Figure 6. Inhibition of fentanyl-induced extracellular signal-regulated kinase (ERK) activation by naloxone. (A) Mu receptor-expressing C6 cells were pretreated with the indicated concentrations of naloxone (or vehicle, for fentanyl treatment alone) for 15 min, and then 100 nM fentanyl was added for 5 min. Activation of ERK above baseline can be seen with 1 nM naloxone pretreatment. (B) Dose-response curve for inhibition of ERK activation caused by naloxone. Activation was induced as in panel A. Activation is expressed as a percentage of the activation caused by 100 nM fentanyl alone in each experiment. Values are means +/- SEM of four independent experiments.