Aggression, defeat and opioid activation in mice: influences of social factors, size and territory.

The aggressive components and opioid-mediated behavioral consequences of various types of intraspecific agonistic interactions between individual male mice were examined. The size of the animals, their previous social history (group or isolation housing) and territory on which the encounter took place were varied to yield 26 different 'resident-intruder' paradigms. In these agonistic encounters the latency to first attack, number of bites and time to defeat, as well as the number of attack bouts present varied according to the 'resident-intruder' paradigm employed. The behavioral consequences of aggression and defeat, including analgesia, increased activity and augmented feeding were determined from the subordinate mice in 5 representative agonistic interactions. These behavioral responses, which had been previously shown to be mediated by endogenous opioid systems also varied according to the 'resident-intruder' paradigm employed. When both mice were group-housed, there was no agonistic behavior, regardless of the size of the mice or the testing arena. In isolated animals the defeat posture was only observed in 1 of the 19 paradigms. It is suggested that various 'resident-intruder' pairings and agonistic interactions can provide a reliable and useful means of examining differential naturalistic stress-induced endogenous opioid activation.