[125I]SCH 23982, a 'selective' D-1 receptor antagonist, labels with high affinity 5-HT1C sites in pig choroid plexus

[125I]SCH 23982, a ligand reported to be very selective for dopamine D-1 receptors, binds with high affinity to membranes of pig choroid plexus (KD = 186 pM, Bmax = 142 fmol/mg protein). Saturation and competition experiments suggested that [125I]SCH 23982 labels a homogeneous population of sites. The rank order for affinity of agonists, 5-HT greater than or equal to DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) much greater than dopamine greater than fenoldopam, and antagonists, metergoline greater than mesulergine = mianserin greater than SCH 23390 greater than methiothepin greater than ketanserin greater than fluphenazine much greater than (-)-sulpiride greater than (+)-sulpiride, was compatible with labelling of 5-HT1C receptors by [125I]SCH 23982. It also correlates very significantly with [3H]mesulergine binding to pig choroid plexus membranes. The effects of SCH 23390 and its analogues should not be systematically attributed to an interaction with D-1 receptors.