2.6. Conditioned place preference (CPP) test

Mice used in the CPP test were male NMU-KO mice (n = 17) and WT (n = 19) mice. The CPP test was conducted with slightly modified procedure as described in a previous report [21]. Briefly, the CPP test was conducted in a three-chamber apparatus (MED Associates, St. Albans, VT, USA) consisting of a small middle chamber that connected two large chambers that differed in floor and wall conditions [21]. On Day 0, mice were allowed to move freely between the three chambers for 20 min. On Days 1–3, mice were confined to one of the large chambers for 20 min immediately after they had received saline. Four hours later, they received 10 mg/kg cocaine and were confined to the other large chamber for 20 min. On Day 4, mice were placed in the middle chamber and allowed to move freely in the three chambers for 20 min. Actual time spent in the cocaine-paired compartment of WT and NMU-KO mice at pre-test and post-test was determined. And CPP score was calculated as a ratio as follows: [time spent in cocaine injection compartment at test day (sec)] – [time spent in cocaine injection compartment at pre-test (sec)]/total time (sec). In addition, we have also examined locomotor activity for 30 min after cocaine treatment. The activity level was recorded by a video camera, and the traveled distance was analyzed using the SMART software system (Panlab, Barcelona, Spain).