CHIMERIC D1 D2 DOPAMINE-RECEPTORS - DISTINCT DETERMINANTS OF SELECTIVE EFFICACY, POTENCY, AND SIGNAL-TRANSDUCTION/

D1/D2 chimeras were constructed that had D1 dopamine receptor sequence at the amino terminal end and D2 dopamine receptor sequence at the ca rboxyl terminal end. The chimeras with the first four, five and six tr ansmembrane domains of the D1 receptor (CH2, CH3, CH4, respectively) b ound the D1 receptor antagonist [H-3]SCH 23390 with high affinity. Rec iprocal chimeras constructed with D2 receptor sequence at the amino te rminal end displayed no detectable specific binding of [H-3]SCH 23390, [I-125]epidepride, or [H-3]spiperone. CH2, CH3, and CH4 had lower aff inity than either D1f or D2 dopamine receptors for the nonselective an tagonists and agonists and D2-selective antagonists tested. The chimer ic receptors had affinities for three D1-selective ligands and the D2- selective agonist, quinpirole, that were intermediate between D1 and D 2 receptor affinities for the drugs. The substantial loss or gain of a ffinity for three ligands upon replacement of D1 transmembrane VII wit h D2 sequence (CH4) suggests an important role for this region in the selectivity of these drugs. Stimulation of adenylyl cyclase activity b y D1 agonists occurred in cells expressing CH3 and CH4, both of which included the D1 third cytoplasmic loop, but not in cells expressing CH 1 or CH2, both with the D2 third cytoplasmic loop. However, only CH3 w as able to mediate stimulation of adenylyl cyclase by quinpirole, impl ying that D2 receptor transmembrane domain VI was an important determi nant of the selective efficacy of quinpirole. On the other hand, trans membrane domain VII was particularly important for the selective poten cy of quinpirole. Inhibition of beta-adrenergic receptor-stimulated ad enylyl cyclase activity by dopamine was seen in cells expressing D2 re ceptors and CH1, but not CH2, CH3, or CH4. Thus, the third cytoplasmic loop of D1 dopamine receptors was crucial for the coupling of the rec eptors to G(s), but inhibition of adenylyl cyclase via G(i) required s tructural features, such as the second cytoplasmic loop of the D2 rece ptor, in addition to the 3rd cytoplasmic loop.