Viewer 3

The fear conditioning training was performed as previously described^{25 (link)}. Briefly, mice were individually placed in the fear conditioning chamber (Coulbourn Instruments) located in the centre of a sound attenuating cubicle, which was cleaned with 10% ethanol to provide a background odour. A ventilation fan provided a background noise at ~55 dB. After a 2-min exploration period, three tone–foot shock pairings separated by 1-min intervals were provided. The 85 dB 2 kHz tone lasted for 30 s, and the foot shocks were at 0.75 mA and lasted for 2 s. The foot shocks were co-terminated with the tone. The mice remained in the training chamber for another 60 s before being returned to the home cages. For the context recall, mice were placed back into the original conditioning chamber for 5 min 16 days after the training. 4-OHT injections were performed immediately (within 30 min) before the recall experiments. For the HC and the NR groups, 4-OHT was injected at a similar time when the other two groups were subjected to recall. The behaviour of the mice was recorded and analysed with the FreezeFrame software (version 4; Coulbourn Instruments). Motionless bouts that lasted more than 1 s were considered as freeze. Data were analysed with the tracking software Viewer III (Biobserve).