Neurotransmitter transporters

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

Receptors and transporter for serotonin in Merkel cell-nerve endings in the rat sinus hair follicle. An immunohistochemical study

Serotonin (5-HT) has been a candidate for neurotransmitters in cutaneous type I mechanoreceptors (i.e., Merkel cell-nerve endings). Although recent electrophysiological studies have suggested the presence of the 5-HT2 and 3 receptors in the Merkel cell-nerve endings, the histological localization of these receptors are obscure. We thus immunohistochemically examined the presence of 5-HT1, 2, 3 receptors in Merkel cell-nerve endings in sinus hair follicles of the rat whisker pad. We also studied the immunohistochemical localization of the 5-HT transporter to confirm the site of 5-HT secretion. For this purpose, we used antibodies for the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C and 5-HT3 receptors, and for the 5-HT transporter, as well as antibodies for cytokeratin 20 (as a marker of Merkel cells) and neurofilament H (a marker of type I sensory nerve terminals). The immuno-stained sections were analyzed under a laser-scanning microscope. It was found that the sensory nerve terminals in the Merkel cell-nerve endings showed strong positive immunoreactions of 5-HT1A and 1B receptors but not 5-HT2A, 2C, and 3 receptors. Furthermore, both the Merkel cells and related axon terminals showed strong immunoreactions of the 5-HT transporter. These findings support the idea that 5-HT molecules are released from the Merkel cells during mechanical reception and indirectly regulate neural actions of sensory neurons via 5-HT1 receptors. The localization of the 5-HT transporter found in this study also suggests a possibility that axon terminals in the Merkel cell-nerve endings also release 5-HT.

Sodium-mediated axonal degeneration in inflammatory demyelinating disease

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS). The mechanisms responsible for the degeneration remain unclear, but evidence suggests that a failure to maintain axonal sodium ion homeostasis may be a key step that underlies at least some of the degeneration. Sodium ions can accumulate within axons due to a series of events, including impulse activity and exposure to inflammatory factors such as nitric oxide. Recent findings have demonstrated that partial blockade of sodium channels can protect axons from nitric oxide-mediated degeneration in vitro, and from the effects of neuroinflammatory disease in vivo. This review describes some of the reasons why sodium ions might be expected to accumulate within axons in MS, and recent observations suggesting that it is possible to protect axons from degeneration in neuroinflammatory disease by partial sodium channel blockade.

Rapid substrate-induced charge movements of the GABA transporter GAT1

The GABA transporter GAT1 removes the neurotransmitter GABA from the synaptic cleft by coupling of GABA uptake to the co-transport of two sodium ions and one chloride ion. The aim of this work was to investigate the individual reaction steps of GAT1 after a GABA concentration jump. GAT1 was transiently expressed in HEK293 cells and its pre-steady-state kinetics were studied by combining the patch-clamp technique with the laser-pulse photolysis of caged GABA, which allowed us to generate GABA concentration jumps within <100 micros. Recordings of transport currents generated by GAT1, both in forward and exchange transport modes, showed multiple charge movements that can be separated along the time axis. The individual reactions associated with these charge movements differ from the well-characterized electrogenic "sodium-occlusion" reaction by GAT1. One of the observed electrogenic reactions is shown to be associated with the GABA-translocating half-cycle of the transporter, in contradiction to previous studies that showed no charge movements associated with these reactions. Interestingly, reactions of the GABA-bound transporter were not affected by the absence of extracellular chloride, suggesting that Cl- may not be co-translocated with GABA. Based on the results, a new alternating access sequential-binding model is proposed for GAT1's transport cycle that describes the results presented here and those by others.

Laser treatment of cultured retinal pigment epithelial cells-evaluation of the cellular damage in vitro

PURPOSE

Evaluation of the effects of laser photocoagulation on cultured primary retinal pigment epithelial cells.

METHODS

Cells were treated by a diode laser (678 nm) with 800 and 1600 mW for 0.186 second. Cell toxicity was tested by the WST-1 assay, and the uptakes of glutamate and gamma-aminobutyric acid (GABA) were measured.

RESULTS

Laser photocoagulation (1600 mW) caused cell damage and the mitochondrial enzyme activity evaluated by a WST-1 test significantly decreased by 20%-30%. Laser treatment caused a dose-dependent decrease in glutamate uptake but increased GABA uptake.

CONCLUSIONS

Laser treatment and the laser-induced increase in temperature influence transport processes in retinal pigment epithelial cells and may cause cell damage in the posterior part of the retina.

Effect of low pH on glutamate uptake and release in isolated presynaptic endings from rat brain

The effect of acidification of the incubation medium on the membrane potential and glutamate uptake and release was studied in isolated presynaptic neuronal endings (synaptosomes) from rat brain. Using the fluorescent probe diS-C3-(5), a rapid depolarization of plasma membrane was detected at pH 6.0, most probably as a result of the inhibition of the sodium pump and potassium channel blockade. The membrane potential decrease did not result in increase of basal efflux of glutamate. Glutamate release following K(+)-induced depolarization was decreased upon lowering pH to 6.0. Acidosis inhibited mainly calcium-dependent (vesicular) release of glutamate and did not significantly reduce [14C]glutamate uptake. This inhibition of glutamate release but not of glutamate uptake may be a mechanism of the protective effect of acidosis during brain ischemia.

Mitochondrial localization of glucocortocoid receptor in glial (Müller) cells in the salamander retina

Glucocorticoid hormones regulate the transcription of nuclear genes by way of their receptors. In addition, these hormones modulate mitochondrial gene transcription by mechanisms that remain poorly understood. Using immunofluorescence labeling in isolated Müller and photoreceptor cells and in intact salamander retina, we found that the glucocorticoid receptor (GR) is localized in both cell types. Confocal laser scanning microscopy and double staining with cytochrome oxidase (COX) showed that GR is localized in the mitochondria of Müller cells, but not in the mitochondria of photoreceptors. GR also colocalizes with glutamine synthetase (GS) in the cytoplasm of Müller cells. GR is also localized in the microvilli of the distal process of Müller cells and in the synaptic terminal of photoreceptors. Pre-incubation of Müller cells with 1 microM dexamethasone (DEX) for 7 h led to greater than 50% inhibition of the glutamate-induced increase in mitochondrial NADH. This late effect of glucocorticoids on glutamate metabolism could be ascribed, in part, to a direct action of steroid hormones on mitochondrial metabolism.

GAT-1 and reversible GABA transport in Bergmann glia in slices

Although glial GABA uptake and release have been studied in vitro, GABA transporters (GATs) have not been characterized in glia in slices. Whole cell patch-clamp recordings were obtained from Bergmann glia in rat cerebellar slices to characterize carrier-mediated GABA influx and efflux. GABA induced inward currents at -70 mV that could be pharmacologically separated into GABA(A) receptor and GAT currents. In the presence of GABA(A/B/C) receptor blockers, mean GABA-induced currents measured -48 pA at -70 mV, were inwardly rectifying between -70 and +50 mV, were inhibited by external Na(+) removal, and were diminished by reduction of external Cl(-). Nontransportable blockers of GAT-1 (SKF89976-A and NNC-711) and a transportable blocker of all the GAT subtypes (nipecotic acid) reversibly reduced GABA-induced transport currents by 68 and 100%, respectively. A blocker of BGT-1 (betaine) had no effect. SKF89976-A and NNC-711 also suppressed baseline inward currents that likely result from tonic GAT activation by background GABA. The substrate agonists, nipecotic acid and beta-alanine but not betaine, induced voltage- and Na(+)-dependent currents. With Na(+) and GABA inside the patch pipette or intracellular GABA perfusion during the recording, SKF89976-A blocked baseline outward currents that activated at -60 mV and increased with more depolarized potentials. This carrier-mediated GABA efflux induced a local accumulation of extracellular GABA detected by GABA(A) receptor activation on the recorded cell. Overall, these results indicate that Bergmann glia express GAT-1 that are activated by ambient GABA. In addition, GAT-1 in glia can work in reverse and release sufficient GABA to activate nearby GABA receptors.

Pharmacology and toxicology of astrocyte-neuron glutamate transport and cycling

The interaction between astrocytes and neurons is examined from the standpoint of glutamate neurotoxicity. The review details 1. the distribution of glutamate transporters on astrocytes and neurons, provoking a reformulation of the interdependence between these two cell types in removing extracellular glutamate and preventing excitotoxic injury; 2. the potential involvement of aberrant glutamate transporter function in the etiology of neuropathological conditions; 3. the role of astrocyte-neuron interaction in widely divergent aspects of brain energetics; 4. the role of astrocytes in the process of glutamate recycling within the context of anesthetic treatment with pentobarbital and thiopental.

Metabolic signaling between neurons and glial cells: a short review

There is convincing evidence that astrocytes transform blood-born glucose to lactate, alpha-Keto-glutarate and alanine and supply the neurons. There is a tight regulation of this metabolic coupling by means of chemical signals released by functioning neurons. Previous, pioneer, studies have explored several signals-candidates the major being K(+), Ca(++) and several neuromodulators. However, recent results of numerous studies identify glutamate as the major signal that traffics between excited neurons and astrocytes. The excited neurons also produce and release NH(4)(+) in the extracellular space. Both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Ammonia fixation by glutamine synthase controls the amount of lactate, glutamine and alanine produced and released by Muller cells in the extracellular space and then taken up by neurons. Thus, there is a tight coupling between function and metabolism in the central neurons system.

Distribution of glutamate transporters in the hippocampus of patients with pharmaco-resistant temporal lobe epilepsy

In patients suffering from temporal lobe epilepsy (TLE), increased extracellular glutamate levels in the epileptogenic hippocampus both during and after clinical seizures have been reported. These increased glutamate levels could be the result of malfunctioning and/or downregulation of glutamate transporters (also known as EAATs; excitatory amino acid transporters). In this study, the distribution of protein and mRNA of EAAT subtypes was examined in the hippocampus of TLE patients with hippocampal sclerosis (HS group) and without hippocampal sclerosis (non-HS group), and in autopsy controls without neurological disorders. EAAT protein localization was studied by immunohistochemistry on paraffin sections using specific poly- and monoclonal antibodies against the glial glutamate transporters EAAT1 and EAAT2 and the neuronal glutamate transporter EAAT3. Antibody specificity was shown by immunoblotting. In the HS group, a small decrease in EAAT1-immunoreactivity (IR) was observed in CA4 and in the polymorphic and supragranular layer of the dentate gyrus, compared with the control group. The strongest changes were found for EAAT2 levels. In the non-HS group, increased EAAT2-IR was detected in the CA1 and CA2 field, compared with non-epileptic controls. EAAT2-IR was decreased in the HS compared with the non-HS group. Fewer EAAT3-positive cells were found in the HS group than in the non-HS and control group. In both TLE groups, increased EAAT3 levels were observed in individual neurones. In the HS group, the percentage of EAAT3-IR neurones was increased in CA2 and in the granule cell layer of the dentate gyrus. Radioactive in situ hybridization for EAAT1-3 confirmed our immunohistochemical results. Non-radioactive in situ hybridization showed that not only astrocytes, but also neurones express EAAT2 mRNA. Taken together, differences in both mRNA and protein levels of glutamate transporter subtypes were found in specific regions in the TLE hippocampus, with most severe changes found for EAAT2 and EAAT3 levels. The results indicate an upregulation of EAAT2 protein expression in CA1 and CA2 in neurones in the non-HS group. This is in line with decreased EAAT2 protein levels in the HS group, since these hippocampi are characterized by severe neuronal cell loss. The functional consequences (glutamate transport capacity) of the reported changes in EAAT2 and EAAT3 remain to be determined.

Histamine H1 receptor activation stimulates mitogenesis in human astrocytoma U373 MG cells

In human astrocytoma U373 MG cells that express histamine H1 receptors (180 +/- 6 fmol/mg protein) but not H2 or H3 receptors, histamine stimulated mitogenesis as assessed by [3H]-thymidine incorporation (173 +/- 2% of basal; EC50, 2.5 +/- 0.4 microM). The effect of 100 microM histamine was fully blocked by the selective H1 antagonist mepyramine (1 microM) and was markedly reduced (93 +/- 4% inhibition) by the phospholipase C inhibitor U73122 (10 microM). The activator of protein kinase C (PKC) phorbol 12-tetradecanoyl-13-acetate (TPA, 100nM) stimulated [3H]-thymidine incorporation (270 +/- 8% of basal), and this response was not additive with that to 100 microM histamine. The incorporation of [3H]-thymidine induced by 100 microM histamine was partially reduced by the PKC inhibitor Ro 31-8220 (57 +/- 7% inhibition at 300 nM) and by the compound PD 098,059 (30 microM, 62 +/- 14% inhibition), an inhibitor of the mitogen-activated kinase (MAPK) kinases MEK1/MEK2. These results show that histamine H1 receptor activation stimulates the proliferation of human astrocytoma U373 MG cells. The action of histamine appears to be partially mediated by PKC stimulation and MAPK activation.

Methylmercury-mediated inhibition of 3H-d-aspartate transport in cultured astrocytes is reversed by the antioxidant catalase

Astrocytes are essential for removal of glutamate from the extracellular space in the central nervous system. The neurotoxic heavy metal methylmercury potently and specifically inhibits the transport of glutamate in cultured astrocytes by an unknown mechanism. Glutamate transport in astrocytes is also inhibited by reactive oxygen species. A glutamate-induced transporter current is inhibited both by reactive oxygen species and thiol oxidizing agents. These observations suggest that oxidation of the transporter might mediate methylmercury-induced inhibition of glutamate transport. In the present study, we examined the ability of thiol reducing or oxidizing agents to inhibit transport of 3H-D-aspartate, a glutamate analog, in primary cultures of neonatal rat astrocytes. To assess if methylmercury-mediated inhibition of 3H-aspartate transport was due to overproduction of reactive oxygen species, we tested the ability of Trolox, alpha-phenyl-tert-butyl nitrone (PBN), or catalase to attenuate the methylmercury-induced inhibition of aspartate uptake. Neither the thiol reducing agent dithiothreitol (DTT), nor the thiol oxidizing agent 5,5'-dithio-bis(2-nitrobenzoic) acid (DTNB) had any effect on 3H-aspartate transport suggesting that the thiol redox state does not alter transporter function. In contrast, the antioxidant catalase (1000 U/ml) significantly attenuated methylmercury-induced inhibition of 3H-aspartate uptake, suggesting that excess reactive oxygen species, specifically H2O2, inhibit the function of an astrocytic excitatory amino acid transporter (EAAT1). Prolonged exposure (6 h) to inhibitors of glutamate transport significantly decreased EAAT1 mRNA levels suggesting that transporter expression is related to function. This study suggests that methylmercury-induced overproduction of H2O2 is a mechanism for inhibition of glutamate transport and transporter expression in cultured astrocytes.

Glycine induced calcium concentration changes in vestibular type I sensory cells

Glutamate is the neurotransmitter of the synapse between vestibular type I hair cells and the afferent nerve calyx. This calyx may also be involved in local feedback, which may modify sensory cell activity via N-methyl-D-aspartate (NMDA) receptors. Glycine is the co-agonist of glutamate in NMDA receptor activation. Both agents have been detected by immunocytochemistry in the nerve calyx. Glutamate and NMDA stimulations cause changes in the intracellular calcium concentration ([Ca(2+)](i)) of isolated type I sensory cells. We investigated the effect of glycine stimulation on [Ca(2+)](i) in guinea pig type I sensory cells by spectrofluorimetry with fura-2. Glycine application to isolated type I sensory cells induced a rapid and transient increase in [Ca(2+)](i). The fluorescence ratio increased by 55% above the resting level. The peak was reached in 9 s and the return to basal level took about 20 s. A specific antagonist of the glycine site on NMDA receptors, 7-chlorokynurenate (10 microM), decreased the calcium response to glycine by 60%. Glycine may activate NMDA receptors. Glycine may also activate the strychnine-sensitive glycine receptor-gated channel. Strychnine (50 microM) decreased the calcium response to glycine by 60%. Thus, glycine probably induces calcium concentration changes in type I vestibular sensory cells via NMDA receptors and/or glycine receptors.

Dendritic GABA release depresses excitatory transmission between layer 2/3 pyramidal and bitufted neurons in rat neocortex

GABAergic, somatostatin-containing bitufted interneurons in layer 2/3 of rat neocortex are excited via glutamatergic excitatory postsynaptic potentials (EPSPs) by pyramidal neurons located in the same cortical layer. Pair recordings showed that short bursts of backpropagating dendritic action potentials (APs) reduced the amplitude of unitary EPSPs. EPSP depression was dependent on a rise in dendritic [Ca2+]. The effect was blocked by the GABA(B) receptor (GABA(B)-R) antagonist CGP55845A and was mimicked by the GABA(B)-R agonist baclofen. As presynaptic GABA(B)-Rs were activated neither by somatostatin nor by GABA released from axon collaterals of the bitufted cell, we conclude that GABA(B)-Rs were activated by a retrograde messenger, most likely GABA, released from the dendrite. Because synaptic depression was prevented by loading bitufted neurons with GDP-beta-S, it is likely to be caused by exocytotic GABA release from dendrites.

Molecular cloning and functional characterization of a GABA transporter from the CNS of the cabbage looper, Trichoplusia ni

A cDNA encoding a GABA transporter in the caterpillar Trichoplusia ni has been cloned and expressed in baculovirus-infected insect cells. The cDNA contains an ORF encoding a 608-residue protein, designated TrnGAT. Hydropathy analysis of the deduced amino acid sequence suggests 12 transmembrane domains, a structure similar to that of all other cloned Na+/Cl(-)-dependent GABA transporters. The deduced amino acid sequence shows high identity with a GABA transporter (MasGAT) expressed in the embryo of Manduca sexta. Expression of TrnGAT mRNA was detected only in the brain. Sf21 cells infected with recombinant baculovirus exhibited a 20- to 30-fold increase in [3H]GABA uptake compared to control-infected cells. Several blockers of GABA uptake were used to determine the pharmacological profile of TrnGAT. Although most similar to mammalian neuronal GABA transporter GAT-1 in its kinetic properties, stoichiometry of ionic dependence and pharmacological properties, TrnGAT may be distinguished from mammalian GAT-1 by the inability of cyclic GABA analogues, such as nipecotic acid and its derivatives, to inhibit GABA uptake by the insect protein. The unique pharmacology of TrnGAT suggests that the GABA transport system in the lepidopteran CNS could be a useful target in the future development of rapidly-acting neuroactive agents used to control agriculturally-important insects.

Characterization and distribution of Hxt1, a Na(+)/Cl(-)-dependent orphan transporter, in the human brain

Rxt1, a transporter-like protein structurally related to the large family of Na(+)/Cl(-)-dependent carriers, was isolated from the rat brain. In the present study, Hxt1, the homologue of Rxt1, was isolated from human cortex cDNA. Comparison of their respective nucleotidic sequences revealed a 96% conservation between Hxt1 and Rxt1. Genetic mapping with human genome radiation hybrids allowed the location of the gene coding for Hxt1 between 323ya5 and 084xb3 AFM markers, on a portion of chromosome 1p which spans over 7 cM or 118 cRay. Northern blot analyses demonstrated that Hxt1 mRNA ( approximately 7.5 Kb) is expressed in the human brain but not in peripheral tissues. The immunodistribution of Hxt1 was determined with antibodies raised against the C-terminus of Rxt1. Hxt1 is concentrated in the cerebral cortex, caudate-putamen, substantia nigra, hippocampus, and cerebellum, appearing as a diffuse or a punctate labeling at the light microscope level. This regional and cellular distribution suggests that Hxt1, as its rat homologue, could be present in axon terminals of glutamatergic neurons. The high pressure of selection exerted upon this protein, its strategic anatomical and subcellular distributions suggest that this orphan transporter could be involved in critical functions in the central nervous system.

Analysis of a gene encoding two glycine transporter variants reveals alternative promoter usage and a novel gene structure

The rat GLYT-1 gene encodes two glycine transporter variants, GLYT-1a and GLYT-1b, that differ in NH2 termini and 5'-noncoding regions as well as in tissue distribution. The GLYT-1 gene contains 15 exons, with the first two specific for GLYT-1a and the third specific for GLYT-1b. By combining RNase protection and rapid amplification of cDNA ends analysis, we have determined transcription start sites for GLYT-1a and GLYT-1b. By using a functional luciferase reporter assay, we demonstrate that distinct promoters regulate the expression of these transporters in several cell lines. Serially truncated GLYT-1b promoter constructs reveal a basal promoter within 304 base pairs of the transcription start site, possible negative regulatory elements between -304 and -1310, and additional positive regulatory elements between -1310 and -5264. The GLYT-1 gene contains three sets of dinucleotide repeats, two AC repeats, and one TG repeat which may form stem-loop structures to either facilitate or interfere with transcription of one of the transporter isoforms. The potential use of dinucleotide repeats in this manner would represent a novel mechanism for gene splicing. The use of distinct promoters for GLYT-1a and GLYT-1b suggests that these transporters have unique regulatory requirements that may reflect the differential tissue-specific expression patterns in white matter (GLYT-1b) and gray matter (GLYT-1a).

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Activation and desensitization properties of native and recombinant kainate receptors

The activation-inactivation properties of membrane currents induced by the rapid application of glutamate or kainate were studied in cultured hippocampal neurons and in HEK cells transfected with a cDNA encoding the GluR6 subunit. The onset of desensitization was rapid and similar in native and recombinant channels (approximately 80 s(-1) of onset rate constant). Recovery from desensitization was slow and agonist-dependent in neurons, proceeding slightly faster in GluR6 receptors. Half-maximal activation (EC50) of native channels was obtained at a glutamate concentration of 330 microM, while the half-maximal steady state desensitization (IC1/2) was attained at 2.8 microM. These values differed from those obtained in recombinant receptors (EC50 = 762 microM and IC1/2 = 0.44 microM). A small window under the crossing point of activation and inactivation curves was observed, indicating that, for some concentrations of either agonist, steady state channel activity could exist. In native receptors, this window presented maximum values at approximately 100 microM for glutamate, which predicted well the potency of glutamate to reduce the GABAergic drive in hippocampal slices. These data indicate that for neuronal kainate receptors, the concentrations for half activation and half inactivation differ by two orders of magnitude such that the maximum response to a maintained concentration of glutamate is small, and the steady state dose response curve is skewed and bell shaped.

Trafficking of molecules and metabolic signals in the retina

Photoreceptors need the support of pigment epithelial (PE) and Müller glial cells in order to maintain visual sensitivity and neurotransmitter resynthesis. In rod outer segments (ROS), all-trans-retinal is transformed to all-trans-retinol by retinol dehydrogenase using NADPH. NADPH is restored in ROS by the pentose phosphate pathway utilizing high amounts of glucose supplied by choriocapillaries. The retinal formed is transported to PE cells where regeneration of 11-cis-retinal occurs. Müller cells take up and metabolize glucose predominantly to lactate which is massively released into the extracellular space (ES). Lactate is taken up by photoreceptors, where it is transformed to pyruvate which, in turn, enters the Krebs cycle in mitochondria of the inner segment. Stimulation of neurotransmitter release by darkness induces 130% rise in the amount of glutamate released into ES. Glutamate is transported into Müller cells where it is predominantly transformed to glutamine. Stimulation of photoreceptors induces an eightfold increase in glutamine formation. It appears, therefore, that there is a signaling function in the transfer of amino acids from Müller cells to photoreceptors. Work on the model-system of the honeybee retina demonstrated that photoreceptors release NH4+ and glutamate in a stimulus-dependent manner which, in turn, contribute to the biosynthesis of alanine in glia. Alanine released into the extracellular space is taken up and used by photoreceptors. Glial cells take glutamate by high-affinity transporters. This uptake induces a transient change in glial cell metabolism. The transformation of glutamate to glutamine is possibly also controlled by the uptake of NH4+ which directly affects cellular metabolism.

Kainate receptors: an interplay between excitatory and inhibitory synapses

The potent excitatory amino acid glutamate mediates its excitatory effects through a great variety of specific ionotropic receptors, including NMDA, AMPA and kainate receptors. Despite the identification, isolation and cloning of several subunits of the kainate receptor, this receptor has been rather elusive and its function remains enigmatic. Recent results indicate that kainate receptors can be reached by synaptically released glutamate and that their activation downregulates GABAergic inhibition by modulating the reliability of GABA synapses. Thus, kainate receptors may have a role in the etiology of epilepsy and could become a target for antiepileptic drugs.

Neurotransmitter transporters as molecular targets for addictive drugs

The neurotransmitter dopamine lies at or near the center of current theories of drug abuse and dependence. Multiple lines of evidence indicate that dopaminergic cells play key roles in a variety of motivated behaviors. Accordingly, it is not surprising that cocaine and amphetamines--some of the most widely used illicit drugs--elevate extraneuronal dopamine concentrations through their actions on the plasma membrane dopamine transporter. From the point of view of developing novel pharmacological interventions for the treatment or prevention of psychostimulant abuse, practical benefits may arise from an improved understanding of how neurotransmitter transporters operate and how drugs interact with them.

Analysis of the 5' flanking sequence of the human norepinephrine transporter gene

The human norepinephrine transporter (NET) gene was cloned and structurally analyzed. The far 5' fragment containing exon 1 (a non-coding exon) and exon 2 was sequenced. The transcription start site of the gene in human brain stem tissue was determined by primer extension analysis. It was found that the gene could be transcribed from multiple starting points. The 5' flanking sequence contains a proximal G-C rich region, one possible GSG element and several SP1 sites. However it does not contain TATA box and CAAT box motifs. Gel shift analysis with nuclear extracts from different tissues of mouse shows that the G-C rich region may be involved in tissue specific expression of the gene.

Glutamine immunoreactivity in Müller cells of monkey eyes with experimental glaucoma

The action of glutamate in retina is largely terminated through rapid uptake by Müller cells and subsequent conversion primarily to glutamine. Glutamine, transferred from Müller cells to neurons, serves as a precursor for the formation of glutamate in neurons completing the glutamate-glutamine cycle. In a monkey model of high-tension glaucoma, we have examined glutamine immunoreactivity in the Müller cell as well as the number of Müller cells to determine whether the activity of these cells in the glutamate-glutamine cycle is affected, particularly since high vitreal glutamate has been reported in glaucoma. Unilateral glaucoma was induced in three monkeys by argon laser application to the trabecular meshwork. LR White sections of retina from the temporal mid-periphery (about 23 degrees) and the parafovea (central 3 degrees) were immunolabeled for glutamine using immunogold and silver intensification. The percentage difference in labeling intensity (darkness) in the glaucomatous retina was determined relative to the labeling found in the control retina by image analysis. Ganglion cell density was estimated from radial sections in the parafovea and from retinal whole mounts in the mid-periphery. The number of Müller cells was estimated from vibratome sections immunolabeled by vimentin antibodies in the temporal mid-periphery (about 30 degrees). Glutamine immunoreactivity was localized predominately in ganglion cells and Müller cells. However, the intensity of glutamine immunolabeling was greater in Müller cells of glaucomatous eyes than in control eyes. This increase in glutamine immunolabeling was 25-32% in the temporal mid-periphery and 27-48% in the parafovea. Müller cell number in the glaucomatous eye was similar to that of the control in the temporal mid-periphery. The data in this study indicate that the increase in glutamine in Müller cells is not a consequence of their loss and that Müller cell function in the glutamate-glutamine cycle continues in glaucomatous eyes. These findings are consistent with a previous report that extracellular/vitreal glutamate concentration is elevated in high-tension glaucoma.

Amino acid neurotransmitters in the retina: a functional overview

Physiological and pharmacological mechanisms of glutamatergic, GABAergic and glycinergic synapses in the tiger salamander retina were studied. We used immunocytochemical and autoradiographic methods to study localizations of these neurotransmitters and their uptake transporters; and electrophysiological methods (intracellular, extracellular and whole cell patch electrode recordings) to study the light responses, miniature postsynaptic currents and neurotransmitter-induced postsynaptic currents in various retinal neurons. Our results are consistent with the following scheme: Glutamate is used by the photoreceptor and bipolar cell output synapses and the release of glutamate is largely mediated by calcium-dependent vesicular processes. The postsynaptic glutamate receptors in DBCs are L-AP4 receptors, in HBCs, HCs and ganglion cells are the kainate/AMPA and NMDA receptors. Subpopulations of HCs make GABAergic synapses on cones and gate chloride condunctance through GABAA receptors. GABAergic HCs do not make feedforward synapses on bipolar cell dendrites and the neurotransmitter identity of the HCs making feedforward synapses is unknown. Subpopulations of amacrine cells make GABAergic synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and GABA gates chloride conductances in theses cells. Glycinergic amacrine cells make synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and glycine opens postsynaptic chloride channels. Glycinergic interplexiform cells make synapses on bipolar cells in the outer retina and glycine released from these cells open chloride channels in bipolar cell dendrites.

Serotonin transporter messenger RNA in the developing rat brain: early expression in serotonergic neurons and transient expression in non-serotonergic neurons

Serotonin has been shown to affect the development of the mammalian nervous system. The serotonin transporter is a major factor in regulating extracellular serotonin levels. Using in situ hybridization histochemistry the rat serotonin transporter messenger RNA was localized during embryogenesis, the first four weeks postnatally and adulthood. Three general classes of serotonin transporter messenger RNA expression patterns were observed: (i) early detection with continued expression through adult age, (ii) transient expression colocalized with vesicular monoamine transporter 2 messenger RNA but with no detectable tryptophan hydroxylase immunoreactivity, and (iii) transient expression in the apparent absence of both vesicular monoamine transporter 2 messenger RNA and tryptophan hydroxylase immunoreactivity. For example, hybridization for serotonin transporter messenger RNA was strong in serotonin cell body-containing areas beginning early in gestation, and remained intense through adulthood. Immunoreactivity for tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, was completely overlapping with the presence of serotonin transporter messenger RNA in raphe nuclei postnatally. Sensory relay systems including the ventrobasal nucleus (somatosensory), lateral and medial geniculate nuclei (visual and auditory, respectively) as well as trigeminal, cochlear and solitary nuclei were representative of the second class of observations. In general, the limbic system expressed serotonin transporter messenger RNA in the third pattern with various limbic structures differing in the timing of expression. Septum, olfactory areas and the developing hippocampus contained serotonin transporter messenger RNA early in the developing brain. Other regions such as cingulate and frontopolar cortex exhibited hybridization peri- and postnatally, respectively. Several hypothalamic nuclei and pituitary transiently expressed serotonin transporter messenger RNA either postnatally or perinatally, respectively. If the observed patterns correlate with functional protein expression, distinct classes of serotonin transporter messenger RNA expression may reflect different functional roles for the serotonin transporter and serotonin, itself. Since the serotonin transporter is a target for a number of addictive substances including cocaine and amphetamine derivatives as well as antidepressants, transient expression of the serotonin transporter might suggest a window of vulnerability of associated cells to fetal drug exposure. Re-uptake, storage and re-release from non-serotonergic neurons might serve as a feedback mechanism from target neurons to serotonergic neurons. Alternatively, the transient expression of serotonin transporter messenger RNA may reflect critical periods important for tight regulation of extracellular serotonin in several brain regions, and may indicate previously unappreciated roles for serotonin as a developmental cue.

Calcium in ischemic cell death

BACKGROUND

This review article deals with the role of calcium in ischemic cell death. A calcium-related mechanism was proposed more than two decades ago to explain cell necrosis incurred in cardiac ischemia and muscular dystrophy. In fact, an excitotoxic hypothesis was advanced to explain the acetylcholine-related death of muscle end plates. A similar hypothesis was proposed to explain selective neuronal damage in the brain in ischemia, hypoglycemic coma, and status epilepticus.

SUMMARY OF REVIEW

The original concepts encompass the hypothesis that cell damage in ischemia-reperfusion is due to enhanced activity of phospholipases and proteases, leading to release of free fatty acids and their breakdown products and to degradation of cytoskeletal proteins. It is equally clear that a coupling exists between influx of calcium into cells and their production of reactive oxygen species, such as .O2, H2O2, and .OH. Recent results have underscored the role of calcium in ischemic cell death. A coupling has been demonstrated among glutamate release, calcium influx, and enhanced production of reactive metabolites such as .O2-, .OH, and nitric oxide. It has become equally clear that the combination of .O2- and nitric oxide can yield peroxynitrate, a metabolite with potentially devastating effects. The mitochondria have again come into the focus of interest. This is because certain conditions, notably mitochondrial calcium accumulation and oxidative stress, can trigger the assembly (opening) of a high-conductance pore in the inner mitochondrial membrane. The mitochondrial permeability transition (MPT) pore leads to a collapse of the electrochemical potential for H+, thereby arresting ATP production and triggering production of reactive oxygen species. The occurrence of an MPT in vivo is suggested by the dramatic anti-ischemic effect of cyclosporin A, a virtually specific blocker of the MPT in vitro in transient forebrain ischemia. However, cyclosporin A has limited effect on the cell damage incurred as a result of 2 hours of focal cerebral ischemia, suggesting that factors other than MPT play a role. It is discussed whether this could reflect the operation of phospholipase A2 activity and degradation of the lipid skeleton of the inner mitochondrial membrane.

CONCLUSIONS

Calcium is one of the triggers involved in ischemic cell death, whatever the mechanism.

Motor discoordination and increased susceptibility to cerebellar injury in GLAST mutant mice

To study the function of GLAST, a glutamate transporter highly expressed in the cerebellar Bergmann astrocytes, the mouse GLAST gene was inactivated. GLAST-deficient mice developed normally and could manage simple coordinated tasks, such as staying on a stationary or a slowly rotating rod, but failed more challenging task such as staying on a quickly rotating rod. Electrophysiological examination revealed that Purkinje cells in the mutant mice remained to be multiply innervated by climbing fibres even at the adult stage. We also found that oedema volumes in the mutant mice increased significantly after cerebellar injury. These results indicate that GLAST plays active roles both in the cerebellar climbing fibre synapse formation and in preventing excitotoxic cerebellar damage after acute brain injury.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Immunocytochemical localization of gamma-aminobutyric acid plasma membrane transporters in the tiger salamander retina

Gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) play an important role in regulating GABA neurotransmission in the nervous system. The distribution of two GATs, GAT 1 and GAT 3, in salamander retina was investigated by using affinity-purified polyclonal antisera directed to the predicted C-terminals of rat GAT 1 and rat GAT 3. GAT 1-immunoreactivity (-IR) was found in type IB and IIB orthotopic bipolar cells (BCs) located in the distal and middle of the inner nuclear layer (INL), respectively; in type IIA and IA amacrine cells (ACs) located in the middle and proximal INL, respectively; and in interplexiform cells and cells in the ganglion cell layer (GCL). No detectable staining was found in horizontal cells (HCs) or in structures resembling Müller cells. GAT 1-immunoreactive fibers were present in the outer plexiform layer (OPL) and inner plexiform layer (IPL) in three bands corresponding to the three bands previously reported to be GABA-IR. GAT 3 antibodies labeled fewer cells and cell types than GAT 1 antibodies. GAT 3-IR was localized to type IIA and IA ACs and cells in the GCL, but not to BCs, HCs, or Müller cell-like structures. There was weak labeling of the OPL and stronger labeling of the IPL, with three distinct bands at the same depth as observed with GAT 1-IR. Double-labeling showed that the majority of GAT 1-IR BCs (88%), ACs (88%), and cells in the GCL (78%) colocalized with GABA-IR. The present study provides the first direct evidence of the expression of two GAT subtypes in neurons of nonmammalian retinas. These transporters could regulate GABA neurotransmission by reuptake and termination of GABA's action and, perhaps, by GABA release mechanisms. The presence of GAT 1-IR/GABA-IR bipolar cells further supports our earlier observations that a subgroup of orthotopic bipolar cells are likely to be GABAergic.

Molecular cloning and functional expression of an insect high-affinity Na+-dependent glutamate transporter

Excitatory amino acid transporters in the central and peripheral nervous systems of insects are thought to assist in maintaining glutamate concentrations in the resting synapse below the activation threshold of glutamate receptors. We have isolated a cDNA from the caterpillar Trichoplusia ni which encodes a high-affinity Na+-dependent glutamate transporter, designated TrnEAAT1. The deduced amino acid sequence shows strong identity with known members of the vertebrate Na+- and K+-dependent amino acid transporter family. Expression of the insect transporter mRNA was predominantly localized in the caterpillar brain. The function of the TrnEAAT1 protein was analyzed in cultured insect cells using a baculovirus expression system. Cells infected with the recombinant virus were found to exhibit a 50-fold increase in ability to accumulate labeled L-glutamate compared to mock-infected cultures, and this activity was shown to be Na+-dependent. Transport activity was further demonstrated by chromatographic identification of various glutamate analogues accumulated by infected cells. Various glutamate uptake inhibitors were used to outline the pharmacological properties of the cloned transporter and to compare it with known mammalian transporters. Despite the significant differences between insect and vertebrate physiology, the characteristics of the respective transporters were found to be remarkably similar.

Neuronal and glial glutamate transporters possess an SH-based redox regulatory mechanism

Glutamate uptake into nerve cells and astrocytes via high-affinity transporters controls the extracellular glutamate concentration in the brain, with major implications for physiological excitatory neurotransmission and the prevention of excitotoxicity. We report here that three recently cloned rat glutamate transporter subtypes, viz. EAAC1 (neuronal), GLT1 and GLAST (glial), possess a redox-sensing property, undergoing opposite functional changes in response to oxidation or reduction of reactive sulphydryls present in their structure. In particular, thiol oxidation with 5,5'-dithio-bis(2-nitrobenzoic) acid (DTNB) and disulphide reduction with dithiothreitol (DTT) result, respectively, in reduced and increased uptake capacity by a preparation of partially purified brain transporters as well as by the three recombinant proteins reconstituted into liposomes. In this model system, EAAC1, GLT1 and GLAST react similarly to DTT/DTNB exposures despite their different contents of cysteines, suggesting that only the conserved residues might be involved in redox modulation. Redox sensitivity is a property of the glutamate transporters also when present in their native cell environment. Thus, by using cultured cortical astrocytes and the whole-cell patch-clamp technique we were able to observe dynamic increase and decrease of the glutamate uptake current in response to application of DTT and DTNB in sequence. Moreover, in the same paradigm, DDT-reversible current inhibition was observed with hydrogen peroxide instead of DTNB, indicating that the SH-based redox modulatory site is targeted by endogenous oxidants and might constitute an important physiological or pathophysiological regulatory mechanism of glutamate uptake in vivo.

Excitatory amino acid transporter 5, a retinal glutamate transporter coupled to a chloride conductance

Although a glutamate-gated chloride conductance with the properties of a sodium-dependent glutamate transporter has been described in vertebrate retinal photoreceptors and bipolar cells, the molecular species underlying this conductance has not yet been identified. We now report the cloning and functional characterization of a human excitatory amino acid transporter, EAAT5, expressed primarily in retina. Although EAAT5 shares the structural homologies of the EAAT gene family, one novel feature of the EAAT5 sequence is a carboxy-terminal motif identified previously in N-methyl-D-aspartate receptors and potassium channels and shown to confer interactions with a family of synaptic proteins that promote ion channel clustering. Functional properties of EAAT5 were examined in the Xenopus oocyte expression system by measuring radiolabeled glutamate flux and two-electrode voltage clamp recording. EAAT5-mediated L-glutamate uptake is sodium- and voltage-dependent and chloride-independent. Transporter currents elicited by glutamate are also sodium- and voltage-dependent, but ion substitution experiments suggest that this current is largely carried by chloride ions. These properties of EAAT5 are similar to the glutamate-elicited chloride conductances previously described in retinal neurons, suggesting that the EAAT5-associated chloride conductance may participate in visual processing.

Histamine H1 receptor activation stimulates [3H]GABA release from human astrocytoma U373 MG cells

In U373 MG cells, a line derived from a human astrocytoma, histamine stimulated the release of [3H]gamma-aminobutyric acid ([3H]GABA) in a concentration-dependent manner (286 +/- 23% of basal release at 1 mM histamine). Neither Ca2+ removal nor Cd2+ (100 microM) affected [3H]GABA release evoked by 100 microM histamine but the response was significantly reduced by 10 microM U-73122 ({1-[6-((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)-amino)-hexyl]-1 H-pyrrole-2,5-dione}), an inhibitor of phospholipase C activation (79 +/- 8% inhibition) and by 10 microM dimethylbenzamil, a selective blocker of plasma membrane Na+/Ca2+ exchange (58 +/- 6% inhibition). In [3H]inositol-labelled cells histamine stimulated [3H]inositol phosphate accumulation (EC50, 17 +/- 2 microM; maximum effect, 203 +/- 4% of basal). Histamine-evoked Ca2+ mobilisation yielded an EC50 of 12 +/- 2 microM and maximum delta[Ca2+]i of 337 +/- 23 nM. Thapsigargin (1 nM) increased [Ca2+]i (delta[Ca2+]i 164 +/- 12 nM) and prevented any further increase by histamine (100 microM). The effects of histamine on [3H]GABA release, [3H]inositol phosphate accumulation and Ca2+ mobilisation were blocked by the selective histamine H1 receptor antagonist mepyramine. Taken together, these results indicate that histamine stimulates [3H]GABA release by increasing [Ca2+]i. The mechanism of release may be related to changes in transmembranal Na+ gradients and reversal of GABA carrier transport due to stimulation of plasma membrane Na+/Ca2+ exchange.

Effects of uptake carrier blockers SK & F 89976-A and L-trans-PDC on in vivo release of amino acids in rat hippocampus

This report describes the in vivo effects of the uptake carrier blockers 1-(4,4-diphenyl-3-butenyl)-3-piperidine carboxylic acid hydrochloride (SK & F 89976-A) and L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) on basal and K(+)-evoked extracellular levels of gamma-aminobutyric acid (GABA), glutamate, aspartate and taurine in the hippocampus of anaesthetised rats, using the microdialysis technique. SK & F 89976-A increased extracellular GABA levels under K(+)-depolarised conditions and did not affect extracellular glutamate, aspartate and taurine levels, indicating its selective effect on GABA uptake L-trans-PDC dose dependently increased basal and K(+)-evoked extracellular glutamate levels, and did not affect extracellular GABA levels, but increased basal aspartate and taurine levels. The K(+)-evoked release of GABA and glutamate, measured in the presence of both SK & F 89976-A and L-trans-PDC, was Ca(2+)-dependent for about 50% and 65%, respectively. In contrast, the release of the putative amino acid transmitters aspartate and taurine was not Ca(2+)-dependent. These results indicate that (1) in rat hippocampus uptake carriers actively regulate extracellular GABA and glutamate levels, (2) the GABA and glutamate released by K+ was derived from both Ca(2+)-dependent (presumably vesicular) and Ca(2+)-independent (presumably cytosolic) pools, whereas aspartate and taurine release was exclusively from Ca(2+)-independent pools.

Channels in transporters

Recent electrophysiological investigations of plasma membrane neurotransmitter transporters have shown that carriers can function in ways similar to ion channels. The results of these studies reveal underlying mechanisms not encompassed by classic carrier models and support an emerging view that transporter-mediated ionic currents may contribute to signaling in the nervous system.

Sodium-dependent GABA-induced currents in GAT1-transfected HeLa cells

1. HeLa cells were infected with recombinant vaccinia virus containing the T7 RNA polymerase gene and transfected with the cDNA for a rat GABA transporter, GAT1, cloned downstream of a T7 RNA polymerase promoter. Six to sixteen hours after transfection, whole-cell recording with a voltage ramp in the range -90 to 50 mV revealed GABA-induced currents (approximately -100 pA at -60 mV in 100 microM GABA, 16 h after transfection at room temperature). No GABA-induced currents were observed in parental HeLa cells or in mock-transfected cells. 2. GABA-induced currents were suppressed by extracellular perfusion with GABA-free solutions or addition of GAT1 inhibitors SKF89976-A or SKF100330-A. At fixed voltage the GABA dependence of the inward current fitted the Michaelis-Menten equation with a Hill coefficient, n, near unity and an equilibrium constant, K(m), near 3 microM. The Na+ dependence of the inward currents fitted the Michaelis-Menten equation with n approximately equal to 2 and K(m) approximately equal to 10 mM. The constants n and K(m) for GABA and Na+ were independent of voltage in the range -90 to -30 mV. 3. GABA-induced currents reverse direction in the range 5-10 mV. The implication of this result is that GAT1 can mediate electrogenic (electrophoretic) influx or efflux of GABA depending on the membrane voltage. The presence of an outward current in our experiments is consistent with radioactive-labelled flux data from resealed vesicle studies. However, it is inconsistent with frog oocyte expression experiments using the sample clone. In oocytes, GAT1 generates no outward current in a similar voltage range. Smaller intracellular volume or higher turnover rates in the mammalian expression system may explain the outward currents. 4. External GABA induces inward current, and internal GABA induces outward current. However, in cells initially devoid of internal GABA, external GABA can also facilitate an outward current. This GAT1-mediated outward current occurs only after applying negative potentials to the cell. These data are consistent with the concept that negative potentials drive GABA and Na+ into the cell, which then leads to electrogenic efflux through GAT1 at positive voltages. 5. Assuming coupled transport, we estimate the number of transporters, N, times the turnover rate, r, to be Nr approximately 10(9) s-1 under nominal conditions (V = -60 mV, 30 microM GABA, 130 mM Na+ and room temperature). This indicates either very high levels of expression (approximately 10(4) microns-2), assuming published turnover rates (approximately 10 s-1), or turnover rates that are significantly greater than previously reported. As an alternative, a channel may exist in the GAT1 protein that is gated by GABA and Na+ and blocked by GAT1 antagonists. The channel mode of conduction would exist in addition to the coupled, fixed-stoichiometry transporter mode of conduction.

Modulation of non-vesicular glutamate release by pH

Glutamate uptake into glial cells helps to keep the brain extracellular glutamate concentration, [glu]o, below levels that kill neurons. Uptake is powered by the transmembrane gradients of Na+, K+ and pH. When the extracellular [K+] rises in brain ischaemia, uptake reverses, releasing glutamate into the extracellular space. Here we show, by monitoring glutamate transport electrically and detecting released glutamate with ion channels in neurons placed outside glial cells, that a raised [H+] inhibits both forward and reversed glutamate uptake. No electroneutral reversed uptake was detected, contradicting the idea that forward and reversed uptake differ fundamentally. Suppression of reversed uptake by the low pH occurring in ischaemia will slow the Ca(2+)-independent release of glutamate with can raise [glu]o to a neurotoxic level, and will thus protect the brain during a transient loss of blood supply.

Molecular characterization of a high-affinity mouse glutamate transporter

The complete coding sequence of a mouse glutamate transporter (mEAAT2) has been cloned by polymerase chain reaction (PCR) from adult whole-brain total RNA. Southern hybridization analysis of PCR products amplified from templates derived from various murine adult tissues demonstrated that the transcript for mEAAT2 was specific to the central nervous system. High-affinity transport of D-aspartate, Km value (17 +/- 5 microM), was determined in a vaccinia/T7 RNA polymerase expression system. The deduced amino-acid sequence of mEAAT2 shares 96 and 93% identity with the rat and human EAAT2 homologues, respectively.

Substrate-charge dependence of stoichiometry shows membrane potential is the driving force for proton-peptide cotransport in rat renal cortex

The proton dependence of the transport of three labelled, hydrolysis-resistant synthetic dipeptides carrying a net charge of -1, 0 or +1 has been investigated in a brush border membrane vesicle preparation obtained from rat renal cortex. Cross-inhibition studies are consistent with the transport of all peptides studied being through a single system. The extent and time course of uptake in response to an inwardly directed electrochemical gradient of protons differed for each peptide. For the cationic peptide D-Phe-L-Lys this gradient did not stimulate the initial rate of uptake, while for the neutral dipeptide D-Phe-L-Ala and the anionic peptide D-Phe-L-Glu stimulation was observed. However, the effect on D-Phe-L-Glu was more marked than that on D-Phe-L-Ala and the proton activation differed for these two peptides. The calculated Hill coefficients for the two proton-dependent peptides were 1.14 +/- 0.16 and 2.15 +/- 0.10 for D-Phe-L-Ala and D-Phe-L-Glu, respectively, providing evidence that the stoichiometry of proton:peptide cotransport is different for each peptide (0:1, 1:1 and 2:1 for D-Phe-L-Lys, D-Phe-L-Ala and D-Phe-L-Glu respectively); studies on energetics are compatible with this conclusion. The physiological and molecular implications of this model are discussed, as are the applicability of the conclusions to secondary active transport systems more generally.