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

Abstract

As NASA prepares for a mission to Mars, concerns regarding the health risks associated with deep space radiation exposure have emerged. Until now, the impacts of such exposures have only been studied in animals after acute exposures, using dose rates ∼1.5×10⁵ higher than those actually encountered in space. Using a new, low dose-rate neutron irradiation facility, we have uncovered that realistic, low dose-rate exposures produce serious neurocognitive complications associated with impaired neurotransmission. Chronic (6 month) low-dose (18 cGy) and dose rate (1 mGy/d) exposures of mice to a mixed field of neutrons and photons result in diminished hippocampal neuronal excitability and disrupted hippocampal and cortical long-term potentiation. Furthermore, mice displayed severe impairments in learning and memory, and the emergence of distress behaviors. Behavioral analyses showed an alarming increase in risk associated with these realistic simulations, revealing for the first time, some unexpected potential problems associated with deep space travel on all levels of neurological function.

Keywords: cognitive dysfunction; electrophysiology; long-term potentiation; low dose-rate; neutrons; space radiation.

Figure 1.

Neutron irradiation alters the electrophysiological properties of CA1 pyramidal neurons. All data are from whole-cell current-clamp recordings of CA1 pyramidal neurons from the superficial layer of the dorsal hippocampus, 6 months following the completion of 18 cGy neutron irradiation. A, RMP was decreased following neutron irradiation. B, Representative examples of responses to a range of brief current injections in neurons from 0 cGy control and 18 cGy mice. C, The rheobase current required to evoke an action potential was greater in neutron-irradiated animals compared with concurrent controls. D, Action potential frequency was attenuated across a range of current injections in 18 cGy neurons, (E) including in a subset of neurons with an RMP within −69.6 ± 3 mV. F, There was no significant alteration in input resistance. N = 8/7 animals, 29/28 cells (0 cGy and 18 cGy, respectively), except for E, where N = 23/16 cells. Gardner–Altman estimation plots show raw data on the left axis and a bootstrapped sampling distribution on the right axis. A black dot depicts the mean difference between groups and the 95% CI is indicated by the ends of the vertical black bars. Data are presented as mean ± SEM for D and E. *p < 0.05, ***p < 0.001 (MLM regression or two-way ANOVA).

Figure 2.

Excitatory synaptic signaling to CA1 pyramidal neurons is decreased following neutron irradiation. All data are from whole-cell recordings of CA1 pyramidal neurons from the superficial layer of the dorsal hippocampus voltage-clamp at −65 mV, 6 months following the completion of 18 cGy neutron irradiation. A, Representative examples of recordings containing spontaneous EPSCs from neurons from neutron-irradiated animals. B, sEPSCs were less frequent in neurons from neutron-irradiated animals. C, Examples of EPSCs in representative neurons from neutron-irradiated animals. Light lines show individual sEPSCs, whereas the darker line displays the average sEPSC during a 200 s recording from that neuron. Neither the sEPSC amplitude (D) nor sEPSC charge transfer (E) was altered by neutron irradiation. N = 8/7 animals, 24/25 cells (0 cGy and 18 cGy, respectively). Gardner–Altman estimation plots show raw data on the left axis and a bootstrapped sampling distribution on the right axis. A black dot depicts the mean difference between groups and the 95% CI is indicated by the ends of the vertical black bars. *p < 0.05 (MLM regression).

Figure 3.

Neutron irradiation alters long-term synaptic plasticity in the hippocampal area CA1 and ventral medial prefrontal cortex. A, B, Extracellular field recordings following stimulation of the Schaffer-commissural projections to the proximal apical dendrites of field CA1b of the dorsal hippocampus, 6 months following completion of the 18 cGy neutron irradiation. A, Following a stable 20 min baseline recording, a single train of TBS was applied and baseline recordings were resumed for an additional 60 min. The time course shows that TBS-induced LTP was markedly reduced in slices from irradiated mice compared with slices from 0 cGy control mice. Inset, Representative traces collected during baseline (solid line) and 60 min post-TBS (dotted line). B, Field EPSP slope was significantly reduced 60 min post-TBS in slices from the neutron-irradiated mice compared with controls. C, D, Field responses recorded in cortical layer III following stimulation of glutamatergic inputs in cortical layer IV of the ventral medial prefrontal cortex. C, Similar results were obtained as shown in A. TBS-induced LTP was nearly completely blocked in slices from neutron-irradiated mice relative to controls. Inset, Representative traces collected during baseline (solid line) and 60 min post-TBS (dotted line). D, Field EPSP slope was significantly reduced 60 min post-TBS in slices from 18 cGy mice compared with controls. N = 9 slices per group. ***p < 0.0001 (two-tailed t test).

Figure 4.

Space-relevant, low dose-rate neutron irradiation disrupts cognition. A, B, Social interaction behavior testing of mice 3 months after the conclusion of neutron irradiation (18 cGy cumulative dose) reveals an increase in avoidance behavior during 10 min trials compared with un-irradiated control mice, whereas the total time spent interacting did not change. C, Disrupted performance on a NOR task by irradiated mice indicates a significant decrement in novelty recognition memory. D, The spatial exploration behavior analyzed for the OiP task demonstrates that neutron irradiation impairs spatial memory retention as manifested in a reduced preference to explore the novel placement of objects. Data are presented as mean ± SEM (A, N = 8 per group; B–D, N = 14 per group). *p < 0.05 (Mann–Whitney’s two-tailed, nonparametric t test) compared with controls.

Figure 5.

Space-relevant, low dose-rate neutron irradiation elicits anxiety-like behavior. A, Neutron irradiation increases anxiety-like behavior as the irradiated mice exhibit reduced numbers of transitions between the light and dark chambers in the LDB test. B, Irradiated mice did not show depression-like behavior on the FST. C, Last, neutron irradiation compromised fear extinction memory function. Mice showed elevated freezing following a series of three tone–shock pairings (0.6 mA, T₁–T₃). Subsequently, 24 h later, fear extinction training was administered every 24 h (20 tones) for 3 d. All mice showed a gradual decrease in the freezing behavior (Days 1–3), however, irradiated mice spent a significantly higher time in freezing compared with controls. Twenty-four hours after extinction training, control mice showed abolished fear memory compared with neutron-exposed mice (C1, inset). Data are presented as mean ± SEM (A–C, C1, N = 14 per group). *p < 0.05, **p < 0.01, ***p < 0.001 (Mann–Whitney’s two-tailed, nonparametric t test) compared with controls.