Dopamine: stimulation-induced release from central neurons.

Dopamine, synthesized in rat brain slices from labeled L-tyrosine or L-dopa, can be released by electrical stimulation of a type known to induce neuronal depolarization. Pretreatment of the animals with 6-hydroxydopamine, which destroys central catecholamine-containing nerve terminals, substantially reduced the release of dopamine synthesized from [(14)C]tyrosine or from a low concentration of [(3)H]dopa, whereas the release of dopamine formed from a high concentration of [(3)H]dopa remained essentially unchanged. The observations that at high concentrations L-dopa may enter noncatecholaminergic cells, undergo decarboxylation to dopamine, and subsequently be liberated in response to depolarization suggest that dopamine may act as a substitute central transmitter, possibly in serotonergic neurons. This mechanism may contribute to L-dopa's clinical effects in parkinsonian patients.