Substrate soiled by an unfamiliar conspecific modifies opioid activity in mice placed in novel environments

Naloxone induces behavioural changes in rodents exposed to novel environments, indicating the involvement of endogenous opioid mechanisms in these situations. The present study investigated whether soiled sawdust substrate from the cage of an unfamiliar, isolated, male conspecific modifies the effect of naloxone (0.5 or 12.5 mg/kg) upon behaviour of mice in an open field test situation. There was little difference between the effects of naloxone upon the frequency of acts or postures shown in the soiled and unsoiled environments. Cluster analysis of the activities according to their position and frequency in behavioural sequences, revealed variations in behavioural organisation in these two situations in control animals, and differential responses to naloxone administration. The data are discussed in terms of an involvement in behaviour of opioid mechanisms which can be modified by non-painful, biologically-relevant, aversive stimuli such as unfamiliar, conspecific-soiled substrates.