The hyperactivity of GluA1−/− mice could have interfered with the expression of conditioned place preference. Unfortunately, locomotor activity is seldom measured or reported in conditioned place preference studies. In the study by Vezina and Stewart [23], in which hyperactivity was suggested to mask the expression of place preference, the testing chamber was greater in size than that used during the conditioning phase. This difference may have predisposed animals to enhanced drive for exploring the testing chamber. Thus, increased exploration due to changed environment might have caused decreased expression of conditioned place preference, not the increased locomotor activity. In the present study, we co-measured locomotor activity during the place preference test trials. Although GluA1−/− mice had higher locomotor activity than the GluA1+/+ mice, no correlation between the locomotor activity and place preference was found in our data. Locomotor activity of GluA1+/+ 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
