Analgesia in selectively bred mice exposed to cold in helium/oxygen atmosphere

In order to evaluate the stressing role of swim hypothermia in producing sw im stress-induced analgesia (SSIA), we examined whether a mere decrease in the animals' core temperature without swimming would be sufficient to elici t analgesia. The subjects were Swiss-Webster mice selectively bred for 37 a nd 40 generations for divergent magnitudes of SSIA. High (HA) and low analg esia (LA) mice were exposed for 15 min to temperatures in the range between -5 and +20 degrees C in 79% He/21% O-2 (Heliox) atmosphere. The Heliox exp osure produced ambient temperature-dependent hypothermia and analgesia, as assessed with a hot-plate test (56 degrees C). The post-Heliox analgesia wa s of much higher magnitude in HA than in LA mice. The steeper slope of regr ession of the magnitude of analgesia upon hypothermia in HA mice indicates that these mice are far more sensitive to the analgesic effect of hypotherm ia than LA mice. Naltrexone HCl (10 mg/kg i.p.) attenuated analgesia in amb ient temperature-dependent manner in HA, but not in LA mice. In view of the apparent similarity of Heliox-induced analgesia and SSIA we suggest that h ypothermia is a powerful component of swim stress to induce SSIA in the mou se. (C) 1999 Elsevier Science Inc.