tokens: D5 / D1 ( CT ) or D5 / D1D ( CT ) tail substitution mutants displayed a rank order of potency and agonist affinities virtually mimicking wild - type ( wt ) D1 receptors , as indexed by both ligand binding and dopamine - stimulated cAMP accumulation assays , and , similar to wt D1 receptors , did not exhibit receptor constitutive activity or responsiveness to inverse agonists .