Endogenous opioids and analgesic effects of ionizing radiation in rats

Some stressors can cause a temporary decrease of sensitivity to pain (stress-induced analgesia, SIA) in mammals. Ionizing radiation belongs to non-specific stressors, too. The aim of this study was to analyze the effects of ionizing radiation on pain sensitivity in laboratory rats and to establish whether the release of endogenous opioids in post-irradiation period is involved in analgesic effect of radiation. Two-month-old (310-340 g) intact male Sprague-Dowley rats divided in four groups (n = 10) housed in groups of five were used in the experiments. Rats were held under an LD 12:12 artificial light regimen at temperature of 22 degreesC and relative air humidity of 60-70%. Food and water were available ad libitum. The rats were tested in a hot plate apparatus with a surface temperature of 55 +/- 0.5 degreesC. It was found that gamma irradiation with a whole body dose of 6 Gy or with a dose of 10 Gy on the head caused significant prolongation of the hind-paw licking latency in the hot plate test. Intraperitoneal administration of naloxone, a blocker of the endogenous opioid g-receptors in a dose of 8 mg(.)kg(-1) b.m. 30 min before testing on the hot plate significantly (p < 0.05) reversed the post-irradiation analgesia in both irradiation models, while a dose of 4 mg(.)kg(-1) was ineffective. The results suggest that effects on the CNS may be involved in post-irradiation SIA and that the endogenous opioids probably play an important role in this phenomenon.