Puzzle box test

The puzzle box test was slightly modified from the one described in⁷¹ and performed between P23 and P26. The home-made puzzle box consisted of two compartments (a brightly-lit start compartment—illuminated with a bright light (320 lx) and a smaller closed, dark compartment made of grey and plastic material, respectively. Both compartments were separated by a black plastic wall that had a narrow door (about 4 cm wide) with an underpass (depth 2 cm). In each trial, the mouse was placed in the start zone, and the task was to enter the goal zone which contained bedding from its home cage. Each mouse underwent a total of 11 trials over 4 consecutive days, with 3 trials per day on day 1–3 and 2 trials on day 4. During trial 1, the door and the underpass were open and the mouse could use the open door to escape from the aversive illuminated light compartment into the home zone. In trials 2 and 3, the door was blocked and the mouse had to use the small underpass to enter the goal zone. Trial 4 was identical to trials 2 and 3. In trials 5 and 6, however, the underpass was filled with sawdust and the mouse had to dig through the sawdust to reach the home zone. Trial 7 was identical to trials 5 and 6. During trials 8 and 9, the mouse had to deal with an underpass that was blocked by a cardboard plug. The mouse had to remove the plug using its teeth and/or paws to enter the goal zone through the underpass. Trial 10 was a repetition of trial 9. In the last trial, trial 11, trial 1 was repeated as a control trial. After trial 1–10, each mouse was left for 1 min inside the goal zone even if the mouse could not reach the home zone on its own.