Methylnalorphinium fails to reverse naloxone-sensitive stress-induced analgesia in mice

Exposure of mice to cold-restraint stress markedly decreased the number of abdominal constrictions induced by IP acetic acid. Naloxone pretreatment significantly attenuated the antinociceptive effect of cold-restraint stress, suggesting a partial mediation by opioid mechanisms. Pretreatment with the quaternary opioid antagonist methylnalorphinium did not reverse analgesia in stressed mice. Also, nociception in both stressed and non-stressed mice was not modified by pretreatment with the selective alpha 2-adrenoceptor blocker yohimbine. The results suggest that cold-restraint stress promotes analgesia in mice which is mediated in part by opioid but not alpha 2-adrenoceptor mechanisms. Furthermore, the results do not substantiate a peripheral analgesic role for circulating opioids in this model of stress.