Exogenous glutamate is metabolized to glutamine and exported by rat primary astrocyte cultures

In vivo nuclear magnetic resonance studies of glutamate-gamma-aminobutyric acid-glutamine cycling in rodent and human cortex: the central role of glutamine

It has been recognized for many years that the metabolism of brain glutamate and gamma-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters, is linked to a substrate cycle between neurons and astrocytes involving glutamine. However, the quantitative significance of these fluxes in vivo was not known. Recent in vivo 13C and 15N NMR studies in rodents and 13C NMR in humans indicate that glutamine synthesis is substantial and that the total glutamate-GABA-glutamine cycling flux, necessary to replenish neurotransmitter glutamate and GABA, accounts for >80% of net glutamine synthesis. In studies of the rodent cortex, a linear relationship exists between the rate of glucose oxidation and total glutamate-GABA-glutamine cycling flux over a large range of cortical electrical activity. The molar stoichiometric relationship (approximately 1:1) found between these fluxes suggests that they share a common mechanism and that the glutamate-GABA-glutamine cycle is coupled to a major fraction of cortical glucose utilization. Thus, glutamine appears to play a central role in the normal functional energetics of the cerebral cortex.

Glutamine as a pathogenic factor in hepatic encephalopathy

Hepatic encephalopathy (HE) results from acute or chronic liver dysfunction and is associated with hyperammonemia. Ammonium ions penetrate from blood to brain, where they form glutamine (Gln) in the reaction with glutamate catalyzed by an astroglia-specific enzyme, glutamine synthetase (GS). Experimental data suggest that many manifestations of HE can be ascribed to increased Gln synthesis and accumulation in the brain. In HE resulting from acute liver failure ("fulminant hepatic failure"), the osmotic action of Gln appears to be in a large degree responsible for cerebral edema and edema-associated disturbances of cerebral blood flow and ionic homeostasis. In chronic HE not accompanied by cerebral edema, Gln contributes to impairment of cerebral energy metabolism, and its increased transport from brain to the periphery accelerates the blood-to-brain transport of aromatic amino acids, of which tryptophen (Trp) is converted to metabolites directly implicated in HE. Most of the evidence that Gln participates in pathological events has been derived from their disappearance or amelioration in HE rats in which the cerebral Gln content was reduced by treatment with a GS inhibitor, methionine sulfoximine.

Ammonium in nervous tissue: transport across cell membranes, fluxes from neurons to glial cells, and role in signalling

Most, but not all, animal cell membranes are permeable to NH3, the neutral, minority form of ammonium which is in equilibrium with the charged majority form NH4+. NH4+ crosses many cell membranes via ion channels or on membrane transporters, and cultured mammalian astrocytes and glial cells of bee retina take up NH4+ avidly, in the latter case on a Cl(-)-cotransporter selective for NH4+ over K+. In bee retina, a flux of ammonium from neurons to glial cells is an essential component of energy metabolism, which involves a flux of alanine from glial cells to neurons. In mammalian brain, both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Glutamine is transferred to neurons where it is deamidated to re-form glutamate; the maintenance of this cycle appears to require a substantial flux of ammonium from neurons to astrocytes. In addition to maintaining the glial cell content of fixed N (a "bookkeeping" function), ammonium is expected to participate in the regulation of glial cell metabolism (a signalling function): it will increase conversion of glutamate to glutamine, and, by activating phosphofructokinase and inhibiting the alpha-ketoglutarate dehydrogenase complex, it will tend to increase the formation of lactate.

The effect of thiopental on glutamate metabolism in mouse cerebellar astrocytes in vitro

The effect of thiopental on [U-(13)C]glutamate metabolism was studied in cerebellar astrocytes from mice using (13)C magnetic resonance spectroscopy. The cells were incubated with 0.5 mM [U-(13)C]glutamate for 2 h in the presence of 1 mM thiopental and 1 mM thiopental plus 0.2 mM gammaamino butyric acid (GABA). Labeled glutamate, glutamine, aspartate and glutathione were observed in cell extracts, and glutamine, aspartate and lactate in the media. Not only uniformly labeled glutamate was present in the medium, but also glutamate derived from the tricarboxylic acid (TCA) cycle. In the presence of thiopental, the amount of unlabeled glucose and [U-(13)C]glutamate removed from the medium was unchanged. This is in contrast to previous results obtained in cortical astrocytes, showing cellular heterogeneity. The concentrations of [1,2,3-(13)C]glutamate and [U-(13)C]glutamine were increased in the cell extracts, but unchanged in the medium, indicating an increased synthesis and an unchanged exchange or release. It should be noted that [U-(13)C]lactate is formed from [U-(13)C]glutamate via the TCA cycle and is released to the medium. In the presence of thiopental less [U-(13)C]lactate was observed in the medium. GABA had no influence on the effects of thiopental on cerebellar astrocytes.

Transfer of glutamine between astrocytes and neurons

The export of glutamine from astrocytes, and the uptake of glutamine by neurons, are integral steps in the glutamate-glutamine cycle, a major pathway for the replenishment of neuronal glutamate. We review here the functional and molecular identification of the transporters that mediate this transfer. The emerging picture of glutamine transfer in adult brain is of a dominant pathway mediated by system N transport (SN1) in astrocytes and system A transport (SAT/ATA) in neurons. The participating glutamine transporters are functionally and structurally related, sharing the following properties: (a) unlike many neutral amino acid transporters which have proven to be obligate exchangers, these glutamine transporters mediate net substrate transfer energized by coupling to ionic gradients; (b) they are sensitive to small pH changes in the physiological range; (c) they are susceptible to adaptive and humoral regulation; (d) they are related structurally to the AAAP (amino acid and auxin permeases) family of transporters. A key difference between SN1 and the SAT/ATA transporters is the ready reversibility of glutamine fluxes via SN1 under physiological conditions, which allows SN1 both to sustain a glutamine concentration gradient in astrocytes and to mediate the net outward flux of glutamine. It is likely that the ASCT2 transporter, an obligate exchanger of neutral amino acids, displaces the SN1 transporter as the main carrier of glutamine export in proliferating astrocytes.

Studies of metabolic compartmentation and glucose transport using in vivo MRS

Organs consist of several types of cells with specialized functions. This cellular localization of function is often referred to as compartmentation. Due to the intrinsic low sensitivity of MR methods it is generally not possible in vivo to obtain images or spectra of single cells. Instead the MRS signal is the sum of the signal from millions of cells and multiple cell types. A major challenge in using MRS to study biological processes such as metabolism and transport is to devise measurements that provide cell-specific information from this mix. Fortunately nature has helped the MR scientist by in several cases nearly completely localizing metabolic pathways and their associated metabolites in specific cell types. The chemical specificity of MRS allows the concentrations and synthesis rates of these metabolites to be measured, providing information about the compartmentation of metabolism and function. In this review examples are presented from MRS studies of metabolic trafficking between neurons and astrocytes in the brain, brain glucose transport, and the role of muscle glucose transport in insulin resistance and diabetes. The concepts and approaches used in these studies are generally applicable for studying cellular metabolic compartmentation in a wide range of systems.

Development of an immobilized brain glutamine synthetase liquid chromatographic stationary phase for on-line biochemical studies

Glutamine synthetase (GS) plays a key role in the regulation of glutamate availability to neurons. In the present study glutamine synthetase was immobilized on a silica-based immobilized artificial membrane liquid chromatographic stationary phase (IAM-SP) to create the GS-IAM. The stability of GS was improved by immobilization, but the enzyme's affinity for the substrates L-glutamate and D-glutamate was significantly decreased. In contrast, immobilization significantly increased GS sensitivity to inhibition by methionine sulfoximine. The GS-IAM was packed into a chromatography column to create an immobilized enzyme reactor (GS-IMER). On-line experiments with the GS-IMER demonstrated that the immobilized enzyme was comparable to the non-immobilized enzyme with regards to retention of activity and selectivity toward substrates and inhibitors and was reusable for several weeks.

Ultrastructural study of rat hippocampus after chronic administration of aluminum L-glutamate: an acceleration of the aging process

An ultrastructural study of rat hippocampus was performed on young (group 1) and old (group 4) rats receiving daily subcutaneous injections of aluminum L-glutamate and on old untreated rats (group 5). Young controls were treated with sodium L-glutamate (group 2) and physiological saline (group 3). Group 1 showed vacuolated astrocytes with numerous lipofuscin deposits, mitochondrial swelling, a thinning of the myelin sheath, and many multivesicular bodies invading the cytoplasm. Cellular structure did not appear to be affected in groups 2 and 3. Group 4 showed swollen mitochondria, a demyelination process in axonal regions, sizable perivascular oedema with vessel retraction and gliofilament bundles. In this group, lipofuscin deposits in astrocytes were associated with multivesicular bodies that thinned the myelin sheath to the breaking point; however, no excitotoxic glutamate-induced effects were observed. In group 5, extreme cytoplasmic vacuolation was observed, with massive mitochondrial swelling, considerable thinning of the myelin sheath (at times to the breaking point), sizable vacuolar degeneration and gliofilament bundles. These results indicate that ultrastructural alterations in the hippocampus, such as cell vacuolization, massive mitochondrial swelling and the demyelination process, occur with aging and independently of aluminum intoxication. Similar alterations were observed in aluminum L-glutamate-intoxicated young rats, but not in controls. These results are consistent with aluminum-induced acceleration of the aging process.

Helium-oxygen pressure induces striatal glutamate increase: a microdialysis study in freely-moving rats

In rat, helium pressures induce locomotor and motor activity which requires dopaminergic and N-methyl-D-aspartate (NMDA) receptor activities at striatal level. However, biochemical studies have suggested that pressure exposure may increase striatal glutamate level. We used microdialysis technique to study the effects of pressure on glutamate level in the striatum and the effects of local administration of D1 (SCH23390) or D2 (sulpiride) on these changes. Pressures increase both glutamate and glutamine levels in striatal microdialysates. Administration of sulpiride (1 microM) or SCH23390 (1 microM) by reverse microdialysis did not affect significantly pressure induced glutamate increase. So, protective effects of D1 and D2 antagonists against locomotor and motor hyperactivity (LMA) are probably independent of the processes involved in the striatal glutamate increase evoked by pressure.

Nerve tissue-specific (GLUD2) and housekeeping (GLUD1) human glutamate dehydrogenases are regulated by distinct allosteric mechanisms: implications for biologic function

Human glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is known to exist in housekeeping and nerve tissue-specific isoforms encoded by the GLUD1 and GLUD2 genes, respectively. As there is evidence that GDH function in vivo is regulated, and that regulatory mutations of human GDH are associated with metabolic abnormalities, we sought here to characterize further the functional properties of the two human isoenzymes. Each was obtained in recombinant form by expressing the corresponding cDNAs in Sf9 cells and studied with respect to its regulation by endogenous allosteric effectors, such as purine nucleotides and branched chain amino acids. Results showed that L-leucine, at 1.0 mM:, enhanced the activity of the nerve tissue-specific (GLUD2-derived) enzyme by approximately 1,600% and that of the GLUD1-derived GDH by approximately 75%. Concentrations of L-leucine similar to those present in human tissues ( approximately 0.1 mM:) had little effect on either isoenzyme. However, the presence of ADP (10-50 microM:) sensitized the two isoenzymes to L-leucine, permitting substantial enzyme activation at physiologically relevant concentrations of this amino acid. Nonactivated GLUD1 GDH was markedly inhibited by GTP (IC(50) = 0.20 microM:), whereas nonactivated GLUD2 GDH was totally insensitive to this compound (IC(50) > 5,000 microM:). In contrast, GLUD2 GDH activated by ADP and/or L-leucine was amenable to this inhibition, although at substantially higher GTP concentrations than the GLUD1 enzyme. ADP and L-leucine, acting synergistically, modified the cooperativity curves of the two isoenzymes. Kinetic studies revealed significant differences in the K:(m) values obtained for alpha-ketoglutarate and glutamate for the GLUD1- and the GLUD2-derived GDH, with the allosteric activators differentially altering these values. Hence, the activity of the two human GDH is regulated by distinct allosteric mechanisms, and these findings may have implications for the biologic functions of these isoenzymes.

Glutamic acid decarboxylase-expressing astrocytes exhibit enhanced energetic metabolism and increase PC12 cell survival under glucose deprivation

Astrocytes play a key role by catabolizing glutamate from extracellular space into glutamine and tricarboxylic acid components. We previously produced an astrocytic cell line that constitutively expressed glutamic acid decarboxylase (GAD67), which converts glutamate into GABA to increase the capacity of astrocytes to metabolize glutamate. In this study, GAD-expressing astrocytes in the presence of glutamate were shown to have increased energy metabolism, as determined by a moderate increase of 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, by an increased ATP level, and by enhanced lactate release. These changes were due to GAD transgene expression because transient expression of a GAD antisense plasmid resulted in partial suppression of the ATP level increase. These astrocytes had an increased survival in response to glucose deprivation in the presence of glutamate compared with the parental astrocytes, and they were also able to enhance survival of a neuronal-like cell line (PC12) under glucose deprivation. This protection may be partially due to the increased lactate release by GAD-expressing astrocytes because PC12 cell survival was enhanced by lactate and pyruvate under glucose deprivation. These results suggest that the establishment of GAD expression in astrocytes enhancing glutamate catabolism could be an interesting strategy to increase neuronal survival under hypoglycemia conditions.

Optic nerve head structure in glaucoma: astrocytes as mediators of axonal damage

Increased intraocular pressure (IOP) is recognised as the principal risk factor for the development of glaucomatous cupping of the optic disc. The hypothesis that it disrupts the function of retinal ganglion cell axons by increasing mechanical forces on the lamina cribrosa of the optic nerve head has received considerable experimental support. However, many patients with glaucoma will have progressive cupping even though the IOPs remain within the normal range, suggesting that mechanical compression is unlikely to be the sole cause of optic nerve damage. Clinical studies have emphasised the role of other factors, such as optic nerve head ischaemia, in generating optic disc cupping. One of the outstanding problems in understanding optic nerve head dysfunction in glaucoma has been the elucidation of the pathways that could integrate the effects of IOP and ischaemia to generate the characteristic changes seen. This review considers the role that optic nerve head astrocytes might play in the initiation of axon damage, based on the hypothesis that these cells are sensitive to mechanical or ischaemic factors and are important for the maintenance of retinal ganglion physiology. It discusses their role in the remodelling of the structure of the lamina cribrosa and the effect that this might have on axon function. Recent evidence has shown that the modulation of astrocyte activity, for example by the reduction of the production of nitric oxide, may prevent retinal ganglion cell death in ocular hypertension. The possibility that astrocyte-axon interactions are important in the development of glaucomatous optic neuropathy suggests new avenues of therapeutic intervention, not related to the control of IOP, that would prevent retinal ganglion cell death in glaucoma.

Compromised glutamate transport in human glioma cells: reduction-mislocalization of sodium-dependent glutamate transporters and enhanced activity of cystine-glutamate exchange

Elevated levels of extracellular glutamate ([Glu](o)) can induce seizures and cause excitotoxic neuronal cell death. This is normally prevented by astrocytic glutamate uptake. Neoplastic transformation of human astrocytes causes malignant gliomas, which are often associated with seizures and neuronal necrosis. Here, we show that Na(+)-dependent glutamate uptake in glioma cell lines derived from human tumors (STTG-1, D-54MG, D-65MG, U-373MG, U-251MG, U-138MG, and CH-235MG) is up to 100-fold lower than in astrocytes. Immunohistochemistry and subcellular fractionation show very low expression levels of the astrocytic glutamate transporter GLT-1 but normal expression levels of another glial glutamate transporter, GLAST. However, in glioma cells, essentially all GLAST protein was found in cell nuclei rather than the plasma membrane. Similarly, brain tissues from glioblastoma patients also display reduction of GLT-1 and mislocalization of GLAST. In glioma cell lines, over 50% of glutamate transport was Na(+)-independent and mediated by a cystine-glutamate exchanger (system x(c)(-)). Extracellular L-cystine dose-dependently induced glutamate release from glioma cells. Glutamate release was enhanced by extracellular glutamine and inhibited by (S)-4-carboxyphenylglycine, which blocked cystine-glutamate exchange. These data suggest that the unusual release of glutamate from glioma cells is caused by reduction-mislocalization of Na(+)-dependent glutamate transporters in conjunction with upregulation of cystine-glutamate exchange. The resulting glutamate release from glioma cells may contribute to tumor-associated necrosis and possibly to seizures in peritumoral brain tissue.

Role of glutamate receptors and glutamate transporters in the regulation of the glutamate-glutamine cycle in the awake rat

In the present study we investigate the effects of a specific glutamate reuptake blocker, L-trans-pyrrolidine-3,4-dicarboxylic acid (PDC), on extracellular concentrations of glutamine and glutamate in the striatum of the freely moving rat. Intracerebral infusions of PDC (1, 2 and 4 mM) produced a dose-related increase in extracellular concentrations of glutamate and a dose-related decrease in extracellular concentrations of glutamine. These increases in extracellular glutamate and decreases in extracellular glutamine were significantly correlated. To investigate the involvement of ionotropic glutamate receptors in the decreases of extracellular glutamine produced by PDC, N-methyl-D-aspartate (NMDA) receptor antagonist and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonist were used. Perfusion of the NMDA receptor antagonist blocked the decrease of extracellular glutamine but had no effect on the increase of extracellular glutamate, both produced by PDC. Perfusion of the AMPA/kainate receptor antagonist attenuated the increase of extracellular glutamate and not only blocked the decrease of extracellular glutamine but also produced a significant increase of extracellular glutamine. The results reported in this study suggest that both NMDA and AMPA/kainate glutamatergic receptors are involved in the regulation of extracellular glutamine.

Effects of anti-epileptic drugs on glutamine synthetase activity in mouse brain

1. Glutamine synthetase (GS) is a key enzyme in the regulation of glutamate neurotransmission in the central nervous system. It is responsible for the conversion of glutamate to glutamine, and for the detoxification of ammonia. 2. We have investigated the effects of single and repeated intraperitoneal administration of a range of established and new anti-epileptic drugs on GS activity in mouse brain. 3. Four hours after the final dose, animals were sacrificed and the brains removed for analysis of GS activity. 4. Both single and repeated doses of phenytoin and carbamazepine were found to reduce enzyme activity (P<0.05). 5. Single doses of phenobarbitone, felbamate and topiramate were without effect, however repeated administration of these drugs dose-dependently reduced GS activity (P<0.05). 6. Single and repeated doses of sodium valproate, vigabatrin, lamotrigine, gabapentin, tiagabine, levetiracetam and desglycinyl-remacemide were found to have no effect on GS activity. 7. The reduction in enzyme activity demonstrated is unlikely to be related to the anti-epileptic actions of these drugs, but may contribute to their toxicity.

The highly integrated dialogue between neurons and astrocytes in brain function

For decades neurons have been regarded as the only cells involved in the generation and control of brain signalling, while the surrounding glia was supposed to provide structural and metabolic support to neuronal function. However, based on a number of recent findings, a new view is emerging: astrocytes, the glial cells ensheathing synaptic specializations, are active and integrated participants of neurotransmission. Not only do astrocytes take up and remove synaptically released glutamate (the major excitatory neurotransmitter), thus ending its stimulatory action and preventing neuronal damage, but also and outstandingly, they are able to undergo rapid bidirectional communication with neurons, based on reciprocal glutamatergic signalling. Thus, release of glutamate from synaptic terminals, in addition to postsynaptic neurons, turns on the astrocytes nearby which respond by liberating the same neurotransmitter via a novel Ca(2+)-dependent mechanism and thereby signal back to neurons. The present review discusses the above findings and their important implications as well as additional evidence supporting the new concept of an integrated neuron-astrocyte communication in brain function.

Corticosterone peak is responsible for stress-induced elevation of glutamate in the hippocampus

Effect of ether stress on dialysate concentration of extracellular amino acids in the hippocampus was studied by microdialysis in freely moving rats that have been either sham operated (SHAM) or adrenalectomized and supplemented with subcutaneous steroid pellets (ADX+CORT) providing constant corticosterone (CORT) plasma levels. In SHAM rats, ether stress resulted in a peak of glutamate and taurine 30 min after stress, while extracellular aspartate concentration was increased 120 min after challenge. These changes in amino acid levels as well as in glutamate/glutamine ratio were paralleled by stress-induced rise of plasma CORT. No significant alterations were detected in the concentration of hippocampal arginine, alanine, glycine, glutamine, threonine or serine. In contrast to SHAM animals, ether stress failed to have an effect on dialysate concentration of amino acid transmitters in the hippocampus of adrenalectomized rats supplemented with 50 mg CORT-pellets. Our results demonstrate that ether stress alters aspartate, glutamate, glutamate/glutamine ratio and taurine concentration in the hippocampus and indicate that stress-induced CORT release in plasma may be responsible for these amino acid alterations. These changes may also contribute to negative feedback effect of CORT on hypothalamo-pituitary-adrenocortical (HPA) axis via the hippocampus during stress.

Glutamate transport and metabolism in astrocytes

Glutamate uptake and metabolism was studied in cerebral cortical astrocytes. The expression of the astrocytic glutamate transporter GLAST was found to be stimulated by extracellular glutamate through activation of kainate receptors on the astrocytes. Energy metabolism and ammonia homeostasis are two important aspects of glutamate handling in astrocytes. It is well known that glutamate transport into astrocytes and glutamine formation are energy consuming processes. Furthermore, ammonia is required for glutamine production. On the other hand, glutamate metabolism through the tricarboxylic acid cycle is an energy and ammonia producing pathway. In the present study it was shown that at an extracellular glutamate concentration of 0.5 mM, high energy phosphates were reduced, and more than 50% of the glutamate carbon skeleton entered the tricarboxylic acid cycle to yield products like lactate, aspartate, and additionally glutamate and glutamine derived from tricarboxylic acid cycle intermediates. Entry into the cycle was not affected by the transaminase inhibitor aminooxyacetic acid, indicating that deamination is the major route for 2-oxoglutarate formation from glutamate. Synthesis of glutamate from 2-oxoglutarate, however, proceeded via transamination. In an earlier study it was shown that at glutamate concentrations at and below 0.2 mM, glutamine appears to be the major product and entry of glutamate into the tricarboxylic acid cycle is decreased 70% by aminooxyacetic acid. In an attempt to unify the above mentioned results, it is suggested that availability of ammonia and energy demands are major factors determining the metabolic fate of glutamate in astrocytes.

Modulation of glutamine synthesis in cultured astrocytes by nitric oxide

1. Previous results suggest that glutamine synthesis in brain could be modulated by nitric oxide. The aim of this work was to assess this possibility. 2. As glutamine synthetase in brain is located mainly in astrocytes, we used primary cultures of astrocytes to assess the effects of increasing or decreasing nitric oxide levels on glutamine synthesis in intact astrocytes. 3. Nitric oxide levels were decreased by adding nitroarginine, an inhibitor of nitric oxide synthase. To increase nitric oxide we used S-nitroso-N-acetylpenicillamine, a nitric oxide generating agent. 4. It is shown that S-nitroso-N-acetylpenicillamine decreases glutamine synthesis in intact astrocytes by approximately 40-50%. Nitroarginine increases glutamine synthesis slightly in intact astrocytes. 5. These results indicate that brain glutamine synthesis may be modulated in vivo by nitric oxide.

Assessment of neurotoxicity and "neuroprotection"

Coronal brain slices allow the study of neurotoxicity and "neuroprotection" under conditions where the differentiation-state and interrelationships of the neurones and glial cells are closer to those occurring in the intact tissue than is the case for co-cultured cell systems. The involvement of glial cells in the excitotoxicity of kainate and the potentiation of this toxicity by inhibition of glutamine synthase can be demonstrated. Longer-term toxicity of kainate may also be compounded by depletion of glutathione levels resulting from inhibition of gamma-glutamylcysteine synthase. The involvement of nitric oxide formation in the toxicity of N-methyl-D-aspartate can also be shown. The neurotoxicity of 1-methyl-4-phenylpyridinium can be readily demonstrated in coronal slice preparations. Taurine affords protection against this neurotoxicity. The possible mechanisms of these effects are considered in terms of the cyclic interrelationships between the different events which can lead to cell death.

Effect of intrahippocampal dexamethasone on the levels of amino acid transmitters and neuronal excitability

Direct effect of type-II corticosteroid receptor agonist dexamethasone on extracellular amino acid levels and neuronal excitability in the hippocampus was studied by simultaneous application of in vivo microdialysis and recording hippocampal evoked responses in freely moving male rats. Microdialysis probes and hippocampal recording electrodes were implanted to the CA1-CA3 regions of dorsal hippocampus. Local dexamethasone infusion via microdialysis resulted in a transient increase in glutamate level at 30 min, while glutamine decreased by 30-40% throughout the 180-min sampling period. Taurine increased by 50% and remained elevated up to 180 min. No significant changes were detected in extracellular concentration of asparagine, arginine, glycine, threonine, alanine and serine. In contrast, dexamethasone infusion to the striatum had no effect on the extracellular levels of amino acid transmitters. Effect of dexamethasone injected via microdialysis on the neural activity elicited by perforant path stimulation was a decrease in population spikes after 60 min starting dexamethasone infusion. Steroid effect on neural excitability was reversible. Our data indicate that local infusion of type-II receptor agonist dexamethasone has a complex effect in the hippocampus, starts with a change in extracellular glutamate and glutamine concentration and followed by a reduced synaptic excitability.

MRS study of glutamate metabolism in cultured neurons/glia

[U-13C]Glutamate metabolism was studied in primary brain cell cultures. Cell extracts as well as redissolved lyophilized media were subjected to nuclear magnetic resonance spectroscopy in order to identify 13C labeled metabolites. Both neurons and astrocytes metabolized glutamate extensively with 13C label appearing in aspartate in all cultures. Additionally, GABA is synthesized in the GABAergic cortical neurons. Labeling of lactate and glutamine was prominent in medium from astrocytes, but not detectable in cerebral cortical neurons. Cerebellar granule neurons showed some labeling of lactate. Glutamate derived from the first turn of the tricarboxylic acid cycle (1,2,3-13C3-isotopomer) is present in all cell types analyzed. However, glutamate derived from the second turn of the cycle was only detected in granule neurons. In astrocytes, the transaminase inhibitor aminooxyacetic acid not only abolished the appearance of aspartate, but also of the 1,2,3-13C3-isotopomer of glutamate, thus showing that transamination is necessary for the conversion of 2-oxoglutarate to glutamate. The entry of glutamate into the tricarboxylic acid cycle was, however, not seriously impaired. 3-nitropropionic acid abolished the appearance of aspartate, the 1,2,3-13C3-isotopomer of glutamate and lactate in cerebellar granule neurons.

Glutamate: a potential mediator of inorganic mercury neurotoxicity

Exposure to mercury vapor (Hg0) produces neurotoxic effects which are for the most part subsequent to its biotransformation in brain to the mercuric cation (Hg2 +), which has an exceptionally strong affinity towards the SH groups in proteins. However, neurologic symptoms are often encountered in subjects in which Hg+ concentration in the brain remains in the submicromolar range, markedly below the anticipated threshold for direct inhibition of cerebral metabolism and function. In this report we review biochemical and morphological evidence obtained in this and other laboratories in tissue culture studies suggesting that in such instances mercury neurotoxicity may be mediated by excitotoxic activity of glutamate (GLU). Mercuric chloride (MC) at 1 microM concentration (or less) inhibits GLU uptake and stimulates GLU release in cultured astrocytes, which in vivo is likely to result in excessive GLU accumulation in the extracellular space of the CNS. Inhibition of GLU uptake and stimulation of GLU release by MC may be attenuated by addition to the cultures of a cell membrane-penetrating agent dithiothreitol (DTT) but not of glutathione (GSH), which is not transported to the inside of the cells. However, MC-stimulated release of GLU is suppressed when the intracellular GSH levels are increased by metabolic manipulation. The results indicate that the MC-vulnerable SH groups critical for GLU transport are located within the astrocytic membranes. Ultrastructural evidence for GLU-mediated MC neurotoxicity came from studies in an organotypic culture of rat cerebellum. We have shown that: 1) 1 microM MC lowers the threshold of GLU neurotoxicity, 2) the combined neurotoxic effect of GLU plus MC is attenuated by DTT but not by GSH, which is consistent with the involvement of impaired astrocytic GLU transport, and 3) neuronal damage induced by GLU plus MC becomes less accentuated in a medium with dizocilpine (MK-801), a noncompetitive NMDA receptor antagonist.

Evaluation of the importance of transamination versus deamination in astrocytic metabolism of [U-13C]glutamate

Glutamate metabolism was studied in primary cultures of cerebral cortical astrocytes to determine the significance of transamination for the oxidative metabolism of glutamate. Cultures were incubated with [U-13C]glutamate (0.5 mM) in the presence and absence of the transaminase inhibitor aminooxyacetic acid (AOAA) and in some cases with methionine sulfoximine, an inhibitor of glutamine synthetase. Perchloric acid extracts of the cells as well as redissolved lyophilized incubation media were subjected to nuclear magnetic resonance spectroscopy to identify 13C-labeled metabolites. Additionally, biochemical analyses were performed to quantify amino acids, lactate, citrate, and ammonia. Glutamine released into the medium and intracellular glutamate were labeled uniformly to a large extent, but the C-3 position showed not only the expected apparent triplet but also a doublet due to 12C incorporation into the C-4 and C-5 positions. Incorporation of 12C into the C-4 and C-5 positions of glutamate and glutamine as well as labeling of lactate, citrate, malate, and aspartate could only arise via metabolism of [U-13C]glutamate through the tricarboxylic acid (TCA) cycle. Entry of the carbon skeleton of glutamate into the TCA cycle must proceed via 2-oxoglutarate. This conversion can occur as a transamination or an oxidative deamination. After blocking transamination with AOAA, metabolism of glutamate through the TCA cycle was still taking place since lactate labeling was only slightly reduced. Glutamate and glutamine synthesis from 2-oxoglutarate could, however, not be detected under this condition. It therefore appears that while glutamate dehydrogenase is important for glutamate degradation, glutamate biosynthesis occurs mainly as a transamination.

Alteration in the glial cell metabolism of glutamate by kainate and N-methyl-D-aspartate

Incubation of coronal slices of rat brain with neurotoxic concentrations of kainate (300 microM) and N-methyl-D-aspartate (NMDA; 500 microM) for 40 min reduced the activity of the glial enzyme, glutamine synthetase, by 33% and 21%, respectively. The immunoreactivity of the neuronal enzyme, gamma gamma-enolase (neuron-specific enolase), was also decreased, but to a lesser extent than glutamine synthetase. Pre-incubation of the slices with L-methionine-S-sulphoximine (500 microM), an irreversible inhibitor of both glutamine synthetase and gamma-glutamylcysteine synthetase, before addition of either kainate or NMDA produced a supra-additive reduction in the activity of the enzyme in both cases. Neither kainate nor NMDA directly inhibited the activity of glutamine synthetase, but kainate did inhibit gamma-glutamylcysteine synthetase, a rate-limiting enzyme of the gamma-glutamyl cycle, which is responsible for maintaining glutathione levels within cells. Pre-incubation of the slices with L-NG-nitroarginine, a competitive inhibitor of nitric oxide synthase, effectively prevented the NMDA-induced reduction in glutamine synthetase and neuron specific enolase, but did not diminish the kainate-induced decrease in the activity of either enzyme. These results provide evidence that NMDA, as well as kainate, indirectly affects the activity of glutamine synthetase in brain slices, yet does so by a different mechanism from kainate. The results are discussed in terms of the possible mode of action of each toxin in inhibiting the glial cell metabolism of glutamate.

Astrocytes as potential modulators of mercuric chloride neurotoxicity

1. MC has been shown to inhibit the uptake of L-glutamate and increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. 2. Two sulfhydryl (SH-)-protecting agents; reduced glutathione (GSH), a cell membrane-nonpenetrating compound, and the membrane permeable dithiothreitol (DTT), have been shown consistently to reverse the above effects. MC-induced D-aspartate release is completely inhibited by the addition of 1 mM DTT or GSH during the actual 5-min perfusion period with MC (5 microM); when added after MC treatment, DTT fully inhibits the MC-induced D-aspartate release, while GSH does not. 3. Neither DTT nor GSH, in the absence of MC, have any effect on the rate of astrocytic D-aspartate release. Other studies demonstrate that although MC treatment (5 microM) does not induce astrocytic swelling, its addition to astrocytes swollen by exposure to hypotonic medium leads to their failure to volume regulate. 4. Omission of calcium from the medium greatly potentiates the effect of MC on astrocytic D-aspartate release, an effect which can be reversed by cotreatment of astrocytes with the dihydropyridine Ca(2+)-channel antagonist nimodipine (10 microM), indicating that one possible route of MC entry into the cells is through voltage-gated L-type channels.

The role of -SH groups in methylmercuric chloride-induced D-aspartate and rubidium release from rat primary astrocyte cultures

Methylmercuric chloride (MeHgCl) was shown to increase D-aspartate and rubidium (Rb; a marker for potassium) release from preloaded astrocytes in a dose- and time-dependent fashion. Two sulfhydryl (-SH) protecting agents: a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MeHgCl on the efflux of radiolabeled D-aspartate as well as Rb. MeHgCl-induced D-aspartate and Rb release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MeHgCl (10 microM). However, when added after MeHgCl treatment, this inhibition could not be fully sustained by GSH, while DTT fully inhibited the MeHgCl-induced release of D-aspartate. Neither DTT or GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MeHgCl on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MeHgCl-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier cannot be invoked to explain MeHgCl-induced D-aspartate release. Omission of Ca2+ from the perfusion solution increased the time-dependent MeHgCl-induced D-aspartate release.

Valproate increases glutaminase and decreases glutamine synthetase activities in primary cultures of rat brain astrocytes

It has been proposed that hyperammonemia may be associated with valproate therapy. As astrocytes are the primary site of ammonia detoxification in brain, the effects of valproate on glutamate and glutamine metabolism in astrocytes were studied. It is well established that, because of compartmentation of glutamine synthetase, astrocytes are the site of synthesis of glutamine from glutamate and ammonia. The reverse reaction is catalyzed by the ubiquitous enzyme glutaminase, which is present in both neurons and astrocytes. In astrocytes exposed to 1.2 mM valproate, glutaminase activity increased 80% by day 2 and remained elevated at day 4; glutamine synthetase activity was decreased 30%. Direct addition of valproate to assay tubes with enzyme extracts from untreated astrocytes had significant effects only at concentrations of 10 and 20 mM. When astrocytes were exposed for 4 days to 0.3, 0.6, or 1.2 mM valproate and subsequently incubated with L-[U-14C]glutamate, label incorporation into [14C]glutamine was decreased by 11, 25, and 48%, respectively, and is consistent with a reduction in glutamine synthetase activity. Label incorporation from L-[U-14C]glutamate into [14C]aspartate also decreased with increasing concentrations of valproate. Following a 4-day exposure to 0.6 mM valproate, the glutamine levels increased 40% and the glutamate levels 100%. These effects were not directly proportional to valproate concentration, because exposure to 1.2 mM valproate resulted in a 15% decrease in glutamine levels and a 25% increase in glutamate levels compared with control cultures. Intracellular aspartate was inversely proportional to all concentrations of extracellular valproate, decreasing 60% with exposure to 1.2 mM valproate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic exposure to ammonia on glutamate and glutamine interconversion and compartmentation in homogeneous primary cultures of mouse astrocytes

Accumulation of radioactivity was studied in primary cultures of mouse astrocytes as a function of time of exposure (4-60 min) to 50 microM glutamate and 200 microM glutamine (initial concentrations), of which either glutamate or glutamine was 14C-labeled. Both the glutamate pool and the glutamine pool were compartmentalized. Initially, by far the major intracellular glutamate pool (> or = 90%) was derived from extracellular glutamate and could be converted to glutamine. This allowed a rather accurate determination of metabolic flux from glutamate to glutamine, which under control conditions amounted to 2.0-2.2 nmol/min per mg protein. After chronic exposure to 3 mM ammonia for 3 days this flux was significantly increased to 3.1-3.6 nmol/min per mg protein. Acute exposure to ammonia caused a smaller, apparent increase, which was not statistically significant. The glutamine content was compartmentalized at all stages of the incubation. It consisted of at least two different pools. One of these was accessible to extracellular glutamine and could be converted to intracellular glutamate (constituting a sizeable fraction of the total glutamate pool after longer incubation), whereas the other constituted endogenously derived glutamine, formed from accumulated glutamate. The specific activity of the precursor pool for glutamate synthesis could not be accurately determined and relatively exact fluxes therefore not be calculated. There was, however, no evidence that chronic exposure to ammonia decreases the rate of glutamine hydrolysis.

Effect of fructose-1,6-bisphosphate on glutamate uptake and glutamine synthetase activity in hypoxic astrocyte cultures

Astrocytes are important in regulating the microenvironment of neurons both by catabolic and synthetic pathways. The glutamine synthetase (GS) activity observed in astrocytes affects neurons by removing toxic substances, NH3 and glutamate; and by providing an important neuronal substrate, glutamine. This glutamate cycle might play a critical role during periods of hypoxia and ischemia, when an increase in extracellular excitatory amino acids is observed. It was previously shown in our laboratory that fructose-1,6-bisphosphate (FBP) protected cortical astrocyte cultures from hypoxic insult and reduced ATP loss following a prolonged (18-30 hrs) hypoxia. In the present study we established the effects of FBP on the level of glutamate uptake and GS activity under normoxic and hypoxic conditions. Under normoxic conditions, [U-14C]glutamate uptake and glutamine production were independent of FBP treatment; whereas under hypoxic conditions, the initial increase in glutamate uptake and an overall increase in glutamine production in astrocytes were FBP-dependent. Glutamine synthetase activity was dependent on FBP added during the 22 hours of either normoxic- or hypoxic-treatment, hence significant increases in activity were observed due to FBP regardless of the oxygen/ATP levels in situ. These studies suggest that activation of GS by FBP may provide astrocytic protection against hypoxic injury.

Stimulation of D-aspartate efflux by mercuric chloride from rat primary astrocyte cultures

Mercuric chloride (HgCl2; MC) was shown to increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. Two sulfhydryl (-SH) protecting agents, a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane-permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MC on the efflux of radiolabeled D-aspartate. MC-induced D-aspartate release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MC (5 microM). However, when added after MC treatment, this inhibition could not be sustained by GSH, while DTT fully inhibited the MC-induced release of D-aspartate. Neither DTT nor GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MC on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MC-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier can not be invoked to explain MC-induced D-aspartate release. Furthermore, MC did not appear to be associated with astrocytic swelling.

Aluminum pretreatment impairs the ability of astrocytes to protect neurons from glutamate mediated toxicity

A number of laboratories have shown that astrocytes protect neurons from glutamate excitotoxicity. The experiments described in this paper were designed to address the question whether prior exposure of astrocytes to aluminum (in the form of aluminum citrate) interfered with the ability of astrocytes to protect neurons from glutamate excitotoxicity. Our culture paradigm consisted of highly enriched cultures of neurons and astrocytes grown on separate coverslips; this design enables one to subject either the neurons or the astrocytes to specific treatments and recombine the cells into the same petri dish simply by moving coverslips from dish to dish. We have confirmed findings of other laboratories that astrocytes could protect neurons from glutamate-induced death when glutamate (100 microM) is added to the culture medium. We have also demonstrated that prior treatment of astrocytes with 100 microM aluminum citrate impairs this ability of astrocytes to promote neuronal survival. No differences, however, were observed in the ability of control and aluminum-treated astrocytes to take up glutamate. These findings suggest that aluminum may cause astrocytes to: (i) secrete a factor that makes neurons more susceptible to glutamate-induced toxicity; (ii) secrete a neuronotoxic factor in the presence of glutamate; or (iii) reduce secretion of a factor that protects neurons from glutamate excitotoxicity.

Metabolism of [U-13C]glutamate in astrocytes studied by 13C NMR spectroscopy: incorporation of more label into lactate than into glutamine demonstrates the importance of the tricarboxylic acid cycle

Primary cultures of cerebral cortical astrocytes were incubated with [U-13C]glutamate (0.5 mM) in modified Dulbecco's medium for 2 h. Perchloric acid (PCA) extracts of the cells as well as redissolved lyophilized media were subjected to NMR spectroscopy to identify 13C-labeled metabolites. NMR spectra of the PCA extracts exhibited distinct multiplets for glutamate, aspartate, glutamine, and malate. The culture medium showed peaks for a multitude of compounds released from the astrocytes, among which lactate, glutamine, alanine, and citrate were readily identifiable. For the first time incorporation of label into lactate from glutamate was clearly demonstrated by doublet formation in the C-3 position and two doublets in the C-2 position of lactate. This labeling pattern can only occur by incorporation from glutamate, because natural abundance will only produce singlets in proton-decoupled 13C spectra. Glutamine, released into the medium, was labeled uniformly to a large extent, but the C-3 position not only showed the expected apparent triplet but also a doublet due to 13C incorporation into the C-4 position of glutamine. The doublet accounted for 11% of the total label in the glutamine synthesized and released within the incubation period. The corresponding labeling pattern of [13C]glutamate in the PCA extracts showed that 19% of the glutamate contained 12C. Labeling of lactate, citrate, malate, and aspartate as well as incorporation of 12C into uniformly labeled glutamate and glutamine could only arise via the tricarboxylic acid cycle. The relative amount of glutamate metabolized via this route is at least 70% as calculated from the areas of the C-3 resonances of these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

A glutamatergic mechanism for aluminum toxicity in astrocytes

The effect of aluminum on the metabolism of glutamate and glutamine in astrocytes was studied to provide information about a possible biochemical mechanism for aluminum neurotoxicity and its potential contribution to neurodegenerative disease. Exposure of cultured rat brain astrocytes for 3-4 d to 5-7.5 mM aluminum lactate increased glutamine synthetase activity by 100-300% and diminished glutaminase activity by 50-85%. Increased glutamine synthetase enzyme activity was accompanied by an elevated level of glutamine synthetase mRNA. Alterations in glutaminase and glutamine synthetase following aluminum exposure caused increased intracellular glutamine levels, decreased intracellular glutamate levels, and increased conversion of glutamate to glutamine and the release of the latter into the extracellular space. The results of these changes may alter the availability of neurotransmitter glutamate in vivo and may be a mechanism for the aluminum neurotoxicity observed in individuals exposed to the metal during dialysis procedures and other situations.

Cerebral aspartate utilization: near-equilibrium relationships in aspartate aminotransferase reaction

The pathways of nitrogen transfer from 50 microM [15N]aspartate were studied in rat brain synaptosomes and cultured primary rat astrocytes by using gas chromatography-mass spectrometry technique. Aspartate was taken up rapidly by both preparations, but the rates of transport were faster in astrocytes than in synaptosomes. In synaptosomes, 15N was incorporated predominantly into glutamate, whereas in glial cells, glutamine and other 15N-amino acids were also produced. In both preparations, the initial rate of N transfer from aspartate to glutamate was within a factor of 2-3 of that in the opposite direction. The rates of transamination were greater in synaptosomes than in astrocytes. Omission of glucose increased the formation of [15N]-glutamate in synaptosomes, but not in astrocytes. Rotenone substantially decreased the rate of transamination. There was no detectable incorporation of 15N from labeled aspartate to 6-amino-15N-labeled adenine nucleotides during 60-min incubation of synaptosomes under a variety of conditions; however, such activity could be demonstrated in glial cells. The formation of 15N-labeled adenine nucleotides was marginally increased by the presence of 1 mM aminooxyacetate, but was unaffected by pretreatment with 1 mM 5-amino-4-imidazolecarboxamide ribose. It is concluded that (1) aspartate aminotransferase is near equilibrium in both synaptosomes and astrocytes under cellular conditions, but the rates of transamination are faster in the nerve endings; (2) in the absence of glucose, use of amino acids for the purpose of energy production increases in synaptosomes, but may not do so in glial cells because the latter possess larger glycogen stores; and (3) nerve endings have a very limited capacity for salvage of the adenine nucleotides via the purine nucleotide cycle.

The role of sulfhydryl groups and calcium in the mercuric chloride-induced inhibition of glutamate uptake in rat primary astrocyte cultures

Inhibition by mercuric chloride (MC) of the astrocytic uptake of the excitotoxic neurotransmitter L-glutamate (L-GLU) has been postulated to contribute to MC neurotoxicity. In the present study, we analyzed the ability of two sulfhydryl (SH)-protecting agents: a cell membrane non-penetrating compound-reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), to reverse the inhibitory action of MC on the initial rate of uptake of radiolabelled GLU (100 microM) in primary cultures of rat astrocytes. MC at 5 microM concentration reduced the uptake to 46% of control when present in the incubation medium during the 5 min of actual uptake, and to 27% of control when astrocytes were preincubated for 30 min in HEPES buffer containing MC prior to GLU uptake measurements. GLU uptake inhibition caused by 30 min preincubation with MC was partly relieved by the addition of 1 mM DTT during the actual 5 min uptake period. However, this inhibition could not be reversed by 1 mM GSH. Accordingly, it is postulated that the inhibitory effect exerted by MC on GLU uptake is associated with vulnerable SH groups located within, but not on the surface of the cell membrane. Neither 5 microM N-ethylmaleimide (NEM) nor 5 microM or 25 microM iodoacetate (IA) affected GLU uptake, indicating steric hindrance of the access of these two sulfhydryl reagents to the SH groups critical for the uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptosomal glutamate uptake declines progressively in the spinal cord of a mutant mouse with motor neuron disease

It has been suggested that the degeneration of motor neurons in amyotrophic lateral sclerosis is a consequence of excitotoxicity resulting from a loss of synaptosomal glutamate uptake. The role of synaptosomal glutamate uptake in the pathogenesis of motor neuron disease was studied in the Mnd mouse. Glutamate uptake in spinal-cord synaptosomes declined in parallel with the onset of behavioral deficits in Mnd mice but lagged considerably behind the appearance of pathology in motor neurons. Glutamate uptake did not decline significantly in corpus striatum, and GABA uptake did not change significantly in either spinal cord or striatum. The presence of pronounced histopathological changes before the loss of glutamate uptake suggests that the decline of glutamate uptake is a consequence rather than the primary cause of motor neuron disease in the Mnd mouse.

Methylmercury-induced alterations in excitatory amino acid transport in rat primary astrocyte cultures

To determine whether methylmercury (MeHg) has the potential to interfere with homeostatic functions in neonatal rat cortical primary astrocyte cultures, the effects of MeHg on the uptake and efflux of both L-glutamate and D-aspartate were examined. Uptake of both of these excitatory amino acids (EAAs) was significantly (P < 0.05) reduced in the presence of MeHg concentrations as low as 10(-5) M. Efflux of both glutamate and aspartate from preloaded astrocytes was also increased by MeHg in a dose- and time-dependent fashion. Since in our earlier studies we had found that MeHg causes dose-dependent astrocytic swelling, which could have been the mechanism of the increased efflux, we examined whether blockage of conductive ion fluxes, which have been implicated in astrocytic swelling, could reverse the MeHg-induced increase in L-glutamate and D-aspartate efflux. Three compounds which inhibit the hypotonic-media-induced efflux of EAA, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), L-644,711 (a derivative of ethacrynic acid) and furosemide were tested at their maximal concentrations for their ability to reverse MeHg-induced EAA efflux. Only furosemide (5 mM) could sustain the reversal for the entire 120 min duration of the efflux measurement. Since hypotonic-media swelling-induced release of EAAs is inhibited by these anion inhibitors (in the following rank order: L-644,711 > SITS > furosemide), we conclude that different mechanisms account for EAA release from primary astrocyte cultures during MeHg exposure as compared to hypotonic media-induced efflux.

Effects of increased gamma-aminobutyric acid levels on GAD67 protein and mRNA levels in rat cerebral cortex

Rats were injected with saline or the gamma-aminobutyric acid (GABA) transaminase inhibitor gamma-vinyl-GABA for 7 days and the effects on GABA content and glutamic acid decarboxylase (GAD) activity, and the protein and mRNA levels of the two forms of GAD (GAD67 and GAD65) in the cerebral cortex were studied. gamma-Vinyl-GABA induced a 2.3-fold increase in GABA content, whereas total GAD activity decreased by 30%. Quantitative immunoblotting showed that the decline in GAD activity was attributable to a 75-80% decrease in GAD67 levels, whereas the levels of GAD65 remained unchanged. RNA slot-blotting with a 32P-labeled GAD67 cDNA probe demonstrated that the change in GAD67 protein content was not associated with a change in GAD67 mRNA levels. Our results suggest that GABA specifically controls the level of GAD67 protein. This effect may be mediated by a decreased translation of the GAD67 mRNA and/or a change in the stability of the GAD67 protein.

Glial glycogen stores affect neuronal survival during glucose deprivation in vitro

Glia perform several energy-dependent functions that may aid neuronal survival under pathological conditions. Glycogen is the major energy reserve in brain, and it is localized almost exclusively to astrocytes. Using murine cortical cell cultures containing both glia and neurons, we examined the effect of altered glial glycogen stores on neuronal survival following glucose deprivation. As previously reported, cultures exposed for several hours to media lacking glucose developed widespread neuronal degeneration without glial degeneration. If glial astrocyte glycogen content was increased to 2-3 times control levels by a 24-h pretreatment with 1 microM insulin or 0.5 mM methionine sulfoximine (MSO), glucose deprivation-induced neuronal degeneration was attenuated. These protective effects were blocked if glycogen levels were reduced back to control levels by a 30-min exposure to 1 mM dibutyryl cyclic AMP or 20 microM norepinephrine prior to glucose deprivation. Astrocyte glycogen stores may be an important factor influencing neuronal survival under conditions of energy substrate limitation.

Effect of pH on Glutamine Content Derived from Exogenous Glutamate in Astrocytes

A shift in pH from 7.4 to 7.8 in the incubation solution caused a 3.4-fold increase in the free glutamine content of mouse cerebral astrocytes that were incubated with glutamate (100 microM) and ammonium (100 microM). This large and reversible steady-state increase in glutamine content was accompanied by smaller transient increases in the following: (a) net formation of glutamine; (b) clearance of glutamate from the incubation solution; and (c) glutamate content. The content of glutamine was reduced markedly by omission of either glutamate or ammonium from the incubation solution, or by inhibition of glutamine synthetase activity with methionine sulfoximine. The rate at which glutamine was exported from the astrocytes was unaffected by the pH change. The effects of pH on the concentration of free ammonia or on glutamate uptake do not appear to mediate the increase in glutamine content. Uptake of exogenous glutamine was little affected by the pH change. Therefore, possible mediation of the effect by an increase in intracellular pH must be considered. The response to altered pH described here may provide a cellular basis for the increased level of brain glutamine observed in hyperammonemia.

Effect of intracellular glutamine on the uptake of large neutral amino acids in astrocytes: concentrative Na(+)-independent transport exhibits metastability

To examine whether the concentration gradient of glutamine (Gln) drives concentrative Na(+)-independent uptake of neutral amino acids (NAA) in mouse cerebral astrocytes, uptake was compared in "Gln-depleted" and "Gln-replete" cultures. Uptake (30 min in Na(+)-free buffer) of histidine, kynurenine, leucine, tyrosine, and a model substrate for System L transport was 70-150% greater in Gln-replete cultures. Phenylalanine uptake was not affected. All of these NAA trans-stimulated the export of Gln from astrocytes. However, the increase in NAA uptake was sustained even though the Gln content of Gln-replete cultures declined. Also, uptake of Gln itself was enhanced in Gln-replete cultures. Thus, countertransport of Gln was insufficient to explain the enhancement of NAA uptake. Enhanced uptake was restored, and could be magnified, by reloading Gln-depleted cultures either with Gln or with histidine. It is suggested that substrate-induced asymmetry and molecular hysteresis in the Na(+)-independent carrier could account for the sustained enhancement of NAA uptake. Only histidine and kynurenine were concentrated comparably to Gln (15- to 29-fold at 1 mM in Na(+)-free buffer). The other NAA were four to six times less concentrated. At least two Na(+)-dependent transport systems also supported the concentration gradient of Gln in regular buffer.

Interactions of trimethyl tin (TMT) with rat primary astrocyte cultures: altered uptake and efflux of rubidium,l-glutamate andD-aspartate

Studies were undertaken to assess the effects of trimethyl tin (TMT) on metabolic functions in primary neonatal rat cultured astrocytes. Concentrations as low as 10(-5) M TMT significantly inhibited the initial rate (1 min) of uptake of 86RbCl, used as a tracer for K+. TMT also markedly inhibited the initial rate (1 min) of Na(+)-dependent uptake of L-[3H]glutamate and D-[3H]aspartate, and stimulated the release of intracellular 86Rb+, -[3H]glutamate and D-[3H]aspartate in a dose-dependent fashion. These observations support the hypothesis that the astrocyte plasma membrane is potentially an important target for TMT's toxic effect and specifically that small concentrations of this organometal can inhibit the ability of astrocytes to maintain a transmembrane K+ gradient. This would be expected to compromise the ability of astrocytes to control extracellular K+ either by spatial buffering or active uptake, and exacerbate on-going swelling. Increased levels of glutamate and aspartate in the extracellular fluid upon release from astrocytes may play an important role in TMT neurotoxicity.

Glutamate metabolism in rat cortical astrocyte cultures

Glutamate metabolism in rat cortical astrocyte cultures was studied to evaluate the relative rates of flux of glutamate carbon through oxidative pathways and through glutamine synthetase (GS). Rates of 14CO2 production from [1-14C]glutamate were determined, as was the metabolic fate of [14C(U)]glutamate in the presence and absence of the transaminase inhibitor aminooxyacetic acid and of methionine sulfoximine, an irreversible inhibitor of GS. The effects of subculturing and dibutyryl cyclic AMP treatment of astrocytes on these parameters were also examined. The vast majority of exogenously added glutamate was converted to glutamine and exported into the extracellular medium. Inhibition of GS led to a sustained and greatly elevated intracellular glutamate level, thereby demonstrating the predominance of this pathway in the astrocytic metabolism of glutamate. Nevertheless, there was some glutamate oxidation in the astrocyte culture, as evidenced by aspartate production and labeling of intracellular aspartate pools. Inhibition of aspartate aminotransferase caused a greater than 70% decrease in 14CO2 production from [1-14C]glutamate. Inhibition of GS caused an increase in aspartate production. It is concluded that transamination of glutamate rather than oxidative deamination catalyzed by glutamate dehydrogenase is the first step in the entry of glutamate carbon into the citric acid cycle in cultured astrocytes. This scheme of glutamate metabolism was not qualitatively altered by subculturing or by treatment of the cultures with dibutyryl cyclic AMP.

Physiological levels of ammonia regulate glutamine synthesis from extracellular glutamate in astrocyte cultures

The effect of ammonia on glutamate accumulation and metabolism was examined in astrocyte cultures prepared from neonatal rat cortices. Intact astrocytes were incubated with 70 microM L-[14C(U)]glutamate and varying amounts of ammonium chloride. The media and cells were analyzed separately by HPLC for amino acids and labelled metabolites. Extracellular glutamate was reduced to 8 microM by 60 min. Removal of glutamate from the extracellular space was not altered by addition of ammonia. The rate of glutamine synthesis was increased from 3.6 to 9.3 nmol/mg of protein/min by addition of 100 microM ammonia, and intracellular glutamate was reduced from 262 to 86 nmol/mg of protein after 30 min. The metabolism of accumulated glutamate was matched nearly perfectly by the synthesis of glutamine, and both processes were proportional to the amount of added ammonia. The transamination and deamination products of glutamate were minor metabolites that either decreased or remained unchanged with increasing ammonia. Thus, ammonia addition stimulates the conversion of glutamate to glutamine in intact astrocyte cultures. At physiological concentrations of ammonia, glutamine synthesis appears to be limited by the rate of glutamate accumulation and the activity of competing reactions and not by the activity of glutamine synthetase.