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 astroc ytic glutamine provides carbon for net synthesis of GABA in the rat neocort ex in vivo. Isotopic labeling of GABA and glutamate from astrocytic glutami ne was followed in halothane anesthetized and ventilated rats during an int ravenous infusion of [2-C-13]glucose. A net increase in GABA was achieved b y administration of the GABA-transaminase inhibitor, gabaculine to suppress catabolism of GABA and recycling of C-13 label. C-13 Percentage enrichment s of GABA, glutamate and glutamine were assessed in tissue extracts using C -13-edited H-1 nuclear magneic resonance at 8.4 T. GABA levels increased 2. 6 mu mol/g at 2 h and 6.1 mu mol/g at 5 h after gabaculine, whereas glutama te and glutamine decreased in toto by 5.6 mu mol/g at 2 h and 3.1 mu mol/g at 5 h. Selective enrichment of glutamine, glutamate, and GABA C3's over ot her carbon positions was observed consistent with a precursor role for astr ocytic glutamine. Between 1 h (control) and 3 h (gabaculine-treated) of [2- C-13]glucose infusion, 13C percentage enrichment increased in glutarnine 0 (from 3.2-_10.5 to 7.0 0.9%), glutamate 0 (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-C-13]GABA concentration (0.15 mu mol/g) was close to the predicted valu e (0.13 mu mol/g) that would be expected if the increase in GABA were produ ced entirely from glutamine compared to glutamate (0.07 mu mol/g) based on the average precursor enrichments between 1 and 3 h. We conclude that gluta mine is the major source of GABA carbon in the rat neocortex produced acute ly following GABA-T inhibition by gabaculine in vivo. (C) 2001 Elsevier Sci ence B.V. All rights reserved.