Effects of internal exposure to neutron-activated (MnO2)-Mn-56 powder on locomotor activity in rats

At the detonation of the atomic bombing in Hiroshima and Nagasaki, a significant amount of radionuclides was produced by the neutron induced activation. The residual radiation from the explosion is crucial to the health risk of the people who entered these cities after the bombing and might have inhaled these radioactive materials. Because Mn-56 is one of the major radionuclides produced in soil and have not been studied until now, we had conducted a series of experiments using rats to investigate the biological impacts of exposure of (MnO2)-Mn-56 particles. In these experiments, the rats' spontaneous locomotor activity was also assessed to examine the possible effects of Mn-56 on their behavior. However, the locomotor activity data obtained from an individual experiment failed to identify radiation effects due to the large variation among animals and the small sample size. In the present study, all available data from our previous studies on (MnO2)-Mn-56 exposure (0.02-0.15 Gy of whole-body doses) as well as Co-60-gamma exposure (at 2-5 Gy of whole-body doses) were pooled. Our statistical method, which takes into account individual differences and daily fluctuations, successfully identified a decrease in locomotor activity caused by (MnO2)-Mn-56 exposure, where the changes were gradual and reached the maximum reduction around 2 weeks after exposure. In contrast, exposure to Co-60-gamma rays produced the highest decline of activity within one day. These results suggest that internal exposure to Mn-56 at whole-body doses of even less than 0.15 Gy may have a long-lasting impact on locomotor activity.