Region-Specific Microglial Remodeling Following Low-Dose Space Radiation in Female Rats

As human spaceflight advances toward long-duration missions beyond low Earth orbit, understanding how space radiation (SR) affects the central nervous system (CNS) is increasingly important. Some cognitive processes relevant to decision-making appear susceptible to SR exposure, although reported effects vary according to the task and experimental conditions. The underlying cellular mechanisms remain unclear. Microglia are important regulators of the brain’s response to radiation, yet the extent to which their responses differ between brain regions remains poorly understood. In this study, adult female rats were exposed to 10 cGy He particles or a simplified GCR simulation (SGS). Microglial number and morphology were quantified using unbiased stereology and morphometric analyses in the dorsomedial striatum, subventricular zone (SVZ), dentate gyrus (DG) granule cell layer (GCL), and the dentate hilus. Neither radiation exposure significantly altered Iba1+ microglial numbers in the regions examined, providing no evidence of generalized microgliosis. In contrast, both radiation paradigms induced significant, region-specific morphological remodeling, with robust changes occurring in the SVZ, dorsomedial striatum, and DG GCL, whereas the dentate hilus remained comparatively unaffected. Even neighboring regions exhibited distinct responses to identical radiation exposures. Together, these findings demonstrate that 10 cGy SR induces region-specific microglial remodeling without detectable changes occurring in microglial numbers, suggesting that microglial morphology may reflect the response of local tissue environments to radiation.