Effects of opiate agonists and antagonists on aggressive encounters and subsequent opioid-induced analgesia, activity and feeding responses in male mice.

The effects of peripheral administration of the mu, kappa and sigma opiate agonists, levorphanol (1.0 mg/kg), U-50,488 (1.0 and 10.0 mg/kg), (+/-) SKF-10,047 (10.0 and 30.0 mg/kg), respectively, as well as the delta opiate antagonists, ICI-154,129 (10.0 mg/kg), and the prototypic antagonist, naloxone (1.0 mg/kg), on the agonistic behaviors and subsequent analgesic, locomotory and ingestive responses of subordinate mice were examined in a "resident-intruder" paradigm. The latter behaviors were examined in both defeated and nondefeated mice that had received an equivalent level of aggression. The mu and delta opiate antagonists decreased, while the mu, kappa, and sigma opiate agonists selectively increased aggressive behavior (number of bouts of aggressive interactions, number of bites to defeat, time to defeat). Both naloxone and the delta antagonist suppressed defeat- and aggression-induced activity and feeding, while only naloxone blocked the analgesic response. Levorphanol enhanced, U-50,488 had variable dose related effects, and SKF-10,047 decreased the defeat and aggressive-induced responses. These results indicate that various opioid systems and opiate receptors are differentially involved in the mediation of various components of the agonistic encounters and in the expression of the consequences of social conflict and defeat-induced opioid activation.