Persistent increase of I.V. cocaine self-administration in a subgroup of C57BL/6J male mice after social defeat stress.

RATIONALE: Stressful life experiences can persistently increase the motivation for, and consumption of, intensely rewarding stimuli, like cocaine, over time. In rodents, intermittent versus continuous exposure to social stress engenders opposing changes to reward-related behavior, as measured by consumption of sucrose and cocaine.
OBJECTIVE: The present study examines if the effects of intermittent versus continuous social stress on cocaine self-administration in mice parallel those seen in rats.
METHODS: Both forms of social stress involve a brief daily physical confrontation with an aggressive resident for 10 consecutive days. Continuous social stress involves constant visual and olfactory exposure to an aggressive resident via habitation in a protected portion of the resident's home cage, while exposure to an aggressive resident during intermittent social stress is limited to a single, physical encounter per day. Implementing a femoral vein catheterization method for the first time in mice, we determined divergent changes to intravenous cocaine self-administration.
RESULTS: Modestly increased cocaine self-administration after intermittent social stress was confirmed. In a subset of animals, continuous social stress in mice substantially increased cocaine self-administration and sucrose intake. By stark contrast, another subpopulation had substantial attenuation of cocaine self-administration and sucrose intake after continuous social stress.
CONCLUSIONS: Bimodal divergence in responding for rewarding stimuli including cocaine after social stress experience likely reflects two opposing forms of coping to continuous social stress that promote either a sensitization or attenuation of reward-seeking.