Stress resilience and vulnerability: The association with rearing conditions, endocrine function, immunology, and anxious behavior

Background: The current study explored the underlying behavioral, endocrine, and immune markers of vulnerability to stress-induced depression, and the impact of rearing environments on adult functioning. Method: Adult Sprague-Dawley rats (n = 195) were reared in either Maternal Separation (MS), Early Weaning and Isolation (EWI), or Non-Handled (NH) conditions. Anxiety behavior was assessed using the emergence test at mean postnatal day (PND) 60. Stress-induced depressive behavior was measured at mean PND 86 using an intermittent cold water swim stress and swim escape test (SET) paradigm. Immediately following the SET, and in a sample of naïve controls (N = 31), trunk blood was collected to assay for serum corticosterone (CORT) and spleens were removed for determination of Concanavalin A (Con-A) stimulated T-cell proliferation. Results: Stress vulnerable rats (top tertile of SET swim time) were characterised by increased anxiety-like behavior, greater post-stress CORT concentrations, and a significantly higher Con-A induced T-cell proliferative response compared to stress resilient rats (bottom tertile of SET swim time). The EWI rearing condition was a contributing factor in predicting total swim escape time, however MS was not. MS offspring did have double the basal level of CORT than NH offspring, suggestive of a hyperfunctioning HPA axis. Conclusion: The swim stress animal model enabled observation of stress vulnerability and resilience; results point towards the existence of distinct behavioral, endocrine, and immunological profiles of the vulnerable and resilient animal, which may have important implications for mental health and stress research.