FUNCTIONAL-DIFFERENTIATION OF MULTIPLE DOPAMINE D1-LIKE RECEPTORS BY NNC-01-0012

Although members of the multiple vertebrate/ mammalian dopamine D1 rec eptor gene family can be selectively classified on the basis of their molecular/phylogenetic, structural, and tissue distribution profiles, no subtype-specific discriminating agents have yet been identified tha t can functionally differentiate these receptors, To define distinct p harmacological/functional attributes of multiple D1-like receptors, we analyzed the ligand binding profiles, affinity, and functional activi ty of 12 novel NNC compounds at mammalian/vertebrate D1/ D1A and D5/D1 B, as well as vertebrate D1C/D1D, dopamine receptors transiently expre ssed in COS-7 cells. Of all the compounds tested, only NNC 01-0012 dis played preferential selectivity for vertebrate D1C receptors, inhibiti ng [H-3]SCH-23390 binding with an estimated affinity (similar to 0.6 n M) 20-fold higher than either mammalian/vertebrate D1/D1A or D5/D1B re ceptors or the D1D receptor. Functionally, NNC 01-0012 is a potent ant agonist at D1C receptors, inhibiting to basal levels dopamine (10 mu M )stimulated adenylyl cyclase activity. In contrast, NNC 01-0012 (10 mu M) exhibits weak antagonist activity at DIA receptors, inhibiting onl y 60% of maximal cyclic: AMP production by dopamine, while acting as a partial agonist at vertebrate D1B and D1D receptors, stimulating aden ylyl cyclase activity by similar to 33% relative to the full agonist d opamine (10 mu M), an effect that was blocked by the selective D1 rece ptor antagonist NNC 22-0010. These data clearly suggest that the benza zepine NNC 01-0012, despite lacking the N-methyl residue in the R3 pos ition, is a selective and potent DIC receptor antagonist. Moreover, th e differential signal transduction properties exhibited by NNC 01-0012 at these receptor subtypes provide further evidence, at least in vert ebrates, for the classification of the D1C receptor as a distinct D1 r eceptor subtype.