Abstract

The mitogen-activated protein kinase (MAPK) cascade is stimulated by both receptor tyrosine kinases and G protein-coupled receptors. We show that recombinant human dopamine D₃ receptors expressed in Chinese hamster ovary cells transiently activate MAPK via pertussis toxin-sensitive Gi and/or Go proteins. The involvement of D₃ receptors was confirmed by use of the D₃agonists PD 128,907 and (+)-7-hydroxy-2-dipropylaminotetralin, which mimicked the response to dopamine (DA). Furthermore, haloperidol and the selective D₃ receptor antagonists S 14297 and GR 218,231 attenuated DA-induced MAPK activation; however, when tested alone, S 14297 weakly stimulated MAPK activity, suggesting partial agonist activity. The transduction mechanisms by which hD₃receptors activate MAPK were explored with specific kinase inhibitors. Genistein and lavendustin A, inhibitors of tyrosine kinase activity, did not reduce DA-induced MAPK activation. In contrast, PD 98059, an inhibitor of MAPK kinase, and Ro 31–8220 and Gö 6983, inhibitors of protein kinase C (PKC), blocked DA-induced MAPK activation. However, MAPK activation was insensitive to PKC down-regulation by phorbol esters, indicating the involvement of an “atypical” PKC. Furthermore, MAPK activation involved phosphatidylinositol 3-kinase inasmuch as its inhibition by LY 294002 and wortmannin reduced DA-induced MAPK activation. In conclusion, this study demonstrates that stimulation of hD₃ receptors activates MAPK. This action is mediated via an atypical isoform of PKC, possibly involving cross-talk with products of phosphatidylinositol 3-kinase activation.