Effects of morphine and cocaine in mice with stable high aggressive and nonaggressive behavioral strategy

The social group experience of mice with opposite aggressive and nonaggressive behavioral strategies was examined to modulate reinforcing effects of morphine and cocaine. Highly aggressive and nonaggressive male mice cohoused for long period in three-member groups were tested to self-administrate the drugs and to develop conditioned place preferring by them. Mouse triads formed by principle of descending aggression were used as a model of linear hierarchical group. The level of mouse aggression was identified previously within the stock group and during encounter with unknown intruder that continued to be stable over the time of experiment. Highly aggressive mice self-administered morphine and cocaine at higher unit concentrations (1.5 and 1.5 mg/ml) as compare with nonaggressive animals (0.5 and 0.25, 0.5, 1.0 mg/ml). Both morphine (2.5, 5.0, 10.0, and 20.0 mg/kg) and cocaine (2.5, 5.0, and 10.0 mg/kg) induced conditioned place preference in nonaggressive mice at all doses. In contrast, morphine had no effect in highly aggressive mice, while cocaine induced place conditioning at the highest doses (10 mg/kg) only. Our results illustrate that social experience in a stable group alter mouse sensitivity to the rewarding properties of drugs of abuse and social state should be taken into account in the experiments when social interactions are present.