Trauma-induced increase of extracellular ascorbate in rat cerebral cortex

Therapies targeting lipid peroxidation in traumatic brain injury

Lipid peroxidation can be broadly defined as the process of inserting a hydroperoxy group into a lipid. Polyunsaturated fatty acids present in the phospholipids are often the targets for peroxidation. Phospholipids are indispensable for normal structure of membranes. The other important function of phospholipids stems from their role as a source of lipid mediators - oxygenated free fatty acids that are derived from lipid peroxidation. In the CNS, excessive accumulation of either oxidized phospholipids or oxygenated free fatty acids may be associated with damage occurring during acute brain injury and subsequent inflammatory responses. There is a growing body of evidence that lipid peroxidation occurs after severe traumatic brain injury in humans and correlates with the injury severity and mortality. Identification of the products and sources of lipid peroxidation and its enzymatic or non-enzymatic nature is essential for the design of mechanism-based therapies. Recent progress in mass spectrometry-based lipidomics/oxidative lipidomics offers remarkable opportunities for quantitative characterization of lipid peroxidation products, providing guidance for targeted development of specific therapeutic modalities. In this review, we critically evaluate previous attempts to use non-specific antioxidants as neuroprotectors and emphasize new approaches based on recent breakthroughs in understanding of enzymatic mechanisms of lipid peroxidation associated with specific death pathways, particularly apoptosis. We also emphasize the role of different phospholipases (calcium-dependent and -independent) in hydrolysis of peroxidized phospholipids and generation of pro- and anti-inflammatory lipid mediators. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Ascorbic acid protects the newborn rat brain from hypoxic-ischemia

Ascorbic acid (AA) is a potent antioxidant, and its neuroprotective effect has not been established yet. Using the Rice-Vannucci model, we examined the effect of AA on hypoxic-ischemic (HI) injury in the immature rat brain. Under isoflurane anesthesia, 7-day-old rat pups received 750 mg/kg of AA by intraperitoneal injection just before hypoxic exposure; 8% oxygen for 90 min. Vehicle controls received an equal volume of saline. AA decreased a macroscopic brain injury score at 48 and 168 h post-HI compared with vehicle controls (48 h post-HI, AA 1.38+/-0.45 vs. controls 2.94+/-0.24, p<0.05; 168 h post-HI, 1.13+/-0.44 vs. 2.50+/-0.25, p<0.05). AA injection significantly decreased the number of both necrotic and apoptotic cells in cortex, caudate putamen, thalamus and hippocampus, and also seemed to reduce the number of TUNEL-positive cells. Western blot analysis showed that AA significantly suppressed 150/145 kDa subunits of alpha-fodrin breakdown products (FBDP) in cortex, striatum, thalamus and hippocampus at 24 and 48 h post-HI, and also 120 kDa subunit of FBDP in all examined regions except for thalamus, which indicated that AA injection inhibited both calpain and caspase-3 activation. Western blot analysis of nitrotyrosine failed to show inhibition of free radical production by AA, however, our results show that AA inhibits both necrotic and apoptotic cell death and that AA is neuroprotective after HI in immature rat brain.

Intraventricular ascorbic acid administration decreases hypoxic-ischemic brain injury in newborn rats

Neuronal cell damage following hypoxic-ischemic (HI) brain injury is partly caused by production of free radicals and reactive oxygen species (ROS). Ascorbic acid (AA) is a potent antioxidant, which scavenges various types of ROS. Some studies have shown that it is neuroprotective, however, the issue is still controversial. In this study, we examined the effect of intraventricular AA administration on immature HI brain using the Rice-Vannucci model. After unilateral carotid artery ligation under isoflurane anesthesia, 7-day-old rat pups received varying concentrations of AA (0.04, 0.2, 1 and 5 mg/kg) by intraventricular injection and were exposed to 8% oxygen for 90 min. Vehicle controls received an equal volume of phosphate saline buffer. We assessed the neuroprotective effect of AA at 7 days post-HI. The percent brain damage measured by comparing the wet weight of the ligated side of hemisphere with that of contralateral one was reduced in both 1 and 5 mg/kg groups but not in either 0.04 or 0.2 mg/kg groups compared to vehicle controls (5 mg/kg 16.0 +/- 4.3%, 1 mg/kg 10.9 +/- 5.0%, vs. controls 36.7 +/- 3.6%, P < 0.05). Macroscopic evaluation of brain injury revealed the neuroprotective effect of AA in both 1 and 5 mg/kg groups (5 mg/kg 1.1 +/- 0.4, 1 mg/kg 0.4 +/- 0.3, vs. controls 2.9 +/- 0.3, P < 0.05). Western blots of fodrin on the ligated side also showed that AA significantly suppressed 150/145-kDa bands of fodrin breakdown products, which suggested that AA suppressed activation of calpain. Neuropathological quantitative analysis of cell death revealed that 1 mg/kg of AA injection significantly reduced the number of necrotic cells in cortex, caudate putamen, thalamus and hippocampus CA1, whereas that of apoptotic cells was only reduced in cortex. These findings show that intraventricular AA injection is neuroprotective after HI in immature rats.

Iron inhibits neurotoxicity induced by trace copper and biological reductants

The extracellular microenvironment of the brain contains numerous biological redox agents, including ascorbate, glutathione, cysteine and homocysteine. During ischemia/reperfusion, aging or neurological disease, extracellular levels of reductants can increase dramatically owing to dysregulated homeostasis. The extracellular concentrations of transition metals such as copper and iron are also substantially elevated during aging and in some neurodegenerative disorders. Increases in the extracellular redox capacity can potentially generate neurotoxic free radicals from reduction of Cu(II) or Fe(III), resulting in neuronal cell death. To investigate this in vitro, the effects of extracellular reductants (ascorbate, glutathione, cysteine, homocysteine or methionine) on primary cortical neurons was examined. All redox agents except methionine induced widespread neuronal oxidative stress and subsequent cell death at concentrations occurring in normal conditions or during neurological insults. This neurotoxicity was totally dependent on trace Cu (>or=0.4 microM) already present in the culture medium and did not require addition of exogenous Cu. Toxicity involved generation of Cu(I) and H(2)O(2), while other trace metals did not induce toxicity. Surprisingly, administration of Fe(II) or Fe(III) (>or=2.5 microM) completely abrogated reductant-mediated neurotoxicity. The potent protective activity of Fe correlated with Fe inhibiting reductant-mediated Cu(I) and H(2)O(2) generation in cell-free assays and reduced cellular Cu uptake by neurons. This demonstrates a novel role for Fe in blocking Cu-mediated neurotoxicity in a high reducing environment. A possible pathogenic consequence for these phenomena was demonstrated by abrogation of Fe neuroprotection after pre-exposure of cultures to the Alzheimer's amyloid beta peptide (Abeta). The loss of Fe neuroprotection against reductant toxicity was greater after treatment with human Abeta1-42 than with human Abeta1-40 or rodent Abeta1-42, consistent with the central role of Abeta1-42 in Alzheimer's disease. These findings have important implications for trace biometal interactions and free radical-mediated damage during neurodegenerative illnesses such as Alzheimer's disease and old-age dementia.

In vivo voltammetry and concomitant electrophysiology at a single micro-biosensor to analyse ischaemia, depression and drug dependence

Electrochemical methods such as voltammetry can be used to understand patho-physiological mechanisms of action and, therefore, develop therapeutic approaches. In particular, voltammetry with treated micro-biosensors (carbon fibre micro-electrodes, mCFE) has been used to study models of (1) ischaemia; (2) drug dependence, and in particular craving; (3) depression. In addition, in studies (1) and (3) concomitant in vivo voltammetric and electrophysiological analysis has been performed by means of the same mCFE. Original data concerning ascorbate release in ischaemia, peptidergic activity during craving for drugs of abuse and concomitant voltammetric and electrophysiological changes of the serotonergic system in rats submitted to forced swimming test or to pharmacological treatment with the selective serotonin reuptake inhibitor fluoxetine are shown and discussed.

Closed head injury increases extracellular levels of antioxidants in rat hippocampus in vivo: an adaptive mechanism?

Reactive oxygen species (ROS) are a major cause of secondary brain injury following head trauma. Low molecular weight antioxidants (LMWA) protect the tissue against oxidative damage caused by ROS. In the present study, we measured the extracellular levels of the LMWA ascorbic acid and uric acid in the rat brain before, during and after experimental closed head injury (CHI). A dialysis probe was inserted into the right ventral hippocampus through a chronically implanted guide. CHI was applied to the left hemisphere using a weight-drop device. CHI induced a rapid but transient increase in ascorbic acid levels. Uric acid levels increased to 250% of baseline shortly after CHI and remained elevated at 2 h after CHI. Previous results show that the overall reducing power of brain tissue decreases following CHI. Together with previous results, the current findings suggest that ascorbic acid and uric acid are mobilized from brain cells to the extracellular space.

Microdialytical monitoring of uric and ascorbic acids in the brains of patients after severe brain injury and during neurovascular surgery

OBJECTIVES

Microdialysis has been extensively used to monitor brain metabolism in the extracellular fluid of patients with severe head injury, to detect the onset of secondary ischaemic damage. The aim was to investigate whether concentrations of uric and ascorbic acids were altered in such patients. Both these compounds play a part in free radical metabolism, which is accelerated after ischaemia and brain injury.

METHODS

Patients with aneurysm or bypass operations were monitored intraoperatively to assess concentrations in minimally disturbed tissue. Afterwards, 13 patients with severe head trauma were monitored for up to 13 days in the intensive care unit.

RESULTS

Intraoperatively, concentrations of both ascorbic and uric acids were significantly higher in the bypass group than in patients with aneurysm, which might be attributed to chronic ischaemic conditions caused by the unilateral occlusion of the carotid artery. In the patients with trauma, mean values of uric acid, varying between 6 microM and 180 microM, did not correlate with type of injury (contusion or diffuse) or duration of monitoring time. Patients who died had significantly higher concentrations of uric acid than those with a good outcome. Ascorbic acid could be detected only intermittently, probably due to technical problems. Concentrations of these two compounds could not be correlated with clinical findings during the course of monitoring.

CONCLUSIONS

Although uric and ascorbic acids are influenced by ischaemic conditions-for example, in bypass patients, neither compound is suitable for monitoring for free radical activity after severe head injury. Patients with a bad outcome tended to have higher concentrations of uric acid.

Increased extracellular ascorbate release reflects glutamate re-uptake during the early stage of reperfusion after forebrain ischemia in rats

Ascorbate is highly concentrated in neuropils, and its extracellular release is closely related to that of the excitatory neurotransmitters. Thus, the extracellular release of ascorbate and glutamate was measured during the early stage of forebrain ischemia-reperfusion in the rat hippocampus using a microdialysis biosensor system. Male Wistar rats were anesthetized with halothane under mechanical ventilation and normothermia. Two probes of the microdialysis biosensor electrode were inserted in the hippocampus bilaterally. One probe was perfused with phosphate-buffered saline (PBS) and the oxidation signal of dialyzed ascorbate was recorded. A second electropolymerized probe was perfused with PBS containing glutamate oxidase for glutamate measurement. Forebrain ischemia-reperfusion was performed by bilateral carotid artery occlusion with hemorrhagic hypotension (MAP=30 mmHg) for 10 min (Group 10, n=10) or 15 min (Group 15, n=10), followed by reperfusion for 60 min. The release of glutamate increased significantly to 294% (Group 10) and 334% (Group 15) during ischemia, and then decreased rapidly. In Group 15, however, it remained significantly higher after reperfusion than in Group 10. The release of ascorbate increased significantly to 504% (Group 10) and 334% (Group 15) after reperfusion. In Group 10, it was significantly higher for 5-15 min after reperfusion than in Group 15. The marked increase of ascorbate during reperfusion was associated with the rapid decrease in glutamate. The extended time of ischemia significantly inhibited glutamate re-uptake and ascorbate release during reperfusion. These findings suggest the extracellular ascorbate release during reperfusion after global ischemia as a marker of glutamate re-uptake.

Oxidative stress following traumatic brain injury in rats: quantitation of biomarkers and detection of free radical intermediates

Oxidative stress may contribute to many pathophysiologic changes that occur after traumatic brain injury. In the current study, contemporary methods of detecting oxidative stress were used in a rodent model of traumatic brain injury. The level of the stable product derived from peroxidation of arachidonyl residues in phospholipids, 8-epi-prostaglandin F(2alpha), was increased at 6 and 24 h after traumatic brain injury. Furthermore, relative amounts of fluorescent end products of lipid peroxidation in brain extracts were increased at 6 and 24 h after trauma compared with sham-operated controls. The total antioxidant reserves of brain homogenates and water-soluble antioxidant reserves as well as tissue concentrations of ascorbate, GSH, and protein sulfhydryls were reduced after traumatic brain injury. A selective inhibitor of cyclooxygenase-2, SC 58125, prevented depletion of ascorbate and thiols, the two major water-soluble antioxidants in traumatized brain. Electron paramagnetic resonance (EPR) spectroscopy of rat cortex homogenates failed to detect any radical adducts with a spin trap, 5,5-dimethyl-1-pyrroline N:-oxide, but did detect ascorbate radical signals. The ascorbate radical EPR signals increased in brain homogenates derived from traumatized brain samples compared with sham-operated controls. These results along with detailed model experiments in vitro indicate that ascorbate is a major antioxidant in brain and that the EPR assay of ascorbate radicals may be used to monitor production of free radicals in brain tissue after traumatic brain injury.

Continuous real-time measurement of extracellular ascorbate release in the rat striatum in vivo during forebrain ischemia-reperfusion

Apart from its physiological role as a major antioxidant, ascorbate is highly concentrated in neuropils and ascorbate-mediated protection from excitotoxins has been demonstrated in vitro. Therefore, extracellular release of ascorbate during the early stage of ischemia-reperfusion was measured using a microdialysis electrode technique. One or two probes of the microdialysis biosensor were inserted into the rat striatum. One probe (n=16) was perfused with phosphate-buffered saline (PBS) for continuous oxidative signal recording. A second electropolymerised probe inserted into the other side of the striatum was perfused with PBS containing ascorbate oxidase in six rats. Forebrain ischemia-reperfusion was performed for 10min, followed by reperfusion for 60min. Ascorbate increased transiently during ischemia, and markedly to a maximum of 247.5+/-55. 8 microM from the baseline of 68.5+/-25.3 microM after reperfusion. The marked increase of extracellular ascorbate may be a marker of the early stage of reperfusion.

Sodium-ascorbate cotransport controls intracellular ascorbate concentration in primary astrocyte cultures expressing the SVCT2 transporter

Expression of the Na(+)-ascorbate cotransporter, SVCT2, was detected in rat brain and in primary cultures of cerebral astrocytes by Northern blot analysis. SVCT2 expression in cultured astrocytes increased in response to the cyclic AMP analog, dibutyryl cyclic AMP. A mathematical model of ascorbic acid transport was developed to evaluate the hypothesis that Na(+)-ascorbate cotransport across the plasma membrane regulates the steady state intracellular concentration of ascorbic acid in these cells. The outcomes predicted by this model were compared to experimental observations obtained with primary cultures of rat cerebral astrocytes exposed to normal and pathologic conditions. Both cotransport activity and intracellular ascorbic acid concentration increased in astrocytes activated by dibutyryl cyclic AMP. Conversely transport activity and ascorbic acid concentration were decreased by hyposmotic cell swelling, low extracellular Na(+) concentration, and depolarizing levels of extracellular K(+). In cells incubated for up to 3 h in medium having an ascorbic acid concentration typical of brain extracellular fluid, the changes in intracellular ascorbic acid concentration actually measured were not significantly different from those predicted by modeling changes in Na(+)-ascorbate cotransport activity. Thus, it was not necessary to specify alterations in vitamin C metabolism or efflux pathways in order to predict the steady state intracellular ascorbic acid concentration. These results establish that SVCT2 regulates intracellular ascorbic acid concentration in primary astrocyte cultures. They further indicate that the intracellular-to-extracellular ratio of ascorbic acid concentration at steady state depends on the electrochemical gradients of Na(+) and ascorbate across the plasma membrane.

Glutamate stimulates ascorbate transport by astrocytes

The concentrations of glutamate and ascorbate in brain extracellular fluid increase following seizure activity, trauma and ischemia. Extracellular ascorbate concentration also rises following intracerebral glutamate injection. We hypothesized that glutamate triggers the release of ascorbate from astrocytes. We observed in primary cultures of rat cerebral astrocytes that glutamate increased ascorbate efflux significantly within 30 min. The half-maximal effective concentration of glutamate was 180+/-30 microM. Glutamate-stimulated efflux of ascorbate was attenuated by hypertonic media. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited both Na(+)-dependent glutamate uptake and ascorbate efflux. Two other inhibitors of volume-sensitive organic anion channels (1, 9-dideoxyforskolin and 5-nitro-2-(3-phenylpropylamino) benzoic acid) did not slow glutamate uptake but prevented stimulation of ascorbate efflux. Glutamate also stimulated the uptake of ascorbate by ascorbate-depleted astrocytes. In contrast, glutamate uptake was not affected by intracellular ascorbate, thus ruling out a putative glutamate-ascorbate heteroexchange mechanism. These results are consistent with activation by glutamate of ascorbate-permeant channels in astrocytes.

The impact of genetic removal of GFAP and/or vimentin on glutamine levels and transport of glucose and ascorbate in astrocytes

The importance of the intermediate filament (IF) proteins glial fibrillary acidic protein (GFAP) and vimentin for astrocyte function was studied by investigating astrocytes prepared from GFAP-/- and/or vimentin-/- mice. The rate of glucose uptake through facilitative hexose transporters was not affected by depletion of GFAP or vimentin. Similarly, the absence of these IF proteins did not affect ascorbate uptake, under control or cyclic AMP-stimulated conditions, or ascorbate efflux through volume-sensitive organic anion channels. However, compared with wild-type astrocytes, glutamine concentrations were increased up to 200% in GFAP-/- astrocytes and up to 150% in GFAP+/- astrocytes and this increase was not dependent on the presence of vimentin. GFAP-/- astrocytes in culture still contain IFs (made of vimentin and nestin), whereas GFAP-/- vim-/- cultured astrocytes lack IFs. Thus, glutamine levels appear to correlate inversely with GFAP, rather than depend on the presence of IFs per se. Furthermore, the effect of GFAP is dose-dependent since the glutamine concentration in GFAP+/- astrocytes falls between those in wild-type and GFAP-/- astrocytes.

Kainic acid causes redox changes in cerebral cortex extracellular fluid: NMDA receptor activity increases ascorbic acid whereas seizure activity increases uric acid

Kainic acid (KA) causes seizures and extensive brain damage in rats. To study the effects of KA on the redox state in cerebral cortex extracellular fluid (ECF), ascorbic and uric acid concentrations were measured in intracerebral microdialysis samples before and after systemic KA administration (ip). During seizures, concentrations of ascorbic and uric acid increased 500 and 100%, respectively. When midazolam was given with KA to prevent seizures, ascorbic acid still increased 400%, but uric acid increased only transiently. When the NMDA receptor antagonist aminophosphonovaleric acid (APV) was included in the microdialysis perfusion media, ascorbic acid levels decreased during baseline perfusion in a concentration-dependent manner. APV then suppressed the KA-induced increase in ascorbic acid levels, without blocking seizure activity. In summary, increased uric acid levels in brain ECF activity after KA administration are related to the induced seizure, but ascorbic acid levels are associated with NMDA receptor activity.

Cerebral penetration injury leads to H2O2 generation in microdialysis samples

Delayed tissue damage is proposed to be caused by reactive oxygen species. We investigated the effects of microdialysis probe penetration into rat piriform cortex on hydrogen peroxide (H2O2) in brain extracellular fluid (ECF). H2O2 decreased immediately after probe insertion into the brain, but increased over 300% in samples within minutes after collection. We assessed H2O2 changes in vitro in microdialysis perfusion media containing various ascorbic acid concentrations and confirmed ascorbic acid is a source of H2O2. We conclude that decreased H2O2 concentrations in perfusion media as it passes through the brain reflect an extracellular antioxidant effect, whereas the increase in H2O2 with time after sample collection indicates that H2O2 generating substances are present in ECF. Thus, the potential for producing reactive oxygen species in brain ECF exists following penetration injury, especially if transition metals are released into the neuronal microenvironment.

Oxidative stress following traumatic brain injury in rats

BACKGROUND

Free radicals may be involved in the pathophysiology of traumatic brain injury (TBI) through oxidative damage of neurovascular structures. Endogenous antioxidants, such as ascorbate and alpha-tocopherol, may play a critical role in combating these oxidative reactions and their oxidized products can serve as an important index of oxidative stress.

METHODS

We used electron spin resonance (ESR) spectroscopy and in vivo spin trapping (reaction of an organic compound with free radical species) to detect the possible generation of free radicals after TBI. Injury was inflicted by a weight drop technique over the head (5.7 kg-cm). Rats were intravenously infused with either 1 mL, 0.1 M of the spin trap, alpha-phenyl-N-tert-butyl nitrone (PBN), or an equivalent volume of saline immediately before TBI or sham-injury. Animals were divided into four groups: (1) Group I: PBN-infused sham-injured, (2) Group II: PBN-infused injured, (3) Group III: saline-infused sham-injured, and (4) Group IV: saline-infused injured. Additional groups of saline-infused uninjured, saline-infused, and PBN-infused injured animals were used for histopathology. Sixty minutes after TBI or sham-injury, rats were again anesthetized and decapitated. The brains were removed within 1 minute, homogenized, and extracted for lipids. The extracts were analyzed by ESR spectroscopy. Brain ascorbic acid (AA) concentration was determined spectrophotometrically, using the ascorbate oxidase assay.

RESULTS

No PBN spin adduct signals (indicating trapped free radical species) were visible 60 minutes after TBI. All groups of rats showed an ascorbyl free radical signal. The ascorbyl signal intensity (AI) was, however, significantly higher in the injured rats, while the brain (AA) was significantly reduced. In addition, the ratio of AI/AA, which eliminates the effect of variable ascorbate concentrations in the brain, was also significantly higher in the injured animals.

CONCLUSIONS

We conclude that 60 minutes following TBI there was a significantly increased level of oxidative stress in the brain. This may reflect formation of free radical species with subsequent interaction with ascorbate (antioxidant) during the 60 minute period. The lack of PBN spin adduct signals 1 hour after TBI may indicate that free radical generation is time dependent and might be detectable earlier or later than the 60 minute period.

Redox changes in perfusates following intracerebral penetration of microdialysis probes

Microdialysis probe insertion into rat cerebral cortex significantly affects the levels of redox-active substances in brain extracellular fluid. Ascorbic acid levels are high immediately after probe insertion, decline rapidly, and then rise as the rat recovers from anesthesia 5-8 hours after surgery. Uric acid is at a low level for 5 hours and then rapidly increases in parallel with ascorbic acid. High ascorbic acid levels immediately after probe insertion are likely due to a shift from intracellular to extracellular fluids, whereas the delayed increase in uric acid may be due to increased enzymatic formation. After removal from the brain, hydrogen peroxide (H2O2) in microdialysis samples produces catalase-sensitive oxidative chemiluminescence. Microdialysis samples also produce high level catalase-resistant chemiluminescence associated with ascorbic acid levels after penetration injury. Although ascorbic acid is likely an antioxidant at concentrations estimated to be in brain extracellular fluid, it may have prooxidant effects when complexed with transition metals released into the neuronal microenvironment during traumatic brain injury.

Ascorbate availability and neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease in which upper and lower motoneurons progressively deteriorate and die. Neuronal damage is most evident in the lower central nervous system, and death generally occurs following central respiratory failure. Proposed and demonstrated mechanisms for amyotrophic lateral sclerosis are diverse, and include altered superoxide dismutase and neurofilament proteins, autoimmune attack, and hyperglutamatergic activity. However, they do not account for the late onset of the disease, its earlier onset in males, and the differential vulnerability of neurons located in the brainstem and spinal cord. It is proposed here that, within the context of a specific defect such as altered superoxide dismutase, age-dependent decline in ascorbate availability triggers the disease. A role for ascorbate, which is found in millimolar levels in neurons, is suggested by a number of consistencies: 1) superoxide radicals being a common substrate for superoxide dismutase and ascorbate; 2) a close association between central nervous system ascorbate levels and injury tolerance; 3) a steady decline in ascorbate plasma levels and cellular availability with age; 4) plasma ascorbate levels being lower in males; 5) an association of ascorbate release with motor activity in central nervous system regions, in vivo; 6) the coupling of brain-cell ascorbate release with glutamate uptake; 7) possible roles for ascorbate modulation of N-methyl-D-aspartate receptor activity; 9) the ability of ascorbate to prevent peroxynitrite anion formation; and 10) evidence supporting the scorbutic guinea pig as a model for amyotrophic lateral sclerosis. Emphasis is placed on the probable competition between superoxide dismutase and ascorbate within the context of a primary defect of metal-binding or metal access in high-concentration proteins such as superoxide dismutase and human heavy neurofilaments. Finally, distinct features of alpha-motoneuronal physiology suggest that cell physiological characteristics such as high metabolic activity and extensive calcium dynamics may render neurons differentially vulnerable in amyotrophic lateral sclerosis.

Concomitant in vivo electrophysiological and voltammetric analysis indicate that ascorbic acid is a biochemical index of early ischaemia

A number of in vitro studies or in vivo cortical microdialysis measurements have observed that changes in the levels of ascorbic acid (AA), uric acid (UA), tryptophan (TRY), indoles and other compounds may be biochemical markers of cerebral ischaemic damages following occlusion of the rat middle cerebral artery (MCAO). The aim of the present work was to study the influence of early ischaemia upon presynaptic and postsynaptic activities in the cerebral cortex of rats. These activities have been studied by means of electrophysiological and electro-biochemical (voltammetric) measurements performed concomitantly every 5 min and applied with the same biosensor. The biosensor was inserted in the cerebral cortex of anaesthetised adult male rats which were then submitted to focal ischaemia via MCAO. Since changes in electrophysiological activity are considered marker of rise of ischaemia, the choice of simultaneous electrophysiological and electrochemical (voltammetric) analysis could allow the observation of specific biochemical(s) correlation(s) with the initial phase of ischaemia. The data obtained indicated that electrophysiological and voltammetric changes can be monitored simultaneously in the same brain region (i.e. effected by MCAO) by means of a single biosensor with an improved time resolution when compared with previous biochemical in vivo studies. In addition, a high correlation was observed between MCAO reduced functional responses of the neurons monitored by electrophysiology and increased levels of AA measured by voltammetry. This original observation suggests that AA is a biochemical marker of the very early stages of focal ischaemia and could be a useful tool for the evaluation of initial ischaemic damage.

Detection of the lipid peroxidation product malonaldehyde in rat brain in vivo

The extracellular concentrations of the lipid peroxidation product malonaldehyde (MDA) and the antioxidants ascorbic and uric acid were measured in rat brain in vivo using microdialysis coupled to HPLC with ultra violet spectrophotometry. Treatment with kainic acid at pH 4.1 (50 nmol) caused a significant increase in the sampled concentration of MDA but no significant changes in the antioxidants. Treatment with the same dose of kainic acid at pH 7.2 did not cause a significant increase in MDA, although some changes were noted in the antioxidants. The paper demonstrates the ability to monitor changes of a lipid peroxidation breakdown product as a measure of oxidative stress in vivo. Furthermore, the data suggests that the toxic action of kainic acid in acute preparations may be due to the elevation of hydrogen ions.

Ascorbate potentiates amphetamine-induced conditioned place preference and forebrain dopamine release in rats

In order to evaluate the effects of ascorbate, which is known to modulate dopamine neurotransmission, on the reinforcing effects of amphetamine, we coadministered ascorbate and amphetamine during the acquisition of conditioned place preference (CPP) in rats. Our results indicate that 100 mg/kg ascorbate potentiates the CPP induced by 0.5 mg/kg, but not 1.0 mg/kg, amphetamine. A higher dose of ascorbate (500 mg/kg) did not influence the CPP induced by either dose of amphetamine. In vitro release assays revealed that, whereas ascorbate alone (0.01-1.0 mM) did not influence striatal dopamine levels, this vitamin potentiated amphetamine-induced dopamine release in both the nucleus accumbens and neostriatum. Collectively, these results raise the possibility that ascorbate potentiates amphetamine-induced CPP by increasing the ability of this psychostimulant to release dopamine.

Biochemical studies of isolated mitochondria from normal and diseased tissues

Isolated mitochondria have served as useful tools for identifying the site(s) of impairment associated with respiratory chain-linked oxidative phosphorylation at the molecular level. Over the last three decades, a number of diseases associated with mitochondrial dysfunction have been identified. The literature is large and diverse. This paper presents a brief survey of the current state of knowledge concerning biochemical studies of mitochondrial diseases associated with skeletal muscle, such as mitochondrial myopathies and, with brain injury such as that induced by ischemia/reperfusion. Various mitochondrial preparations and assay conditions are evaluated. The importance of fresh tissue for the isolation of tightly coupled mitochondria and the selection of suitable assay conditions for characterization have been demonstrated. Appropriate methodologies for isolation and characterization of tightly coupled mitochondria from both skeletal muscle and brain are presented.

Direct effects of reactive oxygen species on cochlear outer hair cell shape in vitro

Reactive oxygen species (ROS) have been implicated in the ototoxicity of various agents. This study examines the effects of superoxide anion (O2), hydroxyl radical (OH.) and hydrogen peroxide (H2O2), on isolated cochlear outer hair cell (OHC) morphology. OHCs were superfused with artificial perilymph (AP) or AP containing a specific ROS scavenger, and then with AP, ROS system or scavenger plus ROS system for 90 min. The generation of ROS as well as the scavenging properties of other agents were confirmed by specific biochemical assays. Control cells decreased 4.8% in mean length, and showed no obvious membrane damage. Generation of O2. or OH. resulted in high rates (85.7 and 42.9%, respectively) of bleb formation at the synaptic pole, and decreased (O2., 15.2%; OH., 17.3%) mean cell length. Length change and bleb formation rate were H2O2 concentration-dependent. 20 mM H2O2 led to 33.3% decreased mean cell length, and only 20% bleb formation; 0.1 mM H2O2 led to 83.3% bleb formation, with no length decrease. Superoxide dismutase, deferoxamine and catalase protected against O2., OH. and H2O2 effects, respectively. Bleb formation and diminished cell length likely represent differential lipid peroxidative outcomes at supra- and infranuclear membranes, and are consistent with effects of certain ototoxicants.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Ascorbic acid in the brain

Ascorbic acid is highly concentrated in the central nervous system. Measurement of the extracellular concentration of ascorbate in animals, mainly by the technique of voltammetry in vivo, has demonstrated fluctuation in release from neuropil, both spontaneously and in response to physical stimulation of the animal and to certain drugs. Although in the adrenal medulla ascorbate is co-released with catecholamines, release of ascorbate from brain cells is associated principally with the activity of glutamatergic neurones, mainly by glutamate-ascorbate heteroexchange across cell membranes of neurones or glia. This phenomenon is discussed in relation to a possible role of ascorbate as a neuromodulator or neuroprotective agent in the brain.

Free radical generation in the brain precedes hyperbaric oxygen-induced convulsions

We tested the hypothesis that hyperbaric oxygenation (HBO) generates free radicals in the brain before the onset of neurological manifestations of central nervous system (CNS) oxygen poisoning. Chronically cannulated, conscious rats were individually placed in a transparent pressure chamber and exposed to (1) 5 atmospheres absolute (ATA) oxygen for 15 min (n = 4); (2) 5 ATA oxygen for 30 min (n = 5), during which no visible convulsions occurred; (3) 5 ATA oxygen for 30 min with recurrent convulsions (n = 6); (4) 5 ATA oxygen until the appearance of the first visible convulsions (n = 5); (5) 4 ATA oxygen for 60 min during which no convulsions occurred (n = 5); and (6) 5 ATA air for 30 min (n = 5, controls). Immediately before compression, 1 mL of 0.1 M of alpha-phenyl-N-tert-butyl nitrone (PBN) was administered intravenously (iv) for spin trapping. At the termination of each experiment, rats were euthanized by pentobarbital iv and decompressed within 1 min. Brains were rapidly removed for preparation of lipid extracts (Folch). The presence of PBN spin adducts in the lipid extracts was examined by electron spin resonance (ESR) spectroscopy. ESR spectra from unconvulsed rats exposed to 5 ATA oxygen for 30 min revealed both oxygen-centered and carbon-centered PBN spin adducts in three of the five brains. One of the five rats in this group showed an ascorbyl signal in the ESR spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

The osmotic/calcium stress theory of brain damage: are free radicals involved?

This overview presents data showing that glucose use increases and that excitatory amino acids (i.e., glutamate, aspartate), taurine and ascorbate increase in the extracellular fluid during seizures. During the cellular hyperactive state taurine appears to serve as an osmoregulator and ascorbate may serve as either an antioxidant or as a pro-oxidant. Finally, a unifying hypothesis is given for seizure-induced brain damage. This unifying hypothesis states that during seizures there is a release of excitatory amino acids which act on glutamatergic receptors, increasing neuronal activity and thereby increasing glucose use. This hyperactivity of cells causes an influx of calcium (i.e., calcium stress) and water movements (i.e., osmotic stress) into the cells that culminate in brain damage mediated by reactive oxygen species.

Extracellular antioxidants and amino acids in the cortex of the rat: monitoring by microdialysis of early ischemic changes

Extracellular concentrations of ascorbic acid, glutathione, cysteine, uric acid, tyrosine, and tryptophan were monitored using intracerebral microdialysis in the left frontoparietal cortex of spontaneous hypertensive rats before, and for 3 h after, either focal ischemia [left middle cerebral artery occlusion (MCAO)] or sham operation. The size of the ischemic area and the position of the microdialysis probe were checked using the enzyme histotopochemical acid phosphatase reaction. The probe was always located in the cortex inside the stained area. Ascorbic acid levels rose immediately after MCAO and remained at about 12-fold for 3 h. There was a transient release of glutathione during 1-1.5 h. Uric acid concentrations were also increased but the differences did not reach significance. The levels of the amino acids tyrosine and tryptophan increased steadily after MCAO. The increases in cysteine were variable but significant. In some experiments, the pH of the dialysate was measured online. The parameters ascorbic acid, glutathione, cysteine, and pH are suitable for the early detection of cortical ischemic events by microdialysis.

Interstitial and cerebrospinal fluid levels of energy-related metabolites after middle cerebral artery occlusion in rats

This investigation was designed to study the dynamics of energy-related metabolites (i.e., lactate, pyruvate, inosine, and hypoxanthine) in the extracellular fluid (ECF) of the striatum and in cisternal cerebrospinal fluid (CSF) during the first 6 h after middle cerebral artery occlusion (MCAO) using microdialysis. Ischemia induced a dramatic increase in the ECF levels of lactate, inosine, and hypoxanthine, while pyruvate did not change significantly. The major part of these changes occurred during the first 10 min after MCAO. Inosine tended to normalize towards the end, while lactate and hypoxanthine remained elevated throughout the experiment. There was no increase of the energy-related metabolites in CSF during the experiment. It was concluded that lactate, inosine, and hypoxanthine appear to be useful ECF markers of the compromised energy state of the brain during ischemia. Because the metabolites did not appear in CSF during the first 6 h after MCAO, such measurements seem not to be useful for early detection of a disturbance in energy metabolism.