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

Assessment of cerebrovascular alterations induced by inflammatory response and oxidative-nitrative stress after traumatic intracranial hypertension and a potential mitigation strategy

The rapid perfusion of cerebral arteries leads to a significant increase in intracranial blood volume, exposing patients with traumatic brain injury to the risk of diffuse brain swelling or malignant brain herniation during decompressive craniectomy. The microcirculation and venous system are also involved in this process, but the precise mechanisms remain unclear. A physiological model of extremely high intracranial pressure was created in rats. This development triggered the TNF-α/NF-κB/iNOS axis in microglia, and released many inflammatory factors and reactive oxygen species/reactive nitrogen species, generating an excessive amount of peroxynitrite. Subsequently, the capillary wall cells especially pericytes exhibited severe degeneration and injury, the blood-brain barrier was disrupted, and a large number of blood cells were deposited within the microcirculation, resulting in a significant delay in the recovery of the microcirculation and venous blood flow compared to arterial flow, and this still persisted after decompressive craniectomy. Infliximab is a monoclonal antibody bound to TNF-α that effectively reduces the activity of TNF-α/NF-κB/iNOS axis. Treatment with Infliximab resulted in downregulation of inflammatory and oxidative-nitrative stress related factors, attenuation of capillary wall cells injury, and relative reduction of capillary hemostasis. These improved the delay in recovery of microcirculation and venous blood flow.

No-reflow after stroke reperfusion therapy: An emerging phenomenon to be explored

In the field of stroke thrombectomy, ineffective clinical and angiographic reperfusion after successful recanalization has drawn attention. Partial or complete microcirculatory reperfusion failure after the achievement of full patency of a former obstructed large vessel, known as the "no-reflow phenomenon" or "microvascular obstruction," was first reported in the 1960s and was later detected in both experimental models and patients with stroke. The no-reflow phenomenon (NRP) was reported to result from intraluminal occlusions formed by blood components and extraluminal constriction exerted by the surrounding structures of the vessel wall. More recently, an emerging number of clinical studies have estimated the prevalence of the NRP in stroke patients following reperfusion therapy, ranging from 3.3% to 63% depending on its evaluation methods or study population. Studies also demonstrated its detrimental effects on infarction progress and neurological outcomes. In this review, we discuss the research advances, underlying pathogenesis, diagnostic techniques, and management approaches concerning the no-reflow phenomenon in the stroke population to provide a comprehensive understanding of this phenomenon and offer references for future investigations.

The neuroprotective role of prolonged normobaric oxygenation applied during ischemia and in the early stage of reperfusion in cerebral ischemic rats

BACKGROUND

Recanalization is the main treatment option for ischemic stroke. However, prognosis remains poor for about half of patients after recanalization, possibly due to the "no-reflow" phenomenon at the early phase of recanalization. Normobaric oxygenation (NBO) during ischemia can reportedly maintain the partial pressure of oxygen and exert a protective effect in ischemic brain tissue.

OBJECTIVES AND METHODS

This study investigated whether prolonged NBO treatment during ischemia and the early phase of reperfusion (i/rNBO) has neuroprotective effects and to elucidate the underlying mechanisms in rats with middle cerebral artery occlusion plus reperfusion.

RESULTS

NBO treatment significantly elevated the level of O2 in the atmosphere and arterial blood without altering the level of CO2. The infarcted cerebral volume was significantly reduced by application of i/rNBO as compared to iNBO (applied during ischemia) or rNBO (applied at the early phase of reperfusion), indicating better protective effects of i/rNBO. i/rNBO more effectively suppressed s-nitrosylation of MMP-2 (amplifying inflammation) as compared to iNBO and rNBO, dramatically downregulated the cleavage of poly(ADP-ribose)polymerase-1 (PARP-1, acting as the substrate of MMP-2), and suppressed neuronal apoptosis, as determined by the TUNEL assay and staining for NeuN. These results demonstrated that application of i/rNBO in the early stage of reperfusion significantly alleviated neuronal apoptosis via suppression of the MMP-2/PARP-1 pathway.

CONCLUSIONS

The mechanism underlying the neuroprotective role of i/rNBO involved prolonged NBO treatment for cerebral ischemia, suggesting that i/rNBO may allow expansion of the time window for NBO application in stroke patients following vascular recanalization.

Worsened Stroke Outcome in a Model of Preeclampsia is Associated With Poor Collateral Flow and Oxidative Stress

BACKGROUND

Preeclampsia increases the incidence of maternal stroke, a devastating condition that is on the rise. We investigated stroke outcome in a model of experimental preeclampsia with and without treatment with clinically relevant doses of magnesium sulfate (experimental preeclampsia+MgSO4) compared to normal late-pregnant and nonpregnant rats.

METHODS

Transient middle cerebral artery occlusion was used to induce focal stroke for either 1.5 or 3 hours. Infarct volume and hemorrhagic transformation were determined as measures of stroke outcome. Changes in core middle cerebral artery and collateral flow were measured by dual laser Doppler. The relationship between middle cerebral artery perfusion deficit and infarction was used as a measure of ischemic tolerance. Oxidative stress and endothelial dysfunction were measured by 3-nitrotyrosine and 8-isoprostane, in brain and serum, respectively.

RESULTS

Late-pregnant animals had robust collateral flow and greater ischemic tolerance of brain tissue, whereas experimental preeclampsia had greater infarction that was related to poor collateral flow, endothelial dysfunction, and oxidative stress. Importantly, pregnancy appeared preventative of hemorrhagic transformation as it occurred only in nonpregnant animals. MgSO4 did not provide benefit to experimental preeclampsia animals for infarction.

CONCLUSIONS

Stroke outcome was worse in a model of preeclampsia. As preeclampsia increases the risk of future stroke and cardiovascular disease, it is worth understanding the influence of preeclampsia on the material brain and factors that might potentiate injury both during the index pregnancy and years postpartum.

Acidosis in arterial blood gas testing is associated with clinical outcomes after endovascular thrombectomy

Background

Despite recanalization, some of the patients undergoing endovascular thrombectomy (EVT) still suffer from unfavorable outcomes. Patients with poor prognoses are often accompanied by acidosis in arterial blood gas (ABG) testing. We, therefore, explored the ABG testing results in the early phase of recanalization and analyzed their association with poor prognosis.

Patients and methods

We identified all patients with ischemic stroke and successful endovascular recanalization for anterior circulation vessel occlusion between June 2019 and May 2022. ABG testing was performed in all patients within 0-30 min and 8 h after endovascular therapy. We investigated the relationship between the ABG testing results with symptomatic intracerebral hemorrhage (sICH), hemicraniectomy, and mortality.

Results

A total of 123 patients with stroke after endovascular thrombectomy were analyzed. Of those, eight (6.5%) patients had postinterventional sICH. Acidosis was associated with sICH. Decreased HCO 3 - levels and HCO 3 - levels at 8 h after EVT were independently related to a higher risk of sICH. Twelve (9.8%) patients underwent hemicraniectomy for postischemic malignant edema and similar results were found for hemicraniectomy. Increased lactate at 8 h after EVT and decreased HCO 3 - levels at 8 h after EVT were closely associated with hemicraniectomy. Twenty-two (17.9%) patients died within 3 months. Decreased HCO 3 - levels were independently related to mortality, as were decreased pH levels at 8 h after EVT and decreased HCO 3 - levels at 8 h after EVT.

Conclusion

Acidosis is associated with clinical outcomes after endovascular therapy and may help to select patients with poor prognosis in the acute early phase of recanalization.

Inhaled nitric oxide improves post-cardiac arrest outcomes via guanylate cyclase-1 in bone marrow-derived cells

RATIONALE

Nitric oxide (NO) exerts its biological effects primarily via activation of guanylate cyclase (GC) and production of cyclic guanosine monophosphate. Inhaled NO improves outcomes after cardiac arrest and cardiopulmonary resuscitation (CPR). However, mechanisms of the protective effects of breathing NO after cardiac arrest are incompletely understood.

OBJECTIVE

To elucidate the mechanisms of beneficial effects of inhaled NO on outcomes after cardiac arrest.

METHODS

Adult male C57BL/6J wild-type (WT) mice, GC-1 knockout mice, and chimeric WT mice with WT or GC-1 knockout bone marrow were subjected to 8 min of potassium-induced cardiac arrest to determine the role of GC-1 in bone marrow-derived cells. Mice breathed air or 40 parts per million NO for 23 h starting at 1 h after CPR.

RESULTS

Breathing NO after CPR prevented hypercoagulability, cerebral microvascular occlusion, an increase in circulating polymorphonuclear neutrophils and neutrophil-to-lymphocyte ratio, and right ventricular dysfunction in WT mice, but not in GC-1 knockout mice, after cardiac arrest. The lack of GC-1 in bone marrow-derived cells diminished the beneficial effects of NO breathing after CPR.

CONCLUSIONS

GC-dependent signaling in bone marrow-derived cells is essential for the beneficial effects of inhaled NO after cardiac arrest and CPR.

Iptakalim improves cerebral microcirculation in mice after ischemic stroke by inhibiting pericyte contraction

Pericytes are present tight around the intervals of capillaries, play an essential role in stabilizing the blood-brain barrier, regulating blood flow and immunomodulation, and persistent contraction of pericytes eventually leads to impaired blood flow and poor clinical outcomes in ischemic stroke. We previously show that iptakalim, an ATP-sensitive potassium (K-ATP) channel opener, exerts protective effects in neurons, and glia against ischemia-induced injury. In this study we investigated the impacts of iptakalim on pericytes contraction in stroke. Mice were subjected to cerebral artery occlusion (MCAO), then administered iptakalim (10 mg/kg, ip). We showed that iptakalim administration significantly promoted recovery of cerebral blood flow after cerebral ischemia and reperfusion. Furthermore, we found that iptakalim significantly inhibited pericytes contraction, decreased the number of obstructed capillaries, and improved cerebral microcirculation. Using a collagen gel contraction assay, we demonstrated that cultured pericytes subjected to oxygen-glucose deprivation (OGD) consistently contracted from 3 h till 24 h during reoxygenation, whereas iptakalim treatment (10 μM) notably restrained pericyte contraction from 6 h during reoxygenation. We further showed that iptakalim treatment promoted K-ATP channel opening via suppressing SUR2/EPAC1 complex formation. Consequently, it reduced calcium influx and ET-1 release. Taken together, our results demonstrate that iptakalim, targeted K-ATP channels, can improve microvascular disturbance by inhibiting pericyte contraction after ischemic stroke. Our work reveals that iptakalim might be developed as a promising pericyte regulator for treatment of stroke.

Endogenous zinc protoporphyrin formation critically contributes to hemorrhagic stroke-induced brain damage

Hemorrhagic stroke is a leading cause of death. The causes of intracerebral hemorrhage (ICH)-induced brain damage are thought to include lysis of red blood cells, hemin release and iron overload. These mechanisms, however, have not proven very amenable to therapeutic intervention, and so other mechanistic targets are being sought. Here we report that accumulation of endogenously formed zinc protoporphyrin (ZnPP) also critically contributes to ICH-induced brain damage. ICH caused a significant accumulation of ZnPP in brain tissue surrounding hematoma, as evidenced by fluorescence microscopy of ZnPP, and further confirmed by fluorescence spectroscopy and supercritical fluid chromatography-mass spectrometry. ZnPP formation was dependent upon both ICH-induced hypoxia and an increase in free zinc accumulation. Notably, inhibiting ferrochelatase, which catalyzes insertion of zinc into protoporphyrin, greatly decreased ICH-induced endogenous ZnPP generation. Moreover, a significant decrease in brain damage was observed upon ferrochelatase inhibition, suggesting that endogenous ZnPP contributes to the damage in ICH. Our findings reveal a novel mechanism of ICH-induced brain damage through ferrochelatase-mediated formation of ZnPP in ICH tissue. Since ferrochelatase can be readily inhibited by small molecules, such as protein kinase inhibitors, this may provide a promising new and druggable target for ICH therapy.

Retinal Oxygen Delivery, Metabolism and Extraction Fraction and Retinal Thickness Immediately Following an Interval of Ophthalmic Vessel Occlusion in Rats

Limited knowledge is currently available about alterations of retinal blood flow (F), oxygen delivery (DO₂), oxygen metabolism (MO₂), oxygen extraction fraction (OEF), or thickness after the ophthalmic blood vessels have been closed for a substantial interval and then reopened. We ligated the ophthalmic vessels for 120 minutes in one eye of 17 rats, and measured these variables within 20 minutes after release of the ligature in the 10 rats which had immediate reflow. F, DO₂ and MO₂ were 5.2 ± 3.1 μL/min, 428 ± 271 nL O₂/min, and 234 ± 133 nL O₂/min, respectively, that is, to 58%, 46% and 60% of values obtained from normal fellow eyes (P < 0.004). OEF was 0.65 ± 0.23, 148% of normal (P = 0.03). Inner and total retinal thicknesses were 195 ± 24 and 293 ± 20 μm, respectively, 117% and 114% of normal, and inversely related to MO₂ (P ≤ 0.02). These results reflect how much energy is available to the retina immediately after an interval of nonperfusion for 120 minutes. Thus, they elucidate aspects of the pathophysiology of nonperfusion retinal injury and may improve therapy in patients with retinal artery or ophthalmic artery obstructions.

Rapamycin in ischemic stroke: Old drug, new tricks?

The significant morbidity that accompanies stroke makes it one of the world's most devastating neurological disorders. Currently, proven effective therapies have been limited to thrombolysis and thrombectomy. The window for the administration of these therapies is narrow, hampered by the necessity of rapidly imaging patients. A therapy that could extend this window by protecting neurons may improve outcome. Endogenous neuroprotection has been shown to be, in part, due to changes in mTOR signalling pathways and the instigation of productive autophagy. Inducing this effect pharmacologically could improve clinical outcomes. One such therapy already in use in transplant medicine is the mTOR inhibitor rapamycin. Recent evidence suggests that rapamycin is neuroprotective, not only via neuronal autophagy but also through its broader effects on other cells of the neurovascular unit. This review highlights the potential use of rapamycin as a multimodal therapy, acting on the blood-brain barrier, cerebral blood flow and inflammation, as well as directly on neurons. There is significant potential in applying this old drug in new ways to improve functional outcomes for patients after stroke.

Transit time homogenization in ischemic stroke - A novel biomarker of penumbral microvascular failure?

Cerebral ischemia causes widespread capillary no-flow in animal studies. The extent of microvascular impairment in human stroke, however, is unclear. We examined how acute intra-voxel transit time characteristics and subsequent recanalization affect tissue outcome on follow-up MRI in a historic cohort of 126 acute ischemic stroke patients. Based on perfusion-weighted MRI data, we characterized voxel-wise transit times in terms of their mean transit time (MTT), standard deviation (capillary transit time heterogeneity - CTH), and the CTH:MTT ratio (relative transit time heterogeneity), which is expected to remain constant during changes in perfusion pressure in a microvasculature consisting of passive, compliant vessels. To aid data interpretation, we also developed a computational model that relates graded microvascular failure to changes in these parameters. In perfusion-diffusion mismatch tissue, prolonged mean transit time (>5 seconds) and very low cerebral blood flow (≤6 mL/100 mL/min) was associated with high risk of infarction, largely independent of recanalization status. In the remaining mismatch region, low relative transit time heterogeneity predicted subsequent infarction if recanalization was not achieved. Our model suggested that transit time homogenization represents capillary no-flow. Consistent with this notion, low relative transit time heterogeneity values were associated with lower cerebral blood volume. We speculate that low RTH may represent a novel biomarker of penumbral microvascular failure.

Improving Microcirculatory Reperfusion Reduces Parenchymal Oxygen Radical Formation and Provides Neuroprotection

BACKGROUND AND PURPOSE

Reperfusion is the most significant determinant of good outcome after ischemic stroke. However, complete reperfusion often cannot be achieved, despite satisfactory recanalization. We hypothesized that microvascular protection was essential for achieving effective reperfusion and, hence, neuroprotection. To test this hypothesis, we have developed an in vivo model to differentially monitor parenchymal and vascular reactive oxygen species (ROS) formation. By comparing the ROS-suppressing effect of N-tert-butyl-α-phenylnitrone (PBN) with its blood-brain barrier impermeable analog 2-sulfo-phenyl-N-tert-butylnitrone (S-PBN), we assessed the impact of vascular ROS suppression alone on reperfusion and stroke outcome after recanalization.

METHODS

The distal middle cerebral artery was occluded for 1 hour by compressing with a micropipette and then recanalized (n=60 Swiss mice). ROS formation was monitored for 1 hour after recanalization by intravital fluorescence microscopy in pial vasculature and cortical parenchyma with topically applied hydroethidine through a cranial window. PBN (100 mg/kg) or S-PBN (156 mg/kg) was administered shortly before recanalization, and suppression of the vascular and parenchymal hydroethidine fluorescence was examined (n=22). Microcirculatory patency, reperfusion, ischemic tissue size, and neurological outcome were also assessed in a separate group of mice 1 to 72 hours after recanalization (n=30).

RESULTS

PBN and S-PBN completely suppressed the reperfusion-induced increase in ROS signal within vasculature. PBN readily suppressed ROS produced in parenchyma by 88%. S-PBN also suppressed the parenchymal ROS by 64% but starting 40 minutes later. Intriguingly, PBN and S-PBN comparably reduced the size of ischemic area by 65% and 48% (P>0.05), respectively. S-PBN restored the microvascular patency and perfusion after recanalization, suggesting that its delayed parenchymal antioxidant effect could be secondary to improved microcirculatory reperfusion.

CONCLUSIONS

Promoting microvascular reperfusion by protecting vasculature can secondarily reduce parenchymal ROS formation and provide neuroprotection. The model presented can be used to directly assess pharmacological end points postulated in brain parenchyma and vasculature in vivo.

Cerebral venous collaterals: A new fort for fighting ischemic stroke?

Stroke therapy has entered a new era highlighted by the use of endovascular therapy in addition to intravenous thrombolysis. However, the efficacy of current therapeutic regimens might be reduced by their associated adverse events. For example, over-reperfusion and futile recanalization may lead to large infarct, brain swelling, hemorrhagic complication and neurological deterioration. The traditional pathophysiological understanding on ischemic stroke can hardly address these occurrences. Accumulating evidence suggests that a functional cerebral venous drainage, the major blood reservoir and drainage system in brain, may be as critical as arterial infusion for stroke evolution and clinical sequelae. Further exploration of the multi-faceted function of cerebral venous system may add new implications for stroke outcome prediction and future therapeutic decision-making. In this review, we emphasize the anatomical and functional characteristics of the cerebral venous system and illustrate its necessity in facilitating the arterial infusion and maintaining the cerebral perfusion in the pathological stroke content. We then summarize the recent critical clinical studies that underscore the associations between cerebral venous collateral and outcome of ischemic stroke with advanced imaging techniques. A novel three-level venous system classification is proposed to demonstrate the distinct characteristics of venous collaterals in the setting of ischemic stroke. Finally, we discuss the current directions for assessment of cerebral venous collaterals and provide future challenges and opportunities for therapeutic strategies in the light of these new concepts.

Cerebral Ischemic Reperfusion Injury Following Recanalization of Large Vessel Occlusions

Although stroke has recently dropped to become the nation's fifth leading cause of mortality, it remains the top leading cause of morbidity and disability in the US. Recent advances in stroke treatment, including intravenous fibrinolysis and mechanical thromboembolectomy, allow treatment of a greater proportion of stroke patients than ever before. While intra-arterial fibrinolysis with recombinant tissue plasminogen is an effective for treatment of a broad range of acute ischemic strokes, endovascular mechanical thromboembolectomy procedures treat severe strokes due to large artery occlusions, often resistant to intravenous drug. Together, these procedures result in a greater proportion of revascularized stroke patients than ever before, up to 88% in 1 recent trial (EXTEND-IA). Subsequently, there is a growing need for neurointensivists to develop more effective strategies to manage stroke patients following successful reperfusion. Cerebral ischemic reperfusion injury (CIRI) is defined as deterioration of brain tissue suffered from ischemia that concomitantly reverses the benefits of re-establishing cerebral blood flow following mechanical or chemical therapies for acute ischemic stroke. Herein, we examine the pathophysiology of CIRI, imaging modalities, and potential neuroprotective strategies. Additionally, we sought to lay down a potential treatment approach for patients with CIRI following emergent endovascular recanalization for acute ischemic stroke.

Progressive accumulation of autofluorescent granules in macrophages in rat striatum after systemic 3-nitropropionic acid: a correlative light- and electron-microscopic study

A variety of tissue biomolecules and intracellular structures are known to be autofluorescent. However, autofluorescent signals in brain tissues often confound analysis of the fluorescent markers used for immunohistochemistry. While investigating tissue and cellular pathologies induced by 3-nitropropionic acid, a mitochondrial toxin selective for striatal neurons, we encountered many autofluorescent signals confined to the lesion core. These structures were excited by blue (wavelength = 488 nm) and yellow-orange (555 nm), but not by red (639 nm) or violet (405 nm) lasers, indicating that this autofluorescence overlaps with the emission spectra of commonly used fluorophores. Almost all of the autofluorescence was localized in activated microglia/macrophages, while reactive astrocytes emitted no detectable autofluorescence. Amoeboid brain macrophages filled with autofluorescent granules revealed very weak expression of the microglial marker, ionized calcium-binding adaptor molecule 1 (Iba1), while activated microglia with evident processes and intense Iba1 immunoreactivity contained scant autofluorescent granules. In addition, immunolabeling with two lysosomal markers, ED1/CD68 and lysosomal-associated membrane protein 1, showed a pattern complementary with autofluorescent signals in activated microglia/macrophages, implying that the autofluorescent structures reside within cytoplasm free of intact lysosomes. A correlative light- and electron-microscopic approach finally revealed the ultrastructural identity of the fluorescent granules, most of which matched to clusters of lipofuscin-like inclusions with varying morphology. Thus, autofluorescence in the damaged brain may reflect the presence of lipofuscin-laden brain macrophages, which should be taken into account when verifying any fluorescent signals that are likely to be correlated with activated microglia/macrophages after brain insults.

Methylene blue and normobaric hyperoxia combination therapy in experimental ischemic stroke

INTRODUCTION

Ischemic stroke is a global burden that contributes to the disability and mortality of millions of patients. This study aimed to evaluate the efficacy of combined MB (methylene blue) and NBO (normobaric hyperoxia) therapy in experimental ischemic stroke.

METHODS

Rats with transient (60 min) MCAO (middle cerebral artery occlusion) were treated with: (1) air + vehicle (N = 8), (2) air + MB (N = 8), (3) NBO + vehicle (N = 7), and (4) NBO + MB (N = 9). MB (1 mg/kg) was administered at 30 min, again on days 2, 7, and 14 after stroke. NBO was given during MRI (30-150 min) on day 0, and again 1 h each during MRI on subsequent days. Serial diffusion, perfusion and T2 MRI were performed to evaluate lesion volumes. Foot-fault and cylinder tests were performed to evaluate sensorimotor function.

RESULTS

The major findings were: (1) NBO + MB therapy showed a greater decrease in infarct volume compared to NBO alone, but similar infarct volume compared to MB alone, (2) NBO + MB therapy accelerated sensorimotor functional recovery compared to NBO or MB alone, (3) Infarct volumes on day 2 did not change significantly from those on day 28 for all four groups, but behavioral function continued to show improved recovery in the NBO + MB group.

CONCLUSIONS

These findings support the hypothesis that combined NBO + MB further improves functional outcome and reduces infarct volume compared to either treatment alone and these improvements extended up to 28 days.

Venous system in acute brain injury: Mechanisms of pathophysiological change and function

Cerebral vascular injury is a major component of acute brain injury. Currently, neuroprotective strategies primarily focus on the recanalization of cerebral arteries and capillaries, and the protection of insulted neurons. Hitherto, the role of vein drainage in the pathophysiology of acute brain injury has been overlooked, due to an under appreciation of the magnitude of the impact of veins in circulation. In this review, we summarize the changes in the vein morphology and functions that are known, or likely to occur related to acute brain injury, and aim to advance the therapeutic management of acute brain injury by shifting the focus from reperfusion to another term: recirculation. Recent progress in the neurobiological understanding of the vascular neural network has demonstrated that cerebral venous systems are able to respond to acute brain injury by regulating the blood flow disharmony following brain edema, blood brain barrier disruption, ischemia, and hemorrhage. With the evidence presented in this review, future clinical management of acutely brain injured patients will expand to include the recirculation concept, establishing a harmony between arterial and venous systems, in addition to the established recanalization and reperfusion strategies.

Revisiting Cerebral Postischemic Reperfusion Injury: New Insights in Understanding Reperfusion Failure, Hemorrhage, and Edema

Cerebral postischemic reperfusion injury is defined as deterioration of ischemic brain tissue that parallels and antagonizes the benefits of restoring cerebral circulation after therapeutic thrombolysis for acute ischemic stroke. To understand the paradox of injury caused by treatment, we first emphasize the phenomenon in which recanalization of an occluded artery does not lead to tissue reperfusion. Additionally, no-reflow after recanalization may be due to injury of the neurovascular unit, distal microthrombosis, or both, and certainly worsens outcome. We examine the mechanism of molecular and sub-cellular damage in the neurovascular unit, notably oxidative stress, mitochondrial dysfunction, and apoptosis. At the level of the neurovascular unit, which mediates crosstalk between the damaged brain and systemic responses in blood, we summarize emerging evidence demonstrating that individual cell components play unique and cumulative roles that lead to damage of the blood–brain barrier and neurons. Furthermore, we review the latest developments in establishing a link between the immune system and microvascular dysfunction during ischemic reperfusion. Progress in assessing reperfusion injury has also been made, and we review imaging studies using various magnetic resonance imaging modalities. Lastly, we explore potential treatment approaches, including ischemic preconditioning, postconditioning, pharmacologic agents, and hypothermia.

Extended normobaric hyperoxia therapy yields greater neuroprotection for focal transient ischemia-reperfusion in rats

Background

Normobaric hyperoxia (NBO) therapy is neuroprotective in acute ischemic stroke. However, how long the NBO should last to obtain optimal outcome is still unclear. Reports show that ischemic penumbra blood supply may remain compromised for a long period after ischemia-reperfusion, which would impair tissue oxygenation in ischemic penumbra. Therefore, we hypothesized that longer-lasting NBO may yield greater neuroprotection.

Methods

The relationship between treatment outcome and NBO duration was examined in this study. Rats were subjected to 90 min middle cerebral artery occlusion followed by reperfusion for 22.5 hours. NBO started at 30 min post ischemia and lasted for 2, 4 or 8 h. Treatment efficacy was evaluated by measuring infarction volume, oxidative stress and apoptosis.

Results

Among 2 h, 4 h and 8 h NBO, 8 h NBO offered the greatest efficacy in reducing 24-hour infarction volume, attenuating oxidative stress that was indicated by decreased production of 8-hydroxydeoxyguanosine and NADPH oxidase catalytic subunit gp91^phox, and alleviating apoptosis that was associated with reduced production of DNA fragment and caspase-3 activity in cortex penumbra.

Conclusions

Under our experimental conditions, longer duration of NBO treatment produced greater benefits in focal transient cerebral ischemia-reperfusion rats.

AKT-related autophagy contributes to the neuroprotective efficacy of hydroxysafflor yellow A against ischemic stroke in rats

Hydroxysafflor yellow A (HSYA) has been approved clinically for treating cardiac patients in China since 2005. Recent studies have indicated that HSYA may be neuroprotective at 24 h in experimental stroke models. Autophagy is a vital degradation pathway of damaged intracellular macromolecules or organelles to maintain homeostasis in physiological or pathological conditions. The purpose of this study is to investigate the neuroprotection of HSYA at 72 h and its mechanism via activating the autophagy pathway using an acute ischemic-reperfusion stroke rat model. Rats were treated with HSYA (2 mg/kg) during 90 min middle cerebral artery occlusion/72 h reperfusion by intravenous administration at four different time points (15 min post-ischemia, 15 min, 24 h, and 48 h post reperfusion), mimicking the potential treatment for acute ischemic stroke. HSYA administration reduced infarction volume and improved various neurological functions at 72 h of reperfusion. The possible molecular mechanism was investigated. We found that HSYA activated the AKT-autophagy pathway in penumbra tissue, which occurred in neuronal-specific cells. Moreover, blocking the AKT-autophagy pathway by an AKT inhibitor abolished HSYA-induced neuroprotection after cerebral ischemia. HSYA may be a promising drug for treating acute ischemic stroke and the AKT-dependent autophagy pathway contributes to the HSYA-afforded neuroprotection.

Perfusion computed tomography thresholds defining ischemic penumbra and infarct core: studies in a rat stroke model

BACKGROUND

Perfusion computed tomography is becoming more widely used as a clinical imaging tool to predict potentially salvageable tissue (ischemic penumbra) after ischemic stroke and guide reperfusion therapies.

AIMS

The study aims to determine whether there are important changes in perfusion computed tomography thresholds defining ischemic penumbra and infarct core over time following stroke.

METHODS

Permanent middle cerebral artery occlusion was performed in adult outbred Wistar rats (n = 6) and serial perfusion computed tomography scans were taken every 30 mins for 2 h. To define infarction thresholds at 1 h and 2 h post-stroke, separate groups of rats underwent 1 h (n = 6) and 2 h (n = 6) of middle cerebral artery occlusion followed by reperfusion. Infarct volumes were defined by histology at 24 h. Co-registration with perfusion computed tomography maps (cerebral blood flow, cerebral blood volume, and mean transit time) permitted pixel-based analysis of thresholds defining infarction, using receiver operating characteristic curves.

RESULTS

Relative cerebral blood flow was the perfusion computed tomography parameter that most accurately predicted penumbra (area under the curve = 0.698) and also infarct core (area under the curve = 0.750). A relative cerebral blood flow threshold of < 75% of mean contralateral cerebral blood flow most accurately predicted penumbral tissue at 0.5 h (area under the curve = 0.660), 1 h (area under the curve = 0.659), 1.5 h (area under the curve = 0.636), and 2 h (area under the curve = 0.664) after stroke onset. A relative cerebral blood flow threshold of < 55% of mean contralateral most accurately predicted infarct core at 1 h (area under the curve = 0.765) and at 2 h (area under the curve = 0.689) after middle cerebral artery occlusion.

CONCLUSIONS

The data provide perfusion computed tomography defined relative cerebral blood flow thresholds for infarct core and ischemic penumbra within the first two hours after experimental stroke in rats. These thresholds were shown to be stable to define the volume of infarct core and penumbra within this time window.

Long-term neuroprotection induced by regional brain cooling with saline infusion into ischemic territory in rats: a behavioral analysis

The neuroprotective effect of hypothermia has long been recognized. Our recent studies have demonstrated the significant therapeutic value of local brain cooling in the ischemic territory prior to reperfusion in stroke, with reduced infarction and inflammatory responses up to 48 hours of reperfusion. The goal of this study was to determine if local brain cooling, produced by infusion of cold saline, could induce long-term functional improvement after stroke. A hollow filament was used to block the middle cerebral artery (MCA) for 3 hours, and then to locally infuse the ischemic territory with 6 ml cold saline (20 degrees C) for 10 minutes prior to reperfusion. This brain cooling infusion induced a significant (p < 0.01) decrease in neurologic deficits and significantly (p < 0.01) improved motor behavior in ischemic rats after 14 days of reperfusion, compared with ischemic rats without local cold saline infusion. This improvement continued for up to 28 days after reperfusion. No significant difference in motor performance was observed between the brain cooling infusion and normal control groups. Significant (p < 0.01) reductions in infarct volume were also evident. In conclusion, a local cerebral hypothermia induced by local saline infusion prior to reperfusion produced a long-term functional recovery after ischemic stroke. A therapeutic procedure, which combines prereperfusion infusion into an ischemic region with coincident cerebral hypothermia and perhaps subsequent recanalization of an occluded intracranial vessel, may improve the outcome for stroke patients.

MicroRNA-124-mediated regulation of inhibitory member of apoptosis-stimulating protein of p53 family in experimental stroke

BACKGROUND AND PURPOSE

p53-mediated neuronal death is a central pathway of stroke pathophysiology, but its mechanistic details remain unclear. Here, we identified a novel microRNA mechanism that downregulation of inhibitory member of the apoptosis-stimulating proteins of p53 family (iASPP) by the brain-specific microRNA-124 (miR-124) promotes neuronal death after cerebral ischemia.

METHODS

In a mouse model of focal permanent cerebral ischemia, the expression of iASPP and miR-124 was quantified by reverse transcription quantitative real-time polymerase chain reaction, immunofluorescence staining, and Western blot. Luciferase reporter assay was used to validate whether miR-124 can directly bind to the 3'-untranslated region of iASPP mRNA. To evaluate the role of miR-124, miR-124 mimic and its inhibitor were transfected into Neuro-2a cells and C57 mice.

RESULTS

There was no change in the iASPP mRNA level in cerebral ischemia. However, iASPP protein was remarkably decreased, with a concurrent elevation in miR-124 level. Furthermore, miR-124 can bind to the 3'-untranslated region of iASPP in 293T cells and downregulate its protein levels in Neuro-2a cells. In vivo, infusion of miR-124 decreased brain levels of iASPP, whereas inhibition of miR-124 enhanced iASPP levels and significantly reduced infarction in mouse focal cerebral ischemia.

CONCLUSIONS

These data demonstrate that p53-mediated neuronal cell death after stroke can be nontranscriptionally regulated by a novel mechanism involving suppression of endogenous cell death inhibitors by miR-124. Further dissection of microRNA regulatory mechanisms may lead to new therapeutic opportunities for preventing neuronal death after stroke.

Can restoring incomplete microcirculatory reperfusion improve stroke outcome after thrombolysis?

Substantial experimental data and recent clinical evidence suggesting that tissue reperfusion is a better predictor of outcome after thrombolysis than recanalization necessitate that patency of microcirculation after recanalization should be reevaluated. If indeed microcirculatory blood flow cannot be sufficiently reinstituted despite complete recanalization as commonly observed in coronary circulation, it may be one of the factors contributing to low efficacy of thrombolysis in stroke. Although microvascular no-reflow is considered an irreversible process that prevents tissue recovery from injury, emerging evidence suggests that it might be reversed with pharmacological agents administered early during recanalization. Therefore, therapeutic approaches aiming at reducing microvascular obstructions may improve success rate of recanalization therapies. Importantly, promoting oxygen delivery to the tissue, where entrapped erythrocytes cannot circulate in capillaries, with ongoing serum flow may improve survival of the underreperfused tissue. Altogether, these developments bring about the exciting possibility that benefit of reperfusion therapies can be further improved by restoring microcirculatory function because survival in the penumbra critically depends on adequate blood supply. Here, we review the available evidence suggesting presence of an 'incomplete microcirculatory reperfusion' (IMR) after focal cerebral ischemia and discuss potential means that may help investigate IMR in stroke patients after recanalization therapies despite technical limitations.

Improvement of oxygen supply by an artificial carrier in combination with normobaric oxygenation decreases the volume of tissue hypoxia and tissue damage from transient focal cerebral ischemia

Tissue hypoxia may play an important role in the development of ischemic brain damage. In the present study we investigated in a rat model of transient focal brain ischemia the neuroprotective effects of increasing the blood oxygen transport capacity by applying a semifluorinated alkane (SFA)-containing emulsion together with normobaric hyperoxygenation (NBO). The spread of tissue hypoxia was studied using pimonidazole given prior to filament-induced middle cerebral artery occlusion (MCAO, 2 h). Treatment consisted of intravenous injection of saline or the SFA-containing emulsion (0.5 or 1.0 ml/100g body weight; [SFA(0.5) or SFA(1.0)]) either upon establishing MCAO (early treatment) or after filament removal (delayed treatment). After injection NBO was administered for 8 h (early treatment) or 6 h (delayed treatment). Experiments were terminated 8 or 24 h after MCAO. In serial brain sections tissue hypoxia and irreversible cell damage were quantitatively determined. Furthermore, we studied hypoxia-related gene expression (VEGF, flt-1). Early treatment significantly (p<0.05) reduced the volumes of tissue damage (8 h after MCAO: SFA(1.0), 57±34 mm³; controls, 217±70 mm³; 24 h after MCAO: SFA(1.0), 189±82 mm³; controls, 317±60 mm³) and of P-Add immunoreactivity (8 h after MCAO: SFA(1.0), 261±37 mm³; controls, 339±26 mm³; 24h after MCAO: SFA(1.0), 274±47 mm³; controls, 364±46 mm³). Delayed treatment was comparably successful. The volume of the hypoxic penumbra was not decreased by the treatment. Similarly, VEGF and flt-1 mRNA levels did not differ between the experimental groups. From these data we conclude that increasing the blood oxygen transport capacity in the plasma compartment provides a neuroprotective effect by alleviating the severity of hypoxia to a level sufficient to prevent cells from transition into irreversible damage.

Visualization of clot lysis in a rat embolic stroke model: application to comparative lytic efficacy

BACKGROUND AND PURPOSE

The purpose of this study was to develop a novel MRI method for imaging clot lysis in a rat embolic stroke model and to compare tissue plasminogen activator (tPA)-based clot lysis with and without recombinant Annexin-2 (rA2).

METHODS

In experiment 1 we used in vitro optimization of clot visualization using multiple MRI contrast agents in concentrations ranging from 5 to 50 μL in 250 μL blood. In experiment 2, we used in vivo characterization of the time course of clot lysis using the clot developed in the previous experiment. Diffusion, perfusion, angiography, and T1-weighted MRI for clot imaging were conducted before and during treatment with vehicle (n=6), tPA (n=8), or rA2 plus tPA (n=8) at multiple time points. Brains were removed for ex vivo clot localization.

RESULTS

Clots created with 25 μL Magnevist were the most stable and provided the highest contrast-to-noise ratio. In the vehicle group, clot length as assessed by T1-weighted imaging correlated with histology (r=0.93). Clot length and cerebral blood flow-derived ischemic lesion volume were significantly smaller than vehicle at 15 minutes after treatment initiation in the rA2 plus tPA group, whereas in the tPA group no significant reduction from vehicle was observed until 30 minutes after treatment initiation. The rA2 plus tPA group had a significantly shorter clot length than the tPA group at 60 and 90 minutes after treatment initiation and significantly smaller cerebral blood flow deficit than the tPA group at 90 minutes after treatment initiation.

CONCLUSIONS

We introduce a novel MRI-based clot imaging method for in vivo monitoring of clot lysis. Lytic efficacy of tPA was enhanced by rA2.

Pericyte contraction induced by oxidative-nitrative stress impairs capillary reflow despite successful opening of an occluded cerebral artery

Here we show that ischemia induces sustained contraction of pericytes on microvessels in the intact mouse brain. Pericytes remain contracted despite successful reopening of the middle cerebral artery after 2 h of ischemia. Pericyte contraction causes capillary constriction and obstructs erythrocyte flow. Suppression of oxidative-nitrative stress relieves pericyte contraction, reduces erythrocyte entrapment and restores microvascular patency; hence, tissue survival improves. In contrast, peroxynitrite application causes pericyte contraction. We also show that the microvessel wall is the major source of oxygen and nitrogen radicals causing ischemia and reperfusion-induced microvascular dysfunction. These findings point to a major but previously not recognized pathophysiological mechanism; ischemia and reperfusion-induced injury to pericytes may impair microcirculatory reflow and negatively affect survival by limiting substrate and drug delivery to tissue already under metabolic stress, despite recanalization of an occluded artery. Agents that can restore pericyte dysfunction and microvascular patency may increase the success of thrombolytic and neuroprotective treatments.

Quantitative assessments of cerebral vascular damage with a silicon rubber casting method in photochemically-induced thrombotic stroke rat models

Previous studies have described microvascular disturbances downstream of occluded large vessels arising during the acute phase (several hours) following cerebral ischemic insult. Prolonged microvascular disturbances may cause delayed neuronal cell death in ischemic penumbral regions, leading to expanded brain infarctions and poor neurological and functional outcomes. The lack of simple and quantitative methods for investigating this microcirculation failure suggests the need to develop a new method for clarifying the precise distribution and persistence of post-ischemic microvascular disturbances. The present study used a silicone rubber casting method in quantitative analyses of microvascular conditions in photochemically-induced thromboembolic (PIT) stroke rat models. After the casting procedure in rats with PIT stroke, a 6 microm-thick coronal section was obtained, and quantitative analyses of microvascular density and measurements of the infarct area in the serial section were performed. The major findings of the present study are as follows: (1) Silicone rubber casting techniques can be applied to precise quantitative analyses of microvessels in the same individual in whom brain infarct volume was measured; (2) the persistence and spatial distribution of microvascular disturbances assessed at the ischemic core, ischemic penumbra, and non-ischemic regions strongly suggest that microvascular disturbances affect brain infarct expansion; (3) the current method demonstrated the protective effects of MK-801 on microvessels, indicating that the technique may be useful in investigating factors that provide vascular protection. The experimental procedure introduced here would facilitate future evaluations of vascular protective agents.

Electron paramagnetic resonance-guided normobaric hyperoxia treatment protects the brain by maintaining penumbral oxygenation in a rat model of transient focal cerebral ischemia

Oxygen therapy for ischemic stroke remains controversial. Too much oxygen may lead to oxidative stress and free radical damage while too little oxygen will have minimal therapeutic effect. In vivo electron paramagnetic resonance (EPR) oximetry, which can measure localized interstitial partial oxygen (pO2), can monitor penumbral changes of pO2. Therefore, we used EPR to study the effects of oxygen therapy in a rat model of 90-mins middle cerebral artery occlusion (MCAO). We found that 95% normobaric O2 given during ischemia was able to maintain penumbral interstitial pO2 levels close to the preischemic value while it may cause a two-fold increase in penumbral pO2 level if given during reperfusion. Elevation of the penumbra pO2 to preischemic physiologic level during MCAO significantly reduced infarction volume, improved neurologic function, decreased the generation of reactive oxygen species (ROS), and reduced matrix metalloproteinase (MMP)-9 expression and caspase-8 cleavage in the penumbra tissue of rats brain treated with oxygen. These results suggest that maintaining penumbral oxygenation by normobaric oxygen treatment during ischemia lead to neuroprotection, which is further reflected by the decreased production of ROS, MMP-9, and caspase-8.

Characterization of Fluoromisonidazole Binding in Stroke

BACKGROUND AND PURPOSE

[18F]fluoromisonidazole (FMISO) positron emission tomography has been used to image hypoxia early after human stroke. To further study the role of hypoxia in stroke and the binding characteristics of FMISO, we aimed to develop [3H]FMISO autoradiography in an animal stroke model. We hypothesized that [3H]FMISO binding is prolonged, allowing correlation with 24-hour histology, and that there is no FMISO binding after effective reperfusion.

METHODS

Temporary middle cerebral artery (MCA) occlusion was performed in rats, followed by [3H]FMISO administration. Tissue preparation for autoradiography and histology (from the same sections) was performed 2.5 hours after MCA occlusion (MCAo; replicating [18F]FMISO studies). Then, otherwise identical cohorts with tissue preparation at 2.5 or 24 hours were prepared. For reperfusion studies, animals had 1-hour MCAo, with [3H]FMISO administered 1 hour after reperfusion.

RESULTS

[3H]FMISO autoradiography provided a high-resolution image of hypoxia throughout the ischemic territory. Delaying animal death from 2.5 to 24 hours allowed histological changes of stroke to develop, without significantly altering either relative intensity (1.88+/-0.06 and 2.02+/-0.11, respectively) or volume (25+/-6 mm3 and 28+/-5 mm3, respectively) of hypoxic binding. [3H]FMISO binding did not occur after effective reperfusion, despite histological injury from the preceding MCAo.

CONCLUSIONS

[3H]FMISO autoradiography of hypoxia in experimental stroke offers several advantages. Bound FMISO is retained in tissues long term, enabling direct correlation with 24-hour histology. It is not bound after effective reperfusion. Therefore, positive [18F]FMISO positron emission tomography studies in stroke patients are indicative of ongoing tissue hypoxia, not merely recent tissue injury.

Pharmacologic cerebral capillary blood flow improvement after deep hypothermic circulatory arrest: An intravital fluorescence microscopy study in pigs

BACKGROUND

Despite meticulous investigation of bypass techniques for deep hypothermic circulatory arrest, unfavorable long-term neurologic deficits have been well documented. Our aim was to improve brain perfusion by reducing platelet plugging with a glycoprotein IIb/IIIa inhibitor (eptifibatide) in an experimental model of deep hypothermic circulatory arrest-reperfusion in pigs.

METHODS

Two groups of 12 piglets each (eptifibatide group [eptifibatide + unfractionated heparin] vs UFH group [only unfractionated heparin]) underwent 10 minutes of normothermic bypass, 40 minutes of cooling during cardiopulmonary bypass (hematocrit, 30%; cardiopulmonary bypass flow, 100 mL x kg(-1) x min(-1)), 60 minutes of circulatory arrest at 15 degrees C, and a 40-minute rewarming period. Intravital fluorescence microscopy of pial vessels at set intervals was performed.

RESULTS

During the cooling period, there was a tendency toward reduced functional capillary density values without statistical significance in both groups. During reperfusion, the eptifibatide group demonstrated a significantly decreased platelet adhesion and aggregation (at 30 minutes of reperfusion: functional capillary density, 104% +/- 3% vs 77% +/- 4% relative to baseline, P = .02; red blood cell velocity, 0.65 vs 0.30 mm/s, P < .004). A more rapid recovery of tissue oxygenation (P < .001) was documented. Furthermore, a significant microvascular permeability reduction was achieved compared with that seen in the UFH group (P < .02). The use of eptifibatide resulted in fewer ultrastructural changes in hippocampal tissue, which is demonstrated by histologic examination.

CONCLUSIONS

Platelet plugging reduction with the glycoprotein IIb/IIIa inhibitor eptifibatide improves cerebral capillary blood flow and reduces cerebral ischemia in the setting of deep hypothermic circulatory arrest. Furthermore, significant endothelial cell injury and perivascular edema reduction can be achieved.

Decomposition and long-lasting downregulation of extracellular matrix in perineuronal nets induced by focal cerebral ischemia in rats

The upregulation of extracellular matrix components, especially chondroitin sulfate proteoglycans, after brain injury and stroke is known to accompany the glial reaction, forming repellent scars that hinder axonal growth and the reorganization of the injured neuronal networks. The extracellular matrix associated with perineuronal nets (PNs) in the primarily injured and remote regions has not yet been systematically analyzed. We use the model of permanent middle cerebral artery occlusion (MCAO) to investigate the acute and long-lasting consequences of ischemia for PNs, related to the damage of neurons and reactions of glial cells, in spontaneously hypertensive rats. Extracellular matrix components associated with PNs around cortical interneurons and neurons in thalamic nuclei were characterized 1, 7, 14, and 35 days after MCAO, using Wisteria floribunda agglutinin (WFA) staining and immunocytochemistry. The degradation of PNs in the infarct core was initiated by loss of WFA-binding matrix components, indicating the cleavage of glycosaminoglycan chains of chondroitin sulfate proteoglycans. Immunostaining showed the subsequent removal of proteoglycan core proteins within the extending microglia/macrophage invasion zone lasting for 2 weeks after MCAO. In the cortical periinfarct region, delineated by an astrocytic scar against the infarct core, the number of WFA-stained and proteoglycan core protein-immunoreactive PNs was permanently reduced. In the homolateral ventroposterior thalamus, the delayed decrease in perineuronal matrix was related to the distribution pattern of activated microglia and massive neuronal degeneration. It can be concluded from these results that complementary to the known upregulation of matrix components in the glial scar, deficits in the expression of the neuron-associated extracellular matrix develop in the periinfarct and remote regions. These deficits may contribute to the long-lasting functional impairments after stroke.

Evolution of a “falx lunatica” in demarcation of critically ischemic myocutaneous tissue

Using intravital microscopy in a chronic in vivo mouse model, we studied the demarcation of myocutaneous flaps and evaluated microvascular determinants for tissue survival and necrosis. Chronic ischemia resulted in a transition zone, characterized by a red fringe and a distally adjacent white falx, which defined the demarcation by dividing the proximally normal from the distally necrotic tissue. Tissue survival in the red zone was determined by hyperemia, as indicated by recovery of the transiently reduced functional capillary density, and capillary remodeling, including dilation, hyperperfusion, and increased tortuosity. Angiogenesis and neovascularization were not observed over the 10-day observation period. The white rim distal to the red zone, appearing as “falx lunatica,” showed a progressive decrease of functional capillary density similar to that of the necrotic distal area but without desiccation, and thus transparency, of the tissue. Development of the distinct zones of the critically ischemic tissue could be predicted by partial tissue oxygen tension (Pt[Formula: see text]) analysis by the time of flap elevation. The falx lunatica evolved at a Pt[Formula: see text] between 6.2 ± 1.3 and 3.8 ± 0.7 mmHg, whereas tissue necrosis developed at <3.8 ± 0.7 mmHg. Histological analysis within the falx lunatica revealed interstitial edema formation and muscle fiber nuclear rarefaction but an absence of necrosis. We have thus demonstrated that ischemia-induced necrosis does not demarcate sharply from normal tissue but develops beside a fringe of tissue with capillary remodeling an adjacent falx lunatica that survives despite nutritive capillary perfusion failure, probably by direct oxygen diffusion.

Interstitial pO2 in ischemic penumbra and core are differentially affected following transient focal cerebral ischemia in rats

Stroke causes heterogeneous changes in tissue oxygenation, with a region of decreased blood flow, the penumbra, surrounding a severely damaged ischemic core. Treatment of acute ischemic stroke aims to save this penumbra before its irreversible damage by continued ischemia. However, effective treatment remains elusive due to incomplete understanding of processes leading to penumbral death. While oxygenation is central in ischemic neuronal death, it is unclear exactly what actual changes occur in interstitial oxygen tension (pO2) in ischemic regions during stroke, particularly the penumbra. Using the unique capability of in vivo electron paramagnetic resonance (EPR) oximetry to measure localized interstitial pO2, we measured both absolute values, and temporal changes of pO2 in ischemic penumbra and core during ischemia and reperfusion in a rat model. Ischemia rapidly decreased interstitial pO2 to 32% +/- 7.6% and 4% +/- 0.6% of pre-ischemic values in penumbra and core, respectively 1 hour after ischemia. Importantly, whilst reperfusion restored core pO2 close to its pre-ischemic value, penumbral pO2 only partially recovered. Hyperoxic treatment significantly increased penumbral pO2 during ischemia, but not in the core, and also increased penumbral pO2 during reperfusion. These divergent, important changes in pO2 in penumbra and core were explained by combined differences in cellular oxygen consumption rates and microcirculation conditions. We therefore demonstrate that interstitial pO2 in penumbra and core is differentially affected during ischemia and reperfusion, providing new insights to the pathophysiology of stroke. The results support normobaric hyperoxia as a potential early intervention to save penumbral tissue in acute ischemic stroke.