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

Abstract

Previous studies have shown that the glutamate͞glutamine (Glu͞ 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 Glu͞Gln 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 C2]glucose and [2-13 C]acetate to separately determine rates of Glu͞Gln and GABA͞Gln cycling and their respective tricarboxylic acid cycles in the rat cortex under conditions of halothane anesthesia and pentobarbital-induced isoelectricity. Under 1% halothane anesthesia, GABA͞Gln 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 G lutamate (Glu) and GABA are the major excitatory and inhibitory neurotransmitters in the mature cerebral cortex and together account for Ϸ90% of total cortical synapses (1). Excitatory synapses outnumber inhibitory synapses Ϸ5:1 (2), suggesting that excitation plays an energetically dominant role in the cortex. The energetic cost of GABAergic neurotransmission remains an open question, however, because current methods used to assess cortical activity are based on changes in local blood f low or metabolism (e.g., glucose or oxygen consumption), which cannot differentiate glutamatergic from GABAergic neurons. Thus, the interpretation of changes in cortical activity in terms of the energetics of excitation and inhibition requires other methods that are sensitive to the synthesis of these neurotransmitters.Glutamatergic neurotransmitter cycling flux and energy consumption have been reported for rat and human cortex by using 13 C NMR spectroscopy during the i.v. infusion of 13 C-labeled glucose (3-10). These studies show that the Glu͞glutamine (Gln) cycling flux (V cyc ) is substantial, from Ϸ30% to 42% of neuronal tricarboxylic acid (TCA) cycle flux in anesthetized rats (V TCAn ) (4, 5, 8) to Ϸ38-50% of V TCAn in resting awake rat and human cortex (7, 9, 10). In the cortex of anesthetized rats, changes in V cyc and V TCAn are proportional (Ϸ1:1) over a large cortical activity range above isoelectricity (4,8).The determination of V cyc from Gln and Glu 13 C turnover, using [1-13 C]glucose as tracer, includes contributions from GABA as well as Glu (4,5,11). This is because Gln is a com...

Energetics of Inhibition: Insights with a Computational Model of the Human GABAergic Neuron–Astrocyte Cellular Complex

“…The metabolism of GABA has been investigated with 13 C NMR (carbon nuclear magnetic resonance) spectroscopy in vitro (Waagepetersen et al, 1998) and in vivo (Patel et al, 2001), but owing to the difficulty in measuring the relative contributions of exogenous glutamate and glutamine to GABA synthesis, quantifications of the GABA-glutamine cycling flux found in the literature differ significantly. Recently, Yang et al (2007) found this flux to be one order of magnitude smaller than previously estimated (Van den Berg and Garfinkel, 1971;Patel et al, 2005). The still largely unknown metabolic interaction between astrocytes and GABAergic neurons adds to the challenge of understanding GABA metabolism and synthesis.…”

Highly Energized Inhibitory Interneurons are a Central Element for Information Processing in Cortical Networks

“…61,80 In contrast to the excitatory projection cells, estimates about energy costs of inhibitory interneurons are widely lacking because of insufficient experimental data and cell-specific neuroenergetic knowledge. 18,76 Few studies from hippocampus and neocortex provide first evidence that (i) glucose metabolism is increased during long-term recurrent inhibition of hippocampal pyramidal cells, 81 (ii) the contribution of GABA to the glutamate/GABA-glutamine cycle might account for 10% to 15% of the total oxidative metabolism 82 and (iii) glucose metabolism might be significantly stronger in GABAergic neurons than in glutamatergic neurons as revealed by combining high-resolution 2-deoxyglucose and immunohistochemistry. 83 For the latter technically advanced study with single-cell resolution, the methodological obstacles such as adequate label retention during immunohistochemical processing that may limit interpretation have been discussed.…”

The Contribution of Ketone Bodies to Basal and Activity-Dependent Neuronal Oxidation in Vivo

“…Sixteen unanesthetized rats ([2,4-13 C 2 ]BHB, n ¼ 16) received infusions of either [2,[4][5][6][7][8][9][10][11][12][13] C 2 ]BHB (1.5 mol/L) for selected lengths of times (7,15,30,60, and 100 minutes; 3 to 4 rats per time point) or [2-13 C]acetate for 2 hours (n ¼ 4), followed by euthanasia. Separate groups of unanesthetized rats were infused with [2,4-13 C 2 ]BHB (3 mol/L, n ¼ 5; or 4.5 mol/L, n ¼ 5) for 7 minutes to assess plasma BHB concentration dependence on brain amino acid labeling or [2-13 C]acetate for B2 hours (isotopic steady state, n ¼ 4) for calculation of the V cyc /V tcaN ratio.…”

“…Four rats were administered high-dose PB sufficient to induce total suppression of cortical electrical activity (isoelectricity) followed by infusion of [2,[4][5][6][7][8][9][10][11][12][13] C 2 ]BHB (1.5 mol/L) for 20 minutes and then euthanasia. The rats were anesthetized under isoflurane (1% to 2%), tracheotomized and mechanically ventilated with a mixture of 30% O 2 /70% N 2 O.…”

“…In the present study we determined the rates of oxidation of glucose and ketone bodies and Glu-Gln cycle in 36-hour fasted rats receiving infusions of [2,[4][5][6][7][8][9][10][11][12][13] C 2 ]-D-b-hydroxybutyrate (BHB) under awake conditions and during isoelectricity (verified by electrophysiology) induced by deep pentobarbital (PB) anesthesia. In addition, data were included from a third group of 36-hour fasted, 13 C-BHB-infused rats under halothane anesthesia (intermediate activity level) and studied in vivo.…”