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.

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.

Similar Loss of Executive Function Performance after Exposure to Low (10 cGy) Doses of Single (4He) Ions and the Multi-Ion GCRSim Beam.

While astronauts are trained to deal with multiple issues that they are likely to encounter during a mission, it is likely that some problems will arise that astronauts have no direct experience in resolving. During International Space Station (ISS) missions, astronauts can rely on Mission Control to help resolve complex problems, however during the long-duration space missions planned to the Moon and Mars, astronauts will have to act more autonomously, thus the ability of astronauts to conduct executive function will be critical for problem solving during deep space missions. Several studies have shown that exposure to space radiation results in decreased executive function performance. However, to date these studies have used single ions, whereas there is a complex mixture of ion species and energies within the space-radiation spectrum that astronauts will be exposed to. Thus, there is some concern that the neurocognitive impairments reported from single ion studies will not be representative of the severity, frequency or nature of cognitive deficits that arise following exposure to more complex space-radiation spectra. The current study has determined the relative impact that isodoses of He ions or the simplified 6-ion-galactic cosmic ray simulation (GCRSim) beams had on the performance of male Wistar rats in executive function tasks, attentional set shifting (ATSET) task and unconstrained cognitive flexibility (UCFlex). Exposure to 10 cGy GCRSim induced performance deficits in the simple discrimination (SD) stage of the ATSET task, which appears to be universally impacted by all space-radiation ions studied to date. The magnitude of the SD performance decrements in the GCRSim-irradiated rats were comparable to those observed in He-irradiated rats. Importantly, space-radiation exposure does not appear to decrease the ability of rats to identify the key cues in the ATSET task, but increased the time/number of iterations required to successfully find the solution. Practice effect (PE) analysis (comparing prescreen to the postirradiation SD performance) revealed that while the sham-treated rats completed the second ATSET task in 30% less time than they did the prescreen ATSET test (despite the perceptual domain of the relevant (rewarded) cue being changed), the space-radiation-exposed rats took 50% longer to do so. The space-radiation-induced delay in problem solving was not confined to the ATSET task, but was also observed when rats were screened for UCFlex performance. Should similar changes occur in astronauts, these data raise the possibility that space-radiation exposure would reduce in-flight improvement in performance in repetitive tasks (PE) and may lead to a reduced ability to utilize transitive inference from "similar" problems to solve issues that have not been previously encountered.

Similar Loss of Executive Function Performance after Exposure to Low (10 cGy) Doses of Single (4He) Ions and the Multi-Ion GCRSim Beam.

While astronauts are trained to deal with multiple issues that they are likely to encounter during a mission, it is likely that some problems will arise that astronauts have no direct experience in resolving. During International Space Station (ISS) missions, astronauts can rely on Mission Control to help resolve complex problems, however during the long-duration space missions planned to the Moon and Mars, astronauts will have to act more autonomously, thus the ability of astronauts to conduct executive function will be critical for problem solving during deep space missions. Several studies have shown that exposure to space radiation results in decreased executive function performance. However, to date these studies have used single ions, whereas there is a complex mixture of ion species and energies within the space-radiation spectrum that astronauts will be exposed to. Thus, there is some concern that the neurocognitive impairments reported from single ion studies will not be representative of the severity, frequency or nature of cognitive deficits that arise following exposure to more complex space-radiation spectra. The current study has determined the relative impact that isodoses of He ions or the simplified 6-ion-galactic cosmic ray simulation (GCRSim) beams had on the performance of male Wistar rats in executive function tasks, attentional set shifting (ATSET) task and unconstrained cognitive flexibility (UCFlex). Exposure to 10 cGy GCRSim induced performance deficits in the simple discrimination (SD) stage of the ATSET task, which appears to be universally impacted by all space-radiation ions studied to date. The magnitude of the SD performance decrements in the GCRSim-irradiated rats were comparable to those observed in He-irradiated rats. Importantly, space-radiation exposure does not appear to decrease the ability of rats to identify the key cues in the ATSET task, but increased the time/number of iterations required to successfully find the solution. Practice effect (PE) analysis (comparing prescreen to the postirradiation SD performance) revealed that while the sham-treated rats completed the second ATSET task in 30% less time than they did the prescreen ATSET test (despite the perceptual domain of the relevant (rewarded) cue being changed), the space-radiation-exposed rats took 50% longer to do so. The space-radiation-induced delay in problem solving was not confined to the ATSET task, but was also observed when rats were screened for UCFlex performance. Should similar changes occur in astronauts, these data raise the possibility that space-radiation exposure would reduce in-flight improvement in performance in repetitive tasks (PE) and may lead to a reduced ability to utilize transitive inference from "similar" problems to solve issues that have not been previously encountered.

Rapid loss of fine motor skills after low dose space radiation exposure.

Sensorimotor function, motivation, and attentional processes are fundamental aspects of behavioral organization during skilled tasks. NASA's planned expedition to Mars will expose astronauts to space radiation (SR) that has the potential to impair performance in mission critical tasks. Impairments in task accuracy and movement kinematics have been previously reported during string-pulling behavior ~7 months after SR exposure. If similar SR-induced sensorimotor deficits emerge at earlier times, then astronauts may have compromised in-flight performance disruptions while performing skilled tasks in critical situations, such as when manipulating controls or performing seat egress. Due to the possibility that such performance losses may compromise mission success, it is critical to determine if sensorimotor, motivation, or attentional deficits occur acutely after SR exposure at a time point that corresponds to in-flight performance. Male Wistar rats were thus exposed to either 10 cGy simplified galactic cosmic radiation (GCRsim), 10 cGy 4Helium (4He), or no radiation at all (Sham), and string-pulling behavior was assessed approximately 72 h later. Following exposure to SR, rats (4He) took more time to approach the string to initiate string-pulling behavior and to pull in the string to reach the Cheerio (4He and GCRsim) relative to Sham rats. 4He-exposed rats also exhibited a greater number of misses and less contacts relative to both Sham and GCRsim-exposed rats. Further, rats exposed to 4He demonstrated less concentrated reach endpoints with both the left and right hands compared to GCR-exposed rats. This work suggests that sensorimotor function and motivation and/or attentional processes were impaired 72 h after 4He-radiation exposure.

Dissecting Differential Complex Behavioral Responses to Simulated Space Radiation Exposures.

This study has established the impact that space radiation exposure has on the capability of rats to successfully negotiate behavioral tasks of increasing complexity. Rats previously exposed to a low dose (10 cGy) of either 4He ions or a cocktail of 6 ions that simulates the galactic cosmic ray spectrum (GCRSim) were screened initially on an attentional set shifting (ATSET) task that provides a measure of executive function. Rats that exhibited superior ATSET performance were then selected for follow up behavioral assessments designed to evaluate how the cohort of "good performers" would fare when presented with a novel behavioral paradigm termed the Associative Recognition Memory and Interference Touchscreen (ARMIT) task. Central to this approach was to discriminate if/how adaptive problem solving would be impacted by changing the options of associative cues presented over several learning sessions to obtain a reward under time constraints using this newly designed touch screen-based task. Data from these studies indicated that when faced with an increased cognitive load, possibly due to interference from prior associative recognition memories, rats exhibited impairments in their capability to negotiate task dynamics and efficiently engage abstract reasoning. Interestingly, while exposure to the GCRSim adversely impacted problem-solving capabilities, single ion exposure did not, pointing to the nuances of space radiation exposure on CNS functionality. Since the selected behavioral paradigms exhibit strong cross-species correlates, data suggest that rodents succumb to increased task rigor as observed in humans, and make similar mistakes when challenged with the interference of overlapping associative memories. Furthermore, data clearly points to the limitations of over-reliance on a single cognitive endpoint that may underestimate global neurocognitive risk due to space radiation exposure.

Exposure to Low (≤10 cGy) Doses of 4He Particles Leads to Increased Social Withdrawal and Loss of Executive Function Performance.

Astronauts on the planned mission to Mars will be exposed to galactic cosmic radiation (GCR), with proton and He particles accounting (in approximately equal amounts) for ∼75% of the equivalent dose. Exposure to ≤15 cGy of space radiation ions with Z ≥ 15 particles has been shown to impair various executive functions, including attentional set shifting and creative problem-solving in rats. Executive functions also regulate social interactions and mood. Should space radiation exposure alter these executive functions as it does cognitive flexibility, there is the possibility of altered interactions among crew members and team cooperativity during prolonged space exploration. This study characterized the effects of ≤10 cGy 400 MeV/n of 4He particles on cognitive flexibility and social interaction (within freely interacting dyads) in male Wistar rats. Exposure to ≥1 cGy 4He ions induced deficits in the SD and/or CD stages of the attentional set shifting (ATSET) task, as reported after exposure to Z ≥ 15 space radiation ions. Should similar effects occur in astronauts, these data suggest that they would have a reduced ability to identify key events in a new situation and would be more easily distracted by extraneous variables. The irradiated rats were also screened for performance in a task for unconstrained cognitive flexibility (UCFlex), often referred to as creative problem-solving. There was a marked dose-dependent change in UCFlex performance with ∼30% of rats exposed to 10 cGy being unable to solve the problem, while the remaining rats took longer than the sham-irradiated animals to resolve the problem. Importantly, performance in the ATSET test was not indicative of UCFlex performance. From a risk assessment perspective, these findings suggest that a value based on a single behavioral end point may not fully represent the cognitive deficits induced by space radiation, even within the cognitive flexibility domain. Rats that received 5 cGy 4He ion irradiation had a significantly lower level of interaction toward their sham-irradiated partners in a non-anxiogenic (uncaged) dyad interactions study. This is consistent with the social withdrawal previously observed in space radiation-exposed male mice in a three-chamber test. 4He-irradiated rats exhibited a significantly higher incidence and duration of self-grooming, which is even more concerning, given that their dyad partners were able to physically interact with the irradiated rats (i.e., touching/climbing over them). This study has established that exposure of male rats to "light" ions such as He affects multiple executive functions resulting in deficits in both sociability and cognitive flexibility, and possibly affective behavior (reward valuation). Further studies are needed to determine if these space radiation-induced co-morbidities are concomitantly induced within individual rats.

Space Radiation-Induced Alterations in the Hippocampal Ubiquitin-Proteome System.

Exposure of rodents to <20 cGy Space Radiation (SR) impairs performance in several hippocampus-dependent cognitive tasks, including spatial memory. However, there is considerable inter-individual susceptibility to develop SR-induced spatial memory impairment. In this study, a robust label-free mass spectrometry (MS)-based unbiased proteomic profiling approach was used to characterize the composition of the hippocampal proteome in adult male Wistar rats exposed to 15 cGy of 1 GeV/n 48Ti and their sham counterparts. Unique protein signatures were identified in the hippocampal proteome of: (1) sham rats, (2) Ti-exposed rats, (3) Ti-exposed rats that had sham-like spatial memory performance, and (4) Ti-exposed rats that impaired spatial memory performance. Approximately 14% (159) of the proteins detected in hippocampal proteome of sham rats were not detected in the Ti-exposed rats. We explored the possibility that the loss of the Sham-only proteins may arise as a result of SR-induced changes in protein homeostasis. SR-exposure was associated with a switch towards increased pro-ubiquitination proteins from that seen in Sham. These data suggest that the role of the ubiquitin-proteome system as a determinant of SR-induced neurocognitive deficits needs to be more thoroughly investigated.

Sleep fragmentation exacerbates executive function impairments induced by protracted low dose rate neutron exposure.

PURPOSE: Astronauts on the planned missions to Mars are expected to have to work more autonomously than on previous missions. Thus mission success may be influenced by the astronauts' ability to respond quickly to unexpected problems, processes that require several executive functions. The purpose of this study was to determine the impact that prolonged low dose and low dose rate exposure to neutrons had on two executive functions, and whether the severity and incidence of cognitive impairment was altered by sleep fragmentation. MATERIALS AND METHODS: In this study we assessed the impact that prolonged (six month) low dose rate neutron exposure had on the ability of male Wistar rats to perform in two executive function tasks (i.e. attentional set shifting (ATSET) - a constrained cognitive flexibility task and the UCFlex assay - an unconstrained cognitive flexibility task). In recognition of the fact that astronauts also have to contend with inadequate sleep quantity and quality for much of their time in space, we determined the impact that relatively mild sleep disruption had on the ability to perform in the ATSET test in sham and neutron-irradiated rats. RESULTS: Chronic low dose (18 cGy) and dose-rate (1 mGy/day) exposure of rats to mixed neutron and photon over the course of six months resulted in significant impairment of simple discrimination (SD) performance. Should similar effects occur in astronauts subjected to low dose rate exposure to Space Radiation, the impairment of SD performance would result in a decreased ability to identify and learn the 'rules' required to respond to a new task or situation. Analysis of the behavioral data by kernel density estimation revealed that 40% of rats had severe ATSET impairments. This value may be a best-case scenario because exposure to neutrons also adversely impacted performance in the UCFlex task. Furthermore, when the good performing rats were reevaluated after they had been subjected to sleep fragmentation, additional ATSET performance decrements were observed in the set shifting stages of the ATSET test, with only 7.4% of the neutron exposed rats able to successfully perform ATSET under normal and sleep fragmented conditions, as opposed to ∼55% of shams. CONCLUSION: Protracted low dose and low dose rate neutron exposures impairs executive functions in a high percentage of rats that were normally rested, however further detriments in performance become evident when the rats are subjected to sleep fragmentation.

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.

Altered Cognitive Flexibility and Synaptic Plasticity in the Rat Prefrontal Cortex after Exposure to Low (≤15 cGy) Doses of 28Si Radiation.

This study has established the impact that 1-15 cGy 600 MeV/n 28Si radiation had on cognitive flexibility performance, glutamatergic synaptic transmission and plasticity in the prelimbic area (PrL) of the medial prefrontal cortex (mPFC) of ∼10-month-old (at the time of irradiation) male Wistar rats. Exposure to 1 cGy 600 MeV/n 28Si ions resulted in significantly impaired performance in the simple (SD) and compound discrimination (CD) stages of the attentional set shifting (ATSET) task. However, there was a pronounced non-linear dose response for cognitive impairment. Should similar effects occur in astronauts, the impairment of SD performance would result in a decreased ability to identify and learn the "rules" required to respond to new tasks/situations, while the impaired CD performance would result in a decreased ability to identify and maintain focus on relevant aspects of the task being conducted. The irradiated rats were also screened for performance in a task for unconstrained cognitive flexibility (UCFlex), often referred to as creative problem solving. Exposure to 1, 5 and 10 cGy resulted in a significant reduction in UCFlex performance, in an apparent all-or-none responsive manner. Importantly, performance in the ATSET test was not indicative of UCFlex performance. From a risk assessment perspective, these findings suggest that a value based on a single behavioral end point may not fully represent the cognitive deficits induced by space radiation, even within the cognitive flexibility domain. After completion of the cognitive flexibility testing, in vitro electrophysiological assessments of glutamatergic synaptic transmission and plasticity were performed in slices of the PrL cortex of 10 cGy irradiated rats. Extracellular recordings of field excitatory postsynaptic potentials revealed that radiation significantly decreased long-term depression in layer L5. Patch-clamp whole cell recordings in pyramidal neurons of the L2-3 revealed reduced frequency of spontaneous excitatory postsynaptic currents indicating alterations in presynaptic glutamate release and impaired neuronal spiking (e.g., decreased action potential amplitudes) in irradiated neurons. However, there was no obvious correlation between magnitudes of these electrophysiological decrements and the cognitive performance status of the irradiated rats. These data suggest that while radiation-induced changes in synaptic plasticity in the PrL cortex may be associated with cognitive impairment, they are most likely not the sole determinant of the incidence and severity of such impairments.

Exposure to ≤15 cGy of 600 MeV/n 56Fe Particles Impairs Rule Acquisition but not Long-Term Memory in the Attentional Set-Shifting Assay.

On future missions into deep space, astronauts will be required to work more autonomously than on previous missions, and thus their ability to perform executive functions could be critical to mission success. In this study, we determined the effect that ≤15 cGy of 600 MeV/n 56Fe particles has on attentional set-shifting (ATSET) performance of ∼10 month-old (at the time of irradiation) male Wistar rats that had been prescreened for their ability to perform the task. Exposure to 1-15 cGy of 56Fe particles leads to a significant impairment in compound discrimination (CD) performance. Should similar effects occur in astronauts, an impaired ability to execute CD would result in a decreased ability to identify and maintain focus on relevant aspects of the task being performed. The use of rats that had been prescreened for ATSET performance helped to establish that working memory of the rules for the food reward remained intact (for at least 100 days) even after 15 cGy irradiation with 600 MeV/n 56Fe particles, but that 56Fe radiation exposure affected associative cue learning/acquisition rather than an intrinsic inability to perform the CD tasks. Our data suggest that declarative memory, and the ability to transitively infer established rules, also remained intact in the irradiated rats. Thus, should similar effects occur in astronauts, 56Fe-induced CD performance deficits may only be manifested in scenarios where astronauts are required to transitively apply their knowledge to solve problems that they have not previously encountered; nevertheless, potentially one-third of astronauts may not be able to perform event-critical tasks correctly. The implication of this data, from a probabilistic risk assessment perspective, is that cognitive performance studies that use naïve rodents, thus requiring task rule acquisition as well as task performance, are likely to overestimate the risk of 56Fe-induced cognitive deficits.