title: The release of [3H]GABA formed from [3H]glutamate in rat hippocampal slices: comparison with endogenous and exogenous labeled GABA.
abstract: to compare the storage and release of endogenous GABA, of [3H]GABA formed endogenously from glutamate, and of exogenous [14C]GABA, hippocampal slices were incubated with 5 microCi/ml [3,4-3H]1-glutamate and 0.5 microCi/ml [U-14C]GABA and then were superfused in the presence or absence of Ca+ with either 50 mM K+ or 50 microM veratridine. Endogenous GABA was determined by high performance liquid chromatography which separated labeled GABA from its precursors and metabolites. Exogenous [14C]GABA content of the slices declined spontaneously while endogenous GABA and endogenously formed [3H]GABA stayed constant over a 48 min period. In the presence of Ca+ 50 mM K+ and in the presence or absence of Ca2+ veratridine released exogenous [14C]GABA more rapidly than endogenous or endogenously formed [3H]GABA, the release of the latter two occurring always in parallel. The initial specific activity of released exogenous [14C]GABA was three times, while that of endogenously formed [3H]GABA was only 50% higher than that in the slices. There was an excess of endogenous GABA content following superfusion with 50 mM K+ and Ca2+, which did not occur in the absence of Ca2+ or after veratridine. The observation that endogenous GABA and [3H]GABA formed endogenously from glutamate are stored and released in parallel but differently from exogenous labelled GABA, suggests that exogenous [3H] glutamate can enter a glutamate pool that normally serves as precursor of GABA.
mesh_terms: D000818:Animals; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D014316:Tritium; D005680:gamma-Aminobutyric Acid / Q000096:biosynthesis* / Q000378:metabolism
chemical_list: D005971:Glutamates; D014316:Tritium; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D011188:Potassium