Correlation between peri-infarct DC shifts and ischaemic neuronal damage in rat

Evaluation of laser speckle flowmetry for imaging cortical perfusion in experimental stroke studies: quantitation of perfusion and detection of peri-infarct depolarisations

Laser speckle imaging of the exposed cerebral cortex allows detailed examination of the time course and topography of perfusion under different experimental conditions. Here we examine the quantitative capacity of the method and its sensitivity for the detection of peri-infarct depolarisations (PIDs). In four cats anaesthetised with chloralose, the right hemisphere was exposed and the right middle cerebral artery was occluded. The brain was illuminated with a laser diode, the speckle pattern was imaged, and images of inverse speckle correlation time (ICT) were derived from the calculated speckle contrast images. We examined the relationship of ICT with perfusion, as imaged quantitatively using umbelliferone clearance (CBF(umb)). Values of ICT and CBF(umb) were compared and regression parameters were calculated for each experiment. In eight cats, cortical surface direct current (DC) potential was monitored at two locations and detection of PIDs by DC potential and ICT change was compared. ICT- and CBF(umb)-derived values of perfusion were closely correlated, with a high degree of significance (P<0.0001). Overall, monitoring of DC potential detected 90% of PIDs, whereas ICT detected 56%. We conclude that (1) laser speckle imaging provides an index of perfusion that has a linear relationship with the clearance rate of umbelliferone within the range of levels of perfusion examined; (2) this relationship is relatively stable between experiments; and (3) the method's ability to detect blood flow changes associated with PIDs likely depends on the noise level of the speckle measurements.

Cortical spreading depression and peri-infarct depolarization in acutely injured human cerebral cortex

Electrocorticographic (ECoG) activity was recorded for up to 129 h from 12 acutely brain-injured human patients using six platinum electrodes placed near foci of damaged cortical tissue. The method probes ECoG activity in the immediate vicinity of the injured cortex and in adjacent supposedly healthy tissue. Six out of twelve patients displayed a total of 73 spontaneous episodes of spreading depression of the ECoG. Of the remaining 6 patients 1 displayed an episode of synchronous depression of ECoG during surgery. Using the same electrodes we also measured the slow potential changes (SPC) (0.005-0.05 Hz) to test the hypothesis that the ECoG depressions were identical to Leao's cortical spreading depression (CSD), and to be able to record peri-infarct depolarisations (PIDs) in electrically 'silent' cortical tissue. Changes in the SPC indicate depolarization of brain tissue. For the analysis, the SPCs were enhanced by calculating the time integral of the ECoG signal. Spreading ECoG depressions were accompanied at every single recording site by stereotyped SPCs, which spread across the cortical mantle at 3.3 (0.41-10) mm/min (median, range), i.e. at the same speed of spread as the depression of the ECoG activity. The amplitude of the SPCs was 0.06-3 mV. In 4 out of 6 patients the ECoG recovered spontaneously. In 2 patients we subsequently recorded recurrent SPCs, but without recovery of the initial ECoG background activity until 2-5 h later. This represents the first direct recording of PIDs in acutely injured human brain. Evidence from this and our previous study of 14 brain-injured patients suggests that CSDs in acute brain disorders occur at higher incidence in patients <30 years (83%) than above (33%). CSD was recorded in 4 out of 5 traumatic brain injury patients, and in 2 out of 7 patients with spontaneous haemorrhages. We conclude that the spreading ECoG depressions recorded in patients are identical to CSDs recorded in animal experiments. We furthermore provide direct electrophysiological evidence for the existence of PIDs and hence a penumbra in the human brain. We hypothesize that the depolarization events might contribute to tissue damage in acute disorders in the human brain.

Arterial hypotension triggers perifocal depolarizations and aggravates secondary damage in focal brain injury

Perifocal depolarizations (PFD) have been observed after traumatic brain injury, are known to disturb cerebrovascular reactivity and thus may contribute to the morphological consequences of brain injury. In this investigation, the role of PFD was studied in focal brain lesions with/without induction of delayed hypotension. Cerebral freeze lesions were induced in anesthetized normotensive rats that underwent perfusion fixation of brains 5 min, 4 h or 24 h after lesioning, respectively, to obtain quantitative histopathology. In additional groups, a 45-min period of moderate hypobaric hypotension was applied 15 min post-trauma and brains were perfusion fixed after 4 h or 24 h. In a second series, the direct current (DC) potential and cortical laser-Doppler flow (LDF) were measured adjacent to lesions under normotensive or hypotensive conditions. Sham procedures were carried out in rats that underwent hypotension alone. Lesioning resulted in a significant LDF decrease to 50% of baseline, further decreased during hypotension to less than 40% of control (P < 0.05). Sham animals had LDF values between 60 and 70% of control when subjected to hypotension. Focal brain injury always induced a negative DC shift shortly after lesioning. In 6 of 8 rats that underwent cold lesion plus hypotension, a second PFD was observed approximately 2.5 min after onset of hypotension accompanied by a relative LDF increase by 25 +/- 12%. Lesion expansion was significantly worsened by hypotension (8.19 +/- 0.56 mm(3) at 24 h) compared with normotensive rats (7.01 +/- 0.3 mm(3) at 24 h, P < 0.01). We conclude that hypotension triggers depolarizations by an ischemic mechanism that contributes to final tissue damage.

Pathophysiology of Stroke Rehabilitation: Temporal Aspects of Neurofunctional Recovery

Stroke almost always causes an impairment of motor activity and function. Clinical recovery, though usually incomplete, is often highly dynamic and reflects the ability of the neuronal network to adapt. Mechanisms that underlie neuro-functional plasticity are now beginning to be understood. Albeit the enormous efforts undertaken to support the natural course of re-convalescence through rehabilitation, little has been done to relate possible effects of these therapeutic approaches to mechanisms of adaptive pathophysiology. The review presented here focuses on these mechanisms during the course of recovery post stroke. Next to an unmasking of latent network representations, other adaptive processes, such as excitatory metabolic stress, an imbalance in activating and inhibiting transmission, leading to salient hyperexcitability or mechanisms that consolidate novel connections prime the system's plastic capabilities. These pathophysiological processes potentially interact with rehabilitative interventions. They therefore form the foundation of positive, but possibly also negative recuperation under therapy.

Transient NMDA receptor-mediated hypoperfusion following umbilical cord occlusion in preterm fetal sheep

Exposure to severe hypoxia leads to delayed cerebral and peripheral hypoperfusion. There is evidence in the very immature brain that transient abnormal glutaminergic receptor activity can occur during this phase of recovery. We therefore examined the role of N-methyl-D-aspartate (NMDA) receptor activity in mediating secondary hypoperfusion in preterm fetal sheep at 70% of gestation. Fetuses received either sham asphyxia or asphyxia and were studied for 12 h recovery. The specific, non-competitive NMDA receptor antagonist dizocilpine maleate (2 mg kg-1 bolus plus 0.07 mg kg h-1i.v.) or saline (vehicle) was infused from 15 min after asphyxia until 4 h. In the asphyxia-vehicle group abnormal epileptiform EEG transients were observed during the first 4 h of reperfusion, the peak of which corresponded approximately to the nadir in peripheral and cerebral hypoperfusion. Dizocilpine significantly suppressed this activity (2.7+/-1.3 versus 11.2+/-2.7 counts min-1 at peak frequency, P<0.05) and markedly delayed and attenuated the rise in vascular resistance in both peripheral and cerebral vascular beds observed after asphyxia, effectively preventing the initial deep period of hypoperfusion in carotid blood flow and femoral blood flow (P<0.01). However, while continued infusion did attenuate subsequent transient tachycardia, it did not prevent the development of a secondary phase of persistent but less profound hypoperfusion. In conclusion, the present studies suggest that in the immature brain the initial phase of delayed cerebral and peripheral hypoperfusion following exposure to severe hypoxia is mediated by NMDA receptor activity. The timing of this effect in the cerebral circulation corresponds closely to abnormal EEG activity, suggesting a pathological glutaminergic activation that we speculate is related to evolving brain injury.

Intrinsic optical signal of retinal spreading depression: Second phase depends on energy metabolism and nitric oxide

Spreading depression (SD) is a wave-like phenomenon that spreads through the gray matter of central nervous tissue. The aim of this work is to investigate how cellular energy supply and nitric oxide (NO) influence the recovery period after SD wave propagation. We have examined the SD wave in chicken retina by registration of the intrinsic optical signal (IOS). The changes of the IOS were observed via a microscope, transferred to a photomultiplier and amplified. The IOS of the SD wave consists of two phases. The first phase of IOS coexists with cellular swelling induced by ion distribution; the second phase is thought to reflect metabolic changes and reflects the refractory (recovery) period. To analyze the IOS, the amplitude, the duration and the front and the back maximal slopes of the both phases were analyzed. To reduce the cellular level of ATP the blocker of glucose transport-dexamethasone (glucocorticoid hormone) and the blocker of the respiratory chain-potassium cyanide were used. Sodium nitroprusside and trinitroglycerine were chosen as NO-donors. Our results show that during and after SD wave propagation (i) increased NO concentration changes the first and the second phases of IOS (duration of both phases is NO independent), (ii) reduced glucose uptake leads to an increased second phase duration and (iii) block of the respiratory chain prolongs the first phase. According to the results here presented, we propose that glycogen synthesis is one of the mechanisms reflected by the second phase of the IOS.

[Controlled mild-to-moderate hypothermia in the intensive care unit]

Controlled hypothermia is used as a therapeutic intervention to provide neuroprotection and (more recently) cardioprotection. The growing insight into the underlying pathophysiology of apoptosis and destructive processes at the cellular level, and the mechanisms underlying the protective effects of hypothermia, have led to improved application and to a widening of the range of potential indications. In many centres hypothermia has now become part of the standard therapy for post-anoxic coma in certain patients, but for other indications its use still remains controversial. The negative findings of some studies may be partly explained by inadequate protocols for the application of hypothermia and insufficient attention to the prevention of potential side effects. This review deals with some of the concepts underlying hypothermia-associated neuroprotection and cardioprotection, and discusses some potential clinical indications as well as reasons why some clinical trials may have produced conflicting results. Practical aspects such as methods to induce hypothermia, as well as the side effects of cooling are also discussed.

Transient changes in cortical glucose and lactate levels associated with peri-infarct depolarisations, studied with rapid-sampling microdialysis

Peri-infarct depolarisations (PIDs) contribute to infarct expansion in experimental focal ischaemia; furthermore, depolarisations propagate in the injured human brain. Glucose utilisation is increased under both conditions, and depletion of brain glucose carries a poor prognosis. We studied dynamics of cerebral glucose and lactate in relation to PID patterns in experimental stroke. The middle cerebral artery was occluded for 3 h in 23 cats under terminal chloralose anaesthesia. We used fluorescence imaging to detect occurrence of PIDs, and rapid-sampling online microdialysis (rsMD), coupled to a flow-injection assay, to examine changes in cerebral cortical extracellular glucose and lactate at intervals of 30 sec each. After 30 min' ischaemia, lactate had increased by 43.6+/-s.d. 45.9 micromol/L, and stabilised in that range for 3 h. In contrast, glucose fell only slightly initially (11.9+/-9.7 micromol/L), but progressively decreased to a reduction of 56.7+/-47.2 micromol/L at 3 h, with no evidence of stabilisation. There was a highly significant inverse relationship of frequency of PIDs with plasma glucose (P<0.001). The results also characterise a metabolic signature for PIDs for possible application in clinical work, and emphasise potential risks in the use of insulin to control plasma glucose in patients with brain injury.

Anoxic depolarization of rat hippocampal slices is prevented by thiopental but not by propofol or isoflurane

BACKGROUND

There is strong evidence to suggest that anoxic depolarization (AD) is an important factor in hypoxia/ischaemia-induced neural damage. Treatments that prevent the occurrence of AD may be useful in providing neuronal protection against hypoxia. The current study was designed to determine whether general anaesthetics which have been suggested to 'induce prophylaxis' against hypoxia can attenuate the incidence of AD.

METHODS

The effects of anoxia (3 min) on evoked extracellularly recorded field potentials of CA1 neurons in rat hippocampal slices were assessed in the absence and presence of the i.v. general anaesthetics thiopental and propofol and the volatile anaesthetic isoflurane.

RESULTS

In the absence of anaesthetics, AD occurred in 81% of the preparations tested. Thiopental (2 x 10(-4) M) significantly reduced the incidence of AD (16%, P=0.0006). In comparison, propofol (2 x 10(-4) M) and isoflurane (1.5 vol%) were ineffective (69% and 60%, respectively). Furthermore, in the presence of thiopental, the population spike amplitude recovered with and without AD (90% and 94% of pre-anoxic value, respectively) following 3 min anoxia.

CONCLUSION

The prophylactic effect of thiopental against hypoxia might be induced, in part, by preventing the generation of AD.

Peri-infarct depolarizations reveal penumbra-like conditions in striatum

Spreading depression-like peri-infarct depolarizations not only characterize but also worsen penumbra conditions in cortical border zones of experimental focal ischemia. We intended to investigate the relevance of ischemic depolarization in subcortical regions of ischemic territories. Calomel electrodes measured DC potentials simultaneously in the lateral and medial portions of the caudate nucleus (CN) of 11 anesthetized cats after permanent occlusion of the middle cerebral artery. Additionally, platinum electrodes measured cerebral blood flow (CBF) in the CN, and laser Doppler probes CBF in the cortex. Depolarizations (negative DC shifts >10 mV) were obtained in 10 of 11 cats. Further differentiation revealed that short-lasting spreading depression-like depolarizations (SDs; 5 of 10 cats: 5.24 +/- 1.22 min total duration; 23.3 +/- 4.2 mV amplitude) were predominantly found in medial and longer depolarizations (LDs; 4 of 10 cats: 64.7 +/- 47.5 min; 25.0 +/- 11.3 mV) in the lateral CN. Terminal depolarizations (TDs; 6 of 10 cats; without repolarization) occurred immediately after occlusion or at later stages, being then accompanied by elevations of intracranial pressure presumably inducing secondary CBF reduction. CBF tended to be lower in regions with TDs (33.3 +/- 29.9% of control) and LDs (37.3 +/- 22.8%) than in regions with SDs (51.5 +/- 48.0%). We conclude that in focal ischemia, transient peri-infarct depolarizations emerge not only in cortical but also in striatal gray matter, thereby demonstrating the existence of subcortical zones of ischemic penumbra. The generation of these ischemic depolarizations is a multifocal process possibly linked to brain swelling and intracranial pressure rise in the later course of focal ischemia, and therefore a relevant correlate of progressively worsening conditions.

Characterization of a New Rat Model of Penetrating Ballistic Brain Injury

Penetrating brain injury (PBI) is a leading cause of mortality and morbidity in modern warfare and accounts for a significant number of traumatic brain injuries worldwide. Here we characterize the pathophysiology of a new rat model of PBI that simulates the large temporary cavity caused by energy dissipation from a penetrating bullet round. Male Sprague-Dawley rats (250-300 g) were subjected to a simulated ballistic wound to the right frontal hemisphere implemented by an inflatable penetrating probe. Three levels of injury severity were compared to control animals. Neurological and physiological outcome was assessed over a 3-day recovery period and brain tissue collected at 72 h for histopathological evaluation. Brain-injured regions included the ipsilateral frontal cortex and striatum with volumetric increases in intracranial hemorrhage (5-18 mm3) and lesion size (9-86 mm3) related to severity. Similarly, hemispheric swelling increased (3-14%) following PBI, associated with a significant rise in intracranial pressure. Astrogliosis was present in regions adjacent to the core-injury along with microglial and leukocyte infiltration. Injury remote to the lesion was observed in the cerebral peduncle that may have accounted, in part, for observed neurological deficits. Neurological and balance beam testing revealed sensorimotor deficits that persisted through 72 h. Severe electroencephalographic disturbances included the occurrence of cortical spreading depression, slow-waves, and brain seizure activity. In conclusion, this rat PBI model replicates diverse, salient features of clinical PBI pathology, generates reproducible and quantifiable measures of outcome, and is scalable by injury severity, rendering it an attractive vehicle for experimental brain trauma research.

Depolarisation phenomena in traumatic and ischaemic brain injury

1. Cortical spreading depression is a non-physiological global depolarisation of neurones and astrocytes that can be initiated with varying degrees of difficulty in the normally perfused cerebral cortex in the experimental laboratory. Induction is typically with electrical stimulation, needling of the cerebral cortex, or superfusion of isotonic or more concentrated potassium chloride solution. The phenomenon propagates across the cerebral cortex at a rate of 2-5 mm per minute, and is accompanied by marked but transient increases in cerebral blood flow, in local tissue oxygen tension, and most probably in metabolic rate. 2. Peri-infarct depolarisation is also a depolarisation event affecting neurones and glia, with an electrophysiological basis similar or identical to CSD, but occurring spontaneously in the ischaemic penumbra or boundary zone in focal cerebral cortical ischaemia. Most such events arise from the edge of the ischaemic core, and propagate throughout the penumbra, at a rate similar to that of cortical spreading depression. 3. Cortical spreading depression in the normally perfused cortex does not result in histological damage whereas peri-infarct depolarisations augment neuronal damage in the penumbra, and are believed by many authors to constitute an important, or the principal, mechanism by which electrophysiological penumbra progressively deteriorates, ultimately undergoing terminal depolarisation and thus recruitment into an expanded core lesion. 4. There is some experimental evidence to suggest that under some circumstances induction of episodes of cortical spreading depression can confer protection against subsequent ischaemic insults. 5. Although cortical spreading depression and peri-infarct depolarisations have been extensively studied in the experimental in vivo models, there is now clear evidence that depolarisations also occur and propagate in the human brain in areas surrounding a focus of traumatic contusion. 6. Whether such events in the injured human brain represent cortical spreading depression or peri-infarct depolarisation is unclear. However, invasive and probably non-invasive monitoring methods are available which may serve to distinguish which event has occurred. 7. Much further work will be needed to examine the relationship of depolarisation events in the injured brain with outcome from cerebral ischaemia or head injury, to examine the factors which influence the frequency of depolarisation events, and to determine which depolarisation events in the human brain augment the injury and should be prevented.

Transient increases in extracellular K+ produce two pharmacological distinct cytosolic Ca2+ transients

Transient increases in extracellular K+ are observed under various conditions, including repetitive neuronal firing, anoxia, ischemia and hypoglycemic coma. We studied changes in cytoplasmic Ca2+ ([Ca2+]cyt) evoked by pulses of KCl in human neuroblastoma SH-SY5Y cells and rat dorsal root ganglia (DRG) neurons at 37 degrees C. A "pulse" of KCl evoked two transient increases in [Ca2+]cyt, one upon addition of KCl (K+on) and the other upon removal of KCl (K+off). The K+on transient has been described in many cell types and is initiated by the activation of voltage-dependent Ca2+ channels followed by Ca2+-evoked Ca2+ release from intracellular Ca2+ stores. The level of KCl necessary to evoke the K+off transient depends on the type of neuron, in SH-SY5Y cells it required 100 mM KCl, in most (but not all) of dorsal root ganglia neurons it could be detected with 100-200 mM KCl and in a very few dorsal root ganglia neurons it was detectable at 20-50 mM KCl. In SH-SY5Y cells, reduction of extracellular Ca2+ inhibited the K+on more strongly than the K+off and slowed the decay of K+off. Isoflurane (1 mM) reduced the K+on)- but not the K+off-peak. However, isoflurane slowed the decay of K+off. The nonspecific cationic channel blocker La3+ (100 microM) had an effect similar to that of isoflurane. Treatment with thapsigargin (TG) at a concentration known to only deplete IP3-sensitive Ca2+ stores did not affect K+on or K+off, suggesting that Ca2+ release from the IP3-sensitive Ca2+ stores does not contribute to K+on and K+off transients and that the thapsigargin-sensitive Ca2+ ATPases do not contribute significantly to the rise or decay rates of these transients. These findings indicate that a pulse of extracellular K+ produces two distinct transient increases in [Ca2+]cyt.

Pathophysiology of stroke: lessons from animal models

The current pathophysiological understanding of stroke is substantially based on experimental studies. Brain injury after cerebral ischemia develops from a complex signaling cascade that evolves in an at least partially unraveled spatiotemporal pattern. Early excitotoxicity can lead to fast necrotic cell death, which produces the core of the infarction. The ischemic penumbra that surrounds the infarct core suffers milder insults. In this area, both mild excitotoxic and inflammatory mechanisms lead to delayed cell death, which shows biochemical characteristics of apoptosis. While brain cells are challenged by these deleterious mechanisms, they activate innate protective programs of the brain, which can be studied by means of experimentally inducing ischemic tolerance (i.e., ischemic preconditioning). Importantly, cerebral ischemia not only affects the brain parenchyma, but also impacts extracranial systems. For example, stroke induces a dramatic immunosuppression via an overactivation of the sympathetic nervous system. As a result, severe bacterial infections such as pneumonia occur. Complex signaling cascades not only decide about cell survival, but also about the neurological deficit and the mortality after stroke. These mechanisms of damage and endogenous protection present distinct molecular targets that are the rational basis for the development of neuroprotective drugs.

Spreading depression activates unfolded protein response

Preconditioning is a process where a preceding non-lethal form of stress activates a stress response that protects cells against an otherwise lethal form of stress. Preconditioning can be induced in various ways including short-term ischemia or spreading depression. Here we investigated the effect of 1 h repetitive spreading depression on the unfolded protein response (UPR), a stress response activated under conditions associated with endoplasmic reticulum (ER) dysfunction. Spreading depression induced processing of xbp1 mRNA, indicative of an activation of UPR. Processing of xbp1 was paralleled by a rise in grp78 mRNA levels resulting from an activation of a signal transduction pathway that depends on protein synthesis. Preconditioning-induced activation of UPR may preserve ER functioning under pathological conditions interfering with ER functions.

Simultaneous recording of eeg and direct current (DC) potential makes it possible to assess functional and metabolic state of nervous tissue

It has been proposed to assess functional and metabolic state of the brain nervous tissue in terms of bioelectrical parameters. Simultaneous recording of the DC potential level and total slow electrical activity of the nervous tissue was performed in the object of study by nonpolarizable Ag/AgCl electrodes with a DC amplifier. The functional and metabolic state of the brain was determined in terms of enhancement or reduction in the total slow electrical activity and positive or negative shifts in the DC potential level.

Estrogen attenuates neuronal excitability in the insular cortex following middle cerebral artery occlusion

The current investigation examined the role of estrogen in the insular cortex (IC) under both normal and ischemic conditions. Experiments were done in anaesthetized male Sprague-Dawley rats. The effect of systemic 17beta-estradiol (estrogen) administration on levels of amino acids and of endogenous estrogen obtained by microdialysis and its effect on neuronal activity of cells located in the insular cortex were measured in the absence of, and following permanent occlusion of, the right middle cerebral artery (MCA). In normal rats, intravenous (i.v.) injection of estrogen resulted in a significant increase (greater than 25 spikes/bin) in the spontaneous activity of neurons located within the insular cortex, while there was a significant decrease in gamma-aminobutyric acid (GABA) levels measured in IC dialysate. Middle cerebral artery occlusion (MCAO) resulted in a biphasic response consisting of a transient increase in the extracellular concentration of glutamate, aspartate, and GABA, followed by sustained elevations in glutamate and aspartate, but reduced GABA levels 4 h post-MCAO. MCAO also resulted in a significant increase in neuronal activity in the IC (from 28 +/- 9 to 120 +/- 88 spikes/bin). This MCAO-induced excitation was completely blocked following the prior intravenous administration of estrogen. Systemic estrogen administration also resulted in a delay in the progression and decrease in the final infarct volume by approximately 56%. Taken together, these results suggest that under normal conditions, estrogen excites neurons in the insular cortex by decreasing GABA release (disinhibition) and it plays a role in attenuating the MCAO-induced excitability and death of these neurons.

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.

Pathophysiology of cerebral ischemia and brain trauma: similarities and differences

Current knowledge regarding the pathophysiology of cerebral ischemia and brain trauma indicates that similar mechanisms contribute to loss of cellular integrity and tissue destruction. Mechanisms of cell damage include excitotoxicity, oxidative stress, free radical production, apoptosis and inflammation. Genetic and gender factors have also been shown to be important mediators of pathomechanisms present in both injury settings. However, the fact that these injuries arise from different types of primary insults leads to diverse cellular vulnerability patterns as well as a spectrum of injury processes. Blunt head trauma produces shear forces that result in primary membrane damage to neuronal cell bodies, white matter structures and vascular beds as well as secondary injury mechanisms. Severe cerebral ischemic insults lead to metabolic stress, ionic perturbations, and a complex cascade of biochemical and molecular events ultimately causing neuronal death. Similarities in the pathogenesis of these cerebral injuries may indicate that therapeutic strategies protective following ischemia may also be beneficial after trauma. This review summarizes and contrasts injury mechanisms after ischemia and trauma and discusses neuroprotective strategies that target both types of injuries.

Delayed secondary phase of peri-infarct depolarizations after focal cerebral ischemia: relation to infarct growth and neuroprotection

In focal cerebral ischemia, peri-infarct depolarizations (PIDs) cause an expansion of core-infarcted tissue into adjacent penumbral regions of reversible injury and have been shown to occur through 6 hr after injury. However, infarct maturation proceeds through 24 hr. Therefore, we studied PID occurrence through 72 hr after both transient and permanent middle cerebral artery occlusion (MCAo) via continuous DC recordings in nonanesthetized rats. PIDs occurred an average 13 times before reperfusion at 2 hr and then ceased for an average approximately 8 hr. After this quiescent period, PID activity re-emerged in a secondary phase, which reached peak incidence at 13 hr and consisted of a mean 52 PIDs over 2-24 hr. This phase corresponded to the period of infarct maturation; rates of infarct growth through 24 hr coincided with changes in PID frequency and peaked at 13 hr. In permanent MCAo, PIDs also occurred in a biphasic pattern with a mean of 78 events over 2-24 hr. Parameters of secondary phase PID incidence correlated with infarct volumes in transient and permanent ischemia models. The role of secondary phase PIDs in infarct development was further investigated in transient MCAo by treating rats with a high-affinity NMDA receptor antagonist at 8 hr after injury, which reduced post-treatment PID incidence by 57% and provided 37% neuroprotection. Topographic mapping with multielectrode recordings revealed multiple sources of PID initiation and patterns of propagation. These results suggest that PIDs contribute to the recruitment of penumbral tissue into the infarct core even after the restoration of blood flow and throughout the period of infarct maturation.

The natural course of lesion development in brain ischemia

Histopathologic and NMR imaging studies show that focally ischemic brain lesions tend to increase in size over time. In animal models of stroke as well as in patients presenting with hemispheric stroke, considerable lesion growth was observed. In focal cerebral ischemia, lesions predominantly enlarge early (<12 hrs postinsult) and show complete ischemic injury due to pan necrosis in the vast majority of affected tissue. In global cerebral ischemia--a condition that is present after cardiac arrest--lesions appear late (>12 hrs) in selectively vulnerable brain regions and neurons are damaged by apoptosis. The short resuscitation time of the brain explains why periods of global ischemia result in widespread and global loss of energy metabolites combined with diffuse brain edema and global damage. Mechanisms involved in lesion growth include excitotoxicity, peri-infarct depolarizations, lactacidosis, microcirculatory disturbances, and flow-metabolism uncoupling among others. Problems involved in the subject under focus are related to maturation phenomena of injury and the different imaging modalities (metabolic imaging, NMR imaging, positron emission tomography) that require a subtly differentiated interpretation of the alterations observed.

New roles for astrocytes: redefining the functional architecture of the brain

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, work over the past decade has revealed that they interact with the vasculature to form a gliovascular network that might organize not only the structural architecture of the brain but also its communication pathways, activation, thresholds and plasticity. The net effect is that astroglia demarcate gray matter regions, both cortical and subcortical, into functional compartments whose internal activation thresholds and external outputs are regulated by single glial cells. The array of these astrocyte-delimited microdomains along the capillary microvasculature allows the formation of higher-order gliovascular units, which serve to match local neural activity and blood flow while regulating neuronal firing thresholds through coordinative glial signaling. By these means, astrocytes might establish the functional as well as the structural architecture of the adult brain.

The free radical spin-trap α-PBN attenuates periinfarct depolarizations following permanent middle cerebral artery occlusion in rats without reducing infarct volume

The effect of the free radical spin-trap alpha-phenyl-butyl-tert-nitrone (alpha-PBN) in permanent focal cerebral ischemia in rats was examined in two series of experiments. In the first, rats were subjected to permanent occlusion of the middle cerebral artery (MCAO) and treated 1 h after occlusion with a single dose of alpha-PBN (100 mg/kg) or saline. Body temperature was measured and controlled for the first 24 h to obtain identical temperature curves in the two groups. Cortical infarct volumes were determined on histological sections 7 days later. alpha-PBN did not significantly reduce infarct volume (control: 28.3+/-16.3 mm3 vs. alpha-PBN 23.7+/-7.4 mm3). In the second series of experiments, periinfarct depolarizations (PIDs) were recorded with an extracellular DC electrode at two locations in the ischemic penumbra for the initial 3 h following MCAO. alpha-PBN (100 mg/kg, single dose in conjunction with occlusion) significantly reduced the total number (median value of 3 PIDs in the control groups vs. 1 PID in alpha-PBN groups, p<0.001) and total duration of the PIDs (median value 662 s in the control groups vs. 162 s in the alpha-PBN groups, p<0.006). In spite of this, cortical infarct volumes determined 7 days later in the same rats were not smaller in alpha-PBN-treated rats. The study thus demonstrates that attenuation of PIDs does not always lead to smaller infarcts if permanent arterial occlusion is followed by long survival time and does not support the hypothesis that PIDs per se are critical determinants of infarct size in this situation.

Neurogenic neuroprotection

1. Stimulation of the rostral-ventromedial pole of the cerebellar fastigial nucleus exerts powerful effects on systemic and cerebral circulation. 2. Excitation of fibers passing through the fastigial nucleus evokes sympathoactivation and increases in arterial pressure. 3. Increase in cerebral blood flow evoked by excitation of fibers passing through the FN is mediated by intrinsic brain mechanisms independently of metabolism. 4. Excitation of the fastigial nucleus neurons in contrast decreases arterial pressure and cerebral blood flow. The latter probably is secondary to the suppression of brain metabolism. 5. Excitation of the fastigial nucleus neurons significantly decreases damaging effects of focal and global ischemia on the brain. 6. The fastigial nucleus-evoked neuroprotection can be conditioned: 1-h stimulation protects the brain for up to 3 weeks. 7. Other brain structures such as subthalamic cerebrovasodilator area and dorsal periaqueductal gray matter also produce long-lasting brain salvage when stimulated. 8. More than one mechanism may account for neurogenic neuroprotection. 9. Early neuroprotection, which develops immediately after the stimulation, involves opening of potassium channels. 10. Delayed long-lasting neuroprotection may involve changes in genes expression resulting in suppression of inflammatory reaction and apoptotic cascade. 11. It is conceivable that intrinsic neuroprotective system exists within the brain, which renders the brain more tolerant to adverse stimuli when activated. 12. Knowledge of the mechanisms of neurogenic neuroprotection will allow developing new neuroprotective approaches.

Atrial natriuretic peptide expression is increased in rat cerebral cortex following spreading depression: possible contribution to sd-induced neuroprotection

Cortical spreading depression (CSD) is characterised by slowly propagating waves of cellular depolarization and depression and involves transient changes in blood flow, ion balance and metabolism. In cerebral ischaemia, peri-infarct CSD-like depolarization potentiates infarct growth, whereas preconditioning with a CSD episode protects against subsequent ischaemic insult. Thus, many of the long-lasting molecular changes that occur in CSD-affected tissue are presumed to be part of a 'neuroprotective cascade.' 3',5'-Cyclic guanosine monophosphate (cGMP) has been shown to be a neuroprotective mediator and the nitric oxide system, which increases cGMP production by soluble guanylate cyclase, is up-regulated by CSD. Atrial and C-type natriuretic peptide (ANP/CNP) are present in cerebral cortex and their actions are mediated via particulate guanylate cyclase receptors and cGMP production. Therefore, in further efforts to characterise the role of cGMP-related systems in CSD and neuroprotection, this study investigated possible changes in cortical natriuretic peptide expression following acute, unilateral CSD in rats. Using in situ hybridisation, significant 20-80% increases in ANP mRNA were detected in layers II and VI of ipsilateral cortex at 6 h and 1-14 days after CSD. Ipsilateral cortical levels were again equivalent to control contralateral values after 28 days. Assessment of cortical concentrations of ANP immunoreactivity by radioimmunoassay revealed a significant 57% increase at 7 days after CSD. Despite using a sensitive signal-amplification protocol, authentic ANP-like immunostaining was readily detected in subcortical nerve fibres, but was not reliably detected in normal or CSD-affected neocortex, suggesting the presence of very low levels, and/or active or differential processing of the peptide. Cortical CNP mRNA levels are not altered by CSD, indicating the specificity of the observed effects.Overall, these novel findings demonstrate a prolonged increase in cortical ANP expression after an acute episode of CSD. The overlap between the described time course of CSD-induced protection against ischaemic insult and demonstrated increases in ANP levels, suggest that ANP (like nitric oxide) may contribute to CSD-induced neuroprotection, via effects on cGMP production and other signal-transduction pathways.

Ischemia and stroke

Cell death following cerebral ischemia is mediated by a complex pathophysiologic interaction of different mechanisms. In this Chapter we will outline the basic principles as well as introduce in vitro and in vivo models of cerebral ischemia. Mechanistically, excitotoxicity, peri-infarct depolarization, inflammation and apoptosis seem to be the most relevant mediators of damage and are promising targets for neuroprotective strategies.

Peripheral nerve injury influences the disinhibition induced by focal ischaemia in the rat motor cortex

Photothrombotic lesions were produced in the rat primary motor cortex, and the brain excitability was assessed in a paired-pulse stimulation protocol by transcranial recording, in parallel at 16 points of the frontal cortex, including the insulted and the surrounding areas. The cortical lesion reduced the inhibition in the extended frontal cortex, with a delay of a few minutes. Unilateral facial nerve transection, however, accelerated the widespread disinhibition. Although the mechanism is not clear in detail, both peripheral and central injury-induced disinhibition may have a significant impact on the recovery of the function.

Uptake of glutamate is impaired in the cortical penumbra of the rat following middle cerebral artery occlusion: an in vivo microdialysis extraction study

By using microdialysis extraction of (3)H-D-aspartate and concomitant recordings of extracellular direct current (DC) potentials, the effect of middle cerebral artery occlusion (MCAO) was studied continuously over a period of 100 min in the cerebral cortex of rats. From analysis of the DC potentials, rats subjected to MCAO could be divided into three groups, one in which the dialysis probe was located in the ischemic core, one in which the probe was in the penumbra, and one in which the probe was in nonischemic tissue. In general, extraction of (3)H-D-aspartate was positively correlated with the DC potential; i.e., changes in the extraction were concurrent with changes in the DC potential. Comparing the different animal groups by integration of all extraction values obtained during MCAO over time, (3)H-D-aspartate extraction was reduced by 40% in the penumbra, and by 58% in the ischemic core, compared with the sham-operated controls. No changes was found in the nonischemic group. In the penumbra group, extraction of (3)H-D-aspartate was reduced initially upon institution of MCAO but recovered to control-like levels over a period of 15-40 min, despite ongoing MCAO. In addition, extraction was reduced transiently during periinfarct depolarizations. A mean of all extraction values obtained during MCAO in the penumbra group was reduced by 47% compared with a mean of values obtained before institution of MCAO. Induction of death resulted in a reduction of (3)H-D-aspartate extraction by 86%. The present results provide direct evidence that uptake of Glu is reduced both in the ischemic core and in the penumbra of the cerebral cortex following MCAO in rats, possibly contributing to the initiation and spread of infarction. The results further indicate that uptake of Glu in the penumbra recovers to control-like levels, despite ongoing MCAO, providing evidence that Glu uptake by the Glu transporter proteins is reinstituted and/or up-regulated.

Astrocytes and brain injury

Astrocytes are the most numerous cell type in the central nervous system. They provide structural, trophic, and metabolic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions during brain ischemia and other insults can critically influence neuron survival. Astrocyte functions that are known to influence neuronal survival include glutamate uptake, glutamate release, free radical scavenging, water transport, and the production of cytokines and nitric oxide. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is influenced by astrocyte surface molecule expression and trophic factor release. In addition, the death or survival of astrocytes themselves may affect the ultimate clinical outcome and rehabilitation through effects on neurogenesis and synaptic reorganization.

Occurrence of nonconvulsive seizures, periodic epileptiform discharges, and intermittent rhythmic delta activity in rat focal ischemia

A significant proportion of neurologic patients suffer electroencephalographic (EEG) seizures in the acute phase following traumatic or ischemic brain injury, including many without overt behavioral manifestations. Although such nonconvulsive seizures may exacerbate neuropathological processes, they have received limited attention clinically and experimentally. Here we characterize seizure episodes following focal cerebral ischemia in the rat as a model for brain injury-induced seizures. Cortical EEG activity was recorded continuously from both hemispheres up to 72 h following middle cerebral artery occlusion (MCAo). Seizure discharges appeared in EEG recordings within 1 h of MCAo in 13/16 (81%) animals and consisted predominantly of generalized 1-3 Hz rhythmic spiking. During seizures animals engaged in quiet awake or normal motor behaviors, but exhibited no motor convulsant activity. Animals had a mean of 10.6 seizure episodes within 2 h, with a mean duration of 60 s per episode. On average, seizures ceased at 1 h 59 min post-MCAo in permanently occluded animals and did not occur following reperfusion at 2 h in transiently occluded animals. In addition to seizures, periodic lateralized epileptiform discharges (PLEDs) appeared over penumbral regions in the injured hemisphere while intermittent rhythmic delta activity (IRDA) recurred in the contralateral hemisphere with frontoparietal dominance. PLEDs and IRDA persisted up to 72 h in permanent MCAo animals, and early onset of the former was predictive of prolonged seizure activity. The presentation of these EEG waveforms, each with characteristic features replicating those in clinical neurologic populations, validates rat MCAo for study of acutely induced brain seizures and other neurophysiological aspects of brain injury.

Targeted tissue oxidation in the cerebral cortex induces local prolonged depolarization and cortical spreading depression in the rat brain

Spreading depression (SD) has been linked to several neurological disorders as epilepsy, migraine aura, trauma, and cerebral ischemia, which were also influenced by disorderliness of the brain redox homeostasis. To investigate whether local tissue oxidation directly induces SD, we oxidized a restricted local area of the rat cerebral cortex using photo-dynamic tissue oxidation (PDTO) technique and examined the cerebral blood flow (CBF) and direct current (DC) potential in and around the oxidized area. Intensive PDTO induced prolonged depolarization only in the photo-oxidized area, which led to global changes of CBF and DC potential: synchronous negative shifts of DC potential (with an amplitude of approximately 20 mV) and hyperperfusion of CBF occurred. The changes in DC potential and CBF spread at a rate of around 3mm/min beyond the oxidized area to the whole hemisphere of the cerebral cortex, indicating that intensive local oxidation induces SD in the rat brain.

Widespread and long-lasting alterations in GABA(A)-receptor subtypes after focal cortical infarcts in rats: mediation by NMDA-dependent processes

Impairment of inhibitory neurotransmission has been reported to occur in widespread, structurally intact brain regions after focal ischemic stroke. These long-lasting alterations contribute to the functional deficit and influence long-term recovery. Inhibitory neurotransmission is primarily mediated by gamma-aminobutyric acid (GABA)A receptors assembled of five subunits that allow a variety of adaptive changes. In this study, the regional distribution of five major GABA(A)-receptor subunits (alpha1, alpha2, alpha3, alpha5, and gamma2) was analyzed immunohistochemically 1, 7, and 30 days after photochemically induced cortical infarcts. When compared with sham-operated controls, a general and regionally differential reduction in immunostaining was found within the cortex, hippocampus, and thalamus of both hemispheres for almost all subunits. Within ipsilateral and contralateral neocortical areas, a specific pattern of changes with a differential decrease of subunits alpha1, alpha2, alpha5, and gamma2 and a significant upregulation of subunit alpha3 was observed in the contralateral cortex homotopic to the infarct. This dysregulation was most prominent at day 7 and still present at day 30. Interestingly, a single application of the noncompetitive N-methyl-D-aspartate-receptor antagonist MK-801 during lesion induction completely blocked these bihemispheric alterations. Cortical spreading depressions induced by topical application of KCl do not change GABA(A)-receptor subunit expression. As alterations in subtype distribution crucially influence inhibitory function, ischemia-induced modifications in GABA(A)-receptor subtype expression may be of relevance for functional recovery after stroke.

Role of magnesium in the reduction of ischemic depolarization and lesion volume after experimental subarachnoid hemorrhage

OBJECT

Ischemia-induced tissue depolarizations probably play an important role in the pathophysiology of cerebral ischemia caused by parent vessel occlusion. Their role in ischemia caused by subarachnoid hemorrhage (SAH) remains to be investigated. The authors determined whether ischemic depolarizations (IDs) or cortical spreading depressions (CSDs) occur after SAH, and how these relate to the extent of tissue injury measured on magnetic resonance (MR) images. In addition, they assessed whether administration of MgSO4 reduces depolarization time and lesion volume.

METHODS

By means of the endovascular suture model, experimental SAH was induced in 52 rats, of which 37 were appropriate for analysis, including four animals that underwent sham operations. Before induction of SAH, serum Mg++ levels were measured and 90 mg/kg intravascular MgSO4 or saline was given. Extracellular direct current potentials were continuously recorded from six Ag/AgCl electrodes, before and up to 90 minutes following SAH, after which serum Mg++ levels were again measured. Next, animals were transferred to the MR imaging magnet for diffusion-weighted (DW) MR imaging. Depolarization times per electrode were averaged to determine a mean depolarization time per animal. No depolarizations occurred in sham-operated animals. Ischemic depolarizations occurred at all electrodes in all animals after SAH. Only two animals displayed a single spreading depression-like depolarization. The mean duration of the ID time was 41 +/- 25 minutes in the saline-treated controls and 31 +/- 30 minutes in the Mg++-treated animals (difference 10 minutes: p = 0.31). Apparent diffusion coefficient (ADC) maps of tissue H2O, obtained using DW images approximately 2.5 hours after SAH induction, demonstrated hypointensities in both hemispheres, but predominantly in the ipsilateral cortex. No ADC abnormalities were found in sham-operated animals. The mean lesion volume, as defined on the basis of a significant ADC reduction, was 0.32 +/- 0.42 ml in saline-treated controls and 0.11 +/- 0.06 ml in Mg++-treated animals (difference 0.21 ml; p = 0.045). Serum Mg++ levels were significantly elevated in the Mg++-treated group.

CONCLUSIONS

On the basis of their data, the authors suggest that CSDs play a minor role, if any, in the acute pathophysiology of SAH. Administration of Mg++ reduces the cerebral lesion volume that is present during the acute period after SAH. The neuroprotective value of Mg++ after SAH may, in part, be explained by a reduction in the duration of the ID of brain cells.

Astrocytic activation and delayed infarct expansion after permanent focal ischemia in rats. Part II: suppression of astrocytic activation by a novel agent (R)-(-)-2-propyloctanoic acid (ONO-2506) leads to mitigation of delayed infarct expansion and early improvement of neurologic deficits

A novel agent, (R)-(-)-2-propyloctanoic acid (ONO-2506), has a unique property in that it modulates functions of activated cultured astrocytes, including pronounced inhibition of S-100beta synthesis. The present study examined whether administration of this agent would mitigate the delayed expansion of infarct volume and the neurologic deficits after permanent middle cerebral artery occlusion (pMCAO) in rats. Daily intravenous administration of ONO-2506 (10 mg/kg) abolished the delayed infarct expansion between 24 and 168 hours after pMCAO, whereas the acute infarct expansion until 24 hours was unaffected. The agent significantly reduced the expression of S-100beta and glial fibrillary acidic protein in the activated astrocytes and the number of terminal deoxynucleotidyl transferase-mediated 2;-deoxyuridine 5;-triphosphate-biotin nick end labeling-positive cells in the periinfarct area. The neurologic deficits were significantly improved, compared with the vehicle-treated groups, as early as 24 hours after the initial administration of ONO-2506. The agent had a wide therapeutic time window of 0 to 48 hours after pMCAO. These results indicate that because of the pharmacologic modulation of astrocytic activation induced by ONO-2506, symptoms can regress whereas delayed expansion of the lesion is arrested. Pharmacologic modulation of astrocytic activation may confer a novel therapeutic strategy against stroke.

Effects of pinealectomy and melatonin on the retrograde degeneration of retinal ganglion cells in a novel model of intraorbital optic nerve transection in mice

The effects of pinealectomy and of intraperitoneally administered melatonin on the retrograde degeneration of retinal ganglion cells (RGCs) were examined in a novel model of optic nerve (ON) transection in C57BL/16J mice. RGCs were prelabeled with the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (Di-I), and the ON was cut inside the orbital cavity 7 days later. The degree of RGC injury was assessed by counting viable Di-I labeled RGCs in various locations of the retina. In unlesioned control eyes, a mean ganglion cell density of 1,891 +/- 30/mm2 (mean +/- S.E.M.) was determined. The cell density markedly declined at 14 days after axotomy (295 +/- 9 cells/mm2; 15.6% of contralateral). Sham-pinealectomy did not influence the density of RGCs at 14 days after ON transection (382 +/- 37 cells/mm2). In pinealectomized animals, on the other hand, the RGC number was significantly reduced as compared with untreated and sham-pinealectomized animals (91 +/- 33 RGCs/mm2). The effect of pinealectomy was reversed after i.p. administration of melatonin (4 mg/kg bw bolus followed by continuous infusion of 8 mg/kg bw/day) (286 +/- 27 cells/mm2). In nonpinealectomized animals, on the contrary, i.p. melatonin did not influence the RGC density (344 +/- 20 cells/mm2). The present results suggest that endogenous melatonin prevents the delayed degeneration of adult central nervous system (CNS) neurons in vivo, and that exogenous substitution of melatonin may be useful to protect injured neurons against cell death under conditions of melatonin deficiency, e.g. in the aged brain, when melatonin synthesis and secretion have decreased.

Cortical spreading depression transiently activates MAP kinases

Cortical spreading depression (CSD) has been shown to have neuroprotective effects when administered in advance of cerebral ischemia. The mechanism by which CSD induces its neuroprotective effect however remains to be elucidated. Since MAP kinases have been shown to impart neuroprotection in ischemic preconditioning paradigms, we attempted to determine the role CSD may have in the activation of MAPK. We show that CSD is capable of increasing the phosphorylation of ERK in a MEK-dependent manner. This phosphorylation is, however, transient, as phosphorylated ERK levels return to control levels 45 min after 2 h of CSD elicitation. Immunohistochemical analysis reveals that the phosphorylated form of ERK is located ubiquitously in cells of the CSD-treated cortex while CSD-elicited MEK phosphorylation resides solely in the nuclei. These data suggest that CSD may act via the MAP kinase pathways to mediate preconditioning.

Penumbral microcirculatory changes associated with peri-infarct depolarizations in the rat

BACKGROUND AND PURPOSE

This study was designed to investigate the influence of peri-infarct depolarization elicited by occlusion of the middle cerebral artery on the dynamics of the microcirculation.

METHODS

The microcirculation in the frontoparietal cortex of 9 rats was visualized in real time through a closed cranial window with the use of laser-scanning confocal fluorescence microscopy combined with intravenous fluorescein isothiocyanate (FITC)-dextran and FITC-labeled erythrocytes. The direct current potential/electrocorticogram was continuously monitored. Intraluminal focal ischemia was induced for 2 hours in 6 rats anesthetized with halothane and mechanically ventilated. Reperfusion was monitored for 1 hour. Three rats underwent sham operation. Brains were removed 24 hours after occlusion and processed for histology.

RESULTS

In control conditions, the velocity of fluorescent erythrocytes through capillaries was 0.51+/-0.19 mm/s (mean+/-SD), and the diameter of the arterioles studied was 33+/-12 microm. Under ischemia, erythrocyte velocity through capillaries was significantly decreased to 0.33+/-0.14 mm/s, while arteriole diameter did not change significantly. During spontaneous peri-infarct depolarizations, arteriole diameter was significantly increased (119+/-23% of baseline), while capillary erythrocyte velocity was further decreased by 14+/-34%. The direction of arteriolar blood flow episodically and transiently reversed during approximately half of the peri-infarct depolarizations. The decrease in capillary erythrocyte velocity was more pronounced (23+/-37%) in these cases. After reperfusion, the microcirculatory variables rapidly returned to baseline. All rats in the ischemic group had infarcts 24 hours after occlusion.

CONCLUSIONS

Peri-infarct depolarization has an adverse influence on penumbral microcirculation, reducing capillary perfusion by erythrocytes, despite dilatation of arterioles. These findings suggest that a steal phenomenon contributes to the deleterious effect of these depolarizations.

Dynamic changes in cortical NADH fluorescence and direct current potential in rat focal ischemia: relationship between propagation of recurrent depolarization and growth of the ischemic core

Forty rats were subjected to 3 hours of focal ischemia by occluding the left middle cerebral and left common carotid arteries. The propagation of recurrent depolarization around the ischemic core was analyzed using direct-current potential and NADH (reduced nicotinamide adenine dinucleotide) fluorescence images by irradiating the parietal-temporal cortex with ultraviolet light. Based on histological evaluation at direct-current recording sites, the total time of depolarization causing 50% neuronal injury was estimated to be 18.2 minutes. The sites showing recurrent depolarizations resulted in 23 +/- 29% neuronal injury due to the short depolarization time, whereas the sites showing recurrent depolarizations and eventually persistent depolarization resulted in infarction. The NADH fluorescence images showed that recurrent depolarizations propagated along the margin of the ischemic core. In 85.9% of the recurrent depolarizations, the fluorescence disappeared without leaving any traces and did not affect the area of the ischemic core. However, in 47.5% of the animals, 14.1% of recurrent depolarizations merged with the ischemic core and increased the area by 6 +/- 4 mm(2). These findings suggest that recurrent depolarization increases the severity of neuronal injury but does not cause infarction by itself if persistent depolarization does not follow, and that the area of persistent depolarization is enlarged with 14.1% of recurrent depolarizations.

Salvaging the ischaemic penumbra: more than just reperfusion?

1. The ischaemic penumbra is defined as a moderately hypoperfused region that retains structural integrity but has lost function. In animal models of ischaemic stroke, this region is prone to recurrent anoxic depolarization and will become infarcted if reperfusion does not occur. In the macaque model, an ischaemic penumbra has been identified for up to 3 h after ischaemic stroke onset, whereas in selected human patients it may exist for up to 48 h. 2. Although most definitions of the ischaemic penumbra stress a time-brain volume concept, few incorporate the idea that selective and delayed neuronal injury plays an important role. Thus, in addition to necrotic cell death caused by acute injury, it is important to also consider delayed death mediated by caspase-dependent and -independent apoptotic pathways. 3. Salvage of penumbral tissue is possible if reperfusion (e.g. after thrombolysis) occurs. However, neurons within this salvaged region may be still at risk of further delayed neuronal injury. 4. In the present review, we aim to revisit the concept of the ischaemic penumbra and explore the role of selective and delayed neuronal injury in enlargement of the volume of infarction, as well as pathogenic mechanisms of white matter ischaemia. Both animal and human models of cerebral ischaemia imaged using magnetic resonance and positron emission tomography techniques will be discussed.

Depolarization induces insulin-like growth factor binding protein-2 expression in vivo via NMDA receptor stimulation

The effect of depolarization and N-methyl-D-aspartate (NMDA) receptor blockade on insulin-like growth factor-I (IGF-I), IGF binding protein-2 (IGFBP-2) and IGFBP-4 expression was analysed in vivo. Depolarization was induced in adult rat brains by applying 3 M KCl to the exposed cortex for 10 min. A subgroup of animals also received daily injections of MK-801. Four days after KCl exposure, the brains were analysed by in situ hybridization, immunohistochemistry and TUNEL. A significant upregulation of IGFBP-2 mRNA and protein was detected in astrocytes after KCl exposure This upregulation was reduced by MK-801 treatment. No alterations in IGF-I or IGFBP-4 mRNA levels were noted. We did not detect TUNEL positive cells, morphological signs of necrosis or apoptosis, or neuronal loss in the depolarized zone. Taken together, these findings indicate that upregulation of IGFBP-2 by depolarization is mediated by NMDA receptors, and, as no neuronal damage was detected, astrocytic NMDA receptors may be responsible for this upregulation.

Induction of astrocytic nestin expression by depolarization in rats

Nestin is expressed in central nervous system (CNS) progenitor cells and its expression in mature cells represents transition to a less differentiated cellular state under cellular stress. This study was performed to corroborate the hypothesis that nestin synthesis is induced by depolarization and dependent on N-methyl-D-aspartate (NMDA)-receptor activation. Depolarization was induced with application of potassium chloride on the exposed rat cortex and nestin expression was evaluated by immunohistochemistry. Depolarization induced astrocytic nestin expression that was local, or evident in the entire ipsilateral cortex depending on the time of exposure. Nestin expression was NMDA-receptor-dependent since MK-801 treatment abolished the response. Understanding the mechanisms for nestin expression is important since this protein is expressed in reactive and less differentiated CNS cell states and also in neural stem cells. Insights into the control of nestin expression may also provide means for controlling differentiation of CNS cells either post-trauma/ischemia or in transplantation strategies.

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

Asymmetric propagation of spreading depression along the anteroposterior axis of the cerebral cortex in mice

The purpose of this study was to ascertain whether or not spreading depression (CSD) propagates symmetrically along the anteroposterior axis of the cortex of mice, and to determine where CSD should be elicited to achieve a uniform exposure of the cortex to this phenomenon. Experiments were performed in halothane-anesthetized mice, with three different locations aligned 1.5 mm from the midline used for either KCl elicitation of CSD or the recording of its propagation. Our results demonstrated that, at least in the mouse cortex, CSD propagated much more effectively from posterior to anterior regions than in the opposite direction. This feature was due to a different efficacy of propagation in the two opposite directions, and not to a reduced susceptibility of occipital regions to CSD elicitation. Heterogeneous CSD propagation constitutes a potential pitfall for neurochemical studies of post-CSD changes in mice, as brain tissue samples collected for this purpose should be uniformly exposed to CSD. Occipital sites for CSD induction are clearly optimal for this purpose. If CSD propagation is confirmed to be more effective from posterior to anterior regions in other species, this may be relevant to the pathophysiology of classical migraine because the most frequent aura symptoms (i.e., visual disturbances) originate in the occipital cortex.

Relationship between metabolic dysfunctions, gene responses and delayed cell death after mild focal cerebral ischemia in mice

The evolution of brain injury was examined in mice subjected to focal cerebral ischemia as induced by 30 min of intraluminar thread occlusion of the middle cerebral artery, followed by 3 h to 3 days of reperfusion. Metabolic dysfunctions were studied by 3H-leucine autoradiography for the measurement of cerebral protein synthesis and by regional ATP bioluminescent imaging. Metabolic changes were compared with responses of the genes c-fos, c-jun, heat-shock protein gene (hsp)72, p53-activated gene (pag)608 and caspase-3, which were investigated by in situ hybridization histochemistry and immunocytochemistry, and correlated with the degree of DNA fragmentation, as assessed by the terminal TdT-mediated dUTP-biotin nick end labeling method. Intraluminar thread occlusion led to a reproducible reduction of cerebral laser Doppler flow to 20-30% of control. Thread withdrawal was followed by a short-lasting post-ischemic hyperperfusion to approximately 120%. In non-ischemic control animals, fractional protein synthesis values of 0.81+/-0.26 and 0.94+/-0.23 were obtained. Thread occlusion resulted in a suppression of protein synthesis throughout the territory of the middle cerebral artery after 3 h of reperfusion (0.04+/-0.08 in caudate-putamen and 0.14+/-0.19 in somatosensory cortex, P<0.05). Protein synthesis partly recovered in the cortex after 24 h and 3 days (0.71+/-0.40 and 0.63+/-0.26, respectively), but remained suppressed in the caudate-putamen (0.14+/-0.22 and 0.28+/-0.28). Regional ATP levels did not show any major disturbances at the reperfusion times examined. Thread occlusion resulted in a transient increase of c-fos mRNA levels in ischemic and non-ischemic parts of the cortex and caudate-putamen at 3 h after ischemia, which suggests that spreading depressions were elicited in the tissue. At the same time, c-jun and hsp72 mRNAs were elevated only in ischemic brain areas showing inhibition of protein synthesis. C-fos and c-jun responses completely disappeared within 24 h of reperfusion. Hsp72 mRNA levels remained elevated in the cortex after 24 h, but decreased to basal values in the caudate-putamen. Twenty-four hours after reperfusion, pag608 and caspase-3 mRNA levels increased in the caudate-putamen, where protein synthesis rates were still reduced, and remained elevated even after 3 days. However, pag608 and caspase-3 mRNA levels did not increase in the cortex, where protein synthesis recovered. After 24 h and 3 days, functionally active p20 fragment of caspase-3 was detected in the caudate-putamen, closely associated with the appearance of DNA fragmented cells. Neither activated caspase-3 nor DNA fragmentation were noticed in the cortex.In summary, the suppression of protein synthesis is reversible in the ischemia-resistant cortex following 30 min of thread occlusion in mice, but persists in the vulnerable caudate-putamen. In the caudate-putamen, apoptotic programs are induced, closely in parallel with the manifestation of delayed cell death. Thus, the recovery of protein synthesis may be a major factor influencing tissue survival after transient focal ischemia.

Different dynamic patterns of extracellular glutamate release in rat hippocampus after permanent or 30-min transient cerebral ischemia and histological correlation

The extent and severity of neuronal damage is different in ischemia with reperfusion compared to ischemia and no reperfusion. To investigate the role of glutamate in cerebral ischemia-reperfusion injury, in vivo microdialysis was performed to examine the dynamic profile of glutamate in the hippocampus in a transient (30 min) or permanent middle cerebral artery occlusion (MCAo) in Wistar rats. The extracellular concentration of glutamate in the cornu ammonis (CA)1 sector of the ipsilateral hippocampus showed a significant but transient elevation of glutamate for both groups immediately following ischemic insult. The initial high peak in glutamate levels in the transient MCAo group was followed by two secondary elevations in glutamate at 50 min and 90 min after initialization of reperfusion. The histopathological outcome was also different in the two groups. The observation that glutamate releases occurred in the early reperfusion phase provided an evidence of additional excitotoxicity of glutamate and thereby a therapeutic base for extended use of glutamate antagonist in the ischemia-reperfusion injury.

Spreading depression induces permanent cell swelling under penumbra conditions

BACKGROUND

Spreading depression (SD) is known to go along with temporary breakdown of ion gradients and cell swelling which spontaneously normalizes. Here, the effects of SD at reduced flow conditions as encountered in the ischemic penumbra are examined.

METHODS

In rats the right carotid artery was permanently occluded. MABP was lowered to 50 mmHg for 30 min. This is sufficient to reduce CBF to penumbra-like conditions in the right hemisphere. The following parameters were assessed: rCBF, DC potential, and tissue impedance. 5 or 15 min after onset of flow reduction one SD wave was initiated by microinjection of KCl. Histology was performed after 7 days.

RESULTS

In animals with hypotension there was depolarization resembling anoxic depolarization after SD induction and an uncoupling of CBF and metabolism only in the right hemisphere. Impedance increased with SD but did not recover spontaneously as long as rCBF remained reduced. 15 min of SD-induced cell swelling was tolerated without permanent damage, whereas 25 min were followed by severe neuron loss in the affected cortex after 7 days.

CONCLUSIONS

The study demonstrates the induction of penumbra conditions in the cortex of one hemisphere. SD is followed by cell swelling which persists as long as flow is critically reduced. The experiments illustrate how peri-infarct depolarizations may detrimentally affect the penumbra.

Effective reduction of neuronal death by inhibiting gap junctional intercellular communication in a rodent model of global transient cerebral ischemia

Gap junctions assemble astrocytes into syncytia, allowing exchange of metabolites, catabolites, and second-messenger molecules. Connexin43 is the predominant connexin of astrocytic gap junctions. The distribution of gap junction protein connexin43 was analyzed in different subfields of the hippocampal formation as a function of time after transient forebrain ischemia. One decisive key step in understanding why an ischemic insult gradually expands may be to establish how gap junction channels permit dying cells in the ischemic focus to communicate, in particular, with viable cells. The role of gap junctional intercellular communication in the hippocampus under ischemic conditions could be decisive for cell death propagation. We found that the vulnerable CA1/CA2 subfields have a higher density of gap junctions than the resistant CA3/CA4 areas, that changes in the distribution of connexin43 immunoreactivity may correlate with the phenomenon of selective vulnerability, and that inhibition of astrocytic gap junction permeability by octanol restricts the flow of undesirable neurotoxins that could potentially exacerbate neuronal damage. This provides a novel perspective for analysis of the pathophysiology of cerebral ischemia.