Presentations

Abstract

Introduction: Space expeditions like the planned Mars missions will expose astronauts to hazards including space radiation (SR) and social isolation (SI). These hazards could impact stress management and the immune response with implications for astronaut health and performance. Astrocytes and microglia play important roles in the neuroimmune response and have high levels of interaction in the stress-regulatory neurocircuit which includes the hippocampus (HPC), basolateral amygdala (BLA) and medial prefrontal cortex (mPFC). We previously demonstrated that SR and SI impact the neuroimmune response including blood brain barrier permeability. However, the effects of spaceflight hazards on neuroimmune cell abundance and morphology in stress-regulatory brain regions is unclear. In this project, we utilized confocal microscopy to determine the effects of SR and SI on the abundance and morphology of astrocytes and microglia in the HPC, BLA, and mPFC.

Methods: Male, retired breeder outbred Wistar strain rats served as subjects. Animals were randomly placed into the following groups: SHAM (single-housed, control group), SI (single-housed with visual barriers between cages), and SR (single-housed and exposed to 15 cGy simulated Galactic Cosmic Radiation ). Approximately 20 hours following completion of behavioral testing, rats were sacrificed and their brains extracted then divided longitudinally. Full left hemispheres were formalin fixed, paraffin embedded and sliced on a vibrating microtome (5µm). Slides containing visible sections of the HPC, BLA, or mPFC from each treatment group were stained for immune cell markers (GFAP for astrocytes, Iba1 for microglia) and imaged under a confocal microscope to assess immune cell abundance and morphology.

Results: Within each treatment group, there were observable differences in astrocyte abundance and morphology in the HPC, BLA, and mPFC. SHAM animals had protoplasmic astrocytes (long, branching projections) in the HPC and fibrous astrocytes (few projections) in the BLA. Very few astrocytes were observed in mPFC of SHAM . SI animals had protoplasmic astrocytes in the HPC and mPFC, but very few astrocytes in the BLA. The SR group only had fibrous astrocytes in BLA. All treatments contained microglia with few projections; however, microglia were more prominent in regions with a low number of astrocytes, and vice versa. Between treatment groups, SR animals had fewer microglia than SHAM and SI animals.

Conclusion: This study indicates that the abundance of astrocytes and microglia differ depending on brain region (HPC, BLA, mPFC) and treatment (SHAM, SI, SR). Interestingly, areas prevalent in astrocytes had fewer microglia, and areas prevalent with microglia had fewer astrocytes. This was seen in all treatment groups. Astrocyte morphology was also impacted by treatment (protoplasmic and fibrous in SHAM; mostly protoplasmic in SI, mostly fibrous in SR). This suggests that the abundance of neuroimmune cells vary between regions of the brain, and that immune cell abundance and morphology is differentially impacted by SR and SI.