Effects of continuous nicotine treatment and subsequent termination on cocaine vs. food choice in male rhesus monkeys

Abstract

One complicating factor in cocaine addiction may be concurrent exposure and potential dependence on nicotine. The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on cocaine vs. food choice in rhesus monkeys. For comparison, we also determined effects of the nicotinic receptor antagonist mecamylamine on cocaine vs. food choice during continuous saline and nicotine treatment. Rhesus monkeys (n=3) responded under a concurrent schedule of food pellet (1g) and intravenous cocaine (0 – 0.1 mg/kg/injection) availability. Saline and ascending nicotine doses (0.1 – 1.0 mg/kg/h, IV) were continuously infused for 7-day treatment periods and separated by 24 h saline treatment periods. Acute effects of mecamylamine (0.32 – 1.8 mg/kg, IM, 15 min pretreatment) were determined during continuous saline and 0.32 mg/kg/h nicotine treatments. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Nicotine treatment did not alter cocaine vs. food choice. In contrast, preference of 0.032 mg/kg/injection cocaine was attenuated 24 h following termination of 0.32 mg/kg/h nicotine treatment despite no somatic abstinence signs being observed. Acute mecamylamine enhanced cocaine choice during saline treatment and mainly suppressed rates of behavior during nicotine treatment. Overall, continuous nicotine exposure, up to 1 mg/kg/h, does not enhance cocaine choice and does not produce nicotine dependence as demonstrated by the lack of abstinence signs.

Introduction

Cocaine addiction is a significant public health problem both in the United States and worldwide. Cocaine-addicted individuals smoke tobacco at approximately 3-fold higher rates than the general population (Budney, Higgins, Hughes, & Bickel, 1993; Gorelick, Simmons, Carriero, & Tashkin, 1997), and clinical studies suggest that concurrent tobacco and cocaine use is associated with more problematic cocaine use (Meier, Lundy, Patkar, & Weinstein, 2005; Roll, Higgins, Budney, Bickel, & Badger, 1996). Moreover, two recent clinical studies suggest that employing smoking cessation strategies should not increase cocaine use and may in fact decrease cocaine use (Winhusen, Brigham, et al., 2014; Winhusen, Kropp, Theobald, & Lewis, 2014). In addition, preclinical models have demonstrated nicotine and cocaine combinations maintain higher rates of intravenous self-administration behavior compared to cocaine alone in both rats (Bechtholt & Mark, 2002) and rhesus monkeys (Freeman & Woolverton, 2009; Mello & Newman, 2011). Overall, this body of literature suggests a potential interaction between cocaine and nicotine and that tobacco dependence may be an important consideration in both nontreatment-seeking cocaine addicts who might participate in human laboratory studies and treatment-seeking cocaine addicts who might participate in clinical trials.

Chronic tobacco exposure in humans (Hughes et al., 1984; Jorenby et al., 1996; Shiffman & Jarvik, 1976) and chronic nicotine exposure in rodents (Corrigall, Herling, & Coen, 1989; LeSage, Keyler, Collins, & Pentel, 2003) produces a dependent state as demonstrated by the emergence of characteristic abstinence signs. In humans, typical signs and symptoms of tobacco withdrawal include irritability, depression, increased appetite, and sleep disturbances (Hughes et al., 1984; Jorenby et al., 1996). Furthermore, the severity of tobacco withdrawal signs and symptoms appears to predict relapse of tobacco smoking (West, Hajek, & Belcher, 1989). In rodents, nicotine abstinence behavioral signs include teeth chattering, head shakes, ptosis, and writhing, and increased intracranial self-stimulation thresholds (see (Kenny & Markou, 2001; Malin, 2001) for review). To the best of our knowledge, abstinence signs following termination of subchronic nicotine exposure have not been previously reported in nonhuman primates, nor have effects of subchronic nicotine exposure and subsequent termination on cocaine self-administration been previously reported.

The aim of the present study was to determine the effects of 7-day continuous intravenous nicotine treatment and subsequent termination on cocaine vs. food choice in rhesus monkeys. For comparison, we also determined the effects of the nicotinic acetylcholine receptor antagonist mecamylamine on cocaine vs. food choice in the presence and absence of continuous nicotine treatment. Mecamylamine administration has been previously used in rodent studies to precipitate nicotine withdrawal (Malin et al., 1994; Watkins, Stinus, Koob, & Markou, 2000). Continuous and noncontingent intravenous nicotine treatment was chosen as the method of nicotine delivery to assure consistent and controlled nicotine exposure across subjects, because intravenous nicotine self-administration is highly variable between rhesus monkeys (Gould, Czoty, Nader, & Nader, 2011; Koffarnus & Winger, 2015; Mello & Newman, 2011; Slifer & Balster, 1985). A choice procedure was utilized to determine the relative reinforcing effects of cocaine in comparison to a nondrug (food pellet) alternative reinforcer during continuous nicotine treatment and subsequent termination for at least two reasons. First, human laboratory cocaine self-administration studies almost exclusively employ choice procedures (Comer et al., 2008; Haney, 2009; Haney & Spealman, 2008). Furthermore, almost all cocaine users enrolled into human laboratory cocaine self-administration studies have a history of significant tobacco cigarette use and are permitted to smoke tobacco cigarettes throughout the experimental study (Hart, Haney, Foltin, & Fischman, 2000; Stoops, Lile, Glaser, Hays, & Rush, 2012; Vosburg, Haney, Rubin, & Foltin, 2010). Thus, the use of preclinical choice procedures may facilitate the translation of results between nonhumans and humans. Second, the primary dependent measure, percent drug choice, may be less sensitive to reinforcement-independent rate-altering effects than other rate-based drug self-administration procedures (Griffiths, Wurster, & Brady, 1975; Negus, 2003). Limiting the influence of rate-altering drug effects on drug reinforcement may be especially useful in studies examining the effects of subchronic drug exposure and termination on drug reinforcement, because drug withdrawal may reduce rates of operant responding (Griffiths et al., 1975; Negus & Rice, 2008; Wade-Galuska, Galuska, & Winger, 2011). We hypothesized that continuous nicotine treatment would enhance cocaine vs. food choice and that termination of nicotine treatment would attenuate cocaine vs. food choice.

Methods

Subjects

Studies were conducted in three adult male rhesus monkeys (Macaca mulatta) surgically implanted with a chronic indwelling double-lumen venous catheter (STI Components, Raleigh, NC) inserted into a major vein as described previously (Negus, 2003). All monkeys had prior cocaine self-administration histories and exposure to primarily monoaminergic compounds. The intravenous catheter was protected by a tether and jacket system (Lomir Biomedical, Malone, NY) that permitted monkeys to move unrestricted within the cage. Catheter patency was periodically evaluated by intravenous (IV) ketamine (3 mg/kg) administration, and the catheter was considered patent if IV ketamine administration produced a loss of muscle tone within 10 s. Monkeys could earn up to 50 1-g banana-flavored pellets (5TUR Grain-based Precision Primate Tablets; Test Diets, Richmond, IL) during daily experimental sessions (see below). In addition, monkeys also received daily rations of monkey biscuits (Lab Diet High Protein Monkey Biscuits; PMI Feeds, St Louis, MO). The size of these daily biscuit rations was individually determined for each monkey to maintain a healthy adult body weight. These biscuit rations were delivered in the afternoons after choice sessions to minimize the effects of biscuit availability and biscuit consumption on food-maintained operant responding during choice sessions. Animals also received fresh fruit 3 or 4 afternoons per week. Water was continuously available in each monkey’s home chamber and which also served as the experimental chamber (see below). A 12-h light/dark cycle was in effect (lights on from 0600 to 1800 h). Environmental enrichment consisting of foraging boards, novel treats, and movies was also provided after behavioral sessions. Facilities were licensed by both the United States Department of Agriculture and the Association for Assessment and Accreditation of Laboratory Animal Care, and protocols were approved by the Institutional Animal Care and Use Committee.

Behavioral Procedures

Daily experimental sessions were conducted from 0900 to 1100 h in each monkey’s home chamber as previously described (Banks, Blough, & Negus, 2011,2013). The front chamber wall was equipped with a response panel and a 1-g pellet dispenser (Med Associates, ENV-203–1000, St Albans, VT). One pump (Med Associates, PHM-108) delivered contingent cocaine injections. The second pump delivered noncontingent 0.1 mL saline or nicotine infusions every 20 min from 1200 h each day until 1100 h the next morning. The terminal choice schedule consisted of five 20-min components separated by 5-min time out periods. During each component, the left, food-associated key was transilluminated red, and completion of the fixed-ratio (FR) requirement FR100 resulted in pellet delivery. In addition, the right, cocaine-associated key was transilluminated green, and completion of an FR10 resulted in delivery of the IV unit cocaine dose available during that component. The unit cocaine dose available during the five successive components was 0, 0.0032, 0.01, 0.032, and 0.1 mg/kg/injection and dose was manipulated by changing the injection volume (0, 0.01, 0.032, 0.1, and 0.3mL/injection, respectively). Discriminative stimulus conditions on the cocaine-associated key were also varied by flashing the green stimulus lights on and off in 3 s cycles, and longer flashes (with shorter inter-flash intervals) were associated with higher cocaine doses. Monkeys could complete up to 10 ratio requirements across both keys. Responding on either key reset the ratio requirement on the other key. Completion of each ratio requirement initiated a 3 s timeout, during which all stimulus lights were turned off, and responding had no scheduled consequences. If the maximum number of ratio requirements was completed before the 20-min component had elapsed, then all stimulus lights were extinguished, and responding had no scheduled consequences for the remainder of that component. Choice behavior was deemed stable when the lowest cocaine dose maintaining at least 80% cocaine vs. food choice varied by 0.5 log units for three consecutive days. Consequently, with the experimental parameters (unit dose cocaine, FR requirements, alternative food reinforcer magnitude) employed in the present study, we could detect both leftward and rightward shifts in the cocaine vs. food choice dose-effect function that might result from experimental variable manipulation.

Once cocaine vs. food choice was stable, test sessions were conducted to determine effects of continuous 7-day nicotine treatment and subsequent termination on cocaine choice. Nicotine doses (0.1–1.0 mg/ kg/h) were examined in an ascending order in all monkeys. First, we were interested in determining whether continuous nicotine treatment produced a dependent state as demonstrated by observations of abstinence signs following nicotine treatment termination. Thus, to minimize any potential carryover effects from one nicotine dose to the next, an ascending dose order was implemented. At the conclusion of each 7-day nicotine dose treatment period, a 24 h saline treatment period was instituted to probe effects of nicotine withdrawal on cocaine vs. food choice and overt somatic abstinence signs. Following 1.0 mg/kg/h nicotine, a saline treatment period was instituted for five days. Once cocaine choice was stable, 0.32 mg/kg/h nicotine treatment was instituted for 7 days followed by a 24h spontaneous withdrawal probe as a replication experiment.

In a second series of experiments, we determined acute mecamylamine (0.32 – 1.8 mg/kg, IM) effects on cocaine choice during continuous saline or 0.32 mg/kg/h nicotine treatment. Saline or nicotine treatment was implemented for seven consecutive days before initiation of mecamylamine testing and continued throughout testing. On test days, which were conducted on Tuesdays and Fridays, a mecamylamine dose was administered 15 min before the experimental session. This mecamylamine pretreatment time was based on previous mecamylamine results in a cocaine discrimination procedure (Banks, 2014). The mecamylamine dose order was counterbalanced across monkeys, and mecamylamine effects during saline treatment were evaluated at least 14 days after mecamylamine effects during nicotine treatment.

Assessment of somatic abstinence signs

Visual observations were conducted at 1200, 1500, and 0900 h (i.e. 1, 4 and 22 h after termination of nicotine treatment) to document somatic abstinence signs following both termination of nicotine treatment and mecamylamine administration during nicotine treatment using a scoring system previously used for documenting opioid abstinence signs (Negus, 2006). Specifically, eight behavioral signs were counted as present or absent during each 5 min visual assessment, and the total number of behavioral signs was counted to yield an Abstinence Score (i.e., maximum withdrawal score was ‘8’). The eight signs were lying on bottom of cage, unusually aggressive or lethargic response to investigator, increased vocalization, retching/emesis, diarrhea, penile erection/masturbation, tremor/convulsion, and a category for other unusual behaviors.

Plasma nicotine and cotinine analysis

Blood was collected from each monkey 30–45 min after the 7^th day of 0.32 and 1.0 mg/kg/h nicotine treatment sessions to determine nicotine and cotinine plasma levels. The blood was centrifuged to separate plasma from blood, and the plasma was analyzed via HPLC/MS/MS as previously described (AlSharari et al., 2013).

Data Analysis

The primary dependent variable was percent cocaine choice, defined as {number of ratio requirements (choices) completed on the cocaine-associated key/total number of choices completed on both the cocaine- and food-associated keys}*100. Mean data from the last 3 days of each nicotine treatment were calculated for each individual monkey and then averaged to yield group data. These variables were plotted as a function of cocaine dose and analyzed using linear mixed effect model analysis (JMP Pro 11, SAS, Cary, NC) with cocaine dose and nicotine treatment or termination conditions as the fixed main effects and subjects as the random effect. Additional dependent variables collected during each session included total choices, food choices, and cocaine choices. These data were analyzed using one-way repeated-measures ANOVA with treatment condition as the main factor (JMP Pro 11, SAS). A significant main effect was followed by the Dunnett multiple comparisons post hoc test. The criterion for significance was p<0.05 and the effect size measure eta-squared (η²) was calculated as the ratio of the between-groups sum of squares to the sum of the between-groups sum of squares and the error sum of squares (Cohen, 1973; Maher, Markey, & Ebert-May, 2013).

Drugs

(−)-Cocaine HCl and (±)-mecamylamine HCl were provided by the National Institute on Drug Abuse Drug Supply Program (Bethesda, MD). (−)-Nicotine hydrogen bitartrate salt was purchased from Sigma Aldrich (St. Louis, MO) and sodium hydroxide was added to the solution to reach a pH of approximately 6. All solutions were passed through a 0.22 micron Millipore sterile filter (Millipore, Billerica, MA) before use. Drug doses were calculated and expressed using the salt forms listed above except for nicotine, which was calculated and expressed as the free base (Matta et al., 2007).

Results

Effects of continuous nicotine treatment and subsequent termination on cocaine choice

During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Figure 1 show that 7-day continuous nicotine (0.1 – 1.0 mg/kg/h) treatment did not significantly alter (A) cocaine vs. food choice, (B) choices per component, or (C) session total, food, or cocaine choices. Higher nicotine doses were not examined because rates of operant responding were decreased in some, but not all, monkeys during 1.0 mg/kg/h nicotine treatment. Figure 2A shows the effect of suspending nicotine treatment for 24h on cocaine choice and Supplemental Figure 1 shows individual monkey results. Pre-termination data from the last 3 nicotine treatment days were averaged across nicotine doses for ease of graphical display (dotted line for “+ Nicotine”) and because no nicotine dose significantly altered cocaine choice. However, statistical analyses were conducted using data from each nicotine dose. Choice of 0.032 mg/kg/injection cocaine was significantly decreased 24h following terminating 0.32 mg/kg/h nicotine treatment (interaction: F_3,14=7.0, p=0.0042; η²: 0.14) and this effect was replicated as shown in Figure 2B (interaction: F_3,13.3=19.6, p<0.001; η²: 0.14). Cocaine choice was not significantly affected following termination of 0.1 or 1.0 mg/kg/h nicotine. Figures 2C – 2F shows that termination of nicotine treatment did not significantly alter choices per component or session total, food, or cocaine choices.

Figure 1.

Effects of continuous 7-day treatment with nicotine (0.1–1.0 mg/kg/h) on choice (Panel A) between cocaine and food in rhesus monkeys (n=3). Panel B shows effects of continuous nicotine treatment on the number of choices completed per component and panel C shows effects of continuous nicotine treatment on session total choices, food choices, and cocaine choices for the entire session. All points and bars represent group mean data SEM. Nicotine treatment effects represent the average of days 5–7 of each 7-day treatment period. Saline points and bars represent mean data SEM. obtained during the 3 days preceding nicotine treatment when saline was infused through the “treatment” lumen of the double lumen catheter.

Figure 2.

Effects of terminating continuous nicotine treatment on choice (Panel A, B) between cocaine and food in rhesus monkeys (n=3). Saline was substituted for nicotine, and cocaine vs. food choice was examined 24 h later (D1 post nicotine treatment). For graphical clarity and because no nicotine dose significantly altered cocaine vs. food choice, data from the last 3 days of treatment with each nicotine dose were averaged and displayed as “+ nicotine” in Panel A. Panel B shows the replication of continuous 0.32 mg/kg/h nicotine treatment and subsequent termination. Filled symbols indicate statistical significance (p<0.05) compared to 0.32 mg/kg/h nicotine treatment. Numbers in parentheses represent the number of subjects contributing to that data point if less than the total number of subjects. Otherwise, details are the same as in Figure 1.

Somatic effects of nicotine treatment termination

Using the scoring system described above, none of the behaviors listed were noted at any time point following nicotine treatment termination or after mecamylamine administration. Thus, all observations were scored “0.”

Nicotine and cotinine plasma levels

Mean plasma nicotine and cotinine levels were 255.7 ± 24.0 and 1767 ± 119 ng/mL at the end of 0.32 mg/kg/h nicotine treatment, and 634.3 ± 30.6 and 9957±1360 ng/mL at the end of 1.0 mg/kg/h nicotine treatment. Plasma samples after 0.1 mg/kg/h nicotine treatment were not collected.

Effects of mecamylamine on cocaine choice

Figure 3 shows mecamylamine (0.32 – 1.8 mg/kg, IM) pretreatment effects on cocaine vs. food choice during saline (3A, 3C, 3E) and 0.32 mg/kg/h nicotine (3B, 3D, 3F) treatment. During saline treatment, 1.8 mg/kg mecamylamine increased preference of 0.0032 and 0.01 mg/kg/injection cocaine (interaction: F_9,30=3.0, p=0.012; η²: 0.14). There was no significant effect of mecamylamine on (2C) choices per component or (2E) session total, food, or cocaine choices. During 0.32 mg/kg/h nicotine treatment, Figure 2D shows 1.0 mg/kg mecamylamine decreased the number of choices completed during the second and third components when 0.0032 and 0.01 mg/kg/injection cocaine was available and 1.8 mg/kg mecamylamine decreased the number of choices completed during availability of 0, 0.0032, and 0.01 mg/kg/injection cocaine (F_12,38=3.5, p=0.0016; η²: 0.20). Furthermore, Figure 2F shows 1.8 mg/kg mecamylamine significantly decreased both total choices (F_3,6=22.2, p=0.0012; η²: 0.90) and food choices (F_3,6=17.0, p=0.0025; η²: 0.87). As a consequence of these rate-decreasing effects, statistical analysis of percent cocaine choice results could not be conducted (Figure 3B).

Figure 3.

Effects of mecamylamine (0.32–1.8 mg/kg, IM) administered 15–20 minutes before the cocaine vs. food choice (Panels A, B) session in rhesus monkeys (n=3) during treatment with either saline (Panels A, C, E) or 0.32 mg/kg/h nicotine (Panels B, D, F). Baseline points and bars represent the group mean SEM of individual mean data obtained during the days preceding each mecamylamine treatment when saline was infused through the “treatment” lumen of the double lumen catheter. Nicotine treatment points and bars represent the group mean SEM of individual mean data obtained during the days preceding each mecamylamine treatment when 0.32 mg/kg/h nicotine was infused through the “treatment” lumen of the double lumen catheter. Filled symbols and asterisks indicate statistical significant (p<0.05) compared to saline or 0.32 mg/kg/h nicotine, respectively. Numbers in parentheses represent the number of subjects contributing to that data point if less than the total number of subjects. Otherwise, details are the same as in Figure 1.

Discussion

The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on choice between cocaine and food in rhesus monkeys. There were three main findings. First, continuous 7-day nicotine treatment, up to 1 mg/kg/h, did not significantly alter cocaine vs. food choice and despite plasma nicotine and cotinine levels higher than those previously reported in nonhuman primates (Cunningham, Javors, & McMahon, 2012; Kassiou et al., 2001; Perez, Ly, McIntosh, & Quik, 2012) and humans (Gourlay, Benowitz, Forbes, & McNeil, 1997). Second, termination of 0.32 mg/kg/h nicotine attenuated 0.032 mg/kg/injection cocaine preference and reciprocally enhanced food preference. Moreover, abstinence signs were not observed following termination of any nicotine dose or acute mecamylamine administration during 0.32 mg/kg/h nicotine treatment. Lastly, mecamylamine enhanced cocaine choice during saline treatment and mainly decreased rates of responding during 0.32 mg/kg/h nicotine treatment. This former result is consistent with mecamylamine partially generalizing to the cocaine discriminative stimulus (Banks, 2014) and with other acute pharmacological pretreatments on cocaine choice that also share discriminative stimulus effects (Thomsen, Barrett, Negus, & Caine, 2013). Overall, the results of the present study suggest that, under the experimental parameters examined and the limited sample size of 3 monkeys, continuous nicotine treatment does not produce nicotine dependence in rhesus monkeys as demonstrated by the lack of abstinence behavioral signs. Moreover, these results may have implications for cocaine self-administration studies in a human laboratory setting.

Consistent with numerous studies in rodents (Kerstetter et al., 2012; Thomsen et al., 2008), nonhuman primates (Banks et al., 2011; Nader & Woolverton, 1991), and humans (Hart et al., 2000; Stoops et al., 2012) cocaine maintained a dose-dependent increase in preference over an alternative nondrug reinforcer. Furthermore, the present results confirm previous reports demonstrating no effect of subchronic nicotine exposure on cocaine self-administration in both humans (Sobel, Sigmon, & Griffiths, 2004) and rodents (LeSage et al., 2003) and extend these results to cocaine self-administration in nonhuman primates. Moreover, the present results are also consistent with previous results demonstrating subchronic nicotine treatment did not alter food-maintained responding under a progressive-ratio procedure (LeSage, Burroughs, & Pentel, 2006).

The lack of somatic abstinence or other behavioral signs following either termination of subchronic nicotine treatment or acute mecamylamine administration during subchronic nicotine treatment appears somewhat inconsistent with previous rodent results (see (Kenny & Markou, 2001) for review). There are at least two possible explanations for these observed differences. First, rhesus monkeys may not be suitable research subjects for interrogating the effects of drug dependence and withdrawal. This explanation seems unlikely given previous studies demonstrating somatic abstinence signs and changes in rates of operant behavior following both spontaneous and precipitated withdrawal from subchronic 9-tetrahydrocannabinol treatment (Stewart & McMahon, 2010), opioid treatment (Gmerek, Dykstra, & Woods, 1987; Negus, 2006), and benzodiazepine treatment (France & Gerak, 1997; Lockard, Levy, Congdon, DuCharme, & Salonen, 1979).

Second, the treatment duration or nicotine dose employed may not have been sufficient to produce nicotine dependence. Despite group average nicotine levels of 634 ng/mL after 1.0 mg/kg/h nicotine treatment, no gross or operant behavioral signs, such as decreased rates of food-maintained responding were systematically observed. Previous studies have indicated that ≥40 ng/mL nicotine levels in rats was sufficient to produce nicotine dependence (Epping-Jordan, Watkins, Koob, & Markou, 1998; LeSage et al., 2006) and up to 100 ng/mL nicotine levels have been reported (LeSage et al., 2003). In human smokers, peak nicotine levels of 50 ng/mL have been reported after smoking one cigarette an hour for 8 consecutive hours (Russell, Feyerabend, & Cole, 1976). Unfortunately, the absence of any published nonhuman primate study reporting somatic abstinence signs or other behavioral measures of nicotine dependence precludes us from ascertaining what nicotine treatment dose or duration would be sufficient to produce nicotine dependence.

Although the present study failed to demonstrate behavioral signs of nicotine dependence, termination of continuous 0.32 mg/kg/h nicotine treatment did attenuate preference of 0.032 mg/kg/injection cocaine and produce a reciprocal increase in alternative food reinforcer preference. There are two potential mechanisms for this post-nicotine treatment effect on cocaine vs. food choice. First, termination of nicotine treatment may have enhanced the reinforcing efficacy of food pellets while producing no effect on the reinforcing efficacy of cocaine injections. This mechanism would be consistent with the reports of tobacco cigarette cessation-induced hyperphagia (Jorenby et al., 1996; Klesges, Meyers, Klesges, & La Vasque, 1989; Wack & Rodin, 1982). Second, termination of nicotine treatment may have had no effect on the reinforcing efficacy of food pellets while attenuating the reinforcing efficacy of cocaine injections. This potential mechanism would be consistent with nicotine withdrawal blunting the efficacy of electrical brain stimulation to maintain behavior (Epping-Jordan et al., 1998; Watkins et al., 2000). Overall, this later mechanism may be consistent with hypotheses related to nicotine withdrawal blunting reinforcer responsiveness (Dawkins, Powell, Pickering, Powell, & West, 2009; Kalamboka, Remington, & Glautier, 2009; Pergadia et al., 2014). Said another way, termination of nicotine treatment may have blunted the responsiveness of the animal to the discriminative stimuli (flashing green lights) associated with the different cocaine doses such that the monkeys did not allocate their behavior away from the food-associated key until the 0.1 mg/kg/injection cocaine availability following termination of 0.32 mg/kg/h nicotine treatment.

The present results may also have implications for the conduct of human laboratory cocaine vs. nondrug alternative reinforcer choice studies. Preclinical cocaine choice studies examining candidate medications have demonstrated high predictive validity to results from human laboratory and clinical trials (Banks, Hutsell, Schwienteck, & Negus, 2015). The majority of cocaine users who participate in human laboratory cocaine self-administration studies also smoke tobacco cigarettes, see Hart et al (2000), Vosburg et al (Vosburg et al., 2010), and Stoops et al (2012) for examples. In these human laboratory studies, enrolled cocaine users are generally allowed to smoke cigarettes outside of the cocaine self-administration sessions. The results of the present study suggest that nicotine exposure outside of the cocaine self-administration session should not alter the reinforcing effects of cocaine. However, the present results suggest that depending on the duration of the cocaine self-administration session, withholding access to tobacco cigarettes may induce an acute withdrawal state that would be predicted to attenuate cocaine reinforcement.

Because of the absence of robust behavioral effects following nicotine treatment termination, the nicotinic acetylcholine receptor antagonist mecamylamine was administered during nicotine treatment with the goal of precipitating nicotine abstinence signs. Although somatic abstinence signs were not observed, mecamylamine administration did significantly attenuate behavior during nicotine treatment at mecamylamine doses that did not disrupt behavior during saline treatment. Interpretation of mecamylamine results is complicated by mecamylamine enhancing cocaine preference during saline treatment. A recent nonhuman primate study reported that termination of nicotine treatment produced mecamylamine-like discriminative stimulus effects in only 2 out of 5 monkeys and nicotine was unable to antagonize the mecamylamine discriminative stimulus (Cunningham, Moerke, & McMahon, 2014). Overall, these mecamylamine results suggest that mecamylamine effects in rhesus monkeys may be mediated by both nicotinic receptor and non-nicotinic receptor mechanisms.