Chronic foot-shock stress potentiates the influx of bone marrow-derived microglia into hippocampus

For several years, a new population of microglia derived from bone marrow has been described in multiple settings such as infection, trauma, and neurodegenerative disease. The aim of this study was to investigate the migration of bone marrow-derived cells to the brain parenchyma after stress exposure. Stress exposure was performed in mice that had received bone marrow transplantation from GFP mice, allowing identification of blood-derived elements within the brain. Electric foot-shock exposure was chosen because of its ability to serve as fundamental and physical stress in mice. Bone marrow-derived GFP(+) cells migrated to the ventral part of the hippocampus and acquired a ramified microglia-like morphology. Microglia marker Iba1 was expressed by 100% of the ramified cells, whereas ramified cells were negative for the astrocyte marker GFAP. Compared with the case in the control group, ramified cells significantly increased after chronic exposure to stress (5 days). One month after 5 days of stress exposure, ramified cells significantly decreased in ventral hippocampus compared with the group examined immediately after the last stress exposure. We report for the first time the migration of bone marrow-derived cells to the ventral hippocampus after stress exposure. These cells have the characteristics of microglia. Mechanisms responsible for this migration and their roles in the brain remain to be determined.