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Appetitive overshadowing is disrupted by systemic amphetamine but not by electrolytic lesions to the nucleus accumbens shell

¹ School of Psychology, Institute of Neuroscience, University of Nottingham, Nottingham, UK; andPsychology Division, Nottingham Trent University, Nottingham, UK.
² School of Psychology, Institute of Neuroscience, University of Nottingham, Nottingham, UK.
³ D School of Psychology, Institute of Neuroscience, University of Nottingham, Nottingham, UK.

^* To whom correspondence should be addressed.

	Abstract

There is evidence that the indirect dopamine (DA) agonist amphetamine(AMP) can disrupt selective learning in an aversive overshadowing task,consistent with a role for the DA system in this form of saliencemanipulation. In the following experiments we assessed in the maleWistar rat: (1) whether amphetamine disruption of overshadowingextends to an appetitively motivated overshadowing task; and (2)whether selective electrolytic lesions to the n.acc (shell versus coresubfields) disrupt appetitively motivated overshadowing. The experimentsused sucrose reward pellets as the unconditioned stimulus (UCS).In each case, a conditioned stimulus (CS, light) was either conditionedalone or in compound together with a more intense CS (noise or tone).The presence of overshadowing was demonstrated as reduced conditioningto the light when it had been previously conditioned in compoundcompared to when it had been conditioned alone. It was predicted thatAMP and lesions to the n.acc shell would disrupt overshadowing. AMPwas found to abolish overshadowing at 0.5mg/kg, but not at 1mg/kg.Contrary to prediction, the shell lesioned animals did not differ fromshams.The results of Experiment 1 add to the evidence that the DA system canmoderate salience processing of weaker predictors, also in cases where CSsalience is manipulated directly via the physical intensities of the stimuli,as here. However, in terms of the brain structures involved,Experiment 2 suggests that, overshadowing is moderated by projectionsof the DA system without n.acc.

Key Words: appetitive conditioning, overshadowing, amphetamine, nucleus accumbens, dopamine, rat

First published on January 21, 2008, doi:10.1177/0269881107079062

Journal of Psychopharmacology 2008;22:172.

A more recent version of this article appeared on March 1, 2008


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