An emotional stress model using witnessing social defeat scenes in mice

Chronic exposure to stress can lead to a variety of mental disorders such as depression. There are many treatment-resistant patients who do not respond adequately to the standard pharmacological treatments. Therefore, the development of novel therapeutic agents is highly expected. In rodents, socially defeated animals that were exposed to repeated physical aggression from other individuals are widely used in this field of research. The social defeat is considered as a stress that mimics human social stress. On the other hand, emotional stress, but not physical stress, is likely to contribute to the pathogenesis and etiology of depression in human. Therefore, there is a gap between the process of pathogenesis and the animal models, and this is one of the reasons why the development of new psychotropic drugs to treat depression has been difficult. Recently, a novel stress model has been introduced, in which mice are subjected to emotional stress without physical distress by witnessing social defeat scenes of their conspecifics. We have investigated the mechanisms by which emotional stress is transmitted by witnessing social defeat in mice, focusing on the insular cortex. In this article, we summarize and discuss the recent advancements in the neural basis of behavioral changes induced by emotional stress.