Late Effects of 16O-Particle Radiation on Female Social and Cognitive Behavior and Hippocampal Physiology

Abstract

The radiation environment in space remains a major concern for manned space exploration, as there is currently no shielding material capable of fully protecting flight crews. Additionally, there is growing concern for the social and cognitive welfare of astronauts, due to prolonged radiation exposure and confinement they will experience on a mission to Mars. In this artice, we report on the late effects of ¹⁶O-particle radiation on social and cognitive behavior and neuronal morphology in the hippocampus of adult female mice. Six-month-old mice received ¹⁶O-particle whole-body irradiation at doses of either 0.25 or 0.1 Gy (600 MeV/n; 18-33 cGy/min) at the NASA's Space Radiation Laboratory in Upton, NY. At nine months postirradiation, the animals underwent behavioral testing in the three-chamber sociability, novel object recognition and Y-maze paradigms. Exposure to 0.1 or 0.25 Gy ¹⁶O significantly impaired object memory, a 0.25 Gy dose impaired social novelty learning, but neither dosage impaired short-term spatial memory. We observed significant decreases in mushroom spine density and dendrite morphology in the dentate gyrus, cornu ammonis 3, 2 and 1 of the hippocampus, which are critical areas for object novelty and sociability processing. Our data suggest exposure to ¹⁶O modulates hippocampal pyramidal and granular neurons and induces behavioral deficits at a time point of nine months after exposure in females.

Figure 1:. Y-maze.

(A-C) All treatment groups spent significantly more time exploring the novel arm during the testing phase of the Y-maze, indicating no short-term memory deficits. 2. D) All cohorts showed positive discrimination ratios with no significant differences, and thus displayed similar novelty discrimination in all groups. Average ± SEM (n = 12); *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 2:. Novel Object Recognition.

(A-C) Both irradiated groups were unable to discern the novel from the familiar object on test day, whereas sham-irradiated animals spent significantly more time exploring the novel object. (D) Radiation induced negative discrimination ratios, representing an inability to discriminate between the novel and familiar objects. (E) Animals who received 0.1 Gy displayed very low habituation as compared other cohorts on habituation day 2. Average ± SEM (n = 12); *P < 0.05, ***P < 0.001.

Figure 3:. Sociability.

(A) Animals of all cohorts displayed no significant chamber-exploration bias. (B) All cohorts displayed normal sociability by spending significantly more time with a novel mouse (stimulus 1). (C) Sham-irradiated and mice receiving 0.1 Gy were able to discriminate between a novel stranger (Stimulus 2), and a now-familiar stranger (Stimulus 1), whereas mice treated with 0.25 Gy failed to distinguish strangers. Average ± SEM (n = 12); *P < 0.05, **P < 0.0001.

Figure 4:. Dentate Gyrus Dendrite Morphology.

(A-B) Dendritic length reduces significantly due to 0.1 and 0.25 Gy of 16O. (C) Irradiation resulted in nearly identical dendritic reductions in the mossy fibers of the dorsal Dentate Gyrus. (D) Dendritic complexity is severely reduced at both radiation dosages. Average ± SEM (n = 5); ****P < 0.0001.

Figure 5:. Dorsal CA1 Dendritic Morphology.

(A-D) The apical and basal CA1 underwent significant reduction in dendritic length beginning at approximately 50 μm from the soma. (E-F) The extent of dendritic remodeling appears to be dosage independent. (G-H) Dendritic complexity was significantly lowered as a result of treatment, in dosage-independent manner. Average ± SEM (n = 5); ****P < 0.0001.

Figure 6:. Dorsal CA2 Dendritic Morphology.

(A-D) Animals irradiated at 0.1 Gy or 0.25 Gy suffered a decrease in dendritic length in both apical and basal dendrites. (E-F) Radiation dosage resulted in generally indistinguishable dendritic reduction in the apical CA2, but a dosage of 0.25 Gy resulted in significantly higher dendritic length reduction. (G-H) Apical complexity was reduced in both treatment groups, whereas only the higher dosage lowered basal dendritic complexity. Average ± SEM (n = 5); *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 7:. Ventral CA3 Dendritic Morphology.

(A-B) A dosage of 0.1 Gy elicited dendritic length reductions at approximately 100–160 μm from the soma in both the basal and apical CA3. (C-D) Dendritic length decreases in response to a dosage of 0.25 Gy. (E-F) Radiation at the dosages of 0.1 and 0.25 Gy induces nearly identical dendritic reduction in the apical and basal CA3. (G-H) Dendritic complexity is only significantly reduced in the basal CA3 at 0.25 Gy. Average ± SEM (n = 5); *P < 0.05.

Figure 8:. ¹⁶O reduces mushroom spine density in the hippocampus.

(A) Dentate Gyrus mushroom spine density lowers by about one third in irradiated animals. (C) The Apical CA2 underwent reduced mushroom spine density in both dosages. (B,D) The basal CA2 overall, and mushroom spine density at only 0.25 Gy. (E,F) Apical and basal CA1 mushroom spines decrease in a dose-dependent manner in irradiated animals. Average ± SEM (n = 5); *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.

Schematic diagram showing experimental design.

Six-month-old C57BL/6J Female mice received whole-body irradiation ¹⁶O radiation at doses of 0, 0.1 or 0.25 Gy (600MeV/n) at NASA’s Space Radiation Laboratory (NSRL) within BNL. 270 Days post irradiation behavioral testing was performed.