Comparison of release of endogenous dopamine and γ-aminobutyric acid from rat caudate synaptosomes

Release of endogenous dopamine (DA) and gamma-aminobutyric acid (GABA) from superfused rat caudate synaptosomes was monitored with liquid chromatography with electrochemical detection. Dopamine was analyzed by oxidative detection following alumina extraction while GABA was analyzed with reductive detection following pre-column derivatization with trinitrobenzenesulfonic acid and extraction. Both spontaneous and K+-stimulated (40 mM) release were examined as well as the effect of several possible neuromodulatory agents (DA, GABA, muscimol, ascorbic acid, acetylcholine). The content of GABA in the sample and the amount released by K+ were approximately fifty times those of DA although the relative amounts released by repetitive K+ stimulations were similar. Muscimol and DA significantly attenuated both the spontaneous and stimulated release of GABA while ascorbate and acetylcholine had no effect. Acetylcholine significantly increased both the stimulated and spontaneous release of DA while the other agents had no effect. Dopamine showed an absolute dependence on calcium for stimulated release while GABA exhibited a significant calcium-independent release. These results indicate that profound differences exist in the factors which modulate the release of endogenous DA and GABA.