CRITICAL REEVALUATION OF SPIPERONE AND BENZAMIDE BINDING TO DOPAMINE D-2 RECEPTORS - EVIDENCE FOR IDENTICAL BINDING-SITES

There are several inconsistencies in the literature as regards the cha racteristics of benzamide and butyrophenone binding to dopamine D-2-li ke receptors. The variations observed in B-max, K-d and K-i values hav e led to hypotheses, such as the existence of a specific 'benzamide bi nding site' and that dopamine D-2 receptors exist in a monomer-dimer e quilibrium, where benzamides are supposed to bind receptor monomers an d butyrophenones receptor dimers. We have previously suggested that th e discrepant results may instead be related to methodological difficul ties associated with the use of very high-affinity radioligands (e.g. ligand depletion and failure to achieve equilibrium). The present stud y was designed to reinvestigate and critically reevaluate the binding characteristics of [H-3]spiperone, [H-3]nemonapride, yl-2-pyrrolidinyl )methyl]-5,6-dimethylsalicylamide ([I-125]NCQ-298) and [H-3]raclopride to cloned human dopamine D-2A and rat striatal dopamine D-2 receptors in order to establish whether they label the same receptor population . We found that the K-d values of [H-3]spiperone, [I-125]NCQ-298 and [ H-3]nemonapride were about 20 pM and that of [H-3]raclopride about 1 n M. We did not find any significant differences between the B-max value s determined with the various radioligands. Furthermore, the K-i value s of spiperone and NCQ-298 (derived from cross-competition studies) fo r dopamine D-2 receptors labelled with either [H-3]spiperone or [I-125 ]NCQ-298 were in good agreement with the corresponding K-d values. in conclusion, our results clearly demonstrate that when studied under co rrect experimental conditions, all four radioligands label an identica l receptor population.