Release and exchange of neurotransmitters in synaptosomes: Effects of the ionophore A23187 and of ouabain

The effects of the ionophore A23187 and of ouabain on the release of [³H]GABA and [³H]norepinephrine were studied in superfused rat brain synaptosomes. Each of the two drugs moderately stimulated the spontaneous release of [³H]GABA, but greatly potentiated the release of [³H]GABA induced by unlabeled GABA. In contrast, the ionophore and norepinephrine showed an additive, but not a supraadditive, releasing effect on synaptosomal [³H]norepinephrine. Ouabain modestly and transiently potentiated the norepinephrine-induced [³H]norepinephrine release, which, however, was inhibited by the drug after a few minutes. It is suggested that in the new intrasynaptosomal ionic conditions determined by the two drugs, the stoichiometry of the basal homoexchange of GABA is changed in a direction favoring net outward transport.