Adaptive pain inhibition in murine resident-intruder interactions.

In dyadic encounters with aggressive resident conspecifics, male intruder mice display an initial acute nonopioid analgesia followed by a more enduring opioid analgesia. The former reaction occurs in association with active defense (flight or fight) and can be seen in response to the scent of an aggressive conspecific or defeat experience per se. In contrast, the latter (opioid) reaction is associated with passive defense (immobility) and occurs in response to extended conspecific attack. The mechanisms underlying these two ecologically-relevant forms of pain inhibition are contrasted and the phenomena are discussed in relation to the question of adaptive significance.