Long-Term Effects of Repeated Social Defeat Stress on Brain Activity during Social Interaction in BALB/c Mice

Understanding the long-term effects of stress on brain function is crucial for understanding the mechanisms of depression. The BALB/c mouse strain has high susceptibility to stress and is thus an effective model for depression. The long-term effects of repeated social defeat stress (SDS) on BALB/c mice, however, are not clear. Here, we investigated the effects of repeated SDS in male BALB/c mice over the subsequent twoweeks. Some defeated mice immediately exhibited social avoidance, whereas anxiety-like behavior was only evident at later periods. Furthermore, defeated mice segregated into two groups based on the level of social avoidance, namely, avoidant and nonavoidant mice. The characteristic of avoidance or nonavoidance in each individual was not fixed over the twoweeks. In addition, we developed a semi-automated method for analyzing c-Fos expression in the mouse brain to investigate the effect of repeated SDS on brain activity more than twoweeks after the end of the stress exposure. Following social interaction, c-Fos expression was reduced in several brain regions in the defeated mice compared with control mice. The correlation of c-Fos expression among these brain areas, with exception of the medial prefrontal cortex (mPFC) and central amygdala (CeA), was increased in defeated mice, suggesting increased synchrony. Notably, c-Fos expression in the lateral habenula (LHb) was different between mice that exhibited social avoidance from immediately after the repeated SDS and those that exhibited social avoidance only at later periods. These observations provide insight into the long-term effects of social stress on behavior and brain activity. Significance Statement Stress stimuli underlying psychological diseases are frequently of a social nature, and therefore elucidating the effects of social stress in animal models is critical to understanding the underlying mechanisms of these diseases. We investigated how repeated social defeat stress (SDS) affects behavior and brain activity over subsequent weeks in BALB/c mice, a strain exhibiting high susceptibility to stress. Some defeated mice immediately exhibited social avoidance whereas anxiety-like behavior was only evident at later periods. A semi-automated c-Fos extraction and quantitation method developed for mice revealed that brain activity during social interaction differ among socially-avoidant individuals depending on the timing of the emergence of social avoidance.