The endogenous amine 1-methyl-1,2,3,4- tetrahydroisoquinoline prevents the inhibition of complex I of the respiratory chain produced by MPP(+).

The endogenous monoamine 1-methyl-1,2,3,4-tetrahydroisoquinoline has been shown to prevent the neurotoxic effect of MPP(+) and other endogenous neurotoxins, which produce a parkinsonian-like syndrome in humans. We have tested its potential protective effect in vivo by measuring the protection of 1-methyl-1,2,3,4-tetrahydroisoquinoline in the neurotoxicity elicited by MPP(+) in rat striatum by tyrosine hydroxylase immunocytochemistry. Because we know that cellular damage caused by MPP(+) is primarily the result of mitochondrial respiratory inhibition at the complex I level, we have extended the study further to understand this protective mechanism. We found that the inhibitory effect on the mitochondrial respiration rate induced by MPP(+) in isolated rat liver mitochondria and striatal synaptosomes was prevented by addition of 1-methyl-1,2,3,4-tetrahydroisoquinoline. This compound has no antioxidant capacity; therefore, this property is not involved in its protective effect. Thus, we postulate that the preventive effect that 1-methyl-1,2,3,4-tetrahydroisoquinoline has on mitochondrial inhibition for MPP(+) could be due to a "shielding effect," protecting the energetic machinery, thus preventing energetic failure. These results suggest that this endogenous amine may protect against the effect of several parkinsonism-inducing compounds that are associated with progressive impairment of the mitochondrial function.