Reunion of separated sibling mice: neurobiological and behavioral aspects.

Outbred albino NMRI male mice encountering a brother in adulthood, after a long period of separation, show an opioid-dependent increase in pain threshold. Unrelated and unfamiliar males show no similar changes in pain sensitivity. This study investigates which kind of stimuli from the partner may be responsible for such a modification at the neural level. The tail-flick test is used as a measure of pain sensitivity. Exposure to the scent of the brother's home cage, as well as exposure to visual, olfactory and auditory stimuli and partial physical contact with the related male, are not sufficient to induce changes in nociception. Physical affiliative contact between males is higher in sib than in nonsib pairs, and a positive correlation exists in sib pairs between huddling behavior and pain sensitivity at the end of a 2-h social session. Siblings injected with naloxone, an opioid receptor blocker, show a decrease in social behaviors involving physical contact. These results suggest that physical affiliative contact between sibling mice may be responsible for the enhancement of nociceptive threshold.