Scattergrams display the proliferative response of mouse embryonic fibroblasts (MEFs) after 48 h in the presence of: (A) vehicle or 0.3 nM to 300 μM morphine, with the maximum proliferative response produced by 3 µM; (B) vehicle or morphine (MS: 3 µM) or morphine (3 µM) + naloxone (Nal: 3 µM) showing that the proliferative effects of morphine were not altered by equimolar concentrations of the opioid antagonist; (C) vehicle or morphine (3 µM) or PZM21 (0.03 to 30 µM) showing no effect of PZM21; (D) vehicle or morphine (3 µM) or DMT-DALDA (0.03 to 30 µM) showing no effect of DMT-DALDA; and (E) 0.3 pM to 300 nM fentanyl for 48 h. (F) Vehicle or morphine (MS: 3 µM) or 48 to 80 (1 µg/ml) or morphine (3 µM)/ 48 to 80 (1µg/ml) + cromolyn (Cro: 10 µM) showing that the proliferative effects of morphine were mimicked by 48 to 80 and prevented by the mast cell stabilizer. Each point represents a single well. For each drug, a one-way ANOVA was performed and a post hoc analysis was performed comparing vehicle. P < 0.05; **P < 0.01; ***P < 0.001. MS, morphine sulfate.