Loss of thalamic serotonin transporters in early drug-naïve Parkinson's disease patients is associated with tremor: an [(123)I]beta-CIT SPECT study.

In vitro studies revealed serotonin transporter (5-HTT) decline in Parkinson's disease (PD). Yet, few studies investigated thalamic 5-HTT in vivo and its effect on PD heterogeneity. We analyzed thalamic [(123)I]beta-CIT binding (mainly reflecting 5-HTT binding) in 32 drug-naïve PD patients and 13 controls with SPECT. Twenty-six patients were examined twice (17 months apart). Based on UPDRS scores, we identified subgroups of patients with moderate/severe tremor (PD(T)) and without tremor (PD(WT)) at the time of clinical diagnosis. Additionally, depressive symptoms were evaluated using the Beck Depression Inventory (BDI) at baseline. Mean thalamic specific to non-specific [(123)I]beta-CIT binding ratio was lower in patients when compared to controls, and further decreased during follow-up. At baseline, average thalamic ratio was significantly lower in the PD(T) than in the PD(WT) subgroup. No correlation was found between BDI scores and thalamic binding ratios. Our findings show decline of [(123)I]beta-CIT binding to thalamic 5-HTT in PD and its possible contribution to tremor onset.