Coupled transport of glutamate and sodium in a cerebellar nerve cell line

Glutamate transporters, EAAT1 and EAAT2, are potentially important in the pathophysiology and treatment of schizophrenia and affective disorders

Glutamate is the predominant excitatory neurotransmitter in the human brain and it has been shown that prolonged activation of the glutamatergic system leads to nerve damage and cell death. Following release from the pre-synaptic neuron and synaptic transmission, glutamate is either taken up into the pre-synaptic neuron or neighbouring glia by transmembrane glutamate transporters. Excitatory amino acid transporter (EAAT) 1 and EAAT2 are Na⁺-dependant glutamate transporters expressed predominantly in glia cells of the central nervous system. As the most abundant glutamate transporters, their primary role is to modulate levels of glutamatergic excitability and prevent spill over of glutamate beyond the synapse. This role is facilitated through the binding and transportation of glutamate into astrocytes and microglia. The function of EAAT1 and EAAT2 is heavily regulated at the levels of gene expression, post-transcriptional splicing, glycosylation states and cell-surface trafficking of the protein. Both glutamatergic dysfunction and glial dysfunction have been proposed to be involved in psychiatric disorder. This review will present an overview of the roles that EAAT1 and EAAT2 play in modulating glutamatergic activity in the human brain, and mount an argument that these two transporters could be involved in the aetiologies of schizophrenia and affective disorders as well as represent potential drug targets for novel therapies for those disorders.

Glutamate transporters: confining runaway excitation by shaping synaptic transmission

Traditionally, glutamate transporters have been viewed as membrane proteins that harness the electrochemical gradient to slowly transport glutamate from the extracellular space into glial cells. However, recent studies have shown that glutamate transporters on glial and neuronal membranes also rapidly bind released glutamate to shape synaptic transmission. In this Review, we summarize the properties of glutamate transporters that influence synaptic transmission and are subject to regulation and plasticity. We highlight how the diversity of glutamate-transporter function relates to transporter location, density and affinity.

Allosteric modulation of neurotransmitter transporters at excitatory synapses

The regulation of glutamate and glycine concentrations within excitatory synapses plays an important role in maintaining a dynamic signalling process between neurones, but the failure to regulate the concentrations of these neurotransmitters has been implicated in the pathogenesis of various neurological disorders. In this review we shall discuss how glutamate and glycine transporters regulate synaptic concentrations of these neurotransmitters and how endogenous allosteric modulators influence transporter function. Whilst glutamate transport inhibitors are unlikely to be of therapeutic value because their potential to cause excitoxicity and cell death, a greater understanding of how endogenous compounds allosterically modulate glutamate transporters may provide alternate drug targets. On the other hand, there are some promising drugs that inhibit glycine transporters, which are being trialled as an alternate treatment for schizophrenia. We shall discuss how the activity of one such compound may be expected to influence excitatory neurotransmission.

A hydrophobic domain in glutamate transporters forms an extracellular helix associated with the permeation pathway for substrates

Recent work has shown that cysteine residues introduced into domain 10, a highly hydrophobic segment in the excitatory amino acid transporter 1, react readily when hydrophilic sulfhydryl-modifying reagents are applied extracellularly. To investigate the functional contributions of this region, we mutated each residue in domain 10 (Ala(446)-Gly(459)) to cysteine and assessed the transport kinetics and inhibitor sensitivities of the mutant carriers. Modification of the introduced sulfhydryl group with membrane-impermeant methanethiosulfonate derivatives inhibited substrate transport by all but one functional cysteine mutant. Substrates and/or non-transported inhibitors block thiol modification of most mutants within this region, implying that access to the domain becomes restricted as a consequence of the binding of substrates and substrate analogs. An examination of the temperature dependence of substrate protection for one mutant (I453C) indicates that substrates prevent modification at a step prior to the large conformational changes associated with translocation. When superimposed on a helical model, mutants with similar attributes are positioned in close proximity. Our data are consistent with a model in which domain 10 exists as an alpha-helix at an aqueous interface of the translocation pathway, which can be directly occluded by substrates and inhibitors at an early step in the transport cycle.

Extracellular glutamate, aspartate and arginine increase in the ventral posterolateral thalamic nucleus during nociceptive stimulation

Although there is evidence that the thalamus plays a remarkable role in pain processing few in vivo studies on the thalamic neurochemical correlates of pain have been done. In the present experiments a combination of capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) and microdialysis in freely moving rats was used to measure extracellular arginine, glutamate and aspartate in the thalamus during the formalin test. Microdialysis probes were implanted in the left ventral posterolateral (VPL) nucleus of the thalamus in rats. Samples were collected every 30 s, derivatized with fluorescein isothyocyanate and injected into a CZE-LIF instrument. After nine baseline samples, a subcutaneous formalin (5%, 50 microl) injection in the right hind paw caused an increase of arginine, glutamate and aspartate that lasted for about 3 min. These increases were calcium and nerve impulse dependent. These results indicate that the release of arginine, glutamate and aspartate may mediate rapid pain neural transmission in the VPL nucleus of the thalamus.

Sulfhydryl modification of V449C in the glutamate transporter EAAT1 abolishes substrate transport but not the substrate-gated anion conductance

Excitatory amino acid transporters (EAATs) buffer and remove synaptically released L-glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. Here, we demonstrate that modification of a cysteine substituted within a C-terminal domain of EAAT1 abolishes transport in both the forward and reverse directions without affecting activation of the anion conductance. EC(50)s for L-glutamate and sodium are significantly lower after modification, consistent with kinetic models of the transport cycle that link anion channel gating to an early step in substrate translocation. Also, decreasing the pH from 7.5 to 6.5 decreases the EC(50) for L-glutamate to activate the anion conductance, without affecting the EC(50) for the entire transport cycle. These findings demonstrate for the first time a structural separation of transport and the uncoupled anion flux. Moreover, they shed light on some controversial aspects of the EAAT transport cycle, including the kinetics of proton binding and anion conductance activation.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Niflumic acid modulates uncoupled substrate-gated conductances in the human glutamate transporter EAAT4

1. The effects of niflumic acid on the substrate-gated currents mediated by the glutamate transporter EAAT4 expressed in Xenopus laevis oocytes were examined using radiolabelled substrate flux measurements and two-electrode voltage clamp techniques. 2. Niflumic acid significantly enhanced the substrate-gated currents in EAAT4, without affecting the affinity of the substrates towards EAAT4. At a concentration of 300 microM, niflumic acid caused a 19 +/- 5 % reduction in L-[(3)H]glutamate uptake and no significant effect on the uptake of DL-[(3)H]aspartate. Thus, enhancement of the substrate-gated currents in EAAT4 does not correlate with the rate of substrate transport and suggests that the niflumic acid-induced currents are not thermodynamically coupled to the transport of substrate. 3. Niflumic acid and arachidonic acid co-applied with substrate to EAAT4-expressing oocytes had similar functional consequences. However, niflumic acid still enhanced the L-glutamate-gated current to the same extent in the presence and absence of a saturating dose of arachidonic acid, which suggests that the sites of action of the two compounds are distinct. 4. The EAAT4-mediated currents for the two substrates, L-glutamate and L-aspartate, were not enhanced equally by addition of the same dose of niflumic acid and the ionic composition of the niflumic acid-induced currents was not the same for the two substrates. Protons carry the L-glutamate-gated niflumic acid-induced current and both protons and chloride ions carry the L-aspartate-gated niflumic acid-induced current. 5. These results show that niflumic acid can be used to probe the functional aspects of EAAT4 and that niflumic acid and other non-steroid anti-inflammatory drugs could be used as the basis for the development of novel modulators of glutamate transporters.

Substrates and temperature differentiate ion flux from serotonin flux in a serotonin transporter

Neurotransmitter transporters couple the transport of transmitter against its concentration gradient to the electrochemical potential of associated ions which are also transported. Recent studies of some neurotransmitter transporters show them to have properties of both traditional carriers and substrate-dependent ion channels, in that ion fluxes are in excess of that predicted from stoichiometric substrate fluxes. Whether these properties are comparable for all transporters, the extent to which these permeation states are independent, and whether the relationship between these two states can be regulated are not well understood. To address these questions, we expressed the Drosophila serotonin (5HT) transporter (dSERT) in Xenopus oocytes and measured both substrate-elicited ion flux and 5HT flux at various temperatures and substrate concentrations. We find that the ion flux and 5HT flux components of the transport process have a significant temperature dependence suggesting that ion flux and transmitter flux arise from a similar thermodynamically-coupled process involving large conformational changes (e.g., gating). These data are in contrast to those shown for glutamate transporters, suggesting a different permeation process for 5HT transporters. The relationship between ion flux and 5HT flux is differentially regulated by chloride and 5HT, suggesting that these permeation states are distinct. The difference in half-maximal 5HT concentration necessary to mediate ion flux and 5HT flux occurs at submicromolar 5HT concentrations suggesting that the relative participation of dSERT in ion flux and 5HT flux will be determined by the synaptic 5HT concentration.

Water transport by the human Na+-coupled glutamate cotransporter expressed in Xenopus oocytes

The water transport properties of the human Na+-coupled glutamate cotransporter (EAAT1) were investigated. The protein was expressed in Xenopus laevis oocytes and electrogenic glutamate transport was recorded by two-electrode voltage clamp, while the concurrent water transport was monitored as oocyte volume changes. Water transport by EAAT1 was bimodal. Water was cotransported along with glutamate and Na+ by a mechanism within the protein. The transporter also sustained passive water transport in response to osmotic challenges. The two modes could be separated and could proceed in parallel. The cotransport modality was characterized in solutions of low Cl- concentration. Addition of glutamate promptly initiated an influx of 436 +/- 55 water molecules per unit charge, irrespective of the clamp potential. The cotransport of water occurred in the presence of adverse osmotic gradients. In accordance with the Gibbs equation, energy was transferred within the protein primarily from the downhill fluxes of Na+ to the uphill fluxes of water. Experiments using the cation-selective ionophore gramicidin showed no unstirred layer effects. Na+ currents in the ionophore did not lead to any significant initial water movements. In the absence of glutamate, EAAT1 contributed a passive water permeability (Lp) of (11.3 +/- 2.0) x 10(-6) cm s(-1) (osmol l(-1))(-1). In the presence of glutamate, Lp was about 50 % higher for both high and low Cl- concentrations. The physiological role of EAAT1 as a molecular water pump is discussed in relation to cellular volume homeostasis in the nervous system.

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

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

Noxious stimulation increases glutamate and arginine in the periaqueductal gray matter in rats: a microdialysis study

The periaqueductal gray matter (PAG) is an important center in the modulation of behavioral responses during nociception and stress. In the present experiment, extracellular excitatory amino acid overflow in the PAG was measured every 30 s during noxious stimulation. A combination of in vivo brain microdialysis in freely moving rats and capillary zone electrophoresis with laser induced-fluorescence detection allowed us to detect short lasting changes of excitatory amino acid in dialysates. A formalin injection in the hindpaw of the rat increased glutamate, arginine and aspartate concentration in PAG dialysates. This increase was calcium and nerve impulse-dependent, suggesting neuronal and glial origin of glutamate and arginine, respectively. Handling, pinching or saline injection in the hind paw did not increase glutamate showing that this neurochemical phenomenon is related to painful and persistent noxious stimulation. The results suggest that a rapid excitation of the PAG occurs during noxious stimulation. The role of glutamate and arginine in analgesia is discussed.

Isolation of current components and partial reaction cycles in the glial glutamate transporter EAAT2

The kinetic properties of the excitatory amino acid transporter EAAT2 were studied using rapid applications of L-glutamate to outside-out patches excised from transfected human embryonic kidney 293 cells. In the presence of the highly permeant anion SCN(-), pulses of glutamate rapidly activated transient anion channel currents mediated by the transporter. In the presence of the impermeant anion gluconate, glutamate pulses activated smaller currents predicted to result from stoichiometric flux of cotransported ions. Both anion and stoichiometric currents displayed similar kinetics, suggesting that anion channel gating and stoichiometric charge movements are linked to early transitions in the transport cycle. Transporter-mediated anion currents were recorded with ion and glutamate gradients favoring either unidirectional influx or exchange. Analysis of deactivation and recovery kinetics in these two conditions suggests that, after binding, translocation of substrate is more likely than unbinding under physiological conditions. The kinetic properties of EAAT2, the dominant glutamate transporter in brain astrocytes, distinguish it as an efficient sink for synaptically released glutamate.

Na(+)-dependent glutamate transporters (EAAT1, EAAT2, and EAAT3) of the blood-brain barrier. A mechanism for glutamate removal

Na(+)-dependent transporters for glutamate exist on astrocytes (EAAT1 and EAAT2) and neurons (EAAT3). These transporters presumably assist in keeping the glutamate concentration low in the extracellular fluid of brain. Recently, Na(+)-dependent glutamate transport was described on the abluminal membrane of the blood-brain barrier. To determine whether the above-mentioned transporters participate in glutamate transport of the blood-brain barrier, total RNA was extracted from bovine cerebral capillaries. cDNA for EAAT1, EAAT2, and EAAT3 was observed, indicating that mRNA was present. Western blot analysis demonstrated all three transporters were expressed on abluminal membranes, but none was detectable on luminal membranes of the blood-brain barrier. Measurement of transport kinetics demonstrated voltage dependence, K(+)-dependence, and an apparent K(m) of 14 microM (aggregate of the three transporters) at a transmembrane potential of -61 mV. Inhibition of glutamate transport was observed using inhibitors specific for EAAT2 (kainic acid and dihydrokainic acid) and EAAT3 (cysteine). The relative activity of the three transporters was found to be approximately 1:3:6 for EAAT1, EAAT2, and EAAT3, respectively. These transporters may assist in maintaining low glutamate concentrations in the extracellular fluid.

Excitatory amino acid transporters: a family in flux

As the most predominant excitatory neurotransmitter, glutamate has the potential to influence the function of most neuronal circuits in the central nervous system. To limit receptor activation during signaling and prevent the overstimulation of glutamate receptors that can trigger excitotoxic mechanisms and cell death, extracellular concentrations of excitatory amino acids are tightly controlled by transport systems on both neurons and glial cells. L-Glutamate is a potent neurotoxin, and the inadequate clearance of excitatory amino acids may contribute to the neurodegeneration seen in a variety of conditions, including epilepsy, ischemia, and amyotrophic lateral sclerosis. To establish the contributions of carrier systems to the etiology of neurological disorders, and to consider their potential utility as therapeutic targets, a detailed understanding of transporter function and pharmacology is required. This review summarizes current knowledge of the structural and functional diversity of excitatory amino acid transporters and explores how they might serve as targets for drug design.

Stoichiometry of the glial glutamate transporter GLT-1 expressed inducibly in a Chinese hamster ovary cell line selected for low endogenous Na+-dependent glutamate uptake

Glutamate transport across the plasma membrane of neurons and glia is powered by the transmembrane electrochemical gradients for sodium, potassium, and pH, but there is controversy over the number of Na+ cotransported with glutamate. The stoichiometry of glutamate transporters is important because it determines a lower limit to the extracellular glutamate concentration, [glu]o, in both normal and pathological conditions. We used whole-cell clamping to study the stoichiometry of the glial transporter GLT-1, the most abundant glutamate transporter in the brain, expressed under control of the Tet-On system in a Chinese hamster ovary (CHO) cell line selected for low endogenous glutamate transport. After the induction of GLT-1 expression with doxycycline, glutamate evoked a Na+-dependent inward current with the voltage dependence and pharmacology of GLT-1 and acidified the cell cytoplasm. Raising [K+]o around cells clamped with electrodes containing sodium and glutamate evoked an outward reversed uptake current. These responses were reduced by the specific GLT-1 blocker dihydrokainate (DHK). DHK evoked an outward current with NO3-, but not with Cl-, as the main intracellular anion, suggesting that the anion conductance of the transporter is active even without external glutamate but generates little current in the absence of highly permeable anions like NO3-. Measuring the reversal potential of the transporter current in various ionic conditions suggested that the transport of one glutamate anion is coupled to the cotransport of three Na+ and one H+ and to the countertransport of one K+. This suggests that in ischemia, when [K+]o rises to 60 mM, the reversal of glutamate transporters will raise [glu]o to >50 microM.

A reentrant loop domain in the glutamate carrier EAAT1 participates in substrate binding and translocation

To investigate the structural determinants underlying transport by the glutamate transporter EAAT1, we mutated each of 24 highly conserved residues (P392 to Q415) to cysteine. A majority of these substituted cysteines react with the sulfhydryl-modifying reagent MTSEA, suggesting that they reside in an aqueous environment. The impermeant reagents MTSES and MTSET react with residues at each end of the domain (A395C and A414C), supporting a model that places these residues near the extracellular surface. Substrates and inhibitors block the reaction between MTS derivatives and A395C, and the cosubstrate, sodium, slows reaction of MTSEA with Y405C and E406C. From these results, we propose that this domain forms a reentrant membrane loop at the cell surface and may comprise part of the translocation pore for substrates and cotransported ions.

Macroscopic and microscopic properties of a cloned glutamate transporter/chloride channel

The behavior of a Cl- channel associated with a glutamate transporter was studied using intracellular and patch recording techniques in Xenopus oocytes injected with human EAAT1 cRNA. Channels could be activated by application of glutamate to either face of excised membrane patches. The channel exhibited strong selectivity for amphipathic anions and had a minimum pore diameter of approximately 5A. Glutamate flux exhibited a much greater temperature dependence than Cl- flux. Stationary and nonstationary noise analysis was consistent with a sub-femtosiemen Cl- conductance and a maximum channel Po << 1. The glutamate binding rate was similar to estimates for receptor binding. After glutamate binding, channels activated rapidly followed by a relaxation phase. Differences in the macroscopic kinetics of channels activated by concentration jumps of L-glutamate or D-aspartate were correlated with differences in uptake kinetics, indicating a close correspondence of channel gating to state transitions in the transporter cycle.

Anion currents and predicted glutamate flux through a neuronal glutamate transporter

Kinetic properties of a native, neuronal glutamate transporter were studied by using rapid applications of glutamate to outside-out patches excised from Purkinje neurons. Pulses of glutamate activated anion currents associated with the transporter that were weakly antagonized by the transporter antagonist kainate. In addition, kainate blocked a resting anion conductance observed in the absence of glutamate. Transporter currents in response to glutamate concentration jumps under a variety of conditions were used to construct a cyclic kinetic model of the transporter. The model simulates both the anion conductance and the glutamate flux through the transporter, thereby permitting several predictions regarding the dynamics of glutamate transport at the synapse. For example, the concentration-dependent binding rate of glutamate to the transporter is high, similar to binding rates suggested for ligand-gated glutamate receptors. At saturating glutamate concentrations, transporters cycle at a steady-state rate of 13/sec. Transporters are predicted to have a high efficiency; once bound, a glutamate molecule is more likely to be transported than to unbind. Physiological concentrations of internal sodium and glutamate significantly slow net transport. Finally, a fixed proportion of anion and glutamate flux is expected over a wide range of circumstances, providing theoretical support for using net charge flux to estimate the amount and time course of glutamate transport.

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.

Serine-O-sulphate transport by the human glutamate transporter, EAAT2

1. Expression of the recombinant human excitatory amino aid transporters, EAAT1 and EAAT2, in Xenopus laevis oocytes allows electrogenic transport to be studied under voltage clamp conditions. 2. We have investigated the transport of the pharmacological substrate, L-serine-O-sulphate transport by EAAT1 and EAAT2. The EC50 values for L-serine-O-sulphate transport by EAAT2 showed a steep voltage-dependence, increasing from 152+/-11 microM at - 100 mV to 1930+/-160 microM at 0 mV. In contrast to EAAT2, EC50 values for L-serine-O-sulphate transport by EAAT1 were relatively constant over the membrane potential range of - 100 mV to 0 mV. The EC50 values for L-glutamate and D-aspartate transport, by EAAT2, were also relatively constant over this membrane potential range. 3. Chloride ions modulated the voltage-dependent changes in EC50 values for transport by EAAT2. This effect was most apparent for L-serine-O-sulphate transport, and to a lesser extent for L-glutamate and not at all for D-aspartate transport by EAAT2. 4. Extracellular sodium and proton concentrations also modulated the voltage-dependence of L-serine-O-sulphate EC50 values for EAAT2. 5. We speculate that these different properties of L-serine-O-sulphate transport by EAAT2 compared to other substrates may be due to the much stronger acidity of the sulphate group of L-serine-O-sulphate compared to carboxyl groups of L-glutamate or D-aspartate. 6. These results highlight some of the differences in the way different glutamate transporter subtypes transport substrates. This may be used to understand further the transport process and develop subtype selective inhibitors of glutamate transport.

Currents evoked in Bergmann glial cells by parallel fibre stimulation in rat cerebellar slices

1. Whole-cell recordings were obtained from Bergmann glial cells in rat cerebellar slices. 2. The cells had low input resistances (70 +/- 38 M omega; n = 13) and a mean resting potential of -82 +/- 6 mV (n = 12) with a potassium-based internal solution. Electrical and dye coupling between Bergmann glia were confirmed. 3. Stimulation of parallel fibres induced a complex, mostly inward current which could be decomposed pharmacologically. 4. The ionotropic glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), but not DL-2-amino-5-phosphonopentanoic acid (DL-APV; 100 microM) consistently blocked an early inward current component that may reflect synaptic activation of AMPA/kainate receptors in Bergmann glia. 5. Addition of cadmium ions (100 microM) to inhibit transmitter release blocked most of the CNQX-APV-insensitive current. This component probably reflects electrogenic uptake of the synaptically released glutamate. 6. Tetrodotoxin (TTX; 1 microM) blocked the remaining inward current: a slow component, possibly produced by the potassium ion efflux during action potential propagation in parallel fibres. An initial triphasic component of the response was also TTX sensitive and reflected passage of the parallel fibre action potential volley. 7. The putative glutamate uptake current was further characterized; it was blocked by the competitive uptake blockers D-aspartate (0.5 mM) and L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC; 0.5 mM), and by replacement of sodium with lithium. Monitoring the triphasic TTX-sensitive component showed that this inhibition did not result from changes of action potential excitation and propagation. 8. Intracellular nitrate ions increased the putative uptake current, consistent with the effect of this anion on glutamate transporters. 9. The putative uptake current was reduced by depolarization, consistent with the voltage dependence of glutamate uptake. 10. It is concluded that a large fraction of the current induced by parallel fibre stimulation reflects the uptake of synaptically released glutamate. The uptake current activated rapidly, with a 20-80% rise time of 2.3 +/- 0.7 ms (n = 10), and decayed with a principal time constant of 25 +/- 6 ms (n = 10).

Anion conductance behavior of the glutamate uptake carrier in salamander retinal glial cells

Glutamate uptake is driven by the cotransport of Na+ ions, the countertransport of K+ ions, and either the countertransport of OH- or the cotransport of H+ ions. In addition, activating glutamate uptake carriers has been shown to lead to activation of an anion conductance present in the carrier structure. Here we characterize the ion selectivity and gating of this anion conductance. The conductance is small with Cl- as the permeant anion, but it is large with NO3- or ClO4- present, undermining the earlier use of NO3- and ClO4- to suggest that OH- countertransport rather than H+ cotransport helps drive uptake. Activation of the anion conductance can be evoked by extra- or intracellular glutamate and can occur even when glutamate transport is inhibited. By running the carrier backward and detecting glutamate release with AMPA receptors in neurons placed near the glial cells, we show that anion flux is not coupled thermodynamically to glutamate movement, but OH-/H+ transport is. The possibility that cell excitability is modulated by the anion conductance associated with glutamate uptake suggests a target for therapeutic drugs to reduce glutamate release in conditions like epilepsy.

ASCT-1 is a neutral amino acid exchanger with chloride channel activity

The ubiquitous transport activity known as system ASC is characterized by a preference for small neutral amino acids including alanine, serine, and cysteine. ASCT-1 and ASCT-2, recently cloned transporters exhibiting system ASC-like selectivity, are members of a major amino acid transporter family that includes a number of glutamate transporters. Here we show that ASCT1 functions as an electroneutral exchanger that mediates negligible net amino acid flux. The electrical currents previously shown to be associated with ASCT1-mediated transport result from activation of a thermodynamically uncoupled chloride conductance with permeation properties similar to those described for the glutamate transporter subfamily. Like glutamate transporters, ASCT1 activity requires extracellular Na+. However, unlike glutamate transporters, which mediate net flux and complete a transport cycle by countertransport of K+, ASCT-1 mediates only homo- and heteroexchange of amino acids and is insensitive to K+. The properties of ASCT-1 suggest that it may function to equilibrate different pools of neutral amino acids and provide a mechanism to link amino acid concentration gradients.

The glutamate transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylate indirectly evokes NMDA receptor mediated neurotoxicity in rat cortical cultures

Because of the well-documented importance of glutamate uptake in protecting neurons against glutamate toxicity, we were interested in testing the effects of L-trans-pyrrolidine-2,4-dicarboxylate (PDC) on rat cortical cultures. This compound is a substrate for glutamate transporters and is a potent glutamate transport inhibitor that does not interact significantly with glutamate receptors. Using a 30 min exposure, and assessing neuronal survival after 20-24 h, PDC was neurotoxic in conventional astrocyte-rich cortical cultures, with an EC50 in these cultures of 320 +/- 157 microM. In astrocyte-poor cultures, an EC50 for PDC of 50 +/- 5 microM was determined. The neurotoxicity of PDC in both astrocyte-rich and astrocyte-poor cultures was blocked by the NMDA antagonist MK-801, but not by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). We tested the possibility that the neurotoxicity of PDC might be due to release of excitatory amino acids using several approaches. After pre-loading cells with the non-metabolizable analogue of glutamate, [3H]-D-aspartate, first we demonstrated that PDC caused significant efflux of [3H]-D-aspartate. This effect of PDC was dependent upon extracellular sodium. In contrast with glutamate neurotoxicity, PDC neurotoxicity was inhibited by removal of extracellular sodium. In the presence of 1 mM PDC, sodium caused neurotoxicity with an EC50 of 18 +/- 7.6 mM. Tetrodotoxin had no effect on either PDC neurotoxicity or on PDC-evoked [3H]-D-aspartate release. PDC-evoked release of [3H]-D-aspartate was demonstrable in astrocyte cultures with no neurons present. PDC also evoked release of endogenous glutamate. Finally, the neurotoxicity of PDC was blocked by coincubation with glutamate-pyruvate transaminase plus pyruvate to degrade extracellular glutamate. These results demonstrate the neurotoxicity of PDC, and suggest that the mechanism of this toxicity is the glutamate transporter-dependent accumulation of glutamate in the extracellular space.

Ion fluxes associated with excitatory amino acid transport

Flux of substrate and charge mediated by three cloned excitatory amino acid transporters widely expressed in human brain were studied in voltage-clamped Xenopus oocytes. Superfusion of L-glutamate or D-aspartate resulted in currents due in part to electrogenic Na+ cotransport, which contributed 1 net positive charge per transport cycle. A significant additional component of the currents was due to activation of a reversible anion flux that was not thermodynamically coupled to amino acid transport. The selectivity sequence of this ligand-activated conductance was NO3- > 1- > Br- > Cl- > F-. The results suggest that these proteins mediate both transporter- and channel-like modes of permeation, providing a potential mechanism for dampening cell excitability, in addition to removal of transmitter.

The effects of a purified toxic extract of Prymnesium patelliferum on transport of ions through the plasma membrane of synaptosomes

Extract of the ichthyotoxic marine alga Prymnesium patelliferum has been shown to have several different effects on the transport of neurotransmitters across nerve membranes. It inhibits the sodium dependent uptake of L-glutamate and GABA and enhances the calcium-dependent release of acetylcholine. We have therefore investigated the effects of a purified toxic extract of P. patelliferum on some membrane properties using rat brain synaptosomes. We found that under conditions where the algal extract inhibited the uptake of L-glutamate, it increased the intracellular concentrations of Na+ and Ca2+, stimulated efflux of K+ determined as 86Rb efflux, and depolarized the synaptosomal membrane. There was no effect on Na+/K(+)-ATPase or ouabain-insensitive ATPase activities. Further, there was no leakage of the cytosolic marker LDH, indicating that the various effects of the algal extract were not due to nonspecific leakage or lysis of the synaptosomes. The rise in the cytosolic concentration of free Ca2+ induced by the algal extract was dependent on extracellular Ca2+, and was inhibited by flunarizine (1-100 microM) but not by the Ca2+ channel blockers omega-conotoxin GVIA (1 microM), diltiazem (100 microM), nifedipine (100 microM) or verapamil (100-500 microM). The increase in Na+ influx induced by the algal extract was insensitive to tetrodotoxin (3 microM) and procaine (100 microM), whereas both the Na+ influx and the membrane depolarization were inhibited by flunarizine (1-100 microM). The increase in K+ efflux was insensitive to flunarizine (5-100 microM). From these results it appears that the toxic extract of P. patelliferum increases the permeability of synaptosomes to Ca2+, Na+ and K+ and that these effects may be responsible for the plasma membrane depolarization and the disturbance of the neurotransmitter transport processes.

Constitutive ion fluxes and substrate binding domains of human glutamate transporters

Application of L-glutamate activates ionic currents in voltage-clamped Xenopus oocytes expressing cloned human excitatory amino acid transporters (EAATs). However, even in the absence of L-glutamate, the membrane conductance of oocytes expressing EAAT1 was significantly increased relative to oocytes expressing EAAT2 or control oocytes. Whereas transport mediated by EAAT2 is blocked by the non-transported competitive glutamate analog kainate (Ki = 14 microM), EAAT1 is relatively insensitive (Ki > 3 mM). Substitution of a block of 76 residues from EAAT2 into EAAT1, in which 18 residues varied from EAAT1, conferred high affinity kainate binding to EAAT1, and application of kainate to oocytes expressing the chimeric transporter blocked a pre-existing monovalent cation conductance that displayed a permeability sequence K+ > Na+ > Li+ >> choline+. The results identify a structural domain of glutamate transporters that influences kainate binding and demonstrate the presence of a constitutive ion-selective pore in the transporter.

Glutamate 404 is involved in the substrate discrimination of GLT-1, a (Na+ + K+)-coupled glutamate transporter from rat brain

Sodium-coupled glutamate transporters, located in the plasma membrane of nerve terminals and glial processes, serve to keep its extracellular glutamate concentration below extracellular levels. Moreover, they help in conjunction with diffusion to terminate the transmitter's action in synaptic transmission. We have investigated the role of negatively charged amino acid residues of GLT-1, a cloned (Na+ + K+)-coupled glutamate transporter from rat brain. Using site-directed mutagenesis we modified these negative residues, which are located in hydrophobic surroundings and are highly conserved within the glutamate transporter family. Out of five residues meeting these criteria, three, aspartate 398, glutamate 404, and aspartate 470, are critical for heterologously expressed glutamate transport. This defective transport cannot be attributed to the mere requirement of a negative charge at these positions. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of all mutant transporters indicates that their expression levels are similar to that of wild type. No cryptic activity was revealed by reconstitution experiments aimed to monitor the activity of transporter molecules not located in the plasma membrane. Significantly, whereas all of the mutants at the glutamate 404 position exhibit impaired transport of glutamate, they possess considerable transport of D- and L-aspartate, up to 80% of wild type values. Binding of glutamate is not impaired in these mutants. Our observations indicate that the glutamate 404 residue may be located in the vicinity of the glutamate-aspartate permeation pathway.

Conformational changes monitored on the glutamate transporter GLT-1 indicate the existence of two neurotransmitter-bound states

Membrane vesicles from rat brain have been subjected to trypsin treatment in the absence and presence of substrates of the (Na+ + K+)-coupled L-glutamate transporter GLT-1. The fragments of this transporter have been detected upon immunoblotting employing several antibodies raised against sequences from this transporter. At the amino terminus, initially a fragment of an apparent molecular mass of 30 kDa is generated. This fragment is subsequently cleaved to one of 16 kDa. The generation of these bands is greatly inhibited in the presence of lithium. Moreover, lithium abolishes the positive cooperative activation of the transporter by sodium. The generation of the 30- and 16-kDa fragments is accelerated in the presence of L-glutamate and other transportable analogues, provided sodium is present as well. The 30-kDa fragment also contains an epitope from the loop connecting the putative membrane-spanning alpha-helices 3 and 4. This epitope, in contrast with the amino-terminal one, is destroyed with time. The carboxyl-terminal epitope is predominantly located on a 43-kDa fragment which is slowly converted to one of 35 kDa. This conversion is not inhibited by lithium. It is, however, stimulated by L-glutamate and other transportable analogues, but only in sodium-containing media. Potassium also stimulates this conversion regardless of the presence of L-glutamate. The stimulation of generation of amino- and carboxyl-terminal fragments by L-glutamate is not mimicked by the nontransportable analogue dihydrokainate. However, the analogue blocks the stimulation exerted by L-glutamate. In addition to new experimental information on the transporters topology, our observations provide novel information on the function of the GLT-1 transporter. Although lithium by itself does not sustain transport, it may occupy one of the sodium sites and be transported. Furthermore, the transporter-glutamate complex appears to exist in at least two states. After the initial binding (suggested to be important for the decay of synaptic glutamate), it undergoes a conformational change which represents, or is tightly associated with, the transport step.

Electrogenic properties of the epithelial and neuronal high affinity glutamate transporter

Active ion-coupled glutamate transport is of critical importance for excitatory synaptic transmission, normal cellular function, and epithelial amino acid metabolism. We previously reported the cloning of the rabbit intestinal high affinity glutamate transporter EAAC1 (Kanai, Y., and Hediger, M. A. (1992) Nature 360, 467-471), which is expressed in numerous tissues including intestine, kidney, liver, heart, and brain. Here, we report a detailed stoichiometric and kinetic analysis of EAAC1 expressed in Xenopus laevis oocytes. Uptake studies of 22Na+ and [14C]glutamate, in combination with measurements of intracellular pH with pH microelectrodes gave a glutamate to charge ratio of 1:1, a glutamate to Na+ ratio of 1:2, and a OH-/H+ to charge ratio of 1:1. Since transport is K+ dependent it can be concluded that EAAC1-mediated glutamate transport is coupled to the cotransport of 2 Na+ ions, the countertransport of one K+ ion and either the countertransport of one OH- ion or the cotransport of 1 H+ ion. We further demonstrate that under conditions where the electrochemical gradients for these ions are disrupted, EAAC1 runs in reverse, a transport mode which is of pathologic importance. 22Na+ uptake studies revealed that there is a low level of Na+ uptake in the absence of extracellular glutamate which appears to be analogous to the Na+ leak observed for the intestinal Na+/glucose cotransporter SGLT1. In voltage clamp studies, reducing extracellular Na+ from 100 to 10 mM strongly increased K0.5L-glutamate and decreased I(max). The data indicate that Na+ binding at the extracellular transporter surface becomes rate-limiting. Studies addressing the cooperativity of the substrate-binding sites indicate that there are two distinct Na(+)-binding sites with different affinities and that Na+ binding is modulated by extracellular glutamate. A hypothetical ordered kinetic transport model for EAAC1 is discussed.

Regional differences in the inhibition of L-glutamate and L-aspartate sodium-dependent high affinity uptake systems in rat CNS synaptosomes by L-trans-pyrrolidine-2,4-dicarboxylate, threo-3-hydroxy-D-aspartate and D-aspartate

The sodium-dependent high affinity transport of L-[3H]glutamate and L-[3H]aspartate into synaptosomal fractions prepared from three different regions was employed to investigate the inhibitors L-trans-pyrrolidine-2,4-dicarboxylate, threo-3-hydroxy-D-aspartate and D-aspartate. These substances showed regional heterogeneity as inhibitors of sodium-dependent high affinity uptake of L-glutamate and L-aspartate. L-trans-Pyrrolidine-2,4-dicarboxylate was a more potent inhibitor of the uptake of L-glutamate than of L-aspartate in the cortex (IC50 8 microM vs L-glutamate and 13 microM vs L-aspartate) and cerebellum (IC50 4 microM v L-glutamate and 8 microM vs L-aspartate). threo-3-Hydroxy-D-aspartate was a more potent inhibitor of the uptake of L-glutamate than of L-aspartate in the cortex (IC50 9 microM vs L-glutamate and 13 microM vs L-aspartate) and hippocampus (IC50 6 microM v L-glutamate and 11 microM v L-aspartate). D-Aspartate was a more potent inhibitor of the uptake of L-glutamate than of L-aspartate only in the cortex (IC50 8 microM vs L-glutamate and 15 microM vs L-aspartate). These results thus support other evidence that there is regional heterogeneity in sodium-dependent high affinity acidic amino acid uptake sites in the brain.

Glutamate transporters from brain. A novel neurotransmitter transporter family

The removal of neurotransmitters by their transporters in presynaptic nerve terminals and glial cells plays an important role in the termination of synaptic transmission. Many neurotransmitter transporters, which are sodium- and chloride-coupled, have been cloned and shown to constitute a large superfamily. Glutamate is the major excitatory neurotransmitter in the central nervous system. If not efficiently removed, it causes death of neuronal cells. Its transporter couples the flow of glutamate to that of sodium and potassium. Recently three different but related glutamate transporters have been cloned, which have no significant homology to the members of the superfamily.

Localization and transport of N-acetylaspartylglutamate in cells of whole murine brain in primary culture

N-Acetylaspartylglutamate (NAAG) is the most abundant neuropeptide in the mammalian nervous system. Considerable data support the hypothesis that NAAG is synaptically released in a manner consistent with neurotransmission. Primary murine brain cultures containing neurons and glia expressed 1.2-3.5 nmol of NAAG/mg of protein. In contrast to conclusions drawn from immunohistochemistry, pure glial cultures also expressed high levels of NAAG (0.6-2.11 nmol/mg of protein). These data suggest that although a subpopulation of neurons contains very high NAAG levels, micromolar concentrations of the peptide also are present in glia. Both culture types demonstrated robust extracellular peptidase activity when incubated with NAAG, as well as peptide transport. Uptake of [3H]NAAG was both temperature and sodium dependent, yet relatively insensitive to the presence of extracellular glutamate. These results indicate that synaptically released NAAG, as well as that which may be released from glia, is removed from the extracellular space by direct uptake as well as the robust enzymatic degradation of the peptide. A kinetic analysis of this NAAG transport (estimated Km = 1.8 microM) suggests a high-affinity NAAG transport system. The balance of the two processes of direct peptide uptake and peptide hydrolysis would markedly influence the sequence of receptor-mediated events that follow NAAG release.

Glutamate iontophoresis induces long-term potentiation in the absence of evoked presynaptic activity

Protocols that induce long-term potentiation (LTP) typically involve afferent stimulation. We tested the hypothesis that LTP induction does not require presynaptic activity. The significance of this hypothesis is underscored by results suggesting that LTP expression may involve activity-dependent presynaptic changes. An induction protocol using glutamate iontophoresis was developed that reliably induced LTP in hippocampal slices without afferent stimulation. Iontophoresis LTP was Ca2+ dependent, was blocked by MK-801, and occluded tetanus-induced LTP. Iontophoresis LTP was induced when excitatory postsynaptic potentials were completely blocked by adenosine plus tetrodotoxin. Our results suggest constraints on the involvement of presynaptic mechanisms and putative retrograde messengers in LTP induction and expression; namely, these processes must function without many forms of activity-dependent presynaptic processes.

Tolerance induction by clonal deletion of CD4+8+ thymocytes in vitro does not require dedicated antigen-presenting cells

The cellular requirements of T cell tolerance induction in the thymus by clonal deletion was investigated by using an in vitro assay: thymocytes from mice expressing a transgenic TcR specific for lymphocytic choriomeningitis virus (LCMV) and H-2Db were co-cultured with various H-2b cell types as antigen-presenting cells in the presence of the antigenic LCMV peptide. The results revealed that all cell lines examined including embryonic and transformed fibroblasts, melanoma cells, cortical thymic epithelial cells, lymphomas and neuronal cells induced an antigen dose-dependent deletion of CD4+8+ thymocytes. Similarly, highly enriched accessory cell populations from thymus and spleen (macrophages, dendritic and cortical epithelial cells, i.e. thymic nurse cells) could induce antigen-specific depletion of immature CD4+8+ thymocytes. Depending on the cell type examined micromolar to picomolar concentration of LCMV peptide were required to induce deletion. The effectiveness of deletion by the different cell types did not correlate with their major histocompatibility class I expression level; it was, however, influenced by the presence of ICAM-1 adhesion molecules.

Heterogeneity of high affinity uptake of L-glutamate and L-aspartate in the mammalian central nervous system

Characteristics of high affinity uptake of L-glutamate are examined in order to evaluate the possible use of the uptake of [3H]L-glutamate, [3H]L-aspartate or any other suitable [3H]-labelled substrate as a marker for glutamatergic and aspartergic synapses in autoradiographic studies in the mammalian brain. Review of data on substrate specificity indicates the presence of at least two high affinity uptake systems specific for acidic amino acids in the central nervous tissue; one which takes up L-glutamate and L-aspartate and the other which is selective for L-glutamate only. Studies on ionic requirements, too, point to the existence of at least two distinct uptake systems with high affinity for L-glutamate. The Na(+)-dependent uptake system(s) handle(s) both L-glutamate and L-aspartate whereas the Na(+)-independent uptake system(s) show(s) selectivity for L-glutamate only. Available data do not favour the Na(+)-dependent binding of [3H]D-aspartate to thaw-mounted sections of frozen brain tissue as a suitable marker for glutamatergic/aspartergic synaptic nerve endings. However, there are reasons--such as the results of lesion studies and the existence of uptake sites which have a higher affinity for L-aspartate than for D-aspartate--to suggest that Na(+)-dependent binding of [3H]L-aspartate, rather than that of [3H]D-aspartate, should be further investigated as a possible marker for the glutamatergic/aspartergic synapses in the autoradiographic studies using sections of frozen brain.

Stimulation of 22Na+ efflux from rat forebrain membrane vesicles by L-glutamic acid, L-aspartic acid and kainic acid

A glass fiber filter assay method is described for measuring 22Na+ efflux stimulated by L-glutamic acid, L-aspartic acid and kainic acid from osmotically sensitive membrane vesicles prepared from rat brain. L-Glutamic acid and L-aspartic acid showed the greatest efficacy for the stimulation of 22Na+ efflux with EC50 values of 3 microM. Kainic acid produced 28% of the maximal efflux seen with L-glutamic acid or L-aspartic acid with an EC50 value of 1.5 microM. Quisqualic acid never showed statistically significant increases in 22Na+ efflux over control experiments. N-Methyl-D-aspartic acid showed no detectable efflux activity in this preparation. DL-2-Amino-4-phosphonobutyric acid (APB) inhibited up to 40% of the 50 microM L-glutamic acid-stimulated or 50 microM L-aspartic acid-stimulated 22Na+ efflux with an IC50 value of 1.5 nM. Calcium was required for the inhibitory action of APB, but not for the stimulatory actions of L-glutamic, L-aspartic, or kainic acids. L-Glutamic, L-aspartic, and kainic acids at concentrations above 100 microM were found to inhibit rather than to stimulate 22Na+ efflux. Veratridine (1 microM) had no influence on the 22Na+ efflux component which was produced by L-glutamic or kainic acids. We are unable to firmly establish the mechanism for the stimulated 22Na+ efflux.

Changing cation levels (Mg2+, Ca2+, Na+) alters the release of glutamate, GABA and other substances from the guinea pig cochlea

We examined the effects of changes in cation levels (increased Mg2+ concentration combined with low Ca2+ concentration, and two low concentrations of Na+) on the perilymph levels of gamma-aminobutyric acid (GABA), glutamate (Glu), aspartate (Asp) and other substances. Artificial perilymph solutions containing normal (5 mM) and high (50 mM) levels of K+ were perfused through the perilymphatic compartment of the guinea pig cochlea to examine basal release (5 mM K+) and depolarization-induced release (50 mM K+). Each of the two K+ concentrations were contained in four different solutions: [I] normal artificial perilymph (NARP; NaCl, 137 mM; CaCl2, 2 mM; MgCl2, 1 mM;); [II] high Mg2+ (20 mM)/low Ca2+ (0.1 mM) (HMgLCa); [III] low Na+ (117 mM; LNa), and [IV] very low Na+ (NaCl, 0 mM; VLNa). The effluent was collected and assayed for eighteen primary amines by HPLC. Compared with NARP, the HMgLCa group had an increase in the high K(+)-induced release of Asp and Glu with no change in GABA. VLNa increased the normal K+ levels of Asp, Glu and GABA up to those observed with high K+ in NARP. VLNa increased the high K+ levels of Asp and Glu over fivefold compared with the high K+ levels in NARP, but decreased GABA. We ascribe the results to an interference with either a Na(+)-dependent uptake processes or a Na+/Ca2+ exchange carrier.

Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina

The effects of excitatory amino acids on the membrane current of isolated retinal glial cells (Müller cells) were investigated using whole-cell patch clamping. 2. L-Glutamate evoked an inward current at membrane potentials between -140 and +50 mV. The current was larger at more negative potentials. 3. The glutamate-evoked current was activated by external cations with relative efficacies: Na+ much greater than Li+ greater than K+ greater than Cs+, choline. It was activated by internal cations with relative efficacies K+ greater than Rb+ greater than Cs+ much greater than choline. Chloride and divalent cations did not affect the glutamate-evoked current. 4. Raising the intracellular sodium or glutamate concentrations, or raising the extracellular potassium concentration, reduced the current evoked by external glutamate. The suppressive effect of internal glutamate was larger when the internal sodium concentration was high. 5. Some analogues of glutamate also evoked an inward current. Responses to L-aspartate resembled those to glutamate, but for aspartate the apparent affinity was higher and the voltage dependence of the current was steeper. In the physiological potential range the current evoked by a saturating dose of aspartate was less than that evoked by a saturating dose of glutamate. 6. The uptake blocker threo-3-hydroxy-DL-aspartate (30 microM) reduced the glutamate-evoked current, but also generated a current itself. Dihydrokainate (510 microMs) weakly inhibited the glutamate-evoked current without generating a current itself. 7. The commonly used blockers of glutamate-gated ion channels, 2-amino-5-phosphonovalerate (APV; 100 microMs), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microMs), and kynurenate (1mM) had no effect on the glutamate-evoked current. 8. The voltage dependence, cation dependence and pharmacological profile of the current evoked by excitatory amino acids indicate that it is caused by activation of the high-affinity glutamate uptake carrier. This carrier appears to transport one glutamate anion into the cell, one K+ ion out of the cell, and two or more Na+ ions into the cell, on each carrier cycle. At the inner membrane surface some or all of the transported Na+ dissociates from the carrier after the transported glutamate has dissociated. 9. In addition to glutamate, the uptake carrier can also transport aspartate and threo-3-hydroxy-DL-aspartate, but not dihydrokainate.

Amino acid-mediated EPSPs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord

1. Fast excitatory postsynaptic potentials (EPSPs) evoked by stimulation of A delta and C fibres were examined by intracellular recording from substantia gelatinosa (SG) neurones in a transverse slice preparation of adult rat spinal cord. 2. Single low-intensity stimuli applied to the dorsal root activated A delta fibres and evoked monosynaptic EPSPs in 70% of SG neurones. In 5% of SG neurones, increasing the intensity and duration of stimulation evoked solely C fibre-mediated EPSPs. About 20% of neurones received both A delta and C fibre input from primary afferents. 3. Low concentrations of tetrodotoxin (TTX, approximately 50 nM) blocked EPSPs evoked by stimulation of A delta fibres without affecting those evoked by C fibre stimulation. Higher concentrations of TTX (500 nM) also blocked C fibre-evoked responses. 4. EPSPs evoked by A delta and C fibre stimulation reversed in polarity at membrane potentials near 0 mV, similar to the reversal potential of spontaneous EPSPs and of the potential change evoked by exogenous glutamate. 5. A delta and C fibre-evoked EPSPs were depressed by kynurenate and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); C fibre-evoked EPSPs appeared to be less sensitive. 6. In the presence of TTX, only 50% of SG neurones were depolarized by L-glutamate. However, neurones which exhibited no direct response to L-glutamate received afferent-evoked EPSPs which were sensitive to CNQX. In sensitive neurones, the depolarization evoked by L-glutamate was depressed by only approximately 15% in the presence of CNQX, whereas afferent-evoked EPSPs recorded from the same neurone were almost completely suppressed. Combined application of DL-2-amino-5-phosphonovaleric acid (APV) and CNQX depressed the response to L-glutamate by only approximately 25%. 7. These findings suggest that A delta and C fibres use L-glutamate or a related amino acid as a transmitter at synapses with substantia gelatinosa neurones. The postsynaptic actions of this transmitter are mediated predominantly by non N-methyl-D-aspartic acid (NMDA) receptors. The failure of CNQX and APV to completely block the L-glutamate-evoked depolarization of substantia gelatinosa neurones raises the possibility that exogenously applied L-glutamate activates a non-NMDA receptor distinct from that which mediates the actions of the synaptically released afferent transmitter.