Early cognitive changes due to whole body γ-irradiation: a behavioral and diffusion tensor imaging study in mice

Radiation-induced aberration in the neuronal integrity and cognitive functions are well known. However, there is a lacuna between sparsely reported immediate effects and the well documented delayed effects of radiation on cognitive functions. The present study was aimed at investigating the radiation-dose dependent incongruities in the early cognitive changes, employing two approaches, behavioral functions and diffusion tensor imaging (DTI). Six-month old female C57BL/6 mice were exposed to whole-body doses of 2, 5 and 8 Gy of γ-radiation and 24 h after exposure, the stress and anxiety levels were examined in the open-field test (OFT). Forty-eight hours after irradiation, the hippocampal dependent recognition memory was observed by the novel object recognition task (NORT), and the cognitive functions related to memory processing and recall were tested using the elevated plus maze (EPM). Magnetic resonance imaging, including diffusion tensor imaging (DTI) was done at 48-hour post-irradiation to visualize microstructural damage in brain parenchyma. Our results indicate a complex dose independent effect on the cognitive functions immediately after exposure to gamma rays. Radiation exposure caused short-term memory dysfunctions at lower doses, which were seen to be abrogated at higher doses, but the long-term memory processing was disrupted at higher doses. The hippocampus emerged as one of the sensitive regions to be affected by whole-body exposure to gamma rays, which led to profound immediate alterations in cognitive functions. Furthermore, the results indicate a cognitive recovery process, which might be dependent on the extent of damage to the hippocampal region. The present study also emphasizes the importance of further research to unravel the complex pattern of neurobehavioral responses immediately following ionizing radiation exposure.