Role of monoamine neural systems in L-dihydroxyphenylalanine-stimulated activity

The potentiation of the stimulatory response to L-dihydroxyphenylalanine (L-dopa) in 6-hydroxydopamine-treated rats given a peripheral decarboxylase inhibitor occurred only after treatments that destroyed dopamine-containing fibers, Destruction of serotonin- or norepinephrine-containing fibers did not potentiate the action of L-dopa. Furthermore, other data indicated thatL-dopa-induced activity could not be attributed to release of serotonin or to the formation of norepinephrine. That the L-dopa must be converted to catecholamines was demonstrated by antagonism of the activity responses to L-dopa with inhibition of decarboxylases in brain. After destruction of serotonin-containing fibers with 5,7-dihydroxytryptamine, L-dopa-induced activity in 6-hydroxydopamine-treated rats was significantly reduced, as was the accumulation of dopamine formed from L-dopa. These latter data provide direct evidence that serotonergic fibers contribute to the accumulation of dopamine in brain after L-dopa administration to 6-hydroxydopamine-treated rats.