2.2. Chronic social defeat stress

The chronic social defeat stress (CSDS) paradigm described for adult male mice (Golden et al., 2011) was used with minor modifications. Thirty-day-old C57BL/6 mice were randomly assigned to control or experimental groups. The experimental mice were individually introduced into the social defeat cage in the compartment of a resident male CD-1 mouse. The aggressor attacked the adolescent intruder for 5 min, and then the experimental mouse was removed and immediately placed in the contiguous empty side for the remaining 24 h. By this procedure, the adolescents were protected from repeated attacks but remained threatened by olfactory, visual, and auditory contact with the aggressor. The CSDS was performed for 10 consecutive days, from 14:00 h to 18:00 h, with the defeated mouse moved into different aggressor cages every day. The pairing between each adolescent and CD-1 mouse was randomized daily to minimize the variability in aggression to which the mice were exposed. Control adolescent mice were paired in identical social defeat cages, but each one was placed in a different compartment; consequently, there was no physical contact between them. This was performed in a distinct experimental room, where control animals were rotated to a new social defeat cage and paired on a daily basis. After the last day of social stress, both defeated and control adolescents were individually housed.