Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “GLUTAMATES”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,117 records · Page 62Linked to original sources
PubMed2002

The origin and neuronal function of in vivo nonsynaptic glutamate.

Basal extracellular glutamate sampled in vivo is present in micromolar concentrations in the extracellular space outside the synaptic cleft, and neither the origin nor the function of this glutamate is known. This report reveals that blockade of glutamate release from the cystine-glutamate antiporter produced a significant decrease (60%) in extrasynaptic glutamate levels in the rat striatum, whereas blockade of voltage-dependent Na+ and Ca2+ channels produced relatively minimal changes (0-30%). This indicates that the primary origin of in vivo extrasynaptic glutamate in the striatum arises from nonvesicular glutamate release by the cystine-glutamate antiporter. By measuring [35S]cystine uptake, it was shown that similar to vesicular release, the activity of the cystine-glutamate antiporter is negatively regulated by group II metabotropic glutamate receptors (mGluR2/3) via a cAMP-dependent protein kinase mechanism. Extracellular glutamate derived from the antiporter was shown to regulate extracellular levels of glutamate and dopamine. Infusion of the mGluR2/3 antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate levels, and previous blockade of the antiporter prevented the APICA-induced rise in extracellular glutamate. This suggests that glutamate released from the antiporter is a source of endogenous tone on mGluR2/3. Blockade of the antiporter also produced an increase in extracellular dopamine that was reversed by infusing the mGluR2/3 agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxlylate, indicating that antiporter-derived glutamate can modulate dopamine transmission via mGluR2/3 heteroreceptors. These results suggest that nonvesicular release from the cystine-glutamate antiporter is the primary source of in vivo extracellular glutamate and that this glutamate can modulate both glutamate and dopamine transmission.

Animals↗
PubMed1996

New insights into the compartmentation of glutamate and glutamine in cultured rat brain astrocytes.

Studies from several groups have provided evidence that glutamate and glutamine are metabolized in different compartments in astrocytes. In the present study we measured the rates of 14CO2 production from U-[14C]glutamate and U-[14C]glutamine, and utilized both substrate competition experiments and the transaminase inhibitor aminooxyacetic acid (AOAA) to obtain more information about the compartmentation of these substrates in cultured rat brain astrocytes. The rates of oxidation of 1 mM glutamine and glutamate were 26.4 +/- 1.4 and 63.0 +/- 7.4 nmol/h/mg protein, respectively. The addition of 1 mM glutamate decreased the rate of oxidation of glutamine to 26.3% of the control rate, demonstrating that glutamate can effectively compete with the oxidation of glutamine by astrocytes. In contrast, the addition of 1 mM glutamine had little or no effect on the rate of oxidation of glutamate by astrocytes, demonstrating that the glutamate produced intracellularly from exogenous glutamine does not dilute the glutamate taken up from the media. The addition of 5 mM AOAA decreased the rate of 14CO2 production from glutamine to 29.2% of the control rate, consistent with earlier studies by our group. The addition of 5 mM AOAA decreased the rate of oxidation of concentrations of glutamate < or = 0.1 mM by approximately 50%, but decreased the oxidation of 0.5-1 mM glutamate by only approximately 20%, demonstrating that a substantial portion of glutamate enters the tricarboxylic acid (TCA) cycle via glutamate dehydrogenase (GDH) rather than transamination, and that as the concentration of glutamate increases the relative proportion entering the TCA cycle via GDH also increases. To determine if the presence of an amino group acceptor (i.e. a ketoacid) would increase the rate of metabolism of glutamate, pyruvate was added in some experiments. Addition of 1 mM pyruvate increased the rate of oxidation of glutamate, and the increase was inhibited by AOAA, consistent with enhanced entry of glutamate into the TCA cycle via transamination in the presence of pyruvate. Enzymatic studies showed that pyruvate increased the activity of mitochondrial aspartate aminotransferase (AAT). Overall, the data demonstrate that glutamate formed intracellularly from glutamine enters the TCA cycle primarily via transamination, but does not enter the same TCA cycle compartment as glutamate taken up from the extracellular milieu. In contrast, extracellular glutamate enters the TCA cycle in astrocytes via both transamination and GDH, and can compete with, or dilute, the oxidation of glutamate produced intracellularly from glutamine.

Aminooxyacetic Acid↗
PubMed1992

Characterization of glutamate efflux from preoptic area synaptosomes.

Treatment of ovariectomized rats in vivo with ovarian steroids has been found to influence the efflux of glutamate and gamma-aminobutyric acid from preoptic area synaptosomes incubated in vitro. Since these studies indicated a possible role of the glutamate carrier in steroid-modulated release of amino acids, the present studies examined the characteristics of efflux of glutamate and of the carrier system for glutamate in synaptosomes of the preoptic area derived from ovariectomized hormone-treated rats. The efflux of [3H]glutamate from preoptic area synaptosomes, was induced by glutamate and by the glutamate carrier agonist, D-aspartate; the putative glutamate carrier antagonist dihydrokainate failed to block this efflux. Dihydrokainate inhibited the uptake of glutamate but it was less effective than D-aspartate. The excitatory amino acid receptor agonists, N-methyl-D-aspartate and kainate were without effect while quisqualate modestly stimulated the efflux of [3H]glutamate. Efflux of [3H]glutamate, induced by glutamate itself or by D-aspartate was not blocked by the excitatory amino acid receptor antagonists, D-2-amino-5-phosphonovaleric acid, 6,7-dinitroquinoxaline-2,3-dione or kynurenate. Glutamate-induced efflux of [3H]glutamate did not require external Ca2+. Glutamate altered neither the basal nor the potassium-induced increases in the intrasynaptosomal concentration of Ca2+ as measured by the fura-2 method. Glutamate-induced efflux of [3H]glutamate was blocked by the putative chloride channel antagonist, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. It is concluded that the glutamate-induced efflux of [3H]glutamate in synaptosomes of the preoptic area is a carrier-mediated process that does not require activation of receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMedDate not supplied

Substrate-induced up-regulation of Na(+)-dependent glutamate transport activity.

Sodium-dependent transporters regulate extracellular glutamate in the CNS. Recent studies suggest that the activity of several different neurotransmitter transporters can be rapidly regulated by a variety of mechanisms. In the present study, we report that pre-incubation of primary 'astrocyte-poor' neuronal cultures with glutamate (100 microM) for 30 min nearly doubled the V(max) for Na(+)-dependent accumulation of L-[(3)H]-glutamate, but had no effect on Na(+)-dependent [(3)H]-glycine transport. Pre-incubation with glutamate also increased the net uptake of non-radioactive glutamate, providing evidence that the increase in accumulation of L-[(3)H]-glutamate was not related to an increase in intracellular glutamate and a subsequent increase in exchange of intracellular non-radioactive glutamate for extracellular radioactive glutamate. The glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate, and (1 S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid did not mimic the effect of pre-incubation with glutamate and the glutamate-induced increase was not blocked by receptor antagonists. However, compounds known to interact with the transporters, including L-aspartate, D-aspartate, L-(-)-threo-3-hydroxyaspartate (L-THA) and L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC), caused variable increases in transport activity and attenuated the increase induced by glutamate, suggesting that the increase is related to the interaction of glutamate with the transporters. Several studies were attempted to define the mechanism of this regulation. We found no evidence for increases in transporter synthesis or cell surface expression. Inhibitors of signaling molecules known to regulate other neurotransmitter transporters had no effect on this stimulation. Using a variety of cultures, evidence is provided to suggest that this substrate-induced up-regulation of glutamate transport is specific for the GLT-1 and GLAST subtypes and does not influence transport mediated by EAAC1. These studies suggest that the interaction of glutamate with some of the subtypes of glutamate transporters causes an increase in transport activity. Conceivably, this phenomenon provides an endogenous mechanism to increase the clearance of glutamate during periods of prolonged elevations in extracellular glutamate.

ATP-Binding Cassette Transporters↗
PubMed1998

Origins of the extracellular glutamate released during total metabolic blockade in the immature retina.

Previous studies have shown that complete blockade of metabolism in embryonic chick retina causes a time-dependent increase in the release of glutamate into the extracellular space. The present study examined the cellular source of this glutamate, i.e., neuronal and/or glial. Pure cultures of retinal neurons or glia were labeled for 10 min at 37 degrees C with [3H]acetate. Retinal glia, but not retinal neurons, were found to selectively and preferentially metabolize acetate, thus producing 3H-labeled amino acids in the glial compartment. This finding provides direct evidence to substantiate findings from several other laboratories that have indirectly determined the preferential metabolism of acetate by glia by using mixed neuronal/glial populations. To study the cellular source of glutamate released during total metabolic blockade, whole retina were prelabeled with [3H]acetate plus [U-14C]glucose (to label the neuronal compartment). Total metabolic blockade was instituted with a combination of iodoacetate (IOA) plus KCN, and the release of glutamate into the medium was followed at 5, 15, and 30 min. During total energy blockade, net extracellular glutamate was not elevated at 5 min [0.17 +/- 0.02 vs. 0.12 +/- 0.01 microM for treated vs. control retina (means +/- SEM), respectively], but was increased significantly at 15 (1.2 +/- 0.26 microM) and 30 min (2.6 +/- 0.22 microM). Total [3H]glutamate in the medium during IOA/KCN treatment was unchanged at 5 min, but was increased 1.5- and threefold above basal levels at 15 and 30 min, respectively. During the time when extracellular glutamate increased, the specific activity of [3H]glutamate remained fairly constant, 731 +/- 134 and 517 +/- 82 dpm/nmol (means +/- SEM) at 15 and 30 min, respectively. In contrast, 14C-labeled glutamate in the medium did not increase during IOA/KCN treatment and paralleled basal levels. Thus, the specific activity of 14C-labeled extracellular glutamate decreased from 309 +/- 87 dpm/nmol at 15 min to 42 +/- 8 dpm/nmol at 30 min. Prior loading of the tissue with 0.5 mM trans-pyrrolidine-2,4-dicarboxylate (t-PDC), a glutamate transport inhibitor, blocked 57% of the glutamate released at 30 min of IOA/KCN exposure, suggesting that reversal of an Na+-dependent glutamate transporter was a key contributor to the appearance of extracellular glutamate during energy deprivation. The increase in extracellular [3H]glutamate, constancy of the specific activity of extracellular [3H]glutamate, decrease in the specific activity of extracellular [14C]glutamate, and attenuation of release by prior loading with t-PDC indicate that glial pools of glutamate released via reversal of the transporter contribute significantly to the rise in extracellular glutamate after metabolic inhibition in this preparation.

Acetates↗
PubMed1989

Hepatocyte heterogeneity in glutamate metabolism and bidirectional transport in perfused rat liver.

1. The metabolic fate of infused [1-14C]glutamate was studied in perfused rat liver. The 14C label taken up by the liver was recovered to 85 +/- 2% as 14CO2 and [14C]glutamine. Whereas 14CO2 production accounted for about 70% of the [1-14C]glutamate taken up under conditions of low endogenous rates of glutamine synthesis, stepwise stimulation of glutamine synthesis by NH4Cl increased 14C incorporation into glutamine at the expense of 14CO2 production. Extrapolation to maximal rates of hepatic glutamine synthesis yielded an about 100% utilization of vascular glutamate taken up by the liver for glutamine synthesis. This was observed in both, antegrade and retrograde perfusions and suggests an almost exclusive uptake of glutamate into perivenous glutamine-synthetase-containing hepatocytes. 2. Glutamate was simultaneously taken up and released from perfused rat liver. At a near-physiological influent glutamate concentration (0.1 mM), the rates of unidirectional glutamate influx and efflux were similar (about 100 and 120 nmol g-1 min-1, respectively). 3. During infusion of [1-14C]oxoglutarate (50 microM), addition of glutamate (2 mM) did not affect hepatic uptake of [1-14C]oxoglutarate. However, it increased labeled glutamate release from the liver about 10-fold (from 9 +/- 2 to 86 +/- 20 nmol g-1 min-1; n = 4), whereas 14CO2 production from labeled oxoglutarate decreased by about 40%. This suggests not only different mechanisms of oxoglutarate and glutamate transport across the plasma membrane, but also points to a glutamate/glutamate exchange. 4. Oxoglutarate was recently shown to be taken up almost exclusively by perivenous glutamine-synthetase-containing hepatocytes [Stoll, B & Häussinger, D. (1989) Eur. J. Biochem. 181, 709-716] and [1-14C]oxoglutarate (9 microM) was used to label selectively the intracellular glutamate pool in this perivenous cell population. The specific radioactivity of this intracellular (perivenous) glutamate pool was assessed by measuring the specific radioactivity of newly synthesized glutamine which is continuously released from these cells into the perfusate. Comparison of the specific radioactivities of glutamine and glutamate released from perivenous cells indicates that about 60% of total glutamate release from the liver is derived from the perivenous glutamine-synthetase-containing cell population. Following addition of unlabeled glutamate (0.1 mM), unidirectional glutamate efflux from perivenous cells increased from about 30 to 80 nmol g-1 min-1, whereas glutamate efflux from non-perivenous (presumably periportal) hepatocytes remained largely unaltered (i.e. 20-30 nmol g-1 min-1). 5. It is concluded that, in the intact liver, vascular glutamate is almost exclusively taken up by the small perivenous hepatocyte population containing glutamine synthetase.

Animals↗
PubMed1972

Uptake and release of glutamate in cerebral-cortex slices from the rat.

1. Cerebral-cortex slices from rat brain, loaded with labelled l-glutamate as a result of aerobic incubation with labelled glucose, lost less than 15% of this glutamate on subsequent incubation in the presence of unlabelled glucose and l-glutamate. This indicates that very little exchange occurs between extracellular l-glutamate and glutamate accumulated in the neurons as a result of glucose metabolism. 2. Slices, loaded with labelled l-glutamate as a result of aerobic incubation in a medium containing unlabelled glucose and labelled l-glutamate, lost more than half of this glutamate on subsequent incubation in the presence of unlabelled l-glutamate. This indicates that exchange occurs between extracellular glutamate and glutamate accumulated in brain slices as a result of its uptake from the incubation medium. 3. Evidence was obtained suggesting that only a part of the glutamate, accumulated in brain slices as a result of its uptake from an incubation medium containing both glucose and l-glutamate, entered the neurons; apparently almost all the rest entered the glia. 4. It is concluded that the slices contain a pool of glutamate, derived from glucose and located in the neurons, which is poorly exchangeable with extracellular glutamate, and another pool of glutamate, derived from extracellular glutamate and located in the glia, which is freely exchangeable with extracellular glutamate.

Animals↗
PubMed2002

Net interorgan transport of L-glutamate in rats occurs via the plasma, not via erythrocytes.

Glutamate is concentrated within RBC, but this intracellular glutamate is often ignored in studies of glutamate metabolism in vivo. The objective of this work was to determine the size of the plasma and cellular glutamate pools in rat blood and to clarify the role of RBC in the interorgan transport of glutamate. Approximately 20% of whole-blood glutamate was associated with isolated RBC membranes, but this was easily removed by washing with high salt solutions. Arterial plasma glutamate levels were relatively stable and did not show marked differences with starvation, streptozotocin diabetes or feeding 60% casein diets. In rats fed 5% casein, the plasma glutamate level was slightly higher (P < 0.05) than in most other groups. In contrast, RBC glutamate levels showed considerable variation. In rats consuming 5% casein, cellular glutamate levels were approximately 100% higher (P < 0.05) than in control, starved, diabetic or 20 or 60% casein-fed rats. Cellular glutamate levels were also higher (P < 0.05) in rats fed 60% casein than in those consuming 20% casein or the control diet. Rat erythrocytes in vitro did not take up or release free glutamate, confirming that they do not possess a glutamate transporter. Arteriovenous difference measurements across the portal drained viscera indicated a net glutamate release into the portal vein in control, 60% casein-fed and diabetic rats. In all cases, the net change in blood glutamate across the tissue occurred via the plasma, with no change in cellular glutamate levels. Therefore analyses of glutamate metabolism in rats in vivo may be made confidently using measurements of either whole-blood or plasma glutamate concentrations.

Animals↗
PubMed1973

Glutamate transport in Escherichia coli K-12: nonidentity of carriers mediating entry and exit.

The exit of glutamate from Escherichia coli K-12 cells preloaded with the radioactive amino acid and its relation to the reaction of entry were studied. Experiments with cells preloaded to different intracellular concentrations of radioactive glutamate confirmed our earlier conclusion that glutamate exit was a first-order reaction. l-Glutamate, competitive inhibitors of glutamate uptake (d-glutamate and l-glutamate-gamma-methyl ester), noncompetitive inhibitors of glutamate uptake (l-serine and l-alanine), and the energy poison NaN(3) all accelerated glutamate exit 2.8-fold. No additive effect was observed in the presence of NaN(3) together with l-glutamate. Preloading with cold l-glutamate did not increase the rate of uptake of radioactive glutamate. It is concluded that the acceleration of glutamate exit in the presence of l-glutamate in the medium is not due to exchange diffusion and that l-glutamate and azide affect exit indirectly by preventing recapture. Sucrose, 25%, slowed down glutamate exit by a factor of about 4.7 and increased the steady-state level of glutamate accumulation to about the same extent. Increasing the intracellular K(+) concentration enhanced glutamate uptake but did not affect the half-time of exit. It is concluded that separate carriers are most probably involved in mediating the entry and exit reactions.

Alanine↗
PubMed1995

Adenosine pre- and postsynaptic modulation of glutamate-dependent calcium activity in hypothalamic neurons.

1. Within the hypothalamus, adenosine has been reported to influence temperature regulation, sleep homeostasis, and endocrine secretions. The effects of adenosine on hypothalamic neurons have not been studied at the cellular level. Adenosine (5 nM-30 microM) showed no influence on intracellular Ca2+ or electrical activity in the presence of glutamate receptor antagonists D-2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2,3-dione; consequently, we examined the role of adenosine in modulating the activity of glutamate in cultured hypothalamic neurons (n > 1,700) with fura-2 Ca2+ digital imaging and whole cell patch-clamp electrophysiology in the absence of glutamate receptor block. 2. When glutamate receptors were not blocked, adenosine (1-30 microM) and the selective adenosine A1 receptor agonist N6-cyclopentyl adenosine (CPA; 5 nM-1 microM) caused a large reduction in intracellular Ca2+ and electrical activity, suggesting that glutamate neurotransmission was critical for an effect of adenosine to be detected. Neuronal Ca2+ levels were reversibly depressed by CPA (50 nM), with a maximum depression of 90%, and these effects were blocked by coadministration of the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). 3. Ca2+ levels in immature neurons before the time of synaptogenesis were not affected by adenosine. Adenosine A1 receptor activation suppressed glutamate-mediated Ca2+ activity in neurons in vitro 8 to 73 days. 4. Adenosine (1 or 10 microM) caused a hyperpolarization of membrane potential and a reduction of large postsynaptic potentials arising from endogenously released glutamate. The administration of low concentrations of CPA (5 nM) decreased the frequency of glutamate-mediated, neuronally synchronized Ca2+ transients and the frequency of postsynaptic potentials. 5. To compare the relative effects of adenosine on hypothalamic neurons with cells from other brain regions, we assayed the effects of CPA on glutamate-mediated Ca2+ in hippocampal and cortical cultures. CPA (50 nM) reversibly depressed glutamate-mediated Ca2+ rises in hypothalamic neurons by 35%, compared with 54% in hippocampal neurons and 46% in cortical neurons. 6. If it does play a functional role, adenosine should be released by hypothalamic cells. In some neurons the adenosine A1 receptor antagonists cyclopentyltheophylline or DPCPX caused an increase in intracellular Ca2+, suggesting that adenosine was secreted by hypothalamic cells, tonically depressing glutamate-enhanced neuronal Ca2+. 7. To determine whether adenosine could exert a postsynaptic effect, we coapplied it with glutamate agonists in the presence of tetrodotoxin. Within subpopulations of hypothalamic neurons, adenosine and CPA either inhibited (18% of total neurons) or potentiated (6% of total neurons) responses to glutamate, N-methyl-D-aspartate, and kainate by > or = 20%. 8. In contrast to the modest effects found in neurons, responses of hypothalamic astrocytes to the application of glutamate or the metabotropic glutamate receptor agonist (+/-)-trans-1-amino-1,3-cyclopentanedicarboxylic acid were strongly potentiated by adenosine (mean +225%) and CPA. 9. Together, these findings suggest that adenosine exerts a major presynaptic effect and a minor postsynaptic effect in the modulation of glutamate neurotransmission in the hypothalamus, where it can play a significant role in blocking a large part of the glutamate-induced Ca2+ rise. In the absence of glutamate transmission, adenosine has relatively little effect on either neuronal intracellular Ca2+ or electrical activity.

Adenosine↗
PubMed1996

Dopamine enhancement and depression of glutamate-regulated calcium and electrical activity in hypothalamic neurons.

1. The neurotransmitter dopamine is found throughout the hypothalamus both in cell bodies and in axons originating from intra- and extrahypothalamic sources. To study the mechanisms of action of dopamine on cultured rat hypothalamic neurons, particularly in relation to Ca2+ regulation, we used Ca2+ digital imaging with fura-2 and whole cell patch-clamp recording. We focused on the modulatory actions of dopamine on glutamate. 2. Dopamine administration had little or no independent effect on intracellular Ca2+. However, in the presence of tetrodotoxin to block action potentials and action-potential-dependent transmitter release, dopamine (10 microM for 2-3 min) caused an increase in glutamate-evoked Ca2+ rises in 22% of 64 neurons and depressed glutamate-evoked Ca2+ rises in an equal number of neurons. Shorter exposure to dopamine reduced the number of responding cells. 3. Dopamine application to neurons with an elevated Ca2+ due to synaptic release of glutamate (in the absence of tetrodotoxin) generally caused a decrease in Ca2+ levels (40% of 106 neurons), but sometimes increased cytosolic Ca2+ (10% of 106 neurons). That dopamine influenced cells differently in conditions of spontaneous activity compared with evoked activity may be due to dopamine effects on presynaptic receptors detected under conditions of ongoing synaptic release of glutamate. 4. Dopamine modulation of glutamate responses was detected at early stages of neuronal development (embryonic day 18 after 2 days in vitro) and also after 60 days in vitro. 5. The D1, D2, and D3 dopamine receptor agonists SKF38393, quinpirole, and 7-OH-DPAT (+/- 7 hydroxy-dipropylaminotetralin) caused a reduction in Ca2+ levels raised by endogenous glutamate release or evoked by exogenous glutamate application. 6. To block the actions of dopamine released by hypothalamic neurons, D1 and D2 dopamine receptor antagonists were used. As with dopamine, dopamine antagonists had no effect on intracellular Ca2+ during glutamate receptor blockade. In the absence of glutamate receptor block, the D1 antagonist SCH23390 (1 microM) reduced Ca2+ in responding cells; in contrast, the D2 antagonist eticlopride (1 microM) generated a delayed increase in Ca2+ levels. 7. Dopamine is known to activate second messengers through G proteins independent of changes in membrane potential or input resistance. Whole cell recording was used to demonstrate that, parallel to the modulation of Ca2+, dopamine exerted a dramatic change in glutamate-mediated electrical activity, generally depressing activity and hyperpolarizing the membrane potential (8 of 15 neurons). In a smaller number of neurons (5 of 15), dopamine enhanced glutamate-mediated excitatory activity. 8. Dopamine-evoked changes in membrane potential were in part mediated through modulation of glutamate actions. Dopamine depressed glutamate-evoked currents in a dose-dependent fashion, with Hill slopes in individual neurons ranging from 0.3 to 0.6. Dopamine could also evoke a direct hyperpolarizing action on hypothalamic neurons in the presence of tetrodotoxin or glutamate receptor blockers, at least in part by opening K+ channels. 9. Glutamate plays an important role as a primary excitatory transmitter within the hypothalamus. Our data support the hypothesis that a major mechanism of dopamine's influence on hypothalamic neurons involves the modulation of glutamate's excitatory action, mostly by inhibition. This is consistent with the hypothesis that modulation of glutamate activity may be an important mechanism of dopamine action throughout the nervous system.

Action Potentials↗
PubMed1965

CONTROL OF GLUTAMATE OXIDATION IN BRAIN AND LIVER MITOCHONDRIAL SYSTEMS.

1. Glutamate oxidation in brain and liver mitochondrial systems proceeds mainly through transamination with oxaloacetate followed by oxidation of the alpha-oxoglutarate formed. Both in the presence and absence of dinitrophenol in liver mitochondria this pathway accounted for almost 80% of the uptake of glutamate. In brain preparations the transamination pathway accounted for about 90% of the glutamate uptake. 2. The oxidation of [1-(14)C]- and [5-(14)C]-glutamate in brain preparations is compatible with utilization through the tricarboxylic acid cycle, either after the formation of alpha-oxoglutarate or after decarboxylation to form gamma-aminobutyrate. There is no indication of gamma-decarboxylation of glutamate. 3. The high respiratory control ratio obtained with glutamate as substrate in brain mitochondrial preparations is due to the low respiration rate in the absence of ADP: this results from the low rate of formation of oxaloacetate under these conditions. When oxaloacetate is made available by the addition of malate or of NAD(+), the respiration rate is increased to the level obtained with other substrates. 4. When the transamination pathway of glutamate oxidation was blocked with malonate, the uptake of glutamate was inhibited in the presence of ADP or ADP plus dinitrophenol by about 70 and 80% respectively in brain mitochondrial systems, whereas the inhibition was only about 50% in dinitrophenol-stimulated liver preparations. In unstimulated liver mitochondria in the presence of malonate there was a sixfold increase in the oxidation of glutamate by the glutamate-dehydrogenase pathway. Thus the operating activity of glutamate dehydrogenase is much less than the ;free' (non-latent) activity. 5. The following explanation is put forward for the control of glutamate metabolism in liver and brain mitochondrial preparations. The oxidation of glutamate by either pathway yields alpha-oxoglutarate, which is further metabolized. Since aspartate aminotransferase is present in great excess compared with the respiration rate, the oxaloacetate formed is continuously removed by the transamination reaction. Thus alpha-oxoglutarate is formed independently of glutamate dehydrogenation, and the question is how the dehydrogenation of glutamate is influenced by the continuous formation of alpha-oxoglutarate. The results indicate that a competition takes place between the alpha-oxoglutarate-dehydrogenase complex and glutamate dehydrogenase, probably for NAD(+), resulting in preferential oxidation of alpha-oxoglutarate.

Adenine Nucleotides↗
PubMed1984

The uptake and release of glutamate at the crayfish neuromuscular junction.

The abdominal slow flexor muscle of the crayfish (Cambarus clarkii) was pre-incubated in deuterium-labelled glutamate (glutamate-d5) solution, and the release of glutamate, glutamate-d5 and aspartate in the bath solution was measured by mass fragmentography. The glutamate-d5 was taken up into the preparation and released together with endogenous glutamate. The resting release of glutamate-d5 was 8.2 +/- 0.96 pmol/10 min after the incubation with 0.5 mM-glutamate-d5, and those of endogenous glutamate and aspartate were 7.4 +/- 1.19 and 2.8 +/- 0.81 pmol/10 min, respectively. The release of glutamate-d5 was not significantly increased by nerve stimulation, while that of endogenous glutamate was increased by about 9.7 pmol/10 min above the resting release. The application of high-K solution induced a net increase of 1.9 pmol/10 min in glutamate-d5 release, 6.1 pmol/10 min in endogenous glutamate release and 7.9 pmol/10 min in aspartate release. The relative amount of glutamate-d5 accumulated into the preparation during pre-incubation with 0.5 mM-glutamate-d5 was a few per cent of the endogenous glutamate in the preparation. It is concluded that the exogenous glutamate is taken up mainly into a compartment which differs from that related to nerve evoked transmitter release and that high-K solution has effects on the amino acid release which differ from those of nerve stimulation.

Animals↗
PubMed1991

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.

Ambystoma↗
PubMed1994

Glutamate metabolism in fetus and placenta of late-gestation sheep.

Glutamate is produced by the fetal liver and taken up by the placenta. To explore the functional meaning of this exchange, the disposal rate (DR), clearance, conversion to glutamine, and decarboxylation rate of fetal plasma glutamate were studied at 129 +/- 2 days of gestation in seven fetal lambs infused via a systemic vein with L-[2,3,3,4,4-2H5]glutamate and L-[1-14C]glutamate. In two experiments, L-[1-13C]glutamate was also infused. The mean glutamate DR and clearance were 11.9 +/- 1.3 mumol.min-1.kg-1 and 200 +/- 8 ml.min-1.kg-1, respectively. The placenta extracted 88.5 +/- 0.8% of the tracer glutamate carried by the umbilical circulation and contributed to 61.3 +/- 3.2% of the glutamate DR. Most of the 14C infused as L-[1-14C]glutamate was converted to 14CO2: 37 +/- 4% by the fetus and 41 +/- 6% by the placenta. Of the labeled glutamate taken up by the placenta, 6.2 +/- 1.5% was returned to the fetus as glutamine. The glutamine-to-glutamate enrichment ratio in fetal arterial plasma was 0.066 +/- 0.008. We conclude that fetal plasma glutamate has an exceptionally high clearance because the flux of glutamate into the placenta is virtually equal to umbilical glutamate delivery rate. The main pathway of fetal plasma glutamate disposal is oxidation by placental and fetal tissues. Placental conversion of glutamate to fetal glutamine is a relatively small component of the placental metabolism of fetal glutamate.

Animals↗
PubMed1990

Patterns of glutamate immunoreactivity in the goldfish retina.

Postembedding silver-intensified immunogold procedures reveal high levels of glutamate immunoreactivity in "vertical" elements of the goldfish retina: (1) Red-sensitive and green-sensitive cones display strong glutamate immunoreactivity, especially in their synaptic terminals, but blue-sensitive cones are poorly immunoreactive. (2) All type Mb (on-center) and Ma (off-center) mixed rod-cone bipolar cells and all identifiable cone bipolar cells are highly glutamate immunoreactive. We find no evidence for bipolar cells that lack glutamate immunoreactivity. (3) The majority of the somas in the ganglion cell layer and certain large cells of the amacrine cell layer resembling displaced ganglion cells are strongly glutamate immunoreactive. (4) Despite their high affinity symport of acidic amino acids, the endogenous levels of glutamate in Müller's cells are among the lowest in the retina. (5) GABAergic neurons possess intermediate levels of glutamate immunoreactivity. Quantitative immunocytochemistry coupled with digital image analysis allows estimates of intracellular glutamate levels. Photoreceptors and bipolar and ganglion cells contain from 1 to 10 mM glutamate. The bipolar and ganglion cell populations maintain high intracellular glutamate concentrations, averaging about 5 mM, whereas red-sensitive and green-sensitive cones apparently maintain lower levels. Importantly, photoreceptor glutamate levels are extremely volatile, and in vitro maintenance is required to preserve cone glutamate immunoreactivity in the goldfish. GABAergic horizontal and amacrine cells contain about 0.3-0.7 mM glutamate, which matches the values predicted from the Km of glutamic acid decarboxylase. Müller's cells and non-GABAergic amacrine cells contain less than 0.1 mM glutamate. Though Müller's cells are known to possess potent glutamate symport, they clearly possess equally potent mechanisms for maintaining low intracellular glutamate concentrations.

Animals↗
PubMedDate not supplied

Responses of neurons in the primate taste cortex to the glutamate ion and to inosine 5'-monophosphate.

To investigate the neural encoding of glutamate taste in the primate, recordings were made from taste responsive neurons in the cortical taste areas in macaques. Most of the neurons were in the orbitofrontal cortex taste area, with a small number in adjacent taste areas. First, it was shown that single neurons that had their best responses to sodium glutamate also had good responses to glutamic acid. The correlation between the responses to these two tastants was higher than between any other pair of tastants, which included glucose (sweet), sodium chloride (salty), HCl (sour), and quinine HCl (bitter). Accordingly, the responsiveness to glutamic acid clustered with the response to monosodium glutamate in a cluster analysis with this set of stimuli, and glutamic acid was close to sodium glutamate in a space created by multidimensional scaling. Second, it was shown that the responses of these neurons to the nucleotide umami tastant inosine 5'-monophosphate were more correlated with their responses to monosodium glutamate than to any prototypical tastant. Third, concentration response curves showed that concentrations of monosodium glutamate as low as 0.001 M were just above threshold for some of these neurons. Fourth, neurons have not yet been found in this cortical region that showed synergism of monosodium glutamate and the nucleotide inosine 5'-monophosphate: it was shown that mixtures of 0.0001 M inosine 5'-monophosphate with different concentrations (0.001, 0.01, and 0.1 M) of monosodium glutamate did not have a greater effect than the monosodium glutamate alone. Fifth, some neurons in the orbitofrontal region, which responded to monosodium glutamate and other food tastes, decreased their responses after feeding with monosodium glutamate to behavioural satiety. In some cases this reduction was sensory-specific. These findings show that the taste neurons activated by monosodium glutamate can also be activated by other umami tastants, including glutamic acid and the nucleotide inosine 5'-monophosphate. The responses to these umami tastants were more similar to each other than to any of the other prototypical tastants, providing evidence that in this system umami is encoded differently from the other tastants. Moreover, the findings with these tastants provide additional evidence that the responses to monosodium glutamate are not due just to activation of a sodium taste channel.

Animals↗
PubMed2005

Tonic release of glutamate by a DIDS-sensitive mechanism in rat hippocampal slices.

Tonic release of glutamate into the extracellular space of the hippocampus and striatum is non-vesicular, and has been attributed largely to a cystine-glutamate exchanger which is blockable by the glutamate analogue (S)-4-carboxyphenylglycine (CPG). Tonic glutamate release may be functionally important: modulation of this release in the striatum has been suggested to underlie relapse in the use of cocaine. We monitored tonic glutamate release in area CA1 of hippocampal slices by measuring the glutamate receptor-mediated current evoked in pyramidal cells on block of Na(+)-dependent glutamate uptake with dl-threo-beta-benzyloxyaspartate (TBOA). Superfused cystine increased tonic glutamate release, and this increase was blocked by CPG, but CPG did not affect tonic glutamate release in the absence of superfused cystine. Tonic glutamate release was not affected by blocking gap junctional hemichannels with 18alpha-glycyrrhetinic acid, blocking ATP receptors with pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), blocking Ca(2)(+)-dependent exocytosis from neurones with Cd(2)(+) or bafilomycin, blocking Ca(2)(+)-dependent release from glia with indomethacin, or blocking anion channels with 5-nitro-2-(3-phenylpropyl amino) benzoic acid (NPPB) or tamoxifen. However tonic glutamate release was reduced by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), and was potentiated by inhibiting astrocytic conversion of glutamate to glutamine with methionine sulfoximine. These data suggest that although cystine-glutamate exchange is present in the hippocampus it does not generate significant tonic release of glutamate when the extracellular [cystine] is at a physiological level, and that tonic glutamate release is at least partly from astrocytes and is mediated by a DIDS-sensitive mechanism. Theoretical calculations suggest that a significant fraction of tonic glutamate release in hippocampal slices could occur via diffusion of glutamate across lipid membranes.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗