mice tested in Mor10-state was equal between groups of mice that were conditioned with either 10 mg/kg or 20 mg/kg morphine, but only 10 mg/kg produced high expression of CPP. It cannot be fully excluded that increased locomotor activity per se affected the expression of CPP, but this is unlikely, since Mead and Stephens [37] have reported the GluA1−/− mice to display both normal cocaine-induced place preference and increased locomotor activity during the preference test trial.
