Turnover and release of GABA in rat cortical slices: Effect of a GABA-T inhibitor, gabaculine

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Abstract
The turnover and release of endogenous and labeled GABA were followed in rat cortical slices after incubation with [3H]GABA. High performance liquid chromatography was used to measure endogenous GABA and to separate [3H]GABA from its metabolites. During superfusion with 3 mM K+ the slices rapidly lost their [3H]GABA content while maintaining constant GABA levels. Exposure to 50 mM K+ for 25 min caused an initial rapid rise in the release of both endogenous and [3H]GABA followed by a more rapid decline in the release of the latter. The specific activity of released GABA was two to four times higher than that in the slices. Depolarization lead to a net synthesis of GABA. The GABA-T inhibitor, gabaculine, (5 μM) in vitro arrested the metabolism of [3H]GABA and rapidly doubled the GABA content but did not significantly increase the high K+ evoked release of endogenous GABA. In vivo pretreatment with 0.5 mM/kg gabaculine quadrupled GABA content and increased both the spontaneous and evoked release of endogenous GABA but while its Ca2+-dependent release increased by 50%, the Ca2+-independent release was enhanced sevenfold. This large Ca2+-independent release of GABA is likely to have different functional significance from the normal Ca2+-dependent release.

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