Mu and kappa opiate receptor involvement in agonistic behaviour in mice.

The influence of opioid drugs on agonistic behaviour is reviewed and an experiment is reported that examines the impact of U-50488 (a kappa agonist) and DAGO (a mu agonist) on the social encounters of male and female mice interacting with anosmic male partners. Although DAGO did not significantly influence inter-male social encounters, U-50488 decreased social investigation, increased timid/defensive behaviour and potently suppressed aggression. In contrast, U-50488 did not significantly influence the behaviour of timid female mice, whereas DAGO decreased social and timid/defensive behaviour. It was concluded that a kappa mechanism increases whereas a mu mechanism decreases submissive behaviour.