On account of the positive findings with respect to 5-HT3 blockade and reduction in signs of physical dependence, we extended our testing to a paradigm probing the reinforcing properties of opioid administration by using CPP. The group of mice that were administered morphine 5 mg/kg and placed in the conditioning chamber 25 min after injection had a mean difference between pre and postconditioning percentage of time spent in the compartment associated with drug of 21.29 percent (95% confidence interval: 16.82−25.75; P < 0.001). The group of mice that received ondansetron 1 mg/kg in addition to morphine 5 mg/kg displayed a mean difference of 0.40 percent (95% confidence interval: −9.666−10.47; P = 0.93) (Fig. 5). Thus, ondansetron completely blocked the place preference associated with morphine administration.
