Increased oxidative stress during hyperglycemic cerebral ischemia

In this review, we summarize the role of hyperglycemia during cerebral ischemia. Hyperglycemia occurring during experimental and clinical stroke has been associated with increased cerebral damage. Increased oxidative stress resulting from hyperglycemia is believed to contribute to the exacerbated damage. More specifically, superoxide, nitric oxide and peroxynitrite are believed to play an important role in cerebral damage. This also involves increased recruitment of various blood cells to the ischemic zone that contribute to inflammation. We present data from our group and others that demonstrate that free radical production is increased during hyperglycemic stroke in rodents. Recent data suggest that inflammation is an important component of ischemic damage under both normo- and hyperglycemic conditions. We summarize numerous studies that indicate that a variety of antioxidant (inhibition of free radical production, scavenging of free radicals and increasing free radical degradation) and anti-inflammatory strategies decrease cerebral infarction. Finally, we compare the success of some of these strategies in clinical trials compared to the animal models.     

Central nervous system trauma and stroke. I. Biochemical considerations for oxygen radical formation and lipid peroxidation

The generation of oxygen radicals and the process of lipid peroxidation have become a focus of attention for investigators in the fields of central nervous system (CNS) trauma and stroke (e.g., ischemia). Considering our level of understanding of free radical and lipid peroxidation chemistry, absolute proof for their involvement in the pathophysiology of traumatic and ischemic damage to the CNS has been meager. While direct, unequivocal evidence for the participation of free radicals and lipid peroxidation as primary contributors to the death of neuronal tissue waits to be established, numerous recent studies have provided considerable support for the occurrence of free radical and lipid peroxidation reactions in the injured or ischemic CNS. In addition, the pharmacological use of antioxidants and free radical scavengers in the treatment of experimental CNS trauma and ischemia has provided convincing, although indirect evidence, for the involvement of oxygen radicals and lipid peroxidation in these conditions. The intent of this and its companion paper is to review: 1) the biochemical processes which may give rise to free radical reactions in the CNS, 2) the environment of the ischemic cell as it may affect the generation of oxygen radicals and the catalysis of lipid peroxidation reactions, 3) the evidence for the involvement of free radical mechanisms in CNS trauma and ischemia, and 4) the pathophysiological consequences of these phenomena.     

Oxygen radical mechanisms of brain injury following ischemia and reperfusion.

This review addresses current understanding of oxygen radical mechanisms as they relate to the brain during ischemia and reperfusion. The mechanism for radical production remains speculative in large part because of the difficulty of measuring radical species in vivo. Breakdown of lipid membranes during ischemia leads to accumulation of free fatty acids. Decreased energy stores during ischemia result in the accumulation of adenine nucleotides. During reperfusion, metabolism of free fatty acids via the cyclooxygenase pathway and metabolism of adenine nucleotides via the xanthine oxidase pathway are the most likely sources of oxygen radicals. Although leukocytes have been found to accumulate in some models of ischemia and reperfusion, their mechanistic role remains in question. Therapeutic strategies aimed at decreasing brain injury have included administration of radical scavengers at the time of reperfusion. Efficacy of traditional oxygen radical scavengers such as superoxide dismutase and catalase may be limited by their inability to cross the blood-brain barrier. Lipid-soluble antioxidants appear more efficacious because of their ability to cross the blood-brain barrier and because of their presence in membrane structures where peroxidative reactions can be halted.     

Oxidative stress and neuronal death/survival signaling in cerebral ischemia

It has been demonstrated by numerous studies that apoptotic cell death pathways are implicated in ischemic cerebral injury in ischemia models in vivo. Experimental ischemia and reperfusion models, such as transient focal/global ischemia in rodents, have been thoroughly studied and the numerous reports suggest the involvement of cell survival/death signaling pathways in the pathogenesis of apoptotic cell death in ischemic lesions. In these models, reoxygenation during reperfusion provides oxygen as a substrate for numerous enzymatic oxidation reactions and for mitochondrial oxidative phosphorylation to produce adenosine triphosphate. Oxygen radicals, the products of these biochemical and physiological reactions, are known to damage cellular lipids, proteins, and nucleic acids and to initiate cell signaling pathways after cerebral ischemia. Genetic manipulation of intrinsic antioxidants and factors in the signaling pathways has provided substantial understanding of the mechanisms involved in cell death/survival signaling pathways and the role of oxygen radicals in ischemic cerebral injury. Future studies of these pathways could provide novel therapeutic strategies in clinical stroke.     

Free radicals, mitochondria, and hypoxia-ischemia in the developing brain

The immature brain is particularly susceptible to free radical injury because of its poorly developed scavenging systems and high availability of iron for the catalytic formation of free radicals. Neurons are more vulnerable to free radical damage than glial cells, but oligodendrocyte progenitors and immature oligodendrocytes in very prematurely born infants are selectively vulnerable to depletion of antioxidants and free radical attack. Reactive oxygen and nitrogen species play important roles in the initiation of apoptotic mechanisms and in mitochondrial permeability transition, and therefore constitute important targets for therapeutic intervention. Oxidative stress is an early feature after cerebral ischemia and experimental studies targeting the formation of free radicals demonstrate various degrees of protection after perinatal insults. Oxidative stress-regulated release of proapoptotic factors from mitochondria appears to play a much more important role in the immature brain. This review will summarize and compare with the adult brain some of the current knowledge of free radical formation in the developing brain and its roles in the pathophysiology after cerebral hypoxia-ischemia.     

自由基和基质金属蛋白酶介导脑缺血血脑屏障损伤的研究进展

血脑屏障的破坏是引起脑缺血损伤及继发水肿、出血、炎症的微观原因。缺血缺氧和再灌注过程产生的自由基,以及后续基质金属蛋白酶的激活,是破坏血脑屏障结构和功能的重要分子机制。因而,在脑缺血早期及时抑制自由基产生并清除自由基,抑制基质金属蛋白酶的活性,是降低脑缺血血脑屏障损伤及其并发症的关键环节。本文将从血脑屏障损伤的角度,概述自由基与基质金属蛋白酶在脑缺血损伤过程中的作用。     

Prospects for the future: the role of free radicals in the treatment of stroke

Studies using animal models of stroke have demonstrated that free radicals are highly reactive molecules generated predominantly during cellular respiration and normal metabolism. Imbalance between cellular production of free radicals and the ability of cells to defend against them is referred to as oxidative stress. After ischemic brain damage introduced by ischemic stroke or reperfusion, production of reactive oxygen species may increase, sometimes drastically, leading to tissue damage via several different cellular molecular pathways. The damage can become more widespread due to weakened cellular antioxidant defense systems after ischemic stroke. These experimental findings have important implications for the treatment of human cerebral ischemia. Agents directed at eliminating oxygen radicals must be administered before, or in the early stages of, reperfusion after ischemia. The therapeutic window seems to be narrow and limited to, at most, a few hours. Future research may clarify the current hypothesis that the accuracy of gene expression could account for the recovery of cellular function after ischemic stroke. This may open the window to the future use of drug combinations that may be rationally administered sequentially. If the phenomenon of ischemic tolerance plays a role in this concept is still a matter of debate.     

Oxidative damage following cerebral ischemia depends on reperfusion - a biochemical study in rat

The extent of brain injury during reperfusion appears to depend on the experimental pattern of ischemia/reperfusion. The goals of this study were: first, to identify the rate of free radicals generation and the antioxidant activity during ischemia and reperfusion by means of biochemical measurement of lipid peroxidation (LPO) and both enzymatic (superoxid dismutase - SOD, catalase - CAT, glutathion peroxidase - GPx) and non-enzymatic antioxidants activity (glutathione - GSH); and second, to try to find out how the pattern of reperfusion may influence the balance between free radical production and clearance. Wistar male rats were subject of four-vessel occlusion model (Pulsinelly & Brierley) cerebral blood flow being controlled by means of two atraumatic arterial microclamps placed on carotid arteries. The level of free radicals and the antioxidant activity were measured in ischemic rat brain tissue homogenate using spectrophotometrical techniques. All groups subjected to ischemia shown an increase of LPO and a reduction of the activity of enzymatic antioxidative systems (CAT, GPx, SOD) and non-enzymatic systems (GSH). For both groups subjected to ischemia and reperfusion, results shown an important increase of LPO but less significant than the levels found in the group with ischemia only. Statistically relevant differences (p<0.01) between continuous reperfusion and fragmented reperfusion were observed concerning the LPO, CAT, SOD and GSH levels, oxidative aggresion during fragmented reperfusion being more important.     

Endogenous antioxidant changes in the myocardium in response to acute and chronic stress conditions

Oxygen is a diradical and because of its unique electronic configuration, it has the potential to form strong oxidants (e.g. superoxide radical, hydrogen peroxide and hydroxyl radical) called oxygen free radicals or partially reduced forms of oxygen (PRFO). These highly reactive oxygen species can cause cellular injury by oxidizing lipids and proteins as well as by causing strand breaks in nucleic acids. PRFO are produced in the cell during normal redox reactions including respiration and there are various antioxidants in the cell which scavenge these radicals. Thus in order to maintain a normal cell structure and function, a proper balance between free radical production and antioxidant levels is absolutely essential. Production of PRFO in the myocardium is increased during variousin vivo as well asin vitro pathological conditions and these toxic radicals are responsible for causing functional, biochemical and ultrastructural changes in cardiac myocytes. Indirect evidence of free radical involvement in myocardial injury is provided by studies in which protection against these alterations is seen in the presence of exogenous administration of antioxidants. Endogenous myocardial antioxidants have also been reported to change under various physiological as well as pathophysiological conditions. It appears that endogenous antioxidants respond and adjust to different stress conditions and failure of these compensatory changes may also contribute in cardiac dysfunction. Thus endogenous and/or exogenous increase in antioxidants might have a therapeutic potential in various pathological conditions which result from increased free radical production.     

Detection of Free Radicals by Microdialysis/Spin Trapping Epr Following Focal Cerebral Ischemia-Reperfusion and a Cautionary Note on the Stability of 5,5-Dimethyl-1-Pyrroline N-Oxide (DMPO)

We have examined free radical production in a rat model of focal cerebral ischemia using microdialysis coupled with EPR analysis. A microdialysis probe was inserted 2 mm into the cerebral cortex, supplied by the right middle cerebral artery (MCA), and after a 2-hour washout period with artificial cerebral spinal fluid (ACSF), the perfusate solution was changed to ACSF containing the spin trapping agent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). No free radicals were detected by DMPO during the pre-ischemia period. Both common carotid arteries and the right MCA were then ligated for 90 minutes. Microdialysate collected every 15 min during the ischemic period demonstrated predominantly superoxide or peroxyl radical production. After release of the occlusive sutures, hydroxyl radical became apparent initially, then thiyl and carbon centered radicals appeared later in samples collected every 15 min for two hours following cortical reperfusion. Careful studies on the purification and stability of DMPO solution were performed to circumvent artifacts and spurious signals.     

Effect of dietary sesame oil as antioxidant on brain hippocampus of rat in focal cerebral ischemia

Oxidative stress may be regarded as an imbalance between free radical production and opposing antioxidant defenses. Free radical oxidative stress is implicated in rat cerebral ischemia and naturaceutical antioxidants are dietary supplements that have been reported to have neuroprotective activity. Many studies have reported dietary sesame oil (SO) as an effective antioxidant. In the present study the neuroprotective effect of dietary SO was evaluated against middle cerebral artery occlusion (MCAO)-induced cerebral ischemia injury in rats. Rats were fed on diet (20% SO) for 15 days. The middle cerebral artery of adult male Wistar rat was occluded for 2 h and reperfused for 22 h. The antioxidant properties of brain were measured as levels of reduced glutathione (GSH), glutathione-S-transferase (GST), glutathione peroxide (GPx), glutathione reductase (GR), catalase (CAT), superoxide dismutase (SOD) and thiobarbituric acid reactive substance (TBARS). A decrease in the activity of all the enzymatic and non-enzymatic antioxidants was observed along with an increase in lipid peroxidation (LPO) in MCAO group. The neurobehavioral activity of rats was also observed by using videopath analyzer. Dietary SO improved the antioxidant status in MCAO+SO group when compared with MCAO group. The results of neurobehavioral activity also support our biochemical data. The results obtained suggest protective effect of SO against cerebral ischemia in rat brain through their antioxidant properties.     

Reperfusion-induced arrhythmias: do free radicals play a critical role?

This article assesses whether oxygen-derived free radicals are one of the molecular causes of life-threatening arrhythmias that arise upon reperfusion of the ischemic myocardium. Evidence supporting this proposition has been obtained from studies of the effects of free radical scavengers and antioxidants, free radical generating systems, inhibition of various sources of free radicals and studies investigating the formation of free radicals and their products during early reperfusion. It has been hypothesized that free radical formation causes localised membrane damage to the sarcolemma that results in focal alterations in transmembrane ionic fluxes, particularly potassium. These changes in ionic fluxes may then lead to electrophysiological abnormalities that culminate in ventricular arrhythmias.     

Serendipitous findings while researching oxygen free radicals

This review is based on the honor of receiving the Discovery Award from the Society of Free Radical Biology and Medicine. The review is reflective and presents our thinking that led to experiments that yielded novel observations. Critical questioning of our understanding of oxygen free radicals in biomedical problems led us to use and develop more direct and extremely sensitive methods. This included nitrone free radical spin trapping and HPLC–electrochemical detection. This technology led to the pioneering use of salicylate to trap hydroxyl free radicals and show increased flux in ischemia/reperfused brain regions and also to first sensitively detect 8-hydroxyl-2-deoxyguanosine in oxidatively damaged DNA and help assess its role in cancer development. We demonstrated that methylene blue (MB) photoinduces formation of 8-hydroxyguanine in DNA and RNA and discovered that MB sensitively photoinactivates RNA viruses, including HIV and the West Nile virus. Studies in experimental stroke led us serendipitously to discover that α-phenyl-tert-butylnitrone (PBN) was neuroprotective if given after the stroke. This led to extensive commercial development of NXY-059, a PBN derivative, for the treatment of stroke. More recently we discovered that PBN nitrones have potent anti-cancer activity and are active in preventing hearing loss caused by acute acoustical trauma.     

Oxygen radicals in the adult respiratory distress syndrome, in myocardial ischemia and reperfusion injury, and in cerebral vascular damage

Recent work suggests that oxygen radicals may be important mediators of damage in a wide variety of pathologic conditions. In this review we consider the evidence supporting the participation of oxygen radicals in the adult respiratory distress syndrome, in ischemia reperfusion injury in the myocardium, and in cerebral vascular injury in acute hypertension and traumatic brain injury. In the adult respiratory distress syndrome there is active sequestration of polymorphonuclear neutrophils in the pulmonary vascular system. There is evidence that activation of these neutrophils results in the production of oxygen radicals which injure the capillary membrane and increase permeability, leading to progressive hypoxia and decreased lung compliance which are hallmarks of the syndrome. In acute arterial hypertension or experimental brain injury oxygen radicals are important mediators of vascular damage. The metabolism of arachidonic acid is the source of oxygen free radical production in these conditions. In myocardial ischemia and reperfusion injury, the ischemic myocyte is "primed" for free radical production. With reperfusion and reintroduction of molecular oxygen there is a burst of oxygen radical production resulting in extensive tissue destruction. Myocardial ischemia--reperfusion injury shares in common with the other two syndromes activation of the arachidonic acid cascade and acute inflammation. Thus it would appear that the generation of toxic oxygen species may represent a final common pathway of tissue destruction in several pathophysiologic states.     

Free radical scavengers suppress the accumulation of platinum in the cerebral cortex

We investigated whether free radical scavengers and antioxidants inhibit the accumulation of platinum (Pt) in the cerebral cortex. Pt was detected in the cerebral cortex of mice afters administration of cisplatin and exposure to short-term hypoxia. When mice were treated with either allopurinol (20 mg/kg) or catalase (100 mg/kg) before cisplatin administration and low oxygen exposure, Pt was not detected in the cerebral cortex. However, Pt was detected in the cerebral cortex of mice pretreated with either a low dosage of allopurinol or heat-denatured catalase. Furthermore, Pt was detected in the cerebral cortex of mice preadministered vitamin C, vitamin E, or deferoxamine. Lipid peroxide levels in the cerebral cortex increased 10 min after the treatment of hypoxia, and peaked 30 min after the treatment. These results suggested that short-term hypoxia produces free radicals, which allows Pt to pass through the blood-brain barrier and accumulate in the cerebral cortex, and that the production of free radicals is reduced by the administration of either allopurinol or catalase, which prevents Pt from passing through the barrier.     

Reactive Oxygen Species and the Central Nervous System

Radicals are species containing one or more unpaired electrons, such as nitric oxide (NO.). The oxygen radical superoxide (O2.-) and the nonradical hydrogen peroxide (H2O2) are produced during normal metabolism and perform several useful functions. Excessive production of O2.- and H2O2 can result in tissue damage, which often involves generation of highly reactive hydroxyl radical (.OH) and other oxidants in the presence of "catalytic" iron or copper ions. An important form of antioxidant defense is the storage and transport of iron and copper ions in forms that will not catalyze formation of reactive radicals. Tissue injury, e.g., by ischemia or trauma, can cause increased metal ion availability and accelerate free radical reactions. This may be especially important in the brain because areas of this organ are rich in iron and CSF cannot bind released iron ions. Oxidative stress on nervous tissue can produce damage by several interacting mechanisms, including increases in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that free radical reactions are involved in the neurotoxicity of aluminum and in damage to the substantia nigra in patients with Parkinson's disease are reviewed. Finally, the nature of antioxidants is discussed, it being suggested that antioxidant enzymes and chelators of transition metal ions may be more generally useful protective agents than chain-breaking antioxidants. Careful precautions must be used in the design of antioxidants for therapeutic use.     

Alpha-Phenyl-Tert-Butyl-Nitrone (PBN) Attenuates Hydroxyl Radical Production During Ischemia-Reperfusion Injury of Rat Brain: An EPR Study

α-phenyl-tert-butyl-nitrone (PBN) a spin adduct forming agent is believed to have a protective action in ischemia-reperfusion injury of brain by forming adducts of oxygen free radicals including ±OH radical. Electron paramagnetic resonance (EPR) has been used to both detect and monitor the time course of oxygen free radical formation in the in vivo rat cerebral cortex. Cortical cups were placed over both cerebral hemispheres of methoxyflurane anesthetized rats prepared for four vessel occlusion-evoked cerebral ischemia. Prior to the onset of sample collection, both cups were perfused with artificial cerebrospinal fluid (aCSF) containing the spin trap agent α-(4-pyridyl-1-oxide)-N-tert butylnitrone (POBN 100 mM) for 20 min. In addition 50 mg/kg BW of POBN was administered intraperitoneally (IP) 20 min prior to ischemia in order to improve our ability to detect free radical adducts. Cup fluid was subsequently replaced every 15 min during ischemia and every 10 min during reperfusion with fresh POBN containing CSF and the collected cortical superfusates were analyzed for radical adducts by EPR spectroscopy. After a basal 10 min collection, cerebral ischemia was induced for 15 or 30 min (confirmed by EEG flattening) followed by a 90 min reperfusion. -OH radical adducts (characterized by six line EPR spectra) were detected during ischemia and 90 min reperfusion. No adduct was detected in the basal sample or after 90 min of reperfusion. Similar results were obtained when diethylenetriaminepenta-acetic acid (100 μM; DETAPAC) a chelating agent was included in the artificial CSF. Systemic administration of PBN (100 mg/kg BW) produced a significant attenuation of radical adduct during reperfusion. A combination of systemic and topical PBN (100 mM) was required to suppress -OH radical adduct formation during ischemia as well as reperfusion. PBN free radical adducts were detected in EPR spectra of the lipid extracts of PBN treated rat brains subjected to ischemia/reperfusion. Thus this study suggests that PBN's protective action in cerebral ischemia/reperfusion injury is related to its ability to prevent a cascade of free radical generation by forming spin adducts.     

Alpha-Phenyl-Tert-Butyl-Nitrone (PBN) Attenuates Hydroxyl Radical Production During Ischemia-Reperfusion Injury of Rat Brain: An EPR Study

-phenyl-tert-butyl-nitrone (PBN) a spin adduct forming agent is believed to have a protective action in ischemia-reperfusion injury of brain by forming adducts of oxygen free radicals including ±OH radical. Electron paramagnetic resonance (EPR) has been used to both detect and monitor the time course of oxygen free radical formation in the in vivo rat cerebral cortex. Cortical cups were placed over both cerebral hemispheres of methoxyflurane anesthetized rats prepared for four vessel occlusion-evoked cerebral ischemia. Prior to the onset of sample collection, both cups were perfused with artificial cerebrospinal fluid (aCSF) containing the spin trap agent -(4-pyridyl-1-oxide)-N-tert butylnitrone (POBN 100 mM) for 20 min. In addition 50 mg/kg BW of POBN was administered intraperitoneally (IP) 20 min prior to ischemia in order to improve our ability to detect free radical adducts. Cup fluid was subsequently replaced every 15 min during ischemia and every 10 min during reperfusion with fresh POBN containing CSF and the collected cortical superfusates were analyzed for radical adducts by EPR spectroscopy. After a basal 10 min collection, cerebral ischemia was induced for 15 or 30 min (confirmed by EEG flattening) followed by a 90 min reperfusion. -OH radical adducts (characterized by six line EPR spectra) were detected during ischemia and 90 min reperfusion. No adduct was detected in the basal sample or after 90 min of reperfusion. Similar results were obtained when diethylenetriaminepenta-acetic acid (100 μM; DETAPAC) a chelating agent was included in the artificial CSF. Systemic administration of PBN (100 mg/kg BW) produced a significant attenuation of radical adduct during reperfusion. A combination of systemic and topical PBN (100 mM) was required to suppress -OH radical adduct formation during ischemia as well as reperfusion. PBN free radical adducts were detected in EPR spectra of the lipid extracts of PBN treated rat brains subjected to ischemia/reperfusion. Thus this study suggests that PBN's protective action in cerebral ischemia/reperfusion injury is related to its ability to prevent a cascade of free radical generation by forming spin adducts.     

Involvement of free radicals in cerebral vascular reperfusion injury evaluated in a transient focal cerebral ischemia model of rat

Free radicals have been suggested to be largely involved in the genesis of ischemic brain damage, as shown in the protective effects of alpha-phenyl-N-tert-butyl nitrone (PBN), a spin trapping agent, against ischemic cerebral injury. In the present study, the effects of PBN as well as MCI-186, a newly-developed free radical scavenger, and oxypurinol, an inhibitor of xanthine oxidase, were evaluated in a rat transient middle cerebral aretery (MCA) occlusion model to clarify the possible role of free radicals in the reperfusion injury of brain. The volume of cerebral infarction, induced by 2-h occlusion and subsequent 2-h reperfusion of MCA in Fisher-344 rats, was evaluated. The administration of PBN (100 mg/kg) and MCI-186 (100 mg/kg) just before reperfusion of MCA significantly reduced the infarction volume. In contrast, oxypurinol (100 mg/kg) failed to show any preventive effect on the infarction. These results suggest that free radical formation is involved in the cerebral damage induced by ischemia-reperfusion of MCA, and that hydroxyl radical is responsible for the reperfusion injury after transient focal brain ischemia. It is also suggested that xanthine oxidase is not a major source of free radicals.