Conditioned locomotor stimulant effects of cocaine in rats do not result from interference with habituation

Rationale: Classical conditioning has been proposed to account fur the hype ractivity observed in drug-free rats when placed in an environment previous ly paired with cocaine administration. However, an alternative explanation is that hyperactivity results from an inability of rats to habituate to the environment under the influence of cocaine. Objectives: In this study, pre conditioning exposure to the test environment was increased from one sessio n (standard procedure) to seven (modified procedure) to test the "antihabit uation" hypothesis. Methods: After preconditioning exposure, six conditioni ng sessions took place over a 10-day to 13-day period. Paired rats received 10 mg/kg cocaine i.p. prior to activity sessions and saline i.p. upon retu rn to the colony room. Unpaired rats received saline prior to and cocaine a fter activity sessions. Time-off rats were withheld from the activity boxes , but were subject to all other procedures during conditioning. On the test day, all rats received saline prior to activity sessions. Results: In the standard procedure, paired rats exhibited significantly greater activity th an unpaired rats on the test day, consistent with previous reports. In the modified procedure, mean activity (all rats) decreased between the first an d last preconditioning sessions. Still, the paired group exhibited granter activity than the unpaired group on the test day, suggesting that a conditi oned stimulant effect developed in habituated rats. Activity in the time-of f group did not significantly differ from the unpaired group demonstrating the habituation had not dissipated over this time period. Conclusions: Thes e results support the conclusion that hyperactivity observed on the test da y was not a result of antihabituation effects of cocaine.