Effects of Proton and Combined Proton and ⁵⁶Fe Radiation on the Hippocampus

The space radiation environment contains protons and ⁵⁶Fe, which could pose a significant hazard to space flight crews during and after missions. The space environment involves complex radiation exposures, thus, the effects of a dose of protons might be modulated by a dose of heavy-ion radiation. The brain, and particularly the hippocampus, may be susceptible to space radiation-induced changes. In this study, we first determined the dose-response effect of proton radiation (150 MeV) on hippocampus-dependent cognition 1 and 3 months after exposure. Based on those results, we subsequently exposed mice to protons alone (150 MeV, 0.1 Gy), ⁵⁶Fe alone (600 MeV/n, 0.5 Gy) or combined proton and ⁵⁶Fe radiations (protons first) with the two exposures separated by 24 h. At one month postirradiation, all animal groups showed novel object recognition. However, at three months postirradiation, mice exposed to either protons or combined proton and ⁵⁶Fe radiations showed impaired novel object recognition, which was not observed in mice irradiated with ⁵⁶Fe alone. The mechanisms in these impairments might involve inflammation. In mice irradiated with protons alone or ⁵⁶Fe alone three months earlier, there was a negative correlation between a measure of novel object recognition and the number of newly born activated microglia in the dentate gyrus. Next, cytokine and chemokine levels were assessed in the hippocampus. At one month after exposure the levels of IL-12 were higher in mice exposed to combined radiations compared with sham-irradiated mice, while the levels of IFN-γ were lower in mice exposed to ⁵⁶Fe radiation alone or combined radiations. In addition, IL-4 levels were lower in ⁵⁶Fe-irradiated mice compared with proton-irradiated mice and TNF-α levels were lower in proton-irradiated mice than in mice receiving combined radiations. At three months after exposure, macrophage-derived chemokine (MDC) and eotaxin levels were lower in mice receiving combined radiations. The levels of MDC and eotaxin correlated and the levels of MDC, but not eotaxin, correlated with the percentage of newly born activated microglia in the blades of the dentate gyrus. Finally, hippocampal IL-6 levels were higher in mice receiving combined radiations compared with mice receiving ⁵⁶Fe radiation alone. These data demonstrate the sensitivity of novel object recognition for detecting cognitive injury three months after exposure to proton radiation alone, and combined exposure to proton and ⁵⁶Fe radiations, and that newly-born activated microglia and inflammation might be involved in this injury.