Rationale: Escalated, binge-like patterns of cocaine self-administration are engendered by repeated, intermittent exposure to episodes of social defeat stress, as well as by extended drug access.
Objectives: The present study investigated if prior exposure to brief episodes of social defeat stress would intensify the escalation of cocaine self-administration associated with extended access conditions. The consequences of both stress sensitization and prolonged access were further assessed with progressive ratio (PR) break points and during a 24-h variable dose "binge".
Methods: Male Long-Evans rats were exposed to four episodes of defeat stress (days 1-4-7-10), and their locomotor response to cocaine was assessed 10 days later. Rats were subsequently implanted with intravenous catheters. After acquisition, stressed and control rats were allowed daily short (1 h/day) or extended (6 h/day) sessions of cocaine self-administration for 14 days (0.75 mg/kg/infusion). In sequence, we determined break points for cocaine on PR tests and assessed drug intake patterns during a 24-h variable dose binge.
Results: Defeat stress induced cross-sensitization to a cocaine challenge, increased break points for cocaine, and produced persistent, escalated cocaine taking during a 24-h binge. Rats with extended access to cocaine-both stressed and controls-similarly escalated their drug intake throughout the 14 days. Extended access conditions accelerated the rate of cocaine self-administration in the first half of the binge, indicated by shorter post-infusion intervals, but failed to amplify the accumulated drug intake in non-stressed controls.
Conclusions: Both social defeat stress and drug access conditions may engender escalated cocaine intake via distinct mechanisms that regulate drug self-administration.