Immunohistochemical localization of superoxide dismutase in the hippocampus following ischemia in a gerbil model of ischemic tolerance

The adaptive effects of hypoxic preconditioning of brain neurons

Prophylactic transient hypoxia (preconditioning) increased neuron resistance to subsequent induction of severe hypoxia. Published data and results obtained by the authors on the molecular-cellular mechanisms of hypoxic preconditioning are presented. The roles of intracellular signal transduction, genome function, stress proteins, and neuromodulatory peptides in this process are discussed. The roles of glutamatergic as well as calcium and phosphoinositide regulatory systems and neuromodulatory factors as components of "volume" signal transmission are analyzed in hypoxic preconditioning-associated induction of functional tolerance mechanisms against the acute harmful effects of hypoxia on neurons in olfactory slices.

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.

Effects of mild hypothermia on superoxide anion production, superoxide dismutase expression, and activity following transient focal cerebral ischemia

Following a transient ischemic insult there is a marked increase in free radical (FR) production within the first 10-15 min of reperfusion and again at the peak of the inflammatory process. Hypothermia decreases lipid peroxidation following global ischemia, raising the possibility that it may act by reducing FR production early on and by maintaining or increasing endogenous antioxidant systems. By means of FR fluorescence, Western blot, immunohistochemistry, and enzymatic assay, we studied the effects of mild hypothermia on superoxide (O(-*)(2)) anion production, superoxide dismutase SOD expression, and activity following focal cerebral ischemia in rats. Mild hypothermia significantly reduced O(-*)(2) generation in the ischemic penumbra and corresponding contralateral region, but did not alter the bilateral SOD expression. SOD enzymatic activity in the ischemic core was slightly reduced in hypothermia-treated animals compared with normothermic controls. Our results suggest that the neuroprotective effect of mild hypothermia may be due, in part, to a reduction in neuronal and endothelial O(-*)(2) production during early reperfusion.

Role of superoxide dismutases in oxidative damage and neurodegenerative disorders

In recent years, oxidative stress has been implicated in a variety of degenerative processes, diseases, and syndromes. Some of these include atherosclerosis, myocardial infarction, stroke, and ischemia/reperfusion injury; chronic and acute inflammatory conditions such as wound healing; central nervous system disorders such as forms of familial amyotrophic lateral sclerosis (ALS) and glutathione peroxidase-linked adolescent seizures; Parkinson's disease and Alzheimer's dementia; and a variety of other age-related disorders. Among the various biochemical events associated with these conditions, emerging evidence suggests the formation of superoxide anion and expression/activity of its endogenous scavenger, superoxide dismutase (SOD), as a common denominator. This review summarizes the function of SOD under normal physiological conditions as well as its role in the cellular and molecular mechanisms underlying oxidative tissue damage and neurological abnormalities. Experimental evidence from laboratory animals that either overexpress (transgenics) or are deficient (knockouts) in antioxidant enzyme/protein levels and the genetic SOD mutations observed in some familial cases of ALS are also discussed.

Chemical preconditioning with 3-nitropropionic acid: lack of induction of neuronal tolerance in gerbil hippocampus subjected to transient forebrain ischemia

Chemical preconditioning using the mitochondrial toxin, 3-nitropropionic acid (3-NP) has been reported to induce neuroprotection against subsequent global ischemia. To investigate the underlying mechanisms, Mongolian gerbils were pretreated with either vehicle or 3-NP at the dose of 3 or 10 mg/kg, intraperitoneal, 3 days prior to a 5-min bilateral carotid artery occlusion followed by either 48 h or 7 days of blood recirculation. Neuronal damage was assessed by a cresyl violet/fuchsin acid staining. Induction of heat shock protein 72 (HSP72) and manganese superoxide dismutase (MnSOD) expression was evaluated by Western blotting. Astroglial and microglial activation was detected by immunohistochemistry (glial fibrillary acid protein) and by histochemistry (isolectin B4 staining), respectively. Present data show that the hippocampal neuronal damage induced by ischemia were of similar extent between the vehicle- and 3-NP-treated gerbils, whatever the dose tested, indicating that 3-NP did not induce tolerance to transient forebrain ischemia under our experimental conditions. The lack of difference in the post-ischemic level of HSP72 and MnSOD protein expression and in the intensity of astroglial and microglial activation represents further indirect indications of the absence of 3-NP preconditioning effect. In conclusion, although chemical preconditioning with 3-NP is a well-established phenomenon at least in vitro and in models of focal ischemia, the relevance of 3-NP as a preconditioning molecule towards global brain ischemia remains an open question.

Ischemic preconditioning acts upstream of GluR2 down-regulation to afford neuroprotection in the hippocampal CA1

Animals subjected to sublethal transient global ischemia (ischemic preconditioning) exhibit neuroprotection against subsequent global ischemia-induced neuronal death in the hippocampal CA1 (ischemic tolerance). The molecular mechanisms underlying ischemic tolerance are unclear. Here we report that ischemic preconditioning induced a small, transient down-regulation of GluR2 mRNA expression and greatly attenuated subsequent ischemia-induced GluR2 mRNA and protein down-regulation and neuronal death. Ischemic preconditioning and GluR2 antisense knockdown acted synergistically to increase cell death. Sublethal antisense knockdown did not protect against subsequent ischemic insults or antisense knockdown. These findings indicate that ischemic preconditioning acts at step(s) upstream from suppression of GluR2 gene expression to afford neuroprotection and implicate transcriptional regulation of GluR2 expression in the adaptive mechanisms associated with ischemic tolerance.

Post-ischemic transcriptional and translational responses of EC-SOD in mouse brain and serum

Extracellular superoxide dismutase (EC-SOD) is neuroprotective, but its role in cerebral ischemia remains to be determined. We herein describe the topographical localization and quantitative changes in EC-SOD and its mRNA expression following cerebral ischemia in mice. Mice were subjected to transient forebrain ischemia and varied intervals of reperfusion. The measurements of EC-SOD using ELISA showed increased brain EC-SOD after 24 h of reperfusion and an increase in EC-SOD brain/serum ratio after 3 h. The immunohistochemical examination in normal mice showed strong EC-SOD immunoreactivity in the choroid plexus, pia mater, and ventral tuberal area of the hypothalamus. EC-SOD immunoreactivity in cortical and striatal capillary wall was conspicuous after 3 h. EC-SOD immunoreactivity was also noted in cortical neurons after 24 h. Northern blot analysis showed an increased EC-SOD mRNA expression in the brain after 24 h. An in situ hybridization study in normal mice demonstrated the mRNA expression of EC-SOD in choroid plexus and neurons through the brain especially in the cortex or ventral tuberal area of the hypothalamus, but demonstrated no mRNA expression of EC-SOD in the capillary wall. These findings suggest that EC-SOD accumulates on endothelial cells in response to this injury by an unknown mechanism, while cortical neurons produce EC-SOD themselves after cerebral ischemia with reperfusion.

Intracellular generation of free radicals and modifications of detoxifying enzymes in cultured neurons from the developing rat forebrain in response to transient hypoxia

To address the influence of oxidative stress and defense capacities in the effects of transient hypoxia in the immature brain, the time course of reactive oxygen species generation was monitored by flow cytometry using dihydrorhodamine 123 and 2',7'-dichlorofluorescein-diacetate in cultured neurons issued from the fetal rat forebrain and subjected to hypoxia/reoxygenation (6 h/96 h). Parallel transcriptional and activity changes of superoxide dismutases, glutathione peroxidase and catalase were analyzed, in line with cell outcome. The study confirmed hypoxia-induced delayed apoptotic death, and depicted increased mitochondrial and cytosolic productions of free radicals (+30%) occurring over the 48-h period after the restoration of oxygen supply, with sequential stimulations of superoxide dismutases. Whereas catalase mRNA levels and activity were augmented by cell reoxygenation, glutathione peroxidase activity was transiently repressed (-24%), along with reduced glutathione reductase activity (-27%) and intracellular glutathione depletion (-19%). Coupled with the neuroprotective effects of the glutathione precursor N-acetyl-cysteine (50 microM), these data suggest that hypoxia/reoxygenation-induced production of reactive oxygen species can overwhelm glutathione-dependent antioxidant capacity, and thus may contribute to the resulting neuronal apoptosis.

Synergistic induction of HSP40 and HSC70 in the mouse hippocampal neurons after cerebral ischemia and ischemic tolerance in gerbil hippocampus

An ischemia-induced gene was screened using a differential display technique in mouse transient forebrain ischemia. One of the ischemia-responsive clones was found to encode mouse hsp40. HSP40 has a critical regulatory function in the HSC70 ATPase activity. Expression of hsp40 mRNA was low in the nonischemic mouse hippocampus, but it was significantly upregulated 4 hr after ischemia by Northern blot analysis. In situ hybridization analysis revealed hsp40 mRNA induction in the neuron. HSP40 protein expression was also enhanced in the pyramidal and dentate granular neurons from 2 to 4 days after ischemia. The temporal expression and distribution profile of HSC70 protein was similar to that of HSP40, and both proteins were colocalized in ischemic hippocampal neurons. In the gerbil transient forebrain ischemia model, both HSP40 and HSC70 proteins were expressed strongly in ischemia-resistant CA3 neurons and dentate granule cells 1 day after 5 min ischemia, but were not expressed in vulnerable CA1 neurons. However, both proteins were in parallel expressed in the tolerance-acquired CA1 neurons. Based on the current observation that both HSP40 and HSC70 proteins were synergistically expressed in the ischemia-resistant and tolerance-acquired neurons, cochaperone HSP40 may play a significant role against postischemic neuronal response and lead to cell survival through interaction with simultaneously induced HSC70.

Preconditioning with cortical spreading depression does not upregulate Cu/Zn-SOD or Mn-SOD in the cerebral cortex of rats

Previous studies have demonstrated that preconditioning the brain with cortical spreading depression (CSD) induces tolerance to a subsequent episode of ischemia. In other models of preconditioning, induction of ischemic tolerance has been associated with increased expression of the antioxidant enzyme, superoxide dismutase (SOD). The objective of the present study was to determine whether CSD upregulates Cu/Zn-SOD or Mn-SOD. CSD was induced in one hemisphere by applying 2 M KCl to the frontal cortex in Wistar rats. After 2 or 24 h of recovery, Cu/Zn-SOD and Mn-SOD mRNA levels were determined in both hemispheres using Northern blot analysis. In separate rats, Cu/Zn-SOD and Mn-SOD protein levels were determined 24 and 72 h after CSD using Western blot analysis. In addition, total SOD, Cu/Zn-SOD and Mn-SOD enzymatic activities were measured 24 and 72 h after CSD using spectrophotometric and zymographic assays. At the times investigated, no significant differences in mRNA or protein levels for Cu/Zn-SOD or Mn-SOD were observed between the ipsilateral and contralateral cortex. Further, there were no significant differences in Cu/Zn-SOD or Mn-SOD enzymatic activities between the two hemispheres at 24 or 72 h after CSD. In addition, CSD did not alter the activities of Cu/Zn-SOD or Mn-SOD in either hemisphere, relative to those in unoperated animals. Taken together, these results fail to support the hypothesis that CSD-induced tolerance is mediated through the upregulation of Cu/Zn-SOD or Mn-SOD.

Mn-SOD and Bcl-2 expression after repeated hyperbaric oxygenation

To clarify the mechanism of ischemic tolerance induced by HBO, we investigated the effect of HBO on immunoreactivity to Bcl-2 and Bax, apoptosis-regulating protein, or Mn-SOD, a radical scavenging system, in the CA1 sector of the gerbil hippocampus. Pretreatment comprising, five sessions at 2 ATA (atmosphere absolute) every other day, but not that comprising, ten sessions at 3 ATA every day, caused significant increases in Bcl-2 and Mn-SOD immunoreactivity in the CA1 sector compared with in the sham pretreatment group. No significant differences in Bax immunoreactivity and neuronal density in the CA1 hippocampal neurons was observed between the groups. These results suggest that protection against mitochondrial alterations after ischemia through Mn-SOD and/or Bcl-2 expression is related to the ischemic tolerance induced by repeated HBO pre-treatment.

Preferential conditions for and possible mechanisms of induction of ischemic tolerance by repeated hyperbaric oxygenation in gerbil hippocampus

OBJECTIVE

We reported previously that repeated hyperbaric oxygenation (HBO) as pretreatment induced ischemic tolerance in the gerbil hippocampus. This study was conducted to determine the preferential conditions for induction of ischemic tolerance by HBO and the mechanism of this induction through immunohistochemical analysis of Bcl-2, Bax, and manganese superoxide dismutase expression.

METHODS

Five-minute forebrain ischemia was produced in gerbils after pretreatment with 2 atmospheres absolute (ATA) HBO once every other day for one, three, or five sessions, 2 ATA hyperbaric air once every other day for five sessions, or 3 ATA HBO once daily for 10 sessions. Histological examinations were then performed. Two days after pretreatment with 2 ATA HBO once every other day for five sessions or with 3 ATA HBO once daily for 10 sessions, sections were analyzed immunohistochemically.

RESULTS

Pretreatment with 2 ATA HBO once every other day for three or five sessions induced ischemic tolerance; however, pretreatment with 2 ATA HBO for one session, 2 ATA hyperbaric air once every other day for five sessions, or 3 ATA HBO once daily for 10 sessions did not. Pretreatment with 2 ATA HBO once every other day for five sessions, but not with 3 ATA HBO once daily for 10 sessions, significantly increased Bcl-2 and manganese superoxide dismutase immunoreactivity in the CA1 sector.

CONCLUSION

These results suggest that protection against mitochondrial alterations after ischemia through manganese superoxide dismutase and/or Bcl-2 expression may be related to induction of ischemic tolerance by repeated HBO pretreatment.

Huperzine A and donepezil protect rat pheochromocytoma cells against oxygen-glucose deprivation

Huperzine A (HupA) and donepezil, two novel selective acetylcholinesterase inhibitors available for Alzheimer's disease, were tested for their ability to alleviate injury from oxygen-glucose deprivation (OGD) in the rat pheochromocytoma line PC12 cells. OGD for 30 min triggered death in more than 50% of cells, along with major changes in morphology and biochemistry including elevated levels of lipid peroxide, superoxide disamutase activity and lactate. Cells pretreated for 2 h with HupA or donepezil showed improved survival and reduced biochemical and morphologic signs of toxicity (statistically significant over the range from 10 microM down to 1.0 and 0.1 microM, respectively). Our results indicated that HupA and donepezil protected PC12 cells against OGD-induced toxicity, most likely by alleviating disturbances of oxidative and energy metabolism.

Cerebral protection by hypoxic preconditioning in a murine model of focal ischemia-reperfusion

Sublethal periods of hypoxia or ischemia can induce adaptive mechanisms to protect against subsequent lethal ischemic insults in a process known as ischemic preconditioning. In the present study, we developed a murine model of cerebral preconditioning using several common strains of adult mice. Animals were exposed to sublethal hypoxia (11% oxygen for 2 h) 48 h prior to a 90 min period of transient focal middle cerebral artery occlusion, induced by an intraluminal filament; injury was assessed 24 h later by TTC staining. Infarct volume in hypoxia-preconditioned animals was reduced 46%, 58%, and 64% in C57Bl/6, 129SvEv, and Swiss-Webster ND4 mice relative to their respective untreated controls. This non-invasive murine model of ischemic tolerance should be useful for elucidating the molecular basis of this protection using transgenic and knockout mice.

Stress response to hypoxia in gerbil brain: HO-1 and Mn SOD expression and glial activation

Hypoxic preconditioning has been shown to induce neuroprotection against a subsequent damaging insult. In order to study the underlying molecular and cellular mechanisms of hypoxic preconditioning, we investigated, in gerbil hippocampus, the effects in vivo of transient exposure to hypoxia (4% O(2) for 6 min followed by either 48 h or 7 days of reoxygenation) (i) on the induction of 72 kDa heat shock protein (HSP72), heme oxygenase-1 (HO-1) and manganese superoxide dismutase (Mn SOD) as assessed by Western immunoblotting and (ii) on the astroglial and microglial activation as detected by both immunohistochemistry and Western immunoblotting for GFAP, and histochemistry for isolectin B4, respectively. Our data show that, although hypoxia and subsequent reoxygenation led to neither neuronal damage nor HSP72 induction in gerbil hippocampus, it induced a progressive and sustained expression of HO-1 and Mn SOD. As expected from the absence of neuronal death, hypoxia was not associated with microglial activation but led to a significant astrocytic activation. These findings demonstrate that transient hypoxia enhances the antioxidative enzymatic defenses of the brain, which are susceptible to increased tolerance against a subsequent damaging insult.

Cell permeable exogenous ceramide reduces infarct size in spontaneously hypertensive rats supporting in vitro studies that have implicated ceramide in induction of tolerance to ischemia

Previous work in primary cell culture has shown that TNF-alpha and ceramide are involved in the signaling that induces tolerance to brain ischemia (Ginis et al., 1999; Liu et al., 2000). To validate the in vitro studies, the authors administered cell permeable analogs of ceramides intracisternally or intravenously to examine their effect on neuroprotection after focal cerebral ischemia. Permanent middle cerebral artery occlusion (MCAO) was performed in spontaneously hypertensive rats. Infarct volumes were assessed at 24 hours after surgery. D-erythro-N-acetylsphingosine (C2-ceramide) or its vehicle was infused intracisternally for 1 hour before MCAO. In a second set of studies, D-erythro-N-octanoylsphingosine (C8-ceramide) or its vehicle was injected intravenously 48 or 24 hours before MCAO to mimic preconditioning (PC) and was also injected 5 minutes after MCAO. C2-ceramide infusion significantly reduced infarct volumes by approximately 14% (P < 0.05). C8-ceramide injection reduced infarct volumes approximately 17% compared with controls. This effect was constant and significant compared with controls over the time periods examined (P < 0.01). This work supports findings in primary brain cell cultures that implicate ceramide as a downstream signal that is proximate to development of tolerance to brain ischemia. Because the degree of protection represents approximately 50% of the maximal infarct reduction observed in this model, there are probably additional signaling pathways that subserve tolerance.

Protective and regenerative response endogenously induced in the ischemic brain

Neuronal cells are highly vulnerable to ischemic insult. Because adult neurons are highly differentiated and cannot self-propagate, loss of neurons often results in functional deficits in mammalian brains. However, it has recently been shown that neurons and neuronal circuits exhibit protective and regenerative responses in a rodent model of experimental ischemia. At first, neurons respond by producing several protective proteins such as heat shock proteins (HSPs) after sublethal ischemia and then acquire tolerance against a subsequent ischemic insult (ischemic tolerance). Once neurons suffer irreversible injury, two repair processes, neurogenesis and synaptogenesis, are endogenously induced. Neuronal stem and (or) progenitor cells can proliferate in two brain areas in adult animals: the subventricular zone and the subgranular zone in the dentate gyrus. After ischemic insult, these stem (progenitor) cells proliferate and differentiate into neurons in the dentate gyrus of the hippocampus. Reactive synaptogenesis has been also observed in the injured brain following a period of long-term infarction, but it is unclear if it can compensate for disconnected circuits. Understanding the molecular mechanism underlying these protective and regenerative responses will be important in developing a new strategy for aimed at the augmentation of resistance against ischemic insult and the replacement of injured neurons and neuronal circuits.Key words: ischemic tolerance, neurogenesis, synaptogenesis.

Accumulation of 4-hydroxynonenal-modified proteins in hippocampal CA1 pyramidal neurons precedes delayed neuronal damage in the gerbil brain

It has been proposed that reactive oxygen species and lipid peroxidation have a role in the delayed neuronal death of pyramidal cells in the CA1 region. To explore the in situ localization and serial changes of 4-hydroxy-2-nonenal-modified proteins, which are major products of membrane peroxidation, we used immunohistochemistry of the gerbil hippocampus after transient forebrain ischemia with or without preconditioning ischemia. The normal gerbil hippocampus showed weak immunoreactivity for 4-hydroxy-2-nonenal-modified proteins in the cytoplasm of CA1 pyramidal cells. 4-hydroxy-2-nonenal immunoreactivity showed no marked changes after preconditioning ischemia. In the early period after ischemia and reperfusion, there was a transient increase of nuclear 4-hydroxy-2-nonenal immunoreactivity in CA1 pyramidal neurons. In contrast, cytoplasmic immunoreactivity transiently disappeared during same period and then increased markedly from 8h to seven days. One week after ischemia, 4-hydroxy-2-nonenal immunoreactivity was observed within reactive astrocytes in the CA1 region. Early nuclear accumulation of 4-hydroxy-2-nonenal in CA1 neurons may indicate a possible role in signal transduction between the nucleus and cytoplasm/mitochondria, while delayed accumulation of 4-hydroxy-2-nonenal-modified proteins in the cytoplasm may be related to mitochondrial damage. We conclude that 4-hydroxy-2-nonenal may be a key mediator of the oxidative stress-induced neuronal signaling pathway and may have an important role in modifying delayed neuronal death.

Attenuation of ischemic brain edema and cerebrovascular injury after ischemic preconditioning in the rat

Ischemic preconditioning (IPC) induces neuroprotection to subsequent severe ischemia, but its effect on the cerebrovasculature has not been studied extensively. This study evaluated the effects of IPC on brain edema formation and endothelial cell damage that follows subsequent permanent focal cerebral ischemia in the rat. Transient (15 minute) middle cerebral artery occlusion (MCAO) was used for IPC. Three days after IPC or a sham operation, permanent MCAO was induced. Twenty-four hours after permanent MCAO, neurologic deficit, infarction volume, and water and ion content were evaluated. Six hours post-ischemia, blood-brain barrier (BBB) permeability was examined using [3H]-inulin. Water, ion contents, and BBB permeability were assessed in three zones (core, intermediate, and outer) depending on their relation to the MCA territory. Heat shock protein 70 (HSP70) was also examined as a potential marker of vascular injury. The model of IPC significantly reduced brain infarction and neurologic deficit. Compared with a sham operation, IPC also significantly attenuated brain edema formation in the intermediate (sham and IPC water contents: 5.99+/-0.65 vs. 4.99+/-0.81 g/g dry weight; P < 0.01) and outer zones (5.02+/-0.48 vs. 4.37+/-0.42 g/g dry weight; P < 0.01) of the ipsilateral hemisphere but not in the core zone. Blood-brain barrier disruption assessed by [3H]-inulin was significantly attenuated in the IPC group and the number of blood vessels that displayed HSP70 immunoreactivity was also reduced. Thus, IPC significantly attenuates ischemic brain edema formation, BBB disruption, and, as assessed by HSP70, vascular injury. Understanding the mechanisms involved in IPC may provide insight into methods for preserving cerebrovascular function during ischemia.

Free radical production and changes in superoxide dismutases associated with hypoxia/reoxygenation-induced apoptosis of embryonic rat forebrain neurons in culture

Following hypoxia/reoxygenation (6h/96h), cultured neurons from the embryonic rat forebrain undergo delayed apoptosis. To evaluate the participation of oxidative stress and defense mechanisms, temporal evolution of intraneuronal free radical generation was monitored by flow cytometry using dihydrorhodamine 123, in parallel with the study of transcriptional, translational, and activity changes of the detoxifying enzymes Cu/Zn-SOD and Mn-SOD. Two distinct peaks of radical generation were depicted, at the time of reoxygenation (+ 27%) and 48 h later (+ 25%), respectively. Radical production was unaffected by caspase inhibitors YVAD-CHO or DEVD-CHO, which prevented neuronal damage, suggesting that caspase activation is not an upstream initiator of radicals in this model. Cell treatment by vitamin E (100 microM) displayed significant neuroprotection, whereas the superoxide generating system xanthine/xanthine oxidase induced apoptosis. Transcript and protein levels of both SODs were reduced 1 h after the onset of hypoxia, but activities were transiently stimulated. Reoxygenation was associated with an increased expression (139%), but a decreased activity (21%) of the inducible Mn-SOD, whereas Cu/Zn-SOD protein and activity were low and progressively increased until 48 h post-hypoxia, when the second rise in radicals occurred. In spite of a temporal regulation of SODs, which parallels radical formation, oxidative stress might account for neurotoxicity induced by hypoxia.

Attenuation and augmentation of ischaemia-related neuronal death by tumour necrosis factor-alpha in vitro

Upregulation of the pro-inflammatory cytokine tumour necrosis factor-alpha (TNF) occurs rapidly in the brain following ischaemia, although it is unclear whether this represents a neurotoxic or neuroprotective response. We have investigated whether TNF has different actions in the pre- and postischaemic periods in a tissue culture model of cerebral ischaemia. Organotypic hippocampal slice cultures were prepared from 8-10-day-old rats and maintained in vitro for 14 days. Neuronal damage was induced by either 1 h oxygen-glucose deprivation or 3 h exposure to NMDA or the superoxide generator duroquinone, and assessed after 24 h by propidium iodide fluorescence. TNF pretreatment was neuroprotective against both oxygen-glucose deprivation and duroquinone. This effect was associated with an activation of the transcription factor NFkappaB and upregulation of manganese superoxide dismutase, and was prevented by a free radical scavenger. When addition of TNF was delayed until the postinsult period, an exacerbation of neurotoxicity occurred, which was also prevented by a free radical scavenger. The actions of TNF are determined by whether TNF is present before or after an ischaemia-related insult. Both actions are mediated through the production of free radicals, and the response to TNF is determined by whether a cell is metabolically competent to respond by synthesis of antioxidant defences.

Molecular mechanisms of ischemic neuronal injury

Brain ischemia triggers a complex cascade of molecular events that unfolds over hours to days. Identified mechanisms of postischemic neuronal injury include altered Ca(2+) homeostasis, free radical formation, mitochondrial dysfunction, protease activation, altered gene expression, and inflammation. Although many of these events are well characterized, our understanding of how they are integrated into the causal pathways of postischemic neuronal death remains incomplete. The primary goal of this review is to provide an overview of molecular injury mechanisms currently believed to be involved in postischemic neuronal death specifically highlighting their time course and potential interactions.

Unilateral upregulation of cyclooxygenase-2 following cerebral, cortical photothrombosis in the rat: suppression by MK-801 and co-distribution with enzymes involved in the oxidative stress cascade

Cyclooxygenase-2 (COX-2) is an essential enzyme for prostaglandin synthesis from arachidonic acid, during which considerable amounts of superoxide are produced. During pathological conditions, superoxide and nitric oxide (NO) rapidly form peroxynitrite, a potent cytotoxin, causing symptoms referred to as oxidative stress response. Superoxide is controlled by enzymes such as manganese- or copper-zinc-dependent superoxide dismutase (Mn-SOD, CuZn-SOD), glutathione peroxidase (GPx) and antioxidants derived from heme oxygenase (HO) activity such as biliverdin and bilirubin. NO derives from 3 NO-synthases (NOS I-III) from which the calcium-dependent NOS-I and III are activated rapidly due to hyperexcitation. We studied the induction of COX-2 by immunohistochemistry at days 1, 2 and 5 following cortical photothrombosis in normal and MK-801 treated rats. The results showed a weak constitutive, neuronal expression of COX-2 in cortex and amygdala. Layers II+III contained considerably more COX-2 than infragranular layers. One and 2 days following injury COX-2 was highly upregulated in the supragranular layers of the whole injured hemisphere compared with sham-operated animals and compared to the contralateral unlesioned hemisphere, whereas at day 5 COX-2 levels had returned to baseline. MK-801 treatment caused a reduction in COX-2 upregulation at day one and by day 2 no significant differences between injured and contralateral hemisphere were measurable. COX-2 positive neurons were found in close association with NOS-I containing neurons and their fibers but were not colocalized. In addition, codistribution of COX-2 was found with HO-1, CuZn-SOD and GPx containing cells, whereas COX-2 was colocalized with HO-2 and/or MnSOD in cortical neurons.

Tolerance-Inducing dose of 3-nitropropionic acid modulates bcl-2 and bax balance in the rat brain: a potential mechanism of chemical preconditioning

Many studies have reported ischemia protection using various preconditioning techniques, including single dose 3-nitropropionic acid (3-NPA), a mitochondrial toxin. However, the cellular signal transduction cascades resulting in ischemic tolerance and the mechanisms involved in neuronal survival in the tolerant state still remain unclear. The current study investigated the mRNA and protein expression of the antiapoptotic bcl-2 and the proapoptotic bax. two antagonistic members of the bcl-2 gene family, in response to a single dose of 3-NPA, to global cerebral ischemia-reperfusion. and to the combination of both 3-NPA-pretreatment and subsequent global cerebral ischemia-reperfusion. Brain homogenates of adult Wistar rats (n = 25) were analyzed for bcl-2 and bax mRNA expression using a new highly sensitive and quantitative polymerase chain reaction (PCR) technique that allows real-time fluorescence measurements of the PCR product (LightCycler; Roche Diagnostics, Mannheim, Germany). Animals for mRNA analysis received 3-NPA (20 mg/kg, intraperitoneal; "chemical preconditioning") or vehicle (normal saline), and were either observed for 24 plus 3 hours or were subjected to 15 minutes of global cerebral ischemia 24 hours after the pretreatment and observed for 3 hours of reperfusion. Immunohistochemistry was applied to serial brain sections of additional rats (n = 68) to determine amount and localization of the respective Bcl-2 and Bax protein expression in various brain areas. One set of animals was injected with 3-NPA and observed for 3, 12, 24, and 96 hours; a second set was exposed to 15 minutes global cerebral ischemia, 3, 12, and 24 hours reperfusion; and a third set was pretreated with 3-NPA or saline 24 hours before the ischemic brain insult and observed for 96 hours of reperfusion. The authors found single dose 3-NPA treatment to be associated with an elevated bcl-2:bax ratio (increased bcl-2 expression, decreased bax expression), both on the transcriptional (mRNA) and the translational (protein) level. The differential influence of 3-NPA was maintained during early recovery from global cerebral ischemia (3 hours), when 3-NPA pretreated animals showed higher bcl-2 and lower bax mRNA levels compared with rats with saline treatment. Respective changes in protein expression were localized predominately in neurons vulnerable to ischemic damage. Compared with baseline, Bcl-2 protein was significantly higher in surviving neurons at 96 hours after the insult, whereas Bax protein remained unchanged. However, at this late time of postischemic recovery (96 hours), the protein expression pattern of surviving neurons was not different between animals with and without 3-NPA pretreatment. To the authors' knowledge, the current study is the first report on the differential expression of pro- and antiapoptotic genes after a single, nonlethal dose of 3-NPA. The current results suggest alterations in the balance between pro- and antiapoptotic proteins as a potential explanation for the reported protection provided by chemical preconditioning using 3-NPA in rats.

Müller cells in the preconditioned retinal ischemic injury rat

The role of Müller cells in the preconditioned retinal ischemic injury rat was investigated. In anesthetized Sprague Dawley rats, retinal ischemia for 5 minutes constituted the preconditioning stimulus for the left eye. After 24 hours, both eyes were clamped for 60 minutes. In 30, 60, 90, and 120, minutes and 1 day, 3 days, and 7 days after ischemia, electroretinograms were recorded, and the eyeballs were enucleated. After fixation with 4% paraformaldehyde, the avidin-biotin-peroxidase technique was applied to show glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP). Furthermore, for the solubilized retinas, Western blot analysis and enzyme-linked immunosorbent assay were performed to detect GS and GFAP in the extracts. Preconditioning performed 24 hours before ischemia significantly improved the recovery of the a-, and b-waves 1 day after 60 minute ischemia. In the 30, 60, 90, and 120 minutes after ischemia, the recovery of the a-wave only was observed. There was a nonsignificant trend toward greater recovery in the first 120 minutes after 60 minute ischemia, especially in the b-wave. GS immunoreactivity had no significant difference between non-preconditioned and preconditioned groups 30, 60, 90, and 120 minutes after ischemia. In 1 day after ischemia, GS immunoreactivity decreased in both groups. In 3 and 7 days after ischemia, GS immunoreactivity recovered only in the preconditioned group. The retinas at 3 and 7 days after 1 hour of ischemia showed increased GFAP immunoreactivity in the non-preconditioned group. In the preconditioned group, only slight GFAP immunoreactivity was observed. These results suggested that the mechanism of preconditioned retinal ischemia may be related to Müller cells in the retina.

Increase in endogenous brain superoxide dismutase as a potential mechanism of lipopolysaccharide-induced brain ischemic tolerance

A low dose (0.5 mg/kg) of lipopolysaccharide (LPS), administered 72 hours before 60-minute middle cerebral artery occlusion, induced a delayed neuroprotection proven by the significant decrease (-35%) of brain infarct volume in comparison with control, whereas infarct volumes remained unchanged in rats treated 12, 24, or 168 hours before ischemia. This delayed neuroprotective effect of LPS was induced only with low doses (0.25 to 1 mg/kg), whereas this effect disappeared with a higher dose (2 mg/kg). The delayed neuroprotection of LPS was induced in the cortical part of the infarcted zone, not in the subcortical part. The beneficial effect of LPS on consequences of middle cerebral artery occlusion was suppressed by dexamethasone (3 mg/kg) and indomethacin (3 mg/ kg) administered 1 hour before LPS, whereas both drugs had no direct effect on infarct volume by themselves, suggesting that activation of inflammatory pathway is involved in the development of LPS-induced brain ischemic tolerance. Preadministration of cycloheximide, an inhibitor of protein synthesis, also blocked LPS-induced brain ischemic tolerance suggesting that a protein synthesis is also necessary as a mediating mechanism. Superoxide dismutase (SOD) could be one of the synthesized proteins because lipopolysaccharide increased SOD brain activity 72 hours, but not 12 hours, after its administration, which paralleled the development of brain ischemic tolerance. In contrast, catalase brain activity remained unchanged after LPS administration. The LPS-induced delayed increase in SOD brain content was suppressed by a previous administration of indomethacin. These data suggest that the delayed neuroprotective effect of low doses of LPS is mediated by an increased synthesis of brain SOD that could be triggered by activation of inflammatory pathway.

Hyperbaric oxygenation induced tolerance against focal cerebral ischemia in mice is strain dependent

SV129 or C57BL/6 mice were exposed to hyperbaric oxygenation (HBO, 5 days, 1 h every day, 100% O(2) at 3 atm absolute). One day after the 5th HBO session focal cerebral ischemia was induced. In SV129 mice, HBO induced tolerance against permanent focal cerebral ischemia (n=42, mean infarct volume reduction 27%, P=0.001), but not against transient (30 or 60 min) focal cerebral ischemia. In the C57BL/6 strain of mice, HBO did not induce tolerance against focal cerebral ischemia, even when the duration of ischemia or the HBO protocol were modified. For the first time we demonstrate that HBO can induce tolerance to focal cerebral ischemia, but this effect is strain dependent.

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.

Possible role of the superoxide anion in the development of neuronal tolerance following ischaemic preconditioning in rats

There is a large body of evidence that reactive oxygen species play a major role in the pathogenesis of ischaemic brain damage. On the other hand, it has recently been suggested that superoxide anions participate in the development of neuronal tolerance against lethal ischaemia following ischaemic preconditioning (PC). The present study aimed to examine whether or not the intravenous administration of human recombinant Cu/Zn superoxide dismutase (hr SOD) prior to PC would affect the subsequent development of neuronal tolerance. Animals were randomly assigned to the following three groups: group 1, sham PC treated with vehicle; group 2, PC treated with hr SOD and group 3, PC treated with vehicle. For PC, 10 min occlusion of the middle cerebral artery (MCA) by a modified intraluminal suture method was followed by 60 min recirculation and this procedure was successively repeated three times. The procedures were similar for sham PC except that the MCA was kept unoccluded. Just prior to PC or sham PC, a bolus of hr SOD (6 x 103 IU/2 ml/kg) was administered intravenously. Seventy-two hours thereafter, rats were subjected to lethal ischaemia, i.e. MCA occlusion for 100 min followed by recirculation for 48 h. The infarct area and volume were assessed with the 2,3,5-triphenyltetrazolium stain. A significant difference in the infarct volume was revealed between the sham PC/vehicle and the PC/vehicle groups (total and cortex P < 0.01; striatum P < 0.05), showing that PC induced a marked neuronal tolerance against lethal ischaemia. The infarct volume in the PC/SOD group was close to that in the sham PC/vehicle group, being significantly greater than that in the PC/vehicle group (total and cortex P < 0.01) and showing that the administration of hr SOD suppressed the development of neuronal tolerance induced by PC. In a parallel experiment, expression of 72-kDa heat-shock protein (hsp 72) at 72 h after PC was considerably reduced in rats treated with hr SOD compared with those treated with vehicle. These results suggest that superoxide anions intraluminally generated within cerebral microvessels participate in the development of neuronal tolerance as well as the induction of hsp 72 following PC.

Cellular distribution of superoxide dismutases in the rat CNS

Superoxide dismutase (SOD) is considered to be a major factor in protection of nervous tissue against excitotoxic and ischemic/hypoxic lesion. Controversial reports about the localization of SOD after such an insult prompted us to re-investigate immunocytochemically the localization of the enzyme in the brain and spinal cord using specific antibodies against the manganese (Mn-SOD) and copper/zinc (Cu/Zn-SOD) containing isoenzyme in combination with cell type specific markers. CNS tissue sections were analyzed by confocal laser scanning microscopy and digital photo imaging. Cu/Zn-SOD immunoreactivity was found to be located predominantly in astrocytes throughout the CNS. The staining was found in the cytoplasm, in cellular processes and, less intensive, in the nucleus sparing the nucleolus. At a lower level the enzyme was also detectable in neuronal perikarya and in structures of the neuropil. Motoneurons of the spinal cord displayed an enhanced Cu/Zn-SOD staining intensity, when compared to brain neurons. In contrast the Mn-containing isoenzyme was predominantly localized to neurons and their processes throughout the brain and the spinal cord. Confirming the mitochondrial localization of the enzyme, a granular staining pattern sparing the nucleus was observed. Mn-SOD stained mitochondria were also seen in astroglial cells but the staining intensity was, on the whole, much lower compared to neurons, and often hardly detectable. It seems reasonable to conclude that differences in the basal content of SOD-isoenzymes may contribute to different cellular susceptibilities in neurodegenerative processes that are accompanied by oxidative stress.

A global hypoxia preconditioning model: neuroprotection against seizure-induced specific gravity changes (edema) and brain damage in rats

Hypoxia preconditioning states that a sublethal hypoxia episode will afford neuroprotection against a second challenge in the near future. We describe and discuss a procedure for the development of global hypoxia preconditioning in adult male Wistar rats, using a mildly hypoxic (9% O(2), 91% N(2)) atmospheric exposure of 8 h. The persistence of neuroprotection was analyzed using a kainic acid (KA) model of brain injury. Rats were challenged with KA (14 mg/kg, i.p.) on 1-14 days post-hypoxia. The effects of hypoxia preconditioning on seizure score, weight loss, brain edema and histopathology were assessed. Brain edema, predominantly of vasogenic origin, was measured 24 h after KA administration using a reproducible and quantitative method based on the specific gravities of tissue samples. A density gradient column (1.0250-1.0650 g/cm(3)) comprised of kerosene and bromobenzene was used to assess the presence of edema in regions involved in seizure initiation and propagation that are normally extensively damaged (i.e., piriform cortex and hippocampus). Specific gravities of tissues were calculated through extrapolation with known NaCl standards. We found that hypoxia preconditioning prevented the formation of edema in these brain regions when KA challenge was given 1, 3, and 7, but not 14 days post-hypoxia exposure. Furthermore, neuroprotection was observed in animals that had robust seizures. The described procedure may be used to examine the neuroprotective mechanisms induced by global hypoxia preconditioning against many subsequent challenges reflecting a variety of experimental models of brain injury, and will provide a better understanding of the brain response to hypoxia and stress.

Respiratory chain inhibition induces tolerance to focal cerebral ischemia

The authors show that the inhibitor of the succinate dehydrogenase, 3-nitroproprionic acid (3-NPA), which in high doses and with chronic administration is a neurotoxin, can induce profound tolerance to focal cerebral ischemia in the rat when administered in a single dose (20 mg/kg) 3 days before ischemia. Infarcts were approximately 70% and 35% smaller in the 3-NPA preconditioned groups of permanent and transient focal cerebral ischemia, respectively. This regimen of 3-NPA preconditioning neither induced necrosis, apoptosis, or any other histologically detectable damage to the brain, nor did it affect behavior of the animals. 3-NPA led to an immediate (1-hour) and long-lasting (3-day) decrease in succinate dehydrogenase activity (30% reduction) throughout the brain, whereas only a short metabolic impairment occurred (ATP decrease of 35% within 30 minutes, recovery within 2 hours). The authors found that 3-NPA induces a burst of reactive oxygen species and the free radical scavenger dimethylthiourea, when administered shortly before the 3-NPA stimulus, completely blocked preconditioning. Inhibition of protein synthesis with cycloheximide given at the time of 3-NPA administration completely inhibited preconditioning. The authors were unsuccessful in showing upregulation of mRNA for the manganese superoxide dismutase, and did not detect increased activities of the copper-zinc and manganese superoxide dismutases, prototypical oxygen free radicals scavenging enzymes, after 3-NPA preconditioning. The authors conclude that it is possible to pharmacologically precondition the brain against focal cerebral ischemia, a strategy that may in principal have clinical relevance. The data show the relevance of protein synthesis for tolerance, and suggests that oxygen free radicals may be critical signals in preconditioning.

Cerebrovascular hemodynamics and ischemic tolerance: lipopolysaccharide-induced resistance to focal cerebral ischemia is not due to changes in severity of the initial ischemic insult, but is associated with preservation of microvascular perfusion

Lipopolysaccharide (LPS), administered 72 hours before middle cerebral artery (MCA) occlusion, confers significant protection against ischemic injury. For example, in the present study, LPS (0.9 mg/kg intravenously) induced a 31% reduction in infarct volume (compared with saline control) assessed 24 hours after permanent MCA occlusion. To determine whether LPS induces true tolerance to ischemia, or merely attenuates initial ischemic severity by augmenting collateral blood flow, local CBF was measured autoradiographically 15 minutes after MCA occlusion. Local CBF did not differ significantly between LPS- and saline-pretreated rats (e.g., 34 +/- 10 and 29 +/- 15 mL x 100 g(-1) x min(-1) for saline and LPS pretreatment in a representative region of ischemic cortex), indicating that the neuroprotective action of LPS is not attributable to an immediate reduction in the degree of ischemia induced by MCA occlusion, and that LPS does indeed induce a state of ischemic tolerance. In contrast to the similarity of the initial ischemic insult between tolerant (LPS-pretreated) and nontolerant (saline-pretreated) rats, microvascular perfusion assessed either 4 hours or 24 hours after MCA occlusion was preserved at significantly higher levels in the LPS-pretreated rats than in controls. Furthermore, the regions of preserved perfusion in tolerant animals were associated with regions of tissue sparing. These results suggest that LPS-induced tolerance to focal ischemia is at least partly dependent on the active maintenance of microvascular patency and hence the prevention of secondary ischemic injury.

Activities of calcineurin and phosphatase 2A in the hippocampus after transient forebrain ischemia

We investigated the changes in the enzyme activity and immunoreactivity of calcineurin in the rat hippocampus after transient forebrain ischemia. Immediately after 20-min transient forebrain ischemia, calcineurin activity decreased to about 40% of the control in the CA1 region and to about 55% in other regions. Protein phosphatase 2A activity showed no remarkable changes. By 12 h after ischemia, calcineurin activity recovered, more in the CA1 region than in other regions. At 24 h it decreased again, but only in the CA1 region. Immunohistochemical- and immunoblot analyses showed no remarkable change in calcineurin in any region of the hippocampus within 12 h after ischemia. Thus, the activity of calcineurin is dissociated from its immunoreactivity and quantity. Several studies have suggested that unknown inhibitory factor(s) and/or reversible changes in calcineurin act to modify enzyme activity after ischemia. In contrast, phosphatase 2A activity underwent no obvious changes during the post-ischemia period we examined. This unique time course of calcineurin activity may contribute to the mechanism of ischemic neuronal injury.

Novel brain ischemic change on MRI. Delayed ischemic hyperintensity on T1-weighted images and selective neuronal death in the caudoputamen of rats after brief focal ischemia

BACKGROUND AND PURPOSE

Specific change of persistent hyperintensity/hypointensity on T1-weighted (T1W) and T2-weighted (T2W) MRI, respectively, has been reported to develop in the human basal ganglia after brief hemispheric ischemia. We investigated whether this ischemic change observed in humans could be reproduced experimentally in rats after brief middle cerebral artery (MCA) occlusion (MCAO), and if so, what the neuroradiological change represented histologically.

METHODS

The origin of the right MCA of male Wistar rats (n=25) was occluded for 15 minutes by inserting a silicon-coated nylon thread from the external carotid artery into the internal carotid artery. After 15 minutes' MCAO, coronal MR images (T1W, T2W, and T1W with fat saturation pulse) were obtained once at 3-day reperfusion (n=5) and twice at 3- and 7-day reperfusion (n=20). Brain specimens were examined histologically immediately after the last MRI study in all rats.

RESULTS

Neither T1W nor T2W MRI showed marked signal changes 3 days after reperfusion following 15-minute MCAO. However, the ischemic change of hyperintensity and hypointensity on T1W and T2W MRI, respectively, appeared in the striatum following 7-day reperfusion after 15-minute MCAO (n=19/20). Histological examination revealed that the specific lesion in the rat striatum on MRI corresponded to selective neuronal death and proliferation of reactive astrocytes and microglia without infarct, hemorrhage, lipid accumulation, or calcification.

CONCLUSIONS

Brief MCAO with reperfusion induces the delayed ischemic changes of hyperintensity and hypointensity on T1W and T2W MRI, respectively, in the rat striatum with high reproducibility. This specific ischemic change on MRI histologically corresponded to selective neuronal death and gliosis with preservation of the macroscopic structure of the brain. A similar MRI pattern reported in patients who have sustained brief ischemia may represent similar histology. We speculate that the ischemic change reflects some biochemical changes affecting the magnetic field as the brain tissue undergoes subtle structural changes.

Hypoxia preconditioning attenuates brain edema associated with kainic acid-induced status epilepticus in rats

Kainic acid (KA)-induced seizures elicit edema associated with necrosis in susceptible brain regions (e.g., piriform cortex and hippocampal CA1 and CA3 regions). To test the hypothesis that hypoxia preconditioning protects against KA-induced edema formation, adult male rats were exposed to a 9% O2, 91% N2 atmosphere for 8 h. KA (14 mg/kg, i.p.) was administered 1, 3, 7, or 14 days later. Regional analysis of edema indicated that hypoxia exposure attenuated edema formation in piriform and frontal cortices and hippocampus when KA was given 1, 3, or 7 days later but not 14 days after hypoxia. Cycloheximide (2 mg/kg s.c.) given 1 h prior to hypoxia prevented the protective effect of hypoxia on KA-induced edema attenuation in the piriform cortex and hippocampus. Thus, hypoxic challenge induces a general adaptive response that protects against the seizure-associated pathophysiology, with no direct relationship to seizure intensity. This response may involve stress-related transcription factors and effector proteins.

Overexpression of thioredoxin in transgenic mice attenuates focal ischemic brain damage

Thioredoxin (TRX) plays important biological roles both in intra- and extracellular compartments, including in regulation of various intracellular molecules via thiol redox control. We produced TRX overexpressing mice and confirmed that there were no anatomical and physiological differences between wild-type (WT) mice and TRX transgenic (Tg) mice. In the present study we subjected mice to focal brain ischemia to shed light on the role of TRX in brain ischemic injury. At 24 hr after middle cerebral artery occlusion, infarct areas and volume were significantly smaller in Tg mice than in WT mice. Moreover neurological deficit was ameliorated in Tg mice compared with WT mice. Protein carbonyl content, a marker of cellular protein oxidation, in Tg mice showed less increase than did that of WT mice after the ischemic insult. Furthermore, c-fos expression in Tg mice was stronger than in WT mice 1 hr after ischemia. Our results suggest that transgene expression of TRX decreased ischemic neuronal injury and that TRX and the redox state modified by TRX play a crucial role in brain damage during stroke.

Anti-death properties of TNF against metabolic poisoning: mitochondrial stabilization by MnSOD

The cytokine tumor necrosis factor (TNF) is toxic to some mitotic cells, but protects cultured neurons from a variety of insults by mechanisms that are unclear. Pretreatment of neurons or astrocytes with TNF caused significant increases in MnSOD activity, and also significantly attenuated 3-nitropropionic acid (3-NP) induced superoxide accumulation and loss of mitochondrial transmembrane potential. In oligodendrocytes, however, MnSOD activity was not increased, and 3-NP toxicity was unaffected by TNF. Genetically engineered PC6 cells that overexpress MnSOD also were resistant to 3-NP-induced damage. TNF pretreatment and MnSOD overexpression prevented 3-NP induced apoptosis, and shifted the mode of death from necrosis to apoptosis in response to high levels of 3-NP. Mitochondria isolated from either MnSOD overexpressing PC6 cells or TNF-treated neurons maintained resistance to 3-NP-induced loss of transmembrane potential and calcium homeostasis, and showed attenuated release of caspase activators. Overall, these results indicate that MnSOD activity directly stabilizes mitochondrial transmembrane potential and calcium buffering ability, thereby increasing the threshold for lethal injury. Additional studies showed that levels of oxidative stress and striatal lesion size following 3-NP administration in vivo are increased in mice lacking TNF receptors.

The antioxidant defences of brain mitochondria during short-term forebrain ischemia and recirculation in the rat

This study evaluated changes in the antioxidant defences of mitochondria induced by 30 min of forebrain ischemia and recirculation up to 24 h in rats. Following treatment, mitochondria were isolated from two brain subregions: the dorsolateral striatum, an area in which there is loss of most neurons, and the paramedian cortex in which most neurons are resistant to damage. During ischemia and the first few hours of recirculation, the mitochondrial defences were largely preserved based on measurements of the activities of the enzymes, superoxide dismutase, glutathione peroxidase and glutathione reductase, as well as the response of the mitochondria to a subsequent exposure to H2O2 in vitro. However, some moderate changes were detected, particularly in the mitochondria from the dorsolateral striatum. A decrease of 30% in the activity of superoxide dismutase was seen at the conclusion of the ischemic period and a small increase in susceptibility to changes induced by H2O2 was detected during early recirculation. This latter change preceded and possibly contributed to the development of an impairment of respiratory function detected in mitochondria from the dorsolateral striatum at 3 h of recirculation. At 24 h of recirculation, larger changes were seen in the activities of all three of the enzymes in mitochondria from the dorsolateral striatum but not the paramedian cortex that was associated with progression to advanced neuronal damage in the former subregion.

Differential expression of superoxide dismutase isoforms in neuronal and glial compartments in the course of excitotoxically mediated neurodegeneration: relation to oxidative and nitrergic stress

To examine the cellular distribution of radical scavenging enzymes in glia, in comparison to that in neurons and their behaviour during excitotoxically induced neurodegenerative processes, protein levels and the cellular localization of cytosolic and mitochondrial superoxide dismutase (Cu/Zn- and Mn-SOD) were investigated in the rat brain undergoing quinolinic acid (Quin)-induced neurodegeneration. Evidence for the specificity of the applied antibodies to detect immunocytochemically these SOD isoforms was obtained from electron microscopy and Western blotting. In control striatum Mn-SOD was clearly confined to neurons, whereas Cu/Zn-SOD was found, rather delicately, only in astrocytes. Microglia failed to stain with antibodies to both SOD isoforms. Quin application resulted in an initial formation of oxygen and nitrogen radicals as determined by the decline in the ratio of ascorbic to dehydroascorbic acid and by increased levels of nitrated proteins, an indicator for elevated peroxynitrite formation. Morphologically, massive neuronal damage was seen in parallel. Astroglia remained intact but showed initially decreased glutamine synthetase activities. The levels of Mn-SOD protein increased 2-fold 24 h after Quin injection (Western blotting) and declined only slowly over the time period considered (10 days). Cu/Zn-SOD levels increased only 1.3-fold. Immunocytochemical studies revealed that the increase in Mn-SOD is confined to neurons, whereas that of Cu/Zn-SOD was observed only in astroglial cells. Quiescent microglial cells were, as a rule, free of immunocytochemically detectable SOD, whereas in activated microglia a few Mn-SOD immunolabeled mitochondria occurred. Our results suggest a differential protective response in the Quin lesioned striatum in that Mn-SOD is upregulated in neurons and Cu/Zn-SOD in astroglia. Both SOD-isoforms are assumed to be induced to prevent oxidative and nitric oxide/peroxynitrite-mediated damage. In the border zone of the lesion core this strategy may contribute to resist the noxious stimulus.

Regional changes of membrane phospholipid concentrations in rabbit spinal cord following brief repeated ischemic insults

Changes in the concentration of membrane-bound phospholipids following single (25-min) spinal cord ischemia and 3 h of reperfusion were determined. These were compared with the changes following brief repeated (8-, 8-, and 9-min) ischemia followed each time by reperfusion for 1 h, or the same periods of ischemia followed by 8 h, 8 h, and 24 h of reperfusion, respectively. Phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and sphingomyelin (SM) were assayed in regions of the spinal cord of the rabbit, including gray matter, white matter, dorsal horns, intermediate zone, and ventral horns. The brief repeated ischemia with 1-h reperfusions produced more extensive degradation of phospholipids in almost all regions compared with the equivalent time of ischemia (25 min) in a single period. After a lengthy reperfusion after repeated ischemia, the phospholipids were resynthesized with the exception of the phosphatidylinositol in the gray matter. The resynthesis was most pronounced in the dorsal horns and in the white matter.

Preconditioning in vivo ischemia inhibits anoxic long-term potentiation and functionally protects CA1 neurons in the gerbil

Preconditioning with sublethal ischemia induces tolerance to subsequent lethal ischemia in neurons. We investigated electrophysiologic aspects of the ischemic tolerance phenomenon in the gerbil hippocampus. Gerbils were subjected to 2 minutes of forebrain ischemia (preconditioning ischemia). Some of them were subjected to a subsequent 5 minutes of forebrain ischemia 2 to 3 days after the preconditioning ischemia (double ischemia). Hippocampal slices were prepared from these gerbils subjected to the preconditioning or double ischemia, and field excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons. Capacity for long-term potentiation triggered by tetanic stimulation (tetanic LTP) was transiently inhibited 1 to 2 days after the double ischemia but then recovered. Latency of anoxic depolarization was not significantly different between slices from preconditioned gerbils and those from sham-operated gerbils when these slices were subjected to in vitro anoxia. Postanoxic potentiation of N-methyl-D-aspartate (NMDA) receptor-mediated transmission (anoxic LTP) was inhibited in slices from gerbils 2 to 3 days after the preconditioning ischemia, whereas it was observed in slices from sham-operated gerbils and gerbils 9 days after the preconditioning ischemia. These results suggest that protection by induced tolerance is (1) not only morphologic but also functional, and (2) expressed in inhibiting postischemic overactivation of NMDA receptor-mediated synaptic responses.

Mitochondrial manganese superoxide dismutase prevents neural apoptosis and reduces ischemic brain injury: suppression of peroxynitrite production, lipid peroxidation, and mitochondrial dysfunction

Oxidative stress is implicated in neuronal apoptosis that occurs in physiological settings and in neurodegenerative disorders. Superoxide anion radical, produced during mitochondrial respiration, is involved in the generation of several potentially damaging reactive oxygen species including peroxynitrite. To examine directly the role of superoxide and peroxynitrite in neuronal apoptosis, we generated neural cell lines and transgenic mice that overexpress human mitochondrial manganese superoxide dismutase (MnSOD). In cultured pheochromocytoma PC6 cells, overexpression of mitochondria-localized MnSOD prevented apoptosis induced by Fe2+, amyloid beta-peptide (Abeta), and nitric oxide-generating agents. Accumulations of peroxynitrite, nitrated proteins, and the membrane lipid peroxidation product 4-hydroxynonenal (HNE) after exposure to the apoptotic insults were markedly attenuated in cells expressing MnSOD. Glutathione peroxidase activity levels were increased in cells overexpressing MnSOD, suggesting a compensatory response to increased H2O2 levels. The peroxynitrite scavenger uric acid and the antioxidants propyl gallate and glutathione prevented apoptosis induced by each apoptotic insult, suggesting central roles for peroxynitrite and membrane lipid peroxidation in oxidative stress-induced apoptosis. Apoptotic insults decreased mitochondrial transmembrane potential and energy charge in control cells but not in cells overexpressing MnSOD, and cyclosporin A and caspase inhibitors protected cells against apoptosis, demonstrating roles for mitochondrial alterations and caspase activation in the apoptotic process. Membrane lipid peroxidation, protein nitration, and neuronal death after focal cerebral ischemia were significantly reduced in transgenic mice overexpressing human MnSOD. The data suggest that mitochondrial superoxide accumulation and consequent peroxynitrite production and mitochondrial dysfunction play pivotal roles in neuronal apoptosis induced by diverse insults in cell culture and in vivo.

Redox control of neuronal damage during brain ischemia after middle cerebral artery occlusion in the rat: immunohistochemical and hybridization studies of thioredoxin

Thioredoxin (TRX) is a small, multifunctional protein with a redox-active site and multiple biological functions that include reducing activity for reactive oxygen intermediates. We assayed TRX and TRX mRNA by immunohistochemical methods and hybridization experiments in the rat brain after middle cerebral artery (MCA) occlusion. During ischemia, the immunoreactivity for TRX decreased; it disappeared after MCA occlusion in the ischemic regions. It rapidly decreased and nearly disappeared at 4 and 16 hours after MCA occlusion in the lateral striatum and frontoparietal cortex, respectively. On the other hand, in the perifocal ischemic region, the penumbra, TRX immunoreactivity began to increase 4 hours after MCA occlusion and continued to increase until 24 hours after occlusion. In hybridization experiments, TRX mRNA decreased and nearly disappeared 4 hours after MCA occlusion in the lateral striatum. In the frontoparietal cortex, it decreased until 24 hours after MCA occlusion. In the perifocal ischemic region, TRX mRNA began to increase 4 hours after MCA occlusion and continued to increase until 24 hours. Northern blot analysis showed that total TRX mRNA in the operated hemispheres was induced from 8 hours and increased until 24 hours after the surgical procedures. We previously reported that recombinant TRX promotes the in vitro survival of primary cultured neurons. We now suggest that TRX in the penumbra has neuroprotective functions and that decreased levels of TRX in the ischemic core modify neuronal damage during focal brain ischemia.

Transient increase of manganese-superoxide dismutase in remote brain areas after focal photothrombotic cortical lesion

BACKGROUND AND PURPOSE

Free radicals including superoxide are responsible for postlesional cytotoxicity. In contrast to the constitutive CuZn-superoxide dismutases (SODs), manganese-superoxide dismutase (Mn-SOD) is inducible and has the potential to protect neurons by its superoxide dismutating activity. Therefore, we studied the presence and the regional changes in Mn-SOD within the brain after focal cortical ischemia.

METHODS

Focal cortical photothrombotic lesions were produced in the hindlimb region of rat brains. Animals were anesthetized and transcardially perfused with Zamboni's fixative. Mn-SOD was immunohistochemically localized using an antiserum against rat-Mn-SOD. Changes in Mn-SOD immunoreactivity were quantified by image analysis.

RESULTS

Focal photothrombosis caused a perilesional increase in Mn-SOD after 24 hours, followed by a further significant increase at 48 hours in perilesional cortex, ipsilateral corpus callosum, hippocampus, and thalamus, as well as in a homotopic cortical area within the nonlesioned hemisphere. At day 2, Mn-SOD was present in neurons and astrocytes. Up to day 7, Mn-SOD increased in the entire ipsilateral and contralateral cortex but remained higher elevated in the ipsilateral hippocampus and thalamus. Thereafter, Mn-SOD decreased globally but remained elevated in some cortical neurons up to day 60.

CONCLUSIONS

The early transient increase of Mn-SOD in distinct brain regions, which are functionally connected via afferents and efferents, suggests that these regions are affected by the injury. It suggests that Mn-SOD protects the cells in these regions from superoxide-induced damage and therefore may limit the retrograde and anterograde spread of neurotoxicity.

Regional and ultrastructural immunolocalization of copper-zinc superoxide dismutase in rat central nervous system

We examined the distribution of copper-zinc superoxide dismutase (CuZnSOD) in adult rat central nervous system by light and electron microscopic immunocytochemistry, using an affinity-purified polyclonal antibody. The enzyme appeared to be exclusively localized in neurons. No immunoreactivity was seen in non-neuronal cells. The staining intensity was variable, depending on the brain region and, within the same region, on the neuron type. Highly immunoreactive elements included cortical neurons evenly distributed in the different layers, hippocampal interneurons, neurons of the reticular thalamic nucleus, and Golgi, stellate, and basket cells of the cerebellar cortex. Other neurons, i.e., pyramidal cells of the neocortex and hippocampus, Purkinje and granule cells of the cerebellar cortex, and the majority of thalamic neurons, showed much weaker staining. In the spinal cord, intense CuZnSOD immunoreactivity was present in many neurons, including motor neurons. Pre-embedding immunoelectron microscopy of the neocortex, hippocampus, reticular thalamic nucleus, and cerebellar cortex showed cytosolic and nucleoplasmic labeling. Moreover, single membrane-limited immunoreactive organelles identified as peroxisomes were often found, even in neurons that appeared weakly stained at the light microscopic level. In double immunogold labeling experiments, particulate CuZnSOD immunoreactivity co-localized with catalase, a marker enzyme for peroxisomes, thus demonstrating that in neural tissue CuZnSOD is also present in peroxisomes.

Ischemic tolerance in hippocampal CA1 neurons studied using contralateral controls

We induced ischemic tolerance unilaterally in gerbil hippocampus using the contralateral hippocampus as control. Ischemia for 2 min of right common carotid occlusion was reversible but sufficient to cause heat-shock protein 70 production in CA1 neurons. This pretreatment given four days prior to occlusion of both common carotids for 5 min, but not at longer preceding intervals, induced tolerance in right CA1 neurons. Neuroprotection was still evident two months after the 5 min occlusion. Adenosine triphosphate content and immunoreactive microtubule associate protein 2 in the hippocampus showed that the 5 min ischemic insult was essentially equal in both hemispheres. Repetitive pretreatments at two day intervals caused almost complete protection of CA1 neurons against subsequent 5 min ischemia, while a single pretreatment showed 80% protection. However, the increase in heat-shock protein 70 with repeated pretreatments was not significantly more than with one pretreatment. We concluded that true ischemic tolerance was induced by ischemic stress itself, was long-lasting, was not due to mitigation of subsequent ischemia, and was augmented by repetition without further increase of heat-shock protein 70.

A near-infrared spectroscopic study of cerebral ischemia and ischemic tolerance in gerbils

BACKGROUND AND PURPOSE

To explore the physiological mechanism of ischemic tolerance, we studied intracerebral oxygenation states noninvasively using near-infrared spectroscopy after bilateral common carotid artery occlusion (BCO) in gerbils with and without ischemic pretreatment.

METHODS

Under ether anesthesia, gerbils with sham operation (S group, n = 8) and those with pretreatment consisting of BCO for 2 minutes, twice at 3 days and 2 days earlier (T group, n = 8), were again subjected to BCO for 5 minutes. Changes in oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and total hemoglobin (HbT) as well as reduction in cytochrome oxidase (cyt.aa3) were calculated from the absorbance changes of the light transmitted through the brain. Seven days after the ischemic study, immunohistochemical examination was performed with an antiserum against microtubule-associated proteins.

RESULTS

In both groups, the increase of Hb and decrease of HbO2 and HbT proceeded rapidly after BCO, and the maximal deoxygenation of hemoglobin occurred within 2.5 minutes. Reduction of cyt.aa3 also ensued rapidly and reached the maximal reduction within 3 minutes in both groups. In the T group, however, both deoxygenation of hemoglobin and reduction of cyt.aa3 progressed more slowly than in the S group. The time (seconds) necessary for a maximal change for cyt.aa3 was significantly longer in the T group (203.8 +/- 34.0 [mean +/- SD]; P < .01) than in the S group (68.0 +/- 14.7). The time necessary for a half-maximal change was also significantly longer in the T group than in the S group for both Hb (22.0 +/- 7.5 and 13.5 +/- 4.0, respectively; P < .05) and cyt.aa3 (23.9 +/- 5.7 and 11.6 +/- 4.3; P < .01). After recirculation for 7 days, all gerbils in the S group were found to have neuronal death in the hippocampus, while those in the T group did not.

CONCLUSIONS

The present study indicated that mild ischemic stress can induce improvement in oxygen metabolism during subsequent ischemia, which might be causally related to the phenomenon known as "ischemic tolerance," in which a protective effect toward ischemic/postischemic injury is induced by earlier mild ischemic pretreatment.