Fig. 3. μ-Opioid receptor (MOR) activation induces AP-1–mediated transcription. Cells were pretreated and stimulated in a manner similar to that described in the figure 2legend. The ratio of normalized luciferase activity to that of nonstimulated control cells is expressed as mean ± SD from three separate experiments done in duplicate (*P < 0.05 vs.  nonstimulated control; #P < 0.05 vs.  cells stimulated by the same agonist without pretreatment). DAMGO =[D-Ala², N -Me-Phe⁴,Gly-ol⁵]enkephalin; PTX = pertussis toxin.