Increase in Dopamine Turnover Occurs Early in Parkinson's Disease: Evidence from a New Modeling Approach to PET 18F-Fluorodopa Data

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Abstract
An increase in dopamine turnover has been hypothesized to occur early in Parkinson's disease (PD) as a compensatory mechanism for dopaminergic neuronal loss. A new approach to the determination of dopamine turnover was developed using 4-hour-long 18F-fluorodopa (FD) positron emission tomography (PET) data. An effective dopamine turnover, an estimate of dopamine turnover, has been measured using its inverse, the effective dopamine distribution volume (EDV). This new method is based on a reversible tracer approach and determines the EDV using a graphical method. Six healthy subjects and 10 subjects with very early PD underwent a 4-hour-long FD scan. The EDV and the plasma uptake rate constant Ki, a marker of dopamine synthesis and storage, were compared according to their ability to separate the PD group from the healthy group. The EDV was the better discriminator (93.8% correct classification versus 81.3% for Ki). Effective dopamine distribution volume decreased by 65% in the PD group relative to the healthy group, whereas the decrease in Ki was 39%. These results show that changes in EDV are measurable with PET earlier than changes in the dopamine synthesis and storage rate, indicating that EDV is a sensitive marker for early PD and that a dopamine turnover increase likely serves as an early compensatory mechanism.

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