GABA exists in at least two different intracellular pools, i.e., a cytoplasmic or metabolic pool and a vesicular pool. This study was performed to gain information about the quantitative role of the tricarboxylic acid (TCA) cycle in biosynthesis of GABA from glutamine when GABA was selectively released from either one of these two pools. Cultured cerebral cortical neurons (GABAergic) were incubated in a medium containing 0.5 mM [U-13C]glutamine and subsequently depolarized for release of GABA from either the vesicular or the cytoplasmic pool. The vesicular release was induced by 55 mM K+ in the presence of tiagabine, a nontransportable inhibitor of the plasma membrane GABA carriers, whereas the cytoplasmic release via a reversal of the GABA carrier was induced by exposure to N-methyl-D-aspartate (NMDA; 50 microM) in the presence of (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate (AMPA; 50 microM). Cell extracts were analyzed by 13C magnetic resonance spectroscopy subsequent to the incubation or depolarization. The percentage of GABA generated from glutamine via the TCA cycle decreased from 60% to 46% during depolarization, inducing GABA release from the cytoplasmic pool, whereas a significant change in this parameter was not observed after release from the vesicular pool. These observations indicate that, during release from the cytoplasmic pool, the fraction of GABA synthesized directly from glutamine without involvement of the TCA cycle is more pronounced than that occurring during resting conditions and when release occurs from the vesicular pool. This might be explained by differences in the regulation of the two isoforms of glutamate decarboxylase (GAD(65) and GAD(67)), which presumably play different roles in the maintenance of GABA in the two pools. Both isoforms were found in the cultured cerebral cortical neurons, as shown by Western blotting employing an antibody recognizing GAD(65) as well as GAD(67).
Copyright 2001 Wiley-Liss, Inc.