ISOLATION IMPAIRS PLACE PREFERENCE CONDITIONING TO MORPHINE BUT NOT AVERSIVE LEARNING IN MICE

Morphine (8-100 mg/kg IP) induces place preference conditioning in mic e. The effect of two different periods of isolation (15 and 30 days) w as examined. Mice isolated for 15 days but not 30 days exhibited place preference conditioning to morphine (8 mg/kg). After 30 days of isola tion morphine could not induce place preference conditioning with the following doses (8, 16, 64, 100 mg/kg). Social regrouping of male mice previously isolated for 30 days with naive female mice for 15 or 30 d ays resulted in a reappearance of the conditioned place preference to morphine (16 mg/kg). The specificity of this associative deficit was e xamined by testing learning in isolated compared to non-isolated mice in two distinct settings: escape learning in the Morris water maze and passive avoidance acquisition and retention. On the Morris water maze isolated mice did not differ from non-isolated mice regarding place l earning, the probe trial or extinction. Isolated mice were unimpaired in passive avoidance acquisition and retention. It was concluded that the deficits ill place preference conditioning were not the result of a global learning impairment in isolated mice.