β-endorphin modulates the acute response to a social conflict in male mice but does not play a role in stress-induced changes in sleep

β-Endorphin is an endogenous opioid peptide that is released during stress and has been associated with many physiological functions. In this experiment β-endorphin deficient mice were used to study the role of endorphins in the acute physiological and behavioral responses to a social conflict, as well as their role in social stress-induced changes in sleep. Adult male β-endorphin deficient and wild type mice were subjected to the stress of a 1 h social conflict with an aggressive dominant conspecific. After the conflict, the β-endorphin deficient mice had higher corticosterone levels but the peak increase in body temperature was not different from that in wild type animals. In fact, body temperature returned to baseline levels faster in the β-endorphin deficient mice. During their interaction with the aggressive conspecific several of the β-endorphin deficient mice showed clear signs of counter aggression whereas this was not seen in any of the wild type mice. Overall, the β-endorphin deficient mice and wild type mice had fairly similar sleep patterns under baseline conditions and also showed similar amounts of NREM sleep, REM sleep and EEG slow-wave energy after the social conflict. In addition, no differences were found in the sleep patterns of mice that showed counter aggression and mice that did not. In conclusion, the results suggest that β-endorphin modulates the acute endocrine, thermoregulatory and behavioral response to a social conflict but the data do not support a major role for β-endorphin in the regulation of sleep or social stress-induced alterations in sleep.