Place preference test was performed two days after the last conditioning trial at 10 a.m. The cage floor was covered with both plastic and grid materials, thereby bisecting it into two distinct equally-sized zones without any vertical borders between them. Thus, during both the place preference test trial and conditioning trials the area of the arena was identical, i.e. the animals had access to the whole apparatus during all phases of the experiment. Spatial orientation of the materials was balanced for the test groups. The animals were injected with saline or a priming dose of morphine (10 mg/kg), thus creating saline-state (Sal-state) or morphine 10 mg/kg-state (Mor10-state), respectively, and placed in the center of the cage. Locomotor activity and location of the mouse were observed for 15 min. Place preference was presented as time spent in the morphine-associated zone.
