Inflammation and vascular remodeling in the ventral hippocampus contributes to vulnerability to stress

During exposure to chronic stress, some individuals engage in active coping behaviors that promote resiliency to stress. Other individuals engage in passive coping that is associated with vulnerability to stress and with anxiety and depression. In an effort to identify novel molecular mechanisms that underlie vulnerability or resilience to stress, we used nonbiased analyses of microRNAs in the ventral hippocampus (vHPC) to identify those miRNAs differentially expressed in active (long-latency (LL)/resilient) or passive (short-latency (SL)/vulnerable) rats following chronic social defeat. In the vHPC of active coping rats, miR-455-3p level was increased, while miR-30e-3p level was increased in the vHPC of passive coping rats. Pathway analyses identified inflammatory and vascular remodeling pathways as enriched by genes targeted by these microRNAs. Utilizing several independent markers for blood vessels, inflammatory processes and neural activity in the vHPC, we found that SL/vulnerable rats exhibit increased neural activity, vascular remodeling and inflammatory processes that include both increased blood-brain barrier permeability and increased number of microglia in the vHPC relative to control and resilient rats. To test the relevance of these changes for the development of the vulnerable phenotype, we used pharmacological approaches to determine the contribution of inflammatory processes in mediating vulnerability and resiliency. Administration of the pro-inflammatory cytokine vascular endothelial growth factor-164 increased vulnerability to stress, while the non-steroidal anti-inflammatory drug meloxicam attenuated vulnerability. Collectively, these results show that vulnerability to stress is determined by a re-designed neurovascular unit characterized by increased neural activity, vascular remodeling and pro-inflammatory mechanisms in the vHPC. These results suggest that dampening inflammatory processes by administering anti-inflammatory agents reduces vulnerability to stress. These results have translational relevance as they suggest that administration of anti-inflammatory agents may reduce the impact of stress or trauma in vulnerable individuals.