Decrease of 1-methyl-1,2,3,4-tetrahydroisoquinoline synthesizing enzyme activity in the brain areas of aged rat.

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ), an endogenous monoamine, which prevents the neurotoxic effect of 1-methyl-4-phenylpyridinium ion (MPP(+)) and other endogenous neurotoxins, has been described as being enzymatically formed in the brain by the 1-MeTIQ synthesizing enzyme (1-MeTIQse). In this paper, we report the brain's regional distribution of this enzyme in 3- and 24-month-old rats. The results show that the activity is spread throughout the brain, the highest activity being in the dopaminergic areas (striatum and substantia nigra) and in the cortex. During aging there was a 1-MeTIQse activity reduction ( approximately 50%) in the areas implicated in the ethyology of Parkinson disease (substantia nigra, striatum) and in the cerebral cortex.