Fluctuations in plasma dopamine level impair memory in Parkinson's disease.

We found a causal relationship between variation in plasma dopamine level and memory impairment in Parkinson's disease. If the level of dopamine was changed between the time of original learning and a later time of attempted memory retrieval, memory performance was impaired when compared with a maintenance of similar dopamine levels on both occasions. The absolute level of dopamine did not influence memory performance. Side-effects of levodopa-carbidopa therapy include variation in dopamine level, parkinsonian symptoms, and possibly "state-dependent" impairment of memory.