Ashdin Publishing Journal of Drug and Alcohol Research ASHDIN Vol. 4 (2015), Article ID 235940, 6 pages publishing doi:10.4303/jdar/235940 Research Article Inhibition of Aldehyde Dehydrogenase-2 (ALDH-2) Suppresses Nicotine Self-Administration in Rats Amir H. Rezvani,1 Edward D. Levin,1 Maria P. Arolfo,2 Corinne Wells,1 Michael Graupe,2 and Ivan Diamond2 1Department of Psychiatry and Behavioral Sciences, Duke University School of Medicine, Durham, NC 27710, USA 2Gilead Sciences, Palo Alto, CA 94304, USA Address correspondence to Amir H. Rezvani, [email protected] Received 31 August 2015; Revised 13 November 2015; Accepted 16 November 2015 Copyright © 2015 Amir H. Rezvani et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Introduction. Aldehyde dehydrogenase-2 (ALDH-2) addiction. Nicotine exerts its reinforcing effects by binding inhibitors have been shown to reduce cocaine and alcohol intake to neuronal nicotinic acetylcholine receptors (nAChRs), in rats. The mechanism of action appears to be due to inhibition of leading to enhanced release of dopamine (DA) and other drug-induced dopamine (DA) production in the ventral tegmental area (VTA) and DA release in the nucleus accumbens. The purpose of this neurotransmitters involved in reward and pleasure [2]. study was to explore the potential of a selective ALDH-2 inhibitor to Among the brain monoaminergic neurotransmitter reduce self-administration of nicotine. Materials and methods. Adult systems, the dopaminergic system has been most clearly male rats were trained to self-administer nicotine intravenously. After implicated in reinforcing properties of addictive drugs acquiring a stable baseline for nicotine intake, rats were given one of the three oral gavage doses (5, 10 or 30 mg eq/kg, calculated based including nicotine [3,4,5,6,7]. A wide range of animal on parent drug) of the prodrug GS-6637 of an ALDH-2 inhibitor or studies as well as in vivo imaging have demonstrated vehicle one hour before a nicotine self-administration session. Results. that DA plays a major role in reinforcing properties of Our data showed that acute administration of GS-6637 at 10 mg eq/kg drugs of abuse including nicotine [8]. Nicotine is not and 30 mg eq/kg significantly reduced nicotine self-administration. Similarly, subchronic administration of GS-6637 for seven days the only psychoactive compound in cigarette smoke. significantly reduced nicotine self-administration at 10 mg eq/kg and In addition to nicotine itself, other natural and added 30 mg eq/kg without inducing tolerance. In order to compare GS-6637 constituents of cigarette smoke might also contribute to with varenicline, rats were given single doses of varenicline at 1.6, 3.2, smoking addiction. For example, condensation products and 6.4 mg/kg. Consistent with previous reports, significant inhibitions of nicotine self-administration was observed at the 3.2 mg/kg and of acetaldehyde with biogenic amines in cigarette smoke 6.4 mg/kg doses but less than observed with GS-6637. Discussion inhibit the activity of the enzyme monoamine oxidase and conclusions. Our data suggest that selective ALDH-2 inhibition (MAO), which leads to a decrease in DA metabolism appears to have therapeutic potentials as novel therapy for smoking and consequently an increase in DA concentration [9]. cessation. Mesolimbic DA neurons express high-affinity nAChRs Keywords nicotine addiction; treatment; smoking cessation; both on their cell bodies and terminals [10]. Moreover, dopamine; inhibition; aldehyde dehydrogenase-2; self-administration presynaptic nAChRs on midbrain DA neurons project from the ventral tegmental area (VTA) to the nucleus accumbens 1. Introduction and prefrontal cortex. These presynaptic nAChRs evoke Nicotine addiction arising from smoking tobacco is a DA release when they are activated by nicotine (tobacco major health problem. Approximately 1.1 billion people smoking) [2] or alcohol administration [11,12,13]. It has worldwide smoke tobacco. Although the prevalence of been reported that acute nicotine administration enhances smoking in North America has been decreasing and striatal and limbic DA turnover and metabolism [14]. currently is about 23%, smoking is much more prevalent Nicotine also elevates extracellular DA in the striatum and in developing countries and continuing to rise. The health in the nucleus accumbens of rats [7,15,16]. Additionally, consequences of smoking and costs to the society are the demonstration that nicotine can block DA reuptake [17] enormous. Tobacco smoking is the most common cause of strengthens the hypothesis that DA mechanisms may be cancer-related deaths and the leading cause of heart disease, important in nicotine reinforcement. emphysema, and bronchitis [1]. Among thousands of Current available therapies to reduce tobacco use include artificial and natural constituents in tobacco smoke, nicotine nicotine replacement, bupropion, and varenicline (Chantix; is the main psychoactive constituent that underlies smoking Pfizer, KS, USA) [18]. However, they have limited success 2 Journal of Drug and Alcohol Research rates in the large majority of people attempting to quit or were calculated as a function of the nicotine base weight. trying to maintain long-term abstinence [19]. Recently, it The pH of the solutions was adjusted to 7.0 using NaOH. has been shown that inhibition of aldehyde dehydrogenase- Next, the solutions were passed through a 0.2 µm filter 2 (ALDH-2) can suppress cocaine intake [20] and alcohol (Millipore Corp, Billerica, MA, USA). Solutions of GS- intake in rats [21]. The mechanism of action of ALDH-2 6637 (Lot #8 & 9) were prepared in pyrogen-free glassware inhibition has been shown to involve suppression of DA pro- in vehicle (Lot #MG-4373-143, Formulation 2B: 25% PEG duction [21]. Furthermore, recently we showed that another 400/5% Vit E TPGS/1% SLS, 0.5% Methocel, 69% water). selective mitochondrial ALDH-2 inhibitor, GS-455534 (pre- GS-6637 and vehicle were provided by Gilead Sciences. viously known as CVT-10216), at low doses significantly Varenicline was purchased from Sigma-Aldrich (Saint reduces alcohol intake in alcohol preferring rats [22]. These Louis, MO, USA) and was dissolved in dH2O. GS-6637 findings suggest that ALDH-2 inhibitors might also reduce and vehicle were administered orally (gavage) in a volume nicotine intake. of 4 mL/kg one hour before testing sessions. Varenicline The aims of the current studies were (1) to determine (4 mL/kg, PO) and its vehicle were administered 15 min the acute oral dose-response of GS-6637, a novel selective before testing. ALDH-2 inhibitor, on nicotine self-administration, (2) to learn whether tolerance develops after repeated oral 2.4. Training and surgery administration of GS-6637, and (3) to compare GS- Rats were placed in dual lever test chambers for behavioral 6637 efficacy to varenicline. It is hypothesized that training. Each chamber was equipped with a tone generator, inhibition of ALDH-2 by GS-6637 will reduce nicotine a house light, a cue light above each lever, and a metal tether self-administration. to cover the drug delivery line. A computer programmed with MED-PC software controlled experimental events and 2. Materials and methods data collection. Each catheter was connected to a micro- 2.1. Animals liter syringe pump and tethers made of polyethylene tubing Male Sprague-Dawley albino rats (225–250 g at arrival) with huber needles for access to ports and catheters. Rats were singly housed in approved standard laboratory wore infusion harnesses during each session to connect them conditions at the Duke University vivarium facility next to the tethers. Initially, the rats were trained daily during to the testing room to minimize any stress induced by 15 min with tutor sessions to press the levers for food pellet transporting the rats. The day-night cycle was reversed so reinforcers. Half the animals were rewarded for responding that rats were in their active phase during behavioral testing. on the right lever and half for responding on the left. Only All rats had ad lib access to water and were fed the same the cue light over the correct lever was illuminated while type of rat chow once daily throughout the study to keep the light over the incorrect lever was off. Responses on the them at approximately 85% ad lib weight with food amounts correct lever were rewarded by pressing a button connected adjusted from 8 g to 16 g per day as they grew to provide to the control panel, which caused immediate delivery of one a lean healthy growth curve. The procedures used in this 45 mg food pellet and activation of the feedback tone for study were approved by the Duke University Animal Care 0.5 s. There were no timeout periods in the tutor sessions. and Use Committee and conform to the Animal Care Guide. These tutor sessions were followed by three daily 45 min pellet sessions on a fixed ratio 1 (FR1) schedule, where rats 2.2. Nicotine self-administration obtained one food pellet after pressing the lever once. The Three sets of experiments were carried out. First, we estab- training procedure took place before rats underwent surgery lished a dose-response function following acute oral admin- for catheter implantation for IV nicotine self-administration. istration of 0, 5, 10, and 30 mg eq/kg of GS-6637 using Following the completion of their final training session four separate groups of rats (n = 13–14/group). Next, we with food reinforcement, animals were anesthetized with a studied the effect of subchronic (seven consecutive days) mixture of ketamine (60 mg/kg) and dormitor (15 mg/kg). oral gavage administration of the compound at 0, 5, 10, and A plastic SoloPort was attached intraoperatively to a 30 mg eq/kg using the same four independent groups used polyurethane catheter (Strategic Application, Libertyville, for the dose response study (n = 12–14/group).