Search

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 Gin and contributes to total Glu/Gln neurotransmitter cycling, although the fraction contributed by GABA is unknown. In the present study, we used [sup.13]C NMR spectroscopy together with i.v. infusions of [1,6-[sup.13][C.sub.2]]glucose and [2-[sup.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. GABAergic neurons | glutamatergic neurons | magnetic resonance spectroscopy | neuronal-glial interactions

Multivolume [sup.1]H-[[sup.13]C] NMR spectroscopy in combination with i.v. [1,6-[sup.13][C.sub.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.sub.1] relaxation mapping. The [sup.13]C turnover curves of [4-[sup.13]C]glutamate, [4-[sup.13]C]glutamine, and [3-[sup.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 [+ or -] 0.15, 0.20 [+ or -] 0.11, and 0.42 [+ or -] 0.09 [micro]mol/min per g, respectively. The glutamate-glutamine neurotransmitter cycle fluxes in cerebral gray matter, white matter, and subcortex were 0.31 [+ or -] 0.07, 0.02 [+ or -] 0.04, and 0.18 [+ or -] 0.12 [micro]mol/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.

Functional MRI (fMRI) is widely assumed to measure neuronal activity, but no satisfactory mechanism for this linkage has been identified. Here we derived the changes in the energetic component from the blood oxygenation level-dependent (BOLD) fMRI signal and related it to changes in the neuronal spiking frequency in the activated voxels. Extracellular recordings were used to measure changes in cerebral spiking frequency ([DELTA]v/v) of a neuronal ensemble during forepaw stimulation in the [alpha]-chloralose anesthetized rat. Under the same conditions localized changes in brain energy metabolism ([DELTA]CM[R.sub.O2]/CM[R.sub.O2]) were obtained from BOLD fMRI data in conjunction with measured changes in cerebral blood flow ([DELTA]CBF/CBF), cerebral blood volume ([DELTA]CBV/CBV), and transverse relaxation rates of tissue water ([T.sup.*.sub.2] and T2) by MRI methods at 7T. On stimulation from two different depths of anesthesia [DELTA]CM[R.sub.O2]/CM[R.sub.O2] [approximately equal to] [DELTA]v/v. Previous [sup.13]C magnetic resonance spectroscopy studies, under similar conditions, had shown that [DELTA]CM[R.sub.O2]/CM[R.sub.O2] was proportional to changes in glutamatergic neurotransmitter flux ([DELTA][V.sub.cyc]/[V.sub.cyc]). These combined results show that [DELTA]CM[R.sub.O2]/CM[R.sub.O2] [approximately equal to] [DELTA][V.sub.cyc]/[V.sub.cyc] [approximately equal to] [DELTA]v/v, thereby relating the energetic basis of brain activity to neuronal spiking frequency and neurotransmitter flux. Because [DELTA]CM[R.sub.O2]/CM[R.sub.O2] had the same high spatial and temporal resolutions of the fMRI signal, these results show how BOLD imaging, when converted to [DELTA]CM[R.sub.O2]/CM[R.sub.O2], responds to localized changes in neuronal spike frequency.

Recent 13C NMR studies in rat models have shown that the glutamate/glutamine cycle is highly active in the cerebral cortex and is coupled to incremental glucose oxidation in an [approximately equal to]1:1 stoichiometry. To determine whether a high level of glutamatergic activity is present in human cortex, the rates of the tricarboxylic acid cycle, glutamine synthesis, and the glutamate/glutamine cycle were determined in the human occipital/parietal lobe at rest. During an infusion of [1-13C]-glucose, in vivo 13C NMR spectra were obtained of the time courses of label incorporation into [4-13C]-glutamate and [4-13C]-glutamine. Using a metabolic model we have validated in the rat, we calculated a total tricarboxylic acid cycle rate of 0.77 [+ or -] 0.07 [[micro]mol]/min/g (mean [+ or -] SD, n = 6), a glucose oxidation rate of 0.39 n 0.04 [[micro]mol]/min/g, and a glutamate/glutamine cycle rate of 0.32 [+ or -] 0.05 [[micro]mol]/min/g (mean [+ or -] SD, n = 6). In agreement with studies in rat cerebral cortex, the glutamate/glutamine cycle is a major metabolic flux in the resting human brain with a rate [approximately equal to]80% of glucose oxidation.

To determine the relationship between cerebral Glc metabolism and glutamatergic neuronal function, we used 13C NMR spectroscopy to measure, simultaneously, the rates of the tricarboxylic acid cycle and Gin synthesis in the rat cortex in vivo. From these measurements, we calculated the rates of oxidative Glc metabolism and glutamate-neurotransmitter cycling between neurons and astrocytes (a quantitative measure of glutamatergic neuronal activity). By measuring the rates of the tricarboxylic acid cycle and Gin synthesis over a range of synaptic activity, we have determined the stoichiometry between oxidative Glc metabolism and glutamate-neurotransmitter cycling in the cortex to be close to 1:1. This finding indicates that the majority of cortical energy production supports functional (synaptic) glutamatergic neuronal activity. Another implication of this result is that brain activation studies, which map cortical oxidative Glc metabolism, provide a quantitative measure of synaptic glutamate release.

NMR spectroscopy was used to test recent proposals that the additional energy required for brain activation is provided through nonoxidative glycolysis. Using localized NMR spectroscopic methods, the rate of C4-glutamate isotopic turnover from infused [1-13C]glucose was measured in the somatosensory cortex of rat brain both at rest and during forepaw stimulation. Analysis of the glutamate turnover data using a mathematical model of cerebral glucose metabolism showed that the tricarboxylic acid cycle flux ([V.sub.TCA]) increased from 0.49 [+ or -] 0.03 at rest to 1.48 [+ or -] 0.82 [[micro]mol]/g/min during stimulation (P < 0.01). The minimum fraction of C4-glutamate derived from Cl-glucose was [approximately equal to]75%, and this fraction was found in both the resting and stimulated rats. Hence, the percentage increase in oxidative cerebral metabolic rate of glucose use ([CMR.sub.glc]) equals the percentage increases in [V.sub.TCA] and cerebral metabolic rate of oxygen consumption ([CMR.sub.O2]). Comparison with previous work for the same rat model, which measured total [CMR.sub.glc] [Ueki, M., Linn, F. & Hossman, K. A. (1988) J. Cereb. Blood Flow Metab. 8, 486-494], indicates that oxidative [CMR.sub.glc] supplies the majority of energy during sustained brain activation.

Localized 'H NMR spectroscopy with J editing helped in assessing the concentration of gamma-aminobutyric acid (GABA) in the occipital lobe of four control and four epileptic human volunteers who were being administered the drug vigabatrin. The NMR measurements of GABA in patients receiving GABA enhancing drugs such as vigabatrin are significant in finding the connection between seizure suppression and the concentration of brain GABA.

Previous 13C magnetic resonance spectroscopy experiments have shown that over a wide range of neuronal activity, approximately one molecule of glucose is oxidized for every molecule of glutamate released by neurons and recycled through astrocytic glutamine. The measured kinetics were shown to agree with the stoichiometry of a hypothetical astrocyte-to-neuron lactate shuttle model, which predicted negligible functional neuronal uptake of glucose. To test this model, we measured the uptake and phosphorylation of glucose in nerve terminals isolated from rats infused with the glucose analog, 2-fluoro-2-deoxy-D-glucose (FDG) in vivo. The concentrations of phosphorylated FDG (FDG6P), normalized with respect to known neuronal metabolites, were compared in nerve terminals, homogenate, and cortex of anesthetized rats with and without bicuculline-induced seizures. The increase in FDG6P in nerve terminals agreed well with the increase in cortical neuronal glucose oxidation measured previously under the same conditions in vivo, indicating that direct uptake and oxidation of glucose in nerve terminals is substantial under resting and activated conditions. These results suggest that neuronal glucose-derived pyruvate is the major oxidative fuel for activated neurons, not lactate-derived from astrocytes, contradicting predictions of the original astrocyte-to-neuron lactate shuttle model under the range of study conditions. [ABSTRACT FROM AUTHOR]

It has been reported that chronic and acute alcohol exposure decreases cerebral glucose metabolism and increases acetate oxidation. However, it remains unknown how much ethanol the living brain can oxidize directly and whether such a process would be affected by alcohol exposure. The questions have implications for reward, oxidative damage, and long-term adaptation to drinking. One group of adult male Sprague–Dawley rats was treated with ethanol vapor and the other given room air. After 3 wk the rats received i.v. [2-13C] ethanol and [1, 2-13C2]acetate for 2 h, and then the brain was fixed, removed, and divided into neocortex and subcortical tissues for measurement of 13C isotopic labeling of glutamate and glutamine by magnetic resonance spectroscopy. Ethanol oxidation was seen to occur both in the cortex and the subcortex. In ethanol-naïve rats, cortical oxidation of ethanol occurred at rates of 0.017 ± 0.002 µmol/min/g in astroglia and 0.014 ± 0.003 µmol/min/g in neurons, and chronic alcohol exposure increased the astroglial ethanol oxidation to 0.028 ± 0.002 µmol/min/g (P = 0.001) with an insignificant effect on neuronal ethanol oxidation. Compared with published rates of overall oxidative metabolism in astroglia and neurons, ethanol provided 12.3 ± 1.4% of cortical astroglial oxidation in ethanol-naïve rats and 20.2 ± 1.5% in ethanol-treated rats. For cortical astroglia and neurons combined, the ethanol oxidation for naïve and treated rats was 3.2 ± 0.3% and 3.8 ± 0.2% of total oxidation, respectively. 13C labeling from subcortical oxidation of ethanol was similar to that seen in cortex but was not affected by chronic ethanol exposure. [ABSTRACT FROM AUTHOR]