DOPAMINE D1B RECEPTOR CHIMERAS REVEAL MODULATION OF PARTIAL AGONIST ACTIVITY BY CARBOXYL-TERMINAL TAIL SEQUENCES

NNC 01-0012, a second-generation benzazepine compound, pharmacological ly differentiates multiple vertebrate D1 receptor subtypes (D1A, D1B, D1C, and D1D) and displays high selectivity and affinity for dopamine D1C receptors, Functionally, whereas NNC 01-0012 acts as a full or poo r antagonist at D1C and D1A receptor-mediated cyclic AMP production, r espectively, it exhibits partial agonist activity at the D1B receptor. To define some of the structural motifs that regulate the pharmacolog ical and functional differentiation of vertebrate dopamine D1 receptor s by NNC 01-0012, a series of receptor chimeras were constructed in wh ich the divergent carboxyl-terminal (CT) receptor tails were replaced with the corresponding sequences of D1A, D1B, or D1C receptors, Substi tution of the vertebrate D1B carboxyl-terminal-tail at position Tyr(34 5) with carboxyl-terminal-tail sequences of the D1A receptor abolished the partial agonist activity of NNC 01-0012 without affecting dopamin e-stimulated cyclic AMP accumulation. At vertebrate D1B/D1CT-tail rece ptor mutants, however, the intrinsic activity of the partial agonist N NC 01-0012 (10 mu M) was markedly enhanced (similar to 60% relative to 10 mu M dopamine) with no concomitant alteration in the molecule's li gand binding affinity or constitutive activity of the chimeric recepto r. Similar results were obtained with other benzazepines such as SKF-3 8393 and SCH-23390, which act as partial agonists at vertebrate D1B re ceptors. Substitution of D1A and D1C receptor carboxyl-terminal tails with sequences encoded by the D1B receptor carboxyl-terminal tail did not, however, produce receptors with functional characteristics signif icantly different from wild type. Taken together, these data clearly s uggest that in addition to well characterized domains and amino acid r esidues in the third cytoplasmic loop, partial agonist activity at the D1B receptor is modulated by sequence-specific motifs within the carb oxyl-terminal tail, a region that may underlie the possible structural basis for functionally divergent roles of multiple dopamine D1-like r eceptors.