Brain cells sensitive to stress and threat remember past social trauma and become overactive when exposed to social situations involving non-threatening targets. According to a study undertaken by experts at the Brain and Body Research Center at Mount Sinai and released on November 30 in Nature, defective social reward perception can lead to psychiatric diseases, and widespread social avoidance is a key factor in the development of both.
New research shows that extreme social avoidance develops after social trauma disrupts brain reward functioning to the point that interacting with others is no longer pleasurable. Few studies have examined the effects of chronic social defeat stress on social reward in rodents, despite its widespread usage in the study of stress susceptibility and resistance.
Previous research has measured how people engage with an adult mouse that mimics the ones employed as aggressors. In these situations, social avoidance is more indicative of fear or submission than of a change in social reward.
The experiment
Adult male and female mice were subjected to chronic social failure stress by being continuously dominated by aggressive mice. This was followed by socializing tests, in which an experimental mouse was put in a cage with a bigger hostile mouse behind a barrier, and the time spent engaging between the two was measured.
Based on their responses to the stressor in social interactions, the mice were labeled as either resilient or vulnerable. A 4-6 week-old same-sex mouse was then brought into the individuals’ homecages and permitted to freely interact as part of a second social interaction test termed the resident-intruder test. Then, the adult mice were conditioned alongside the adolescent mice to determine which of the social objectives they preferred.
While interacting with the juvenile in the resident-intruder test, both control and resilient mice displayed similarly high levels of social activity. Individuals in these groups of mice were far less likely to avoid the juveniles and more likely to openly approach and study them. The opposite was true for the stress-prone mice, who were far less interested in exploring their social environments.
Furthermore, research with an aggressive adult mouse revealed a correlation between measures of social investigation duration, social avoidance, and delay to explore. In addition to avoiding potentially dangerous adult male mice, these findings reveal that vulnerable animals also avoid potentially harmless young mice of the same species.
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