Saline drinking and naloxone: lightcycle dependent effects on social behaviour in male mice.

Male mice (TO strain) were observed in the resident-intruder test and several behaviours monitored. Treatments were: isotonic saline drinking ad lib for 48 hours before testing (SAL), IP injections of 1 mg/kg naloxone 30 minutes before testing (NLX) or both treatments combined (NLX + SAL). The same sequence of tests were carried out both in the light and the dark phase of the 24-hour light cycle. The single treatments (SAL and NLX) both increased fighting (SAL, p less than 0.02) in the light phase. The combined treatment (NLX + SAL), which reduced behavioural activity overall, caused a marked increase in the proportion of behaviour time spent on aggression (p less than 0.002). These effects were either reversed (significant reductions in aggression with SAL and NLX, p less than 0.05) or cancelled (minor reduction, NLX + SAL) in the dark phase. Partner body sniffing showed a trend towards lower levels following treatments in the light, and higher levels in the dark. No other behaviour changed systematically with treatments. Plasma sodium levels were monitored: there was a trend of treatment group mean sodium levels changing in proportion to the aggressogenic effect of the treatment, but it was not confirmed by correlation tests on the plasma sodium and aggression levels of individuals. The data are discussed in terms of a possible interaction between opioid receptors and physiological sodium chloride loads.