In the present study, we found that morphine state-dependency was altered in the mouse line lacking the GluA1 subunit of the AMPA-type glutamate receptor. State-dependency was probed in three conditions: Firstly, when adult GluA1+/+ mice were tested in a state (Mor10-state) equal to the conditioning state, they expressed a significant place preference. However, GluA1−/− mice failed to show place preference in this condition. Secondly, both lines conditioned with 20 mg/kg morphine, and tested in Mor10-state (now mice received half of the conditioning dose and thus were in a different state) failed to express place preference. Thirdly, some expression of morphine-induced place preference was detected in both mouse lines after being conditioned with morphine (10 or 20 mg/kg) and tested in Sal-state. These data indicate that a testing dose equal to conditioning dose (i.e. state-dependency) produces the greatest expression of place preference, also demonstrated by others in the place preference paradigm [24], [25]. Importantly, these data suggest a deficit in opioid state-dependency in the GluA1−/− mouse line.
