Levodopa and the Progression of Parkinson's Disease

To the Editor: The measurements of motor responses after treatment with levodopa reported by the Parkinson Study Group (Dec. 9 issue) 1 were taken too soon. The brief half-life of blood-borne levodopa is not related to the very slow rates of accumulation and depletion of dopamine retained in the limbic system. Ignoring paired facts that both the full effectiveness and the full limbic depletion of ingested levodopa require nearly 10 weeks, this study obtained skewed results by measuring movement only 2 weeks after the withdrawal of levodopa therapy. The full decline in motor function as the level of limbic dopamine drops . . .