Resident's scent: a critical factor in acute analgesic reaction to defeat experience in male mice.

Although it has recently been reported that defeated male mice evidence an acute non-opioid analgesia, little is currently known about the specific features of the defeat experience with which the analgesic reaction is associated. The present experiments not only confirm that defeat experience reliably induces acute antinociception in intruder mice, but show that a similar reaction also occurs as a consequence of exposure to an aggressive resident which does not attack during the brief test period, a well-characterized non-aggressive resident and the 'unoccupied' soiled home cage of an aggressive resident. Results also indicated that, with appropriate exposure duration, scent alone can give rise to a quantitatively similar analgesia to that observed in defeated mice. Furthermore, time-course comparisons and the absence of naloxone antagonism suggest that 'scent' and 'defeat' analgesias are mediated via a common non-opioid mechanism. Data are discussed in relation to the ecological significance of urinary odours in social communication in mice.