Social behaviour in pairs of C57BL/6 mice of both sexes in the open field: effects of saline drinking and of naloxone.

We have previously found that saline drinking increases fighting in male pairs and decided to test this treatment (0.9% NaCl for 24 hours before test day 1; SAL) on social behaviour of both males and females. Paired C57BL/6 mice (same-sex pairs) were observed in the open field in daily sessions for three days. One member of each pair (test mouse) was given either SAL treatment, a control injection of saline (SI), an injection of naloxone (1 mg/kg IP; NLX) or a combination of both treatments (NLX + SAL). NLX alone had previously been found to increase aggression in resident/intruder tests at the dose used. Open field testing is not associated with aggressive encounters in our experience. SAL had little effect on (unaggressive) social behaviour in males, but increased social contact seeking in females. The NLX and NLX + SAL treatments had essentially the same effects, irrespective of sex; the treated animals showed behavioural inhibition (reduced social and ambulatory behaviour), while their untreated partners showed significantly more than normal interest in the naloxone treated mice. The results are discussed in terms of opioid involvement in social behaviour.