INTERACTIONS OF LIGANDS WITH ACTIVE AND INACTIVE CONFORMATIONS OF THEDOPAMINE D-2 RECEPTOR

The affinities of 19 pharmacologically diverse dopamine D-2 receptor l igands were determined for the active and inactive conformations of cl oned human dopamine D? receptors expressed in Ltk(-) cells. The agonis t [H-3]quinpirole was used to selectively label the guanine nucleotide -binding protein-coupled, active receptor conformation. The antagonist [3H]raclopride, in the presence of the non-hydrolysable GTP-analogue Gpp(NH)p and sodium ions and in the absence of magnesium ions, was use d to label the free inactive receptor conformation. The intrinsic acti vities of the ligands were determined in a forskolin-stimulated cyclic AMP assay using the same cells. An excellent correlation was shown be tween the affinity ratios (K-R/K-RG) of the ligands for the two recept or conformations and their intrinsic activity (r= 0.96). The ligands i ncluded eight structurally related and enantiopure 2-aminotetralin der ivatives; the enantiomers of 5-hydroxy-2-(dipropylamino)tetralin, 5-me thoxy-2-(dipropylamino)tetralin, 5-fluoro-2-(dipropylamino)tetralin an d 2-(dipropylamino)tetralin. The (S)-enantiomers behaved as full agoni sts in the cyclic AMP assay and displayed a large K-R/K-RG ratio. The (R)-enantiomers were classified as partial agonists and had lower rati os. The structure-affinity relationships of these compounds at the act ive and the inactive receptor conformations were analysed separately, and used in conjunction with a homology based receptor model of the do pamine D-2 receptor. This led to proposed binding modes for agonists, antagonists and partial agonists in the 2-aminotetralin series. The co ncepts used in this study should be of value in the design of ligands with predetermined affinity and intrinsic activity. (C) 1998 Elsevier Science B.V.