Conditioned Place Preference as a Function of Individual Differences in Impulsivity

1. Conditioned Place Preference as a Function of Individual Differences in Impulsivity Justin Yates & Michael Bardo University of Kentucky, Dept of Psychology Figure 4. Figure 3. • Introduction • Impulsivity is a multifaceted construct that includes lack of inhibitory control, lack of forethought, and inability to delay gratification (Evenden, 1999). • Amphetamine (AMPH) is a potent psychostimulant used to treat attention deficit hyperactivity disorder (ADHD), narcolepsy, and obesity and has high abuse potential (Berman et al., 2009). • Past research has indicated a link between impulsivity and drug abuse (Olmstead, 2006). • The present experiment examined amphetamine conditioned place preference (CPP) in high impulsive rats and low impulsive rats. • The purpose of this study was to determine if rats high in impulsivity are more sensitive to the rewarding properties of amphetamine measured by the CPP task. • The hypothesis of this study was that rats high in impulsivity would show greater CPP than rats low in impulsivity. • Results • Across the 21 days of delay discounting sessions, there was an overall increase in MAD scores that reached a plateau for high and low impulsives after approximately 14 sessions (Figure 1). • During CPP trials, both high and low impulsive rats experienced increased locomotor activity when given amphetamine, but there were no significant differences between groups collapsed across trials (Figure 2). • There was no significant difference between high and low impulsives based on the CPP difference scores (Figure 3). • However, when the CPP results were expressed as absolute values, high impulsives spent significantly more time in the amphetamine-paired compartment compared to the low impulsives (t = 2.857, p < .05)(Figure 4). * Figure 1. • Methods • Eighteen male Sprague-Dawley rats served as subjects. • To measure individual differences in impulsivity, rats were tested on a delay discounting procedure for 21 days. • Rats were able to choose between a small, immediate reward (1 food pellet) and a larger, delayed reward (3 food pellets). • Mean adjusted delay (MAD) scores were calculated, and the average MAD scores for the last 10 days were analyzed. • Rats with MAD scores below the median were considered high impulsives; rats with MAD scores above the median were considered low impulsives. • Following delay discounting, rats went through 10 days of CPP. • On day 1, rats were allowed to explore all three compartments of the CPP chamber for 15 minutes (pretest), and the time spent in each compartment was calculated. • On days 2 through 9, rats received injections of d-amphetamine (0.5 mg/kg, s.c.) and saline on alternating days, and were confined to either the white or black compartment (counterbalanced for drug) for 30 minutes. Activity was measured by photobeams on each conditioning trial. • After conditioning, rats were allowed to explore all three compartments for 15 minutes (posttest) on day 10. • A difference score was calculated as the difference in time spent in the drug compartment on the pretest and posttest sessions. *p < .05 compared to Low AMPH • Discussion • AMPH produced a similar increase in locomotor activity in both high and low impulsive rats. • High impulsive rats spent significantly more time in the compartment paired with AMPH during the posttest compare to low impulsive rats. • Even though there was no significant difference in difference scores between the two groups, this effect could be attributed to an outlier in the low impulsive group. When this rat’s score was removed, there is a significant difference between these groups • These results indicate that impulsivity in a delay discounting task does not predict individual differences in the locomotor stimulant effect of AMPH. • These results also indicate that impulsivity in a delay discounting task does predict the rewarding property of AMPH as measured by CPP. • Thus, these findings provide further evidence indicating that impulsivity is an antecedant risk factor for drug abuse vulnerability. • References • Berman, SM, Kuczenski, R, McCracken, JT, London, ED. (2009). Potential adverse effects of amphetamine treatment on brain and behavior: a review. Molecular Psychiatry, 14, 123-142. • Evenden, JL. (1999). Varieties of impulsivity. Psychopharmacology, 146, 348-361. • Olmstead, MC. (2006). Animal models of drug addiction: Where do we go from here? The Quarterly Journal of Experimental Psychology, 59, 625-653. Figure 2. • Acknowledgements • Funding provided by NIH grant P50 DA05312. • Thanks to Josh Beckmann, Julie Marusich, and Cassie Gipson. • Future Directions • Another experiment will repeat this experiment using a higher dose of AMPH (1.0 mg/kg). • We plan to determine if dopamine and serotonin systems in the orbitofrontal and medial prefrontal cortices play a role in these behavioral effects.