Glutamate uptake and glutamate content in primary cultures of mouse astrocytes during anoxia, substrate deprivation and simulated ischemia under normothermic and hypothermic conditions

Effect of mild hyponatremia on in-hospital falls of elderly hospitalized patients: A retrospective, cohort study

OBJECTIVES

This study investigated the association between hyponatremia and falls in elderly hospitalized patients, focusing on mild hyponatremia as a potential risk factor.

MATERIALS AND METHODS

A retrospective analysis of 16,952 patients admitted to Kochi Medical School Hospital from 2012 to 2021 was performed. Serum sodium levels were categorized, and falls during a 30-day observation period were recorded. A Cox proportional hazards model and a machine learning model were used to estimate risk and explore interactions.

RESULTS

Mild hyponatremia (130-134 mEq/L) was identified as an independent risk factor for falls (hazard ratio: 1.42, 95 % confidence interval 1.16-1.74), especially in patients with higher activities of daily living. The fall prediction model showed an area under the curve (AUC) of 0.780 (95 % confidence interval 0.751-0.806).

CONCLUSION

A significant association between mild hyponatremia and falls in elderly hospitalized patients was found. The findings highlight the need for targeted fall prevention and further research into the underlying mechanisms. Mild hyponatremia may serve as a clinical marker for fall risk, especially in patients with independent activities of daily living.

Excitatory amino acid changes in the brains of rhesus monkeys following selective cerebral deep hypothermia and blood flow occlusion

Selective cerebral deep hypothermia and blood flow occlusion can enhance brain tolerance to ischemia and hypoxia and reduce cardiopulmonary complications in monkeys. Excitotoxicity induced by the release of a large amount of excitatory amino acids after cerebral ischemia is the major mechanism underlying ischemic brain injury and nerve cell death. In the present study, we used selective cerebral deep hypothermia and blood flow occlusion to block the bilateral common carotid arteries and/or bilateral vertebral arteries in rhesus monkey, followed by reperfusion using Ringer's solution at 4°C. Microdialysis and transmission electron microscope results showed that selective cerebral deep hypothermia and blood flow occlusion inhibited the release of glutamic acid into the extracellular fluid in the brain frontal lobe and relieved pathological injury in terms of the ultrastructure of brain tissues after severe cerebral ischemia. These findings indicate that cerebral deep hypothermia and blood flow occlusion can inhibit cytotoxic effects and attenuate ischemic/hypoxic brain injury through decreasing the release of excitatory amino acids, such as glutamic acid.

Use of hypothermia in the intensive care unit

Used for over 3600 years, hypothermia, or targeted temperature management (TTM), remains an ill defined medical therapy. Currently, the strongest evidence for TTM in adults are for out-of-hospital ventricular tachycardia/ventricular fibrillation cardiac arrest, intracerebral pressure control, and normothermia in the neurocritical care population. Even in these disease processes, a number of questions exist. Data on disease specific therapeutic markers, therapeutic depth and duration, and prognostication are limited. Despite ample experimental data, clinical evidence for stroke, refractory status epilepticus, hepatic encephalopathy, and intensive care unit is only at the safety and proof-of-concept stage. This review explores the deleterious nature of fever, the theoretical role of TTM in the critically ill, and summarizes the clinical evidence for TTM in adults.

Spatial and temporal correlation in progressive degeneration of neurons and astrocytes in contusion-induced spinal cord injury

BACKGROUND

Traumatic spinal cord injury (SCI) causes acute neuronal death followed by delayed secondary neuronal damage. However, little is known about how microenvironment regulating cells such as microglia, astrocytes, and blood inflammatory cells behave in early SCI states and how they contribute to delayed neuronal death.

METHODS

We analyzed the behavior of neurons and microenvironment regulating cells using a contusion-induced SCI model, examining early (3-6 h) to late times (14 d) after the injury.

RESULTS

At the penumbra region close to the damaged core (P1) neurons and astrocytes underwent death in a similar spatial and temporal pattern: both neurons and astrocytes died in the medial and ventral regions of the gray matter between 12 to 24 h after SCI. Furthermore, mRNA and protein levels of transporters of glutamate (GLT-1) and potassium (Kir4.1), functional markers of astrocytes, decreased at about the times that delayed neuronal death occurred. However, at P1 region, ramified Iba-1+ resident microglia died earlier (3 to 6 h) than neurons (12 to 24 h), and at the penumbra region farther from the damaged core (P2), neurons were healthy where microglia were morphologically activated. In addition, round Iba-1/CD45-double positive monocyte-like cells appeared after neurons had died, and expressed phagocytic markers, including mannose receptors, but rarely expressed proinflammatory mediators.

CONCLUSION

Loss of astrocyte function may be more critical for delayed neuronal death than microglial activation and monocyte infiltration.

Increased concentrations of both NMDA receptor co-agonists D-serine and glycine in global ischemia: a potential novel treatment target for perinatal asphyxia

Worldwide, perinatal asphyxia is an important cause of morbidity and mortality among term-born children. Overactivation of the N-methyl-d-aspartate receptor (NMDAr) plays a central role in the pathogenesis of cerebral hypoxia–ischemia, but the role of both endogenous NMDAr co-agonists d-serine and glycine remains largely elusive. We investigated d-serine and glycine concentration changes in rat glioma cells, subjected to oxygen and glucose deprivation (OGD) and CSF from piglets exposed to hypoxia–ischemia by occlusion of both carotid arteries and hypoxia. We illustrated these findings with analyses of cerebrospinal fluid (CSF) from human newborns affected by perinatal asphyxia. Extracellular concentrations of glycine and d-serine were markedly increased in rat glioma cells exposed to OGD, presumably through increased synthesis from l-serine. Upon reperfusion glycine concentrations normalized and d-serine concentrations were significantly lowered. The in vivo studies corroborated the finding of initially elevated and then normalizing concentrations of glycine and decreased d-serine concentrations upon reperfusion These significant increases of both endogenous NMDAr co-agonists in combination with elevated glutamate concentrations, as induced by global cerebral ischemia, are bound to lead to massive NMDAr activation, excitotoxicity and neuronal damage. Influencing these NMDAr co-agonist concentrations provides an interesting treatment target for this common, devastating and currently poorly treatable condition.

Investigational therapies for ischemic stroke: neuroprotection and neurorecovery

Stroke is one of the leading causes of death and disability worldwide. Current treatment strategies for ischemic stroke primarily focus on reducing the size of ischemic damage and rescuing dying cells early after occurrence. To date, intravenous recombinant tissue plasminogen activator is the only United States Food and Drug Administration approved therapy for acute ischemic stroke, but its use is limited by a narrow therapeutic window. The pathophysiology of stroke is complex and it involves excitotoxicity mechanisms, inflammatory pathways, oxidative damage, ionic imbalances, apoptosis, angiogenesis, neuroprotection, and neurorestoration. Regeneration of the brain after damage is still active days and even weeks after a stroke occurs, which might provide a second window for treatment. A huge number of neuroprotective agents have been designed to interrupt the ischemic cascade, but therapeutic trials of these agents have yet to show consistent benefit, despite successful preceding animal studies. Several agents of great promise are currently in the middle to late stages of the clinical trial setting and may emerge in routine practice in the near future. In this review, we highlight select pharmacologic and cell-based therapies that are currently in the clinical trial stage for stroke.

Hypothermia as a cytoprotective strategy in ischemic tissue injury

Hypothermia is a well established cytoprotectant, with remarkable and consistent effects demonstrated across multiple laboratories. At the clinical level, it has recently been shown to improve neurological outcome following cardiac arrest and neonatal hypoxia-ischemia. It is increasingly being embraced by the medical community, and could be considered an effective neuroprotectant. Conditions such as brain injury, hepatic encephalopathy and cardiopulmonary bypass seem to benefit from this intervention. It's role in direct myocardial protection is also being explored. A review of the literature has demonstrated that in order to appreciate the maximum benefits of hypothermia, cooling needs to begin soon after the insult, and maintained for relatively long period periods of time. In the case of ischemic stroke, cooling should ideally be applied in conjunction with the re-establishment of cerebral perfusion. Translating this to the clinical arena can be challenging, given the technical challenges of rapidly and stably cooling patients. This review will discuss the application of hypothermia especially as it pertains to its effects neurological outcome, cooling methods, and important parameters in optimizing hypothermic protection.

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.

Glutamate, a neurotransmitter--and so much more. A synopsis of Wierzba III

It appears almost incredible that the first indications that glutamate excites brain tissue were obtained during the second half of the 20th century, that vesicles containing glutamate were demonstrated in glutamatergic neurons less than 25 years ago, and that glutamate was not accepted as the major excitatory transmitter until about the same time. During this span of time it has also become realized that glutamate is so much more than a conventional neurotransmitter: (1) astrocytes express vesicles accumulating glutamate by vesicular transporters akin to the vesicular glutamate transporters in glutamatergic neurons, and they release glutamate by exocytosis; (2) a series of metabolic processes in astrocytes (glutamate uptake, glutamine synthetase activity, glutamine release) are involved in neuronal reutilization of transmitter glutamate; (3) glutamine may also be utilized for synthesis of GABA, the major inhibitory transmitter; (4) de novo synthesis of glutamate accounts for 20% of cerebral glucose metabolism, all of which initially occurs in astrocytes, and at steady state a corresponding amount of glutamate is oxidatively degraded, mainly or exclusively in astrocytes; (5) tissue contents of glutamate/glutamine increase during enhanced glutamatergic activity, i.e., astrocytic de novo synthesis exceeds astrocytic metabolic degradation of glutamate.

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.

Enhanced release of synaptic glutamate underlies the potentiation of oxygen-glucose deprivation-induced neuronal injury after induction of NOS-2

Reactive nitrogen oxide species (RNOS) may contribute to the progression/enhancement of ischemic injury by augmentation of glutamate release, reduction of glutamate uptake, or a combination of both. Consistent with this, induction of nitric oxide synthase (NOS-2) in murine neocortical cell cultures potentiated neuronal cell death caused by combined oxygen-glucose deprivation in association with a net increase in extracellular glutamate accumulation. However, uptake of glutamate via high affinity, sodium-dependent glutamate transporters was unimpaired by induction of NOS-2 under either aerobic or anaerobic conditions. Further, blocking possible routes of extra-synaptic glutamate release with NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a volume-sensitive organic anion channel blocker, or TBOA (d,l-threo-beta-benzyloxyaspartate), an inhibitor of glutamate transport, exacerbated rather than ameliorated injury. Finally, treatment with riluzole or tetanus toxin attenuated the enhancement in both glutamate accumulation and oxygen-glucose deprivation-induced neuronal injury supporting the idea that increased synaptic release of glutamate underlies, at least in part, the potentiation of neuronal injury by RNOS after NOS-2 induction.

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.

Hypothermia after cardiac arrest: feasibility and safety of an external cooling protocol

BACKGROUND

No proven neuroprotective treatment exists for ischemic brain injury after cardiac arrest. Mild-to-moderate induced hypothermia (MIH) is effective in animal models.

METHODS AND RESULTS

A safety and feasibility trial was designed to evaluate mild-to-moderate induced hypothermia by use of external cooling blankets after cardiac arrest. Inclusion criteria were return of spontaneous circulation within 60 minutes of advanced cardiac life support, hypothermia initiated within 90 minutes, persistent coma, and lack of acute myocardial infarction or unstable dysrhythmia. Hypothermia to 33 degrees C was maintained for 24 hours followed by passive rewarming. Nine patients were prospectively enrolled. Mean time from advanced cardiac life support to return of spontaneous circulation was 11 minutes (range 3 to 30); advanced cardiac life support to initiation of hypothermia was 78 minutes (range 40 to 109); achieving 33 degrees C took 301 minutes (range 90 to 690). Three patients completely recovered, and 1 had partial neurological recovery. One patient developed unstable cardiac dysrhythmia. No other unexpected complications occurred.

CONCLUSIONS

Mild-to-moderate induced hypothermia after cardiac arrest is feasible and safe. However, external cooling is slow and imprecise. Efforts to speed the start of cooling and to improve the cooling process are needed.

Influence of hypothermia on cell volume and cytotoxic swelling of glial cells in vitro

In view of the increasing significance of mild hypothermia (32 degrees C) as an efficient procedure of neuroprotection, the present study was performed to examine the influence of this level of hypothermia on the volume of glial cells under physiological as well as under pathological conditions. The influence of mild (32 degrees C) and moderate (27 degrees C) hypothermia on cell volume and cell viability of C6 glioma cells was studied for 60 minutes in vitro. Cells were suspended in an incubation chamber under continuous control of temperature, pH and pO2. Cell volume was measured by an advanced Coulter system. Hypothermia itself was causing significant cell swelling in a dose-dependent manner, which could be prevented by omission of Na(+)-ions from the suspension medium, while the replacement of Cl(-)-ions failed to prevent cell swelling from hypothermia. Inhibition of the Na+/H(+)-antiporter with EIPA (5N-ethyl-n-isopropyl-amiloride, 50 microM) was significantly reducing the hypothermia induced cell swelling, indicating activation of the Na+/H(+)-antiporter. Conversely, mild or moderate hypothermia failed to prevent cell swelling from lactic acid, arachidonic acid or glutamate, i.e. agents which are mediating the development of cytotoxic brain edema in vivo in cerebral trauma, ischemia and other acute insults. The findings indicate that cerebral protection by hypothermia in vivo is most likely not attributable to an inhibition of cytotoxic brain edema. Further investigations, however, are required in vivo and in vitro to elucidate the hypothermia-induced swelling of glial cells in more detail, e.g. as to the role of the Na+/H(+)-antiporter.

Perinatal brain injury: from pathogenesis to neuroprotection

Brain injury secondary to hypoxic-ischemic disease is the predominant form of all brain injury encountered in the perinatal period. The focus of this article is the most recent research developments in this field and especially those developments that should lead to the most profound effects on interventions in the first years of the new millennium. Neuronal injury is the predominant form of cellular injury in the term infant. The principal mechanisms leading to neuronal death after hypoxia-ischemia/reperfusion are initiated by energy depletion, accumulation of extracellular glutamate, and activation of glutamate receptors. The cascade of events that follows involves accumulation of cytosolic calcium and activation of a variety of calcium-mediated deleterious events. Notably this deleterious cascade, which evolves over many hours, may be interrupted even if interventions are instituted after termination of the insult, an important clinical point. Of the potential interventions, the leading candidates for application to the human infant in the relative short-term are mild hypothermia, inhibitors of free radical production, and free radical scavengers. Promising clinical data are available for the use of mild hypothermia.

Role of excitatory amino acid transporter 1 in neonatal rat neuronal damage induced by hypoxia-ischemia

The role of excitatory amino acid transporter 1 in neonatal rat neuronal damage was studied following hypoxia-ischemia. To induce hypoxia-ischemia injury, rats on postnatal day 7 were exposed to 8 % oxygen for 2 h following unilateral common carotid artery ligation. According to brain damage scoring based on Cresyl Violet staining, the neuronal damage time-dependently changed in the ischemic regions following hypoxia-ischemia. Immunohistochemical studies showed that excitatory amino acid transporter 1 expression was mainly observed in the cerebral cortex ipsilateral to common carotid artery ligation and markedly increased at 24 h and 48 h following hypoxia-ischemia. Combined with confocal laser scanning microscopic analysis, double staining showed that excitatory amino acid transporter 1 positive staining appeared in neurons as well as astrocytes after hypoxia-ischemia. Most excitatory amino acid transporter 1 positive staining cells exhibited regular morphological characteristics and only a few were double-stained by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick-end labeling. Down-regulation of excitatory amino acid transporter 1 expression by intraventricular administration of specific antisense oligonucleotide exacerbated neuronal damage in hypoxia-ischemia brain. These results suggest that the increase of excitatory amino acid transporter 1 expression may be involved in a pathophysiological process of hypoxia-ischemia brain damage and may reflect a self-compensative mechanism for protecting neurons from further injury.

Effect of moderate hypothermia on experimental severe subarachnoid hemorrhage, as evaluated by apparent diffusion coefficient changes

OBJECTIVE

The aims of this study were to investigate the early changes in the mean apparent diffusion coefficient (ADC) after severe subarachnoid hemorrhage (SAH), as a marker of ischemic damage, and to examine the effects of moderate hypothermia, induced at various time points, on ADC changes.

METHODS

ADC maps were calculated from diffusion-weighted, blipped-epi, spin echo, magnetic resonance imaging sequences (2.35-T BIOSPEC 24/40 scanner; Bruker Medizin Technik GmbH, Karlsruhe, Germany) for 21 anesthetized (0.45-1% halothane, temperature-adjusted/30% oxygen/69% nitrogen) and ventilated Wistar rats. After baseline scanning, bolus injection of 0.5 ml of autologous arterial blood or artificial cerebrospinal fluid (control group), into the cisterna magna, was performed. Serial scanning was performed for 3 hours after injection, using normothermic or hypothermic (32 degrees C) rats. In an additional series of experiments, hypothermia was initiated either immediately or 60 minutes after normothermic SAH. The water contents of the removed brains were calculated using the wet/dry weight method.

RESULTS

The ADC values did not change in the control group but decreased to 88.6+/-5.2% (P < 0.05 versus baseline) after SAH and remained significantly decreased throughout the experiment in normothermia. An injection of blood during hypothermia caused an initial decrease in ADC to 96.1+/-5.6% (P < 0.05 versus baseline); values continuously increased and reached normal levels within 60 minutes. Delayed hypothermia also normalized ADC values within the observation period. The brain water content in the control group was 80.3+/-0.1%, that after SAH in normothermia was 81.1+/-0.7%, and that after SAH in hypothermia was 79.3+/-0.5%.

CONCLUSION

This model of severe SAH in rats causes significant ADC changes, which are reversible by application of moderate hypothermia even when it is induced after a 60-minute delay. These findings support the concept of moderate hypothermia exerting a neuroprotective effect in severe SAH.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Neuronal release of vasoactive intestinal peptide is important to astrocytic protection of neurons from glutamate toxicity

Astrocytes regulate clearance of glutamate from the vicinity of neurons. This helps to protect neurons directly from glutamate toxicity. Recent findings have indicated that a complex molecular interaction between neurons and astrocytes that is necessary for this protection occurs. In the present investigation the role of vasoactive intestinal peptide (VIP) in signaling between neurons and astrocytes was investigated. VIP was found to be necessary for the protective effects of astrocytes in a coculture system. VIP in combination with neuronal-conditioned medium enhanced glutamate uptake by astrocytes. Also, VIP enhanced the expression of the high-affinity VIP receptor, increased astrocytic release of interleukin-6, and indirectly reduced the toxicity of glutamate in neuronal-conditioned astrocyte medium. These results indicate that VIP is essential to the molecular interaction of neurons and astrocytes and is involved in the regulation of the protective effects of astrocytes for neurons.

The glutathione content of retinal Müller (glial) cells: effect of pathological conditions

Maintenance of isolated retinal Müller (glial) cells in glutamate-free solutions over 7 h causes a significant loss of their initial glutathione content; this loss is largely prevented by the blockade of glutamine synthesis using methionine sulfoximine (5 mM). Anoxia does not reduce the glutathione content of Müller cells when glucose (11 mM), glutamate and cystine (0.1 mM each) are present. In contrast, simulation of total ischemia (i.e., anoxia plus removal of glucose) decreases the glutathione levels dramatically, even in the presence of glutamate and cystine. Less severe effects are caused by high extracellular K+ (40 mM). Reactive oxygen species are generated in the retina under various conditions, such as anoxia, ischemia, and reperfusion. One of the crucial substances protecting the retina against reactive oxygen species is glutathione, a tripeptide constituted of glutamate, cysteine and glycine. It was recently shown that glutathione can be synthesized in retinal Müller glial cells and that glutamate is the rate-limiting substance. In this study, glutathione levels were determined in acutely isolated guinea-pig Müller cells using the glutathione-sensitive fluorescent dye monochlorobimane. The purpose was to find out how the glial glutathione content is affected by anoxia/ischemia and accompanying pathophysiological events such as depolarization of the cell membrane. Our results further strengthen the view that glutamate is rate-limiting for the glutathione synthesis in glial cells. During glutamate deficiency, as caused by e.g., impaired glutamate uptake, this amino acid is preferentially delivered to the glutamate-glutamine pathway, at the expense of glutathione. This mechanism may contribute to the finding that total ischemia (but not anoxia) causes a depletion of glial glutathione. In situ depletion may be accelerated by the ischemia-induced increase of extracellular K+, decreasing the driving force for glutamate uptake. The ischemia-induced lack of glutathione is particularly fatal considering the increased production of reactive oxygen species under this condition. Therefore the therapeutic application of exogenous free radical scavengers is greatly recommended.

Taurine release is enhanced in cell-damaging conditions in cultured cerebral cortical astrocytes

The release of preloaded [3H]taurine from cultured cerebral cortical astrocytes was studied under various cell-damaging conditions, including hypoxia, ischemia, aglycemia and oxidative stress, and in the presence of free radicals. Astrocytic taurine release was enhanced by K+ (50 mM), veratridine (0.1 mM) and the ionotropic glutamate receptor agonist kainate (1.0 mM). Metabotropic glutamate receptor agonists had only weak effects on taurine release. Similarly to the swelling-induced taurine release the efflux in normoxia seems to be mediated mainly by DIDS-(diisothiocyanostilbene-2,2'-disulphonate) and SITS-(4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate) sensitive CI- channels, since these blockers were able to reduce both basal and K+ -stimulated release. The basal release of taurine was moderately enhanced in hypoxia and ischemia, whereas the potentiation in the presence of free radicals was marked. The small basal release from astrocytes signifies that taurine release from brain tissue in ischemia may originate from neurons rather than glial cells. On the other hand, the release evoked by K+ in hypoxia and ischemia was greater than in normoxia, with a very slow time-course. The enhanced release of the inhibitory amino acid taurine from astrocytes in ischemia may be beneficial to surrounding neurons, outlasting the initial stimulus and counteracting overexcitation.

Selective induction of glial glutamate transporter GLT-1 by hypertonic stress in C6 glioma cells

Glial glutamate transporter GLT-1 mRNA was selectively induced in C6 glioma cells exposed to hypertonic stress (HS), while the expression of two other subtypes, GLAST and EAAC1, was suppressed. HS increased phosphorylation of the MAPK family, ERK, p38 MAPK, and JNK. Treatment with a PKC inhibitor showed that phosphorylation of both p38 MAPK and JNK is PKC-dependent but ERK phosphorylation is independent. Inhibition of either ERK or p38 MAPK did not abolish GLT-1 mRNA induction. Inhibition of PKC also had no effect. These findings indicate that the induction of GLT-1 mRNA by HS is independent of the MAPK pathways. This is the first report that the expression of glial glutamate transporters is osmotically regulated.

Glial cell swelling--effect of hypothermia

The effect of mild to moderate hypothermia (32/27 degrees C) was analyzed on the cell volume of C6 glioma cells and primary cultured astrocytes at normal pH, during lactacidosis (pH6.2) and during exposure to glutamate or arachidonic acid in vitro. The cells were suspended in an incubation chamber under continuous control of pH, pO2 and temperature. Cell swelling was quantified by an advanced Coulter-system. Following a control period at 37 degrees C, the ambient temperature was decreased to 27 and 32 degrees C for 30 min. Hypothermia alone led to an immediate and significant cell volume increase of 107.3 +/- 0.4% (mean +/- SEM) of control after 30 min at 32 degrees C. Yet, hypothermia (27 degrees C) afforded partial protection against the acidosis-induced cell swelling at pH 6.2, attaining 120.4 +/- 0.9% in the normothermic control group after 60 min, while only 111.3 +/- 0.9% at 27 degrees C. Hypothermia, however, was not associated with a reduction of the glutamate- or arachidonic acid-induced cell swelling. The results demonstrate that mild hypothermia per se induces glial cell swelling, but simultaneously inhibits cell swelling from acidosis, while not from glutamate- or arachidonic acid.

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.

Neurons depend on astrocytes in a coculture system for protection from glutamate toxicity

Glutamate can be toxic to neurons although it is a neurotransmitter. Regulation of extracellular glutamate levels is essential for prevention of glutamate neurotoxicity. Astrocytes play a major role in clearance of glutamate released by neurons. A coculture system combining cerebellar cells and astrocytes was employed to investigate the astrocytic control of glutamate toxicity. Coculture of astrocytes with cerebellar neurons enhanced uptake of glutamate by astrocytes. Inhibition of glutamate uptake in a coculture system led to death of cerebellar cells. This toxicity could be inhibited by MK801. However, in the presence of the glutamate uptake inhibitor, there was no increase in glutamate in the cultures compared to when the neurons were not cocultured. This indicated that neurons become more susceptible to glutamate toxicity in the presence of astrocytes and thus become dependent on astrocytes for prevention of glutamate toxicity. Astrocytes treated with conditioned medium from cerebellar cells did not show an increase in glutamate uptake but medium from astrocytes exposed to neuron conditioned medium was toxic to cerebellar cells. This toxicity was due to glutamate present in the medium. This suggests that a soluble factor released by neurons signals to astrocytes that neurons are present and stimulates a signal back to neurons which causes an increased sensitivity to glutamate toxicity.

Astrocytic glutamate uptake and prion protein expression

Factors influencing glutamate uptake by astrocytes may indirectly influence neuronal survival. Elevated extracellular glutamate may be excitotoxic or may exacerbate neurodegeneration in various neurological diseases. By using a cell culture model, we have investigated the influence of astrocytic prion protein (PrPc) expression on glutamate uptake. Type 1 astrocytes expressing PrPc have a higher rate of Na+-dependent glutamate uptake than PrPc-deficient type 1 astrocytes. This difference is exacerbated when serum free media is used to culture the astrocytes. Further analysis suggested that a decrease in substrate affinity is responsible for the sensitivity of PrP-deficient astrocytic glutamate uptake to culture conditions. PrPc has been shown to bind copper. Greater sensitivity of cells to copper concentrations may be responsible for the decreased substrate affinity observed. PrPc-deficient cerebellar cells are more sensitive to glutamate toxicity in the presence of copper. These results show that glutamate uptake from astrocytes is dependent on PrPc expression which in turn may be related to copper metabolism.

Glutamate, glutamine, and GABA as substrates for the neuronal and glial compartments after focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

Even though the utilization of substrates alternative to glucose may play an important role in the survival of brain cells under ischemic conditions, evidence on changes in substrate selection by the adult brain in vivo during ischemic episodes remains very limited. This study investigates the utilization of glutamate, glutamine, and GABA as fuel by the neuronal and glial tricarboxylic acid cycles of both cerebral hemispheres after partially reversible focal cerebral ischemia (FCI).

METHODS

Right hemisphere infarct was induced in adult Long-Evans rats by permanent occlusion of the right middle cerebral artery and transitory occlusion of both common carotid arteries. (1,2-13C2) acetate was infused for 60 minutes in the right carotid artery immediately after carotid recirculation had been re-established (1-hour group) or 23 hours later (24-hour group). Extracts from both cerebral hemispheres were prepared and analyzed separately by 13C nuclear magnetic resonance and computer-assisted metabolic modeling.

RESULTS

FCI decreased the oxidative metabolism of glucose in the brain in a time-dependent manner. Reduced glucose oxidation was compensated for by increased oxidations of (13C) glutamate and (13C) GABA in the astrocytes of the ipsilateral hemispheres of both groups. Increased oxidative metabolism of (13C) glutamine in the neurons was favored by increased activity of the neuronal pyruvate recycling system in the 24-hour group.

CONCLUSIONS

Data were obtained consistent with time-dependent changes in the utilization of glutamate and GABA or glutamine as metabolic substrates for the glial or neuronal compartments of rat brain after FCI.

Depressed ventilatory response to hypoxia in hypothermic newborn piglets: role of glutamate

To evaluate whether changes in extracellular glutamate (Glu) levels in the central nervous system could explain the depressed hypoxic ventilatory response in hypothermic neonates, 12 anesthetized, paralyzed, and mechanically ventilated piglets <7 days old were studied. The Glu levels in the nucleus tractus solitarius obtained by microdialysis, minute phrenic output (MPO), O2 consumption, arterial blood pressure, heart rate, and arterial blood gases were measured in room air and during 15 min of isocapnic hypoxia (inspired O2 fraction = 0.10) at brain temperatures of 39.0 +/- 0.5 degrees C [normothermia (NT)] and 35.0 +/- 0.5 degrees C [hypothermia (HT)]. During NT, MPO increased significantly during hypoxia and remained above baseline. However, during HT, there was a marked decrease in MPO during hypoxia (NT vs. HT, P < 0.03). Glu levels increased significantly in hypoxia during NT; however, this increase was eliminated during HT (P < 0.02). A significant linear correlation was observed between the changes in MPO and Glu levels during hypoxia (r = 0.61, P < 0.0001). Changes in pH, arterial PO2, O2 consumption, arterial blood pressure, and heart rate during hypoxia were not different between the NT and HT groups. These results suggest that the depressed ventilatory response to hypoxia observed during HT is centrally mediated and in part related to a decrease in Glu concentration in the nucleus tractus solitarius.

Effect of mild and moderate hypothermia on the acidosis-induced swelling of glial cells

The effect of mild (32 degrees C) and moderate (27 degrees C) hypothermia was analyzed on the cell volume and intracellular pH (pHi) of C6 glioma cells at normal pH and during lactacidosis at pH 6.2 in vitro. The cells were suspended in an incubation chamber under continuous control of pH, PO2 and temperature. Cell swelling was quantified by an advanced Coulter-system. pHi was measured by flow cytometry using the fluorescent dye bis-carboxyethyl carboxyfluorescein (BCECF). Following a control period at 37 degrees C, the ambient temperature was decreased to 32 degrees C for 30 min, and subsequently to 27 degrees C for another 30 min. Hypothermia alone led to an immediate and significant cell volume increase of 107.3 +/- 0.4% (mean +/- SEM) of control after 30 min at 32 degrees C, and further swelling to 110.5 +/- 0.9% after 30 min at 27 degrees C. Yet, hypothermia (27 degrees C) afforded partial protection against the acidosis-induced cell swelling at pH 6.2, which was reaching to 120.4 +/- 0.9% in the normothermic control group after 60 min, while only to 111.3 +/- 0.9% at 27 degrees C. Hypothermia, however, was associated with a more pronounced decrease of the pHi during acidosis (6.3 +/- 0.04) as compared to that of the normothermic control falling then to 6.5 +/- 0.03. The results demonstrate that mild and moderate hypothermia induce glial cell swelling, but simultaneously inhibit cell swelling from acidosis. The protection against cell swelling, however, has its price as indicated by the enhancement of the intracellular acidification.

Expression of glutamate uptake transporters after dibutyryl cyclic AMP differentiation and traumatic injury in cultured astrocytes

Our findings indicate that differentiation of primary astrocytes by dibutyryl cyclic adenosine monophosphate (dBcAMP) and scratch injury together resulted in increased glutamate transporter gene expression. Confluent primary cultures were prepared from cerebral cortex of normal new born rat pups. The primary cultures were then divided into four groups each: control and scratch-injured, and dBcAMP-treated control and scratch-injured cultures. Total RNA was extracted at 0, 1, 2, 4, and 7 days after injury. Expression of the electrogenic glutamate transporters, GLAST, GLT-1, and EAAC-1, was quantitated by the reverse transcriptase-polymerase chain reaction method (RT-PCR) and slot blot hybridization followed by densitometric scanning. Triplicate cultures were analyzed for each time-point. Our studies indicate that all these astrocyte cultures expressed the two glial transporters, GLAST and GLT-1, while none of the cultures expressed the neuronal transporter, EAAC-1. The expression of the two transporters in the dBcAMP-treated primary cultures were markedly increased from the non-treated cultures. The dBcAMP-treated cultures had 2- to 4-times increase in levels of GLAST and GLT-1-mRNA expression both before and after scratch injury, as compared to untreated non-injured and injured primary cultures. All of the cultures expressed GLAST in greater proportion than GLT-1.

Glutamate and potassium stimulation of hippocampal slices metabolizing glucose or glucose and pyruvate

Using 2-deoxyglucose phosphorylation as an index of glucose use and concentrations of selected intermediates to monitor metabolic pathways, responses of rat hippocampal slices to glutamate and K+ stimulation were examined. With glutamate, the glucose phosphorylation rate (GPR) increased, and the slices accumulated glutamate at a constant rate, for 10 min. The uptake rate at each glutamate level was matched, approximately, by the increase in GPR at that level, with 4 or 5 glutamate molecules accumulated for every glucose molecule phosphorylated. Phosphocreatine and ATP levels fell abruptly, and lactate rose, probably reflecting neuronal activity, found by others to be very brief in the presence of glutamate. K+ stimulation produced responses of phosphocreatine, ATP and lactate levels and of GPR similar to those due to glutamate. There were also prolonged changes in the levels of other metabolites: with both stimulants glucose 6-phosphate fell, and malate rose. The changes in malate may be the result of the participation of mitochondrial malate dehydrogenase in both citrate cycle and malate shuttle. Citrate and alpha-ketoglutarate rose only with K+. When pyruvate was added to the medium, resting GPR was reduced, but for both stimulants the relative increases in GPR with stimulation were the same as without pyruvate. The changes in metabolic intermediates in response to K+ were like those with glucose alone. But with glutamate, the rise in lactate was greatly diminished, and malate fell instead of rising. Glutamate interference with the transfer of both 3-carbon as well as 4- and 5-carbon intermediates from glia to neurons may explain these results. If so, this interference is greater with pyruvate supplementation than with glucose alone.

Apoptotic death in cerebral hemisphere cells is density dependent and modulated by transient oxygen and glucose deprivation

Flow cytometry, light and fluorescence microscopy, and designated biochemical techniques were used to examine the type of death which occurs in cerebral cortex cells when grown under crowded vs. sparse conditions or after brief anoxia/hypoglycemia. A 4 hr episode of anoxia combined with glucose deprivation enhanced apoptotic cell death as assessed by 4',6-diamidino-2-phenylindole (DAPI) staining and reduced neutral red eye uptake. An additional form of cell death involving exclusion of the nucleus was recorded by time lapse cinematography and DAPI stain. The presence of the endonuclease inhibitor aurintricarboxylic acid (0.1 mM) reduced cell death by 56.6%, while the protein and RNA synthesis inhibitors actinomycin D and cycloheximide (each at 5 micrograms/ml) effectively decreased cell death by 83.3% and 90.6%, respectively. In contrast, 5 mM glutamate had no effect on cell death in accord with the immature state of the cells. Growth of cells under crowded conditions improved cell survival; after 2 h or 4 days in culture, cells seeded at high density (34 microgram cellular DNA/cm2) showed a nearly 3-fold decline in the amount of cell death in comparison to cells seeded at low density (5 micrograms cellular DNA/cm2). At high cell density, anoxic episodes enhanced cell death most likely by preventing a cell density-mediated rescue. Neutral red dye uptake, an index for cell viability, was enhanced with increasing cell density and in vitro maturation, but was reduced in dense cultures exposed to anoxic/hypoglycemic conditions. The data suggest that cell density may play a critical role in brain organogenesis and that anoxic stress is more deleterious in dense than sparse cell assemblies.

Stimulation of glutamate uptake and Na,K-ATPase activity in rat astrocytes exposed to ischemia-like insults

The postsynaptic actions of glutamate are rapidly terminated by high affinity glutamate uptake into glial cells. In this study we demonstrate the stimulation of both glutamate uptake and Na,K-ATPase activity in rat astrocyte cultures in response to sublethal ischemia-like insults. Primary cultures of neonatal rat cortical astrocytes were subjected to hypoxia, or to serum- and glucose-free medium, or to both conditions (ischemia). Cell death was assessed by propidium iodide staining of cell nuclei. To measure sodium pump activity and glutamate uptake, 3H-glutamate and 86Rb were both simultaneously added to the cell culture in the presence or absence of 2 mM ouabain. Na,K-ATPase activity was defined as ouabain-sensitive 86Rb uptake. Concomitant transient increases (2-3 times above control levels) of both Na,K-ATPase and glutamate transporter activities were observed in astrocytes after 4-24 h of hypoxia, 4 h of glucose deprivation, and 2-4 h of ischemia. A 24 h ischemia caused a profound loss of both activities in parallel with significant cell death. The addition of 5 mM glucose to the cells after 4 h ischemia prevented the loss of both sodium pump activity and glutamate uptake and rescued astrocytes from death observed at the end of 24 h ischemia. Reoxygenation after the 4 h ischemic event caused the selective inhibition of Na,K-ATPase activity. The observed increases in Na,K-ATPase activity and glutamate uptake in cultured astrocytes subjected to sublethal ischemia-like insults may model an important functional response of astrocytes in vivo by which they attempt to maintain ion and glutamate homeostasis under restricted energy and oxygen supply.

Neurochemical mechanisms in brain injury and treatment: a review

This article reviews cellular energy transformation processes and neurochemical events that take place at the time of brain injury and shortly thereafter emphasizing hypoxia-ischemia, cerebrovascular accident, and traumatic brain injury. New interpretations of established concepts, such as diffuse axonal injury, are discussed; specific events, such as free radical production, excess production of excitatory amino acids, and disruption of calcium homeostasis, are reviewed. Neurochemically-based interventions are also presented: calcium channel blockers, excitatory amino acid antagonists, free radical scavengers, and hypothermia treatment. Concluding remarks focus on the role of clinical neuropsychologists in validation of treatment interventions.

Protective effect of MK-801 on the anoxia-aglycemia induced damage in the fluorocitrate-treated hippocampal slice of the rat

We investigated electrophysiological responses induced by ischemia-like insult (anoxia and aglycemia, AA) in the rat hippocampal CA1 pyramidal cells in an in vitro slice preparation devoid of glial metabolism. In the slice treated with fluorocitrate (100 microM), a glia-specific metabolic inhibitor, 10 min AA induced hyperexcitation as evidenced by an appearance of multiple population spikes evoked by stimulation of the Schaffer collateral/commissural pathway in the CA1 region prior to elimination of the response. Readministration of oxygen and glucose failed to restore the population spike amplitude. Intracellular recordings revealed that 10 min AA induced slow EPSPs with relative long duration. The induction of the slow EPSPs was followed by a rapid membrane depolarization with a large amplitude. When the fluorocitrate-treated slice was exposed to MK-801 (10 microM), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, 10 min AA failed to induce either the hyperexcitation of synaptic responses or the rapid depolarization. Furthermore, synaptic responses were fully restored after readministration of oxygen and glucose. In contrast, neither the synaptic hyperexcitation nor the rapid depolarization was observed during 10 min AA in the hippocampal CA1 pyramidal cells of the control slice. In addition, an irreversible synaptic failure associated with AA was not induced in the control slice. These results strongly suggest that fluorocitrate increases NMDA receptor-dependent AA-induced damage in the hippocampal slice by interfering glial spatial buffering of K+.

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.

Astrocyte-neuron interaction during one-trial aversive learning in the neonate chick

During two specific stages of the Gibbs-Ng model of one-trial aversive learning in the neonate chick, we have recently found unequivocal evidence for a crucial involvement of astrocytes. This evidence is metabolic (utilization of the astrocyte-specific energy store, glycogen, during normal learning and inhibition of memory formation by the astrocyte specific metabolic inhibitors, fluoroacetate and methionine sulfoximine) as well as physiological (abolition of memory formation in the presence of ethacrynic acid, an astrocyte-specific inhibitor of cellular reaccumulation of potassium ions). These findings are discussed in the present review in the framework of a more comprehensive description of metabolic and physiological neuronal-astrocytic interactions across an interstitial (extracellular) space bounded by minute processes from either cell type.

Pharmacological characteristics of potassium-induced, glycogenolysis in astrocytes

Elevated extracellular concentrations of the potassium ion ([K+]o) stimulate glycogenolysis in primary cultures of mouse astrocytes that have been grown in the presence of dibutyryl cyclic AMP but not in corresponding cultures which have not been treated in this manner. The response is potently inhibited by nifedipine, suggesting that it is evoked by entry of calcium ions through voltage dependent L-channels. The benzodiazepine midazolam, which is known to enhance calcium entry at concentrations of [K+]o causing submaximum calcium entry, increases the glycogenolytic effect by such levels of [K+]o.

Acidosis causes failure of astrocyte glutamate uptake during hypoxia

Failure of glutamate uptake during ischemia can lead to neurotoxic accumulations of glutamate in brain extracellular space. Hypoxia and acidosis are metabolic consequences of ischemia that may individually or in combination impair glutamate uptake. We used primary rat astrocyte cultures to study the effects of acidosis, chemical hypoxia, and the combination of acidosis plus chemical hypoxia on glutamate uptake. Chemical hypoxia alone reduced uptake by 35-45%. Reduction in pH from 7.4 to 5.8 also caused a significant but incomplete inhibition of glutamate uptake, and this effect was more pronounced in medium buffered with CO2/bicarbonate. However, the combination of chemical hypoxia plus acidosis reduced glutamate uptake to below 10% of controls. Astrocyte ATP levels, like glutamate uptake, were significantly reduced by chemical hypoxia and further reduced by the combination of hypoxia plus acidosis. Acidosis under normoxic conditions had no significant effect on astrocyte ATP levels. These results suggest two mechanisms by which acidosis may contribute to failure of astrocyte glutamate uptake during ischemia: Acidosis may act in concert with hypoxia to cause ATP depletion, and acidosis may also have direct effects on glutamate transporters unrelated to effects on cellular ATP levels. pH effects on glutamate uptake may be an important factor affecting neuronal survival during incomplete ischemia.

Neuroprotective effect of phenylsuccinate, an inhibitor of cytosolic glutamate formation from glutamine, under anoxic conditions but not during exposure to exogenous glutamate

Phenylsuccinate is an inhibitor of cytosolic glutamate formation from extracellular glutamine in cultured cerebellar granule cell neurons, a glutamatergic preparation. It prevents anoxic cell death in these cells as indicated by decreased lactate dehydrogenase (LDH) release and by the morphological appearance of the cells after the insult. In contrast, it does not prevent neurotoxicity by added glutamate because it is not an antagonist of the glutamate receptor.

Fluorocitrate and fluoroacetate effects on astrocyte metabolism in vitro

The Krebs cycle inhibitor fluorocitrate (FC) and its precursor fluoroacetate (FA) are taken up in brain preferentially by glia. These compounds are used experimentally to inhibit glial metabolism in situ. The actions of these agents have been attributed to both the disruption of carbon flux through the Krebs cycle and to impairment of ATP production. We used primary astrocyte cultures to evaluate these two possible modes of action. Astrocyte ATP levels exhibited little or no reduction during incubation with 0.5 mM FC or 25 mM FA. Correspondingly, FC and FA caused less than 30% reductions in glutamate uptake (P > 0.05), an important energy-dependent astrocyte function. Carbon flux through the Krebs cycle was assessed by measuring astrocyte glutamine production in the absence of exogenous glutamate or aspartate. Under these conditions, glutamine production was reduced 65 +/- 5% by 0.5 mM FC and 61 +/- 3% by 25 mM FA (P < 0.01). In contrast, FC and FA had no effect on glutamine production when 50 microM glutamate was provided in the media. These findings suggest that the metabolic effects of FC and FA on astrocytes in vivo result from impairment of carbon flux through the Krebs cycle, and not from impairment of oxidative ATP production.

Effect of anoxia on glutamate formation from glutamine in cultured neurons: dependence on neuronal subtype

Synthesis and release of glutamate formed from labeled glutamine were studied in primary cultures of the glutamatergic cerebellar granule cells and of the mainly GABAergic cerebral cortical neurons under anoxic conditions and under normoxic control conditions. Under both control and anoxic conditions cerebellar granule cells synthesized and released glutamate more intensely than cerebral cortical neurons, but this difference was enhanced under anoxic conditions. Thus, under normoxic conditions synthesis of intracellular labeled glutamate from glutamine was twice as high in cerebellar granule cell neurons as in cerebral cortical neurons during 30 min of incubation, but the release of newly synthesized labeled glutamate to the extracellular medium from cerebellar granule cell neurons was more than 4 times higher than the release from cerebral cortical neurons during 30 min of incubation. Based on these observations it is suggested that a major reason for the increase in extracellular glutamate concentration during brain ischemia may be enhanced production and release of glutamate, especially in glutamatergic neurons.

Astrocytic glycogenolysis energizes memory processes in neonate chicks

In previous pharmaco-behavioural experiments, we have shown that three sequential stages can be distinguished in discrimination memory for a single trial passive avoidance experience in neonate chicks: a short-term (STM) stage, available for 10 min following learning; an intermediate (ITM) stage, operating between 20 and 50 min (ITMB) post-learning; and a long-term (LTM) stage formed by 60 min after learning. The ITM stage can be divided into two parts: a first phase (ITMA) which is susceptible to inhibition by the uncoupler of oxidative phosphorylation (and thus of oxidative metabolism), 2,4-dinitrophenol (DNP), and a second DNP-insensitive phase (ITMB). ITMA occurs between 20 and 30 min post-training and ITMB between 30 and 50 min. In the present study we have shown: (1) that day-old chicks trained in the passive avoidance task and immediately thereafter injected with the glycolytic inhibitor iodoacetate show retention deficits that are first evident 30 min post-training, and (2) that glycogenolysis, i.e. breakdown of glycogen, a high-molecular carbohydrate energy store localized in astrocytes, occurs in the forebrains of trained, but otherwise untreated birds, between 35 and 55 min after learning. These findings strongly suggest that glycolysis, including astrocytically localized glycogenolysis, is essential to provide energy for active processes occurring during ITMB and that these processes are indispensable for subsequent development of long-term memory.