Analgesia induced by repeated exposure to low dose x-rays in mice, and involvement of the accessory olfactory system in modulation of the radiation effects.

The effects of low-dose x-rays on mouse nociceptive behavior were examined using a formalin injected test that rated the amount of time the animals spent licking the injected hind paw. Male ICR White Swiss mice showed a marked suppression of licking behavior after repeated low-dose x-irradiation (5 cGy/day, 6 consecutive days). The most profound effect was observed on the day 30 after irradiation. The decline of licking behavior, however, was not observed at all following olfactory bulbectomy or vomeronasal tract cut. The analgesic effects could be observed in writhing animals administered acetic acid intraperitoneally. Moreover, analgesia was totally blocked by the administration of N-nitro-L-arginine, a nitric oxide synthase inhibitor, to accessory olfactory bulbs prior to the exposure. The present results indicate that the olfactory system plays an important role in modulation of radiation-induced analgesia, and a possible involvement of nitric oxide in the formation of recognition memory subjected to repeated x-rays. Relatively higher doses (5 cGy x 9 days, 5 cGy x 12 days), however, did not induce such effects, namely, the decline of nociceptive response was limited to the animals irradiated with the smaller dose.