The tetrahydroisoquinoline nucleus is present in a number of compounds and drugs. Tetrahydroisoquinolines (THIQs), which belong to a group of cyclized condensation adducts of biogenic amines with aldehydes, are referred to as mammalian alkaloids. They include salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline; SAL) and tetrahydropapaveroline (6,7-dihydroxy-1-(3',4'-dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline; THP) that are derived from dopamine through condensation with acetaldehyde and dopaldehyde (3,4-dihydroxyphenylacetaldehyde), respectively. THP is a putative dopaminergic neurotoxin that is implicated in the pathology of Parkinson's disease. It is known that THP is detected at a high level in the brain and the urine of parkinsonian patients under 3,4-dihydroxyphenylalanine (L-DOPA) therapy. In addition, significant levels of THP have also been detected in the brain after ethanol uptake and/or L-DOPA treatment, and this endogenous alkaloid is considered to account for the neurobehavioral abnormalities associated with alcoholism and may act as a neurotransmitter