Longitudinal decline in striatal dopamine transporter binding in Parkinson's disease: Associations with apathy and anhedonia

BackgroundMotivational symptoms such as apathy and anhedonia are common in Parkinsons disease, respond poorly to treatment, and have been hypothesised to share underlying neural mechanisms. Striatal dopaminergic dysfunction is considered central to motivational symptoms in Parkinsons disease, but the association has never been examined longitudinally. We investigated whether the progression of dopaminergic neurodegeneration was associated with emergent apathy and anhedonia symptoms in Parkinsons disease. MethodsLongitudinal cohort study of 412 newly diagnosed Parkinsons disease patients followed over five years as part of the Parkinsons Progression Markers Initiative (PPMI) cohort. Apathy and anhedonia were measured using a composite score derived from relevant items of the 15-item geriatric depression scale (GDS-15) and part I of the MDS-Unified Parkinsons Disease Rating Scale (MDS-UPDRS). Dopaminergic neurodegeneration was measured using repeated ioflupane [123-I] single photon emission computed tomography imaging of striatal dopamine transporters (DAT). ResultsLinear mixed-effects modelling across all contemporaneous data points identified a significant negative relationship between striatal DAT specific binding ratio(SBR) and apathy/anhedonia symptoms, which emerged as Parkinsons disease progressed (interaction: {beta}=-0.09, 95%CI[-0.15 -0.03], p=0.002). Appearance and subsequent worsening of apathy/anhedonia symptoms began on average two years after diagnosis and below a threshold striatal DAT SBR level. The interaction between striatal DAT SBR and time was specific to apathy/anhedonia symptoms, with no evidence of a similar interaction for general depressive symptoms from the GDS-15 (excluding apathy/anhedonia items) ({beta}=-0.06, 95%CI[-0.13 0.01]) or motor symptoms indexed by the MDS-UPDRS part III ({beta}=0.20, 95%CI[-0.25 0.65]). ConclusionThe relationship between the progression of dopaminergic neurodegeneration and emergent apathy/anhedonia symptoms supports a central role for dopaminergic dysfunction in motivational symptoms in Parkinsons disease. Striatal DAT imaging may be a useful indicator of apathy/anhedonia risk that could inform intervention strategies.