Determination of the rate of the glutamate/glutamine cycle in the human brain by in vivo 13C NMR

Diffusion-weighted NMR spectroscopy allows probing of 13C labeling of glutamate inside distinct metabolic compartments in the brain

In the present work, diffusion-weighted (DW)-NMR spectroscopy of glutamate was performed during a (13)C-labeled glucose infusion in monkey brain (six experiments). It is shown that glutamate (13)C labeling occurs significantly faster at higher diffusion weightings-slightly for glutamate in position C4, and more markedly for glutamate in position C3. This demonstrates the existence of different diffusion compartments for glutamate, associated with different metabolic rates. Metabolic modeling of (13)C enrichment time-courses suggests that these compartments might be gray and white matter, each having a specific oxidative metabolism rate possibly paralleled by a specific glutamate diffusion coefficient.

Astrocytes as the glucose shunt for glutamatergic neurons at high activity: an in silico study

The question of the preferred substrate of glutamatergic neurons at high neural activity has been vibrantly debated for over a decade since the classical hypothesis (CH) of the primacy of glucose has been challenged by the astrocyte-neuron lactate shuttle hypothesis (ANLSH), which replaces the primacy of glucose with astrocyte produced lactate. We perform Bayesian Flux Balance Analysis (BFBA) with a new mathematical model of cellular brain energetics, comprising detailed biochemical pathways in and between astrocytes and glutamatergic neurons and partitioning of each cell type into cytosol and mitochondria. Supported by the results of our in silico studies, which are in remarkable agreement with previously published results, we posit the Glucose Shunt Hypothesis (GSH) that during high activity, the inhibition of the phosphofructokinase (PFK) enzyme in neuron impairs neuronal glycolysis, enabling the process by which lactate effluxed by astrocytes is taken up by glutamatergic neurons, whereas at low activity, glucose remains the preferred substrate for neurons. We postulate that the ANLS is a shunt utilized by glutamatergic neurons to bypass their glycolysis impaired by the inhibition of PFK in connection with increased oxidative phosphorylation at high neuronal activity.

Localized sensitivity enhanced in vivo 13C MRS to detect glucose metabolism in the mouse brain

The application of in vivo 13C MR spectroscopy to mouse brain models is potentially valuable for improving the understanding of cerebral carbohydrate metabolism and glutamatergic neurotransmission in various neuropathologies. However, the low sensitivity of 13C nuclei and contaminating signals of lipids in the relatively small mouse brain make this application rather challenging. To meet these technical challenges, localized semi-adiabatic distortionless enhanced polarization transfer (DEPT) MR spectroscopy in combination with a continuous intravenous [1,6-13C2] glucose infusion was implemented to detect glucose metabolism in isoflurane-anesthetized mice at 7T. The signal enhancement and high spectral resolution obtained in these experiments enabled the separate determination of 13C label incorporation into as much as 13 metabolites from a 175 microL volume. Signal increases of glucose (C6), glutamine (C3, C4), and glutamate (C3, C4) were determined with a time resolution of 8.6 min. This study demonstrates an optimized MR method for the application of in vivo 13C MRS in mouse brain.

Adaptation of brain glutamate plus glutamine during abstinence from chronic methamphetamine use

Methamphetamine (METH) is a stimulant drug that is toxic primarily to dopaminergic and serotonergic neurons and may lead to inflammatory changes in the brain. Additionally, the glutamatergic system is altered following METH exposure. Therefore, concentrations of brain glutamate + glutamine (GLX) were assessed during abstinence from chronic METH abuse. Twenty-five subjects with a history of METH dependence (age 31.8 +/- 7.4 years, 14 women and 11 men) and 28 control subjects without a history of drug abuse (age 32.6 +/- 8.8 years, 14 women) were enrolled. Twelve of the METH subjects were followed and rescanned 5 months later. METH users had used the drug 5.9 +/- 1.7 times per week, for 109 +/- 69 months, and had been abstinent for 2.1 +/- 3.0 months. GLX was measured in the basal ganglia and frontal gray and white matter, using proton magnetic resonance spectroscopy. While overall GLX concentrations at baseline were similar between METH and control subjects, METH users with less than or equal to 1 month of abstinence showed reduced frontal gray matter GLX (p = 0.01). Time of abstinence correlated positively with GLX in frontal gray (p < 0.0001) and white matter (p = 0.03). After 5 months, changes in frontal gray matter GLX showed a trend to correlate inversely with the duration of abstinence (p = 0.07). Subjects with craving symptoms had lower frontal gray matter GLX than those without craving (-8%, p = 0.05). These findings suggest dynamic abnormalities in brain GLX in recently abstinent METH users, with a depletion of the glutamatergic system in METH users within the first 2 months of abstinence and some normalization during prolonged abstinence. Since craving may contribute to relapse, medications that normalize GLX may minimize craving in METH users.

The micro-architecture of the cerebral cortex: functional neuroimaging models and metabolism

In order to interpret/integrate data obtained with different functional neuroimaging modalities (e.g. fMRI, EEG/MEG, PET/SPECT, fNIRS), forward-generative models of a diversity of brain mechanisms at the mesoscopic level are considered necessary. For the cerebral cortex, the brain structure with possibly the most relevance for functional neuroimaging, a variety of such biophysical models has been proposed over the last decade. The development of technological tools to investigate in vitro the physiological, anatomical and biochemical principles at the microscopic scale in comparative studies formed the basis for such theoretical progresses. However, with the most recent introduction of systems to record electrical (e.g. miniaturized probes chronically/acutely implantable in the brain), optical (e.g. two-photon laser scanning microscopy) and atomic nuclear spectral (e.g. nuclear magnetic resonance spectroscopy) signals using living laboratory animals, the field is receiving even greater attention. Major advances have been achieved by combining such sophisticated recording systems with new experimental strategies (e.g. transgenic/knock-out animals, high resolution stereotaxic manipulation systems for probe-guidance and cellular-scale chemical-delivery). Theoreticians may now be encouraged to re-consider previously formulated mesoscopic level models in order to incorporate important findings recently made at the microscopic scale. In this series of reviews, we summarize the background at the microscopic scale, which we suggest will constitute the foundations for upcoming representations at the mesoscopic level. In this first part, we focus our attention on the nerve ending particles in order to summarize basic principles and mechanisms underlying cellular metabolism in the cerebral cortex. It will be followed by two parts highlighting major features in its organization/working-principles to regulate both cerebral blood circulation and neuronal activity, respectively. Contemporary theoretical models for functional neuroimaging will be revised in the fourth part, with particular emphasis in their applications, advantages/limitations and future prospects.

On the reliability of (13)C metabolic modeling with two-compartment neuronal-glial models

Metabolic modeling of (13)C NMR spectroscopy ((13)C MRS) data using two-compartment neuronal-glial models enabled non-invasive measurements of the glutamate-glutamine cycle rate (V(NT)) in the brain in vivo. However, the reliability of such two-compartment metabolic modeling has not been examined thoroughly. This study uses Monte-Carlo simulations to investigate the reliability of metabolic modeling of (13)C positional enrichment time courses measured in brain amino acids such as glutamate and glutamine during [1-(13)C]- or [1,6-(13)C(2)]glucose infusion. Results show that the determination of V(NT) is not very precise under experimental conditions typical of in vivo NMR studies, whereas the neuronal TCA cycle rate V(TCA(N)) is determined with a much higher precision. Consistent with these results, simulated (13)C positional enrichment curves for glutamate and glutamine are much more sensitive to the value of V(TCA(N)) than to the value of V(NT). We conclude that the determination of the glutamate-glutamine cycle rate V(NT) using (13)C MRS is relatively unreliable when fitting (13)C positional enrichment curves obtained during [1-(13)C] or [1,6-(13)C(2)]glucose infusion. Further developments are needed to improve the determination of V(NT), for example using additional information from (13)C-(13)C isotopomers and/or using glial specific substrates such as [2-(13)C]acetate.

Effect of exogenous glutamate and N-Methyl-D-aspartic acid on spontaneous activity of isolated human ureter

OBJECTIVES

While the neurotransmitter role of glutamate in the gastrointestinal tract has been shown, its effects on smooth muscle of the human ureter have not previously been investigated. In our study we have investigated the effects of exogenous glutamate on the spontaneous activity of isolated human ureter, taken from 14 adult patients after nephrectomy.

METHODS

The segment of ureter, excised 3 cm distal from the pyeloureteral junction, was isolated in an organ bath. Both longitudinal tension and intraluminal pressure of the segment were recorded simultaneously.

RESULTS

Glutamate administered in the lumen of the isolated ureteral segments (7.8 x 10(-7) M/L-3.5 x 10(-2) M/L) was ineffective. When added to the isolated organ bath from the serous side of the ureteral segment, glutamate (7.9 x 10(-6) M/L-10.6 x 10(-3) M/L) and N-Methyl-D-aspartic acid (NMDA) (9.1 x 10(-8) M/L-3.1 x 10(-5) M/L) produced a concentration-dependent increase in spontaneous activity of the isolated preparations, while kainic acid (6.3 x 10(-8) M/L-10.5 x 10(-5) M/L) and (+/-)-trans-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) (7.7 x 10(-8) M/L -6.5 x 10(-5) M/L) were ineffective.

CONCLUSIONS

The results of our study suggest that an excitatory neurotransmitter glutamate stimulates spontaneous activity of the human ureter through activation of NMDA ionotropic receptors, located on smooth muscle cells or intramural nerve fibers.

Inverse polarization transfer for detecting in vivo 13C magnetization transfer effect of specific enzyme reactions in 1H spectra

The wide chemical shift dispersion and long T(1) of (13)C have allowed determination of in vivo magnetization transfer effects caused by aspartate aminotransferase and lactate dehydrogenase reactions using (13)C magnetic resonance spectroscopy. In this report, we demonstrate that these effects can be observed in the proton spectra by transferring the equilibrium magnetization of (13)C via the one-bond scalar coupling between (13)C and (1)H using an inverse insensitive nuclei enhanced by polarization transfer-based heteronuclear polarization transfer method. This inverse method allows a combination of the advantages of the long (13)C T(1) for maximum magnetization transfer and the high sensitivity of proton detection. The feasibility of this in vivo inverse polarization transfer approach was evaluated for detecting the (13)C magnetization transfer effect of aspartate aminotransferase and lactate dehydrogenase reactions from a 72.5-microl voxel in the rat brain at 11.7 T.

Reconstruction and flux analysis of coupling between metabolic pathways of astrocytes and neurons: application to cerebral hypoxia

BACKGROUND

It is a daunting task to identify all the metabolic pathways of brain energy metabolism and develop a dynamic simulation environment that will cover a time scale ranging from seconds to hours. To simplify this task and make it more practicable, we undertook stoichiometric modeling of brain energy metabolism with the major aim of including the main interacting pathways in and between astrocytes and neurons.

MODEL

The constructed model includes central metabolism (glycolysis, pentose phosphate pathway, TCA cycle), lipid metabolism, reactive oxygen species (ROS) detoxification, amino acid metabolism (synthesis and catabolism), the well-known glutamate-glutamine cycle, other coupling reactions between astrocytes and neurons, and neurotransmitter metabolism. This is, to our knowledge, the most comprehensive attempt at stoichiometric modeling of brain metabolism to date in terms of its coverage of a wide range of metabolic pathways. We then attempted to model the basal physiological behaviour and hypoxic behaviour of the brain cells where astrocytes and neurons are tightly coupled.

RESULTS

The reconstructed stoichiometric reaction model included 217 reactions (184 internal, 33 exchange) and 216 metabolites (183 internal, 33 external) distributed in and between astrocytes and neurons. Flux balance analysis (FBA) techniques were applied to the reconstructed model to elucidate the underlying cellular principles of neuron-astrocyte coupling. Simulation of resting conditions under the constraints of maximization of glutamate/glutamine/GABA cycle fluxes between the two cell types with subsequent minimization of Euclidean norm of fluxes resulted in a flux distribution in accordance with literature-based findings. As a further validation of our model, the effect of oxygen deprivation (hypoxia) on fluxes was simulated using an FBA-derivative approach, known as minimization of metabolic adjustment (MOMA). The results show the power of the constructed model to simulate disease behaviour on the flux level, and its potential to analyze cellular metabolic behaviour in silico.

CONCLUSION

The predictive power of the constructed model for the key flux distributions, especially central carbon metabolism and glutamate-glutamine cycle fluxes, and its application to hypoxia is promising. The resultant acceptable predictions strengthen the power of such stoichiometric models in the analysis of mammalian cell metabolism.

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.

A combined spectroscopic and functional MRI investigation of the dorsal anterior cingulate region in opiate addiction

Converging neuropsychological and functional neuroimaging evidence indicates that the dorsal anterior cingulate cortex (dACC) is dysfunctional in drug-addicted populations. Few studies, however, have investigated the biochemical and physiological properties of the dACC in such populations. We used proton magnetic resonance spectroscopy ((1)H-MRS) together with functional magnetic resonance imaging (fMRI) to probe dACC biochemistry and physiological activity during performance of a behavioural control task in 24 opiate-dependent individuals (maintained on a stable dose of methadone or buprenorphine at the time of study) and 24 age, gender, intelligence and performance-matched healthy subjects. While both groups activated the dACC to comparable levels, the opiate-using group displayed relatively increased task-related activation of frontal, parietal and cerebellar regions, as well as reduced concentrations of dACC N-acetylaspartate and glutamate/glutamine. In addition, the opiate-using group failed to show the expected correlations between dACC activation and behavioural measures of cognitive control. These findings suggest that the dACC is biochemically and physiologically abnormal in long-term opiate-dependent individuals. Furthermore, opiate addicts required increased, perhaps compensatory, involvement of the fronto-parietal and cerebellar behavioural regulation network to achieve normal levels of task performance/behavioural control. These neurobiological findings may partly underpin key addiction-related phenomena, such as poor inhibitory control of drug-related behaviour in the face of adverse consequences, and may be of relevance to the design of future treatment studies.

Cerebral glutamine and glutamate levels in relation to compromised energy metabolism: a microdialysis study in subarachnoid hemorrhage patients

Astrocytic glutamate (Glt) uptake keeps brain interstitial Glt levels low. Within the astrocytes Glt is converted to glutamine (Gln), which is released and reconverted to Glt in neurons. The Glt-Gln cycle is energy demanding and impaired energy metabolism has been suggested to cause low interstitial Gln/Glt ratios. Using microdialysis (MD) measurements from visually noninjured cortex in 33 neurointensive care patients with subarachnoid hemorrhage, we have determined how interstitial Glt and Gln, as a reflection of the Glt-Gln cycle turnover, relate to perturbed energy metabolism. A total of 3703 hourly samples were analyzed. The lactate/pyruvate (L/P) ratios correlated to the Gln/Glt ratios (r=-0.66), but this correlation was not stronger than the correlation between L/P and Glt (r=0.68) or the correlation between lactate and Glt (r=0.65). A novel observation was a linear relationship between interstitial pyruvate and Gln (r=0.52). There were 13 periods (404 h) of 'energy crisis', defined by L/P ratios above 40. All were associated with high interstitial Glt levels. Periods with L/P ratios above 40 and low pyruvate levels were associated with decreased interstitial Gln levels, suggesting ischemia and failing astrocytic Gln synthesis. Periods with L/P ratios above 40 and normal or high pyruvate levels were associated with increased interstitial Gln levels, which may represent an astrocytic hyperglycolytic response to high interstitial Glt levels. The results imply that moderately elevated L/P ratios cannot always be interpreted as failing energy metabolism and that interstitial pyruvate levels may discriminate whether or not there is sufficient astrocytic capacity for Glt-Gln cycling in the brain.

Brain energy metabolism and neurotransmission at near-freezing temperatures: in vivo (1)H MRS study of a hibernating mammal

The brain of a hibernating mammal withstands physiological extremes that would result in cerebral damage and death in a non-hibernating species such as humans. To examine the possibility that this neuroprotection results from alterations in cerebral metabolism, we used in vivo(1)H NMR spectroscopy at high field (9.4 T) to measure the concentration of 18 metabolites (neurochemical profile) in the brain of 13-lined ground squirrels (Spermophilus tridecemlineatus) before, during, and after hibernation. Resolved in vivo(1)H NMR spectra were obtained even at low temperature in torpid hibernators ( approximately 7 degrees C). The phosphocreatine-to-creatine ratio was increased during torpor (+143%) indicating energy storage, and remained increased to a lesser extent during interbout arousal (IBA) (+83%). The total gamma-aminobutyric acid concentration was increased during torpor (+135%) and quickly returned to baseline during IBA. Glutamine (Gln) was decreased (-54%) during torpor but quickly returned to normal levels during IBA and after terminal arousal in the spring. Glutamate (Glu) was also decreased during torpor (-17%), but remained decreased during IBA (-20% compared with fall), and returned to normal level in the spring. Our observation that Glu and Gln levels are depressed in the brain of hibernators suggests that the balance between anaplerosis and loss of Glu and Gln (because of glutamatergic neurotransmission or other mechanisms) is altered in hibernation.

Novel strategy for cerebral 13C MRS using very low RF power for proton decoupling

One of the major difficulties of in vivo 13C MRS is the need to decouple the large, one-bond, 1H-13C scalar couplings in order to obtain useful signal-to-noise ratios (SNRs) and spectral resolution at magnetic field strengths that are accessible to clinical studies. In this report a new strategy for in vivo cerebral 13C MRS is proposed. We realized that the turnover kinetics of glutamate (Glu) C5 from exogenous [2-(13)C]glucose (Glc) is identical to that of Glu C4 from exogenous [1-(13)C]Glc. The carboxylic/amide carbons are only coupled to protons via very weak long-range 1H-13C scalar couplings. As such, they can be effectively decoupled at very low RF power. Therefore, decoupling of the large 1H-13C scalar couplings can be avoided by the use of [2-(13)C]Glc. An additional advantage of this strategy is the lack of contamination from subcutaneous lipids because there are no overlapping fat signals in the vicinity of the Glu C5 and glutamine (Gln) C5 peaks. The feasibility of this strategy was demonstrated using 13C MRS on rhesus monkey brains at 4.7T.

Glia mechanisms in mood regulation: a novel model of mood disorders

INTRODUCTION

Recent evidence in clinical and preclinical studies has implicated glutamate neurotransmissions in pathophysiology of mood disorders. The regulation of amino acid neurotransmission, i.e., glutamate and gamma-aminobutyric acid (GABA) involves coordinated mechanisms of uptake and transport within a tripartite synaptic system that includes neurons and glia. Newly appreciated role of the glia, more specifically astrocytes on neuronal functions combined with reported postmortem abnormalities of glia in patients with mood disorders further supports the role of glia in mood disorders.

MATERIALS AND METHODS

This report presents some of our preliminary results utilizing glia-selective toxins and other pharmacological tools to suppress glial function within the limbic system to study the resulting behavioral abnormalities, and thus, elucidate glial involvement in the development of mood disorders.

RESULTS AND DISCUSSION

We demonstrate that chronic blockade of glutamate uptake by a glial/neuronal transporter antagonist L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) within the amygdala, a key area implicated in mood regulation, results in dose-dependent reduction in social exploratory behavior and disrupts circadian activity patterns consistent with symptoms of mood disorders. Similarly, the selective astrocytic glutamate transporter type 1 (GLT-1) blocker dihydrokainic acid (DHK) injected into the amygdala also results in reduced social interaction that is blocked by selective glutamate N-methyl-D-aspartate (NMDA) type receptor antagonist AP5. The results are discussed in the context of glial and glutamate mechanisms in mood disorders and potential therapeutic avenues to address these mechanisms.

Modelling the role of excitatory and inhibitory neuronal activity in the generation of the BOLD signal

A biophysical model of the coupling between neuronal activity and the BOLD signal that allows for explicitly evaluating the role of both excitatory and inhibitory activity is formulated in this paper. The model is based on several physiological assumptions. Firstly, in addition to glycolysis, the "glycogen shunt" is assumed for excitatory synapses as a mechanism for energy production in the astrocytes. As a result, oxygen-to-glucose index (OGI) is not constant but varies with excitatory neuronal activity. In contrast, a constant OGI=6 (glycolysis) is assumed for inhibitory synapses. Finally we assume that cerebral blood flow is not directly controlled by energy usage, but it is only related to excitatory activity. Simulations' results show that increases in excitatory activity amplify the oscillations associated with the transient BOLD response, by increasing the initial dip, the maximum, and the post-stimulus undershoot of the signal. In contrast, increasing the inhibitory activity evoked an overall decrease of the BOLD signal along the whole time interval of the response. Simultaneous increase of both types of activity is then expected to reinforce the initial dip and the post-stimulus undershoot, while respective effects on the maximum tend to counteract each other. Two mechanisms for negative BOLD response (NBS) generation were predicted by the model: (i) when inhibition was present alone or together with low activation levels and (ii) when deactivation occurred independently of the accompanying inhibition level. Interestingly, NBS was associated with negative oxygen consumption changes only for the case of mechanism (ii).

Neuron-astrocyte interactions in the regulation of brain energy metabolism: a focus on NMR spectroscopy

An adequate and timely production of ATP by brain cells is of cardinal importance to support the major energetic cost of the rapid processing of information via synaptic and action potentials. Recently, evidence has been accumulated to support the view that the regulation of brain energy metabolism is under the control of an intimate dialogue between astrocytes and neurons. In vitro studies on cultured astrocytes and in vivo studies on rodents have provided evidence that glutamate and Na(+) uptake in astrocytes is a key triggering signal regulating glucose use in the brain. With the advent of NMR spectroscopy, it has been possible to provide experimental evidence to show that energy consumption is mainly devoted to glutamatergic neurotransmission and that glutamate-glutamine cycling is coupled in a approximately 1 : 1 molar stoichiometry to glucose oxidation, at least in the cerebral cortex. This improved understanding of neuron-astrocyte metabolic interactions offers the potential for developing novel therapeutic strategies for many neurological disorders that include a metabolic deficit.

Measurements of the anaplerotic rate in the human cerebral cortex using 13C magnetic resonance spectroscopy and [1-13C] and [2-13C] glucose

Recent studies in rodent and human cerebral cortex have shown that glutamate-glutamine neurotransmitter cycling is rapid and the major pathway of neuronal glutamate repletion. The rate of the cycle remains controversial in humans, because glutamine may come either from cycling or from anaplerosis via glial pyruvate carboxylase. Most studies have determined cycling from isotopic labeling of glutamine and glutamate using a [1-(13)C]glucose tracer, which provides label through neuronal and glial pyruvate dehydrogenase or via glial pyruvate carboxylase. To measure the anaplerotic contribution, we measured (13)C incorporation into glutamate and glutamine in the occipital-parietal region of awake humans while infusing [2-(13)C]glucose, which labels the C2 and C3 positions of glutamine and glutamate exclusively via pyruvate carboxylase. Relative to [1-(13)C]glucose, [2-(13)C]glucose provided little label to C2 and C3 glutamine and glutamate. Metabolic modeling of the labeling data indicated that pyruvate carboxylase accounts for 6 +/- 4% of the rate of glutamine synthesis, or 0.02 micromol/g/min. Comparison with estimates of human brain glutamine efflux suggests that the majority of the pyruvate carboxylase flux is used for replacing glutamate lost due to glial oxidation and therefore can be considered to support neurotransmitter trafficking. These results are consistent with observations made with arterial-venous differences and radiotracer methods.

In vivo dynamic turnover of cerebral 13C isotopomers from [U-13C]glucose

An INEPT-based (13)C MRS method and a cost-effective and widely available 11.7 Tesla 89-mm bore vertical magnet were used to detect dynamic (13)C isotopomer turnover from intravenously infused [U-(13)C]glucose in a 211 microL voxel located in the adult rat brain. The INEPT-based (1)H-->(13)C polarization transfer method is mostly adiabatic and therefore minimizes signal loss due to B(1) inhomogeneity of the surface coils used. High quality and reproducible data were acquired as a result of combined use of outer volume suppression, ISIS, and the single-shot three-dimensional localization scheme built in the INEPT pulse sequence. Isotopomer patterns of both glutamate C4 at 34.00 ppm and glutamine C4 at 31.38 ppm are dominated first by a doublet originated from labeling at C4 and C5 but not at C3 (with (1)J(C4C5) = 51 Hz) and then by a quartet originated from labeling at C3, C4, and C5 (with (1)J(C3C4) = 35 Hz). A lag in the transition of glutamine C4 pattern from doublet-dominance to quartet dominance as compared to glutamate C4 was observed, which provides an independent verification of the precursor-product relationship between neuronal glutamate and glial glutamine and a significant intercompartmental cerebral glutamate-glutamine cycle between neurons and glial cells.

The interaction of neuroactive steroids and GABA in the development of neuropsychiatric disorders in women

A growing literature suggests that hormonal fluctuations occurring across the menstrual cycle, during and after pregnancy, and during the menopausal transition are associated with onset of affective disorders or exacerbation of existing disorders. This influence of the neuroendocrine system on psychiatric disorders is thought to be mediated by an abnormality in central nervous system response to neuroactive steroids such as estradiol, progesterone, and the progesterone derivative allopregnanolone (ALLO). This interplay is considerably complex as neuroactive steroids modulate the function of multiple neurotransmitter systems throughout various stages of development. While one could choose to study any number of steroid-neurotransmitter interactions, our group in addition to others has focused our investigative efforts on unraveling the contribution of neuroactive steroids to psychiatric syndromes and disorders via their modulation of gamma aminobutyric acid (GABA), the brain's major inhibitory neurotransmitter. The goal of this article is two-fold: to synthesize the clinical and preclinical research focusing on the interplay between neuroactive steroids and GABA as they relate to neuropsychiatric and substance use disorders in women and to integrate data from our laboratory using proton magnetic resonance spectroscopy into this context.

Neuronal-glial glucose oxidation and glutamatergic-GABAergic function

Prior 13C magnetic resonance spectroscopy (MRS) experiments, which simultaneously measured in vivo rates of total glutamate-glutamine cycling (V(cyc(tot))) and neuronal glucose oxidation (CMR(glc(ox), N)), revealed a linear relationship between these fluxes above isoelectricity, with a slope of approximately 1. In vitro glial culture studies examining glutamate uptake indicated that glutamate, which is cotransported with Na+, stimulated glial uptake of glucose and release of lactate. These in vivo and in vitro results were consolidated into a model: recycling of one molecule of neurotransmitter between glia and neurons was associated with oxidation of one glucose molecule in neurons; however, the glucose was taken up only by glia and all the lactate (pyruvate) generated by glial glycolysis was transferred to neurons for oxidation. The model was consistent with the 1:1 relationship between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) measured by 13C MRS. However, the model could not specify the energetics of glia and gamma-amino butyric acid (GABA) neurons because quantitative values for these pathways were not available. Here, we review recent 13C and 14C tracer studies that enable us to include these fluxes in a more comprehensive model. The revised model shows that glia produce at least 8% of total oxidative ATP and GABAergic neurons generate approximately 18% of total oxidative ATP in neurons. Neurons produce at least 88% of total oxidative ATP, and take up approximately 26% of the total glucose oxidized. Glial lactate (pyruvate) still makes the major contribution to neuronal oxidation, but approximately 30% less than predicted by the prior model. The relationship observed between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) is determined by glial glycolytic ATP as before. Quantitative aspects of the model, which can be tested by experimentation, are discussed.

Increased oxygen consumption in the somatosensory cortex of alpha-chloralose anesthetized rats during forepaw stimulation determined using MRS at 11.7 Tesla

The significance of changes in cerebral oxygen consumption in focally activated brain tissue is still controversial. Since the rate of cerebral oxygen consumption is tightly coupled to that of tricarboxylic acid cycle which can be measured from the turnover kinetics of [4-(13)C]glutamate using in vivo (1)H{(13)C} magnetic resonance spectroscopy, changes in tricarboxylic acid cycle flux rate were assessed in primary somatosensory cortex of alpha-chloralose anesthetized rats during electrical forepaw stimulation. With markedly improved (1)H{(13)C} magnetic resonance spectroscopy technique and the use of high magnetic field strength of 11.7 T accessible to the current study, [4-(13)C]glutamate at 2.35 ppm was spectrally resolved from overlapping resonances of [4-(13)C]glutamine at 2.46 ppm and [2-(13)C]GABA at 2.28 ppm as well as the more distal [3-(13)C]glutamate and [3-(13)C]glutamine. The results showed a significantly increased V(TCA) in focally activated primary somatosensory cortex during forepaw stimulation, corresponding to approximately 51 +/- 27% (n = 6, mean +/- SD) increase in cerebral oxygen consumption rate. Considering the high efficiency in producing adenosine triphosphate by oxidative metabolism of glucose, the results demonstrate that aerobic oxidative metabolism provides the majority of energy required for cerebral focal activation in alpha-chloralose anesthetized rats subjected to forepaw stimulation.

13C magnetic resonance spectroscopy studies of alterations in glutamate neurotransmission

Over the past a few years, significant progress has been made in refining the in vivo 13C magnetic resonance spectroscopy technique and in applying it to studying the alterations in the glutamate-glutamine cycling flux. Meanwhile, the details of the metabolic modeling are being rigorously debated. Recent evidence against fast alpha-ketoglutarate-glutamate exchange across the mitochondrial membrane is examined. Previous reports have indicated that glutamate release or 13C label incorporation into glutamine is attenuated at elevated concentrations of endogenous gamma-aminobutyric acid (GABA). A recent study has shown that phenelzine administration reduces the glutamate-glutamine cycling flux while raising endogenous GABA levels in vivo. Effects of several metabotropic glutamate receptor agonists and antagonists and brain disorders on the glutamate-glutamine cycle are also summarized.

Advances in the treatment of anxiety: targeting glutamate

Our current psychopharmacological treatments for anxiety disorders evince a number of shortcomings, including troublesome side effects and lack of primary effects. Whereas many new drugs have been developed in the past few decades, most are based on outmoded theories of the pathogenesis of these disorders (i.e., monoamine hypotheses), thus frustrating our ability to create more specific and effective interventions. Recently, however, the neurobiological literature has shown a convergence of findings focusing on the glutamatergic system in anxiety disorders, and the growth of pharmacological tools targeting these receptors has led to the development of novel treatments having anxiolytic effects in humans and animals alike. Additionally, as this system is showing promise as a final common pathway in the pathogenesis of anxiety disorders, we may be able to employ glutamate-specific neuroimaging techniques (e.g., N-acetyl-aspartate, GLX) to both guide treatment decisions and present reliable objective biomarkers for treatment efficacy.

In vivo 13C NMR spectroscopy and metabolic modeling in the brain: a practical perspective

In vivo 13C NMR spectroscopy has the unique capability to measure metabolic fluxes noninvasively in the brain. Quantitative measurements of metabolic fluxes require analysis of the 13C labeling time courses obtained experimentally with a metabolic model. The present work reviews the ingredients necessary for a dynamic metabolic modeling study, with particular emphasis on practical issues.

Increased brain monocarboxylic acid transport and utilization in type 1 diabetes

We hypothesized that increased capacity for brain utilization of nonglucose substrates (monocarboxylic acids [MCAs]) by upregulation of the MCA transporters may contribute metabolic substrates during hypoglycemia. To test this hypothesis, we assessed brain acetate metabolism in five well-controlled type 1 diabetic subjects and six nondiabetic control subjects using 13C magnetic resonance spectroscopy during infusions of [2-(13)C]acetate during hypoglycemia (approximately 55 mg/dl). Acetate is transported into the brain through MCA transporters that are also used for lactate and ketones. Brain acetate concentrations were over twofold higher in the subjects with diabetes than the control subjects (P = 0.01). The fraction of oxidative metabolism from acetate (P = 0.015) and the rate of MCA transport (P = 0.01) were also approximately twofold higher in the diabetic subjects. We conclude that during hypoglycemia MCA transport in the brain was increased by approximately twofold in patients with well-controlled type 1 diabetes, as reflected by higher brain acetate concentrations and rates of acetate oxidation. This upregulation would potentially allow a similar twofold increase in the transport of other MCAs, including lactate, during insulin-induced hypoglycemia. These data are consistent with the hypothesis that upregulation of MCA transport may contribute to the maintenance of brain energetics during hypoglycemia in patients with type 1 diabetes.

Planning optimal measurements of isotopomer distributions for estimation of metabolic fluxes

MOTIVATION

Flux estimation using isotopomer information of metabolites is currently the most reliable method to obtain quantitative estimates of the activity of metabolic pathways. However, the development of isotopomer measurement techniques for intermediate metabolites is a demanding task. Careful planning of isotopomer measurements is thus needed to maximize the available flux information while minimizing the experimental effort.

RESULTS

In this paper we study the question of finding the smallest subset of metabolites to measure that ensure the same level of isotopomer information as the measurement of every metabolite in the metabolic network. We study the computational complexity of this optimization problem in the case of the so-called positional enrichment data, give methods for obtaining exact and fast approximate solutions, and evaluate empirically the efficacy of the proposed methods by analyzing a metabolic network that models the central carbon metabolism of Saccharomyces cerevisiae.

Sensitivity-enhanced13C MR spectroscopy of the human brain at 3 Tesla

A new coil design for sensitivity-enhanced 13C MR spectroscopy (MRS) of the human brain is presented. The design includes a quadrature transmit/receive head coil optimized for 13C MR sensitivity. Loss-less blocking circuits inside the coil conductors allow this coil to be used inside a homogeneous circularly polarized 1H B1 field for 1H decoupled 13C MRS. A quadrature 1H birdcage coil optimized for minimal local RF heating makes broadband 1H decoupling in the entire human brain possible at 3 Tesla while remaining well within international safety guidelines for RF absorption. Apart from a substantial increase in sensitivity compared to conventional small linear coils, the quadrature 13C coil combined with the quadrature 1H birdcage coil allows efficient cross polarization (CP) in the brain, resulting in an additional 3.5-fold sensitivity improvement compared to direct 13C measurements without nuclear Overhauser enhancement (NOE) or polarization transfer. Combined with the gain in power efficiency, this setup allows broadband 1H to 13C CP over large areas of the brain. Clear 13C resonances from glutamate (Glu), glutamine (Gln), aspartate (Asp), lactate (Lac), and gamma-aminobutyrate (GABA) carbon spins in the human brain demonstrate the quality of 13C MR spectra obtained in vivo with this coil setup.

Effects of noncompetitive NMDA receptor blockade on anterior cingulate cerebral blood flow in volunteers with schizophrenia

Schizophrenia may be related to dysfunctional glutamatergic activity, specifically hypofunction of the N-methyl-D-aspartate receptor (NMDAR). In addition, it has been proposed that NMDAR hypofunction may paradoxically cause an increase in glutamate release and hypermetabolism in corticolimbic regions. If a state of partial, chronic NMDAR blockade underlies schizophrenia, then schizophrenic volunteers (SV) may have greater glutamate release and associated elevations in regional cerebral blood flow (rCBF) than normal volunteers (NV), following drug-induced NMDAR antagonism. Therefore, we have given acute ketamine, a noncompetitive NMDAR antagonist, to NV (n=13) and medicated volunteers with schizophrenia (n=10) in conjunction with serial positron emission tomography blood flow studies. Drug administration caused marked rCBF elevations in frontal and cingulate regions in both groups. Contrasts between NV and SV ketamine groups showed that SV had greater relative blood flow increases in the anterior cingulate than NV. Maximum blood flow, and the area under the curve for blood flow in the anterior cingulate cortex, significantly correlated with changes in psychosis ratings in SV and NV (maximum rCBF only). These changes are consistent with a relatively hypoactive thalamic NMDAR and increased cortical glutamate neurotransmission at non-NMDARs in schizophrenia. We hypothesize that ketamine antagonizes an NMDAR-dependent inhibitory system that is partially compromised in subjects with schizophrenia. The ketamine-induced reduction of inhibition leads to a marked increase in glutamate release and hypermetabolism (elevated rCBF) in frontal and cingulate cortical regions. The loss of inhibition and increased glutamate release may cause the distorted thoughts and diminished cognitive abilities elicited by NMDAR blockade.

In vivo single-shot, proton-localized 13C MRS of rhesus monkey brain

A single-shot, proton-localized, polarization transfer (13)C spectroscopic method was proposed and implemented on a 4.7 T scanner for studying rhesus monkey brains. The polarization transfer sequence was mostly adiabatic, minimizing signal loss due to B(1) inhomogeneity. RF pulses in polarization transfer were also used for voxel selection of protons with gradient fields. The transferred (13)C magnetization was refocused by additional refocusing adiabatic pulses. With the intravenous infusion of D-[1-(13)C]glucose solution, (13)C NMR spectra from a 30 mL voxel were acquired for the resonances of C1 of glucose, C2,3,4 of glutamate and glutamine. The time-resolved turnover of glutamate, glutamine and aspartate from intravenously infused D-[1-(13)C]glucose at a temporal resolution of 12 min was demonstrated with excellent spectral resolution and signal-to-noise ratio. Typically, the half-height linewidth of the decoupled (13)C peaks was approximately 4 Hz. Data obtained with infusion of sodium [2-(13)C]acetate using the proposed polarization transfer method and data from the carboxylic carbon region using non-localized acquisition are also presented.

About being BOLD

The last decade has seen an unprecedented increase in the use of functional magnetic resonance imaging (fMRI) to understand the neural basis of cognition and behavior. Being non-invasive and relatively easy to use, most studies relied on changes in the blood oxygenation level dependent (BOLD) contrast as an indirect marker of variations in brain activity. However, the fact that BOLD fMRI is dependent on the blood flow response that follows neural activity and does not measure neural activity per se is seen as an inherent cause for concern while interpreting data from these studies. In order to characterize the BOLD signal correctly, it is imperative that we have a better understanding of neural events that lead to the BOLD response. A review of recent studies that addressed several aspects of BOLD fMRI including events at the level of the synapse, the nature of the neurovascular coupling, and some parameters of the BOLD signal is provided. This is intended to serve as background information for the interpretation of fMRI data in normal subjects and in patients with compromised neurovascular coupling. One of the aims is also to encourage researchers to interpret the results of functional imaging studies in light of the dynamic interactions between different brain regions, something that often is neglected.

Reduced anterior cingulate glutamate in pediatric major depression: a magnetic resonance spectroscopy study

BACKGROUND

Anterior cingulate cortex has been implicated in the pathogenesis of major depressive disorder (MDD). With single voxel proton magnetic resonance spectroscopy, we reported reductions in anterior cingulate glutamatergic concentrations (grouped value of glutamate and glutamine) in 14 pediatric MDD patients versus 14 case-matched healthy control subjects. These changes might reflect a change in glutamate, glutamine, or their combination.

METHODS

Fitting to individually quantify anterior cingulate glutamate and glutamine was performed in these subjects with a new basis set created from data acquired on a 1.5 Tesla General Electric Signa (GE Healthcare, Waukesha, Wisconsin) magnetic resonance imaging scanner with LCModel (Version 6.1-0; Max-Planck-Institute, Gottingen, Germany).

RESULTS

Reduced anterior cingulate glutamate was observed in MDD patients versus control subjects (8.79 +/- 1.68 vs. 11.46 +/- 1.55, respectively, p = .0002; 23% decrease). Anterior cingulate glutamine did not differ significantly between patients with MDD and control subjects.

CONCLUSIONS

These findings provide confirmatory evidence of anterior cingulate glutamate alterations in pediatric MDD.

Rapid modulation of GABA concentration in human sensorimotor cortex during motor learning

Movement representations within the human primary motor and somatosensory cortices can be altered by motor learning. Decreases in local GABA concentration and its release may facilitate this plasticity. Here we use in vivo magnetic resonance spectroscopy (MRS) to noninvasively measure serial changes in GABA concentration in humans in a brain region including the primary sensorimotor cortex contralateral to the hand used for an isometric motor sequence learning task. Thirty minutes of motor sequence learning reduced the mean GABA concentration within a 2 x 2 x 2-cm3 voxel by almost 20%. This reduction was specific to motor learning: 30 min of similar, movements with an unlearnable, nonrepetitive sequence were not associated with changes in GABA concentration. No significant changes in GABA concentration were found in the primary sensorimotor cortex ipsilateral to the hand used for learning. These changes suggest remarkably rapid, regionally specific short-term presynaptic modulation of GABAergic input that should facilitate motor learning. Although apparently confined to the contralateral hemisphere, the magnitude of changes seen within a large spectroscopic voxel suggests that these changes occur over a wide local neocortical field.

Metabolic alterations in focally activated primary somatosensory cortex of alpha-chloralose-anesthetized rats measured by 1H MRS at 11.7 T

Previously, magnetic resonance spectroscopy studies of alterations in cerebral metabolite concentration during functional activation have been focused on phosphocreatine using 31P MRS and lactate using 1H MRS with controversial results. Recently, significant improvements on the spectral resolution and sensitivity of in vivo spectroscopy have been made at ultrahigh magnetic field strength. Using highly resolved localized short-TE 1H MRS at 11.7 T, we report metabolic responses of rat somatosensory cortex to forepaw stimulation in alpha-chloralose-anesthetized rats. The phosphocreatine/creatine ratio was found to be significantly decreased by 15.1 +/- 4.6% (mean +/- SEM, P < 0.01). Lactate remained very low (approximately <0.3 micromol/g w/w) with no statistically significant changes observed during forepaw stimulation at a temporal resolution of 10.7 min. An increase in glutamine and a decrease in glutamate and myo-inositol were also detected in the stimulated state. Our results suggest that, under the experimental conditions used in this study, increased energy consumption due to focal activation causes a shift in the creatine kinase reaction towards the direction of adenosine triphosphate production. At the same time, metabolic matching prevails during increased energy consumption with no significant increase in the glycolytic product lactate in the focally activated primary somatosensory cortex of alpha-chloralose-anesthetized rats.

In vivo evidence for reduced cortical glutamate-glutamine cycling in rats treated with the antidepressant/antipanic drug phenelzine

Converging evidence has indicated that hyperglutamatergic activity and GABAergic dysfunction may play important roles in the neurobiology and treatment of depression and other mood disorders. In this study, in vivo 1H[13C] magnetic resonance spectroscopy was used to quantify the effects of acute phenelzine administration on cortical energetics, glutamate neurotransmission, and GABA synthesis flux. The time-resolved kinetics of cortical [4-13C]glutamate, [4-13C]glutamine, and [2-13C]GABA turnover from i.v.-infused [1,6-13C2]glucose was measured at 11.7 T in alpha-chloralose anesthetized rats four hours after phenelzine treatment (10 mg/kg, i.p.) and in non-treated controls. The rate of the tricarboxylic acid cycle flux was not affected by phenelzine treatment compared with the non-treated group (0.46+/-0.05 vs. 0.50+/-0.05 micromol/g/min, respectively). The rate of the glutamate-glutamine cycling flux between neurons and glia in the phenelzine-treated group was significantly reduced (from 0.16+/-0.04 to 0.10+/-0.03 micromol/g/min), providing in vivo evidence that phenelzine attenuates glutamate neurotransmission. Following phenelzine treatment, the cortical GABA concentration increased significantly (from 1.02+/-0.17 to 2.30+/-0.26 micromol/g), while the GABA synthesis flux was unchanged (from 0.07+/-0.02 to 0.06+/-0.02 micromol/g/min). The possible role of augmented GABAergic function resulting from elevated GABA levels in the observed modulatory effect of phenelzine on the glutamate-glutamine cycling flux was discussed. The reduced glutamate-glutamine cycling flux observed in this study suggests that, in addition to its effects on monoaminergic and GABAergic systems, the attenuation of glutamate neurotransmission resulting from phenelzine administration may also contribute to its efficacy in the treatment of depression. This study is the first demonstration that the glutamate-glutamine cycling flux, which can be measured non-invasively in the human brain in vivo, was altered due to the action of a psychotropic drug.

Riluzole augmentation in treatment-resistant obsessive-compulsive disorder: an open-label trial

BACKGROUND

Most patients with obsessive-compulsive disorder (OCD) show only partial reduction of symptoms with standard therapy. Recent imaging data suggests glutamatergic dysfunction in the corticostriatal pathway in OCD. We investigated the efficacy of augmentation therapy with riluzole, a glutamate-modulating agent, in treatment-resistant OCD.

METHODS

Thirteen patients aged between 18 and 65 years with a primary diagnosis of OCD that had proven resistant to standard treatment were treated with the addition of riluzole to their existing pharmacotherapy. Yale-Brown Obsessive Compulsive Scale (Y-BOCS), Hamilton Depression Inventory (HAM-D), and Hamilton Anxiety Inventory (HAM-A) scores were obtained weekly.

RESULTS

Thirteen treatment-resistant OCD patients received riluzole 50 mg twice a day. Y-BOCS scores improved significantly over time. Of 13 patients, 7 (54%) demonstrated a >35% reduction in Y-BOCS scores, and 5 (39%) were categorized as treatment responders. HAM-D and HAM-A scores for the group also significantly improved over time. Riluzole was well tolerated with no serious adverse effects noted.

CONCLUSIONS

Riluzole appears to have significant antiobsessional, antidepressant, and antianxiety properties. The addition of this agent may be of practical clinical benefit in patients with OCD.

Neuroglial metabolism in the awake rat brain: CO2 fixation increases with brain activity

Glial cells are thought to supply energy for neurotransmission by increasing nonoxidative glycolysis; however, oxidative metabolism in glia may also contribute to increased brain activity. To study glial contribution to cerebral energy metabolism in the unanesthetized state, we measured neuronal and glial metabolic fluxes in the awake rat brain by using a double isotopic-labeling technique and a two-compartment mathematical model of neurotransmitter metabolism. Rats (n = 23) were infused simultaneously with 14C-bicarbonate and [1-13C]glucose for up to 1 hr. The 14C and 13C labeling of glutamate, glutamine, and aspartate was measured at five time points in tissue extracts using scintillation counting and 13C nuclear magnetic resonance of the chromatographically separated amino acids. The isotopic 13C enrichment of glutamate and glutamine was different, suggesting significant rates of glial metabolism compared with the glutamate-glutamine cycle. Modeling the 13C-labeling time courses alone and with 14C confirmed significant glial TCA cycle activity (V(PDH)((g)), approximately 0.5 micromol x gm(-1) x min(-1)) relative to the glutamate-glutamine cycle (V(NT)) (approximately 0.5-0.6 micromol x gm(-1) x min(-1)). The glial TCA cycle rate was approximately 30% of total TCA cycle activity. A high pyruvate carboxylase rate (V(PC), approximately 0.14-0.18 micromol x gm(-1) x min(-1)) contributed to the glial TCA cycle flux. This anaplerotic rate in the awake rat brain was severalfold higher than under deep pentobarbital anesthesia, measured previously in our laboratory using the same 13C-labeling technique. We postulate that the high rate of anaplerosis in awake brain is linked to brain activity by maintaining glial glutamine concentrations during increased neurotransmission.

Integrated RF probe for in vivo multinuclear spectroscopy and functional imaging of rat brain using an 11.7 Tesla 89 mm bore vertical microimager

To acquire high quality in vivo NMR data from rat brain using a vertical 89-mm bore magnet, specially designed NMR probes with integrated RF coils and animal handling capability are required. An RF probe design that is also capable of rat head fixation, body support and suitable for physiology monitoring and maintenance was constructed for an 89 mm bore, 11.7 T, vertical microimager which is equipped with a 57-mm i.d. gradient insert. Design concept and practical aspects of probe construction are described in detail. The device allows accurate and highly reproducible positioning of rat head inside the magnet while providing excellent RF performance. Typical results from fMRI, localized in vivo proton and multinuclear spectroscopy using this probe system are presented.

The contribution of GABA to glutamate/glutamine cycling and energy metabolism in the rat cortex in vivo

Previous studies have shown that the glutamate/glutamine (Glu/Gln) neurotransmitter cycle and neuronal glucose oxidation are proportional (1:1), with increasing neuronal activity above isoelectricity. GABA, a product of Glu metabolism, is synthesized from astroglial Gln and contributes to total Glu/Gln neurotransmitter cycling, although the fraction contributed by GABA is unknown. In the present study, we used (13)C NMR spectroscopy together with i.v. infusions of [1,6-(13)C(2)]glucose and [2-(13)C]acetate to separately determine rates of Glu/Gln and GABA/Gln cycling and their respective tricarboxylic acid cycles in the rat cortex under conditions of halothane anesthesia and pentobarbital-induced isoelectricity. Under 1% halothane anesthesia, GABA/Gln cycle flux comprised 23% of total (Glu plus GABA) neurotransmitter cycling and 18% of total neuronal tricarboxylic acid cycle flux. In isoelectric cortex, glucose oxidation was reduced >3-fold in glutamatergic and GABAergic neurons, and neurotransmitter cycling was below detection. Hence, in both cell types, the primary energetic costs are associated with neurotransmission, which increase together as cortical activity is increased. The contribution of GABAergic neurons and inhibition to cortical energy metabolism has broad implications for the interpretation of functional imaging signals.

Prefrontal atrophy in first episodes of schizophrenia associated with limbic metabolic hyperactivity

Reduced volume and activity of the prefrontal (PF) cortical gray matter (GM) and hippocampal hypermetabolism are repeated findings in schizophrenia. There is still an information deficit about the significance of reduction of PF GM in schizophrenia, and a simultaneous study of PF anatomy and activity and limbic metabolism can contribute to fill that deficit. In order to do so, we used positron emission tomography (PET) with 18-fluoro-deoxy-glucose (FDG) during an attention task and magnetic resonance imaging (MRI) to study a sample of first episodes of psychosis. We included 21 first episodes (FE) of psychosis and 16 healthy controls. A diagnosis of schizophrenia was confirmed in the follow-up in eleven of these patients and ruled out in the remaining 10 cases. Volumes of PF GM were determined and also activity in the same region and in the hippocampus. Residual GM was estimated in the PF region as a quantitative measurement of the degree of atrophy in each individual, using age and intracranial volume data from a set of 45 healthy controls and linear regression. Patients with schizophrenia had lower PF metabolic activation and greater hippocampal activity than controls. FE patients without schizophrenia were no different in any parameter as compared to controls. Patients with schizophrenia presented an inverse and significant association between GM deficit and hippocampal activity that was not observed in controls or in patients without schizophrenia. The same association was previously described by our group using PET in the resting state in recent-onset and chronic patients with schizophrenia. These findings support a loss in PF inhibitory capacity as a possible link between anatomical and functional alterations in schizophrenia.

Effect of strong homonuclear proton coupling on localized13C detection using PRESS

The effect of strong homonuclear proton coupling on (13)C incorporation measurements by either indirect or direct means was investigated (and illustrated with glutamate) both numerically and experimentally at 3.0 T. In particular, two sequences were considered, each using a proton PRESS sequence for localization. The indirect (13)C detection method incorporated the POCE (proton observe carbon edited) technique onto PRESS, and for direct (13)C detection a DEPT (distortionless enhancement by polarization transfer) sequence was appended to the PRESS localization. Both analysis and experiment demonstrate that when strong homonuclear coupling of protons is additional to heteronuclear coupling with (13)C spins, the (13)C measures derived from either the indirect PRESS-POCE sequence or the direct-but-enhanced PRESS-DEPT sequence are significantly modified. Specifically, the MR lineshapes of both (13)C-bonded and nonbonded protons are changed during (13)C incorporation, giving rise, for example, to a potential cross-contamination of < or =30% between glutamate (13)C(3) and (13)C(4) measures from the PRESS-POCE indirect method. During direct-but-enhanced detection, the DEPT enhancement is reduced for glutamate (13)C(2), (13)C(3), and (13)C(4) but not equally, and the reduction is further exacerbated by proton PRESS localization, which gives rise to enhancements that are strong functions of PRESS TE(1) and TE(2).

Theoretical and experimental evaluation of broadband decoupling techniques for in vivo nuclear magnetic resonance spectroscopy

A theoretical and experimental evaluation of existing broadband decoupling methods with respect to their utility for in vivo (1)H-(13)C NMR spectroscopy is presented. Simulations are based on a modified product operator formalism, while an experimental evaluation is performed on in vitro samples and human leg and rat brain in vivo. The performance of broadband decoupling methods was evaluated with respect to the required peak and average RF powers, decoupling bandwidth, decoupling side bands, heteronuclear scalar coupling constant, and sensitivity toward B(2) inhomogeneity. In human applications only the WALTZ and MLEV decoupling methods provide adequate decoupling performance at RF power levels that satisfy the FDA guidelines on local tissue heating. For very low RF power levels (B(2max) < 300 Hz) one should verify empirically whether the experiment will benefit from broadband decoupling. At higher RF power levels acceptable for animal studies additional decoupling techniques become available and provide superior performance. Since the average RF power of adiabatic RF pulses is almost always significantly lower than the peak RF power, it can be stated that for average RF powers suitable for animal studies it is always possible to design an adiabatic decoupling scheme that outperforms all other schemes. B(2) inhomogeneity degrades the decoupling performance of all methods, but the decoupling bandwidths for WALTZ-16 and especially adiabatic methods are still satisfactory for useful in vivo decoupling with a surface coil.

Cultures of rat astrocytes challenged with a steady supply of glutamate: New model to study flux distribution in the glutamate-glutamine cycle

Glutamate metabolism in astrocytes was studied using an experimental setup that simulates the role of neurons (glutamate producers and glutamine consumers) by the addition of glutaminase to the culture medium. Thereby, a steady supply of glutamate was imposed at the expense of glutamine, and the stress intensity was manipulated by changing the glutaminase concentration. Glutamate supply rates in the range 8-23 nmol/min/mg protein were examined for periods of up to 48 h. When the glutamate supply rate exceeded the uptake rate of this amino acid, a transient increase in the extracellular concentration of glutamate was observed. In response to this stress, the fluxes through the glutamate transporter and glutamine synthetase were increased considerably, and the extracellular concentration of glutamate was eventually restored to a low level. The increased levels of glutamine synthetase were demonstrated by immunoblotting analysis. The effect on glutamate metabolism of the transaminase inhibitor, aminooxyacetic acid (AOAA), and of NH4Cl was also investigated. The supply of glutamate caused a concomitant reduction in the levels of phosphocreatine, phosphoethanolamine, and phosphocholine without affecting the ATP pool. Glutamine synthetase was shown to be is a key element in the control of glutamate metabolism in astrocytic cultures. The metabolic fate of glutamate depends greatly on the time of endurance to the challenge: in naive cells, glutamate was primarily metabolized through the transaminase pathway, while in well-adapted cells glutamate was converted almost exclusively through glutamine synthetase.

In vivo detection of cortical GABA turnover from intravenously infused [1-13C]D-glucose

In this study [2-(13)C] gamma-aminobutyric acid (GABA) was spectrally resolved in vivo and detected simultaneously with [4-(13)C]glutamate (Glu) and [4-(13)C]glutamine (Gln) in the proton spectra obtained from a localized 40 microL voxel in rat neocortex with the use of an adiabatic (1)H-observed, (13)C-edited (POCE) spectroscopy method and an 89-mm-bore vertical 11.7 Tesla microimager. The time-resolved kinetics of (13)C label incorporation from intravenously infused [1-(13)C]glucose into [4-(13)C]Glu, [4-(13)C]Gln, and [2-(13)C]GABA were measured after acute administration of gabaculine, a potent and specific inhibitor of GABA-transaminase. In contrast to previous observations of a rapid turnover of [2-(13)C]GABA from [1-(13)C]glucose in intact rat brain, the rate of (13)C incorporation from [1-(13)C]glucose into [2-(13)C]GABA in the gabaculine-treated rats was found to be significantly reduced as a result of the blockade of the GABA shunt.

NMR measurement of brain oxidative metabolism in monkeys using 13C-labeled glucose without a 13C radiofrequency channel

We detected glutamate C4 and C3 labeling in the monkey brain during an infusion of [U-13C6]glucose, using a simple 1H PRESS sequence without 13C editing or decoupling. Point-resolved spectroscopy (PRESS) spectra revealed decreases in 12C-bonded protons, and increases in 13C-bonded protons of glutamate. To take full advantage of the simultaneous detection of 12C- and 13C-bonded protons, we implemented a quantitation procedure to properly measure both glutamate C4 and C3 enrichments. This procedure relies on LCModel analysis with a basis set to account for simultaneous signal changes of protons bound to 12C and 13C. Signal changes were mainly attributed to 12C- and 13C-bonded protons of glutamate. As a result, we were able to measure the tricarboxylic acid (TCA) cycle flux in a 3.9 cm3 voxel centered in the monkey brain on a whole-body 3 Tesla system (VTCA = 0.55 +/- 0.04 micromol x g(-1) x min(-1), N = 4). This work demonstrates that oxidative metabolism can be quantified in deep structures of the brain on clinical MRI systems, without the need for a 13C radiofrequency (RF) channel.

Glutamatergic neurotransmission and neuronal glucose oxidation are coupled during intense neuronal activation

13C nuclear magnetic resonance (NMR) experiments have previously shown that glutamatergic neurotransmitter flux (Vcycle(Glu/Gln)) changes proportionately with neuronal glucose oxidation (CMRglc(ox)N) in the nonactivated cortex of anesthetized rats. Positron Emission Tomography measurements of glucose and oxygen uptake during sensory stimulation had shown that the incremental glucose utilization is greater than oxygen leading to the suggestion that the energy required for stimulated neuronal activity arises from nonoxidative glucose metabolism. In this study, the authors used spatially localized 1H-observed, 13C-edited NMR spectroscopy during an infusion of [1,6-13C2]glucose to assess the relationship between changes in Vcycle(Glu/Gln) and glucose utilization (CMRglc(ox)N and CMRglc(nonox)) during the intense cortical activity associated with bicuculline-induced seizures. Metabolic fluxes were determined by model-based analysis of the 13C-enrichment time courses of glutamate-C4 and glutamine-C4 (CMRglc(ox)N, Vcycle(Glu/Gln)) and lactate-C3 (CMRglc(nonox)). The exchange rate between alpha-ketoglutarate and glutamate was found to be significantly faster than TCA cycle flux both for control (41 micromol.g(-1).min(-1); 95% CI, 5 to 109 micromol.g(-1).min(-1)) and during seizures (21 micromol.g(-1).min(-1); 95% CI, 4.4 to 51.8 micromol.g(-1).min(-1)). During seizures, total glucose utilization (CMRglc(ox+nonox)) increased substantially (466% between 0 and 6 minutes; 277% between 6 and 55 minutes). Glucose oxidation (CMRglc(ox)N) also increased (214%; from 0.26 +/- 0.02 to 0.57 +/- 0.07 micromol.g(-1).min(-1)) but to a lesser degree, resulting in a large increase in cortical lactate concentration. Vcycle(Glu/Gln) increased 233% (from 0.22 +/- 0.04 to 0.52 +/- 0.07 micromol.g(-1).min(-1)), which was similar to the increase in glucose oxidation. The value of Vcycle(Glu/Gln) and CMRglc(ox)N obtained here lie on the line predicted in a previous study. These results indicate that neuronal glucose oxidation and not total glucose utilization is coupled to the glutamate/glutamine cycle during intense cortical activation.

Regional glucose metabolism and glutamatergic neurotransmission in rat brain in vivo

Multivolume (1)H-[(13)C] NMR spectroscopy in combination with i.v. [1,6-(13)C(2)]glucose infusion was used to detect regional glucose metabolism and glutamatergic neurotransmission in the halothane-anesthetized rat brain at 7 T. The regional information was decomposed into pure cerebral gray matter, white matter, and subcortical structures by means of tissue segmentation based on quantitative T(1) relaxation mapping. The (13)C turnover curves of [4-(13)C]glutamate, [4-(13)C]glutamine, and [3-(13)C]glutamate + glutamine were fitted with a two-compartment neuronal-astroglial metabolic model. The neuronal tricarboxylic acid cycle fluxes in cerebral gray matter, white matter, and subcortex were 0.79 +/- 0.15, 0.20 +/- 0.11, and 0.42 +/- 0.09 micromol/min per g, respectively. The glutamate-glutamine neurotransmitter cycle fluxes in cerebral gray matter, white matter, and subcortex were 0.31 +/- 0.07, 0.02 +/- 0.04, and 0.18 +/- 0.12 micromol/min per g, respectively. The exchange rate between the mitochondrial and cytosolic metabolite pools was fast relative to the neuronal tricarboxylic acid cycle flux for all cerebral tissue types.

Energetic basis of brain activity: implications for neuroimaging

The complex activities of the brain need not distract us from the certainty that it uses energy and performs work very efficiently. The human brain, which claims approximately 2% of our body mass, is responsible for approximately 20% of our body oxygen consumption. In vivo magnetic resonance spectroscopy (MRS) follows the metabolic pathways of energy production (as glucose oxidation) and work (as monitored by the cycling of glutamate and GABA neurotransmitters). In the resting awake state, approximately 80% of energy used by the brain supports events associated with neuronal firing and cycling of GABA and glutamate neurotransmitters. Small differences in neuronal activity between stimulation and control conditions can be measured and localized using functional magnetic resonance imaging (fMRI). MRS and fMRI experiments show that the majority of cerebral activity, which is often disregarded in imaging experiments, is ongoing even when the brain appears to be doing nothing.

A13C isotopomer kinetic analysis of cardiac metabolism: influence of altered cytosolic redox and [Ca2+]o

Rat hearts were perfused with mixtures of [3-(13)C]pyruvate and [3-(13)C]lactate (to alter cytosolic redox) at low (0.5 mM) or high (2.5 mM) Ca(2+) concentrations to alter contractility. Hearts were frozen at various times after exposure to these substrates, were extracted, and were then analyzed by (13)C NMR spectroscopy. The time-dependent multiplets observed in the (13)C NMR resonances of glutamate in all hearts and in malate and aspartate in hearts perfused with high-pyruvate/low-lactate concentrations were analyzed using a kinetic model of the tricarboxylic acid (TCA) cycle. The analysis showed that TCA cycle flux (V(TCA)) and exchange flux (V(X)) that involved cycle intermediates were both sensitive to cell redox and altered Ca(2+) concentration, and the ratio of these fluxes (V(X)/V(TCA)) varied >10-fold.