Multiple signaling routes involved in the regulation of adenylyl cyclase and extracellular regulated kinase by dopamine D₂ and D₃ receptors

Most G protein coupled receptors (GPCR) regulate multiple cellular processes by coupling to more than one kind of G protein. Furthermore, recent studies have reported G protein-independent/β-arrestin-dependent signaling pathway for some GPCRs. Dopamine D₂ and D₃ receptors (D₂R, D₃R), the major targets of currently used antipsychotic drugs, are co-expressed in some of the same dopaminergic neurons and regulate the same overlapping effectors. However, the specific subunits of G proteins that regulate each signaling pathway are not clearly identified. In addition, the existence of β-arrestin-dependent/G protein-independent signaling is not clear for these receptors. In this study, we determined the G protein subtypes and β-arrestin dependency involved in the signaling of D₂R and D₃R, which was measured by inhibition of adenylyl cyclase and extracellular signal-regulated kinase (ERK) activation. For the inhibition of cAMP production in HEK-293 cells, D₂R used the Gαo subunit but D₃R used the βγ subunit of Gi family proteins. For the regulation of ERK activation, D₂R used the α subunits of Gi/o proteins both in HEK-293 cells and COS-7 cells, but D ₃R used Gαo and Gβγ in HEK-293 cells and COS-7 cells, respectively. β-Arrestin-dependent/G protein-independent ERK activation was not observed for both D₂R and D₃R. Agonist-induced β-arrestin translocation was observed with D₂R but not with D₃R, and β-arrestins exerted inhibitory influences on G protein-dependent ERK activation by D₂R, but not D₃R. These results show that the D₂R and D₃R, which have overlapping cellular expressions and functional roles, employ distinct G protein subunits depending on the cell types and the effectors they control.