An endogenous substance of the brain, tetrahydroisoquinoline, produces parkinsonism in primates with decreased dopamine, tyrosine hydroxylase and biopterin in the nigrostriatal regions.

An endogenous substance of the human brain, tetrahydroisoquinoline (TIQ), that had been increased in the parkinsonian brain, produced parkinsonism in marmosets after daily injection of TIQ (50 mg/kg per day, s.c. for 11 days). Tyrosine hydroxylase activity, total biopterin and dopamine concentrations were also decreased in TIQ-treated marmosets. The results suggest that TIQ is one of the candidates of neurotoxins to produce parkinsonism.