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The Journal of Neuroscience, January 3, 2007, 27(1):245-250; doi:10.1523/JNEUROSCI.4080-06.2007

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Brief Communications
Previous Cocaine Exposure Makes Rats Hypersensitive to Both Delay and Reward Magnitude

Matthew R. Roesch,¹ Yuji Takahashi,¹ Nishan Gugsa,³ Gregory B. Bissonette,² and Geoffrey Schoenbaum^1,2,3

¹Departments of Anatomy and Neurobiology and Psychiatry, and ²Program in Neuroscience, University of Maryland School of Medicine, Baltimore, Maryland 21201, and ³Department of Psychology, University of Maryland Baltimore County, Baltimore, Maryland 21228

Correspondence should be addressed to Matthew Roesch, Departments of Anatomy and Neurobiology, 20 Penn Street HSF-2, Room 5251, University of Maryland School of Medicine, Baltimore, MD 21201. Email: mroes001{at}umaryland.edu

Animals prefer an immediate over a delayed reward, just as they prefer a large over a small reward. Exposure to psychostimulants causes long-lasting changes in structures critical for this behavior and might disrupt normal time-discounting performance. To test this hypothesis, we exposed rats to cocaine daily for 2 weeks (30 mg/kg, i.p.). Approximately 6 weeks later, we tested them on a variant of a time-discounting task, in which the rats responded to one of two locations to obtain reward while we independently manipulated the delay to reward and reward magnitude. Performance did not differ between cocaine-treated and saline-treated (control) rats when delay lengths and reward magnitudes were equal at the two locations. However, cocaine-treated rats were significantly more likely to shift their responding when we increased the delay or reward size asymmetrically. Furthermore, they were slower to respond and made more errors when forced to the side associated with the lower value. We conclude that previous exposure to cocaine makes choice behavior hypersensitive to differences in the time to and size of available rewards, consistent with a general effect of cocaine exposure on reward valuation mechanisms.

Key words: time-discounting; behavior; cocaine; delayed gratification; rat; reward

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Received Sept. 18, 2006; revised Dec. 1, 2006; accepted Dec. 1, 2006.

Correspondence should be addressed to Matthew Roesch, Departments of Anatomy and Neurobiology, 20 Penn Street HSF-2, Room 5251, University of Maryland School of Medicine, Baltimore, MD 21201. Email: mroes001{at}umaryland.edu

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