Strategies for studies of neurotoxic mechanisms involving deficient transport of L-glutamate: antisense knockout in rat brain in vivo and changes in the neurotransmitter metabolism following inhibition of glutamate transport in guinea pig brain slices

AMP-activated protein kinase activators have compound and concentration-specific effects on brain metabolism

AMP-activated protein kinase (AMPK) is a key sensor of energy balance playing important roles in the balancing of anabolic and catabolic activities. The high energy demands of the brain and its limited capacity to store energy indicate that AMPK may play a significant role in brain metabolism. Here, we activated AMPK in guinea pig cortical tissue slices, both directly with A769662 and PF 06409577 and indirectly with AICAR and metformin. We studied the resultant metabolism of [1-13C]glucose and [1,2-13C]acetate using NMR spectroscopy. We found distinct activator concentration-dependent effects on metabolism, which ranged from decreased metabolic pool sizes at EC50 activator concentrations with no expected stimulation in glycolytic flux to increased aerobic glycolysis and decreased pyruvate metabolism with certain activators. Further, activation with direct versus indirect activators produced distinct metabolic outcomes at both low (EC50) and higher (EC50 × 10) concentrations. Specific direct activation of β1-containing AMPK isoforms with PF 06409577 resulted in increased Krebs cycle activity, restoring pyruvate metabolism while A769662 increased lactate and alanine production, as well as labelling of citrate and glutamine. These results reveal a complex metabolic response to AMPK activators in brain beyond increased aerobic glycolysis and indicate that further research is warranted into their concentration- and mechanism-dependent impact.

Impact of Inhibition of Glutamine and Alanine Transport on Cerebellar Glial and Neuronal Metabolism

The cerebellum, or “little brain”, is often overlooked in studies of brain metabolism in favour of the cortex. Despite this, anomalies in cerebellar amino acid homeostasis in a range of disorders have been reported. Amino acid homeostasis is central to metabolism, providing recycling of carbon backbones and ammonia between cell types. Here, we examined the role of cerebellar amino acid transporters in the cycling of glutamine and alanine in guinea pig cerebellar slices by inhibiting amino acid transporters and examining the resultant metabolism of [1-¹³C]d-glucose and [1,2-¹³C]acetate by NMR spectroscopy and LCMS. While the lack of specific inhibitors of each transporter makes interpretation difficult, by viewing results from experiments with multiple inhibitors we can draw inferences about the major cell types and transporters involved. In cerebellum, glutamine and alanine transfer is dominated by system A, blockade of which has maximum effect on metabolism, with contributions from System N. Inhibition of neural system A isoform SNAT1 by MeAIB resulted in greatly decreased metabolite pools and reduced net fluxes but showed little effect on fluxes from [1,2-¹³C]acetate unlike inhibition of SNAT3 and other glutamine transporters by histidine where net fluxes from [1,2-¹³C]acetate are reduced by ~50%. We interpret the data as further evidence of not one but several glutamate/glutamine exchange pools. The impact of amino acid transport inhibition demonstrates that the cerebellum has tightly coupled cells and that glutamate/glutamine, as well as alanine cycling, play a major role in that part of the brain.

L-Aspartate, L-Ornithine and L-Ornithine-L-Aspartate (LOLA) and Their Impact on Brain Energy Metabolism

L-Ornithine-L-aspartate (LOLA), a crystalline salt, is used primarily in the management of hepatic encephalopathy. The degree to which it might penetrate the brain, and the effects it might have on metabolism in brain are poorly understood. Here, to investigate the effects of LOLA on brain energy metabolism we incubated brain cortical tissue slices from guinea pig (Cavea porcellus) with the constituent amino acids of LOLA, L-ornithine or L-aspartate, as well as LOLA, in the presence of [1-¹³C]D-glucose and [1,2-¹³C]acetate; these labelled substrates are useful indicators of brain metabolic activity. L-Ornithine produced significant "sedative" effects on brain slice metabolism, most likely via conversion of ornithine to GABA via the ornithine aminotransferase pathway, while L-aspartate showed concentration-dependent excitatory effects. The metabolic effects of LOLA reflected a mix of these two different processes and were concentration-dependent. We also investigated the effect of an intraperitoneal bolus injection of L-ornithine, L-aspartate or LOLA on levels of metabolites in kidney, liver and brain cortex and brain stem in mice (C57Bl6J) 1 h later. No significant changes in metabolite levels were seen following the bolus injection of L-aspartate, most likely due to rapid metabolism of aspartate before reaching the target tissue. Brain cortex glutamate was decreased by L-ornithine but no other brain effects were observed with any other compound. Kidney levels of aspartate were increased after injection of L-ornithine and LOLA which may be due to interference by ornithine with the kidney urea cycle. It is likely that without optimising chronic intravenous infusion, LOLA has minimal impact on healthy brain energy metabolism due to systemic clearance and the blood - brain barrier.

Acetate metabolism does not reflect astrocytic activity, contributes directly to GABA synthesis, and is increased by silent information regulator 1 activation

[¹³ C]Acetate is known to label metabolites preferentially in astrocytes rather than neurons and it has consequently been used as a marker for astrocytic activity. Recent discoveries suggest that control of acetate metabolism and its contributions to the synthesis of metabolites in brain is not as simple as first thought. Here, using a Guinea pig brain cortical tissue slice model metabolizing [1-¹³ C]D-glucose and [1,2-¹³ C]acetate, we investigated control of acetate metabolism and the degree to which it reflects astrocytic activity. Using a range of [1,2-¹³ C]acetate concentrations, we found that acetate is a poor substrate for metabolism and will inhibit metabolism of itself and of glucose at concentrations in excess of 2 mmol/L. By activating astrocytes using potassium depolarization, we found that use of [1,2-¹³ C]acetate to synthesize glutamine decreases significantly under these conditions showing that acetate metabolism does not necessarily reflect astrocytic activity. By blocking synthesis of glutamine using methionine sulfoximine, we found that significant amount of [1,2-¹³ C]acetate are still incorporated into GABA and its metabolic precursors in neurons, with around 30% of the GABA synthesized from [1,2-¹³ C]acetate likely to be made directly in neurons rather than from glutamine supplied by astrocytes. Finally, to test whether activity of the acetate metabolizing enzyme acetyl-CoA synthetase is under acetylation control in the brain, we incubated slices with the AceCS1 deacetylase silent information regulator 1 (SIRT1) activator SRT 1720 and showed consequential increased incorporation of [1,2-¹³ C]acetate into metabolites. Taken together, these data show that acetate metabolism is not directly nor exclusively related to astrocytic metabolic activity, that use of acetate is related to enzyme acetylation and that acetate is directly metabolized to a significant degree in GABAergic neurons. Changes in acetate metabolism should be interpreted as modulation of metabolism through changes in cellular energetic status via altered enzyme acetylation levels rather than simply as an adjustment of glial-neuronal metabolic activity.

Toluene inhalation in adolescent rats reduces flexible behaviour in adulthood and alters glutamatergic and GABAergic signalling

Toluene is a commonly abused inhalant that is easily accessible to adolescents. Despite the increasing incidence of use, our understanding of its long-term impact remains limited. Here, we used a range of techniques to examine the acute and chronic effects of toluene exposure on glutameteric and GABAergic function, and on indices of psychological function in adult rats after adolescent exposure. Metabolomics conducted on cortical tissue established that acute exposure to toluene produces alterations in cellular metabolism indicative of a glutamatergic and GABAergic profile. Similarly, in vitro electrophysiology in Xenopus oocytes found that acute toluene exposure reduced NMDA receptor signalling. Finally, in an adolescent rodent model of chronic intermittent exposure to toluene (10 000 ppm), we found that, while toluene exposure did not affect initial learning, it induced a deficit in updating that learning when response-outcome relationships were reversed or degraded in an instrumental conditioning paradigm. There were also group differences when more effort was required to obtain the reward; toluene-exposed animals were less sensitive to progressive ratio schedules and to delayed discounting. These behavioural deficits were accompanied by changes in subunit expression of both NMDA and GABA receptors in adulthood, up to 10 weeks after the final exposure to toluene in the hippocampus, prefrontal cortex and ventromedial striatum; regions with recognized roles in behavioural flexibility and decision-making. Collectively, our data suggest that exposure to toluene is sufficient to induce adaptive changes in glutamatergic and GABAergic systems and in adaptive behaviour that may underlie the deficits observed following adolescent inhalant abuse, including susceptibility to further drug-use.

Metabolomic Approaches to Defining the Role(s) of GABAρ Receptors in the Brain

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) acts through various types of receptors in the central nervous system. GABAρ receptors, defined by their characteristic pharmacology and presence of ρ subunits in the channel structure, are poorly understood and their role in the cortex is ill-defined. Here, we used a targeted pharmacological, NMR-based functional metabolomic approach in Guinea pig brain cortical tissue slices to identify a distinct role for these receptors. We compared metabolic fingerprints generated by a range of ligands active at GABAρ and included these in a principal components analysis with a library of other metabolic fingerprints obtained using ligands active at GABAA and GABAB, with inhibitors of GABA uptake and with compounds acting to inhibit enzymes active in the GABAergic system. This enabled us to generate a metabolic "footprint" of the GABAergic system which revealed classes of metabolic activity associated with GABAρ which are distinct from other GABA receptors. Antagonised GABAρ produce large metabolic effects at extrasynaptic sites suggesting they may be involved in tonic inhibition.

Silent information regulator 1 modulator resveratrol increases brain lactate production and inhibits mitochondrial metabolism, whereas SRT1720 increases oxidative metabolism

Silent information regulators (SIRTs) have been shown to deacetylate a range of metabolic enzymes, including those in glycolysis and the Krebs cycle, and thus alter their activity. SIRTs require NAD(+) for their activity, linking cellular energy status to enzyme activity. To examine the impact of SIRT1 modulation on oxidative metabolism, this study tests the effect of ligands that are either SIRT-activating compounds (resveratrol and SRT1720) or SIRT inhibitors (EX527) on the metabolism of (13)C-enriched substrates by guinea pig brain cortical tissue slices with (13)C and (1)H nuclear magnetic resonance spectroscopy. Resveratrol increased lactate labeling but decreased incorporation of (13)C into Krebs cycle intermediates, consistent with effects on AMPK and inhibition of the F0/F1-ATPase. By testing with resveratrol that was directly applied to astrocytes with a Seahorse analyzer, increased glycolytic shift and increased mitochondrial proton leak resulting from interactions of resveratrol with the mitochondrial electron transport chain were revealed. SRT1720, by contrast, stimulated incorporation of (13)C into Krebs cycle intermediates and reduced incorporation into lactate, although the inhibitor EX527 paradoxically also increased Krebs cycle (13)C incorporation. In summary, the various SIRT1 modulators show distinct acute effects on oxidative metabolism. The strong effects of resveratrol on the mitochondrial respiratory chain and on glycolysis suggest that caution should be used in attempts to increase bioavailability of this compound in the CNS.

The Rho kinase inhibitor Fasudil up-regulates astrocytic glutamate transport subsequent to actin remodelling in murine cultured astrocytes

BACKGROUND AND PURPOSE

Glutamate transporters play a major role in maintaining brain homeostasis and the astrocytic transporters, EAAT1 and EAAT2, are functionally dominant. Astrocytic excitatory amino acid transporters (EAATs) play important roles in various neuropathologies wherein astrocytes undergo cytoskeletal changes. Astrocytic plasticity is well documented, but the interface between EAAT function, actin and the astrocytic cytoskeleton is poorly understood. Because Rho kinase (ROCK) is a key determinant of actin polymerization, we investigated the effects of ROCK inhibitors on EAAT activity and astrocytic morphology.

EXPERIMENTAL APPROACH

The functional activity of glutamate transport was determined in murine cultured astrocytes after exposure to the ROCK inhibitors Fasudil (HA-1077) and Y27632 using biochemical, molecular and morphological approaches. Cytochemical analyses assessed changes in astrocytic morphology, F-/G-actin, and localizations of EAAT1/2.

RESULTS

Fasudil and Y27632 increased [(3)H]-D-aspartate (D-Asp) uptake into astrocytes, and the action of Fasudil was time-dependent and concentration-related. The rapid stellation of astrocytes (glial fibrillary acidic protein immunocytochemistry) induced by Fasudil was accompanied by reduced phalloidin staining of F-actin and increased V(max) for [(3)H]-D-Asp uptake. Immunoblotting after biotinylation demonstrated that Fasudil increased the expression of EAAT1 and EAAT2 on the cell surface. Immunocytochemistry indicated that Fasudil induced prominent labelling of astrocytic processes by EAAT1/2.

CONCLUSION AND IMPLICATIONS

These data show for the first time that ROCK plays a major role in determining the cell surface expression of EAAT1/2, providing new evidence for an association between transporter function and astrocytic phenotype. ROCK inhibitors, via the actin cytoskeleton, effect a consequent elevation of glutamate transporter function - this activity profile may contribute to their beneficial actions in neuropathologies.

γ-Hydroxybutyrate and the GABAergic footprint: a metabolomic approach to unpicking the actions of GHB

Gamma-hydroxybutyrate is found both naturally in the brain and self-administered as a drug of abuse. It has been reported to act at endogenous γ-hydroxybutyrate (GHB) receptors and GABA(B) receptors [GABA(B)R], and may also be metabolized to GABA. Here, the metabolic fingerprints of a range of concentrations of GHB were measured in brain cortical tissue slices and compared with those of ligands active at GHB and GABA-R using principal components analysis (PCA) to identify sites of GHB activity. Low concentrations of GHB (1.0 μM) produced fingerprints similar to those of ligands active at GHB receptors and α4-containing GABA(A)R. A total of 10 μM GHB clustered proximate to mainstream GABAergic synapse ligands, such as 1.0 μM baclofen, a GABA(B)R agonist. Higher concentrations of GHB (30 μM) clustered with GABA(C)R agonists and the metabolic responses induced by blockade of the GABA transporter-1 (GAT1). The metabolic responses induced by 60 and 100 μM GHB were mimicked by simultaneous blockade of GAT1 and GAT3, addition of low concentrations of GABA(C)R antagonists, or increasing cytoplasmic GABA concentrations by incubation with the GABA transaminase inhibitor vigabatrin. These data suggest that at concentrations > 30 μM, GHB may be active via metabolism to GABA, which is then acting upon an unidentified GABAergic master switch receptor (possibly a high-affinity extrasynaptic receptor), or GHB may itself be acting directly on an extrasynaptic GABA-R, capable of turning off large numbers of cells. These results offer an explanation for the steep dose-response curve of GHB seen in vivo, and suggest potential target receptors for further investigation.

Cardiac glycosides ouabain and digoxin interfere with the regulation of glutamate transporter GLAST in astrocytes cultured from neonatal rat brain

Glutamate transport (GluT) in brain is mediated chiefly by two transporters GLT and GLAST, both driven by ionic gradients generated by (Na⁺, K⁺)-dependent ATPase (Na⁺/K⁺-ATPase). GLAST is located in astrocytes and its function is regulated by translocations from cytoplasm to plasma membrane in the presence of GluT substrates. The phenomenon is blocked by a naturally occurring toxin rottlerin. We have recently suggested that rottlerin acts by inhibiting Na⁺/K⁺-ATPase. We now report that Na⁺/K⁺-ATPase inhibitors digoxin and ouabain also blocked the redistribution of GLAST in cultured astrocytes, however, neither of the compounds caused detectable inhibition of ATPase activity in cell-free astrocyte homogenates (rottlerin inhibited app. 80% of Pi production from ATP in the astrocyte homogenates, IC50 = 25 μM). Therefore, while we may not have established a direct link between GLAST regulation and Na⁺/K⁺-ATPase activity we have shown that both ouabain and digoxin can interfere with GluT transport and therefore should be considered potentially neurotoxic.

Rottlerin inhibits (Na+, K+)-ATPase activity in brain tissue and alters D-aspartate dependent redistribution of glutamate transporter GLAST in cultured astrocytes

The naturally occurring toxin rottlerin has been used by other laboratories as a specific inhibitor of protein kinase C-delta (PKC-delta) to obtain evidence that the activity-dependent distribution of glutamate transporter GLAST is regulated by PKC-delta mediated phosphorylation. Using immunofluorescence labelling for GLAST and deconvolution microscopy we have observed that D-aspartate-induced redistribution of GLAST towards the plasma membranes of cultured astrocytes was abolished by rottlerin. In brain tissue in vitro, rottlerin reduced apparent activity of (Na+, K+)-dependent ATPase (Na+, K+-ATPase) and increased oxygen consumption in accordance with its known activity as an uncoupler of oxidative phosphorylation ("metabolic poison"). Rottlerin also inhibited Na+, K+-ATPase in cultured astrocytes. As the glutamate transport critically depends on energy metabolism and on the activity of Na+, K+-ATPase in particular, we suggest that the metabolic toxicity of rottlerin and/or the decreased activity of the Na+, K+-ATPase could explain both the glutamate transport inhibition and altered GLAST distribution caused by rottlerin even without any involvement of PKC-delta-catalysed phosphorylation in the process.

Metabolic effects of blocking lactate transport in brain cortical tissue slices using an inhibitor specific to MCT1 and MCT2

A novel inhibitor of lactate transport, AR-C122982, was used to study the effect of inhibiting the monocarboxylate transporters MCT1 and MCT2 on cortical brain slice metabolism. We studied metabolism of L-[3-13C]lactate, and D-[1-13C]glucose under a range of conditions. Experiments using L-[3-13C]lactate showed that the inhibitor AR-C122982 altered exchange of lactate. Under depolarizing conditions, net flux of label from D-[1-13C]glucose was barely altered by 10 or 100 nM AR-C122982. In the presence of AMPA or glutamate there were increases in net flux of label and metabolic pool sizes. These data suggest lactate may supply compartments in the brain not usually directly accessed by glucose. In general, it would appear that movement of lactate between cell types is not essential for metabolic activity, with the heavy metabolic workloads imposed being unaffected by inhibition of MCT1 and MCT2. Further experiments investigating the mechanism of operation of AR-C122982 are necessary to corroborate this finding.

Understanding your inhibitions: modulation of brain cortical metabolism by GABA(B) receptors

Although the impact of neuronal excitation on the functional activity of brain is well understood, the nature of functional responses to inhibitory modulation is far from clear. In this work, we investigated the effects of modulation of the metabotropic GABA(B) receptor on brain metabolism using a targeted neuropharmacological, (1)H/(13)C nuclear magnetic resonance spectroscopy, and metabolomic approach. While agonists at GABA(B) receptors (Baclofen and SKF 97541) generally decreased metabolic activity, mild agonist action could also stimulate metabolism. Less potent antagonists (CGP 35348, Phaclofen) significantly decreased metabolic activity, while more potent antagonists (CGP 52432 and SCH 50911) had opposite, stimulatory, effects. Examination of the data by principal components analysis showed clear divisions of the effects into excitatory and inhibitory components. GABAergic modulation can, therefore, have stimulatory, inhibitory, or even neutral net effects on metabolic activity in brain tissue. This is consistent with GABAergic activity being context dependent, and this conclusion should be taken into account when evaluating functional imaging data involving modulation of neuronal inhibition.

Alanine metabolism, transport, and cycling in the brain

Brain glutamate/glutamine cycling is incomplete without return of ammonia to glial cells. Previous studies suggest that alanine is an important carrier for ammonia transfer. In this study, we investigated alanine transport and metabolism in Guinea pig brain cortical tissue slices and prisms, in primary cultures of neurons and astrocytes, and in synaptosomes. Alanine uptake into astrocytes was largely mediated by system L isoform LAT2, whereas alanine uptake into neurons was mediated by Na(+)-dependent transporters with properties similar to system B(0) isoform B(0)AT2. To investigate the role of alanine transport in metabolism, its uptake was inhibited in cortical tissue slices under depolarizing conditions using the system L transport inhibitors 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and cycloleucine (1-aminocyclopentanecarboxylic acid; cLeu). The results indicated that alanine cycling occurs subsequent to glutamate/glutamine cycling and that a significant proportion of cycling occurs via amino acid transport system L. Our results show that system L isoform LAT2 is critical for alanine uptake into astrocytes. However, alanine does not provide any significant carbon for energy or neurotransmitter metabolism under the conditions studied.

Inhibitors of glutamate transport modulate distinct patterns in brain metabolism

High affinity uptake of glutamate plays a major role in the termination of excitatory neurotransmission. Identification of the ramifications of transporter function is essential to understand the diseases in which defective excitatory amino acid transporters (EAAT) have been implicated. In this work we incubated Guinea pig cortical tissue slices with [3-(13)C]pyruvate and major currently available glutamate uptake inhibitors and studied the resultant metabolic sequelae by (13)C and (1)H NMR spectroscopy using a multivariate statistical approach. Perturbation of glutamate uptake produced significant effects on metabolic flux through the Krebs cycle, and on glutamate/glutamine cycling rates, with this effect accounting for 76% of the variation in the total data set. The effects of all inhibitors were separable from each other along three major principal components. The competitive inhibitor L-CCG III ((2S,1'S,2'R)-2-carboxycyclopropyl)glycine) differed most from the other inhibitors, showing negative weightings on both the first and second principal components, whereas the EAAT2-specific inhibitor dihydrokainate (DHK) showed metabolic patterns similar to that of anti-endo-3,4-methanopyrolidine dicarboxylate but separate from those of DL-threo-beta-benzyloxyaspartate (TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (L-tPDC). This indicates that different inhibition mechanisms or different colocalisation of the separate transporter subtypes with glutamate receptors can produce significantly different metabolic and functional outcomes for the brain.

Modulation of brain metabolism by very low concentrations of the commonly used drug delivery vehicle dimethyl sulfoxide (DMSO)

Dimethyl sulfoxide (DMSO) has long been used in studies as a vehicle to enhance the solubility and transport of ligands in biological systems. The effects of this drug on the outcomes of such studies are still unclear, with concentrations of DMSO reported as "safe" varying considerably. In the present work, we investigated the effects of very low concentrations of DMSO on the brain metabolism of [3-(13)C]pyruvate and D-[1-(13)C]glucose using (1)H/(13)C NMR spectroscopy and a guinea pig cortical brain slice model. Our results show that DMSO is accumulated by brain slices. DMSO at all concentrations [0.000025%-0.25% (v/v)] increased the metabolic rate when [3-(13)C]pyruvate was used as a substrate and also in the presence of D-[1-(13)C]glucose (0.00025%-0.1% DMSO). These results are consistent with DMSO stimulating respiration, which it may do through altering the kinetics of ATP-requiring reactions. Our results also emphasize that there is no practical concentration of DMSO that can be used in metabolic experiments without effect. Therefore, care should be taken when evaluating the actions of drugs administered in combination with DMSO.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

A metabolomic approach to ionotropic glutamate receptor subtype function: a nuclear magnetic resonance in vitro investigation

A range of behaviours are elucidated via ionotropic glutamate receptors (iGluR). In this work, we examined the acute activation of iGluRs by a range of receptor ligands and effectors to see whether distinguishable metabolic sequelae were elucidated by the activity. We used a guinea-pig brain cortical tissue slice model using targeted receptor ligands ((RS)-(tetrazol-5-yl)glycine (TZG), (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801, dizocilpine), cis-4-[phosphomethyl]-piperidine-2-carboxylic acid (CGS 19755), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, (2S, 3S, 4S)-2-carboxy-4-(1-methylethenyl)-3-pyrrolidineacetic acid (kainate) and D-serine (D-Ser), as well as compounds (quinolinic acid and kynurenic acid (KynA)) involved in some neuroinflammatory responses. The data were derived using 13C and 1H NMR spectroscopy, and analysed by metabolomic approaches and multivariate statistics. The metabolic effects of agonists at the three major classes of iGluR were easily separated from each other using this method. The classical N-methyl-D-aspartate receptor agonist TZG and the antagonist CGS 19755 produced excitatory and inhibitory metabolic responses, respectively, while the blocker MK-801 resulted in a significant decrease in net metabolism and produced the largest decrease in all metabolite pool sizes seen by any glutamatergic ligand we have studied. Quinolinic acid and KynA produced similar acute metabolic responses, which were unlike those to TZG or CGS 19755, but similar to that of D-Ser. D-Ser was highly stimulatory of net flux into the Krebs cycle. These data show that the metabolic response to iGluR perturbation in vitro is a sensitive discriminator of function.

Impact of the gliotoxin l-serine-O-sulphate on cellular metabolism in cultured rat astrocytes

L-serine-O-sulphate is a member of a group of amino acids collectively called gliotoxins and is a substrate for the high affinity sodium-dependent glutamate transporters. Previous studies have shown that it is toxic to primary cultures of astrocytes but the mode of toxicity is unknown. The current study demonstrates that L-serine-O-sulphate, at a sub-toxic concentration (400 microM), causes significant disruption to glucose and alanine metabolism in cultures of rat cortical astrocytes. More specifically, using (13)C NMR spectroscopy a significant reduction in labelled end products from [1-(13)C]glucose and [3-(13)C]alanine was found in the presence of L-serine-O-sulphate. Additionally, using [2-(13)C]glycine a 27% reduction in de novo glutathione synthesis was observed in the presence of the gliotoxin. Incubation of the cells with L-serine-O-sulphate reduced the activity of alanine and aspartate aminotransferase by 53% and 67%, respectively. Collectively these results show that the gliotoxin, L-serine-O-sulphate, causes major disruptions to metabolic pathways in primary cultures of astrocytes.

Glutamatergic hypothesis of schizophrenia: involvement of Na+/K+-dependent glutamate transport

Hypothetical model based on deficient glutamatergic neurotransmission caused by hyperactive glutamate transport in astrocytes surrounding excitatory synapses in the prefrontal cortex is examined in relation to the aetiology of schizophrenia. The model is consistent with actions of neuroleptics, such as clozapine, in animal experiments and it is strongly supported by recent findings of increased expression of glutamate transporter GLT in prefrontal cortex of patients with schizophrenia. It is proposed that mechanisms regulating glutamate transport be investigated as potential targets for novel classes of neuroactive compounds with neuroleptic characteristics. Development of new efficient techniques designed specifically for the purpose of studying rapid activity-dependent translocation of glutamate transporters and associated molecules such as Na+, K+-ATPase is essential and should be encouraged.

Effects of glutamate transport substrates and glutamate receptor ligands on the activity of Na-/K(+)-ATPase in brain tissue in vitro

1. It has been suggested that Na+/K(+)-ATPase and Na(+)-dependent glutamate transport (GluT) are tightly linked in brain tissue. In the present study, we have investigated Na+/K(+)-ATPase activity using Rb+ uptake by 'minislices' (prisms) of the cerebral cortex. This preparation preserves the morphology of neurons, synapses and astrocytes and is known to possess potent GluT that has been well characterized. Uptake of Rb+ was determined by estimating Rb+ in aqueous extracts of the minislices, using atomic absorption spectroscopy. 2. We determined the potencies of several known substrates/inhibitors of GluT, such as L-trans-pyrrolidine-2,4-dicarboxylate (LtPDC), DL-threo-3-benzyloxyaspartic acid, (2S,3S,4R)-2-(carboxycyclopropyl)-glycine (L-CCG III) and L-anti,endo-3,4-methanopyrrolidine dicarboxylic acid, as inhibitors of [3H]-L-glutamate uptake by cortical prisms. In addition, we established the susceptibility of GluT, measured as [3H]-L-glutamate uptake in brain cortical prisms, to the inhibition of Na+/K(+)-ATPase by ouabain. Then, we tested the hypothesis that the Na+/K(+)-ATPase (measured as Rb+ uptake) can respond to changes in the activity of GluT produced by using GluT substrates as GluT-specific pharmacological tools. 3. The Na+/K(+)-ATPase inhibitor ouabain completely blocked Rb+ uptake (IC50 = 17 micromol/L), but it also potently inhibited a fraction of GluT (approximately 50% of [3H]-L-glutamate uptake was eliminated; IC50 < 1 micromol/L). 4. None of the most commonly used GluT substrates and inhibitors, such as L-aspartate, D-aspartate, L-CCG III and LtPDC (all at 500 micromol/L), produced any significant changes in Rb+ uptake. 5. The N-methyl-D-aspartate (NMDA) receptor agonists (R,S)-(tetrazol-5-yl)-glycine and NMDA decreased Rb+ uptake in a manner compatible with their known neurotoxic actions. 6. None of the agonists or antagonists for any of the other major classes of glutamate receptors caused significant changes in Rb+ uptake. 7. We conclude that, even if a subpopulation of glutamate transporters in the rat cerebral cortex may be intimately linked to a fraction of Na+/K(+)-ATPase, it is not possible, under the present experimental conditions, to detect regulation of Na+/K(+)-ATPase by GluT.

Quinolinic acid induces NMDA receptor-mediated lipid peroxidation in rat brain microvessels

Quinolinic acid increased the generation of lipid peroxidation products by isolated rat brain microvessels in vitro. The effect was inhibited both by a specific NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid and by reduced glutathione (GSH). Furthermore, quinolinic acid displaced specific binding of [(3)H]-L-glutamate by cerebral microvessel membranes, particularly in the presence of NMDA receptor co-agonist (glycine) and modulator (spermidine). We conclude that quinolinic acid can cause potentially cytotoxic lipid peroxidation in brain microvessels via an NMDA receptor mediated mechanism.

Group I and II metabotropic glutamate receptors alter brain cortical metabolic and glutamate/glutamine cycle activity: a 13C NMR spectroscopy and metabolomic study

Metabotropic glutamate receptors (mGluR) modulate neuronal function. Here, we tested the effect on metabolism of a range of Group I and II mGluR ligands in Guinea pig brain cortical tissue slices, applying 13C NMR spectroscopy and metabolomic analysis using multivariate statistics. The effects of Group I agonists (S)-3,5-dihydroxyphenylglycine (DHPG) and (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) depended upon concentration and were mostly stimulatory, increasing both net metabolic flux through the Krebs cycle and glutamate/glutamine cycle activity. Only the higher (50 microm) concentrations of CHPG had the opposite effect. The Group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), consistent with its neuroprotective role, caused significant decreases in metabolism. With principal components analysis of the metabolic profiles generated by these ligands, the effects could be separated by two principal components. Agonists at Group II mGluR [(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC)] generally stimulated metabolism, including glutamate/glutamine cycling, although this varied with concentration. The antagonist (2S)-alpha-ethylglutamic acid (EGLU) stimulated astrocyte metabolism with minimal impact on glutamate/glutamine cycling. (RS)-1-Aminophosphoindan-1-carboxylic acid (APICA) decreased metabolism at 5 microm but had a stimulatory effect at 50 microm. All ligand effects were separated from control and from each other using two principal components. The ramifications of these findings are discussed.

Gliotoxins disrupt alanine metabolism and glutathione production in C6 glioma cells: a 13C NMR spectroscopic study

Gliotoxins are a group of amino acids that are toxic to astrocytes, and are substrates of high-affinity sodium-dependent glutamate transporters. In the present study, C6 glioma cells were preincubated for 20 h in the presence of 400 microM L-alpha-aminoadipate, L-serine-O-sulphate, D-aspartate or L-cysteate, as well as in the presence of the poorly transported L-glutamate uptake inhibitor, L-anti-endo-methanopyrrolidine dicarboxylate. In experiments following [3-13C]alanine metabolism, all toxins caused a decreased incorporation of label into glutamate. Production of labelled lactate changed only when cells were incubated in the presence of L-alpha-aminoadipate or L-serine-O-sulphate. Incubation with L-anti-endo-methanopyrrolidine dicarboxylate caused no change in the amount of label incorporated into either glutamate or lactate. When glutathione production was followed using 1 mM [2-13C]glycine, differential effects of the gliotoxins were revealed. Most notably, both L-serine-O-sulphate and L-alpha-aminoadipate caused significant increases in labelling of glutathione. Once again, L-anti-endo-methanopyrrolidine dicarboxylate was without effect. Overall, we have shown that the gliotoxins cause disruption to alanine metabolism and glutathione production in C6 glioma cells, but that there are notable differences in their mechanisms of action. In the absence of any disruption to metabolism by L-anti-endo-methanopyrrolidine dicarboxylate, it is concluded that their mode of action involves more than inhibition of glutamate transport.

Brain gene expression, metabolism, and bioenergetics: interrelationships in murine models of cerebral and noncerebral malaria

Malaria infection can cause cerebral symptoms without parasite invasion of brain tissue. We examined the relationships between brain biochemistry, bioenergetics, and gene expression in murine models of cerebral (Plasmodium berghei ANKA) and noncerebral (P. berghei K173) malaria using multinuclear NMR spectroscopy, neuropharmacological approaches, and real-time RT-PCR. In cerebral malaria caused by P. berghei ANKA infection, we found biochemical changes consistent with increased glutamatergic activity and decreased flux through the Krebs cycle, followed by increased production of the hypoxia markers lactate and alanine. This was accompanied by compromised brain bioenergetics. There were few significant changes in expression of mRNA for metabolic enzymes or transporters or in the rate of transport of glutamate or glucose. However, in keeping with a role for endogenous cytokines in malaria cerebral pathology, there was significant up-regulation of mRNAs for TNF-alpha, interferon-gamma, and lymphotoxin. These changes are consistent with a state of cytopathic hypoxia. By contrast, in P. berghei K173 infection the brain showed increased metabolic rate, with no deleterious effect on bioenergetics. This was accompanied by mild up-regulation of expression of metabolic enzymes. These changes are consistent with benign hypermetabolism whose cause remains a subject of speculation.

Morphology and ultrastructure of rat hippocampal formation after i.c.v. administration of N-acetyl-L-aspartyl-L-glutamate

N-Acetyl-L-aspartyl-L-glutamate (NAAG) is one of the most abundant neuroactive compounds in the mammalian CNS. Our recent observations have suggested that NAAG administered into rat cerebral ventricles can cause neuronal death by apparently excitotoxic mechanisms that can be antagonized by the N-methyl-D-aspartate-receptor blockers and by ligands of metabotropic glutamate receptor of Group II. Therefore, the principal aim of the present study has been to use quantitative morphology, electron microscopy and terminal deoxynucleotidyl transferase-mediated biotin dUTP nick-end labeling to study a dose- and time-dependence as well as regional distribution of neurodegeneration in hippocampi of rats after the intraventricular infusion of 0.25 micromol NAAG/ventricle and of equimolar doses of L-glutamate (L-GLU) and N-acetyl-L-aspartate (NAA), breakdown products of NAAG. The degenerative changes were observed after the infusion of 0.25 and 1.25 micromol of NAAG/ventricle, but not when a dose of 0.05 micromol of NAAG/ventricle was injected into each lateral cerebral ventricle. With a dose of 0.25 micromol of NAAG/ventricle the number of degenerated neurons reached a maximum on the fourth day after the infusion. The neuronal damage following bilateral administration of 0.25 micromol of NAAG/lateral cerebral ventricle exhibited features of a delayed neuronal degeneration, expressed mainly in the layer of dentate granule neurons. The degeneration was characterized on the basis of ultrastructural appearance and DNA-fragmentation. The morphological changes caused by L-glutamate and NAA were much smaller than those observed after the administration of NAAG and displayed a different pattern of regional distribution. The present findings suggest that NAAG can cause a loss of hippocampal neurons in vivo, apparently resulting from the neurotoxicity of NAAG itself.

Avian Purkinje neuronal cultures: extrinsic control of morphology by cell type and glutamate

An in vitro coculture system is described to study the avian Purkinje neuron and the interactions occurring with astrocytes and granule cells during development in the cerebellum. Astrocytes initially and granule cells later regulate Purkinje neuron morphology. The coculture system presented here provides an excellent system for investigating the morphological, immunocytochemical, and electrophysiological differentiation of Purkinje neurons under controlled conditions and for studying cell-cell interactions and extrinsic factors, e.g., glutamate in normal and neuropathological conditions.

Acetoacetate protects hippocampal neurons against glutamate-mediated neuronal damage during glycolysis inhibition

Glucose is the main substrate that fulfills energy brain demands. However, in some circumstances, such as diabetes, starvation, during the suckling period and the ketogenic diet, brain uses the ketone bodies, acetoacetate and beta-hydroxybutyrate, as energy sources. Ketone body utilization in brain depends directly on its blood concentration, which is normally very low, but increases substantially during the conditions mentioned above. Glutamate neurotoxicity has been implicated in neurodegeneration associated with brain ischemia, hypoglycemia and cerebral trauma, conditions related to energy failure, and to elevation of glutamate extracellular levels in brain. In recent years substantial evidence favoring a close relation between glutamate neurotoxic potentiality and cellular energy levels, has been compiled. We have previously demonstrated that accumulation of extracellular glutamate after inhibition of its transporters, induces neuronal death in vivo during energy impairment induced by glycolysis inhibition. In the present study we have assessed the protective potentiality of the ketone body, acetoacetate, against glutamate-mediated neuronal damage in the hippocampus of rats chronically treated with the glycolysis inhibitor, iodoacetate, and in hippocampal cultured neurons exposed to a toxic concentration of iodoacetate. Results show that acetoacetate efficiently protects against glutamate neurotoxicity both in vivo and in vitro probably by a mechanism involving its role as an energy substrate.

An NMR study of alterations in [1-13C]glucose metabolism in C6 glioma cells by gliotoxic amino acids

A series of glutamate analogues, known as gliotoxins, are toxic to astrocytes in culture, and are inhibitors or substrates of high affinity sodium-dependent glutamate transporters. The mechanisms by which these gliotoxins cause toxicity are not fully understood. The effects of a series of gliotoxic amino acids (L-alpha-aminoadipate, L-serine-O-sulphate, D-aspartate and L-cysteate) on metabolism of [1-13C]glucose were examined in C6 glioma cells using 13C nuclear magnetic resonance (NMR) spectroscopy. The cells were preincubated in the presence of sub toxic concentrations of each gliotoxin (400 micromol/l) for 20 h. This was followed by incubation (4 h) with [1-13C]glucose (5.5 mmol/l) in the presence and absence of each gliotoxin. The incorporation of 13C label into the observed metabolites was analysed. Following preincubation with L-alpha-aminoadipate, D-aspartate, and L-serine-O-sulphate there was a significant decrease in the incorporation of 13C label into glutamate, alanine and lactate from [1-13C]glucose. In the presence of L-cysteate production of labelled glutamate was decreased, while there was no significant effect on the concentrations of labelled lactate and alanine. There was no change in the quantity of LDH released into the medium after incubation of the cells with any of the gliotoxins. Overall these results indicate that the presence of gliotoxins profoundly alters the flux of glucose to lactate, alanine, aspartate and glutamate.

Inhibition of glutamine transport depletes glutamate and GABA neurotransmitter pools: further evidence for metabolic compartmentation

The role of glutamine and alanine transport in the recycling of neurotransmitter glutamate was investigated in Guinea pig brain cortical tissue slices and prisms, and in cultured neuroblastoma and astrocyte cell lines. The ability of exogenous (2 mm) glutamine to displace 13C label supplied as [3-13C]pyruvate, [2-13C]acetate, l-[3-13C]lactate, or d-[1-13C]glucose was investigated using NMR spectroscopy. Glutamine transport was inhibited in slices under quiescent or depolarising conditions using histidine, which shares most transport routes with glutamine, or 2-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of the neuronal system A. Glutamine mainly entered a large, slow turnover pool, probably located in neurons, which did not interact with the glutamate/glutamine neurotransmitter cycle. This uptake was inhibited by MeAIB. When [1-13C]glucose was used as substrate, glutamate/glutamine cycle turnover was inhibited by histidine but not MeAIB, suggesting that neuronal system A may not play a prominent role in neurotransmitter cycling. When transport was blocked by histidine under depolarising conditions, neurotransmitter pools were depleted, showing that glutamine transport is essential for maintenance of glutamate, GABA and alanine pools. Alanine labelling and release were decreased by histidine, showing that alanine was released from neurons and returned to astrocytes. The resultant implications for metabolic compartmentation and regulation of metabolism by transport processes are discussed.

Roles and regulation of glutamate transporters in the central nervous system

1. Glutamate transporters (also known as excitatory amino acid transporters or EAAT) are solely responsible for the removal of the excitatory neurotransmitter l-glutamate (Glu) from the extracellular space and, thus, permit normal transmission, as well as preventing cell death due to the excessive activation of Glu receptors. 2. Five subtypes of glutamate transporter (EAAT1-5) exist, possessing distinct pharmacology, cellular localization and modulatory mechanisms. 3. Experimental inhibition of EAAT activity in vitro and in vivo results in increased extracellular concentrations of Glu and in neuronal death via excitotoxicity, highlighting the importance of EAAT in normal excitatory neurotransmission. 4. Dysfunction of EAAT may contribute to the pathology of both acute neuronal injury and chronic neurodegenerative conditions, so correction of EAAT function under these conditions may provide a valuable therapeutic strategy. 5. The present review describes basic pharmacological studies that allow new insights into EAAT function and suggest possible strategies for the therapeutic modulation of EAAT.

Molecular pharmacology of the Na+-dependent transport of acidic amino acids in the mammalian central nervous system

The Na+-dependent transport of L-glutamate (GluT) has been identified in brain tissue more than thirty years ago. Neurochemical studies, performed in various experimental models during 1970's, defined the basic rules for the selection or synthesis of GluT-specific substrates and inhibitors. The protein molecules (transporters) that mediate the translocation of the substrates across the plasma membrane have been cloned and studied during the last ten years. The sites on the transporters that bind the substrates favour glutamate-like or aspartate-like molecules with one positively charged and two negatively charged ionised groups. Substituents at C3 and C4 are often tolerated but substitutions at C2 or alterations of the ionisable groups usually impede the binding. The substrate binding sites display an "anomalous" selectivity towards stereoisomers. These structural requirements are shared by all Na+-dependent glutamate transporters thus making the design of transporter-selective ligands a challenging task. Moreover, the molecular mechanisms of the transport have not yet been adequately elucidated. Data from a wide variety of experimental studies strongly indicate that Na+-dependent GluT regulates the functioning of the glutamatergic excitatory synapses-the most important rapid inter-neuronal signalling system in the mammalian brain. Altered structural and/or functional properties of the Na+-dependent glutamate transporters have been implicated in the damage to the brain tissue following cerebral ischaemia and in the progressive loss of neurons in conditions such as Alzheimer dementia and amyotrophic lateral sclerosis. Furthermore, it seems that fine-tuning of glutamatergic neurotransmission by regulating the Na+-dependent GluT could be useful in the therapy of schizophrenia.

Evaluation of drugs acting at glutamate transporters in organotypic hippocampal cultures: new evidence on substrates and blockers in excitotoxicity

Removal of L-glutamate (Glu) from the synapse is critical to maintain normal transmission and to prevent excitotoxicity, and is performed exclusively by excitatory amino acid transporters (EAATs). We investigated the effects of substrates and blockers of EAATs on extracellular Glu and cellular viability in organotypic cultures of rat hippocampus. Seven-day treatment with a range of drugs (L-trans-pyrrolidine-2,4-dicarboxylate, (2S,4R)-4-methyl-glutamate, (+/-)-threo-3-methylglutamate and DL-threo-beta-benzyloxyaspartate), in the presence of 300 microM added Glu, resulted in increased extracellular Glu and a significant correlation between Glu concentration and cellular injury (as indicated by lactate dehydrogenase release). In contrast, (2S,3s,4R)-2-(carboxycyclopropyl)glycine (L-CCG-III) exerted a novel neuroprotection against this toxicity, and elevations in extracellular Glu were not toxic in the presence of this compound. Similar results were obtained following two-week treatment of cultures without added Glu. Whilst blockade of GLT-1 alone was relatively ineffective in producing excitotoxic injury, heteroexchange of Glu by EAAT substrates may exacerbate excitotoxicity.

Effects of L-glutamate transport inhibition by a conformationally restricted glutamate analogue (2S,1'S,2'R)-2-(carboxycyclopropyl)glycine (L-CCG III) on metabolism in brain tissue in vitro analysed by NMR spectroscopy

(2S,1'S,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG III) was a substrate of Na(+)-dependent glutamate transporters (GluT) in Xenopus laevis oocytes (IC50 to approximately 13 and to approximately 2 microM for, respec tively, EAAT 1 and EAAT 2) and caused an apparent inhibition of [3H]L-glutamate uptake in "mini-slices" of guinea pig cerebral cortex (IC50 to approximately 12 microM). In slices (350 microM) of guinea pig cerebral cortex, 5 microM L-CCG III increased both the flux of label through pyruvate carboxylase and the fractional enrichment of glutamate, GABA, glutamine and lactate, but had no effect on total metabolite pool sizes. At 50 microM L-CCG III decreased incorporation of 13C from [3-13C]-pyruvate into glutamate C4, glutamine C4, lactate C3 and alanine C3. The total metabolite pool sizes were also decreased with no change in the fractional enrichment. Furthermore, L-CCG III was accumulated in the tissue, probably via GluT. At lower concentration, L-CCG III would compete with L-glutamate for GluT and the changes probably reflect a compensation for the "missing" L-glutamate. At 50 microM, intracellular L-CCG III could reach > 10 mM and metabolism might be affected directly.

Abnormalities in brain biochemistry associated with lack of dystrophin: studies of the mdx mouse

Biochemical abnormalities have been reported in dystrophin-deficient muscle of boys with Duchenne (severe Xp21) muscular dystrophy or in the murine (mdx) model of the disease. These abnormalities include altered energy metabolism and responses to osmotic shock. In contrast, the situation in brain is less well understood and it is probable that dystrophin is playing a different role (or roles) in this organ. In this study we conclude that the elevation in choline-containing compounds reported in mdx brain is confined to cerebellum and hippocampus in older (> 6 months) mice. We report alterations in glucose metabolism in mdx brain under normal, awake conditions, and a reduced response of brain metabolism to the gamma-aminobutyric acid(A) receptor agonist muscimol. Using brain cortical slices we found no difference in the response of dystrophic tissue to hypoosmotic shock, but increased, substrate-dependent oxygen consumption rates at low oxygen partial pressures.

N-Acetyl-L-aspartyl-L-glutamate changes functional and structural properties of rat blood-brain barrier

Intracerebroventricular administration of N-acetyl-L-aspartyl-L-glutamate (NAAG), an agonist at group II metabotropic and NR1/NR2D-containing N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors, increased the permeability of the blood-brain barrier (BBB) to serum albumin in the striatum, but produced no similar effects in the entorhinal cortex or in the hippocampal formation. Electron microscopy showed that NAAG, but not its hydrolytic products L-glutamate and N-acetyl-L-aspartate, increased the number of transport vesicles in the hippocampal endothelial cells. Furthermore, immunocytochemistry detected NR2D subunits on hippocampal capillaries. Consequently, NAAG may have influenced the vesicular transport via NMDA receptors. There was, however, no correlation with the regional pattern of BBB changes (increased permeability in the striatum) that, in turn, could not be directly related to the NAAG-induced neurodegeneration described previously in the hippocampus where no significant changes in BBB permeability were detected.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.