Simultaneous determination of in vivo hydroxylation of tyrosine and tryptophan in rat striatum by microdialysis-HPLC: relationship between dopamine and serotonin biosynthesis

Summary 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-hydroxytrytophan (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 mol/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.Abbreviations AADC aromatic L-aminoacid decarboxylase - ANOVA analysis of variance - DA dopamine - DOPAC 3,4-dihydroxyphenylacetic acid - ECD electrochemicaldetection - 5-HIAA 5-hydroxyindoleacetic acid - HPLC high performance liquid chromatography - 5-HT 5-hydroxytryptamine (serotonin) - 5-HTP 5-hydroxytryptophan - HVA homovanillic acid - Km Michaelis constant - L-DOPA L-3,4-dihydroxyphenylalanine - (6 R)BH₄ (6R)-L-erythro-tetrahydrobiopterin - TH tyrosine hydroxylase - TRH tryptophan hydroxylase - TRP tryptophan - TYR tyrosine