Effects of head-only or whole-body exposure to very low doses of 4He (1000 MeV/n) particles on neuronal function and cognitive performance

Effects of HZE-Particle Exposure Location and Energy on Brain Inflammation and Oxidative Stress in Rats

Astronauts on exploratory missions will be exposed to particle radiation of high energy and charge (HZE particles), which have been shown to produce neurochemical and performance deficits in animal models. Exposure to HZE particles can produce both targeted effects, resulting from direct ionization of atoms along the particle track, and non-targeted effects (NTEs) in cells that are distant from the track, extending the range of potential damage beyond the site of irradiation. While recent work suggests that NTEs are primarily responsible for changes in cognitive function after HZE exposures, the relative contributions of targeted and non-targeted effects to neurochemical changes after HZE exposures are unclear. The present experiment was designed to further explore the role of targeted and non-targeted effects on HZE-induced neurochemical changes (inflammation and oxidative stress) by evaluating the effects of exposure location and particle energy/linear energy transfer (LET). Forty-six male Sprague-Dawley rats received head-only or body-only exposures to 56Fe particles [600 MeV/n (75 cGy) or 1,000 MeV/n (100 cGy)] or 48Ti particles [500 MeV/n (50 cGy) or 1,100 MeV/n (75 cGy)] or no irradiation (0 cGy). Twenty-four h after irradiation, rats were euthanized, and the brain was dissected for analysis of HZE-particle-induced neurochemical changes in the hippocampus and frontal cortex. Results showed that exposure to 56Fe and 48Ti ions produced changes in measurements of brain inflammation [glial fibrillary astrocyte protein (GFAP)], oxidative stress [NADPH-oxidoreductase-2 (NOX2)] and antioxidant enzymes [superoxide dismutase (SOD), glutathione S-transferase (GST), nuclear factor erythroid 2-related factor 2 (Nrf2)]. However, radiation effects varied depending upon the specific measurement, brain region, and exposure location. Although overall exposures of the head produced more detrimental changes in neuroinflammation and oxidative stress than exposures of the body, body-only exposures also produced changes relative to no irradiation, and the effect of particle energy/LET on neurochemical changes was minimal. Results indicate that both targeted and non-targeted effects are important contributors to neurochemical changes after head-only exposure. However, because there were no consistent neurochemical changes as a function of changes in track structure after head-only exposures, the role of direct effects on neuronal function is uncertain. Therefore, these findings, although in an animal model, suggest that NTEs should be considered in the estimation of risk to the central nervous system (CNS) and development of countermeasures.

Out-of-field effects: lessons learned from partial body exposure

Partial body exposure and inhomogeneous dose delivery are features of the majority of medical and occupational exposure situations. However, mounting evidence indicates that the effects of partial body exposure are not limited to the irradiated area but also have systemic effects that are propagated outside the irradiated field. It was the aim of the "Partial body exposure" session within the MELODI workshop 2020 to discuss recent developments and insights into this field by covering clinical, epidemiological, dosimetric as well as mechanistic aspects. Especially the impact of out-of-field effects on dysfunctions of immune cells, cardiovascular diseases and effects on the brain were debated. The presentations at the workshop acknowledged the relevance of out-of-field effects as components of the cellular and organismal radiation response. Furthermore, their importance for the understanding of radiation-induced pathologies, for the discovery of early disease biomarkers and for the identification of high-risk organs after inhomogeneous exposure was emphasized. With the rapid advancement of clinical treatment modalities, including new dose rates and distributions a better understanding of individual health risk is urgently needed. To achieve this, a deeper mechanistic understanding of out-of-field effects in close connection to improved modelling was suggested as priorities for future research. This will support the amelioration of risk models and the personalization of risk assessments for cancer and non-cancer effects after partial body irradiation.

Multi-Domain Touchscreen-Based Cognitive Assessment of C57BL/6J Female Mice Shows Whole-Body Exposure to 56Fe Particle Space Radiation in Maturity Improves Discrimination Learning Yet Impairs Stimulus-Response Rule-Based Habit Learning

Astronauts during interplanetary missions will be exposed to galactic cosmic radiation, including charged particles like 56Fe. Most preclinical studies with mature, "astronaut-aged" rodents suggest space radiation diminishes performance in classical hippocampal- and prefrontal cortex-dependent tasks. However, a rodent cognitive touchscreen battery unexpectedly revealed 56Fe radiation improves the performance of C57BL/6J male mice in a hippocampal-dependent task (discrimination learning) without changing performance in a striatal-dependent task (rule-based learning). As there are conflicting results on whether the female rodent brain is preferentially injured by or resistant to charged particle exposure, and as the proportion of female vs. male astronauts is increasing, further study on how charged particles influence the touchscreen cognitive performance of female mice is warranted. We hypothesized that, similar to mature male mice, mature female C57BL/6J mice exposed to fractionated whole-body 56Fe irradiation (3 × 6.7cGy 56Fe over 5 days, 600 MeV/n) would improve performance vs. Sham conditions in touchscreen tasks relevant to hippocampal and prefrontal cortical function [e.g., location discrimination reversal (LDR) and extinction, respectively]. In LDR, 56Fe female mice more accurately discriminated two discrete conditioned stimuli relative to Sham mice, suggesting improved hippocampal function. However, 56Fe and Sham female mice acquired a new simple stimulus-response behavior and extinguished this acquired behavior at similar rates, suggesting similar prefrontal cortical function. Based on prior work on multiple memory systems, we next tested whether improved hippocampal-dependent function (discrimination learning) came at the expense of striatal stimulus-response rule-based habit learning (visuomotor conditional learning). Interestingly, 56Fe female mice took more days to reach criteria in this striatal-dependent rule-based test relative to Sham mice. Together, our data support the idea of competition between memory systems, as an 56Fe-induced decrease in striatal-based learning is associated with enhanced hippocampal-based learning. These data emphasize the power of using a touchscreen-based battery to advance our understanding of the effects of space radiation on mission critical cognitive function in females, and underscore the importance of preclinical space radiation risk studies measuring multiple cognitive processes, thereby preventing NASA's risk assessments from being based on a single cognitive domain.

Machine Learning Models to Predict Cognitive Impairment of Rodents Subjected to Space Radiation

This research uses machine-learned computational analyses to predict the cognitive performance impairment of rats induced by irradiation. The experimental data in the analyses is from a rodent model exposed to ≤15 cGy of individual galactic cosmic radiation (GCR) ions: 4He, 16O, 28Si, 48Ti, or 56Fe, expected for a Lunar or Mars mission. This work investigates rats at a subject-based level and uses performance scores taken before irradiation to predict impairment in attentional set-shifting (ATSET) data post-irradiation. Here, the worst performing rats of the control group define the impairment thresholds based on population analyses via cumulative distribution functions, leading to the labeling of impairment for each subject. A significant finding is the exhibition of a dose-dependent increasing probability of impairment for 1 to 10 cGy of 28Si or 56Fe in the simple discrimination (SD) stage of the ATSET, and for 1 to 10 cGy of 56Fe in the compound discrimination (CD) stage. On a subject-based level, implementing machine learning (ML) classifiers such as the Gaussian naïve Bayes, support vector machine, and artificial neural networks identifies rats that have a higher tendency for impairment after GCR exposure. The algorithms employ the experimental prescreen performance scores as multidimensional input features to predict each rodent's susceptibility to cognitive impairment due to space radiation exposure. The receiver operating characteristic and the precision-recall curves of the ML models show a better prediction of impairment when 56Fe is the ion in question in both SD and CD stages. They, however, do not depict impairment due to 4He in SD and 28Si in CD, suggesting no dose-dependent impairment response in these cases. One key finding of our study is that prescreen performance scores can be used to predict the ATSET performance impairments. This result is significant to crewed space missions as it supports the potential of predicting an astronaut's impairment in a specific task before spaceflight through the implementation of appropriately trained ML tools. Future research can focus on constructing ML ensemble methods to integrate the findings from the methodologies implemented in this study for more robust predictions of cognitive decrements due to space radiation exposure.

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.

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.

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.

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.

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.