Dopamine D2 receptors labeled with [3H]raclopride in rat and rabbit brains. Equilibrium binding, kinetics, distribution and selectivity.

The binding properties of the substituted benzamide raclopride to dopamine D2 receptors were studied with membrane preparations from rat and rabbit neostriatum. An analysis of the association kinetics suggested a single binding site but the data from the dissociation experiments were better described by a two-site model. Examination of saturation curves at equilibrium revealed a single class of binding sites in the neostriatum from both species (rat: maximum binding capacity (Bmax) = 247 fmol/mg of protein; rabbit: Bmax = 337 fmol/mg of protein). In cortical regions known to possess a distinct dopaminergic innervation (piriform-entorhinal areas and cingulate cortex) the Bmax values ranged between 9 and 22 fmol/mg of protein. [3H]Raclopride binding sites (less than 12 fmol/mg of protein) were also detectable in the dorsal and ventral hippocampus as well as in the somatosensory and visual cortices. The selectivity in the neostriatum was examined by competition experiments with dopaminergic drugs. The rank of potency of agonists and antagonists to displace [3H]raclopride binding revealed its selectivity for the dopamine D2 receptor and was essentially the same for both species. Antagonist competition curves could be fitted to a single site but inhibition by agonists was better described assuming a two-site model. The stereospecificity of binding was demonstrated by the use of the enantiomer pairs. These results validate the utilization of the novel benzamide [3H]raclopride as a selective marker of dopamine D2 receptors.