Sensitization facilitates habit formation: implications for addiction.

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see Habits and compulsions have long been thought to relate to drug addiction. On the one hand, the increasing negative consequences faced by drug-taking addicts have led to the characterization of addic-tive behavior as compulsive, and the development of addiction has been hypothesized to involve the progressive control of drug taking by inflexible habits (Everitt et al., 2001). More broadly, drug addicts are also more likely to develop obsessive compulsive disorder and often display elaborate and repetitive stereotypical behaviors that are not exclusively drug related. Conversely, people who have obsessive compulsive disorder have a higher risk of developing addiction, which, according to one intriguing report, can further increase dramatically after psycho-surgical lesions that include the ventral striatum and the prelimbic prefrontal cortex (Irle et al., 1998). The striatal dopamine system has been extensively implicated in the development of habits and addiction. Because repeated administration of drugs of abuse can lead to an increased responsiveness of me-solimbic and nigrostriatal dopamine systems (sensitization), and because dopa-mine may drive the learning that underlies instrumental conditioning through enabling striatal plasticity, repeated self-administration of drugs of abuse could accelerate the formation of habitual drug-taking behavior. In their recent Journal of Neuroscience paper, Nelson and Killcross (2006) investigated the effect of amphetamine sensiti-zation on the development of habitual instrumental responding for a natural reward. In instrumental conditioning, the response is initially controlled by a goal-directed process in which the instrumental behavior is sensitive to the motiva-tional value of the outcome. With overtraining, behavior comes to be dominated by inflexible stimulus–response associations. This transition is marked by the appearance of insensitivity to outcome devaluation when tested in extinction (Balleine, 2005). Nelson and Killcross induced amphet-amine sensitization in rats over 7 d by daily injection of 2 mg/kg D-amphetamine or saline. During the subsequent 7 d, the animals were left undisturbed except that they were reduced to 80% of their ad libi-tum weight. After that, the rats were trained instrumentally on a random-interval schedule over 5 d in either the morning or afternoon. At the other time of the day, an alternative reinforcer was presented noncontingently on a matched random …