Social stress and escalated drug self-administration in mice II. Cocaine and dopamine in the nucleus accumbens.

RATIONALE: Social defeat stress results in escalation of cocaine taking and long-term neural adaptations in rats. How the intensity and timing of social defeat stress determine these effects, particularly in mice, have not been well characterized.
OBJECTIVE: This study investigated the effects of mild vs. moderate intensities and durations of social stress on intravenous cocaine self-administration as well as on dopamine (DA) release in the nucleus accumbens shell (NAcSh) by using in vivo microdialysis.
METHODS: Adult male Swiss Webster (CFW) mice experienced 10 days of social defeat stress, either mild (15 attack bites in ca. 1.8 min) or moderate (30 attack bites in ca. 3.6 min), and compared to controls that were handled daily. Subsequently, the socially stressed mice were assessed for either (1) intravenous cocaine self-administration, using several unit doses (0, 0.3, 0.6, 1.0 mg/kg/infusion) under limited access conditions, or (2) neural sensitization, as determined by in vivo microdialysis of DA in the NAcSh in response to acute d-amphetamine challenge.
RESULTS: Social defeat stress resulted in escalated cocaine self-administration in both mild and moderate socially stressed groups. In addition, social defeat stress led to increased DA release after d-amphetamine challenge.
CONCLUSIONS: These data suggest that both mild and moderate socially stressed mice exhibit increased cocaine taking compared to controls, and this increase is associated with escalated dopaminergic responses in the NAcSh.