Robust escalation of nicotine intake with extended access to nicotine self-administration and intermittent periods of abstinence

Intermittent nicotine access is as effective as continuous access in promoting nicotine seeking and taking in rats

RATIONALE

Nicotine is a principal psychoactive agent in tobacco, contributing to tobacco's addictive potential. Preclinical studies on the effects of voluntary nicotine intake typically use self-administration procedures that provide continuous nicotine access during each self-administration session. However, many smokers consume cigarettes intermittently rather than continuously throughout each day. For drugs including cocaine and opioids, research in laboratory rats shows that intermittent intake can be more effective than continuous intake in producing patterns of drug use relevant to addiction.

OBJECTIVE

We determined how intermittent versus continuous nicotine self-administration influences nicotine seeking and taking behaviours.

METHODS

Female and male rats had continuous (i.e., Long Access; LgA, 6 h/day) or intermittent (IntA; 12 min ON, 60 min OFF, for 6 h/day) access to intravenous nicotine (15 µg/kg/infusion), for 12 daily sessions. We then assessed intake, responding for nicotine under a progressive ratio schedule of drug reinforcement and cue- and nicotine-induced reinstatement of drug seeking. We also estimated nicotine pharmacokinetic parameters during LgA and IntA self-administration.

RESULTS

Overall, LgA rats took twice more nicotine than did IntA rats, yielding more sustained increases in estimated brain concentrations of the drug. However, the two groups showed similar motivation to seek and take nicotine, as measured using reinstatement and progressive ratio procedures, respectively.

CONCLUSIONS

Intermittent nicotine use is just as effective as continuous use in producing addiction-relevant behaviours, despite significantly less nicotine exposure. This has implications for modeling nicotine self-administration patterns in human smokers and resulting effects on brain and behaviour.

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Single-nucleus genomics in outbred rats with divergent cocaine addiction-like behaviors reveals changes in amygdala GABAergic inhibition

The amygdala processes positive and negative valence and contributes to addiction, but the cell-type-specific gene regulatory programs involved are unknown. We generated an atlas of single-nucleus gene expression and chromatin accessibility in the amygdala of outbred rats with high and low cocaine addiction-like behaviors following prolonged abstinence. Differentially expressed genes between the high and low groups were enriched for energy metabolism across cell types. Rats with high addiction index (AI) showed increased relapse-like behaviors and GABAergic transmission in the amygdala. Both phenotypes were reversed by pharmacological inhibition of the glyoxalase 1 enzyme, which metabolizes methylglyoxal-a GABAA receptor agonist produced by glycolysis. Differences in chromatin accessibility between high and low AI rats implicated pioneer transcription factors in the basic helix-loop-helix, FOX, SOX and activator protein 1 families. We observed opposite regulation of chromatin accessibility across many cell types. Most notably, excitatory neurons had greater accessibility in high AI rats and inhibitory neurons had greater accessibility in low AI rats.

Dopamine D1-like receptor activation decreases nicotine intake in rats with short or long access to nicotine

The use of nicotine and tobacco products is highly addictive. The dopaminergic system plays a key role in the initiation and maintenance of nicotine intake. Dopamine D1-like receptor blockade diminishes nicotine intake in rats with daily short (1 h) access to nicotine, but little is known about the effects of dopamine receptor antagonists or agonists on nicotine intake in rats with intermittent long (23 h) access. Because of the extended access conditions and high nicotine intake, the intermittent long access procedure might model smoking and vaping better than short access models. We investigated the effects of the dopamine D1-like receptor antagonist SCH 23390 and the D1-like receptor agonist A77636 on nicotine intake in male rats with intermittent short or long access to nicotine. The rats self-administered nicotine for 5 days (1 h/day) and were then given 15 intermittent short (1 h/day) or long (23 h/day) access sessions (3 sessions/week, 0.06 mg/kg/inf). The D1-like receptor antagonist SCH 23390 decreased nicotine intake to a similar degree in rats with short or long access to nicotine. The D1-like receptor agonist A77636 induced a greater decrease in nicotine intake in the rats with long access to nicotine than in rats with short access. Treatment with A77636 induced a prolonged decrease in nicotine intake that lasted throughout the dark and light phase in the long access rats. These findings indicate that blockade and stimulation of D1-like receptors decrease nicotine intake in an intermittent long access animal model that closely models human smoking and vaping.

Development of Dependence in Smokers and Rodents With Voluntary Nicotine Intake: Similarities and Differences

INTRODUCTION

Smoking and vaping throughout adolescence and early adulthood lead to nicotine dependence. Nicotine withdrawal is associated with somatic and affective withdrawal symptoms that contribute to smoking and relapse. Affective nicotine withdrawal symptoms in humans include craving for cigarettes, depression, anxiety, trouble sleeping, and cognitive deficits.

METHODS

Herein, we review clinical studies that investigated nicotine dependence in people who smoke or vape. We also discuss studies that investigated the development of dependence in animals with oral nicotine intake, nicotine aerosol self-administration, and intravenous nicotine self-administration.

RESULTS

Clinical studies report that adolescents who smoke daily develop nicotine dependence before those who smoke infrequently, but ultimately all smokers become dependent in adulthood. Preclinical studies indicate that rats that self-administer nicotine also become dependent. Rats that self-administer nicotine display somatic withdrawal signs and affective withdrawal signs, including increased anxiety and depressive-like behavior, cognitive deficits, and allodynia. Most nicotine withdrawal signs were observed in rodents with daily (7 days/week) or intermittent long access (23-hour) to nicotine. Clinical smoking studies report symptoms of nicotine dependence in adolescents of both sexes, but virtually all preclinical nicotine self-administration studies have been done with adult male rats.

CONCLUSIONS

The role of sex and age in the development of dependence in nicotine self-administration studies remains under-investigated. However, the role of sex and age in nicotine withdrawal has been thoroughly evaluated in studies in which nicotine was administered noncontingently. We discuss the need for volitional nicotine self-administration studies that explore the gradual development of dependence during adolescence and adulthood in rodents of both sexes.

IMPLICATIONS

The reviewed clinical studies investigated the development of nicotine dependence in male and female adolescent and young adult smokers and vapers. These studies indicate that most adolescent smokers and vapers gradually become nicotine dependent. Preclinical studies with rodents show that nicotine intake in widely used self-administration models also leads to dependence. However, almost all animal studies that investigated the development of nicotine dependence have been conducted with adult male rats. To better model smoking and vaping, it is important that nicotine intake in rats or mice starts during adolescence and that both sexes are included.

Altered neuronal activity in the ventromedial prefrontal cortex drives nicotine intake escalation

Nicotine addiction develops after prolonged drug use and escalation of drug intake. However, because of difficulties in demonstrating escalation of nicotine use in rats, its underlying neuroadaptations still remain poorly understood. Here we report that access to unusually high doses of nicotine (i.e., from 30 µg to 240 µg/kg/injection) for self-administration precipitated a rapid and robust escalation of nicotine intake and increased the motivation for the drug in rats. This nicotine intake escalation also induced long-lasting changes in vmPFC neuronal activity both before and during nicotine self-administration. Specifically, after escalation of nicotine intake, basal vmPFC neuronal activity increased above pre-escalation and control activity levels, while ongoing nicotine self-administration restored these neuronal changes. Finally, simulation of the restoring effects of nicotine with in vivo optogenetic inhibition of vmPFC neurons caused a selective de-escalation of nicotine self-administration.

Depressive-like phenotype enhances relapse of nicotine seeking after forced abstinence in rats

OBJECTIVE

Comorbidity of depression and drug addiction is common, but effective treatment is missing. A rat model combining the olfactory bulbectomy (OBX) model and IV drug self-administration has provided evidence of differential reactivity of the OBX rats towards drugs of abuse. This study evaluates nicotine taking and seeking behaviour in this model.

METHODS

Adult male Wistar rats were used; in one group, the OBX was performed while the other group was sham-operated. After three weeks of nicotine self-administration (fixed ratio-1 schedule), rats underwent two weeks of forced abstinence followed by a drug-free relapse-like session. Two doses of nicotine were studied: 0.019 and 0.030 mg/kg per infusion. The locomotor test took place before the self-administration protocol and on the first day of abstinence.

RESULTS

OBX induced characteristic hyperactive locomotor phenotype. OBX rats self-administered more nicotine in the experiment using 0.019 mg/kg per infusion, but they reached lower drug intake in the study using 0.030 mg/kg per infusion. However, relapse of nicotine seeking after forced abstinence was significantly higher in the OBX groups in both cohorts.

CONCLUSION

These results are in line with previous studies showing OBX-induced dissimilarities in drug-seeking and drug-taking and represent complementary information to reports on other substances.

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Logical fallacies and misinterpretations that hinder progress in translational addiction neuroscience

Substance use disorders (SUDs) are heterogeneous and complex, making the development of translationally predictive rodent and nonhuman primate models to uncover their neurobehavioral underpinnings difficult. Neuroscience-focused outcomes have become highly prevalent, and with this, the notion that SUDs are disorders of the brain embraced as a dominant theoretical orientation to understand SUD etiology and treatment. These efforts, however, have led to few efficacious pharmacotherapies, and in some cases (as with cocaine or methamphetamine), no pharmacotherapies have translated from preclinical models for clinical use. In this theoretical commentary, we first describe the development of animal models of substance use behaviors from a historical perspective. We then define and discuss three logical fallacies including 1) circular explanation, 2) affirming the consequent, and 3) reification that can apply to developed models. We then provide three case examples in which conceptual or logical issues exist in common methods (i.e., behavioral economic demand, escalation, and reinstatement). Alternative strategies to refocus behavioral models are suggested for the field to better bridge the translational divide between animal models, the clinical condition of SUDs, and current and future regulatory pathways for intervention development.

Activation of trace amine-associated receptor 1 attenuates nicotine withdrawal-related effects

Nicotine addiction is a leading avoidable brain disorder globally. Although nicotine induces a modest reinforcing effect, which is important for the initial drug use, the transition from nicotine use to nicotine addiction involves the mechanisms responsible for the negative consequences of drug abstinence. Recent study suggested that trace amine-associated receptor 1 (TAAR1) is a promising pharmacological target for the modulation of positive reinforcing effects of nicotine. However, whether TAAR1 plays a part in the negative reinforcement of nicotine withdrawal remains to be determined. Here, using a long-access (LA) self-administration model, we investigated whether LA rats show increased nicotine intake and withdrawal symptoms in comparison with saline and ShA rats and then tested the effect of TAAR1 partial agonist RO5263397 on nicotine withdrawal effects. We found that rats from long-access group showed significant abstinence-induced anxiety-like behaviour, mechanic hypersensitivity, increased number of precipitated withdrawal signs and higher motivation for the drug, while rats from short-access did not differ from saline group. TAAR1 partial agonist RO5263397 significantly reduced the physical and motivational withdrawal effects of nicotine in LA rats, as reflected by increased time spent on the open arm in the elevated plus maze (EPM) test, normalized paw withdrawal threshold, decreased withdrawal signs and motivation to self-administer nicotine. This study indicates that activation of TAAR1 attenuates the negative-reinforcing effects of nicotine withdrawal and further suggests TAAR1 as a promising target to treat nicotine addiction.

NBQX attenuates relapse of nicotine seeking but not nicotine and methamphetamine self-administration in rats

OBJECTIVE

Pharmacological manipulations of glutamatergic ionotropic receptors have been suggested as a promising target for addiction treatment. Antagonists of AMPA/kainate receptors were shown to reduce alcohol intake or alcohol-seeking in various animal models. In this study, we evaluated the effect of NBQX, an AMPA/kainate receptor antagonist, on methamphetamine (METH) and nicotine self-administration in rats.

METHODS

Male Wistar rats were trained to self-administer METH (0.08 mg/kg per infusion, session of 90 min) and nicotine (0.03 mg/kg per infusion, session of 60 min) under the fixed ratio 1 schedule of reinforcement. The maintenance training was 2 weeks. During the second week, NBQX was injected subcutaneously at doses of 5 or 10 mg/kg 20 min before the session or intravenously (IV) at doses of 1 and 5 mg/kg 10 min before the session. Following the maintenance training, rats were subjected to forced abstinence for 2 weeks and 1 day of the drug-free relapse-like session with IV NBQX treatment performed as before.

RESULTS

Although NBQX did not affect nicotine maintenance, it significantly suppressed the drug-paired responding in the relapse session. Regarding METH, NBQX did not exert a significant effect at either phase of the study.

CONCLUSIONS

These findings suggest selective involvement of AMPA/kainate receptors in the relapse of nicotine seeking after a period of forced abstinence.

Natural and synthetic estrogens specifically alter nicotine demand and cue-induced nicotine seeking in female rats

Women have more difficulty maintaining smoking cessation than men, and experience greater withdrawal symptomatology as well as higher prevalence of relapse. Further, currently available treatments for smoking cessation, such as the nicotine patch and varenicline, have been shown to be less effective in women. Fluctuations in ovarian hormones across the menstrual cycle can affect craving and smoking relapse propensity. In addition, many women who smoke use some form of oral contraceptives, which most often contain ethinyl estradiol (EE), a synthetic, orally bio-available estrogen that is currently prescribed to women chronically and has been shown to alter smoking reward in women. The current study examined the impact of 17β-estradiol (E2), the prominent endogenous form of the steroid hormone estrogen, as well as EE, on nicotine self-administration, demand, and reinstatement following ovariectomy (OVX) or sham surgery. OVX vehicle-treated female rats consumed less nicotine, had lower intensity of demand, and reinstated less compared to sham vehicle-treated female rats. OVX-E2 and OVX-EE treatment groups showed a rebound of nicotine intake later in training, and Q₀ levels of consumption were partially rescued in both groups. Further, E2 but not EE reversed the abolishment of reinstated nicotine seeking induced by OVX. Taken together, these results demonstrate that natural and synthetic estrogens play a critical role in mediating the neurobehavioral effects of nicotine, and future studies are essential for our understanding of how synthetic hormones contained within oral contraceptives interact with smoking.

Paradoxical changes in brain reward status during oxycodone self-administration in a novel test of the negative reinforcement hypothesis

BACKGROUND AND PURPOSE

The extra medical use of, and addiction to, prescription opioid analgesics is a growing health problem. To characterize how prescription opioid abuse develops, this study investigated the affective consequences of escalating prescription opioid use using intracranial self-stimulation (ICSS) reward and oxycodone intravenous self-administration (IVSA) models.

EXPERIMENTAL APPROACH

Male Wistar rats were given access to oxycodone IVSA (0.15 mg·kg-1 per infusion, i.v.) in short-access (ShA; 1 h) or long-access (LgA; 12 h) sessions for five sessions per week followed by intermittent 60-h discontinuations from drug access, a novel explicit test of the negative reinforcement hypothesis. Separate groups were first trained in the ICSS procedure and then in oxycodone IVSA in 11-h LgA sessions.

KEY RESULTS

Rats given LgA to oxycodone escalated their responding more than ShA rats, with further significant increases observed following each 60-h discontinuation. Presession brain reward thresholds increased with sequential daily LgA IVSA sessions, consistent with a growing negative affective state consequent to successive daily intoxication/abstinence cycles. A 1-h oxycodone IVSA interval was sufficient to normalize these elevated reward thresholds, as was, paradoxically, a 60-h weekend abstinence. The increase in ICSS thresholds was attenuated in a group treated with the long-acting κ-opioid antagonist norbinaltorphimine prior to IVSA training.

CONCLUSION AND IMPLICATIONS

Changes in brain reward function during escalation of oxycodone self-administration are driven by an interplay between κ-opioid receptor-mediated negative affective state associated with escalated oxycodone intake and dynamic restoration of brain reward status during longer periods of abstinence.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

Strain and sex matters: Differences in nicotine self-administration between outbred and recombinase-driver transgenic rat lines

Preclinical studies of nicotine self-administration provide important value for the field as they are highly rigorous, controlled, can be conducted quickly, and are generalizable to humans. Given the translational value of the nicotine self-administration model, and the relatively new guidelines of the National Institutes of Health to include sex as a biological variable, strain and sex differences in nicotine acquisition were examined here in two outbred rat strains. Sprague-Dawley (SD) and Long-Evans (LE; wildtype and cholinergic acetyltransferase cre-recombinase transgenic) rats of each sex were implanted with indwelling intravenous jugular catheters. Rats were trained to self-administer nicotine (0.02 mg/kg per infusion, paired with contingent light + tone stimuli). Acquisition criteria were set at a minimum active:inactive response ratio of 2:1 and a minimum of 10 infusions per session, both of which had to be met for a minimum of 10 sessions. Across 10 sessions, male SD rats self-administered significantly more nicotine than female SD rats (p < .05), indicating a sex difference in this strain. LE females self-administered more nicotine than SD females indicative of a strain difference between females (p < .05). SD males increased nicotine infusions across sessions compared to LE males and SD females (p < .05). No strain or sex differences were observed in the number of sessions to reach criteria. No differences between wildtype and transgenic LE rats were observed. These results demonstrate sex and strain differences in nicotine self-administration between SD and LE rats and may lend insight into development of other nicotine self-administration models, where sex and strain may impact acquisition. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Flavor additives facilitate oral self-administration of nicotine solution in mice

RATIONALE

Tobacco products are very addictive, partly because they contain nicotine which is reinforcing, but also because they include appealing aromas and tastes. Flavor additives are such sensory stimuli which enhance attractiveness, as well as use and abuse of tobacco and vaping products. Yet, the interaction between these flavor additives and nicotine remains poorly understood.

OBJECTIVES

We want to understand how flavors may reduce nicotine' aversive taste and how it may enhance its voluntary oral self-administration in mice.

METHODS

We first studied the effect of flavor additives on nicotine solution palatability in a free bottle choice paradigm. Second, we investigated the effect of vanilla flavoring on the different stages of nicotine (40 μg/ml) oral self-administration in mice.

RESULTS

We show that adding flavors increase nicotine palatability and facilitate acquisition and maintenance of oral self-administration when compared to nicotine-alone group. Mice adapt their operant behavior depending on changes in nicotine concentration. All mice reinstate nicotine seeking upon presentation of associated cues. Nevertheless, vanilla-flavored nicotine was not more reinforcing than vanilla-flavored water which was reinforcing enough to drive similar operant response rates.

CONCLUSIONS

Flavor additives increase nicotine oral consumption and help maintaining operant behavior in mice. Moreover, flavors can be very attractive and can have high reinforcing value by themselves. Thus, it is crucial that the investigation on how taste signals play an important role in modulating oral nicotine intake in rodent models remains explored.

Converging vulnerability factors for compulsive food and drug use

Highly palatable foods and substance of abuse have intersecting neurobiological, metabolic and behavioral effects relevant for understanding vulnerability to conditions related to food (e.g., obesity, binge eating disorder) and drug (e.g., substance use disorder) misuse. Here, we review data from animal models, clinical populations and epidemiological evidence in behavioral, genetic, pathophysiologic and therapeutic domains. Results suggest that consumption of highly palatable food and drugs of abuse both impact and conversely are regulated by metabolic hormones and metabolic status. Palatable foods high in fat and/or sugar can elicit adaptation in brain reward and withdrawal circuitry akin to substances of abuse. Intake of or withdrawal from palatable food can impact behavioral sensitivity to drugs of abuse and vice versa. A robust literature suggests common substrates and roles for negative reinforcement, negative affect, negative urgency, and impulse control deficits, with both highly palatable foods and substances of abuse. Candidate genetic risk loci shared by obesity and alcohol use disorders have been identified in molecules classically associated with both metabolic and motivational functions. Finally, certain drugs may have overlapping therapeutic potential to treat obesity, diabetes, binge-related eating disorders and substance use disorders. Taken together, data are consistent with the hypotheses that compulsive food and substance use share overlapping, interacting substrates at neurobiological and metabolic levels and that motivated behavior associated with feeding or substance use might constitute vulnerability factors for one another. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Non-genetic factors that influence methamphetamine intake in a genetic model of differential methamphetamine consumption

RATIONALE

Genetic and non-genetic factors influence substance use disorders. Our previous work in genetic mouse models focused on genetic factors that influence methamphetamine (MA) intake. The current research examined several non-genetic factors for their potential influence on this trait.

OBJECTIVES

We examined the impact on MA intake of several non-genetic factors, including MA access schedule, prior forced MA exposure, concomitant ethanol (EtOH) access, and gamma-aminobutyric acid type B (GABA_B) receptor activation. Selectively bred MA high drinking (MAHDR) and low drinking (MALDR) mice participated in this research.

RESULTS

MAHDR, but not MALDR, mice increased MA intake when given intermittent access, compared with continuous access, with a water choice under both schedules. MA intake was not altered by previous exposure to forced MA consumption. Male MAHDR mice given simultaneous access to MA, EtOH, and an EtOH+MA mixture exhibited a strong preference for MA over EtOH and EtOH+MA; MA intake was not affected by EtOH in female MAHDR mice. When independent MAHDR groups were given access to MA, EtOH, or EtOH+MA vs. water in each case, MA intake was reduced in the water vs. EtOH+MA group, compared with the water vs. MA group. The GABA_B receptor agonist R(+)-baclofen (BAC) not only reduced MA intake but also reduced water intake and locomotor activity in MAHDR mice. There was a residual effect of BAC, such that MA intake was increased after termination of BAC treatment.

CONCLUSIONS

These findings demonstrate that voluntary MA intake in MAHDR mice is influenced by non-genetic factors related to MA access schedule and co-morbid EtOH exposure.

Relationship Between Nicotine Intake and Reward Function in Rats With Intermittent Short Versus Long Access to Nicotine

INTRODUCTION

Tobacco use improves mood states and smoking cessation leads to anhedonia, which contributes to relapse. Animal studies have shown that noncontingent nicotine administration enhances brain reward function and leads to dependence. However, little is known about the effects of nicotine self-administration on the state of the reward system.

METHODS

To investigate the relationship between nicotine self-administration and reward function, rats were prepared with intracranial self-stimulation electrodes and intravenous catheters. The rats were trained on the intracranial self-stimulation procedure and allowed to self-administer 0.03 mg/kg/infusion of nicotine. All rats self-administered nicotine daily for 10 days (1 hour/day) and were then switched to an intermittent short access (ShA, 1 hour/day) or long access (LgA, 23 hour/day) schedule (2 days/week, 5 weeks).

RESULTS

During the first 10 daily, 1-hour sessions, nicotine self-administration decreased the reward thresholds, which indicates that nicotine potentiates reward function. After switching to the intermittent LgA or ShA schedule, nicotine intake was lower in the ShA rats than the LgA rats. The LgA rats increased their nicotine intake over time and they gradually consumed a higher percentage of their nicotine during the light phase. The nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine induced a larger increase in reward thresholds (ie, anhedonia) in the LgA rats than the ShA rats. In the LgA rats, nAChR blockade with mecamylamine decreased nicotine intake for 2 hours and this was followed by a rebound increase in nicotine intake.

CONCLUSIONS

A brief period of nicotine self-administration enhances reward function and a high level of nicotine intake leads to dependence.

IMPLICATIONS

These animal studies indicate that there is a strong relationship between the level of nicotine intake and brain reward function. A high level of nicotine intake was more rewarding than a low level of nicotine intake and nicotine dependence was observed after long, but not short, access to nicotine. This powerful combination of nicotine reward and withdrawal makes it difficult to quit smoking. Blockade of nAChRs temporarily decreased nicotine intake, but this was followed by a large rebound increase in nicotine intake. Therefore, nAChR blockade might not decrease the use of combustible cigarettes or electronic cigarettes.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Genetic susceptibility to nicotine addiction: Advances and shortcomings in our understanding of the CHRNA5/A3/B4 gene cluster contribution

Over the last decade, robust human genetic findings have been instrumental in elucidating the heritable basis of nicotine addiction (NA). They highlight coding and synonymous polymorphisms in a cluster on chromosome 15, encompassing the CHRNA5, CHRNA3 and CHRNB4 genes, coding for three subunits of the nicotinic acetylcholine receptor (nAChR). They have inspired an important number of preclinical studies, and will hopefully lead to the definition of novel drug targets for treating NA. Here, we review these candidate gene and genome-wide association studies (GWAS) and their direct implication in human brain function and NA-related phenotypes. We continue with a description of preclinical work in transgenic rodents that has led to a mechanistic understanding of several of the genetic hits. We also highlight important issues with regards to CHRNA3 and CHRNB4 where we are still lacking a dissection of their role in NA, including even in preclinical models. We further emphasize the use of human induced pluripotent stem cell-derived models for the analysis of synonymous and intronic variants on a human genomic background. Finally, we indicate potential avenues to further our understanding of the role of this human genetic variation. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Role of corticotropin-releasing factor in alcohol and nicotine addiction

The two most prevalent substance use disorders involve alcohol and nicotine, which are often co-abused. Robust preclinical and translational evidence indicates that individuals initiate drug use for the acute rewarding effects of the substance. The development of negative emotional states is key for the transition from recreational use to substance use disorders as subjects seek the substance to obtain relief from the negative emotional states of acute withdrawal and protracted abstinence. The neuropeptide corticotropin-releasing factor (CRF) is a major regulator of the brain stress system and key in the development of negative affective states. The present review examines the role of CRF in preclinical models of alcohol and nicotine abuse and explores links between CRF and anxiety-like, dysphoria-like, and other negative affective states. Finally, the present review discusses preclinical models of nicotine and alcohol use with regard to the CRF system, advances in molecular and genetic manipulations of CRF, and the importance of examining both males and females in this field of research.

Chronic voluntary caffeine intake in male Wistar rats reveals individual differences in addiction-like behavior

Caffeine is the most widely consumed psychoactive substance in the world. However, there is controversy about whether becoming addicted to caffeine is possible and a lack of well-established animal models to examine caffeine consumption. The present study sought to establish a model of caffeine consumption in Wistar rats, identify different rat populations based on caffeine preference, and determine whether extended voluntary caffeine consumption produces compulsive-like caffeine intake and withdrawal symptoms. Male Wistar rats were used throughout the experiment. The optimal concentration of caffeine to maximize caffeine consumption and caffeine preference was determined. Rats were then given continuous access to caffeine, followed by intermittent access. Rats were tested for signs of withdrawal-like behavior by measuring mechanical nociception and irritability-like behavior. Rats were further examined for compulsive-like caffeine consumption using quinine adulteration. Dose-response testing indicated an optimal caffeine concentration of 0.3 mg/mL. During intermittent access to caffeine, the rats did not escalate their caffeine intake and instead exhibited a decrease in intake over sessions. Three groups of rats were identified based on caffeine preference (high, medium, and low) across continuous and intermittent access. These three groups of rats matched low (1 cup), medium (2 cups), and high (4 cups) levels of daily coffee consumption in humans. Caffeine-consuming rats did not exhibit differences in mechanical nociception or irritability-like behavior compared with controls. In high caffeine-preferring rats but not in medium or low caffeine-preferring rats, compulsive-like caffeine consumption was observed. The present study established a rodent model of caffeine consumption that resulted in large individual differences in caffeine intake, similar to humans. Compulsive-like caffeine consumption in high caffeine-preferring rats and differences in caffeine preference between groups suggest that caffeine may result in compulsive-like intake in a subpopulation of subjects. Further testing is necessary to determine the factors that contribute to differences in caffeine preference and compulsive-like intake.

The therapeutic potential of exercise for neuropsychiatric diseases: A review

Exercise is known to have a myriad of health benefits. There is much to be learned from the effects of exercise and its potential for prevention, attenuation and treatment of multiple neuropsychiatric diseases and behavioral disorders. Furthermore, recent data and research on exercise benefits with respect to major health crises, such as, that of opioid and general substance use disorders, make it very important to better understand and review the mechanisms of exercise and how it could be utilized for effective treatments or adjunct treatments for these diseases. In addition, mechanisms, epigenetics and sex differences are examined and discussed in terms of future research implications.

Nicotine e-cigarette vapor inhalation effects on nicotine & cotinine plasma levels and somatic withdrawal signs in adult male Wistar rats

RATIONALE

Non-contingent chronic nicotine exposure procedures have evolved rapidly in recent years, culminating in electronic nicotine delivery systems (ENDS or e-cigarettes) to deliver vaporized drugs to rodents in standard housing chambers.

OBJECTIVES

The aim of the current work was to use ENDS to test concentration-dependent effects of nicotine e-cigarette vapor inhalation on blood-nicotine concentrations, blood-cotinine concentrations, and somatic withdrawal signs over time in rats.

METHODS

Male Wistar rats were exposed to vapor containing various concentrations of nicotine (20, 40, 80 mg/mL) for 11 days through ENDS, and blood concentrations of nicotine and cotinine, the major proximate metabolite of nicotine, as well as spontaneous and precipitated somatic withdrawal signs, were measured over time (across days of exposure and over hours after termination of vapor exposure).

RESULTS

Exposing male Wistar rats to non-contingent nicotine vapor inhalation through ENDS produces somatic withdrawal symptoms and measurable blood-nicotine and blood-cotinine levels that change according to (1) concentration of nicotine in vape solution, (2) number of days of nicotine vapor exposure, (3) time since termination of nicotine vapor exposure, and (4) relative to the withdrawal signs, whether withdrawal was spontaneous or precipitated (by mecamylamine).

CONCLUSIONS

The data presented here provide parameters that can be used as a reasonable starting point for future work that employs ENDS to deliver non-contingent nicotine vapor in rats, although many parameters can and should be altered to match the specific goals of future work.

Comparison of the Relative Abuse Liability of Electronic Cigarette Aerosol Extracts and Nicotine Alone in Adolescent Rats: A Behavioral Economic Analysis

Background: Characterizing the determinants of the abuse liability of electronic cigarettes (ECs) in adolescents is needed to inform product regulation by the United States Food and Drug Administration (FDA). We recently reported that Vuse Menthol EC aerosol extract containing nicotine and a range of non-nicotine constituents (e.g., menthol, propylene glycol) had reduced aversive effects compared to nicotine alone in adolescent rats, whereas Aroma E-Juice EC aerosol extract did not. The current study used a behavioral economic approach to compare the relative abuse liability of these EC extracts and nicotine alone in an i.v. self-administration (SA) model in adolescents. Methods: Adolescents were tested for the SA of EC extracts prepared using an ethanol (ETOH) solvent or nicotine and saline, with and without 4% ETOH (i.e., the same concentration in the EC extracts) in 23 h/day sessions. Results. Although acquisition of SA was faster for nicotine + ETOH compared to all other formulations, the elasticity of demand for all nicotine-containing formulations was similar. Conclusions: EC aerosol extracts did not have greater abuse liability than nicotine alone in adolescents. These data suggest that nicotine may be the primary determinant of the abuse liability of these ECs in youth, at least in terms of the primary reinforcing effects of ECs mediated within the central nervous system.

Increases in compulsivity, inflammation, and neural injury in HIV transgenic rats with escalated methamphetamine self-administration under extended-access conditions

The abuse of stimulants, such as methamphetamine (METH), is associated with treatment non-compliance, a greater risk of viral transmission, and the more rapid clinical progression of immunological and central nervous system human immunodeficiency virus (HIV) disease. The behavioral effects of METH in the setting of HIV remain largely uncharacterized. We used a state-of-the-art paradigm of the escalation of voluntary intravenous drug self-administration in HIV transgenic (Tg) and wildtype rats. The rats were first allowed to self-administer METH under short-access (ShA) conditions, which is characterized by a nondependent and more "recreational" pattern of METH use, and then allowed to self-administer METH under long-access (LgA) conditions, which leads to compulsive (dependent) METH intake. HIV Tg and wildtype rats self-administered equal amounts of METH under ShA conditions. HIV Tg rats self-administered METH under LgA conditions following a 4-week enforced abstinence period to model the intermittent pattern of stimulant abuse in humans. These HIV Tg rats developed greater motivation to self-administer METH and self-administered larger amounts of METH. Impairments in function of the medial prefrontal cortex (mPFC) contribute to compulsive drug and alcohol intake. Gene expression profiling of the mPFC in HIV Tg rats with a history of escalated METH self-administration under LgA conditions showed transcriptional evidence of increased inflammation, greater neural injury, and impaired aerobic glucose metabolism than wildtype rats that self-administered METH under LgA conditions. The detrimental effects of the interaction between neuroHIV and escalated METH intake on the mPFC are likely key factors in the greater vulnerability to excessive drug intake in the setting of HIV.

Assessing nicotine dependence using an oral nicotine free-choice paradigm in mice

Models to assess the addictive-like properties of nicotine in mice are limited. Therefore, we aimed to characterize and validate an addiction index by using an oral nicotine free-choice paradigm in mice. Adult C57BL/6J, DBA/2J, or genetically modified mice carrying deletions for nicotinic acetylcholine receptor (nAChR) subunits, (n = 8-10/sex/group) were given a choice of water or nicotine (10-960 μg/ml) solution using a two-bottle free-choice (2BC) paradigm. In general, oral nicotine intake and preference were higher in female mice compared to males. Absence of nicotine led to withdrawal, and intermittent access resulted in an escalation in consumption and greater nicotine withdrawal than continuous exposure. Additionally, oral nicotine consumption increased nucleus accumbens tyrosine hydroxylase levels. While β2 and α6 KO mice showed a significant decrease in nicotine intake, deletion of α5 nAChRs increased nicotine consumption at high concentrations. Deletion of the α7 subunit altered the observed sex difference in nicotine consumption, with females consuming less than males. The α4β2 partial agonist varenicline decreased oral nicotine consumption. Although addition of quinine to the nicotine solution lowered nicotine intake, mice primed with nicotine did not lower their intake after quinine addition. Nicotine deprivation followed by re-exposure showed increased nicotine consumption, and DBA/2J mice consumed less nicotine compared to C57BL/6J. We validated the mouse 2BC paradigm to study nicotine's addictive-like properties including nicotine intake, preference, withdrawal, and escalation of nicotine consumption during binge drinking or after reinstatement of a deprivation period.

Nicotine Self-Administration as Paradigm for Medication Discovery for Smoking Cessation: Recent Findings in Medications Targeting the Cholinergic System

Tobacco kills every year approximately six million people as a direct result of direct use, and it is still considered one of the most excruciating threats for human health worldwide. The low successful rates of the currently available pharmacotherapies to assist in quitting tobacco use suggest there is a need for more effective treatments.The intravenous self-administration (IVSA) paradigm is considered the gold standard to study voluntary drug intake in animal models, including nicotine. The IVSA paradigm has been used to identify key mechanisms involved in the addictive properties of nicotine in both rodents and nonhuman primates. In this chapter we describe how the IVSA paradigm has served to further investigate the role of nicotinic acetylcholine receptors (nAChRs) in the reinforcing properties of nicotine. Notably, this review will cover recent advances (i.e., research carried out during the past decade) using the IVSA paradigm, with a focus on the status of research on current smoking cessation medications (such as varenicline and bupropion) and of other nAChR ligands.The combination of the IVSA paradigm with pharmacological and genetic tools (e.g., knockout animals) has greatly contributed to our understanding of the role of specific subtype nAChRs in nicotine reinforcement processes. We also discuss some of the limitations of the IVSA paradigm so these can be taken into consideration when interpreting and designing new studies.

Chronic treatment with N-acetylcysteine decreases extinction responding and reduces cue-induced nicotine-seeking

N-acetylcysteine (NAC), a promising glutamatergic therapeutic agent, has shown some clinical efficacy in reducing nicotine use in humans and has been shown to reverse drug-induced changes in glutamatergic neurophysiology. In rats, nicotine-seeking behavior is associated with alterations in glutamatergic plasticity within the nucleus accumbens core (NAcore). Specifically, cue-induced nicotine-seeking is associated with rapid, transient synaptic plasticity (t-SP) in glutamatergic synapses on NAcore medium spiny neurons. The goal of the present study was to determine if NAC reduces nicotine-seeking behavior and reverses reinstatement-associated NAcore glutamatergic alterations. Rats were extinguished from nicotine self-administration, followed by subchronic NAC administration (0 or 100 mg/kg/d) for 4 days prior to cue-induced reinstatement. NAcore synaptic potentiation was measured via dendritic spine morphology and mRNA and protein of relevant glutamatergic genes were quantified. Nicotine-seeking behavior was not reduced by subchronic NAC treatment. Also, NAcore transcript and protein expression of multiple glutamatergic genes, as well as spine morphological measures, were unaffected by subchronic NAC. Finally, chronic NAC treatment (15 days total) during extinction and prior to reinstatement significantly decreased extinction responding and reduced reinstatement of nicotine-seeking compared to vehicle. Together, these results suggest that chronic NAC treatment is necessary for its therapeutic efficacy as a treatment strategy for nicotine addiction and relapse.

An enzymatic approach reverses nicotine dependence, decreases compulsive-like intake, and prevents relapse

Tobacco use disorder is the leading cause of disease and preventable death worldwide, but current medications that are based on pharmacodynamics have low efficacy. Novel pharmacokinetic approaches to prevent nicotine from reaching the brain have been tested using vaccines, but these efforts have failed because antibody affinity and concentration are not sufficient to completely prevent nicotine from reaching the brain. We provide preclinical evidence of the efficacy of an enzymatic approach to reverse nicotine dependence, reduce compulsive-like nicotine intake, and prevent relapse in rats with a history of nicotine dependence. Chronic administration of NicA2-J1, an engineered nicotine-degrading enzyme that was originally isolated from Pseudomonas putida S16, completely prevented nicotine from reaching the brain and reversed somatic signs of withdrawal, hyperalgesia, and irritability-like behavior in nicotine-dependent rats with a history of escalation of nicotine self-administration. NicA2-J1 also decreased compulsive-like nicotine intake, reflected by responding despite the adverse consequences of contingent footshocks, and prevented nicotine- and stress (yohimbine)-induced relapse. These results demonstrate the efficacy of enzymatic therapy in treating nicotine addiction in advanced animal models and provide a strong foundation for the development of biological therapies for smoking cessation in humans.

The duration of intermittent access to preferred sucrose-rich food affects binge-like intake, fat accumulation, and fasting glucose in male rats

Many people restrict their palatable food intake. In animal models, time-limiting access to palatable foods increases their intake while decreasing intake of less preferred alternatives; negative emotional withdrawal-like behavior is sometimes reported. In drug addiction models, intermittent extended access drives greater changes in use than brief access. When it comes to palatable food, the impact of briefer vs. longer access durations within intermittent access conditions remains unclear. Here, we provided male rats with chow or with weekday access to a preferred, sucrose-rich diet (PREF) (2, 4, or 8 h daily) with chow otherwise available. Despite normal energy intake, all restricted access conditions increased weight gain by 6 weeks and shifted diet acceptance within 1 week. They increased daily and 2-h intake of PREF with individual vulnerability and decreased chow intake. Rats with the briefest access had the greatest binge-like (2-h) intake, did not lose weight on weekends despite undereating chow, and were fattier by 12 weeks. Extended access rats (8 h) showed the greatest daily intake of preferred food and corresponding undereating of chow, slower weight gain when PREF was unavailable, and more variable daily energy intake from week to week. Increased fasting glucose was seen in 2-h and 8-h access rats. During acute withdrawal from PREF to chow diet, restricted access rats showed increased locomotor activity. Thus, intermittent access broadly promoted weight gain, fasting hyperglycemia and psychomotor arousal during early withdrawal. More restricted access promoted greater binge-like intake and fat accumulation, whereas longer access promoted evidence of greater food reward tolerance.

Intermittent, extended access to preferred food leads to escalated food reinforcement and cyclic whole-body metabolism in rats: Sex differences and individual vulnerability

Compulsive binge eating is a hallmark of binge eating disorder and bulimia nervosa and is implicated in some obesity cases. Eating disorders are sexually dimorphic, with females more often affected than males. Animal models of binge-like eating based on intermittent access to palatable food exist; but, little is known regarding sex differences or individual vulnerability in these models with respect to the reinforcing efficacy of food, the development of compulsive- and binge-like eating, or associated changes in whole-body metabolism or body composition. Adolescent male (n = 24) and female (n = 32) Wistar rats were maintained on chow or a preferred, high-sucrose, chocolate-flavored diet in continuous or intermittent, extended access conditions. Body weight and composition, intake, fixed- and progressive-ratio operant self-administration, and whole body energy expenditure and respiratory exchange ratios were measured across an 11-week study period. Subgroup analyses were conducted to differentiate compulsive-like "high responder" intermittent access rats that escalated to extreme progressive-ratio self-administration performance vs. more resistant "low responders." Female rats had greater reinforcing efficacy of food than males in all diet conditions and were more often classified as "high responders". In both sexes, rats with intermittent access showed cycling of fuel substrate utilization and whole-body energy expenditure. Further, "high-responding" intermittent access female rats had especially elevated respiratory exchange ratios, indicating a fat-sparing phenotype. Future studies are needed to better understand the molecular and neurobiological basis of the sex and individual differences we have observed in rats and their translational impact for humans with compulsive, binge eating disorders.

Utility of Nonhuman Primates in Substance Use Disorders Research

Substance use disorders (i.e., drug addiction) constitute a global and insidious public health issue. Preclinical biomedical research has been invaluable in elucidating the environmental, biological, and pharmacological determinants of drug abuse and in the process of developing innovative pharmacological and behavioral treatment strategies. For more than 70 years, nonhuman primates have been utilized as research subjects in biomedical research related to drug addiction. There are already several excellent published reviews highlighting species differences in both pharmacodynamics and pharmacokinetics between rodents and nonhuman primates in preclinical substance abuse research. Therefore, the aim of this review is to highlight three advantages of nonhuman primates as preclinical substance abuse research subjects. First, nonhuman primates offer technical advantages in experimental design compared to other laboratory animals that afford unique opportunities to promote preclinical-to-clinical translational research. Second, these technical advantages, coupled with the relatively long lifespan of nonhuman primates, allows for pairing longitudinal drug self-administration studies and noninvasive imaging technologies to elucidate the biological consequences of chronic drug exposure. Lastly, nonhuman primates offer advantages in the patterns of intravenous drug self-administration that have potential theoretical implications for both the neurobiological mechanisms of substance use disorder etiology and in the drug development process of pharmacotherapies for substance use disorders. We conclude with potential future research directions in which nonhuman primates would provide unique and valuable insights into the abuse of and addiction to novel psychoactive substances.

Sex differences in nicotine intravenous self-administration: A meta-analytic review

OBJECTIVE

This report reflects a meta-analysis that systematically reviewed the literature on intravenous self-administration (IVSA) of nicotine in female and male rats. The goal was to determine if sex differences in nicotine IVSA exist, estimate the magnitude of the effect, and identify potential moderators of the relationship between sex differences and nicotine consumption.

METHODS

Extensive search procedures identified 20 studies that met the inclusion criteria of employing both female and male rats in nicotine IVSA procedures. The meta-analysis was conducted on effect size values that were calculated from mean total intake or nicotine deliveries using the Hedges' unbiased g^u statistic.

RESULTS

A random effects analysis revealed that overall females self-administered more nicotine than males (weighted g^u=0.18, 95% CI [0.003, 0.34]). Subsequent moderator variable analyses revealed that certain procedural conditions influenced the magnitude of sex differences in nicotine IVSA. Specifically, higher reinforcement requirements (>FR1) and extended-access sessions (23h) were associated with greater nicotine IVSA in females versus males. Females also displayed higher nicotine intake than males when the experiment included a light cue that signaled nicotine delivery. Sex differences were not influenced by the diurnal phase of testing, dose of nicotine, or prior operant training.

CONCLUSION

Overall, the results revealed that female rats display higher levels of nicotine IVSA than males, suggesting that the strong reinforcing effects of nicotine promote tobacco use in women.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

Neuropeptide systems and new treatments for nicotine addiction

RATIONALE

The mildly euphoric and cognitive enhancing effects of nicotine play a role in the initiation of smoking, while dysphoria and anxiety associated with smoking cessation contribute to relapse. After the acute withdrawal phase, smoking cues, a few cigarettes (i.e., lapse), and stressors can cause relapse. Human and animal studies have shown that neuropeptides play a critical role in nicotine addiction.

OBJECTIVES

The goal of this paper is to describe the role of neuropeptide systems in the initiation of nicotine intake, nicotine withdrawal, and the reinstatement of extinguished nicotine seeking.

RESULTS

The reviewed studies indicate that several drugs that target neuropeptide systems diminish the rewarding effects of nicotine by preventing the activation of dopaminergic systems. Other peptide-based drugs diminish the hyperactivity of brain stress systems and diminish withdrawal-associated symptom severity. Blockade of hypocretin-1 and nociceptin receptors and stimulation of galanin and neurotensin receptors diminishes the rewarding effects of nicotine. Both corticotropin-releasing factor type 1 and kappa-opioid receptor antagonists diminish dysphoria and anxiety-like behavior associated with nicotine withdrawal and inhibit stress-induced reinstatement of nicotine seeking. Furthermore, blockade of vasopressin 1b receptors diminishes dysphoria during nicotine withdrawal, and melanocortin 4 receptor blockade prevents stress-induced reinstatement of nicotine seeking. The role of neuropeptide systems in nicotine-primed and cue-induced reinstatement is largely unexplored, but there is evidence for a role of hypocretin-1 receptors in cue-induced reinstatement of nicotine seeking.

CONCLUSION

Drugs that target neuropeptide systems might decrease the euphoric effects of smoking and improve relapse rates by diminishing withdrawal symptoms and improving stress resilience.

Antireward, compulsivity, and addiction: seminal contributions of Dr. Athina Markou to motivational dysregulation in addiction

RATIONALE AND OBJECTIVES

Addiction is defined as a chronically relapsing disorder characterized by compulsive drug seeking that is hypothesized to derive from multiple sources of motivational dysregulation.

METHODS AND RESULTS

Dr. Athina Markou made seminal contributions to our understanding of the neurobiology of addiction with her studies on the dysregulation of reward function using animal models with construct validity. Repeated overstimulation of the reward systems with drugs of abuse decreases reward function, characterized by brain stimulation reward and presumbably reflecting dysphoria-like states. The construct of negative reinforcement, defined as drug taking that alleviates a negative emotional state that is created by drug abstinence, is particularly relevant as a driving force in both the withdrawal/negative affect and preoccupation/anticipation stages of the addiction cycle.

CONCLUSIONS

The negative emotional state that drives such negative reinforcement is hypothesized to derive from the dysregulation of key neurochemical circuits that drive incentive-salience/reward systems (dopamine, opioid peptides) in the ventral striatum and from the recruitment of brain stress systems (corticotropin-releasing factor, dynorphin) within the extended amygdala. As drug taking becomes compulsive-like, the factors that motivate behavior are hypothesized to shift to drug-seeking behavior that is driven not only by positive reinforcement but also by negative reinforcement. This shift in motivation is hypothesized to reflect the allostatic misregulation of hedonic tone such that drug taking makes the hedonic negative emotional state worse during the process of seeking temporary relief with compulsive drug taking.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Rats quit nicotine for a sweet reward following an extensive history of nicotine use

Drug use may be exacerbated in environments which lack alternative means of engaging in rewarding behaviour. When alternative rewards are available, drug use may decrease-an effect that can be harnessed for therapeutic benefit. This idea is particularly well-supported by recent preclinical evidence demonstrating that a majority of rats will readily choose a potent non-drug reward over cocaine or heroin. Here we examine whether the same holds true for nicotine, a drug considered to have one of the highest addiction liabilities amongst drugs of abuse. Rats were trained to nose-poke separately for saccharin or nicotine on alternate days. Using a discrete-trial, forced-choice procedure, rats were then allowed to choose between nicotine and saccharin. This was followed by choice testing after a decrease in saccharin concentration (0.2-0%), omission of the fluid reward, an increase in nicotine concentration and following an extended nicotine self-administration history. All rats demonstrated a clear and immediate preference for saccharin at all times. This was despite variations in reward concentrations, or after an extensive nicotine history. Notably, rats preferred to nose-poke for water over nicotine and would omit responses when no fluid was delivered, rather than resume responding for nicotine. Overall, this study confirms and extends to nicotine previous research on other drugs of abuse, including cocaine and heroin. The ease with which rats quit nicotine in the present study contrasts with the well-known difficulty of humans to quit tobacco smoking. Possible factors that could explain this apparent discrepancy are discussed.

Nicotine self-administration research: the legacy of Steven R. Goldberg and implications for regulation, health policy, and research

BACKGROUND AND RATIONALE

Steven R. Goldberg was a pioneering behavioral pharmacologist whose intravenous drug self-administration studies advanced the understanding of conditioned stimuli and schedules of reinforcement as determinants of pattern and persistence of drug-seeking behavior, and in particular, the importance of nicotine in tobacco use. His passing in 2014 led to invitations to contribute articles to psychopharmacology dedicated to his work.

OBJECTIVES

The objectives of this review are to summarize and put into historical perspective Goldberg's contributions to elucidate the reinforcing effects of nicotine and to summarize the implications of his research for medication development, tobacco regulation, and potential tobacco control policy options. This includes a review of intravenous nicotine self-administration research from the 1960s to 2016.

RESULTS

Goldberg's application of behavioral pharmacology methods to investigate nicotine reinforcement and the influence of schedule of reinforcement and conditioned stimuli on nicotine administration contributed to the conclusions of the US National Institute on Drug Abuse, and the Surgeon General, that nicotine met the criteria as a dependence-producing drug and cigarette smoking as a prototypic drug dependency or "addiction." Equally important, this work has been systematically extended to other species and applied to address a range of factors relevant to tobacco use, medication development, regulation, and public health policy.

CONCLUSIONS

Steven R. Goldberg was a pioneering scientist whose systematic application of the science of behavioral pharmacology advanced the understanding of tobacco and nicotine use and contributed to the scientific foundation for tobacco product regulation and potential public health tobacco control policy development.

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.

Voluntary co-consumption of alcohol and nicotine: Effects of abstinence, intermittency, and withdrawal in mice

Alcohol and nicotine are often used together, and there is a high rate of co-occurrence between alcohol and nicotine addiction. Most animal models studying alcohol and nicotine interactions have utilized passive drug administration, which may not be relevant to human co-addiction. In addition, the interactions between alcohol and nicotine in female animals have been understudied, as most studies have used male animals. To address these issues, we developed models of alcohol and nicotine co-consumption in male and female mice that utilized voluntary, oral consumption of unsweetened alcohol, nicotine and water. We first examined drug consumption and preference in single-drug, sequential alcohol and nicotine consumption tests in male and female C57BL/6 and DBA/2J mice. We then tested chronic continuous and intermittent access alcohol and nicotine co-consumption procedures. We found that male and female C57BL/6 mice readily co-consumed unsweetened alcohol and nicotine. In our continuous co-consumption procedures, we found that varying the available nicotine concentration during an alcohol abstinence period affected compensatory nicotine consumption during alcohol abstinence, and affected rebound alcohol consumption when alcohol was re-introduced. Consumption of alcohol and nicotine in an intermittent co-consumption procedure produced higher alcohol consumption levels, but not nicotine consumption levels, compared with the continuous co-consumption procedures. Finally, we found that intermittent alcohol and nicotine co-consumption resulted in physical dependence. Our data show that these voluntary co-consumption procedures can be easily performed in mice and can be used to study behavioral interactions between alcohol and nicotine consumption, which may better model human alcohol and nicotine co-addiction.

A key role for the N/OFQ-NOP receptor system in modulating nicotine taking in a model of nicotine and alcohol co-administration

Alcohol and nicotine are often co-abused. Although the N/OFQ-NOP receptor system is considered a potential target for development of drug abuse pharmacotherapies, especially for alcoholism, little is known about the role of this system in nicotine dependence. Furthermore, the effect of prior history of nicotine dependence on subsequent nicotine and alcohol taking is understudied. Using an operant co-administration paradigm, in which rats concurrently self-administer nicotine and alcohol, we found that nicotine dependent rats increased nicotine self-administration over time as compared to non-dependent animals, while patterns of alcohol lever pressing did not change between groups. Pretreatment with the potent NOP receptor agonist AT-202 (0.3–3 mg/kg) increased nicotine lever pressing of both dependent and non-dependent groups, whereas the selective antagonist SB612111 (1–10 mg/kg) elicited a clear reduction of nicotine responses, in both dependent and non-dependent rats. In parallel, AT-202 only produced minor changes on alcohol responses and SB612111 reduced alcohol taking at a dose that also reduced locomotor behavior. Results indicate that a history of nicotine dependence affects subsequent nicotine- but not alcohol-maintained responding, and that NOP receptor antagonism, rather than agonism, blocks nicotine self-administration, which strongly suggests a critical role for the endogenous N/OFQ in the modulation of nicotine reinforcement processes.

Cognitive Dysfunction, Affective States, and Vulnerability to Nicotine Addiction: A Multifactorial Perspective

Although smoking prevalence has declined in recent years, certain subpopulations continue to smoke at disproportionately high rates and show resistance to cessation treatments. Individuals showing cognitive and affective impairments, including emotional distress and deficits in attention, memory, and inhibitory control, particularly in the context of psychiatric conditions, such as attention-deficit hyperactivity disorder, schizophrenia, and mood disorders, are at higher risk for tobacco addiction. Nicotine has been shown to improve cognitive and emotional processing in some conditions, including during tobacco abstinence. Self-medication of cognitive deficits or negative affect has been proposed to underlie high rates of tobacco smoking among people with psychiatric disorders. However, pre-existing cognitive and mood disorders may also influence the development and maintenance of nicotine dependence, by biasing nicotine-induced alterations in information processing and associative learning, decision-making, and inhibitory control. Here, we discuss the potential forms of contribution of cognitive and affective deficits to nicotine addiction-related processes, by reviewing major clinical and preclinical studies investigating either the procognitive and therapeutic action of nicotine or the putative primary role of cognitive and emotional impairments in addiction-like features.

Effects of MAO inhibition and a combination of minor alkaloids, β-carbolines, and acetaldehyde on nicotine self-administration in adult male rats

INTRODUCTION

Although nicotine is the primary reinforcing constituent in cigarettes, there is evidence that other constituents in cigarette smoke may interact with nicotine to reinforce smoking behavior.

METHODS

The present experiments investigated whether a novel combination of these cigarette smoke constituents would increase nicotine self-administration in adult male rats. The constituents included five minor alkaloids (anabasine, nornicotine, cotinine, myosmine, and anatabine), two β-carbolines (harman and norharman), and acetaldehyde. All doses were indexed to be proportional to concentrations in cigarette smoke given a standard dose of nicotine used in rodent self-administration, or ten times higher than this standard. To model MAO inhibition seen in chronic smokers, some groups received separate injections of tranylcypromine prior to each self-administration session.

RESULTS

Tranylcypromine increased low-dose nicotine self-administration independent of other smoke constituents, which had no effect on self-administration behavior. The effect of tranylcypromine was confirmed across a large range of reinforcement schedules. The effect of tranylcypromine on low-dose nicotine self-administration was observed regardless of whether the injection was delivered 1-h or 23-h prior to the self-administration session, consistent with the interpretation that MAO inhibition was responsible for the increase in self-administration, instead of acute off-target effects.

CONCLUSIONS

These data suggest that this cocktail of constituents does not significantly alter the primary reinforcing effects of nicotine, but constituents that inhibit MAO may increase the primary reinforcing effects of nicotine, especially at low doses.