Glutamine is the major precursor for GABA synthesis in rat neocortex in vivo following acute GABA-transaminase inhibition

The objective of the present study was to assess the degree to which astrocytic glutamine provides carbon for net synthesis of GABA in the rat neocortex in vivo. Isotopic labeling of GABA and glutamate from astrocytic glutamine was followed in halothane anesthetized and ventilated rats during an intravenous infusion of [2-13C]glucose. A net increase in GABA was achieved by administration of the GABA-transaminase inhibitor, gabaculine to suppress catabolism of GABA and recycling of 13C label. 13C Percentage enrichments of GABA, glutamate and glutamine were assessed in tissue extracts using 13C-edited 1H nuclear magneic resonance at 8.4 T. GABA levels increased 2.6 μmol/g at 2 h and 6.1 μmol/g at 5 h after gabaculine, whereas glutamate and glutamine decreased in toto by 5.6 μmol/g at 2 h and 3.1 μmol/g at 5 h. Selective enrichment of glutamine, glutamate, and GABA C3’s over other carbon positions was observed consistent with a precursor role for astrocytic glutamine. Between 1 h (control) and 3 h (gabaculine-treated) of [2-13C]glucose infusion, 13C percentage enrichment increased in glutamine C3 (from 3.2±0.5 to 7.0±0.9%), glutamate C3 (from 1.8±0.5 to 3.4±0.9%), and GABA C3 (from 2.7±1.6 to 4.8±0.4%). The measured incremental [3-13C]GABA concentration (0.15 μmol/g) was close to the predicted value (0.13 μmol/g) that would be expected if the increase in GABA were produced entirely from glutamine compared to glutamate (0.07 μmol/g) based on the average precursor enrichments between 1 and 3 h. We conclude that glutamine is the major source of GABA carbon in the rat neocortex produced acutely following GABA-T inhibition by gabaculine in vivo.