Behavioral effects of a chronic envy-like stress paradigm in mice using an adjacent cage model

Social comparison and envy are significant psychosocial stressors in humans and are known to be involved in the onset and persistence of psychiatric disorders. However, animal models capable of experimentally reproducing the effects of indirect social comparison without physical contact are limited. In this study, we used a newly developed "adjacent-cage paradigm" to investigate whether chronic vicarious exposure to conspecifics in different environments induces envy-like stress in mice.

Male C57BL/6 N mice served as observers, while demonstrator mice were assigned to one of four conditions: (1) an environment enriched with objects, (2) an igloo, (3) a tube, or (4) social isolation. Observers were continuously exposed to these adjacent cages for 21 days. Subsequently, a comprehensive battery of behavioral tests was conducted to assess general health, anxiety-like behavior, spatial memory, social behavior, and depression-like behavior.

In the objects condition, a decrease in time spent in the light compartment of the light/dark box indicated an increase in anxiety-like behavior. In the isolation condition, the mean duration per social interaction was shortened, suggesting a qualitative change in social behavior. The igloo condition resulted in reduced immobility time in the forced swim test, suggesting a possible alteration in stress coping behavior. Furthermore, increased nociceptive sensitivity was observed in the hot plate test under both the objects and isolation conditions.

Although the envy-like stress paradigm did not affect many behavioral indices, it did cause condition-dependent and limited behavioral changes. This suggests that the paradigm may serve as a novel model for capturing psychological and context-dependent social stress, which differs from conventional physical stress models. Elucidating the neural basis of this paradigm is expected to contribute to the understanding of how social comparison affects mental health in modern society.