In the conditioning phase, two 30-min trials were performed per day on four consecutive days resulting in total of eight conditioning trials. Each day, in the first trial, saline injection was paired with either plastic or grid floor (CS− trial, conditioning stimulus with saline). In the second trial 4 h after the first one, morphine (10 or 20 mg/kg) was paired with the other floor type (CS+ trial, conditioning stimulus with morphine), so that half of the animals received morphine paired with plastic and the other half with grid. Immediately after the injections the mice were placed to the cages for 30 min. To ascertain that mice expressed no preference for either floor material, also control animals of both mouse lines were included, always receiving saline paired with both materials throughout the trials.
