Figure 1.

Schematic diagram of the experimental procedures. Mice were intraperitoneally injected with fentanyl (0.03 or 0.3 mg/kg) or saline (control) once a day. As fentanyl treatment period 1, we continued fentanyl treatment for 28 days. In the first 7 days, no behavioral tests were performed as pretreatment. After the first 7 days, the behavioral test battery was performed. The fentanyl administration was interrupted for 14 days as a withdrawal period, and fentanyl treatment was resumed as treatment period 2