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

- Exploratory behavior increases basal spine density in the CA1 hippocampal region.
- Exploratory behavior increases basal spine density in the enclosed blade of the DG.
- These effects were not seen in 56Fe-irradiated mice.
- 56Fe irradiation affects the ability of exploratory behavior to alter spine measures.

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.