Inc.). C57BL/6J mice (8−12 weeks old) were used in all experiments, which were carried out during the second half of the light phase between 13 : 00 and 18 : 00 h. On Day 0, the mice were acclimated to the test room and the time spent in each compartment was recorded (preconditioning report). Any mouse that spent more than 75% of the time in either of the association compartments was excluded. On the next day, each mouse was randomized to a control or drug group and assigned to either the white or black association compartments in a counter-balanced manner (n = 10−15 per group). On Days 1, 3, and 5 the drug group received intraperitoneal injections of the drug(s): vehicle (0.9% saline), morphine 5 mg/kg or ondansetron 1 mg/kg plus morphine 5 mg/kg (separate injections). Twenty-five minutes after injection(s), each mouse was placed into its assigned association compartment for 25 min without access to the other compartments. On Days 2, 4, and 6, the Mice received saline injections and were placed in the alternative compartment after 25 min. On Day 7, mice were placed in the middle neutral compartment of the place preference apparatus with full access to the
