Effect of anions on the uptake and release of γ-aminobutyric acid by isolated synaptic plasma membranes

Abstract

The influence of various anions on the uptake and release of γ-aminobutyric acid (GABA) was investigated in synaptic plasma membrane (SPM) vesicles isolated from sheep brain cortex. We observed that substitution of Cl − by various anions greatly reduces [ 3H]GABA accumulation by SPM vesicles. The magnitude of the effect is about 30, 85, 95 and 100% when Cl − is replaced by Br −, NO 3 −, CH 3COO − and SO 4 2−, respectively. However, no effect was observed when these anions were added together with Cl −, which indicates that they do not inhibit the [ 3H]GABA uptake mechanism by SPM vesicles. On the other hand, we observed that [ 3H]GABA release, either by homoexchange or induced by K + depolarization, is maximal in the presence of Cl − or Br −, whereas the other anions (NO 3 −, CH 3COO − and SO 4 2−) caused a 50% reduction in the two processes of [ 3H]GABA release. We also observed that the basal release of [ 3H]GABA is not greatly altered by Br − and NO 3 −, but it is greatly enhanced by CH 3COO − and SO 4 2− in substitution of Cl −. In contrast to these alterations in [ 3H]GABA movements, the membrane potential is not significantly affected by any of the anions tested. The results confirm the idea that GABA uptake implies Cl − co-transport and they demonstrate that the maximal release of [ 3H]GABA through its carrier (homoexchange or K + depolarization-induced release) requires the presence of small anions (Cl − or Br −) at the opposite side of the membrane from where the neurotransmitter is translocated. Furthermore, it appears that CH 3COO − and SO 4 2− uncouple the system by inducing basal release, whereas it remains coupled in the presence of Cl −, Br − and NO 3 −.