Changes in network activity with the progression of Parkinson's disease

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Abstract
Parkinson's disease (PD) is associated with abnormal activity in spatially distributed neural systems mediating the motor and cognitive manifestations of this disorder. Metabolic PET studies have demonstrated that this illness is characterized by a set of reproducible functional brain networks that correlate with these clinical features. The time at which these abnormalities appear is unknown, as is their relationship to concurrent clinical and dopaminergic indices of disease progression. In this longitudinal study, 15 early stage PD patients (age 58.0 ± 10.2 years; Hoehn and Yahr Stage 1.2 ± 0.3) were enrolled within 2 years of diagnosis. The subjects underwent multitracer PET imaging at baseline, 24 and 48 months. At each timepoint they were scanned with [18F]-fluorodeoxyglucose (FDG) to assess longitudinal changes in regional glucose utilization and in the expression of the PD-related motor (PDRP) and cognitive metabolic covariance patterns (PDCP). At each timepoint the subjects also underwent PET imaging with [18F]-fluoropropyl βCIT (FP-CIT) to quantify longitudinal changes in caudate and putamen dopamine transporter (DAT) binding. Regional metabolic changes across the three timepoints were localized using statistical parametric mapping (SPM). Longitudinal changes in regional metabolism and network activity, caudate/putamen DAT binding, and Unified Parkinson's Disease Rating Scale (UPDRS) motor ratings were assessed using repeated measures analysis of variance (RMANOVA). Relationships between these measures of disease progression were assessed by computing within-subject correlation coefficients. We found that disease progression was associated with increasing metabolism in the subthalamic nucleus (STN) and internal globus pallidus (GPi) (P < 0.001), as well as in the dorsal pons and primary motor cortex (P < 0.0001). Advancing disease was also associated with declining metabolism in the prefrontal and inferior parietal regions (P < 0.001). PDRP expression was elevated at baseline relative to healthy control subjects (P < 0.04), and increased progressively over time (P < 0.0001). PDCP activity also increased with time (P < 0.0001). However, these changes in network activity were slower than for the PDRP (P < 0.04), reaching abnormal levels only at the final timepoint. Changes in PDRP activity, but not PDCP activity, correlated with concurrent declines in striatal DAT binding (P < 0.01) and increases in motor ratings (P < 0.005). Significant within-subject correlations (P < 0.01) were also evident between the latter two progression indices. The early stages of PD are associated with progressive increases and decreases in regional metabolism at key nodes of the motor and cognitive networks that characterize the illness. Potential disease-modifying therapies may alter the time course of one or both of these abnormal networks.