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

General In Situ Engineering of Carbon-Based Materials on Carbon Fiber for In Vivo Neurochemical Sensing

Developing real-time, dynamic, and in situ analytical methods with high spatial and temporal resolutions is crucial for exploring biochemical processes in the brain. Although in vivo electrochemical methods based on carbon fiber (CF) microelectrodes are effective in monitoring neurochemical dynamics during physiological and pathological processes, complex post modification hinders large-scale productions and widespread neuroscience applications. Herein, we develop a general strategy for the in situ engineering of carbon-based materials to mass-produce functional CFs by introducing polydopamine to anchor zeolitic imidazolate frameworks as precursors, followed by one-step pyrolysis. This strategy demonstrates exceptional universality and design flexibility, overcoming complex post-modification procedures and avoiding the delamination of the modification layer. This simplifies the fabrication and integration of functional CF-based microelectrodes. Moreover, we design highly stable and selective H+, O2, and ascorbate microsensors and monitor the influence of CO2 exposure on the O2 content of the cerebral tissue during physiological and ischemia-reperfusion pathological processes.

Vitamin C Deficiency in the Young Brain-Findings from Experimental Animal Models

Severe and long-term vitamin C deficiency can lead to fatal scurvy, which is fortunately considered rare today. However, a moderate state of vitamin C (vitC) deficiency (hypovitaminosis C)-defined as a plasma concentration below 23 μM-is estimated to affect up to 10% of the population in the Western world, albeit clinical hallmarks in addition to scurvy have not been linked to vitC deficiency. The brain maintains a high vitC content and uniquely high levels during deficiency, supporting vitC's importance in the brain. Actions include both antioxidant and co-factor functions, rendering vitamin C deficiency likely to affect several targets in the brain, and it could be particularly significant during development where a high cellular metabolism and an immature antioxidant system might increase sensitivity. However, investigations of a non-scorbutic state of vitC deficiency and effects on the developing young brain are scarce. This narrative review provides a comprehensive overview of the complex mechanisms that regulate vitC homeostasis in vivo and in the brain in particular. Functions of vitC in the brain and the potential consequences of deficiency during brain development are highlighted, based primarily on findings from experimental animal models. Perspectives for future investigations of vitC are outlined.

Ischemic Postconditioning Recovers Cortex Ascorbic Acid during Ischemia/Reperfusion Monitored with an Online Electrochemical System

As a promising therapeutic treatment, ischemic postconditioning has recently received considerable attention. Although the neuroprotection effect of postconditioning has been observed, a reliable approach that can evaluate the neuroprotective efficiency of postconditioning treatment during the acute period after ischemia remains to be developed. This study investigates the dynamics of cortex ascorbic acid during the acute period of cerebral ischemia before and after ischemic postconditioning with an online electrochemical system (OECS). The cerebral ischemia/reperfusion injury and the neuronal functional outcome are evaluated with triphenyltetrazolium chloride staining, immunohistochemistry, and electrophysiological recording techniques. Electrochemical recording results show that cortex ascorbic acid sharply increases 10 min after middle cerebral artery occlusion and then reaches a plateau. After direct reperfusion following ischemia (i.e., without ischemic postconditioning), the cortex ascorbic acid further increases and then starts to decrease slowly at a time point of about 40 min after reperfusion. In striking contrast, the cortex ascorbic acid drops and recovers to its basal level after ischemic postconditioning followed by reperfusion. With the recovery of cortex ascorbic acid, ischemic postconditioning concomitantly promotes the recovery of neural function and reduces the oxidative damage. These results demonstrate that our OECS for monitoring cortex ascorbic acid can be used as a platform for evaluating the neuroprotective efficiency of ischemic postconditioning in the acute phase of cerebral ischemia, which is of great importance for screening proper postconditioning parameters for preventing ischemic damages.

Adding an orange to the banana bag: vitamin C deficiency is common in alcohol use disorders

BACKGROUND

At least a third of the world's population consumes alcohol regularly. Patients with alcohol use disorders (AUDs) are frequently hospitalized for both alcohol-related and unrelated medical conditions. It is well recognized that patients with an AUD are thiamine deficient with thiamine replacement therapy being considered the standard of care. However, the incidence of vitamin C deficiency in this patient population has been poorly defined.

METHODS

In this retrospective, observational study, we recorded the admission vitamin C level in patients with an AUD admitted to our medical intensive care unit (MICU) over a 1-year period. In addition, we recorded relevant clinical and laboratory data including the day 2 and day 3 vitamin C level following empiric treatment with vitamin C. Septic patients were excluded from this study.

RESULTS

Sixty-nine patients met the inclusion criteria for this study. The patients' mean age was 53 ± 14 years; 52 patients (75%) were males. Severe alcohol withdrawal syndrome was the commonest admitting diagnosis (46%). Eighteen patients (26%) had cirrhosis as the admitting diagnosis with 18 (13%) patients admitted due to alcohol/drug intoxication. Forty-six patients (67%) had evidence of acute alcoholic hepatitis. The mean admission vitamin C level was 17.0 ± 18.1 μmol/l (normal 40-60 μmol/l). Sixty-one (88%) patients had a level less than 40 μmol/l (subnormal) while 52 patients (75%) had hypovitaminosis C (level < 23 μmol/l). None of the variables recorded predicted the vitamin C level. Various vitamin C replacement dosing strategies were used. A 1.5-g loading dose, followed by 500-mg PO q 6, was effective in restoring blood levels to normal by day 2.

CONCLUSION

Our results suggest that hypovitaminosis C is exceedingly common in patients with an AUD admitted to an intensive care unit and that all such patients should receive supplementation with vitamin C in addition to thiamine. Additional studies are required to confirm the findings of our observational study and to determine the optimal vitamin C dosing strategy.

Therapeutic hypothermia effectively reduces elevated extracellular ascorbate concentrations caused by acute spinal cord injury

In recent years, systemic hypothermia has taken the spotlight for its use in spinal cord injury (SCI) research fields, but detailed molecular mechanisms are still not fully understood. In this study, we use an online-electrochemical system (OECS) to in vivo continuously monitor the ascorbate of the rats' spinal cord. We find that the basal level of ascorbate in rat spinal cord is 1.85 ± 0.88 μmol L^-1 (n = 20). It increased immediately after SCI and reached 2.36 ± 0.65 μmol L^-1 (164.90% ± 7.99% of the basal level) (n = 5) at 60 min after the injury. The SCI-induced extracellular ascorbate increase is obviously attenuated by therapeutic hypothermia (28 °C) after injury and ascorbate returns to 3.01 ± 0.59 μmol L^-1 (100.24% ± 5.02% of the basal level) (n = 5), at 60 min after SCI. These results substantially manifest that the OECS for ascorbate detection could be employed as a platform for understanding the pathological changes during spinal cord injury. This study provides experimental evidence for the essential roles of ascorbate in SCI which could serve as a biomarker for SCI. Our findings also raise the possibility that therapeutic hypothermia can effectively exert neuroprotection in the acute phase of SCI.

Ascorbic Acid Ameliorates Gestational Lead Exposure-Induced Developmental Alteration in GAD67 and c-Kit Expression in the Rat Cerebellar Cortex

In the present study, we investigated the effects of ascorbic acid on lead-exposed developing cerebellum. Female rats were divided into the following three groups: control (distilled water), lead (0.2% lead acetate), and lead plus ascorbic acid (100 mg/kg/day, 10% solution). To evaluate the effect of lead exposure and ascorbic acid treatment accurately on the cerebellar development for the gestational period, we halted further treatment with lead and ascorbic acid in the dams after delivery of the pups. Although the ascorbic acid slightly decreased the lead level in pups, lead level was still high in the group treated with lead plus ascorbic acid group compared with the control group. The blood lead levels indicated that the ascorbic acid could facilitate both the excretion and transfer of lead from a dam to its pups via milk. At postnatal day 21, lead exposure significantly reduced the number of Purkinje cells in the cerebellar cortex of pups. Additionally, lead treatment induced degenerative changes such as reduction of glutamic acid decarboxylase (GAD67) and c-kit expressions are observed in the developing cerebellar cortex. In the cerebellum of the pups from the lead plus ascorbic acid group, reduction of the number of Purkinje cells, GAD67 expression, and c-kit immunopositivity were remarkably restored compared with the lead group. Our present results suggested that ascorbic acid treatment to lead-exposed dam exerted protective effects on the developing cerebellum against lead-induced neurotoxicity.

Old Things New View: Ascorbic Acid Protects the Brain in Neurodegenerative Disorders

Ascorbic acid is a key antioxidant of the Central Nervous System (CNS). Under brain activity, ascorbic acid is released from glial reservoirs to the synaptic cleft, where it is taken up by neurons. In neurons, ascorbic acid scavenges reactive oxygen species (ROS) generated during synaptic activity and neuronal metabolism where it is then oxidized to dehydroascorbic acid and released into the extracellular space, where it can be recycled by astrocytes. Other intrinsic properties of ascorbic acid, beyond acting as an antioxidant, are important in its role as a key molecule of the CNS. Ascorbic acid can switch neuronal metabolism from glucose consumption to uptake and use of lactate as a metabolic substrate to sustain synaptic activity. Multiple evidence links oxidative stress with neurodegeneration, positioning redox imbalance and ROS as a cause of neurodegeneration. In this review, we focus on ascorbic acid homeostasis, its functions, how it is used by neurons and recycled to ensure antioxidant supply during synaptic activity and how this antioxidant is dysregulated in neurodegenerative disorders.

Does vitamin C deficiency affect cognitive development and function?

Vitamin C is a pivotal antioxidant in the brain and has been reported to have numerous functions, including reactive oxygen species scavenging, neuromodulation, and involvement in angiogenesis. Absence of vitamin C in the brain has been shown to be detrimental to survival in newborn SVCT2(−/−) mice and perinatal deficiency have shown to reduce hippocampal volume and neuron number and cause decreased spatial cognition in guinea pigs, suggesting that maternal vitamin C deficiency could have severe consequences for the offspring. Furthermore, vitamin C deficiency has been proposed to play a role in age-related cognitive decline and in stroke risk and severity. The present review discusses the available literature on effects of vitamin C deficiency on the developing and aging brain with particular focus on in vivo experimentation and clinical studies.

Vitamin C Transporters, Recycling and the Bystander Effect in the Nervous System: SVCT2 versus Gluts

Vitamin C is an essential micronutrient in the human diet; its deficiency leads to a number of symptoms and ultimately death. After entry into cells within the central nervous system (CNS) through sodium vitamin C transporters (SVCTs) and facilitative glucose transporters (GLUTs), vitamin C functions as a neuromodulator, enzymatic cofactor, and reactive oxygen species (ROS) scavenger; it also stimulates differentiation. In this review, we will compare the molecular and structural aspects of vitamin C and glucose transporters and their expression in endothelial or choroid plexus cells, which form part of the blood-brain barrier and blood-cerebrospinal fluid (CSF) barrier, respectively. Additionally, we will describe SVCT and GLUT expression in different cells of the brain as well as SVCT2 distribution in tanycytes and astrocytes of the hypothalamic region. Finally, we will describe vitamin C recycling in the brain, which is mediated by a metabolic interaction between astrocytes and neurons, and the role of the "bystander effect" in the recycling mechanism of vitamin C in both normal and pathological conditions.

Vitamin C transport and its role in the central nervous system

Vitamin C, or ascorbic acid, is important as an antioxidant and participates in numerous cellular functions. Although it circulates in plasma in micromolar concentrations, it reaches millimolar concentrations in most tissues. These high ascorbate cellular concentrations are thought to be generated and maintained by the SVCT2 (Slc23a2), a specific transporter for ascorbate. The vitamin is also readily recycled from its oxidized forms inside cells. Neurons in the central nervous system (CNS) contain some of the highest ascorbic acid concentrations of mammalian tissues. Intracellular ascorbate serves several functions in the CNS, including antioxidant protection, peptide amidation, myelin formation, synaptic potentiation, and protection against glutamate toxicity. The importance of the SVCT2 for CNS function is supported by the finding that its targeted deletion in mice causes widespread cerebral hemorrhage and death on post-natal day 1. Neuronal ascorbate content as maintained by this protein also has relevance for human disease, since ascorbate supplements decrease infarct size in ischemia-reperfusion injury models of stroke, and since ascorbate may protect neurons from the oxidant damage associated with neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. The aim of this review is to assess the role of the SVCT2 in regulating neuronal ascorbate homeostasis and the extent to which ascorbate affects brain function and antioxidant defenses in the CNS.

Cellular pathways for transport and efflux of ascorbate and dehydroascorbate

The mechanisms allowing the cellular transport of ascorbic acid represent a primary aspect for the understanding of the roles played by this vitamin in pathophysiology. Considerable research effort has been spent in the field, on several animal models and different cell types. Several mechanisms have been described to date, mediating the movements of different redox forms of ascorbic acid across cell membranes. Vitamin C can enter cells both in its reduced and oxidized form, ascorbic acid (AA) and dehydroascorbate (DHA), utilizing respectively sodium-dependent transporters (SVCT) or glucose transporters (GLUT). Modulation of SVCT expression and function has been described by cytokines, steroids and post-translational protein modification. Cellular uptake of DHA is followed by its intracellular reduction to AA by several enzymatic and non-enzymatic systems. Efflux of vitamin C has been also described in a number of cell types and different pathophysiological functions were proposed for this phenomenon, in dependence of the cell model studied. Cellular efflux of AA is mediated through volume-sensitive (VSOAC) and Ca(2+)-dependent anion channels, gap-junction hemichannels, exocytosis of secretory vesicles and possibly through homo- and hetero-exchange systems at the plasma membrane level. Altogether, available data suggest that cellular efflux of ascorbic acid - besides its uptake - should be taken into account when evaluating the cellular homeostasis and functions of this important vitamin.

Corticostriatal dysfunction underlies diminished striatal ascorbate release in the R6/2 mouse model of Huntington's disease

A behavior-related deficit in the release of ascorbate (AA), an antioxidant vitamin, occurs in the striatum of R6/2 mice expressing the human mutation for Huntington's disease (HD), a dominantly inherited condition characterized by striatal dysfunction. To determine the role of corticostriatal fibers in AA release, we combined slow-scan voltammetry with electrical stimulation of cortical afferents to measure evoked fluctuations in extracellular AA in wild-type (WT) and R6/2 striatum. Although cortical stimulation evoked a rapid increase in AA release in both groups, the R6/2 response had a significantly shorter duration and smaller magnitude than WT. To determine if corticostriatal dysfunction also underlies the behavior-related AA deficit in R6/2s, we measured striatal AA release in separate groups of mice treated with d-amphetamine (5 mg/kg), a psychomotor stimulant known to release AA from corticostriatal terminals independently of dopamine. Relative to WT, both AA release and behavioral activation were diminished in R6/2 mice. Collectively, our results show that the corticostriatal pathway is directly involved in AA release and that this system is dysfunctional in HD. Moreover, because AA release requires glutamate uptake, a failure of striatal AA release in HD is consistent with an overactive glutamate system and diminished glutamate transport, both of which are thought to be central to HD pathogenesis.

A metabolic switch in brain: glucose and lactate metabolism modulation by ascorbic acid

In this review, we discuss a novel function of ascorbic acid in brain energetics. It has been proposed that during glutamatergic synaptic activity neurons preferably consume lactate released from glia. The key to this energetic coupling is the metabolic activation that occurs in astrocytes by glutamate and an increase in extracellular [K(+)]. Neurons are cells well equipped to consume glucose because they express glucose transporters and glycolytic and tricarboxylic acid cycle enzymes. Moreover, neuronal cells express monocarboxylate transporters and lactate dehydrogenase isoenzyme 1, which is inhibited by pyruvate. As glycolysis produces an increase in pyruvate concentration and a decrease in NAD(+)/NADH, lactate and glucose consumption are not viable at the same time. In this context, we discuss ascorbic acid participation as a metabolic switch modulating neuronal metabolism between rest and activation periods. Ascorbic acid is highly concentrated in CNS. Glutamate stimulates ascorbic acid release from astrocytes. Ascorbic acid entry into neurons and within the cell can inhibit glucose consumption and stimulate lactate transport. For this switch to occur, an ascorbic acid flow is necessary between astrocytes and neurons, which is driven by neural activity and is part of vitamin C recycling. Here, we review the role of glucose and lactate as metabolic substrates and the modulation of neuronal metabolism by ascorbic acid.

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.

Vitamin C function in the brain: vital role of the ascorbate transporter SVCT2

Ascorbate (vitamin C) is a vital antioxidant molecule in the brain. However, it also has a number of other important functions, participating as a cofactor in several enzyme reactions, including catecholamine synthesis, collagen production, and regulation of HIF-1 alpha. Ascorbate is transported into the brain and neurons via the sodium-dependent vitamin C transporter 2 (SVCT2), which causes accumulation of ascorbate within cells against a concentration gradient. Dehydroascorbic acid, the oxidized form of ascorbate, is transported via glucose transporters of the GLUT family. Once in cells, it is rapidly reduced to ascorbate. The highest concentrations of ascorbate in the body are found in the brain and in neuroendocrine tissues such as adrenal, although the brain is the most difficult organ to deplete of ascorbate. Combined with regional asymmetry in ascorbate distribution within different brain areas, these facts suggest an important role for ascorbate in the brain. Ascorbate is proposed as a neuromodulator of glutamatergic, dopaminergic, cholinergic, and GABAergic transmission and related behaviors. Neurodegenerative diseases typically involve high levels of oxidative stress and thus ascorbate has been posited to have potential therapeutic roles against ischemic stroke, Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Evaluation of hydrogel-coated glutamate microsensors

Glutamate microsensors form a promising analytical tool for monitoring neuronally derived glutamate directly in the brain. However, when a microsensor is implanted in brain tissue, many factors can diminish its performance. Consequently, a thorough characterization and evaluation of a microsensor is required concerning all factors that may possibly be encountered in vivo. The present report deals with the validation of a hydrogel-coated glutamate microsensor. This microsensor is constructed by coating a carbon fiber electrode (10-microm diameter; 300-500 microm long) with a five-component redox hydrogel, in which L-glutamate oxidase, horseradish peroxidase, and ascorbate oxidase are wired via poly(ethylene glycol) diglycidyl ether to an osmium-containing redox polymer. A thin Nafion coating completes the construction. Although this microsensor was previously used in vivo, information concerning its validation is limited. In the present study, attention was given to its selectivity, specificity, calibration, oxygen dependency, biofouling, operating potential dependency, and linear range. In addition, successful microsensor experiments in microdialysate, in vitro (in organotypic hippocampal slice cultures), and in vivo (in anesthesized rats) are shown.

Primary graft nonfunction and Kupffer cell activation after liver transplantation from non-heart-beating donors in pigs

More extensive use of non-heart-beating donors (NHBD) could reduce mortality on liver transplantation waiting lists, but this is associated with more primary nonfunction (PNF). We assessed which parameters are involved in the development of PNF in livers from NHBD in a previously validated pig liver transplantation model, in which livers were transplanted after exposure to incremental periods of warm ischemia. The risk of PNF was unacceptably high (>50%) when livers were exposed to >30 minutes' warm ischemia before a short cold ischemic period. This study examined how PNF is affected by Kupffer cell activation (beta-galactosidase), the generation of cytokines tumor necrosis factor alpha and interleukin 6, antioxidant mechanisms (ascorbic acid, alpha-tocopherol, reduced glutathione), circulating redox-active iron, and sinusoidal endothelial cell function (hyaluronic acid clearance). Kupffer cells were more activated in PNF recipients, as suggested by higher beta-galactosidase levels (15 minutes after reperfusion), and secondarily, by higher production of tumor necrosis factor alpha and interleukin 6 (180 minutes after reperfusion). In addition, alpha-tocopherol and reduced glutathione were lower, and ascorbic acid and redox-active iron higher in PNF recipients. Finally, PNF grafts displayed progressively decreasing hyaluronic acid clearance (suggesting sinusoidal endothelial cell dysfunction) and parenchymal edema. Consequently, a reduced-flow phenomenon was documented. In grafts from NHBD that are destined to fail, beta-galactosidase activity (a surrogate of Kupffer cell activation) is higher, proinflammatory cytokines are overproduced, some antioxidant mechanisms fail, and circulating redox-active iron is more rapidly released. A no-flow phenomenon is eventually observed in these failing grafts.

Ethanol Similarly Induces Ascorbic Acid Release in the Prefrontal Cortex and Striatum of Freely Moving Mice

Previous studies have shown that acute systemic administration of ethanol induced striatal ascorbic acid (AA) release in mice and rats. Undercutting the prefrontal cortex completely eliminated ethanol-induced AA release in rat striatum. In the present study, in vivo brain dialysis coupled with high performance liquid chromatography (HPLC)-electrochemical detection was used to evaluate the effect of ethanol on the release of AA in the prefrontal cortex, compared to that in the striatum of freely moving mice. The results showed that ethanol (4.0 g/kg i.p.) similarly induced AA release in the prefrontal cortex and striatum of freely moving mice.

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.

Sepsis inhibits recycling and glutamate-stimulated export of ascorbate by astrocytes

Sepsis causes brain dysfunction. Because neurotransmission requires high ascorbate and low dehydroascorbic acid (DHAA) concentrations in brain extracellular fluid, the effect of septic insult on ascorbate recycling (i.e., uptake and reduction of DHAA) and export was investigated in primary rat and mouse astrocytes. DHAA raised intracellular ascorbate to physiological levels but extracellular ascorbate only slightly. Septic insult by lipopolysaccharide and interferon-gamma increased ascorbate recycling in astrocytes permeabilized with saponin but decreased it in those with intact plasma membrane. The decrease was due to inhibition of the glucose transporter (GLUT1) that translocates DHAA because septic insult slowed uptake of the nonmetabolizable GLUT1 substrate 3-O-methylglucose. Septic insult also abolished stimulation by glutamate of ascorbate export. Specific nitric oxide synthase (NOS) inhibitors and nNOS and iNOS deficiency failed to alter the effects of septic insult. Inhibitors of NADPH oxidase generally did not protect against septic insult, because only one of those tested (diphenylene iodonium) increased GLUT1 activity and ascorbate recycling. We conclude that astrocytes take up DHAA and use it to synthesize ascorbate that is exported in response to glutamate. This mechanism may provide the antioxidant on demand to neurons under normal conditions, but it is attenuated after septic insult.

Regulation of vitamin C transport

Ascorbic acid and dehydroascorbic acid (DHAA, oxidized vitamin C) are dietary sources of vitamin C in humans. Both nutrients are absorbed from the lumen of the intestine and renal tubules by, respectively, enterocytes and renal epithelial cells. Subsequently vitamin C circulates in the blood and enters all of the other cells of the body. Concerning flux across the plasma membrane, simple diffusion of ascorbic acid plays only a small or negligible role. More important are specific mechanisms of transport and metabolism that concentrate vitamin C intracellularly to enhance its function as an enzyme cofactor and antioxidant. The known transport mechanisms are facilitated diffusion of DHAA through glucose-sensitive and -insensitive transporters, facilitated diffusion of ascorbate through channels, exocytosis of ascorbate in secretory vesicles, and secondary active transport of ascorbate through the sodium-dependent vitamin C transporters SVCT1 and SVCT2 proteins that are encoded by the genes Slc23a1 and Slc23a2, respectively. Evidence is reviewed indicating that these transport pathways are regulated under physiological conditions and altered by aging and disease.

Improving Glutamate Microsensors by Optimizing the Composition of the Redox Hydrogel

Amperometric hydrogel-coated glutamate microsensors form a promising concept to detect glutamate levels directly in brain tissue. These microsensors are constructed by coating a carbon fiber electrode (CFE) (10 microm diameter; 300-500 microm long) with a five-component redox-hydrogel, in which L-glutamate oxidase, horseradish peroxidase, and ascorbate oxidase are wired via poly(ethylene glycol) diglycidyl ether to an osmium-containing redox polymer. Coating with a thin Nafion film completes the construction. Prior to use in vivo, a reliable and reproducible construction of microsensors with a high performance is required. For an optimal microsensor performance, the balance between the five individual hydrogel components is critical. However, due to their small size, hydrogel application to CFE's need to be performed by dip-coating. Dip-coating is a difficult procedure to control and does not allow individual application of hydrogel constituents. To improve the microsensor construction and to better control the dip-coating procedure, we have recently developed an automated device. Throughout this study, automatic dip-coating was performed with premixed solutions, in which the amount of a single component was varied. This allowed us to optimize the hydrogel composition, which resulted in a significant improvement of the microsensor properties in terms of sensitivity, current density, linearity, detection limit, and interference by ascorbic acid.

Experimental spinal cord injury induced an increase of extracellular ascorbic acid concentration in anesthetized rats: a microdialysis study

Ascorbic acid plays important roles in mammalian central nervous system. We employed an on-line analytical system to monitor the extracellular ascorbic acid concentrations in anesthetized rat spinal cord before and after the experimental injury. A microdialysis probe (216 microm od, 200 microm id, 3 mm in length) was implanted into an anesthetized rat spinal cord (Thoratic-12). Microdialysis perfusate (2 microl/min) was collected in the sample loop (20 microl) of an on-line injector for direct injection onto a High Performance Liquid Chromatography (HPLC) system equipped with an electrochemical detector. Normal ascorbic acid concentrations in the spinal cord extracellular fluids ranged from 1.8 microM to 10.8 microM (mean +/- S.D. 5.6 +/- 2.4 microM, n = 8). The experimental spinal cord injury, induced by a lesion at T-10, gradually yet significantly increased the extracellular ascorbic acid levels. The effect of exogenous glutamate perfusion (0.2 mM, 2 mM, and 20 mM) through the microdialysis probe also increased the extracellular ascorbic acid concentrations in a dose dependent manner. These results suggested that the injury-induced ascorbic acid accumulation may result from elevated extracellular glutamate levels that are commonly observed in spinal cord injury. This on-line, continuous and automatic monitoring system can be applied to future investigations on the roles of ascorbic acid in spinal cord injuries.

Ischemic preconditioning increases antioxidants in the brain and peripheral organs after cerebral ischemia

BACKGROUND AND PURPOSE

Low molecular weight antioxidants (LMWA), which reflect tissue reducing power, are among the endogenous mechanisms for neutralizing reactive oxygen species (ROS). Ischemic preconditioning (IPC) was associated with decreased oxidative stress. We examined the effect of focal ischemia on LMWA and on prostaglandin E(2) (PGE(2), a product of arachidonic acid oxidation) in the brain, heart, liver, and lungs of rats subjected to 90 min of ischemia and in IPC rats subjected to similar insult.

METHODS

Transient right middle cerebral artery occlusion (MCAO) was performed for 90 min and at 0, 5, 30, 60, or 240 min of reperfusion, LMWA and PGE(2) were evaluated by cyclic voltametry (CV) and radioimmunoassay, respectively. IPC was induced by 2 min of MCAO, 24 h prior to the major ischemic episode.

RESULTS

LMWA decreased at 5 min of reperfusion in the brain, heart, liver, and lung and rose 4 h later only in the brain. PGE(2) levels increased three to fivefold in all tissues examined. Surprisingly, in IPC rats a dramatic increase of LMWA occurred at 5 min of reperfusion in the brain and in the peripheral organs. Uric acid, but not ascorbic, is the major LMWA increased.

CONCLUSIONS

We propose that after ischemia, ROS rapidly consume the antioxidants reserves in the brain and also in peripheral organs, suggesting that the whole body is under oxidative stress. Moreover, part of the neuroprotection afforded by IPC is mediated by the brain's ability to mobilize antioxidants, especially uric acid, that attenuate the massive ROS-mediated oxidative stress.

Effect of glutamate on inflammatory responses of intestine and brain after focal cerebral ischemia

AIM: To study the modulation of glutamate on post-ischemic intestinal and cerebral inflammatory responses in a ischemic and excitotoxic rat model.

METHODS: Adult male rats were subjected to bilateral carotid artery occlusion for 15 min and injection of monosodium glutamate intraperitoneally, to decapitate them at selected time points. Tumor necrosis factor alpha (TNF-α) level and nuclear factor kappa B (NF-κB) activity were determined by enzyme-linked immunosorbant assay (ELISA) and electrophoretic mobility shift assay (EMSA), respectively. Hemodynamic parameters were monitored continuously during the whole process of cerebral ischemia and reperfusion.

RESULTS: Monosodium glutamate (MSG) treated rats displayed statistically significant high levels of TNF-α in cerebral and intestinal tissues within the first 6 h of ischemia. The rats with cerebral ischemia showed a minor decrease of TNF-α production in cerebral and intestinal tissues. The rats with cerebral ischemia and treated with MSG displayed statistically significant low levels of TNF-α in cerebral and intestinal tissues. These results correlated significantly with NF-κB production calculated at the same intervals. During experiment, the mean blood pressure and heart rates in all groups were stable.

CONCLUSION: Glutamate is involved in the mechanism of intestinal and cerebral inflammation responses. The effects of glutamate on cerebral and intestinal inflammatory responses after ischemia are up-regulated at the transcriptional level, through the NF-κB signal transduction pathway.

Role of nitric oxide in ethanol-induced ascorbic acid release in striatum of freely moving mice

In the present study, in vivo brain microdialysis coupled with high performance liquid chromatography (HPLC) and electrochemical detection were used to evaluate the effects of either L-arginine (L-Arg), the substrate of nitric oxide synthase (NOS), Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME), a non-selective NOS inhibitor, or sodium nitroprusside (SNP), a donor of NO, on the ethanol-induced release of ascorbic acid (AA) in the striatum of freely moving mice. Drugs were administered intrastriatally via the microdialysis probe and ethanol (2-4 g/kg) was administered intraperitoneally. The results showed that L-arginine (1-10 mg/ml) had no effect on either the basal AA contents in striatal extracellular fluid or the ethanol-induced release of AA. L-NAME (10(-4) to 10(-3) mg/ml) and SNP (10(-4) to 10(-3) mg/ml) both reduced the basal AA concentrations in striatal extracellular fluid. L-NAME significantly inhibited ethanol-induced release of AA, while SNP only had a transient inhibitory effect on the ethanol-induced release of AA. SNP significantly increased dehydroascorbic acid (DHAA) contents and DHAA/AA ratio but had no effect on the total AA contents (AA and DHAA contents) in striatal extracellular fluid, while L-NAME had no effect on DHAA contents but decreased the total AA contents in striatal extracellular fluid. Only high concentration L-NAME induced a transient increase in DHAA/AA ratio. Our results suggest that nitric oxide (NO) might not directly be involved in the mechanism of ethanol-induced release of AA in mouse striatum.

Effect of middle cerebral artery occlusion on mRNA expression for the sodium-coupled vitamin C transporter SVCT2 in rat brain

The sodium-vitamin C co-transporter SVCT2 is primarily responsible for the accumulation of the important antioxidant ascorbate into brain cells. In vitro studies have demonstrated strong expression of this transporter in cultured astrocytes, whereas in situ hybridization analysis has so far detected SVCT2 only in neurons. In the present study, we examined the response of SVCT2 mRNA expression in the brain to focal ischemia induced for 2 h by unilateral middle cerebral artery occlusion. The mRNA expression patterns of SVCT2 and the glutamate-activated immediate early gene Arc were investigated at 2 and 22 h after ischemia. SVCT2 and Arc mRNA expression was lost in the ischemic core at both time points. In areas outside the core, Arc was strongly up-regulated, primarily at 2 h, whereas SVCT2 showed an increase at 2 and 22 h. SVCT2 expression was increased in neurons as well as in astrocytes, providing the first evidence for SVCT2 expression in astrocytes in situ. These findings underscore the importance of ascorbate as a neuroprotective agent and may have implications for therapeutic strategies. In addition, the increase of SVCT2 in astrocytes after ischemia suggests that cultured astrocytes are exposed to chronic oxidative stress.

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.