Quantitative Analysis of Effects of a Single 60Co Gamma Ray Point Exposure on Time-Dependent Change in Locomotor Activity in Rats

Fatigue is one of the earliest nonspecific symptoms of radiation exposure in humans, but its etiology, mechanism, and dose dependency remain unexplained. Investigating initial behavioral changes caused by irradiation of animals might provide important information to aid understanding of early health effects of radiation exposure and clinical features of radiation injury. Although previous studies in rodents suggested that radiation exposure leads to reduced activity, detailed properties of the effects were unrevealed due to a lack of proper statistical analysis, which is needed to better elucidate details of changes in locomotor activity. Ten-week-old male Wistar rats were subjected to single point external whole-body irradiation with 60Co gamma rays at 0, 2.0, 3.5, and 5.0 Gy (4 rats per group). Infrared sensors were used to continuously record locomotor activity of each rat. Cumulative number of movements during the night was defined as “activity” for each day. A non-linear mixed effects model accounting for individual differences and daily fluctuation of activity was applied to analyze the rats’ longitudinal locomotor data. Despite a small number of animals per group, our statistical method successfully revealed characteristics of the changes in locomotor activity after radiation exposure, showing that 1) reduction in activity occurred immediately—and in a dose-dependent manner—after irradiation and 2) recovery to pre-irradiation levels required almost one week, with the same recovery rate in each dose group. In addition to improving our understanding of radiation effects on locomotor activity, this statistical framework should be useful to analyze other data with similar structure.