Effects of social isolation and galactic cosmic radiation on fine motor skills and behavioral performance

Future NASA missions will require astronauts to travel farther and spend longer durations in space than ever before. This will also expose astronauts to longer periods of several physical and psychological challenges, including exposure to space radiation (SR) and periods of social isolation (SI), which could have unknown negative effects on physical and mental health. Each also has the potential to negatively impact sleep which can reduce the ability to cope with stressful experiences and lead to sensorimotor, neurocognitive, and physical deficits. The effects of SI and SR on gross motor performance has been shown to vary, and depend on, individual differences in stress resilience and vulnerability based on our established animal model in which stress produces different effects on sleep. In this study, the impact that SI and SR, either alone or together, had on fine motor skill performance (bilateral tactile adhesive removal task (BTAR)) was assessed in male rats. We also examined emotional, exploratory, and other off-task behavioral responses during testing and assessed whether sensorimotor performance and emotion varied with individual differences in resilience and vulnerability. BTAR task performance was differentially impacted by SI and SR, and were further influenced by the stress resilience/vulnerability phenotype of the rats. These findings further demonstrate that identifying individual responses to stressors that can impact sensorimotor ability and behavior necessary to perform mission-related tasks will be of particular importance for astronauts and future missions. Should similar effects occur in humans, there may be considerable inter-individual variability in the impact that inflight stressors have on astronauts and their ability to perform mission-related tasks.

Complex 33-beam simulated galactic cosmic radiation exposure impacts cognitive function and prefrontal cortex neurotransmitter networks in male mice

Astronauts will encounter extended exposure to galactic cosmic radiation (GCR) during deep space exploration, which could impair brain function. Here, we report that in male mice, acute or chronic GCR exposure did not modify reward sensitivity but did adversely affect attentional processes and increased reaction times. Potassium (K+)-stimulation in the prefrontal cortex (PFC) elevated dopamine (DA) but abolished temporal DA responsiveness after acute and chronic GCR exposure. Unlike acute GCR, chronic GCR increased levels of all other neurotransmitters, with differences evident between groups after higher K+-stimulation. Correlational and machine learning analysis showed that acute and chronic GCR exposure differentially reorganized the connection strength and causation of DA and other PFC neurotransmitter networks compared to controls which may explain space radiation-induced neurocognitive deficits.

Behavioral performance and microglial status in mice after moderate dose of proton irradiation

Cognitive impairment is a remote effect of gamma radiation treatment of malignancies. The major part of the studies on the effect of proton irradiation (a promising alternative in the treatment of radio-resistant tumors and tumors located close to critical organs) on the cognitive abilities of laboratory animals and their relation to morphological changes in the brain is rather contradictory. The aim of this study was to investigate cognitive functions and the dynamics of changes in morphological parameters of hippocampal microglial cells after 7.5 Gy of proton irradiation. Two months after the cranial irradiation, 8- to 9-week-old male SHK mice were tested for total activity, spatial learning, as well as long- and short-term hippocampus-dependent memory. To estimate the morphological parameters of microglia, brain slices of control and irradiated animals each with different time after proton irradiation (24 h, 7 days, 1 month) were stained for microglial marker Iba-1. No changes in behavior or deficits in short-term and long-term hippocampus-dependent memory were found, but an impairment of episodic memory was observed. A change in the morphology of hippocampal microglial cells, which is characteristic of the transition of cells to an activated state, was detected. One day after proton exposure in the brain tissue, a slight decrease in cell density was observed, which was restored to the control level by the 30th day after treatment. The results obtained may be promising with regard to the future use of using high doses of protons per fraction in the irradiation of tumors.

Loss of Cognitive Flexibility Practice Effects in Female Rats Exposed to Simulated Space Radiation

During the planned missions to Mars, astronauts will be faced with many potential health hazards including prolonged exposure to space radiation. Ground-based studies have shown that exposure to space radiation impairs the performance of male rats in cognitive flexibility tasks which involve processes that are essential to rapidly and efficiently adapting to different situations. However, there is presently a paucity of information on the effects of space radiation on cognitive flexibility in female rodents. This study has determined the impact that exposure to a low (10 cGy) dose of ions from the simplified 5-ion galactic cosmic ray simulation [https://www.bnl.gov/nsrl/userguide/SimGCRSim.php (07/2023)] (GCRSim) beam or 250 MeV/n 4He ions has on the ability of female Wistar rats to perform in constrained [attentional set shifting (ATSET)] and unconstrained cognitive flexibility (UCFlex) tasks. Female rats exposed to GCRSim exhibited multiple decrements in ATSET performance. Firstly, GCRSim exposure impaired performance in the compound discrimination (CD) stage of the ATSET task. While the ability of rats to identify the rewarded cue was not compromised, the time the rats required to do so significantly increased. Secondly, both 4He and GCRSim exposure reduced the ability of rats to reach criterion in the compound discrimination reversal (CDR) stage. Approximately 20% of the irradiated rats were unable to complete the CDR task; furthermore, the irradiated rats that did reach criterion took more attempts to do so than did the sham-treated animals. Radiation exposure also altered the magnitude and/or nature of practice effects. A comparison of performance metrics from the pre-screen and post-exposure ATSET task revealed that while the sham-treated rats completed the post-exposure CD stage of the ATSET task in 30% less time than for completion of the pre-screen ATSET task, the irradiated rats took 30-50% longer to do so. Similarly, while sham-treated rats completed the CDR stage in ∼10% fewer attempts in the post-exposure task compared to the pre-screen task, in contrast, the 4He- and GCRSim-exposed cohorts took more (∼2-fold) attempts to reach criterion in the post-exposure task than in the pre-screen task. In conclusion, this study demonstrates that female rats are susceptible to radiation-induced loss of performance in the constrained ATSET cognitive flexibility task. Moreover, exposure to radiation leads to multiple performance decrements, including loss of practice effects, an increase in anterograde interference and reduced ability or unwillingness to switch attention. Should similar effects occur in humans, astronauts may have a compromised ability to perform complex tasks.

New Radiobiological Principles for the CNS Arising from Space Radiation Research

Traditionally, the brain has been regarded as a relatively insensitive late-reacting tissue, with radiologically detectable damage not being reported at doses < 60 Gy. When NASA proposed interplanetary exploration missions, it was required to conduct an intensive health and safety evaluation of cancer, cardiovascular, and cognitive risks associated with exposure to deep space radiation (SR). The SR dose that astronauts on a mission to Mars are predicted to receive is ~300 mGy. Even after correcting for the higher RBE of the SR particles, the biologically effective SR dose (<1 Gy) would still be 60-fold lower than the threshold dose for clinically detectable neurological damage. Unexpectedly, the NASA-funded research program has consistently reported that low (<250 mGy) doses of SR induce deficits in multiple cognitive functions. This review will discuss these findings and the radical paradigm shifts in radiobiological principles for the brain that were required in light of these findings. These included a shift from cell killing to loss of function models, an expansion of the critical brain regions for radiation-induced cognitive impediments, and the concept that the neuron may not be the sole critical target for neurocognitive impairment. The accrued information on how SR exposure impacts neurocognitive performance may provide new opportunities to reduce neurocognitive impairment in brain cancer patients.

Multiple decrements in switch task performance in female rats exposed to space radiation

Astronauts on the Artemis missions to the Moon and Mars will be exposed to unavoidable Galactic Cosmic Radiation (GCR). Studies using male rats suggest that GCR exposure impairs several processes required for cognitive flexibility performance, including attention and task switching. Currently no comparable studies have been conducted with female rats. Given that both males and females will travel into deep space, this study determined whether simulated GCR (GCRsim) exposure impairs task switching performance in female rats. Female Wistar rats exposed to 10cGy GCRsim (n = 12) and shams (n=14) were trained to perform a touchscreen-based switch task that mimics a switch task used to evaluate pilots' response times. In comparison to sham rats, three-fold more GCRsim-exposed rats failed to complete the stimulus response stage of training, a high cognitive loading task. In the switch task, 50% of the GCRsim-exposed rats failed to consistently transition between the repeated and switch blocks of stimuli, which they completed during lower cognitive loading training stages. The GCRsim-exposed rats that completed the switch task only performed at 65% of the accuracy of shams. Female rats exposed to GCRsim thus exhibit multiple decrements in the switch task under high, but not low, cognitive loading conditions. While the operational significance of this performance decrement is unknown, if GCRSim exposure was to induce similar effects in astronauts, our data suggests there may be a reduced ability to execute task switching under high cognitive loading situations.

Effects of preexposure to a subthreshold dose of helium particles on the changes in performance produced by exposure to helium particles

On exploratory class missions, such as a mission to Mars, astronauts will be exposed to doses of particles of high energy and charge and protons up to 30 - 40 cGy. These exposures will most likely occur at random intervals across the estimated 3-yr duration of the mission. As such, the possibility of an interaction between particles must be taken into account: a prior subthreshold exposure to one particle may prevent or minimize the effect of a subsequent exposure (adaptation), or there may be an additive effect such that the prior exposure may sensitize the individual to a subsequent exposure of the same or different radiations. Two identical replications were run in which rats were exposed to a below threshold dose of ⁴He particles and 2, 24 or 72 h later given either a second below threshold or an above threshold dose of ⁴He particles and tested for performance on an operant task. The results indicate that preexposure to a subthreshold dose of ⁴He particles can either sensitize or attenuate the effects of the subsequent dose, depending upon the interval between exposures and the doses. These results suggest that exposure to multiple doses of heavy particles may have implications for astronaut health on exploratory class missions.

Sleep and Core Body Temperature Alterations Induced by Space Radiation in Rats

Sleep problems in astronauts can arise from mission demands and stress and can impact both their health and ability to accomplish mission objectives. In addition to mission-related physical and psychological stressors, the long durations of the proposed Mars missions will expose astronauts to space radiation (SR), which has a significant impact on the brain and may also alter sleep and physiological functions. Therefore, in this study, we assessed sleep, EEG spectra, activity, and core body temperature (CBT) in rats exposed to SR and compared them to age-matched nonirradiated rats. Male outbred Wistar rats (8-9 months old at the time of the study) received SR (15 cGy GCRsim, n = 15) or served as age- and time-matched controls (CTRL, n = 15) without irradiation. At least 90 days after SR and 3 weeks prior to recording, all rats were implanted with telemetry transmitters for recording EEG, activity, and CBT. Sleep, EEG spectra (delta, 0.5-4 Hz; theta, 4-8 Hz; alpha, 8-12 Hz; sigma, 12-16 Hz; beta, 16-24 Hz), activity, and CBT were examined during light and dark periods and during waking and sleeping states. When compared to the CTRLs, SR produced significant reductions in the amounts of dark period total sleep time, total nonrapid eye movement sleep (NREM), and total rapid eye movement sleep (REM), with significant decreases in light and dark period NREM deltas and dark period REM thetas as well as increases in alpha and sigma in NREM and REM during either light or dark periods. The SR animals showed modest increases in some measures of activity. CBT was significantly reduced during waking and sleeping in the light period. These data demonstrate that SR alone can produce alterations to sleep and temperature control that could have consequences for astronauts and their ability to meet mission demands.

Differential Impact of Social Isolation and Space Radiation on Behavior and Motor Learning in Rats

Future missions to Mars will expose astronauts to several physical and psychological challenges, including exposure to space radiation (SR) and periods of social isolation (SI). Each of these stressors, in addition to mission demands, can affect physical and mental health and potentially negatively impact sleep. The effects of inflight stressors may vary with duration and time course, may be additive or compounding, and may vary with individual differences in stress resilience and vulnerability. Determining how individual differences in resilient and vulnerable phenotypes respond to these mission-related stressors and their interactions with sleep will be crucial for understanding and mitigating factors that can impair performance and damage health. Here, we examined the single and compound effects of ground-based analogs of SI and SR on sensorimotor performance on the balance beam (BB) in rats. We also assessed emotional responses during testing on the BB and assessed whether sensorimotor performance and emotion varied with individual differences in stress resiliency using our established animal model in which stress produces different effects on sleep. Results showed differential motor performance and emotion in the BB task between SI and SR, and these varied based on resilient and vulnerable phenotypes. These findings demonstrate that identifying individual responses to stressors that can impact sensorimotor ability and behavior necessary to perform mission-related tasks will be of particular importance for astronauts and future missions. Should similar effects occur in humans, there may be considerable inter-individual variability in the impact that flight stressors have on the mental health of astronauts and their ability to perform mission-related tasks.

The Effects of Galactic Cosmic Rays on the Central Nervous System: From Negative to Unexpectedly Positive Effects That Astronauts May Encounter

Galactic cosmic rays (GCR) pose a serious threat to astronauts’ health during deep space missions. The possible functional alterations of the central nervous system (CNS) under GCR exposure can be critical for mission success. Despite the obvious negative effects of ionizing radiation, a number of neutral or even positive effects of GCR irradiation on CNS functions were revealed in ground-based experiments with rodents and primates. This review is focused on the GCR exposure effects on emotional state and cognition, emphasizing positive effects and their potential mechanisms. We integrate these data with GCR effects on adult neurogenesis and pathological protein aggregation, forming a complete picture. We conclude that GCR exposure causes multidirectional effects on cognition, which may be associated with emotional state alterations. However, the irradiation in space-related doses either has no effect or has performance enhancing effects in solving high-level cognition tasks and tasks with a high level of motivation. We suppose the model of neurotransmission changes after irradiation, although the molecular mechanisms of this phenomenon are not fully understood.

High-Energy, Whole-Body Proton Irradiation Differentially Alters Long-Term Brain Pathology and Behavior Dependent on Sex and Alzheimer's Disease Mutations

Whole-body exposure to high-energy particle radiation remains an unmitigated hazard to human health in space. Ongoing experiments at the NASA Space Radiation Laboratory and elsewhere repeatedly show persistent changes in brain function long after exposure to simulations of this unique radiation environment, although, as is also the case with proton radiotherapy sequelae, how this occurs and especially how it interacts with common comorbidities is not well-understood. Here, we report modest differential changes in behavior and brain pathology between male and female Alzheimer's-like and wildtype littermate mice 7-8 months after exposure to 0, 0.5, or 2 Gy of 1 GeV proton radiation. The mice were examined with a battery of behavior tests and assayed for amyloid beta pathology, synaptic markers, microbleeds, microglial reactivity, and plasma cytokines. In general, the Alzheimer's model mice were more prone than their wildtype littermates to radiation-induced behavior changes, and hippocampal staining for amyloid beta pathology and microglial activation in these mice revealed a dose-dependent reduction in males but not in females. In summary, radiation-induced, long-term changes in behavior and pathology, although modest, appear specific to both sex and the underlying disease state.

Fine motor deficits exhibited in rat string-pulling behavior following exposure to sleep fragmentation and deep space radiation

Deep space flight missions will expose astronauts to multiple stressors, including sleep fragmentation and space radiation. There is debate over whether sleep disruptions are an issue in deep space. While these stressors independently impair sensorimotor function, the combined effects on performance are currently unknown. String-pulling behavior involves highly organized bimanual reach-to-grasp and withdraw movements. This behavior was examined under rested wakeful conditions and immediately following one session of sleep fragmentation in Sham and irradiated rats 3 months after exposure (10 cGy ⁴Helium or 5-ion simulated Galactic Cosmic Radiation). Sleep fragmentation disrupted several aspects of string-pulling behavior, such that rats' ability to grasp the string was reduced, reach endpoint concentration was more variable, and distance traveled by the nose increased in the Y-range compared to rested wakeful performance. Overall, irradiated rats missed the string more than Sham rats 3 months post-exposure. Irradiated rats also exhibited differential impairments at 3 months, with additional deficits unveiled after sleep fragmentation. ⁴Helium-exposed rats took longer to approach the string after sleep fragmentation. Further, rats exposed to ⁴Helium traveled shorter withdraw distances 3 months after irradiation, while this only emerged in the other irradiated group after sleep fragmentation. These findings identify sleep fragmentation as a risk for fine motor dysfunction in Sham and irradiated conditions, in addition to radiation exposure. There may be complex temporal alterations in performance that are stressor- and ion-dependent. Thus, it is critical to implement appropriate models of multi-flight stressors and performance assessments in preparation for future deep space flight missions.

Galactic cosmic ray simulation at the NASA space radiation laboratory - Progress, challenges and recommendations on mixed-field effects

For missions beyond low Earth orbit to the moon or Mars, space explorers will encounter a complex radiation field composed of various ion species with a broad range of energies. Such missions pose significant radiation protection challenges that need to be solved in order to minimize exposures and associated health risks. An innovative galactic cosmic ray simulator (GCRsim) was recently developed at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). The GCRsim technology is intended to represent major components of the space radiation environment in a ground analog laboratory setting where it can be used to improve understanding of biological risks and serve as a testbed for countermeasure development and validation. The current GCRsim consists of 33 energetic ion beams that collectively simulate the primary and secondary GCR field encountered by humans in space over the broad range of particle types, energies, and linear energy transfer (LET) of interest to health effects. A virtual workshop was held in December 2020 to assess the status of the NASA baseline GCRsim. Workshop attendees examined various aspects of simulator design, with a particular emphasis on beam selection strategies. Experimental results, modeling approaches, areas of consensus, and questions of concern were also discussed in detail. This report includes a summary of the GCRsim workshop and a description of the current status of the GCRsim. This information is important for future advancements and applications in space radiobiology.

Comparative Analysis of Behavioral Reactions and Morphological Changes in the Rat Brain After Exposure to Ionizing Radiation with Different Physical Characteristics

The aim of this research was to study behavioral reactions and morphological changes in the brain of adult female Sprague Dawley rats after exposure to 170 MeV and 70 MeV protons and gamma radiation (60Co) at a dose of 1 Gy. The analysis of the behavioral reactions in the T-maze showed that exposure to ionizing radiation with different LETs led to an increase in number of repeated entries into the arms of the maze in the spontaneous alternation test. In the Open Field test a decrease in overall motor activity in the group of irradiated animals (70 MeV protons at the Bragg peak) was observed. A decrease in the number of standing positions was seen in all groups of irradiated animals. Morphological analysis showed the development of early amyloidosis, autolysis of the ependymal layer, an increase in the number of neurodegenerative changes in various structures of the brain, and the development of neuronal hypertrophy on the 30th day after irradiation in the cerebellum and hippocampal hilus. Exposure to protons at a dose of 1 Gy leads to the development of structural and functional disorders of the central nervous system of animals on the 30th day after irradiation. These data indicate a damage of short-term memory, a decrease in motor activity and exploratory behavior of animals. With an increase in LET, there is an increase in the number of amyloid plaques in the forebrain of rats, autolysis of the ependymal layer of the ventricles, and the development of dystrophic changes. Investigations of behavioral reactions and morphological changes in various parts of the brain of adult rats on the 30th day after influence of ionizing radiation with different physical characteristics at a dose of 1 Gy. Various negative patho-morphological and cognitive-behavioral changes observed.

Cognitive and behavioral effects of whole brain conventional or high dose rate (FLASH) proton irradiation in a neonatal Sprague Dawley rat model

Recent studies suggest that ultra-high dose rates of proton radiation (>40 Gy/s; FLASH) confer less toxicity to exposed healthy tissue and reduce cognitive decline compared with conventional radiation dose rates (~1 Gy/s), but further preclinical data are required to demonstrate this sparing effect. In this study, postnatal day 11 (P11) rats were treated with whole brain irradiation with protons at a total dose of 0, 5, or 8 Gy, comparing a conventional dose rate of 1 Gy/s vs. a FLASH dose rate of 100 Gy/s. Beginning on P64, rats were tested for locomotor activity, acoustic and tactile startle responses (ASR, TSR) with or without prepulses, novel object recognition (NOR; 4-object version), striatal dependent egocentric learning ([configuration A] Cincinnati water maze (CWM-A)), prefrontal dependent working memory (radial water maze (RWM)), hippocampal dependent spatial learning (Morris water maze (MWM)), amygdala dependent conditioned freezing, and the mirror image CWM [configuration B (CWM-B)]. All groups had deficits in the CWM-A procedure. Weight reductions, decreased center ambulation in the open-field, increased latency on day-1 of RWM, and deficits in CWM-B were observed in all irradiated groups, except the 5 Gy FLASH group. ASR and TSR were reduced in the 8 Gy FLASH group and day-2 latencies in the RWM were increased in the FLASH groups compared with controls. There were no effects on prepulse trials of ASR or TSR, NOR, MWM, or conditioned freezing. The results suggest striatal and prefrontal cortex are sensitive regions at P11 to proton irradiation, with reduced toxicity from FLASH at 5 Gy.

Simulated Space Radiation Exposure Effects on Switch Task Performance in Rats

BACKGROUND: Astronauts on the mission to Mars will be subjected to galactic cosmic radiation (GCR) exposures. While ground-based studies suggest that simulated GCR (GCRsim) exposure impairs performance in multiple cognitive tasks, the impact of such exposures on task switching performance (an important skill for all aviators) has not yet been determined.METHODS: Male Wistar rats previously exposed to 10 cGy of 4He ions or GCRsim and their sham littermates were trained to perform a touchscreen-based switch task designed to mimic warning light response tests used to evaluate pilots’ response times.RESULTS: Irradiated rats failed to complete a high cognitive task load training task threefold more frequently than shams. There were 18 (4 Sham, 7 He-, and 7 GCR-exposed) rats that successfully completed initial training and underwent switch task testing. Relative to the sham rats in the switch task, the GCRsim-exposed rats had significantly slower response times in switch but not repeat trials. The GCRsim-exposed rats had significantly (P < 0.01) higher switch response ratios (switch/repeat trial response time) and absolute switch costs (switch minus repeat trial response time) than either the sham or He-exposed rats.DISCUSSION: Rats exposed to GCRsim have significantly impaired performance in the switch task manifested as an absolute switch cost of ∼700 ms. The operational significance of such an increase requires further investigation, but a 1000-ms switch cost results in a twofold increase in cockpit error rates in pilots. If exposure to GCR in space results in similar effects in humans, the operational performance of astronauts on the Mars mission may be suboptimal.Stephenson S, Britten R. Simulated space radiation exposure effects on switch task performance in rats. Aerosp Med Hum Perform. 2022; 93(9):673–680.

Quantitative proteomic analytic approaches to identify metabolic changes in the medial prefrontal cortex of rats exposed to space radiation

NASA’s planned mission to Mars will result in astronauts being exposed to ∼350 mSv/yr of Galactic Cosmic Radiation (GCR). A growing body of data from ground-based experiments indicates that exposure to space radiation doses (approximating those that astronauts will be exposed to on a mission to Mars) impairs a variety of cognitive processes, including cognitive flexibility tasks. Some studies report that 33% of individuals may experience severe cognitive impairment. Translating the results from ground-based rodent studies into tangible risk estimates for astronauts is an enormous challenge, but it would be germane for NASA to use the vast body of data from the rodent studies to start developing appropriate countermeasures, in the expectation that some level of space radiation (SR) -induced cognitive impairment could occur in astronauts. While some targeted studies have reported radiation-induced changes in the neurotransmission properties and/or increased neuroinflammation within space radiation exposed brains, there remains little information that can be used to start the development of a mechanism-based countermeasure strategy. In this study, we have employed a robust label-free mass spectrometry (MS) -based untargeted quantitative proteomic profiling approach to characterize the composition of the medial prefrontal cortex (mPFC) proteome in rats that have been exposed to 15 cGy of 600 MeV/n28Si ions. A variety of analytical techniques were used to mine the generated expression data, which in such studies is typically hampered by low and variable sample size. We have identified several pathways and proteins whose expression alters as a result of space radiation exposure, including decreased mitochondrial function, and a further subset of proteins differs in rats that have a high level of cognitive performance after SR exposure in comparison with those that have low performance levels. While this study has provided further insight into how SR impacts upon neurophysiology, and what adaptive responses can be invoked to prevent the emergence of SR-induced cognitive impairment, the main objective of this paper is to outline strategies that can be used by others to analyze sub-optimal data sets and to identify new information.

Dynamics of Dopamine and Other Monoamines Content in Rat Brain after Single Low-Dose Carbon Nuclei Irradiation

Space radiation, presented primarily by high-charge and -energy particles (HZEs), has a substantial impact on the central nervous system (CNS) of astronauts. This impact, surprisingly, has not only negative but also positive effects on CNS functions. Despite the fact that the mechanisms of this effect have not yet been elucidated, several studies indicate a key role for monoaminergic networks underlying these effects. Here, we investigated the effects of acute irradiation with 450 MeV/n carbon (12C) nuclei at a dose of 0.14 Gy on Wistar rats; a state of anxiety was accessed using a light–dark box, spatial memory in a Morris water maze, and the dynamics of monoamine metabolism in several brain morphological structures using HPLC. No behavioral changes were observed. Irradiation led to the immediate suppression of dopamine turnover in the prefrontal cortex, hypothalamus, and striatum, while a decrease in the level of norepinephrine was detected in the amygdala. However, these effects were transient. The deferred effect of dopamine turnover increase was found in the hippocampus. These data underscore the ability of even low-dose 12C irradiation to affect monoaminergic networks. However, this impact is transient and is not accompanied by behavioral alterations.

Impact of spaceflight stressors on behavior and cognition: A molecular, neurochemical, and neurobiological perspective

The response of the human body to multiple spaceflight stressors is complex, but mounting evidence implicate risks to CNS functionality as significant, able to threaten metrics of mission success and longer-term behavioral and neurocognitive health. Prolonged exposure to microgravity, sleep disruption, social isolation, fluid shifts, and ionizing radiation have been shown to disrupt mechanisms of homeostasis and neurobiological well-being. The overarching goal of this review is to document the existing evidence of how the major spaceflight stressors, including radiation, microgravity, isolation/confinement, and sleep deprivation, alone or in combination alter molecular, neurochemical, neurobiological, and plasma metabolite/lipid signatures that may be linked to operationally-relevant behavioral and cognitive performance. While certain brain region-specific and/or systemic alterations titrated in part with neurobiological outcome, variations across model systems, study design, and the conspicuous absence of targeted studies implementing combinations of spaceflight stressors, confounded the identification of specific signatures having direct relevance to human activities in space. Summaries are provided for formulating new research directives and more predictive readouts of portending change in neurobiological function.

Dynamic ensemble prediction of cognitive performance in spaceflight

During spaceflight, astronauts face a unique set of stressors, including microgravity, isolation, and confinement, as well as environmental and operational hazards. These factors can negatively impact sleep, alertness, and neurobehavioral performance, all of which are critical to mission success. In this paper, we predict neurobehavioral performance over the course of a 6-month mission aboard the International Space Station (ISS), using ISS environmental data as well as self-reported and cognitive data collected longitudinally from 24 astronauts. Neurobehavioral performance was repeatedly assessed via a 3-min Psychomotor Vigilance Test (PVT-B) that is highly sensitive to the effects of sleep deprivation. To relate PVT-B performance to time-varying and discordantly-measured environmental, operational, and psychological covariates, we propose an ensemble prediction model comprising of linear mixed effects, random forest, and functional concurrent models. An extensive cross-validation procedure reveals that this ensemble outperforms any one of its components alone. We also identify the most important predictors of PVT-B performance, which include an individual's previous PVT-B performance, reported fatigue and stress, and temperature and radiation dose. This method is broadly applicable to settings where the main goal is accurate, individualized prediction of human behavior involving a mixture of person-level traits and irregularly measured time series.

Particle Radiation Side-Effects: Intestinal Microbiota Composition Shapes Interferon-γ-Induced Osteo-Immunogenicity

Microbiota can both negatively and positively impact radiation-induced bone loss. Our prior research showed that compared to mice with conventional gut microbiota (CM), mice with restricted gut microbiota (RM) reduced inflammatory tumor necrosis factor (TNF) in bone marrow, interleukin (IL)-17 in blood, and chemokine (C-C motif) ligand 20 (CCL20) in bone marrow under anti-IL-17 treatment. We showed that Muribaculum intestinale was more abundant in intestinal epithelial cells (IECs) from the small intestine of female RM mice and positively associated with augmented skeletal bone structure. Female C57BL/6J pun RM mice, which were injected with anti-IL-17 antibody one day before exposure to 1.5 Gy 28Si ions of 850 MeV/u, showed high trabecular numbers in tibiae at 6 weeks postirradiation. Irradiated CM mice were investigated for lower interferon-γ and IL-17 levels in the small intestine than RM mice. IL-17 blockage resulted in bacterial indicator phylotypes being different between both microbiota groups before and after irradiation. Analysis of the fecal bacteria were performed in relation to bone quality and body weight, showing reduced tibia cortical thickness in irradiated CM mice (-15%) vs. irradiated RM mice (-9.2%). Correlation analyses identified relationships among trabecular bone parameters (TRI-BV/TV, Tb.N, Tb.Th, Tb.Sp) and Bacteroides massiliensis, Muribaculum sp. and Prevotella denticola. Turicibacter sp. was found directly correlated with trabecular separation in anti-IL-17 treated mice, whereas an unidentified Bacteroidetes correlated with trabecular thickness in anti-IL-17 neutralized and radiation-exposed mice. We demonstrated radiation-induced osteolytic damage to correlate with bacterial indicator phylotypes of the intestinal microbiota composition, and these relationships were determined from the previously discovered dose-dependent particle radiation effects on cell proliferation in bone tissue. New translational approaches were designed to investigate dynamic changes of gut microbiota in correlation with conditions of treatment and disease as well as mechanisms of systemic side-effects in radiotherapy.

Examining the Effects of 4He Exposure on the Gut-Brain Axis

Beyond low-Earth orbit, space radiation poses significant risks to astronaut health. Previous studies have shown that the microbial composition of the gastrointestinal (GI) microbiome changes upon exposure to high-linear energy transfer radiation. Interestingly, radiation-induced shifts in GI microbiota composition are linked to various neuropsychological disorders. Herein, we aimed to study changes in GI microbiota and behaviors of rats exposed to whole-body radiation (0, 5 or 25 cGy 4He, 250 MeV/n) at approximately 6 months of age. Fecal samples were collected 24 h prior to 4He irradiation and 24 h and 7 days postirradiation for quantitative PCR analyses to assess fecal levels of spore-forming bacteria (SFB), Bifidobacterium, Lactobacillus and Akkermansia. Rats were also tested in the social odor recognition memory (SORM) test at day 7 after 4He exposure. A subset of rats was euthanized 90 min after completion of the SORM test, and GI tissue from small intestine to colon were prepared for examining overall histological changes and immunohistochemical staining for serotonin (5-HT). No notable pathological changes were observed in GI tissues. Akkermansia spp. and SFB were significantly decreased in the 25 cGy group at 24 h and 7 days postirradiation compared to pre-exposure, respectively. Bifidobacterium and Lactobacillus spp. showed no significant changes. 5-HT production was significantly higher in the proximal small intestine and the cecum in the 25 cGy group compared to the sham group. The 25 cGy group exhibited deficits in recognition in SORM testing at day 7 postirradiation. Taken together, these results suggest a connection between GI microbiome composition, serotonin production, and neurobehavioral performance, and that this connection may be disrupted upon exposure to 25 cGy of 4He ions.

Review of Conventional and High Dose Rate Brain Radiation (FLASH): Neurobehavioural, Neurocognitive and Assessment Issues in Rodent Models

Ionising radiation causes secondary tumours and/or enduring cognitive deficits, especially in children. Proton radiotherapy reduces exposure of the developing brain in children but may still cause some lasting effects. Recent observations show that ultra-high dose rate radiation treatment (≥40 Gy/s), called the FLASH effect, is equally effective at tumour control but less damaging to surrounding tissue compared with conventional dose rate protons (0.03-3 Gy/s). Most studies on the FLASH effect in brain and other tissues with different radiation modalities (electron and photon radiation), show FLASH benefits in these preclinical rodent models, but the data are limited, especially for proton FLASH, including for dose, dose rate and neurochemical and neurobehavioural outcomes. Tests of neurocognitive outcomes have been limited despite clinical evidence that this is the area of greatest concern. The FLASH effect in the context of proton exposure is promising, but a more systematic and comprehensive approach to outcomes is needed.

Nonhuman primate models in the study of spaceflight stressors: Past contributions and future directions

Studies in rodents suggest that exposure to distinct spaceflight stressors (e.g., space radiation, isolation/confinement, microgravity) may have a profound impact on an astronaut's ability to perform both simple and complex tasks related to neurocognitive performance, central nervous system (CNS) and vestibular/sensorimotor function. However, limited information is currently available on how combined exposure to the spaceflight stressors will impact CNS-related neurocognitive and neurobiological function in-flight and, as well, terrestrial risk of manifesting neurodegenerative conditions when astronauts return to earth. This information gap has significantly hindered our ability to realistically estimate spaceflight hazard risk to the CNS associated with deep space exploration. Notwithstanding a significant body of work with rodents, there have been very few direct investigations of the impact of these spaceflight stressors in combination and, to our knowledge, no such investigations using nonhuman primate (NHP) animal models. In view of the widely-recognized translational value of NHP data in advancing biomedical discoveries, this research deficiency limits our understanding regarding the impact of individual and combined spaceflight stressors on CNS-related neurobiological function. In this review, we address this knowledge gap by conducting a systematic and comprehensive evaluation of existing research on the impact of exposure to spaceflight stressors on NHP CNS-related function. This review is structured to: a) provide an overarching view of the past contributions of NHPs to spaceflight research as well as the strengths, limitations, and translational value of NHP research in its own right and within the existing context of NASA-relevant rodent research; b) highlight specific conclusions based on the published literature and areas needed for future endeavors; c) describe critical research gaps and priorities in NHP research to facilitate NASA's efforts to bridge the key knowledge gaps that currently exist in translating rodent data to humans; and d) provide a roadmap of recommendations for NASA regarding the availability, validity, strengths, and limitations of various NHP models for future targeted research.

All for one, though not one for all: team players in normal tissue radiobiology

PURPOSE

As part of the special issue on 'Women in Science', this review offers a perspective on past and ongoing work in the field of normal (non-cancer) tissue radiation biology, highlighting the work of many of the leading contributors to this field of research. We discuss some of the hypotheses that have guided investigations, with a focus on some of the critical organs considered dose-limiting with respect to radiation therapy, and speculate on where the field needs to go in the future.

CONCLUSIONS

The scope of work that makes up normal tissue radiation biology has and continues to play a pivotal role in the radiation sciences, ensuring the most effective application of radiation in imaging and therapy, as well as contributing to radiation protection efforts. However, despite the proven historical value of preclinical findings, recent decades have seen clinical practice move ahead with altered fractionation scheduling based on empirical observations, with little to no (or even negative) supporting scientific data. Given our current appreciation of the complexity of normal tissue radiation responses and their temporal variability, with tissue- and/or organ-specific mechanisms that include intra-, inter- and extracellular messaging, as well as contributions from systemic compartments, such as the immune system, the need to maintain a positive therapeutic ratio has never been more urgent. Importantly, mitigation and treatment strategies, whether for the clinic, emergency use following accidental or deliberate releases, or reducing occupational risk, will likely require multi-targeted approaches that involve both local and systemic intervention. From our personal perspective as five 'Women in Science', we would like to acknowledge and applaud the role that many female scientists have played in this field. We stand on the shoulders of those who have gone before, some of whom are fellow contributors to this special issue.

The Effect of Helium Ion Radiation on the Material Properties of Bone

Ionizing radiation, from both space and radiation therapy, is known to affect bone health. While there have been studies investigating changes in bone density and microstructure from radiation exposure, the effects of radiation on material properties are unknown. The current study addresses this gap by assessing bone material property changes in rats exposed to helium-4 radiation through spherical micro-indentation. Rats were exposed to a single dose of 0, 5, and 25 cGy whole body helium-4 radiation. Animals were euthanized at 7, 30, 90, or 180-days after exposure. Spherical micro-indentation was performed on axial cross sections of the femur cortical bone to determine instantaneous and relaxed shear moduli. At 90-days after exposure, the 25 cGy exposure caused a significant decline in shear modulus compared to control and 5 cGy groups. The instantaneous modulus decreased 33% and the relaxed modulus decreased 32% as compared to the sham group. This decline was followed by a recovery of both moduli, which was observed by 180-days after exposure; at 180 days, the moduli were no longer statistically different from those at 7 or 30 days. The observed decrease at 90 days, followed by recovery to baseline levels, can be attributed to the biological mechanisms involved in bone formation that were affected by radiation, bone turnover, and systemic changes in hormones due to radiation exposure. Continued assessment of the mechanisms that drive such a response in material properties may enable identification of pathways for therapeutic countermeasures against radiation exposure.

Chronic Low Dose Neutron Exposure Results in Altered Neurotransmission Properties of the Hippocampus-Prefrontal Cortex Axis in Both Mice and Rats

The proposed deep space exploration to the moon and later to Mars will result in astronauts receiving significant chronic exposures to space radiation (SR). SR exposure results in multiple neurocognitive impairments. Recently, our cross-species (mouse/rat) studies reported impaired associative memory formation in both species following a chronic 6-month low dose exposure to a mixed field of neutrons (1 mGy/day for a total dose pf 18 cGy). In the present study, we report neutron exposure induced synaptic plasticity in the medial prefrontal cortex, accompanied by microglial activation and significant synaptic loss in the hippocampus. In a parallel study, neutron exposure was also found to alter fluorescence assisted single synaptosome LTP (FASS-LTP) in the hippocampus of rats, that may be related to a reduced ability to insert AMPAR into the post-synaptic membrane, which may arise from increased phosphorylation of the serine 845 residue of the GluA1 subunit. Thus, we demonstrate for the first time, that low dose chronic neutron irradiation impacts homeostatic synaptic plasticity in the hippocampal-cortical circuit in two rodent species, and that the ability to successfully encode associative recognition memory is a dynamic, multicircuit process, possibly involving compensatory changes in AMPAR density on the synaptic surface.

Progressive increase in the complexity and translatability of rodent testing to assess space-radiation induced cognitive impairment

Ground-based rodent models have established that space radiation doses (approximately those that astronauts will be exposed to on a mission to Mars) significantly impair performance in a wide range of cognitive tasks. Over the last 40 years there has been a progressive increase in both the complexity and the translatability (to humans) of the cognitive tasks investigated. This review outlines technical and conceptual advances in space radiation rodent testing approaches, along with the advances in analytical approaches, that will make data from ground based studies more amenable to probabilistic risk analysis. While great progress has been made in determining the impact of space radiation on many advanced cognitive processes, challenges remain that need to be addressed prior to commencing deep space missions. A summary of on-going attempts to address existing knowledge gaps and the critical role that rodent studies will have in establishing the impact of space radiation on even more complex (human) cognitive tasks are presented and discussed.

Short-term metabolic disruptions in urine of mouse models following exposure to low doses of oxygen ion radiation

Molecular alterations as a result of exposure to low doses of high linear energy transfer (LET) radiation can have deleterious short- and long-term consequences on crew members embarking on long distance space missions. Oxygen ions (¹⁶O) are among the high LET charged particles that make up the radiation environment inside a vehicle in deep space. We used mass spectrometry-based metabolomics to characterize urinary metabolic profiles of male C57BL/6J mice exposed to a single dose of 0.1, 0.25 and 1.0 Gy of ¹⁶O (600 MeV/n) at 10 and 30 days post-exposure to delineate radiation-induced metabolic alterations. We recognized a significant down regulation of several classes of metabolites including cresols and tryptophan metabolites, ketoacids and their derivatives upon exposure to 0.1 and 0.25 Gy after 10 days. While some of these changes reverted to near normal by 30 days, some metabolites including p-Cresol sulfate, oxalosuccinic acid, and indoxylsulfate remained dysregulated at 30 days, suggesting long term prognosis on metabolism. Pathway analysis revealed a long-term dysregulation in multiple pathways including tryptophan and porphyrin metabolism. These results suggest that low doses of high-LET charged particle irradiation may have long-term implications on metabolic imbalance.

Consequences of space radiation on the brain and cardiovascular system

Staying longer in outer space will inevitably increase the health risks of astronauts due to the exposures to galactic cosmic rays and solar particle events. Exposure may pose a significant hazard to space flight crews not only during the mission but also later, when slow-developing adverse effects could finally become apparent. The body of literature examining ground-based outcomes in response to high-energy charged-particle radiation suggests differential effects in response to different particles and energies. Numerous animal and cellular models have repeatedly demonstrated the negative effects of high-energy charged-particle on the brain and cognitive function. However, research on the role of space radiation in potentiating cardiovascular dysfunction is still in its early stages. This review summarizes the available data from studies using ground-based animal models to evaluate the response of the brain and heart to the high-energy charged particles of GCR and SPE, addresses potential sex differences in these effects, and aims to highlight gaps in the current literature for future study.

The individual and combined effects of spaceflight radiation and microgravity on biologic systems and functional outcomes

Both microgravity and radiation exposure in the spaceflight environment have been identified as hazards to astronaut health and performance. Substantial study has been focused on understanding the biology and risks associated with prolonged exposure to microgravity, and the hazards presented by radiation from galactic cosmic rays (GCR) and solar particle events (SPEs) outside of low earth orbit (LEO). To date, the majority of the ground-based analogues (e.g., rodent or cell culture studies) that investigate the biology of and risks associated with spaceflight hazards will focus on an individual hazard in isolation. However, astronauts will face these challenges simultaneously Combined hazard studies are necessary for understanding the risks astronauts face as they travel outside of LEO, and are also critical for countermeasure development. The focus of this review is to describe biologic and functional outcomes from ground-based analogue models for microgravity and radiation, specifically highlighting the combined effects of radiation and reduced weight-bearing from rodent ground-based tail suspension via hind limb unloading (HLU) and partial weight-bearing (PWB) models, although in vitro and spaceflight results are discussed as appropriate. The review focuses on the skeletal, ocular, central nervous system (CNS), cardiovascular, and stem cells responses.

Sleep Fragmentation Exacerbates Executive Function Impairments Induced by Low Doses of Si Ions

Astronauts on deep space missions will be required to work autonomously and thus their ability to perform executive functions could be critical to mission success. Ground-based rodent experiments have shown that low (<25 cGy) doses of several space radiation (SR) ions impair various aspects of executive function. Translating ground-based rodent studies into tangible risk estimates for astronauts remains an enormous challenge, but should similar neurocognitive impairments occur in astronauts exposed to low-SR doses, a Numbers-Needed-to-Harm analysis (of the rodent data) predicts that approximately 30% of the astronauts could develop severe cognitive flexibility decrements. In addition to the health risks associated with SR exposure, astronauts have to contend with other stressors, of which inadequate sleep quantity and quality are considered to be major concerns. We have shown that a single session of fragmented sleep uncovered latent attentional set-shifting (ATSET) performance deficits in rats exposed to protracted neutron radiation that had no obvious defects in performance under rested wakefulness conditions. It is unclear if the exacerbating effect of sleep fragmentation (SF) only occurs in rats receiving protracted low-dose-rate-neutron radiation. In this study, we assessed whether SF also unmasks latent ATSET deficits in rats exposed to 5 cGy 600 MeV/n 28Si ions. Only sham and Si-irradiated rats that had good ATSET performance (passing every stage of the test on their first attempt) were selected for study. Sleep fragmentation selectively impaired performance in the more complex IDR, EDS and EDR stages of the ATSET test in the Si-irradiated rats. Set-shifting performance has rarely been affected by SR exposure in our studies conducted with rats tested under rested wakefulness conditions. The consistent SF-related unmasking of latent set-shifting deficits in both Si- and neutron-irradiated rats suggests that there is a unique interaction between sleep fragmentation and space radiation on the functionality of the brain regions that regulate performance in the IDR, EDS and EDR stages of ATSET. The uncovering of these latent SR-induced ATSET performance deficits in both Si- and neutron-irradiated rats suggests that the true impact of SR-induced cognitive impairment may not be fully evident in normally rested rats, and thus cognitive testing needs to be conducted under both rested wakefulness and sleep fragmentation conditions.

Whole brain proton irradiation in adult Sprague Dawley rats produces dose dependent and non-dependent cognitive, behavioral, and dopaminergic effects

Proton radiotherapy causes less off-target effects than X-rays but is not without effect. To reduce adverse effects of proton radiotherapy, a model of cognitive deficits from conventional proton exposure is needed. We developed a model emphasizing multiple cognitive outcomes. Adult male rats (10/group) received a single dose of 0, 11, 14, 17, or 20 Gy irradiation (the 20 Gy group was not used because 50% died). Rats were tested once/week for 5 weeks post-irradiation for activity, coordination, and startle. Cognitive assessment began 6-weeks post-irradiation with novel object recognition (NOR), egocentric learning, allocentric learning, reference memory, and proximal cue learning. Proton exposure had the largest effect on activity and prepulse inhibition of startle 1-week post-irradiation that dissipated each week. 6-weeks post-irradiation, there were no effects on NOR, however proton exposure impaired egocentric (Cincinnati water maze) and allocentric learning and caused reference memory deficits (Morris water maze), but did not affect proximal cue learning or swimming performance. Proton groups also had reduced striatal levels of the dopamine transporter, tyrosine hydroxylase, and the dopamine receptor D1, effects consistent with egocentric learning deficits. This new model will facilitate investigations of different proton dose rates and drugs to ameliorate the cognitive sequelae of proton radiotherapy.

Skilled movement and posture deficits in rat string-pulling behavior following low dose space radiation (28Si) exposure

Deep space flight missions beyond the Van Allen belt have the potential to expose astronauts to space radiation which may damage the central nervous system and impair function. The proposed mission to Mars will be the longest mission-to-date and identifying mission critical tasks that are sensitive to space radiation is important for developing and evaluating the efficacy of counter measures. Fine motor control has been assessed in humans, rats, and many other species using string-pulling behavior. For example, focal cortical damage has been previously shown to disrupt the topographic (i.e., path circuity) and kinematic (i.e., moment-to-moment speed) organization of rat string-pulling behavior count to compromise task accuracy. In the current study, rats were exposed to a ground-based model of simulated space radiation (5 cGy ²⁸Silicon), and string-pulling behavior was used to assess fine motor control. Irradiated rats initially took longer to pull an unweighted string into a cage, exhibited impaired accuracy in grasping the string, and displayed postural deficits. Once rats were switched to a weighted string, some deficits lessened but postural instability remained. These results demonstrate that a single exposure to a low dose of space radiation disrupts skilled hand movements and posture, suggestive of neural impairment. This work establishes a foundation for future studies to investigate the neural structures and circuits involved in fine motor control and to examine the effectiveness of counter measures to attenuate the effects of space radiation on fine motor control.

Can a comparison of clinical and deep space irradiation scenarios shed light on the radiation response of the brain?

Not surprisingly, our knowledge of the impact of radiation on the brain has evolved considerably. Decades of work have struggled with identifying the critical cellular targets in the brain, the latency of functional change and understanding how irradiation alters the balance between excitatory and inhibitory circuits. Radiation-induced cell kill following clinical fractionation paradigms pointed to both stromal and parenchymal targets but also defined an exquisite sensitivity of neurogenic populations of newly born cells in the brain. It became more and more apparent too, that acute (days) events transpiring after exposure were poorly prognostic of the late (months-years) waves of radiation injury believed to underlie neurocognitive deficits. Much of these gaps in knowledge persisted as NASA became interested in how exposure to much different radiation types, doses and dose rates that characterize the space radiation environment might impair central nervous system functionality, with possibly negative implications for deep space travel. Now emerging evidence from researchers engaged in clinical, translational and environmental radiation sciences have begun to fill these gaps and have uncovered some surprising similarities in the response of the brain to seemingly disparate exposure scenarios. This article highlights many of the commonalities between the vastly different irradiation paradigms that distinguish clinical treatments from occupational exposures in deep space.

Effects of partial- or whole-body exposures to 56Fe particles on brain function and cognitive performance in rats

On exploratory class missions, such as a mission to Mars, astronauts will be exposed to particles of high energy and charge (HZE particles). Exposure to HZE particles produces changes in neuronal function and can disrupt cognitive performance. Cells throughout the entire body, not just the brain, will be impacted by these particles. To determine the possible effects that irradiation of the body might have on neuronal function and cognitive performance, rats were given head-only, body-only or whole-body exposures to ⁵⁶Fe particles. Cognitive performance (novel object recognition, operant responding) was tested in one set of animals; changes in brain function (oxidative stress, neuroinflammation) was tested in a second set of rats. The results indicated that there were no consistent differences in either behavioral or neurochemical endpoints as a function of the location of the irradiation. These results suggest that radiation to the body can impact the brain, therefore it may be necessary to re-evaluate the estimates of the risk of HZE particle-induced changes in neuronal function and cognitive performance.

Sex-Specific Cognitive Deficits Following Space Radiation Exposure

The radiation fields in space define tangible risks to the health of astronauts, and significant work in rodent models has clearly shown a variety of exposure paradigms to compromise central nervous system (CNS) functionality. Despite our current knowledge, sex differences regarding the risks of space radiation exposure on cognitive function remain poorly understood, which is potentially problematic given that 30% of astronauts are women. While work from us and others have demonstrated pronounced cognitive decrements in male mice exposed to charged particle irradiation, here we show that female mice exhibit significant resistance to adverse neurocognitive effects of space radiation. The present findings indicate that male mice exposed to low doses (≤30 cGy) of energetic (400 MeV/n) helium ions (⁴He) show significantly higher levels of neuroinflammation and more extensive cognitive deficits than females. Twelve weeks following ⁴He ion exposure, irradiated male mice demonstrated significant deficits in object and place recognition memory accompanied by activation of microglia, marked upregulation of hippocampal Toll-like receptor 4 (TLR4), and increased expression of the pro-inflammatory marker high mobility group box 1 protein (HMGB1). Additionally, we determined that exposure to ⁴He ions caused a significant decline in the number of dendritic branch points and total dendritic length along with the hippocampus neurons in female mice. Interestingly, only male mice showed a significant decline of dendritic spine density following irradiation. These data indicate that fundamental differences in inflammatory cascades between male and female mice may drive divergent CNS radiation responses that differentially impact the structural plasticity of neurons and neurocognitive outcomes following cosmic radiation exposure.

Effects of single-dose protons or oxygen ions on function and structure of the cardiovascular system in male Long Evans rats

PURPOSE

Studies are required to determine whether exposures to radiation encountered during manned missions in deep space may have adverse effects on the cardiovascular system. Most of the prior studies on effects of simulated space radiation on the heart and vasculature have been performed in mouse models. To provide data from a second animal species, two studies were performed to assess effects of high-energy charged particle radiation on the heart and abdominal aorta in a rat model.

MATERIALS AND METHODS

In study A, male Long Evans rats were exposed to whole-body protons (250 MeV, 0.5 Gy) or oxygen ions (¹⁶O, 600 MeV/n, 0.5 Gy), and ultrasonography was used to measure in vivo cardiac function and blood flow parameters at 3, 5, 9 and 12 months after radiation, followed by tissue collection at 12 months. In study B, male Long Evans rats were exposed to ¹⁶O (1 GeV/n, 0.01-0.25 Gy), and hearts collected at 6 to 7 and 12 months for histology and western-blots.

RESULTS

Both protons (250 MeV) and ¹⁶O (600 MeV/n) caused a decrease in left ventricular posterior wall thickness at 3-5 months, but did not change echocardiographic measures of cardiac function. In Pulsed-wave Doppler assessment of the abdominal aorta, an increase was seen in mean velocity, peak velocity, and velocity time integral at 12 months after ¹⁶O (600 MeV/n), suggesting a change in vascular function. There were no significant changes in histopathology or histological quantification of total collagens in heart or aorta. On the other hand, an increase was seen in a 75 kDa peptide of collagen type III in the left ventricle of rats exposed to protons (250 MeV) and ¹⁶O (600 MeV/n and 1 GeV/n), suggesting that radiation caused remodeling of existing collagens in the heart. ¹⁶O (600 MeV/n and 1 GeV/n) caused increases in left ventricular protein levels of immune cell markers CD2, CD4, CD8, and CD68.

CONCLUSION

A single low dose of whole body protons or ¹⁶O in male Long Evans rats did not change cardiac function or induce gross pathological changes in the heart or aorta, but induced mild changes in vascular function and remodeling of existing collagens in the heart. Altogether, studies in prior mouse models and the current work in rats indicate minor changes in cardiac function and structure after a low dose of single-ion radiation.

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

Astronauts on interplanetary missions - such as to Mars - will be exposed to space radiation, a spectrum of highly-charged, fast-moving particles that includes 56Fe and 28Si. Earth-based preclinical studies show space radiation decreases rodent performance in low- and some high-level cognitive tasks. Given astronaut use of touchscreen platforms during training and space flight and given the ability of rodent touchscreen tasks to assess functional integrity of brain circuits and multiple cognitive domains in a non-aversive way, here we exposed 6-month-old C57BL/6J male mice to whole-body space radiation and subsequently assessed them on a touchscreen battery. Relative to Sham treatment, 56Fe irradiation did not overtly change performance on tasks of visual discrimination, reversal learning, rule-based, or object-spatial paired associates learning, suggesting preserved functional integrity of supporting brain circuits. Surprisingly, 56Fe irradiation improved performance on a dentate gyrus-reliant pattern separation task; irradiated mice learned faster and were more accurate than controls. Improved pattern separation performance did not appear to be touchscreen-, radiation particle-, or neurogenesis-dependent, as 56Fe and 28Si irradiation led to faster context discrimination in a non-touchscreen task and 56Fe decreased new dentate gyrus neurons relative to Sham. These data urge revisitation of the broadly-held view that space radiation is detrimental to cognition.

Cranial irradiation mediated spine loss is sex-specific and complement receptor-3 dependent in male mice

Cranial irradiation is the main therapeutic treatment for primary and metastatic malignancies in the brain. However, cranial radiation therapy produces long-term impairment in memory, information processing, and attention that contribute to a decline in quality of life. The hippocampal neural network is fundamental for proper storage and retrieval of episodic and spatial memories, suggesting that hippocampal signaling dysfunction could be responsible for the progressive memory deficits observed following irradiation. Previous rodent studies demonstrated that irradiation induces significant loss in dendritic spine number, alters spine morphology, and is associated with behavioral task deficits. Additionally, the literature suggests a common mechanism in which synaptic elimination via microglial-mediated phagocytosis is complement dependent and associated with cognitive impairment in aging as well as disease. We demonstrate sexual dimorphisms in irradiation-mediated alterations of microglia activation markers and dendritic spine density. Further, we find that the significant dendritic spine loss observed in male mice following irradiation is microglia complement receptor 3 (CR3)-dependent. By identifying sex-dependent cellular and molecular factors underlying irradiation-mediated spine loss, therapies can be developed to counteract irradiation-induced cognitive decline and improve patient quality of life.

Space-like 56Fe irradiation manifests mild, early sex-specific behavioral and neuropathological changes in wildtype and Alzheimer's-like transgenic mice

Space travel will expose people to high-energy, heavy particle radiation, and the cognitive deficits induced by this exposure are not well understood. To investigate the short-term effects of space radiation, we irradiated 4-month-old Alzheimer’s disease (AD)-like transgenic (Tg) mice and wildtype (WT) littermates with a single, whole-body dose of 10 or 50 cGy ⁵⁶Fe ions (1 GeV/u) at Brookhaven National Laboratory. At ~1.5 months post irradiation, behavioural testing showed sex-, genotype-, and dose-dependent changes in locomotor activity, contextual fear conditioning, grip strength, and motor learning, mainly in Tg but not WT mice. There was little change in general health, depression, or anxiety. Two months post irradiation, microPET imaging of the stable binding of a translocator protein ligand suggested no radiation-specific change in neuroinflammation, although initial uptake was reduced in female mice independently of cerebral blood flow. Biochemical and immunohistochemical analyses revealed that radiation reduced cerebral amyloid-β levels and microglia activation in female Tg mice, modestly increased microhemorrhages in 50 cGy irradiated male WT mice, and did not affect synaptic marker levels compared to sham controls. Taken together, we show specific short-term changes in neuropathology and behaviour induced by ⁵⁶Fe irradiation, possibly having implications for long-term space travel.

Meta-analysis of Cognitive Performance by Novel Object Recognition after Proton and Heavy Ion Exposures

Experimental studies of cognitive detriments in mice and rats after proton and heavy ion exposures have been performed by several laboratories to investigate possible risks to astronauts exposed to cosmic rays in space travel and patients treated for brain cancers with proton and carbon beams in Hadron therapy. However, distinct radiation types and doses, cognitive tests and rodent models have been used by different laboratories, while few studies have considered detailed dose-response characterizations, including estimates of relative biological effectiveness (RBE). Here we report on the first quantitative meta-analysis of the dose response for proton and heavy ion rodent studies of the widely used novel object recognition (NOR) test, which estimates detriments in recognition or object memory. Our study reveals that linear or linear-quadratic dose-response models of relative risk (RR) do not provide accurate descriptions. However, good descriptions for doses up to 1 Gy are provided by exponentially increasing fluence or dose-response models observed with an LET dependence similar to a classical radiation quality response, which peaks near 100-120 keV/µm and declines at higher LET values. Exponential models provide accurate predictions of experimental results for NOR in mice after mixed-beam exposures of protons and ⁵⁶Fe, and protons, ¹⁶O and ²⁸Si. RBE estimates are limited by available X-ray or gamma-ray experiments to serve as a reference radiation. RBE estimates based on use of data from combined gamma-ray and high-energy protons of low-LET experiments suggest modest RBEs, with values <8 for most heavy ions, while higher values <20 are based on limited gamma-ray data. In addition, we consider a log-normal model for the variation of subject responses at defined dose levels. The log-normal model predicts a heavy ion dose threshold of approximately 0.01 Gy for NOR-related cognitive detriments.

Effects of exposure to 12C and 4He particles on cognitive performance of intact and ovariectomized female rats

Exposure to the types of radiation encountered outside the magnetic field of the earth can disrupt cognitive performance. Exploratory class missions to other planets will include both male and female astronauts. Because estrogen can function as a neuroprotectant, it is possible that female astronauts may be less affected by exposure to space radiation than male astronauts. To evaluate the effectiveness of estrogen to protect against the disruption of cognitive performance by exposure to space radiation intact and ovariectomized female rats with estradiol or vehicle implants were tested on novel object performance and operant responding on an ascending fixed-ratio reinforcement schedule following exposure to ¹²C (290 MeV/n) or ⁴He (300 MeV/n) particles. The results indicated that exposure to carbon or helium particles did not disrupt cognitive performance in the intact rats. Estradiol implants in the ovariectomized subjects exacerbated the disruptive effects of space radiation on operant performance. Although estrogen does not appear to function as a neuroprotectant following exposure to space radiation, the present data suggest that intact females may be less responsive to the deleterious effects of exposure to space radiation on cognitive performance, possibly due to the effects of estrogen on cognitive performance.

Risks of cognitive detriments after low dose heavy ion and proton exposures

Purpose: Heavy ion and proton brain irradiations occur during space travel and in Hadron therapy for cancer. Heavy ions produce distinct patterns of energy deposition in neuron cells and brain tissues compared to X-rays leading to large uncertainties in risk estimates. We make a critical review of findings from research studies over the last 25 years for understanding risks at low dose. Conclusions: A large number of mouse and rat cognitive testing measures have been reported for a variety of particle species and energies for acute doses. However, tissue reactions occur above dose thresholds and very few studies were performed at the heavy ion doses to be encountered on space missions (<0.04 Gy/y) or considered dose-rate effects, such that threshold doses are not known in rodent models. Investigations of possible mechanisms for cognitive changes have been limited by experimental design with largely group specific and not subject specific findings reported. Persistent oxidative stress and activated microglia cells are common mechanisms studied, while impairment of neurogenesis, detriments in neuron morphology, and changes to gene and protein expression were each found to be important in specific studies. Future research should focus on estimating threshold doses carried out with experimental designs aimed at understating causative mechanisms, which will be essential for extrapolating rodent findings to humans and chronic radiation scenarios, while establishing if mitigation are needed.

Behavioral effects of space radiation: A comprehensive review of animal studies

As NASA prepares for the first manned mission to Mars in the next 20 years, close attention has been placed on the cognitive welfare of astronauts, who will likely endure extended durations in confinement and microgravity and be subjected to the radioactive charged particles travelling at relativistic speeds in interplanetary space. The future of long-duration manned spaceflight, thus, depends on understanding the individual hazards associated with the environment beyond Earth's protective magnetosphere. Ground-based single-particle studies of exposed mice and rats have, in the last 30 years, overwhelmingly reported deficits in their cognitive behaviors. However, as particle-accelerator technologies at NASA's Space Radiation Laboratory continue to progress, more realistic representations of space radiation are materializing, including multiple-particle exposures and, eventually, at multiple energy distributions. These advancements help determine how to best mitigate possible hazards due to space radiation. However, risk models will depend on delineating which particles are most responsible for specific behavioral outcomes and whether multiple-particle exposures produce synergistic effects. Here, we review the literature on animal exposures by particle, energy, and behavioral assay to inform future mixed-field radiation studies of possible behavioral outcomes.