Hypothermic protection in rat focal ischemia models: strain differences and relevance to "reperfusion injury"

Flinders sensitive line rats are resistant to infarction following transient occlusion of the middle cerebral artery

BACKGROUND

Depression is a common complication of stroke and increases the risk of mortality and disability. Pre-stroke depression is a possible risk factor for stroke and has also been linked to adverse outcomes. The underlying mechanisms linking depression and stroke remain unclear. Preclinical models may provide novel insights, but models reflecting both conditions are lacking.

METHODS

In this study, we investigated the effects of a 45-min transient middle cerebral artery occlusion (MCAo) on infarct size in male adult Flinders Sensitive Line rats, a genetic animal model of depression, and their control strains Flinders Resistant Line and Sprague-Dawley rats. Infarct size was assessed by tetrazolium chloride (TTC) and microtubule-associated protein 2 (MAP2) staining after 48 h of reperfusion. Angiograms of the vascular structure of naïve animals were produced with a µ-CT scanner.

RESULTS

Both Flinders strains had significantly smaller infarcts following MCAo compared to Sprague-Dawley rats. This effect does not appear to be due to changes in cerebrovascular architecture, as indicated by an initial exploration of vascular organization using angiograms, or body temperature regulation.

CONCLUSIONS

Our study suggests that the rat strain does not influence infarct volumes following MCAo.

Hypothermia revisited: Impact of ischaemic duration and between experiment variability

To assess the true effect of novel therapies for ischaemic stroke, a positive control that can validate the experimental model and design is vital. Hypothermia may be a good candidate for such a positive control, given the convincing body of evidence from animal models of ischaemic stroke. Taking conditions under which substantial efficacy had been seen in a meta-analysis of hypothermia for focal ischaemia in animal models, we undertook three randomised and blinded studies examining the effect of hypothermia induced immediately following the onset of middle cerebral artery occlusion on infarct volume in rats (n = 15, 23, 264). Hypothermia to a depth of 33℃ and maintained for 130 min significantly reduced infarct volume compared to normothermia treatment (by 27-63%) and depended on ischaemic duration (F(3,244) = 21.242, p < 0.05). However, the protective effect varied across experiments with differences in both the size of the infarct observed in normothermic controls and the time to reach target temperature. Our results highlight the need for sample size and power calculations to take into account variations between individual experiments requiring induction of focal ischaemia.

Peri-infarct depolarizations during focal ischemia in the awake Spontaneously Hypertensive Rat. Minimizing anesthesia confounds in experimental stroke

Anesthesia profoundly impacts peri-infarct depolarizations (PIDs), but only one prior report has described their monitoring during experimental stroke in awake animals. Since temporal patterns of PID occurrence are model specific, the current study examined PID incidence during focal ischemia in the awake Spontaneously Hypertensive Rat (SHR), and documented the impact of both prior and concurrent isoflurane anesthesia. For awake recordings, electrodes were implanted under isoflurane anesthesia 1day to 5weeks prior to occlusion surgery. Rats were then subjected to permanent or transient (2h) tandem occlusion of the middle cerebral and ipsilateral common carotid arteries, followed by PID monitoring for up to 3days. Comparison perfusion imaging studies evaluated PID-associated hyperemic transients during permanent ischemia under anesthesia at varied intervals following prior isoflurane exposure. Prior anesthesia attenuated PID number at intervals up to 1week, establishing 2weeks as a practical recovery duration following surgical preparation to avoid isoflurane preconditioning effects. PIDs in awake SHR were limited to the first 4h after permanent occlusions. Maintaining anesthesia during this interval reduced PID number, and prolonged their occurrence through several hours following anesthesia termination. Although PID number otherwise correlated with infarct size, PID suppression by anesthesia was not protective in the absence of reperfusion. PIDs persisted up to 36h after transient occlusions. These results differ markedly from the one previous report of such monitoring in awake Sprague-Dawley rats, which found an extended biphasic PID time course during 24h after both permanent and transient filament occlusions. PID occurrence closely reflects the time course of infarct progression in the respective models, and may be more useful than absolute PID number as an index of ongoing pathology.

Retracted: Reversal of Spinal Cord Ischemia Following Endovascular Thoracic Aortic Aneurysm Repair with Hyperbaric Oxygen and Therapeutic Hypothermia

This article has been officially retracted. The senior author, Emmanuel Urquieta, of the article entitled, "Reversal of Spinal Cord Ischemia Following Endovascular Thoracic Aortic Aneurysm Repair with Hyperbaric Oxygen and Therapeutic Hypothermia," has requesed that the article, published online ahead of print (DOI: 10.1089/ther.2015.0025), be retracted because he discovered one of his coauthors mistakenly submitted the same article to the Journal of Vascular Surgery due to a miscommunication between them. The editorial leadership of Therapeutic Hypothermia and Temperature Management agree that the article must be retracted as a matter of proper scientific publishing protocol whereby an article may not be simultaneously submitted to two journals. The Editors commend Dr. Urquieta for willingly bringing this situation to their attention. Dr. Urquieta and his coauthors sincerely apologize to the Editors and the readership of Therapeutic Hypothermia and Temperature Management.

Dodecafluoropentane Emulsion Extends Window for tPA Therapy in a Rabbit Stroke Model

Dodecafluoropentane emulsion (DDFPe) nanodroplets are exceptional oxygen transporters and can protect ischemic brain in stroke models 24 h without reperfusion. Current stroke therapy usually fails to reach patients because of delays following stroke onset. We tested using DDFPe to extend the time window for tissue plasminogen activator (tPA). Longer treatment windows will allow more patients more complete stroke recovery. We test DDFPe to safely extend the time window for tPA thrombolysis to 9 h after stroke. With IACUC approval, randomized New Zealand white rabbits (3.4-4.7 kg, n = 30) received angiography and 4-mm blood clot in the internal carotid artery for flow-directed middle cerebral artery occlusion. Seven failed and were discarded. Groups were IV tPA (n = 11), DDFPe + tPA (n = 7), and no therapy controls (n = 5). DDFPe (0.3 ml/kg, 2 % emulsion) IV dosing began at 1 h and continued at 90 min intervals for 6 doses in one test group; the other received saline injections. Both got standard IV tPA (0.9 mg/kg) therapy starting 9 h post stroke. At 24 h, neurological assessment scores (NAS, 0-18) were determined. Following brain removal percent stroke volume (%SV) was measured. Outcomes were compared with Kruskal-Wallis analysis. For NAS, DDFPe + tPA was improved overall, p = 0.0015, and vs. tPA alone, p = 0.0052. For %SV, DDFPe + tPA was improved overall, p = 0.0003 and vs. tPA alone, p = 0.0018. NAS controls and tPA alone were not different but %SV was, p = 0.0078. With delayed reperfusion, DDFPe + tPA was more effective than tPA alone in preserving functioning brain after stroke. DDFPe significantly extends the time window for tPA therapy.

Dodecafluoropentane Emulsion (DDFPe) Decreases Stroke Size and Improves Neurological Scores in a Permanent Occlusion Rat Stroke Model

Background:

Dodecafluoropentane emulsion (DDFPe), given IV one hour after stroke, has been shown to greatly reduce the percent stroke volume (%SV) in rabbits. With repeated doses its effect continued for 24 hours.

Purpose:

Test DDFPe as neuroprotective agent in permanent occlusion rat stroke models in Sprague Dawley (SD) and Spontaneously Hypertensive Rats (SHR) measuring both %SV and neurological assessment scores (NAS).

Methods:

The male rats received either saline (control), or one or four doses (1x or 4x) of DDFPe (0.6ml/kg IV) one hour post stroke. Treatment groups were SD (n=26) (control, 1x and 4x; n=12, 7 and 7) and SHR (n=14) (control, 1x and 4x; n=7, 3 and 4). The 4x doses were given at 1.5 hour intervals. At six hours post stroke, the rats received a NAS using standard tests for balance, reflexes, and motor performance. Then rats were euthanized and brains removed for TTC evaluation of %SV.

Results:

For %SV analysis strain differences were not significant therefore strains were combined. DDFPe significantly decreased %SV in 1x and 4xDDFPe groups compared to control groups (2.59±1.81 and 0.98±0.88 vs. 9.24±6.06, p≤0.001 each; p≤0.0001 for the overall test for treatment effect). The 1x versus 4xDDFPe groups were not significantly different (p=0.40). In NAS analysis both strains showed significant improvement with 4xDDFPe therapy vs. controls, (SD: 5.00+2.45 vs. 9.36+3.56, p=0.01; SHR: 7.75+4.43 vs. 12.14+3.08, p=0.05). Differences between the 1x DDFPe group and controls were not significant (SD: 8.43+3.69; SHR: 9. 33+3.51).

Conclusion:

DDFPe treatment provides significant neuroprotection when assessed six hours post stroke.

Standards and pitfalls of focal ischemia models in spontaneously hypertensive rats: with a systematic review of recent articles

We reviewed the early development of various focal ischemia models in spontaneously hypertensive rats (SHR), and summarized recent reports on this topic. Among 6 focal ischemia models established in divergent substrains of SHR, distal middle cerebral artery occlusion is the most frequently used and relevant method of focal ischemia in the light of penumbra concept. We performed an online PubMed search (2001–2010), and identified 118 original articles with focal ischemia in SHR. Physiological parameters such as age, body weight, and even blood pressure were often neglected in the literature: the information regarding the physiological parameters of SHR is critical, and should be provided within the methodology section of all articles related to stroke models in SHR. Although the quality of recent studies on neuroprotective strategy is improving, the mechanisms underlying the protection should be more clearly recognized so as to facilitate the translation from animal studies to human stroke. To overcome the genetic heterogeneity in substrains of SHR, new approaches, such as a huge repository of genetic markers in rat strains and the congenic strategy, are currently in progress.

Mild hypothermia of 34°C reduces side effects of rt-PA treatment after thromboembolic stroke in rats

BACKGROUND

Hypothermia is neuroprotective in experimental stroke and may extend the so far limited therapeutic time window for thrombolysis. Therefore, hypothermia of 34°C and its effects on delayed thrombolysis including reperfusion-associated injury were investigated in a model of thromboembolic stroke (TE).

METHODS

Male Wistar rats (n = 48) were subjected to TE. The following treatment groups were investigated: control group - normothermia (37°C); thrombolysis group - rt-PA 90 min after TE; hypothermia by 34°C applied 1.5 to 5 hours after TE; combination therapy- hypothermia and rt-PA. After 24 hours infarct size, brain edema and neuroscore were assessed. Protein markers for inflammation and adhesion, gelatinase activity, and blood brain barrier (BBB) disruption were determined. MRI-measurements investigated infarct evolution and blood flow parameters.

RESULTS

The infarct volume and brain swelling were smaller in the hypothermia group compared to the other groups (p < 0.05 to p < 0.01). Thrombolysis resulted in larger infarct and brain swelling than all others. Hypothermia in combination with thrombolysis reduced these parameters compared to thrombolysis (p < 0.05). Moreover, the neuroscore improved in the hypothermia group compared to control and thrombolysis. Animals of the combination therapy performed better than after thrombolysis alone (p < 0.05). Lower serum concentration of sICAM-1, and TIMP-1 were shown for hypothermia and combination therapy. Gelatinase activity was decreased by hypothermia in both groups.

CONCLUSIONS

Therapeutic hypothermia reduced side-effects of rt-PA associated treatment and reperfusion in our model of TE.

Systemic inflammatory challenges compromise survival after experimental stroke via augmenting brain inflammation, blood- brain barrier damage and brain oedema independently of infarct size

BACKGROUND

Systemic inflammation impairs outcome in stroke patients and experimental animals via mechanisms which are poorly understood. Circulating inflammatory mediators can activate cerebrovascular endothelium or glial cells in the brain and impact on ischaemic brain injury. One of the most serious early clinical complications of cerebral ischaemia is brain oedema, which compromises survival in the first 24-48 h. It is not understood whether systemic inflammatory challenges impair outcome after stroke by increasing brain injury only or whether they have direct effects on brain oedema, cerebrovascular inflammation and blood-brain barrier damage.

METHODS

We used two different systemic inflammatory stimuli, acute endotoxin treatment and anaphylaxis to study mechanisms of brain injury after middle cerebral artery occlusion (MCAo). Ischaemic brain injury, blood-brain barrier damage and oedema were analysed by histological techniques. Systemic cytokine responses and inflammatory changes in the brain were analysed by cytometric bead array, immunofluorescence, in situ hibridization and quantitative real-time PCR.

RESULTS

Systemic inflammatory challenges profoundly impaired survival in the first 24 h after experimental stroke in mice, independently of an increase in infarct size. Systemic lipopolysaccharide (LPS) dose-dependently increased mortality (50-100%) minutes to hours after cerebral ischaemia. Acute anaphylactic challenge in ovalbumin-sensitised mice affected stroke more seriously when induced via intraperitoneal administration compared to intravenous. Both LPS and anaphylaxis induced inflammatory changes in the blood and in the brain prior to experimental stroke. Plasma cytokine levels were significantly higher after LPS, while increased IL-10 levels were seen after anaphylaxis. After MCAo, both LPS and anaphylaxis increased microglial interleukin-1α (IL-1α) expression and blood-brain barrier breakdown. LPS caused marked granulocyte recruitment throughout the ipsilateral hemisphere. To investigate whether reduction of ischaemic damage can improve outcome in systemic inflammation, controlled hypothermia was performed. Hypothermia reduced infarct size in all treatment groups and moderately improved survival, but failed to reduce excess oedema formation after anaphylaxis and LPS-induced neuroinflammation.

CONCLUSIONS

Our results suggest that systemic inflammatory conditions induce cerebrovascular inflammation via diverse mechanisms. Increased brain inflammation, blood-brain barrier injury and brain oedema formation can be major contributors to impaired outcome in mice after experimental stroke with systemic inflammatory stimuli, independently of infarct size.

Therapeutic applications of hypothermia in cerebral ischaemia

There is considerable experimental evidence that hypothermia is neuroprotective and can reduce the severity of brain damage after global or focal cerebral ischaemia. However, despite successful clinical trials for cardiac arrest and perinatal hypoxia-ischaemia and a number of trials demonstrating the safety of moderate and mild hypothermia in stroke, there are still no established guidelines for its use clinically. Based upon a review of the experimental studies we discuss the clinical implications for the use of hypothermia as an adjunctive therapy in global cerebral ischaemia and stroke and make some suggestions for its use in these situations.

Experimental and clinical use of therapeutic hypothermia for ischemic stroke: opportunities and limitations

Stroke remains a disease with a serious impact on quality of life but few effective treatments exist. There is an urgent need to develop and/or improve neuroprotective strategies to combat this. Many drugs proven to be neuroprotective in experimental models fail to improve patient outcome in a clinical setting. An emerging treatment, therapeutic hypothermia (TH), is a promising neuroprotective therapy in stroke management. Several studies with TH in experimental models and small clinical trials have shown beneficial effects. Despite this, implementation into the clinical setting is still lacking due to methodological considerations as well as hypothermia-related complications. This paper discusses the possible opportunities and limitations of the use of TH in animal models and the translation into the clinic.

An experimental model of focal ischemia using an internal carotid artery approach

Animal models of cerebral ischemia represent an important contribution to both our understanding of stroke mechanism and the development of new therapies. The technique of MCAO (middle cerebral artery occlusion) via ECA (external carotid artery) occlusion is widely utilized. Disruption of the ECA and its branches leads to impaired mastication and oral intake, post-surgical body weight loss, and poor neurological recovery which can possibly confound one's interpretation of rats' neurological outcome. Here, we developed a novel modified technique for MCAO without ligation or coagulation of the ECA and its branches using an approach via the internal carotid artery (ICA). In our modified technique, we perform an additional fixation of the filament in the ICA which improves the stability of the model and increases the homogeneity in stroke size. Compared with the original MCAO technique via the ECA, our modified technique via the ICA demonstrated decreased variability in the percent infarcted volume and brain edema, as well as a decreased mortality. Additionally, we observed that with our modified technique, rats gained more weight after surgery and there was less initial weight loss after the surgical preparation. Our new approach may serve as an effective model for stroke, and may lead to a better understanding of stoke pathophysiology and to the future development of new drugs and other neuroprotective agents.

Therapeutic hypothermia for acute ischemic stroke: ready to start large randomized trials?

Therapeutic hypothermia is a means of neuroprotection well established in the management of acute ischemic brain injuries such as anoxic encephalopathy after cardiac arrest and perinatal asphyxia. As such, it is the only neuroprotective strategy for which there is robust evidence for efficacy. Although there is overwhelming evidence from animal studies that cooling also improves outcome after focal cerebral ischemia, this has not been adequately tested in patients with acute ischemic stroke. There are still some uncertainties about crucial factors relating to the delivery of hypothermia, and the resolution of these would allow improvements in the design of phase III studies in these patients and improvements in the prospects for successful translation. In this study, we discuss critical issues relating first to the targets for therapy including the optimal depth and duration of cooling, second to practical issues including the methods of cooling and the management of shivering, and finally, of factors relating to the design of clinical trials. Consideration of these factors should inform the development of strategies to establish beyond doubt the place of hypothermia in the management of acute ischemic stroke.

The protective effect of early hypothermia on PTEN phosphorylation correlates with free radical inhibition in rat stroke

We recently showed that intraischemic moderate hypothermia (30 degrees C) reduces ischemic damage through the Akt pathway after permanent distal middle cerebral artery occlusion in rats. The only Akt pathway component preserved by hypothermia is phosphorylated phosphatase and tensin homolog deleted on chromosome 10 (p-PTEN), which suggests that p-PTEN may have a central role in neuroprotection. Reactive oxygen species (ROS) are critically involved in mediating ischemic damage after stroke by interacting with signaling molecules, including Akt, PTEN, and delta-protein kinase C (PKC). We investigated the protective mechanisms of moderate hypothermia on these signaling proteins after transient focal ischemia in rats. Early moderate hypothermia (3 h) was administered 15 mins before reperfusion, and delayed moderate hypothermia (3 h) was applied 15 mins after reperfusion. Our results indicate that early hypothermia reduced infarction, whereas delayed hypothermia did not. However, both early and delayed hypothermia maintained levels of Mn-SOD (superoxide dismutase) and phosphorylated Akt and blocked delta-PKC cleavage, suggesting that these factors may not be critical to the protection of hypothermia. Nevertheless, early hypothermia preserved p-PTEN levels after reperfusion, whereas delayed hypothermia did not. Furthermore, ROS inhibition maintained levels of p-PTEN after stroke. Together, these findings suggest that phosphorylation levels of PTEN are closely associated with the protective effect of early hypothermia against stroke.

Early brain temperature elevation and anaerobic metabolism in human acute ischaemic stroke

Early after acute ischaemic stroke, elevation of brain temperature might augment tissue metabolic rate and conversion of ischaemic but viable tissue to infarction. This might explain the observed link between pyrexia, severe stroke and poor outcome. We tested this hypothesis by measuring brain temperature and lactate concentration with multi-voxel magnetic resonance spectroscopic imaging across the acute ischaemic stroke lesion and normal brain as determined on diffusion imaging. We compared patterns of lactate concentration (reported in 'institutional units') and temperature elevation in diffusion lesion core, potential penumbra, ipsilateral and contralateral normal brain and with stroke severity. Amongst 40 patients with moderate to severe acute stroke imaged up to 26 h after onset, lactate concentration was highest in the ischaemic lesion core (42 versus 26 units in potential penumbra, P < 0.05), whereas temperature was highest in the potential penumbra (37.7 versus 37.3 degrees C in lesion core, P < 0.05). Neither sub-regional temperature nor lactate concentration correlated with stroke severity. With increasing time after stroke, ipsilateral brain temperature did not change, but contralateral hemisphere temperature was higher in patients scanned at later times; lactate remained elevated in the lesion core, but declined in potential penumbral and ipsilateral normal tissue at later times. We conclude that early brain temperature elevation after stroke is not directly related to lactate concentration, therefore augmented metabolism is unlikely to explain the relationship between early pyrexia, severe stroke and poor outcome. Early brain temperature elevation may result from different mechanisms to those which raise body temperature after stroke. Further studies are required to determine why early brain temperature elevation is highest in potential penumbral tissue.

Methamphetamine preconditioning: differential protective effects on monoaminergic systems in the rat brain

Pretreatment with methamphetamine (METH) can attenuate toxicity due to acute METH challenges. The majority of previous reports have focused mainly on the effects of the drug on the striatal dopaminergic system. In the present study, we used a regimen that involves gradual increases in METH administration to rats in order to mimic progressively larger doses of the drug used by some human METH addicts. We found that this METH preconditioning was associated with complete protection against dopamine depletion caused by a METH challenge (5 mg/kg x 6 injections given 1 h apart) in the striatum and cortex. In contrast, there was no preconditioning-mediated protection against METH-induced serotonin depletion in the striatum and hippocampus, with some protection being observed in the cortex. There was also no protection against METH-induced norepinephrine (NE) depletion in the hippocampus. These results indicate that, in contrast to the present dogmas, there might be differences in the mechanisms involved in METH toxicity on monoaminergic systems in the rodent brain. Thus, chronic injections of METH might activate programs that protect against dopamine toxicity without influencing drug-induced pathological changes in serotoninergic systems. Further studies will need to evaluate the cellular and molecular bases for these differential responses.

Dynamic changes in cortical NADH fluorescence in rat focal ischemia: evaluation of the effects of hypothermia on propagation of peri-infarct depolarization by temporal and spatial analysis

Suppression of peri-infarct depolarizations (PIDs) is one of the major mechanisms of hypothermic protection against transient focal cerebral ischemia. Previous studies have shown the lack of hypothermic protection against permanent focal ischemia. We hypothesized the lack of hypothermic protection was due to the poor efficacy in suppression of PIDs. To examine the hypothesis, we elucidated the effects of hypothermia on the manner of propagation of PIDs with temporal and spatial resolutions using NADH (reduced nicotinamide adenine dinucleotide) fluorescence images by illuminating the parietal-temporal cortex with ultraviolet light. Spontaneously hypertensive rats (n=14) were subjected to permanent focal ischemia by occlusion of the middle cerebral and left common carotid arteries. 2-h hypothermia (30 degrees C) was initiated before ischemia. Although hypothermia delayed the appearance of PIDs, it did not suppress their appearance. Furthermore, 54% of the PIDs enlarged the high-intensity area of NADH fluorescence in the hypothermia group, similar to the normothermia group (53%). The high-intensity area of NADH fluorescence widened by each PID was larger in the hypothermia group than in the normothermia group. These findings suggest that PIDs even in hypothermia are one of the major factors causing growth of infarction, emphasizing the importance of therapy that targets suppression of PIDs even during hypothermia.

Therapeutic hypothermia in experimental models of focal and global cerebral ischemia and intracerebral hemorrhage

Experimental evidence shows that therapeutic hypothermia (TH) protects the brain from cerebral injury in multiple ways. In different models of focal and global cerebral ischemia, mild-to-moderate hypothermia reduces mortality and neuronal injury and improves neurological outcome. In models of experimental intracerebral hemorrhage (ICH), TH reduces edema formation but does not show consistent benefi cial effects on functional outcome parameters. However, the number of studies of hypothermia on ICH is still limited. TH is most effective when applied before or during the ischemic event, and its neuroprotective properties vary according to species, strains and the model of ischemia used. Intrinsic changes in body and brain temperature frequently occur in experimental models of focal and global cerebral ischemia, and may have infl uenced studies on other neuroprotectants. This might be one explanation for the failure of a large amount of translational clinical neuroprotective trials. Hypothermia is the only neuroprotective therapeutic agent for cerebral ischemia that has successfully managed the transfer from bench to bedside, and it is an approved therapy for patients after cardiac arrest and children with hypoxic-ischemic encephalopathy. However, the implementation of hypothermia in the treatment of stroke patients is still far from routine clinical practice. In this article, the authors describe the development of TH in different models of focal and global cerebral ischemia, point out why hypothermia is so efficient in experimental cerebral ischemia, explain why temperature regulation is essential for further neuroprotective studies and discuss why TH for acute ischemic stroke still remains a promising but controversial therapeutic option.

Comparison of 12, 24 and 48 h of systemic hypothermia on outcome after permanent focal ischemia in rat

Mild hypothermia reduces injury in models of global and focal cerebral ischemia even when initiated after the insult. Neuroprotection depends critically upon the duration of hypothermia with longer treatments often being more efficacious. However, the ideal treatment duration is not known for most insults and this knowledge would facilitate clinical studies. Thus, we compared 12, 24 and 48 h of systemic hypothermia (33 degrees C vs. normothermia) initiated 1 h after permanent middle cerebral artery occlusion (pMCAO), which was produced by permanent occlusion of the carotid arteries and cauterization of the distal MCA in rat. Behavioral recovery and lesion volume were determined 7 days after pMCAO. All three treatments significantly and equally attenuated neurological deficits (e.g., forelimb placing response). Conversely, stepping error rate in the horizontal ladder test was significantly reduced only by the 24-h (18.7%) and 48-h treatments (11.7%) compared to normothermic rats (34.4%), and the 48-h treatment was significantly better than the 12-h treatment (28.8%). Similarly, brain injury was significantly reduced by 24-h (78.8 mm(3) lesion volume) and 48-h (66.8 mm(3)) treatments compared to normothermia (142.6 mm(3)), and the 48-h treatment was significantly better than the 12-h duration (114.6 mm(3)). In separate experiments cerebral edema was measured via wet-dry weight measurements and significantly reduced by hypothermia (e.g., from 83.7% water in the injured cortex of normothermic rats to 81.4% in rats cooled for one day), but for this there were no significant duration effects. In summary, prolonged hypothermia treatment provides superior protection overall, but this is not explained by reductions in edema.

Temporal thresholds for infarction and hypothermic protection in Long-Evans rats: factors affecting apparent 'reperfusion injury' after transient focal ischemia

BACKGROUND AND PURPOSE

Some previous studies in Long-Evans rats noted larger infarcts after transient middle cerebral artery (MCA) occlusions than after permanent occlusions, interpreted to demonstrate "reperfusion injury." Recent experiments failed to reproduce this phenomenon, prompting an investigation of the sources of variability in this animal model.

METHODS

Male Long-Evans rats were subjected to surgical occlusion of the right MCA and ipsilateral common carotid artery. Variables tested included duration of occlusion and halothane anesthesia exposure and targeting of proximal or distal MCA occlusion sites. The temporal window for hypothermic protection was also investigated.

RESULTS

MCA occlusions at the level of the rhinal fissure produced graded increases in infarct volume with ischemia duration, and lesion size did not differ between 3-hour and permanent occlusions independent of anesthesia duration. Occlusions at a more distal site produced infarcts of comparable size after transient 3-hour occlusions and after permanent occlusions accompanied by prolonged anesthesia, but significantly smaller infarcts were seen when permanent occlusions were followed by rapid anesthesia termination. Hypothermia conferred protection only when initiated before reperfusion after transient proximal occlusions.

CONCLUSIONS

These results indicate that previously described "reperfusion injury" after transient MCA occlusions conversely reflects unexpected injury reduction when rats with permanent occlusions experience early anesthesia termination. More rapid blood pressure recovery under such conditions permits improved collateral perfusion. The absence of a detectable postischemic window for hypothermic protection further argues against a significant component of delayed postreperfusion injury in this model.

Transient cooling during early reperfusion attenuates delayed edema and infarct progression in the Spontaneously Hypertensive Rat. Distribution and time course of regional brain temperature change in a model of postischemic hypothermic protection

The temperature threshold for protection by brief postischemic cooling was evaluated in a model of transient focal ischemia in the Spontaneously Hypertensive Rat, using an array of epidural probes to monitor regional brain temperatures. Rats were subjected to 90 mins tandem occlusion of the right middle cerebral artery (MCA) and common carotid artery. Systemic cooling to 32 degrees C was initiated 5 mins before recirculation, with simultaneous brain cooling to temperatures ranging from 28 degrees C to 32 degrees C within the MCA territory by means of a temperature-controlled saline drip. Rewarming was initiated at 2 h recirculation and was complete within 30 mins. Tissue damage and edema volume showed clear temperature-dependent reductions when evaluated at 3 days survival, with no protection evident in the group at 32 degrees C but progressive effects on both parameters after deeper cooling. A particularly striking effect was the essentially complete elimination of edema progression between 1 and 3 days. Temperature at distal sites within the MCA territory better predicted reductions in lesion volume, indicating that protection required effective cooling of the penumbral regions destined to be spared. These results show that even brief cooling can be highly protective when initiated at the time of recirculation after focal ischemia, but indicate a substantially lower temperature threshold for hypothermic protection than has been reported for other strains, occlusion methods, and cooling durations.

General versus specific actions of mild-moderate hypothermia in attenuating cerebral ischemic damage

Mild or moderate hypothermia is generally thought to block all changes in signaling events that are detrimental to ischemic brain, including ATP depletion, glutamate release, Ca(2+) mobilization, anoxic depolarization, free radical generation, inflammation, blood-brain barrier permeability, necrotic, and apoptotic pathways. However, the effects and mechanisms of hypothermia are, in fact, variable. We emphasize that, even in the laboratory, hypothermic protection is limited. In certain models of permanent focal ischemia, hypothermia may not protect at all. In cases where hypothermia reduces infarct, some studies have overemphasized its ability to maintain cerebral blood flow and ATP levels, and to prevent anoxic depolarization, glutamate release during ischemia. Instead, hypothermia may protect against ischemia by regulating cascades that occur after reperfusion, including blood-brain barrier permeability and the changes in gene and protein expressions associated with necrotic and apoptotic pathways. Hypothermia not only blocks multiple damaging cascades after stroke, but also selectively upregulates some protective genes. However, most of these mechanisms are addressed in models with intraischemic hypothermia; much less information is available in models with postischemic hypothermia. Moreover, although it has been confirmed that mild hypothermia is clinically feasible for acute focal stroke treatment, no definite beneficial effect has been reported yet. This lack of clinical protection may result from suboptimal criteria for patient entrance into clinical trials. To facilitate clinical translation, future efforts in the laboratory should focus more on the protective mechanisms of postischemic hypothermia, as well as on the effects of sex, age and rewarming during reperfusion on hypothermic protection.

Neuroprotection in focal cerebral ischemia owing to delayed treatment with melanocortins

In gerbils subjected to transient global cerebral ischemia, melanocortin peptides produce long-lasting protection with a broad time window, and through the activation of central nervous system melanocortin MC(4) receptors. Here we aimed to investigate whether melanocortins are neuroprotective also in a rat model of focal cerebral ischemia induced by intrastriatal microinjection of endothelin-1. The vasoconstrictor agent endothelin-1 caused a significant impairment in spatial learning and memory, as well as in sensory-motor orientation and limb use, associated with severe striatal morphological damage including intense neuronal death and an almost complete myelin degradation. Treatment of ischemic rats with a nanomolar dose (340 microg/kg/day i.p. for 11 days, beginning 3 h or 9 h after endothelin-1 microinjection) of the melanocortin analog [Nle(4), D-Phe(7)]alpha-melanocyte-stimulating hormone (NDP-alpha-MSH) significantly reduced striatal damage, and improved subsequent functional recovery, with all scheduled NDP-alpha-MSH treatments. Pharmacological blockade of melanocortin MC(4) receptors prevented the protective effect of NDP-alpha-MSH. Our findings give evidence that melanocortins are neuroprotective, with a broad time window, also in a severe model of focal cerebral ischemia, and suggest that melanocortin MC(4) receptor agonists could produce neuroprotection in different experimental models of ischemic stroke.

Selective, reversible occlusion of the middle cerebral artery in rats by an intraluminal approach

These studies optimized design and application of an intraluminal filament method to achieve selective middle cerebral artery (MCA) occlusion in rats. Silicone plugs of 300 microm diameter and 700-800 microm length were molded onto 6-0 suture. These were introduced into Wistar rats previously fitted with telemetric probes, using established placement procedures, with and without heparinization. Temperature and activity were monitored for 3 days, after which lesion volumes were assessed by triphenyltetrazolium chloride staining. Optimized filaments entered the MCA in 85% of Wistar rats, failures being attributable to anatomical variation at its origin from the internal carotid artery. Infarcts restricted to the MCA territory were apparent after 90 min occlusion, and maximal after 3 h occlusion. Intraischemic hyperthermia was noted in a third of occlusions performed without heparin, but never with anticoagulant treatment. Permanent occlusions were also evaluated in Fisher, Lewis, Long-Evans, Spontaneously Hypertensive and Sprague-Dawley rats, and Wistar rats from a second supplier, and compared with data for surgical MCA occlusions. Success rates varied among strains, but infarct volumes correlated with those obtained after surgical occlusions in respective populations. These studies demonstrate the feasibility and limitations of reversible and selective intraluminal filament occlusion of the MCA in rats.

CBF changes associated with focal ischemic preconditioning in the spontaneously hypertensive rat

Experimental stroke models exhibit robust protection after prior preconditioning (PC) insults. This study comprehensively examined cerebral blood flow (CBF) responses to permanent middle cerebral artery (MCA) occlusion in spontaneously hypertensive rats preconditioned by noninjurious transient focal ischemia, using [(14)C]iodoantipyrine autoradiography at varied occlusion intervals. Preconditioning was produced by 10-min occlusion of the MCA and ipsilateral common carotid artery under halothane anesthesia. These vessels were permanently coagulated 24 h later in naïve, PC, and sham-operated rats. Infarct volumes were determined from hematoxylin-eosin-stained frozen sections after 1 or 3 days. Edema-corrected infarct volume was reduced from 127+/-21 in naïve rats to 101+/-31 and 52+/-28 mm(3) in sham and PC groups, respectively, at 1 day, with similar results at 3 days. All animals exhibited a consistent CBF threshold for infarction (approximately 30 mL/100 g/min). Tissue volumes below this threshold were identical in naïve and PC groups after 15-min occlusion. However, by 3 h the volume of ischemic cortex decreased in the PC group but remained unchanged in naïve rats, predicting final infarct volumes. Cerebral blood flow recovery was confirmed in brains of individual rats evaluated by repeated laser Doppler perfusion imaging during the same 3-h interval. Modest sham protection correlated with better-maintained global perfusion, detectable also in the contralateral cortex, apparently reflecting the PC effects of prior anesthesia. These results establish that timely reperfusion of penumbra, achieved by synergistic mechanisms, is a primary determinant of PC-induced protection in experimental stroke.

The proteasome inhibitor VELCADE® reduces infarction in rat models of focal cerebral ischemia

The potential neuroprotective effects of VELCADE were investigated in two different models of focal cerebral ischemia. For time-window assessment, male Wistar-Kyoto rats were treated with 0.2 mg/kg VELCADE at 1, 2, or 3 h after the induction of permanent middle cerebral artery occlusion (MCAO) using the suture occlusion method (experiment 1). To evaluate effects in a different model, male Sprague-Dawley rats received 0.2 mg/kg VELCADE after embolic MCAO (experiment 2). Infarct volume was calculated based on TTC-staining 24 h postischemia and whole blood proteasome activity was fluorometrically determined in both experiments at baseline, 1 and 24 h post-MCAO. In experiment 1, a dose of 0.2 mg/kg inhibited proteasome activity by 77% and infarct volume was reduced to 175.7+/-59.9 mm3 and 205.9+/-83.9 mm3 (1 and 2 h group, respectively; p<0.05) compared to 306.5+/-48.5 mm3 (control). Treatment at 3 h was not neuroprotective (293.0+/-40.1 mm3). After embolic MCAO, infarct volume was 167.5+/-90.7 mm3 (treatment group) and 398.9+/-141.3 mm3 (control; p=0.002). In conclusion, VELCADE treatment inhibited whole blood proteasome activity and achieved significant neuroprotection in two rat models of focal cerebral ischemia at various time points poststroke.

Broad therapeutic treatment window of [Nle(4), D-Phe(7)]alpha-melanocyte-stimulating hormone for long-lasting protection against ischemic stroke, in Mongolian gerbils

Melanocortin peptides have been shown to produce neuroprotection in experimental ischemic stroke. The aim of the present investigation was to identify the therapeutic treatment window of melanocortins, and to determine whether these neuropeptides chronically protect against damage consequent to brain ischemia. A 10-min period of global cerebral ischemia in gerbils, induced by occluding both common carotid arteries, caused impairment in spatial learning and memory (Morris test: four sessions from 4 to 67 days after the ischemic episode), associated with neuronal death in the hippocampus. Treatment with a nanomolar dose (340 microg/kg i.p., every 12 h for 11 days) of the melanocortin analog [Nle(4), D-Phe(7)]alpha-melanocyte-stimulating hormone (NDP-alpha-MSH), starting 3-18 h after the ischemic episode, reduced hippocampal damage with improvement in subsequent functional recovery. The protective effect was long-lasting (67 days, at least) with all schedules of NDP-alpha-MSH treatment; however, in the latest treated (18 h) gerbils, some spatial memory deficits were detected. Pharmacological blockade of melanocortin MC(4) receptors prevented the protective effects of NDP-alpha-MSH. Our findings indicate that, in conditions of brain ischemia, melanocortins can provide strong and long-lasting protection with a broad therapeutic treatment window, and with involvement of melanocortin MC(4) receptors, 18 h being the approximately time-limit for stroke late treatment to be effective.

Histopathological effects of delayed reperfusion after middle cerebral artery occlusion in the anesthetized baboon

In patients with middle cerebral artery (MCA) territory stroke, attempts to recanalize the brain are currently being extended beyond the classic 3-h time window. Mechanical thrombectomy is particularly attractive as it may carry lesser risks of severe hemorrhagic transformation than thrombolysis. However, whether late reperfusion per se promotes hemorrhagic transformation and increases infarct volume as compared to permanent occlusion is unclear. There is no study of the histopathologic sequelae of late reperfusion following MCA occlusion (MCAo) in the non-human primate. Five young adult baboons completed a specially designed protocol of 20-h MCAo (under etomidate anaesthesia), followed by 4-week survival and finally perfusion-fixation. Infarct volume was measured histologically using validated stereological methods. The results were compared to our previously published series of 6 h and permanent MCAo performed with identical experimental and post mortem procedures. An infarct was present in each baboon, consistently involving the caudate head, internal capsule and putamen; the adjacent inferior frontal cortex was involved in one subject. Infarct volume was significantly larger than with 6 h MCAo, as expected, but did not differ from permanent MCAo. There was no evidence of hemorrhage around the infarcted area in any animal. We found that following a 20 h ischemic episode, the infarct volume was similar to that found with permanent occlusion, with no evidence of hemorrhagic transformation. Cautiously extrapolating to the human situation, our findings suggest that even late mechanical recanalization may not promote brain damage and could be considered in selected cases.

Differences in ischemic lesion evolution in different rat strains using diffusion and perfusion imaging

BACKGROUND AND PURPOSE

Interstrain differences in the temporal evolution of ischemia after middle cerebral artery occlusion (MCAO) in rats may considerably influence the results of experimental stroke research. We investigated, in 2 commonly used rat strains (Sprague-Dawley [SD] and Wistar-Kyoto [WK]), the spatiotemporal evolution of ischemia after permanent suture MCAO using diffusion and perfusion imaging.

METHODS

Serial measurements of quantitative cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) were performed up to 210 min after MCAO. Lesion volumes were calculated by using previously established viability thresholds and correlated with infarct volume defined by 2,3,5-triphenyltetrazolium chloride staining 24 hours after MCAO.

RESULTS

While the ADC-derived lesion volume increased rapidly during the first 120 min after MCAO and essentially stopped growing after 3 hours in SD rats, ADC lesion in WK rats increased progressively during the entire 210-min period and was significantly smaller at all time points (P<0.05). The abnormal perfusion volume correlated highly with the TTC-defined infarct size in both groups. In WK rats, the abnormal perfusion volume was significantly larger than the abnormal diffusion volume up to 90 min after MCAO (P<0.001), whereas the diffusion/perfusion mismatch was significant (P<0.001) only at 45 min in SD rats. ADC-CBF scatterplots analysis revealed a slower and less robust ADC decline over time in WK rats in pixels with severe (<20% of normal) and moderate (21 to 40% of normal) CBF reduction.

CONCLUSIONS

This study demonstrated substantial differences in acute ischemic lesion evolution between SD and WK rats. These interstrain variations must be taken into account when assessing new therapeutic approaches on ischemic lesion evolution in the rat MCAO model.

In vivo protective effects of urocortin on ischemia-reperfusion injury in rat heart via free radical mechanisms

The aim of this study was to investigate the effects of urocortin (UCN) on oxidative stress and the mechanisms of urocortin on ischemia–reperfusion injury in vivo in the rat model. Thirty-six Sprague–Dawley rats were divided into 6 groups, including sham, control (normal saline solution), UCN1, UCN2, UCN3, and verapamil groups. The left anterior descending coronary artery of all rats except those in the sham group was treated with a 30-min occlusion followed by a 60-min reperfusion. Just before the occlusion, normal saline solution, UCN (5, 10, and 20 µg/kg body mass), or verapamil (1 mg/kg body mass) was administered. Heart rates, beating rhythm, and S-T segments were constantly monitored using an ECG. At the completion of the drug adminstration, blood samples were taken to measure the activity of superoxide dismutase (SOD), malonaldehyde (MDA), glutathione peroxidase (GSH-PX), and nitric oxide (NO) to evaluate the effects of UCN on oxidative stress. Finally, the size of infarction was measured. Arrhythmia rates were significantly lower, and the infarction size was significantly smaller (p < 0.01), in the UCN groups vs. the control group. Verapamil also significantly reduced arrhythmia rates and infarction size. The MDA activities were remarkably diminished, whereas the SOD, GSH-PX, and NO activities were significantly higher in the UCN and VER groups (p < 0.01). MDA, SOD, and NO activities were strongly correlated with UCN doses. These results suggest that UCN may play a protective role in ischemia–reperfusion injury in rat hearts against the oxidative stress by inhibiting free radicals' activities. Key words: urocortin, ischemia–reperfusion injury, arrhythmias, free radical anti-oxidative enzymes, oxidative stress.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

Effects of temperature on calcium transients and Ca2+-dependent afterhyperpolarizations in neocortical pyramidal neurons

In neocortical pyramidal neurons, the medium (mAHP) and slow AHP (sAHP) have different relationships with intracellular [Ca2+]. To further explore these differences, we varied bath temperature and compared passive and active membrane properties and Ca2+ transients in response to a single action potential (AP) or trains of APs. We tested whether Ca(2+)-dependent events are more temperature sensitive than voltage-dependent ones, the slow rise time of the sAHP is limited by diffusion, and temperature sensitivity differs between the mAHP and sAHP. The onset and decay kinetics of the sAHP were very temperature sensitive (more so than diffusion). We found that the decay time course of Ca2+ transients was also very temperature sensitive. In contrast, the mAHP (amplitude, time to peak, and exponential decay) and sAHP peak amplitude were moderately sensitive to temperature. The amplitudes of intracellular Ca2+ transients evoked either by a single spike or a train of spikes showed modest temperature sensitivities. Pyramidal neuron input resistance was increased by cooling. With the exception of threshold, which remained unchanged between 22 and 35 degrees C, action potential parameters (amplitude, half-width, maximum rates of rise and fall) were modestly affected by temperature. Collectively, these data suggest that temperature sensitivity was higher for the Ca(2+)-dependent sAHP than for voltage-dependent AP parameters or for the mAHP, diffusion of Ca2+ over distance cannot explain the slow rise of the sAHP in these cells, and the kinetics of the sAHP and mAHP are affected differently by temperature.

Therapeutic hypothermia for acute ischemic stroke: what do laboratory studies teach us?

BACKGROUND AND PURPOSE

The significance of brain temperature to outcome in cerebral ischemia is recognized. Numerous variations of depth, duration, and delay of cooling have been studied in animal models. It is important to become familiar with these studies to design appropriate clinical trials. With that in mind, a critical review of the pertinent literature is presented, taking into consideration potential limitations in translating such laboratory work to the clinical level.

METHODS

Hypothermia is an especially robust neuroprotectant in the laboratory and has been shown to alter many of the damaging effects of cerebral ischemia. Most laboratory research on therapeutic cooling in cerebral ischemia has been conducted in rodent models of temporary and permanent middle cerebral artery occlusion and report the effects of mild or moderate hypothermia arranged during or after ischemia.

RESULTS

Intraischemic cooling vastly reduces infarct size in most occlusion models. Tissue salvage with delayed onset of cooling is less dramatic but is commonly observed when cooling is begun within 60 minutes of stroke onset in permanent and 180 minutes of stroke onset in temporary occlusion models. Prolonged postischemic cooling further enhances efficacy.

CONCLUSIONS

Laboratory studies have shown that intraischemic hypothermia is more protective than postischemic hypothermia and more benefit is conferred with temporary occlusion than permanent occlusion models. The efficacy of postischemic hypothermia is critically dependent on the duration and depth of hypothermia and its timing relative to ischemia.