The role of spreading depression in focal ischemia evaluated by diffusion mapping

Sodium ion apparent diffusion coefficient in living rat brain

The apparent diffusion coefficient (ADC) of Na(+) was determined in live rat brain. The brain extracellular-to-intracellular Na(+) content ratio is approximately 8:2, which is the inverse of that for water in these spaces. Consequently, the ADC of Na(+) is primarily affected by motion in the extracellular space, and Na(+) can be viewed as a reporter molecule for motion in that space. Likewise, water ADC is dominated by intracellular motion. The brain Na(+) ADC was 1.15 +/- 0.09 microm(2)/ms, which is 61% of the aqueous Na(+) free diffusion coefficient (D(free)) at 37 degrees C (1.9 microm(2)/ms), while the ADC for brain water is 28% of the water D(free) at 37 degrees C (3 microm(2)/ms). This suggests that the ADC of molecular species within the extracellular space is roughly twofold that within the intracellular space. In postmortem brain, both Na(+) and water decrease to 17% of the respective D(free) values. These results are consistent with Na(+) and water ADC values sharing the same biophysical determinants in postmortem brain. The observed difference between Na(+) and water ADC/D(free) ratios in living brain tissue may be attributable to the extracellular environment hindering molecular displacements twofold less than the intracellular environment.

Dr. Bernice Grafstein’s paper on the mechanism of spreading depression

This essay looks at the historical significance of one APS classic paper that is freely available online:

Grafstein B. Mechanism of spreading cortical depression. J Neurophysiol 19: 154–171, 1956 ( http://jn.physiology.org/cgi/reprint/19/2/154 ).

Bilateral induction of the S-100A9 gene in response to spreading depression is modulated by the cyclooxygenase-2 activity

Cyclooxygenase-2 (COX-2) was reported to be induced in the infarcted human brain. Spreading depression (SD) is thought to play a role in this induction. In this study, we correlated the expression of SD-associated genes with COX-2 production in brains after SD. Rats were divided into 3 groups: rats that did not undergo SD (group I saline controls, n=7), rats that underwent unilateral SD as a result of KCl application (group II, n=9), and rats that were pretreated with the selective COX-2 inhibitor, JTE-522 3 h before the induction of SD (group III, n=7). The expression of the SD-associated genes, S-100A9, and mitogen-activated proteinkinase phosphatase (cpg21) was analyzed 2 h later using a cDNA array. In group II, COX-2 and cpg21 mRNA expression, as determined by RT-PCR, were significantly upregulated in the hemisphere undergoing SD. While the expression of S-100A9 mRNA was bilaterally upregulated in these animals, this expression was significantly reduced in group III, and was accompanied by reduced bilateral production of PGE(2). Thus, the bilateral induction of expression of the S-100A9 gene in response to SD was associated with COX-2 activation.

Spreading depression in vivo potentiates electrically-driven responses in frog optic tectum

This is the first description of an in vivo potentiation phenomenon associated to spreading depression (SD) in the frog optic tectum. Field potential responses electrically-elicited from the optic tract and recorded in the optic tectum disappeared during KCl-elicited SD and recovered 10-20 min thereafter. Post-SD responses reached amplitudes 10-30% higher than their pre-SD values (P<0.05), indicating a potentiation effect. Current source density analysis of the tectal depth profiles of field-potential responses, as well as the calculation of the post-SD intratectal conductance changes, also supported the potentiation phenomenon. This in vivo potentiation lasted for 40-90 min, suggesting a post-SD enhancement of synaptic transmission, which may be important in understanding mechanisms of brain disfunctions like epilepsy.

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.

Dynamic changes in brain glucose and lactate in pericontusional areas of the human cerebral cortex, monitored with rapid sampling on-line microdialysis: relationship with depolarisation-like events

The pathophysiology of peri-lesion boundary zones in acute brain injury is highly dynamic, and it is now clear that spreading-depression-like events occur frequently in areas of cerebral cortex adjacent to contusions in the injured human brain. An automated method to assay microdialysate from peri-lesion cerebral cortex in 11 patients with intracranial haematomas requiring surgery was used. Perfusate (2 microL/min) flowed directly into a flow-injection system for assay of glucose and lactate at intervals typically of 30 secs each. Four channels of electrocorticogram (ECoG) were recorded from a subdural strip adjacent to the catheter. Several patterns of change in metabolites were identified in different time domains. Overall, the number of transient lactate events was significantly correlated with the number of glucose events (r2=0.48, P=0.027, n=10). Progressive reduction in dialysate glucose was very closely correlated with the aggregate number of ECoG events (r2=0.76, P=0.0004, n=11). It is proposed that the recently documented adverse impact of low dialysate glucose on clinical outcome may be because of recurrent, spontaneous spreading-depression-like events in the perilesion cortex.

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.

Combination therapy of moderate hypothermia and thrombolysis in experimental thromboembolic stroke--an MRI study

Thrombolysis (T) is limited by reperfusion-associated injury and the short therapeutic window after stroke onset. The present study investigates whether hypothermia alone or in combination with thrombolysis has beneficial effects after experimental thromboembolic stroke. Wistar rats (n = 60) were subjected to thromboembolic occlusion (TE) of the middle cerebral artery (MCA). Thrombolysis (T) was performed with intravenous recombinant tissue-plasminogen activator (rt-PA) 1 h (early T) or 3 h (late T) after TE. Hypothermia (Hy) was applied for 4 h at 33 degrees C started 1 h after TE. Experimental groups included control (C), early thrombolysis (ET), late thrombolysis (LT), hypothermia (Hy), early thrombolysis plus hypothermia (ET+Hy), and late thrombolysis plus hypothermia (LT+Hy). Animals were investigated by MRI and silver infarct staining (SIS) to assess the cerebral infarct size. All animals of group Hy survived, in contrast to 40% in group C (P < 0.05). ET+HY and LT+Hy showed a trend towards better survival as compared to ET and LT alone. PWI parameters were not significantly different between ET versus ET+HY and LT versus LT+Hy, but rt-PA administration led to improved cerebral perfusion in MRI. Significant differences in infarct volumes (T2/SIS) were found after 24 h in all treatment groups versus the control group (P < 0.05). The lesion volume calculated from T2 was significantly smaller in ET (16% +/- 5%), ET+Hy (10 +/- 4%), and LT+Hy (20% +/- 9%) after 5.5 h (10.8% +/- 4.8%) versus C (42% +/- 15%), (P < 0.05). These data indicate that hypothermia improves survival and decreases infarct volume. However, there were no significant differences between the use of rt-PA alone or in combination with hypothermia. Further studies are needed to confirm these effects, also several days after stroke onset.

Cortical spreading depression produces long-term disruption of activity-related changes in cerebral blood volume and neurovascular coupling

Cortical spreading depression (CSD) is a pronounced depolarization of neurons and glia that spreads slowly across the cortex followed by a period of depressed electrophysiological activity. The vascular changes associated with CSD are a large transient increase in blood flow followed by a prolonged decrease lasting greater than 1 h. Currently, the profile of functional vascular activity during this hypovolemic period has not been well characterized. Perfusion-based imaging techniques such as functional magnetic resonance imaging (fMRI) assume a tight coupling between changes in neuronal and vascular activity. Under normal conditions, these variables are well correlated. Characterizing the effect of CSD on this relationship is an important step to understand the impact acute pathophysiological events may have on neurovascular coupling. We examine the effect of CSD on functional changes in cerebral blood volume (CBV) evoked by cortical electrophysiological activity for 1 h following CSD induction. CBV signal amplitude, duration, and time to peak show little recovery at 60 min post-induction. Analysis of spontaneous vasomotor activity suggests a decrease in vascular reactivity may play a significant role in the disruption of normal functional CBV responses. Electrophysiological activity is also attenuated but to a lesser degree. CBV and evoked potentials are not well correlated following CSD, suggesting a breakdown of the neurovascular coupling relationship.

Effects of physiologic challenge on the ADC of intracellular water in the Xenopus oocyte

The biophysical determinants of the intracellular water apparent diffusion coefficient (ADC) in mammalian tissues are poorly understood. Model systems that are more amenable to physical measurements may provide insights into the behavior of more complex systems. Toward that end, we used MRI to evaluate the effects of altered microtubule concentration, nuclear breakdown, and ATP depletion on intracellular water ADC in the Xenopus oocyte. Water ADC did not change in response to polymerization of microtubules with taxol or depolymerization with nocodazole. Water ADC did not change following the breakdown of the nucleus in healthy cells. Short-term depletion of ATP (approximately 20% of normal levels following 4 hr of exposure to sodium azide and 2-deoxy-D-glucose) was not associated with a change in intracellular ADC. Long-term depletion of ATP (approximately 20% of normal levels following 2 days of exposure to antimycin A) was associated with a significant decrease in intracellular water ADC. These findings suggest that intracellular water diffusion in oocytes is not dependent on the state of microtubule polymerization or short-term ATP depletion, although long-term ATP depletion is associated with changes that lead to a decrease in intracellular water ADC.

Pronounced hypoperfusion during spreading depression in mouse cortex

We studied unique cerebral blood flow (CBF) responses to cortical spreading depression in mice using a novel two-dimensional CBF imaging technique, laser speckle flowmetry. Cortical spreading depression caused a triphasic CBF response in both rat and mouse cortex. In rats, mild initial hypoperfusion (approximately 75% of baseline) was followed by a transient hyperemia reaching approximately 220% of baseline. In mice, the initial hypoperfusion was pronounced (40-50% of baseline), and the anticipated hyperemic phase barely reached baseline. The duration of hypoperfusion significantly correlated with the duration of the DC shift. As a possible explanation for the pronounced hypoperfusion, mouse cerebral vessels showed enhanced resistance to relaxation by acetylcholine (3 microM) after K+ -induced preconstriction (20, 40, and 80 mM) but dilated normally in response to acetylcholine after preconstriction with U46619, a synthetic thromboxane A2 analog. By contrast, rat vessels dilated readily to acetylcholine after preconstriction by K+. The transient normalization of CBF after hypoperfusion in the mouse was abolished by L-NA but not 7-NI. In summary, the CBF response to cortical spreading depression in mice contrasts with the rat in that the initial hypoperfusion is pronounced, and the hyperemic phase is markedly diminished. The differences in CBF response between species may be in part caused by an increased sensitivity of mouse cerebral vessels to elevated extracellular K+.

Nuclear magnetic resonance (NMR) measurement of the apparent diffusion coefficient (ADC) of tissue water and its relationship to cell volume changes in pathological states

Diffusion-weighted nuclear magnetic resonance (NMR) imaging (DWI) is sensitive to the random translational motion of water molecules due to Brownian motion. Although the mechanism is still not completely understood, the cellular swelling that accompanies cell membrane depolarization results in a reduction in the net displacement of diffusing water molecules and thus a concomitant reduction in the apparent diffusion coefficient (ADC) of tissue water. Cerebral regions of reduced ADC appear hyperintense in a DWI and this technique has been used extensively to study acute stroke. In addition to cerebral ischemia, reductions in the ADC of cerebral water have been observed following cortical spreading depression, ischemic depolarizations (IDs), transient ischemic attack (TIA), status epilepticus, and hypoglycemia. Although the mechanism responsible for initiating membrane depolarization varies in each case, the ensuing cell volume changes follow a similar pattern. Water ADC values are also affected by the presence and orientation of barriers to translational motion (such as cell membranes and myelin fibers) and thus NMR measures of anisotropic diffusion are sensitive to more chronic pathological states where the integrity of these structures is modified by disease. Both theoretical prediction and experimental evidence suggest that the ADC of tissue water is related to the volume fraction of the interstitial space via the electrical conductivity of the tissue. The implication is that acute neurological disorders that exhibit electrical conductivity changes should also exhibit ADC changes that are detectable by DWI. A qualitative correlation between electrical conductivity and the ADC of water has been demonstrated in a number of animal model studies and the results indicate that reduced ADC values are associated with reductions in the extracellular volume fraction and increased extracellular tortuosity. The close relationship between ADC changes and cell volume changes in various pathological states suggests that NMR measurements are also sensitive to chemical communication between cells through the extracellular space (i.e., extrasynaptic or volume transmission, VT).

Multiplexed cytokine protein expression profiles from spreading depression in hippocampal organotypic cultures

Cytokines are involved in ischemic tolerance, including that triggered by spreading depression (SD), yet their roles in neuroprotection remain incompletely defined. The latter may stem from the pleiotropic nature of these signaling molecules whose complexities for interaction might be better deciphered through simultaneous measurement of multiple targeted proteins. Accordingly, the authors used microsphere-based flow cytometric immunoassays and hippocampal organotypic cultures (HOTCs) to characterize the magnitude, time course, and diversity of cytokine (interleukin [IL] 1alpha, IL-1beta, IL-2, IL-4, IL-6, IL-10, granulocyte-macrophage colony-stimulating factor [GM-CSF], interferon-gamma [IFN-gamma], and tumor necrosis factor-alpha [TNF-alpha]) response to SD. GM-CSF was not detected in HOTCs or media. However, SD triggered a significant, generalized increase in seven cytokines evident in HOTCs 6 hours later, with the remaining cytokine, IL-1beta, becoming significantly different at 1 and 3 days. Additionally, these changes extended to include surrounding media for IL-6 and TNF-alpha by 1 and 3 days. This increase was localized to microglia via immunostaining for IL-1alpha, IL-1beta, and interferon-y. IL-10, although significantly more abundant in HOTCs 6 hours after SD, was significantly less abundant in surrounding media at that time and at 1 day. Finally, the generalized early increase in tissue cytokines later settled to a pattern at 3 days of recovery centering on changes in IL-1alpha, IL-1beta, and TNF-alpha, cytokines capable of modulating ischemic injury.

The effect of rotational angle and experimental parameters on the diffraction patterns and micro-structural information obtained from q-space diffusion NMR: implication for diffusion in white matter fibers

Diffusion NMR may provide, under certain experimental conditions, micro-structural information about confined compartments totally non-invasively. The influence of the rotational angle, the pulse gradient length and the diffusion time on the diffusion diffraction patterns and q-space displacement distribution profiles was evaluated for ensembles of long cylinders having a diameter of 9 and 20 microm. It was found that the diffraction patterns are sensitive to the rotational angle (alpha) and are observed only when diffusion is measured nearly perpendicular to the long axis of the cylinders i.e., when alpha= 90 degrees +/- 5 degrees under our experimental conditions. More importantly, we also found that the structural information extracted from the displacement distribution profiles and from the diffraction patterns are very similar and in good agreement with the experimental values for cylinders of 20 microm or even 9 microm, when data is acquired with parameters that satisfy the short gradient pulse (SGP) approximation (i.e., delta -->0) and the long diffusion time limit. Since these experimental conditions are hardly met in in vitro diffusion MRI of excised organs, and cannot be met in clinical MRI scanners, we evaluated the effect of the pulse gradient duration and the diffusion time on the structural information extracted from q-space diffusion MR experiments. Indeed it was found that, as expected, accurate structural information, and diffraction patterns are observed when Delta is large enough so that the spins reach the cylinders' boundaries. In addition, it was found that large delta results in extraction of a compartment size, which is smaller than the real one. The relevance of these results to q-space MRI of neuronal tissues and fiber tracking is discussed.

Investigation of techniques to quantify in vivo lesion volume based on comparison of water apparent diffusion coefficient (adc) maps with histology in focal cerebral ischemia of rats

Stroke lesion-volume estimates derived from calculated water apparent diffusion coefficient (ADC) maps provide a quantitative surrogate end-point for investigating the efficacy of drug treatment or studying the temporal evolution of cerebral ischemia. Methodology is described for estimating ischemic lesion volumes in a rat model of permanent middle cerebral artery occlusion (MCAO) based on absolute and percent-reduction threshold values of the water ADC at 3 h post-MCAO. Volume estimates derived from average ADC (ADC(av)) maps were compared with those derived from post-mortem histological sections. Optimum ADC thresholds were established as those that provided the best correlation and one-to-one correspondence between ADC- and histologically derived lesion-volume estimates. At 3 h post-MCAO, an absolute-ADC(av) threshold of 47 x 10(-5) mm(2)/s (corresponding to a 33% reduction in ADC(av) based on a contralateral hemisphere comparison) provided the most accurate estimate of percent hemispheric lesion volume (%HLV). Experimental and data analysis issues for improving and validating the usefulness of DWI as a surrogate endpoint for the quantification of ischemic lesion volume are discussed.

Cerebral perfusion and stroke

Stroke is a heterogeneous syndrome caused by multiple disease mechanisms, but all result in a disruption of cerebral blood flow with subsequent tissue damage. This review covers the mechanisms responsible for regulation of the normal cerebral circulation, and how they are disrupted in disease states. A central concept in treating patients with acute ischaemic stroke is the existence of an ischaemic penumbra of potentially salvageable tissue, and the evidence for its existence in humans is reviewed.

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.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Relationship between Intracranial Pressure and Cortical Spreading Depression following Fluid Percussion Brain Injury in Rats

Traumatic brain injury (TBI) is known to be accompanied by an increase in intracranial pressure (ICP) and in some cases, by spontaneous generation of cortical spreading depression (CSD) cycles. However, the role of CSD in the pathophysiology of cerebral contusion is still unknown. A multiparametric monitoring assembly was placed on the right hemisphere of the rat brain to evaluate ICP, DC potential, extracellular K(+), cerebral blood flow (CBF), and electrocorticogram in 27 rats during 5 h. Fluid percussion brain injury (FPBI) with the magnitude of the impact 2.9, 3.3, 4.1, and 5.0 atmospheres was induced to the left parietal cortex in animal groups A, B, C, and D, respectively. A slow increase in ICP was evident, and was pronounced in group C and especially in group D, where four of nine animals died during the monitoring. At the end of the 5 h experiment, the mean ICP levels were 6.75 +/- 2.87, 8.40 +/- 2.70, 12.75 +/- 4.03, 29.56 +/- 9.25, and the mean total number of CSD cycles was 2.00 +/- 1.41, 4.29 +/- 4.23, 11.71 +/- 13.29, and 20.11 +/- 19.26 in groups A, B, C, and D, respectively. The maximal level of intensity of CSD cycle generation after FPBI was obtained in group D, where almost constant activity was maintained until the end of the experiment. A significant coefficient of correlation between ICP level and total number of CSD cycles was found for all ICP measurements (r = 0.47-0.63, p < 0.05, n = 27), however more significant (p < 0.001) was the coefficient during the period of monitoring between 2 and 4 h after FPBI. Our results suggest that numerous repeating CSD cycles are typical phenomena in moderate and especially severe forms of FPBI. The rising number of CSD cycles under condition of an ICP level >/=20 mm Hg may demonstrate, with high probability, the unfavorable development of TBI, caused by growing secondary hypoxic insult.

Cyclooxygenase-2 expression associated with spreading depression in a primate model

The authors previously provided evidence that spreading depression (SD) can be evoked in primates. Cyclooxygenase-2 (COX-2) expression has been found to increase in the rodent cortex undergoing SD, and the authors sought to determine whether this association exists in primate brain. In the present study, neuronal COX-2 expression was induced during SD in the primate cortex. The mean expression ratio of COX-2 messenger RNA in animals with SD was significantly higher than that measured in controls (1.69 vs. 0.5; P = 0.02). Induction of COX-2 in these animals was also detected by human microarray analysis. Results show that, as in rodents, neuronal COX-2 is induced in the primate cortex in response to SD.

MR imaging of ischemic penumbra

Cerebral ischemic stroke is one of the most fatal diseases despite current advances in medical science. Recent demonstration of efficacy using intravenous and intra-arterial thrombolysis demands therapeutic intervention tailored to the physiologic state of the individual tissue and stratification of patients according to the potential risks for therapies. In such an era, the role of the neuroimaging becomes increasingly important to evaluate the extent and location of tissues at risk of infarction (ischemic penumbra), to distinguish it from unsalvageable infarcted tissues or doomed hemorrhagic parenchyma. In this review, we present briefly the current role and limitation of computed tomography and conventional magnetic resonance imaging (MRI). We also present the possible applications of advanced MR techniques, such as diffusion and perfusion imaging, concentrating on the delineation or detection of ischemic penumbra.

Astrocytes and stroke: networking for survival?

Astrocytes are now known to be involved in the most integrated functions of the central nervous system. These functions are not only necessary for the normally working brain but are also critically involved in many pathological conditions, including stroke. Astrocytes may contribute to damage by propagating spreading depression or by sending proapoptotic signals to otherwise healthy tissue via gap junction channels. Astrocytes may also inhibit regeneration by participating in formation of the glial scar. On the other hand, astrocytes are important in neuronal antioxidant defense and secrete growth factors, which probably provide neuroprotection in the acute phase, as well as promoting neurogenesis and regeneration in the chronic phase after injury. A detailed understanding of the astrocytic response, as well as the timing and location of the changes, is necessary to develop effective treatment strategies for stroke patients.

The macrosphere model: evaluation of a new stroke model for permanent middle cerebral artery occlusion in rats

BACKGROUND AND PURPOSE

The suture middle cerebral artery occlusion (MCAO) model is widely used for the simulation of focal cerebral ischemia in rats. This technique causes hypothalamic injury resulting in hyperthermia, which can worsen outcome and obscure neuroprotective effects. Herein, we introduce a new MCAO model that avoids these disadvantages.

METHODS

Permanent MCAO was performed by intraarterial embolization using six TiO(2) macrospheres (0.3-0.4 mm in diameter) or by the suture occlusion technique. Body temperature was monitored, functional and histologic outcome was assessed after 24 h. Additional 16 rats were subjected to macrosphere or suture MCAO. Lesion progression was evaluated using magnetic resonance imaging (MRI).

RESULTS

The animals subjected to suture MCAO developed hyperthermia (>39 degrees C), while the temperature remained normal in the macrosphere MCAO group. Infarct size, functional outcome and model failure rate were not significantly different between the groups. Lesion size on MRI increased within the first 90 min and remained unchanged thereafter in both groups.

CONCLUSIONS

The macrosphere MCAO model provides reproducible focal cerebral ischemia, similar to the established suture technique, but avoids hypothalamic damage and hyperthermia. This model, therefore, may be more appropriate for the preclinical evaluation of neuroprotective therapies and can also be used for stroke studies under difficult conditions, e.g., in awake animals or inside the MRI scanner.

Neuroimaging of animal models of brain disease

The main aim of this review is to describe some of the many animal models that have proved to be valuable from a neuroimaging perspective. This paper complements other articles in this volume, with a focus on animal models of the pathology of human brain disorders for investigations with modern non-invasive neuroimaging techniques. The use of animal model systems forms a fundamental part of neuroscience research efforts to improve the prevention, diagnosis, understanding and treatment of neurological conditions. Without such models it would be impossible to investigate such topics as the underlying mechanisms of neuronal cell damage and death, or to screen compounds for possible anticonvulsant properties. The adequacy of any one particular model depends on the suitability of information gained during experimental conditions. It is important, therefore, to understand the various types of animal model available and choose an appropriate model for the research question.

High b-value q-space analyzed diffusion-weighted MRS and MRI in neuronal tissues - a technical review

This review deals with high b-value q-space diffusion-weighted MRI (DW-MRI) of neuronal tissues. It is well documented that at sufficiently high b-values (and high q-values) neuronal water signal decay in diffusion experiments is not mono-exponential. This implies the existence of more than one apparent diffusing component or evidence for restriction. The assignment of the different apparent diffusing components to real physical entities is not straightforward. However, the apparent slow diffusing component that was found to be restricted to a compartment of a few microns, if originating mainly from a specific pool and if assigned correctly, may potentially be used to obtain more specific MR images with regard to specific pathologies of the CNS. This review examines the utility of analyzing high b-value diffusion MRS and MRI data using the q-space approach introduced by Callaghan and by Cory and Garroway. This approach provides displacement probability maps that emphasize, at long diffusion times, the characteristics of the apparent slow diffusing component. Examples from excised spinal cord, where the experimental conditions for which the q-space analysis of MR diffusion data was developed can be met or approached will be presented. Then examples from human MS patients, where q-space requirement for the short gradient pulse is clearly violated, are presented. In the excised spinal cord studies, this approach was used to study spinal cord maturation and trauma, and was found to be more sensitive than other conventional methods in following spinal cord degeneration in an experimental model of vascular dementia (VaD). High b-value q-space DWI was also recently used to study healthy and MS diseased human brains. This approach was found to be very sensitive to the disease load in MS, compared with other conventional MRI methods, especially in the normal appearing white matter (NAWM) of MS brains. Finally, the potential diagnostic capacity embedded in high b-value q-space analyzed diffusion MR images is discussed. The potentials and caveats of this approach are outlined and experimental data are presented that show the effect of violating the short gradient pulse (SGP) approximation on the extracted parameters from the q-space analysis.

Multiwavelength optical intrinsic signal imaging of cortical spreading depression

Cortical spreading depression (CSD) is an important disease model for migraine and cerebral ischemia. In this study, we exploit the high temporal and spatial resolution of optical imaging to characterize perfusion-dependent and -independent changes in response to CSD and to investigate the etiology of reflectance changes during CSD. In this experiment, we characterized the optical response to CSD at wavelengths that emphasize perfusion-related changes (610 and 550 nm), and we compared these results with 850 nm and blood volume data. Blood volume changes during CSD were recorded using an intravascular fluorescent dye, Texas Red dextran. We observed triphasic optical signals at 850 and 550 nm characterized by spreading waves of increased, decreased, then increased reflectance (Fig. 1) which expanded at a rate of approximately 3-5 mm/min. The signal at 610 nm had a similar initial phase, but the phase 2 response was slightly more complex, with a parenchymal decrease in reflectance but a vascular increase in reflectance. Reflectance values decreased in phase three. Blood volume signals were delayed relative to the optical intrinsic signals and corresponded temporally to phases 2 and 3. This is the first study to characterize optical imaging of intrinsic signal responses to CSD, in vivo, at multiple wavelengths. The data presented here suggest that changes in light scattering precede perfusion responses, the blood volume increase (phase 2) is accompanied by a reduction in deoxyhemoglobin, and the blood volume decrease (phase 3) is accompanied by an increase in deoxyhemoglobin. Previous studies have suggested the oligemia of spreading depression was a result of decreased metabolic demand. This study suggests that during the oligemic period there is a greater reduction in oxygen delivery than in demand.

Unique profile of spreading depression in a primate model

Spreading depression (SD) is considered to play a role in pathologic conditions of humans such as in the evolution of ischemic brain injury and migraine aura. Because many studies have demonstrated spreading hypoperfusion in patients with migraine and persistent hypoperfusion in nonprimate animal models of SD, these changes in cerebral blood flow (CBF) were regarded as an epiphenomenon of SD. However, there is no direct evidence of the occurrence of SD in primates. The authors attempted to elicit SD by applying 3.3 mol/L potassium chloride to the cerebral cortex of nine male cynomolgus monkeys. The CBF was monitored by positron emission tomography in five animals. Propagated direct-current shifts were found by the two neighboring microelectrodes only in one animal. The direct-current wave propagated at a speed of 4 mm/min and its amplitude was 20 mV, being consistent with the SD findings. Except in one animal with 6 SD episodes, SD waves were recorded infrequently at the rostral site (none in three animals, once in three, and twice in two). Focal hyperemia accompanied SD. Neither spreading hypoperfusion nor persistent hypoperfusion was found. These unique features of SD in primates raise a doubt as to whether the role of SD in nonprimate animals is the same as that in stroke and migraine in humans.

Repetitive concentric wave-ring spread of oligemia/hyperemia in the sensorimotor cortex accompanying K(+)-induced spreading depression in rats and cats

Vascular changes accompanying spreading depression (SD) remain controversial. We examined dynamic alterations of local cerebral blood volume (CBV) during SD by observing light transmission at an isosbestic point of hemoglobin (550 nm) in seven rats and five cats under alpha-chloralose/urethane anesthesia. The two species were used for comparison between the lissencephalic and gyrencephalic brains. We found that a concentrated K(+) solution microinjected into the sensorimotor cortex provoked CBV changes that appeared as a repetitive propagation of concentric wave-rings of ischemia followed by hyperemia expanding peripherally from the injection site at speeds of 1.9-3.2 mm/min. The dynamic CBV changes continued repeatedly every 1-5 min for more than 30 min in three rats, ceased within 30 min in three rats and remained at the site of K(+) injection in one rat. Similar repeated CBV changes occurred in two out of five cats.

Early thrombolysis inhibits peri-infarct depolarizations in embolic MCA occlusion

Rats submitted to middle cerebral artery (MCA) clot embolism were treated with tissue plasminogen activator (TPA) 1.5 and 3.5 h post-occlusion. Reperfusion patterns were monitored by measuring cortical laser-Doppler flow; the direct current potential was measured to detect peri-infarct depolarizations (PID), a known mechanism of ischemic injury. TPA treatment induced reperfusion in 58% of treated animals that was delayed by 41 +/- 7 min (mean +/- s.e.m.) from treatment onset. The probability of reperfusion did not differ significantly between the two treatment groups. TPA treatment led to a 3-fold reduced frequency of PID if administered early or if successful reperfusion was observed (each p < 0.001). Early thrombolysis inhibits, but does not block, PID as an important mechanism of ischemic injury in embolic stroke.

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.

A glycine site antagonist ZD9379 reduces number of spreading depressions and infarct size in rats with permanent middle cerebral artery occlusion

Spreading depressions (SDs) occur in experimental focal ischemia and contribute to lesion evolution. N-methyl-D-aspartate (NMDA) antagonists inhibit SDs and reduce infarct size. The glycine site on the NMDA receptor complex offers a therapeutic target for acute focal ischemia, potentially devoid of many side effects associated with competitive and non-competitive NMDA antagonists. We evaluated the effect of the glycine antagonist, ZD9379, on SDs and brain infarction. Male Sprague-Dawley rats (n = 18) weighing 290 to 340 g undergoing permanent middle cerebral artery occlusion (MCAO) were randomly and blindly assigned to receive drug or placebo: Group 1 (pre-MCAO treatment group, n = 5) a 5 mg/kg bolus of ZD9379 over 5 minutes followed by 5 mg/kg/hour drug infusion for 4 hours beginning 30 minutes before MCAO; Group 2 (post-MCAO treatment group, n = 7) a 5 mg/kg bolus of ZD9379 30 minutes after MCAO followed by 5 mg/kg/hour drug infusion for 4 hours; and Group 3 (control group, n = 6) vehicle for 5 hours beginning 30 minutes before MCAO. SDs were monitored electrophysiologically for 4.5 hours following MCAO by continuous recording of cortical direct current (DC) potentials and electrocorticogram (ECoG). Infarct volume was measured 24 hours after MCAO by 2,3,5 triphenyltetrazolium chloride (TTC) staining. Corrected infarct volume was 90 +/- 72 mm3 (mean +/- standard deviation) in Group 1, 105 +/- 46 mm3 in Group 2, and 226 +/- 41 mm3 in Group 3 (P < .001). The corresponding numbers of SDs in the 3 groups were 8.2 +/- 5.8, 8.1 +/- 2.5, and 16.0 +/- 5.1, respectively (P < .01). When all animals (n = 18) were analyzed, infarct volumes and the number of SDs were significantly correlated (r = .68, P = .002). This study demonstrated that ZD9379 initiated before or after MCAO significantly reduced the number of SDs and infarct volume in a permanent focal ischemia model, implying that ZD9379 is neuroprotective and its neuroprotective effect may be related to inhibiting ischemia-related SDs.

Delayed rt-PA treatment in a rat embolic stroke model: diagnosis and prognosis of ischemic injury and hemorrhagic transformation with magnetic resonance imaging

The authors characterized effects of late recombinant tissue plasminogen activator (rt-PA) administration in a rat embolic stroke model with magnetic resonance imaging (MRI), to assess potential MRI correlates, or predictors, or both, of rt-PA-induced hemorrhage. Diffusion-, perfusion-, and postcontrast T1-weighted MRI were performed between 4 and 9 hours and at 24 hours after embolic stroke in spontaneously hypertensive rats. Treatment with either rt-PA or saline was started 6 hours after stroke. A spectrophotometric hemoglobin assay quantified hemorrhage severity. Before treatment, relative cerebral blood flow index (rCBFi) and apparent diffusion coefficient (ADC) in the ischemic territory were 30% +/- 23% and 60% +/- 5% (of contralateral), respectively, which increased to 45% +/- 39% and 68% +/- 4% 2 hours after rt-PA. After 24 hours, rCBFi and ADC were 27% +/- 27% and 59 +/- 5%. Hemorrhage volume after 24 hours was significantly greater in rt-PA-treated animals than in controls (8.7 +/- 3.7 microL vs. 5.1 +/- 2.4 microL, P < 0.05). Before rt-PA administration, clear postcontrast T1-weighted signal intensity enhancement was evident in areas of subsequent bleeding. These areas had lower rCBFi levels than regions without hemorrhage (23% +/- 22% vs. 36% +/- 29%, P < 0.05). In conclusion, late thrombolytic therapy does not necessarily lead to successful reperfusion. Hemorrhage emerged in areas with relatively low perfusion levels and early blood-brain barrier damage. Magnetic resonance imaging may be useful for quantifying effects of thrombolytic therapy and predicting risks of hemorrhagic transformation.

Extracellular apparent diffusion in rat brain

The apparent diffusion coefficients (ADCs) of a series of markers concentrated in the extracellular space of normal rat brain were measured to evaluate, by inference, the ADC of water in the extracellular space. The markers (mannitol, phenylphosphonate, and polyethylene glycols) are defined as "compartment selective" because tissue culture experiments demonstrate some leakage into the intracellular space, making them less "compartment specific" than commonly believed. These primarily extracellular markers have ADCs similar to those of intracellular metabolites of comparable hydrodynamic radius, suggesting that water ADC values in the intra- and extracellular spaces are similar. If this is the case, then it is unlikely that a net shift of water from the extra- to the intracellular space contributes significantly to the reduction in water ADC detected following brain injury. Rather, this reduction is more likely due primarily to a reduction of the ADC of intracellular water associated with injury.

Mild hypothermia decreases the incidence of transient ADC reduction detected with diffusion MRI and expression of c-fos and hsp70 mRNA during acute focal ischemia in rats

The effects of mild hypothermia on the apparent diffusion coefficient of water (ADC) and expression of c-fos and hsp70 mRNA were examined during acute focal cerebral ischemia. Young adult rats were subjected to 60-min middle cerebral artery occlusion under either normothermia (37.5 degrees C) or hypothermia (33 degrees C). Diffusion-weighted echo-planar magnetic resonance imaging was used to monitor changes in ADC throughout the ischemic period. Perfusion MRI with dysprosium contrast was used at the end of the ischemic period to verify that the occlusion was successful. C-fos and hsp70 mRNA expression were examined with in situ hybridization at the end of the ischemic period. The results indicate that the size of the region that exhibited reduced ADC was smaller during hypothermia than during normothermia. Hypothermia also decreased the frequency of occurrence of transient ADC reductions, especially in dorsal aspects of cortex. Expression of both c-fos and hsp70 mRNA were markedly reduced by hypothermia. Transient ADC reduction and c-fos expression are associated with spreading depression, which is believed to contribute to lesion expansion during acute focal ischemia. The results suggest that part of the neuroprotective effect of hypothermia may be due to a reduced incidence of spreading depression.

Diffusion- and perfusion-weighted magnetic resonance imaging of focal cerebral ischemia and cortical spreading depression under conditions of mild hypothermia

In a model of experimental stroke, we characterize the effects of mild hypothermia, an effective neuroprotectant, on fluid shifts, cerebral perfusion and spreading depression (SD) using diffusion- (DWI) and perfusion-weighted MRI (PWI). Twenty-two rats underwent 2 h of middle cerebral artery (MCA) occlusion and were either kept normothermic or rendered mildly hypothermic shortly after MCA occlusion for 2 h. DWI images were obtained 0.5, 2 and 24 h after MCA occlusion, and maps of the apparent diffusion coefficient (ADC) were generated. SD-like transient ADC decreases were also detected using DWI in animals subjected to topical KCl application (n=4) and ischemia (n=6). Mild hypothermia significantly inhibited DWI lesion growth early after the onset of ischemia as well as 24 h later, and improved recovery of striatal ADC by 24 h. Mild hypothermia prolonged SD-like ADC transients and further decreased the ADC following KCl application and immediately after MCA occlusion. Cerebral perfusion, however, was not affected by temperature changes. We conclude that mild hypothermia is neuroprotective and suppresses infarct growth early after the onset of ischemia, with better ADC recovery. The ADC decrease during SD was greater during mild hypothermia, and suggests that the source of the ADC is more complex than previously believed.

High speed diffusion magnetic resonance imaging of ischemia and spontaneous periinfarct spreading depression after thromboembolic stroke in the rat

Spontaneous episodes of transient cell membrane depolarization (spreading depression [SD]) occur in the surroundings of experimental stroke lesions and are believed to contribute to infarct growth. Diffusion-weighted imaging (DWI) is capable of detecting the water shifts from extracellular to intracellular space associated with SD waves and ischemia, and can make in vivo measurements of these two features on a pixel-by-pixel basis with good temporal resolution. Using continuous high speed DWI with a temporal resolution of 12 seconds over a period of 3 hours, the in vivo contribution of spontaneous SDs to the development of ischemic tissue injury was examined in 8 rats using a thromboembolic stroke model. During the observation period, the initial lesion volume increased in 4 animals, remained unchanged in 1 animal, and decreased in 3 animals (most likely because of spontaneous clot lysis). Irrespective of the lesion evolution patterns, animals demonstrated 6.5 +/- 2.1 spontaneous SDs outside of the ischemic core. A time-to-peak analysis of apparent diffusion coefficient (ADC) changes for each SD wave demonstrated multidirectional propagation patterns from variable initiation sites. Maps of the time constants of ADC recovery, reflecting the local energy supply and cerebral blood flow, revealed prolonged recovery times in areas close to the ischemic core. However, repetitive SD episodes in the periinfarct tissue did not eventually lead to permanent ADC reductions. These results suggest that spontaneous SD waves do not necessarily contribute to the expansion of the ischemic lesion volume in this model.

Failure to demonstrate peri-infarct depolarizations by repetitive MR diffusion imaging in acute human stroke

BACKGROUND AND PURPOSE

Peri-infarct depolarizations (PIDs) have been demonstrated with diffusion-weighted MRI (DWI) in experimental stroke and are regarded as an important mechanism of ischemic injury. We tested the hypothesis that PIDs are of relevance for the early enlargement of human brain infarcts.

METHODS

Ten stroke patients were investigated by repetitive imaging of the apparent diffusion coefficient (ADC) in the acute phase (7 patients) or subacute phase (3 patients) of developing cortical infarction. In each patient, 20 ADC maps were obtained from serially measured echo-planar DWI (interval of 45 seconds). Data analysis focused on the potential spatial and temporal ADC changes, including structured qualitative analysis, calculation of subtraction images, serial analysis of regions of interest positioned in the infarct core and border, and calculation of hemispheric lesion areas, depending on various ADC thresholds ranging between 0 and 800 microm(2)/s.

RESULTS

Data analysis was unable to disclose any time-dependent changes in ADC that would resemble PID. In ischemic regions, the ADC reduction significantly progressed from the infarct border (555+/-96 microm(2)/s) to the infarct core (431+/-104 microm(2)/s, P:<0.01).

CONCLUSIONS

By using an MRI protocol with high temporal resolution and elaborated postprocessing, we were unable to demonstrate a pattern of diffusion changes that would be indicative of PID in human stroke. Experimental infarction and human stroke may differ in the detectability of PID.

Inapparent hemodynamic insufficiency exacerbates ischemic damage in a rat microembolic stroke model

BACKGROUND AND PURPOSE

Patients with severe carotid artery stenosis may have more severe ischemic damage after embolic stroke than patients without this abnormality. Unilateral proximal carotid occlusion (UCO) alone typically does not induce infarction in normotensive rats. The aim of this study was to investigate whether UCO increases infarct size after microembolic, experimental stroke.

METHODS

Microembolic infarction was induced in 2 groups of Sprague-Dawley rats by injecting 2000 microspheres (50-micrometer diameter) intracranially from the external carotid artery. The common carotid artery (CCA) was either ligated just after the injection (CCA occlusion group, n=8) or left intact (CCA open group, n=8). In the control group (n=4), vehicle without microspheres was injected and the CCA was ligated. Twenty-four hours later, the brains were removed and infarct volumes measured. Perfusion-weighted imaging was used to evaluate the cerebral circulation before and after CCA occlusion with and without microsphere injection in a separate group of animals (n=16).

RESULTS

All animals in the microemboli groups survived and had only a slight hemiparesis 24 hours after occlusion. No neurological deficits were observed in the control group. Infarct volumes were 145+/-57 mm(3) in the CCA occlusion group and 45+/-26 mm(3) in the CCA open group (P <0.01). There were no infarctions detected in the control group. Perfusion-weighted imaging showed that cerebral blood flow decreased after the CCA occlusion in both experiments with and without the microsphere injection.

CONCLUSIONS

UCO alone does not induce ischemic damage, but it worsens ischemic lesion size after multiple microemboli. This is probably due to the slight cerebral perfusion insufficiency caused by UCO. These results suggest that patients with cerebral hemodynamic insufficiency, such as those with severe carotid stenosis, may have increased ischemic damage after microembolic events.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Spreading waves of transient and prolonged decreases in water diffusion after subarachnoid hemorrhage in rats

Diffusion-weighted MRI (DWI), which can detect cortical spreading depressions (SDs) as propagating waves of reduced apparent diffusion coefficient (ADC) of water, was used to investigate whether spreading depression occurs after subarachnoid hemorrhage (SAH) induced by endovascular perforation in the rat. Eleven rats underwent SAH while positioned in the magnet. The ADC measurements had a temporal resolution of 12 sec. Transient decreases in ADC to 74 +/- 5% of pre-SAH values were observed in three rats after SAH, which propagated over the cortex with an average speed of 4.2 +/- 0. 6 mm/min, consistent with an SD wave. Furthermore, in all 11 rats, a wavefront of reduced ADC, which did not resolve within the 12 min observation period, spread at a speed of 3.2 +/- 1.7 mm/min in the ipsilateral cortex, and again is consistent with the speed of SD propagation. Therefore, spreading depression-like cellular depolarization is a consequence of acute subarachnoid hemorrhage in rats. Magn Reson Med 44:110-116, 2000.

Effects of single and repetitive spreading depression on cerebral blood flow and glucose metabolism in cats: a PET study

To clarify the effects of spreading depression (SD) on cerebral circulation and metabolism, we elicited a single or repetitive episode of SD and evaluated CBF and CMRglc three-dimensionally in normal cats (n=4, in each group) using a high-resolution positron emission tomography (PET) scanner. SD was evoked by applying KCl to the left occipital cortex. We then monitored DC potential changes with tungsten electrodes inserted into the left temporal cortex. CBF was measured twice before and three times (immediately, 30-60 min, and 60-120 min) following KCl application using [15O]H(2)O, and CMRglc was determined using 2-[18F]fluoro-2-deoxy-D-glucose immediately following the last CBF measurement. The following results were obtained: (1) a single episode of SD produced a temporary CBF increase, followed by a long-lasting hypoperfusion in the cortex, with no significant changes to CBF observed in the subcortex; (2) no significant CMRglc changes were observed in either cortical or subcortical regions following a single episode of SD; (3) a flow-metabolism uncoupling was observed in the cortical regions concurrently with persistent hypoperfusion; (4) repetitive SD produced significant CBF changes in the cortex; and (5) the cortical CMRglc increased as a result of repeated episodes of SD, with no significant changes observed in the subcortex. Thus, we succeeded in determining three-dimensionally the effects of single and repetitive SD on CBF and CMRglc in cats using a high-resolution PET scanner. The present study provides the first direct evidence of CBF-CMRglc uncoupling occurring concurrently with persistent hypoperfusion following SD.

CP-101,606, an NR2B subunit selective NMDA receptor antagonist, inhibits NMDA and injury induced c-fos expression and cortical spreading depression in rodents

(1S, 2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP-101,606) is a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors containing the NR2B subunit. This compound was used to investigate the role of NR2B containing receptors in three responses to NMDA receptor activation in vivo. In mouse, CP-101,606 completely inhibited increases in fos-like immunoreactivity in dentate gyrus caused by a subconvulsant intraperitoneal dose of NMDA. In rat, the compound completely blocked cortical c-fos mRNA induction following focal injury in parietal cortex and the initiation and propagation of electrically induced cortical spreading depression. Inhibition of these responses by CP-101,606 indicates that c-fos induction and cortical spreading depression are dependent on activation of NMDA receptors containing the NR2B subunit. Since NMDA receptor dependent c-fos induction and cortical spreading depression may contribute to neuron loss after focal CNS injury, inhibition of these responses by CP-101,606 may contribute to the neuroprotective efficacy of the compound.

Diffusion and high resolution MRI of traumatic brain injury in rats: time course and correlation with histology

Although widely employed in studies of cerebral ischemia, the use of diffusion-weighted imaging (DWI) for traumatic brain injury (TBI) has been both limited and primarily confined to the first few hours after injury. Therefore, the present study examined the temporal evolution of magnetic resonance imaging (MRI) signal changes from hours to weeks after moderate fluid-percussion TBI in rats. We used isotropic diffusion along three directions and high resolution (HR) spin-echo pulse sequences to visualize DWI and HR MRI changes, respectively. Late changes were compared to histopathological and neurological outcome. A significant decrease (P<0.05) in the apparent diffusion coefficients (ADC) below preinjury levels was found in the left cortex and left hippocampus (ipsilateral to injury) at 1-2 h post-TBI. At 2 weeks post-TBI, ADCs were significantly elevated (P<0.05) above preinjury levels in both cortex and hippocampus. Regions of hypo- and hyperintensity detected in HR MRI scans also showed evidence of tissue damage by histological evaluation. Neurological assessment indicated that such changes were observed at a level of injury which produced moderate impairment 2 weeks after the insult. These results indicate that alterations in DWI and HR MRI signals occur both early (hours) and late (weeks) after lateral fluid-percussion injury. Furthermore, the study showed that DWI was sensitive to MR signal change at 1-2 h post TBI (in select ROIs), whereas HR scans showed MR signal change primarily at later time points (3-4 h and later). Moreover, regions which demonstrate late changes are associated with histological damage and neurological impairment. The study demonstrates the utility of MRI to detect early changes, in some cases, that are predictive of long-lasting damage verified histologically.

Prolonged induction of neuronal NOS expression and activity following cortical spreading depression (SD): implications for SD- and NO-mediated neuroprotection

Cortical spreading depression (CSD) is associated with various short- and long-term physiological and neurochemical changes and has been shown to confer an increased susceptibility to accompanying ischemic injury or provide protection against a subsequent experimental ischemia. Nitric oxide is involved in the processes of ischemic injury and under certain conditions mediates cellular protection. To investigate the possibility that CSD-induced alterations in nitric oxide synthase (NOS) expression and activity occur and might be associated with the time-dependent enhancement or prevention by CSD of ischemic damage, this study examined the spatiotemporal changes in nNOS expression and activity in cerebral cortex following CSD. Anesthetized rats had unilateral CSD induced by a 10-min topical application of KCl and were killed at various times thereafter. CSD increased both nNOS mRNA and protein levels throughout layers II-III of cortex. nNOS mRNA in the affected neocortex was significantly increased by 30-90% at 2, 7, and 14 days (P < or = 0.05) compared with contralateral levels, but was not significantly above control values at 1-6 h, 1 day, and 28 days after CSD induction. Levels of [3H]-L-N(G)-nitroarginine binding to NOS were increased by 40-170% 7, 14, and 28 days (P < or = 0.01) after CSD in a similar, but delayed, profile to nNOS mRNA. Levels of nNOS-immunoreactivity were also increased in both neurons and astrocytes of ipsilateral cortex 7 and 14 days after CSD--confirmed by double-immunofluorescence localization. Ex vivo NOS activity in layers I-III of ipsilateral cortex was also increased by 30-50% (P < or = 0.01) at 7 and 14 days after CSD, times coincident with reported maximal ischemic protection. These results demonstrate that nNOS is up-regulated by cellular depolarization/depression occurring during CSD, or by resultant stimuli and suggest that "CSD-conditioned" cortex may be capable of producing appropriate levels of NO to mediate or contribute to protective/adaptive responses to subsequent physical ischemic injury.

Mild hypothermia improves recovery of cortical extracellular potassium ion activity and excitability after middle cerebral artery occlusion in the rat

BACKGROUND AND PURPOSE

Mild brain hypothermia significantly alleviates damage after focal ischemia, although the mechanism of this protection remains poorly defined. In the present study, we tested the hypothesis that mild hypothermia would protect cortex from early deterioration of ion homeostasis and loss of excitability associated with reperfusion after focal ischemia.

METHODS

Cortical extracellular potassium ion activity ([K+]o) and the response of [K+]o to direct cortical stimulation was measured both in the ischemic core and in the ischemic penumbra of normothermic and mildly hypothermic (31.5 degrees C to 32 degrees C) rats after distal middle cerebral artery occlusion (MCAO) and reperfusion.

RESULTS

The response of [K+]o during MCAO was similar in normothermic and hypothermic animals. However, within 1 hour of reperfusion, [K+]o in the ischemic core region of normothermic animals showed incomplete recovery and was refractory to direct cortical stimulation. [K+]o in hypothermic animals returned to preischemic levels on reperfusion and continued to respond to direct cortical stimulation. Mild hypothermia prevented extensive infarction 24 hours after transient MCAO.

CONCLUSIONS

The data suggest that transient focal ischemia is accompanied by early disturbances of potassium ion homeostasis during reperfusion, which are accompanied by loss of excitability and which may contribute ultimately to cortical infarction.