Mouse-killing in rats induces a naloxone-blockable increase in nociceptive threshold.

Mouse-killing behaviour in rats increased the nociceptive threshold for paw licking, as measured in the hot-plate test. This effect was prevented by the opiate antagonist naloxone, which, on the other hand, had no influence on mouse-killing behavior per se. It is concluded that opioid mechanisms may be involved in changes of nociceptive threshold associated with aggressive behavior.