γ-Aminobutyric Acid Can Both Inhibit and Facilitate Dopamine Release in the Caudate Nucleus of the Rabbit

Abstract: Slices from rabbit caudate nucleus were preincubated with ³II]dopamine and then superfused and stimulated electrically. y -Aminobutyric acid IW4 and moYL increased both the basal and the stimulation-evoked overflow of tritium. The effects were not changed by picrotoxin and were only slightly reduced by bicuculline. In the presence of nipecotate mol/L, y-aminobutyric acid decreased rather than enhanced the basal and the evoked overflow. The inhibition persisted in the presence of bicuculline. Muscimol did not affect, whereas baclofen decreased, the evoked overflow of tritium. Similar results were obtained with synaptosomes that were stimulated by 30 mmoVL K'. The results indicate that y -aminobutyric acid can both facilitate and depress the release of dopamine. Facilitation occurs after entry of y -aminobutyric acid into the dopaminergic terminal axons, whereas inhibition is probably mediated by a receptor site located in the membrane of these terminals.