Fig. 5. Signal transduction from μ-opioid receptor (MOR) activation to gene expression. The activated Gior Goproteins induce extracellular signal–regulated kinase (ERK) activation through the cascade including Ras, Raf-1, and the mitogen-activated protein kinase/ERK kinase (MEK). It is suggested that ERK induces c-fos and junB transcription by phosphorylating a transcription factor Elk-1. The products of c-fos and junB participate in formation of AP-1 complex, and induce transcription of other genes. PTX (pertussis toxin) and PD98059 block the signal transduction pathway by affect-ing the G-protein and MEK, respectively. SRF = serum response factor; SRE = serum response element.