The contribution of (R)-enantiomer of N-methyl-
salsolinol (1,2-dimethyl-6,7-dihydroxy-1,2,3,4-
tetrahydroisoquinoline;
NMSal) to the degeneration of dopaminergic neurons in the course of
Parkinson's disease (PD) has been predominantly suggested by in vitro experiments in cell culture and by an in vivo study in which this compound has been directly injected into the rat striatum. The aim of the present study was to examine the effects of racemic
NMSal (50 and 100 mg/kg) administered systemically, acutely and chronically for 21 days to rats, on the neurochemical and behavioral markers of PD. Our results showed that racemic
NMSal easily penetrated the blood-brain barrier. Its brain level was relatively high 2-6 h after a single injection than gradually decreased.
NMSal was quickly eliminated from the rat brain, its concentration 24 h after withdrawal from chronic treatment was very low.
NMSal at both examined doses did not affect striatal and nigral levels of
dopamine (DA) 2 h after the first and last chronic
injections, however, it markedly changed DA catabolism. In the striatum both its doses evoked a significant acceleration of the total and oxidative,
monoamine oxidase (
MAO)-dependent DA catabolism without affecting the
catechol-O-methyltransferase (COMT)-dependent O-methylation. In the substantia nigra (SN), only the higher dose of
NMSal produced such effect. DA catabolism in either structure was the same as in control 24 h after cessation of chronic treatment. The second characteristic marker of PD, the number of
tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the SN, was not affected by chronic
NMSal treatment as revealed by the stereological counting. In the behavioral study, it was found that racemic
NMSal significantly suppressed spontaneous locomotor activity and effectively prevented that stimulated by
apomorphine. Our results suggest that
NMSal may play an important role in the regulation of dopaminergic activity rather than in inducing changes of parkinsonian type.