Social factors in conditioned place preference with morphine in mice.

Using a conditioned place preference (CPP) protocol, I previously demonstrated that social interactions enhance the reinforcing properties of methamphetamine in mice. Here, I examined the influence of social interactions on morphine-induced CPP. Paired and control-paired groups both received CPP training with a cage mate. In the paired group, both mice received the same CPP training and the same type of injections (either morphine (0.1-3.0mg/kg) or saline, depending on the day). In the control-paired group, both mice received the same CPP training, but the injections were reversed. When one mouse was injected with morphine, the other was injected with saline. CPP training in the absence of a cage mate resulted in a conditioned preference when subjects were injected with 1.0 or 3.0mg/kg of morphine (but not with 0.1mg/kg). A cage mate that was not administered morphine enhanced the reinforcing effects of the 3.0mg/kg dose. In contrast, a cage mate that received morphine reduced the reinforcing property of the 1.0mg/kg dose. Results from these experiments differ from previous results concerning methamphetamine, in which a drug-free partner did not enhance the drug's effect.