In vitro and in vivo comparison of [³H](+)-PHNO and [³H]raclopride binding to rat striatum and lobes 9 and 10 of the cerebellum: a method to distinguish dopamine D₃ from D₂ receptor sites.

In vitro binding characteristics of the dopamine D₃/D₂ antagonist [³H]raclopride were compared to the D₃/D₂ agonist ³H-PHNO in membrane preparations from rat striatum, cerebellum Lobules 9 and 10 (CB L9,10), and other cerebellar regions. In striatum, both radioligands labeled a single binding site. ³H-PHNO showed higher affinity, though lower B(max) , compared with [³H]raclopride and was sensitive to inhibition by Gpp(NH)p. ³H-PHNO showed significant specific binding to CB L9,10 membranes with higher affinity compared to striatal membranes. ³H-PHNO binds to a high- and a low-affinity binding site in CB L9,10 membranes; the high-affinity site was not Gpp(NH)p-sensitive. ³H-PHNO did not significantly bind cerebellum left hemisphere membranes. Very low specific binding of [³H]raclopride was found in CB L9,10. The selective dopamine D₃ antagonist SB-277011 did not displace the binding of either ligand to striatal membranes but potently inhibited the binding of ³H-PHNO in CB L9,10 membranes. The highly selective D₂ antagonist SV-156 showed the opposite profile. In vivo experiments were consistent with and supported by in vitro results. In summary, ³H-PHNO and [³H]raclopride mainly label dopamine D₂ receptors in rat striatum, with ³H-PHNO labeling a D₂(High) population. In vitro and in vivo, ³H-PHNO labels CB L9,10 dopamine D₃ receptors that are apparently in a high affinity state whereas [³H]raclopride gave only very low signal in this region. The present approaches appear useful for selectively labeling dopamine D₃ and D₂ receptors in different rat brain regions and offer the possibility to demonstrate D₃ versus D₂ receptor selectivity of compounds using native rat brain tissue.