Repeated social-defeat stress, cocaine or morphine. Effects on behavioral sensitization and intravenous cocaine self-administration "binges".

RATIONALE: Repeated social stress experiences engender "behavioral sensitization" and may also increase the potential for abuse of psychomotor stimulants, particularly cocaine use during "binges."
OBJECTIVE: Experimental protocols were designed to induce behavioral sensitization through exposures to social-defeat stress or injections of cocaine or morphine. The impact of stress, cocaine or morphine sensitization on cocaine self-administration was assessed using several protocols.
METHODS: Behavioral sensitization was induced in male Long-Evans rats by four social-defeat stress episodes, each separated by 72 h. Expression was assessed following a challenge of D-amphetamine (1.0 mg/kg) or cocaine (7.5 mg/kg or 10.0 mg/kg), 10-15 days after the last defeat. Sensitization to cocaine (15.0 mg/kg) or morphine (10.0 mg/kg) was induced via daily administrations for 10 days and later assessed by challenges with cocaine or morphine. Subsequently, i.v. self-administration of cocaine was analyzed for (i) rates of acquisition, (ii) sensitivity to various doses, (iii) "breaking points" during a progressive ratio schedule of cocaine reinforcement, and (iv) patterns of intake during a 24-h binge, in sensitized and control rats.
RESULTS: Social-defeat stress, cocaine or morphine administrations increased the locomotor response to stimulant challenges. During 24-h cocaine self-administration binges, sensitization to social-defeat stress or to cocaine prolonged responding, resulting in more cocaine intake. In addition, cocaine sensitization increased the rate of acquisition to cocaine self-administration and the breaking point during a progressive ratio schedule.
CONCLUSION: The process of sensitization to social-defeat stress or cocaine intensifies cocaine intake during a binge, supporting the hypothesis that sensitization may facilitate the transition to compulsive drug taking.