Regional H2O2 concentration in rat brain after hyperoxic convulsions

GAT inhibition preserves cerebral blood flow and reduces oxidant damage to mitochondria in rodents exposed to extreme hyperbaric oxygen

Oxygen breathing at elevated partial pressures (PO2's) at or more than 3 atmospheres absolute (ATA) causes a reduction in brain γ-aminobutyric acid (GABA) levels that impacts the development of central nervous system oxygen toxicity (CNS-OT). Drugs that increase brain GABA content delay the onset of CNS-OT, but it is unknown if oxidant damage is lessened because brain tissue PO2 remains elevated during hyperbaric oxygen (HBO2) exposures. Experiments were performed in rats and mice to measure brain GABA levels with or without GABA transporter inhibitors (GATs) and its influence on cerebral blood flow, oxidant damage, and aspects of mitochondrial quality control signaling (mitophagy and biogenesis). In rats pretreated with tiagabine (GAT1 inhibitor), the tachycardia, secondary rise in mean arterial blood pressure, and cerebral hyperemia were prevented during HBO2 at 5 and 6 ATA. Tiagabine and the nonselective GAT inhibitor nipecotic acid similarly extended HBO2 seizure latencies. In mice pretreated with tiagabine and exposed to HBO2 at 5 ATA, nuclear and mitochondrial DNA oxidation and astrocytosis was attenuated in the cerebellum and hippocampus. Less oxidant injury in these regions was accompanied by reduced conjugated microtubule-associated protein 1A/1B-light chain 3 (LC3-II), an index of mitophagy, and phosphorylated cAMP response element binding protein (pCREB), an initiator of mitochondrial biogenesis. We conclude that GABA prevents cerebral hyperemia and delays neuroexcitation under extreme HBO2, limiting oxidant damage in the cerebellum and hippocampus, and likely lowering mitophagy flux and initiation of pCREB-initiated mitochondrial biogenesis.

A Biomimetic Plasmonic Nanoreactor for Reliable Metabolite Detection

Reliable monitoring of metabolites in biofluids is critical for diagnosis, treatment, and long-term management of various diseases. Although widely used, existing enzymatic metabolite assays face challenges in clinical practice primarily due to the susceptibility of enzyme activity to external conditions and the low sensitivity of sensing strategies. Inspired by the micro/nanoscale confined catalytic environment in living cells, the coencapsulation of oxidoreductase and metal nanoparticles within the nanopores of macroporous silica foams to fabricate all-in-one bio-nanoreactors is reported herein for use in surface-enhanced Raman scattering (SERS)-based metabolic assays. The enhancement of catalytical activity and stability of enzyme against high temperatures, long-time storage or proteolytic agents are demonstrated. The nanoreactors recognize and catalyze oxidation of the metabolite, and provide ratiometric SERS response in the presence of the enzymatic by-product H2O2, enabling sensitive metabolite quantification in a "sample in and answer out" manner. The nanoreactor makes any oxidoreductase-responsible metabolite a candidate for quantitative SERS sensing, as shown for glucose and lactate. Glucose levels of patients with bacterial infection are accurately analyzed with only 20 µL of cerebrospinal fluids, indicating the potential application of the nanoreactor in vitro clinical testing.

Transition to 37°C reveals importance of NADPH in mitigating oxidative stress in stored RBCs

The RBC storage lesion is a multiparametric response that occurs during storage at 4°C, but its impact on transfused patients remains unclear. In studies of the RBC storage lesion, the temperature transition from cold storage to normal body temperature that occurs during transfusion has received limited attention. We hypothesized that multiple deleterious events might occur in this period of increasing temperature. We show dramatic alterations in several properties of therapeutic blood units stored at 4°C after warming them to normal body temperature (37°C), as well as febrile temperature (40°C). In particular, the intracellular content and redox state of NADP(H) were directly affected by post-storage incubation at 37°C, as well as by pro-oxidant storage conditions. Modulation of the NADPH-producing pentose phosphate pathway, but not the prevention of hemoglobin autoxidation by conversion of oxyhemoglobin to carboxyhemoglobin, provided protection against storage-induced alterations in RBCs, demonstrating the central role of NADPH in mitigating increased susceptibility of stored RBCs to oxidative stress. We propose that assessing RBC oxidative status after restoration of body temperature constitutes a sensitive method for detecting storage-related alterations that has the potential to improve the quality of stored RBCs for transfusion.

Seizure frequency in more than 180,000 treatment sessions with hyperbaric oxygen therapy - a single centre 20-year analysis

INTRODUCTION

Hyperbaric oxygen therapy (HBOT) involves the risk of central nervous system oxygen toxicity (CNS-OT), including seizures in patients breathing oxygen at pressures ≥ 2 atmospheres absolute. This study aimed to determine the seizure frequency and assess the clinical benefit of a 5-minute air-break (5´-AIRBK).

METHODS

Twenty-year (1999-2018) retrospective analysis of all consecutive treatments with HBOT. Medical records were reviewed to determine patient demographics, comorbidities, HBOT indications, and seizure characteristics and timing. Seizure frequency was compared before and after incorporating a 5´-AIRBK in the treatment protocol. Chi-square testing was performed using SPSS (version 24.0); P < 0.05 was accepted as statistically significant.

RESULTS

We evaluated 188,335 HBOT sessions (74,255 before versus 114,080 after introducing a 5´-AIRBK). A total of 43 seizures were observed: 29 before and 14 after the 5´-AIRBK introduction (3.9 versus 1.2 per 10,000 treatments; P < 0.0001). Seizures occurred after a median of 57 (range 15-85) minutes following compression and after a median of 21 HBOT sessions (1-126). Patients experiencing seizures were undergoing treatment for: diabetic ulcer (n = 11); acute traumatic peripheral ischaemia (ATPI) (n = 6); non-diabetic ulcer (n = 5); sudden sensorineural hearing loss (n = 5); chronic refractory osteomyelitis (n = 5); radionecrosis (n = 3); necrotising fasciitis (NF) (n = 2); and haemorrhagic cystitis after allogeneic bone marrow transplantation (n = 1). ATPI and NF had a considerably higher relative frequency of seizures compared to other indications.

CONCLUSIONS

A statistically significant lower seizure frequency was achieved with a 5´-AIRBK. Assessing and defining the appropriate patient/treatment profile can be useful to minimise the risk of CNS-OT.

CNS function and dysfunction during exposure to hyperbaric oxygen in operational and clinical settings

Hyperbaric oxygen (HBO2) is breathed during hyperbaric oxygen therapy and during certain undersea pursuits in diving and submarine operations. What limits exposure to HBO2 in these situations is the acute onset of central nervous system oxygen toxicity (CNS-OT) following a latent period of safe oxygen breathing. CNS-OT presents as various non-convulsive signs and symptoms, many of which appear to be of brainstem origin involving cranial nerve nuclei and autonomic and cardiorespiratory centers, which ultimately spread to higher cortical centers and terminate as generalized tonic-clonic seizures. The initial safe latent period makes the use of HBO2 practical in hyperbaric and undersea medicine; however, the latent period is highly variable between individuals and within the same individual on different days, making it difficult to predict onset of toxic indications. Consequently, currently accepted guidelines for safe HBO2 exposure are highly conservative. This review examines the disorder of CNS-OT and summarizes current ideas on its underlying pathophysiology, including specific areas of the CNS and fundamental neural and redox signaling mechanisms that are thought to be involved in seizure genesis and propagation. In addition, conditions that accelerate the onset of seizures are discussed, as are current mitigation strategies under investigation for neuroprotection against redox stress while breathing HBO2 that extend the latent period, thus enabling safer and longer exposures for diving and medical therapies.

The Effects of Hyperbaric Oxygen at Different Pressures on Oxidative Stress and Antioxidant Status in Rats

Background: The optimal use of oxygen at greater than atmospheric pressures in any operational or therapeutic application (hyperbaric oxygen, HBO₂) requires awareness of the fact that the beneficial effects of oxygen coexist with toxic effects depending on the pressure and duration of exposure. In this study, we aimed to investigate the effect of HBO₂ therapy on oxidative stress and antioxidant status in commonly used protocol for acute HBO₂ indications, such as carbon monoxide intoxication, central retinal artery occlusion, crush injury, gas gangrene, and to compare it with normobaric oxygen (NBO₂) in healthy rats. Materials and Methods: Fifty-six male, young adult Wistar albino rats were randomly divided into seven groups and named as Group I through Group VII. Plasma malondialdehyde (MDA), superoxide dismutase (SOD), and erythrocyte glutathione (GSH) levels in control group were compared to the levels in other groups. Results: The increases in MDA levels and the decrease in SOD activities were statistically significant in HBO₂ groups at the end of the first 24 h when compared to the control group, and the significant decrease in erythrocyte GSH level was only at 2.4 atmospheres absolute. Conclusions: The present study showed that pressure and frequency of exposure are important factors to consider when investigating HBO₂-induced oxidative stress and antioxidant response.

Acute Hyperbaric Oxygenation, Contrary to Intermittent Hyperbaric Oxygenation, Adversely Affects Vasorelaxation in Healthy Sprague-Dawley Rats due to Increased Oxidative Stress

The present study was aimed at assessing endothelium-dependent vasorelaxation, at measuring superoxide production in the aorta and femoral artery, and at determining antioxidative enzyme expression and activity in aortas of male Sprague-Dawley rats (N = 135), randomized to an A-HBO₂ group exposed to a single hyperbaric oxygenation session (120′ of 100% O₂ at 2.0 bars), a 24H-HBO₂ group (single session, examined 24 h after exposure), a 4D-HBO₂ group (4 consecutive days of single sessions), and a CTRL group (untreated group). Vasorelaxation of aortic rings in response to acetylcholine (AChIR) and to reduced pO₂ (HIR) was tested in vitro in the absence/presence of NOS inhibitor L-NAME and superoxide scavenger TEMPOL. eNOS, iNOS, antioxidative enzyme, and NADPH oxidase mRNA expression was assessed by qPCR. Serum oxidative stress markers and enzyme activity were assessed by spectrometry, and superoxide production was determined by DHE fluorescence. Impaired AChIR and HIR in the A-HBO₂ group were restored by TEMPOL. L-NAME inhibited AChIR in all groups. Serum oxidative stress and superoxide production were increased in the A-HBO₂ group compared to all other groups. The mRNA expression of iNOS was decreased in the A-HBO₂ and 24H-HBO₂ groups while SOD1 and 3 and NADPH oxidase were increased in the 4D-HBO₂ group. The expression and activity of catalase and glutathione peroxidase were increased in the 4D-HBO₂ group as well. AChIR was NO dependent. Acute HBO₂ transiently impaired vasorelaxation due to increased oxidative stress. Vasorelaxation was restored and oxidative stress was normalized 24 h after the treatment.

Central Nervous System Oxygen Toxicity and Hyperbaric Oxygen Seizures

INTRODUCTION

The use of hyperbaric oxygen (O2) as a therapeutic agent carries with it the risk of central nervous system (CNS) O2 toxicity.

METHODS

To further the understanding of this risk and the nature of its molecular mechanism, a review was conducted on the literature from various fields.

RESULTS

Numerous physiological changes are produced by increased partial pressures of oxygen (Po2), which may ultimately result in CNS O2 toxicity. The human body has several equilibrated safeguards that minimize effects of reactive species on neural networks, believed to play a primary role in CNS O2 toxicity. Increased partial pressure of oxygen (Po2) appears to saturate protective enzymes and unfavorably shift protective reactions in the direction of neural network overstimulation. Certain regions of the CNS appear more susceptible than others to these effects. Failure to decrease the elevated Po2 can result in a tonic-clonic seizure and death. Randomized, controlled studies in human populations would require a multicenter trial over a long period of time with numerous endpoints used to identify O2 toxicity.

CONCLUSIONS

The mounting scientific evidence and apparent increase in the number of hyperbaric O2 treatments demonstrate a need for further study in the near future.

Baroreceptor afferents modulate brain excitation and influence susceptibility to toxic effects of hyperbaric oxygen

Unexplained adjustments in baroreflex sensitivity occur in conjunction with exposures to potentially toxic levels of hyperbaric oxygen. To investigate this, we monitored central nervous system, autonomic and cardiovascular responses in conscious and anesthetized rats exposed to hyperbaric oxygen at 5 and 6 atmospheres absolute, respectively. We observed two contrasting phases associated with time-dependent alterations in the functional state of the arterial baroreflex. The first phase, which conferred protection against potentially neurotoxic doses of oxygen, was concurrent with an increase in baroreflex sensitivity and included decreases in cerebral blood flow, heart rate, cardiac output, and sympathetic drive. The second phase was characterized by baroreflex impairment, cerebral hyperemia, spiking on the electroencephalogram, increased sympathetic drive, parasympatholysis, and pulmonary injury. Complete arterial baroreceptor deafferentation abolished the initial protective response, whereas electrical stimulation of intact arterial baroreceptor afferents prolonged it. We concluded that increased afferent traffic attributable to arterial baroreflex activation delays the development of excessive central excitation and seizures. Baroreflex inactivation or impairment removes this protection, and seizures may follow. Finally, electrical stimulation of intact baroreceptor afferents extends the normal delay in seizure development. These findings reveal that the autonomic nervous system is a powerful determinant of susceptibility to sympathetic hyperactivation and seizures in hyperbaric oxygen and the ensuing neurogenic pulmonary injury.

Effects of hydrogen peroxide on diazepam and xylazine sedation in chicks

Oxidative stress may cause various neuronal dysfunctions and modulate responses to many centrally acting drugs. This study examines the effects of oxidative stress produced by hydrogen peroxide (H₂O₂) on sedation induced by diazepam or xylazine as assessed in 7–14 day-old chicks. Day-old chicks were provided with either plane tap water (control group) or H₂O₂ in tap water as 0.5% v/v drinking solution for two weeks in order to produce oxidative stress. Spectrophotometric methods were used to determine glutathione and malondialdehyde concentrations in plasma and whole brain. Drug-induced sedation in the chicks was assessed by monitoring the occurrence of signs of sedation manifested as drooping of the head, closed eyelids, reduced motility or immotility, decreased distress calls, and recumbency. The latency to onset of sedation and its duration were also recorded. H₂O₂ treatment for two weeks significantly decreased glutathione and increased malondialdehyde concentrations in plasma and whole brain of the chicks on days 7, 10 and 14 as compared with respective age-matched control groups. H₂O₂ decreased the median effective doses of diazepam and xylazine for the induction of sedation in chicks by 46% and 63%, respectively. Injection of diazepam at 2.5, 5 and 10 mg/kg, i.m. or xylazine at 2, 4 and 8 mg/kg, i.m. induced sedation in both control and H₂O₂-treated chicks in a dose dependent manner, manifested by the above given signs of sedation. H₂O₂ significantly decreased the latency to onset of sedation in chicks treated with diazepam at 5 and 10 mg/kg, increased the duration of sedation and prolonged the total recovery time in comparison with respective non-stressed control chicks. A similar trend occurred with xylazine in the H₂O₂-treated chicks, though the differences from control counterparts did not attain the statistical significance, except for the recovery time of the lowest dose of the drug. The data suggest that H₂O₂-induced oxidative stress sensitizes the chicks to the depressant action of the sedatives diazepam and xylazine. Further studies are needed to examine the potential role of oxidative stress in modulating the actions of therapeutic agents on the brain.

Sleep/wake dependent changes in cortical glucose concentrations

Most of the energy in the brain comes from glucose and supports glutamatergic activity. The firing rate of cortical glutamatergic neurons, as well as cortical extracellular glutamate levels, increase with time spent awake and decline throughout non rapid eye movement sleep, raising the question whether glucose levels reflect behavioral state and sleep/wake history. Here chronic (2-3 days) electroencephalographic recordings in the rat cerebral cortex were coupled with fixed-potential amperometry to monitor the extracellular concentration of glucose ([gluc]) on a second-by-second basis across the spontaneous sleep-wake cycle and in response to 3 h of sleep deprivation. [Gluc] progressively increased during non rapid eye movement sleep and declined during rapid eye movement sleep, while during wake an early decline in [gluc] was followed by an increase 8-15 min after awakening. There was a significant time of day effect during the dark phase, when rats are mostly awake, with [gluc] being significantly lower during the last 3-4 h of the night relative to the first 3-4 h. Moreover, the duration of the early phase of [gluc] decline during wake was longer after prolonged wake than after consolidated sleep. Thus, the sleep/wake history may affect the levels of glucose available to the brain upon awakening.

Acute hyperoxia increases lipid peroxidation and induces plasma membrane blebbing in human U87 glioblastoma cells

Atomic force microscopy (AFM), malondialdehyde (MDA) assays, and amperometric measurements of extracellular hydrogen peroxide (H(2)O(2)) were used to test the hypothesis that graded hyperoxia induces measurable nanoscopic changes in membrane ultrastructure and membrane lipid peroxidation (MLP) in cultured U87 human glioma cells. U87 cells were exposed to 0.20 atmospheres absolute (ATA) O(2), normobaric hyperoxia (0.95 ATA O(2)) or hyperbaric hyperoxia (HBO(2), 3.25 ATA O(2)) for 60 min. H(2)O(2) (0.2 or 2 mM; 60 min) was used as a positive control for MLP. Cells were fixed with 2% glutaraldehyde immediately after treatment and scanned with AFM in air or fluid. Surface topography revealed ultrastructural changes such as membrane blebbing in cells treated with hyperoxia and H(2)O(2). Average membrane roughness (R(a)) of individual cells from each group (n=35 to 45 cells/group) was quantified to assess ultrastructural changes from oxidative stress. The R(a) of the plasma membrane was 34+/-3, 57+/-3 and 63+/-5 nm in 0.20 ATA O(2), 0.95 ATA O(2) and HBO(2), respectively. R(a) was 56+/-7 and 138+/-14 nm in 0.2 and 2 mM H(2)O(2). Similarly, levels of MDA were significantly elevated in cultures treated with hyperoxia and H(2)O(2) and correlated with O(2)-induced membrane blebbing (r(2)=0.93). Coapplication of antioxidant, Trolox-C (150 microM), significantly reduced membrane R(a) and MDA levels during hyperoxia. Hyperoxia-induced H(2)O(2) production increased 189%+/-5% (0.95 ATA O(2)) and 236%+/-5% (4 ATA O(2)) above control (0.20 ATA O(2)). We conclude that MLP and membrane blebbing increase with increasing O(2) concentration. We hypothesize that membrane blebbing is an ultrastructural correlate of MLP resulting from hyperoxia. Furthermore, AFM is a powerful technique for resolving nanoscopic changes in the plasma membrane that result from oxidative damage.

Hyperbaric oxygenation reduces overexpression of c-Fos and oxidative stress in the brain stem of experimental endotoxemic rats

OBJECTIVE

Septic encephalopathy is associated with an increased mortality rate in septic patients. We have previously shown that a peripheral lipopolysaccharide (LPS) injection induces neuronal activation in the brain-stem nuclei of rats. Nitric oxide (NO) and superoxide are involved in LPS-induced brain damage. Hyperbaric oxygenation (HBO) provides protective effects against systemic oxidative stress and mortality in animals with septic shock. We examined the effects of HBO on neuronal activation and oxidative stress in the brain-stem nuclei of LPS-treated rats.

DESIGN AND INTERVENTIONS

Wistar rats were randomly distributed into six groups for the following treatments:(a) normal saline injection (NS); (b) HBO; (c) LPS; (d) LPS-HBO; (e) LPS-aminoguanidine (AG, an inhibitor of inducible nitric oxide synthase); or (f) hydralazine (HYD, a direct vasodilator). The HYD induces prolonged hypotension and was used as a comparison for LPS stimulation. The AG was used as a comparison for HBO treatment. Two HBO sessions were administered, 1 and 4[Symbol: see text]h after LPS.

RESULTS

HBO and AG significantly reversed the overproduction of c-Fos induced by LPS in the brain stems of rats, with greater reversal in the nucleus tractus solitarii (NTS) by HBO. Although AG did not reduce the superoxide level, HBO significantly abolished superoxide production and NADPH diaphorase expression in the brain stems of LPS-treated rats. The HYD induced much lower c-Fos expression in the brain-stem nuclei than that in LPS-treated animals and caused no significant increase in NADPH diaphorase expression or superoxide formation.

CONCLUSION

HBO protects against endotoxin-related neuronal activation and oxidative stress in the brain-stem nuclei of rats.

A comparison of hyperbaric oxygen versus hypoxic cerebral preconditioning in neonatal rats

The potency of hyperbaric preconditioning (HBO-PC) is uncertain compared to well-validated ischemic or hypoxic models and no studies have directly compared HBO-PC to hypoxic preconditioning (HPC). We subjected rat pups to unilateral carotid cauterization followed by 90 min (min) of hypoxia using 8% O(2). Three HBO-PC regimes (maximum 2.5 atmospheres for 150 min) were compared to HPC (150 min of 8% O(2)) for changes in mortality and brain weight. Preconditioning-induced oxidative stress was assessed using aconitase activity and manganese superoxide dismutase (MnSOD) transcript levels. Initial brain weight data revealed a large coefficient of variation and compelled an examination of the temperature sensitivity of the model that revealed a narrow optimal range of 35 to 37 degrees C of variability in brain injury and mortality. With rigorous temperature control, high dose HBO-PC and HPC showed comparable anatomic (mean hemispheric weight decrease: control 42%, HPC 25% (P=0.01), HBO-PC 26% (P=0.01) and mortality protection (control 14.7%, HPC 5.9% HBO-PC 5.7%, P=0.001). High dose HBO-PC, but not HPC, suppressed aconitase activity by 65% at 24 h after the preconditioning stimulus (P=0.001). In contrast, MnSOD mRNA increased 2.5-fold at 24 h after HPC (P=0.007) but not after high dose HBO-PC. Thus, when temperature variability is eliminated, HBO-PC and HPC elicit similar preconditioning efficacy in neonatal brain but invoke different defenses against oxidative stress.

Oxygen-induced mitochondrial biogenesis in the rat hippocampus

The hypothesis that damage to mitochondrial DNA by reactive oxygen species increases the activity of nuclear and mitochondrial transcription factors for mitochondrial DNA replication was tested in the in vivo rat brain. Mitochondrial reactive oxygen species generation was stimulated using pre-convulsive doses of hyperbaric oxygen and hippocampal mitochondrial DNA content and neuronal and mitochondrial morphology and cell proliferation were evaluated at 1, 5 and 10 days. Gene expression was subsequently evaluated to assess nuclear and mitochondrial-encoded respiratory genes, mitochondrial transcription factor A, and nuclear respiratory transcription factors-1 and -2. After 1 day, a mitochondrial DNA deletion emerged involving Complex I and IV subunit-encoding regions that was independent of overt neurological or cytological O(2) toxicity, and resolved before the onset of cell proliferation. This damage was attenuated by blockade of neuronal nitric oxide synthase. Compensatory responses were found in nuclear gene expression for manganese superoxide dismutase, mitochondrial transcription factor A, and nuclear respiratory transcription factor-2. Enhanced nuclear respiratory transcription factor-2 binding activity in hippocampus was accompanied by a nearly three-fold boost in mitochondrial DNA content over 5 days. The finding that O(2) activates regional mitochondrial DNA transcription, replication, and mitochondrial biogenesis in the hippocampus may have important implications for maintaining neuronal viability after brain injury.

Effects of 7-nitroindazole and N-nitro-l-arginine methyl ester on changes in cerebral blood flow and nitric oxide production preceding development of hyperbaric oxygen-induced seizures in rats

Hyperbaric oxygen (HBO(2)) exposure induces increases in cerebral blood flow (CBF) and extracellular concentrations of nitric oxide (NO) that precede the appearance of central nervous system toxicity, which may manifest as convulsions. To elucidate the origins of NO production during HBO(2) exposure, we examined the effects of the selective neuronal NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), and the non-selective NOS inhibitor, N-nitro-l-arginine methyl ester (l-NAME), on changes in CBF and NO metabolites (NO(x), nitrite and nitrate) using a laser Doppler flow probe and in vivo microdialysis techniques, respectively. Rats were anesthetized, artificially ventilated, and pressurized to 5 atmosphere absolute (ATA) with pure oxygen for 60 min. In rats treated with vehicle, CBF and NO(x) levels in the cortex increased to 201% and 239% of basal levels, respectively, before the onset of electrical discharges, measured by electroencephalogram. The increase in CBF and NO(x) was completely inhibited by 7-NI and l-NAME. Both drugs also inhibited the appearance of electrical discharges for 60 min. Dynamic changes in CBF and NO(x) were not significantly different between 7-NI and l-NAME. These findings suggest that neuronal NOS is the main mediator of NO production associated with increase in CBF leading to the appearance of electrical discharge during HBO(2) exposure.

Methods for detection of reactive metabolites of oxygen and nitrogen: in vitro and in vivo considerations

Facile detection of reactive oxygen and nitrogen species in biologic systems is often problematic. This is a result of the numerous cellular mechanisms, both enzymatic and nonenzymatic involved in their catabolism/decomposition, the complex and overlapping nature of their reactivities, as well as the often limited intracellular access of detector systems. This review describes approaches to the direct and indirect measurement of different reactive metabolites of oxygen and nitrogen. Particular attention to a method's applicability for in vivo determinations will be addressed.

Hyperbaric oxygen therapy prevents vascular derangement during zymosan-induced multiple-organ-failure syndrome

OBJECTIVE

This study investigated the effects of hyperbaric oxygen (HBO) therapy on the cardiovascular alteration (e.g. mean arterial pressure, vascular reactivity of thoracic aorta rings changes) caused by zymosan in rats.

DESIGN

Rats.

SETTING

University research laboratory.

INTERVENTION AND MEASUREMENTS

We investigated the effects of HBO therapy (2 ATA at the fourth and eleventh hours after study onset) on the cardiovascular alteration caused by zymosan (500 mg/kg, administered i.p. as a suspension in saline) in rats. Cardiovascular alterations were assessed 18 h after administration of zymosan and/or HBO therapy.

RESULTS

Treatment of rats with HBO therapy attenuated the vasoplegic response to zymosan. In fact, the analysis of arterial pressure curves revealed no signs of vasoplegic shock. The aorta rings of animals treated with zymosan and HBO had a significantly increased contraction to norepinephrine (NE) and endothelin-1 (ET-1) and dilation to acetylcholine (ACh) compared with the zymosan group. The HBO therapy also attenuated the increase of malondialdehyde (MDA) levels caused by zymosan in the aorta. Immunohistochemical analysis for nitrotyrosine and for iNOS revealed positive staining in the aorta from zymosan-treated rats. The degree of staining for nitrotyrosine and iNOS was markedly reduced in tissue sections obtained from zymosan-rats treated with HBO therapy.

CONCLUSION

This study provides the first evidence that HBO therapy attenuates the degree of zymosan-induced cardiovascular derangement in the rat.

Hyperoxia, reactive oxygen species, and hyperventilation: oxygen sensitivity of brain stem neurons

Hyperoxia is a popular model of oxidative stress. However, hyperoxic gas mixtures are routinely used for chemical denervation of peripheral O2 receptors in in vivo studies of respiratory control. The underlying assumption whenever using hyperoxia is that there are no direct effects of molecular O2 and reactive O2 species (ROS) on brain stem function. In addition, control superfusates used routinely for in vitro studies of neurons in brain slices are, in fact, hyperoxic. Again, the assumption is that there are no direct effects of O2 and ROS on neuronal activity. Research contradicts this assumption by demonstrating that O2 has central effects on the brain stem respiratory centers and several effects on neurons in respiratory control areas; these need to be considered whenever hyperoxia is used. This mini-review summarizes the long-recognized, but seldom acknowledged, paradox of respiratory control known as hyperoxic hyperventilation. Several proposed mechanisms are discussed, including the recent hypothesis that hyperoxic hyperventilation is initiated by increased production of ROS during hyperoxia, which directly stimulates central CO2 chemoreceptors in the solitary complex. Hyperoxic hyperventilation may provide clues into the fundamental role of redox signaling and ROS in central control of breathing; moreover, oxidative stress may play a role in respiratory control dysfunction. The practical implications of brain stem O2 and ROS sensitivity are also considered relative to the present uses of hyperoxia in respiratory control research in humans, animals, and brain stem tissues. Recommendations for future research are also proposed.

Neuronal sensitivity to hyperoxia, hypercapnia, and inert gases at hyperbaric pressures

As ambient pressure increases, hydrostatic compression of the central nervous system, combined with increasing levels of inspired Po2, Pco2, and N2 partial pressure, has deleterious effects on neuronal function, resulting in O2 toxicity, CO2 toxicity, N2 narcosis, and high-pressure nervous syndrome. The cellular mechanisms responsible for each disorder have been difficult to study by using classic in vitro electrophysiological methods, due to the physical barrier imposed by the sealed pressure chamber and mechanical disturbances during tissue compression. Improved chamber designs and methods have made such experiments feasible in mammalian neurons, especially at ambient pressures <5 atmospheres absolute (ATA). Here we summarize these methods, the physiologically relevant test pressures, potential research applications, and results of previous research, focusing on the significance of electrophysiological studies at <5 ATA. Intracellular recordings and tissue Po2 measurements in slices of rat brain demonstrate how to differentiate the neuronal effects of increased gas pressures from pressure per se. Examples also highlight the use of hyperoxia (<or=3 ATA O2) as a model for studying the cellular mechanisms of oxidative stress in the mammalian central nervous system.

Hyperoxia-induced NAD(P)H oxidase activation and regulation by MAP kinases in human lung endothelial cells

Hyperoxia increases reactive oxygen species (ROS) production in vascular endothelium; however, the mechanisms involved in ROS generation are not well characterized. We determined the role and regulation of NAD(P)H oxidase in hyperoxia-induced ROS formation in human pulmonary artery endothelial cells (HPAECs). Exposure of HPAECs to hyperoxia for 1, 3, and 12 h increased the generation of superoxide anion, which was blocked by diphenyleneiodonium but not by rotenone or oxypurinol. Furthermore, hyperoxia enhanced NADPH- and NADH-dependent and superoxide dismutase- or diphenyleneiodonium-inhibitable ROS production in HPAECs. Immunohistocytochemistry and Western blotting revealed the presence of gp91, p67 phox, p22 phox, and p47 phox subcomponents of NADPH oxidase in HPAECs. Transfection of HPAECs with p22 phox antisense plasmid inhibited hyperoxia-induced ROS production. Exposure of HPAECs to hyperoxia activated p38 MAPK and ERK, and inhibition of p38 MAPK and MEK1/2 attenuated the hyperoxia-induced ROS generation. These results suggest a role for MAPK in regulating hyperoxia-induced NAD(P)H oxidase activation in HPAECs.

Polyamine oxidase activity in sera of depressed and schizophrenic patients after ECT treatment

High level of polyamine oxidase activity is detected in sera of depressed as well as in schizophrenic patients. ECT treatment of depressed and schizophrenic patients reduced significantly the level of polyamine oxidase activity in their sera. After ECT treatment, clinically improved depressed and schizophrenic subjects were found to have sera polyamine oxidase activity not significantly differ from that of normal subjects. Possible biochemical mechanisms, which link polyamine oxidase activity, schizophrenia, depression and ECT effect are discussed here.

Temporal profiles of the levels of endogenous antioxidants after four-vessel occlusion in rats

Although it is known that development of lipid peroxidation after ischemia occurs predominantly in vulnerable regions, temporal profiles of antioxidants after ischemia have not been regionally elucidated. After reperfusion periods of 0, 3, 24, and 72 hours following 20 minutes of four-vessel occlusion (n = 6 in each group), the concentration of total glutathione (GSH) and the activities of superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px) were assayed in the hippocampus, parietal cortex, striatum, thalamus, and brain stem. The levels of all antioxidants were unchanged in all regions without reperfusion; however, the concentration of total GSH significantly decreased in the hippocampus at 3 hours after the onset of reperfusion, and showed a maximum decrease in the hippocampus (68% of the sham-control level), parietal cortex (78% of the sham-control level), and striatum (76% of the sham-control level) after 24 hours of reperfusion. After 72 hours of reperfusion, these regions and the thalamus showed restoration and an increase in the total GSH concentration, respectively. The activities of SOD, GSH-Px, and catalase were stable during the reperfusion period, but the hippocampus showed significant increases in these enzyme activities and the parietal cortex and striatum showed significant increases in SOD activities at 72 hours after the onset of reperfusion. These results indicate that endogenous antioxidants take 72 hours for restoration in vulnerable regions after 20 minutes of four-vessel occlusion in rats.

Effect of hyperbaric oxygen treatment on nitric oxide and oxygen free radicals in rat brain

Oxygen (O(2)) at high pressures acts as a neurotoxic agent leading to convulsions. The mechanism of this neurotoxicity is not known; however, oxygen free radicals and nitric oxide (NO) have been suggested as contributors. This study was designed to follow the formation of oxygen free radicals and NO in the rat brain under hyperbaric oxygen (HBO) conditions using in vivo microdialysis. Male Sprague-Dawley rats were exposed to 100% O(2) at a pressure of 3 atm absolute for 2 h. The formation of 2,3-dihydroxybenzoic acid (2, 3-DHBA) as a result of perfusing sodium salicylate was followed as an indicator for the formation of hydroxyl radicals. 2,3-DHBA levels in hippocampal and striatal dialysates of animals exposed to HBO conditions were not significantly different from controls. However, rats treated under the same conditions showed a six- and fourfold increase in nitrite/nitrate, break down products of NO decomposition, in hippocampal and striatal dialysates, respectively. This increase was completely blocked by the nitric oxide synthase (NOS) inhibitor L-nitroarginine methyl ester (L-NAME). Using neuronal NOS, we determined the NOS O(2) K(m) to be 158 +/- 28 (SD) mmHg, a value which suggests that production of NO by NOS would increase approximately four- to fivefold under hyperbaric O(2) conditions, closely matching the measured increase in vivo. The increase in NO levels may be partially responsible for some of the detrimental effects of HBO conditions.

Distribution and kinetics of ethanol metabolism in rat brain

It was found that the accumulation of acetaldehyde produced from 50 mM ethanol in rat brain homogenates takes place in all major brain regions. The velocity varied between 3.5 to 7.1 nmol/mg of protein/hr. The rate increased in the following order: brain hemispheres, striatum, brainstem, hypothalamus, and cerebellum. Significant regional differences in this process were found: in the initial period of incubation (5 min), acetaldehyde accumulation was maximal in the brain hemispheres; but, in the 30- to 60-min period, it became significantly higher in the cerebellum. Inhibition of this process by the catalase inhibitor, 3-amino-1,2,4-triazole (8 mM), was minimal in the brainstem (27%) and maximal (57%) in the cerebellum, despite nearly complete inhibition of catalase. This would indicate that processes other than catalase activity must contribute to acetaldehyde accumulation.

Pulmonary vascular stress from carbon monoxide

Studies were conducted with rats to investigate whether exposure to carbon monoxide (CO) at concentrations frequently found in the environment caused lung injury mediated by nitric oxide (*NO)-derived oxidants. Lung capillary leakage was significantly increased 18 h after rats had been exposed to CO at concentrations of 50 ppm or more for 1 h. An elevation of *NO during CO exposure was demonstrated by electron paramagnetic resonance spectroscopy. There was a 2.6-fold increase of *NO over control in the lungs of rats exposed to 100 ppm CO. A qualitative increase in the concentration of H2O2 was also detected in lungs during CO exposure, and this change was caused by *NO as it was inhibited in rats pretreated with the nitric oxide synthase inhibitor, Nomega nitro-l-arginine methyl ester (l-NAME). Production of *NO-derived oxidants during CO exposure was indicated by an elevated concentration of nitrotyrosine in lung homogenates. The CO-associated elevations in lung capillary leakage and nitrotyrosine concentration did not occur when rats were pretreated with l-NAME. CO exposure did not change the concentrations of endothelial or inducible nitric oxide synthase in lung and leukocyte sequestration was not detected as a consequence of CO exposure. CO-mediated lung leak and nitrotyrosine elevation were not affected by neutropenia. We conclude that CO exposure elevates the steady-state concentration of *NO in lungs. Consequences from this change include increases in the concentration of reactive oxygen species, production of *NO-derived oxidants such as peroxynitrite, and physiological evidence of lung injury.

Nitro blue tetrazolium inhibits but does not mimic hypoxic vasoconstriction in isolated rabbit lungs

It has been suggested that hypoxic pulmonary vasoconstriction (HPV) may mainly proceed via loss of normoxic vasodilation, forwarded by tonic O2-dependent formation of nitric oxide and superoxide (23). Both agents may stimulate guanylate cyclase, the latter via conversion to hydrogen peroxide and formation of compound I with catalase. We probed this hypothesis in perfused rabbit lungs, employing the superoxide scavengers superoxide dismutase (SOD), 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), and nitro blue tetrazolium (NBT) and the catalase inhibitor aminotriazole (AT). NBT turned out to be a potent dose-dependent inhibitor of HPV in a concentration range of 200 nM to 1 microM, and superimposable dose-inhibition curves were obtained when lung synthesis of nitric oxide and vasodilatory prostanoids was preblocked by NG-monomethyl-L-arginine (L-NMMA) and acetylsalicylic acid (ASA). The NBT effect was specific because no inhibition in the vasoconstrictor responses to the stable thromboxane analog U-46619 and angiotensin II was observed. In contrast, SOD and Tiron were ineffective. AT exerted nonspecific inhibition of the hypoxia- and chemical vasoconstrictor-induced pressor responses. When applied under normoxic conditions, however, NBT alone or coapplied with L-NMMA or ASA, both for blockage of parallel vasodilatory pathways, did not mimic the hypoxia-induced vasoconstrictor response. In conclusion, the present study supports an important role for superoxide in the basic mechanism of HPV, but it questions the concept that loss of tonic vasorelaxation via this pathway is the underlying event in rabbit lungs. The mechanisms relating O2 tension-dependent superoxide and hydrogen peroxide generation to the vasoconstrictor event occurring in HPV remain to be further elucidated.

Ethanol metabolism in the brain

Acetaldehyde is suspected of being involved in the central mechanism of central nervous system depression and addiction to ethanol, but in contrast to ethanol, it can not penetrate easily from blood into the brain because of metabolic barriers. Therefore, the possibility of ethanol metabolism and acetaldehyde formation inside the brain has been one of the crucial questions in biomedical research of alcoholism. This article reviews the recent progress in this area and summarizes the evidence on the first stage of ethanol oxidation in the brain and the specific enzyme systems involved. The brain alcohol dehydrogenase and microsomal ethanol oxidizing systems, including cytochrome P450 II E1 and catalase are considered. Their physicochemical properties, the isoform composition, substrate specificity, the regional and subcellular distribution in CNS structures, their contribution to brain ethanol metabolism, induction under ethanol administration and the role in the neurochemical mechanisms of psychopharmacological and neurotoxic effects of ethanol are discussed. In addition, the nonoxidative pathway of ethanol metabolism with the formation of fatty acid ethyl esters and phosphatidylethanol in the brain is described.

Effect of ascorbic acid on hyperoxic rat astrocytes

The effect of ascorbic acid on cell size and ascorbic acid transport was studied in hyperoxic astrocytes. Subcultured rat astrocytes plated on poly-L-lysine-coated coverslips or on plastic dishes were exposed to serum-free culture medium and 20% or 42% ambient oxygen for 48 h. Vehicle (homocysteine) or L-ascorbic acid was added to the medium at 0 and 24 h. Cell size and relative optical density of glial fibrillary acidic protein-positive astrocytes were measured by a computerized imaging system. Cells on the dishes were used for ascorbic acid transport studies. Hyperoxia significantly increased the cell size of astrocytes, and this effect was inhibited by ascorbic acid. The rate of L-[14C]ascorbic acid Na(+)-dependent uptake was also inhibited by hyperoxia in vehicle-treated cultures but not in ascorbic acid-supplemented cultures. These results indicate that the presence of ascorbic acid during the hyperoxic episode can prevent astrocytic cell swelling and preserve membrane transport function.

Regional lipid peroxidation and protein oxidation in rat brain after hyperbaric oxygen exposure

Reactive oxygen species may participate in development of neurological toxicity resulting from hyperbaric oxygen exposure. To explore the possibility that increased reactive O2 metabolite generation may result in oxidative modification of lipids and proteins, rats were exposed to five atmospheres (gauge pressure) of O2 until development of an electroencephalographic seizure. Lipid peroxidation (as thiobarbituric acid-reactive substances) and protein oxidation (as 2,4-dinitrophenyl-hydrazones) were measured in five brain regions. Oxidized and reduced glutathione were also determined because of their role in regulating lipid peroxidation. Lipid peroxidation was confined to the frontal cortex and hippocampus, while protein oxidation (in both cytoplasmic and membranous fractions) and increased oxidized glutathione was evident throughout the brain. These results support a role for formation of reactive O2 metabolites from hyperbaric O2 exposure and suggest that protein oxidation, especially in soluble proteins, may be one of the most sensitive measures.

Nitration of tyrosine by hydrogen peroxide and nitrite

Peroxynitrite anion is a powerful oxidant which can initiate nitration and hydroxylation of aromatic rings. Peroxynitrite can be formed in several ways, e.g. from the reaction of nitric oxide with superoxide or from hydrogen peroxide and nitrite at acidic pH. We investigated pH dependent nitration and hydroxylation resulting from the reaction of hydrogen peroxide and nitrite to determine if this reaction proceeds at pH values which are known to occur in vivo. Nitration and hydroxylation products of tyrosine and salicyclic acid were separated with an HPLC column and measured using ultraviolet and electrochemical detectors. These studies revealed that this reaction favored hydroxylation between pH 2 and pH 4, while nitration was predominant between pH 5 and pH 6. Peroxynitrite is presumed to be an intermediate in this reaction as the hydroxylation and nitration profiles of authentic peroxynitrite showed similar pH dependence. These findings indicate that hydrogen peroxide and nitrite interact at hydrogen ion concentrations present under some physiologic conditions. This interaction can initiate nitration and hydroxylation of aromatic molecules such as tyrosine residues and may thereby contribute to the biochemical and toxic effects of the molecules.

Modulation of hydrogen peroxide release from vascular endothelial cells by oxygen

We have investigated factors that regulate hydrogen peroxide (H2O2) release from vascular endothelial cells. Endothelial cells produce H2O2 at an intracellular site in the vicinity of peroxisomes and at a second site near the cell surface that is inaccessible to intracellular catalase or glutathione peroxidase. Regulation of H2O2 generation at the intracellular site was studied using aminotriazole, which inactivates catalase in the presence of H2O2. Regulation of H2O2 generation at the second site was studied by measuring H2O2 release into the medium. The rate of H2O2 release was constant over 2 h when cells were incubated in room air. Changing O2 levels in the atmosphere from 0% to 10% O2 resulted in a threefold increase in the rate of H2O2 release. Elevation of O2 levels from 10% to 95% O2 produced no further enhancement in the rate of release. Preincubation of cells under hypoxic conditions did not lead to an exaggerated rate of H2O2 release when cells were returned to room air. Pretreatment of cells with exogenous H2O2 inhibited subsequent H2O2 release while pretreatment with catalase enhanced H2O2 release. Although arachidonic acid transiently enhanced the rate of H2O2 release through a mechanism dependent on PGH synthase, basal H2O2 release was independent of this enzyme. Neither hypoxia, hyperoxia, or hypoxia followed by reoxygenation altered H2O2 generation at the intracellular site accessible to peroxisomal catalase. These data demonstrate that H2O2 release from endothelial cells is responsive to changes in O2 concentrations over a narrow range. The mechanisms involved are subject to product inhibition and appear to be saturated at 10% O2 in the atmosphere.

Generation and detection of hydroxyl radical in vivo in rat spinal cord by microdialysis administration of Fenton's reagents and microdialysis sampling

We developed a double microdialysis fiber technique to generate hydroxyl radicals (OH.) in rat spinal cord. H2O2 and FeCl2/EDTA were pumped through two parallel microdialysis fibers inserted into the spinal cord such that the reactants mix in the tissue to generate OH. by the Fenton reaction. Generated OH. was detected by administering phenylalanine through one fiber and measuring o-, m- and p-hydroxyphenylalanine in collected dialysates by high pressure liquid chromatography and fluorescence detection. The hydroxyphenylalanines are produced by OH. attacking the phenylalanine. OH. generation was also accomplished in in vitro experiments and the results were consistent with in vivo experiments. This novel method to generate and measure OH. radical in vivo overcomes difficulties in studying damage to tissue by short-lived OH.. Although developed to study the role of OH. in spinal cord injury, this method could be used to study other diseases involving OH. damage.

The use of salicylate hydroxylation to detect hydroxyl radical generation in ischemic and traumatic brain injury. Reversal by tirilazad mesylate (U-74006F)

Oxygen free radicals have been implicated as a causal factor in posttraumatic neuronal cell loss following cerebral ischemia and head injury. The conversion of salicylate to dihydroxybenzoic acid (DHBA) in vivo was employed to study the formation of hydroxyl radical (.OH) following central nervous system (CNS) injury. Bilateral carotid occlusion (BCO) in gerbils and concussive head trauma in mice were selected as models of brain injury. The lipid peroxidation inhibitor, tirilazad mesylate (U-74006F), was tested for its ability to attenuate hydroxyl radical formation in these models. In addition, U-74006F was studied as a scavenger of hydroxyl radical in an in vitro assay based on the Fenton reaction. For in vivo experimentation, hydroxyl radical formation was expressed as the ratio of DHBA to salicylate (DHBA/SAL) measured in brain. In the BCO model, hydroxyl radical formation increased in whole brain with 10 min of occlusion followed by 1 min of reperfusion. DHBA/SAL was also found to increase in the mouse head injury model at 1 h postinjury. In both models, U-74006F (1 or 10 mg/kg) blocked the increase in DHBA/SAL following injury. In vitro, reaction of U-74006F with hydroxyl radical gave a product with a mol wt that was 16 greater than U-74006F, indicative of hydroxyl radical scavenging. We speculate that U-74006F may function by blocking oxyradical-dependent cell damage, and thereby maintaining free iron (which catalyzes hydroxyl radical formation) concentrations at normal levels.

Effect of hyperbaric oxygen on prostaglandin and thromboxane synthesis in the cortex and the striatum of rat brain

The effect of a previous exposure to hyperbaric oxygen (HBO) on the synthesis capacity of prostaglandin (PG) and thromboxane (TX) was investigated in the brain of male rats. Three groups of rats were used: 1. Neurotoxic HBO (n = 11): The rats were exposed to sixfold the atmospheric pressure (101.3 kPa), i.e., 6 absolute atmospheres (ATA), of pure O2 up to the first convulsion (6 ATA O2); 2. Mild hyperoxia (n = 10): The rats were exposed to compressed air at the same absolute pressure and for a similar time than that of the neurotoxic HBO group (here PO2 is 1.26 ATA); 3. Normoxia at atmospheric pressure (PO2 is 0.21 ATA) for control. There was no convulsion in groups 2 and 3. Decompression of the high pressure groups lasted 15 min. After decapitation, samples of the frontal cortex and the striatum were taken, weighed, washed, and then incubated in Krebs-Ringer bicarbonate for 1 h. The release of eicosanoids in the medium was determined by enzyme immunoassay. Mild hyperoxia only significantly reduced in the striatum the release of 6-keto-PGF1 alpha (1.3 +/- 2.4 vs 10.9 +/- 6.6 pg/mg wet tissue, p < 0.001; mean +/- SD) and PGE2 (3.2 +/- 2.7 vs 7.8 +/- 6.5 pg/mg wet tissue, p < 0.05), whereas TXB2 did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral amino acid, norepinephrine and nitric oxide metabolism in CNS oxygen toxicity

CNS oxygen (O2) toxicity is complex, and the etiology of its most severe manifestation, O2 convulsions, is yet to be determined. A role for depletion of the brain GABA pool has been proposed, although recent data have implicated production of reactive O2 species, e.g. H2O2, in this process. We hypothesized that the production of H2O2 and NH3 produced by monoamine oxidase (MAO) would lead to depletion of GABA and production of nitric oxide (NO.) respectively, and thereby enhance CNS O2 toxicity. In this study, rats treated with an MAO inhibitor (pargyline) or a nitric oxide synthase inhibitor (LNNA) were protected against O2-induced convulsions. Selected cerebral amino acids including arginine were measured in control and O2 treated rats (6 ATA, 20 min) with or without drug pretreatment. After O2 exposure, the cerebral pools of glutamate, aspartate, and GABA decreased significantly while glutamine content increased relative to control (P < 0.05). After treatment with either enzyme inhibitor, glutamine, glutamate and aspartate concentrations were maintained near control levels. Remarkably, GABA depletion by O2 was not prevented despite protection from seizures by both pargyline and LNNA. The NO. precursor, arginine, was increased significantly in the brain by toxic O2 exposure, but both pargyline and LNNA inhibited this effect. Simultaneous norepinephrine measurements indicated that its storage substantially decreased during hyperoxia (P < 0.05), but this effect too was blocked by either pargyline or LNNA. These data indicate that protection against O2 by these inhibitors is not related to preservation of the GABA pool. More importantly, O2 dependent norepinephrine metabolism and NO. synthesis appear to be interactive during CNS O2 toxicity.

Hydrogen peroxide production by monoamine oxidase during ischemia-reperfusion in the rat brain

Monoamine oxidase (MAO) as a source of hydrogen peroxide (H2O2) was evaluated during ischemia-reperfusion in vivo in the rat brain. H2O2 production was assessed with and without inhibition of MAO during and after 15 min of ischemia. Metabolism of H2O2 by catalase during ischemia and reperfusion was measured in forebrain homogenates using aminotriazole (ATZ), an irreversible H2O2-dependent inhibitor of catalase. Catecholamine and glutathione concentrations in forebrain were measured with and without MAO inhibitors. During ischemia, forebrain blood flow was reduced to 8% of baseline and H2O2 production decreased as measured at the microperoxisome. During reperfusion, a rapid increase in H2O2 generation occurred within 5 min as measured by a threefold increase in oxidized glutathione (GSSG). The H2O2-dependent rates of ATZ inactivation of catalase between control and ischemia-reperfusion were similar, indicating that H2O2 was more available to glutathione peroxidase than to catalase in this model. MAO inhibitors eliminated the biochemical indications of increased H2O2 production and increased the catecholamine concentrations. Mortality was 67% at 48 h after ischemia-reperfusion, and there was no improvement in survival after inhibition of MAO. We conclude that MAO is an important source of H2O2 generation early in brain reperfusion, but inhibition of the enzyme does not improve survival in this model despite ablating H2O2 production.

Mitochondrial oxidative stress after carbon monoxide hypoxia in the rat brain

To better understand the mechanisms of tissue injury during and after carbon monoxide (CO) hypoxia, we studied the generation of partially reduced oxygen species (PROS) in the brains of rats subjected to 1% CO for 30 min, and then reoxygenated on air for 0-180 min. By determining H2O2-dependent inactivation of catalase in the presence of 3-amino-1,2,4-triazole (ATZ), we found increased H2O2 production in the forebrain after reoxygenation. The localization of catalase to brain microperoxisomes indicated an intracellular site of H2O2 production; subsequent studies of forebrain mitochondria isolated during and after CO hypoxia implicated nearby mitochondria as the source of H2O2. In the mitochondria, two periods of PROS production were indicated by decreases in the ratio of reduced to oxidized glutathione (GSH/GSSG). These periods of oxidative stress occurred immediately after CO exposure and 120 min after reoxygenation, as indicated by 50 and 43% decreases in GSH/GSSG, respectively. The glutathione depletion data were supported by studies of hydroxyl radical generation using a salicylate probe. The salicylate hydroxylation products, 2,3 and 2,5-dihydroxybenzoic acid (DHBA), were detected in mitochondria from CO exposed rats in significantly increased amounts during the same time intervals as decreases in GSH/GSSG. The DHBA products were increased 3.4-fold immediately after CO exposure, and threefold after 120 min reoxygenation. Because these indications of oxidative stress were not prominent in the postmitochondrial fraction, we propose that PROS generated in the brain after CO hypoxia originate primarily from mitochondria. These PROS may contribute to CO-mediated neuronal damage during reoxygenation after severe CO intoxication.