2.6. Active (shuttle) avoidance test
The mouse was placed in the shuttle box (35×17×20 cm, width×length×height,
model E10-15, Coulbourn Instruments), which was equally divided into two
compartments that were connected by a gate (7.5×6.5 cm). Each mouse was allowed
a 10-s brief orientation period and then the gate, which was controlled by a computer,
was opened. At the same time, the conditional stimulus (CS), which consisted of a tone,
and red, yellow and green lights, was presented. Following this stimulus, an
unconditional stimulus consisting of a 0.3-mA, 0.5-s scrambled foot shock was carried
out if the mouse had not escaped by entering the other compartment during the CS.
Each mouse was exposed to 5 CS per session and two sessions per day, and the total
experiment was performed for four consecutive days. Success on the first day of test
meant acquisition of successful avoidance, and other avoidance responses signified
retention. The avoidance responses of the mice were recorded automatically