2 INTRACELLULAR SIGNALING PATHWAYS FOR THE DOPAMINE D-3-RECEPTOR - OPPOSITE AND SYNERGISTIC INTERACTIONS WITH CYCLIC-AMP

As cerebral neurons express the dopamine D-1 receptor positively coupl ed with adenylyl cyclase, together with the D-3 receptor, we have inve stigated in a heterologous cell expression system the relationships of cyclic AMP with D-3 receptor signaling pathways. In NG108-15 cells tr ansfected with the human D-3 receptor cDNA, dopamine, quinpirole, and other dopamine receptor agonists inhibited cyclic AMP accumulation ind uced by forskolin. Quinpirole also increased mitogenesis, assessed by measuring [H-3]thymidine incorporation. This effect was blocked partia lly by genistein, a tyrosine kinase inhibitor. Forskolin enhanced by 5 0-75% the quinpirole-induced [H-3]thymidine incorporation. This effect was maximal with 100 nM forskolin, occurred after 6-16 h, was reprodu ced by cyclic AMP-permeable analogues, and was blocked by a protein ki nase A inhibitor. Forskolin increased D-3 receptor expression up to 13 5%, but only after 16 h and at concentrations of >1 mu M. Thus, in thi s cell line, the D-3 receptor uses two distinct signaling pathways: it efficiently inhibits adenylyl cyclase and induces mitogenesis, an eff ect possibly involving tyrosine phosphorylation. Activation of the cyc lic AMP cascade potentiates the D-3 receptor-mediated mitogenic respon se, through phosphorylation by a cyclic AMP-dependent kinase of a yet unidentified component. Hence, transduction of the D-3 receptor can in volve both opposite and synergistic interactions with cyclic AMP.