Social conflict activates status-dependent endogenous analgesic or hyperalgesic mechanisms in male mice: effects of naloxone on nociception and behaviour.

The concept of environmentally-induced activation of endogenous analgesia mechanisms rests, almost exclusively, upon studies which have involved the use of rather intense artificial stimuli. The current study was therefore designed to assess the validity of this concept under the more naturalistic conditions of social conflict between isolated resident mice and group-housed intruders. Agonistic experience was found to result in a potent, naloxone-reversible (10 mg/kg) analgesia in intruder mice while, in residents, it produced a moderate hyperalgesic reaction which was very sensitive to naloxone antagonism (0.1 mg/kg). Detailed videotape analyses revealed that only the behaviour of residents was significantly altered by naloxone treatment, with a highly selective inhibition of attack observed at 10 mg/kg. These data suggest that (1) social conflict in mice is a potent, and biologically-relevant, stimulus in the activation of endogenous naloxone-sensitive pain control mechanisms, (2) social status is an important determinant of nociceptive response to such experience and (3) inescapability from attack may be a critical factor in the development of encounter-induced analgesia.