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

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

The Role of Excitatory Amino Acid Transporters in Cerebral Ischemia

Glutamate is an excitatory neurotransmitter that plays a major role in the pathogenesis of ischemia brain injury. The regulation of glutamate neurotransmission is carried out by excitatory amino acid transporters (EAATs) that act through reuptake of glutamate into cells. EAATs may also release glutamate into the extracellular space in a calcium-independent manner during ischemia and dysfunction of EAATs is specifically implicated in the pathology of cerebral ischemia. Recent studies show that up-regulation of EAAT2 provides neuroprotection during ischemic insult. This review summarizes current knowledge regarding the role of EAATs in cerebral ischemia.     

Toward a new role for plasma membrane sodium-dependent glutamate transporters of astrocytes: maintenance of antioxidant defenses beyond extracellular glutamate clearance

The primary function assigned to the sodium-dependent glutamate transporters, also known as excitatory amino acid transporters (EAATs), is to maintain the extracellular glutamate concentration in the low micromolar range, allowing glutamate to be used as a signaling molecule in the brain and preventing its cytotoxic effects. However, glutamate and cyst(e)ine, that is also a substrate of EAATs, are also important metabolites used for instance in the synthesis of the main antioxidant glutathione. This review describes the evidence suggesting that EAATs, by providing glutathione precursors, are crucial to prevent oxidative death in particular cells of the nervous system while being dispensable in others. This differential importance may depend on the way antioxidant defenses are maintained in each cell type and on the metabolic fate of transported substrates, both being probably controlled by EAAT interacting proteins. As oxidative stress invariably contributes to various forms of cell death, a better understanding of how antioxidant defenses are maintained in particular brain cells will probably help to develop protective strategies in degenerative insults specifically affecting these cells.     

Neuroprotective properties of the excitatory amino acid carrier 1 (EAAC1)

Extracellular glutamate should be maintained at low levels to conserve optimal neurotransmission and prevent glutamate neurotoxicity in the brain. Excitatory amino acid transporters (EAATs) play a pivotal role in removing extracellular glutamate in the central nervous system (CNS). Excitatory amino acid carrier 1 (EAAC1) is a high-affinity Na⁺-dependent neuronal EAAT that is ubiquitously expressed in the brain. However, most glutamate released in the synapses is cleared by glial EAATs, but not by EAAC1 in vivo. In the CNS, EAAC1 is widely distributed in somata and dendrites but not in synaptic terminals. The contribution of EAAC1 to the control of extracellular glutamate levels seems to be negligible in the brain. However, EAAC1 can transport not only extracellular glutamate but also cysteine into the neurons. Cysteine is an important substrate for glutathione (GSH) synthesis in the brain. GSH has a variety of neuroprotective functions, while its depletion induces neurodegeneration. Therefore, EAAC1 might exert a critical role for neuroprotection in neuronal GSH metabolism rather than glutamatergic neurotransmission, while EAAC1 dysfunction would cause neurodegeneration. Despite the potential importance of EAAC1 in the brain, previous studies have mainly focused on the glutamate neurotoxicity induced by glial EAAT dysfunction. In recent years, however, several studies have revealed regulatory mechanisms of EAAC1 functions in the brain. This review will summarize the latest information on the EAAC1-regulated neuroprotective functions in the CNS.     

Cooperative action of glutamate transporters and cystine/glutamate antiporter system Xc- protects from oxidative glutamate toxicity

Oxidative glutamate toxicity in the neuronal cell line HT22 is a model for cell death by oxidative stress. In this paradigm, an excess of extracellular glutamate blocks the glutamate/cystine-antiporter system Xc-, depleting the cell of cysteine, a building block of the antioxidant glutathione. Loss of glutathione leads to the accumulation of reactive oxygen species and eventually cell death. We selected cells resistant to oxidative stress, which exhibit reduced glutamate-induced glutathione depletion mediated by an increase in the antiporter subunit xCT and system Xc- activity. Cystine uptake was less sensitive to inhibition by glutamate and we hypothesized that glutamate import via excitatory amino acid transporters and immediate re-export via system Xc- underlies this phenomenon. Inhibition of glutamate transporters by l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) and DL-threo-beta-benzyloxyaspartic acid (TBOA) exacerbated glutamate-induced cell death. PDC decreased intracellular glutamate accumulation and exacerbated glutathione depletion in the presence of glutamate. Transient overexpression of xCT and the glutamate transporter EAAT3 cooperatively protected against glutamate. We conclude that EAATs support system Xc- to prevent glutathione depletion caused by high extracellular glutamate. This knowledge could be of use for the development of novel therapeutics aimed at diseases associated with depletion of glutathione like Parkinson's disease.     

Glutamate Transporters: Expression and Function in Oligodendrocytes

Glutamate, the main excitatory neurotransmitter of the vertebrate central nervous system (CNS), is well known as a regulator of neuronal plasticity and neurodevelopment. Such glutamate function is thought to be mediated primarily by signaling through glutamate receptors. Thus, it requires a tight regulation of extracellular glutamate levels and a fine-tuned homeostasis that, when dysregulated, has been associated with a wide range of central pathologies including neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. In the mammalian CNS, extracellular glutamate levels are controlled by a family of sodium-dependent glutamate transporters belonging to the solute carrier family 1 (SLC1) that are also referred to as excitatory amino acid transporters (EAATs). The presumed main function of EAATs has been best described in the context of synaptic transmission where EAATs expressed by astrocytes and neurons effectively regulate extracellular glutamate levels so that synapses can function independently. There is, however, increasing evidence that EAATs are expressed by cells other than astrocytes and neurons, and that they exhibit functions beyond glutamate clearance. In this review, we will focus on the expression and functions of EAATs in the myelinating cells of the CNS, oligodendrocytes. More specifically, we will discuss potential roles of oligodendrocyte-expressed EAATs in contributing to extracellular glutamate homeostasis, and in regulating oligodendrocyte maturation and CNS myelination by exerting signaling functions that have traditionally been associated with glutamate receptors. In addition, we will provide some examples for how dysregulation of oligodendrocyte-expressed EAATs may be involved in the pathophysiology of neurologic diseases.

     

胶质细胞谷氨酸转运体及其在脑缺血和脑缺血预适应中的变化

兴奋性氨基酸转运体(excitatory amino acid transporters,EAATs)是摄取细胞外液谷氨酸、保持细胞外谷氨酸低浓度的主要机制,已发现了五种EAATs,其中胶质细胞谷氨酸转运体在终止谷氨酸能神经传递、维持细胞外液谷氨酸浓度处于低水平方面发挥更重要作用。胶质细胞谷氨酸转运体的表达和功能受谷氨酸及其受体、垂体腺苷酸环化酶激活多肽、生长因子、内皮素、一氧化氮等许多因素的影响,其表达减少及功能降低与脑缺血损害的发生和发展密切相关,脑缺血预适应可通过调控其表达或改善其功能而诱导脑缺血耐受。     

Excitatory amino acid transporters: keeping up with glutamate

Excitatory amino acid transporters (EAATs) are the primary regulators of extracellular glutamate concentrations in the central nervous system. Among the five known human EAAT subtypes, the glial carriers, EAAT1 and EAAT2 have the greatest impact on clearance of glutamate released during neurotransmission. Studies of carriers expressed on neurons, Purkinje cells and photoreceptor cells (EAAT3, EAAT4 and EAAT5, respectively) suggest more subtle roles for these subtypes in regulating excitability and signalling. The data suggest that EAA transporters may influence glutamatergic transmission by regulating the amount of glutamate available to activate pre- and post-synaptic metabotropic receptors and by altering neuronal excitability through a transporter-associated anion conductance that is activated by carrier substrates. Recent studies on structural, mechanistic and physiological aspects of carrier function in a variety of model systems and organisms have led to surprising insights into how excitatory amino acid transporters shape cellular communication in the nervous system.     

高亲和力谷氨酸转运体结构和功能的研究进展

真核生物高亲和力谷氨酸转运体(excitatory amino acid transporters,EAATs)分为GLAST(EAAT1)、GLT-1(EAAT2)、EAAC1(EAAT3)、EAAT4和EAAT5等5个亚型.高亲和力谷氨酸转运体结构学的研究,揭示了谷氨酸转运体的跨膜拓扑结构、真核和原核生物EAATs结构的差异,以及在底物转运过程中的一些底物和协同转运离子的结合位点.其功能学的研究发现,EAATs在参与突触的传递,避免兴奋性氨基酸的毒性效应中发挥重要作用,同时也参与了对学习、记忆以及运动行为的调控.结合我们既往的工作,就近几年EAATs的结构和功能研究做一综述.     

Metabotropic glutamate receptor 3 protects neurons from glucose-induced oxidative injury by increasing intracellular glutathione concentration

High glucose concentrations cause oxidative injury and programmed cell death in neurons, and can lead to diabetic neuropathy. Activating the type 3 metabotropic glutamate receptor (mGluR3) prevents glucose-induced oxidative injury in dorsal root ganglion neurons co-cultured with Schwann cells. To determine the mechanisms of protection, studies were performed in rat dorsal root ganglion neuron-Schwann cell co-cultures. The mGluR3 agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate prevented glucose-induced inner mitochondrial membrane depolarization, reactive oxygen species accumulation, and programmed cell death, and increased glutathione (GSH) concentration in co-cultured neurons and Schwann cells, but not in neurons cultured without Schwann cells. Protection was diminished in neurons treated with the GSH synthesis inhibitor l-buthionine-sulfoximine, suggesting that mGluR-mediated protection requires GSH synthesis. GSH precursors and the GSH precursor GSH-ethyl ester also protected neurons from glucose-induced injury, indicating that GSH synthesis in Schwann cells, and transport of reaction precursors to neurons, may underlie mGluR-mediated neuroprotection. These results support the conclusions that activating glial mGluR3 protects neurons from glucose-induced oxidative injury by increasing free radical scavenging and stabilizing mitochondrial function, through increased GSH antioxidant defense.     

Amyloid-beta peptide decreases expression and function of glutamate transporters in nervous system cells

Glutamate is an essential excitatory neurotransmitter that regulates brain functions, and its activity is tightly regulated by glutamate transporters. Excess glutamate in the synaptic cleft and dysfunction of excitatory amino acid transporters have been shown to be involved in development of Alzheimer’s disease, but the precise regulatory mechanism is poorly understood. Using a D-[³H]-aspartic acid uptake assay, we found that Aβ_1-42 oligomers impaired glutamate uptake in astrocytes and neurons. In astrocytes, this process was accompanied by reduced expression of GLT-1 and GLAST as detected by Western blot and immunocytofluorescence. However, mRNA levels of EAATs detected by qPCR in astrocytes and neurons were not altered, which suggests that this process is post-translational. Co-localization analysis using immunocytofluorescence showed that ubiquitylation of GLT-1 significantly increased. Therefore, we hypothesized that Aβ_1-42 oligomers-induced endocytosis of astrocytic GLT-1 may be involved in ubiquitylation. In addition, Aβ_1-42 oligomers enhanced secretion of IL-1β, TNF-α, and IL-6 into culture supernatant, which may be correlated with an inflammatory response and altered EAATs expression or function in Alzheimer’s disease. These findings support the idea that dysregulation of the glutamatergic system may play a significant role in pathogenesis of Alzheimer’s disease. Furthermore, enhancing expression or function of EAATs in astrocytes and neurons might be a new therapeutic approach in treatment of Alzheimer’s disease.     

中枢神经系统谷氨酸转运体的研究进展

在中枢神经系统,谷氨酸转运体在谷氨酸一谷氨酰胺循环中发挥着重要作用。谷氨酸转运体有高亲和力转运体,即兴奋性氨基酸转运体（excitatory amino acid transporters,EAATs）和低亲和力转运体,即囊泡谷氨酸转运体（vesicular glutamate transporters,VGLUTs）两种类型。其中,VGLUTs的功能是特异地将突触囊泡外的谷氨酸转运进入突触囊泡内,它包括三个成员,分别是VGLUT1、VGLUT2和VGLUT3。一方面,VGLUT1和VGLUT2标记了所有的谷氨酸能神经元,是谷氦酸能神经元和它们轴突末端高度特异的标志;另一方面,VGLUT1标志着皮质一皮质投射,而VGLUT2则标志着丘脑一皮层投射,VGLUT3则位于抑制性突触末端。     

Na+-dependent high-affinity glutamate transport in macrophages

Excessive accumulation of glutamate in the CNS leads to excitotoxic neuronal damage. However, glutamate clearance is essentially mediated by astrocytes through Na+-dependent high-affinity glutamate transporters (excitatory amino acid transporters (EAATs)). Nevertheless, EAAT function was recently shown to be developmentally restricted in astrocytes and undetectable in mature astrocytes. This suggests a need for other cell types for clearing glutamate in the brain. As blood monocytes infiltrate the CNS in traumatic or inflammatory conditions, we addressed the question of whether macrophages expressed EAATs and were involved in glutamate clearance. We found that macrophages derived from human blood monocytes express both the cystine/glutamate antiporter and EAATs. Kinetic parameters were similar to those determined for neonatal astrocytes and embryonic neurons. Freshly sorted tissue macrophages did not possess EAATs, whereas cultured human spleen macrophages and cultured neonatal murine microglia did. Moreover, blood monocytes did not transport glutamate, but their stimulation with TNF-alpha led to functional transport. This suggests that the acquisition of these transporters by macrophages could be under the control of inflammatory molecules. Also, monocyte-derived macrophages overcame glutamate toxicity in neuron cultures by clearing this molecule. This suggests that brain-infiltrated macrophages and resident microglia may acquire EAATs and, along with astrocytes, regulate extracellular glutamate concentration. Moreover, we showed that EAATs are involved in the regulation of glutathione synthesis by providing intracellular glutamate. These observations thus offer new insight into the role of macrophages in excitotoxicity and in their response to oxidative stress.     

Modulation of neuronal glutathione synthesis by EAAC1 and its interacting protein GTRAP3-18

Glutathione (GSH) plays essential roles in different processes such as antioxidant defenses, cell signaling, cell proliferation, and apoptosis in the central nervous system. GSH is a tripeptide composed of glutamate, cysteine, and glycine. The concentration of cysteine in neurons is much lower than that of glutamate or glycine, so that cysteine is the rate-limiting substrate for neuronal GSH synthesis. Most neuronal cysteine uptake is mediated through the neuronal sodium-dependent glutamate transporter, known as excitatory amino acid carrier 1 (EAAC1). Glutamate transporters are vulnerable to oxidative stress and EAAC1 dysfunction impairs neuronal GSH synthesis by reducing cysteine uptake. This may start a vicious circle leading to neurodegeneration. Intracellular signaling molecules functionally regulate EAAC1. Glutamate transporter-associated protein 3-18 (GTRAP3-18) activation down-regulates EAAC1 function. Here, we focused on the interaction between EAAC1 and GTRAP3-18 at the plasma membrane to investigate their effects on neuronal GSH synthesis. Increased level of GTRAP3-18 protein induced a decrease in GSH level and, thereby, increased the vulnerability to oxidative stress, while decreased level of GTRAP3-18 protein induced an increase in GSH level in vitro. We also confirmed these results in vivo. Our studies demonstrate that GTRAP3-18 regulates neuronal GSH level by controlling the EAAC1-mediated uptake of cysteine.     

Glutamate induces release of glutathione from cultured rat astrocytes – a possible neuroprotective mechanism?

Glutamate is the major excitatory amino acid of the mammalian brain but can be toxic to neurones if its extracellular levels are not tightly controlled. Astrocytes have a key role in the protection of neurones from glutamate toxicity, through regulation of extracellular glutamate levels via glutamate transporters and metabolic and antioxidant support. In this study, we report that cultures of rat astrocytes incubated with high extracellular glutamate (5 mM) exhibit a twofold increase in the extracellular concentration of the tripeptide antioxidant glutathione (GSH) over 4 h. Incubation with glutamate did not result in an increased release of lactate dehydrogenase, indicating that the rise in GSH was not because of membrane damage and leakage of intracellular pools. Glutamate-induced increase in extracellular GSH was also independent of de novo GSH synthesis, activation of NMDA and non-NMDA glutamate receptors or inhibition of extracellular GSH breakdown. Dose–response curves indicate that GSH release from rat astrocytes is significantly stimulated even at 0.1 mM glutamate. The ability of astrocytes to increase GSH release in the presence of extracellular glutamate could be an important neuroprotective mechanism enabling neurones to maintain levels of the key antioxidant, GSH, under conditions of glutamate toxicity.     

Neuronal Expression of Splice Variants of “Glial” Glutamate Transporters in Brains Afflicted by Alzheimer’s Disease: Unmasking an Intrinsic Neuronal Property

Anomalies in glutamate homeostasis may contribute to the pathological processes involved in Alzheimer’s disease (AD). Glutamate released from neurons or glial cells is normally rapidly cleared by glutamate transporters, most of which are expressed at the protein level by glial cells. However, in some patho-physiological situations, expression of glutamate transporters that are normally considered to be glial types, appears to be evoked in populations of distressed neurons. This study analysed the expression of exon-skipping forms of the three predominant excitatory amino acid (glutamate) transporters (EAATs1-3) in brains afflicted with AD. We demonstrate by immunocytochemistry in temporal cortex, the expression of these proteins particularly in limited subsets of neurons, some of which appeared to be dys-morphic. Whilst the neuronal expression of the “glial” glutamate transporters EAAT1 and EAAT2 is frequently considered to represent the abnormal and ectopic expression of such transporters, we suggest this may be a misinterpretation, since neurons such as cortical pyramidal cells normally express abundant mRNA for these EAATs (but little if any EAAT protein expression). We hypothesize instead that distressed neurons in the AD brain can turn on the translation of pre-existent mRNA pools, or suppress the degradation of alternately spliced glutamate transporter protein, leading to the “unmasking” of, rather than evoked expression of “glial” glutamate transporters in stressed neurons. Special issue article in honor of Dr. Graham Johnston.     

A Mutation in Transmembrane Domain 7 (TM7) of Excitatory Amino Acid Transporters Disrupts the Substrate-dependent Gating of the Intrinsic Anion Conductance and Drives the Channel into a Constitutively Open State

In the mammalian central nervous system, excitatory amino acid transporters (EAATs) are responsible for the clearance of glutamate after synaptic release. This energetically demanding activity is crucial for precise neuronal communication and for maintaining extracellular glutamate concentrations below neurotoxic levels. In addition to their ability to recapture glutamate from the extracellular space, EAATs exhibit a sodium- and glutamate-gated anion conductance. Here we show that substitution of a conserved positively charged residue (Arg-388, hEAAT1) in transmembrane domain 7 with a negatively charged amino acid eliminates the ability of glutamate to further activate the anion conductance. When expressed in oocytes, R388D or R388E mutants show large anion currents that display no further increase in amplitude after application of saturating concentrations of Na⁺ and glutamate. They also show a substantially reduced transport activity. The mutant transporters appear to exist preferentially in a sodium- and glutamate-independent constitutive open channel state that rarely transitions to complete the transport cycle. In addition, the accessibility of cytoplasmic residues to membrane-permeant modifying reagents supports the idea that this substrate-independent open state correlates with an intermediate outward facing conformation of the transporter. Our data provide additional insights into the mechanism by which substrates gate the anion conductance in EAATs and suggest that in EAAT1, Arg-388 is a critical element for the structural coupling between the substrate translocation and the gating mechanisms of the EAAT-associated anion channel.     

Reticulon RTN2B regulates trafficking and function of neuronal glutamate transporter EAAC1

Excitatory amino acid transporters (EAATs) are the primary regulators of extracellular glutamate concentrations in the central nervous system. Their dysfunction may contribute to several neurological diseases. To date, five distinct mammalian glutamate transporters have been cloned. In brain, EAAC1 (excitatory amino acid carrier 1) is the primary neuronal glutamate transporter, localized on the perisynaptic membranes that are near release sites. Despite its potential importance in synaptic actions, little is known concerning the regulation of EAAC1 trafficking from the endoplasmic reticulum (ER) to the cell surface. Previously, we identified an EAAC1-associated protein, GTRAP3-18, an ER protein that prevents ER exit of EAAC1 when induced. Here we show that RTN2B, a member of the reticulon protein family that mainly localizes in the ER and ER exit sites interacts with EAAC1 and GTRAP3-18. EAAC1 and GTRAP3-18 bind to different regions of RTN2B. Each protein can separately and independently form complexes with EAAC1. RTN2B enhances ER exit and the cell surface composition of EAAC1 in heterologous cells. Expression of short interfering RNA-mediated knockdown of RTN2B decreases the EAAC1 protein level in neurons. Overall, our results suggest that RTN2B functions as a positive regulator in the delivery of EAAC1 from the ER to the cell surface. These studies indicate that transporter exit from the ER controlled by the interaction with its ER binding partner represents a critical regulatory step in glutamate transporter trafficking to the cell surface.     

High-density presynaptic transporters are required for glutamate removal from the first visual synapse

Reliable synaptic transmission depends not only on the release machinery and the postsynaptic response mechanism but also on removal or degradation of transmitter from the synaptic cleft. Accumulating evidence indicates that postsynaptic and glial excitatory amino acid transporters (EAATs) contribute to glutamate removal. However, the role of presynaptic EAATs is unclear. Here, we show in the mouse retina that glutamate is removed from the synaptic cleft at the rod to rod bipolar cell (RBC) synapse by presynaptic EAATs rather than by postsynaptic or glial EAATs. The RBC currents evoked by electrical stimulation of rods decayed slowly after pharmacological blockade of EAATs. Recordings of the evoked RBC currents from EAAT subtype-deficient mice and the EAAT-coupled anion current reveal that functional EAATs are localized to rod terminals. Model simulations suggest that rod EAATs are densely packed near the release site and that rods are equipped with an almost self-sufficient glutamate recollecting system.     

Effects of Ionotropic Excitatory Amino Acid Receptor Antagonists on Glutamate Transport and Transport-Mediated Changes in Extracellular Excitatory Amino Acids in the Rat Striatum

Abstract: This study examined the effects of intrastriatal administration of ionotropic excitatory amino acid receptor antagonists on biochemical markers of excitatory amino acid transmission in the rat striatum. High-affinity glutamate uptake was measured ex vivo on striatal homogenates 15 min after the local administration of either 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA receptor antagonist, or dl -2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA antagonist, at various doses (10–500 pmol injected). DNQX induced a dose-dependent increase in glutamate uptake rate, related to an increase in the V _max of the transport process, whereas no significant change in glutamate uptake was detected after AP5 administration. Similar results were obtained from animals subjected to excitotoxic lesion of striatal neurons by kainate administration 15 days before the injection of DNQX or AP5. In a parallel series of experiments using in vivo microdialysis we showed that DNQX (10⁻⁵ M ) in the dialysis probe diminished by ∼30–40% the increases in the concentrations of glutamate and aspartate elicited by l - trans -pyrrolidine-2,4-dicarboxylic acid (1 m M ). These data suggest that presynaptic glutamate transmission in the rat striatum may undergo facilitatory autoregulatory processes involving ionotropic non-NMDA receptors and highlight the view that transporters for glutamate may be potent regulatory sites for glutamatergic transmission.