Hormetic effects of gamma radiation on the stress axis of natural populations of meadow voles (Microtus pennsylvanicus)

We tested the hypothesis that low doses of gamma radiation have beneficial, hormetic effects on the stress axis (the hypothalamic‐pituitary‐adrenocortical axis) of free‐ranging meadow vole populations (Microtus pennsylvanicus). Voles were exposed to chronic gamma radiation from a 137Cs field irradiator. In isolated populations, voles received one of three treatments over a four‐year period: Controls (0.19–0.42 μGy/h — levels that were 2–5X above background levels [0.1 μGy/h] and live‐trapped in all years — 1982–1985), low doses (22.6 μGy/h — 50–200X background, live‐trapped from November 1982–April 1985), or high doses (3,840 μGy/h — 40,000X background, live‐trapped from November 1983–April 1985). Voles exposed to a low dose had levels of free and total corticosterone that were significantly higher than those in the control or high‐dose groups. Differences in response to radiation between the sexes were apparent for maximum corticosterone‐binding capacity, with females exposed to low doses having higher binding capacity than control or high‐dose females, whereas males exposed to low doses had lower binding capacity than control or high‐dose males. Low‐dose voles had higher counts of neutrophils than either the controls or high‐dose voles; hematocrit was greater in the controls than in irradiated voles. These results indicate that voles display a hormetic response to radiation, wherein low doses of an otherwise harmful agent produce a beneficial effect. The stimulation of the stress axis resulting in the increased secretion of glucocorticoids, which may protect against the excessive actions of the immune and inflammatory responses, may be a key mechanism producing this effect.