QUANTIFICATION OF THE GABA SHUNT AND THE IMPORTANCE OF THE GABA SHUNTVERSUS THE 2-OXOGLUTARATE DEHYDROGENASE PATHWAY IN GABAERGIC NEURONS

We investigated the activity of the cerebral GABA shunt relative to th e overall cerebral tricarboxylic acid (TCA) cycle and the importance o f the GABA shunt versus 2-oxoglutarate dehydrogenase for the conversio n of 2-oxoglutarate into succinate in GABAergic neurons. Awake mice we re dosed with [1-C-13]glucose, and brain extracts were analyzed by C-1 3 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 flux thro ugh the GABA shunt relative to the overall cerebral TCA cycle flux equ als 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 C-13 labeling of aspartate from [1-C-13]-glucose gl ucose specifically reflects the metabolic activity of GABAergic neuron s. In the present study, the reduction in the formation of [C-13]aspar tate during inhibition of the GABA shunt by gamma-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 t hrough the GABA shunt and 2-oxoglutarate dehydrogenase in GABAergic ne urons 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 gamma-vinyl-GABA, which increase d GABA levels dramatically, caused changes in the C-13 labeling of glu tamate and glutamine, which indicated a reduction in the transfer of g lutamate from neurons to glia, implying reduced glutamatergic neurotra nsmission. In the most severely affected animals these alterations wer e associated with convulsions.