In the Sal-state, both GluA1+/+ and GluA1−/− mice expressed place preference that was dependent on the morphine conditioning dose (Fig. 3A; conditioning dose effect, F2,158 = 4.26, p<0.05). After the conditioning dose of 10 mg/kg, GluA1+/+ mice did not display place preference in the Sal-state (p = 0.3), whereas GluA1−/− mice showed a significant preference (however, no significant genotype difference here). After the conditioning dose of 20 mg/kg, slightly less place preference was observed in GluA1−/− mice compared to GluA1+/+ mice (p<0.05, t-test). In the Mor10-state, the place preference was either increased or decreased depending on the conditioning dose (conditioning dose×testing state interaction, F2,158 = 3.82, p<0.05). In GluA1+/+ mice, testing in Mor10-state increased place preference as compared to testing in Sal-state in mice conditioned with 10 mg/kg morphine, but decreased the preference in mice conditioned with 20 mg/kg morphine. However, GluA1−/− mice expressed no place preference when tested in Mor10-state independent of the conditioning dose (10 and 20 mg/kg) (genotype×conditioning dose×testing state interaction, F2,158 = 5.40, p<0.01).
