1521-0103/366/2/397–409$35.00 https://doi.org/10.1124/jpet.118.248070 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS J Pharmacol Exp Ther 366:397–409, August 2018 Copyright ª 2018 by The American Society for Pharmacology and Experimental Therapeutics Involvement of Nicotinic Receptor Subtypes in the Behavioral Effects of Nicotinic Drugs in Squirrel Monkeys Sarah L. Withey,1 Michelle R. Doyle,1,2 Jack Bergman, and Rajeev I. Desai Preclinical Pharmacology Laboratory, McLean Hospital/Harvard Medical School, Belmont, Massachusetts Received January 27, 2018; accepted May 17, 2018 ABSTRACT Evidence suggests that the a4b2, but not the a7, subtype of the except for lobeline, the nicotinic agonists produced either full nicotinic acetylcholine receptor (nAChR) plays a key role in [(1)-epibatidine, (2)-epibatidine, and nicotine] or partial (vare- Downloaded from mediating the behavioral effects of nicotine and related drugs. nicline, cytisine, anabaseine, and isoarecolone) substitution for However, the importance of other nAChR subtypes remains (1)-epibatidine. In interaction studies with antagonists differing unclear. The present studies were conducted to examine the in selectivity, (1)-epibatidine discrimination was substan- involvement of nAChR subtypes by determining the effects of tively antagonized by mecamylamine, slightly attenuated selected nicotinic agonists and antagonists in squirrel monkeys by hexamethonium (peripherally restricted) or dihydro- b a either 1) responding for food reinforcement or 2) discriminating the -erythroidine, and not altered by methyllycaconitine ( 7 jpet.aspetjournals.org nicotinic agonist (1)-epibatidine (0.001 mg/kg) from vehicle. In selective). Varenicline and cytisine enhanced (1)-epibati- food-reinforcement studies, nicotine, (1)-epibatidine, varenicline dine’s discriminative-stimulus effects. Correlational analysis and cytisine all produced dose-dependent decreases in rates of revealed a close correspondence between relative behavioral food-maintained responding. The rate-decreasing effects of potencies of nicotinic agonists in both studies and their nicotine were antagonized by mecamylamine (nonselective), not published relative binding affinities at a4b2anda3b4, but not appreciably altered by dihydro-b-erythroidine (a4b2 selective), a7 nAChR, subtypes. Collectively, these results are consistent and exacerbated by the nicotinic partial agonists, varenicline and with the idea that the a4b2anda3b4, but not a7nAChR cytisine. Results from discrimination studies show that non- subtypes play a role in the behavioral effects of nicotinic at ASPET Journals on September 25, 2021 nicotinic drugs did not substitute for (1)-epibatidine, and that agonists. Introduction Several lines of evidence in rats and mice support a prominent role for a4b2 nicotinic acetylcholine receptor (nAChR) mech- ’ Several pharmacological agents that may reduce nicotine s anisms in the behavioral effects of nicotine. First, the a4b2- addictive effects are currently available for smoking cessation selective nicotinic antagonist dihydro-b-erythroidine (DHbE) in the United States [varenicline (Chantix, Pfizer, Groton, CT) has been reported to block nicotine self-administration in rats and bupropion (Wellbutrin XL, Valeant Pharmaceuticals, (Watkins et al., 1999). Moreover, nicotine fails to maintain Bridgewater, NJ)] and Central/Eastern Europe [cytisine self-administration behavior in genetically modified mice lack- (Tabex, Sopharma, Sofia, Bulgaria or Desmoxan, Aflofarm ing the b2 subunit (Picciotto et al., 1998; Epping-Jordan et al., Farmacja, Pabianice, Poland)] (Etter et al., 2008; Benowitz, 1999), whereas selective activation of a4* subunits by nicotine 2010; Walker et al., 2014). Administered alone or in combina- suffices to produce rewarding effects (Tapper et al., 2004). tion with behavioral therapy, these medications produce Second, neurobiological data suggest that nicotine acts at short-term increases in quit rates but display low anti- a4b2 nAChRs in the ventral tegmental area to concomi- — relapse efficacy and may produce adverse effects factors tantly modulate excitatory (e.g., glutamate) and inhibitory that limit their widespread clinical application (Hughes (e.g., gamma-aminobutyric acid) neurotransmission (Picciotto et al., 2003; Hays et al., 2008). These issues highlight the et al., 1998; Quarta et al., 2007), and consequently elevate and strong need for better understanding of the neurobiological diminish dopamine levels in the nucleus accumbens (D’Souza and behavioral mechanisms that mediate the addiction- and Markou, 2013). Finally, the a4b2 nAChR subtype has been related effects of nicotine, and for novel medication strat- implicated in the discriminative-stimulus effects of nicotine. For egies to combat them. example, a4b2subtype–selective nicotinic agonists and antag- onists mimic and attenuate nicotine’s effects, respectively, whereas drugs acting exclusively at other subtypes of nAChRs This work was supported by the National Institutes of Health National (e.g., a7anda3b4) or other pharmacological classes (e.g., Institute on Drug Abuse [Grant K01DA031231 (to R.I.D.) and Grant DA026892 (J.B.)]. muscarinic agents) fail to engender nicotine-like stimulus effects 1S.L.W and M.R.D contributed equally to this work. 2 (for review, see Smith and Stolerman, 2009). Taken together, Current affiliation: Department of Pharmacology, University of Texas a b Health Science Center at San Antonio, San Antonio, Texas. these converging lines of evidence suggest the 4 2subtype https://doi.org/10.1124/jpet.118.248070. of nAChR plays a key role in nicotine’sabuse-relatedactions. ABBREVIATIONS: DHbE, dihydro-b-erythroidine; FR, fixed ratio; MA, methamphetamine; nAChR, nicotinic acetylcholine receptor; TO, timeout. 397 398 Withey et al. In addition to the a4b2 nAChR, other subtypes of nAChRs fed a daily allotment of high-protein primate chow (Monkey Chow; also have been proposed to play a role in nicotine’s behavioral Purina, St. Louis, MO) supplemented with fruit and multivitamins. effects (McGehee et al., 1995; Gotti et al., 1997). For example, Diets were adjusted as needed to maintain stable body weights. selective antagonists at a7 (Markou and Paterson, 2001; Behavioral experiments were conducted 5 days a week (Monday to Brunzell and McIntosh, 2012), a6 (Beckmann et al., 2015), Friday between 8:00 AM and 6:00 PM) under protocols approved by the Institutional Animal Care and Use Committee at McLean and a3b4 (e.g., Glick et al., 2002; Toll et al., 2012) nAChRs Hospital. All subjects had previously participated in behavioral have been reported to attenuate nicotine self-administration studies involving drugs from differing pharmacological classes but or conditioned place preference. Moreover, based on results in had not participated in experiments for at least one month prior to knockout mice, a role has been proposed for a5 and, in this study. separate studies, a6 nAChR subtypes in the abuse-related behavioral effects of nicotine, (Pons et al., 2008; Fowler et al., Apparatus 2013; Faure et al., 2014; Sanjakdar et al., 2015). Finally, the During experimental sessions, subjects sat in customized Plexiglas a b b 4 2-selective nicotinic receptor antagonist DH E antago- chairs that were enclosed in ventilated, sound- and light-attenuating nized the effects of nicotine but not the a4b2 partial agonists chambers. Subjects faced a panel with two sets of red stimulus varenicline and cytisine, in studies investigating their lights and below each set, a response lever. Each lever press with a response-rate decreasing and discriminative-stimulus effects force .0.2 N produced an audible click, and was recorded as a (Cunningham and McMahon, 2011; de Moura and McMahon, response. In food-reinforced behavior studies, a Plexiglas recepta- 2017). cle for liquid delivery was positioned between the response levers. Downloaded from The present studies were conducted to further explore the A syringe pump (Model PHM-100-10; Med Associates Inc., Georgia, role of nAChR subtypes in the behavioral profile of nicotine VT) could be operated to deliver sweetened condensed milk via Tygon tubing into the Plexiglas receptacle (0.15 ml in 0.84 seconds). by examining its effects and those of other nicotinic drugs in In drug discrimination studies, a shaved portion of the monkey’stail squirrel monkeys. First, we examined the rate-altering was secured under two brass electrodes by a small stock for the effects of nicotine and nicotine-related drugs on food- delivery of brief, low-intensity electrical stimulation (3 mA for maintained operant performance, and in interaction studies 200 milliseconds). Electrode gel was applied to the shaved area of jpet.aspetjournals.org we examined whether nicotine’s effects on operant perfor- the tail before each session to ensure low-resistance electrical mance are altered by pretreatment with nAChR antagonists contact with the electrodes. These parameters are below values or partial agonists. Next, we employed drug discrimina- associated with painful stimuli in human subjects (Vierck et al., tion methodology in a separate group of squirrel mon- 2008). All experimental events and data collection were recorded and keys to determine whether agonists that varied in efficacy controlled using commercially available interfacing equipment and and selectivity at nAChR subtypes and non-nicotinic drugs operating software (MED-PC, MedState Notation; Med Associates Inc.,St.Albans,VT). [methamphetamine (MA; indirect monoaminergic