Presence of radiolabelled metabolites in release studies using [3H]gamma-aminobutyric acid

Most studies on gamma-aminobutyric acid (GABA) release from nervous tissue have been conducted using radiolabelled GABA in the presence of aminooxyacetic acid (AOAA) to inhibit GABA: 2-oxoglutarate aminotransferase (GABA-T) to prevent conversion of labelled GABA to labelled catabolites. Here we present data showing that even in the presence of 10 microM-AOAA the spontaneous release of tritium from rat cortical synaptosomes prelabelled with 2,3-[3H]GABA is mainly in the form of tritiated water but that the increase in tritium release in the presence of unlabelled GABA or high potassium-ion concentrations is in the form of authentic [3H]GABA. Interpretation of results should take these facts into account.