Competing processes of cell death and recovery of function following ischemic preconditioning

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.

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.

Ischemic preconditioning upregulates expression of phospholipase D2 in the rat hippocampus

To investigate the possible involvement of phospholipase D2 (PLD2) in the induction of ischemic tolerance, we analyzed the distribution and time course of PLD2 expression in the rat hippocampus after a sublethal period of ischemia. Forebrain ischemia was induced by four-vessel occlusion for 3 min. Increased PLD2 immunoreactivity after this sublethal ischemia was observed in CA1 pyramidal neurons of the rat hippocampus. In tolerance-acquired CA1 neurons, PLD2 immunoreactivity was upregulated as early as 12 h post-ischemia and was most prominent at 1-3 days, with expression sustained for at least 7 days, as shown by a time course of immunoblotting and measurement of the enzymatic activity of PLD. PLD2 expression was also increased in ischemia-resistant CA3 neurons and dentate granule cells, although weaker staining intensity was noted. Further, we showed that, in cultured SK-N-BE(2)C human neuroblastoma cells, overexpression of PLD2 inhibited cell death by chemical hypoxia induced with potassium cyanide and deoxyglucose. These data suggest that upregulation of PLD2 might be involved in the neuroprotective mechanism of ischemic tolerance in the rat hippocampus.

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'.

The changes in endogenous antioxidant enzyme activity after postconditioning

1. The aim of this work was to study potential mechanisms participating in postischemic protection of selectively vulnerable CA1 neurons in the hippocampus. Experiments were focused on measuring changes in endogenous antioxidant enzyme activity. 2. Forebrain cerebral ischemia was induced in a rat by four-vessel occlusion. Ten minutes of ischemia induces so-called delayed neuronal death in selectively vulnerable CA1 region 3 days later. After 7 days of reperfusion, 71.6% of neurons succumb to neurodegeneration. When 5 min of ischemia was used as postconditioning, 2 days after 10 min of cerebral ischemia, delayed neuronal death in CA1 was almost completely (89.9%) prevented. 3. Searching for mechanisms of protection, we measured the activity of endogenous antioxidant enzymes. Activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) were measured in the hippocampus, striatum and cortex by spectrophotometric methods after 10 min of ischemia used as the preconditioning. Two days after the preconditioning or the sham operation, second ischemia was induced for 5 min. We observed significant increase of total SOD activity in all studied regions of the brain 5 h after postconditioning (5 min of ischemia). SOD activity decreased to control values after 24 h. 4. In some experiments, we used intraperitoneal injections of norepinephrine (3.1 microM/kg) or 3-nitropropionic acid (20 mg/kg) as postconditioning, instead of ischemia. All three treatments resulted in significant increase of SOD activity, but norepinephrine was the most effective. The same effect as was seen for total SOD activity could be observed for CuZn-SOD as well as Mn-SOD activity. Similarly, considerable increase in the activity of catalase was detected 5 h after postconditioning (5 min of ischemia). It is interesting that the greatest changes were established in selectively vulnerable hippocampus and striatum. As in the case of SOD, the highest levels of CAT activity were induced by norepinephrine, while lower but significant increase in CAT activity was induced by 3-nitropropionic acid.5. Our results suggest that endogenous antioxidants SOD and CAT could play considerable neuroprotective role after postconditioning.

NMDA-induced preconditioning attenuates synaptic plasticity in the rat hippocampus

It was recently demonstrated that glutamate could precondition hippocampal slices against the damaging effects of hypoxia, and we have now extended this observation by investigating (i) the ability of glutamate receptor agonists to act as preconditioning agents and (ii) the effects of preconditioning on synaptic plasticity. Using rat hippocampal slices, 15 microM NMDA applied for 10 min (chemical insult) caused abolition of the population spike potentials (PS) followed by approximately 33% recovery at 60 min post-insult. In comparison, a 5 min preconditioning exposure of 10 microM NMDA given 30 min prior to the insult significantly improved the recovery to 69%. Preconditioning did not alter paired pulse facilitation; however, it significantly enhanced paired pulse depression and reduced population spike long-term potentiation (PS-LTP) and LTP in field recordings. This effect on PS-LTP appeared to be NMDA receptor dependent and was blocked by the nitric oxide synthase inhibitors nitro-L-arginine methyl ester (L-NAME) and 7-nitro indazole (7-NI) but not by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). We conclude that preconditioning by NMDA can improve recovery following acute insults but may have deleterious effects on neuronal plasticity.

Expression of prostaglandin E2 receptor subtypes, EP2 and EP4, in the rat hippocampus after cerebral ischemia and ischemic tolerance

We investigated the distribution and time course of expression of two subtypes of prostaglandin E(2) (PGE(2)) receptors, EP2 and EP4, in a rat model of cerebral ischemia and ischemic tolerance. Adult male Sprague-Dawley rats were subjected to either lethal global ischemia (10 min) with or without sublethal ischemic preconditioning (3 min), or ischemia only (3 min). A short 3-min cerebral ischemia and a 3-min ischemia followed by a second lethal ischemia enhanced the expression of EP2 and EP4 receptors in CA1 pyramidal neurons of the hippocampus. In tolerance-acquired CA1 neurons, the immunoreactivities of EP2 and EP4 were upregulated after 4 h and 12 h, respectively. The immunoreactivities were most prominent at 3 days and were sustained for at least 14 days, consistent with results of immunoblotting experiments. However, immunoreactivities for these PGE(2) receptors increased in reactive glial cells in the vulnerable CA1 and hilar regions of rats subjected to lethal ischemia without ischemic preconditioning. Most of the EP2 immunoreactivity occurred in microglial cells and some astrocytes, whereas increased immunoreactivity for EP4 was found only in astrocytes. These data suggest that ischemia and the induction of ischemia tolerance have different regulatory effects on the expression of EP2 and EP4 receptors. Moreover, PGE(2) may exert its unique pathophysiological functions in relation to delayed neuronal death and ischemic tolerance induction in the rat hippocampus via specific PGE(2) receptors.

HPLC method for the determination and pharmacokinetic studies on puerarin in cerebral ischemia reperfusion rat plasma after intravenous administration of puerariae radix isoflavone

A new HPLC method for the determination of puerarin in cerebral ischemia reperfusion rat plasma is introduced. Puerarin, the principal bioactive component of puerariae radix isoflavone, was extracted from plasma by methanol. The HPLC separation was then performed on a reversed-phase C18 column using water-acetonitrile (89:11, v/v) as eluting solvent system, and UV detection at 252 nm to measure the analyte with a limit of quantitation about 9.44 ng ml(-1). The calibration curve for puerarin was linear (r=0.9998) in the concentration range of 9.44-1208.00 ng ml(-1), both intra- and inter-day precision of the puerarin were determined and their coefficient of variation did not exceed 10%. The validated method has been successfully applied for pharmacokinetic studies of puerarin from rat plasma after intravenous administration of puerariae radix isoflavone. Another novel finding of this study was that the elimination rate of puerarin was significantly slower in the cerebral ischemia reperfusion rat than in the normal rat, judging by the pharmacokinetic parameters obtained. Since puerariae radix isoflavone was mainly administrated to the patients suffering from cerebralvascular diseases, the pharmacokinetic studies performed on the pathological animal models were suitable references for clinical application.

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.

Mild hypoxia preconditioning prevents impairment of passive avoidance learning and suppression of brain NGFI-A expression induced by severe hypoxia

The aim of this work was to study effects of mild preconditioning hypobaric hypoxia (380 Torr for 2 h, repeated 3 or 6 times spaced at 24 h) on brain NGFI-A immunoreactivity and passive avoidance (PA) behavior in rats exposed to severe hypoxia (160 Torr for 3 h). Severe hypobaric hypoxia produced extensive neuronal loss in hippocampal CA1, while the preceding hypoxic preconditioning had clear protective effect on neuronal viability of vulnerable hippocampal cells. Besides, the hypoxic preconditioning prevented impairment of acquisition and retention of PA caused by severe hypoxia. The six-trial hypobaric preconditioning was more effective in protection against PA learning deficits in severe hypoxia exposed rats than the three-trial preconditioning. The preconditioning up-regulated severe hypoxia-suppressed neocortical and hippocampal expression of NGFI-A, suggesting a possible role for NGFI-A in the neuroprotective mechanisms activated by hypoxic preconditioning.

Ischemic preconditioning-induced expression of gp130 and STAT3 in astrocytes of the rat hippocampus

We investigated the distribution and time course of expression of both gp130 mRNA and signal transducer and activator of transcription factor-3 (STAT3) in a rat model of ischemic tolerance induction. Forebrain ischemia was induced by four-vessel occlusion for 3 min as an ischemic preconditioning. Ischemic preconditioning 3 days before a 10-min lethal ischemia preserved neuronal signal. Expression of gp130 mRNA was induced 12 h after ischemic preconditioning, and was most prominent in the CA1 and the hilar region at 3 days, with expression sustained for at least 7 days. Ischemic preconditioning-induced STAT3 activation, as revealed by nuclear translocation, resembled that of gp130 expression. Nuclear STAT3 immunoreactivity occurred in the CA1 and the hilar region within 12 h after ischemic preconditioning, and was sustained for at least 7 days. Double-labeling experiments revealed that the cells expressing gp130 or STAT3 were glial fibrillary acidic protein (GFAP) immunoreactive astrocytes. These results demonstrate upregulation of gp130 and STAT3 in reactive astrocytes following ischemic preconditioning, indicating that this signal pathway is involved in the astroglial reaction to ischemic preconditioning in the rat hippocampus.

17 Beta-oestradiol attenuates dexamethasone-induced lethal and sublethal neuronal damage in the striatum and hippocampus

Abnormal corticosteroid release is extensively associated with mood disorders. This association may result from the toxic actions of endogenous corticosteroids which can induce apoptosis of hippocampal neurons. Similarly, dexamethasone, a synthetic corticosteroid, can induce lethal and sublethal damage to rat hippocampal and striatal neurons and can result in steroid-induced psychoses in humans. The experiments reported here tested the hypothesis that pre-treatment with oestrogen would also attenuate dexamethasone-induced neuronal damage as oestrogens have neuroprotective actions against a variety of insults and falling levels of oestrogen are associated with increased vulnerability to mood disorders. Male Sprague-Dawley rats received three systemic injections which were a combination of vehicle, 17-beta-oestradiol (0.2 mg/kg, s.c.), the oestrogen receptor antagonist tamoxifen (10 mg/kg, s.c.) and dexamethasone (0.7 mg/kg, i.p.) and were killed 24 h after the final injection. Injections of dexamethasone (when preceded by vehicle injections) resulted in elevated levels of apoptosis and sub-lethal damage, as demonstrated by reduced levels of microtubule-associated protein-2-immunopositive neurons, in the striatum and hippocampus. This damage was regional with the dorsomedial caudate putamen and the dentate gyrus and CA1 and CA3 hippocampal sub-fields being particularly affected. Pretreatment with oestrogen substantially attenuated the dexamethasone-induced neuronal damage. This oestrogen-induced neuronal protection was in turn virtually eliminated by giving an initial injection of tamoxifen. These results suggest, therefore, that oestrogens can protect from corticosteroid-induced neuronal damage via an oestrogen receptor-mediated process.

Sleep deprivation prior to transient global cerebral ischemia attenuates glial reaction in the rat hippocampal formation

This study was aimed to ascertain the effect of sleep deprivation on subsequent cerebral ischemia in the rat hippocampal formation. Seven days after transient global cerebral ischemia induced by four-vessel occlusion method, most of the pyramidal cells in the hippocampal CA1 subfield underwent disruption and pyknosis as detected by cresyl violet staining. With OX-42, OX-18, OX-6 and ED1 immunohistochemistry, robust microglia/macrophage reactions were observed in the CA1 and dentate hilus. The majority of reactive microglia was rod-shaped, bushy or amoeboidic cells bearing hypertrophic processes. Astrocytes also displayed hypertrophic processes, whose immunostaining for glial fibrillary acidic protein was markedly enhanced. The ischemia-induced neuronal damage and glial reactions, however, were noticeably attenuated in rats subjected to pretreatment with sleep deprivation for five consecutive days. The most drastic effect was the diminution of OX-18, OX-6 and ED1 immunoreactivities, suggesting that the immune potentiality and/or phagocytosis of these cells was suppressed by prolonged sleep deprivation prior to ischemic insult. It is postulated that sleep deprivation may have a preconditioning influence on subsequent lethal cerebral ischemia. Hence, sleep deprivation may be considered as a therapeutic strategy in brain ischemic damage.

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.

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.

Mild postischemic hypothermia is neuroprotective in the immature rat neocortex slice

Mild hypothermia as an intervention after perinatal asphyxia may prevent neurological damage in the newborn. We used stimulus-induced field potentials to monitor recovery from oxygen and glucose deprivation (OGD) in neocortex slices of 6-8-day-old wistar rats. OGD after a latency of 10.7+/-2.1 min (mean+/-S.E.) resulted in an anoxic depolarisation with an amplitude of 5.4+/-2.4 mV. Mild hypothermia of 31 degrees C (vs. 35 degrees C in the control group) was applied for 60 min after end of OGD. The 20, 40, 60 and 80% recovery of the field potential amplitude was significantly faster in the hypothermia group in comparison to the control group. These data indicate that mild postischemic hypothermia may have neuroprotective effects after perinatal asphyxia.

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.

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.

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.

Diazepam-induced neuroprotection: dissociating the effects of hypothermia following global ischemia

Global cerebral ischemia produces hippocampal CA1 neuronal loss which in turn leads to deficits in memory related tasks. Previous studies have shown that the benzodiazepine diazepam is effective at attenuating this cell death and the related behavioural impairments. However these studies have been confounded by diazepam-induced hypothermia. In this study we sought to determine the neuroprotective efficacy of diazepam in the absence of hypothermia. Diazepam (10 mg/kg) was administered to two groups of gerbils at 30 and 90 min following a 5-min ischemic insult. In one group the brain temperature was monitored for 24 h post-ischemically but not regulated. In the second group, post-ischemic brain temperature was maintained at 36.5 degrees C to counteract the hypothermia produced by diazepam. Both behaviour (open field performance) and CA1 cell counts from these groups were compared to those from sham/normal, no drug ischemic and vehicle ischemic groups at 10 days survival. In animals treated with diazepam without temperature regulation, there was significant histological and behavioural protection at 10 days compared to untreated ischemic animals. Preventing hypothermia in diazepam-treated animals resulted in a decrease in the number of cells surviving (from 41.2 to 31.6% of sham) and abolished behavioural protection. Diazepam appears to have limited ability to attenuate neuronal loss and its neuroprotective efficacy is augmented by the concurrent hypothermic actions of the drug itself.