Heroin addicts display poorer impulse control than non-addicts, however it is not known if high impulsivity is a function of chronic heroin intake or a pre-disposing vulnerability for heroin addiction. Using animal models, relatively few studies have examined changes in impulsive choice as a function of chronic drug. The objective of this study was to measure alterations in impulsive choice through a delay discounting paradigm, as a function of chronic heroin administration. Animals were trained on a series of delay discounting sessions. Each session contained 5 blocks of trials. Blocks started with 2 forced, followed by 6 free choice trials. Pressing one lever resulted in the delivery of a small immediate (1 food pellet) reward and another lever in a large delayed (5 pellets) reward. Sessions consisted of the 3 ascending delay sequences in seconds. On the terminal sequence (0, 10, 20, 40, and 60s) animals exhibited a reversal of reward choice pattern of responding that allowed for the calculation of an indifference point (IP). After animals showed stable IPs they were treated with either heroin or saline for 12 days. Three days after the last injection animals were again placed in operant chambers and experienced the terminal delay discounting sequence at which time IPs were reassessed. Heroin-treated animals exhibited significant progressive increases in locomotor activity. Groups did not differ in IPs or performance across delay conditions during either before or after chronic treatment periods. These results indicate that chronic heroin intake does not impact later impulsive responding for natural (food) reward.
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