1. ¹Department of Psychology, University of Pennsylvania, Philadelphia, PA, USA
2. ²Department of Psychiatry, University of Pennsylvania School of Medicine, Philadelphia, PA, USA
3. ³Neuroscience Graduate Group, University of Pennsylvania School of Medicine, Philadelphia, PA, USA

Correspondence: Dr LL Peoples, Department of Psychology, University of Pennsylvania, 3720 Walnut St, Philadelphia, PA 19104, USA. Tel: +1 215 898 4253, Fax: +1 215 898 7301, E-mail: lpeoples@psych.upenn.edu

Received 28 April 2006; Revised 22 September 2006; Accepted 26 September 2006; Published online 13 December 2006.

Abstract

Exposure to psychomotor stimulants, during conditioning sessions, can lead to a persistent increase in the strength of conditioned behaviors and the effects of conditioned stimuli, which can be detected in subsequent drug-free periods. It is possible that the effects are selective for the behaviors and stimuli conditioned during drug exposure. The present study was designed to test this prediction. Animals were trained to discriminate two sets of stimuli. For each set, lever pressing during the presentation of one stimulus (S+) was reinforced and responding during the presentation of the other stimulus (S-) had no programmed consequences. Following an initial acquisition phase, training with one set of stimuli continued during sessions of amphetamine exposure, whereas training with the second set continued during saline exposure (20 intermixed sessions). The findings of subsequent drug-free choice tests showed that the drug history selectively enhanced the propensity of animals to engage in the drug-assigned behavior relative to the saline-assigned behavior. This change in behavior was evident in S+, but not S- trials and was potentially mediated by an acute effect of amphetamine on stimulus conditioning. The findings provide novel evidence that the facilitative effects of coincident conditioning and acute psychomotor stimulant exposure can be selective for the stimuli and behaviors conditioned during the drug exposure. These findings are relevant to hypotheses regarding the etiology of drug addiction.