Figure 2A–B displays the effects of morphine and saline on locomotor activity during the four CS+ conditioning trials. Morphine increased locomotor activity in a dose-dependent manner in both genotypes (conditioning dose effect, F2,170 = 49.34, p<0.001). Morphine effect on locomotor activity was increased progressively after each conditioning trial (day effect, F3,510 = 41.00, p<0.001). The dose of 20 mg/kg produced a greater sensitization than the dose of 10 mg/kg (day×conditioning dose interaction, F6,510 = 27.90, p<0.001).
