Emergence and development of stress-induced analgesia and concomitant behavioral changes in mice exposed to social...

Mice of the inbred strain DBA/2, when exposed to a social conflict, developed a low intensity, naloxone-insensitive analgesia after 15 bites, and a more pronounced naloxone-sensitive analgesia after 45 bites. The effective inhibition of the antinociceptive response following low and high number of bites by the alkylating opiate antagonist beta-chlornaltrexamine suggests participation of opioid mechanisms at both stress levels. Emergence of an increased tail-flick latency was indicated by the occurrence of defensive upright postures upon contact with the opponent, while animals displaying full analgesic response during the period of bite 31-45 increased their escape reactions without being in contact with the aggressor. Suppression of social conflict analgesia in mice by pretreatment with opiate antagonists facilitated the occurrence of these escape reactions. The display of panic escape responses is discussed in the context of increased fear and helplessness that developed under conditions of sustained attacks.