Characterization of [3H]quinpirole binding to human dopamine D2A and D3 receptors: effects of ions and guanine nucleotides.

The in vitro receptor binding characteristics of [3H]quinpirole to cloned human dopamine D2A (long isoform) and D3 receptors were investigated and compared with those of rat striatal dopamine receptors. [3H]Quinpirole selectively labeled the high-affinity state of cloned dopamine D2A and striatal D2 receptors with an affinity of about 4 nM. In the striatum, [3H]quinpirole bound to 70% of the receptors labeled by the antagonist [3H]raclopride, whereas the corresponding value for cloned dopamine D2A receptors was 26%. [3H]Quinpirole labeled both the high- and "low-"affinity states of the dopamine D3 receptor with the affinities of 0.6 nM (36% of the receptors) and 7.3 nM, respectively. At all three receptors, sodium decreased the proportion of receptors labeled by [3H]quinpirole, whereas its affinity for the remaining high-affinity sites was not changed. Further addition of guanine nucleotides completely converted the high-affinity binding into low. Thus, even the dopamine D3 receptor was regulated by sodium and guanine nucleotides. Competition studies for [3H]quinpirole and [3H]raclopride binding revealed that the agonists (+)-(R)-7-hydroxy-2-dipropylaminotetralin and quinpirole, previously claimed to be highly dopamine D3-selective (approximately 100-fold), displayed high affinity for the high-affinity agonist states of both dopamine D2 and D3 receptors. When these values were compared, instead of the apparent affinities from the one-site analysis, the dopamine D3-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 agonist states when receptor selectivities are evaluated.