Carriers for GABA and noradrenaline uptake coexist on the same nerve terminal in rat hippocampus.

gamma-Aminobutyric acid (GABA; 1-300 microM) increased the basal release of [3H]noradrenaline ([3H]NA) from rat hippocampal synaptosomes. The effect of GABA at low concentrations (below 10 microM) was largely bicuculline-sensitive while the sensitivity to bicuculline decreased at higher concentrations. Muscimol mimicked GABA but only below 10 microM; bicuculline antagonized the effect of muscimol. Up to 300 microM (-)baclofen did not modify [3H]NA release. The effect of GABA was potently counteracted by SK & F 89976A, SK & F 100330A and SK & F 100561, three novel inhibitors of neuronal GABA uptake. These compounds could not entirely prevent the effect of GABA, being least effective at the lowest GABA concentrations (below 10 microM) and becoming progressively more effective when the concentrations of GABA were increased. The effect of muscimol was insensitive to SK & F 89976A. The effect of 100 microM GABA was totally prevented when bicuculline and uptake inhibitor were added together to the superfusion medium. The results suggest that the basal release of [3H]NA can be enhanced by GABA through two mechanisms: GABAA receptor activation and penetration into NA terminals by a GABA uptake process. Thus a carrier for the uptake of NA and a carrier for the uptake of GABA appear to coexist on the same nerve terminal in rat hippocampus.