Ischemic preconditioning: a long term survival study using behavioural and histological endpoints

Neuroprotection by Remote Ischemic Conditioning in Rodent Models of Focal Ischemia: a Systematic Review and Meta-Analysis

Remote ischemic conditioning (RIC) is a promising neuroprotective therapy for ischemic stroke. Preclinical studies investigating RIC have shown RIC reduced infarct volume, but clinical trials have been equivocal. Therefore, the efficacy of RIC in reducing infarct volume and quality of current literature needs to be evaluated to identify knowledge gaps to support future clinical trials. We performed a systematic review and meta-analysis of preclinical literature involving RIC in rodent models of focal ischemia. This review was registered with PROSPERO (CRD42019145441). Eligibility criteria included rat or mice models of focal ischemia that received RIC to a limb either before, during, or after stroke. MEDLINE and Embase databases were searched from 1946 to August 2019. Risk of bias was assessed using the SYRCLE risk of bias tool along with construct validity. Seventy-two studies were included in the systematic review. RIC was shown to reduce infarct volume (SMD - 2.19; CI - 2.48 to - 1.91) when compared to stroke-only controls and no adverse events were reported with regard to RIC. Remote ischemic conditioning was shown to be most efficacious in males (SMD - 2.26; CI - 2.58 to - 1.94) and when delivered poststroke (SMD - 1.34; CI - 1.95 to - 0.73). A high risk of bias was present; thus, measures of efficacy may be exaggerated. A limitation is the poor methodological reporting of many studies, resulting in unclear construct validity. We identified several important, but under investigated topics including the efficacy of RIC in different stroke models, varied infarct sizes and location, and potential sex differences.

Protective effects of ischemic preconditioning against neuronal apoptosis and dendritic injury in the hippocampus are age-dependent

Ischemic preconditioning with non-lethal ischemia can be protective against lethal forebrain ischemia. We hypothesized that aging may aggravate ischemic susceptibility and reduce brain plasticity against preconditioning. Magnetic resonance diffusion tensor imaging (DTI) is a sensitive tool to detect brain integrity and white matter architecture. This study used DTI and histopathology to investigate the effect of aging on ischemic preconditioning. In this study, adult and middle-aged male Mongolian gerbils were subjected to non-lethal 5-min forebrain ischemia (ischemic preconditioning) or sham-operation, followed by 3 days of reperfusion, and then lethal 15-min forebrain ischemia. A 9.4-Tesla MR imaging system was used to study DTI indices, namely fractional anisotropy (FA), mean diffusivity (MD), and intervoxel coherence (IC) in the hippocampal CA1 and dentate gyrus (DG) areas. In situ expressions of microtubule-associated protein 2 (MAP2, dendritic marker protein) and apoptosis were also examined. The 5-min ischemia did not cause dendritic and neuronal injury and any significant change in DTI indices and MAP2 in adult and middle-aged gerbils. The 15-min ischemia-induced significant delayed neuronal apoptosis and early dendritic injury evidenced by DTI and MAP2 studies in both CA1 and DG areas with more severe injury in middle-aged gerbils than adult gerbils. Ischemic preconditioning could improve neuronal apoptosis in CA1 area and dendritic integrity in both CA1 and DG areas with better improvement in adult gerbils than middle-aged gerbils. This study thus suggests an age-dependent protective effect of ischemic preconditioning against both neuronal apoptosis and dendritic injury in hippocampus after forebrain ischemia.

Ischemic preconditioning provides long-lasting neuroprotection against ischemic stroke: The role of Nrf2

BACKGROUND AND PURPOSE

A major gap in the field of ischemic preconditioning (IPC) is whether or not long-lasting neuroprotection can be achieved. Moreover, the specific mechanisms underlying IPC and how they can be translated into the clinic remain uncertain. To fill these gaps, we tested the hypothesis that IPC exerts long-lasting structural and functional neuroprotection against ischemic stroke through the master gatekeeper of antioxidant defenses, nuclear factor erythroid 2-related factor 2 (Nrf2). We also tested whether the brain could be pharmaceutically preconditioned with a potent and blood-brain barrier-permeable Nrf2 activator, 2-cyano-3,12-dioxo-oleana-1,9(11)-dien-28-trifluoethyl amide (CDDO-TFEA).

METHODS

IPC was induced by transient middle cerebral artery occlusion (MCAO) for 12 min, and ischemic stroke was generated by MCAO for 60 min in wild-type (WT) or Nrf2 knockout (KO) mice. Sensorimotor function, learning/memory skills, and brain tissue loss were measured up to 35 days after stroke. Primary rodent cortical neurons from wildtype (WT) and Nrf2 KO mice were subjected to lethal oxygen-glucose deprivation (OGD) or a brief OGD episode as a preconditioning (PC) stimulus before OGD. Cell viability/death, lipid electrophile generation, and Nrf2 activation were measured. CDDO-TFEA or its vehicle was administered in vivo for three consecutive days before MCAO. Tissue loss and neurological tests were performed 35 days after stroke.

RESULTS

IPC significantly reduced sensorimotor deficits, post-stroke cognitive impairments, and brain tissue loss, 35 days after MCAO in WT mice. These enduring protective effects of IPC were inhibited in Nrf2 KO mice. In neuronal cultures, PC also endowed primary neurons with ischemic tolerance against OGD-induced cell death, an effect that was abolished by loss of Nrf2 expression in KO neurons. PC induced the generation of low levels of lipid electrophiles and led to activation of the Nrf2 pathway. The mechanism underlying IPC may be translatable, as exogenous administration of the Nrf2 activator CDDO-TFEA significantly reduced neurological dysfunction and ischemic brain damage after MCAO.

CONCLUSIONS

IPC provides long-lasting neuroprotection against ischemic brain injury and post-stroke cognitive dysfunction. Nrf2 activation plays a key role in this beneficial outcome and is a promising therapeutic target for the attenuation of ischemic brain injury.

Ethyl-acetate fraction of Trichilia catigua restores long-term retrograde memory and reduces oxidative stress and inflammation after global cerebral ischemia in rats

We originally reported that an ethyl-acetate fraction (EAF) of Trichilia catigua prevented the impairment of water maze learning and hippocampal neurodegeneration after transient global cerebral (TGCI) in mice. We extended that previous study by evaluating whether T. catigua (i) prevents the loss of long-term retrograde memory assessed in the aversive radial maze (AvRM), (ii) confers hippocampal and cortical neuroprotection, and (iii) mitigates oxidative stress and neuroinflammation in rats that are subjected to the four vessel occlusion (4-VO) model of TGCI. In the first experiment, naive rats were trained in the AvRM and then subjected to TGCI. The EAF was administered orally 30min before and 1h after TGCI, and administration continued once per day for 7days post-ischemia. In the second experiment, the EAF was administered 30min before and 1h after TGCI, and protein carbonylation and myeloperoxidase (MPO) activity were assayed 24h and 5days later, respectively. Retrograde memory performance was assessed 8, 15, and 21days post-ischemia. Ischemia caused persistent retrograde amnesia, and this effect was prevented by T. catigua. This memory protection (or preservation) persisted even after the treatment was discontinued, despite the absence of histological neuroprotection. Protein carbonyl group content and MPO activity increased around 43% and 100%, respectively, after TGCI, which were abolished by the EAF of T. catigua. The administration of EAF did not coincide with the days of memory testing. The data indicate that antioxidant and/or antiinflammatory actions in the early phase of ischemia/reperfusion contribute to the long-term antiamnesic effect of T. catigua.

Alterations in the corticotropin-releasing hormone (CRH) neurocircuitry: Insights into post stroke functional impairments

Although it is well accepted that changes in the regulation of the hypothalamic-pituitary adrenal (HPA) axis may increase susceptibility to affective disorders in the general population, this link has been less examined in stroke patients. Yet, the bidirectional association between depression and cardiovascular disease is strong, and stress increases vulnerability to stroke. Corticotropin-releasing hormone (CRH) is the central stress hormone of the HPA axis pathway and acts by binding to CRH receptors (CRHR) 1 and 2, which are located in several stress-related brain regions. Evidence from clinical and animal studies suggests a role for CRH in the neurobiological basis of depression and ischemic brain injury. Given its importance in the regulation of the neuroendocrine, autonomic, and behavioral correlates of adaptation and maladaptation to stress, CRH is likely associated in the pathophysiology of post stroke emotional impairments. The goals of this review article are to examine the clinical and experimental data describing (1) that CRH regulates the molecular signaling brain circuit underlying anxiety- and depression-like behaviors, (2) the influence of CRH and other stress markers in the pathophysiology of post stroke emotional and cognitive impairments, and (3) context and site specific interactions of CRH and BDNF as a basis for the development of novel therapeutic targets. This review addresses how the production and release of the neuropeptide CRH within the various regions of the mesocorticolimbic system influences emotional and cognitive behaviors with a look into its role in psychiatric disorders post stroke.

Progesterone improves long-term functional and histological outcomes after permanent stroke in older rats

Previous studies have shown progesterone to be beneficial in animal models of central nervous system injury, but less is known about its longer-term sustained effects on recovery of function following stroke. We evaluated progesterone's effects on a panel of behavioral tests up to 8 weeks after permanent middle cerebral artery occlusion (pMCAO). Male Sprague-Dawley rats 12m.o. were subjected to pMCAO and, beginning 3h post-pMCAO, given intraperitoneal injections of progesterone (8mg/kg) or vehicle, followed by subcutaneous injections at 8h and then every 24h for 7 days, with tapering of the last 2 treatments. The rats were then tested on functional recovery at 3, 6 and 8 weeks post-stroke. We observed that progesterone-treated animals showed attenuation of infarct volume and improved functional outcomes at 8 weeks after stroke on grip strength, sensory neglect, motor coordination and spatial navigation tests. Progesterone treatments significantly improved motor deficits in the affected limb on a number of gait parameters. Glial fibrillary acidic protein expression was increased in the vehicle group and considerably lowered in the progesterone group at 8 weeks post-stroke. With repeated post-stroke testing, sensory neglect and some aspects of spatial learning performance showed spontaneous recovery, but on gait and grip-strength measres progesterone given only in the acute stage of stroke (first 7 days) showed sustained beneficial effects on all other measures of functional recovery up to 8 weeks post-stroke.

Robust and enduring atorvastatin-mediated memory recovery following the 4-vessel occlusion/internal carotid artery model of chronic cerebral hypoperfusion in middle-aged rats

Chronic cerebral hypoperfusion (CCH) is a common condition associated with the development and/or worsening of age-related dementia.We previously reported persistent memory loss and neurodegeneration after CCH in middle-aged rats. Statin-mediated neuroprotection has been reported after acute cerebral ischemia. Unknown, however, is whether statins can alleviate the outcome of CCH. The present study investigated whether atorvastatin attenuates the cognitive and neurohistological outcome of CCH. Rats (12–15 months old) were trained in a non-food-rewarded radial maze, and then subjected to CCH. Atorvastatin (10 mg/kg, p.o.) was administered for 42 days or 15 days, beginning 5 h after the first occlusion stage. Retrograde memory performance was assessed at 7, 14, 21, 28, and 35 days of CCH, and expressed by “latency,” “number of reference memory errors” and “number of working memory errors.” Neurodegeneration was then examined at the hippocampus and cerebral cortex. Compared to sham, CCH caused profound and persistent memory loss in the vehicle-treated groups, as indicated by increased latency (91.2% to 107.3%) and number of errors (123.5% to 2508.2%), effects from which the animals did not spontaneously recover across time. This CCH-induced retrograde amnesia was completely prevented by atorvastatin (latency: −4.3% to 3.3%; reference/working errors: −2.5% to 45.7%), regardless of the treatment duration. This effect was sustained during the entire behavioral testing period (5 weeks), even after discontinuing treatment. This robust and sustained memory-protective effect of atorvastatin occurred in the absence of neuronal rescue (39.58% to 56.45% cell loss). We suggest that atorvastatin may be promising for the treatment of cognitive sequelae associated with CCH.

Fish oil provides robust and sustained memory recovery after cerebral ischemia: influence of treatment regimen

We previously reported that long-term treatment with fish oil (FO) facilitates memory recovery after transient, global cerebral ischemia (TGCI), despite the presence of severe hippocampal damage. The present study tested whether this antiamnesic effect resulted from an action of FO on behavioral performance itself, or whether it resulted from an anti-ischemic action. Different treatment regimens were used that were distinguished from each other by their initiation or duration with regard to the onset of TGCI and memory assessment. Naive rats were trained in an eight-arm radial maze, subjected to TGCI (4-VO model, 15 min), and tested for memory performance up to 6 weeks after TGCI. Fish oil (docosahexaenoic acid, 300 mg/kg/day) was given orally according to one of the following regimens: regimen 1 (from 3 days prior to ischemia until 4 weeks post-ischemia), regimen 2 (from 3 days prior to ischemia until 1 week post-ischemia), and regimen 3 (from week 2 to week 5 post-ischemia). When administered according to regimens 1 and 2, FO abolished amnesia completely. This effect persisted for at least 5 weeks after discontinuing the treatment. Such an effect did not occur, however, in the group treated according to regimen 3. Hippocampal and cortical damage was not alleviated by FO. The present results demonstrate that FO-mediated memory recovery (or preservation) following TGCI is a reproducible, robust, and long-lasting effect. Moreover, such an effect was found with a relatively short period of treatment, provided it covered the first days prior to and after ischemia. This suggests that FO prevented amnesia by changing some acute, ischemia/reperfusion-triggered process and not by stimulating memory performance on its own.

Sildenafil provides sustained neuroprotection in the absence of learning recovery following the 4-vessel occlusion/internal carotid artery model of chronic cerebral hypoperfusion in middle-aged rats

In this study, we tested whether the phosphodiesterase-5 inhibitor sildenafil protects against neurodegeneration and facilitates recovery from learning deficits examined long after chronic cerebral hypoperfusion (CCH) induced by the 4-vessel occlusion/internal carotid artery (4-VO/ICA) model in middle-aged rats. Male Wistar rats (12-15 months of age) were subjected to permanent 3-stage 4-VO/ICA with an interstage interval of 4 days. Sildenafil (3 mg/kg, p.o.) was administered at one dose per day for 10 days, beginning soon after the first occlusion stage. Three months later, learning in a non-food-rewarded, eight-arm radial maze task was tested. Learning performance is expressed as the latency to find a goal box and the number of reference or working memory errors. Histological examination was performed 1-3 days after behavioral testing. In the vehicle-treated group, permanent 4-VO/ICA markedly disrupted learning performance and caused moderate-to-severe neurodegeneration in the CA1-CA4 subfields of the hippocampus (56.2%), dentate gyrus (DG; 19.2%), retrosplenial cortex (RS cortex; 47.4%), and parietal association cortex (PtA cortex; 38.2%). Sildenafil treatment did not prevent 4-VO/ICA-induced learning deficits, whereas neurodegeneration was significantly reduced in the CA1-CA4 subfields (30.5%), DG (7.2%), RS cortex (11.8%), and PtA cortex (6.5%). Advancing previous findings from our laboratory, this study suggests that while sildenafil can provide important neuroprotection in different brain regions of middle-aged rats subjected to CCH, such histological effect does not translate into cognitive recovery.

Effect of pre-ischaemic conditioning on hypoxic depolarization of dopamine efflux in the rat caudate brain slice measured in real-time with fast cyclic voltammetry

Fast cyclic voltammetry can be used to measure dopamine release after oxygen and glucose deprivation (OGD) induced anoxic depolarization in vitro. Here we measure dopamine efflux with 1s time resolution, which is appropriate to measure OGD-evoked dopamine efflux accurately. In the present study, we examined whether OGD-evoked dopamine efflux could be used to show pre-ischaemic conditioning in the rat caudate brain slice. Caudate slices were exposed to 0, 2, or 10 min OGD pre-ischaemic conditioning, then 60 min later exposed to a second OGD event of 15 min duration. We measured the OGD-evoked dopamine efflux using fast cyclic voltammetry and in some experiments caudate dopamine and DOPAC tissue levels were measured using HPLC and 20 μm cryostat sections were Nissl stained to indicate neuronal loss. We found that 10 but not 2 min OGD pre-ischaemic conditioning resulted in a longer time to onset of OGD-evoked dopamine efflux on the main OGD event (475 ± 31 and 287 ± 30 s for 10 Vs 0 min pre-ischaemic conditioning respectively). Further, 10 min OGD pre-ischaemic conditioning resulted in less dopamine efflux on the second OGD event (4.23 ± 1.12 and 8.14 ± 0.82 μM for 10 Vs 0 min pre-ischaemic conditioning respectively), despite these slices having similar tissue dopamine content and DOPAC/DA ratio, and the rate of dopamine release was slower in the main OGD event (21 ± 5 and 74 ± 8 nM/s for 10 Vs 0 min pre-ischaemic conditioning respectively). These data suggest that 10 min OGD pre-ischaemic conditioning can evoke tolerance to a second OGD event and that voltammetric recording of OGD-evoked dopamine efflux is a useful model of pre-ischaemic conditioning in neuronal tissue.

Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection

Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells.

Changes in the NPY immunoreactivity in gerbil hippocampus after hypoxic and ischemic preconditioning

Preconditioning with sublethal ischemia or hypoxia may reduce the high susceptibility of CA1 pyramidal neurons to ischemic injury. In this study, we tested the hypothesis that enhanced level of neuropeptide Y (NPY) might play a role in the mechanisms responsible for this induced tolerance. Changes in NPY immunoreactivity in the hippocampal formation of preconditioned Mongolian gerbils were compared with the level of tolerance to test ischemia. Tolerance was induced by preconditioning with 2-min of ischemia or with three trials of mild hypobaric hypoxia (360 Torr, 2 h), separated by 24 h, that were completed 48 h before the 3-min test ischemia. The number of NPY-positive neurons in the gerbil hippocampal formation was assessed 2, 4 and 7 days after preconditioning. Survival of the CA1 pyramidal neurons was examined 14 days after the insult. Our experiments demonstrated that ischemic and hypoxic preconditioning produced equal attenuation of the damage evoked by 3-min ischemia, although the pattern of NPY immunoreactivity in the hippocampus differed. Preconditioning ischemia resulted in a 20% rise in the number of NPY-positive neurons 2 days later that disappeared 4 days after the ischemic episode, while mild hypobaric hypoxia induced a twofold increase in the number of NPY-positive neurons that lasted for at least 7 days. Although induced tolerance to ischemia 2 days after ischemic or hypoxic preconditioning was accompanied by increased immunoreactivity of NPY, there was no correlation between its intensity and the level of neuroprotection.

Protective effect of minocycline treatment on striatal ischemia

Minocycline reduces infarct volume measured up to 1 week after focal cerebral ischemia, but it has not been shown that this results in lasting improvement in functional outcome. This study examined behavioral outcome in rats out to 3 weeks after focal ischemia induced by injection of the vasoconstrictor endothelin (ET)-1 (400 pmol in 1 microL of saline) into the striatum. Magnetic resonance imaging confirmed reduced blood flow after administration of ET-1, and was used to determine lesion volumes at 1 and 21 days postischemia. In control rats, intraperitoneal injection of minocycline resulted in plasma levels of 6.6 +/- 2.7 microg mL(-1) between 1 and 8 hours after administration. Based on these results, intraperitoneal minocycline treatment was started either 1 hour before or 3 hours after ET-1 administration, and was repeated daily for 5 days. Outcome, assessed using a composite behavioral deficit score (days 2, 4, 7, 14, and 21) and a test of asymmetric forelimb use (days 7 and 21), was significantly better in both groups of rats treated with minocycline, and the improvement was maintained for the 3-week study period. No differences were found in infarct volumes between groups.

Ischemic preconditioning: postischemic structural changes in the brain

Ischemic brain damage can be prevented or at least significantly reduced when there is a preceding brief ischemic period that does not exceed the threshold for tissue damage--a phenomenon termed "ischemic preconditioning" (ischemic PC). Experimental PC in rodents is now considered to be a model for transient ischemic attacks in humans, and there is increasing hope for translating the knowledge of underlying mechanisms in the animal models into the clinic to enhance endogenous neuroprotective mechanisms in patients with stroke. However, although PC was originally defined as a subtoxic stimulus without any morphologic damage, there is a growing body of evidence from studies using sensitive techniques that postischemic structural alterations of brain tissue manifest not only after ischemia with prior PC but also after the PC stimulus itself. Furthermore, it has become evident over time that the primary shortcomings of many experimental studies on PC are the short observation intervals. The few studies with extended postischemic survival periods done to date provide clear evidence of considerable structural changes and even cell death, which may only be postponed by PC. Therefore, further studies are needed to elucidate structural long-term changes after PC and to validate the persistence of the neuroprotective effects.

The Akt signaling pathway contributes to postconditioning's protection against stroke; the protection is associated with the MAPK and PKC pathways

We previously reported that ischemic postconditioning with a series of mechanical interruptions of reperfusion reduced infarct volume 2 days after focal ischemia in rats. Here, we extend this data by examining long-term protection and exploring underlying mechanisms involving the Akt, mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) signaling pathways. Post-conditioning reduced infarct and improved behavioral function assessed 30 days after stroke. Additionally, postconditioning increased levels of phosphorylated Akt (Ser473) as measured by western blot and Akt activity as measured by an in vitro kinase assay. Inhibiting Akt activity by a phosphoinositide 3-kinase inhibitor, LY294002, enlarged infarct in postconditioned rats. Postconditioning did not affect protein levels of phosphorylated-phosphatase and tensin homologue deleted on chromosome 10 or -phosphoinositide-dependent protein kinase-1 (molecules upstream of Akt) but did inhibit an increase in phosphorylated-glycogen synthase kinase 3beta, an Akt effector. In addition, postconditioning blocked beta-catenin phosphorylation subsequent to glycogen synthase kinase, but had no effect on total or non-phosphorylated active beta-catenin protein levels. Furthermore, postconditioning inhibited increases in the amount of phosphorylated-c-Jun N-terminal kinase and extracellular signal-regulated kinase 1/2 in the MAPK pathway. Finally, postconditioning blocked death-promoting deltaPKC cleavage and attenuated reduction in phosphorylation of survival-promoting epsilonPKC. In conclusion, our data suggest that postconditioning provides long-term protection against stroke in rats. Additionally, we found that Akt activity contributes to postconditioning's protection; furthermore, increases in epsilonPKC activity, a survival-promoting pathway, and reductions in MAPK and deltaPKC activity; two putative death-promoting pathways correlate with postconditioning's protection.

Food restriction attenuates ischemia-induced spatial learning and memory deficits despite extensive CA1 ischemic injury

The purpose of the present study was to examine whether short-term food restriction (40% less food over a 3-month period) can attenuate ischemia-induced CA1 neuronal degeneration, and whether this attenuation translated into improved recovery of functional impairments following global ischemia. There was a significant loss of pyramidal CA1 neurons in ischemic compared to sham-operated rats but no difference between the ad lib and food-restricted ischemic animals. Although the diet did not influence neuronal damage in ischemic animals, the performance of food-restricted ischemic rats in spatial task such as the radial arm maze was significantly better than that of ad lib fed ischemic rats. Food-restricted ischemic rats made equivalent numbers of working memory errors as sham-operated animals and took the same time to complete a standard 8-arm radial arm maze task. They also displayed higher activity level in the open field compared to ad libitum fed ischemic rats, and spent considerably more time in the open arms of the elevated plus maze compared to the other groups, suggesting decreased anxiety in these ischemic rats. The relative sparing of spatial memory performance in food-restricted ischemic animals suggests that food restriction facilitates functional recovery.

Short-term ischemia usually used for ischemic preconditioning causes loss of dendritic integrity after long-term survival in the gerbil hippocampus

Ischemic preconditioning has been established as a powerful experimental neuroprotective strategy, both after global and focal cerebral ischemia. Little is known, however, about the structural and functional long-term outcome. Therefore, our present study was designed to check for potential subtle alterations in the hippocampus after long-term survival. Gerbils were subjected either to short-term ischemia of 2.5 min duration usually used for ischemic preconditioning (n=8) or to sham operation (n=6) and allowed to survive for 6 weeks. Hippocampi with neuronal densities comparable to those of sham-operated control animals were analyzed for dendritic marker proteins MAP2, MAP1B and synaptopodin, respectively. Although MAP2 immunoreactivity was widely unchanged in all hippocampal subfields, both MAP1B and synaptopodin protein expression was decreased to about 80% to 90% of sham controls. A significant reduction, however, was only seen for synaptopodin immunoreactivity in stratum oriens and pyramidale of hippocampal CA1 subfield. In conclusion, our data indicate a dissociation of neuronal survival and dendritic integrity 6 weeks after short-term ischemia usually used for ischemic preconditioning. Analysis of postischemic synaptopodin protein expression is the most sensitive method to detect subtle dendritic changes compared to the classical dendritic marker molecules MAP2 and MAP1B.

Cerebral preconditioning and ischaemic tolerance

Adaptation is one of physiology's fundamental tenets, operating not only at the level of species, as Darwin proposed, but also at the level of tissues, cells, molecules and, perhaps, genes. During recent years, stroke neurobiologists have advanced a considerable body of evidence supporting the hypothesis that, with experimental coaxing, the mammalian brain can adapt to injurious insults such as cerebral ischaemia to promote cell survival in the face of subsequent injury. Establishing this protective phenotype in response to stress depends on a coordinated response at the genomic, molecular, cellular and tissue levels. Here, I summarize our current understanding of how 'preconditioning' stimuli trigger a cerebroprotective state known as cerebral 'ischaemic tolerance'.

Antagonists of group I metabotropic glutamate receptors do not inhibit induction of ischemic tolerance in gerbil hippocampus

In this study we tested the effect of antagonists of two subtypes of the group I metabotropic glutamate receptors (mGluRs GI) on the induction of ischemic tolerance in relation to brain temperature. These experiments were prompted by indications that glutamate receptors may participate in the mechanisms of ischemic preconditioning. The role of NMDA receptors in the induction of ischemic tolerance has been debated while there is lack of information concerning the involvement of mGluRs GI in this phenomenon. The tolerance to injurious 3 min forebrain ischemia in Mongolian gerbils was induced 48 h earlier by 2 min preconditioning ischemia. Brain temperature was measured using telemetry equipment. EMQMCM and MTEP, antagonists of mGluR1 and mGluR5, respectively, were injected i.p. at a dose of 5 mg/kg. They were administered either before preconditioning ischemia in a single dose or after 2 min ischemia three times every 2 h. Both antagonists did not inhibit the induction of ischemic tolerance. Thus, our data indicate that group I metabotropic glutamate receptors do not play an essential role in the induction of ischemic tolerance.

Ischaemic preconditioning: therapeutic implications for stroke?

Ischaemic preconditioning (IPC), also known as ischaemic tolerance (IT), is a phenomenon whereby tissue is exposed to a brief, sublethal period of ischaemia, which activates endogenous protective mechanisms, thereby reducing cellular injury that may be caused by subsequent lethal ischaemic events. The first description of this phenomenon was in the heart, which was reported by Murry and co-workers in 1986. Subsequent studies demonstrated IPC in lung, kidney and liver tissue, whereas more recent studies have concentrated on the brain. The cellular mechanisms underlying the beneficial effects of IPC remain largely unknown. This phenomenon, which has been demonstrated by using various injury paradigms in both cultured neurons and animal brain tissue, may be utilised to identify and characterise therapeutic targets for small-molecule, antibody, or protein intervention. This review will examine the experimental evidence demonstrating the phenomenon termed IPC in models of cerebral ischaemia, the cellular mechanisms that may be involved and the therapeutic implications of these findings.

Differences in infarct evolution between lipopolysaccharide-induced tolerant and nontolerant conditions to focal cerebral ischemia

OBJECT

Although brain tissue may be protected by previous preconditioning, the temporal evolution of infarcts in such preconditioned brain tissue during focal cerebral ischemia is largely unknown. Therefore, in this study the authors engaged in long-term observation with magnetic resonance (MR) imaging to clarify the difference in lesion evolution between tolerant and nontolerant conditions.

METHODS

Bacterial lipopolysaccharide (LPS; 0.9 mg/kg) was administered intravenously to induce cross-ischemic tolerance. Focal cerebral ischemia was induced 72 hours later in spontaneously hypertensive rats. Serial brain MR images were obtained 6 hours, 24 hours, 4 days, 7 days, and 14 days after ischemia by using a 7.05-tesla unit. Lesion-reducing effects were evident 6 hours after ischemia in the LPS group. Preconditioning with LPS does not merely delay but prevents ischemic cell death by reducing lesion size. Lesion reduction was a sustained effect noted up to 14 days after ischemia. Reduction of local cerebral blood flow (ICBF) in the periinfarct area was significantly inhibited in the LPS group, which was correlated with endothelial nitric oxide synthase (eNOS) expression.

CONCLUSIONS

Significant preservation of ICBF in the periinfarct area, which is relevant to sustained upregulation of eNOS, could be a candidate for the long-term inhibiting effect on infarct evolution in the LPS-induced tolerant state.

Protective preconditioning by transient global ischemia in the rat: components of delayed injury progression and lasting protection distinguished by comparisons of depolarization thresholds for cell loss at long survival times

Robust ischemic preconditioning has been shown in rodent brain, but there are concerns regarding the persistence of neuron protection. This issue was examined in rat hippocampus following 4-vessel occlusion (4-VO) ischemia, using DC shifts characteristic of ischemic depolarization to reproducibly define insult severity. Preconditioning ischemia producing 2 to 3.5 mins depolarization was followed at intervals of 2, 5, or 7 days by test insults of varied duration, after which CA1 counts were obtained at 1, 2, 4, or 12 weeks. Neuron loss in naive animals increased with depolarization time longer than 4 mins regardless of postischemic survival interval. Preconditioning 2, 5, or 7 days before test insults prolonged the injury threshold evaluated at 1 week survival to 15, 9, or 6 mins, respectively, showing robust protection and a rapid decay of the protected state. However, by 2 weeks survival after preconditioning at a 2-day interval, the injury threshold dramatically regressed from 15 to 9 mins. Thereafter protection remained relatively stable through 1 month, but slight progression of neuron injury was evident at 3 months. Inflammatory responses were seen in both naive and preconditioned hippocampi throughout this interval, appropriate to the extent of neuron injury. These studies show distinct components of transient and lasting protection after ischemic preconditioning. Finally, it was found that ischemic depolarization was delayed by approximately 1 min in optimally preconditioned rat hippocampus, in contrast to previous results in the gerbil, identifying one specific mechanism by which insult severity is reduced in this model.

Preconditioning enhances the expression of mitochondrial antioxidant thioredoxin-2 in the forebrain of rats exposed to severe hypobaric hypoxia

The impact of severe hypoxia and preconditioning on the expression of the mitochondrial antioxidant thioredoxin-2 (Trx-2) in rat hippocampus (CA1, CA2, CA3 fields, and dentate gyrus) and neocortex was studied by immunocytochemistry. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 hr) spaced at 24 hr. The last trial was followed by severe hypobaric hypoxia (180 Torr, 3 hr) 24 hr later. Both in hippocampus and in neocortex, severe hypobaric hypoxia resulted in enhanced Trx-2 expression at 3 hr, followed by a slight decline in Trx-2 levels, which nevertheless remained increased at 24 hr elsewhere except for the CA1 region. The preconditioning considerably augmented severe hypoxia-induced Trx-2 immunoreactivity, affecting both the number of immunoreactive cells and the intensity of immunostaining. The findings suggest a role for Trx-2 in the formation of brain hypoxic/ischemic tolerance accomplished by the preconditioning.

Hypoxic preconditioning confers long-term reduction of brain injury and improvement of neurological ability in immature rats

Exposure to preconditioning (PC) hypoxia 24 h before a severe hypoxic-ischemic (HI) insult reduces development of injury in the immature brain. Several protective regimens have proved effective in the short-term but not in the long-term perspective. The aim of the present study, therefore, was to evaluate the PC effect on long-term morphologic and neurologic outcome in the developing brain. Six-day-old rats were subjected to hypoxia (36 degrees C, 8.0% O2; PC/HI group) and sham controls to normoxia (36 degrees C; HI group) for 3 h. Twenty-four hours later, all rats were exposed to cerebral HI produced by unilateral carotid artery occlusion combined with 1 h, 15 min of hypoxia (36 degrees C, 7.7% O2). A cylinder test was used to evaluate forelimb asymmetry to determine sensorimotor function at 4, 6, and 8 wk of age. Spatial/cognitive ability was assessed by Morris water maze trials at 7 wk of recovery. Neuropathologic analysis was performed 8 wk after insult. Brain damage was reduced (p<0.0001) in PC/HI (45.0+/-11.1 mm3) in comparison with HI (159.3+/-12.2 mm3) rats. A bias for using the ipsilateral forelimb in wall movements was observed in the cylinder test in HI compared with PC/HI rats at 4 (p<0.001), 6 (p<0.01), and 8 (p<0.0001) wk of age. Results of the Morris water maze test revealed differences (p<0.0001) in average path length between groups on the third and fourth day of trials. Hypoxic PC before HI reduced brain injury by 72% at 8 wk after the insult and provided long-term improvement of sensorimotor and spatial/cognitive functions.

NMDA receptor antagonism does not inhibit induction of ischemic tolerance in gerbil brain in vivo

Effects of high and moderate affinity uncompetitive NMDA receptor antagonists (+)MK-801 and memantine on ischemic tolerance were compared in relation to telemetrically controlled brain temperature. The tolerance to an injurious 3 min test of global forebrain ischemia in Mongolian gerbils was induced 48 h earlier by 2 min preconditioning ischemia. Normothermic preconditioning was virtually harmless, and greatly reduced neurodegeneration evoked by test ischemia. In hyperthermic animals it was injurious and failed to induce tolerance. Memantine (5 mg/kg) and (+)MK-801 (3 mg/kg) injected i.p. 1 h before preconditioning did not inhibit ischemic tolerance in the normothermic gerbils, while in hyperthermic animals treated with (+)MK-801 ischemic tolerance was partially restored. Subchronic 3 day infusion of memantine (30 mg/kg/day) significantly decreased neurodegeneration, and preconditioning in the normothermic gerbils further reduced neuronal damage. Hyperthermia exacerbated preconditioning ischemia and in this way reduced expression of tolerance, while (+)MK-801 partially reversed this effect. Our results do not confirm previous reports on the role of NMDA receptors in the induction of ischemic tolerance in gerbils.

Dietary supplementation of omega-3 polyunsaturated fatty acids worsens forelimb motor function after intracerebral hemorrhage in rats

Dietary intake of omega-3 polyunsaturated fatty acids has been associated with decreased clotting ability and increased risk of hemorrhagic stroke. The aim of the current study was to assess the effect of dietary supplementation of omega-3 polyunsaturated fatty acid on functional outcome after hemorrhagic stroke. Rats were maintained on a diet containing approximately 30% of energy as either fish oil (rich in omega-3 fatty acids) or safflower oil (rich in omega-6 fatty acids) and subjected to either intracerebral hemorrhage or sham surgery. Behavioral tests, infarct measurement, and MR imaging techniques were used to assess outcome. While there was no significant difference in infarct volume between rats on different diets, animals maintained on a diet enriched with fish oil exhibited increased cerebral blood flow after surgery. These animals were significantly more impaired than rats fed the safflower-oil-enriched diet in tests of forelimb dexterity and fine motor control. These results suggest that high intake of omega-3 polyunsaturated fatty acids may not only increase the risk of hemorrhagic stroke as shown in previous studies, but most importantly may lead to a more severe motor impairment and a poorer functional outcome after such an event.

Preconditioning and the developing brain

Preconditioning occurs when a subinjurious exposure renders the brain less vulnerable to a subsequent damaging exposure. In this essay, various models of preconditioning in the immature brain are discussed. Adenosine, excitatory amino acids, nitric oxide, hypoxia-inducible factor, ATP-sensitive K+ channels, caspases, heat shock proteins, inflammatory mediators and gene expression all seem to be involved in sensing, transducing and executing preconditioning resistance. Also reviewed in this essay is evidence that some subinjurious exposures render the brain more vulnerable to a subsequent damaging exposure. We believe that unraveling the mechanisms of how the developing brain becomes inherently resilient or vulnerable will offer important insights into the pathogenesis of injury. Preconditioning of the brain or induction of tolerance of the immune system might be utilized in the future to decrease CNS vulnerability and the occurrence of perinatal brain injury.

The release of tumor necrosis factor-alpha is associated with ischemic tolerance in human stroke

Tumor necrosis factor (TNF)-alpha overexpression has been related to experimental ischemic tolerance when transient ischemia precedes cerebral infarction. We investigated TNF-alpha and interleukin (IL)-6 plasma concentrations in 283 patients with an acute stroke within 24 hours after symptom onset. An ipsilateral transient ischemic attack (TIA) within 72 hours before stroke was recorded in 38 patients. The infarct volume measured on computed tomography on days 4 to 7 and the frequency of poor outcome (Barthel Index score < 85) at 3 months were significantly lower in patients with prior TIA. Plasma concentrations of TNF-alpha were higher (42.5 +/- 9.9 vs 13.1 +/- 6.4pg/ml, p < 0.0001) and IL-6 levels were lower (10.1 +/- 6.2 vs 28.3 +/- 17.3pg/ml, p < 0.0001) in patients with prior TIA. A new variable termed TNF-alpha/IL-6 index was considered positive when TNF-alpha was greater than 30pg/ml and IL-6 was less than 30pg/ml. Positive TNF-alpha/IL-6 index was found in 92% of patients with prior TIA and in 1% of those without. TNF-alpha/IL-6 index (p = 0.0003) and TIA (p = 0.0001) were associated with good outcome in logistic regression analysis after adjusting for potential confounding factors. Ischemic tolerance in acute stroke is associated with increased plasma levels of TNF-alpha in the presence of reduced concentrations of IL-6.

Induced hippocampal neuron protection in an optimized gerbil ischemia model: insult thresholds for tolerance induction and altered gene expression defined by ischemic depolarization

Preconditioning of hippocampal CA1 neurons was evaluated in a gerbil model of transient global ischemia using extracellular recording of DC potential shifts characteristic of ischemic depolarization to precisely define the duration of both priming and test insults. Brief ischemia resulting in depolarizations of 2.5 to 3.5 minutes consistently induced maximal tolerance (95% protection) against subsequent challenges 2 days later with an approximate doubling of the insult duration required for complete CA1 neuron loss from 6 to 12 minutes depolarization when evaluated 1 week after the test insult. Significant protection persisted at 2 months survival, although the apparent injury threshold regressed to approximately 8 minutes, indicating delayed progression of injury after longer test insults. In situ hybridization was used to evaluate depolarization thresholds for induction of mRNAs encoding the 70 kDa heat shock/stress protein, hsp72, as well as several immediate-early genes (c-fos, c-jun, junB, and junD). Immediate-early genes were prominently expressed after short insults inducing tolerance, whereas appreciable hsp72 induction only occurred after insults approaching the threshold for neuron injury. These results establish an ischemic preconditioning model with the predictability needed for mechanistic studies and demonstrate that prior transcriptional activation of the postischemic heat shock response is not required for expression of delayed tolerance.

Ischemic preconditioning in the brain

PURPOSE OF REVIEW

Brain ischemia is responsible for significant morbidity and mortality associated with cardiovascular surgery, and is the end result of multiple disease states, including cardiac arrest, stroke, and traumatic brain injury. Despite significant resources dedicated to developing neuroprotective strategies, little progress has been made in this regard. Neuronal ischemic preconditioning is an endogenous neuroprotective strategy that provides sustained and robust ischemic tolerance. Identification of the mechanisms responsible for mediating the preconditioning response may offer novel therapeutic targets and further our understanding of the natural adaptations to brain injury.

RECENT FINDINGS

Recent research efforts have elucidated many intracellular signaling pathways that ultimately lead to ischemic tolerance after a preconditioning stimulus. Most of these are associated with glutamate receptor signal transduction, the intracellular kinases, and several transcription regulators. Microarray analysis has identified several gene families that warrant further investigation to identify novel candidates for neuroprotective therapies. These include genes involved in synaptic architecture and signal propagation, cell cycle and transcription regulators, and mediators of apoptosis such as the heat shock proteins and anti-apoptotic mitochondrial proteins.

SUMMARY

Neuronal ischemic preconditioning is an endogenous mechanism that leads to robust neuroprotection from ischemia. Identification of the upstream pathways that initiate preconditioning and candidate genes that mediate this phenomenon may offer novel therapeutic targets, with applicability to a variety of disease states and perioperative complications.

Long-term effects of clomethiazole in a model of global ischemia

The failure of neuroprotective drugs in clinical trials has raised questions about the predictive value of animal models. To address this issue we reexamined the efficacy of clomethiazole using functional and histological outcome measures in combination with long-term survival times. Gerbils were exposed to 5 min of global ischemia and received 400 mg/ml clomethiazole (via osmotic minipump) plus a bolus injection (60 mg/kg) 30 min after ischemia. Brain temperature was maintained at approximately 36.5 degrees C during ischemia and for the first 30 min after ischemia, and was monitored in all groups for 24 h. Subgroups of clomethiazole-treated gerbils had their temperatures regulated in the normothermic range while in other animals temperature was not controlled. Open-field habituation tests were conducted 5, 10, 30, and 60 days after occlusion. CA1 cell counts and CA1 slice recordings were done at the conclusion of behavioral testing. Clomethiazole significantly attenuated CA1 cell loss at 10-, 30-, and 60-day survival. A modest reduction in habituation deficits was evident only on Day 10 (P < 0.05). Similarly, field potential amplitude was not maintained in the rostral CA1 region. Clomethiazole produced mild hypothermia that developed over several hours. Based on short-term CA1 cell counts, clomethiazole provided significant histological protection with limited functional preservation. Neuroprotection disappeared when longer survival times (60 day) were employed and temperature confounds eliminated. These data demonstrate the necessity of utilizing more clinically relevant survival times and carefully monitoring/regulating postischemic temperature when assessing potential neuroprotective compounds.

Changes in heat shock protein 27 phosphorylation and immunocontent in response to preconditioning to oxygen and glucose deprivation in organotypic hippocampal cultures

Organotypic hippocampal cultures have been recently used to study in vitro ischaemic neuronal death. Sub-lethal periods of ischaemia in vivo confer resistance to lethal insults and many studies have demonstrated the involvement of heat shock proteins in this phenomenon. We used organotypic hippocampal cultures to investigate the involvement of heat shock protein (HSP) 27 in preconditioning to oxygen and glucose deprivation. Neuronal damage was assessed using propidium iodide fluorescence; HSP27 phosphorylation and immunocontent were obtained using (32)Pi labelling followed by sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting. We observed that immunocontent of HSP27 was increased after lethal or sub-lethal treatment, indicating it is a response to metabolic stress. Treatments with 5 or 10 min of oxygen and glucose deprivation (OGD) or 1- microM N-methyl-D-aspartate (NMDA) induced tolerance to 40 min of OGD associated with an increase in HSP27 immunocontent and phosphorylation. These data suggest that, in vitro, phosphorylated HSP27 might be involved in preconditioning, probably acting as a modulator of actin filaments or by the blockage of neurodegenerative processes.

Delayed transient ischemic attacks kill some CA1 neurons previously salvaged with postischemic hypothermia: neuroprotection undone

Delayed hypothermia reduces ischemic hippocampal CA1 injury. However, there are residual structural and functional abnormalities. Therefore, we studied whether these apparently vulnerable rescued neurons are susceptible to secondary insults. All gerbils were subjected to normothermic forebrain ischemia (ISC, 5 min) or SHAM operation. Gerbils were treated with mild hypothermia (HYPO; 33 degrees C for 24 h+35 degrees C for 24 h) beginning 12 h after surgery, or they remained normothermic (NORMO). Then 5 and 6 days following ISC/SHAM operation gerbils received sublethal transient ischemic attacks (TIA, 1.5 min) or sham (SH) surgeries. Behavioral testing was done and animals survived for 30 days for quantification of medial, middle and lateral CA1 sector cell death. The SHAM groups were not significantly different. The ISC+NORMO+SH group lost 87.3% (of SHAM) of medial CA1 neurons, which was not significantly exacerbated in the ISC+NORMO+TIA group (91.1%, P=0.633). However, the ISC+HYPO+TIA group (58.8% loss) had significantly more cell death than the ISC+HYPO+SH group (42.8%; P=0.035), although CA1 protection was still better than in ISC+NORMO groups (P<0.001). Trends were similar in middle and lateral CA1, but the deleterious effects of TIAs were not statistically significant. Behavioral testing did not distinguish groups with or without TIA, but did reveal deficits in ISC+NORMO groups and protection in ISC+HYPO groups. These data, like previous ultrastructural findings, show that while most hypothermia-rescued CA1 neurons are healthy, some are susceptible. Perhaps other neuroprotectants, especially weaker ones, might be undone by delayed insults (e.g. TIA, fever).

Ischemic tolerance

A brief period of cerebral ischemia confers transient tolerance to a subsequent ischemic challenge in the brain. This phenomenon of ischemic tolerance has been confirmed in various animal models of forebrain ischemia and focal cerebral ischemia. Since the ischemic tolerance afforded by preceding ischemia can bring about robust protection of the brain, the mechanism of tolerance induction has been extensively studied. It has been elucidated that ischemic tolerance protects neurons, and at the same time, it preserves brain function. Further experiments have shown that metabolic and physical stresses can also induce cross-tolerance to cerebral ischemia, but the protection by cross-tolerance is relatively modest. The underlying mechanism of ischemic tolerance still is not fully understood. Potential mechanisms may be divided into two categories: (1) A cellular defense function against ischemia may be enhanced by the mechanisms inherent to neurons. They may arise by posttranslational modification of proteins or by expression of new proteins via a signal transduction system to the nucleus. These cascades of events may strengthen the influence of survival factors or may inhibit apoptosis. (2) A cellular stress response and synthesis of stress proteins may lead to an increased capacity for health maintenance inside the cell. These proteins work as cellular "chaperones" by unfolding misfolded cellular proteins and helping the cell to dispose of unneeded denatured proteins. Recent experimental data have demonstrated the importance of the processing of unfolded proteins for cell survival and cell death. The brain may be protected from ischemia by using multiple mechanisms that are available for cellular survival. If tolerance induction can be manipulated and accelerated by a drug treatment that is safe and effective enough, it could greatly improve the treatment of stroke.

Environmental enrichment enhances recovery of function but exacerbates ischemic cell death

Prior exposure to brief 'conditioning' episodes of ischemia protects hippocampal CA1 neurons against a subsequent more severe ischemic insult. However, protected cells exhibit abnormal function and as survival times are extended this ischemic tolerance dissipates and cells begin to die. In this study, we sought to determine whether environmental enrichment could alter the above pattern of delayed cell death and functional impairment in a gerbil model of ischemic tolerance. Gerbils received either ischemic preconditioning, 5 min of ischemia without preconditioning or sham surgery. Three days after ischemia, gerbils were placed in either an enriched environment or standard laboratory housing. Open field habituation was assessed 3, 7, 10, 30 and 60 days after ischemia. Subsequently, animals were trained in two versions (win-shift and win-stay) of a T-maze task. Following behavioral testing, extracellular CA1 field potential amplitudes and CA1 cell counts were determined. Initial open field activity was significantly higher in all experimental groups compared to sham animals (P<0.001). By 60 days, enriched ischemic preconditioned and enriched ischemic gerbils were not different than shams whereas non-enriched, ischemic preconditioned and ischemic gerbils continued to have higher activity scores (P<0.05). Preconditioned and enriched ischemic animals learned the win-shift T-maze problem as quickly as shams while non-enriched ischemic gerbils were severely impaired compared with all other groups (P<0.001). Only the sham and enriched preconditioned groups readily acquired the win-stay paradigm. CA1 field potential amplitudes were lower (P<0.05) in ischemic than sham gerbils irrespective of treatment. Surprisingly, CA1 cell counts were significantly lower (P<0.01) in enriched versus non-enriched ischemic preconditioned animals. These data demonstrate that early, intensive intervention after ischemia can improve functional outcome but that this is accompanied by increased brain damage. Careful consideration needs to be given to the timing of rehabilitation after stroke and related types of brain injury.

Effects of global cerebral ischemia and preconditioning on heat shock protein 27 immunocontent and phosphorylation in rat hippocampus

Global cerebral ischemia, with or without preconditioning, leads to an increase in heat shock protein 27 (HSP27) immunocontent and alterations in HSP27 phosphorylation in CA1 and dentate gyrus areas of the hippocampus. We studied different times of reperfusion (1, 4, 7, 14, 21 and 30 days) using 2 min, 10 min or 2+10 min of ischemia. The results showed an increase in HSP27 immunocontent of about 300% after 10 min of ischemia in CA1 and dentate gyrus. CA1, a hippocampal vulnerable area, showed an increase in HSP27 phosphorylation, parallel with immunocontent. In dentate gyrus, a resistant area, the increase in HSP phosphorylation was lower than immunocontent. After preconditioned ischemia (2+10 min), when CA1 neurons are protected to a lethal, 10 min insult, we observed an increase in HSP immunocontent and a decrease in phosphorylation in both regions of the hippocampus, suggesting that, when there is no neuronal death, HSP27 in a vulnerable area responds similarly to the resistant area.When dephosphorylated, HSP27 acts as a chaperone, protecting other proteins from denaturation. As it is markedly expressed in astrocytes, we suggest that HSP27 could be protecting hippocampal astrocytes, which could then be helping neurons to resist to the insult, maintaining tissue normal homeostasis.

Intravenous anesthetics differentially reduce neurotransmission damage caused by oxygen-glucose deprivation in rat hippocampal slices in correlation with N-methyl-d-aspartate receptor inhibition

OBJECTIVE

To examine the relation between the effect of intravenous anesthetics on ischemic neurotransmission damage and their actions on N-methyl-d-aspartate (NMDA) receptors in an in vitro cerebral ischemic model.

DESIGN

Prospective, randomized study in freshly prepared rat hippocampal slices.

SETTING

University research laboratory.

SUBJECTS

Hippocampal slices were prepared from male Wistar rats (4-5 wks old).

INTERVENTIONS AND MEASUREMENTS

In vitro ischemia was induced by exposing slices to glucose-free Krebs solution gassed with 95% N2 /5% CO2 at 37.1-37.3 degrees C. Ischemic neurotransmission damage was indicated by the amplitudes of population spikes (PS) recorded from the CA1 pyramidal layer after stimulation of the Schaffer collaterals. The effect of anesthetics on NMDA receptors was determined by measuring the NMDA-mediated changes in intracellular calcium in the CA1 pyramidal layer with a calcium indicator, fura-2.

RESULTS

Following 4, 6, and 7.5 mins ischemia in vitro, the recoveries of PS (% control) were 100%, 17.5 +/- 21.8%, and 5.4 +/- 2.1%, respectively. 3-(R)-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 5 microM), an NMDA receptor antagonist, increased the recovery of PS to 88.3 +/- 24.5% after 6 mins ischemia, and to 42.1 +/- 18.7% after 7.5 mins ischemia. Thiopental (400 microM), thiamylal (400 microM), and ketamine (100 microM), but not propofol (100 microM) and etomidate (10 microM), improved the recovery of PS after 6 and 7.5 mins ischemia; the degrees of their protection were comparable to that of 5 microM CPP. The NMDA-mediated increases in intracellular calcium were almost completely inhibited by thiamylal, reduced to half by ketamine and thiopental, augmented by propofol, and not affected by etomidate.

CONCLUSIONS

The results indicate that the efficacy of intravenous anesthetics in attenuating ischemic neuronal damage varies among agents, relating to their effects on NMDA receptors.

Therapeutic implications of hypothermic and hyperthermic temperature conditions in stroke patients

Brain temperature is an important variable in determining the outcome of cerebral ischemia; increases in core temperature escalate neural damage whereas decreases in core temperature reduce damage. Fever induction often occurs in patients prior to or as a direct or indirect result of the ischemic insult, with a worsened stroke outcome, compared with non-febrile ischemic patients. Most importantly, post-ischemic hypothermia reduces long term neural damage and associated behavioral deficits in animals studied for up to a year after the ischemic insult. This review discusses the importance of monitoring the brain temperature of stroke patients and implemention of therapeutic thermoregulatory strategies to reduce the temperature of ischemic patients.Key words: hypothermia, neuroprotection, fever, neural and behavioral outcomes.

Electrophysiological properties of CA1 neurons protected by postischemic hypothermia in gerbils

BACKGROUND AND PURPOSE

Recent studies show that prolonged (eg, 24-hour) postischemic hypothermia confers lasting histological and behavioral protection against severe global cerebral ischemia. However, functional abnormalities may be compensated for by undamaged brain regions and thus not detected by behavioral tests. To determine whether hypothermia preserves CA1 functional integrity, we measured synaptic and membrane properties of CA1 neurons in ischemic gerbils treated with postischemic hypothermia.

METHODS

Gerbils were subjected to 5 minutes of forebrain ischemia and were either left untreated or exposed to 2 days of hypothermia (32 degrees C for 24 hours and then 34 degrees C for 24 hours). Sham animals were operated on but not made ischemic, then either allowed to recover at room temperature or subjected to hypothermia for 2 days. Approximately 5 weeks after ischemia or sham surgery, patch-clamp recordings were obtained from the CA1 region of hippocampal slices.

RESULTS

There was approximately 95% CA1 cell loss in untreated ischemic animals, whereas ischemic gerbils treated with hypothermia had cell counts similar to sham animals. Resting membrane potential, action potential amplitude and duration, input resistance, and synaptic currents evoked by Schaffer collateral stimulation were similar between pyramidal cells obtained from ischemic gerbils treated with hypothermia and sham-operated animals (P:>0.05).

CONCLUSIONS

These data demonstrate that postischemic hypothermia preserves the measured electrophysiological properties of CA1 neurons in the absence of any apparent functional abnormalities. This study provides further support for the use of hypothermia as a treatment for cerebral ischemia.

Prolonged but delayed postischemic hypothermia: a long-term outcome study in the rat middle cerebral artery occlusion model

Delayed but prolonged hypothermia persistently decreases cell death and functional deficits after global cerebral ischemia in rodents. Postischemic hypothermia also reduces infarction after middle cerebral artery occlusion (MCAO) in rat. Because initial neuroprotection is sometimes transient and may not subserve functional recovery, especially on demanding tasks, the authors examined whether postischemic cooling would persistently reduce infarction and forelimb reaching deficits after MCAO. Male spontaneously hypertensive rats were trained to retrieve food pellets in a staircase test that measures independent forelimb reaching ability. Later, rats underwent 90 minutes of normothermic MCAO, through a microclip, or sham operation. In some rats, prolonged cooling (33 degrees C for 24 hours and then 35 degrees C for 24 hours) began 2.5 hours after the onset of ischemia (60 minutes after the start of reperfusion; n = 17 with subsequently 1 death) or sham procedures (n = 4), whereas untreated sham (n = 4) and ischemic (n = 16 with subsequently 1 death) rats maintained normothermia. An indwelling abdominal probe continually measured core temperature, and an automated fan and water spray system was used to produce hypothermia. One month later rats were reassessed in the staircase test over five days and then killed. The contralateral limb impairment in food pellet retrieval was completely prevented by hypothermia (P = 0.0001). Hypothermia reduced an infarct volume of 67.5 mm3 after untreated ischemia to 35.8 mm3 (P < 0.0001). These findings of persistent benefit encourage the clinical assessment of hypothermia.

Preconditioning the Brain and Heart: Implications for Cardiac Surgery

Despite many recent advances in emboli detection, aortic imaging, myocardial preservation, and perfusion equipment, ischemic injury to the heart and brain remains a serious complications after cardiac surgery. Hypoperfusion (particularly in the heart) and microem boli (particularly in the brain) during cardiopulmonary bypass constitute the etiology of ischemia. Although hypothermia has traditionally been the mainstay for systemic protection from transient ischemia, there has been a general trend to accept warmer heart and core temperatures during bypass, which increases the poten tial for ischemic injury to various organs. This article discusses recent advances in the understanding of myocardial and brain preconditioning and their poten tial role to provide additional protection during cardiac surgery.

Effect of propentofylline (HWA 285) on focal ischemia in rats: effect of treatment and posttreatment duration on infarct size

BACKGROUND AND PUROSEe: in this study we tested the potentially neuroprotective properties of propentofylline in a model of focal ischemia with long-term, repeated treatment.

METHODS

37 male Wistar rats (280-300 g) underwent permanent occlusion of the middle cerebral artery (MCA). Infusion was started 30 min after occlusion of the MCA over a period of 2 h with a dosage of 0.01 mg/kg body weight. Immediately after the termination of infusion repetitive intraperitoneal injections were started. Animals were assigned to four groups: continuous treatment for a period of 12 h with 24-h survival (group A, n=9) or 48-h survival (group B, n=10), continuous treatment for a period of 48 h with 48-h survival (group C, n=9) and placebo (group D, n=9). Infarct size was calculated from brain slices stained with 2,3,5-triphenyltetrazolium chloride.

RESULTS

the infarct size was significantly reduced in group C (treatment for 48 h) (163.9+/-30.5 mm(3)) compared to the placebo group (297.4+/-17. 7 mm(3)). No effect on infarct size was observed in group A (196. 8+/-37.3 mm(3)) and group B (239.6+/-42.9 mm(3)) compared to placebo.

CONCLUSION

continuous i.p. injections of propentofylline over a period of 48 h significantly reduces infarct size in an animal model of focal cerebral ischemia. With shorter periods of continuous administration of the drug and delayed postmortem analysis, reductions in the infarct size did not reach a level of significance. These data show the importance of continuous long-term administration after ischemic stroke in clinical trials to achieve the beneficial effects of neuroprotection by propentofylline.

Hypothermia-induced ischemic tolerance

Delayed resistance to ischemic injury can be induced by a variety of conditioning stimuli. This phenomenon, known as delayed ischemic tolerance, is initiated over several hours or a day, and can persist for up to a week or more. The present paper describes recent experiments in which transient hypothermia was used as a conditioning stimulus to induce ischemic tolerance. A brief period of hypothermia administered 6 to 48 hours prior to focal ischemia reduces subsequent cerebral infarction. Hypothermia-induced ischemic tolerance is reversed by 7 days postconditioning, and is blocked by the protein synthesis inhibitor anisomycin. Electrophysiological studies utilizing in vitro brain slices demonstrate that hypoxic damage to synaptic responses is reduced in slices prepared from hypothermia-preconditioned animals. Taken together, these findings indicate that transient hypothermia induces tolerance in the brain parenchyma, and that increased expression of one or more gene products contributes to this phenomenon. Inasmuch as hypothermia is already an approved clinical procedure for intraischemic and postischemic therapy, it is possible that hypothermia could provide a clinically useful conditioning stimulus for limiting injury elicited by anticipated periods of ischemia.

Focal ischemic preconditioning induces rapid tolerance to middle cerebral artery occlusion in mice

In a process called ischemic preconditioning, a brief, sublethal ischemic insult protects tissue from subsequent, more severe injury. There have been no reports of rapidly induced ischemic preconditioning. The authors sought to develop a model of cerebral ischemic preconditioning in the mouse that can be applied to transgenic and knockout animals. They found that brief middle cerebral artery (MCA) occlusion only minutes before a severe ischemic insult can induce protection from that insult. Here the investigators describe a mouse model of preconditioning using intraluminal MCA occlusion as both the conditioning and the test stimulus. One or three 5-minute episodes of ischemia given 30 minutes before MCA occlusion for 1 or 24 hours (permanent occlusion) confer significant protection as assessed by infarct volume measurements 24 hours later.

Ischemic preconditioning in 18- to 20-month-old gerbils: long-term survival with functional outcome measures

BACKGROUND AND PURPOSE

In young animals, ischemic preconditioning protects CA1 hippocampal neurons against global ischemia. However, cerebral ischemia occurs most frequently in individuals aged >/=65 years. This study examined the protection provided by ischemic preconditioning in a population of aged (18- to 20-month-old) gerbils.

METHODS

One group of animals was exposed to two 1.5-minute episodes of global ischemia separated by 24 hours and followed 72 hours later by a 5-minute occlusion of both carotid arteries. A second group was given 2 episodes of preconditioning only. Two other groups were exposed to 5 minutes of ischemia or sham surgery. The animals survived 10, 30, or 60 days. Functional and histological assessments were used to determine the extent of protection.

RESULTS

Ten days after ischemia there was >80% protection of CA1 neurons in ischemic preconditioned animals compared with 6% in ischemic gerbils. Nevertheless, these preconditioned animals were impaired in open-field tests of habituation. In addition, CA1 dendritic field potentials were smaller in amplitude compared with those in sham animals. While there was a complete loss of staining for CA1 microtubule-associated protein-2 in ischemic animals, staining in ischemic preconditioned animals was normal. This suggests that dendritic abnormalities per se were not responsible for the observed functional deficits. CA1 cell survival declined to approximately 75% of sham values (P<0.05) at 60 days after ischemia.

CONCLUSIONS

Ischemic preconditioning provided substantial neuroprotection in aged gerbils. Nonetheless, the striking dissociation between histological and functional protection provided by ischemic preconditioning in aged animals emphasizes the need to use functional end points and long-term survival when assessing neuroprotection. Although functional recovery was evident with increasing survival time, CA1 cell death continued, thereby raising the possibility that the level of neuroprotection attained was not permanent.