New Concerns for Neurocognitive Function during Deep Space Exposures to Chronic, Low Dose-Rate, Neutron Radiation

Abstract

As NASA prepares for a mission to Mars, concerns regarding the health risks associated with deep space radiation exposure have emerged. Until now, the impacts of such exposures have only been studied in animals after acute exposures, using dose rates ∼1.5×10 5 higher than those actually encountered in space. Using a new, low dose-rate neutron irradiation facility, we have uncovered that realistic, low dose-rate exposures produce serious neurocognitive complications associated with impaired neurotransmission. Chronic (6 month) low-dose (18 cGy) and dose rate (1 mGy/d) exposures of mice to a mixed field of neutrons and photons result in diminished hippocampal neuronal excitability and disrupted hippocampal and cortical long-term potentiation. Furthermore, mice displayed severe impairments in learning and memory, and the emergence of distress behaviors. Behavioral analyses showed an alarming increase in risk associated with these realistic simulations, revealing for the first time, some unexpected potential problems associated with deep space travel on all levels of neurological function.

Multi-domain cognitive assessment of male mice shows space radiation is not harmful to high-level cognition and actually improves pattern separation

“…Most preclinical data that led to the Probabilistic Risk Assessment of HZE particles being detrimental to cognition were from tests performed on young adult rodents (~2-3 months [mon] at exposure) 14 ; in many cases, age at testing was not reported 14 . To more accurately reflect the average age of astronauts, NASA now requires ground-based space studies to be performed in mature animals (~6-mon-old at irradiation) [14][15][16][17][18][19][20][21][22][23][24] . Indeed, some studies now directly compare the cognitive impact of HZE irradiation in early life vs. maturity [25][26][27][28] , although the results are mixed.…”

Space Radiation Biology for “Living in Space”

“…The long-term effects of these doses of radiation on the CNS are largely unknown. Acharya et al exposed mice to chronic, low-dose-rate (1 mGy/day) radiation for 6 months to investigate how deep space travel could affect the CNS [152]. They found that the radiation exposure impaired cellular signaling in the hippocampus, a part tied to learning and memory, and the prefrontal cortex, which plays a role in higher cognitive functions, resulting in learning and memory impairments.…”

Multi-domain cognitive assessment of male mice reveals whole body exposure to space radiation is not detrimental to high-level cognition and actually improves pattern separation

“…Statistical Analyses 26 Data are reported as mean ± s.e.m. Testing of data assumptions (for example, normal distribution, similar variation between control and experimental groups, etc.)…”

“…Most preclinical data that led to the Probabilistic Risk Assessment of HZE particles being detrimental to cognition and related behavior were from tests performed on young adult rodents (~2-3 months [mon] old at exposure) (23,25) ; in many cases, age of the animals tested was not even reported (23) . To more accurately reflect the average age of astronauts, NASA now requires ground-based space studies to be performed in mature animals (~6 mon old at start of irradiation) (23,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37) . Second, type of behavioral test matters.…”

“…Second, type of behavioral test matters. Recent work with mature rodents shows HZE particle exposure decreases performance in some -but not all -behavioral tests, and even tests that engage similar neural circuits produce distinct results (23,(26)(27)(28)(29)(30)(31)38) . A potential contribution to these test-dependent discrepancies is the testing environments used for each task.…”