[H]Spiperone binds selectively to rat striatal D2 dopamine receptors in vivo: a kinetic and pharmacological analysis

We have determined the kinetic, equilibrium saturation, and pharmacological characteristics of ³H]spiperone (³H]SPIP) binding to rat brain regional particulate fractions following i.v. injections of ³H]SPIP and compared these parameters to those determined in vitro with traditional ligand-homogenate binding assays. ³H]SPIP binding to rat striatum in vivo and in vitro occurs to a single class of non-interacting binding sites which possess the pharmacological properties of a D₂ dopamine (DA) receptor. The potencies of neuroleptic drugs in inhibiting DA receptor-mediated behaviors correlate with their potencies at displacing striatal ³H]SPIP binding in vivo. While striatum possesses a similar density of ³H]SPIP binding sites in vivo (34 pmol/g) and in vitro (31 pmol/g), binding affinity in vivo is about 200 times lower than in vitro. This difference in binding affinities appears to arise from alterations of ³H]SPIP association and dissociation rate constants brought about by tissue homogenization. The implications of our findings for external imaging of DA receptors and studies of DA receptor function in human brain homogenates are discussed.