Long-term stability of neurotransmitter activity investigated with 11C-raclopride PET

There is evidence for the shift of regulatory setpoints of functionally linked neurotransmitter systems as a basis of psychiatric disorders. 11C-raclopride PET, which has been shown to be sensitive to changes in endogenous dopamine and has a high short-term test-retest reliability, can be used to investigate different regulatory states of the dopaminergic system with respect to psychiatric diseases and pharmacological influences. Prior to these studies, the reliability of the method over time has to be established. The current study was performed in order to evaluate the long-term stability of the striatal dopaminergic system. Eight normal healthy subjects (mean age: 48.1 years; range: 24-75) were studied twice with 11C-raclopride PET two times under resting conditions with a mean time interval between the scans of 11.3 months (range: 1-19). The ratio of basal ganglia (BG) to cerebellar (CB) distribution volumes (DVs) revealed a mean absolute change of 6.94 (range: 0.0-12.87%) between study A and B. BG DVs mean absolute change was 6.30% (range: 0.55-30.46%), CB DVs mean absolute change was 8.65% (range: 3.51-16.33%). The mean change of the BG/CB ratio was -0.33% (range: 12.87-12.34%). BG DVs mean change was 4.55% (range: 4.2-30.46%), CB DVs mean change was 5.10% (range: -10.71-16.33%). The intraindividual differences between the two scans in our study were not significantly different as compared to the 24 hour interval test-retest data, which have been published earlier (repeated measures ANOVA with df = 11; F = 0.49; P = 0.50) [Volkow et al. (1993) J. Nucl. Med., 34:609-613]. The intraclass correlation of the DV ratio index was r = 0.81. The binding potential in the baseline scans and repeated scans showed a non-significant correlation with age (r = -0.58, P = 0.13). Interindividually, the DV ratio index revealed a mean of 3.18 (range = 2.55-3.68, SD = 0.42 in study A and of 3.16 (range 2.37-3.57, SD = 0.41) in study B. The intrasubject stability of the 11C-raclopride binding over a long-term period in normal human subjects suggests the feasibility of study designs investigating the long-term changes of the dopaminergic responsivity after pharmacological challenges. The baseline stability will also serve as a necessary reference for further dose-response studies and investigations of subchronical pharmacological interventions.