Altered serotonin metabolism in depressed patients with parkinson's disease.

Depression is a common symptom in patients with Parkinson's disease. It is not related to the severity of the motor symptoms or changes in dopamine metabolism and does not improve on treatment with dopamine agonists. Alterations in serotonin metabolism are found in primary (endogenous) depression. The brain content of serotonin in Parkinson's disease is also reduced, but this has not been related to any manifestation of the disorder. We found that the CSF content of the major metabolite of serotonin, 5-hydroxyindoleacetic acid, was lower in depressed than in nondepressed parkinsonians. The data suggest that the alterations in serotonin metabolism in Parkinson's disease identify a subgroup of patients who are prone to depression.