main pathogenetic forms of hypoxic-ischaemic brain damage and its consequences for causally orientated therapy and monitoring is given. Different pathogenetic components act in several combinations. In these processes disturbances of the cardio-vascular system, of the haemorheology mainly at the level of microvessels, and disturbances of the blood brain barrier are involved. Furthermore, there are relevant disturbances of the neuronal metabolism such as accumulation of cytosolic Ca++ or disturbances caused by the effect of radicals, lipid peroxidation and changes of protein synthesis. Nowadays, chances of a cause-related therapy are increased by improved evaluation of main pathogenetic components especially regarding secondary brain damage. To evaluate these components in detail, animal models must be more clinically related, especially regarding long-term studies. The increasing knowledge about critical thresholds of reversible and irreversible brain changes also favours effective cerebral function monitoring. The main conclusions from previous experimental as well as clinically related studies indicate the necessity of very early therapeutical interventions.

A reversible component of cerebral injury as identified by the histochemical stain 2,3,5-triphenyltetrazolium chloride (TTC)

The extent of histochemical change following middle cerebral artery occlusion was quantitatively determined in three groups of Sprague-Dawley rats with 2,3,5-triphenyltetrazolium chloride (a marker of mitochondrial oxidative enzyme function). In group I (n = 7) occlusion was maintained for 3 h, with immediate sacrifice. In group II (n = 7) occlusion was maintained for 5 h, with immediate sacrifice. In group III (n = 7) occlusion was maintained for 3 h, followed by a 2-h period of reperfusion prior to sacrifice. The area of injury was significantly larger (P less than 0.05) in the 5-h occlusion group [15 +/- 4% (mean +/- SD)] compared to the 3-h occlusion group (9 +/- 2%); indicating a time-dependent worsening of the histochemical detection of injury. However, the area of injury was significantly less in the reperfusion group (5 +/- 2%) compared to the group that was evaluated after 3 h of occlusion without reperfusion (9 +/- 2%); indicating that some component of the injury revealed by 2,3,5-triphenyltetrazolium chloride is potentially reversible. These data suggest that contrary to previous understanding, the histochemical abnormality revealed by 2,3,5-triphenyltetrazolium chloride is reversible in some circumstances and does not necessarily represent inevitable infarction.

The evolution of focal ischemic damage: a metabolic analysis

Focal cerebral ischemia in the rat was induced by left middle cerebral artery occlusion. The area of ischemia was determined by infusion of a qualitative perfusion indicator, neutral red. The temporal evolution of alterations in regional energy metabolism was assessed by direct microquantitative histochemical analysis of high-energy phosphates, glucose, glycogen, and lactate content of the tissue. Perfusion analyses demonstrated a perifocal region of diminished, but not absent perfusion up to 6 hr after occlusion. By 24 hr, there was an abrupt demarcation between perfused and nonperfused regions. Profound metabolic alterations were seen as early as 20 min after occlusion. Although there was an area of intermediate metabolic derangement in the more medial portions of the lateral ipsilateral cortex up to 6 hr, by 24 hr there was an abrupt transition from normal to abnormal cortex. No evidence of metabolic recovery was seen in this model of permanent occlusion.

Rodent models of cerebral ischemia

The use of physiologically regulated, reproducible animal models is crucial to the study of ischemic brain injury--both the mechanisms governing its occurrence and potential therapeutic strategies. Several laboratory rodent species (notably rats and gerbils), which are readily available at relatively low cost, are highly suitable for the investigation of cerebral ischemia and have been widely employed for this purpose. We critically examine and summarize several rodent models of transient global ischemia, resulting in selective neuronal injury within vulnerable brain regions, and focal ischemia, typically giving rise to localized brain infarction. We explore the utility of individual models and emphasize the necessity for meticulous experimental control of those variables that modulate the severity of ischemic brain injury.