Background A direct antidepressant effect has been reported for certain dopaminergic medications used in the treatment of Parkinson's disease (PD). Objective To examine whether dopaminergic medications may exert differential effects on mood in early PD. Methods We analyzed prospectively-collected 5-year data on 405 early, drug-naive (at baseline) PD patients enrolled in the Parkinson's Progression Markers Initiative (PPMI) cohort study, initiated on levodopa, dopamine agonists (DAs), or monoamine-oxidase type B inhibitors (iMAO-B) under naturalistic conditions. The outcome for depressive symptoms was the 15-item Geriatric Depression Scale (GDS-15) score. Potential motor and cognitive confounders were measured using the Unified Parkinson's disease Rating Scale (MDS-UPDRS-III) and the Montreal Cognitive Assessment (MoCA). Three statistical models were used to determine medication effects on GDS-15 scores: unadjusted, adjusted, and a marginal structural model. Results One-third of patients in this cohort met GDS-15 threshold for clinically-significant depressive symptoms (GDS-15 ≥ 5). There was a marginal positive effect on GDS-15 scores after iMAO-B treatment initiation (−0.35 95%; CI: −0.73, 0.04; p = 0.08). There were no significant interactions between any of the three medication groups, but robust interactions between MoCA scores and both DAs (p = 0.005) and iMAO-B (p = 0.03) use on GDS-15 scores. Specifically, as MoCA scores worsened, DAs yielded a steeper worsening of GDS-15 scores while iMAO-B a moderating effect on GDS-15. Conclusion Dopaminergic medications have no direct effect on mood in early, unselected PD patients.
