The impact of low-dose ionizing radiation (IR) on the human brain has recently attracted attention due to the increased use of IR for diagnostic purposes. The aim of this study was to investigate low-dose radiation response in the hippocampus. Female B6C3F1 mice were exposed to total body irradiation with 0 (control), 0.063, 0.125, or 0.5 Gy. Quantitative label-free proteomic analysis of the hippocampus was performed after 24 months. CREB signaling and CREB-associated pathways were affected at all doses. The lower doses (0.063 and 0.125 Gy) induced the CREB pathway, whereas the exposure to 0.5 Gy deactivated CREB. Similarly, the lowest dose (0.063 Gy) was antiinflammatory, reducing the number of activated microglia. In contrast, induction of activated microglia and reactive astroglia was found at 0.5 Gy, suggesting increased inflammation and astrogliosis, respectively. The apoptotic markers BAX and cleaved CASP-3 and oxidative stress markers were increased only at the highest dose. Since the activated CREB pathway plays a central role in learning and memory, these data suggest neuroprotection at the lowest dose (0.063 Gy) but neurodegeneration at 0.5 Gy. The response to 0.5 Gy resembles alterations found in healthy aging and thus may represent radiation-induced accelerated aging of the brain.

The increasing use of low-dose ionizing radiation in medicine requires a systematic study of its long-term effects on the brain, behaviour and its possible association with neurodegenerative disease vulnerability. Therefore, we analysed the long-term effects of a single low-dose irradiation exposure at 10 weeks of age compared to medium and higher doses on locomotor, emotion-related and sensorimotor behaviour in mice as well as on hippocampal glial cell populations. Materials and methods: We determined the influence of radiation dose (0, 0.063, 0.125 or 0.5 Gy), time postirradiation (4, 12 and 18 months p.i.), sex and genotype (wild type versus mice with Ercc2 DNA repair gene point mutation) on behaviour. Results: The high dose (0.5 Gy) had early-onset adverse effects at 4 months p.i. on sensorimotor recruitment and lateonset negative locomotor effects at 12 and 18 months p.i. Notably, the low dose (0.063 Gy) produced no early effects but subtle late-onset (18 months) protective effects on sensorimotor recruitment and exploratory behaviour. Quantification and morphological characterization of the microglial and the astrocytic cells of the dentate gyrus 24 months p.i. indicated heightened immune activity after high dose irradiation (0.125 and 0.5 Gy) while conversely, low dose (0.063 Gy) induced more neuroprotective features. Conclusion: This is one of the first studies demonstrating such long-term and late-onset effects on brain and behaviour after a single radiation event in adulthood.

A better alignment of preclinical and clinical neurobiological measures could help improve neuropsychiatric disease therapeutic development. This unit describes a compendium of hypothesis-driven neuroanatomical phenotyping strategies to be employed in genetic mouse models. Using neuropsychiatric disease-based alterations as a guide, these are histological and immunohistochemical methodologies also applied to human tissue. They include quantification assays of neurochemical-, newly born neuron- and glial-cell markers, synaptic proteins, regional volumetrics, dendritic complexity and spine number as well as an index of excitation/inhibition balance. The techniques can be implemented in isolation or to complement concordant behavioral and electrophysiological analyses. Each outcome will provide functional detail necessary to decipher underlying neural circuit abnormalities associated with a brain-related phenotype in mice. Experimental design, timing, anticipated results and potential pitfalls are discussed. © 2018 by John Wiley & Sons, Inc.

Garrett, L. et al. Complex long-term effects of radiation on adult mouse behaviour. Radiat.Res.

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