False positive in the intravenous drug self-administration test in C57BL/6J mice

The objective of this study was to examine C57BL/6J (B6) mice during extinction conditions, after food training, and for rates and patterns of operant behavior that seems similar to behavior maintained by intravenous cocaine injections. The rationale was to evaluate the potential for false positives in the intravenous self-administration test using protocols common in studies of knockout mice backcrossed to B6. An additional aim was to assess the influence of food-associated and drug-associated cues and mouse strain. Mice were allowed to acquire lever pressing reinforced by sweetened condensed milk under a fixed ratio 1 then fixed ratio 2 schedule of reinforcement accompanied by a flashing light. A catheter base was then implanted for simulation of intravenous self-administration conditions. Mice were allowed to lever press with cues remaining the same as during food training but without further scheduled consequences (i.e. no drug or food reinforcers delivered). All mice sustained lever pressing for several weeks, and over half met commonly used criteria for 'self-administration behavior.' Thus, B6 mice showed perseveration of a previously reinforced behavior that closely resembled rates and patterns of drug self-administration. This effect in B6 mice was greater than with A/J mice, and the lack of extinction was even more robust in the presence of cocaine-associated cues than with food-associated cues. We suggest that a necessary criterion for positive results in the intravenous drug self-administration test include an increase in responding when cocaine is made available after extinction with saline self-administration.