Objective To determine the stress intensity in a mouse model of depression induced by social defeat stress and observe the effects of reward on depression-like behaviors of the mice exposed to different stress intensities. Methods We assessed the depression-like behaviors of male wild-type C57 BL/6 J mice(9-12 weeks old) in response to attacks by CD-1 mice

and investigated the effects of different stress intensities controlled by the attack duration(10 min vs 5 min per day) and the escape space(in a small vs large cage) on depression-like behaviors of the mice to determine the optimal method for modeling. The mouse models of social stress established using the optimal method were given rewards by providing them with company by female C57 BL/6 J mice(9-12 weeks old) for 24 h during the modeling

and the effects of the reward on depression-like behaviors were evaluated in the mice exposed to different stress intensities. Results The attacks by CD-1 mice induced depression-like behaviors in C57 BL/6 J mice. Controlling the escape space significantly affected the social interaction indexed in the social interaction zone(P<0.05) and the corner zone(P<0.05) and the forced swimming time of mice(P<0.05)

while the other two methods did not well distinguish the effects of different stress intensities. Under a high stress intensity

the mice in the reward group showed significant changes in the social interaction indexes in the social interaction zone(P<0.01) and the corner zone(P<0.05) and the immobility time during forced swimming(P<0.05); under a low stress intensity

the reward only changed the social interaction index in the social interaction zone(P<0.05). The reward-induced changes in the social interaction index in the social interaction zone(P<0.05) and the corner zone(P<0.05) and the immobility time during forced swimming(P<0.01) were significantly more obvious under a high stress intensity than under a low stress intensity. Conclusion In preparing mouse models of social defeat stress

controlling the escape space allows more efficient determination of the stress intensity. Reward can reduce stress-induced depression-like behaviors in mice

especially when a high stress intensity is applied.