STRIATAL BINDING OF THE PET LIGAND C-11 RACLOPRIDE IS ALTERED BY DRUGS THAT MODIFY SYNAPTIC DOPAMINE LEVELS

Bilateral decreases in striatal C-11-raclopride binding were observed in adult female baboons with high resolution PET following administration of drugs that act centrally on dopaminergic neurons. At baseline and following administration of d-amphetamine (a dopamine-releasing drug), GBR-12909 (a potent dopamine reuptake inhibitor), or tetrabenazine (a biogenic amine depleting drug) PET scans of C-11-raclopride binding were obtained in a CTI 931 positron tomograph. In all studies, the ratio of the distribution volumes for the striatum to the cerebellum for C-11-raclopride binding decreased significantly by an average of 16.2% for d-amphetamine, 22.1% for GBR-12909, and 28.3% for tetrabenazine while there were no significant changes observed in the cerebellum or in the rate of systemic metabolism of the radiotracer. These decreases exceed the test/retest variability of striatal C-11-raclopride binding measured in the same animals under identical experimental conditions (Dewey et al., 1992b). Together these studies demonstrate that PET measurements of striatal C-11-raclopride binding can be used to indirectly and non-invasively monitor changes in synaptic dopamine concentrations that result from a variety of neurophysiologic mechanisms.