Pharmacology of human dopamine D 3 receptor expressed in a mammalian cell line: comparison with D 2 receptor

Abstract

Two cell lines were created by transfecting cDNAs of the human D 2 receptor or the recently cloned human D 3 receptor to CHO cells, and the properties of [ 125I]iodosulpride binding to membranes of these cells were compared. In cell lines expressing the D 2 receptor subtype where the selectable marker, a phleomycin-resistance gene, was cotransfected in a different plasmid, a stable expression could be maintained for only few passages. In cell lines expressing the D 3 receptor subtype, the selectable marker, a dihydrofolate reductase gene, was cotransfected in the same plasmid and a stable expression could be obtained. In addition, the D 3 receptor gene could be amplified in these latter cell lines and a high expression level reached (up to 10 6 binding sites per cell). Sodium and, to a lesser extent, lithium similarly increased [ 125I]iodosulpride binding to D 2 and D 3 receptors. In the absence of guanylnucleotide, dopamine had a 24-fold higher apparent affinity D 3 than at D 2 receptors. Gpp(NH)p induced rightward shift and steepening of dopamine competition curves at either subtype but the effects were more marked at D 2 than at D 3 receptors. Several agonists and antagonists, previously regarded as autoreceptor-selective, displayed higher affinities at D 3 than at D 2 receptors. Although most antagonists used as antipsychotics displayed high affinities at the D 3 receptor, all were more potent at the D 2 receptor. However, the ratio of K i values varied over about 10-fold among these compounds, suggesting that they realize differential dopamine receptor subtype occupancy during treatments and that this might be reflected in their clinical profile.