Cosmic radiation exposure and persistent cognitive dysfunction

Abstract

The Mars mission will result in an inevitable exposure to cosmic radiation that has been shown to cause cognitive impairments in rodent models, and possibly in astronauts engaged in deep space travel. Of particular concern is the potential for cosmic radiation exposure to compromise critical decision making during normal operations or under emergency conditions in deep space. Rodents exposed to cosmic radiation exhibit persistent hippocampal and cortical based performance decrements using six independent behavioral tasks administered between separate cohorts 12 and 24 weeks after irradiation. Radiation-induced impairments in spatial, episodic and recognition memory were temporally coincident with deficits in executive function and reduced rates of fear extinction and elevated anxiety. Irradiation caused significant reductions in dendritic complexity, spine density and altered spine morphology along medial prefrontal cortical neurons known to mediate neurotransmission interrogated by our behavioral tasks. Cosmic radiation also disrupted synaptic integrity and increased neuroinflammation that persisted more than 6 months after exposure. Behavioral deficits for individual animals correlated significantly with reduced spine density and increased synaptic puncta, providing quantitative measures of risk for developing cognitive impairment. Our data provide additional evidence that deep space travel poses a real and unique threat to the integrity of neural circuits in the brain.

Figure 1. Cognitive deficits evaluated 12 weeks after cosmic radiation exposure.

(a) Analysis of preference for novelty on a Novel Object Recognition (NOR) task shows that 30 cGy ⁴⁸Ti particle irradiation significantly reduced recognition memory. (b) Performance on an Object in Place (OiP) task shows decrements in spatial memory retention for mice exposed to 30 cGy ⁴⁸Ti particles as manifested in a reduced preference to explore an object found in a novel location. (c) All ⁴⁸Ti and ¹⁶O irradiations significantly impaired recency memory as evident by a reduced preference for the less recently explored object in the Temporal Order task (TO). *P < 0.05, **P < 0.01, ***P < 0.001; one-way ANOVA followed by Bonferroni’s multiple comparison post hoc analysis. (d) Attentional set shifting performance of adult male Wistar rats at 12 weeks post irradiation. Number of attempts required to reach criterion in the Simple Discrimination (SD); Compound Discrimination (CD) and Compound Discrimination Reversal (CDR) paradigms. Graphs show means ± SEM for control rats or rats exposed to 5 cGy 1 GeV/n ⁴⁸Ti ions. *P = 0.048 (Mann-Whitney, compared to respective control values).

Figure 2. Cognition remains significantly impaired 24 weeks following exposure to cosmic radiation.

(a) Analysis of the preference for novelty in the Novel Object Recognition (NOR) task demonstrates that charged particle irradiation continues to impair object recognition memory 6 months following exposure to low doses of ⁴⁸Ti particles, while animals exposed to ¹⁶O remain unaffected. (b) Performance on the Object in Place (OiP) task, however, shows significant decrements in spatial memory retention following exposure to 30 cGy ¹⁶O, and 5 and 30 cGy ⁴⁸Ti particles when compared to controls. (c) Analysis of preference for the Temporal Order (TO) task shows all ⁴⁸Ti and ¹⁶O irradiations significantly impaired recency memory as shown by a reduced preference for the less recently explored object. (d) Irradiation using 30 cGy of ⁴⁸Ti particles did not impair the acquisition of conditioned fear as demonstrated by similar freezing times observed by tone-shock trial 5 for both control and exposed mice. All mice showed a gradual decrease in freezing behavior over the extinction training on day 3, however the time spent freezing was significantly greater for the irradiated mice as compared to controls. Control mice successful abolish fear memory as demonstrated by reduced freezing behavior in the memory retrieval test when compared to irradiated mice. (e) Irradiated mice showed robust freezing between the first and last extinction training session as compared to controls, demonstrating that irradiated mice have a compromised ability to relearn. (f) Elevated Plus Maze (EPM) testing reveals that charged particle irradiation enhances anxiety-like behavior as demonstrated by reduced numbers of entries and time spent to open arms when compared controls. (g) Irradiated mice exhibiting severe extinction impairment also had increased anxiety as compared to control mice that were able extinguished fear memories. *P < 0.05, **P < 0.01, ***P < 0.001, one-way ANOVA followed by Bonferroni’s multiple comparison post hoc analysis; ^###P = 0.001 compared to 1^st extinction training, paired t test.

Figure 3. Reduced dendritic complexity of neurons in the prelimbic layer of the mPFC 15 weeks following exposure to cosmic radiation.

Digitally reconstructed EGFP-positive neurons from control and irradiated mice showing dendrites (green) and spines (red). Quantification of dendritic parameters (bar charts) shows that dendritic branching and length are significantly reduced 15 weeks after exposure to 5 or 30 cGy of ¹⁶O or ⁴⁸Ti particles. Data are expressed as mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001; one-way ANOVA followed by Bonferroni’s multiple comparison post hoc analysis.

Figure 4. Reduced dendritic spine density in the mPFC 15 weeks following exposure to cosmic radiation.

(a) Representative digital images of 3D reconstructed dendritic segments (green) containing spines (red) in unirradiated (top left panel) and irradiated (bottom panels) brains. Multiple comparisons show that total spine numbers (left bar chart) and spine density (right bar chart) are significantly reduced after exposure to 5 or 30 cGy of ¹⁶O or ⁴⁸Ti particles. Data are expressed as mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001 versus control; ANOVA. (b) Memory deficits correlate with reduced spine density in irradiated mice. Dendritic spine density (per 1.2 mm²) is plotted against the corresponding performance of each animal on the OiP task. Radiation-induced reductions in spine number correlate with reduced DI for novelty after exposure to ¹⁶O (5 cGy, P = 0.01; left panel) and ⁴⁸Ti (5 and 30 cGy, P = 0.01; right panel).

Figure 5. Differential radiosensitivity of dendritic spines.

Representative dendritic segments (green) showing immature filopodia (white), long (pink) and mushroon (red) spine types along with more mature stubby (blue) spines. Exposure to ¹⁶O (upper panel) or ⁴⁸Ti (lower panel) particles leads to significant reductions in the number of immature spines with no effect on mature spines. Quantification of each morphological type of dendritic spine are expressed as the total number of spines for each class. Data are expressed as mean ± SEM. *P < 0.05, **P < 0.01 versus control; ANOVA.

Figure 6. Cosmic radiation exposure induces persistent increases in PSD-95 puncta in the mPFC.

(a) Representative fluorescence micrographs showing PSD-95 puncta (red). (b) Quantitative analyses show that exposure to 5 or 30 cGy of ¹⁶O or ⁴⁸Ti particles leads to increased numbers of PSD-95 puncta in mPFC neurons as compared to control (15 weeks, left panel; 27 weeks, right panel). Data are expressed as mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001 versus control; ANOVA. (c) Overexpression of PSD-95 correlates with cognitive decrements in irradiated mice. PSD-95 puncta (per 400 μm²) are plotted against the corresponding performance of each animal on the OiP task. Increased levels of PSD-95 puncta are associated with decreased behavioral performance following exposure to ¹⁶O (30 cGy, P = 0.01) and ⁴⁸Ti (5 and 30 cGy, P = 0.01).

Figure 7. Exposure to cosmic radiation leads to increased neuroinflammation in the prelimbic layer of the mPFC.

Representative images from left to right illustrate immunohistochemical visualization of ED-1+ cells in control, 5 cGy ¹⁶O or 5 cGy ⁴⁸Ti in the PL subfield 15 weeks post-exposure (ED1+ red and toto-3 counterstain; top). Quantitative analysis demonstrates that, compared to respective controls, irradiation leads to increased numbers of activated microglia 15 and 27 weeks later (left and right lower panels, respectively). Data are expressed as mean ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001 versus control; ANOVA.