The conditioned place preference (CPP) protocol was carried out as previously described (Blednov et al., 2003). Four identical acrylic boxes (30 × 15 × 15 cm) were separately enclosed in ventilated light- and sound-attenuating chambers (Med Associates, St. Albans, VT). Each box has two compartments separated by a wall with a door. The two compartments each have a different type of floor (either bars – [GRID−] or wire – [GRID+]). Infrared light sources and photodetectors were mounted opposite each other at 2.5-cm intervals along the length of each box, 2.2 cm above the floor. Occlusion of the infrared light beams was used to measure general activity and location of the animal (left or right) within the box. Total activity counts and location of the animal (left or right compartment) within the box were recorded by computer. The floors and the insides of the boxes were wiped with a damp sponge and the litter paper beneath the floors was changed between animals. The main principles of CPP have been described earlier (Cunningham, 1993). Briefly, the place-conditioning study involved one habituation session,
