Sex differences in morphine-induced behavioral sensitization and social behaviors in ICR mice.
Social/peer influences are among the strongest predictors of adolescent drug use. However, this important subject does not get much attention in pre-clinical studies. We recently observed that exposure to different social partners modulates morphine locomotor sensitization. Sensitivity to the hyper-locomotor response of drugs of abuse is a predictor of sensitivity to other drug-induced behaviors. Thus, this study examined how exposure to different social partners affected the rewarding properties of morphine. All animals were group-housed four per cage in one of two conditions referred to as 'only' and 'cage-mates'. In the mixed treatment condition, morphine- and saline-treated mice were housed together. These groups are referred to as 'morphine cage-mates' and 'saline cage-mates', respectively. In the separated treatment conditions, all mice in the cage received morphine (i.e. 'morphine only') or saline (i.e. 'saline only'), and cages were visually separated from each other. All animals were subsequently individually tested for the acquisition of morphine conditioned place preference (CPP) following one conditioning session with 10, 20 or 40 mg/kg morphine or saline. As expected, one conditioning session established morphine CPP in the morphine only animals, but not in the saline only animals. Notably, morphine CPP was not acquired by the morphine cage-mate animals. Additionally, 40 mg/kg morphine was sufficient to establish morphine CPP in the saline cage-mate animals. These results indicate that social environment has an effect on the rewarding properties of morphine. It suggests that exposure to different peers can alter the abuse potential of opioids and potentially other illicit drugs.