Extinguishing the rewarding value of smoke cues: pharmacological and behavioral treatments

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

Molecular mechanisms underlying the motivational effects of nicotine

In addition to the primary rewarding properties of nicotine and the alleviation of withdrawal symptoms, cues associated with smoking are critical contributors to maintenance of smoking behavior. Nicotine-paired cues are also critical for precipitating relapse after smoking cessation. An accumulation of evidence suggests that repeated exposure to tobacco, including the primary psychoactive ingredient, nicotine, changes brain neurochemistry in a way that promotes the control that cues associated with smoking or other rewards have over behavior. This chapter will consider the neurochemical mechanisms underlying these neuroadaptations. Targeting these molecular alterations may provide novel treatments for smoking cessation.

Increases in impulsivity following smoking abstinence are related to baseline nicotine intake and boredom susceptibility

Trait impulsivity and response inhibition have been shown to be related to smoking behavior. One measure of response inhibition - antisaccade performance, or the ability to inhibit looking at a novel stimulus - has been shown to be worsened by smoking abstinence, improved by nicotine administration and predictive of smoking cessation outcomes. However, relations between antisaccade performance and measures of trait impulsivity have not been extensively evaluated in smokers. In the present study, twelve dependent smokers (n=12) completed an eye tracking task following smoking as usual and overnight abstinence; and they completed baseline measures of trait impulsivity, smoking history and provided biological samples. As expected, overnight abstinence significantly increased antisaccade errors (p<0.002) while having no effect on prosaccade performance. Abstinence-induced increases in antisaccade errors were positively correlated with baseline plasma cotinine and Sensation Seeking Scale Boredom Susceptibility, and negatively correlated with IQ. These results suggest that smoking abstinence significantly increases errors of response inhibition and that the magnitude of this increase is related to trait impulsivity and nicotine intake variables.

Selectively reduced responses to smoking cues in amygdala following extinction-based smoking cessation: results of a preliminary functional magnetic resonance imaging study

Preliminary studies suggest an extinction-based smoking cessation treatment using reduced nicotine content (RNC) cigarettes decreases self-report craving for cigarettes prior to quitting and may be an effective smoking cessation treatment. The aims of this study was to evaluate the effect of an extinction-based smoking cessation treatment on brain responses to smoking cues using blood-oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI). Sixteen (n = 16) dependent smokers were scanned using BOLD fMRI at baseline, following 2-4 weeks of smoking RNC cigarettes while wearing a 21-mg nicotine patch, and 2-4 weeks following quitting smoking. During scanning, participants viewed smoking-related pictures (e.g. lit cigarette) and pictures of people engaged in everyday activities (e.g. using a stapler). Event-related BOLD responses to smoking and control cues were analyzed in regions of interest (ROIs) known to subserve reward, attention, motivation and emotion. The extinction-based treatment simultaneously attenuated responses to smoking cues in amygdala while potentiating responses to control cues. Exploratory analysis indicated that this pattern was also observed in the thalamus of future abstinent but not relapsing smokers. The results of this preliminary study suggest that an extinction-based treatment for smoking cessation alters brain responses to smoking and control cues in amygdala--a region previously associated with drug cue reactivity and extinction.

Rationale, pharmacology and clinical efficacy of partial agonists of alpha4beta2 nACh receptors for smoking cessation

Most smokers repeatedly fail in their attempts to stop smoking because of the addictive nature of the nicotine in tobacco products. Nicotine dependence is probably mediated through the activation of multiple subtypes of neuronal nicotinic acetylcholine receptor (nAChR), among which the mesolimbic alpha(4)beta(2) subtype has a pivotal role. Here, we discuss the rationale for and the design of alpha(4)beta(2) nAChR partial agonists as novel treatments for tobacco addiction. Such agents are expected to exhibit a dual action by sufficiently stimulating alpha(4)beta(2)-nAChR-mediated dopamine release to reduce craving when quitting and by inhibiting nicotine reinforcement when smoking. Potent and selective alpha(4)beta(2) nAChR partial agonists that exhibit dual agonist and antagonist activity in preclinical models can be identified. The validity of this approach is demonstrated by the clinical efficacy of the alpha(4)beta(2) nAChR partial agonist varenicline, which has significantly better quit rates than do other treatments and offers a new option for smoking cessation pharmacotherapy.

Mecamylamine attenuates cue-induced reinstatement of nicotine-seeking behavior in rats

Mecamylamine, a noncompetitive nicotinic cholinergic antagonist, inhibits nicotine self-administration in animals and may attenuate tobacco smoking in humans trying to quit. Our preliminary data suggested that this agent, at a dose of 2 mg/kg (subcutaneous (s.c.)), also attenuates cue-induced relapse to nicotine-seeking behavior in rats. This study determined whether mecamylamine-induced attenuation can be obtained at doses lower than the high 2 mg/kg dose used in the first study, and whether it is specific to nicotine-associated cues. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. Each infusion was accompanied by a visual cue (1 s onset of a lever light followed by offset of a house light for 20 s during which time no infusions could be obtained). After the nicotine-maintained responding was extinguished by withholding the delivery of nicotine (saline substitution) and its associated cue, reinstatement tests were conducted. Response-contingent re-presentation of the cue without further availability of nicotine significantly reinstated extinguished responding at the previously nicotine-reinforced lever. Pretreatment with mecamylamine (0.5, 1, and 2 mg/kg, s.c.) dose-dependently attenuated the cue-induced reinstatement of lever responding. Mecamylamine did not change food-taking and -seeking responses, whereas the highest dose (2 mg/kg) decreased nicotine self-administration behavior. The results confirm previous findings that stimuli conditioned to nicotine self-administration effectively elicit reinstatement of nicotine-seeking behavior after extinction and demonstrate that mecamylamine, besides suppressing self-administration of nicotine, effectively attenuates cue-induced nicotine-seeking behavior. These findings suggest that the response-reinstatement procedures used in this study may be useful for studying neurobiological mechanisms of nicotine-seeking behavior and that mecamylamine-like drugs may be potential candidates for pharmacological treatment and prevention of relapse to tobacco smoking in abstinent smokers.

Effects of acute abstinence, reinstatement, and mecamylamine on biochemical and behavioral measures of cigarette smoking in schizophrenia

BACKGROUND

Schizophrenics have higher rates of smoking than the general population, and more difficulty with smoking cessation. However, there has been little study of differences between schizophrenics and controls with respect to biochemical and behavioral indices of smoking. We compared smokers with schizophrenia (SS; n=27) and control smokers (CS; n=26) on smoking and psychiatric outcomes at baseline, during acute smoking abstinence and reinstatement, and with pre-treatment using the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (MEC) in a human laboratory setting.

METHODS

Biochemical (e.g., plasma nicotine) and behavioral (e.g., craving, withdrawal) outcomes were assessed at baseline, after overnight abstinence, and after smoking reinstatement during three consecutive test weeks. Each week, participants received one of three doses of MEC (0.0, 5.0, or 10.0 mg/dayx3 days) in a randomized, counterbalanced manner.

RESULTS

Compared to CS, SS displayed similar levels of craving and withdrawal, but higher plasma nicotine and cotinine levels, and cotinine/CPD ratio. During reinstatement, SS consumed significantly more cigarettes than CS, but MEC did not significantly alter indices of smoking, psychiatric symptoms, or cigarette consumption during reinstatement.

CONCLUSIONS

1) The reinforcing effects of smoking may be increased in SS versus CS after overnight abstinence; 2) the lack of effects of nAChR antagonism may suggest that non-nicotinic components of cigarettes may contribute to the behavioral effects of smoking in both SS and CS; and 3) consistent with previous studies, SS may exhibit higher baseline levels of nicotine and cotinine, and greater extraction of nicotine per cigarette than CS.

Smoking in the absence of nicotine: behavioral, subjective and physiological effects over 11 days

AIMS

Sensorimotor stimuli associated with tobacco smoking influence smoking behavior; however, current research has focused almost exclusively on the effects of brief, laboratory-based exposure to smoking-related stimuli. The purpose of this experiment was to characterize the effects of smoking stimuli delivered in the absence of nicotine over an extended (11-day) exposure.

DESIGN, SETTING AND PARTICIPANTS

Thirty adult regular smokers participated in an in-patient study. After assessing preferred brand smoking, participants were assigned randomly to one of three groups corresponding to subsequent smoking conditions: nicotine-containing cigarettes, de-nicotinized cigarettes or no smoking.

MEASUREMENTS

Measures of smoking reinforcement, subjective effects, physiological effects, withdrawal/craving and puff topography were taken repeatedly during both periods of free access and controlled assessments during abstinence.

FINDINGS

Daily de-nicotinized cigarette use declined immediately by 1.7 cigarettes/day compared to the preferred brand baseline and declined by another 3.5 cigarettes over time; participants smoking de-nicotinized cigarettes also demonstrated a 31% decline in the number of puffs earned on a progressive ratio, a measure of the motivation to smoke, during the study. Subjective ratings of smoking were largely negative throughout the study in the de-nicotinized group, while the nicotine-containing condition reported increasingly positive subjective effects with repeated exposure. Acute craving suppression following smoking remained evident throughout the study regardless of nicotine content.

CONCLUSIONS

These effects highlight the importance of non-nicotine sensorimotor stimuli as determinants of the maintenance of smoking behavior and suggests that extinction of conditioned reinforcement in the absence of nicotine progresses slowly.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Appetitive conditioning: neural bases and implications for psychopathology

Appetitive conditioning is the process through which new rewards are learned and acquire their motivational salience. Although it has the same evolutionary survival significance as aversive conditioning, appetitive conditioning has rarely been studied in humans. This gap may be explained by the difficulty to find in humans suitable appetitive stimuli that can elicit physiological responses similar to those elicited by aversive stimuli. To help remedy this gap, we review the literature on conditioning, with emphasis on appetitive conditioning. This review comprises three parts. First, we examine the different forms of conditioning. Second, we review the neural basis of appetitive conditioning, particularly from a functional neuroimaging perspective. And third, we demonstrate how perturbations in processes involved in appetitive conditioning can contribute to implicated psychopathologies and suggest neurobiological models underlying these pathologies. The ultimate goal of this review is to stimulate new avenues of research that have direct links to molecular biology, and thus could prove to be invaluable to progress in the understanding and treatment of psychiatric disabilities.

The role of sensory perception in the development and targeting of tobacco products

AIMS

To examine tobacco industry research on smoking-related sensory effects, including differences in sensory perception across smoker groups, and to determine whether this research informed targeted product development and impacted the development of commercial tobacco products.

DESIGN

We searched previously secret internal tobacco industry documents available online through document databases housed at Tobacco Documents Online, the British American Tobacco Document Archive and the Legacy Tobacco Documents Library. We identified relevant documents using a snowball sampling method to first search the databases using an initial set of key words and to then establish further search terms.

FINDINGS

Sensory research is a priority within the tobacco industry directly impacting commercial markets both in the United States and internationally. Sensory factors contribute to smoker satisfaction and product acceptance, and play an important role in controlling puffing behavior. Cigarette manufacturers have capitalized on distinct sensory preferences across gender, age and ethnic groups by tailoring products for specific populations.

CONCLUSIONS

Regulation of tobacco products is needed to address product changes that are used to reinforce or contribute to tobacco dependence; for instance, the incorporation of additives that target attributes such as smoothness, harshness and aftertaste. Greater understanding of the role of sensory effects on smoking behavior may also help to inform the development of tobacco treatment options that support long-term tobacco abstinence.

How research in behavioral pharmacology informs behavioral science

Behavioral pharmacology is a maturing science that has made significant contributions to the study of drug effects on behavior, especially in the domain of drug-behavior interactions. Less appreciated is that research in behavioral pharmacology can have, and has had, implications for the experimental analysis of behavior, especially its conceptualizations and theory. In this article, I outline three general strategies in behavioral pharmacology research that have been employed to increase understanding of behavioral processes. Examples are provided of the general characteristics of the strategies and of implications of previous research for behavior theory. Behavior analysis will advance as its theories are challenged.

Drugs and genotypes: how pharmacogenetic information could improve smoking cessation treatment

Current smoking cessation treatments are not optimal as the long-term abstinence rates are low. Nicotine replacement therapy and bupropion are the only pharmacotherapies widely available to smokers and there is a need to improve current cessation treatments and to develop new drug therapies. Our goal is to use pharmacogenetic information to improve smoking cessation treatments. Candidate genes include those involved in central mechanisms (such as genes encoding the nicotinic acetylcholine receptors, dopamine receptors, dopamine transporters and opioid receptors) and peripheral mechanisms (such as genes encoding the drug-metabolizing enzymes CYP2A6 and CYP2B6). Pharmacogenetics could be used to improve current smoking cessation treatments through two general approaches. The first would be to identify novel drug targets based on genetic variants that are associated with a natural protection against nicotine dependence, decreased smoking behaviour and/or increased smoking cessation. This knowledge could be used to develop drugs that can mimic these advantages, reducing the risk for acquisition, block the rewarding effects of smoking, decreasing the amount smoked and increasing cessation. The second would be to identify smokers with particular genetic variants who are likely to respond better to one treatment (or dose) versus another and to individualize their treatment to maximize long-term abstinence. This review will highlight examples of how pharmacogenetic information from central and peripheral candidate genes could contribute to improving smoking cessation outcomes by these two approaches.

Complex interactions between nicotine and nonpharmacological stimuli reveal multiple roles for nicotine in reinforcement

RATIONALE

Although considerable progress has been made, we do not yet fully understand the behavioral and neurobiological basis of nicotine reinforcement, and without this knowledge, treatment strategies aimed at reducing smoking remain deficient.

OBJECTIVES

This review describes an original perspective on nicotine reinforcement, which arises from substantial evidence of complex interactions between nicotine and nonpharmacological stimuli. We hypothesize that nicotine reinforcement derives from at least two sources: (1) primary reinforcement, an action that requires response-dependent drug administration and is capable of conveying secondary reinforcing effects on associated stimuli, and (2) the reinforcement-enhancing effect of nicotine, which directly enhances behavior maintained by salient nonnicotine stimuli and does not require a contingent relationship between drug administration and reinforced operant responding. Although novel for nicotine, this hypothesis has origins in an extensive literature on the reinforcing effects of psychostimulants. Empirical support for this hypothesis, based largely on animal models of reinforcement, will be presented.

CONCLUSIONS

Animal models of drug reinforcement have evolved to reflect our growing awareness of the multidimensional nature of drug dependence in humans. Investigating the interaction between nicotine and nonpharmacological stimuli within the context of the drug self-administration paradigm in rats has generated new insights into the paradox of how nicotine, an apparently weak primary reinforcer, can sustain the robust behavior observed in self-administration and in smoking. The hypothesis presented in this paper--that nicotine acts as both a primary reinforcer and an enhancer of other nonnicotine reinforcers--provides important direction for future investigations into the neurobiology of nicotine reinforcement and treatments for smoking cessation.

Nicotine and nonnicotine factors in cigarette addiction

RATIONALE

A great deal of research supports the role of nicotine in cigarette addiction. However, the effectiveness of nicotine replacement therapy (NRT) as a smoking cessation treatment has fallen short of initial hopes. A key reason may be that NRT does not address nonnicotine components of smoking reinforcement. These include constituents that provide reinforcing sensory stimulation, components that minimize excessive irritation from inhaled nicotine and other pharmacologically active compounds in cigarette smoke.

OBJECTIVE

Studies using various paradigms to dissociate nicotine from other components of smoking are summarized.

RESULTS

Nonnicotine components provide many rewarding effects, often surpassing the direct effects of nicotine. Substitutes for the sensory effects of smoking may be effective in relieving craving for cigarettes and in facilitating smoking cessation. Moreover, techniques for devaluing smoking-related cues may decrease craving and enhance subsequent abstinence. Promising approaches for devaluing smoke cues include extinction-based treatments employing denicotinized cigarettes and the use of nicotinic agonist and/or antagonist treatment during the weeks leading up to a quit attempt. Recent studies suggest that incorporating these approaches into a treatment program may significantly increase smoking abstinence rates. Preliminary findings also suggest that replacement of the effects of monoamine oxidase inhibitors contained in cigarette smoke may enhance quit rates.

CONCLUSIONS

While current NRT methods have been the mainstay of smoking cessation treatment and will likely continue to serve a useful role, the next stage of progress will likely entail the development of tools designed with recognition of the importance of nonnicotine components of cigarette smoking.

Expanding treatment of tobacco dependence

Nicotine dependence is the leading preventable cause of adult morbidity and mortality in the world. New research on the treatment of this disorder ranges from studies evaluating access to treatment to studies elucidating the molecular mechanisms of nicotine addiction. As our understanding of the neurobiology of tobacco addiction grows, the number of potential therapeutic targets by which we can intervene in this pernicious disorder also increases. This paper presents an overview of recent research trends in the treatment of tobacco dependence. We review several novel mechanisms of action that may serve as therapeutic targets for the pharmacologic treatment of tobacco dependence, including drugs that affect monamine oxidase, selective nicotinic receptors, glutamate and gamma-aminobutyric acid receptors, and the endocannabinoid system. For each of these therapeutic targets, we discuss medications in development that affect these pathophysiologic mechanisms.

Introduction of unsaturation into the N-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: effect on affinity and selectivity

N-n-octylnicotinium iodide (NONI) and N-n-decylnicotinium iodide (NDNI) are selective nicotinic receptor (nAChR) antagonists mediating nicotine-evoked striatal dopamine (DA) release, and inhibiting [3H]nicotine binding, respectively. This study evaluated effects of introducing unsaturation into the N-n-alkyl chains of NONI and NDNI on inhibition of [3H]nicotine and [3H]methyllycaconitine binding (alpha4beta2* and alpha7* nAChRs, respectively), (86)Rb+ efflux and [3H]DA release (agonist or antagonist effects at alpha4beta2* and alpha6beta2*-containing nAChRs, respectively). In the NONI series, introduction of a C3-cis- (NONB3c), C3-trans- (NONB3t), C7-double-bond (NONB7e), or C3-triple-bond (NONB3y) afforded a 4-fold to 250-fold increased affinity for [3H]nicotine binding sites compared with NONI. NONB7e and NONB3y inhibited nicotine-evoked 86Rb+ efflux, indicating alpha4beta2* antagonism. NONI analogs exhibited a 3-fold to 8-fold greater potency inhibiting nicotine-evoked [3H]DA overflow compared with NONI (IC50 = 0.62 microM; Imax = 89%), with no change in Imax, except for NONB3y (Imax = 50%). In the NDNI series, introduction of a C4-cis- (NDNB4c), C4-trans-double-bond (NDNB4t), or C3-triple-bond (NDNB3y) afforded a 4-fold to 80-fold decreased affinity for [3H]nicotine binding sites compared with NDNI, whereas introduction of a C9 double-bond (NDNB9e) did not alter affinity. NDNB3y and NDNB4t inhibited nicotine-evoked 86Rb+ efflux, indicating antagonism at alpha4beta2* nAChRs. Although NDNI had no effect, NDNB4t and NDNB9e potently inhibited nicotine-evoked [3H]DA overflow (IC50 = 0.02-0.14 microM, Imax = 90%), as did NDNB4c (IC50 = 0.08 microM; Imax = 50%), whereas NDNB3y showed no inhibition. None of the analogs had significant affinity for alpha7* nAChRs. Thus, unsaturated NONI analogs had enhanced affinity at alpha4beta2*- and alpha6beta2*-containing nAChRs, however a general reduction of affinity at alpha4beta2* and an uncovering of antagonist effects at alpha6beta2*-containing nAChRs were observed with unsaturated NDNI analogs.

The airway sensory impact of nicotine contributes to the conditioned reinforcing effects of individual puffs from cigarettes

Puffs from cigarettes are the fundamental unit of smoking reward. Here, we examined the extent to which reward from puffs can be derived from the airway sensory effect of nicotine, in the absence of a direct central nervous system effect of nicotine. We did this by assessing the self-reported reward obtained from individual puffs from nicotinized, denicotinized and unlit cigarettes within 7 s of inhalation, which is before nicotine had an opportunity to reach the brain. We also assessed the self-reported strength of airway sensations elicited by the puffs. We found that nicotinized puffs were rated as both stronger and more rewarding than denicotinized and unlit puffs. We also found that the extent to which nicotine elicited reward was directly correlated with the extent to which nicotine elicited airway sensations. This indicates that the airway sensory effects of nicotine contribute to the reward from puffs, above and beyond the reward derived from the airway sensory effects of non-nicotine constituents. These findings have implications for the interpretation of studies that use puffs as experimental units to examine nicotine reward. They also have implications for the use of denicotinized and low nicotine cigarettes as aids to smoking cessation.