The findings, published in the latest issue of Environmental Toxicology and Chemistry, found that low, chronic doses of gamma radiation at 50 to 200 times background levels had beneficial effects on the stress axis and the immune axis of natural populations of meadow voles. The paper provides evidence of hormesis from the only large-scale, long-term experimental field test ever conducted on the chronic effects of gamma radiation on mammals.

Hormesis is defined as a phenomenon in which low doses of an otherwise harmful agent can result in stimulatory or beneficial effects. This phenomenon has been observed in a broad range of chemicals including alcohol and its metabolites, antibiotics, hydrocarbons, herbicides, insecticides and fungicides, as well as physical processes such as radiation exposure. The effects of hormesis have been observed in a wide range of organisms, from microbes and fungi to plants and animals. Hormetic responses are varied in form and include increased longevity; growth, reproductive and physiological responses; and metabolic effects.

"Exactly how low-level radiation causes a hormetic response remains uncertain because few laboratories have studied the pathology or physiology of mammals exposed throughout life to dose rates below those causing detrimental effects," said Professor Rudy Boonstra of the Centre for the Neurobiology of Stress and Department of Zoology. “This study provides a potential mechanism to explain the benefical effects.”

In the study, Boonstra, along with researchers Richard Manzon, Steve Mihok and Julie Helson, studied the meadow vole populations at the Whiteshell Nuclear Research Establishment at Pinawa, Manitoba, Canada. The experiment, entitled ZEUS (Zoological Environment Under Stress), was set up by Atomic Energy of Canada to test the effects of chronic gamma radiation on natural populations. In isolated populations, voles received one of three radiation treatments over a four-year period.

"Our findings suggest that a moderate increase in glucocorticoid levels, associated with low-level radiation, could be an important factor underlying the increase in longevity that has been observed in other shorter studies on small mammals exposed to low-level radiation," said Boonstra.

The ZEUS experiment was funded by Atomic Energy of Canada and the hormonal analysis was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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