Glutamine and alpha-ketoglutarate uptake and metabolism by nerve terminal enriched material from mouse cerebellum

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain β-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

Acute and sub-acute metabolic change in different brain regions induced by moderate blunt traumatic brain injury

The objective of the study was to observe the effect of moderate closed-head injury on hippocampal, thalamic, and striatal tissue metabolism with time. Closed head injury is responsible for metabolic changes. These changes can be permanent or temporary, depending on the injury's impact. For the experiment, 20 rats were randomly divided into four groups, each containing five animals. Animals were subjected to injury using a modified Marmarou's weight drop device; hippocampal, thalamic, and striatal tissue samples were collected after 1 day, 3 days, and 7 days of injury. NMR spectra were acquired following sample processing. Changes in myo-inositol, creatine, glutamate, succinate, lactate, and N-acetyl aspartic acid in hippocampal tissues were observed at day 3 PI. The tyrosine level in the hippocampus was altered at day 7 PI. While thalamic and striatal tissue samples showed altered levels of branched-chain amino acids and myo-inositol at day 1PI. Taurine, gamma amino butyric acid (GABA), choline, and alpha keto-glutarate levels were found to be significantly altered in striatal tissues at days 1 and 3PI. Acetate and GABA levels were altered in the thalamus on day 1 PI. The choline level in the thalamus was found to alter at all-time points after injury. The alteration in these metabolites may be due to the alteration in their respective pathways. Neurotransmitter and energy metabolism pathways were found to be altered in all three brain regions after TBI. This study may help better understand the effect of injury on the metabolic balance of a specific brain region and recovery.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Transendothelial Transport and Its Role in Therapeutics

Present review paper highlights role of BBB in endothelial transport of various substances into the brain. More specifically, permeability functions of BBB in transendothelial transport of various substances such as metabolic fuels, ethanol, amino acids, proteins, peptides, lipids, vitamins, neurotransmitters, monocarbxylic acids, gases, water, and minerals in the peripheral circulation and into the brain have been widely explained. In addition, roles of various receptors, ATP powered pumps, channels, and transporters in transport of vital molecules in maintenance of homeostasis and normal body functions have been described in detail. Major role of integral membrane proteins, carriers, or transporters in drug transport is highlighted. Both diffusion and carrier mediated transport mechanisms which facilitate molecular trafficking through transcellular route to maintain influx and outflux of important nutrients and metabolic substances are elucidated. Present review paper aims to emphasize role of important transport systems with their recent advancements in CNS protection mainly for providing a rapid clinical aid to patients. This review also suggests requirement of new well-designed therapeutic strategies mainly potential techniques, appropriate drug formulations, and new transport systems for quick, easy, and safe delivery of drugs across blood brain barrier to save the life of tumor and virus infected patients.

Comparative study of various delivery methods for the supply of alpha-ketoglutarate to the neural cells for tissue engineering

Delivery of growth factors or bioactive molecules plays an important role in tissue engineering, as the duration to which these are supplied can modulate the cell fate. Thus, the delivery method plays an important role, and the same is presented in this work wherein the exogenous supply of alpha-ketoglutarate (α-KG) gave better results for fast proliferating cells as compared to delivery by microspheres or microspheres incorporated scaffolds which can be used while culturing slow growing cells. All these studies were performed in two dimensional (2D) and three dimensional (3D) setups in which chitosan-gelatin-polypyrrole has been used as 3-D scaffolds. Chitosan and gelatin microspheres alone as well as incorporated in the cryogels were characterized. MTT assay done using neuro-2a cell line showed approximately 42% and 70% increment in cellular proliferation when gelatin and chitosan microspheres were added in a 3-D setup, respectively, as compared to the control. Biochemical analysis of ammonia showed 6-fold reductions in ammonia level in a 3-D setup compared to the control. We also studied the synthesis of a neurotransmitter-like glutamate and found that its concentration increased up to 0.25 mg/ml when the microspheres were added exogenously in a 3-D system.

Is there in vivo evidence for amino acid shuttles carrying ammonia from neurons to astrocytes?

The high in vivo flux of the glutamate/glutamine cycle puts a strong demand on the return of ammonia released by phosphate activated glutaminase from the neurons to the astrocytes in order to maintain nitrogen balance. In this paper we review several amino acid shuttles that have been proposed for balancing the nitrogen flows between neurons and astrocytes in the glutamate/glutamine cycle. All of these cycles depend on the directionality of glutamate dehydrogenase, catalyzing reductive glutamate synthesis (forward reaction) in the neuron in order to capture the ammonia released by phosphate activated glutaminase, while catalyzing oxidative deamination of glutamate (reverse reaction) in the astrocytes to release ammonia for glutamine synthesis. Reanalysis of results from in vivo experiments using (13)N and (15)N labeled ammonia and (15)N leucine in rats suggests that the maximum flux of the alanine/lactate or branched chain amino acid/branched chain amino acid transaminase shuttles between neurons and astrocytes are approximately 3-5 times lower than would be required to account for the ammonia transfer from neurons to astrocytes needed for glutamine synthesis (amide nitrogen) to sustain the glutamate/glutamine cycle. However, in the rat brain both the total ammonia fixation rate by glutamate dehydrogenase and the total branched chain amino acid transaminase activity are sufficient to support a branched chain amino acid/branched chain keto acid shuttle, as proposed by Hutson and coworkers, which would support the de novo synthesis of glutamine in the astrocyte to replace the ~20 % of neurotransmitter glutamate that is oxidized. A higher fraction of the nitrogen needs of total glutamate neurotransmitter cycling could be supported by hybrid cycles in which glutamate and tricarboxylic acid cycle intermediates act as a nitrogen shuttle. A limitation of all in vivo studies in animals conducted to date is that none have shown transfer of nitrogen for glutamine amide synthesis, either as free ammonia or via an amino acid from the neurons to the astrocytes. Future work will be needed, perhaps using methods for selectively labeling nitrogen in neurons, to conclusively establish the rate of amino acid nitrogen shuttles in vivo and their coupling to the glutamate/glutamine cycle.

Proposed cycles for functional glutamate trafficking in synaptic neurotransmission

To date, the glutamate-glutamine cycle has been the dominant paradigm for understanding the coordinated, compartmentalized activities of phosphate-activated glutaminase (PAG) and glutamine synthetase (GS) in support of functional glutamate trafficking in vivo. However, studies in cell cultures have repeatedly challenged the notion that functional glutamate trafficking is accomplished via the glutamate-glutamine cycle alone. The present study introduces and elaborates alternative cycles for functional glutamate trafficking that integrate glucose metabolism, glutamate anabolism, transport, and catabolism, and trafficking of TCA cycle intermediates from astrocytes to presynaptic neurons. Detailed stoichiometry for each of these alternative cycles is established by strict application of the principle of conservation of atomic species to cytosolic and mitochondrial compartments in both presynaptic neurons and astrocytes. In contrast to the glutamate-glutamine cycle, which requires ATP, but not necessarily oxidative metabolism, to function, cycles for functional glutamate trafficking based on intercellular transport of TCA cycle intermediates require oxidative processes to function. These proposed alternative cycles are energetically more efficient than, and incorporate an inherent mechanism for transporting nitrogen from presynaptic neurons to astrocytes in support of the coordinated activities of PAG and GS that is absent in, the glutamate-glutamine cycle. In light of these newly elaborated alternative cycles, it is premature to presuppose that functional glutamate trafficking in synaptic neurotransmission in vivo is sustained by the glutamate-glutamine cycle alone.

Functional and molecular identification of sodium-coupled dicarboxylate transporters in rat primary cultured cerebrocortical astrocytes and neurons

Na+-coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na+-coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na+-coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na+-dependent succinate and citrate transport in primary cultures of astrocytes and neurons from rat cerebral cortex. Uptake of succinate was Na+ dependent, Li+ sensitive and saturable with a Michaelis constant (Kt) value of 28.4 microM in rat astrocytes. Na+ activation kinetics revealed that the Na+ to succinate stoichiometry was 3:1 and the concentration of Na+ necessary for half-maximal transport was 53 mM. Although uptake of citrate in astrocytes was also Na+ dependent and saturable, its Kt value was significantly higher (approximately 1.2 mM) than that of succinate. Unlabeled succinate (2 mM) inhibited Na+-dependent [14C]succinate (18 microM) and [14C]citrate (4.5 microM) transport completely, whereas unlabeled citrate inhibited Na+-dependent [14C]succinate uptake more weakly. Interestingly, N-acetyl-L-aspartate, which is the second most abundant amino acid in the nervous system, also completely inhibited Na+-dependent succinate transport in rat astrocytes. The inhibition constant (Ki) for the inhibition of [14C]succinate uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 microM respectively. In primary cultures of neurons, uptake of citrate was also Na+ dependent and saturable with a Kt value of 16.2 microM, which was different from that observed in astrocytes, suggesting that different Na+-dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.

Functional characterization of Na+-coupled citrate transporter NaC2/NaCT expressed in primary cultures of neurons from mouse cerebral cortex

Neurons are known to express a high-affinity Na+ -coupled dicarboxylate transporter(s) for uptake of tricarboxylic acid cycle intermediates, such as alpha-ketoglutarate and malate, which are precursors for neurotransmitters including glutamate and gamma-aminobutyric acid. There is, however, little information available on the molecular identity of the transporters responsible for this uptake process in neurons. In the present study, we investigated the characteristics of Na+ -dependent citrate transport in primary cultures of neurons from mouse cerebral cortex and established the molecular identity of this transport system as the Na+ -coupled citrate transporter (NaC2/NaCT). Reverse transcriptase (RT)-PCR and immunocytochemical analyses revealed that only NaC2/NaCT was expressed in mouse cerebrocortical neurons but not in astrocytes. Uptake of citrate in neurons was Na+ -dependent, Li+ -sensitive, and saturable with the Kt value of 12.3 microM. This Kt value was comparable with that in the case of Na+ -dependent succinate transport (Kt = 9.2 microM). Na+ -activation kinetics revealed that the Na+ -to-citrate stoichiometry was 3.4:1 and concentration of Na+ necessary for half-maximal activation (K0.5(Na)) was 45.7 mM. Na+ -dependent uptake of [14C]citrate (18 microM) was significantly inhibited by unlabeled citrate as well as dicarboxylates such as succinate, malate, fumarate, and alpha-ketoglutarate. This is the first report demonstrating the molecular identity of the Na+ -coupled di/tricarboxylate transport system expressed in neurons as NaC2/NaCT, which can transport the tricarboxylate citrate as well as dicarboxylates such as succinate, alpha-ketoglutarate, and malate.

Cerebral pyruvate carboxylase flux is unaltered during bicuculline-seizures

Glutamine synthesis in the astroglia reflects the sum of neurotransmitter cycling (glutamate and gamma-aminobutyric acid [GABA]) and de novo synthesis (anaplerosis), the latter catalyzed by pyruvate carboxylase. Previous studies have shown that the glutamate plus GABA cycling flux is correlated strongly with neuronal activity; however, the relationship between pyruvate carboxylase flux and neuronal activity is not known. In this study, pyruvate carboxylase flux was assessed during intravenous infusion of [2-(13)C]glucose using localized (1)H-[(13)C] NMR spectroscopy at 7 Tesla in vivo in halothane-anesthetized and ventilated adult Wistar rats during 85 min of bicuculline-induced seizures (1 mg/kg, intravenously) and in nontreated controls. During seizures, concentrations of lactate, alanine, glutamine, GABA, and succinate increased whereas glutamate and aspartate decreased such that the decrease in glutamate plus aspartate equaled the increase in glutamine plus GABA. Pyruvate carboxylase flux was assessed by the sum of [2-(13)C] and [3-(13)C] of glutamine and glutamate (Glx(2+3)) labeling during [2-(13)C]glucose infusion. During seizures the initial rate of Glx(2+3) synthesis (0.069 +/- 0.013 micromol/g/min) was not significantly different (P = 0.68) from that of the controls (0.059 +/- 0.010 micromol/g/min), indicating that anaplerotic flow through pyruvate carboxylase was unaltered. Intense neuronal activation of seizures did not seem to increase anaplerosis through pyruvate carboxylase, despite the substantial increase in neuronal activity and glutamate/glutamine cycling shown in a previous study (Patel et al., 2004b).

Regulation of sodium-dicarboxylate cotransporter-3 from winter flounder kidney by protein kinase C

The sodium dicarboxylate cotransporter located at the basolateral side supplies renal proximal tubule cells with Krebs cycle intermediates and maintains the driving force for the exchange of organic anions like PAH against alpha-ketoglutarate through the organic anion transporter-1. Recently, we cloned sodium dicarboxylate cotransporter-3 from winter flounder kidney (fNaDC-3). To understand the regulation of fNaDC-3, we preincubated fNaDC-3-expressing oocytes with PMA, a PKC activator. PMA dose and time dependently inhibited fNaDC-3-mediated succinate uptake. Simultaneous preincubation of fNaDC-3-expressing oocytes with 50 nM PMA and either staurosporine or RO 31-8220 for 30 min attenuated PKC-mediated inhibition of succinate uptake. Site-directed mutagenesis of the five putative PKC sites (S7, T167, S174, T188, and S396) resulted in no change in PKC-mediated inhibition of the transporter. In electrophysiological studies performed at -60 mV, the K0.5 for succinate was not significantly affected (56 +/- 13 vs. 42 +/- 19 microM), but DeltaImax was reduced from -139 +/- 49 to -20 +/- 8 nA by PMA (50 nM, 30 min). Immunofluorescence analysis of fNaDC-3-expressing oocytes revealed that PMA leads to an endocytosis of fNaDC-3 protein. In conclusion, fNaDC-3 expressed in oocytes is downregulated by PMA through endocytosis. PKC consensus sites appear not to be important for this process.

Glutamine is the major precursor for GABA synthesis in rat neocortex in vivo following acute GABA-transaminase inhibition

The objective of the present study was to assess the degree to which astrocytic glutamine provides carbon for net synthesis of GABA in the rat neocortex in vivo. Isotopic labeling of GABA and glutamate from astrocytic glutamine was followed in halothane anesthetized and ventilated rats during an intravenous infusion of [2-(13)C]glucose. A net increase in GABA was achieved by administration of the GABA-transaminase inhibitor, gabaculine to suppress catabolism of GABA and recycling of (13)C label. (13)C Percentage enrichments of GABA, glutamate and glutamine were assessed in tissue extracts using (13)C-edited (1)H nuclear magnetic resonance at 8.4 T. GABA levels increased 2.6 micromol/g at 2 h and 6.1 micromol/g at 5 h after gabaculine, whereas glutamate and glutamine decreased in toto by 5.6 micromol/g at 2 h and 3.1 micromol/g at 5 h. Selective enrichment of glutamine, glutamate, and GABA C3's over other carbon positions was observed consistent with a precursor role for astrocytic glutamine. Between 1 h (control) and 3 h (gabaculine-treated) of [2-(13)C]glucose infusion, (13)C percentage enrichment increased in glutamine C3 (from 3.2+/-0.5 to 7.0+/-0.9%), glutamate C3 (from 1.8+/-0.5 to 3.4+/-0.9%), and GABA C3 (from 2.7+/-1.6 to 4.8+/-0.4%). The measured incremental [3-(13)C]GABA concentration (0.15 micromol/g) was close to the predicted value (0.13 micromol/g) that would be expected if the increase in GABA were produced entirely from glutamine compared to glutamate (0.07 micromol/g) based on the average precursor enrichments between 1 and 3 h. We conclude that glutamine is the major source of GABA carbon in the rat neocortex produced acutely following GABA-T inhibition by gabaculine in vivo.

Cloning and functional characterization of a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain

Neurons contain a high-affinity Na(+)/dicarboxylate cotransporter for absorption of neurotransmitter precursor substrates, such as alpha-ketoglutarate and malate, which are subsequently metabolized to replenish pools of neurotransmitters, including glutamate. We have isolated the cDNA coding for a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain, called mNaDC-3. The mRNA coding for mNaDC-3 is found in brain and choroid plexus as well as in kidney and liver. The mNaDC-3 transporter has a broad substrate specificity for dicarboxylates, including succinate, alpha-ketoglutarate, fumarate, malate, and dimethylsuccinate. The transport of citrate is relatively insensitive to pH, but the transport of succinate is inhibited by acidic pH. The Michaelis-Menten constant for succinate in mNaDC-3 is 140 microM in transport assays and 16 microM at -50 mV in two-electrode voltage clamp assays. Transport is dependent on sodium, although lithium can partially substitute for sodium. In conclusion, mNaDC-3 likely codes for the high-affinity Na(+)/dicarboxylate cotransporter in brain, and it has some unusual electrical properties compared with the other members of the family.

Synaptosomal uptake of alpha-ketoglutarate and glutamine in thioacetamide-induced hepatic encephalopathy in rats

The kinetics of uptake of two astroglia-derived glutamate (GLU) precursors, alpha-ketoglutarate (alpha-KG) and glutamine (GLN) were determined in synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with a hepatotoxin, thioacetamide (TAA). TAA treatment increased by 33% Vmax for high affinity, low capacity alpha-KG uptake, without influencing its Km. The increase of the uptake capacity for alpha-KG may represent a response of the GLUergic nerve terminals to the decreased cerebral alpha-KG content, which during HE is associated with depressed activity of pyruvate carboxylase, an enzyme that replenishes alpha-KG in astrocytes. The result is thus consistent with the notion that HE affects the astroglial control of GLUergic neurotransmission. The Km and Vmax for the low affinity, high capacity GLN uptake was not affected by TAA treatment.

Glutamine transport in cerebellar granule cells in culture

In the present study, uptake of glutamine by rat cerebellar granule cells, a predominantly glutamatergic nerve cell population, has been investigated. Glutamine is taken up by granule cells via at least three transport systems, A, ASC and L. The L-type low affinity system (K(m) = 2.6 mM) is the major transport system in the absence of Na+. The systems A and ASC represent the Na(+)-dependent transport routes, both with almost identical high affinity for glutamine (K(m) = 0.26 mM). Similar transport systems for glutamine are also found in cerebral cortical neurons, a predominantly GABAergic nerve cell population, and cerebral cortical astrocytes. The glutamine transport properties in granule cells, however, show a series of differences from that of cortical neurons and astrocytes: (1) uptake of glutamine by granule cells is primarily mediated by system A (54%), while contributions by system A in cortical neurons and astrocytes are less than 30%; (2) granule cells exhibit strikingly higher transport efficiency for glutamine (V(max)/K(m) = 20 min(-1) for system A as compared to the V(max)/K(m) ratio of 5 min(-1) in cortical neurons and astrocytes), and (3) the initial uptake rates and the steady-state accumulation levels of glutamine are two- to threefold higher in granule cells than that of cortical neurons and astrocytes. These results taken together suggest that in accordance with the important need to replenish the neurotransmitter pool of glutamate, glutamatergic neurons exhibit highly efficient transport systems to accumulate glutamine, one of the major precursors of glutamate.

Glutamate metabolic pathways in displaced ganglion cells of the chicken retina

Glutamate (E) is the putative amino acid neurotransmitter used by ganglion cells, photoreceptors, and bipolar cells. Aspartate (D) and glutamine (Q) are potential precursors of glutamate, and glutamate-utilizing neurons may use one or more of these amino acids to sustain production of glutamate. We used post-embedding immunocytochemistry for several amino acid neurotransmitters to characterize the amino acid signatures for displaced ganglion cells of the avian retina. We found two neurochemical signatures for displaced ganglion cells, EQ and EDQ, in mid-peripheral and far-peripheral retina, respectively. Differences in neurochemical signatures cannot be explained by the existence of two ganglion cell populations, and we propose that the two signature categories for the large-diameter displaced ganglion cells reflect variations in the aspartate precursor pool. The transamination reaction involved in glutamate production, aspartate/oxaloacetate and alpha-ketoglutarate/glutamate, requires an active TCA cycle, since the carbon skeleton of glutamate is derived from alpha-ketoglutarate, a TCA intermediary. We hypothesized that aspartate levels vary in the normal chicken retina because eccentricity-dependent differences in oxygen availability result in changes of alpha-ketoglutarate levels, and hence, alterations in the equilibrium of the transamination reaction. We tested this hypothesis by incubating isolated chicken retinas in anaerobic conditions and found elevated aspartate immunoreactivity in subpopulations of glutamate-utilizing neurons in the central retina. Under aerobic conditions, or in retinas placed directly into fixative, retinal samples from the central edge of the pecten did not show differential cellular staining for aspartate. We have, therefore, identified differences in neurochemical signatures for retinal neurons involving changes in active maintenance of precursor pools.

Glutamate-synthesizing enzymes in GABAergic neurons of the neocortex: a double immunofluorescence study in the rat

L-Glutamate is the immediate precursor of the inhibitory transmitter GABA, and considered to be supplied from alpha-ketoglutarate through a transamination reaction or from glutamine through a glutaminase reaction. In the present study, the localization of aspartate aminotransferase and glutaminase in GABAergic neurons was investigated in the rat neocortex by a double immunofluorescence method. Immunoreactivities for both soluble and mitochondrial aspartate aminotransferases were detected in more than 90% of GABA-positive neurons, whereas glutaminase immunoreactivity was not found in GABA-positive neurons. All neocortical neurons with soluble aspartate aminotransferase immunoreactivity were immunopositive for GABA, but none for glutaminase. Neurons with mitochondrial aspartate aminotransferase immunoreactivity showed either glutaminase or GABA immunoreactivity. Under confocal laser scan microscopy, immunoreactivity for soluble aspartate aminotransferase was observed in many axons and axon terminals showing immunoreactivity for glutamic acid decarboxylase, whereas immunoreactivity for mitochondrial aspartate aminotransferase was seen in only a few axons displaying immunoreactivity for glutamic acid decarboxylase. The present results indicate that soluble aspartate aminotransferase is selectively localized to cell bodies and axon terminals of GABAergic non-pyramidal neurons in the cerebral neocortex. This suggests that glutamate is supplied from alpha-ketoglutarate via transamination and works as the immediate precursor for GABA in axon terminals of GABAergic neurons. The absence of glutaminase immunoreactivity in GABAergic neurons indicates that glutamine is a "metabolically remote" precursor for GABA. Mitochondrial aspartate aminotransferase was located in perikarya, rather than in axon terminals of GABAergic neurons, suggesting a transmitter-irrelevant role of this enzyme in neurons.

Cerebral alanine transport and alanine aminotransferase reaction: alanine as a source of neuronal glutamate

Alanine transport and the role of alanine amino-transferase in the synthesis and consumption of glutamate were investigated in the preparation of rat brain synaptosomes. Alanine was accumulated rapidly via both the high- and low-affinity uptake systems. The high-affinity transport was dependent on the sodium concentration gradient and membrane electrical potential, which suggests a cotransport with Na+. Rapid accumulation of the Na(+)-alanine complex by synaptosomes stimulated activity of the Na+/K+ pump and increased energy utilization; this, in turn, activated the ATP-producing pathways, glycolysis and oxidative phosphorylation. Accumulation of Na+ also caused a small depolarization of the plasma membrane, a rise in [Ca2+]i, and a release of glutamate. Intra-synaptosomal metabolism of alanine via alanine amino-transferase, as estimated from measurements of N fluxes from labeled precursors, was much slower than the rate of alanine uptake, even in the presence of added oxoacids. The velocity of [15N]alanine formation from [15N]glutamine was seven to eight times higher than the rate of [15N]-glutamate generation from [15N]alanine. It is concluded that (a) overloading of nerve endings with alanine could be deleterious to neuronal function because it increases release of glutamate; (b) the activity of synaptosomal alanine aminotransferase is much slower than that of glutaminase and hence unlikely to play a major role in maintaining [glutamate] during neuronal activity; and (c) alanine amino-transferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the alanine concentration rises and that of glutamate falls.

Uptake, release, and metabolism of citrate in neurons and astrocytes in primary cultures

Synthesis, uptake, release, and oxidative metabolism of citrate were investigated in neurons and astrocytes cultured from cerebral cortex or cerebellum. In addition, the possible role of citrate as a donor of the carbon skeleton for biosynthesis of neurotransmitter glutamate was studied. All cell types expressed the enzyme citrate synthase at a high activity, the cerebellar granule neurons containing the enzyme at a higher activity than that found in the astrocytes from the two brain regions or the cortical neurons. Saturable citrate uptake could not be detected in any of the cell types, but the astrocytes, and, in particular, those of cerebellar origin, had a very active de novo synthesis and release of citrate (approximately 70 nmol x h-1 x mg of protein-1). The rate of release of citrate from neurons was < 5% of this value. Using [14C]citrate it could be shown that citrate was oxidatively metabolized to 14CO2 at a modest rate (approximately 1 nmol x h-1 x mg-1 of protein) with slightly higher rates in astrocytes compared with neurons. Experiments designed to investigate the ability of exogenously supplied citrate to serve as a precursor for synthesis of transmitter glutamate in cerebellar granule neurons failed to demonstrate this. Rather than citrate serving this purpose it may be suggested that astrocytically released citrate may regulate the extracellular concentration of Ca2+ and Mg2+ by chelation, thereby modulating neuronal excitability.

Uptake, release, and metabolism of alanine in neurons and astrocytes in primary cultures

The uptake, release, and metabolism of alanine were studied in primary cultures of cerebral cortical neurons or astrocytes and cerebellar granule neurons. All three cell types exhibited a saturable, sodium-dependent uptake of alanine with Km values (microM) of 256 +/- 30, 463 +/- 39, and 292 +/- 39, respectively, and Vmax values (nmol/min/mg) of 15.9 +/- 0.7, 7.9 +/- 0.01, and 17.4 +/- 0.8, respectively. The corresponding values (nmol/min/mg) for the specific activity of alanine aminotransferase were 4.7 +/- 0.4, 17.1 +/- 2.5, and 4.5 +/- 0.9 (all values represent the mean +/- SEM). Release of alanine from the cells was rectilinear with time over a 10 hr period in case of astrocytes (40 nmol/hr/mg) and cerebellar granule neurons (21 nmol/hr/mg). In cortical neurons the release rate declined from an initial value of 19 nmol/hr/mg during the first 3 hr to a value of less than 3 nmol/hr/mg during the subsequent 7 hr of incubation. Metabolism of [14C]alanine to 14CO2 was found to have a lag period of 15 min and subsequently the rate of CO2 production was constant over a 45 min period with a value of 0.5 nmol/min/mg in granule neurons and about 0.3 nmol/min/mg in the other two cell types. Altogether the results show that alanine is preferentially produced in and released from astrocytes and accumulated into both GABAergic cortical neurons and glutamatergic cerebellar granule neurons.

In situ measurements of enzyme activities in the brain

The present review focuses on enzymes involved in the metabolism of amino acid neurotransmitters and the microphotometric determinations of their activities in various layers of the rat hippocampus. The enzymes are NAD-linked isocitrate dehydrogenase (NAD-ICDH), glutamate dehydrogenase (GDH), and GABA transaminase (GABAT), all of which are localized in mitochondria. GDH seems to be restricted to astrocytes, whereas NAD-ICDH and GABAT are localized in neurons as well as in astrocytes. NAD-ICDH is an important enzyme of the tricarboxylic acid cycle and may deliver alpha-ketoglutarate for the formation of glutamate and GABA, which serve as neurotransmitters in the hippocampus. GDH catalyses the interconversion of alpha-ketoglutarate and glutamate, whereas GABAT is the important GABA-degrading enzyme and requires alpha-ketoglutarate for its activity. While differing in their cellular distribution and activity levels, NAD-ICDH, GDH and GABAT are significantly correlated in their hippocampal distribution. Furthermore, developmental and pharmacohistochemical studies suggest that the distribution and activity of astrocytic GDH is correlated with amino-acidergic neurotransmission in the hippocampus. The data reported give further evidence for a metabolic relationship between neurons and astrocytes in the turnover and metabolism of glutamate and GABA.

2-Oxoglutarate transport: a potential mechanism for regulating glutamate and tricarboxylic acid cycle intermediates in neurons

2-Oxoglutarate (alpha-ketoglutarate) is transported into synaptosomal and synaptoneurosomal preparations by a Na(+)-dependent, high-affinity process that exhibits complex kinetics, and is differentially modulated by glutamate, glutamine, aspartate, malate, and a soluble, heat-labile substance of high molecular weight present in rat brain extracts. Glutamate and aspartate generally inhibit 2-oxoglutarate uptake, but under certain conditions may increase uptake. Glutamine generally increases 2-oxoglutarate uptake, but under certain conditions may inhibit uptake. One interpretation of our results is that 2-oxoglutarate uptake is mediated primarily by a transporter that exhibits negative cooperativity and possesses three regulatory sites that differentially modulate substrate affinity, Vmax, and negative cooperatively. Glutamate, aspartate, malate, and 2-oxoglutarate itself may interact with a site that reduces substrate affinity; whereas glutamine, and possibly glutamate and aspartate, appear to interact with another site that increases Vmax. A putative regulatory protein appears to abolish negative cooperativity and increases substrate affinity in the absence of glutamine. Based on the evidence that glutamatergic and GABAergic neurons depend on astrocytes to supply precursors to replenish their neurotransmitter and tricarboxylic acid cycle pools, the uptake of 2-oxoglutarate, presumably into synaptic terminals, may reflect a role for this metabolite in replenishing the transmitter and tricarboxylic acid pools, and a role for the transporter as a site at which these pools are regulated.

Mitochondrial enzymes related to glutamate and GABA metabolism in the hippocampus of young and aged rats: a quantitative histochemical study

Quantitative histochemistry (scanning microphotometry) was used to determine the activities of the mitochondrial enzymes NAD-linked isocitrate dehydrogenase (EC 1.1.1.41), L-glutamate dehydrogenase (EC 1.4.1.3) and GABA transaminase (EC 2.6.1.19) in various layers of the hippocampus (middle one third) of young (3-4 months old) and memory-impaired aged rats (28-30 months old). For comparison, determinations of cytochrome c oxidase (EC 1.9.3.1) as a marker for mitochondria and energy metabolism were also performed. The study showed that there was a layered reaction pattern in the hippocampus and that the cellular distribution and the levels of enzyme activity were different. However, the activities of the different enzymes (excepting GABA transaminase and cytochrome c oxidase) were significantly correlated in the hippocampus in both age groups. Age-dependent changes were only observed for NAD-linked isocitrate dehydrogenase and GABA transaminase (significant increases of activities in some layers of the hippocampus, preferentially in the terminal field of the perforant path). From the present study it is concluded that, 1. the enzymatic complement of mitochondria in neurons and glia depends upon layer specific metabolic processes of the hippocampus (also with respect to glutamatergic and GABAergic terminal fields) indicating a layer specific interaction of the enzymes studied to produce or catabolize glutamate and GABA, and 2. the age dependent changes of the studied enzymes are very restricted.

Transport of asparagine by rat brain synaptosomes: an approach to evaluate glutamine accumulation

Isolated rat brain synaptosomes accumulated L-asparagine with a Km value of 348 microM and a Vmax value of 3.7 nmol/mg of protein/min at 28 degrees C. Uptake of L-asparagine was inhibited by the presence of L-glutamine, whereas transport of L-glutamine was blocked by L-asparagine. Alanine, serine, cysteine, threonine, and, in particular, leucine were also inhibitory whereas alpha-(methylamino)isobutyrate, ornithine, lysine, arginine, and glutamate were much less effective blockers. Transport of L-asparagine had a substantial sodium-dependent component, whereas that of the D-stereoisomer was almost unaffected by the presence or absence of the cation. L-Asparagine was accumulated to a maximal gradient, [L-Asn]i/[L-Asn]o, of 20-30, and this value was reduced to 5-6 by withdrawal of sodium or addition of high [KCI]. A plot of log [Na+]o/[Na+]i against the log [L-Asn]i/[L-Asn]o had a slope close to I, which indicates that a single sodium ion is transported inward with each asparagine molecule. It is postulated that uptake of L-asparagine occurs, to a large extent, in cotransport with Na+ and that it utilizes the sodium chemical gradient and the membrane electrical potential as the source of energy. The similarity between the L-asparagine and L-glutamine transport systems and the reciprocal inhibition of influx of the two amino acids suggest that the same mechanism is responsible for glutamine accumulation. This could explain the high [Gln]i maintained by the brain in vivo.

Glutamate dehydrogenase reaction as a source of glutamic acid in synaptosomes

The role of the glutamate dehydrogenase reaction as a pathway of glutamate synthesis was studied by incubating synaptosomes with 5 mM 15NH4Cl and then utilizing gas chromatography-mass spectrometry to measure isotopic enrichment in glutamate and aspartate. The rate of formation of [15N]glutamate and [15N]aspartate from 5 mM 15NH4Cl was approximately 0.2 nmol/min/mg of protein, a value much less than flux through glutaminase (4.8 nmol/min/mg of protein) but greater than flux through glutamine synthetase (0.045 nmol/min/mg of protein). Addition of 1 mM 2-oxoglutarate to the medium did not affect the rate of [15N]glutamate formation. O2 consumption and lactate formation were increased in the presence of 5 mM NH3, whereas the intrasynaptosomal concentrations of glutamate and aspartate were unaffected. Treatment of synaptosomes with veratridine stimulated reductive amination of 2-oxoglutarate during the early time points. The production of ([15N]glutamate + [15N]aspartate) was enhanced about twofold in the presence of 5 mM beta-(+/-)-2-aminobicyclo [2.2.1]heptane-2-carboxylic acid, a known effector of glutamate dehydrogenase. Supplementation of the incubation medium with a mixture of unlabelled amino acids at concentrations similar to those present in the extracellular fluid of the brain had little effect on the intrasynaptosomal [glutamate] and [aspartate]. However, the enrichment in these amino acids was consistently greater in the presence of supplementary amino acids, which appeared to stimulate modestly the reductive amination of 2-oxoglutarate. It is concluded: (a) compared with the phosphate-dependent glutaminase reaction, reductive amination is a relatively minor pathway of synaptosomal glutamate synthesis in both the basal state and during depolarization; (b) NH3 toxicity, at least in synaptosomes, is not referable to energy failure caused by a depletion of 2-oxoglutarate in the glutamate dehydrogenase reaction; and (c) transamination is not a major mechanism of glutamate nitrogen production in nerve endings.

Utilization of alpha-ketoglutarate as a precursor for transmitter glutamate in cultured cerebellar granule cells

Alpha-ketoglutarate together with an amino group donor (alanine) was shown to be able to serve as a precursor for the glutamate pool which is released by potassium-induced depolarization (i.e., transmitter glutamate) in cerebellar granule cells. However, these compounds could not be utilized as precursors for intracellular glutamate or for release of transmitter aspartate. The formation of transmitter glutamate was inhibited by the transamination inhibitor aminooxyacetic acid but not by phenylsuccinate, an inhibitor of the dicarboxylate carrier in the mitochondrial membrane. Both of these inhibitors have previously been found to inhibit synthesis of transmitter glutamate from glutamine. The results support the hypothesis that alpha-ketoglutarate and alanine undergo transmination in the cytosol to form pyruvate and glutamate, and that this glutamate pool is available for transmitter release of glutamate but does not constitute the major intracellular pool of glutamate.

The metabolism of malate by cultured rat brain astrocytes

Since malate is known to play an important role in a variety of functions in the brain including energy metabolism, the transfer of reducing equivalents and possibly metabolic trafficking between different cell types; a series of biochemical determinations were initiated to evaluate the rate of 14CO2 production from L-[U-14C]malate in primary cultures of rat brain astrocytes. The 14CO2 production from labeled malate was almost totally suppressed by the metabolic inhibitors rotenone and antimycin A suggesting that most of malate metabolism was coupled to the electron transport system. A double reciprocal plot of the 14CO2 production from the metabolism of labeled malate revealed biphasic kinetics with two apparent Km and Vmax values suggesting the presence of more than one mechanism of malate metabolism in these cells. Subsequent experiments were carried out using 0.01 mM and 0.5 mM malate to determine whether the addition of effectors would differentially alter the metabolism of high and low concentrations of malate. Effectors studied included compounds which could be endogenous regulators of malate metabolism and metabolic inhibitors which would provide information regarding the mechanisms regulating malate metabolism. Both lactate and aspartate decreased 14CO2 production from 0.01 mM and 0.5 mM malate equally. However, a number of effectors were identified which selectively altered the metabolism of 0.01 mM malate including aminooxyacetate, furosemide, N-acetylaspartate, oxaloacetate, pyruvate and glucose, but had little or no effect on the metabolism of 0.5 mM malate. In addition, alpha-ketoglutarate and succinate decreased 14CO2 production from 0.01 mM malate much more than from 0.5 mM malate. In contrast, a number of effectors altered the metabolism of 0.5 mM malate more than 0.01 mM. These included methionine sulfoximine, glutamate, malonate, alpha-cyano-4-hydroxycinnamate and ouabain. Both the biphasic kinetics and the differential action of many of the effectors on the 14CO2 production from 0.01 mM and 0.5 mM malate provide evidence for the presence of more than one pool of malate metabolism in cultured rat brain astrocytes.

Neuronal glutamine utilization: glutamine/glutamate homeostasis in synaptosomes

The synaptosomal metabolism of glutamine was studied under in vitro conditions that simulate depolarization in vivo. With [2-15N]glutamine as precursor, the [glutamine]i was diminished in the presence of veratridine or 50 mM KCl, but the total amounts of [15N]glutamate and [15N]aspartate formed were either equal to those of control incubations (veratridine) or higher (50 mM [KCl]). This suggests that depolarization decreases glutamine uptake and independently augments glutaminase activity. Omission of sodium from the medium was associated with low internal levels of glutamine which indicates that influx occurs as a charged Na(+)-amino acid complex. It is postulated that a reduction in membrane potential and a collapse of the Na+ gradient decrease the driving forces for glutamine accumulation and thus inhibit its uptake and enhance its release under depolarizing conditions. Inorganic phosphate stimulated glutaminase activity, particularly in the presence of calcium. At 2 mM or lower [phosphate] in the medium, calcium inhibited glutamine utilization and the production of glutamate, aspartate, and ammonia from glutamine. At a high (10 mM) medium [phosphate], calcium stimulated glutamine catabolism. It is suggested that a veratridine-induced increase in intrasynaptosomal inorganic phosphate is responsible for the enhancement of flux through glutaminase; calcium affects glutaminase indirectly by modulating the level of free intramitochondrial [phosphate]. Because phosphate also lowers the Km of glutaminase for glutamine, augmentation of the amino acid breakdown may occur even when depolarization lowers [glutamine]i. Reducing the intrasynaptosomal glutamate to 26 nmol/mg of protein had little effect on glutamine catabolism, but raising the pH to 7.9 markedly increased formation of glutamate and aspartate. It is concluded that phosphate and H+ are the major physiologic regulators of glutaminase activity.

Activation of glutamate dehydrogenase by leucine and its nonmetabolizable analogue in rat brain synaptosomes

Leucine and beta-(+/-)-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH) stimulated, in a dose-dependent manner, reductive amination of 2-oxoglutarate in rat brain synaptosomes treated with Triton X-100. The concentration dependence curves were sigmoid, with 10-15-fold stimulations at 15 mM leucine (or BCH); oxidative deamination of glutamate also was enhanced, albeit less. In intact synaptosomes, leucine and BCH elevated oxygen uptake and increased ammonia formation, consistent with stimulation of glutamate dehydrogenase (GDH). Enhancement of oxidative deamination was seen with endogenous as well as exogenous glutamate and with glutamate generated inside synaptosomes from added glutamine. With endogenous glutamate, the stimulation of oxidative deamination was accompanied by a decrease in aspartate formation, which suggests a concomitant reduction in flux through aspartate aminotransferase. Activation of reductive amination of 2-oxoglutarate by BCH or leucine could not be demonstrated even in synaptosomes depleted of internal glutamate. It is suggested that GDH in synaptosomes functions in the direction of glutamate oxidation, and that leucine may act as an endogenous activator of GDH in brain in vivo.

Release of [3H]L-glutamate and [3H]L-glutamine in rat cerebellum slices: a comparison of the effect of veratridine and electrical stimulation

Depolarization-elicited release of neurotransmitter glutamate was studied in rat cerebellar slices previously loaded with either [3H]L-glutamate or [3H]L-glutamine. Both depolarization conditions used (e.g. long-lasting tonic depolarization elicited by veratridine, or short repetitive electrical pulses) increased 6 to 8 folds the release of labelled glutamate and of another compound, presumably alpha-ketoglutarate, without modifying the release of labeled glutamine. Because of the position of the label in the precursor radioactive molecules, GABA was weakly labeled and aspartate was unlabeled. The properties of the evoked glutamate release from cerebellar slices were those of a neurotransmitter since it was inhibited by tetrodotoxin and was Ca2+-dependent. Alpha-ketoglutarate is either coreleased from nerve terminals or is released from astrocytes and could participate in glutamate recycling. The data confirm the generally accepted model implying the presence of two neurotransmitter glutamate pools, a neuronal pool of newly synthesized glutamate and an astrocytic storage pool, but in addition indicate that the former is in rapid isotopic equilibrium with the extracellular compartment. Our present results also indicate that the glutamate/glutamine cycle is not activated in depolarizing conditions.

High-affinity transport of L-glutamine by a plasma membrane preparation from rat brain

Plasma membrane vesicles prepared from rat brain contain a saturable, high-affinity transport system for L-glutamine that exhibits the following characteristics: (1) The rate of L-glutamine transport is linear up to 200 micrograms/mL membrane protein. (2) Transport of [3H]-L-glutamine is linear with time for at least 10 min, is significantly reduced by lowering the assay temperature to 4 degrees C, and is essentially abolished by the addition of excess unlabeled L-glutamine. (3) The transport rate is optimal in the range of pH 7.4-8.2. (4) The system exhibits a Km for L-glutamine of approximately 1.7 microM and a Vmax of approximately 46 pmol/(min.mg of protein). (5) The system is not highly dependent upon the addition of monovalent or divalent cations. (6) Inhibitor studies reveal that the amino acid amides exhibit the highest affinity for the system and that there is a high specificity for the L-isomers.

13C nuclear magnetic resonance evidence for gamma-aminobutyric acid formation via pyruvate carboxylase in rat brain: a metabolic basis for compartmentation

The compartmentation of amino acid metabolism is an active and important area of brain research. 13C labeling and 13C nuclear magnetic resonance (NMR) are powerful tools for studying metabolic pathways, because information about the metabolic histories of metabolites can be determined from the appearance and position of the label in products. We have used 13C labeling and 13C NMR in order to investigate the metabolic history of gamma-aminobutyric acid (GABA) and glutamate in rat brain. [1-13C]Glucose was infused into anesthetized rats and the 13C labeling patterns in GABA and glutamate examined in brain tissue extracts obtained at various times after infusion of the label. Five minutes after infusion, most of the 13C label in glutamate appeared at the C4 position; at later times, label was also present at C2 and C3. This 13C labeling pattern occurs when [1-13C]glucose is metabolized to pyruvate by glycolysis and enters the pool of tricarboxylic acid (TCA) intermediates via pyruvate dehydrogenase. The label exchanges into glutamate from the TCA cycle pool through glutamate transaminases or dehydrogenase. After 30 min of infusion, approximately 10% of the total 13C in brain extracts appeared in GABA, primarily (greater than 80%) at the amino carbon (C4), indicating that the GABA detected is labeled through pyruvate carboxylase. The different labeling patterns observed for glutamate and GABA show that the large detectable glutamate pool does not serve as the precursor to GABA. Our NMR data support previous experiments suggesting compartmentation of metabolism in brain, and further demonstrate that GABA is formed from a pool of TCA cycle intermediates derived from an anaplerotic pathway involving pyruvate carboxylase.

Neuronal glutamine utilization: pathways of nitrogen transfer studied with [15N]glutamine

Gas chromatography-mass spectrometry was used to evaluate the metabolism of [15N]glutamine in isolated rat brain synaptosomes. In the presence of 0.5 mM glutamine, synaptosomes accumulated this amino acid to a level of 25-35 nmol/mg protein at an initial rate greater than 9 nmol/min/mg of protein. The metabolism of [15N]glutamine generated 15N-labelled glutamate, aspartate, and gamma-aminobutyric acid (GABA). An efflux of both [15N]glutamate and [15N]aspartate from synaptosomes to the medium was observed. Enrichment of 15N in alanine could not be detected because of a limited pool size. Elimination of glucose from the incubation medium substantially increased the rate and amount of [15N]aspartate formed. It is concluded that: (1) With 0.5 mM external glutamine, the glutaminase reaction, and not glutamine transport, determines the rate of metabolism of this amino acid. (2) The primary route of glutamine catabolism involves aspartate aminotransferase which generates 2-oxoglutarate, a substrate for the tricarboxylic acid cycle. This reaction is greatly accelerated by the omission of glucose. (3) Glutamine has preferred access to a population of synaptosomes or to a synaptosomal compartment that generates GABA. (4) Synaptosomes maintain a constant internal level of glutamate plus aspartate of about 70-80 nmol/mg protein. As these amino acids are produced from glutamine in excess of this value, they are released into the medium. Hence synaptosomal glutamine and glutamate metabolism are tightly regulated in an interrelated manner.

Glutamine and 2-oxoglutarate as metabolic precursors of the transmitter pools of glutamate and GABA: correlation of regional uptake by rat brain synaptosomes

To more clearly define the roles of glutamine and 2-oxoglutarate as metabolic precursors of the transmitter pools of glutamate and GABA we have determined the relative rates at which these four substances, and adenosine and serotonin are accumulated by synaptosomes derived from twelve regions of the rat brain. Initial transport conditions and low substrate concentrations were used to maximize uptake by high-affinity systems, except the uptake of glutamine was determined at both low and high concentrations. Because the uptake of 2-oxoglutarate is markedly enhanced by glutamine, 2-oxoglutarate uptake was determined with and without glutamine (0.2 mM) added to the incubation medium. For each substrate, regional differences in uptake ranged from approximately two- to fourteen-fold. An anaylsis of uptake kinetics revealed that the regional differences were due primarily to differences in transport capacity rather than substrate affinities, at least for glutamate, GABA, and 2-oxoglutarate. Thirty-four correlation analyses of relative uptake values were performed. Strong correlations were found between 2-oxoglutarate and glutamate, and between glutamine and glutamate, whereas no strong correlations occurred between these substrates and GABA. Our results support the view that both glutamine and 2-oxoglutarate are major precursors of the transmitter pool of glutamate throughout the rat brain, but their relative contributions toward replenishing the transmitter pool of GABA are less certain.

Importance of glutamine for gamma-aminobutyric acid synthesis in rat neostriatum in vivo

This work was carried out to evaluate the importance of glial cells in providing precursors for the in vivo synthesis of gamma-aminobutyric acid (GABA). Fluorocitrate, which selectively inhibits the tricarboxylic acid cycle in glial cells, was administered locally in rat neostriatum. Inhibition of the glial cell tricarboxylic acid cycle led to a decrease both in glutamine level and in gamma-vinyl GABA (GVG)-induced GABA accumulation, an observation indicating reduced GABA synthesis. The role of glutamine, which is synthesized in glial cells as a precursor for GABA, was further investigated by inhibition of glutamine synthetase with intrastriatally administered methionine sulfoximine. In this case, the glutamine level was reduced to near zero values, and the GVG-induced GABA accumulation was only half that of normal. The results show that glutamine is an important precursor for GABA synthesis, but it cannot be the sole precursor because it was not possible to depress the GVG-induced GABA accumulation completely.

The effect of fluorocitrate on transmitter amino acid release from rat striatal slices

In order to study the role of glutamine from glial cells for the synthesis of transmitter amino acids, the effect of the gliotoxic-substance fluorocitrate on amino acid release from slices was investigated. In vivo treatment with 1 nmol fluorocitrate reduced the Ca2+ dependent K+ evoked release of endogenous glutamate and GABA from the slices, whereas the glutamine efflux decreased and alanine efflux increased. The K+ evoked release of [3H]D-aspartate increased during fluorocitrate treatment. The latter is consistent with an inhibited uptake of D-aspartate into glial cells. Incubation of striatal slices with fluorocitrate (0.1 mM) decreased the glutamine efflux and increased the alanine efflux. Similar to the in vivo condition, fluorocitrate increased the K+ evoked [3H]D-aspartate release, but the K+ evoked release of endogenous glutamate and GABA increased rather than decreased. The ratio between the K+ evoked release of exogenous D-aspartate to endogenous glutamate increased in both cases. The results suggest an important role of glial cells in the synthesis and inactivation of transmitter amino acids.

Regulation of transmitter gamma-aminobutyric acid (GABA) synthesis and metabolism illustrated by the effect of gamma-vinyl GABA and hypoglycemia

The effect of different treatments on amino acid levels in neostriatum was studied to throw some light on the synthesis and metabolism of gamma-aminobutyric acid (GABA). Irreversible inhibition of GABA transaminase by microinjection of gamma-vinyl GABA (GVG) led to a decrease in aspartate, glutamate, and glutamine levels and an increase in the GABA level, such that the nitrogen pool remained constant. The results indicate that a large part of brain glutamine is derived from GABA. Hypoglycemia led to an increase in the aspartate level and a decrease in glutamate, glutamine, and GABA levels. The total amino acid pool was decreased compared with amino acid levels in normoglycemic rats. GVG treatment of hypoglycemic rats led to a decrease in the aspartate level and a further reduction in glutamate and glutamine levels. In this case, GABA accumulation continued, although the glutamine pool was almost depleted. The GABA level increased postmortem, but there were no detectable changes in levels of the other amino acids. Pretreatment of the rats with hypoglycemia reduced both glutamate and glutamine levels with a subsequent decreased postmortem GABA accumulation. The half-maximal GABA synthesis rate was obtained when the glutamate level was reduced by 50% and the glutamine level was reduced by 80%.

Ion dependence of neurotransmitter uptake: inhibitory effects of ion substitutes

Several ions commonly used as substitutes for Na+ or Cl- were found to inhibit directly the high-affinity uptake of norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid, but not glutamate or glutamine. When Na+ was partially replaced by any of several different cations or sucrose the uptake of all neurotransmitters studied except that of serotonin was reduced more than could be accounted for by just the inhibitory effect of the cation substitute. In contrast, when Cl- was partially replaced by any of several anions only the uptake of dopamine was reduced more than could be accounted for by the inhibitory effect of the anion substitute. These results suggest that for most neurotransmitters the electrochemical potential for Na+, but not for Cl-, contributes to the uptake driving force. When either Na+ or Cl- was totally replaced by an ion substitute or by sucrose the high-affinity uptake was virtually abolished, an exception being that glutamate uptake was not affected when isethionate was substituted for Cl-. The lack of uptake in the absence of either Na+ or Cl- may reflect a specific role for these ions in either increasing the affinity between the substrate and the carrier, or facilitating the translocation process. Alternatively, the transport carriers may undergo a nonspecific conformational change to an inactive form in the absence of Na+ or Cl-. A partial substitution of Na+ with Li+ or sucrose differentially affected the kinetics of uptake in that replacement with Li+, but not sucrose, usually resulted in a marked increase in the Km values.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical characterization of glutamate dehydrogenase in the cerebellum of the rat

The immunocytochemical distribution of glutamate dehydrogenase was studied in the cerebellum of the rat using antibodies made in rabbit and guinea pig against antigen purified from bovine liver. Antiserum was found to block partially enzymatic activity both of the purified enzyme and of extracts of the rat cerebellum. Using immunoblots of proteins of rat cerebellum, a major immunoreactive protein and several minor immunoreactive proteins were detected with antiserum. Only a single immunoreactive protein was detected using affinity-purified antibody preparations. This protein migrates with a molecular weight identical to that of the subunit of glutamate dehydrogenase. Further evidence that the antibodies were selective for glutamate dehydrogenase in rat cerebellum was obtained through peptide mapping. Purified glutamate dehydrogenase and the immunoreactive protein from rat cerebellum generated similar patterns of immunoreactive peptides. No significant cross-reaction was observed with glutamine synthetase. Immunocytochemistry was done on cryostat- and Vibratome-cut sections of the cerebellum of rats that had been perfused with cold 4% paraformaldehyde. Glial cells were found to be the most immunoreactive structures throughout the cerebellum. Most apparent was the intense labeling of Bergmann glial cell bodies and fibers. In the granule cell layer, heavy labeling of astrocytes was seen. Purkinje and granule cell bodies were only lightly immunoreactive, whereas stellate, basket, and Golgi cells were unlabeled. Labeling of presynaptic terminals was not apparent. These findings suggest that glutamate dehydrogenase, like glutamine synthetase, is enriched in glia relative to neurons.

2-Oxo-[14C]glutarate is taken up by glutamatergic nerve terminals in the rat striatum

High affinity uptake of [14C]glutamate into rat striatal synaptosomes was reduced by 33% after bilateral cortical ablation. The lesion had no effect on striatal [14C]GABA uptake, but reduced 2-oxo-[14C]glutarate uptake by 67%. The results demonstrate the existence of a high-affinity uptake site for 2-oxoglutarate on glutamatergic nerve terminals and support the contention that this Krebs cycle intermediate may be used to replenish the neuronal pool of neurotransmitter glutamate. 2-Oxo-[14C]glutarate uptake may serve as a selective marker for glutamatergic neurones.

Preferential glutamine uptake in rat brain synaptic mitochondria

Glutamine uptake has been studied in purified rat brain mitochondria of synaptic or non-synaptic origin. It was taken up by an active saturable transport mechanism, with an affinity two-times higher in synaptic than in non-synaptic mitochondria (Km = 0.45 and 0.94 mM, respectively). Vmax of uptake was 7-times higher in synaptic mitochondria (Vmax = 9.2 and 1.3 nmol/min per mg protein, respectively). Glutamine transport was found to be inhibited by L-glutamate (IC50 = 0.64 mM) as well as thiol reagents (mersalyl, N-ethylmaleimide). It is suggested that differential uptake of glutamine in mitochondria of synaptic or non-synaptic origin may be a major mechanism in the regulation of the synthesis of the neurotransmitter glutamate.

Immunocytochemical localization of glutaminase-like and aspartate aminotransferase-like immunoreactivities in the rat and guinea pig hippocampus

There is considerable evidence that pathways of the hippocampus use an excitatory amino acid as transmitter. We have attempted to immunocytochemically identify excitatory amino acid neurons in the hippocampus of the rat and guinea pig using antiserum to glutaminase and antiserum to aspartate aminotransferase, which have been proposed as markers for aspartergic/glutamergic neurons. Glutaminase-like immunoreactivity was seen in granule cells in the dentate gyrus and fibers and puncta associated with the mossy fiber pathway in the hilus and stratum lucidum of the hippocampus. At the ultrastructural level, glutaminase-like immunoreactivity was observed in mossy fiber terminals in the stratum lucidum. Glutaminase-like immunoreactivity was also seen in pyramidal cells in regio inferior and regio superior and in cells in layer two of the entorhinal cortex. Schaffer collateral terminals, commissural fiber terminals and perforant pathway terminals were not seen at the light microscopic level. Glutaminase-like immunoreactivity is thus found in the cell bodies of proposed excitatory amino acid neurons of hippocampal pathways, but does not appear to label all terminals. Aspartate aminotransferase-like immunoreactivity was not seen in any cells, fibers or terminals in the rat or guinea pig hippocampus.

Alteration of striatal glutamate release after glutamine synthetase inhibition

The effect of the glutamine synthetase (GS) inhibitor, methionine sulfoximine (MSO), on glutamate levels in, and glutamate release from, rat striatal tissue was examined. Tissue levels of glutamate were unchanged 24 h after an intraventricular injection of MSO, but tissue glutamine levels were decreased 50%. Calcium-dependent, potassium-stimulated glutamate release was diminished in tissue prisms from animals pretreated with MSO compared to controls. The decreased release of glutamate correlated over time with the inhibition of GS following an intraventricular injection of MSO. The maximum diminution of calcium-dependent, potassium-stimulated glutamate release (50%) and the maximum inhibition of GS activity (51%) were observed 24 h after MSO. The addition of 0.5 mM glutamine to the perfusion medium completely reversed the effects of MSO pretreatment on calcium-dependent, potassium-stimulated glutamate release. Since GS is localized in glial cells and the measured glutamate release is presumed to occur from neurons, the data support the contention that astroglial glutamine synthesis is an important contributor to normal neuronal neurotransmitter release.