Calcium-dependent and -independent release of endogenous dopamine from rat striatal synaptosomes.

We examined the role of calcium in the stimulus-secretion coupling process of brain neurons by measuring the potassium-stimulated release of endogenous dopamine from striatal synaptosomes in the presence and absence of extracellular calcium. Intracellular free calcium levels were also monitored under these conditions using the fluorescent calcium chelator, fura-2. The fast-phase (less than 3 s) of potassium-stimulated dopamine release was completely blocked by removing calcium from the external medium. Elimination of calcium from the medium with EGTA only partially blocked the slow phase (60 s) of K+-stimulated dopamine release. Depolarization of synaptosomes in the presence of extracellular calcium significantly increased intracellular calcium levels as measured by fura-2. No changes in intracellular calcium were observed during depolarization in calcium free-medium. Reductions in the sodium concentration of the extracellular medium produced a significant increase in the basal release of dopamine under calcium-free conditions. Depolarization of synaptosomes under these conditions markedly enhanced the release of dopamine. These results suggest that the slow-phase of dopamine release from synaptosomes does not require calcium but may be mediated via the reversal of the sodium-linked dopamine transport system.