Chronic social defeat stress (CSDS) induces Gadd45b in the nucleus accumbens (NAc) of susceptible mice. (a) Schematic diagram depicting the experimental procedure for CSDS. (b) Repeated CSDS induces social avoidance in susceptible but not resilient mice [One-way analysis of variance (ANOVA), F(2,27) = 38.22, p < 0.001, Control n = 10, Susceptible n = 10, Resilient n = 10]. Social avoidance was expressed as social interaction ratio (SI ratio) calculated by time spent in the interaction zone with a social target/time in the interaction zone without a social target. c, CSDS induces Gadd45b mRNA levels in NAc of susceptible but not resilient mice (F(2,27) = 3.365, p < 0.05, Control n = 10, Susceptible n = 10, Resilient n = 10). mRNA expression levels were expressed as fold change (FC) compared to control group. (d,e) Phasic stimulation of the VTA-NAc DA pathway (d, Student’s t-test, t(15) = 2.405, *p < 0.05, EYFP n = 8, ChR2 n = 9), or intra-NAc BDNF infusion (e, Student’s t-test, t(17) = 2.281, *p < 0.05, PBS n = 9, BDNF n = 10), induces Gadd45b expression in the NAc. *p < 0.05, ***p < 0.001. Bar graphs show mean ± SEM.