Social defeat alters the acquisition of cocaine self-administration in rats: role of individual differences in cocaine-taking behavior

Rationale: It is known that social defeat can modulate cocaine self-administration. However, it is unclear whether this psychosocial stressor affects drug-taking behavior to the same extent across all individual animals, particularly those with differing propensities to self-administer psychostimulants. Objective: This study examined the effect of social defeat on cocaine self-administration in animals that differ in novelty-seeking behavior that predicts differences in drug self-administration. Methods: Male Sprague-Dawley rats were first classified into high-responder (HR) and low-responder (LR) groups. HR and LR rats were categorized based on their locomotor activity in a novel environment, with HR rats exhibiting higher locomotor activity than LR rats. Then, male rats were exposed on four occasions to an aggressive Long Evans male rat over the course of 4 days. Control rats were not exposed to the social defeat. All rats were subsequently implanted with jugular catheters and 3 days later placed into the self-administration box to study the acquisition of cocaine self-administration (0.25 mg per infusion). Results: HR non-defeated animals self-administered more cocaine than the LR non-defeated animals. Following social defeat, the acquisition of cocaine self-administration is significantly delayed in HR rats and enhanced in LR rats. Conclusion: The unique patterns of responsiveness in the HR and LR animals suggest that social defeat plays a role of equalizer of individual differences in drug-taking behavior.