Expression and resilience of a cocaine-conditioned locomotor response after brief and extended drug-free periods.

Conditioned associations between drug experience and its context are maintained long after drug use ceases, and may contribute to relapse after extended abstinence. These include operantly conditioned associations directed toward seeking drug, but also Pavlovian conditioned associations between drug reward and contextual cues present at the time of drug administration. The present study sought to determine whether expression of a Pavlovian conditioned locomotor (CL) response to a cocaine-associated context increases over time in the same manner observed for instrumental responses, and whether the CL memory is differentially susceptible to extinction and recovery after brief versus extended abstinence. Male rats received injections of cocaine (30mg/kg, i.p.) or vehicle once per day for 6 days. In Exp. 1, CL activity was measured 1, 7, 21, or 42 days later. Rats that had received cocaine injections displayed robust CL, regardless of when testing occurred. In Exp. 2, extinction and recovery of CL were assessed after 1 or 42 days. The CL response was more readily extinguished in rats tested 1 day after drug exposure, as compared to rats tested 42 days later. Exp. 3 confirmed that conditioning in the testing context was necessary for expression of CL. Overall, our results indicate that Pavlovian associations underlying the CL response are maintained long after drug experience. Although the expression of CL does not change with the passage of time, as has been observed for instrumental drug-related responses, the memory trace does appear to become more resilient over time.