Long-term environmental enrichment affects microglial morphology in middle age mice

Aging is associated with increased central nervous system inflammation, in large part due to dysfunctional microglia. Environmental enrichment (EE) provides a model for studying the dynamics of lifestyle factors in the development of age-related neuroinflammation and microglial dysfunction. EE results in improvements in learning and memory, metabolism, and mental health in a variety of animal models. We recently reported that implementing EE in middle age promotes healthy aging. In the present study, we investigated whether EE influences microglial morphology, and whether EE is associated with changes in expression of microglial and neuroinflammatory markers. Inflammatory cytokines and MHC-II were reduced following 12-month EE in 10-month-old mice. Long-term EE for 7.5 months resulted in broad increases in Iba1 expression in hippocampus, hypothalamus, and amygdala detected by immunohistochemistry. Quantification of microglial morphology reveal both hypertrophy and ramification in these three brain regions, without increases in microglial cell density. These data indicate that long-term EE implemented in middle age results in a microglial state distinct from that of normal aging in standard laboratory housing, in specific brain regions, associated with reduced neuroinflammatory markers and improvement of systemic metabolism.