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

Excitatory/inhibitory neuronal metabolic balance in mouse hippocampus upon infusion of [U-13C6]glucose

Hippocampus plays a critical role in linking brain energetics and behavior typically associated to stress exposure. In this study, we aimed to simultaneously assess excitatory and inhibitory neuronal metabolism in mouse hippocampus in vivo by applying ¹⁸FDG-PET and indirect ¹³C magnetic resonance spectroscopy (¹H-[¹³C]-MRS) at 14.1 T upon infusion of uniformly ¹³C-labeled glucose ([U-¹³C₆]Glc). Improving the spectral fitting by taking into account variable decoupling efficiencies of [U-¹³C₆]Glc and refining the compartmentalized model by including two γ-aminobutyric acid (GABA) pools permit us to evaluate the relative contributions of glutamatergic and GABAergic metabolism to total hippocampal neuroenergetics. We report that GABAergic activity accounts for ∼13% of total neurotransmission (V_NT) and ∼27% of total neuronal TCA cycle (V_TCA) in mouse hippocampus suggesting a higher V_TCA/V_NT ratio for inhibitory neurons compared to excitatory neurons. Finally, our results provide new strategies and tools for bringing forward the developments and applications of ¹³C-MRS in specific brain regions of small animals.

Experimental strategies for in vivo13C NMR spectroscopy

In vivo carbon-13 (¹³C) MRS opens unique insights into the metabolism of intact organisms, and has led to major advancements in the understanding of cellular metabolism under normal and pathological conditions in various organs such as skeletal muscles, the heart, the liver and the brain. However, the technique comes at the expense of significant experimental difficulties. In this review we focus on the experimental aspects of non-hyperpolarized ¹³C MRS in vivo. Some of the enrichment strategies which have been proposed so far are described; the various MRS acquisition paradigms to measure ¹³C labeling are then presented. Finally, practical aspects of ¹³C spectral quantification are discussed.

(13)C NMR spectroscopy applications to brain energy metabolism

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

Spectral editing for in vivo ¹³C magnetic resonance spectroscopy

In vivo detection of carboxylic/amide carbons is a promising technique for studying cerebral metabolism and neurotransmission due to the very low RF power required for proton decoupling. In the carboxylic/amide region, however, there is severe spectral overlap between acetate C1 and glutamate C5, complicating studies that use acetate as an astroglia-specific substrate. There are no known in vivo MRS techniques that can spectrally resolve acetate C1 and glutamate C5 singlets. In this study, we propose to spectrally separate acetate C1 and glutamate C5 by a two-step J-editing technique after introducing homonuclear (13)C-(13)C scalar coupling between carboxylic/amide carbons and aliphatic carbons. By infusing [1,2-(13)C(2)]acetate instead of [1-(13)C]acetate the acetate doublet can be spectrally edited because of the large separation between acetate C2 and glutamate C4 in the aliphatic region. This technique can be applied to studying acetate transport and metabolism in brain in the carboxylic/amide region without spectral interference.

Detection of reduced GABA synthesis following inhibition of GABA transaminase using in vivo magnetic resonance signal of [13C]GABA C1

Previous in vivo magnetic resonance spectroscopy (MRS) studies of gamma-aminobutyric acid (GABA) synthesis have relied on (13)C label incorporation into GABA C2 from [1-(13)C] or [1,6-(13)C(2)]glucose. In this study, the [(13)C]GABA C1 signal at 182.3 ppm in the carboxylic/amide spectral region of localized in vivo (13)C spectra was detected. GABA-transaminase of rat brain was inhibited by administration of gabaculine after pre-labeling of GABA C1 and its metabolic precursors with exogenous [2,5-(13)C(2)]glucose. A subsequent isotope chase experiment was performed by infusing unlabeled glucose, which revealed a markedly slow change in the labeling of GABA C1 accompanying the blockade of the GABA shunt. This slow labeling of GABA at elevated GABA concentration was attributed to the relatively small intercompartmental GABA-glutamine cycling flux that constitutes the main route of (13)C label loss during the isotope chase. Because this study showed that using low RF power broadband stochastic proton decoupling is feasible at very high field strength, it has important implications for the development of carboxylic/amide (13)C MRS methods to study brain metabolism and neurotransmission in human subjects at high magnetic fields.

Fast isotopic exchange between mitochondria and cytosol in brain revealed by relayed 13C magnetization transfer spectroscopy

In vivo 13C magnetic resonance spectroscopy has been applied to studying brain metabolic processes by measuring 13C label incorporation into cytosolic pools such as glutamate and aspartate. However, the rate of exchange between mitochondrial alpha-ketoglutarate/oxaloacetate and cytosolic glutamate/aspartate (Vx) extracted from metabolic modeling has been controversial. Because brain fumarase is exclusively located in the mitochondria, and mitochondrial fumarate is connected to cytosolic aspartate through a chain of fast exchange reactions, it is possible to directly measure Vx from the four-carbon side of the tricarboxylic acid cycle by magnetization transfer. In isoflurane-anesthetized adult rat brain, a relayed 13C magnetization transfer effect on cytosolic aspartate C2 at 53.2 ppm was detected after extensive signal averaging with fumarate C2 at 136.1 ppm irradiated using selective radiofrequency pulses. Quantitative analysis using Bloch-McConnell equations and a four-site exchange model found that Vx approximately 13-19 micromol per g per min (>>VTCA, the tricarboxylic acid cycle rate) when the longitudinal relaxation time of malate C2 was assumed to be within +/-33% of that of aspartate C2. If Vx approximately VTCA, the isotopic exchange between mitochondria and cytosol would be too slow on the time scale of 13C longitudinal relaxation to cause a detectable magnetization transfer effect.

Brain lithium, N-acetyl aspartate and myo-inositol levels in older adults with bipolar disorder treated with lithium: a lithium-7 and proton magnetic resonance spectroscopy study

OBJECTIVES

We investigated the relationship between brain lithium levels and the metabolites N-acetyl aspartate (NAA) and myo-inositol (myo-Ino) in the anterior cingulate cortex of a group of older adults with bipolar disorder (BD).

METHODS

This cross-sectional assessment included nine subjects (six males and three females) with bipolar I disorder and currently treated with lithium, who were examined at McLean Hospital's Geriatric Psychiatry Research Program and Brain Imaging Center. The subjects' ages ranged from 56 to 85 years (66.0 +/- 9.7 years) and all subjects had measurements of serum and brain lithium levels. Brain lithium levels were assessed using lithium magnetic resonance spectroscopy. All subjects also had proton magnetic resonance spectroscopy to obtain measurements of NAA and myo-Ino.

RESULTS

Brain lithium levels were associated with higher NAA levels [df = (1, 8), Beta = 12.53, t = 4.09, p < 0.005] and higher myo-Ino levels [df = (1, 7), F = 16.81, p < 0.006]. There were no significant effects of serum lithium levels on any of the metabolites.

CONCLUSION

Our findings of a relationship between higher brain lithium levels and elevated NAA levels in older adult subjects with BD may support previous evidence of lithium's neuroprotective, neurotrophic, and mitochondrial function-enhancing effects. Elevated myo-Ino related to elevated brain lithium levels may reflect increased inositol monophosphatase (IMPase) activity, which would lead to an increase in myo-Ino levels. This is the first study to demonstrate alterations in NAA and myo-Ino in a sample of older adults with BD treated with lithium.

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.

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.

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.

Acute regulation of steady-state GABA levels following GABA-transaminase inhibition in rat cerebral cortex

Cellular GABA levels are determined by the dynamic balance between synthesis and catabolism and are regulated at the level of glutamate decarboxylase, precursor availability (e.g., glutamate and glutamine), and possibly GABA degradation. GABA levels rise and stabilize within hours in human cortex following orally administered vigabatrin, an irreversible inhibitor of GABA-T, suggesting potential product inhibition of GABA synthesis or enhanced GABA degradation through the non-inhibited GABA-T fraction. In this study time courses of the rise in cortical GABA were measured in anesthetized rats in vivo after vigabatrin treatment using localized (1)H magnetic resonance spectroscopy and the times to reach steady-state for a given dose were determined. Rates of GABA synthesis were estimated for the period of constant GABA level from the accumulation of [2-(13)C]GABA following a short intravenous infusion (20 min) of either [1,6-(13)C(2)]glucose or [2-(13)C]acetate. No evidence of product inhibition of glutamate decarboxylase by the increased GABA concentration or reduced synthesis from [1,6-(13)C(2)]glucose (control, 0.031+/-0.010; vigabatrin-treated, 0.037+/-0.004 micromol/g/min, P=0.30) or [2-(13)C]acetate (control, 0.078+/-0.010; vigabatrin-treated, 0.084+/-0.006 micromol/g/min, P=0.42) was found. Fractional changes in steady-state GABA levels and GABA-T activities 5-6 h after vigabatrin treatment were approximately equal. The lack of change in GABA synthesis (and GABA catabolic flux for constant GABA levels) suggests that GABA-T has a near-zero flux control coefficient in vivo-capable of greatly altering the steady-state GABA concentration but exerting little or no control on GABA synthesis or GABA/glutamine cycling flux. The findings are consistent with a Michaelis-Menten kinetic model whereby cellular GABA levels increase until flux through the remaining (uninhibited) transaminase equals the rate of GABA synthesis. The findings suggest that astroglia may be the site of continuing GABA catabolism after acute vigabatrin treatment.

Evidence that GAD65mediates increased GABA synthesis during intense neuronal activityin vivo

In this study we tested the hypothesis that the 65-kDa isoform of glutamate decarboxylase (GAD(65)) mediates activity-dependent GABA synthesis as invoked by seizures in anesthetized rats. GABA synthesis was measured following acute GABA-transaminase inhibition by gabaculine using spatially localized (1)H NMR spectroscopy before and after bicuculline-induced seizures. Experiments were conducted with animals pre-treated with vigabatrin 24 h earlier in order to reduce GAD(67) protein and also with non-treated controls. GAD isoform content was quantified by immunoblotting. GABA was higher in vigabatrin-treated rats compared to non-treated controls. In vigabatrin-treated animals, GABA synthesis was 28% lower compared to controls [p < 0.05; vigabatrin-treated, 0.043 +/- 0.011 micromol/(g min); non-treated, 0.060 +/- 0.014 micromol/(g min)] and GAD(67) was 60% lower. No difference between groups was observed for GAD(65). Seizures increased GABA synthesis in both control [174%; control, 0.060 +/- 0.014 micromol/(g min) vs. seizures, 0.105 +/- 0.043 micromol/(g min)] and vigabatrin-treated rats [214%; control, 0.043 +/- 0.011 micromol/(g min); seizures, 0.092 +/- 0.018 micromol/(g min)]. GAD(67) could account for at least half of basal GABA synthesis but only 20% of the two-fold increase observed in vigabatrin-treated rats during seizures. The seizure-induced activation of GAD(65) in control cortex occurs concomitantly with a 2.3-fold increase in inorganic phosphate, known to be a potent activator of apoGAD(65)in vitro. Our results are consistent with a major role for GAD(65) in activity-dependent GABA synthesis.

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.

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.