Sex-Dependent Cross-Resilience to Social Defeat and Learned Helplessness: The Role of BDNF-ERK Signaling and Norepinephrine

Major depressive disorder (MDD) and post-traumatic stress disorder (PTSD) are two of the most prevalent and disabling psychiatric conditions, both closely tied to stress exposure. While they have distinct diagnostic criteria, MDD and PTSD share considerable comorbidity, overlapping behavioral symptoms, and common neurobiological pathways. However, not all individuals exposed to chronic or traumatic stress develop these disorders, highlighting the importance of resilience, the capacity to maintain psychological and physiological stability under stress. An emerging yet understudied concept is cross-resilience: the idea that resilience to one form of stress may confer protection against another. While animal models have effectively captured individual differences in stress responses, few have explored whether resilience generalizes across distinct stress modalities or differs by sex. We examined cross-resilience using two validated rodent models: chronic social defeat stress (CSDS) and learned helplessness (LH). These paradigms model complementary aspects of stress vulnerability. We assessed how prior CSDS experience influenced subsequent responses to LH. We also evaluated the role of the noradrenergic system using wild-type and norepinephrine (NE)-deficient (VMAT2loxDBHcre KO) mice. Behavioral phenotyping was combined with molecular analyses in key brain regions involved in emotion, motivation, and fear processing: the ventral tegmental area (VTA), nucleus accumbens (NAc), and amygdala (AMY). We focused on BDNF and ERK/MAPK signaling pathways, known to mediate neuroplasticity and stress resilience. Our findings reveal sex-specific patterns of cross-resilience, with prior CSDS resilience predicting LH resilience in males but not females. Molecular results indicate distinct adaptations across brain regions and sexes, underscoring the biological complexity of resilience. These insights may inform personalized strategies for preventing and treating stress-related disorders.