IMMUNOHISTOCHEMICAL EVIDENCE THAT CENTRAL SEROTONIN NEURONS PRODUCE DOPAMINE FROM EXOGENOUS L-DOPA IN THE RAT, WITH REFERENCE TO THE INVOLVEMENT OF AROMATIC L-AMINO-ACID DECARBOXYLASE

The aim of the present study is to examine whether aromatic L-amino ac id decarboxylase (AADC) catalyzes the conversion of exogenous L-3,4-di hydroxyphenylalanine (L-DOPA) to dopamine in serotonin neurons of the rat dorsal raphe nucleus. First, in order to confirm the localization of AADC in central serotonin neurons, we used an immunoperoxidase meth od for AADC and demonstrated that the distribution of AADC-containing neurons in the dorsal raphe nucleus corresponds very closely to the pr evious description on the distribution of serotonin-immunoreactive neu rons. Second, in the rat that received intraperitoneally L-DOPA plus a peripheral AADC inhibitor, we used a double-labeling immunofluorescen ce method and showed that serotonin-stained neurons of the dorsal raph e nucleus were also immunoreactive to dopamine. The present result sug gests that AADC decarboxylating L-5-hydroxytryptophan to serotonin in physiological conditions is also able to catalyze the in vivo decarbox ylation of exogenous L-DOPA.