Specific in vitro and in vivo binding of 3H-raclopride. A potent substituted benzamide drug with high affinity for dopamine D-2 receptors in the rat brain.

The substituted benzamide drug raclopride, [((-)-(S)-3,5-dichloro-N-((1-ethyl-2-pyrrolidinyl) methyl)-6-methoxy-salicylamide tartrate; FLA 870(-); A40664] was shown to be a potent and selective antagonist of dopamine D-2 receptors by its high affinity for striatal 3H-spiperone binding sites and low potency to block dopamine stimulated adenylate cyclase in vitro. In vitro studies showed that 3H-raclopride binds with a high affinity (KD = 1.2 nM) and a low proportion of non-specific binding to rat striatal homogenates. The binding of 3H-raclopride is saturable (Bmax = 23.5 pmoles/g wet wt) and reversible (dissociation half-time = 30 min) with a regional distribution of the specifically bound drug showing the following rank-order: striatum greater than nucleus accumbens greater than olfactory tubercle greater than septum greater than hypothalamus greater than hippocampus greater than frontal cortex. After in vivo administration, 3H-raclopride accumulates preferentially in dopamine rich brain areas with approximately 10 times higher levels in the striatum than in the cerebellum, when examined 30 min after injection. The in vivo binding of 3H-raclopride was saturable, reversible and showed a low component of non-specific binding. More than 90% of the drug reached the brain in a non-metabolized form as judged by thin-layer chromatography. Pharmacological analysis of 3H-raclopride binding showed that it could be displaced by dopamine agonists and antagonists but not by serotoninergic or noradrenergic drugs. Taken together, the results suggest that 3H-raclopride labels dopamine D-2 receptors with high specificity in the rat brain both in vitro and in vivo, and thus, that it should be a useful tool in studies of central dopamine D-2 receptors.