Age-dependent decline of steady state dopamine storage capacity of human brain: An FDOPA PET study

Conventional indices of the utilization of FDOPA in living human brain have not consistently revealed important declines in dopamine function with normal aging. However, most methods of kinetic analysis have assumed irreversible trapping of decarboxylated FDOPA metabolites in brain, an assumption that is violated even in PET recordings of short duration. Therefore, we have developed methods for the calculation of steady-state storage of FDOPA together with its decarboxylated metabolites (Vd, ml g−1), based upon improved kinetic analysis of 120-min emission recordings. In a group of 28 normal male subjects, of age ranging from 23 to 73 years, the magnitude of Vd in the striatum and in extrastriatal regions declined by approximately 10% with each decade. The utilization of FDOPA was also calculated by several conventional methods assuming irreversible trapping, i.e. the net blood brain clearance (Kinapp, ml g−1 min−1), the DOPA decarboxylase activity relative to a reference tissue input (k3S, min−1), and relative to the arterial input (k3D, min−1). None of these methods revealed an age-related decline in FDOPA utilization in the extended striatum, although the magnitude of Kinapp did decline in cerebral cortex. Thus, the capacity to synthesize [18F]fluorodopamine remained largely intact in striatum of the elderly subjects, but in the presence of a substantially increased rate of washout (kloss), which was evident in all brain regions examined. Consequently, the magnitude of Vd declined with healthy aging, possibly reflecting impaired vesicular storage capacity, resulting in enhanced exposure of cytosolic [18F]fluorodopamine to monoamine oxidase.