Pierre Gilles Henry

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 (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(V_{\mathrm{PDH}}^{(\mathrm{g})},{\sim}0.5\) \end{document} μmol · gm-1 · min-1) relative to the glutamate-glutamine cycle (VNT) (∼0.5-0.6 μmol · gm-1 · min-1). The glial TCA cycle rate was ∼30% of total TCA cycle activity. A high pyruvate carboxylase rate (VPC, ∼0.14-0.18 μmol · gm-1 · 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.

Highly resolved in vivo 1H NMR spectroscopy of the mouse brain at 9.4 T

An efficient shim system and an optimized localization sequence were used to measure in vivo 1H NMR spectra from cerebral cortex, hippocampus, striatum, and cerebellum of C57BL/6 mice at 9.4 T. The combination of automatic first‐ and second‐order shimming (FASTMAP) with strong custom‐designed second‐order shim coils (shim strength up to 0.04 mT/cm2) was crucial to achieve high spectral resolution (water line width of 11–14 Hz). Requirements for second‐order shim strengths to compensate field inhomogeneities in the mouse brain at 9.4 T were assessed. The achieved spectral quality (resolution, S/N, water suppression, localization performance) allowed reliable quantification of 16 brain metabolites (LCModel analysis) from 5–10‐μL brain volumes. Significant regional differences (up to 2‐fold, P < 0.05) were found for all quantified metabolites but Asp, Glc, and Gln. In contrast, 1H NMR spectra measured from the striatum of C57BL/6, CBA, and CBA/BL6 mice revealed only small (<13%, P < 0.05) interstrain differences in Gln, Glu, Ins, Lac, NAAG, and PE. It is concluded that 1H NMR spectroscopy at 9.4 T can provide precise biochemical information from distinct regions of the mouse brain noninvasively that can be used for monitoring of disease progression and treatment as well as phenotyping in transgenic mice models. Magn Reson Med 52:478–484, 2004.

Early N-Acetylaspartate Depletion Is a Marker of Neuronal Dysfunction in Rats and Primates Chronically Treated with the Mitochondrial Toxin 3-Nitropropionic Acid:

N-acetylaspartate (NAA) quantification by 1H-magnetic resonance spectroscopy has been commonly used to assess in vivo neuronal loss in neurodegenerative disorders. Here, the authors used ex vivo and in vivo1H-magnetic resonance spectroscopy in rat and primate models of progressive striatal degeneration induced by the mitochondrial toxin 3-nitropropionate (3NP) to determine whether early NAA depletions could also be associated with neuronal dysfunction. In rats that were treated for 3 days with 3NP and had motor symptoms, the authors found a significant decrease in NAA concentrations, specifically restricted to the striatum. No cell loss or dying cells were found at this stage in these animals. After 5 days of 3NP treatment, a further decrease in striatal NAA concentrations was observed in association with the occurrence of dying neurons in the dorsolateral striatum. In 3NP-treated primates, a similar striatal-selective and early decrease in NAA concentrations was observed after only a few weeks of neurotoxic treatment, without any sign of ongoing cell death. This early decrease in striatal NAA was partially reversed after 4 weeks of 3NP withdrawal. These results demonstrate that early NAA depletions reflect a reversible state of neuronal dysfunction preceding cell degeneration and suggest that in vivo quantification of NAA 1H-magnetic resonance spectroscopy may become a valuable tool for assessing early neuronal dysfunction and the effects of potential neuroprotective therapies in neurodegenerative disorders.

Metabolic and Hemodynamic Events after Changes in Neuronal Activity: Current Hypotheses, Theoretical Predictions and in vivo NMR Experimental Findings

Unraveling the energy metabolism and the hemodynamic outcomes of excitatory and inhibitory neuronal activity is critical not only for our basic understanding of overall brain function, but also for the understanding of many brain disorders. Methodologies of magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are powerful tools for the noninvasive investigation of brain metabolism and physiology. However, the temporal and spatial resolution of in vivo MRS and MRI is not suitable to provide direct evidence for hypotheses that involve metabolic compartmentalization between different cell types, or to untangle the complex neuronal microcircuitry, which results in changes of electrical activity. This review aims at describing how the current models of brain metabolism, mainly built on the basis of in vitro evidence, relate to experimental findings recently obtained in vivo by 1H MRS, 13C MRS, and MRI. The hypotheses related to the role of different metabolic substrates, the metabolic neuron—glia interactions, along with the available theoretical predictions of the energy budget of neurotransmission will be discussed. In addition, the cellular and network mechanisms that characterize different types of increased and suppressed neuronal activity will be considered within the sensitivity-constraints of MRS and MRI.

Brain gaba editing without macromolecule contamination

A new scheme is proposed to edit the 3.0 ppm GABA resonance without macromolecule (MM) contamination. Like previous difference spectroscopy approaches, the new scheme manipulates J‐modulation of this signal using a selective editing pulse. The elimination of undesirable MM contribution at 3.0 ppm is obtained by applying this pulse symmetrically about the J‐coupled MM resonance, at 1.7 ppm, in the two steps of the editing scheme. The effectiveness of the method is demonstrated in vitro, using lysine to mimic MM, and in vivo. As compared to the most commonly used editing scheme, which necessitates the acquisition and processing of two distinct difference spectroscopy experiments, the new scheme offers a reduction in experimental time (–33%) and an increase in accuracy. Magn Reson Med 45:517–520, 2001.

Measurement of reduced glutathione (GSH) in human brain using LCModel analysis of difference-edited spectra.

The concentration of reduced glutathione (GSH), an antioxidant, may be altered in various brain diseases. MEGA‐PRESS was used to edit for the 1H NMR signal from GSH in the occipital lobe of 12 normal humans. In all studies, GSH was clearly detected with a spectral pattern consistent with spectra acquired from a phantom containing GSH. Retention of singlet resonances in the subspectra, a key advantage of this difference‐editing technique, provided an unambiguous reference for the offset and phase of the edited signal. Linear combination model (LCModel) analysis provided an unbiased means for quantifying signal contribution from edited metabolites. GSH concentration was estimated from the in vivo spectra as 1.3 ± 0.2 μmol/g (mean ± SD, n = 12). Magn Reson Med 50:19–23, 2003.

Proton Echo-Planar Spectroscopic Imaging of J-Coupled Resonances in Human Brain at 3 and 4 Tesla

In this multicenter study, 2D spatial mapping of J‐coupled resonances at 3T and 4T was performed using short‐TE (15 ms) proton echo‐planar spectroscopic imaging (PEPSI). Water‐suppressed (WS) data were acquired in 8.5 min with 1‐cm3 spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. Constrained spectral fitting in reference to a non‐WS (NWS) scan was performed with LCModel using correction for relaxation attenuation and partial‐volume effects. The concentrations of total choline (tCho), creatine + phosphocreatine (Cr+PCr), glutamate (Glu), glutamate + glutamine (Glu+Gln), myo‐inositol (Ins), NAA, NAA+NAAG, and two macromolecular resonances at 0.9 and 2.0 ppm were mapped with mean Cramer‐Rao lower bounds (CRLBs) between 6% and 18% and ∼150‐cm3 sensitive volumes. Aspartate, GABA, glutamine (Gln), glutathione (GSH), phosphoethanolamine (PE), and macromolecules (MMs) at 1.2 ppm were also mapped, although with larger mean CRLBs between 30% and 44%. The CRLBs at 4T were 19% lower on average as compared to 3T, consistent with a higher signal‐to‐noise ratio (SNR) and increased spectral resolution. Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter (GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies. Magn Reson Med, 2007.

Adaptive mechanisms regulate preferred utilization of ketones in the heart and brain of a hibernating mammal during arousal from torpor

Hibernating mammals use reduced metabolism, hypothermia, and stored fat to survive up to 5 or 6 mo without feeding. We found serum levels of the fat-derived ketone, D-beta-hydroxybutyrate (BHB), are highest during deep torpor and exist in a reciprocal relationship with glucose throughout the hibernation season in the thirteen-lined ground squirrel (Spermophilus tridecemlineatus). Ketone transporter monocarboxylic acid transporter 1 (MCT1) is upregulated at the blood-brain barrier, as animals enter hibernation. Uptake and metabolism of 13C-labeled BHB and glucose were measured by high-resolution NMR in both brain and heart at several different body temperatures ranging from 7 to 38 degrees C. We show that BHB and glucose enter the heart and brain under conditions of depressed body temperature and heart rate but that their utilization as a fuel is highly selective. During arousal from torpor, glucose enters the brain over a wide range of body temperatures, but metabolism is minimal, as only low levels of labeled metabolites are detected. This is in contrast to BHB, which not only enters the brain but is also metabolized via the tricarboxylic acid (TCA) cycle. A similar situation is seen in the heart as both glucose and BHB are transported into the organ, but only 13C from BHB enters the TCA cycle. This finding suggests that fuel selection is controlled at the level of individual metabolic pathways and that seasonally induced adaptive mechanisms give rise to the strategic utilization of BHB during hibernation.

Direct, noninvasive measurement of brain glycogen metabolism in humans

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

Decreased TCA cycle rate in the rat brain after acute 3-NP treatment measured by in vivo 1H-[13C] NMR spectroscopy.

Inhibition of succinate dehydrogenase (SDH) by the mitochondrial toxin 3‐nitropropionic acid (3‐NP) has gained acceptance as an animal model of Huntingtons disease. In this study 13C NMR spectroscopy was used to measure the tricarboxylic acid (TCA) cycle rate in the rat brain after 3‐NP treatment. The time course of both glutamate C4 and C3 13C labelling was monitored in vivo during an infusion of [1‐13C]glucose. Data were fitted by a mathematical model to yield the TCA cycle rate (Vtca) and the exchange rate between α‐ketoglutarate and glutamate (Vx). 3‐NP treatment induced a 18% decrease in Vtca from 0.71 ± 0.02 µmol/g/min in the control group to 0.58 ± 0.02 µmol/g/min in the 3‐NP group (p < 0.001). Vx increased from 0.88 ± 0.08 µmol/g/min in the control group to 1.33 ± 0.24 µmol/g/min in the 3‐NP group (p < 0.07). Fitting the C4 glutamate time course alone under the assumption that Vx is much higher than Vtca yielded Vtca=0.43 µmol/g/min in both groups. These results suggest that both Vtca and Vx are altered during 3‐NP treatment, and that both glutamate C4 and C3 labelling time courses are necessary to obtain a reliable measurement of Vtca.