Cocaine Administration but Not Morphine Leads to Increases in Impulsivity as Measured in a Delay Discounting Task

Cocaine Administration but Not Morphine Leads to Increases in Impulsivity as Measured in a Delay Discounting Task

Jeri Keitzer, Olivia Anchondo, Kaylee Brunetti, Samia Shuchi, and Luis Colon-Perez

Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, TX, 76107

Purpose: Inhibitory and cognitive control are mental process susceptible to repeated exposure to drugs of abuse. Specifically, impulsivity refers to the tendency to act prematurely without regard of the future. Substance use disorders patients (SUD) show impaired impulsivity; however, it is unclear whether psychostimulant use precedes impulsive behavior or vice versa. The delay discounting task is an assessment of impulsive choice used in behavioral neuroscience research. In this task, subjects are given a choice between an immediate small reward or a delayed large reward in which the time of delay increases across trials. Subjects that discount the delayed reward earlier are determined to act with more impulsivity. In this current study, we looked at impulsivity as it relates to performance in a delay discounting task before and after IP injection drug administration of either morphine or cocaine. We hypothesized that the subjects that were administered drugs would devalue the delayed reward earlier within the delay discounting task than control subjects.

Methods: For this study, we used Sprague Dawley rats split into 4 different groups: cocaine, morphine, saline, and naïve. All subjects were food restricted to increase motivation to obtain the reward food pellets. Rats began training on the delay discounting task after a training phase in which the rats were introduced to the behavioral chambers and exposed to the levers within the chambers in which they were trained to press to receive pellets. The delay discounting task consisted of a daily session in which the subject went through 5 phases (a total of 60 total trials) choosing between two levers to receive pellets. Each phase is characterized by two forced trials and ten choice trials. In the forced choice trials, the animal has only one option: either the immediate small reward or delayed choice reward. At each phase the delay increases from no delay to 4s, 8s, 16s, and 32s. During choice trials, rats choose between the immediate and delayed levers. Once a decision has been made, both levers retract until the next trial. First, baseline data of each subject’s impulsive choice was acquired, then the subjects underwent IP injection of either cocaine, morphine, or saline. A naïve group did not receive any injections. Rats were injected with drugs according to their weight (5 mg/kg) once per day for seven days, then they completed the delay discounting task again under experimental conditions.

Results: The cocaine drug group showed a significant decrease in delayed lever presses compared to all other groups during the delay discounting task following drug administration. No other significant differences were seen across any of the other groups.

Conclusion: The results of the study suggest that passive administration of cocaine in rats may lead to an increase in impulsivity but not morphine.