A mathematical model of compartmentalized neurotransmitter metabolism in the human brain

Measurement of unidirectional Pi to ATP flux in human visual cortex at 7 T by using in vivo 31P magnetic resonance spectroscopy

Taking advantage of the high NMR detection sensitivity and the large chemical shift dispersion offered by ultra-high field strength of 7 T, the effect of magnetization transfer on inorganic phosphate (Pi) resonance during saturation of gamma-ATP resonance, mediated by the ATP synthesis reaction, was observed noninvasively in the human primary visual cortex by using in vivo 31P magnetic resonance spectroscopy. The unidirectional flux from Pi to ATP was measured by using progressive saturation transfer experiments. The cerebral ATP synthesis rate in the human primary visual cortex measured by 31P magnetic resonance spectroscopy in this study was 12.1 +/- 2.8 micromol ATP/g per min, which agreed well with the value that was calculated indirectly from the cerebral metabolic rate of glucose consumption reported previously.

Cataplerotic TCA cycle flux determined as glutamate-sustained oxygen consumption in primary cultures of astrocytes

Utilization of glucose by adult brain as its metabolic substrate does not mean that glutamate cannot be synthesized from glucose and subsequently oxidatively degraded. Between 10 and 20% of total pyruvate metabolism in brain occurs as formation of oxaloacetate (OAA), a tricarboxylic acid (TCA) cycle intermediate, from pyruvate plus CO(2). This anaplerotic ('pool-filling') process occurs in astrocytes, which in contrast to neurons express pyruvate carboxylase (PC) activity. Equivalent amounts of pyruvate are converted to acetylcoenzyme A and condensed with oxaloacetate to form citrate (Cit), which is metabolized to alpha-ketoglutarate (generating oxidatively-derived energy), glutamate and glutamine and transferred to neurons in the glutamate-glutamine cycle and used as precursor for transmitter glutamate. Since the blood-brain barrier is poorly permeable to glutamate and its metabolites, net synthesis of glutamate must be followed by degradation of equivalent amounts of glutamate, a cataplerotic ('pool-emptying') process, in which glutamate is converted in the TCA cycle to malate or oxaloacetate (generating additional energy), which exit the cycle to form one molecule pyruvate. To obtain an estimate of the rate of astrocytic oxidation of glutamate the rate of oxygen consumption was measured in primary cultures of mouse astrocytes metabolizing glutamate in the absence of other metabolic substrates. The observed rate is compatible with complete oxidative degradation of glutamate.

Direct, noninvasive measurement of brain glycogen metabolism in humans

The concentration and metabolism of the primary carbohydrate store in the brain, glycogen, is unknown in the conscious human brain. This study reports the first direct detection and measurement of glycogen metabolism in the human brain, which was achieved using localized 13C NMR spectroscopy. To enhance the NMR signal, the isotopic enrichment of the glucosyl moieties was increased by administration of 80 g of 99% enriched [1-13C]glucose in four subjects. 3 h after the start of the label administration, the 13C NMR signal of brain glycogen C1 was detected (0.36+/-0.07 micromol/g, mean+/-S.D., n=4). Based on the rate of 13C label incorporation into glycogen and the isotopic enrichment of plasma glucose, the flux through glycogen synthase was estimated at 0.17+/-0.05 micromol/(gh). This study establishes that brain glycogen can be measured in humans and indicates that its metabolism is very slow in the conscious human. The noninvasive detection of human brain glycogen opens the prospect of understanding the role and function of this important energy reserve under various physiological and pathophysiological conditions.

Effects of ammonia on high affinity glutamate uptake and glutamate transporter EAAT3 expression in cultured rat cerebellar granule cells

Increased levels of extracellular glutamate are a consistent feature of hepatic encephalopathy (HE) associated with liver failure and other hyperammonemic pathologies. Reduction of glutamate uptake has been described in ammonia-exposed cultured astrocytes, synaptosomes, and in animal models of hyperammonemia. In the present study, we examine the effects of pathophysiological concentrations of ammonia on D-aspartate (a non-metabolizable analog of glutamate) uptake by cultured rat cerebellar granule neurons. Exposure of these cells to ammonia resulted in time-dependent (24% reduction at 24h and 60% reduction at 5 days, P<0.001) and dose-dependent (21, 37, and 57% reduction at 1, 2.5, and 5mM for 5 days, P<0.01) suppression of D-aspartate uptake. Kinetic analyses revealed significant decreases in the velocity of uptake (V(max)) (37% decrease at 2.5mM NH(4)Cl, P<0.05 and 52% decrease at 5mM NH(4)Cl, P<0.001) as well as significant reductions in K(m) values (25% reduction at 2.5mM NH(4)Cl, P<0.05 and 45% reduction at 5mM NH(4)Cl, P<0.001). Western blotting, on the other hand, showed no significant changes in the neuronal glutamate transporter EAAC1/EAAT3 protein, the only glutamate transporter currently known to be expressed by these cells. In addition, 1H combined with 13C-NMR spectroscopy studies using the stable isotope [1-13C]-glucose demonstrated a significant increase in intracellular glutamate levels derived from the oxidative metabolism of glucose, rather than from the deamidation of exogenous glutamine in cultured granule neurons exposed to ammonia. The present study provides evidence that the effects of ammonia on glutamate uptake are not solely an astrocytic phenomenon and that unlike the astrocytic glutamate transporter counterpart, EAAT3 protein expression in cultured cerebellar granule cells is not down-regulated when exposed to ammonia. Decrease of glutamate uptake in these cellular preparations may afford an additional regulatory mechanism aimed at controlling intracellular levels of glutamate and ultimately the releasable pool of glutamate in neurons.

In vivo NMR studies of neurodegenerative diseases in transgenic and rodent models

In vivo magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide unique quality to attain neurochemical, physiological, anatomical, and functional information non-invasively. These techniques have been increasingly applied to biomedical research and clinical usage in diagnosis and prognosis of diseases. The ability of MRS to detect early yet subtle changes of neurochemicals in vivo permits the use of this technology for the study of cerebral metabolism in physiological and pathological conditions. Recent advances in MR technology have further extended its use to assess the etiology and progression of neurodegeneration. This review focuses on the current technical advances and the applications of MRS and MRI in the study of neurodegenerative disease animal models including amyotrophic lateral sclerosis, Alzheimer's, Huntington's, and Parkinson's diseases. Enhanced MR measurable neurochemical parameters in vivo are described in regard to their importance in neurodegenerative disorders and their investigation into the metabolic alterations accompanying the pathogenesis of neurodegeneration.

In vivo NMR studies of the glutamate neurotransmitter flux and neuroenergetics: implications for brain function

Until very recently, non-invasive measurement of the glutamate-glutamine cycle in the intact mammalian brain had not been possible. In this review, we describe some studies that have led to quantitative assessment of the glutamate-glutamine cycle (Vcyc), as well as other important metabolic fluxes (e.g., glucose oxidation, CMRglc(ox)), with (13)C magnetic resonance spectroscopy (MRS) in vivo. These (13)C MRS studies clearly demonstrate that glutamate released from presynaptic neurons is taken up by the astrocyte for subsequent glutamine synthesis. Contrary to the earlier concept of a small, metabolically inactive neurotransmitter pool, in vivo (13)C MRS studies demonstrate that glutamate release and recycling is a major metabolic pathway that cannot be distinguished from its actions of neurotransmission. Furthermore, the in vivo (13)C MRS studies demonstrate in the rat cerebral cortex that increases in Vcyc and neuronal CMRglc(ox) are linearly related with a close to 1:1 slope. Measurements in human cerebral cortex are in agreement with this result. This relationship is consistent with more than two thirds of the energy yielded by glucose oxidation being used to support events associated with glutamate neurotransmission, and it supports a molecular model of a stoichiometric coupling between glutamate neurotransmission and functional glucose oxidation. (13)C MRS measurements of resting human cerebral cortex have found a high level of glutamate-glutamine cycling. This high resting neuronal activity, which is subtracted away in brain mapping studies by positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has significant implications for the interpretations of functional imaging data. Here we review and discuss the importance of neurotransmission and neuroenergetics as measured by (13)C MRS for understanding brain function and interpreting fMRI.

Energy metabolism in astrocytes and neurons treated with manganese: relation among cell-specific energy failure, glucose metabolism, and intercellular trafficking using multinuclear NMR-spectroscopic analysis

A central question in manganese neurotoxicity concerns mitochondrial dysfunction leading to cerebral energy failure. To obtain insight into the underlying mechanism(s), the authors investigated cell-specific pathways of [1-13C]glucose metabolism by high-resolution multinuclear NMR-spectroscopy. Five-day treatment of neurons with 100-micro mol/L MnCl(2) led to 50% and 70% decreases of ATP/ADP and phosphocreatine-creatine ratios, respectively. An impaired flux of [1-13C]glucose through pyruvate dehydrogenase, which was associated with Krebs cycle inhibition and hence depletion of [4-13C]glutamate, [2-13C]GABA, and [13C]glutathione, hindered the ability of neurons to compensate for mitochondrial dysfunction by oxidative glucose metabolism and further aggravated neuronal energy failure. Stimulated glycolysis and oxidative glucose metabolism protected astrocytes against energy failure and oxidative stress, leading to twofold increased de novo synthesis of [3-13C]lactate and fourfold elevated [4-13C]glutamate and [13C]glutathione levels. Manganese, however, inhibited the synthesis and release of glutamine. Comparative NMR data obtained from cocultures showed disturbed astrocytic function and a failure of astrocytes to provide neurons with substrates for energy and neurotransmitter metabolism, leading to deterioration of neuronal antioxidant capacity (decreased glutathione levels) and energy metabolism. The results suggest that, concomitant to impaired neuronal glucose oxidation, changes in astrocytic metabolism may cause a loss of intercellular homeostatic equilibrium, contributing to neuronal dysfunction in manganese neurotoxicity.

Automated data processing of [1H-decoupled] 13C MR spectra acquired from human brain in vivo

In clinical 13C infusion studies, broadband excitation of 200 ppm of the human brain yields 13C MR spectra with a time resolution of 2-5 min and generates up to 2000 metabolite peaks over 2h. We describe a fast, automated, observer-independent technique for processing [1H-decoupled] 13C spectra. Quantified 13C spectroscopic signals, before and after the administration of [1-13C]glucose and/or [1-13C]acetate in human subjects are determined. Stepwise improvements of data processing are illustrated by examples of normal and pathological results. Variation in analysis of individual 13C resonances ranged between 2 and 14%. Using this method it is possible to reliably identify subtle metabolic effects of brain disease including Alzheimer's disease and epilepsy.

Effect of hypoglycemia on brain glycogen metabolism in vivo

The brain contains a small but significant amount of glycogen, which has long been considered to play an insignificant role in the brain. In this study, brain glycogen metabolism was measured using (13)C NMR spectroscopy at 9.4 T. Brain glycogen metabolism was modulated by hyperinsulinemia resulting in a net accumulation. The role of glycogen in maintaining brain function is unknown; one possibility is that it may serve as an endogenous glucose reservoir to protect the brain against severe hypoglycemia. To address this possibility, rats were subjected to insulin-induced moderate hypoglycemia and when the level of brain glucose approached zero, brain glycogen content began to decrease gradually, demonstrating utilization of this glucose reservoir. The brain glycogen signal never became undetectable, however, even during 2 hr of hypoglycemia. When plasma and brain glucose concentrations were restored, glycogen increased and the concentration exceeded the pre-hypoglycemic level by several-fold. The data suggest that brain glycogen can provide fuel for extended periods of time when glucose supply is inadequate. Furthermore, brain glycogen can rebound (super-compensate) after a single episode of hypoglycemia. We postulate that brain glycogen serves as an energy store during hypoglycemia and that it may participate in the creation of reduced physiological responses to hypoglycemia that are involved in a symptom often observed in patients with diabetes, hypoglycemia unawareness.

Cerebral metabolism and consciousness

In vivo 13C magnetic resonance spectroscopy studies of the brain have measured rates of glutamate-glutamine cycle (Vcyc) and glucose oxidation (CMRglc(ox)) by detecting 13C label turnover from glucose to glutamate and glutamine. In both the awake human and in the anesthetized rat brains Vcyc and CMRglc(ox) are stoichiometrically related, and form a major pathway in which approximately 80% of the energy from glucose oxidation supports events associated with glutamate neurotransmission. The high energy consumption of the brain at rest and its quantitative usage for neurotransmission reflect a high level of neuronal activity for the non-stimulated brain. This high activity supports a reinterpretation of functional imaging data, e.g., where the large baseline signal has commonly been discarded. Independent measurements of energy consumption (delta CMRO2%) obtained from calibrated fMRI equaled percentage changes in neuronal spiking rate (delta nu %) measured by electrodes during sensory stimulation at two depths of anesthesia. These quantitative biophysical relationships between energy consumption and neuronal activity provide novel insights into the nature of brain function. The high resting brain activity is proposed to include the global interactions constituting the subjective aspects of consciousness. Anesthesia by lowering the total firing rates correlates with the loss of consciousness. These results, which measure the localized neuronal response and distinguish inputs of peripheral neurons from inputs of neurons from other brain regions, fit comfortably into the neuronal scheme of a global workspace proposed by Dehaene and Changeux.

Detection of [1,6-13C2]-glucose metabolism in rat brain by in vivo 1H-[13C]-NMR spectroscopy

Localized, water-suppressed (1)H-[(13)C]-NMR spectroscopy was used to detect (13)C-label accumulation in cerebral metabolites following the intravenous infusion of [1,6-(13)C(2)]-glucose (Glc). The (1)H-[(13)C]-NMR method, based on adiabatic RF pulses, 3D image-selected in vivo spectroscopy (ISIS) localization, and optimal shimming, yielded high-quality (1)H-[(13)C]-NMR spectra with optimal NMR sensitivity. As a result, the (13)C labeling of [4-(13)C]-glutamate (Glu) and [4-(13)C]-glutamine (Gln) could be detected from relatively small volumes (100 microL) with a high temporal resolution. The formation of [n-(13)C]-Glu, [n-(13)C]-Gln (n = 2 or 3), [2-(13)C]-aspartate (Asp), [3-(13)C]-Asp, [3-(13)C]-alanine (Ala), and [3-(13)C]-lactate (Lac) was also observed to be reproducible. The (13)C-label incorporation curves of [4-(13)C]-Glu and [4-(13)C]-Gln provided direct information on metabolic pathways. Using a two-compartment metabolic model, the tricarboxylic acid (TCA) cycle flux was determined as 0.52 +/- 0.04 micromol/min/g, while the glutamatergic neurotransmitter flux equaled 0.25 +/- 0.05 micromol/min/g, in good correspondence with previously determined values.

Effect of deep pentobarbital anesthesia on neurotransmitter metabolism in vivo: on the correlation of total glucose consumption with glutamatergic action

The effect of deep barbiturate anesthesia on brain glucose transport, TCA cycle flux, and aspartate, glutamate, and glutamine metabolism was assessed in the rat brain using 13C nuclear magnetic resonance spectroscopy at 9.4 T in conjunction with [1-13C] glucose infusions. Brain glucose concentrations were elevated, consistent with a twofold reduced cerebral metabolic rate for glucose (CMRglc) compared with light alpha-chloralose anesthesia. Using a mathematical model of neurotransmitter metabolism, several metabolic reaction rates were extracted from the rate of label incorporation. Total oxidative glucose metabolism, CMRglc(ox), was 0.33 +/- 0.03 micromol x g(-1) x min(-1). The neuronal TCA cycle rate was similar to that in the glia, 0.35 +/- 0.03 micromol x g(-1) x min(-1) and 0.26 +/- 0.06 micromol x g(-1) x min(-1), respectively, suggesting that neuronal energy metabolism was mainly affected. The rate of pyruvate carboxylation was 0.03 +/- 0.01 micromol x g(-1) x min(-1). The exchange rate between cytosolic glutamate and mitochondrial 2-oxoglutarate, Vx, was equal to the rate of neuronal pyruvate dehydrogenase flux. This indicates that Vx is coupled to CMRglc(ox), implying that the malate-aspartate shuttle is the major mechanism that facilitates label exchange across the inner mitochondrial membrane. The apparent rate of glutamatergic neurotransmission, V(NT), was 0.04 +/- 0.01 micromol x g x min, consistent with strong reductions in electrical activity. However, the rates of cerebral oxidative glucose metabolism and glutamatergic neurotransmission, CMRglc(ox)/V(NT), did not correlate with a 1:1 stoichiometry.

Glial uptake of neurotransmitter glutamate from the extracellular fluid studied in vivo by microdialysis and (13)C NMR

Glial uptake of neurotransmitter glutamate (GLU) from the extracellular fluid was studied in vivo in rat brain by (13)C NMR and microdialysis combined with gas-chromatography/mass-spectrometry. Brain GLU C5 was (13)C enriched by intravenous [2,5-(13)C]glucose infusion, followed by [(12)C]glucose infusion to chase (13)C from the small glial GLU pool. This leaves [5-(13)C]GLU mainly in the large neuronal metabolic pool and the vesicular neurotransmitter pool. During the chase, the (13)C enrichment of whole-brain GLU C5 was significantly lower than that of extracellular GLU (GLU(ECF)) derived from exocytosis of vesicular GLU. Glial uptake of neurotransmitter [5-(13)C]GLU(ECF) was monitored in vivo through the formation of [5-(13)C,(15)N]GLN during (15)NH(4)Ac infusion. From the rate of [5-(13)C,(15)N]GLN synthesis (1.7 +/- 0.03 micromol/g/h), the mean (13)C enrichment of extracellular GLU (0.304 +/- 0.011) and the (15)N enrichment of precursor NH(3) (0.87 +/- 0.014), the rate of synthesis of GLN (V'(GLN)), derived from neurotransmitter GLU(ECF), was determined to be 6.4 +/- 0.44 micromol/g/h. Comparison with V(GLN) measured previously by an independent method showed that the neurotransmitter provides 80-90% of the substrate GLU pool for GLN synthesis. Hence, under our experimental conditions, the rate of 6.4 +/- 0.44 micromol/g/h also represents a reasonable estimate for the rate of glial uptake of GLU(ECF), a process that is crucial for protecting the brain from GLU excitotoxicity.

A model of the coupling between brain electrical activity, metabolism, and hemodynamics: application to the interpretation of functional neuroimaging

In order to improve the interpretation of functional neuroimaging data, we implemented a mathematical model of the coupling between membrane ionic currents, energy metabolism (i.e., ATP regeneration via phosphocreatine buffer effect, glycolysis, and mitochondrial respiration), blood-brain barrier exchanges, and hemodynamics. Various hypotheses were tested for the variation of the cerebral metabolic rate of oxygen (CMRO(2)): (H1) the CMRO(2) remains at its baseline level; (H2) the CMRO(2) is enhanced as soon as the cerebral blood flow (CBF) increases; (H3) the CMRO(2) increase depends on intracellular oxygen and pyruvate concentrations, and intracellular ATP/ADP ratio; (H4) in addition to hypothesis H3, the CMRO(2) progressively increases, due to the action of a second messenger. A good agreement with experimental data from magnetic resonance imaging and spectroscopy (MRI and MRS) was obtained when we simulated sustained and repetitive activation protocols using hypotheses (H3) or (H4), rather than hypotheses (H1) or (H2). Furthermore, by studying the effect of the variation of some physiologically important parameters on the time course of the modeled blood-oxygenation-level-dependent (BOLD) signal, we were able to formulate hypotheses about the physiological or biochemical significance of functional magnetic resonance data, especially the poststimulus undershoot and the baseline drift.

Development of (17)O NMR approach for fast imaging of cerebral metabolic rate of oxygen in rat brain at high field

A comprehensive technique was developed for using three-dimensional (17)O magnetic resonance spectroscopic imaging at 9.4T for rapidly imaging the cerebral metabolic rate of oxygen consumption (CMRO(2)) in the rat brain during a two-min inhalation of (17)O(2). The CMRO(2) value (2.19 +/- 0.14 micromol/g/min, n = 7) was determined in the rat anesthetized with alpha-chloralose by independent and concurrent (17)O NMR measurements of cerebral H(2)17O content, arterial input function, and cerebral perfusion. CMRO(2) values obtained were consistent with the literature results for similar conditions. Our results reveal that, because of its superior sensitivity at ultra-high fields, the (17)O magnetic resonance spectroscopic imaging approach is capable of detecting small dynamic changes of metabolic H(2)17O during a short inhalation of (17)O(2) gas, and ultimately, for imaging CMRO(2) in the small rat brain. This study provides a crucial step toward the goal of developing a robust and noninvasive (17)O NMR approach for imaging CMRO(2) in animal and human brains that can be used for studying the central role of oxidative metabolism in brain function under normal and diseased conditions, as well as for understanding the mechanisms underlying functional MRI.

Magnetic resonance spectroscopic approaches to studying neuronal: glial interactions

In vivo magnetic resonance spectroscopy (MRS) is a noninvasive technique for the measurement of the concentration and synthesis of metabolites in the brain. Application of the state-of-the-art in vivo (13)C and (15)N MRS techniques to studying the synthesis of glutamate and glutamine has revealed that the glutamate-glutamine cycle between neurons and glia is a major metabolic flux, with a flux rate of 60%-80% relative to neuronal oxidative glucose metabolism in the resting human cerebral cortex. The MRS studies leading to the quantification of the glutamate-glutamine cycling flux are reviewed here. The advantages and limitations of different strategies are also discussed.

In vivo 13C NMR studies of compartmentalized cerebral carbohydrate metabolism

Localized 13C nuclear magnetic resonance (NMR) spectroscopy provides a unique window for studying cerebral carbohydrate metabolism through, e.g. the completely non-invasive measurement of cerebral glucose and glycogen metabolism. In addition, label incorporation into amino acid neurotransmitters such as glutamate (Glu), GABA and aspartate can be measured providing information on Krebs cycle flux and oxidative metabolism. Given the compartmentation of key enzymes such as pyruvate carboxylase and glutamine synthetase, the detection of label incorporation into glutamine indicated that neuronal and glial metabolism can be measured in vivo. The purpose of this paper is to provide a critical overview of these recent advances into measuring compartmentation of brain energy metabolism using localized in vivo 13C NMR spectroscopy. The studies reviewed herein showed that anaplerosis is significant in brain, as is oxidative ATP generation in glia and the rate of glial glutamine synthesis attributed to the replenishment of the neuronal Glu pool and that brain glycogen metabolism is slow under resting conditions. This new modality promises to provide a new investigative tool to study aspects of normal and diseased brain hitherto unaccessible, such as the interplay between glutamatergic action, glucose and glycogen metabolism during brain activation, and the derangements thereof in patients with hepatic encephalopathy, neurodegenerative diseases and diabetes.

Glutamate-glutamine cycling in the epileptic human hippocampus

PURPOSE

Several findings suggest that energy metabolism and the glutamate-glutamine cycle may be impaired in epilepsy. Positron emission tomography often shows interictal hypometabolism of the epileptogenic hippocampus. In vivo microdialysis studies show that seizure-associated glutamate release is doubled, and clearance is slowed. We hypothesized that the glutamate-glutamine cycle between neurons and glia may be decreased in the epileptic human hippocampus.

METHODS

A 20% solution of 2-13C-glucose was infused before resection of the epileptogenic hippocampus. Blood glucose isotopic fractions were measured every 30 min. Blood and brain specimens were frozen quickly; perchloric acid extracts of the small metabolites were prepared and analyzed by proton and carbon magnetic resonance spectroscopy (MRS) at 11.75 Tesla.

RESULTS

Standard histology showed 12 with hippocampal sclerosis and five with minimal neuron loss. The relative rates of glutamate-glutamine cycling with respect to glutamate synthesis were decreased in biopsies affected by hippocampal sclerosis (mean, 0.08; 95% confidence interval, 0.04-0.12) compared with those with minimal neuron loss (0.52; 95% CI, 0.30-0.75). Mean cellular glutamate concentrations were higher in minimal neuron loss (8.9 mM; 95% CI, 7.4-10.4) than hippocampal sclerosis (7.3 mM; 95% CI, 5.9-8.7). Cellular glutamine concentrations (mean, 2.8 mM; 95% CI, 2.4-3.2; n = 17) were the same in all groups.

CONCLUSIONS

The epileptogenic, gliotic human hippocampus appears to be characterized metabolically by slow rates of glutamate-glutamine cycling, decreased glutamine content, and a relative increase in glutamate content. We hypothesize that the low rate of glutamate-glutamine cycling that results from a failure of glial glutamate detoxification could account for slow glutamate clearance from synapses and continuing low-grade excitotoxicity.

Time-dependence of the contribution of pyruvate carboxylase versus pyruvate dehydrogenase to rat brain glutamine labelling from [1-(13) C]glucose metabolism

[1-(13) C]glucose metabolism in the rat brain was investigated after intravenous infusion of the labelled substrate. Incorporation of the label into metabolites was analysed by NMR spectroscopy as a function of the infusion time: 10, 20, 30 or 60 min. Specific enrichments in purified mono- and dicarboxylic amino acids were determined from (1) H-observed/(13) C-edited and (13) C-NMR spectroscopy. The relative contribution of pyruvate carboxylase versus pyruvate dehydrogenase (PC/PDH) to amino acid labelling was evaluated from the enrichment difference between either C2 and C3 for Glu and Gln, or C4 and C3 for GABA, respectively. No contribution of pyruvate carboxylase to aspartate, glutamate or GABA labelling was evidenced. The pyruvate carboxylase contribution to glutamine labelling varied with time. PC/PDH decreased from around 80% after 10 min to less than 30% between 20 and 60 min. This was interpreted as reflecting different labelling kinetics of the two glutamine precursor glutamate pools: the astrocytic glutamate and the neuronal glutamate taken up by astrocytes through the glutamate-glutamine cycle. The results are discussed in the light of the possible occurrence of neuronal pyruvate carboxylation. The methods previously used to determine PC/PDH in brain were re-evaluated as regards their capacity to discriminate between astrocytic (via pyruvate carboxylase) and neuronal (via malic enzyme) pyruvate carboxylation.

[2,4-13 C2 ]-beta-Hydroxybutyrate metabolism in human brain

Infusions of [2,4-13C2]-beta-hydroxybutyrate and 1H-13C polarization transfer spectroscopy were used in normal human subjects to detect the entry and metabolism of beta-hydroxybutyrate in the brain. During the 2-hour infusion study, 13C label was detectable in the beta-hydroxybutyrate resonance positions and in the amino acid pools of glutamate, glutamine, and aspartate. With a plasma concentration of 2.25 +/- 0.24 mmol/L (four volunteers), the apparent tissue beta-hydroxybutyrate concentration reached 0.18 +/- 0.06 mmol/L during the last 20 minutes of the study. The relative fractional enrichment of 13C-4-glutamate labeling was 6.78 +/- 1.71%, whereas 13C-4-glutamine was 5.68 +/- 1.84%. Steady-state modeling of the 13C label distribution in glutamate and glutamine suggests that, under these conditions, the consumption of the beta-hydroxybutyrate is predominantly neuronal, used at a rate of 0.032 +/- 0.009 mmol. kg-1. min-1, and accounts for 6.4 +/- 1.6% of total acetyl coenzyme A oxidation. These results are consistent with minimal accumulation of cerebral ketones with rapid utilization, implying blood-brain barrier control of ketone oxidation in the nonfasted adult human brain.

Effect of endogenous beta-hydroxybutyrate on brain glucose metabolism in fetuses of diabetic rabbits, studied by (13)C magnetic resonance spectroscopy

The aim of our research was to uncover perturbations in in-utero fetal cerebral metabolism resulting from hyperglycemia and hyperketonemia, which occur during maternal diabetes. Therefore, we examined the effects of glucose overload and hyperketonemia on glucose metabolism in the diabetic fetal brain; more specifically, the effect of diabetes on the glucose flux via pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) and subsequent metabolism in the fetal cerebral tricarboxylic acid (TCA) cycle were examined, as well as the effect of diabetes on energy fuel utilization in the neurons and glia. Diabetes was induced in pregnant rabbits, and towards term, [U-(13)C(6)]glucose was infused into maternal circulation, and [(13)C]glucose metabolites were subsequently studied in fetal brain extracts by (13)C MRS isotopomer analysis. Significantly elevated maternal and fetal plasma glucose levels (three- and up to fivefold, respectively) and fetal brain glucose levels (up to eightfold) accompanied by an increase of beta-hydroxybutyrate (beta-HBA) levels (approximately 20-fold) were found in the hyperketonemic diabetic animals, whereas fetal cerebral lactate levels were decreased. Alterations in the (13)C labeling patterns, mainly of glutamine, led us to suggest that the entry of beta-HBA-derived acetyl-CoA inhibits formation and entry of labeled glucose-derived acetyl-CoA into the TCA cycle, mainly in glia. Accumulation of glucose and the decrease in lactate levels in the fetal brain are most likely the result of an inhibitory effect of beta-HBA on glycolysis. In addition, loss of (13)C enrichment of TCA cycle intermediates and products, glutamate and glutamine, in the hyperketonemic diabetic fetal brain may be attributed to the effect of beta-HBA fuel utilization by the fetal brain.

Astroglial contribution to brain energy metabolism in humans revealed by 13C nuclear magnetic resonance spectroscopy: elucidation of the dominant pathway for neurotransmitter glutamate repletion and measurement of astrocytic oxidative metabolism

Increasing evidence supports a crucial role for glial metabolism in maintaining proper synaptic function and in the etiology of neurological disease. However, the study of glial metabolism in humans has been hampered by the lack of noninvasive methods. To specifically measure the contribution of astroglia to brain energy metabolism in humans, we used a novel noninvasive nuclear magnetic resonance spectroscopic approach. We measured carbon 13 incorporation into brain glutamate and glutamine in eight volunteers during an intravenous infusion of [2-13C] acetate, which has been shown in animal models to be metabolized specifically in astroglia. Mathematical modeling of the three established pathways for neurotransmitter glutamate repletion indicates that the glutamate/glutamine neurotransmitter cycle between astroglia and neurons (0.32 +/- 0.07 micromol x gm(-1) x min(-1)) is the major pathway for neuronal glutamate repletion and that the astroglial TCA cycle flux (0.14 +/- 0.06 micromol x gm(-1) x min(-1)) accounts for approximately 14% of brain oxygen consumption. Up to 30% of the glutamine transferred to the neurons by the cycle may derive from replacement of oxidized glutamate by anaplerosis. The further application of this approach could potentially enlighten the role of astroglia in supporting brain glutamatergic activity and in neurological and psychiatric disease.

The first in vivo observation of (13)C-(15)N coupling in mammalian brain

[5-(13)C,(15)N]Glutamine, with (1)J((13)C-(15)N) of 16 Hz, was observed in vivo in the brain of spontaneously breathing rats by (13)C MRS at 4.7 T. The brain [5-(13)C]glutamine peak consisted of the doublet from [5-(13)C,(15)N]glutamine and the center [5-(13)C,(14)N]glutamine peak, resulting in an apparent triplet with a separation of 8 Hz. The time course of formation of brain [5-(13)C,(15)N]glutamine was monitored in vivo with a time resolution of 20-35 min. This [5-(13)C,(15)N]glutamine was formed by glial uptake of released neurotransmitter [5-(13)C]glutamate and its reaction with (15)NH(3) catalyzed by the glia-specific glutamine synthetase. The neurotransmitter glutamate C5 was selectively (13)C-enriched by intravenous [2,5-(13)C]glucose infusion to (13)C-label whole-brain glutamate C5, followed by [(12)C]glucose infusion to chase (13)C from the small and rapidly turning-over glial glutamate pool, leaving (13)C mainly in the neurotransmitter [5-(13)C]glutamate pool, which is sequestered in vesicles until release. Hence, the observed [5-(13)C,(15)N]glutamine arises from a coupling between (13)C of neuronal origin and (15)N of glial origin. Measurement of the rate of brain [5-(13)C,(15)N]glutamine formation provides a novel noninvasive method of studying the kinetics of neurotransmitter uptake into glia in vivo, a process that is crucial for protecting the brain from glutamate excitotoxicity.

Glucose and lactate metabolism during brain activation

The dependence of brain function on blood glucose as a fuel does not exclude the possibility that lactate within the brain might be transferred between different cell types and serve as an energy source. It has been recently suggested that 1) about 85% of glucose consumption during brain activation is initiated by aerobic glycolysis in astrocytes, triggered by demand for glycolytically derived energy for Na+ -dependent accumulation of transmitter glutamate and its amidation to glutamine, and 2) the generated lactate is quantitatively transferred to neurons for oxidative degradation. However, astrocytic glutamate uptake can be fueled by either glycolytically or oxidatively derived energy, and the extent to which "metabolic trafficking" of lactate might occur during brain function is unknown. In this review, the potential for an astrocytic-neuronal lactate flux has been estimated by comparing rates of glucose utilization in brain and in cultured neurons and astrocytes with those for lactate release and uptake. Working brain tissue and isolated brain cells release large amounts of lactate. Cellular lactate uptake occurs by carrier-mediated facilitated diffusion and is normally limited by its dependence on metabolism of accumulated lactate to maintain a concentration gradient. The rate of this process is similar in cultured astrocytes and glutamatergic neurons, and, at physiologically occurring lactate concentrations, lactate uptake corresponds at most to 25% of the rate of glucose oxidation, which accordingly is the upper limit for "metabolic trafficking" of lactate. Because of a larger local release than uptake of lactate and the necessity for rapid lactate clearance to maintain the intracellular redox state to support lactate production in the presence of normal oxygen levels, brain activation in vivo is probably, in many cases, accompanied by a substantial overflow of glycolytically generated lactate, both to different brain areas and under some conditions (spreading depression, hyperammonemia) to circulating blood.