Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in a wide range of mental abilities, including modulation of emotions, motivation, and impulse control. Pathological changes in serotonin levels are associated with mental disorders such as depression, anxiety, and aggression. Chronic stress or traumatic experiences are thought to trigger the development of depression. However, only a subset of individuals experiencing chronic stress or traumatic life events develop depressive-like behavior (susceptible), while others remain resilient. In order to investigate multiple aspects of depression, the Chronic Social Defeat Stress (CSDS) test is used to generate both resilient and vulnerable animals. This project addresses the question of how serotonin can influence different behaviors and how global activity patterns in the brain are regulated by serotonin. We use fiber optic photometry to study the spatiotemporal dynamics of serotonin release in the DR and in target areas of the prefrontal cortex during social behavior in resilient and vulnerable mice.