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.
Malmberg
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Department of Molecular Pharmacology, Preclinical R & D, Astra Arcus AB, Södertälje, Sweden.
Mohell
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D000818:Animals; D001667:Binding, Competitive; D004873:Ergolines / Q000378:metabolism*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D051379:Mice; D019257:Quinpirole; D020891:Raclopride; D051381:Rats; D017207:Rats, Sprague-Dawley; D011954:Receptors, Dopamine / Q000378:metabolism*; D017448:Receptors, Dopamine D2 / Q000378:metabolism*; D050637:Receptors, Dopamine D3; D012457:Salicylamides / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology; D013764:Tetrahydronaphthalenes / Q000378:metabolism
C493593:DRD3 protein, human; C493594:Drd3 protein, mouse; C493595:Drd3 protein, rat; D004873:Ergolines; D011954:Receptors, Dopamine; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D012457:Salicylamides; D013764:Tetrahydronaphthalenes; D019257:Quinpirole; D006165:Guanylyl Imidodiphosphate; D020891:Raclopride; D012964:Sodium; D008274:Magnesium; C031349:7-hydroxy-2-N,N-dipropylaminotetralin