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Examination of dopamine-D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine-D2 (D2) receptors. The two types co-localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [(11)C]-(+)-PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB-277011 to dissect regional fractions of the [(11)C]-(+)-PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with (3)H-(+)-PHNO in wild-type, D2-knock-out, and D3-knock-out mice examined at baseline and following administration of SB-277011. Both sets of results converged to indicate a predominant D3-related component to (+)-PHNO binding in extra-striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [(11)C]-(+)-PHNO PET in vivo.

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