Reinforcement enhancing effect of nicotine and its attenuation by nicotinic antagonists in rats

Individual variations in motives for nicotine self-administration in male rats: evidence in support for a precision psychopharmacology

The significant heterogeneity in smoking behavior among smokers, coupled with the inconsistent efficacy of approved smoking cessation therapies, supports the presence of individual variations in the mechanisms underlying smoking. This emphasizes the need to shift from standardized to personalized smoking cessation therapies. However, informed precision medicine demands precision fundamental research. Tobacco smoking is influenced and sustained by diverse psychopharmacological interactions between nicotine and environmental stimuli. In the classical experimental rodent model for studying tobacco dependence, namely intravenous self-administration of nicotine, seeking behavior is reinforced by the combined delivery of nicotine and a discrete cue (nicotine+cue). Whether self-administration behavior is driven by the same psychopharmacological mechanisms across individual rats remains unknown and unexplored. To address this, we employed behavioral pharmacology and unbiased cluster analysis to investigate individual differences in the mechanisms supporting classical intravenous nicotine self-administration (0.04 mg/kg/infusion) in male outbred Sprague-Dawley rats. Our analysis identified two clusters: one subset of rats sought nicotine primarily for its reinforcing effects, while the second subset sought nicotine to enhance the reinforcing effects of the discrete cue. Varenicline (1 mg/kg i.p.) reduced seeking behavior in the former group, whereas it tended to increase in the latter group. Crucially, despite this fundamental qualitative difference revealed by behavioral manipulation, the two clusters exhibited quantitatively identical nicotine+cue self-administration behavior. The traditional application of rodent models to study the reinforcing and addictive effects of nicotine may mask individual variability in the underlying motivational mechanisms. Accounting for this variability could significantly enhance the predictive validity of translational research.

Elucidating the reinforcing effects of nicotine: a tribute to Nadia Chaudhri

Nadia Chaudhri worked with us as a graduate student in the Center for Neuroscience at the University of Pittsburgh from 1999 until she earned her PhD in 2005, a time that coincided with the discovery in our lab of the dual reinforcing actions of nicotine, a concept that she played an important role in shaping. The research that was described in her doctoral thesis is among the foundational pillars of the now well-accepted notion that nicotine acts as both a primary reinforcer and an amplifier of other reinforcer stimuli. This reinforcement-enhancing action of nicotine is robust and likely to be a powerful driver of nicotine use. Below, we discuss the evidence that these two actions of nicotine - primary reinforcement and reinforcement enhancement - are distinct and dissociable, a finding that Nadia was closely associated with. We go on to address two other topics that greatly interested Nadia during that time, the generalizability of the reinforcement-enhancing action of nicotine to multiple classes of reinforcing stimuli and potential sex differences in the dual reinforcing actions of nicotine. The research has greatly expanded since Nadia's involvement, but the core ideas that she helped to develop remain central to the concept of the dual reinforcing actions of nicotine and its importance for understanding the drivers of nicotine use.

β2* nicotinic acetylcholine receptor subtypes mediate nicotine-induced enhancement of Pavlovian conditioned responding to an alcohol cue

Nicotine enhances Pavlovian conditioned responses to reward-associated cues. We investigated through which nicotinic acetylcholine receptor (nAChR) subtypes nicotine acts to produce this behavioral effect to an alcohol-associated cue. Male Long-Evans rats with freely available food and water were first accustomed to drinking 15% ethanol in their home cages using an intermittent access, two-bottle choice procedure. Then the rats were given 15 Pavlovian conditioning sessions in which a 15-s audiovisual conditioned stimulus (CS) predicted the delivery of 0.2 ml of ethanol, the unconditioned stimulus (US). Each session contained 12 CS-US trials. A control group received explicitly unpaired presentations of the CS and US. We measured Pavlovian conditioned approach to the site of US delivery during presentations of the CS, accounting for pre-CS baseline activity. Before each conditioning session, rats were injected subcutaneously with nicotine (0.4 mg/kg) or saline (1 ml/kg). During nAChR antagonist test sessions, rats were first injected systemically with the β2*-selective nAChR antagonist dihydro-beta-erythroidine (DHβE; 3 mg/kg) or the α7-selective nAChR antagonist methyllycaconitine (MLA; 6 mg/kg), followed by their assigned nicotine or saline injection before assessing their conditioned response to the alcohol-associated cue. Consistent with previous reports, nicotine enhanced the Pavlovian conditioned response to the alcohol-paired cue. DHβE attenuated this enhancement, whereas MLA did not. These results suggest that nicotine acts via β2*, but not α7, nAChRs to amplify Pavlovian conditioned responding to an alcohol cue. These findings contribute to a growing literature that identifies nAChRs as potential targets for pharmacological treatment of co-morbid alcohol and tobacco use disorders.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Investigating sex differences and the effect of drug exposure order in the sensory reward-enhancing effects of nicotine and d-amphetamine alone and in combination

Nicotine enhances the rewarding effects of other environmental stimuli; this reward-enhancement encourages and maintains nicotine consumption. Nicotine use precedes other psychostimulant use, but receiving a stimulant prescription also predicts future smoking. Previously, no study has investigated effects of drug exposure order in reward-enhancement, nor with nicotine and d-amphetamine. Thus, we aimed to investigate how drug exposure order impacted the reward-enhancing effects of nicotine and d-amphetamine, alone and in combination. We used 20 male and 20 female Sprague-Dawley rats. Enhancement was investigated within-subjects by examining responding maintained by a visual stimulus reinforcer following a pre-session injection of either d-amphetamine (Sal, 0.1, 0.3, or 0.6 mg/kg) or nicotine (Sal, 0.03, 0.06, 0.1, 0.3 mg/kg). Twenty rats (10 M, 10 F) completed enhancement testing with nicotine before d-amphetamine. The other 20 rats (10 M, 10 F) completed testing with d-amphetamine before nicotine. Following these phases, rats were then given two pre-session injections: one of d-amphetamine (Sal, 0.1, 0.3, or 0.6 mg/kg) and another of nicotine (Sal, 0.03, 0.06, 0.1, or 0.3 mg/kg). Experiencing amphetamine before nicotine increased reward-enhancing effects of nicotine. Females exhibited greater effects of d-amphetamine on reward-enhancement, with no effect of exposure order. During the interaction phase, receiving nicotine before amphetamine enhanced the interaction between nicotine and d-amphetamine for females whereas amphetamine before nicotine heightened this interaction for males. From this, prior and current amphetamine use, in addition to sex, should be considered when treating nicotine dependency and when examining factors driving poly-substance use involving nicotine and d-amphetamine. Keywords: Adderall, ADHD, Dexedrine, operant, smoking, polysubstance use.

Nicotine-induced enhancement of a sensory reinforcer in adult rats: antagonist pretreatment effects

RATIONALE AND OBJECTIVES

The reinforcement-enhancing effect (REE) of nicotine refers to the drug's ability to enhance the strength of other primary and conditioned reinforcers. The main aim was to investigate neuropharmacological mechanisms underlying nicotine's strengthening of a primary visual reinforcer (i.e., a light cue), using a subcutaneous (SC) dose previously shown to provide plasma nicotine levels associated with habitual smoking.

METHODS

Adult male rats pressed an "active" lever to illuminate a brief cue light during daily 60-min sessions. Rats that showed a clear REE were tested with systemically administered pretreatment drugs followed by nicotine (0.1 mg/kg SC) or saline challenge, in within-subject counterbalanced designs. Pretreatments were mecamylamine (nicotinic, 0.1-1 mg/kg SC), SCH 39166 (D1-like dopaminergic, 0.003-0.2 mg/kg SC), naloxone (opioid, 1 and 5 mg/kg SC), prazosin (alpha1-adrenergic antagonist, 1 and 2 mg/kg IP), rimonabant (CB1 cannabinoid inverse agonist, 3 mg/kg IP), sulpiride (D2-like dopaminergic antagonist, 40 mg/kg SC), or propranolol (beta-adrenergic antagonist, 10 mg/kg IP).

RESULTS

The nicotine REE was abolished by three antagonists at doses that did not impact motor output, i.e., mecamylamine (1 mg/kg), SCH 39166 (0.01 and 0.03 mg/kg), and naloxone (5 mg/kg). Prazosin and rimonabant both attenuated the nicotine REE, but rimonabant also suppressed responding more generally. The nicotine REE was not significantly altered by sulpiride or propranolol.

CONCLUSIONS

In adult male rats, the reinforcement-enhancing effect of low-dose nicotine depends on nicotinic receptor stimulation and on neurotransmission via D1/D5 dopaminergic, opioid, alpha1-adrenergic, and CB1 cannabinoid receptors.

Varenicline rescues nicotine-induced decrease in motivation for sucrose reinforcement

Varenicline is one of the top medications used for smoking cessation and is often prescribed before termination of nicotine use. The effect of this combined nicotine and varenicline use on the reward system and motivation for primary reinforcement is underexplored. The goal of this study was to assess the effects of nicotine and varenicline on motivation for a food reinforcer. In Experiment 1, we first assessed the responding for sucrose after pretreatment with nicotine (0, 0.1, or 0.4 mg/kg) and varenicline (0.0, 0.1, 1.0 mg/kg) using a behavioral economics approach. The responding for sucrose was then assessed using a progressive ratio schedule of reinforcement after pretreatment with all possible combinations of nicotine and varenicline doses. In Experiment 2, rats were assessed for the consumption of sucrose in home cages after pretreatment with nicotine and varenicline. We found that (a) nicotine decreased economic demand for sucrose, (b) varenicline rescued nicotine-induced reduction in economic demand for sucrose, and (c) history of varenicline treatment predicted responding for sucrose on a progressive ratio schedule of reinforcement where rats with a history of varenicline treatment responded significantly lower for sucrose across nicotine doses than rats that had not been exposed to varenicline. The results of Experiment 2 largely confirmed that nicotine decreases motivation for sucrose using a passive consumption protocol and that varenicline rescues this effect. Overall, these findings suggest that varenicline interacts with the effects of nicotine by restoring nicotine-induced reduction in motivation for appetitive rewards.

Phasic Dopamine Release Magnitude Tracks Individual Differences in Sensitization of Locomotor Response following a History of Nicotine Exposure

Smoking remains the primary cause of preventable death in the United States and smoking related illness costs more than $300 billion annually. Nicotine (the primary reinforcer in cigarettes) causes changes in behavior and neurochemistry that lead to increased probability of relapse. Given the role of mesolimbic dopamine projections in motivation, substance use disorder, and drug relapse, we examined the effect of repeated nicotine on rapid dopamine signals in the nucleus accumbens (NAc) of rats. Adult, male Sprague-Dawley rats were exposed to nicotine (0.2 or 0.4 mg/kg, subcutaneous) once daily for 7 days. On day 8, dopamine release and uptake dynamics, and their modulation by nicotinic receptor agonists and antagonists, were assessed using fast scan cyclic voltammetry in the NAc core. Nicotine exposure decreased electrically-stimulated dopamine release across a range of stimulation frequencies and decreased α6β2-containing nicotinic receptor control over dopamine release. Additionally, nicotine locomotor sensitization correlated with accumbal dopamine modulation by nicotine and mecamylamine. Taken together, our study suggests that repeated exposure to nicotine blunts dopamine release in the NAc core through changes in α6β2 modulation of dopamine release and individual differences in the sensitivity to this outcome may predict variation in behavioral models of vulnerability to substance use disorder.

Varenicline Targets the Reinforcing-Enhancing Effect of Nicotine on Its Associated Salient Cue During Nicotine Self-administration in the Rat

Nicotine is acknowledged as the key addictive compound of tobacco. Varenicline (Champix^® or Chantix^®), mainly acting as a partial agonist at the α4β2 nicotinic receptor, is an approved smoking cessation pharmacotherapy, although with efficacy limited to a portion of smokers. Smokers differ in the motives that drive their drug seeking and Varenicline might be more efficient in some groups more than others. Studies in rodents revealed that nicotine-seeking is strongly supported by complex interactions between nicotine and environmental cues, and notably the ability of nicotine to enhance the reinforcing properties of salient environmental stimuli. It is not yet understood whether the decrease of nicotine-seeking by acute Varenicline in rats results from antagonism of the primary reinforcing effects of nicotine, of the reinforcement-enhancing effect of nicotine on cues, or of a combination of both. Thanks to a protocol that allows assessment of the reinforcement-enhancing effect of nicotine on cues during self-administration in rats, we showed that Varenicline targets both nicotine reinforcing effects and reinforcement-enhancing effect of nicotine on cues. Importantly, individual variations in the latter determined the amplitude of acute Varenicline-induced decrease in seeking. These results suggest that Varenicline might be more beneficial in smokers who are more sensitive to nicotine effects on surrounding stimuli.

Not all smokers appear to seek nicotine for the same reasons: implications for preclinical research in nicotine dependence

Tobacco use leads to 6 million deaths every year due to severe long-lasting diseases. The main component of tobacco, nicotine, is recognized as one of the most addictive drugs, making smoking cessation difficult, even when 70 percent of smokers wish to do so. Clinical and preclinical studies have demonstrated consistently that nicotine seeking is a complex behavior involving various psychopharmacological mechanisms. Evidence supports that the population of smokers is heterogeneous, particularly as regards the breadth of motives that determine the urge to smoke. Here, we review converging psychological, genetic and neurobiological data from clinical and preclinical studies supporting that the mechanisms controlling nicotine seeking may vary from individual to individual. It appears timely that basic neuroscience integrates this heterogeneity to refine our understanding of the neurobiology of nicotine seeking, as tremendous progress has been made in modeling the various psychopharmacological mechanisms driving nicotine seeking in rodents. For a better understanding of the mechanisms that drive nicotine seeking, we emphasize the need for individual-based research strategies in which nicotine seeking, and eventually treatment efficacy, are determined while taking into account individual variations in the mechanisms of nicotine seeking.

Targeting the α4β2- and α7-Subtypes of Nicotinic Acetylcholine Receptors for Smoking Cessation Medication Development

Nicotine exerts its reinforcing actions via activating the nicotinic acetylcholine receptors (nAChRs). Among an increasing number of nAChR subtypes, the α4β2 and α7 nAChRs are the two major ones, accounting for about 95% of the whole nAChR population in brain. Research findings from our own laboratory, together with other reports in the field, suggest critical and differential involvement of the α4β2 and α7 nAChRs in the process of nicotine dependence and tobacco addiction. Specifically, rat models of nicotine consumption and cue-induced relapse were used to examine the effects of selective antagonism of these two nAChR subtypes on the primary reinforcement of nicotine and the conditioned reinforcing actions of nicotine-associated environmental stimuli (cues). Results demonstrated that blockade of the α4β2 but not α7 subtype effectively reduced nicotine intake, whereas α7 but not α4β2 nAChR blockade reversed cue-triggered nicotine relapse behavior. These findings lend support for the continued effort to develop cholinergic agents aiming at the α4β2 nAChRs for reducing or stopping smoking. However, it is suggested that manipulation of α7 nAChR activity would be a promising target for preventing smoking relapse triggered by exposure to environmental cues.

An extended history of drug self-administration results in multiple sources of control over drug seeking behavior

It is widely recognized that across the development of drug addiction, cues associated with drug use come to exert increasing control over drug seeking and taking behaviors. However, there remain gaps in our knowledge regarding how the different types of drug related cues affect drug seeking and taking behaviors, and how the emergence of cue control over these behaviors relates to the onset of drug seeking compulsions. This paper reviews the literature on drug self-administration in animals to address these gaps. It first identifies the different types of cues that acquire control over reward seeking behavior generally, and examines whether the same types of cues acquire control over drug seeking behavior specifically. It then examines how the role of drug related cues in motivating and reinforcing drug seeking behavior changes across an extended drug-taking history, with a particular focus on the case of nicotine. The evidence reviewed shows that, after an extended history of drug taking, drug seeking behaviors are controlled by contextual cues associated with the development of drug seeking habits, response contingent cues that accompany delivery of the drug, as well as internal states that correlate with levels of drug intake. These multiple sources of control over drug seeking are discussed in relation to the generation of an addicted phenotype in animal models and the hypothesized progression from internal control over drug use to compulsive drug seeking.

A behavioral economic analysis of the value-enhancing effects of nicotine and varenicline and the role of nicotinic acetylcholine receptors in male and female rats

Reinforcement value enhancement by nicotine of non-nicotine rewards is believed to partially motivate smoking behavior. Recently, we showed that the value-enhancing effects of nicotine are well characterized by reinforcer demand models and that the value-enhancing effects of the smoking-cessation aid bupropion (Zyban) are distinct from those of nicotine and differ between the sexes. The present study evaluated potential sex differences in the enhancement effects of nicotine and varenicline (Chantix) using a reinforcer demand methodology. The role of α4β2* and α7 nicotinic acetylcholine receptors (nAChRs) in the enhancing effects of nicotine and varenicline is also evaluated. Male and female rats (n=12/sex) were trained to lever press maintained by sensory reinforcement by visual stimulus (VS) presentations. Changes in the VS value following nicotine and varenicline administration were assessed using an established reinforcer demand approach. Subsequently, the effects of antagonism of α4β2* and α7 nAChRs on varenicline and nicotine-induced enhancement active lever-pressing were assessed using a progressive ratio schedule. Nicotine and varenicline enhanced VS demand equivalently between the sexes as evaluated by reinforcer demand. However, α4β2* receptor antagonism attenuated value enhancement by nicotine and varenicline in females, but only of nicotine in males.

Status and Future Directions of Preclinical Behavioral Pharmacology in Tobacco Regulatory Science

Behavioral pharmacology is a branch of the experimental analysis of behavior that has had great influence in drug addiction research and policy. This paper provides an overview of recent behavioral pharmacology research in the field of tobacco regulatory science, which provides the scientific foundation for the Food and Drug Administration Center for Tobacco Products (FDA CTP) to set tobacco control policies. The rationale and aims of tobacco regulatory science are provided, including the types of preclinical operant behavioral models it deems important for assessing the abuse liability of tobacco products and their constituents. We then review literature relevant to key regulatory actions being considered by the FDA CTP, including regulations over nicotine and menthol content of cigarettes, and conclude with suggesting some directions for future research. The current era of tobacco regulatory science provides great opportunities for behavioral pharmacologists to address the leading cause of preventable death and disease worldwide.

Reinforcement enhancement by nicotine in adult rats: behavioral selectivity and relation to mode of delivery and blood nicotine levels

RATIONALE

Reinforcement-enhancing effects of nicotine occur in human subjects and laboratory rats. However, the doses used in animal studies typically exceed smoking-associated levels of exposure, and generalized behavioral activation by nicotine can potentially confound data interpretation.

METHODS

During daily 60-min sessions, male adult rats pressed an "active" lever to illuminate a brief cue light. Pressing on either the active or inactive lever retracted both levers for 60 s. Nicotine (0.025-0.2 mg/kg) was given either by continuous intravenous (IV) infusion, or spaced IV pulses (3-s or 30-s/pulse), or pre-session subcutaneous (SC) injection.

RESULTS

Almost all rats responded preferentially for the cue light for several weeks. After several home-cage nicotine injections, reinforcement enhancement occurred even within the first nicotine test session. Nicotine increased active lever responding without altering inactive lever responding, with effects reliably observed at doses as low as 0.1 mg/kg SC or 0.1 mg/kg/session IV. Within the session, the 0.1 mg/kg dose maximally increased active lever responding by 2-3-fold, coinciding with serum levels of 25 ng/ml. Intravenous nicotine (tested at 0.1 mg/kg/60-min session) was equally effective whether delivered by continuous infusion or in a series of equally spaced 0.003 mg/kg pulses each of 3-s or 30-s duration.

CONCLUSIONS

Low doses of nicotine can potentiate responding for a primary sensory reinforcer without producing a generalized increase in lever pressing. Reinforcer enhancement by nicotine generalized to several modes of drug delivery, appeared to track circulating levels of drug, and occurred even at serum levels within the daytime range of moderate cigarette smokers.

Reduction in delay discounting due to nicotine and its attenuation by cholinergic antagonists in Lewis and Fischer 344 rats

RATIONALE

Nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs), and mecamylamine, a nonselective nAChR antagonist, attenuates effects of nicotine on delay discounting in some rat strains; whether nicotine's attenuation is specific to nAChR antagonism is unknown.

OBJECTIVE

During experiment 1, we evaluated dose-dependent effects of nicotine on delay discounting of pair-housed Lewis (LEW) and Fischer 344 (F344) rats. During experiment 2, we examined the sensitivity of nicotine's effects on delay discounting to pharmacological antagonism of nAChRs or muscarinic AChRs (mAChRs).

MATERIALS AND METHODS

Male LEW and F344 were trained to choose between one food pellet delivered immediately and three food pellets delivered after an increasing delay. During experiment 1, saline and nicotine (0.1-1.0 mg/kg) were tested acutely. During experiment 2, mecamylamine (0.25-1.0 mg/kg) or a nonselective mAChR antagonist, scopolamine (0.01-0.056 mg/kg), was administered prior to nicotine administration.

RESULTS

Nicotine dose dependently reduced delay discounting for both rat strains, and no strain differences were observed (ΔAUC = + 107% for 1.0 mg/kg and + 69.6% for 0.3 mg/kg relative to saline). At some doses, pretreatment with mecamylamine (range ΔAUC = - 27.6 to - 7.3%) or scopolamine (range ΔAUC = - 0.74 to - 51.6%) significantly attenuated the nicotine-induced reduction in some measures of delay discounting for both strains.

CONCLUSIONS

Results from experiment 1 suggest that when LEW and F344 are pair housed, there are no strain differences in delay discounting in response to nicotine. Results from experiment 2 suggest that attenuation of nicotine's effects on delay discounting may not be specific to nAChR antagonism.

Effects of the phencyclidine model of schizophrenia and nicotine on total and categorized ultrasonic vocalizations in rats

Patients with schizophrenia smoke cigarettes at a higher rate than the general population. We hypothesized that a factor in this comorbidity is sensitivity to the reinforcing and reinforcement-enhancement effects of nicotine. Phencyclidine (PCP) was used to model behavioral changes resembling negative symptoms of schizophrenia in rats. Ultrasonic vocalizations (USVs) in rats have been used to measure emotional states, with 50 kHz USVs indicating positive states and 22 kHz USVs indicating negative states. Total and categorized numbers of 22 and 50 kHz USVs and USVs during a visual stimulus (e.g. a potential measure of reinforcement-enhancement) were examined in rats following injection of PCP (2.0 mg/kg) and/or nicotine (0.2 or 0.4 mg/kg) daily for 7 days. PCP was then discontinued and all rats received nicotine (0.2 and 0.4 mg/kg) and PCP (2.0 mg/kg) on three challenge days. PCP acutely decreased 50 kHz vocalizations, whereas repeated nicotine potentiated rates of vocalizations, with similar patterns during light presentations. Rats in the PCP and nicotine combination groups made more 50 kHz vocalizations compared with rats in the control groups on challenge days. We conclude that PCP may produce a reward deficit, which is shown by decreased 50 kHz USVs, and behaviors post-PCP exposure may best model the comorbidity between schizophrenia and nicotine.

Nicotine-induced enhancement of Pavlovian alcohol-seeking behavior in rats

RATIONALE

Nicotine enhances responding elicited by Pavlovian cues that predict positive outcomes.

OBJECTIVES

We tested the hypothesis that nicotine acting at nicotinic acetylcholine receptors (nAChRs) would augment Pavlovian alcohol-seeking.

METHODS

Male, Long-Evans rats with unrestricted access to food and water were acclimated to drinking 15% ethanol in their home cages and then given Pavlovian conditioning sessions in which each trial of a 15-s conditioned stimulus (CS, 12 trials/session) was paired with 0.2 ml of ethanol (unconditioned stimulus, US, 2.4 ml/session). Entries into a port where ethanol was delivered were used to assess conditioning. Control groups received explicitly unpaired trials of the CS and US. In experiment 1, systemic injections of saline (1 ml/kg) or nicotine (0.4 mg/kg, freebase) were administered before each session. In experiments 2 and 3, an identical regimen of saline or nicotine injections was administered before the start of Pavlovian conditioning sessions.

RESULTS

All paired groups acquired conditioned port-entry responding to the CS, indicative of Pavlovian alcohol-seeking, whereas unpaired control group did not. Pre-session nicotine injections increased CS port-entries relative to saline, only in the paired group. This nicotine-induced enhancement of Pavlovian alcohol-seeking was blocked by pre-treatment with the nAChR antagonist mecamylamine. Prior exposure to nicotine did not influence the subsequent acquisition of Pavlovian alcohol-seeking.

CONCLUSIONS

These findings highlight for the first time that nicotine acting at nAChRs augments Pavlovian alcohol-seeking, specifically in non-restricted rats. Individuals who smoke and drink may thus be particularly susceptible to alcohol cues that could trigger further drinking.

Sex differences and the role of dopamine receptors in the reward-enhancing effects of nicotine and bupropion

RATIONALE

Nicotine and bupropion have been demonstrated to enhance the value of other reinforcers, and this may partially account for nicotine reward and dependence. Evidence suggests that the sexes differ in their sensitivity to the primary and secondary reinforcing effects of nicotine and nicotine-associated stimuli. Whether the sexes also differ in sensitivity to the reward-enhancing effects of nicotine (and bupropion) is yet unclear.

OBJECTIVES

The present study evaluated potential sex differences in the enhancement effects of nicotine and bupropion using a reinforcer demand approach. Furthermore, we sought to investigate the role that D1- and D2-type dopamine receptors play in the reward-enhancing effects of nicotine and bupropion.

METHODS

Demand for sensory reinforcement was assessed in male and female rats responding on a progression of fixed ratio schedules. The effects of nicotine and 10 or 20 mg/kg bupropion on reinforcer demand were assessed within subjects. Subsequently, the effects of SCH-23390 and eticlopride were assessed on the enhancing effects of nicotine and bupropion on progressive ratio responding.

RESULTS

Nicotine and bupropion enhanced demand metrics of reinforcement value in both sexes. Females were more sensitive to the enhancement effects of bupropion assessed by reinforcer demand and progressive ratio performance. D2-like dopamine receptor antagonism by eticlopride attenuated the enhancement effects of bupropion, but not of nicotine.

CONCLUSIONS

Nicotine and bupropion both enhance reinforcement value in both sexes, though females may be more sensitive to the reward-enhancing effects of bupropion. D2- and possibly D1-type receptors appear to be involved in the reward-enhancing effects of bupropion, but not necessarily nicotine.

Nicotine Enhances Footshock- and Lithium Chloride-Conditioned Place Avoidance in Male Rats

INTRODUCTION

Numerous studies have shown that nicotine (NIC) can enhance the reinforcing effects of non-NIC stimuli through a nonassociative mechanism. To date, it is unclear whether NIC reinforcement enhancement serves to increase behaviors motivated by rewarding stimuli only, or whether NIC potentiates behavior motivated by all stimuli, regardless of valence.

METHODS

The current study used a place conditioning procedure to examine whether acute NIC injection modulates avoidance of an environment previously associated with an aversive stimulus. Separate groups of rats underwent place conditioning using either lithium chloride (125mg/kg/ml, i.p.) or footshock (0.75 mA) as the aversive stimulus. Other rats served as nonconditioned controls. The magnitude of place avoidance was assessed after acute NIC (0.1 or 0.4mg/kg/ml, s.c.) or saline.

RESULTS

Rats avoided chambers previously paired with either lithium chloride or footshock, and conditioned place avoidance was significantly enhanced by NIC pre-treatment.

CONCLUSIONS

These results demonstrate that the ability of NIC to enhance motivated behavior extends to behaviors elicited by aversive stimuli, evidence that NIC affects behavior motivated by a broader range of stimuli than previously appreciated.

IMPLICATIONS

The current study examined whether the reinforcement enhancement properties of NIC apply to aversive stimuli by testing NIC enhancement of conditioned place avoidance in rats. The results demonstrate that NIC enhances the motivational impact of these distinct aversive stimuli, providing novel evidence that NIC affects behavior motivated by a broader range of stimuli than has previously been demonstrated.

Blockade of cholinergic transmission elicits somatic signs in nicotine-naïve adolescent rats

High doses of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine can elicit somatic signs resembling those associated with nicotine withdrawal in nicotine-naïve adult rats. Understanding this phenomenon, and its possible modulation by acute nicotine and age, could inform the use of mecamylamine as both an experimental tool and potential pharmacotherapy for tobacco dependence and other disorders. This study evaluated the ability of high-dose mecamylamine to elicit somatic signs in adolescent rats, and the potential for acute nicotine pretreatment to potentiate this effect as previously reported in adults. Single or repeated injections of mecamylamine (1.5 or 3.0 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents, but this effect was not influenced by 2 h pretreatment with acute nicotine (0.5 mg/kg, s.c.). In an initial evaluation of the effects of age in this model, mecamylamine (2.25 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents and adults. This effect was modestly enhanced following acute nicotine injections in adults but not in adolescents, even when a higher nicotine dose (1.0 rather than 0.5 mg/kg, s.c.) was used in adolescents to account for age differences in nicotine pharmacokinetics. These studies are the first to show that mecamylamine elicits somatic signs in nicotine-naïve adolescent rats, an effect that should be considered when designing and interpreting studies examining effects of high doses of mecamylamine in adolescents. Our findings also provide preliminary evidence that these signs may be differentially modulated by acute nicotine pretreatment in adolescents versus adults.

Neuroscience of learning and memory for addiction medicine: from habit formation to memory reconsolidation

Identifying effective pharmacological treatments for addictive disorders has remained an elusive goal. Many different classes of drugs have shown some efficacy in preclinical models, but the number of effective clinical therapeutics has remained stubbornly low. The persistence of drug use and the high frequency of relapse is at least partly attributable to the enduring ability of environmental stimuli associated with drug use to maintain behavioral patterns of drug use and induce craving during abstinence. We propose that stimuli associated with drug use exert such powerful control over behavior through the development of abnormally strong memories, and their ability to initiate subconscious sequences of motor actions (habits) that promote uncontrolled drug use. In this chapter, we will review the evidence suggesting that drugs of abuse strengthen associations with cues in the environment and facilitate habit formation. We will also discuss potential mechanisms for disrupting memories associated with drug use to help improve treatments for addiction.

Individual variation in the motivational properties of a nicotine cue: sign-trackers vs. goal-trackers

RATIONALE

Individuals vary in the extent to which they attribute incentive salience to reward cues. Discrete food and drug (cocaine and opioid) cues become more attractive, eliciting approach toward them, and more "wanted," in that they serve as more effective conditioned reinforcers, in some rats (sign-trackers, STs), than in others (goal-trackers, GTs).

OBJECTIVES

We asked whether there is similar variation in the extent to which a cue associated with a drug from another class, nicotine, acquires incentive motivational properties.

METHODS

First, a Pavlovian conditioned approach procedure was used to identify rats that attribute incentive salience to a food cue (i.e., STs and GTs). We then measured the extent to which a cue (a light) paired with intravenous nicotine injections acquired two properties of an incentive stimulus: (1) the ability to elicit approach toward it, and (2) the ability to act as a conditioned reinforcer.

RESULTS

In contrast to previous findings with food, cocaine, and opioid cues, we found that the nicotine cue was equally attractive in STs and GTs, eliciting dose-dependent approach behavior in both. However, the nicotine cue was a more effective conditioned reinforcer in STs than in GTs.

CONCLUSIONS

We suggest the dissociation between these two measures of incentive salience attribution may be related to the fact that when present (as in the test of Pavlovian approach), nicotine can act as a potent "incentive amplifier," and by this action, nicotine may render cues especially salient for all animals.

Behavioral effects of phencyclidine on nicotine self-administration and reinstatement in the presence or absence of a visual stimulus in rats

RATIONALE

Tobacco use is a serious health problem in the USA, and this problem is potentiated in patients with schizophrenia. The reward system is implicated in schizophrenia and may contribute to the high comorbidity between nicotine use and schizophrenia, but very little research has been done on the topic. The reward-enhancement effect of nicotine has been shown to be important in nicotine use, but there have been no studies on this effect in animal models of schizophrenia.

OBJECTIVES

This study was designed to determine the effects of phencyclidine, used to model negative symptoms of schizophrenia, on self-administration of nicotine with or without a co-occurring sensory reinforcer [i.e., visual stimulus (VS)] in rats.

METHODS

Phencyclidine (2.0 mg/kg) was administered before each of seven nicotine self-administration sessions (0.01 mg/kg/inf) after which rats (n = 8-9 per group) were given 7 days of extinction without phencyclidine pretreatment. Reinstatement using phencyclidine (2.0 mg/kg), nicotine (0.2 mg/kg), and yohimbine (1.25 mg/kg, a pharmacological stressor) was tested after extinction to determine if previous exposure to phencyclidine would alter reinstatement of active lever pressing.

RESULTS

Phencyclidine initially decreased nicotine self-administration but only in the groups with a concurrent VS. This decrease in self-administration dissipated after 5 days. During reinstatement, rats that had previously received phencyclidine during self-administration with a VS were more sensitive to stress-induced reinstatement than any other group.

CONCLUSIONS

These results show a transitory effect of phencyclidine on nicotine self-administration. Phencyclidine may induce a potential sensitivity to pharmacological stressors contributing to reinstatement of nicotine.

Reinforcer devaluation as a consequence of acute nicotine exposure and withdrawal

RATIONALE

Nicotine discontinuation produces behaviors in rats that are congruent with anhedonia, and these symptoms may be related to the devaluation of non-nicotine reinforcers.

OBJECTIVE

Four separate experiments were performed to explore the parameters surrounding nicotine-induced reinforcer devaluation.

METHODS

In Experiments 1 and 2, nicotine (0.1 or 0.3 mg/kg) or 0.3 mg/kg nicotine plus 1.0 mg/kg mecamylamine was delivered to rats prior to progressive ratio (PR) schedule sessions in which sucrose was used as a reinforcer. In order to evaluate (a) reinforcer enhancement by nicotine and (b) reinforcer devaluation in the absence of nicotine, all rats experienced two PR schedule sessions per day for 10 days. Experiment 3 involved nicotine (0.3 mg/kg) and a visual stimulus in place of sucrose reinforcement. In Experiment 4, rats received nicotine (0.3 mg/kg) either before or after a single PR schedule session for 10 days.

RESULTS

Experiments 1 and 2 demonstrate that reinforcer devaluation is related to the occupation of nicotinic-acetylcholine receptors. Results from Experiment 3 provide some evidence that devaluation occurs with either sucrose or visual-stimulus reinforcement. Experiment 4 demonstrates that a necessary condition for reinforcer devaluation to occur is the concurrent exposure to the reinforcer and nicotine.

CONCLUSIONS

Reinforcer devaluation in rats emerges rapidly in a progressive, orderly fashion that coincides with accumulated exposure to nicotine. These results suggest that reinforcer devaluation may be a feature of nicotine that contributes to the abuse liability of tobacco products.

Differentiating the primary reinforcing and reinforcement-enhancing effects of varenicline

RATIONALE

Varenicline (VAR), a smoking cessation aid that is a partial agonist at nicotinic receptors, mimics the reinforcement-enhancing effects of nicotine. Varenicline, when accompanied by non-drug cues, is self-administered by rats, though it is unclear whether this results from varenicline acting as a primary reinforcer or a reinforcement enhancer of the cues.

OBJECTIVES

This study sought to disentangle these two potential actions.

METHODS

Rats were allowed to self-administer intravenous nicotine, saline, or varenicline during 1-h sessions in operant chambers equipped with two levers. Five groups had concurrent access to drug infusions and a moderately reinforcing visual stimulus (VS) for responding on separate levers. Meeting the reinforcement schedule on one lever was reinforced with VAR (0.01, 0.06, 0.1 mg/kg/infusion), nicotine (0.06 mg/kg/infusion), or saline, while meeting the same schedule on the other lever delivered the VS. Additional groups were reinforced for pressing a single "active" lever and received VAR paired with the VS, the VS with response-independent infusions of VAR, or VAR alone (0.1 mg/kg/infusion).

RESULTS

Rats readily responded for VAR paired with VS on a single lever. However, when VAR was the only reinforcer contingent on a response, rats did not respond more than for saline.

CONCLUSIONS

These findings show that VAR does not serve as a primary reinforcer in rats at doses that increase responding for non-drug reinforcers. These data are consistent with research showing that the primary reinforcing effects of VAR are weak, at best, and that the primary reinforcing and reinforcement-enhancing actions of nicotinic drugs are pharmacologically distinct.

Animal models to assess the abuse liability of tobacco products: effects of smokeless tobacco extracts on intracranial self-stimulation

BACKGROUND

Preclinical models are needed to inform regulation of tobacco products by the Food and Drug Administration (FDA). Typically, animal models of tobacco addiction involve exposure to nicotine alone or nicotine combined with isolated tobacco constituents (e.g. minor alkaloids). The goal of this study was to develop a model using extracts derived from tobacco products that contain a range of tobacco constituents to more closely model product exposure in humans.

METHODS

This study compared the addiction-related effects of nicotine alone and nicotine dose-equivalent concentrations of aqueous smokeless tobacco extracts on intracranial self-stimulation (ICSS) in rats. Extracts were prepared from Kodiak Wintergreen, a conventional product, or Camel Snus, a potential "modified risk tobacco product". Binding affinities of nicotine alone and extracts at various nicotinic acetylcholine receptor (nAChR) subtypes were also compared.

RESULTS

Kodiak and Camel Snus extracts contained levels of minor alkaloids within the range of those shown to enhance nicotine's behavioral effects when studied in isolation. Nonetheless, acute injection of both extracts produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, as well as similar reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at high nicotine doses. Extracts and nicotine alone also had similar binding affinity at all nAChRs studied.

CONCLUSIONS

Relative nicotine content is the primary pharmacological determinant of the abuse liability of Kodiak and Camel Snus as measured using ICSS. These models may be useful to compare the relative abuse liability of other tobacco products and to model FDA-mandated changes in product performance standards.

Revisiting the effect of nicotine on interval timing

This paper reviews the evidence for nicotine-induced acceleration of the internal clock when timing in the seconds-to-minutes timescale, and proposes an alternative explanation to this evidence: that nicotine reduces the threshold for responses that result in more reinforcement. These two hypotheses were tested in male Wistar rats using a novel timing task. In this task, rats were trained to seek food at one location after 8s since trial onset and at a different location after 16s. Some rats received the same reward at both times (group SAME); some received a larger reward at 16s (group DIFF). Steady baseline performance was followed by 3 days of subcutaneous nicotine administration (0.3mg/kg), baseline recovery, and an antagonist challenge (mecamylamine, 1.0mg/kg). Nicotine induced a larger, immediate reduction in latencies to switch (LTS) in group DIFF than in group SAME. This effect was sustained throughout nicotine administration. Mecamylamine pretreatment and nicotine discontinuation rapidly recovered baseline performance. These results support a response-threshold account of nicotinic disruption of timing performance, possibly mediated by nicotinic acetylcholine receptors. A detailed analysis of the distribution of LTSs suggests that anomalous effects of nicotine on LTS dispersion may be due to loss of temporal control of behavior.

The incentive amplifying effects of nicotine are reduced by selective and non-selective dopamine antagonists in rats

Nicotine is a psychomotor stimulant with 'reinforcement enhancing' effects--the actions of nicotine in the brain increase responding for non-nicotine rewards. We hypothesized that this latter effect of nicotine depends on increased incentive properties of anticipatory cues; consistent with this hypothesis, multiple laboratories have reported that nicotine increases sign tracking, i.e. approach to a conditioned stimulus (CS), in Pavlovian conditioned-approach tasks. Incentive motivation and sign tracking are mediated by mesolimbic dopamine (DA) transmission and nicotine facilitates mesolimbic DA release. Therefore, we hypothesized that the incentive-promoting effects of nicotine would be impaired by DA antagonists. To test this hypothesis, separate groups of rats were injected with nicotine (0.4mg/kg base) or saline prior to Pavlovian conditioning sessions in which a CS (30s illumination of a light or presentation of a lever) was immediately followed by a sweet reward delivered in an adjacent location. Both saline and nicotine pretreated rats exhibited similar levels of conditioned approach to the reward location (goal tracking), but nicotine pretreatment significantly increased approach to the CS (sign tracking), regardless of type (lever or light). The DAD1 antagonist SCH-23390 and the DAD2/3 antagonist eticlopride reduced conditioned approach in all rats, but specifically reduced goal tracking in the saline pretreated rats and sign tracking in the nicotine pretreated rats. The non-selective DA antagonist flupenthixol reduced sign-tracking in nicotine rats at all doses tested; however, only the highest dose of flupenthixol reduced goal tracking in both nicotine and saline groups. The reductions in conditioned approach behavior, especially those by SCH-23390, were dissociated from simple motor suppressant effects of the antagonists. These experiments are the first to investigate the effects of dopaminergic drugs on the facilitation of sign-tracking engendered by nicotine and they implicate dopaminergic systems both in conditioned approach as well as the incentive-promoting effects of nicotine.

The effects of nicotine exposure during Pavlovian conditioning in rats on several measures of incentive motivation for a conditioned stimulus paired with water

RATIONALE

Nicotine enhances approach toward and operant responding for conditioned stimuli (CSs), but the effect of exposure during different phases of Pavlovian incentive learning on these measures remains to be determined.

OBJECTIVES

These studies examined the effects of administering nicotine early, late or throughout Pavlovian conditioning trials on discriminated approach behavior, nicotine-enhanced responding for conditioned reinforcement, extinction, and the reinstatement of responding for conditioned reinforcement. We also tested the effect of nicotine on approach to a lever-CS in a Pavlovian autoshaping procedure and for this CS to serve as a conditioned reinforcer.

METHODS

Thirsty rats were exposed to 13 conditioning sessions where a light/tone CS was paired with the delivery of water. Nicotine was administered either prior to the first or last seven sessions, or throughout the entire conditioning procedure. Responding for conditioned reinforcement, extinction, and the reinstatement of responding by the stimulus and nicotine were compared across exposure groups. Separately, the effects of nicotine on conditioned approach toward a lever-CS during autoshaping, and responding for that CS as a conditioned reinforcer, were examined.

RESULTS

Nicotine exposure was necessary for nicotine-enhanced responding for conditioned reinforcement and the ability for nicotine and the stimulus to additively reinstate responding on the reinforced lever. Nicotine increased contacts with a lever-CS during autoshaping, and removal of nicotine abolished this effect. Prior nicotine exposure was necessary for nicotine-enhanced responding reinforced by the lever.

CONCLUSIONS

Enhancements in the motivating properties of CSs by nicotine occur independently from duration and timing effects of nicotine exposure during conditioning.

Effects of systemically applied nAChRα7 agonists and antagonists on light-induced phase shifts of hamster circadian activity rhythms

Many physiological systems in mammals are linked to the body's master circadian rhythm in the sleep/wake cycle and dysfunctions in this rhythm has been associated with neurological diseases such as major depression, Alzheimer's Disease and schizophrenia. There is some evidence that nicotinic cholinergic input to the master circadian pacemaker, the suprachiasmatic nucleus, may modulate circadian activity rhythms, but data employing in vivo preparations is sparse. Therefore we examined the ability of intraperitoneally applied nicotinic agonists and antagonists relatively selective for the α7 nicotinic receptor to modulate light-induced phase shifts of hamster circadian wheel running rhythms. Hamsters were maintained in constant darkness and exposed to light pulses early and late in their active period, mimicking dusk and dawn respectively, which elicited phase delays and advances of their circadian wheel running rhythms. The α7 receptor antagonists bPiDDB (0.03-3mg/kg) and methyllacaconitine (0.1-1mg/kg) inhibited both light- induced phase advances and delays of circadian wheel running rhythms by as much as 75% versus vehicle injections. In contrast, systemic injections of the α7 agonists PHA 543613 and ABT107, both at 0.156-2.5mg/kg, had no effect on light induced phase advances or delays. Further, α7 nicotinic receptors were identified in the hamster suprachiasmatic nucleus using an antibody that recognizes α7 nicotinic receptors. These results clearly identify the ability of α7 nicotinic receptor antagonists to inhibit light-entrainment of the hamster circadian pacemaker. Therefore, nicotinic compounds may be useful for the treatment of circadian dysfunction associated with neurological diseases.

Effects of blockade of α4β2 and α7 nicotinic acetylcholine receptors on cue-induced reinstatement of nicotine-seeking behaviour in rats

Exposure to environmental stimuli conditioned to nicotine consumption critically contributes to the high relapse rates of tobacco smoking. Our previous work demonstrated that non-selective blockade of nicotinic acetylcholine receptors (nAChRs) reversed the cue-induced reinstatement of nicotine seeking, indicating a role for cholinergic neurotransmission in the mediation of the conditioned incentive properties of nicotine cues. The present study further examined the relative roles of the two major nAChR subtypes, α4β2 and α7, in the cue-induced reinstatement of nicotine seeking. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, free base) on a fixed-ratio 5 schedule of reinforcement. A nicotine-conditioned cue was established by associating a sensory stimulus with each nicotine infusion. After nicotine-maintained responding was extinguished by withholding the nicotine infusion and its paired cue, reinstatement test sessions were conducted with re-presentation of the cue but without the availability of nicotine. Thirty minutes before the tests, the rats were administered the α4β2-selective antagonist dihydro-β-erythroidine (DHβE) and α7-selective antagonist methyllycaconitine (MLA). Pretreatment with MLA, but not DHβE, significantly reduced the magnitude of the cue-induced reinstatement of responses on the active, previously nicotine-reinforced lever. In different sets of rats, MLA altered neither nicotine self-administration nor cue-induced reinstatement of food seeking. These results demonstrate that activation of α7 nAChRs participates in the mediation of the conditioned incentive properties of nicotine cues and suggest that α7 nAChRs may be a promising target for the development of medications for the prevention of cue-induced smoking relapse.

Erythrina mulungu alkaloids are potent inhibitors of neuronal nicotinic receptor currents in mammalian cells

Crude extracts and three isolated alkaloids from Erythrina mulungu plants have shown anxiolytic effects in different animal models. We investigated whether these alkaloids could affect nicotinic acetylcholine receptors and if they are selective for different central nervous system (CNS) subtypes. Screening experiments were performed using a single concentration of the alkaloid co-applied with acetylcholine in whole cell patch-clamp recordings in three different cell models: (i) PC12 cells natively expressing α3* nicotinic acetylcholine receptors; (ii) cultured hippocampal neurons natively expressing α7* nicotinic acetylcholine receptors; and (iii) HEK 293 cells heterologoulsy expressing α4β2 nicotinic acetylcholine receptors. For all three receptors, the percent inhibition of acetylcholine-activated currents by (+)-11á-hydroxyerysotrine was the lowest, whereas (+)-erythravine and (+)-11á-hydroxyerythravine inhibited the currents to a greater extent. For the latter two substances, we obtained concentration-response curves with a pre-application protocol for the α7* and α4β2 nicotinic acetylcholine receptors. The IC₅₀ obtained with (+)-erythravine and (+)-11á-hydroxyerythravine were 6 µM and 5 µM for the α7* receptors, and 13 nM and 4 nM for the α4β2 receptors, respectively. Our data suggest that these Erythrina alkaloids may exert their behavioral effects through inhibition of CNS nicotinic acetylcholine receptors, particularly the α4β2 subtype.

Patterns of nicotinic receptor antagonism II: cardiovascular effects in rats

BACKGROUND

Tobacco cessation pharmacotherapies currently are limited to nicotine itself, the partial nicotine agonists varenicline and cytisine, and the antidepressant bupropion. Compared with agonists, nicotinic antagonists such as the noncompetitive, nonselective compound mecamylamine, and the competitive, α4β2-preferring antagonist dihydro-β-erythroidine (DHβE) may be a novel approach to the treatment of tobacco smoking as both are effective antagonists of nicotine's central effects. Considering nicotinic acetylcholine receptors mediate critical peripheral effects of acetylcholine, such as cardiovascular effects, it is important to study how nicotinic antagonists would alter the cardiovascular system and the cardiovascular changes induced by nicotine.

METHODS

The effects of several nicotinic agonists and antagonists on blood pressure and heart rate were measured in conscious, unrestrained rats following parenteral administration using a telemetry system.

RESULTS

Nicotine and other nicotinic receptor agonists (epibatidine, varenicline, and cytisine) produced similar increases in blood pressure, whereas their effects on heart rate were biphasic. The cardiovascular changes were attenuated by the nonselective nicotine antagonist, mecamylamine, but the peripherally restricted antagonist hexamethonium blocked only the agonist-induced changes in blood pressure. The α7-preferring antagonist, MLA, and the α4β2-preferring antagonist, DHβE, were much less effective in blocking the agonist-induced cardiovascular changes, indicating that nicotine's cardiovascular effects, are due to activation at autonomic ganglia involving nicotinic receptor subtypes other than α4, α7, or β2.

CONCLUSIONS

The data indicate that the cardiovascular effects of nicotine and nicotine-like agents are mediated through receptor mechanisms that are distinct from those that mediate the central effects of nicotine.

Nominal association with CHRNA4 variants and nicotine dependence

Nicotine binds to nicotinic acetylcholine receptors and studies in animal models have shown that α4β2 receptors mediate many behavioral effects of nicotine. Human genetics studies have provided support that variation in the gene that codes for the α4 subunit influences nicotine dependence (ND), but the evidence for the involvement of the β2 subunit gene is less convincing. In this study, we examined the genetic association between variation in the genes that code for the α4 (CHRNA4) and β2 (CHRNB2) subunits of the nicotinic acetylcholine receptor and a quantitative measure of lifetime DSM-IV ND symptom counts. We performed this analysis in two longitudinal family-based studies focused on adolescent antisocial drug abuse: the Center on Antisocial Drug Dependence (CADD, N = 313 families) and Genetics of Antisocial Drug Dependence (GADD, N = 111 families). Family-based association tests were used to examine associations between 14 single nucleotide polymorphisms (SNPs) in CHRNA4 and CHRNB2 and ND symptoms. Symptom counts were corrected for age, sex and clinical status prior to the association analysis. Results, when the samples were combined, provided modest evidence that SNPs in CHRNA4 are associated with ND. The minor allele at both rs1044394 (A; Z = 1.988, P = 0.047, unadjusted P-value) and rs1044396 (G; Z = 2.398, P = 0.017, unadjusted P-value) was associated with increased risk of ND symptoms. These data provide suggestive evidence that variation in the α4 subunit of the nicotinic acetylcholine receptor may influence ND liability.

Nicotine-induced enhancement of responding for conditioned reinforcement in rats: role of prior nicotine exposure and α4β2 nicotinic receptors

RATIONALE

Stimuli associated with nicotine can become motivationally significant and may play a role in tobacco dependence. Previous work indicates that nicotine enhances responding for a conditioned reinforcer (CR).

OBJECTIVES

These studies examined the effects of prior exposure to nicotine on responding for a CR, persistence of this response, and the role of α4β2 or α7 nicotinic receptor subtypes.

METHODS

Water deprived rats were given 13 Pavlovian conditioning sessions where a light/tone conditioned stimulus (CS) was paired with the delivery of water. Then, rats were presented with two levers: one delivered the CS (now a CR), the other was inactive. Experiments examined the effect of nicotine administered prior to Pavlovian conditioning sessions on approach behavior during CS presentations, operant responding for CR in the presence and absence of nicotine, and the persistence of responding for CR. The effects of nicotinic acetylcholine receptor (nAChR) antagonism with mecamylamine and α4β2 or α7 nAChR antagonism with dihydro-beta-erythroidine (DHβE) or methyllycaconitine (MLA) on nicotine-enhanced responding for CR were examined.

RESULTS

Nicotine enhanced approach behavior during CS presentations and potentiated operant responding for CR, an effect sensitized as a result of nicotine exposure during conditioning. Responding for CR and its potentiation by nicotine was stable over multiple tests. Enhanced responding for the CR induced by nicotine was blocked by mecamylamine and DHβE, but not MLA.

CONCLUSIONS

Nicotine enhances Pavlovian discriminated approach and shows sensitized nicotine-induced enhancements in responding for CR, an effect depending on α4β2 nAChRs.

Effects of economy type and nicotine on the essential value of food in rats

The exponential demand equation proposed by Hursh and Silberberg (2008) provides an estimate of the essential value of a good as a function of price. The model predicts that essential value should remain constant across changes in the magnitude of a reinforcer, but may change as a function of motivational operations. In Experiment 1, rats' demand for food across a sequence of fixed-ratio schedules was assessed during open and closed economy conditions and across one- and two-pellet per reinforcer delivery conditions. The exponential equation was fitted to the relation between fixed-ratio size and the logarithm of the absolute number of reinforcers. Estimates of the rate of change in elasticity of food, the proposed measure of essential value, were compared across conditions. Essential value was equivalent across magnitudes during the closed economy, but showed a slight decrease across magnitudes during the open economy. Experiment 2 explored the behavioral mechanisms of nicotine's effects on consumption with the results from Experiment 1 serving as a within-subject frame of reference. The same subjects were administered nicotine via subcutaneously implanted osmotic minipumps at a dose of 3 mg/kg/day and exposed to both the one- and two-pellet conditions under a closed economy. Although nicotine produced large decreases in demand, essential value was not significantly changed. The data from the present experiments provide further evidence for the adequacy of the exponential demand equation as a tool for quantifying the rate of change in elasticity of a good and for assessing behavioral mechanisms of drug action.

Natural rewards, neuroplasticity, and non-drug addictions

There is a high degree of overlap between brain regions involved in processing natural rewards and drugs of abuse. "Non-drug" or "behavioral" addictions have become increasingly documented in the clinic, and pathologies include compulsive activities such as shopping, eating, exercising, sexual behavior, and gambling. Like drug addiction, non-drug addictions manifest in symptoms including craving, impaired control over the behavior, tolerance, withdrawal, and high rates of relapse. These alterations in behavior suggest that plasticity may be occurring in brain regions associated with drug addiction. In this review, I summarize data demonstrating that exposure to non-drug rewards can alter neural plasticity in regions of the brain that are affected by drugs of abuse. Research suggests that there are several similarities between neuroplasticity induced by natural and drug rewards and that, depending on the reward, repeated exposure to natural rewards might induce neuroplasticity that either promotes or counteracts addictive behavior.

[Pharmacotherapeutic treatment strategies for smoking cessation]

Regular tobacco smoking occurs in about 35% of the male and 25% of the female German population. Individual attempts to independently quit smoking and to remain abstinent for 1 year have been shown to be successful in less than 5% of cases. This rate can be doubled by means of individual consulting and cognitive-behavioral interventions and additional pharmacological treatment might increase abstinence rates up to 25%. Apart from nicotine substitution (e.g. transdermal, oral and inhalative applications) and bupropion, recent studies have shown beneficial effects of varenicline for smoking cessation and abstinence. Varenicline, a selective partial nicotinergic agonist, has been specifically developed for the purpose of smoking cessation. Currently available data suggest that varenicline is more effective compared to nicotine substitution therapy and bupropion, increasing the abstinence likelihood by a factor of 2.3 compared to a placebo. Recent data regarding anti-nicotine vaccines suggest that this approach might yield a comparable treatment outcome and probably even better relapse-preventing effects than conventional psychopharmacological strategies. The first anti-nicotine vaccines are expected to be approved by national authorities within the forthcoming 1-2 years.

Patterns of nicotinic receptor antagonism: nicotine discrimination studies

Evaluation of the discriminative stimulus effects of drugs is a useful procedure for identification of receptor mediation of in vivo drug effects. This assay can be enhanced when the stimulus effects of different doses of agonist are evaluated. In the present study, rats were trained to discriminate small or large doses of nicotine from saline, and interactions of these effects with nicotinic receptor antagonists and partial agonists were determined. The insurmountable nicotine antagonist mecamylamine blocked both the discriminative stimulus and response rate-reducing effects of nicotine but was less effective against the large dose of nicotine. The α4β2*-selective, competitive antagonist dihydro-β-erythrodine (DHβE) antagonized the discriminative stimulus effects of both doses but was less effective against the larger training dose of nicotine. Schild analyses of DHβE suggested that different nicotinic receptor populations may be mediating the stimulus effects of large and small doses of nicotine. This suggestion was supported by observations that the discriminative stimulus effects of the partial agonist cytisine were more like those of the large dose than of the small dose of nicotine and that cytisine antagonized the effects of only the small nicotine dose. Varenicline produced nicotine-like effects in both training dose groups but reduced the discriminative stimulus effects of intermediate doses of nicotine in the group trained to the small dose of nicotine. Overall, these results suggest that small doses of nicotine produce their stimulus effects via α4β2* nicotine receptors, whereas larger doses of nicotine recruit additional nicotine receptor subtypes, as revealed by drug discrimination assays in rats.

Activation of the opioid μ1, but not δ or κ, receptors is required for nicotine reinforcement in a rat model of drug self-administration

There has long been an interest in examining the involvement of opioid neurotransmission in nicotine rewarding process and addiction to nicotine. Over the past 3 decades, however, clinical effort to test the effectiveness of nonselective opioid antagonists (mainly naloxone and naltrexone) for smoking cessation has yielded equivocal results. In light of the fact that there are three distinctive types of receptors mediating actions of the endogenous opioid peptides, this study, using a rat model of nicotine self-administration, examined involvement of different opioid receptors in the reinforcement of nicotine by selective blockade of the μ1, the δ, and the κ opioid receptors. Male Sprague-Dawley rats were trained in daily 1h sessions to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. After establishment of stable nicotine self-administration behavior, the effects of the opioid antagonists were tested. Separate groups of rats were used to test the effects of naloxanazine (selective for μ1 receptors, 0, 5 and 15 mg/kg), naltrindole (selective for δ receptors, 0, 0.5 and 5mg/kg), and 5'-guanidinonaltrindole (GNTI, selective for κ receptors, 0, 0.25 and 1mg/kg). In each individual drug group, the 3 drug doses were tested by using a within-subject and Latin-Square design. The effects of these antagonists on food self-administering behavior were also examined in the same rats in each respective drug group after retrained for food self-administration. Pretreatment with naloxonazine, but not naltrindole or GNTI, significantly reduced responses on the active lever and correspondingly the number of nicotine infusions. None of these antagonists changed lever-pressing behavior for food reinforcement. These results indicate that activation of the opioid μ1, but not the δ or the κ, receptors is required for the reinforcement of nicotine and suggest that opioid neurotransmission via the μ1 receptors would be a promising target for the development of opioid ligands for smoking cessation.

A second-order schedule of food reinforcement in rats to examine the role of CB1 receptors in the reinforcement-enhancing effects of nicotine

Nicotine is believed to enhance the motivational value of reinforcers. Although endogenous cannabinoids acting on CB1 receptors have been implicated in the motivational effects of nicotine, their role in the 'reinforcement-enhancing' properties of nicotine is unknown. This study compared the effect of acute and chronic non-contingent nicotine administration on responding for an unconditioned reinforcing stimulus (UCS) and a visual conditioned stimulus (CS) and the role of CB1 receptors was examined. Male hooded Lister rats were trained on a second-order schedule [FI 15' (FR5: S)] under which presentation of the CS (5s/5Hz light oscillation) was intermittently reinforced by the UCS (food). The rats were treated with daily saline or nicotine (0.4 mg/kg, subcutaneous [s.c.]) throughout the study. The effect of the acute nicotine challenge (0.05, 0.1 and 0.2 mg/kg, s.c.) and the CB1 receptor antagonist AM251 (0.1, 0.3 and 1 mg/kg, intraperitoneal [i.p.]) on responding for the CS and/or UCS was examined. The acute nicotine challenge increased responding for both the UCS and CS in the rats chronically treated with nicotine, an effect which was less robust in the nicotine-naive rats. AM251 significantly reduced responding for the UCS and CS, and an interaction with the nicotine challenge was found. These data support and extend the hypothesis that nicotine can enhance the motivational value of reinforcing stimuli and suggest the increases in responding produced by nicotine involve CB1 receptors. Furthermore, this study highlights the utility of second-order schedules of reinforcement for investigation of the neural circuits underlying the reinforcement-enhancing effects of nicotine.

Nicotine's enhancing effects on responding maintained by conditioned reinforcers are reduced by pretreatment with mecamylamine, but not hexamethonium, in rats

Several studies have indicated that nicotine increases responding maintained by conditioned reinforcers. We assessed the effects of subcutaneous injections of 0.3 mg/kg nicotine and two nicotinic antagonists on responding maintained by conditioned and primary reinforcers and responding during extinction in 8 Long Evans rats. Mecamylamine, a central and peripheral nicotinic antagonist, and hexamethonium, a peripheral nicotinic antagonist, were administered prior to a subset of the experimental sessions. Nicotine selectively increased responding maintained by conditioned reinforcers and mecamylamine, but not hexamethonium, attenuated this effect. These results suggest that nicotine's enhancing effect on responding maintained by conditioned reinforcers is mediated in the central nervous system.

The effects of response operandum and prior food training on intravenous nicotine self-administration in rats

RATIONALE

Nicotine intravenous self-administration (IVSA) in rats has been conducted using a variety of methodological procedures with equally variable results.

OBJECTIVES

Here, we addressed the importance of the type of response operandum and prior instrumental training with a natural reinforcer on nicotine IVSA and reinstatement.

METHODS

Rats were tested for spontaneous acquisition of IVSA using either nose poke (NP) or lever press (LVR) operandum. A dose-response test was then conducted, followed by extinction and cue- and nicotine-induced reinstatement.

RESULTS

The use of the NP operandum resulted in markedly higher levels of IVSA across acquisition and across dose-response testing compared with the LVR group. Whereas both groups reinstated following a nicotine prime, only the LVR group demonstrated cue-induced reinstatement. As a positive control, the experiment was repeated with cocaine as the reinforcer: equivalent levels of IVSA were observed across all tests, irrespective of operandum. When rats self-administering nicotine received instrumental training with a sucrose reinforcer prior to IVSA, a facilitated acquisition of IVSA was observed in both LVR and NP groups to a similar extent (the effect of operandum remained), but had little effect on responding thereafter. During reinstatement testing, both groups now displayed cue- and nicotine-induced reinstatement, but this was also evident in saline control animals that had never received nicotine.

CONCLUSIONS

These results suggest that, unlike cocaine, an increased physical response requirement can decrease nicotine intake. It also indicates that operandum and prior sucrose training may influence the role that visual cues play in nicotine dependence.

Facilitation of intravenous nicotine self-administration in rats by a motivationally neutral sensory stimulus

RATIONALE AND OBJECTIVE

Intravenous infusions of nicotine appear to exert little primary reinforcing effects in adult rats but, instead, maintain self-administration behavior at least, in part, by increasing the intrinsic reinforcing effects of drug-paired sensory stimuli. The present study examined instead the impact of a motivationally neutral cue on self-administration.

METHODS

Adult male Long-Evans rats were permitted to self-administer nicotine (0.015 mg/kg IV given over 30 s, 2 h/day) or saline presented with or without a sensory stimulus (light, white noise). Fixed and progressive ratio reinforcement schedules of nicotine reinforcement were tested. Experiment 2 determined whether noncontingent nicotine or mecamylamine (nicotinic antagonist) would induce lever pressing for either sensory stimulus. Experiment 3 tested whether the white noise stimulus alone could maintain responding after repeated pairing with self-administered nicotine. Finally, the sensory stimuli were assessed for possible aversive properties.

RESULTS

Nicotine infusions alone were at best weakly reinforcing. The white noise stimulus, presented alone, was neither reinforcing nor aversive, whereas the white light appeared marginally reinforcing. Both stimuli, however, facilitated intravenous nicotine self-administration. Neither nicotine nor mecamylamine challenge rendered the white noise reinforcing. The white noise, after being self-administered with nicotine, failed to maintain self-administration behavior on its own.

CONCLUSIONS

Even a motivationally neutral sensory stimulus, lacking detectable primary or secondary reinforcing properties, can facilitate self-administration of nicotine. Possibly, drug-paired stimuli provide a "response marker" or serve as a temporal bridge between the operant response and drug effect. Motivationally neutral stimuli may therefore serve to isolate primary reinforcing effects of nicotine.

Bupropion and nicotine enhance responding for nondrug reinforcers via dissociable pharmacological mechanisms in rats

RATIONALE

Nicotine serves as a primary reinforcer but also potently enhances responding for nonnicotine stimuli with reinforcing properties. One of the most successful pharmacotherapies for smoking cessation, bupropion, also increases responding for nondrug reinforcers such as food and brain stimulation rewards.

OBJECTIVE

The present studies investigated whether treatment with bupropion and nicotine had similar effects on responding for a reinforcing visual stimulus (VS). They also investigated whether the effects of bupropion and nicotine depended on common pharmacological substrates.

RESULTS

Nicotine (0.4 mg/kg base) enhanced responding for the VS, and this enhancing effect increased across testing sessions, replicating our previous findings. Bupropion (3, 10, and 30 mg/kg salt) dose-dependently increased responding for the VS. Treatment with 10 and 30 mg/kg bupropion resulted in a profile similar to nicotine; operant responding increased over repeated drug treatments. The reinforcement enhancing effect of nicotine, but not bupropion, was blocked by pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine. In contrast, the reinforcement enhancing effect of bupropion, but not nicotine, was blocked by pretreatment with the alpha noradrenergic antagonist prazosin.

CONCLUSION

The reinforcement enhancing effects of nicotine and bupropion increased over time and repeated treatments suggesting a shared mechanism of action. However, the reinforcement enhancing effects of nicotine are mediated by nicotinic acetylcholine receptors, whereas the reinforcement enhancing effects of bupropion were mediated by alpha noradrenergic receptors.

Correlates of individual differences in compensatory nicotine self-administration in rats following a decrease in nicotine unit dose

RATIONALE

The ability of tobacco harm reduction strategies to produce significant reductions in toxin exposure is limited by compensatory increases in smoking behavior. Characterizing factors contributing to the marked individual variability in compensation may be useful for understanding this phenomenon and assessing the feasibility of harm reduction interventions.

OBJECTIVE

The objective of the study was to use an animal model of human compensatory smoking that involves a decrease in unit dose supporting nicotine self-administration (NSA) to examine potential contributors to individual differences in compensation.

METHODS

Rats were trained for NSA during daily 23-h sessions at a unit dose of 0.06 mg/kg/inf until responding was stable. The unit dose was then reduced to 0.03 mg/kg/inf for at least 10 sessions. Following reacquisition of NSA at the training dose and extinction, single-dose nicotine pharmacokinetic parameters were determined.

RESULTS

Decreases in nicotine intake following dose reduction were proportionally less than the decrease in unit dose, indicating partial compensation. Compensatory increases in infusion rates were observed across the course of the 23-h sessions. The magnitude of compensation differed considerably between rats. Rats exhibiting the highest baseline infusion rates exhibited the lowest levels of compensation. Nicotine pharmacokinetic parameters were not significantly correlated with compensation. Infusion rates immediately returned to pre-reduction levels when baseline conditions were restored.

CONCLUSIONS

These findings provide initial insights into correlates of individual differences in compensation following a reduction in nicotine unit dose. The present assay may be useful for characterizing mechanisms and potential consequences of the marked individual differences in compensatory smoking observed in humans.

The neuropharmacological substrates of nicotine reward: reinforcing versus reinforcement-enhancing effects of nicotine

Compulsive nicotine use is thought to be maintained by the acute reinforcing effects of nicotine and the reinforcement-enhancing effects of nicotine, in addition to the negative consequences of nicotine abstinence. Nicotine self-administration and nicotine-induced enhancement of non-nicotine reinforcers such as intracranial self-stimulation provide measures of these dual rewarding properties of nicotine. First, pharmacological manipulations that modulate the reinforcing and reinforcement-enhancing effects of nicotine are identified and discussed. Second, the interpretation and implications of data that identified shared and specific pharmacological substrates underlying the dual rewarding effects of nicotine are discussed, including implications for the preclinical testing of putative antismoking medications. In conclusion, reinforcement-related behaviors that are mediated by central reinforcement processes are likely to, and generally do, exhibit a number of common pharmacological substrates. Interestingly, however, a few pharmacological classes of compounds seem to exert selective effects on components of the dual nicotine reward mechanisms, indicating differences in the pharmacological substrates of the reinforcing and reinforcement-enhancing effects of nicotine. Further characterization of such compounds may ultimately lead to the identification of novel medications for nicotine dependence in humans.

The generality of nicotine as a reinforcer enhancer in rats: effects on responding maintained by primary and conditioned reinforcers and resistance to extinction

RATIONALE

Nicotine may enhance the reinforcing value of other reinforcers. It is unclear whether nicotine enhances responding maintained by all reinforcers or whether there are limits to this role.

OBJECTIVE

The objective of the study is to test the generality of nicotine-induced increases in reinforced responding by using an observing response procedure, which generated measures of responding maintained by food reinforcers, conditioned reinforcers, and responding during extinction. We also examined whether nicotine increased resistance to extinction and whether nicotine's effects could be characterized as rate-dependent.

MATERIALS AND METHODS

Rats received presession subcutaneous injections of Vehicle (n=5), 0.3 (n=6), or 0.56 (n=6) mg/kg nicotine for 70 sessions. Resistance to extinction was also assessed by removing food for five sessions.

RESULTS

Nicotine did not consistently affect food or extinction responding. Both doses of nicotine produced increases in responding maintained by conditioned reinforcers, but did not increase resistance to extinction. Predrug response rates accounted for a small but significant percentage of the variance in the drug effect.

CONCLUSION

Although there was a tendency for nicotine to increase low predrug response rates (i.e., response rates just prior to nicotine administration), 0.3 and 0.56 mg/kg nicotine systematically increased responding maintained by conditioned reinforcers. The results are consistent with a reinforcer-enhancing role of nicotine. However, nicotine did not increase resistance to extinction, nor did it increase food-maintained responses. Nicotine may selectively increase responding maintained by moderately reinforcing stimuli, such as the conditioned reinforcers used in the present study.