Conditioning and place preference testing was performed in eight polycarbonate cages (45×22.5×15 cm; Tecniplast, Buguggiate, Italy) covered with transparent lids with ventilation holes. The cage floors were covered with either one of the two materials, which acted as conditioning stimuli: plastic material consisting of 1.2-cm-wide flat bars separated by 0.5 cm from each other and metal grid material of stainless steel wire mesh with 1-mm holes. These materials were selected from several different materials in 15-min preliminary pre-conditioning preference tests using a separate batch of naïve GluA1−/− and GluA1+/+ mice so that the animals showed no preference for either of them (data not shown). Locomotor activity (distance traveled in meters) and location of each mouse in place preference test (time spent in the morphine-associated side) was determined by Ethovision 3.0 video tracking system (Noldus Information Technology BV., Wageningen, The Netherlands) based on video image analysis of the center-point of the mouse. Between the trials all cages and floor materials were thoroughly washed with water and dried to remove the odors. The whole experiment was repeated 5 times using different animals with all groups being present in each sub-experiment, and the results were finally combined.
