The relationship of serotonin to depression in Parkinson's disease

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Abstract
We have previously reported a correlation between depression in patients with idiopathic Parkinson's disease and decreased concentrations of the cerebrospinal fluid content of the serotonin metabolite, 5‐HIAA. To further examine this relationship, we repeated the study in a new cohort of patients while they remained on dopaminergic medications, conducted follow‐up interviews and examinations in our original cohort, and conducted an open trial of the serotonin precursor, 5‐hydroxytryptophan in a group of new patients with depression. We were again able to demonstrate a significant reduction in cerebrospinal 5‐HIAA in depressed patients in comparison to controls and patients without depression. Demented patients with Parkinson's disease, particularly those with concurrent depression, had the lowest values of 5‐HIAA. No new cases of depression occurred in our orignial cohort after 21/2 years of follow‐up, and depression remitted following conventional or experimental treatment in four patients. Depression improved in six of the seven new patients following oral 5‐hydroxytryptophan. Three of these patients allowed a repeat lumbar puncture, and the concentration of 5‐HIAA increased following 5‐hydroxytryptophan. These three studies support our hypothesis that depression in idiopathic Parkinson's disease is associated with a reduction in brain serotonin. However, it also suggests that other factors, biological or environmental, may be causal factors.