Introduction

1. Individual differences in impulsive action and impulsive choice are associated with dopamine and serotonin transporter function in rat orbitofrontal cortex M. Darna1, J.Yates2, M. T. Bardo2, L. P. Dwoskin1 1 Department of Pharmaceutical Sciences, College of Pharmacy,and 2 Department of Psychology,University of Kentucky, Lexington, Kentucky, USA mPFC OFC VMAT VMAT DA 5-HT 5-HT MAO MAO 5-HTreceptors D2 LikeAutoreceptors DOPAC 5HIAA DA 5-HT 5-HT DA DAT SERT 3-MT 3-MT COMT DA receptors 5-HT receptors Introduction Results Summary Impulsive action and impulsive choice by cued go/no-go and delay discounting tasks Recent studies suggest that impulsivity has a prominent role in drug abuse vulnerability. Impulsivity is a multifaceted construct, broadly divided into impulsive action and impulsive choice. Dysregulation of dopamine (DA) and 5-hydroxytryptamine (5-HT or serotonin) systems in prefrontal cortex, such as medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC), have been implicated in impulsivity. Extracellular concentrations of DA and 5-HT are dependent on both release and uptake processes. Thus, the DA transporter (DAT) and 5-HT transporter (SERT) may be important molecular targets contributing to individual differences in impulsivity. The current aim was to determine the role of DAT and SERT in mPFC and OFC in individual differences in impulsive action and choice. • Vmax for[3H]DA uptake by DAT in OFC, but not in mPFC, was positively correlated with the ratio of VI responses/EXT responses in cued go/no-go task. • Vmaxfor[3H]5-HT uptake by SERT in OFC, but not in mPFC, was negatively correlated with MAD determined in the delay discounting task. Fig 2: A. Average (n=36) ratio of responses during variable interval phase and during extinction phase (VI responses/EXT responses) plotted for 14 days of cued go/no-go. B. Average (n=36) of MADs plotted for 21 days of delay discounting. Conclusions • OFC plays an important role in impulsive action and choice. • A positive correlation between DAT function in OFC and ratio of VI responses/EXT responses suggests that increased impulsive action may be associated with high extracellular levels of DA in OFC. • A negative correlation between SERT function in OFC and MAD suggests that increased impulsive choice may be associated with low extracellular levels of 5-HT in OFC. • Impulsive action and DAT Impulsive choice and SERT • Fig 6: schematic of dopamine and serotonin release and uptake • Current results suggest that the prefrontal mechanisms of impulsive action and choice are dissociable, with impulsive action associated with decreased DAT function in OFC and impulsive choice associated with increased SERT function in OFC. Methods Fig. 3 Representative saturation analyses of [3H]DA and [3H]5-HT uptake into synaptosomes of mPFC (A, C) and OFC (B, D) obtained from individual rats. • Animals: Male 45-day old Sprague-Dawley rats were used. • Across 21 day periods, 2 sets of rats (n=18/set) were tested for impulsive action (cued go/no-go task) and impulsive choice (delay discounting task) in a counter-balanced order. • Cued go/no-go task: Rats were reinforced for responding during a go cue, and not during a no-go cue. The primary dependent measure was the ratio of variable interval (VI) responses during go trials compared to extinction (EXT) responses during no-go trials, averaged across the last 7 sessions. High VI/EXT ratios indicate low impulsive action. • Delay discounting task: Rats were allowed to choose between a small immediate reward and a larger, delayed reward. The mean adjusted delay (MAD) score was calculated by averaging the delays across the last 10 sessions. Low MAD scores indicate high impulsive choice. • Positive correlation between VI/EXT responses and DAT function in OFC OFC Fig 4: Correlations between VI/EXT responses measured in cued go/no-go task and Vmaxvalues for [3H]DA uptake in mPFC (A) and OFC (B); and correlations between MAD measured in delay discounting task and Vmaxvalues for [3H]DA uptake in mPFC (C) and OFC (D). Positive correlation was obtained between VI/EXT responses and Vmaxvalues for [3H]DA uptake in OFC (figure 4B in red box). Fig1: Regions of Interest:Rat brain section showing mPFC and OFC. • Negative correlationbetween MAD and SERT function in OFC [3H]DA and [3H]5-HT Uptake:mPFC and OFC were homogenized in ice-cold 0.32 M sucrose solution. Homogenates were centrifuged at 2,000g for 10 min at 4oC. Supernatants were centrifuged at 20,000g for 17 min at 4oC. Maximal velocity (Vmax) and affinity (Km ) of [3H]DA or [3H]5-HT uptake were determined. [3H]DA uptake was determined in presence of desipramine (5 nM) and paroxetine (5 nM) to prevent uptake into norepinephrine (NE)- and 5-HT-containing nerve terminals, respectively. [3H]5-HT uptake was determined using buffer containing desipramine (1 µM) and GBR 12909 (50 nM) in order to prevent [3H]5-HT uptake into NE and DA nerve terminals, respectively. Nonspecific uptake for [3H]DA or [3H]5-HT was determined using 10 µM nomifensine and 10 µM fluoxetine, respectively. Acknowledgements Supported by P50 DA05312 (UK Center for Drug Abuse Research Translation) Fig 5: Correlations between MAD scores measured in delay discounting task and Vmaxvalues for [3H]5-HT uptake in mPFC (A) and OFC (B); and correlations between VI/EXT responses measured in cued go/no-go task and Vmaxvalues for [3H]5-HT uptake in mPFC (C) and OFC (D). Negative correlation was obtained between MAD and Vmaxvalues for [3H]5-HT uptake in OFC (figure 5D in red box).