Background: We have recently observed an unforeseen dissociation in the effect of environmental context on heroin versus cocaine self-administration in rats. Rats housed in the self-administration chambers (Residents) took more heroin than rats that were transferred to the self-administration chambers only for the test sessions (Nonresidents). The contrary was found for cocaine. The twofold aim of the present study was to investigate: 1) drug choice as a function of ambience in rats given access to both cocaine and heroin, and 2) ambience of choice for cocaine- versus heroin-taking in human addicts.
Methods: Resident and Nonresident rats with double-lumen intrajugular catheters were trained to self-administer cocaine (400 microg/kg/infusion) and heroin (25 microg/kg/infusion) on alternate days and then given the opportunity to choose between the two drugs during seven daily sessions. In the human study, we asked heroin and cocaine abusers where they preferred to take these drugs.
Results: Approximately 46.7% of Resident rats exhibited a preference for heroin over cocaine; 33.3% preferred cocaine, and 20% expressed no preference. In contrast, only 8.3% of Nonresident rats preferred heroin, whereas 66.7% preferred cocaine, and 25% expressed no preference. In the human study, 73% of co-abusers reported that they used heroin exclusively or mostly at home (22% used it outside the home), whereas only 25% reported using cocaine at home (67% took it outside their homes).
Conclusions: Environmental context plays an important role in drug choice in both humans and rats self-administering heroin and cocaine.