Dissociation of Novelty- and Cocaine-Conditioned Locomotor Activity From Cocaine Place Conditioning

Abstract

High locomotor response to novelty is associated with ease of drug self-administration but does not predict greater place-conditioning effects of drugs. Yet, the latter reflects context conditioning and high responders (HR), compared to low responders (LR), show greater conditioned locomotor effects. Conditioned locomotor effects may occur in place conditioning, perhaps confounding its measure. To examine whether conditioned locomotor effects occur in place conditioning, the present study classified rats as HR vs. LR by using approximately the two extreme 15% percentiles of the distributions. The place conditioning and locomotor sensitizing effects of cocaine were tested. In Experiment 1, HR rats exhibited more crossings between compartments but did not differ from LR rats in cocaine place conditioning. Further, both groups showed increased crossings at test compared to baseline, indicative of a conditioned locomotor effect. In Experiment 2, HR rats showed greater acute locomotor activation to cocaine, whereas LR rats tend to show greater locomotor sensitization. Finally, in Experiment 3, HR rats showed habituation in locomotor responses, whereas LR rats did not. Results of these studies suggest that inherent and conditioned locomotor activity levels are dissociated from place-conditioning effects.