Quantification of the GABA Shunt and the Importance of the GABA Shunt Versus the 2‐Oxoglutarate Dehydrogenase Pathway in GABAergic Neurons

Abstract

We investigated the activity of the cerebral GABA shunt relative to the overall cerebral tricarboxylic acid (TCA) cycle and the importance of the GABA shunt versus 2-oxoglutarate dehydrogenase for the conversion of 2-oxoglutarate into succinate in GABAergic neurons. Awake mice were dosed with [1-13C]glucose, and brain extracts were analyzed by 130 NMR spectroscopy. The percent enrichments of GABA C-2 and glutamate C-4 were the same: 5.0 ±1 .6 and 5.1 ±0.2%, respectively (mean ±SD). This, together with previous data, indicates that the fluxthrough the GABA shunt relative to the overall cerebral TCA cycle flux equals the GABA/glutamate pool size ratio, which in the mouse is 17%. It has previously been shown that under the experimental conditions used in this study, the 130 labeling of aspartate from [1~13C]g lucose specifically reflects the metabolic activity of GABAergic neurons. In the present study, the reduction in the formation of [13C]aspartate during inhibition of the GABA shunt by y-vinyl-GABA indicated that not more than half the flux from 2-oxoglutarate to succinate in GABAergic neurons goes via the GABA shunt. Therefore, because fluxes through the GABA shunt and 2-oxoglutarate dehydrogenase in GABAergic neurons are approximately the same, the TCA cycle activity of GABAergic neurons could account for one-third of the overall cerebral TCA cycle activity in the mouse. Treatment with yvinyl-GABA, which increased GABA levels dramatically, caused changes in the 130 labeling of glutamate and glutamine, which indicated a reduction in the transfer of glutamate from neurons to glia, implying reduced glutamatergic neurotransmission. In the most severely affected animals these alterations were associated with convulsions. Key Words: GABA shunt-2-Oxoglutarate dehydrogenase-GABA-Glutamate-Glutamine-Neuron-glia interaction.

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

“…V GAD can be estimated if the shunt f lux in GABAergic neurons is known. In a 13 C NMR labeling study of mouse brain, Hassel et al (32) reported that in GABAergic neurons, glucose oxidation in the pathway from ␣-KG to succinate is divided about equally between the TCA cycle and the GABA shunt. In our study, V TCA(GABA) sums all pathways from ␣-KG to succinate, which includes the GABA shunt in GABAergic neurons (25).…”

Energy Metabolism in Astrocytes and Neurons Treated with Manganese: Relation among Cell-Specific Energy Failure, Glucose Metabolism, and Intercellular Trafficking Using Multinuclear NMR-Spectroscopic Analysis

“…First, as glutamine synthesized in astrocytes serves as an important neuronal energy substrate and precursor of glutamate (Hertz et al, 1999), a limited supply of glutamine to neurons would additionally impair neuronal oxidative metabolism and deplete the neurotransmitter pool of glutamate as well. Second, the inability to convert glutamate to glutamine may represent one mechanism contributing to impaired reuptake of glutamate into astrocytes after manganese exposure (Hazell and Norenberg, 1997) C]glutamate rather than from astrocytic glutamine, may be the result of impaired reuptake via the GABA shunt, an important pathway for the reentry of GABA into the TCA cycle (Hassel et al, 1998).…”

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

“…2 and 4a. Note that due to the marked reduction in the cerebral level and labeling of aspartate accompanied by acute GABA-T inhibition (14) (Fig. 4a), the weaker [3- 13 C]Asp signals at 2.66 and 2.79 ppm were not distinguishable from noise in the 40 L POCE spectra averaged over 11.5 min.…”

“…The disruption of the recycling of endogenous carbon skeleton in GABAergic neurons caused by the acute inhibition of the GABA shunt is accompanied by a decrease in the total concentration of Glu and Gln. 13 C NMR studies have shown that astrocytic Gln acts as the primary precursor for GABA following acute GABA-T inhibition (3,13,14). Since astrocytes do not synthesize GABA (1), a direct in vivo measurement of the turnover of [2][3][4][5][6][7][8][9][10][11][12][13] C]GABA from [1-13 C]Glc following acute GABA-T inhibition should be useful for characterizing the metabolic flux between astrocytes and GABAergic neurons in the living brain.…”

“…GABA turns over rapidly with normal GABA-T activity in the intact brain (8,14 -16). Acute GABA-T inhibition rapidly causes a dramatic reduction in the rate of turnover of 13 C-labeled GABA (14), which makes it impossible to follow its temporal changes in vivo using direct 13 C MRS due to the low concentration of 13 C-labeled GABA. Inverse detection of 1 H bound to 13 C using the 1 H-observed, 13 C-edited (POCE) spectroscopy method offers greatly improved sensitivity (17)(18)(19)(20).…”