Continuous administration of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate produces striatal lesion

Ciliary neurotrophic factor protects striatal neurons against excitotoxicity by enhancing glial glutamate uptake

Ciliary neurotrophic factor (CNTF) is a potent neuroprotective cytokine in different animal models of glutamate-induced excitotoxicity, although its action mechanisms are still poorly characterized. We tested the hypothesis that an increased function of glial glutamate transporters (GTs) could underlie CNTF-mediated neuroprotection. We show that neuronal loss induced by in vivo striatal injection of the excitotoxin quinolinic acid (QA) was significantly reduced (by ∼75%) in CNTF-treated animals. In striatal slices, acute QA application dramatically inhibited corticostriatal field potentials (FPs), whose recovery was significantly higher in CNTF rats compared to controls (∼40% vs. ∼7%), confirming an enhanced resistance to excitotoxicity. The GT inhibitor dl-threo-β-benzyloxyaspartate greatly reduced FP recovery in CNTF rats, supporting the role of GT in CNTF-mediated neuroprotection. Whole-cell patch-clamp recordings from striatal medium spiny neurons showed no alteration of basic properties of striatal glutamatergic transmission in CNTF animals, but the increased effect of a low-affinity competitive glutamate receptor antagonist (γ-d-glutamylglycine) also suggested an enhanced GT function. These data strongly support our hypothesis that CNTF is neuroprotective via an increased function of glial GTs, and further confirms the therapeutic potential of CNTF for the clinical treatment of progressive neurodegenerative diseases involving glutamate overflow.

Reverse glial glutamate uptake triggers neuronal cell death through extrasynaptic NMDA receptor activation

Evidence have accumulated that reverse glutamate uptake plays a key role in the pathophysiology of cerebral ischemia. Here, we investigated the effects of glial glutamate transporter dysfunction on neuronal survival using the substrate inhibitor of glutamate transporters, L-trans-pyrrolidine,2-4,dicarboxylate (PDC), that partly mimics reverse glutamate uptake. On mice primary cortical co-cultures of neurons and astrocytes, PDC treatment triggered an elevation of extracellular glutamate concentration, induced neuronal calcium influx and a massive NMDA receptor (NMDAR) mediated-neuronal death without having any direct agonist activity on NMDARs. We investigated the NMDAR subpopulation activated by PDC-induced glutamate release. PDC application led to the activation of both subtypes of NMDARs but the presence of astrocytes was required to activate NMDARs located extra-synaptically. Extrasynaptic NMDAR activation was also confirmed by the loss of neuronal mitochondrial membrane potential and the inhibition of pro-survival p-ERK signalling pathway. These data suggest that reverse glial glutamate uptake may trigger neuronal death through preferential activation of extrasynaptic NMDAR-related pathways.

Specific knock-down of GAD67 in the striatum using naked small interfering RNAs

Small interfering RNAs (siRNA) are double-stranded RNAs of 9-29 nucleotides designed to reduce the expression of homologous genes by a process known as RNA interference (RNAi). Most studies using siRNA in neurons have been performed in mammalian cell cultures. Only few reports have reported the effects of in vivo infusion of siRNA into the brain. In the present study, we performed local intracerebral infusions of naked siRNA against glutamic acid decarboxylase 67 (GAD67) mRNA to engineer specific knock-down of GAD67 protein in the striatum of adult rats. Directly injecting a mix of GAD67 siRNAs into the striatum decreased the levels of corresponding mRNA, evaluated by quantitative real-time PCR. In particular, we show that GAD67 mRNA expression is reduced in the striatum for 3, 6, and 24h following intrastriatal injection of GAD67 siRNA and is restored at 72h. Relative to controls, the levels of GAD67 protein were also lower in the striatum for 6 and 24h after injection. No changes in GAD65 expression, one of the two isoforms of GAD, were detected in the striatum, which further validates the specificity of the siRNA. We demonstrate the efficiency of the RNAi strategy for producing a specific and selective down-regulation of GAD67 in the adult rat brain. This suggests that siRNA-mediated gene knock-down constitute a valid methodological approach for studying the functional consequences of a transient decrease of a gene expression in a brain structure.

Preferential vulnerability of mesencephalic dopamine neurons to glutamate transporter dysfunction

Nigral depletion of the main brain antioxidant GSH is the earliest biochemical event involved in Parkinson's disease pathogenesis. Its causes are completely unknown but increasing number of evidence suggests that glutamate transporters [excitatory amino acid transporters (EAATs)] are the main route by which GSH precursors may enter the cell. In this study, we report that dopamine (DA) neurons, which express the excitatory amino acid carrier 1, are preferentially affected by EAAT dysfunction when compared with non-DA neurons. In rat embryonic mesencephalic cultures, l-trans-pyrrolidine-2,4-dicarboxylate, a substrate inhibitor of EAATs, is directly and preferentially toxic for DA neurons by decreasing the availability of GSH precursors and lowering their resistance threshold to glutamate excitotoxicity through NMDA-receptors. In adult rat, acute intranigral injection of l-trans-pyrrolidine-2,4-dicarboxylate induces a large regionally selective and dose-dependent loss of DA neurons and alpha-synuclein aggregate formation. These data highlight for the first time the importance of excitatory amino acid carrier 1 function for the maintenance of antioxidant defense in DA neurons and suggest its dysfunction as a candidate mechanism for the selective death of DA neurons such as occurring in Parkinson's disease.

Glia mechanisms in mood regulation: a novel model of mood disorders

INTRODUCTION

Recent evidence in clinical and preclinical studies has implicated glutamate neurotransmissions in pathophysiology of mood disorders. The regulation of amino acid neurotransmission, i.e., glutamate and gamma-aminobutyric acid (GABA) involves coordinated mechanisms of uptake and transport within a tripartite synaptic system that includes neurons and glia. Newly appreciated role of the glia, more specifically astrocytes on neuronal functions combined with reported postmortem abnormalities of glia in patients with mood disorders further supports the role of glia in mood disorders.

MATERIALS AND METHODS

This report presents some of our preliminary results utilizing glia-selective toxins and other pharmacological tools to suppress glial function within the limbic system to study the resulting behavioral abnormalities, and thus, elucidate glial involvement in the development of mood disorders.

RESULTS AND DISCUSSION

We demonstrate that chronic blockade of glutamate uptake by a glial/neuronal transporter antagonist L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) within the amygdala, a key area implicated in mood regulation, results in dose-dependent reduction in social exploratory behavior and disrupts circadian activity patterns consistent with symptoms of mood disorders. Similarly, the selective astrocytic glutamate transporter type 1 (GLT-1) blocker dihydrokainic acid (DHK) injected into the amygdala also results in reduced social interaction that is blocked by selective glutamate N-methyl-D-aspartate (NMDA) type receptor antagonist AP5. The results are discussed in the context of glial and glutamate mechanisms in mood disorders and potential therapeutic avenues to address these mechanisms.

Cerebellar Granule Neurons are More Vulnerable to Transient Transport-Mediated Glutamate Release than to Glutamate Uptake Blockade. Correlation with Excitatory Amino Acids Levels

The extracellular concentration of glutamate is highly regulated due to its excitotoxic nature. Failure of glutamate uptake or reversed activation of its transporters contributes to neurodegeneration related to some pathological conditions. We have compared the neurotoxicity of the substrate glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylate (PDC), which promotes glutamate release by hetero-exchange, with that of DL-threo-beta-benzyloxyaspartate (DL-TBOA), a non-substrate inhibitor, in cerebellar granule cell cultures. PDC substantially increases the extracellular concentration of glutamate during 30 min exposure and causes neuronal death at high concentrations, while DL-TBOA neurotoxicity is only observed after long-term exposure (8-24 h). During mitochondrial inhibition by 3-nitropropionic acid (3-NP), PDC-induced neuronal death is facilitated, but not that of DL-TBOA. In cultures containing a higher population of astrocytes DL-TBOA-induced increase in glutamate levels is more pronounced, but neuronal death is only triggered in the presence of 3-NP. Results suggest that cerebellar granule neurons are more vulnerable to acute transport-mediated glutamate release than to uptake blockade, which correlates with the extracellular excitatory amino acids levels.

Dissociation between hippocampal neuronal loss, astroglial and microglial reactivity after pharmacologically induced reverse glutamate transport

The inflammatory central nervous system response that involves activated microglia and reactive astrocytes may both heal and harm neurons, as inflammatory mediators lead to neuroprotection or excitation at one dose but to injury at a different concentration. To investigate these complex cellular interactions, L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a selective substrate inhibitor of glutamate transport, was infused during 14 days in the rat hippocampus at three different rates: 5, 10 and 25 nmol/h. A microglial reaction appeared at the 5 nmol/h PDC rate in absence of astroglial reaction and neuronal loss. Microgliosis and neuronal death were observed at PDC 10 nmol/h in absence of astrogliosis and calcium precipitation, whereas all four aspects were present at the highest rate. This dissociation between neuronal loss and astroglial reactivity took place in presence of a permanent microglial reaction. These data suggest a specific response of microglia to PDC whose neuronal effects may differ with the infused dose.

The neuronal excitatory amino acid transporter EAAC1/EAAT3: does it represent a major actor at the brain excitatory synapse?

EAAC1/EAAT3 is a transporter of glutamate (Glu) present at the post-synaptic neuronal element, in opposition to the two other main transporters, GLAST/EAAT1 and GLT1/EAAT2, expressed at the excitatory amino acid (EAA) synapse by surrounding astrocytes. Although, in the adult, EAAC1/EAAT3 exhibits a rather low expression level and is considered to make a minor contribution to Glu removal from the synapse, its early expression during brain development, before the astrocytes are functional, suggests that such a neuronal transporter is involved in the developmental effects of EAA and, possibly, in the biosynthesis and trophic role of GABA, which is excitatory in nature in different brain regions during the earlier stages of brain development. This neuronal Glu transporter is considered to have a dual action as it is apparently involved in the neuronal uptake of cysteine, which acts as a key substrate for the synthesis of glutathione, a major anti-oxidant, because the neurones do not express the Xc(-) transport system in the mature brain. Interestingly, EAAC1/EAAT3 activity/expression was shown to be highly regulated by neuronal activity as well as by intracellular signalling pathways involving primarily alpha protein kinase C (alphaPKC) and phosphatidylinositol-3-kinase (PI3K). Such regulatory processes could act either at the post-traductional level or at the transcriptional level. It is worth noting that EAAC1/EAAT3 exhibits specificity, compared with other EAA transporters, because it is present mainly in the intracellular compartment and only for about 20% at the plasma membrane. Variations in neuronal Glu uptake were shown to be associated with rapid changes in the trafficking of the transporter protein altering the membranar location of the transporter. More recent data show that astrocyte-secreted factors such as cholesterol could also influence rapid changes in the location of EAAC1/EAAT3 between the plasma membrane and the cytoplasmic compartment. Such a highly regulated process of EAAC1/EAAT3 activity/expression may have implications in the physiopathology of major diseases affecting EAA brain signalling, which is further supported by data obtained in animal models of hypoxia-anoxia, for example.

Similar calcification process in acute and chronic human brain pathologies

Cellular microcalcification observed in a diversity of human pathologies, such as vascular dementia, Alzheimer's disease, Parkinson's disease, astrogliomas, and posttraumatic epilepsy, also develops in rodent experimental models of central nervous system (CNS) neurodegeneration. Central to the neurodegenerative process is the inability of neurons to regulate intracellular calcium levels properly, and this is extensible to fine regulation of the CNS. This study provides evidence of a common pattern of brain calcification taking place in several human pathologies, and in the rat with glutamate-derived CNS lesions, regarding the chemical composition, physical characteristics, and histological environment of the precipitates. Furthermore, a common physical mechanism of deposit formation through nucleation, lineal growth, and aggregation is presented, under the modulation of protein deposition and elemental composition factors. Insofar as calcium precipitation reduces activity signals at no energy expense, the presence in human and rodent cerebral brain lesions of a common pattern of calcification may reflect an imbalance between cellular signals of activity and energy availability for its execution. If this is true, this new step of calcium homeostasis can be viewed as a general cellular adaptative mechanism to reduce further brain damage.

Differential effects of the substrate inhibitor l-trans-pyrrolidine-2,4-dicarboxylate (PDC) and the non-substrate inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) of glutamate transporters on neuronal damage and extracellular amino acid levels in rat brain in vivo

The extracellular concentration of glutamate is highly regulated by transporter proteins, due to its neurotoxic properties. Dysfunction or reverse activation of these transporters is related to the extracellular accumulation of excitatory amino acids and neuronal damage associated with ischemia and hypoglycemia. We have investigated by microdialysis the effects of the substrate and the non-substrate inhibitors of glutamate transporters, l-trans-2,4-pyrrolidine dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA), respectively, on the extracellular levels of amino acids in the rat hippocampus in vivo. In addition, we have studied the effect of both inhibitors on neuronal damage after direct administration into the hippocampus and striatum. Electroencephalographic activity was recorded after the intrahippocampal infusion of DL-TBOA or PDC. Microdialysis administration of 500 microM DL-TBOA into the hippocampus increased 3.4- and nine-fold the extracellular levels of aspartate and glutamate, respectively. Upon stereotaxic administration it induced neuronal damage dose-dependently in CA1 and dentate gyrus, and convulsive behavior. Electroencephalographic recording showed the appearance of limbic seizures in the hippocampus after DL-TBOA infusion. In the striatum it also induced dose-dependent neuronal damage. These effects were prevented by the i.p. administration of the glutamate receptor antagonists (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-iminemaleate and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline. In contrast to dl-TBOA, PDC (500 microM) induced a more discrete elevation of excitatory amino acids levels (2.6- and three-fold in aspartate and glutamate, respectively), no neuronal damage or behavioral changes, and no alterations in electroencephalographic activity. The differential results obtained with DL-TBOA and PDC might be attributed to their distinct effects on the extracellular concentration of amino acids. Results are relevant to the understanding of the role of glutamate transporters in amino acid removal or release and the induction of excitotoxic cell death.

Differential effects of prolonged high frequency stimulation and of excitotoxic lesion of the subthalamic nucleus on dopamine denervation-induced cellular defects in the rat striatum and globus pallidus

This study examined the effects of prolonged (4 days) high frequency stimulation (HFS) of the subthalamic nucleus (STN), in comparison with those of STN lesion, on the dopamine denervation-mediated cellular changes in the basal ganglia in a Wistar rat model of Parkinson's disease. STN HFS counteracted the dopamine lesion-induced increase in GAD67 mRNA expression in the output structures of the basal ganglia, as shown previously after STN lesion, providing cellular support for the similar antiparkinsonian benefits produced by the two surgical procedures. The dopamine denervation-induced increase in GAD67 mRNA levels in the globus pallidus was partially antagonized after HFS and totally reversed after ibotenate-induced STN lesion. The overexpression of striatal enkephalin mRNA tended to be further increased by HFS but was antagonized by STN lesion. The decrease in striatal substance P mRNA levels was affected neither by STN HFS nor lesion. As STN HFS for two hours was previously found not to interfere with the effects of dopamine lesion in the globus pallidus and striatum, the present data provide strong evidence that the effects of STN surgery in these structures involve long-term adaptive processes and that the rearrangements mediated by HFS and lesion are, at least in part, different.

In vivo neuroprotective adaptation of the glutamate/glutamine cycle to neuronal death

Synaptic increase of glutamate level, when not coupled to a heightened energy production, renders neurons susceptible to death. Astrocyte uptake and recycling of synaptic glutamate as glutamine is a major metabolic pathway dependent on energy metabolism, which inter-relationships are not fully understood and remain controversial. We examine how the glutamate-glutamine cycle and glucose metabolism are modified in two in vivo models of severe and mild brain injury. Graded reductions of glutaminase, the glutamate synthetic enzyme, were evidenced combined with increases in glutamine synthetase, the inactivating glutamate enzyme. Increased lactate dhydrogenase (LDH) activity was only present after a more severe injury. These results indicate an in vivo adaptation of the glutamate-glutamine cycle in order to increase the net glutamine output, reduce glutamate excitotoxicity, and avoid neuronal death. We conclude that the graded modification of the glutamate-glutamine correlation and neuronal lactate availability may be key factors in the apoptotic and necrotic neuronal demise, whose control may prove highly useful to potentiate neuronal survival.

Serotonergic regulation of the GABAergic transmission in the rat basal ganglia

The GABAergic neurons represent a major neuronal population in the basal ganglia. Although alterations in serotonin (5-HT) transmission are associated with neurodegenerative diseases involving these regions, the influence exerted by 5-HT afferents on GABAergic populations remains poorly understood. Here, we examined the consequences of 5,7-dihydroxytryptamine-induced lesion of 5-HT neurons on glutamic acid decarboxylase (GAD) activity, mRNA expression of the two isoforms of the enzyme, GAD65 and GAD67, GABA uptake, and extracellular GABA levels in the striatum. The 5-HT depletion produced an increase in GAD activity without modifying GAD65 and GAD67 mRNA levels, suggesting that 5-HT acts at the posttranscriptional level to regulate striatal GABA synthesis. No change in GAD activity was measured in the main striatal target structures, the globus pallidus and substantia nigra. Striatal GABA uptake and extracellular levels of GABA measured under basal conditions in freely moving rats were maintained in a normal range following 5-HT deprivation. By contrast, depolarization-induced increases in extracellular levels of GABA were larger in the striatum of 5-HT-deprived rats than in controls, which may be accounted for by an increase in a releasable pool of GABA due to increased synthesis rate. Together, these results suggest that 5-HT afferents may exert a phasic inhibitory control on striatal GABA transmission. Therefore, a decrease in striatal 5-HT transmission in disease states, such as Parkinson's disease, may contribute to pathological changes in striatal GABA neuron activity by increasing their reactivity to depolarizing stimuli.

Metabotropic glutamate 5 receptor blockade alleviates akinesia by normalizing activity of selective basal-ganglia structures in parkinsonian rats

Glutamate overactivity within the basal ganglia has been shown to be central to the expression of motor symptoms in advanced stages of Parkinson's disease, and metabotropic glutamate receptors (mGluRs) represent promising targets for new therapeutic strategies in this pathology. Little is known, however, about the cellular and behavioral changes occurring in the early stages of the disease when dopamine depletion is moderate. Here, we report that rats with partial bilateral dopamine lesions exhibit akinetic deficits associated with dramatically increased neuronal metabolic activity in selective structures of the basal ganglia such as the subthalamic nucleus and the substantia nigra pars reticulata, but not in the entopeduncular nucleus. Furthermore, chronic treatment with the mGluR5 antagonist 2-methyl-6-(phenylethylnyl)-pyridine alleviated the akinesia and was associated with a normalization of the activity of these two overactive structures. These data stress the therapeutic potential of mGluR5 antagonists in the treatment of parkinsonian patients in the early stages of the disease.

Glutamate transport alteration triggers differentiation-state selective oxidative death of cultured astrocytes: a mechanism different from excitotoxicity depending on intracellular GSH contents

Recent evidence has been provided for astrocyte degeneration in experimental models of neurodegenerative insults associated with glutamate transport alteration. To determine whether astrocyte death can directly result from altered glutamate transport, we here investigated the effects of L-trans-pyrrolidine-2,4-dicarboxylate (PDC) on undifferentiated or differentiated cultured rat striatal astrocytes. PDC induced death of differentiated astrocytes without affecting undifferentiated astrocyte viability. Death of differentiated astrocytes was also triggered by another substrate inhibitor but not by blockers of glutamate transporters. The PDC-induced death was delayed and apoptotic, and death rate was dose and treatment duration-dependent. Although preceded by extracellular glutamate increase, this death was not mediated through glutamate receptor stimulation, as antagonists did not provide protection. It involves oxidative stress, as a decrease in glutathione contents and a dramatic raise in reactive oxygen species preceded cell loss, and as protection was provided by antioxidants. PDC induced a similar percentage of GSH depletion in the undifferentiated astrocytes, but only a slight increase in reactive oxygen species. Interestingly, undifferentiated astrocytes exhibited twofold higher basal GSH content compared with the differentiated ones, and depleting their GSH content was found to render them susceptible to PDC. Altogether, these data demonstrate that basal GSH content is a critical factor of astrocyte vulnerability to glutamate transport alteration with possible insights onto concurrent death of astrocytes and gliosis in neurodegenerative insults.

Oxidative neuronal death caused by glutamate uptake inhibition in cultured hippocampal neurons

Glutamate transporters are coupled with cystine/glutamate antiporters to supply cystine as a component of glutathione, an important antioxidant. We sought evidence that L-trans-pyrrolidine-2,4-dicarboxylate (PDC) enhances glutamate-induced neuronal damage not only via the N-methyl-D-aspartate (NMDA) receptor mediated pathway, but also through induction of oxidative stress. Cultured hippocampal cells were exposed to glutamate (100 microM) for 5 min, washed and incubated for 18 hr with PDC (200 microM). PDC, increasing the neuronal death to 147% of that induced by glutamate alone, depleted glutathione in the culture, and produced dichloro-dihydro-fluorescein-diacetate-positive reactive oxygen species in neurons. N-acetylcysteine (2 mM) not only reduced PDC-enhanced neuronal death but also recovered glutathione and abolished the reactive oxygen species in these neurons. Threo-beta-benzyloxyaspartate, another type of glutamate transporter inhibitor, also induced glutathione depletion in the glutamate-preloaded cells, suggesting the involvement of glutamate transporter blocking in glutathione depletion. The NMDA receptor antagonist MK-801, although partially effective in reducing PDC toxicity, slightly recovered glutathione level but did not reduce the reactive oxygen species even at a high concentration (100 microM). N-acetylcysteine, dimethylsulfoxide, alpha-phenyl-N-butyl nitrone and glutathione ethylester prevented neuronal death enhanced by PDC, but superoxide dismutase and catalase did not. Our study provides evidence that the block of glutamate uptake by PDC exerts toxicity on glutamate-pretreated neurons not only through the accumulation of extracellular glutamate and subsequent activation of the NMDA receptor but also through depletion of glutathione and generation of reactive oxygen species.

High-frequency stimulation of the subthalamic nucleus selectively reverses dopamine denervation-induced cellular defects in the output structures of the basal ganglia in the rat

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is now recognized as an effective treatment for advanced Parkinson's disease, but the molecular basis of its effects remains unknown. This study examined the effects of unilateral STN HFS (2 hr of continuous stimulation) in intact and hemiparkinsonian awake rats on STN neuron metabolic activity and on neurotransmitter-related gene expression in the basal ganglia, by means of in situ hybridization histochemistry and immunocytochemistry. In both intact and hemiparkinsonian rats, this stimulation was found to induce c-fos protein expression but to decrease cytochrome oxidase subunit I mRNA levels in STN neurons. STN HFS did not affect the dopamine lesion-mediated overexpression of enkephalin mRNA or the decrease in substance P in the ipsilateral striatum. The lesion-induced increases in intraneuronal glutamate decarboxylase 67 kDa isoform (GAD67) mRNA levels on the lesion side were reversed by STN HFS in the substantia nigra, partially antagonized in the entopeduncular nucleus but unaffected in the globus pallidus. The stimulation did not affect neuropeptide or GAD67 mRNA levels in the side contralateral to the dopamine lesion or in intact animals. These data furnish the first evidence that STN HFS decreases the metabolic activity of STN neurons and antagonizes dopamine lesion-mediated cellular defects in the basal ganglia output structures. They provide molecular substrate to the therapeutic effects of this stimulation consistent with the current hypothesis that HFS blocks STN neuron activity. However, the differential impact of STN HFS on the effects of dopamine lesion among structures receiving direct STN inputs suggests that this stimulation may not cause simply interruption of STN outflow.

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.

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.

Differing effects of substrate and non-substrate transport inhibitors on glutamate uptake reversal

Na(+)-dependent excitatory amino acid transporters (EAATs) normally function to remove extracellular glutamate from brain extracellular space, but EAATs can also increase extracellular glutamate by reversal of uptake. Effects of inhibitors on EAATs can be complex, depending on cell type, whether conditions favor glutamate uptake or uptake reversal and whether the inhibitor itself is a substrate for the transporters. The present study assessed EAAT inhibitors for their ability to inhibit glutamate uptake, act as transporter substrates and block uptake reversal in astrocyte and neuron cultures. L-threo-beta-hydroxyaspartate (L-TBHA), DL-threo-beta-benzyloxyaspartate (DL-TBOA), L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-2,4-PDC) (+/-)-cis-4-methy-trans-pyrrolidine-2,4-dicarboxylic acid (cis-4-methy-trans-2,4-PDC) and L-antiendo-3,4-methanopyrrolidine-2,4-dicarboxylic acid (L-antiendo-3,4-MPDC) inhibited L-[14C]glutamate uptake in astrocytes with equilibrium binding constants ranging from 17 microM (DL-TBOA and L-TBHA) - 43 microM (cis-4-methy-trans-2,4-PDC). Transportability of inhibitors was assessed in astrocytes and neurons. While L-TBHA, L-trans-2,4-PDC, cis-4-methy-trans-2,4-PDC and L-antiendo-3,4-MPDC displayed significant transporter substrate activities in neurons and astrocytes, DL-TBOA was a substrate only in astrocytes. This effect of DL-TBOA was concentration-dependent, leading to complex effects on glutamate uptake reversal. At concentrations low enough to produce minimal DL-TBOA uptake velocity (< or = 10 microM), DL-TBOA blocked uptake reversal in ATP-depleted astrocytes; this blockade was negated at concentrations that drove substantial DL-TBOA uptake (> 10 microM). These findings indicate that the net effects of EAAT inhibitors can vary with cell type and exposure conditions.

Inhibition of the glutamate transporter by L-trans-PDC in the nucleus accumbens prevents the locomotor response to amphetamine

Infusion in the nucleus accumbens of the glutamate uptake inhibitor L-trans-PDC prevented the amphetamine-induced locomotor response. Since L-trans-PDC has been shown to block the amphetamine-induced increase in glutamate but not in DA release, our result indicates that the glutamate transporter is an obligatory target for the activating properties of amphetamine.

Characterization of striatal lesions produced by glutamate uptake alteration: cell death, reactive gliosis, and changes in GLT1 and GADD45 mRNA expression

This study investigated the time course of the striatal lesions produced by continuous local injection of the glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylate (PDC) at the rate of 25 nmol/h in rats. The extent of the neurodegeneration area (defined as the lesion area) did not significantly vary with the duration of the PDC treatment between 3 and 14 days, but was markedly reduced 3 months after cessation of the 14-day treatment, probably reflecting striatal atrophy. After the 3-day treatment, the lesion zone showed calcium precipitates and marked microglial reaction contrasting with the reduction of astroglial labeling and loss of the glutamate transporter GLT1 mRNA expression; however reactive astrocytes were observed around the lesion. After the 14-day treatment, the lesion zone presented reactive astrocytes and microglia without calcification, and a partial recovery of GLT1 mRNA expression. Interestingly, the growth arrest DNA damage-inducible GADD45 mRNA expression was induced around the lesion after 3 days but inside the lesion after 14 days of treatment. Three months after the 14-day treatment, the astroglial reactivity persisted within the lesion whereas most of the other markers examined tended to normalize. These data suggest that defective glutamate transport induces primary death of neurons and dysfunction of astrocytes. They strongly implicate reactive astrocytes with GLT1 and GADD45 transcripts in preventing secondary neuronal death.

Reversible activation of glutamate transport in rat brain glia by protein kinase C and an okadaic acid-sensitive phosphoprotein phosphatase

High-affinity L-glutamate (GLU) transport is an important regulator of excitatory amino acid (EAA) concentrations in brain extracellular fluid and may play a key role in excitatory synaptic transmission. In view of evidence that EAA transporters (EAAT) are heterogenous and contain consensus sites for phosphorylation, this investigation was undertaken to contrast the effects of transporter phosphorylation in fractions derived from glia and neurons (synaptosomes) of the adult rat forebrain. Treatment with phorbol-12,13-dibutyrate (PDBu), an activator of protein kinase C (PKC), increased the maximal rate of GLU transport in glial plasmalemmal vesicles by greater than 50 percent (237+/-18 vs. 365+/-27 pmol/mg protein/90s, p < 0.05) but caused no change in synaptosomes. The effect by PDBu was concentration and time-dependent and was inhibited completely by the PKC inhibitor calphostin C. Inhibition of serine-threonine phosphoprotein phosphatases with okadaic acid produced similar effects which were not additive with PDBu. Together, these results demonstrate that glial EAAT can be regulated by multiple phosphorylation processes.

Concentration-dependent isoflurane effects on depolarization-evoked glutamate and GABA outflows from mouse brain slices

The synaptic concentrations of glutamate and gamma-aminobutyric acid (GABA) are modulated by their release and re-uptake. The effects of general anaesthetics on these two processes remain unclear. This study evaluates the effects of isoflurane, a clinically important anaesthetic, on glutamate and GABA release and re-uptake in superfused mouse cerebrocortical slices. Experiments consisted of two 1.5-min exposures to 40 mM KCl in 30 min intervals. During the second exposure, different concentrations of isoflurane with and without 0.3 mM L-transpyrrolidine-2,4-dicarboxylic acid (PDC, a competitive inhibitor of glutamate uptake transporter) or 1 mM nipecotic acid (a competitive inhibitor of GABA uptake transporter) were introduced. The ratios of the second to first KCl-evoked increases in glutamate and GABA were used to determine the isoflurane concentration-response curves. The results can be described as a sum of two independent processes, corresponding to the inhibitions of release and re-uptake, respectively. The EC50 values for the inhibitions of release and re-uptake were 295+/-16 and 805+/-43 microM for glutamate, and 229+/-13 and 520+/-25 microM for GABA, respectively. Addition of PDC did not significantly affect glutamate release but shifted the re-uptake curve to the left (EC50= 315+/-20 microM). Nipecotic acid completely blocked GABA uptake, rendering isoflurane inhibition of GABA re-uptake undetectable. Our data suggest that isoflurane inhibits both the release and re-uptake of neurotransmitters and that the inhibitions occur at different EC50's. For GABA, both EC50's are within the clinical concentration range. The net anaesthetic effect on extracellular concentrations of neurotransmitters, particularly GABA, depends on the competition between inhibition of release and that of re-uptake.

Transient inhibition of glutamate uptake in vivo induces neurodegeneration when energy metabolism is impaired

Impairment of glutamate transport during ischemia might be related to the elevation of the extracellular concentration of glutamate and ischemic neuronal damage. Additionally, impairment of energy metabolism in vivo leads to neurodegeneration apparently mediated by a secondary excitotoxic mechanism. In vitro observations show that glucose deprivation and inhibition of energy metabolism exacerbate the toxic effects of glutamate. We have previously shown that glutamate uptake inhibition in vivo by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) leads to a substantial elevation in the extracellular concentration of excitatory amino acids that is not associated with cell death. These observations suggest that energy depletion during ischemia might be determinant of ischemic neuronal damage. To investigate whether impairment of energy metabolism in vivo increases neuronal susceptibility to glutamate uptake inhibition, we studied the effect of glutamate accumulation induced by the intrahippocampal or intrastriatal administration of PDC in energy-deficient rats chronically treated with 3-nitropropionic acid (3-NP), which irreversibly inhibits the tricarboxylic acid cycle and electron transport chain. Extracellular glutamate levels were monitored by HPLC from fractions collected from microdialysis probes, and neuronal damage was evaluated by histological analysis. Our results show that glutamate uptake inhibition leads to marked neuronal damage in energy-deficient rats but not in intact animals, which apparently is not related to an additional elevation of glutamate levels induced by 3-NP.