L-dopa is the major drug used to treat Parkinson's disease (PD). A low efficacy or lack of efficacy is seen in some patients, and a loss of efficacy (wear-off) often occurs after long-term treatments; the causes of which are unknown. L-dopa and its product, dopamine (DA) occur in high levels during therapy, and avidly react with S-adenosylmethionine (SAM), the methyl donor. In the process they are O-methylated, they deplete SAM and they create great demands for the methyl group. This interaction may be the key to understanding the loss of efficacy and the lack of efficacy of L-dopa. The loss of efficacy (wearing-off effect) may be due to the induction of methionine adenosyl transferase (MAT), the enzyme that produces SAM and also to catechol-O-methyltransferase (COMT), the enzyme that transfers the methyl group of SAM to L-dopa and DA. This, in turn, will increase the methylation and the depletion of L-dopa and DA. Therefore, the therapeutic DA levels usually achieved will no longer be attainable after the long-term use of L-dopa and MAT and COMT induction. During the last grant cycle we investigated this scenario in normal mice, and found that L-dopa increased the activities and induced the production of MAT and COMT, and that the levels of DA that were attained during the early phases of L-dopa administration were not attainable during the latter phases. The results, therefore, suggests that the wearing-off effect may be due to the induction of MAT and COMT. The lack, or low efficacy, of L-dopa may be caused by a constitutive hypermethylation state (high activities of MAT and COMT).
The aims of this application are to (1) develop a less stress chronic L- dopa administration protocol, using osmotic mini-pump model or pellet implantation model, adaptable for drug screening, (2) study the induction of MAT and COMT in 1-methyl-4-phenyl-12,3,6-tetrahydropyridine (MPTP) mouse model for PD, (3) study the kinetic and significance of the enzymes induction, (4) identify strains of mice or rats with high to low constitutive activities of MAT and COMT i.e. find models that are resistant (high activity) or sensitive (low activity) to the effects of L-dopa and (5) study the tolerance to L-dopa and whether cross-tolerance to apomorphine and bromocriptine occurs in animals with induced MAT and COMT. The goal is to understand the mechanisms for the tolerance developed to L-dopa and its lack of efficacy in some patients. Agents that block the induction and activities of MAT and COMT may have therapeutic values.
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