SIMULTANEOUS DETERMINATION OF IN-VIVO HYDROXYLATION OF TYROSINE AND TRYPTOPHAN IN RAT STRIATUM BY MICRODIALYSIS-HPLC - RELATIONSHIP BETWEENDOPAMINE AND SEROTONIN BIOSYNTHESIS

Using a microdialysis technique,the rat striatum was perfused with NSD -1015, an inhibitor of aromatic L-amino acid decarboxylase, and the am ount of L-3,4-dihydroxyphenylalanine (L-DOPA) and 5-hydroxytrytophan ( 5-HTP) accumulating in dialysate was measured as an index of in vivo a ctivities 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 rela tionship between dopaminergic and serotonergic neurons in the striatum , either 5-HTP or L-DOPA was injected intraperitoneally to rats pretre ated with benserazide, an inhibitor of peripheral decarboxylase. 5-HTP administration increased 5-HTP, but decreased L-DOPA in a dose-depend ent manner. Conversely, 5-HTP concentration decreased in an associatio n with the increased content of L-DOPA following L-DOPA administration . The decrease of 5-HTP caused by L-DOPA administration was not as rem arkable as that of L-DOPA by 5-HTP injection. These results suggest th at L-DOPA and 5-HTP, the precursor amino acids for catecholamines and indoleamines, could affect mutually each other neuronal activity throu gh the inhibition of their rate-limiting enzymes.