Previous exposure to psychostimulants enhances the reinstatement of cocaine seeking by nucleus accumbens AMPA.

The effect of previous exposure to psychostimulants on the subsequent self-administration of cocaine as well as reinstatement of this behavior by priming infusions of AMPA into the nucleus accumbens (NAcc) was examined. Rats were exposed to five injections, one injection every third day, of either saline or amphetamine (AMPH: 1.5 mg/kg, i.p.). Starting 10 days later, they were trained to self-administer cocaine (0.3 mg/kg/infusion, i.v.) and subsequently tested under a progressive ratio (PR) schedule for 4 consecutive days. As expected, rats exposed to AMPH worked more and obtained more cocaine infusions than saline exposed controls on the PR test sessions. Following daily extinction sessions during which saline was substituted for cocaine, the effect of priming infusions of AMPA (0.0, 0.08, or 0.8 nmol/0.5 microl/side) into the NAcc was then examined on two tests: one conducted 4 days after the last cocaine PR test session (2-3 weeks after the last AMPH exposure injection) and the next 4 weeks later. Consistent with previous reports, NAcc AMPA dose-dependently reinstated cocaine seeking on both tests regardless of exposure condition. Importantly, this priming effect of NAcc AMPA was significantly enhanced in AMPH compared to saline exposed rats on the first test conducted 2-3 weeks after AMPH. On the second test, conducted 4 weeks after cocaine, reinstatement was similarly enhanced in both groups to levels observed on the first test in AMPH exposed rats. These results indicate that both noncontingent (AMPH) and contingent (cocaine) exposure to psychostimulants enhances the reinstatement of cocaine seeking by NAcc AMPA and appears to do so in a time-dependent manner.