Using a microdialysis technique, the rat striatum was perfused with NSD-1015, an 
inhibitor of aromatic L-amino acid decarboxylase, and the amount of 
L-3,4-dihydroxyphenylalanine (L-DOPA) and 5-hydroxytryptophan (5-HTP) 
accumulating in dialysate was measured as an index of in vivo activities of 
tyrosine hydroxylase and tryptophan hydroxylase. NSD-1015 increased the 
concentration of L-DOPA much higher than that of 5-HTP in a dose-related manner 
(1-100 mumol/L). In order to examine the relationship between dopaminergic and 
serotonergic neurons in the striatum, either 5-HTP or L-DOPA was injected 
intraperitoneally to rats pretreated with benserazide, an inhibitor of 
peripheral decarboxylase. 5-HTP administration increased 5-HTP, but decreased 
L-DOPA in a dose-dependent manner. Conversely, 5-HTP concentration decreased in 
an association with the increased content of L-DOPA following L-DOPA 
administration. The decrease of 5-HTP caused by L-DOPA administration was not as 
remarkable as that of L-DOPA by 5-HTP injection. These results suggest that 
L-DOPA and 5-HTP, the precursor amino acids for catecholamines and indoleamines, 
could affect mutually each other neuronal activity through the inhibition of 
their rate-limiting enzymes.