Pathophysiology of the Ischemic Penumbra—Revision of a Concept

1. The original concept of the ischemic penumbra surrounding a focus of dense cerebral ischemia is based on electrophysiological observations. In the cortex of baboons following middle cerebral artery occlusion, complete failure of the cortical evoked potential was observed at a cerebral blood flow (CBF) threshold level of approx. 0.15 ml/g/min—a level at which extracellular potassium ion activity was only mildly elevated. With a greater CBF decrement to the range of 0.06–0.10 ml/g/min, massive increases in extracellular potassium occurred and were associated with complete tissue infarction. Thus, the ischemic penumbra has been conceptualized as a region in which CBF reduction has exceeded the threshold for failure of electrical function but not that for membrane failure.2. Recent studies demonstrate that the penumbra as defined classically by the flow thresholds does not survive prolonged periods of ischemia. The correlation of CBF autoradiograms with diffusion-weighted MR images and the regional distribution of cerebral metabolites reveals that the ischemic core region enlarges when adjacent, formerly penumbral, areas undergo irreversible deterioration during the initial hours of vascular occlusion. At the same time, the residual penumbra becomes restricted to the periphery of the ischemic territory, and its fate may depend critically upon early therapeutic intervention.3. In the border zone of brain infarcts, marked uncoupling of local CBF and glucose utilization is consistently observed. The correlation with electrophysiological measurements shows that metabolism-flow uncoupling is associated with sustained deflections of the direct current (DC) potential resembling transient depolarizations. Such penumbral cell depolarizations, which are associated with an increased metabolic workload, induce episodes of tissue hypoxia due to the constrained collateral flow, stimulate anaerobic glycolysis leading to lactacidosis, suppress protein synthesis, and, finally, compromise energy metabolism. The frequency of their occurrence correlates with the final volume of ischemic injury. Therefore, penumbral depolarizations are regarded as a key event in the pathogenesis of ischemic brain injury. Periinfarct DC deflections can be suppressed by NMDA and non-NMDA antagonists, resulting in a significant reduction of infarct size.4. The histopathological sequelae within the penumbra consist of various degrees of scattered neuronal injury, also termed incomplete infarction. The reduction of neuronal density at the infarct border is a flow- and time-dependent event which is accompanied by an early response of glial cells. As early as 3 hr after vascular occlusion a generalized microglial activation can be detected throughout the ipsilateral cortex. Astrocytic activation is observed in the intact parts of the ischemic hemisphere from 6 hr postocclusion onward. Thus, the penumbra is a spatially dynamic brain region of limited viability which is characterized by complex pathophysiological changes involving neuronal function as well as glial activation in response to local ischemic injury.     

Neuroprotection in acute ischemic stroke – current status

With the growing understanding of the mechanism of cell death in ischemia, new approaches for treatment such as neuroprotection have emerged. The basic aim of this strategy is to interfere with the events of the ischemic cascade, blocking the pathological processes and preventing the death of nerve cells in the ischemic penumebra. This concept involves inhibition of the pathological molecular events which eventually leads to the influx of calcium, activation of free radicals and neuronal death. Despite encouraging data from experimental animal models, all clinical trials of neuroprotective therapies have to date been unsuccessful. This article reviews some of the reasons for the failure of neuroprotection in the clinical trials so far. Despite all the negative reports, we believe it would be wrong to give up at this point, since there is still reasonable hope of finding an effective neuroprotection for stroke.     

Investigations of line scanning proton therapy with dynamic multi-leaf collimator

PurposeScanning proton therapy has dosimetric advantage over passive treatment, but has a large penumbra in low-energy region. This study investigates the penumbra reduction when multi-leaf collimators (MLCs) are used for line scanning proton beams and secondary neutron production from MLCs.MethodsScanning beam plans with and without MLC shaping were devised. Line scanning proton plan of 36 energy layers between 71.2 and 155.2 MeV was generated. The MLCs were shaped according to the cross-sectional target shape for each energy layer. The two-dimensional doses were measured through an ion-chamber array, depending on the presence of MLC field, and Monte Carlo (MC) simulations were performed. The plan, measurement, and MC data, with and without MLC, were compared at each depth. The secondary neutron dose was simulated with MC. Ambient neutron dose equivalents were computed for the line scanning with 10 × 10 × 5 cm³ volume and maximum proton energy of 150 MeV, with and without MLCs, at lateral distances of 25–200 cm from the isocenter. The neutron dose for a wobbling plan with 10 × 10 × 5 cm³ volume was also evaluated.ResultsThe lateral penumbra width using MLC was reduced by 23.2% on average, up to a maximum of 32.2%, over the four depths evaluated. The ambient neutron dose equivalent was 18.52% of that of the wobbling beam but was 353.1% larger than the scanning open field.ConclusionsMLC field shaping with line scanning reduced the lateral penumbra and should be effective in sparing normal tissue. However, it is important to investigate the increase in neutron dose.     

Inhibition of the group I mGluRs reduces acute brain damage and improves long-term histological outcomes after photothrombosis-induced ischaemia

Group I mGluRs (metabotropic glutamate receptors), including mGluR1 and mGluR5, are GPCRs (G-protein coupled receptors) and play important roles in physiology and pathology. Studies on their role in cerebral ischaemia have provided controversial results. In this study, we used a PT (photothrombosis)-induced ischaemia model to investigate whether antagonists to the group I mGluRs may offer acute and long-term protective effects in adult mice. Our results demonstrated that administration with mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 antagonist {"type":"entrez-nucleotide","attrs":{"text":"LY367385","term_id":"1257996803","term_text":"LY367385"}}LY367385 by intraperitoneal injection at 3 h after PT decreased brain infarct volume evaluated one day after ischaemia. Additive effects on infarct volume were observed upon co-injection with MPEP and {"type":"entrez-nucleotide","attrs":{"text":"LY367385","term_id":"1257996803","term_text":"LY367385"}}LY367385. These antagonists also significantly alleviated neurodegeneration and apoptosis in the penumbra. In addition, when evaluated 2 weeks after PT, they reduced infarct volume and tissue loss, attenuated glial scar formation, and inhibited cell proliferation in the penumbra. Importantly, co-injection with MPEP and {"type":"entrez-nucleotide","attrs":{"text":"LY367385","term_id":"1257996803","term_text":"LY367385"}}LY367385 reduced the expression levels of calpain, a Ca²⁺-activated protease known to mediate ischaemia-induced neuronal death. Injection of calpeptin, a calpain inhibitor, could inhibit neuronal death and brain damage after PT but injection of calpeptin together with MPEP and {"type":"entrez-nucleotide","attrs":{"text":"LY367385","term_id":"1257996803","term_text":"LY367385"}}LY367385 did not further improve the protective effects mediated by MPEP and {"type":"entrez-nucleotide","attrs":{"text":"LY367385","term_id":"1257996803","term_text":"LY367385"}}LY367385. These results suggest that inhibition of group I mGluRs is sufficient to protect ischaemic damage through the calpain pathway. Taken together, our results demonstrate that inhibition of group I mGluRs can mitigate PT-induced brain damage through attenuating the effects of calpain, and improve long-term histological outcomes.     

Optimized System for Cerebral Perfusion Monitoring in the Rat Stroke Model of Intraluminal Middle Cerebral Artery Occlusion

The translational potential of pre-clinical stroke research depends on the accuracy of experimental modeling. Cerebral perfusion monitoring in animal models of acute ischemic stroke allows to confirm successful arterial occlusion and exclude subarachnoid hemorrhage. Cerebral perfusion monitoring can also be used to study intracranial collateral circulation, which is emerging as a powerful determinant of stroke outcome and a possible therapeutic target. Despite a recognized role of Laser Doppler perfusion monitoring as part of the current guidelines for experimental cerebral ischemia, a number of technical difficulties exist that limit its widespread use. One of the major issues is obtaining a secure and prolonged attachment of a deep-penetration Laser Doppler probe to the animal skull. In this video, we show our optimized system for cerebral perfusion monitoring during transient middle cerebral artery occlusion by intraluminal filament in the rat. We developed in-house a simple method to obtain a custom made holder for twin-fibre (deep-penetration) Laser Doppler probes, which allow multi-site monitoring if needed. A continuous and prolonged monitoring of cerebral perfusion could easily be obtained over the intact skull.     

Reversibility of Nimodipine Binding to Brain in Transient Cerebral Ischemia

Using autoradiography, we have measured the in vivo binding of [3H]nimodipine to brain in a rat model of reversible cerebral ischemia. Ischemia was induced by simultaneous occlusion of the middle cerebral artery (MCA) and ipsilateral common carotid artery by microaneurysm clips. Rats were studied after 15 min of ischemia (ischemic group) or after 45 min of reperfusion following 15 min of ischemia (reperfused group). Regional cerebral blood flow (CBF) was determined autoradiographically using [14C]iodoantipyrine in both ischemic (n = 6) and reperfused (n = 6) groups. During ischemia blood flow in the territory of the MCA was depressed and recovered to normal only in the distal territory of the MCA following reperfusion. [3H]Nimodipine binding in the ischemic group (n = 12) was elevated in ischemic brain regions and declined significantly (p < 0.01) in these regions in the reperfused group (n = 11). The ratio of the volume of cortex showing increased binding to the total volume of the forebrain was 0.113 +/- 0.025 (mean +/- SD) in the ischemic group and declined to 0.080 +/- 0.027 following reperfusion (p < 0.005). In general, infarct was only observed in regions showing persistent elevation of nimodipine binding following reperfusion as determined by histology performed in a separate group of rats (n = 8) after 24 h of reperfusion. We conclude that increased nimodipine binding to ischemic tissue is initially reversible with prompt reestablishment of CBF and is a sensitive indicator of early and reversible ischemia-induced cerebral dysfunction.     

落叶阔叶林内的光合作用和气孔传导率特征(英文)

本文根据Wang和BMdocchi(1989)最近提出的冠层辐射模型,进一步给出了一个模拟冠层光合作用速率和气孔传导率的模式.模式将冠层中每一层的叶面积分为向光叶、半影叶、和全遮荫叶三种,并分别计算其光合作用速率和气孔传导率。计算得到的光合速率廓线表明,在落叶阔叶林内,冠层下部的叶片常处于光照不足状态;半影效应使得透过林冠达于底部的辐射量增大,这对于林下植物的光合作用是有利的。 模式计算值与实测值之间的微弱差别应归因于纯辐射模型无法考虑湍流输送机制造成的CO_2传输和冠层底部耐荫性叶对于低光照的适应能力。     

Dynamics of morphological changes after transplantation of mesenchymal stem cells in rat brain provoked by stroke

The study of the dynamic of morphological changes in the brain after ischemic stroke is very important for the preclinical trial of mesenchymal stem cell (MSC) therapy for this widespread disease. Experiments were carried out in inbred Wistar-Kyoto rats. MSCs were isolated, expanded in culture, and labeled with the vital fluorescent dye PKH-26. Animals were subjected to middle cerebral artery occlusion (MCAO), followed by an injection of 5 × 10⁶ rat MSCs into the tail vein on the day of MCAO. Control group animals received PBS injection (negative control). Animals were sacrificed at 1, 2, 3, and 5 days and 1, 2, 4, and 6 weeks after the operation. MSCs were revealed in the brain on the third day after transplantation as being distributed around brain vessels both in the ipsilateral and contralateral hemispheres. This pattern of distribution remained unchanged throughout six weeks of observation. It was demonstrated that the inflammation process and scar formation in the cell therapy group were progressing at a rate 25–30% faster than in the control group. MSC transplantation stimulated endogenous stem cell proliferation in the subependimal zone of lateral ventricles (subventricular zone). In addition, MSC injection caused a neuroprotecting effect; most penumbra neurons retained their structure in cell therapy group, whereas in control group, animal penumbra neurons died or showed signs of serious damage.     

Bcl-2 overexpression protects against neuron loss within the ischemic margin following experimental stroke and inhibits cytochrome c translocation and caspase-3 activity

Bcl-2 protects against both apoptotic and necrotic death induced by several cerebral insults. We and others have previously demonstrated that defective herpes simplex virus vectors expressing Bcl-2 protect against various insults in vitro and in vivo, including cerebral ischemia. Because the infarct margin may be a region that is most amenable to treatment, we first determined whether gene transfer to the infarct margin is possible using a focal ischemia model. Since ischemic injury with and without reperfusion may occur by different mechanisms, we also determined whether Bcl-2 protects against focal cerebral ischemic injury either with or without reperfusion in rats. Bax expression, cytochrome c translocation and activated caspase-3 expression were also assessed. Viral vectors overexpressing Bcl-2 were delivered to the infarct margin. Reperfusion resulted in larger infarcts than permanent occlusion. Bcl-2 overexpression significantly improved neuron survival in both ischemia models. Bcl-2 overexpression did not alter overall Bax expression, but inhibited cytosolic accumulation of cytochrome c and caspase-3 activation. Thus, we provide the first evidence that gene transfer to the infarct margin is feasible, that overexpression of Bcl-2 protects against damage to the infarct margin induced by ischemia with and without reperfusion, and that Bcl-2 overexpression using gene therapy attenuates apoptosis-related proteins. This suggests a potential therapeutic strategy for stroke.