Abstract

Dopamine D3 receptor pharmacology differs from that of the dopamine D2 receptor despite a high degree of receptor sequence similarity. The greatest divergence of the primary sequences of D3 and D2 receptors occurs in the predicted third intracellular loops of the receptors, a region implicated in G protein binding and function. To determine whether this domain specifies the distinct ligand binding and signal transduction characteristics of the D3 receptor, we developed a chimeric receptor, replacing the third intracellular loop of the human D3 receptor with the third intracellular loop of the human D2 receptor. The pharmacology of the chimeric receptor expressed in Chinese hamster ovary cells was examined and compared with that of human dopamine D2 and D3 receptors expressed in the same cell line. The chimeric receptor retained characteristic human D3 receptor binding; the D2 third intracellular loop present in the chimeric receptor did not reduce high affinity agonist binding, characteristic of the D3 receptor, or make high affinity sites sensitive to GTP analogs. Unlike the native human D3 receptor, the chimeric receptor was negatively coupled to adenylyl cyclase through a pertussis toxin-sensitive pathway, apparently mediated by the D2 third intracellular loop. The ability of D3 ligand binding domains to produce a D2 functional response implies that the third intracellular loop of the D3 receptor is unable to mediate this D2 response in Chinese hamster ovary cells. The inhibition of adenylyl cyclase seen with the chimeric receptor is less than the inhibition produced by D2 receptor coupling, suggesting that additional sequences in the D2 receptor contribute to normal G protein coupling.