Genetic differences in cocaine-induced conditioned place preference in mice depend on conditioning trial duration

Rationale: In previous comparisons with C57BL/6J mice, DBA/2J mice have been characterized as 'hyporesponsive' to cocaine's rewarding effect in the conditioned place-preference paradigm. This finding contrasts with other studies showing greater sensitivity of DBA/2J mice to the rewarding effects of ethanol and morphine in the place conditioning task. Objectives: The purpose of the present study was to examine cocaine-induced place conditioning in both strains using apparatus and procedures similar to those used previously to assess ethanol and morphine preference conditioning. Methods: Mice from both strains were exposed to an unbiased place- conditioning procedure using 1, 10, or 30 mg/kg cocaine. Conditioning trial duration was 15, 30, or 60 min. Results: In general, C57BL/6J mice displayed a significant conditioned place preference that was relatively unaffected by cocaine dose or trial duration. In contrast, DBA/2J mice showed no place conditioning at the shortest trial duration, but an increasing level of preference as trial duration increased. At the longest trial duration, both strains showed similar levels of place preference. Conclusions: Genetic differences in sensitivity to cocaine's rewarding effect depend critically on temporal parameters of the place-conditioning procedure. One possible interpretation of these findings is that short trial durations produce conditioned activity responses that interfere more with expression of conditioned place preference in DBA/2J mice than in C57BL/6J mice. More generally, these findings underscore the need for caution when drawing conclusions about genetic differences in place conditioning, especially when using this paradigm to evaluate the effects of gene knockouts or insertions on drug reward.