Behavioral effects of morphine in mice: role of experimental housing

Behavioral effects of morphine were assessed in isolated-timid Swiss mice, and were compared with those observed following morphine administration in nonaggressive-grouped subjects. For this purpose saline- and morphine- (0.5, 1.0, 2.5, and 5.0 mg/kg, IP) injected isolated-timid and nonaggressive-grouped mice interacting with a social partner were observed during a 4-min test. Three main points emerged from the results: a) in basal conditions, compared with social mice, in timid mice the offensive ambivalent behaviors were significantly less pronounced, while the defensive ambivalent behaviors (and all flight behaviors) were significantly more evident; b) 2.5 mg/kg of morphine increased offensive and decreased defensive ambivalent behaviors in timid mice; c) in social mice morphine (2.5 mg/kg) treatment increased defensive ambivalent behaviors and time spent in crouch. The results, which show that the behavioral effects of morphine depend on the state of the individual, are interpreted on the basis of the antiemotional properties of this opiate.