Common experience modifies the reinforcing properties of methamphetamine-injected cage mates but not morphine-injected cage mates in C57 mice

The aim of this study was to determine whether previous exposure to a drug affects the social facilitation of conditioned place preference (CPP) for a drug-injected cage mate. Twenty-two male C57/BL6J mice received drug injections (methamphetamine or morphine) and 22 male C57/BL6J mice received saline injections. All 44 mice then received CPP training, during which one compartment of a conventional CPP apparatus was associated with a drug-injected cage mate (stimulus mouse) and the other compartment was associated with a saline-injected cage mate (stimulus mouse). The subject mice did not receive any drug injection during this CPP training. Time spent in the compartment associated with the drug-injected cage mate was measured before and after training. Subject mice that had previously received methamphetamine injections showed an increase in the time spent in the compartment associated with the methamphetamine-injected cage mate after CPP training. This effect was not observed in subject mice that had previously received saline injections. Subject mice did not show an increase in the time spent in the compartment associated with the morphine-injected cage mate irrespective of whether they had previously received morphine or saline injections. Therefore, in agreement with previous reports, common experience with methamphetamine induced reinforcing properties, but that with morphine did not.