International Journal of Neuropsychopharmacology (2012), 15, 1473–1487. f CINP 2011 doi:10.1017/S1461145711001635
ARTICLE
Impulsive action and impulsive choice are mediated by distinct neuropharmacological substrates in rat Neil E. Paterson, Caitlin Wetzler, Adrian Hackett and Taleen Hanania PsychoGenics Inc., 765 Old Saw Mill River Road, Tarrytown, USA
Abstract Impulsivity is a heterogeneous construct according to clinical and preclinical behavioural measures and there is some preliminary evidence indicating distinct neurobiological substrates underlying the sub-components of impulsivity. Two preclinical assays, the five-choice serial reaction time task (5-CSRTT) and the delayed discounting task (DDT), are hypothesized to provide measures of impulsive action (premature responding) and impulsive choice (percent choice for delayed reward), respectively. In the present studies, we show that the norepinephrine reuptake inhibitor atomoxetine attenuated premature responding in the 5-CSRTT, but was ineffective in the DDT. The mixed dopamine/norepinephrine reuptake inhibitor methylphenidate exhibited an opposite profile of effects. In addition, blockade of 5-HT2A/C receptors via ketanserin decreased premature responding but had no effects on percent choice for delayed reward ; blockade of 5-HT2C receptors via SB 242084 had opposite effects. Follow-up studies provided some limited evidence of additive effects of 5-HT2A/C receptor blockade on the effects of atomoxetine on impulsive action. These studies demonstrate dissociable profiles of stimulant vs. nonstimulant attention deficit hyperactivity disorder medications and 5-HT subtype-selective ligands, in the 5-CSRTT and DDT assays. Thus, the present findings support the sub-categorization of impulsivity and suggest that 5-HT receptor subtype-selective antagonists may provide therapeutic targets for disorders characterized by different forms of impulsivity. Received 13 June 2011 ; Reviewed 21 July 2011 ; Revised 12 September 2011 ; Accepted 30 September 2011 ; First published online 18 November 2011 Key words : Atomoxetine, 5-CSRTT, delayed discounting task, methylphenidate, serotonin.
Introduction Impulsivity is currently thought to be a heterogeneous construct that comprises two subcomponents : impulsive action and impulsive choice (see Winstanley et al. 2006 for a review of animal models of impulsivity). There is increasing evidence for dissociable neurobiological substrates underlying these constructs. The current studies explore the neuropharmacological substrates of impulsive action and impulsive choice, each of which can be assessed with distinct translational assays. Premature responding in the five-choice serial reaction time task (5-CSRTT), which has been described in detail elsewhere (e.g. Bari et al. 2008), is Address for correspondence : Dr N. E. Paterson, Behavioral Pharmacology, PsychoGenics, Inc., 765 Old Saw Mill River Road, Tarrytown, NY 10591, USA. Tel. : 914-406-8058 Fax : 914-406-8090 Email :
[email protected]
hypothesized to provide a measure of impulsive action, i.e. a failure to withhold an inappropriate response (e.g. Bizarro et al. 2004 ; Day et al. 2007). The 5-CSRTT is a preclinical analogue of the continuous performance test in humans, which requires subjects to monitor a succession of stimuli and respond only when a specific stimulus is presented and to withhold responding in the absence of a stimulus (see Young et al. 2009 for a recent adaptation of the rodent 5-CSRTT). The delay discounting task (DDT) is a cross-species task used to assess impulsive choice, defined as intolerance of reward delay (Winstanley et al. 2006), where high impulsivity detected in the task is thought to be a trait of attention deficit hyperactivity disorder (ADHD) patients and other patient populations. In animals, the DDT has been used to assess preclinical efficacy of ADHD medications and additional explorations of neuropharmacological substrates of impulsivity.
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Atomoxetine (Strattera1 ; Lilly, USA), a norepinephrine transporter (NET) inhibitor (Wong et al. 1982), attenuates impulsivity in preclinical studies (for review, see Eagle & Baunez, 2010) and is an efficacious ADHD medication (Chamberlain et al. 2007 ; Michelson et al. 2003). Specifically, atomoxetine was effective in 5-CSRTT (Blondeau & Dellu-Hagedorn, 2007 ; Navarra et al. 2008 ; Paterson et al. 2011 ; Robinson et al. 2008), DDT (Robinson et al. 2008) and the stop-signal reaction time task (Bari et al. 2009) in rats. The anti-impulsive effects of atomoxetine may be due to enhanced cortical noradrenergic and/or dopaminergic neurotransmission (Bymaster et al. 2002 ; Swanson et al. 2006). The mixed dopamine (DA)/ norepinephrine (NE) reuptake inhibitor methylphenidate (Ritalin1 ; Novartis, Switzerland ; Han & Gu, 2006) increased NE and DA overflow in the prefrontal cortex and striatum, unlike atomoxetine, which has no striatal effects (Bymaster et al. 2002) and is an effective ADHD medication (Shiels et al. 2009 ; Simpson & Plosker, 2004). It is the distinct effects of atomoxetine and methylphenidate on striatal DA (and perhaps NE) that results in non-stimulant vs. stimulant effects, respectively. Based on selective neurochemical effects in striatum and contrasting behavioural properties in the 5-CSRTT (Navarra et al. 2008 ; Paterson et al. 2011), methylphenidate and atomoxetine can be distinguished as stimulant and non-stimulant ADHD medications. Although Robinson et al. (2008) reported efficacy of atomoxetine in the 5-CSRTT and the DDT, we hypothesized that stimulant and non-stimulant medications may exhibit different efficacy profiles in 5-CSRTT vs. DDT. In addition to NE and DA, serotonin has also been implicated in aspects of impulsivity. Specifically, administration of the 5-HT2C/B antagonist SER-082 was effective in decreasing impulsivity in the 5-CSRTT but not the DDT ; the 5-HT2A/C antagonist ketanserin exhibited opposite effects (Passetti et al. 2003 ; Ruotsalainen et al. 1997 ; Talpos et al. 2006). Earlier work exploring the effects of 5-HT2 subtype-selective ligands on impulsive behaviours indicated that 5-HT2A blockade attenuated impulsivity (Fletcher et al. 2007 ; Higgins et al. 2003 ; Winstanley et al. 2004), whereas 5-HT2C blockade increased premature responding in the 5-CSRTT (Fletcher et al. 2007 ; Winstanley et al. 2004). Most relevant to the 5-CSRTT task variant used in the present study is the recent finding that 5-HT2A blockade attenuated high levels of premature responding observed under prolonged inter-trial interval (ITI) (9 s) conditions (Fletcher et al. 2011). The current studies sought to replicate the effects of ketanserin in a variable ITI 5-CSRTT and
DDT and extend the effects of the selective 5-HT2C antagonist SB 242084 from the 5-CSRTT to the DDT in rats. In summary, impulsivity can be subdivided into several distinct sub-components that, based on current preclinical evidence, likely exhibit some degree of distinct and shared neurobiological substrates. Based on previous studies with ADHD medications and selective 5-HT receptor subtype ligands, the present studies assessed the effects of atomoxetine, methylphenidate, ketanserin and SB 242084 in the 5-CSRTT, hypothesized to provide a measure of impulsive action, and the DDT, hypothesized to provide a measure of impulsive choice. Based on the observed dissociable effects of these ligands, follow-up studies determined whether co-administration of 5-HT receptor subtype ligands and NE/DA reuptake inhibitors yielded additive effects on therapeutic efficacy. Method Subjects Male Long–Evans rats (275–300 g) were obtained from Harlan Laboratories (USA). Upon arrival, the rats were assigned unique identification numbers (tail marked). Rats were single-housed in standard or OptiRAT cages (Animal Care Systems Inc., USA) and acclimated for 7 d prior to commencing a food restriction regimen : rats were held at 85 % of agematched free-feeding control body weights, receiving approximately 10–20 g of rat chow daily. Water was provided ad libitum, except during testing. Animals were maintained on a 12-h light/dark cycle (lights on 07 : 00 hours EST) with room temperature maintained at 22¡2 xC and the relative humidity maintained at approximately 50 %. All animals were examined, handled and weighed prior to initiation of the study (during the week of habitation) to ensure adequate health and suitability and to minimize non-specific stress associated with testing. All efforts were made to minimize discomfort of any sort at all times during the conduct of the studies. Behavioural test sessions were performed during the animal’s light cycle phase. All experiments were approved by the Institutional Animal Care and Use Committee of PsychoGenics, Inc. (USA) in AAALAC-accredited facilities and in accordance with the Guide to the Care and Use of Laboratory Animals (NIH, 2010). Apparatus The test apparatus consisted of 10 aluminium and Plexiglas chambers with grid floors (width 31.5 cm,
Pharmacologic dissociation of impulsivity subcategories depth 25.0 cm, height 33.0 cm), housed in soundattenuating cabinets (Med Associates, USA). Each cabinet was fitted with a low-level noise extractor fan. The left wall of each chamber was concavely curved, with five apertures evenly spaced, located approximately 2.5 cm from the floor. Each aperture contained a standard 3 W LED to serve as stimulus lights. The opposite wall contained a food magazine, located approximately 3.0 cm from the floor. A retractable lever (approximately 2 cm long) was positioned on either side of the food magazine (approximately 2 cm from the grid floor). Each chamber was illuminated with a 1.5 W house-light located above the food magazine, 1 cm from the chamber ceiling. After each test session, the apparatus was cleaned with 70 % ethanol ; at least 20 min elapsed before the next test session. The test session functions were controlled and all data were captured, by a computer using K-Limbic software (Conclusive Software Ltd, distributed by Med Associates, USA). Experimental procedures 5-CSRTT Animals were trained to monitor the five apertures for stimulus light illumination. Each session was initiated by the illumination of the house-light and the delivery of a food reward into the magazine. The first trial began when the rat nose-poked into the magazine to obtain the food pellet. After the ITI (5 s duration), one of the stimulus lights was illuminated for 500 ms. The rat should nose-poke in the illuminated aperture either during or within 5 s of the stimulus light illumination. Such a response was defined as a correct response and was rewarded with delivery of a food pellet. Collection of the pellet initiated the next trial. A nose-poke response in a non-illuminated aperture (incorrect response) or a nose-poke after the 5 s limited hold (missed trial) resulted in termination of the trial with extinction of the house-light and imposition of a time-out period (5 s duration). Nose-pokes made during the ITI (premature responses) and nose-pokes made after the first response to the stimulus presentation (perseverative response) were punished by imposition of the time-out ; premature responses resulted in a reset of the ITI. Rats took 40–60 sessions to acquire the task. After acquisition of the 5-CSRTT [>70 % correct,
