Parkinson's disease-like syndrome in rats induced by 2,9-dimethyl-beta-carbolinium ion, a beta-carboline occurring in the human brain.

Regarding the pathogenesis of Parkinson's disease, a neurotoxin hypothesis was proposed following the discovery that 1 -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a Parkinson-like syndrome in humans and primates. Since then, researchers have searched for endogenous and exogenous compounds that are structurally similar to this neurotoxin. Such compounds include p-carbolines, formed from tryptophan and its derivatives. β-carbolines are present naturally in the human brain and cerebrospinal fluid. The present study examined the effect of bilateral, intranigral administration of 2,9-dimethyl-β-carbolinium ion on muscle tone, electromyographic activity, dopamine metabolism in the striatum, and the number of tyrosine hydroxylase-immunoreactive neurons and volume of the substantia nigra in rats. We found that the p-carbolinium ion (15 or 40 nmol per side) caused a significant decrease in the striatal levels of dopamine and its metabolites, which was accompanied by an enhancement of muscle tone and electromyographic activity. Stereological counting revealed that the p-carbolinium caused a significant decrease in the total number of tyrosine hydroxylase-immunoreactive neurons and shrinkage of the substantia nigra. The findings suggest that the methylated p-carbolinium ion produces a dose-dependent degeneration of nigrostriatal neurons, leading to deficits in dopaminergic neurotransmission and an increase of muscle resistance and electromyographic activity, a syndrome equivalent to muscle rigidity in Parkinson's disease.