Contextual Fear Conditioning Test

Previously reported methods were used to conduct the conditional fear experiment (Bolton et al., 2012; Zhou et al., 2012). In short, the fear regulator consists of a conditioning chamber. The sound chamber is closed by silencers and the noise is shielded by exhaust fans. The infrared digital camera is placed at the top of the enclosure, about 50 cm from the chamber. On the first day of training, the mice explored freely for 2 min without any stimulation. The loudspeaker (4 Hz, 80 dB) was started and stopped after 30 s. Electrical stimulation (0.5 mA) was started 2.5 s later and ended after 1 s. The mice were exposed to no stimulation for 2 min and then the entire procedure was repeated three times. The chamber was cleaned with 70% alcohol 24 h after training. The mice were exposed to the changed environment for 11 min. The training included 2 min of free exploration, 60 s of speaker (4 Hz, 80 dB), and three repetitions. Fourty-eight hours after the training in the original training room. The mice were left in the same room and allow to explore for 6 min without any electrical or sound stimulation.