Figure 2: Naloxone (0.1-0.4 mg/kg) was given intraperitoneally 10 min pre-testing of morphine response (7.5 mg/kg, subcutaneously). Data are expressed as number of behavioral signs/15 min ± standard error of mean. Tukey post hoc analysis showed the differences: *P < 0.05, **P < 0.01, *** P < 0.001 difference to control that passed the morphine conditioning but only received saline (1 mL/kg) instead of antagonist before testing