Storage and release of endogenous and labelled GABA formed from [3H]glutamine and [14C]glucose in hippocampal slices: effect of depolarization.

To study the effect of depolarization on the synthesis, storage and release of GABA, hippocampal slices were incubated in 0.25 mM [3H]glutamine and 2.5 mM [14C]glucose in the presence of 3 or 50 mM K+. Total and labelled glutamine, glutamate and GABA contents were measured by high-performance liquid chromatography. Depolarization in the presence of Ca2+ led to a two-fold increase of labelled glutamate and a 3-fold increase of labelled GABA content originating from both labelled precursors. In the absence of Ca2+ and in the presence of 10 mM Mg2+, depolarization failed to increase labelled glutamate content and labelled GABA formation was increased by only 30%. Following superfusion with unlabelled 0.25 mM glutamine and 2.5 mM glucose a second depolarization with 50 mM K+ released twice as much labelled GABA from slices that had been incubated in the presence of 50 mM K+, than from those incubated in 3 mM K+. This difference remained unchanged in slices that were superfused with 1 mM aminooxyacetic acid, an inhibitor of GABA synthesis. The contribution of labelled GABA, especially of GABA derived from [3H]glutamine, to released GABA was significantly higher than to GABA stored in the slices. Results suggest that depolarization in the presence of Ca2+ results in increased glutamate and GABA synthesis from both glutamine and glucose and that part of GABA released by high K+ originates from preformed GABA stores.