Social defeat stress, sensitization, and intravenous cocaine self-administration in mice

Behavioral sensitization has been proposed as a process that is important in compulsive drug use and in psychotic disorders. The present experiments examine the relationship between behavioral sensitization, induced by either social defeat or amphetamine, and intravenous cocaine self-administration in mice. Male CFW mice were exposed either to defeat experiences, amphetamine (2.5 mg/kg, i.p.) or saline (i.p.) every day for 10 days. Ten days after the last defeat or injection, mice were challenged with varying doses of amphetamine (1.0-2.5 mg/kg i.p). Mice were then trained to nose poke for intravenous cocaine (1.0 mg/kg/inf) during daily 3-h sessions. Following this acquisition phase, the animals self-administered varying doses of cocaine (0.3-1.8 mg/kg/inf) or were allowed to self-administer cocaine (0.3 mg/kg/inf) according to a progressive ratio schedule of reinforcement. Repeated social defeat produced a sensitized motor response to a single challenge of 1.5 mg/kg amphetamine and to a cumulative dosing of amphetamine. Amphetamine-pretreated mice exhibited increased cocaine self-administration during acquisition and elevated break points during performance on a progressive ratio schedule of reinforcement relative to stress-sensitized and control animals. These data extend the evidence from rats to mice for the process of sensitization leading to more cocaine taking. Contrary to what is seen in rats, increased levels of cocaine self-administration were seen only in the amphetamine-pretreated mice and not after repeated defeat stress, suggesting that the sensitized response to defeat stress may not be as robust as it is in rats in this particular strain of mice.