Acute non-opioid analgesia in defeated male mice

Exposure to repeated attack induces a long-lasting analgesia in male mice. Although this reaction has been linked to the special biological significance of defeat, earlier research has confounded defeat and exposure to further attack. In the present studies, DBA/2 intruder mice were individually placed into the home cages of aggressive conspecifics and removed immediately upon display of the species-characteristic upright submissive posture. Under these test conditions, intruders did indeed show a profound analgesia. However, in marked contrast to the antinociceptive effects of repeated attack, this reaction was of short duration (less than 10 min), was not blocked by naloxone (1-10 mg/kg, IP) and did not show cross-tolerance either to or from morphine (5 mg/kg, IP). These findings are discussed in relation to multiple endogenous pain inhibitory systems and their possible adaptive significance in murine social behaviour.