Novel endogenous 1,2,3,4-tetrahydroisoquinoline derivatives: Uptake by dopamine transporter and activity to induce parkinsonism

We designed as candidate metabolites and synthesized two 1-benzyl-1,2,3,4-tetrahydroisoquinoline derivatives containing a dopamine moiety: 1-(3',4'-dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline (3',4'-DHBnTIQ) and 1-benzyl-6,7-dihydroxy-1,2,3,4-'tetrahydroisoquinoline (6,7DHBnTIQ). Both were detected in mouse brain as endogenous amines by gas chromatography/mass spectrometry. 3',4'DHBnTIQ induced parkinsonism in mice when chronically administered intraperitoneally, whereas 6,7DHBnTIQ did not despite the structural similarity of the two compounds. This difference may be related to cellular uptake: In rat striatal synaptosomes, these compounds were intracellularly transported by the dopamine transporter with K-m values of 6.14 and 7.82 mu M and V-max values of 214.3 and 112.2 pmol/min/mg of protein, respectively. Thus, endogenous 3',4'DHBnTIQ may be actively transported into dopaminergic neurons and accumulated there, contributing at least in part to the induction of idiopathic Parkinson's disease.