Detrimental Effects of Helium Ion Irradiation on Cognitive Performance and Cortical Levels of MAP-2 in B6D2F1 Mice

Abstract

The space radiation environment includes helium (⁴He) ions that may impact brain function. As little is known about the effects of exposures to ⁴He ions on the brain, we assessed the behavioral and cognitive performance of C57BL/6J × DBA2/J F1 (B6D2F1) mice three months following irradiation with ⁴He ions (250 MeV/n; linear energy transfer (LET) = 1.6 keV/μm; 0, 21, 42 or 168 cGy). Sham-irradiated mice and mice irradiated with 21 or 168 cGy showed novel object recognition, but mice irradiated with 42 cGy did not. In the passive avoidance test, mice received a slight foot shock in a dark compartment, and latency to re-enter that compartment was assessed 24 h later. Sham-irradiated mice and mice irradiated with 21 or 42 cGy showed a higher latency on Day 2 than Day 1, but the latency to enter the dark compartment in mice irradiated with 168 cGy was comparable on both days. ⁴He ion irradiation, at 42 and 168 cGy, reduced the levels of the dendritic marker microtubule-associated protein-2 (MAP-2) in the cortex. There was an effect of radiation on apolipoprotein E (apoE) levels in the hippocampus and cortex, with higher apoE levels in mice irradiated at 42 cGy than 168 cGy and a trend towards higher apoE levels in mice irradiated at 21 than 168 cGy. In addition, in the hippocampus, there was a trend towards a negative correlation between MAP-2 and apoE levels. While reduced levels of MAP-2 in the cortex might have contributed to the altered performance in the passive avoidance test, it does not seem sufficient to do so. The higher hippocampal and cortical apoE levels in mice irradiated at 42 than 168 cGy might have served as a compensatory protective response preserving their passive avoidance memory. Thus, there were no alterations in behavioral performance in the open filed or depressive-like behavior in the forced swim test, while cognitive impairments were seen in the object recognition and passive avoidance tests, but not in the contextual or cued fear conditioning tests. Taken together, the results indicate that some aspects of cognitive performance are altered in male mice exposed to ⁴He ions, but that the response is task-dependent. Furthermore, the sensitive doses can vary within each task in a non-linear fashion. This highlights the importance of assessing the cognitive and behavioral effects of charged particle exposure with a variety of assays and at multiple doses, given the possibility that lower doses may be more damaging due to the absence of induced compensatory mechanisms at higher doses.

Keywords: MAP-2; fear conditioning; galactic cosmic radiation; helium ions; object recognition; passive avoidance.

Figure 1

Behavioral testing schedule. The numbers indicated reflect dates and illustrate the time intervals between the distinct tests. The first behavioral test was performed three months following irradiation or sham-irradiation. BNL, Brookhaven National Laboratory.

Figure 2

Behavioral performance in the open field and object recognition. (A) Habituation to the open field. All mice habituated to the open field (effect of trial, p < 0.001), but there was no effect of irradiation. (B) Object recognition. Sham-irradiated mice and mice irradiated with 21 cGy or 168 cGy showed object recognition and spent more time exploring the novel object than the familiar object, but mice irradiated with 42 cGy showed impaired object recognition. Control: n = 13 mice; 21 cGy: n = 23 mice; 42 cGy: n = 20 mice; 168 cGy: n = 16 mice. * p < 0.05 versus familiar object; ** p < 0.01 versus familiar object.

Figure 3

Passive avoidance learning and memory. There were effects of irradiation when the time to enter the dark compartments on Days 1 and 2 were compared using a repeated-measures design within each dose condition. In sham-irradiated mice and mice irradiated with 42 cGy, the latency to enter the dark compartment was significantly higher on Day 2 than Day 1. This did not reach significance in mice irradiated with 21 cGy, and the latency to enter the dark compartment in mice irradiated with 168 cGy was comparable on both days (p = 0.27). Control: n = 13 mice; 21 cGy: n = 23 mice; 42 cGy: n = 20 mice; 168 cGy: n = 16 mice. * p < 0.05; ^# p = 0.078; ⁰ p = 0.0693.

Figure 4

MAP-2 levels in the cortex and hippocampus. There was a radiation × brain region interaction (F(3,64) = 4.577, p = 0.0058). When the cortex and hippocampus were analyzed separately, in the cortex, there was an effect of irradiation (F(3,32) = 3.827, p = 0.0189, ANOVA) with mice irradiated with 42 or 168 cGy showing lower cortical level of MAP-2 than sham-irradiated mice. In the hippocampus, there was an effect of irradiation (F(3,32) = 4.395, p = 0.01, Brown–Forsythe test), but none of the post hoc tests reached significance. n = 9 mice/radiation condition/brain region. * p < 0.05.

Figure 5

(A) ApoE levels in the cortex and hippocampus. There was an effect of irradiation on apoE levels (F(3,32) = 3.810, p = 0.019, Figure 5A). ApoE levels were higher in mice irradiated with 42 than 168 cGy (p = 0.035, Bonferroni’s correction for multiple comparisons, indicated by “a” in the figure), and there was a trend towards higher apoE levels in mice irradiated at 21 than 168 cGy (p = 0.065, Bonferroni’s correction for multiple comparisons, indicated by “b” in the figure). (B) Relationship between hippocampal MAP-2 and apoE levels (r = −0.3277, p = 0.0511, two-tailed Spearman correlation, 36 data points). Inspecting the data, there was one data point that seemed removed from the other ones (indicated in green). Therefore, we also performed the analysis without that data point included. The analysis without that data point revealed: r = −0.4123, p = 0.0138, Spearman correlation, 35 data points.