Computer controlled fear conditioning system

The fear conditioning experiments were performed as previously described [11 (link),13 (link)] using a computer-controlled fear conditioning system (TSE, Bad Homburg, Germany). The training conditioning consisted of a single trial. The mouse was exposed to the conditioning context (180 sec) followed by a tone (30 sec) serving as conditional stimulus. During the last 2 sec of the tone, a foot-shock (0.5 or 0.7 mA) serving as unconditional stimulus was delivered through a stainless steel grid floor. Different animals were used for the 0.5 and 0.7 mA foot-shock. Mice remained in the fear conditioning box for 3 min following the foot-shock in order to determine post-shock freezing. In some experiments mice were preexposed to the training context for 5 min. In context preexposure effect experiment the mice were trained using a 20 sec placement to shock interval 24 h later. Animals were placed in the training context and received a single shock (0.5 mA) 20 sec later. Conditioned freezing was assessed as the behavioral parameter of the learned reaction of mice by a time-sampling procedure every 10 s throughout memory tests. Memory tests were performed 24 h after fear conditioning procedures.