Correlation between content of high-energy phosphates and hypoxic-ischemic damage in immature and mature astrocytes

Consequences of a 2-Deoxyglucose Exposure on the ATP Content and the Cytosolic Glucose Metabolism of Cultured Primary Rat Astrocytes

The glucose analogue 2-deoxyglucose (2DG) has frequently been used as a tool to study cellular glucose uptake and to inhibit glycolysis. Exposure of primary cultured astrocytes to 2DG caused a time- and concentration-dependent cellular accumulation of 2-deoxyglucose-6-phosphate (2DG6P) that was accompanied by a rapid initial decline in cellular ATP content. Inhibitors of mitochondrial respiration as well as inhibitors of mitochondrial uptake of pyruvate and activated fatty acids accelerated the ATP loss, demonstrating that mitochondrial ATP regeneration contributes to the partial maintenance of the ATP content in 2DG-treated astrocytes. After a 30 min exposure to 10 mM 2DG the specific content of cellular 2DG6P had accumulated to around 150 nmol/mg, while cellular ATP was lowered by 50% to around 16 nmol/mg. Following such a 2DG6P-loading of astrocytes, glycolytic lactate production from applied glucose was severely impaired during the initial 60 min of incubation, but was reestablished during longer incubation concomitant with a loss in cellular 2DG6P content. In contrast to glycolysis, the glucose-dependent NADPH regeneration via the pentose phosphate pathway (PPP) was only weakly affected in 2DG6P-loaded astrocytes and in cells that were coincubated with glucose in the presence of an excess of 2DG. Additionally, in the presence of 2DG PPP-dependent WST1 reduction was found to have doubled compared to hexose-free control incubations, indicating that cellular 2DG6P can serve as substrate for NADPH regeneration by the astrocytic PPP. The data presented provide new insights on the metabolic consequences of a 2DG exposure on the energy and glucose metabolism of astrocytes and demonstrate the reversibility of the inhibitory potential of a 2DG-treatment on the glucose metabolism of cultured astrocytes.

Modulation of Cellular Levels of Adenosine Phosphates and Creatine Phosphate in Cultured Primary Astrocytes

Adenosine triphosphate (ATP) is the main energy currency of all cells, while creatine phosphate (CrP) is considered as a buffer of high energy-bond phosphate that facilitates rapid regeneration of ATP from adenosine diphosphate (ADP). Astrocyte-rich primary cultures contain ATP, ADP and adenosine monophosphate (AMP) in average specific contents of 36.0 ± 6.4 nmol/mg, 2.9 ± 2.1 nmol/mg and 1.7 ± 2.1 nmol/mg, respectively, which establish an adenylate energy charge of 0.92 ± 0.04. The average specific cellular CrP level was found to be 25.9 ± 10.8 nmol/mg and the CrP/ATP ratio was 0.74 ± 0.28. The specific cellular CrP content, but not the ATP content, declined with the age of the culture. Absence of fetal calf serum for 24 h caused a partial loss in the cellular contents of both CrP and ATP, while application of creatine for 24 h doubled the cellular CrP content and the CrP/ATP ratio, but did not affect ATP levels. In glucose-deprived astrocytes, the high cellular ATP and CrP contents were rapidly depleted within minutes after application of the glycolysis inhibitor 2-deoxyglucose and the respiratory chain inhibitor antimycin A. For those conditions, the decline in CrP levels always preceded that of ATP contents. In contrast, incubation of glucose-fed astrocytes for up to 30 min with antimycin A had little effect on the high cellular ATP content, while the CrP level was significantly lowered. These data demonstrate the importance of cellular CrP for maintaining a high cellular ATP content in astrocytes during episodes of impaired ATP regeneration.

Endogenous Energy Stores Maintain a High ATP Concentration for Hours in Glucose-Depleted Cultured Primary Rat Astrocytes

Adenosine triphosphate (ATP) is the central energy currency of all cells. Cultured primary rat astrocytes contain a specific cellular ATP content of 27.9 ± 4.7 nmol/mg. During incubation in a glucose- and amino acid-free incubation buffer, this high cellular ATP content was maintained for at least 6 h, while within 24 h the levels of ATP declined to around 30% of the initial value without compromising cell viability. In contrast, cells exposed to 1 mM and 5 mM glucose maintained the initial high cellular ATP content for 24 and 72 h, respectively. The loss in cellular ATP content observed during a 24 h glucose-deprivation was fully prevented by the presence of glucose, fructose or mannose as well as by the mitochondrial substrates lactate, pyruvate, β-hydroxybutyrate or acetate. The high initial specific ATP content in glucose-starved astrocytes, was almost completely abolished within 30 min after application of the respiratory chain inhibitor antimycin A or the mitochondrial uncoupler BAM-15, while these inhibitors lowered in glucose-fed cells the ATP content only to 60% (BAM-15) and 40% (antimycin A) within 5 h. Inhibition of the mitochondrial pyruvate carrier by UK5099 alone or of mitochondrial fatty acid uptake by etomoxir alone hardly affected the high ATP content of glucose-deprived astrocytes during an incubation for 8 h, while the co-application of both inhibitors depleted cellular ATP levels almost completely within 5 h. These data underline the importance of mitochondrial metabolism for the ATP regeneration of astrocytes and demonstrate that the mitochondrial oxidation of pyruvate and fatty acids strongly contributes to the maintenance of a high ATP concentration in glucose-deprived astrocytes.

Cell-specific metabolomic responses to injury: novel insights into blood-brain barrier modulation

On one hand blood-brain barrier (BBB) disturbance aggravates disease progression, on the other it prevents drug access and impedes therapeutic efficacy. Effective ways to modulate barrier function and resolve these issues are sorely needed. Convinced that better understanding of cell-oriented BBB responses could provide valuable insight, and the fact that metabolic dysregulation is prominent in many vascular-related pathological processes associated with BBB disturbance, we hypothesized that differential cell-specific metabolic adaptation majorly influences physiological and pathological barrier functionality. Untargeted liquid chromatography-mass spectrometry (LC-MS) metabolomic profiling was used to obtain individual biochemical fingerprints of primary astrocytes (AC) and brain endothelial cells (EC) during normoxic conditions and increasing hypoxic/ischemic injury and thus a functional readout of cell status. Bioinformatic analyses showed each cell had a distinct metabolic signature. Corroborating their roles in BBB and CNS protection, AC showed an innate ability to dynamically alter their metabolome depending on the insult. Surprisingly, in complete contrast, EC largely maintained their normoxic characteristics in injury situations and their profiles diverged from those of non-brain origin. Tissue specificity/origin is clearly important when considering EC responses. Focusing on energy capacity and utilization we discuss how cell-specific metabolic adaptive capabilities could influence vascular stability and the possibility that altering metabolite levels may be an effective way to modulate brain EC function. Overall this work novel insight into cell-associated metabolic changes, and provides a powerful resource for understanding BBB changes during different injury scenarios.

Cellular Prion Protein (PrPc) and Hypoxia: True to Each Other in Good Times and in Bad, in Sickness, and in Health

The cellular prion protein (PrP^c) and hypoxia appear to be tightly intertwined. Beneficial effects of PrP^c on neuronal survival under hypoxic conditions such as focal cerebral ischemia are strongly supported. Conversely, increasing evidence indicates detrimental effects of increased PrP^c expression on cancer progression, another condition accompanied by low oxygen tensions. A switch between anaerobic and aerobic metabolism characterizes both conditions. A cellular process that might unite both is glycolysis. Putative role of PrP^c in stimulation of glycolysis in times of need is indeed thought provoking. A significance of astrocytic PrP^c expression for neuronal survival under hypoxic conditions and possible association of PrP^c with the astrocyte-neuron lactate shuttle is considered. We posit PrP^c-induced lactate production via transactivation of lactate dehydrogenase A by hypoxia inducible factor 1α as an important factor for survival of both neurons and tumor cells in hypoxic microenvironment. Concomitantly, we discuss a cross-talk between Wnt/β-catenin and PI3K/Akt signaling pathways in executing PrP^c-induced activation of glycolysis. Finally, we would like to emphasize that we see a great potential in joining expertise from both fields, neuroscience and cancer research in revealing the mechanisms underlying hypoxia-related pathologies. PrP^c may prove focal point for future research.

Glucose replaces glutamate as energy substrate to fuel glutamate uptake in glutamate dehydrogenase-deficient astrocytes

Cultured astrocytes treated with siRNA to knock down glutamate dehydrogenase (GDH) were used to investigate whether this enzyme is important for the utilization of glutamate as an energy substrate. By incubation of these cells in media containing different concentrations of glutamate (range 100-500 µM) in the presence or in the absence of glucose, the metabolism of these substrates was studied by using tritiated glutamate or 2-deoxyglucose as tracers. In addition, the cellular contents of glutamate and ATP were determined. The astrocytes were able to maintain physiological levels of ATP regardless of the expression level of GDH and the incubation condition, indicating a high degree of flexibility with regard to regulatory mechanisms involved in maintaining an adequate energy level in the cells. Glutamate uptake was found to be increased in these cells when exposed to increasing levels of extracellular glutamate independently of the GDH expression level. Moreover, increased intracellular glutamate content was observed in the GDH-deficient cells after a 2-hr incubation in the presence of 100 µM glutamate. It is significant that GDH-deficient cells exhibited an increased utilization of glucose in the presence of 250 and 500 µM glutamate, monitored as an increase in the accumulation of tritiated 2-deoxyglucose-6-phosphate. These findings underscore the importance of the expression level of GDH for the ability to utilize glutamate as an energy source fueling its own energy-requiring uptake.

Antioxidant and protective mechanisms against hypoxia and hypoglycaemia in cortical neurons in vitro

In the present work, we have studied whether cell death could be induced in cortical neurons from rats subjected to different period of O₂ deprivation and low glucose (ODLG). This “in vitro” model is designed to emulate the penumbra area under ischemia. In these conditions, cortical neurons displayed loss of mitochondrial respiratory ability however, nor necrosis neither apoptosis occurred despite ROS production. The absence of cellular death could be a consequence of increased antioxidant responses such as superoxide dismutase-1 (SOD1) and GPX3. In addition, the levels of reduced glutathione were augmented and HIF-1/3α overexpressed. After long periods of ODLG (12–24 h) cortical neurons showed cellular and mitochondrial membrane alterations and did not recuperate cellular viability during reperfusion. This could mean that therapies directed toward prevention of cellular and mitochondrial membrane imbalance or cell death through mechanisms other than necrosis or apoptosis, like authophagy, may be a way to prevent ODLG damage.

A sensitive HPLC-based method to quantify adenine nucleotides in primary astrocyte cell cultures

In mono-layered primary cell cultures baseline AMP and ADP levels are found nominally in the mid to low picomolar range and are thus difficult to measure with conventional HPLC methods that often require the pooling of samples or require indirect detection methods using radiotracers or enzyme coupled assays. To address this issue, we developed a highly sensitive and selective ion-pairing HPLC method with fluorescence detection to quantify adenine nucleotides and the adenylate energy charge in primary astrocyte cell cultures. To accomplish this, we optimized the fluorescence derivatization conditions and the HPLC parameters to achieve baseline separation and quantification of all adenine nucleotides. Nucleotides were converted to their respective 1, N(6)-etheno derivatives by incubating with chloroacetaldehyde at pH 4.5 and 60°C for 60 min. Under these conditions, the loss of the adenine nucleotides due to hydrolysis was minimized with a derivatization yield of 94.1% for 1, N(6)-ethenoadenosine. The optimal concentration of tetrabutylammonium phosphate, the ion-pairing reagent, required to achieve a reproducible separation of the adenine nucleotides was found to be 0.8mM. Calibration curves of nucleotide standards were linear within the range of 0.16-10.4 pmol for adenosine, 0.16-20.6 pmol for AMP, 0.15-19.2 pmol for ADP, and 0.15-19.5 pmol for ATP. The limits of detection and quantification for all adenine nucleotides were approximately 0.08 and 0.16 pmol, respectively. The intra- and inter-day variability for this method was less than 5.1 and 3.4%, respectively. This method was successfully used to measure all adenine nucleotides and an adenylate energy charge of 0.92±0.02 in primary astrocyte cell cultures.

Metabolic alterations induced by ischemia in primary cultures of astrocytes: merging 13C NMR spectroscopy and metabolic flux analysis

Disruption of brain energy metabolism is the hallmark of cerebral ischemia, a major cause of death worldwide. Astrocytes play a key role in the regulation of brain metabolism and their vulnerability to ischemia has been described. Aiming to quantify the effects of an ischemic insult in astrocytic metabolism, primary cultures of astrocytes were subjected to 5 h of oxygen and glucose deprivation in a bioreactor. Flux distributions, before and after ischemia, were estimated by metabolic flux analysis using isotopic information and the consumption/secretion rates of relevant extracellular metabolites as constraints. During ischemia and early recovery, 30% of cell death was observed; several metabolic alterations were also identified reflecting a metabolic response by the surviving cells. In the early recovery ( approximately 10 h), astrocytes up-regulated glucose utilization by 30% and increased the pentose phosphate pathway and tricarboxylic acid cycle fluxes by three and twofold, respectively. Additionally, a two to fivefold enhancement in branched-chain amino acids catabolism suggested the importance of anaplerotic molecules to the fast recovery of the energetic state, which was corroborated by measured cellular ATP levels. Glycolytic metabolism was predominant in the late recovery. In summary, this work demonstrates that changes in fluxes of key metabolic pathways are implicated in the recovery from ischemia in astrocytes.

Effects of continuous hypoxia on energy metabolism in cultured cerebro-cortical neurons

Mechanisms underlying hypoxia-induced neuronal adaptation have not been fully elucidated. In the present study we investigated glucose metabolism and the activities of glycolytic and TCA cycle enzymes in cerebro-cortical neurons exposed to hypoxia (3 days in 1% of O2) or normoxia (room air). Hypoxia led to increased activities of LDH (194%), PK (90%), and HK (24%) and decreased activities of CS (15%) and GDH (34%). Neurons were incubated with [1-(13)C]glucose for 45 and 120 min under normoxic or hypoxic (120 min only) conditions and 13C enrichment determined in the medium and cell extract using 1H-{13C}-NMR. In hypoxia-treated neurons [3-(13)C]lactate release into the medium was 428% greater than in normoxia-treated controls (45-min normoxic incubation) and total flux through lactate was increased by 425%. In contrast glucose oxidation was reduced significantly in hypoxia-treated neurons, even when expressed relative to total cellular protein, which correlated with the reduced activities of the measured mitochondrial enzymes. The results suggest that surviving neurons adapt to prolonged hypoxia by up-regulation of glycolysis and down-regulation of oxidative energy metabolism, similar to certain other cell types. The factors leading to adaptation and survival for some neurons but not others remain to be determined.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

Exacerbation of excitotoxic neuronal death induced during mitochondrial inhibition in vivo: relation to energy imbalance or ATP depletion?

During the past two decades a close relationship between the energy state of the cell and glutamate neurotoxicity has been suggested. We have previously shown that increasing the extracellular concentration of glutamate does not cause neuronal death unless a deficit in energy metabolism occurs. The mechanisms of glutamate-induced neuronal death have been extensively studied in vitro and it has been associated with a rapid and severe decrease in ATP levels, accompanied with mitochondrial dysfunction. In this study we aimed to investigate the time course of the changes in energy metabolites during glutamate-induced neuronal death, in the presence of a moderate inhibition of mitochondrial metabolism in the rat striatum in vivo. We also aimed to study whether or not, as reported in vitro, changes in ATP levels are related to the extension of neuronal death. Results show that glutamate-induced lesions are exacerbated when rats are previously treated with a subtoxic dose of the mitochondrial toxin 3-nitropropionic acid (3-NP). However, changes in nucleotide levels were similar in rats injected with glutamate alone and in rats injected with glutamate and previously treated with 3-NP. In spite of the presence of an extensive striatal lesion, nucleotide levels were recovered in 3-NP-treated rats 24 h after glutamate injection. Results show that 3-NP pre-treatment induced an imbalance in nucleotide levels that predisposed cells to glutamate toxicity; however it did not influence the bioenergetic changes induced by glutamate alone. Enhancement of glutamate neurotoxicity in 3-NP pre-treated rats is more related to a sustained nucleotide imbalance than just to a rapid decrease in ATP levels.

Intracerebroventricular administration of creatine protects against damage by global cerebral ischemia in rat

Although a large body of evidence shows that pretreatment of brain tissue with creatine protects against anoxic injury in vitro, only a couple of papers have investigated creatine protection in vivo, and they yielded conflicting results. We attempted to clarify how creatine may be protective in an in vivo model of global cerebral ischemia (GCI). We administered creatine either before of after GCI. We decided to administer it by intracerebroventricular infusion, to maximize its bioavailability to the brain. Our findings show that creatine is clearly protective in vivo when administered before ischemia. In that case, histological evaluation of damage was consistently improved in all regions examined, and neurological score was better in creatine-treated rats than in controls. When administered after ischemia, histology was improved in the hippocampus, while only a not significant trend toward improvement was observed in the cerebral cortex and in the caudo-putamen. Neurological score was not improved by creatine administration after GCI. Our findings show that creatine administration is protective in vivo. Such protection was clear in the case of pretreatment, and was present, to a lesser degree, when treatment was started after ischemia. Our results should encourage further research in the possible role of creatine therapy in neuroprotection.

Consumption of redox energy by glutathione metabolism contributes to hypoxia/ reoxygenation-induced injury in astrocytes

The role of glutathione during ischemia/reperfusion is still a controversial issue. Glutathione should exert beneficial effects in the situation of ischemia/reperfusion due to its antioxidative potency. However, increasing survival time after transient ischemia and hypoxia has been reported for glutathione depleted cells. This work was aimed to analyse whether glutathione metabolism essentially contributes to redox energy failure and subsequent cell damage during ischemia/reperfusion. For this purpose, primary astrocyte rich cell cultures were subjected to 1 h hypoxia followed by up to 4 h reoxygenation in combination with substrate deprivation and glutathione depletion. The ability of the cells to reduce MTT was used to quantify the redox power of the cells. Inhibition of glutathione synthesis by L-buthionine-(S,R)-sulfoximine (BSO) caused depletion of cellular glutathione within 24 h and increase in MTT reduction by about 10% under normoxic conditions. Reoxygenation following 1 h of hypoxia was associated with decrease in MTT reduction which was enhanced by substrate deprivation. Glutathione depletion reduced hypoxia-induced decrease in MTT reduction. Three hours of substrate deprivation prior hypoxia resulted in lower levels of MTT reduction during reoxygenaton. Our data suggest that in situations of oxidative stress such as ischemia/reperfusion, glutathione metabolism may causes decrease of the cellular redox energy below a threshold level required for basic cellular functions finally resulting in cell injury.

Differential metabolic adaptation to acute and long-term hypoxia in rat primary cortical astrocytes

Brain astrocytes provide structural and metabolic support to surrounding cells during ischemia. Glucose and oxygen are critical to brain function, and glucose uptake and metabolism by astrocytes are essential to their metabolic coupling to neurons. To examine astrocyte metabolic response to hypoxia, cell survival and metabolic parameters were assessed in rat primary cortical astrocytes cultured for 3 weeks in either normoxia or in either 1 day or 3 weeks sustained hypoxia (5% O2). Although cell survival and proliferation were not affected by the mildly hypoxic environment, substantial differences in glucose consumption and lactate release after either acute or prolonged hypoxia suggest that astrocyte metabolism may contribute to their adaptation. Hypoxia over a period of 1 day increased glucose uptake, lactate release, and glucose transporter 1 (GLUT1) and monocarboxylate transporter 1 (MCT1) expression, whereas hypoxia over a period of 3 weeks resulted in a decrease of all parameters. Furthermore, increased glucose uptake at 1 day of hypoxia was not inhibited by cytochalasin B suggesting the involvement of additional glucose transporters. We uncovered hypoxia-regulated expression of sodium-dependent glucose transporters (SGLT1) in astrocytes indicating a novel adaptive strategy involving both SGLT1 and GLUT1 to regulate glucose intake in response to hypoxia. Overall, these findings suggest that although increased metabolic response is required for the onset of astrocyte adaptation to hypoxia, prolonged hypoxia requires a shift to an energy conservation mode. These findings may contribute to the understanding of the relative tolerance of astrocytes to hypoxia compared with neurons and provide novel therapeutic strategies aimed at maintaining brain function in cerebral pathologies involving hypoxia.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Seasonal differences and protection by creatine or arginine pretreatment in ischemia of mammalian and molluscan neurons in vitro

We investigated the dose-response relationship of protection by creatine against ischemic damage, and we asked whether or not such protection may be observed in invertebrate neurons that might provide a simpler experimental model. Rat isolated pyramidal neurons from the CA3 region of hippocampus subjected to ischemia ("in vitro ischemia") showed anoxic depolarization (AD) after 3-7 min of incubation in anoxic medium. Membrane potential (MP) was reduced 25-78% from preanoxic value. Inward current was decreased by an average 49%. Supplementation with creatine protected against these changes, with 1 mM being the minimal effective concentration, 2 mM providing a near-maximal protection, a maximal effect being obtained with 5 mM creatine. No additional protection was provided by up to 20 mM creatine. Isolated giant neurons of Lymnaea stagnalis showed a similar response to in vitro ischemia. However, a clear seasonal dependence of sensitivity of these cells was detected. In cells obtained during summertime (May-August), AD latency ranged from 3 to 10 min; during wintertime (December-March), this response did not occur even after 25-50 min. The addition of creatine to the medium did not cause changes in AD latency, probably because these neurons rely on a phosphoarginine/arginine energy system. However, treatment of the cells, harvested during summertime, with 2 mM arginine did provide clear protection against anoxic-aglycaemic changes. Summing up, besides confirming previous findings on creatine protection in mammalian neurons, we (1) better characterized their dose-response relationship and extended the findings to the CA3 region and to isolated neurons, (2) found that invertebrate neurons are not protected by creatine but by arginine supplementation and (3) reported a novel mechanism of seasonal dependence in sensitivity of in vitro ischemia by invertebrate neurons.

Improving the survival of human CNS precursor-derived neurons after transplantation

We have examined the effects of predifferentiation and energy substrate deprivation on long-term expanded human neural precursor cells (HNPCs). The pre-differentiation of HNPC cultures produced large numbers of neurons (>60%) and mature glial cells capable of generating glycogen stores that protected the neuronal population from experimental metabolic stress. When predifferentiated HNPCs were transplanted into intact adult rat hippocampus, fewer cells survived compared to undifferentiated HNPC transplants. This cell death was completely attenuated, however, when predifferentiated HNPC cultures were pretreated to boost glial energy stores and resulted in greatly increased neuronal survival in vivo. The transplanted cells primarily engrafted within the granular layer of the dentate gyrus, where a large proportion of the predifferentiated HNPCs co-expressed neuronal markers whereas most HNPCs outside of the neuronal layer did not, indicating that the predifferentiated cells remained capable of responding to local cues in the adult brain. Undifferentiated HNPCs migrated more widely in the brain after grafting than did the predifferentiated cells, which generally remained within the hippocampus.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Prolonged neonatal seizures exacerbate hypoxic-ischemic brain damage: correlation with cerebral energy metabolism and excitatory amino acid release

BACKGROUND

Perinatal hypoxia-ischemia (HI) is the most common precipitant of seizures in the first 24-48 h of a newborn's life. In a previous study, our laboratory developed a model of prolonged, continuous electrographic seizures in 10-day-old rat pups using kainic acid (KA) as a proconvulsant. Groups of animals included those receiving only KA, or HI for 15 or 30 min, followed by KA infusion. Our results showed that prolonged electrographic seizures following 30 min of HI resulted in a marked exacerbation of brain damage. We have undertaken studies to determine alterations in hippocampal high-energy phosphate reserves and the extracellular release of hippocampal amino acids in an attempt to ascertain the underlying mechanisms responsible for the damage promoted by the combination of HI and KA seizures.

METHODS

All studies were performed on 10-day-old rats. Five groups were identified: (1) group I--KA alone, (2) group II--15 min of HI plus KA, (3) group III--15 min of HI alone, (4) group IV--30 min of HI plus KA, and (5) group VI--30 min of HI alone. HI was induced by right common carotid artery ligation and exposure to 8% oxygen/balance nitrogen. Glycolytic intermediates and high-energy phosphates were measured. Prior to treatment, at the end of HI (both 15 and 30 min), prior to KA injection, and at 1 (onset of seizures), 3, 5 (end of seizures), 7, 24 and 48 h, blood samples were taken for glucose, lactate and beta-hydroxybutyrate. At the same time points, animals were sacrificed by decapitation and brains were rapidly frozen for subsequent dissection of the hippocampus and measurement of glucose, lactate, beta-hydroxybutyrate, adenosine triphosphate (ATP) and phosphocreatine (PCr). In separate groups of rats as defined above, microdialysis probes (CMA) were stereotactically implanted into the CA2-3 region of the ipsilateral hippocampus for measurement of extracellular amino acid release. Dialysate was collected prior to any treatment, at the end of HI (15 and 30 min), prior to KA injection, and at 1 (onset of seizures), 3, 5 (end of seizures), 7 and 9 h. Determination of glutamate, serine, glutamine, glycine, taurine, alanine, and GABA was accomplished using high-performance liquid chromatography with EC detection.

RESULTS

Blood and hippocampal glucose concentrations in all groups receiving KA were significantly lower than control during seizures (p < 0.05). beta-Hydroxybutyrate values displayed the inverse, in that values were significantly higher (p < 0.01) in all KA groups compared with pretreatment controls during seizure activity. Values returned to control by 2 h following the cessation of seizures. Lactate concentrations in brain and blood mimicked those of beta-hydroxybutyrate. ATP values declined to 0.36 mmol/l in both the 15 and 30 min hypoxia groups compared with 1.85 mmol/l for controls (p < 0.01). During seizures, ATP and PCr values declined significantly below their homologous controls. Following seizures, ATP values only for those animals receiving KA plus HI for 30 min remained below their homologous controls for at least 24 h. Determination of amino acid release revealed elevations of glutamate, glycine, taurine, alanine and GABA above pretreatment control during HI, with a return to normal prior to KA injections. During seizures and for the 4 h of recovery monitored, only glutamate in the combined HI and KA group rose significantly above both the 15 min of HI plus KA and the KA alone group (p < 0.05).

CONCLUSION

Under circumstances in which there is a protracted depletion of high-energy phosphate reserves, as occurs with a combination of HI- and KA-induced seizures, excess amounts of glutamate become toxic to the brain. The latter may account for the exacerbation of damage to the newborn hippocampus, and serve as a target for future therapeutic intervention.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

Changes of ATP and ADP in cultured astrocytes under and after in vitro ischemia

A very large body of evidence from in vivo studies has been accumulated on a link between the change of energy and cell survival/apoptosis. Using an in vitro ischemia model, we have previously shown that ischemia could induce apoptosis in astrocytes. In this study, we utilized the same in vitro model to investigate changes in ATP and ADP levels in cultured astrocytes and attempted to demonstrate an energy-cell death linkage. Astrocytes remained unaltered after 2 hr of ischemia but were moderately or severely damaged after 4 or 6-8 hr, respectively. The astrocytes that survived various lengths of in vitro ischemic incubation retained their ability to produce ATP after ischemia. Both ATP and ADP levels were increased in astrocytes that remained alive under in vitro ischemia for over 6 hr. The largest decline in the percent of viable astrocytes during ischemia corresponded well to the reduction in ATP and ADP levels in these cultures.

Adenosine and purine nucleosides protect rat primary astrocytes from peroxynitrite-potentiated, glucose deprivation-induced death: preservation of intracellular ATP level

Previously we have reported that immunostimulated astrocytes became highly vulnerable to glucose deprivation. In the present study we examined the effect of various kinds of nucleosides on the augmented death of glucose-deprived immunostimulated astrocytes. Preincubation with interferon-gamma (100 U/ml) and lipopolysaccharide (1 microg/ml) for 48 h and continuous exposure to glucose deprivation (4 h) significantly induced the lactate dehydrogenase (LDH) release, as a marker of cell injury or death, from astrocytes. The glucose deprivation-induced augmented cell death in immunostimulated astrocytes was mimicked by exogenous peroxynitrite generator 3-morpholinosydnonimine (SIN-1). The increased death in immunostimulated or SIN-1-treated astrocytes deprived of glucose was blocked by adenosine and ATP. Other purine nucleos(t)ides, not pyrimidine nucleotides, also showed similar protective effects. Adenosine receptor agonist R(-)-N-(2-phenylisopropyl)-adenosine or N-cyclohexyladenosine did not alter the augmented cell death. Adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, xanthine amine congener or 3,7-dimethyl-1-propargylxanthine also did not reverse the protective effect of adenosine. Intracellular ATP levels rapidly decreased prior to the LDH release in glucose-deprived immunostimulated astrocytes. The loss of intracellular ATP was prevented by adenosine and other purine nucleotides. The present results suggest that adenosine and their metabolites may protect astrocytes from peroxynitrite-potentiated, glucose deprivation-induced death by serving as substrates for intracellular ATP generation.

Prolonged seizures exacerbate perinatal hypoxic-ischemic brain damage

This study was undertaken to clarify whether seizures in the newborn cause damage to the healthy brain and, more specifically, to determine the extent to which seizures may contribute to the brain-damaging effects of hypoxia-ischemia (HI). Seizures were induced in 10-d-old rat pups with kainic acid (KA). Seizure duration was determined electrographically. HI was induced by common carotid artery ligation followed by exposure to 8% oxygen for either 15 or 30 min. Six groups of animals were assessed: 1) controls [neither KA nor HI (group I)]; 2) group II, KA alone; 3) group III, 15 min HI alone; 4) group IV,15 min HI plus KA; 5) group V, 30 min HI alone; and 6) group VI, 30 min HI plus KA. Animals were assessed neuropathologically at 3 (early) and 20 (late) d of recovery. KA injection without hypoxia resulted in continuous clinical and electrographic seizures lasting a mean of 282 min. No neuropathologic injury was seen in groups I (no HI or KA), II (KA alone), III (15 min HI alone), or IV (15 min HI and KA). Animals in group V (30 min HI alone) displayed brain damage with a mean score of 2.3 and 0.60 at 3 and 20 d of recovery, respectively. Animals in group VI (30 min HI and KA) had a mean score of 12.1 and 3.65 at 3 and 20 d of recovery, respectively. Compared with group V, the increased damage as a result of the seizure activity in group VI occurred exclusively in the hippocampus. Status epilepticus in the otherwise "healthy" neonatal brain does not cause neuropathologic injury. However, seizures superimposed on HI significantly exacerbate brain injury in a topographically specific manner.

Anoxic ATP depletion in neonatal mice brainstem is prevented by creatine supplementation

BACKGROUND

Sufficient ATP concentrations maintain physiological processes and protect tissue from hypoxic damage. With decreasing oxygen concentration, ATP synthesis relies increasingly on the presence of phosphocreatine.

AIM

The effect of exogenously applied creatine on phosphocreatine and ATP concentrations was studied under control and anoxic conditions.

METHODS

Pregnant mice were fed orally with creatine monohydrate (2 g/kg body weight/day). Brainstem slices from these mice pups were compared with those from pups of non-creatine supplemented pregnant mice. Measurements were performed under normoxic and anoxic conditions. In addition, brainstem slices from non-creatine treated mice pups were incubated for 3 hours in control artificial cerebrospinal fluid (CSF) (n = 10) or in artificial CSF containing 200 microM creatine (n = 10). ATP and phosphocreatine contents were determined enzymatically in single brainstem slices.

RESULTS

ATP concentrations were in the same range in all preparations. However, there was a significant increase of phosphocreatine in the brainstems from pups of creatine fed mice when compared with the brainstems of pups from non-creatine treated mice or in non-incubated brainstems of control animals. After 30 minutes anoxia, ATP as well as phosphocreatine concentrations remained significantly higher in creatine pretreated slices compared with controls.

CONCLUSION

The data indicate that exogenous application of creatine is effective in neuroprotection.

Astrocytes respond to hypoxia by increasing glycolytic capacity

Astrocytes cope more readily with hypoxic insults than do neurons. We hypothesized that astrocytes can upregulate their glycolytic capacity, allowing anaerobic glycolysis to provide sufficient ATP for cell survival as well as for carrying out critical functions such as taking up glutamate. To test this hypothesis, astrocytes were subjected to hypoxia for 5 hr. Lactate dehydrogenase (LDH) and pyruvate kinase activities increased 3- to 4-fold. Examination of LDH isoenzyme patterns determined that it was the anaerobic isoenzymes that were upregulated. To determine whether increase in enzyme activity translates into increased glycolytic capacity, astrocytes were subjected to varying time periods of hypoxia, and glucose uptake was measured under conditions where astrocytes were forced to consume more ATP. This demonstrated that 8 hr of hypoxia resulted in a doubling of glycolytic capacity. We suggest that how quickly astrocytes upregulate glycolytic capacity may determine whether or not neurons within the stroke penumbra survive.

Inosine and guanosine preserve neuronal and glial cell viability in mouse spinal cord cultures during chemical hypoxia

Murine spinal cord primary mixed cultures were treated with the respiratory inhibitor, rotenone, to mimic hypoxic conditions. Under these conditions neurons rapidly underwent oncosis (necrosis) with a complete loss in viability occurring within 260 min; however, astrocytes, which accounted for most of the cell population, died more slowly with 50% viability occurring at 565 min. Inosine preserved both total cell and neuronal viability in a concentration-dependent manner. The time of inosine addition relative to hypoxic insult was critical with the most effective protection occurring when inosine was added just prior to or within 5 min after insult. Inosine was ineffective when added 30 min after hypoxic insult. The effect of guanosine was similar to that of inosine. Treatment of cultures with BCX-34, a purine nucleoside phosphorylase inhibitor, prevented protection by inosine or guanosine, suggesting involvement of a purine nucleoside phosphorylase in the nucleoside protective effect.

Magnetic resonance imaging during cerebral hypoxia-ischemia: T2 increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T2-weighted (T2W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8+/-4.9% and 30.5+/-2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0+/-8.3%) also appeared in T2W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T2, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T2W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T2W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T2 within 2-wk-old but not 4-wk-old animals.

Oxygen and ion concentrations in normoxic and hypoxic brain cells

The goal of the present contribution is to discuss the relationships among brain oxygen tension, energy (ATP) level, and ion gradients and movements. The function of the CNS, the generation and transmission of impulses, is determined to a large extent by the movements of ions. Hence elucidation of these relationships is necessary to the understanding of how brain works. Moreover, such knowledge is indispensable for the design of rational therapies for treatment of a large group of pathological states caused by lack of oxygen. This paper is partly a review and partly an original contribution although the former involves to a considerable extent, results obtained in our laboratories. It is divided into 3 parts: a) a very brief general introduction which reminds the reader some well-known facts; b) presentation and discussion of data; and c) conclusions and/or predictions.

The effects of glucose, mannose, fructose and lactate on the preservation of neural activity in the hippocampal slices from the guinea pig

Using hippocampal slices from guinea pigs, we investigated the effect of different concentrations of glucose and replacement of glucose with mannose, fructose and lactate on neural activity. As an index of neural activity, the population spikes (PS) were recorded in the granule cell layer of the dentate gyrus (DG) and the pyramidal cell layer of the CA3 area in the hippocampal slices. Lowering the concentration of glucose from 10 mM to 5, 3, 2, 1 and 0 mM caused a reduction in the PS amplitude. There were differences in the decay times of the PS evoked in these two regions. PS evoked in CA3 region decayed faster even at a concentration of 3 mM glucose at which PS in granule cell layer was well maintained. The decay time of the PS in the CA3 region in the presence of glucose up to a concentration of 3 mM was shorter than that evoked in the DG. After the replacement of glucose with mannose, fructose or lactate, the PS disappeared within 35 min and there were no significant differences between the decay times in the two regions of slices incubated in the same medium. ATP, creatine phosphate (CrP) and lactate levels in each slice were determined. To investigate whether mannose and fructose could be metabolized or not in the tissue slice, anaerobic production of lactate from glucose, mannose and fructose were measured during oxygen and glucose deprivation. Under anaerobic conditions for 60 min, the levels of high-energy phosphates decreased to 50% of the initial level and lactate was produced from glucose, mannose or fructose. However, there were significant differences in the rate of lactate production between the DG and CA3 areas during application of 3 mM glucose, 10 mM mannose and 10 mM fructose. These results indicate that mannose, fructose and lactate can be metabolized and are available for maintaining the levels of high-energy phosphates but not for neural activity in the tissue slices and that the presence of glucose is indispensable for the maintenance of neural activity.

Increasing vulnerability of astrocytes to oxidative injury with age despite constant antioxidant defenses

This paper investigates the vulnerability of astrocytes to oxidative injury as a function of age in culture in mice. Primary murine cortical astrocyte cultures of different ages were exposed to H2O2, combined oxygen-glucose deprivation or glucose deprivation. Astrocytes became more vulnerable to damage from each injury paradigm with age, showing transitions between 15 and 22 days. Both the antioxidant glutathione and superoxide dismutase activity increased after 30 days in culture, while catalase activity did not change up to 34 days. When the decrease in glutathione with injury was measured, young cells showed no change with H2O2 and decreases of < 20% after oxygen-glucose deprivation or glucose deprivation, while older cultures lost > 50% of their glutathione with the same insults. Since iron can be a catalyst for hydroxyl radical formation, we stained cultures and found both iron staining and ferritin immunoreactivity increased with age. Increased iron correlated with protection by deferoxamine against H2O2 injury. The three injury paradigms each had a unique pattern of protection by antioxidants. Dimethylthiourea, a hydrophilic antioxidant, protected from all three insults. Trolox, a lipophilic antioxidant, protected older astrocytes from oxygen-glucose deprivation and glucose deprivation. Deferoxamine provided near complete protection from H2O2, partial protection from oxygen-glucose deprivation and no protection from glucose deprivation. As evidence of increasing oxidative stress, lipid peroxidation resulting from oxygen-glucose deprivation increased with age, assessed with cis-parinaric acid. The increasing sensitivity of ageing astrocytes to oxidative injury occurs while antioxidant defenses are maintained. Increased sensitivity to H2O2 or oxygen-glucose deprivation correlates with iron accumulation.

The effects of lactate and beta-hydroxybutyrate on the energy metabolism and neural activity of hippocampal slices from adult and immature rat

We investigated the correlation between energy metabolism and neural activity during glucose deprivation and during replacement of glucose with lactate and beta-hydroxybutyrate (OHBA) in neural tissue from rats of different ages. Hippocampal slices were prepared from 4-, 7-, 10-, 13- and 16-day-old and adult rats. The population spikes (PS) were recorded in the pyramidal cell layer of the CA3 area as the index of neural activity. ATP and creatine-phosphate (CrP) levels in each slice were determined during glucose deprivation and during replacement of glucose with lactate or OHBA. After deprivation of glucose, the PS of the slices from 4-, 7- and 10-day-old and adult rats decayed and extinguished in 30 min and the decay time was shortened according to the age of the rat. The levels of ATP and CrP in the slices also decreased, but to a lesser extent than the amplitudes of PS. After substitution of lactate or beta-hydroxybutyrate (OHBA) for glucose, PS of the adult rat disappeared as was the case with glucose deprivation, although the levels of high energy phosphates were well maintained. In the case of the immature rat, however, PS decayed more slowly. Especially in the case of 4-day-old rat, ATP and CrP in the slices were maintained as high as those under the initial concentrations and PS amplitude showed no decay even after 60 min. These results indicate that the presence of glucose is essential for neural activity in the adult rat, and lactate or OHBA cannot replace it for the maintenance of neural activity. In the immature rat, glucose metabolites such as lactate and OHBA are available for both neural activity as well as maintaining the levels of high-energy phosphates in the tissue slice.

Ion homeostasis in brain cells: differences in intracellular ion responses to energy limitation between cultured neurons and glial cells

Intracellular concentrations of sodium, potassium and calcium together with membrane potentials were measured in cultured murine cortical neurons and glial cells under conditions which mimicked in vivo hypoxia, ischemia and hypoglycemia. These included; glucose omission with and without added pyruvate, addition of rotenone in the presence and absence of glucose and substitution of 2-deoxyglucose for glucose with and without rotenone. Cellular energy levels ([ATP], [ADP], [phosphocreatine], [creatine]) were measured in suspensions of C6 cells incubated in parallel under identical conditions. [Na+]i and [Ca2+]i rose while [K+]i fell and plasma membrane depolarized when energy production was limited. Intracellular acidification was observed when glycolysis was the sole source for ATP synthesis. There was a positive correlation between the extent of energy depletion in glial cells and the magnitude and velocity of alterations in ion levels. Neither glycolysis alone nor oxidative phosphorylation alone were able to ensure unaltered ion gradients. Since oxidative phosphorylation is much more efficient in generating ATP than glycolysis, this finding suggests a specific requirement of the Na pump for ATP generated by glycolysis. Changes in [Na+]i and [K+]i observed during energy depletion were gradual and progressive whereas those in [Ca2+]i were initially slow and moderate with large elevations occurring only as a late event. Increases in [Na+]i were usually smaller than reductions in [K+]i, particularly in the glia, suggestive of cellular swelling. Glia were less sensitive to identical insults than were neurons under all conditions. Results presented in this study lead to the conclusion that the response to energy deprivation of the two main types of brain cells, neurons and astrocytes, is a complex function of their capacity to produce ATP and the activities of various pathways which are involved in ion homeostasis.

Effect of mild hypothermia on cerebral energy metabolism during the evolution of hypoxic-ischemic brain damage in the immature rat

BACKGROUND AND PURPOSE

Intraischemic hypothermia (34 degrees C and 31 degrees C) has a profound neuroprotective effect on the brain of the immature rat. Hypothermia immediately after hypoxia-ischemia is not beneficial. To determine the mechanisms by which mild to moderate hypothermia affects cerebral energy metabolism of the brain of the newborn rat pup, we examined alterations in cerebral glycolytic intermediates and high-energy phosphate compounds during intraischemic and postischemic hypothermia and correlated these findings with known neuropathologic injury.

METHODS

Seven-day-old rat pups underwent unilateral common carotid artery ligation and exposure to hypoxia in 8% oxygen at either 37 degrees C, 34 degrees C, or 31 degrees C for 3.0 hours. Separate groups were exposed to hypoxia-ischemia at 37 degrees C for 3 hours but recovered at either 37 degrees C, 34 degrees C, or 31 degrees C. At 60, 120, and 180 minutes of intraischemic hypothermia and at 10, 30, 60, and 240 minutes of postischemic hypothermia, individual rat pups were quick-frozen in liquid nitrogen for later determination of cerebral concentrations of glucose, lactate, ATP, and phosphocreatine.

RESULTS

Cerebral glucose was significantly higher and lactate significantly lower in the 31 degrees C animals during hypoxia-ischemia than either the 34 degrees C or 37 degrees C groups. Brain ATP concentrations were completely preserved during hypoxia-ischemia at 31 degrees C, whereas 34 degrees C of hypothermia had no effect on preserving high-energy phosphate compounds compared with those animals in the 37 degrees C group. Postischemic hypothermia of either 34 degrees C or 31 degrees C had no effect on the rate or extent of recovery of glycolytic intermediates or high-energy phosphate compounds compared with the normothermic 37 degrees C rat pups.

CONCLUSIONS

Moderate hypothermia of 31 degrees C completely inhibits the depletion of ATP during hypoxia-ischemia, a mechanism that likely accounts for its neuroprotective effect. No preservation of ATP was seen, however, during intraischemic mild hypothermia of 34 degrees C despite the relatively profound neuroprotective effect of this degree of temperature reduction. Thus, the mechanisms by which mild hypothermia is neuroprotective are temperature dependent and may act at more than one point along the cascade of events eventually leading to hypoxic-ischemic brain damage in the immature rat.

Glutathione release and catabolism during energy substrate restriction in astrocytes

This study examined the effect of simulated ischemia (deprivation of both oxygen and substrate) on astrocyte reduced-glutathione (GSH). We have demonstrated that under normoxic conditions there is no GSH efflux from living astrocytes; this suggests that the high levels of GSH in astrocytes in vivo are not available for neighbouring neural cells. Under simulated ischemia there is release of GSH from astrocytes only when astrocytes die. Furthermore, when astrocytic energy stores are depleted GSH is catabolized, such that after 12 h of simulated ischemia approximately 20% of GSH is catabolized. This GSH catabolism can be increased at an earlier time by causing increased ATP utilization through activating the sodium pump either by introducing glutamate into the culture medium or by raising medium potassium. Since GSH is catabolized into glycine, glutamate and cysteine, the latter two amino acids being neurotoxic, our findings indicate that the high levels of GSH in astrocytes may be used by these cells to survive ischemic insults, but the catabolism of GSH may result in increased neuronal damage.

Oligodendroglial precursor cell susceptibility to hypoxia is related to poor ability to cope with reactive oxygen species

Oligodendrocyte precursors and astrocytes in 2-week-old rat primary glial cultures survived 24 h of anoxia, suggesting both cell types could survive using glycolysis for ATP synthesis; however, when the hypoxia developed gradually, the majority of oligodendrocyte precursor cells died within 24 h of the beginning of the experiment but astrocytes survived. Similarly when cultures were exposed to an atmosphere of 1% oxygen, but not 2% or greater, oligodendrocyte precursors died within 24 h. Much more lipid peroxidation was seen under conditions of hypoxia than under conditions of anoxia suggesting that oligodendrocyte precursors died under the former condition because of free radical-induced damage. Using 5-(and -6)-carboxy-2',7'-dichlorodihydrofluorescein (DCFH) as an intracellular probe of oxidative stress, we have demonstrated directly on living cells that oligodendrocyte precursors have a poorer ability to scavenge free radicals than astrocytes. Furthermore, when free radicals were induced to form in the cells either by cysteine auto-oxidation or menadione redox cycling, oligodendrocyte precursors were more readily damaged than astrocytes. We conclude that oligodendroglial precursor cells are exquisitively sensitive to reactive oxygen species.

Astrocyte survival in the absence of exogenous substrate: comparison of immature and mature cells

Astrocyte cultures prepared from newborn mouse neopallium were grown for either one or three weeks (representing, respectively, immature and mature astrocytes) and then exposed to deprivation of substrate (glucose and amino acids) for up to 48 hr. Cultures which had been deprived of metabolic substrates for either 24, 30, 36 or 48 hr were examined for lactate dehydrogenase efflux into the medium (an indicator of cell death) and ATP content. Significant cell death in mature astrocytes began after 30 hr of incubation in the substrate-deprived medium, a time when ATP had fallen to approximately 10% of its initial value. Immature astrocytes survived on a substrate-free medium for 48 hr before there was any indication at all of cell death, and this corresponded to a time when ATP values had fallen to 5% of the initial values. These findings are compared to previous observations during simulated ischemia (substrate deprivation plus anoxia) when (1) there was a faster cell death and (2) cell death occurred at higher ATP levels.