title: Disruption of endogenous opioid activity during instrumental learning enhances habit acquisition.
abstract: Considerable evidence suggests that in instrumental conditioning rats learn the relationship between actions and their consequences, or outcomes. Such goal-directed actions are sensitive to changes in outcome value. The present study assessed the role of the endogenous opioid system in goal-directed reward learning. In two experiments, rats were trained to lever press for food pellets either under vehicle or naloxone-induced opioid receptor blockade. Specific satiety procedures were used for outcome devaluation, and the effect of this devaluation on instrumental responding was then tested in extinction. In Experiment 1 outcome devaluation resulted in a reduction in lever pressing in rats that were trained after vehicle injections, indicating that actions in these rats were goal-directed. In contrast, actions in rats trained under naloxone were insensitive to outcome devaluation when tested off drug, suggesting that lever pressing had become habitual in these rats. Interestingly, in Experiment 2 naloxone-induced habitual behavior was shown to be specific to the context in which the training occurred under naloxone; rats showed normal sensitivity to outcome devaluation when tested in an alternate vehicle-trained context. Additionally, in Experiment 2 we found that the acute administration of naloxone on test had no effect in itself, indicating that opioid receptor-related processes contribute to the acquisition of goal-directed actions and not to their general performance. These data suggest that an intact endogenous opioid system is necessary for normal goal-directed learning and more importantly, reveal that a compromised endogenous opioid system during learning enhances the habitual control of actions.
authors.affiliation: Department of Psychiatry and Biobehavioral Sciences, Semel Institute for Neuroscience and Human Behavior, University of California Los Angeles, Los Angeles, CA 90024, USA. kwassum@ucla.edu
authors.forename: K M
authors.initials: KM
authors.lastname: Wassum
authors.forename: I C
authors.initials: IC
authors.lastname: Cely
authors.forename: N T
authors.initials: NT
authors.lastname: Maidment
authors.forename: B W
authors.initials: BW
authors.lastname: Balleine
mesh_terms: D000818:Animals; D003216:Conditioning, Operant / Q000187:drug effects / Q000502:physiology*; D004192:Discrimination, Psychological / Q000187:drug effects / Q000502:physiology; D006185:Habituation, Psychophysiologic / Q000187:drug effects / Q000502:physiology*; D007858:Learning / Q000187:drug effects / Q000502:physiology*; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists; D051381:Rats; D017207:Rats, Sprague-Dawley; D011957:Receptors, Opioid / Q000378:metabolism*
chemical_list: D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D009270:Naloxone