Extinction of the response to the female's urinary odour as a consequence of low-dose X-irradiation in male hamster, occurs in conjunction with a marked suppression of aggressive behaviour.

In recent years concern over the biological effects of low-dose X-rays has been growing. I have noticed that fighting injuries usually observed among male Syrian hamsters tend to decrease in animals irradiated with low-dose X-rays. Therefore, quantitatively the effects of low-dose X-irradiation on aggressive behaviour were examined using a resident-intruder paradigm. Hamsters became gradually calm, and showed remarkably quiet behaviour 7-10 days following whole-body 5 or 15 cGy X-irradiation. Only the anterior part of the head (olfactory system including orbits) exposure also induced the remarkable suppression of the aggressive behaviour. The calm behaviour induced by low-dose X-irradiation might be related to the changes in the olfactory function. In an additional experiment, the effects of low-dose X-irradiation on the olfactory system was examined using operant conditioning techniques, in which male hamsters were trained to discriminate between a conspecific female's urine and a water stimulus in return for a food reward. Following a 5 cGy dose of X-rays, the irradiated hamsters made slightly fewer correct discriminations relative to non-irradiated controls. A significant difference was found when the radiation dose was increased to 15 cGy. Similar results were obtained when the radiation exposure was restricted to the anterior portion of the head. Significant effects were not observed prior to the 8th day post-exposure, and the effect was most pronounced 10 days after the exposure to X-rays. Possible relationships between the two effects are discussed.