1-Benzyl-1,2,3,4-Tetrahydroisoquinoline, an Endogenous Parkinsonism-Inducing Toxin, Strongly Potentiates MAO-Dependent Dopamine Oxidation and Impairs Dopamine Release: Ex vivo and In vivo Neurochemical Studies

1-Benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ), an endogenous neurotoxin, is known to cause a parkinsonism-like syndrome in rodents and primates. In this study we evaluated the effects of single and multiple 1BnTIQ (50 mg/kg i.p.) administration on the concentrations of dopamine, serotonin, and respective metabolites (homovanillic acid, HVA; 3,4-dihydroxyphenylacetic acid, DOPAC; 3-methoxytyramine, 3-MT; and 5-hydroxyindolacetic acid, 5-HIAA), in substantia nigra, striatum (STR), and nucleus accumbens of Wistar rats. In addition, the effect of 1BnTIQ on locomotor activity and dopamine release in vivo was also estimated in rat STR. In a behavioral study, acute administration of 1BnTIQ (50 mg/kg i.p.) produced a significant decrease in exploratory locomotor activity. A high-performance liquid chromatography with electrochemical detection ex vivo study showed that a single injection of 1BnTIQ produced a dramatic fall in the dopamine concentration in the noted brain regions (similar to 65%; P < 0.01), but not in striatal serotonin. Moreover, 1BnTIQ reduced the content of the extraneuronal dopamine metabolite 3-MT by 70% (P < 0.01). Conversely, levels of DOPAC, HVA, and 5-HIAA were elevated by 220, 320, and 185%, respectively (P < 0.01). Interestingly, multiple 1BnTIQ treatments (50 mg/kg/day i.p. x 10 days) resulted in development of tolerance to its dopamine depressing effect, while the impairment of dopamine synthesis was persisted. An in vivo microdialysis study demonstrated that 1BnTIQ (50 mg/kg i.p.) produced a profound and long-lasting decrease in extraneuronal striatal dopamine. Concurrently, however, DOPAC and HVA were elevated. This comparison between ex vivo and in vivo effects of 1BnTIQ provides greater insight into the neurotoxic actions of 1BnTIQ specific to dopamine neurons. 1BnTIQ neurotoxicity may be related to an impairment of dopamine storage, leading to a fall in intraneuronal dopamine and enhanced dopamine catabolism through a monoamine oxidize-dependent oxidative pathway that results in free radical production and ultimate cell death. Because 1BnTIQ is an endogenous compound, it may be one of the factors responsible for idiopathic Parkinson's disease.