Effects of Monoamine Oxidase Inhibition on the Reinforcing Properties of Low-Dose Nicotine

Chronic exposure to cigarette smoke extract increases nicotine withdrawal symptoms in adult and adolescent male rats

The aim of the current study was to determine whether non-nicotine constituents of cigarette smoke contribute to nicotine dependence in adolescent and adult male Sprague Dawley rats. For 10 days animals were given three times daily intravenous injections of nicotine (1.5 mg/kg/day) or cigarette smoke extract (CSE) containing an equivalent dose of nicotine. Both spontaneous and mecamylamine-precipitated withdrawal were then measured. Chronic treatment with CSE induced significantly greater somatic and affective withdrawal signs than nicotine in both adolescents and adults. Mecamylamine-precipitated somatic signs were similar at both ages. In contrast, animals spontaneously withdrawn from chronic drug treatment exhibited significant age differences: whereas adolescents chronically treated with nicotine did not show somatic signs, those treated with CSE showed similar physical withdrawal to those of adults. Mecamylamine did not precipitate anxiety-like behavior at either age. However, both adolescents and adults showed significant anxiety in a light-dark box test 18 h after spontaneous withdrawal. Anxiety-like behavior was still evident in an open field test 1 month after termination of drug treatment, with adolescents showing significantly greater affective symptoms than adults. Our findings indicate that non-nicotine constituents of cigarette smoke do contribute to dependence in both adolescents and adults and emphasize the importance of including smoke constituents with nicotine in animal models of tobacco dependence.

Nicotine Enhances Intravenous Self-administration of Cannabinoids in Adult Rats

INTRODUCTION

Nicotine and cannabis are commonly used together, yet few studies have investigated the effects of concurrent administration. Nicotine exhibits reinforcement enhancing effects by promoting the reinforcing properties of stimuli including other drugs. As many studies of this effect used non-contingent nicotine, we implemented a dual-self-administration model where rats have simultaneous access to two drugs and choose which to self-administer throughout a session. Here, we investigated the effect of self-administered or non-contingently delivered nicotine on cannabinoid self-administration.

METHODS

Adult male rats were allowed to self-administer the synthetic cannabinoid WIN 55,212-2 (WIN) intravenously, with or without subcutaneous nicotine injections before each session. A separate group of animals were allowed to self-administer WIN, nicotine, or saline using a dual-catheter procedure, where each solution was infused independently and associated with a separate operant response. A third group of male and female rats were allowed to self-administer delta-9-tetrahydrocannabinol (THC) with or without pre-session injections of nicotine.

RESULTS

Nicotine injections increased self-administration of WIN and THC. During dual self-administration, nicotine availability increased saline and WIN infusions but nicotine intake was not changed by WIN or saline availability. Rats preferred nicotine over saline, but preferred nicotine and WIN equally when both were available. The effect of nicotine on cannabinoid self-administration was acute and reversible when nicotine was no longer present.

CONCLUSIONS

These results expand our understanding of the ability of nicotine to enhance reinforcement of other drugs and suggest that co-use of nicotine and cannabinoids promotes cannabinoid use beyond what would be taken alone.

IMPLICATIONS

This study utilizes a dual intravenous self-administration model to investigate the ability of nicotine to enhance cannabinoid intake. Our results demonstrate that the reinforcement enhancing properties of nicotine on drug use extend to include cannabinoids, but that this effect occurs specifically when nicotine is administered alongside the cannabinoid. Interestingly, cannabinoid use did not promote nicotine intake, suggesting this mechanism of reinforcement is specific to nicotine.

Non-nicotine constituents in cigarette smoke extract enhance nicotine addiction through monoamine oxidase A inhibition

Tobacco addiction has been largely attributed to nicotine, a component in tobacco leaves and smoke. However, extensive evidence suggests that some non-nicotine components of smoke should not be overlooked when considering tobacco dependence. Yet, their individual effect and synergistic effect on nicotine reinforcement remain poorly understood. The study herein focused on the role of non-nicotine constituents in promoting the effects of nicotine and their independent reinforcing effects. Denicotinized cigarettes were prepared by chemical extracting of cut tobacco, and the cigarette smoke extracts (CSE, used as a proxy for non-nicotine ingredients) were obtained by machine-smoking the cigarettes and DMSO extraction. The compositions of harmful components, nicotine, and other minor alkaloids in both cut tobacco and the CSE of experimental denicotinized cigarettes were examined by GC-MS, and compared with 3R4F reference cigarettes. individually and in synergy with nicotine were determined by conditioned place preference (CPP), dopamine (DA) level detection, the open field test (OFT), and the elevated plus maze (EPM). Finally, the potential enhancement mechanism of non-nicotinic constituents was investigated by nicotine metabolism and monoamine oxidase A (MAOA) activity inhibition in the striatum of mice and human recombinant MAOA. Thenicotine content in smoke from the experimental denicotinized cigarettes (under ISO machine-smoking conditions) was reduced by 95.1% and retained most minor alkaloids, relative to the 3R4F reference cigarettes. It was found that non-nicotine constituents increased acute locomotor activities. This was especially pronounced for DA levels in NAc and CPP scores, decreased the time in center zone. There were no differences in these metrics with DNC group when compared to the NS group. Non-nicotine constituents alone did not show reinforcing effects in CPP or striatum DA levels in mice. However, in the presence of nicotine, non-nicotine constituents further increased the reinforcing effects. Furthermore, non-nicotine constituents may enhance nicotine's reinforcing effects by inhibiting striatum MAOA activity rather than affecting nicotine metabolism or total striatum DA content in mice. These findings expand our knowledge of the effect on smoking reinforcement of non-nicotine constituents found in tobacco products.

A review of monoamine oxidase (MAO) inhibitors in tobacco or tobacco smoke

Tobacco smoking is reputed to be the most difficult addiction of all to give up, and nicotine has been noted as the major addictive agent in tobacco smoke. However, research shows that nicotine addiction is due to more than nicotine alone. One hypothesis is that monoamine oxidase (MAO) inhibition from non-nicotinic components in, or derived from, tobacco smoke contributes to nicotine addiction. Harman and norharman, have been recognised as major and potent MAO inhibitors in tobacco smoke, but these two inhibitors together comprise perhaps less than 10% of the total MAO A inhibitory activity in cigarette smoke suggesting other unidentified components may make significant contributions to total inhibitory activity. Therefore, we reviewed an index of the chemical components of tobacco and tobacco smoke and identified those known to be MAO inhibitors. Amongst these inhibitors, phenols and phenolic acids with MAO inhibitory activity are commonly reversible and selective MAO A inhibitors, whereas trans,trans-farnesol, 2-methyl-1,4-naphthoquinone (menadione), 1,4-naphthoquinone, scopoletin, and diosmetin with MAO inhibitory activity are reversible and selective MAO B inhibitors. The compound, 1,4-benzoquinone is an irreversible MAO A inhibitor and to the best of our knowledge, this is the first irreversible MAO A inhibitor to be reported in tobacco smoke. MAO inhibitors have been used clinically to treat depression, anxiety, and Parkinson's disease. The MAO inhibitors identified from tobacco and tobacco smoke and summarized in this review, are potential pharmacological candidates to be investigated further. This review will enhance our knowledge of the way tobacco smoke affects MAO activity in smokers and will also be important in helping to understand nicotine addiction.

Monoamine oxidase inhibition in cigarette smokers: From preclinical studies to tobacco product regulation

Monoamine oxidase (MAO) activity is reduced in cigarette smokers and this may promote the reinforcing actions of nicotine, thereby enhancing the addictive properties of cigarettes. At present, it is unclear how cigarette smoking leads to MAO inhibition, but preclinical studies in rodents show that MAO inhibition increases nicotine self-administration, especially at low doses of nicotine. This effect of MAO inhibition develops slowly, likely due to plasticity of brain monoamine systems; studies relying on acute MAO inhibition are unlikely to replicate what happens with smoking. Given that MAO inhibition may reduce the threshold level at which nicotine becomes reinforcing, it is important to consider this in the context of very low nicotine content (VLNC) cigarettes and potential tobacco product regulation. It is also important to consider how this interaction between MAO inhibition and the reinforcing actions of nicotine may be modified in populations that are particularly vulnerable to nicotine dependence. In the context of these issues, we show that the MAO-inhibiting action of cigarette smoke extract (CSE) is similar in VLNC cigarettes and cigarettes with a standard nicotine content. In addition, we present evidence that in a rodent model of schizophrenia the effect of MAO inhibition to enhance nicotine self-administration is absent, and speculate how this may relate to brain serotonin systems. These issues are relevant to the MAO-inhibiting effect of cigarette smoking and its implications to tobacco product regulation.

Harmane Potentiates Nicotine Reinforcement Through MAO-A Inhibition at the Dose Related to Cigarette Smoking

Nicotine is the primary addictive component in cigarette smoke, and dopamine release induced by nicotine is considered a significant cause of persistent smoking and nicotine dependence. However, the effects of nicotine replacement therapy on smoking cessation were less effective than expected, suggesting that other non-nicotine constituents may potentiate the reinforcing effects of nicotine. Harmane is a potent, selective monoamine oxidase A (MAO-A) inhibitor found in cigarette smoke, but showed no effect on nicotine self-administration in previous studies, possibly due to the surprisingly high doses used. In the present study, we found that harmane potentiated nicotine self-administration on the fixed ration schedule at the dose related to human cigarette smoking by the synergistic effects in up-regulating genes in addiction-related pathways, and the effect was reduced at doses 10 times higher or lower than the smoking-related dose. The smoking-related dose of harmane also enhanced the increase of locomotor activity induced by nicotine, accompanied by increased dopamine basal level and dopamine release in the nucleus accumbens through MAO-A inhibition. Our findings provided new evidence for the important role of non-nicotine ingredients of tobacco products in smoking addiction.

Cigarette Smoke Extract, but Not Electronic Cigarette Aerosol Extract, Inhibits Monoamine Oxidase in vitro and Produces Greater Acute Aversive/Anhedonic Effects Than Nicotine Alone on Intracranial Self-Stimulation in Rats

Conventional tobacco cigarettes appear to have greater abuse liability than non-combusted products such as electronic cigarettes (ECs) and nicotine replacement therapy (NRT). This may be due to the higher levels of behaviorally active non-nicotine constituents [e.g., monoamine oxidase (MAO) inhibitors such as β-carbolines] in cigarette smoke (CS) compared to non-combusted products. To evaluate this hypothesis, the current studies compared the relative abuse liability of CS and EC aerosol extracts containing nicotine and a range of non-nicotine constituents to that of nicotine alone (NRT analog) using intracranial self-stimulation (ICSS) in rats. Effects of formulations on brain MAO activity in vitro and ex vivo were also studied to evaluate the potential role of MAO inhibition in the ICSS study. CS extract contained higher levels of several behaviorally active non-nicotine constituents (e.g., the β-carbolines norharmane and harmane) than EC extract. Nicotine alone reduced ICSS thresholds at a moderate nicotine dose, suggesting a reinforcement-enhancing effect that may promote abuse liability, and elevated ICSS thresholds at a high nicotine dose, suggesting an aversive/anhedonic effect that may limit abuse liability. CS extract elevated ICSS thresholds to a greater degree than nicotine alone at high nicotine doses. Effects of EC extract on ICSS did not differ from those of nicotine alone. Finally, CS extract significantly inhibited MAO-A and MAO-B activity in vitro, whereas EC extract and nicotine alone did not. None of the formulations inhibited MAO measured ex vivo. These findings indicate greater acute aversive/anhedonic effects for CS extract compared to nicotine alone, suggesting lower abuse liability. Although confirmation of our findings using other dosing regimens, preclinical addiction models, and tobacco product extracts is needed, these findings suggest that the centrally-mediated effects of MAO inhibitors and other non-nicotine constituents may not account for the greater abuse liability of cigarettes compared to non-combusted products. Nonetheless, identifying the specific constituent(s) mediating the effects of CS extracts in this study could help clarify mechanisms mediating tobacco addiction and inform FDA product standards.

Biologically Active Compounds Present in Tobacco Smoke: Potential Interactions Between Smoking and Mental Health

Tobacco dependence remains one of the major preventable causes of premature morbidity and mortality worldwide. There are well over 8,000 compounds present in tobacco and tobacco smoke, but we do not know what effect, if any, many of them have on smokers. Major interest has been on nicotine, as well as on toxic and carcinogenic effects and several major and minor components of tobacco smoke responsible for the negative health effects of smoking have been elucidated. Smokers themselves report a variety of positive effects from smoking, including effects on depression, anxiety and mental acuity. Smoking has also been shown to have protective effects in Parkinson’s Disease. Are the subjective reports of a positive effect of smoking due to nicotine, of some other components of tobacco smoke, or are they a manifestation of the relief from nicotine withdrawal symptoms that smoking provides? This mini-review summarises what is currently known about the components of tobacco smoke with potential to have positive effects on smokers.

Tobacco and ADHD: A Role of MAO-Inhibition in Nicotine Dependence and Alleviation of ADHD Symptoms

Attention-deficit/hyperactivity disorder (ADHD) is a relatively commonly occurring neurodevelopmental disorder affecting approximately 5% of children and young people. The neurobiological mechanisms of ADHD are proposed to particularly center around increased dopamine receptor availability related to associated symptoms of reduced attention regulation and impulsivity. ADHD is also persistent across the lifespan and associated with a raft of impulsive and health-risk behaviors including substance abuse and smoking. Research highlighting the potentially significant levels of monoamine oxidase (MAO) inhibitory properties in tobacco smoke and e-cigarettes may provide a mechanism for increased tobacco smoke dependence among those with ADHD, in addition to the role of nicotine.

Aim

This scoping review aimed to establish evidence for the above neurobiological pathway between smoking and ADHD symptom-alleviation or “self-medication” with the inclusion of the mechanism of MAO-inhibitors indirect increasing dopamine in the brain.

Methodology

Scoping review methodologies were employed in this review selected to synthesize multiple sources of empirical research to identify current gaps in the knowledge base and identify key characteristics of research data related to a phenomenon. Databases searched included OVID MEDLINE(R), Embase, Cochrane, PsycINFO and SCOPUS limited to 2000 onward and empirically validated, peer-reviewed research.

Findings

There is support for the role of MAO-inhibition on greater reinforcement of smoking for individuals with ADHD through a greater impact on dopaminergic availability than nicotine; potentially moderating ADHD symptoms.

Conclusion

Greater support for a “self-medication” model of ADHD and smoking includes not only nicotine but also MAO-inhibitors as dopamine agonists contained in cigarettes and e-cigarettes.

A review of the evidence on cigarettes with reduced addictiveness potential

BACKGROUND

In May 2018, the Secretariat for the World Health Organization Framework Convention on Tobacco Control convened a meeting to discuss the potential for reducing the addictiveness of tobacco products. A central focus was to review research findings on the behavioral effects of reducing the addictiveness of cigarettes.

METHODS

This manuscript reports the results of a review of the behavioral science literature, updated through April 2021, with special attention to both the potential benefits and unintended consequences of reducing nicotine in cigarettes.

RESULTS

Available evidence suggests that reducing nicotine content in cigarettes to very low levels could benefit public health in three primary ways, by 1) decreasing uptake of regular smoking, 2) decreasing the amount people smoke, and 3) increasing the likelihood of smoking cessation. Current evidence also suggests that reducing nicotine in cigarettes may produce similar benefits across many important subpopulations of people who smoke, including those with psychiatric comorbidities, those who use other substances, those with low socioeconomic status, young people, people who smoke infrequently and people who prefer menthol cigarettes. Cigarette nicotine reduction could also lead to some undesirable outcomes, such as experiencing withdrawal, product manipulation, an illicit market, and harm misperceptions; strategies that may mitigate each are discussed.

CONCLUSION

Overall, behavioral research suggests product standards that limit the nicotine content of combusted tobacco products could render cigarettes and similar products less addictive. The availability of legal, non-combusted products that effectively substitute for cigarettes and the dissemination of public health campaigns that clarify misperceptions about the relationship between nicotine, tobacco and disease may facilitate the extent to which a nicotine reduction policy reduces smoking.

Self-administration by female rats of low doses of nicotine alone vs. nicotine in tobacco smoke extract

BACKGROUND

Nicotine has reinforcing effects, but there are thousands of other compounds in tobacco, some of which might interact with nicotine reinforcement.

AIMS

This rat study was conducted to determine if nicotine self-administration is altered by co-administration of the complex mixture of compounds in tobacco smoke extract (TSE).

METHODS

Female Sprague-Dawley rats were tested for self-administration of low doses of nicotine (3 or 10 µg/kg/infusion) at three different rates of reinforcement (FR1, FR3 and FR5) over three weeks either alone or together with the complex mixture of tobacco smoke extract (TSE).

RESULTS

Rats self-administering 3 µg/kg/infusion of nicotine alone showed a rapid initiation on an FR1 schedule, but declined with FR5. Rats self-administering nicotine in TSE acquired self-administration more slowly, but increased responding over the course of the study. With 10 µg/kg/infusion rats self-administered significantly more nicotine alone than rats self-administering the same nicotine dose in TSE. Rats self-administering nicotine alone took significantly more infusions with the 10 than the 3 µg/kg/infusion dose, whereas rats self-administering nicotine in TSE did not. Nicotine in TSE led to a significantly greater locomotor hyperactivity at a dose of 0.1 mg/kg compared to rats that received nicotine alone. Rats self-administering nicotine alone had significantly more responding on the active vs. inactive lever, but rats self-administering the same nicotine doses in TSE did not.

CONCLUSIONS

Self-administration of nicotine in a purer form appears to be more clearly discriminated and dose-related than nicotine self-administered in the complex mixture of TSE.

β-Carbolines found in cigarette smoke elevate intracranial self-stimulation thresholds in rats

Identifying novel constituents that contribute to tobacco addiction is essential for developing more effective treatments and informing FDA regulation of tobacco products. While preclinical data indicate that monoamine oxidase (MAO) inhibitors can have abuse liability or potentiate the addiction-related effects of nicotine, most of these studies have used clinical MAO inhibitors (e.g., tranylcypromine) that are not present in cigarette smoke. The primary goal of this study was to evaluate the abuse potential of the β-carbolines harmane, norharmane, and harmine - MAO inhibitors that are found in cigarette smoke - in an intracranial self-simulation (ICSS) model in rats. A secondary goal was to evaluate the ability of norharmane to influence nicotine's acute effects on ICSS. None of the β-carbolines lowered ICSS thresholds at any dose studied when administered alone, suggesting a lack of abuse liability. Rather, all three β-carbolines produced dose-dependent elevations in ICSS thresholds, indicating aversive/anhedonic effects. Harmane and harmine also elevated ICSS response latencies, suggesting a disruption of motor function, albeit with reduced potency compared to their ICSS threshold-elevating effects. Norharmane (2.5 mg/kg) modestly attenuated the effects of nicotine on ICSS thresholds. Our findings indicate that these β-carbolines produced only aversive/anhedonic effects in an ICSS model when administered alone, and that norharmane unexpectedly attenuated nicotines acute effects on ICSS. Future work evaluating the addiction-related effects of nicotine combined with these and other MAO inhibitors present in smoke may be useful for understanding the role of MAO inhibition in tobacco addiction and informing FDA tobacco regulation.

Dopamine and Addiction

Addiction is commonly identified with habitual nonmedical self-administration of drugs. It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Such addictions can also be defined in terms of the brain mechanisms they activate; most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig; they have only unconditioned consummatory reflexes. Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.

Monoamine oxidase inhibitory activity of flavoured e-cigarette liquids

BACKGROUND AND AIMS

Monoamine oxidase inhibitors have been hypothesised to be important in tobacco dependence, reinforcing the brain's response to nicotine by delaying the degradation of neurotransmitters by monoamine oxidases. The development of electronic cigarettes has provided an alternative nicotine delivery system, which is widely viewed as less toxic than tobacco smoke. However, significant data gaps remain. This paper reports the results of measurements of monoamine oxidase inhibitory activity in a small sample of commercially available, flavoured e-liquids.

METHODS

Twelve e-liquids were tested for monoamine oxidase inhibitory activity, using the kynuramine assay and monoamine oxidase enzymes (human, recombinant). Control samples of carrier liquids, propylene glycol and glycerol, and nicotine were also tested.

RESULTS

Four e-liquids contained high levels of inhibitory activity, four more were moderately inhibitory. The remaining four e-liquids were mildly inhibitory, while the carrier liquids, and nicotine were inactive at relevant concentrations. The active compounds in the e-liquids were subsequently identified as vanillin and ethyl vanillin. Under some conditions of use, the sampled e-liquids with the highest concentrations of monoamine oxidase inhibitory activity have the potential to expose consumers to physiologically significant levels of MAO inhibitory activity.

CONCLUSIONS

While only a small sample of e-liquids was tested, the findings suggest that some flavours have pharmacological actions, with potential to enhance the response to nicotine or to other drugs. The public health implications of these preliminary findings on addiction and smoking cessation warrant exploration and further research.

Basic Science and Public Policy: Informed Regulation for Nicotine and Tobacco Products

Introduction

Scientific discoveries over the past few decades have provided significant insight into the abuse liability and negative health consequences associated with tobacco and nicotine-containing products. While many of these advances have led to the development of policies and laws that regulate access to and formulations of these products, further research is critical to guide future regulatory efforts, especially as novel nicotine-containing products are introduced and selectively marketed to vulnerable populations.

Discussion

In this narrative review, we provide an overview of the scientific findings that have impacted regulatory policy and discuss considerations for further translation of science into policy decisions. We propose that open, bidirectional communication between scientists and policy makers is essential to develop transformative preventive- and intervention-focused policies and programs to reduce appeal, abuse liability, and toxicity of the products.

Conclusions

Through these types of interactions, collaborative efforts to inform and modify policy have the potential to significantly decrease the use of tobacco and alternative nicotine products and thus enhance health outcomes for individuals.

Implications

This work addresses current topics in the nicotine and tobacco research field to emphasize the importance of basic science research and provide examples of how it can be utilized to inform public policy. In addition to relaying current thoughts on the topic from experts in the field, the article encourages continued efforts and communication between basic scientists and policy officials.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

The effect of nicotine and nicotine+monoamine oxidase inhibitor on the value of alcohol

Alcohol is the most commonly abused drug in the USA and many people suffer from alcohol use disorder. Many factors are associated with alcohol use disorder, but the causal role of comorbid nicotine use has not been extensively considered. Nicotine has reward-enhancing properties and may increase the value of alcohol. Monoamine oxidase inhibition increases nicotine self-administration and may increase the reward-enhancing effects of nicotine. We assessed the effect of nicotine and nicotine in combination with a commonly used monoamine oxidase inhibitor (tranylcypromine) on the value of alcohol using a progressive ratio schedule of reinforcement in rats. Nicotine administration increased the breakpoint for alcohol, but nicotine in combination with tranylcypromine decreased the breakpoint for alcohol. The current study adds to previous research showing that nicotine increases the value of alcohol. This finding has important implications for the etiology of addiction because of the comorbidity of smoking with many drugs of abuse. The finding that nicotine in combination with tranylcypromine reduces the value of alcohol warrants further investigation.

The nicotine-degrading enzyme NicA2 reduces nicotine levels in blood, nicotine distribution to brain, and nicotine discrimination and reinforcement in rats

BACKGROUND

The bacterial nicotine-degrading enzyme NicA2 isolated from P. putida was studied to assess its potential use in the treatment of tobacco dependence.

RESULTS

Rats were pretreated with varying i.v. doses of NicA2, followed by i.v. administration of nicotine at 0.03 mg/kg. NicA2 had a rapid onset of action reducing blood and brain nicotine concentrations in a dose-related manner, with a rapid onset of action. A 5 mg/kg NicA2 dose reduced the nicotine concentration in blood by > 90% at 1 min after the nicotine dose, compared to controls. Brain nicotine concentrations were reduced by 55% at 1 min and 92% at 5 min post nicotine dose. To evaluate enzyme effects at a nicotine dosing rate equivalent to heavy smoking, rats pretreated with NicA2 at 10 mg/kg were administered 5 doses of nicotine 0.03 mg/kg i.v. over 40 min. Nicotine levels in blood were below the assay detection limit 3 min after either the first or fifth nicotine dose, and nicotine levels in brain were reduced by 82 and 84%, respectively, compared to controls. A 20 mg/kg NicA2 dose attenuated nicotine discrimination and produced extinction of nicotine self-administration (NSA) in most rats, or a compensatory increase in other rats, when administered prior to each daily NSA session. In rats showing compensation, increasing the NicA2 dose to 70 mg/kg resulted in extinction of NSA. An enzyme construct with a longer duration of action, via fusion with an albumin-binding domain, similarly reduced NSA in a 23 h nicotine access model at a dose of 70 mg/kg.

CONCLUSIONS

These data extend knowledge of NicA2's effects on nicotine distribution to brain and its ability to attenuate addiction-relevant behaviors in rats and support its further investigation as a treatment for tobacco use disorder.

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.

Animal Research on Nicotine Reduction: Current Evidence and Research Gaps

A mandated reduction in the nicotine content of cigarettes may improve public health by reducing the prevalence of smoking. Animal self-administration research is an important complement to clinical research on nicotine reduction. It can fill research gaps that may be difficult to address with clinical research, guide clinical researchers about variables that are likely to be important in their own research, and provide policy makers with converging evidence between clinical and preclinical studies about the potential impact of a nicotine reduction policy. Convergence between clinical and preclinical research is important, given the ease with which clinical trial participants can access nonstudy tobacco products in the current marketplace. Herein, we review contributions of preclinical animal research, with a focus on rodent self-administration, to the science of nicotine reduction. Throughout this review, we highlight areas where clinical and preclinical research converge and areas where the two differ. Preclinical research has provided data on many important topics such as the threshold for nicotine reinforcement, the likelihood of compensation, moderators of the impact of nicotine reduction, the impact of environmental stimuli on nicotine reduction, the impact of nonnicotine cigarette smoke constituents on nicotine reduction, and the impact of nicotine reduction on vulnerable populations. Special attention is paid to current research gaps including the dramatic rise in alternative tobacco products, including electronic nicotine delivery systems (ie, e-cigarettes). The evidence reviewed here will be critical for policy makers as well as clinical researchers interested in nicotine reduction.

IMPLICATIONS

This review will provide policy makers and clinical researchers interested in nicotine reduction with an overview of the preclinical animal research conducted on nicotine reduction and the regulatory implications of that research. The review also highlights the utility of preclinical research for research questions related to nicotine reduction.

Reduced-Nicotine Cigarettes in Young Smokers: Impact of Nicotine Metabolism on Nicotine Dose Effects

The use of cigarettes delivering different nicotine doses allows evaluation of the contribution of nicotine to the smoking experience. We compared responses of 46 young adult smokers to research cigarettes, delivering 0.027, 0.110, 0.231, or 0.763 mg nicotine, and conventional cigarettes. On five separate days, craving, withdrawal, affect, and sustained attention were measured after overnight abstinence and again after smoking. Participants also rated each cigarette, and the nicotine metabolite ratio (NMR) was used to identify participants as normal or slow metabolizers. All cigarettes equally alleviated craving, withdrawal, and negative affect in the whole sample, but normal metabolizers reported greater reductions of craving and withdrawal than slow metabolizers, with dose-dependent effects. Only conventional cigarettes and, to a lesser degree, 0.763-mg nicotine research cigarettes increased sustained attention. Finally, there were no differences between ratings of lower-dose cigarettes, but the 0.763-mg cigarettes and (even more so) conventional cigarettes were rated more favorably than lower-dose cigarettes. The findings indicate that smoking-induced relief of craving and withdrawal reflects primarily non-nicotine effects in slow metabolizers, but depends on nicotine dose in normal metabolizers. By contrast, relief of withdrawal-related attentional deficits and cigarette ratings depend on nicotine dose regardless of metabolizer status. These findings have bearing on the use of reduced-nicotine cigarettes to facilitate smoking cessation and on policy regarding regulation of nicotine content in cigarettes. They suggest that normal and slow nicotine metabolizers would respond differently to nicotine reduction in cigarettes, but that irrespective of metabolizer status, reductions to <0.763 mg/cigarette may contribute to temporary attentional deficits.

Monoamine oxidase inhibitory activity in tobacco particulate matter: Are harman and norharman the only physiologically relevant inhibitors?

Monoamine oxidase inhibition is significant in smokers, but it is still unclear how the inhibition that is seen in the brains and bodies of smokers is brought about. Our aim was to test the contribution of the harman and norharman in tobacco smoke to MAO-A inhibition from tobacco smoke preparations, as part of a re-examination of harman and norharman as the cause of the inhibition of MAO-A inhibition in the brain. Tobacco smoke particulate matter and cigarette smoke particulate matter were prepared and the amounts of harman and norharman measured. The results were compared with the total monoamine oxidase-A inhibitory activity. At a nicotine concentration of 0.6μM (a "physiological" concentration in blood) the total monoamine oxidase-A inhibitory activity measured in these samples was sufficient to inhibit the enzyme by approximately 10%. Of this inhibitory activity, only a small proportion of the total was found to be due to harman and norharman. These results show that harman and norharman provide only a moderate contribution to the total monoamine oxidase-A inhibitory activity of tobacco smoke, perhaps under 10%. This suggests that other inhibitors (either known or unknown) may be more significant contributors to total inhibitory activity than has yet been established, and deserve closer examination.

Self-Administration of Smokeless Tobacco Products in Rats

OBJECTIVE

Preclinical abuse liability assessment is an essential component of tobacco regulatory science. The goal of this project was to evaluate the relative abuse liability of smokeless tobacco products in rats using aqueous extracts of those products. These extracts provide exposure to an extensive range of nicotine and non-nicotine tobacco constituents as occurs in humans.

METHODS

Rats were trained to self-administer either nicotine alone or extracts of Camel Snus or Kodiak smokeless tobacco at an equivalent nicotine unit dose. In Experiment 1, the relative reinforcing efficacy of these formulations was assessed in adults and adolescents using a progressive ratio schedule under limited-access conditions. In Experiment 2, relative reinforcing efficacy was assessed in adolescents under unlimited-access conditions using behavioral economic demand curve analysis.

RESULTS

The reinforcing efficacy of nicotine formulations was higher in adolescents than adults, but no difference was observed between formulations in either age group. Similarly, there was no difference in elasticity of demand between formulations in adolescents.

CONCLUSIONS

The present findings suggest that the abuse liability of these smokeless tobacco products is similar to nicotine alone, and that nicotine dose is the primary determinant of the reinforcing efficacy of systemic exposure to these products.