Effect of behavioral testing on spine density of basal dendrites in the CA1 region of the hippocampus modulated by ⁵⁶Fe irradiation

A unique feature of the space radiation environment is the presence of high-energy charged particles, including ⁵⁶Fe ions, which can present a significant hazard to space flight crews during and following a mission. ⁵⁶Fe irradiation-induced cognitive changes often involve alterations in hippocampal function. These alterations might involve changes in spine morphology and density. In addition to irradiation, performing a cognitive task can also affect spine morphology. Therefore, it is often hard to determine whether changes in spine morphology and density are due to an environmental challenge or group differences in performance on cognitive tests. In this study, we tested the hypothesis that the ability of exploratory behavior to increase specific measures of hippocampal spine morphology and density is affected by ⁵⁶Fe irradiation. In sham-irradiated mice, exploratory behavior increased basal spine density in the CA1 region of the hippocampus and the enclosed blade of the dentate gyrus. These effects were not seen in irradiated mice. In addition, following exploratory behavior, there was a trend toward a decrease in the percent stubby spines on apical dendrites in the CA3 region of the hippocampus in ⁵⁶Fe-irradiated, but not sham-irradiated, mice. Other hippocampal regions and spine measures affected by ⁵⁶Fe irradiation showed comparable radiation effects in behaviorally naïve and cognitively tested mice. Thus, the ability of exploratory behavior to alter spine density and morphology in specific hippocampal regions is affected by ⁵⁶Fe irradiation.