Constitutive activity of a chimeric D2/D1 dopamine receptor

Chimeric D1/D2 receptors were constructed to identify structural determinants of drug affinity and efficacy. We previously reported that chimeras that had D1 receptor transmembrane domain VII together with amino-terminal sequence from the D2 receptor were nonfunctional. D2/D1 chimeras were constructed that contained D2 receptor sequence at the amino- and carboxyl-terminal ends and D1 receptor sequence in the intervening region. Chimeric receptors with D2 sequence from transmembrane domain 7 to the carboxyl terminus together with D2 receptor sequence from the amino terminus through transmembrane helix 4 (D21-4,7]) and 5 (D21-5,7]) bound 3H]spiperone with high affinity, consistent with the hypothesis that D2 receptor transmembrane domain I or II is incompatible with D1 receptor transmembrane domain VII. D21-4,7] and D21-5,7] had affinities similar to D1 and D2 receptors for most nonselective dopamine antagonists and had affinities for most of the selective antagonists that were intermediate between those of the parent receptors. D21-4,7] and D21-5,7] mediated dopamine receptor agonist-induced stimulation and inhibition, respectively, of cAMP accumulation. The more efficient coupling of D21-5,7] to inhibition of cAMP accumulation, compared with the coupling of D25-7] and D23-7], supports the view that multiple D2 receptor cytoplasmic domains acting in concert are necessary for receptor activation of Gi. In contrast, D21-4,7], which contains only one cytoplasmic loop (the third) from the D1 receptor, is capable of activating Gs. D21-4,7] exhibited several characteristics of a constitutively active receptor, including enhanced basal (unliganded) stimulation of cAMP accumulation, high affinity for agonists even in the presence of GTP, and blunted agonist-stimulated cAMP accumulation. A number of dopamine receptor antagonists were inverse agonists at D21-4,7], inhibiting basal cAMP accumulation. Some of these drugs were also inverse agonists at the D1 receptor. Interestingly, several antagonists also potentiated forskolin-stimulated cAMP accumulation via D21-5,7] and via the D2 receptor, which could reflect inverse agonist inhibition of native constitutive activity of this receptor.