Pathogenesis of dyskinesias in Parkinson's disease.

Abnormal involuntary movements complicate the management of a majority of patients with advanced Parkinson's disease. The ability of levodopa to induce dyskinesias and alleviate parkinsonism has generally been considered a continuous dose-dependent pharmacological spectrum. In this study, the acute dose-response profile of intravenously administered levodopa for both inducing dyskinesia and alleviating parkinsonism, and its duration of action on these motor manifestations were evaluated in 52 parkinsonian patients. The minimum dose of levodopa required to produce mild dyskinetic movements was significantly lower in patients with fluctuations in motor response compared with those who had a stable response to standard oral therapy; the minimum dose for antiparkinsonian benefit, however, failed to show significant differences. The rate of disappearance of dyskinetic movements was faster than the rate of reappearance of parkinsonian signs following withdrawal of a steady-state infusion of levodopa. The dissociation of the pharmacodynamic profile of the two major motor effects of levodopa suggests their mediation through two different central pharmacological mechanisms, perhaps involving the two classes of dopamine receptors or other transmitter systems, and could have important implications for the design of future antiparkinsonian agents.