Social isolation increases morphine intake: behavioral and psychopharmacological aspects.

Environmental and situational factors are important determinants of recreational drug use in humans. We aimed to develop a reliable animal model for studying the effects of environmental variables on drug-seeking behavior using the 'social isolation/social restriction' paradigm. Adult Wistar rats housed in short-term isolation (21 days) consumed significantly more morphine solution (0.5 mg/ml) than rats living in pairs, both in one-bottle and in two-bottle tests. No differences were found in their water consumption. This effect was observed in both males and females and the results were also replicated after reversal of housing conditions. We also found that as little as 60-min of daily social-physical interaction with another rat was sufficient to completely abolish the increase in morphine consumption in socially restricted animals. We discuss some possible interpretations for these effects. These results indicate that environmental and situational factors influence drug intake in laboratory rats as they do in humans, and thus may be of interest in studying drug-seeking behavior in humans.