1. Introduction Impulsivity has been linked to drug abuse, although it is unclear whether it is a determinant or consequence of drug abuse (de Wit, 2009). Impulsive choice, or preference for immediate over delayed gratification, has been indicated in stimulant abuse. Delay discounting is a task used to measure impulsive choice in human and nonhuman animals in which subjects choose between a smaller sooner and a larger later reward. One type of delay discounting task includes an adjusting delay, in which the delay to the larger, delayed reward adjusts according to the animal’s behavior. In this task, mean adjusted delays (MADs), or the average delay (sec) to reward across trials, are generated. A higher MAD score indicates less impulsive choice, whereas a lower MAD score indicates more impulsive choice. Previous research has found that temporary inactivation of portions of medial prefrontal cortex (mPFC) impairs performance in delay discounting (thus making rats more impulsive; Evenden & Ryan, 1996) and decreases reinstatement of cocaine-seeking behavior (McLaughlin & See, 2003; Fuchs et al., 2005). Thus, the mPFC plays a role in impulsive choice and drug abuse vulnerability, although the role of specific neurotransmitter systems in these processes is still unclear. The goal of the current studies was to examine the role of neurotransmitter systems in both mPFC and orbitofrontal cortex (OFC) in delay discounting, thus determining if there is region-specificity in the role of these neurotransmitter systems in impulsive choice. Method Rats were initially trained in an adjusting delay discounting task. Following training, rats underwent intracranial surgery in which guide cannulae were implanted bilaterally in either mPFC (AP: +2.7, ML: ±1.2, DV: -2.6 ) or OFC (AP: +4.2, ML: ±2.4, DV: -3.4; coordinates according to Paxinos & Watson, 1998). Following recovery, rats were allowed to stabilize in MADs. They were then given a session in which a PBS infusion was administered intracranially prior to delay discounting. Rats in Experiment 1 were then given methylphenidate, d-amphetamine, and atomoxetine infusions into either mPFC or OFC prior to their sessions, in a latin-square design. There was always at least one day washout between infusions. Rats in Experiment 2 were given an additional PBS+HCl vehicle infusion for ketanserin, as ketanserin did not readily go into solution with PBS. They were then given infusions of 5-HT selective drugs 8OHDPAT, WAY-100635, DOI, or ketanserin. Rats in Experiment 3 were given intracranial infusions of DA selective drugs SKF 81297, SCH23390, quinpirole, or eticlopride into either mPFC or OFC. Following the last infusion, probe placements from each rat were checked using histological methods (see Figure 1). MADs, response latencies, and total nonreinforced responses were analyzed for Experiments 1 (Figures 2a-f), 2 (Figures 3a-f), and 3 (Figures 4a-f). Cassandra D. Gipson 1, Jennifer L. Perry 2, Justin Yates 3, Andrew Meyer 3, Joshua S. Beckmann 3, & Michael T. Bardo 3 1 Medical University of South Carolina, 2 Kalamazoo College, 3 University of Kentucky Results and Discussion In mPFC, dopamine D2 (DA D2) receptors appear to play an important role in the relationship between impulsive choice and drug abuse, as methylphenidate (6.25 and 100 µg) increased MADs, and the DA D2 receptor antagonist eticlopride (1.0 µg) decreased MADs in the mPFC. In OFC, although methylphenidate (6.25 µg) increased nonreinforced responses, this was not significant. No other significant effects were found. It appears that there is region-specificity in the prefrontal cortex with how methylphenidate, an ADHD medication, affects impulsive choice in delay discounting. Although methylphenidate has more than one mechanism of action, it has been found to inhibit reuptake of DA, similar to cocaine (Fleckenstein et al., 2009). The non-monotonic dose-effect function found in mPFC in the current study may reflect a differential ability of methylphenidate to alter multiple cellular targets. No differences in MAD scores, however, were found in the OFC following intracranial infusions of various doses of methylphenidate. Although d-amphetamine and atomoxetine have clinical efficacy in treating ADHD, these drugs did not significantly alter impulsive choice in the current study in mPFC or OFC. Systemic administration of atomoxetine has been found to dose-dependently decrease impulsive choice (Robinson et al., 2007). While it is unclear why intracranial injections of atomoxetine did not alter impulsive choice, it is possible that regions other than mPFC or OFC may be involved in its theapeutic effect. Additionally, serotonergic drugs failed to alter impulsive choice in mPFC and OFC in the current studies. The relationship between 5-HT and impulsive choice is unclear, as there are mixed results. For example, 5- HT depletion has been found to both increase (Wogar et al., 1993; Richards & Seiden, 1995) or not affect (Winstanley et al. 2003) impulsive choice. In conclusion, dopaminergic circuitry in mPFC, but not OFC, may be the underlying neurobiological link between impulsive choice and drug abuse. References de Wit (2009) Impulsivity as a determinant and consequence of drug use: a review of underlying processes. Addiction Biology, 14(1), 22-31. Evenden, J.L., & Ryan, C.N. (1996) The pharmacology of impulsive behaviour in rats: the effects of drugs on response choice with varying delays of reinforcement. Psychopharmacology, 128(2), 161-170. Fleckenstein, A.E., Volz, T.J., & Hanson, G.R. (2009) Psychostimulant-induced alterations in vesicular monoamine transporter-2 function: neurotoxic and therapeutic implications. Neuropharmacology, 56(1), 133-138. Fuchs, R.A., Evans, K.A., Ledford, C.C., Parker, M.P., Case, J.M., Mehta, R.H., & See, R.E. (2005) The role of dorsomedial prefrontal cortex, basolateral amygdala, and dorsal hippocampus in contextual reinstatement of cocaine seeking in rats. Neuropsychopharmacology, 30, 296-309. McLaughlin, J., & See, R.E. (2003). Selective inactivation of the dorsomedial prefrontal cortex and the basolateral amygdala attenuates conditioned-cued reinstatement of extinguished cocaine-seeking behavior in rats. Psychopharmacology, 168(1-2), 57-65. Paxinos, G., & Watson, C. (1998) The rat brain in stereotaxic coordinates. New York: Academic Press. Richards, J.B., & Seiden, L.S. (1995) Serotonin depletion increases impulsive behavior in rats. Society for Neuroscience abstracts. Robinson, E.S., Eagle, D.M., Mar, A.C., Bari, A., Banarjee, G., Jiang, X., Dalley, J.W., & Robbins T.W. (2007) Similar effects of the selective noradrenaline reuptake inhibitor atomoxetine on three distinct forms of impulsivity in the rat. Neuropsychopharmacology, 33, 1028-1037. Winstanley, C.A. Dalley, J.S., Theobald, D.E.H &Robbins, T.W.(2003) Global 5-HT depletion attenuates the ability of amphetamine to decrease impulsive choice on a delay-discounting task in rats. Psychopharmacology, 170(3), 320-331. Wogar, M.A., Bradshaw, C.M., & Szabadi, E. (1993) Effect of lesions of the ascending 5- hydroxytryptaminergic pathways on choice between delayed reinforcers. Psychopharmacology, 111(2), 239-243. Acknowledgements We would like to thank Emily Denehy, Julie Marusich, Kate Fischer, William T. McCuddy, Lindsay Pilgrim, Josh Cutshall, Blake Dennis, and Jason Ross for technical assistance. Supported by UPSHS grants F31 DA028018, P50 DA12964, and T32 DA 007304. We have no financial conflicts of interest to disclose. Role of Dopamine and Serotonin Receptors in Medial Prefrontal and Orbitofrontal Cortex on Impulsive Choice in Rats d.e. f. d. e. f. d.e. f. a. b. c. a. b. c. Figure 4. mPFC (a) MADs, (b) response latencies, (c) nonreinforced responses, and OFC (d) MADs, (e) response latencies, and (f) nonreinforced responses for drug treatment groups in Exp. 3, expressed as % vehicle control. a. b. c. Figure 2. mPFC (a) MADs, (b) response latencies, (c) nonreinforced responses, and OFC (d) MADs, (e) response latencies, and (f) nonreinforced responses for drug treatment groups in Exp. 1, expressed as % vehicle control. Figure 3. mPFC (a) MADs, (b) response latencies, (c) nonreinforced responses, and OFC (d) MADs, (e) response latencies, and (f) nonreinforced responses for drug treatment groups in Exp. 2, expressed as % vehicle control. Figure 1. Probe placements for (a) mPFC and (b) OFC. Bregma 5.16 mm Bregma 4.68 mm a. Bregma 5.16 mm Bregma 4.68 mm b. n = 9 n = 11 n = 9 n = 12 n = 9 n = 11