A new method using high-performance liquid chromatography with electrochemical 
detection (HPLC-ED) for the simultaneous determination of monoamines, their 
precursor amino acids, and related major metabolites in small samples of brain 
tissue weighing from 0.5 to 50 mg is described. The method is based on the 
preliminary isolation of monoamines (dopamine, norepinephrine, epinephrine, and 
serotonin), their precursor amino acids (tyrosine, 3,4-dihydroxyphenylalanine, 
tryptophan and 5-hydroxytryptophan), and their major metabolites 
(3-methoxytyramine, normetanephrine, 3,4-dihydroxyphenylacetic acid, 
homovanillic acid, vanillylmandelic acid, 
3-methoxy-4-hydroxyphenylethyleneglycol, and 5-hydroxyindoleacetic acid) by 
chromatography on small columns of Amberlite CG-50 and Dowex 50W, and by ethyl 
acetate extraction. All the compounds in the four isolated fractions were 
measured by HPLC-ED on a reversed-phase column under four different conditions. 
The sensitivity was from 0.1 to 40 pmol, depending on the substances analysed. 
This newly established method was applied to the study of the effects of an 
aromatic L-amino acid decarboxylase inhibitor (NSD-1015) and a monoamine oxidase 
inhibitor (pargyline) on the levels of monoamines, their precursor amino acids 
and their major metabolites in brain regions of mice.