CHARACTERIZATION OF [H-3] QUINPIROLE BINDING TO HUMAN DOPAMINE D-2A AND D-3 RECEPTORS - EFFECTS OF IONS AND GUANINE-NUCLEOTIDES

The in vitro receptor binding characteristics of [H-3]quinpirole to cl oned human dopamine D-2A(long isoform) and D-3 receptors were investig ated and compared with those of Pat striatal dopamine receptors. [H-3] Quinpirole selectively labeled the high-affinity state of cloned dopam ine D-2A and striatal D-2 receptors with an affinity of about 4 nM. In the striatum, [H-3]quinpirole bound to 70% of the receptors labeled b y the antagonist [H-3]raclopride, whereas the corresponding value for cloned dopamine D-2A receptors was 26%. [H-3]Quinpirole labeled both t he high- and ''low-''affinity states of the dopamine D-3 receptor with the affinities of 0.6 nM (36% of the receptors) and 7.3 nM, respectiv ely. At all three receptors, sodium decreased the proportion of recept ors labeled by [(3)]quinpirole, whereas ifs affinity for the remaining high-affinity sites was not changed. Further addition of guanine nucl eotides completely converted the high-affinity binding into low. Thus, even the dopamine D-3 receptor was regulated by sodium and guanine nu cleotides. Competition studies for [H-3]quinpirole and [(3)]raclopride binding revealed that the agonists (+)-(R)-7-hydroxy-2-dipropylaminot etralin and quinpirole, previously claimed to be highly dopamine D-3-s elective (approximate to 100-fold), displayed high affinity for the hi gh-affinity agonist states of both dopamine D-2 and D-3 receptors. Whe n these values were compared, instead of the apparent affinities from the one-site analysis, the dopamine D-3-selectivities were 20-fold for (+)-(R)-7-hydroxy-2-dipropylaminotetralin and 8-fold for quinpirole. Thus, it is of importance to consider both high- and low-affinity agon ist states when receptor selectivities are evaluated.