Nicotine physical dependence and tolerance in the mouse following chronic oral administration

Nicotine Ameliorates α-Synuclein Preformed Fibril-Induced Behavioral Deficits and Pathological Features in Mice

Epidemiologic study suggests that nicotine reduces the risk of Parkinson's disease (PD) and thus could serve as a potential treatment. In this study, we aimed to investigate the effect of nicotine on the behavioral phenotypes and pathological characteristics of mice induced by human alpha-synuclein preformed fibers (α-syn-PFF). Mice were injected with 5 µg of human α-syn-PFF in the hippocampus while administering nicotine-containing drinking water (200 µg/mL). After 1 month, the motor ability, mood, spatial learning, and memory ability of the PD phenotype-like model mice were detected using open field, rotarod, Y maze, and O maze tests. The expression of pathological α-syn and apoptotic proteins, as well as the number of glial and neural stem cells in the hippocampus of mice, was detected using western blot and immunofluorescence. The results demonstrated that nicotine significantly reduced pathological α-syn accumulation, α-syn serine 129 phosphorylation, and apoptosis induced by α-syn-PFF injection in the hippocampus of mice. Nicotine also inhibited the increase in the number of glia, microglia, and neuronal apoptotic cells, and it decreased the expression of PI3K and Akt while also exhibiting significant memory impairment, motor deficits, and anxiety-like behavior. In conclusion, our findings suggest that nicotine ameliorates behavioral deficits and pathological changes in mice by inhibiting human α-syn-PFF-induced neuroinflammation and apoptosis.

Characterization of a novel oronasal-restricted nicotine vaping self-administration model in mice

Nicotine use remains one of the leading causes of preventable deaths in the United States and, while the prevalence of combustible cigarette use has declined over the past few years, the popularity of electronic nicotine delivery systems continues to rise. Vaping is not without risks, and its long-term effects, particularly in vulnerable populations, remain largely unknown. This study introduces a novel, oronasal-restricted, nicotine vapor self-administration mouse model to investigate the impact of nicotine concentration, genotype, sex, and age on self-administration and behavioral response to nicotine. Our studies show that male and female young adult mice respond to nicotine, demonstrating notable sex-related differences in intake, locomotor sensitization, and somatic withdrawal signs. In addition, we characterized intake in adolescent mice, showing sex differences as well. Finally, we showed genotype-related differences when using β2 knock-out mice, emphasizing the role of the β2 nAChR in nicotine reward and nicotine intake. This new model offers a more targeted approach to studying the potential risks of nicotine vaping in a more relevant and face-valid model compared to traditional whole-body nicotine vapor exposure in rodents.

The bidirectional relationship between pain and tobacco use: Insights from the longitudinal Population Assessment of Tobacco and Health (PATH) study

BACKGROUND

Tobacco smoking remains the leading preventable cause of death, whereas chronic pain is the leading cause of disability. Chronic pain and tobacco smoking are closely interrelated. We investigated whether pain predicts daily cigarette smoking and if daily cigarette smoking predicts the development of pain.

METHODS

We analyzed eight years of longitudinal data from the Population Assessment of Tobacco and Health study. We examined whether baseline pain (scored ≥4 on a 0-10 scale) predicts transition to daily smoking among individuals who do not smoke, and whether baseline daily smoking predicts the development of pain among those with no/low pain (scored 0-3). The pain and smoking outcomes were only included in the survival analysis if they persisted consistently throughout all subsequent years after the initial onset. Our analysis incorporated survey population controlled for alternative tobacco product use, gender, age categories, and race/ethnicity.

RESULTS

Among the 32,320 participants, 49.5 % were women, the largest age cohort was 18-24 (28.2 %). Racial and ethnic distribution was 73.9 % White, 15.6 % Black, and 17.2 % Hispanic. Survival analysis revealed that baseline pain (scored ≥4) significantly increased the risk of transitioning to daily smoking (HR=2.40, 95 % CI=1.62-3.55, p < 0.001, n = 5731). Further, daily smoking at baseline was associated with an increased risk of developing persistent pain (scored ≥4) over the study period (HR=2.00, 95 % CI=1.86-2.16, p < 0.001, n = 12,099).

CONCLUSION

This study provides evidence for the existence of a bidirectional relationship between chronic pain and cigarette smoking. Treatment strategies should consider this relationship early, aiming to prevent development of persistent pain at its earliest stages.

Sex differences in nicotine intake and relapse behavior in nicotine-dependent adult wistar rats

Introduction

Tobacco use is highly addictive and the leading cause of premature mortality in the world. Long-access nicotine self-administration procedures in rats closely model human smoking behavior. However, significant gaps remain in our understanding of sex differences in the development of dependence and relapse in adult rats.

Methods

In the present study, we investigated operant responding for both nicotine and saline and the development of dependence in adult rats of both sexes. The rats had daily access to nicotine or saline for 6 h per day, 7 days per week. Dependence was assessed by evaluating precipitated and spontaneous somatic withdrawal signs, measuring locomotor activity in the small open field test, and assessing anxiety-like behavior in the large open field and elevated plus maze test. The sucrose preference test was used to determine if cessation of nicotine intake leads to anhedonia. It was also investigated if a period of forced abstinence affects nicotine-seeking behavior.

Results

This study showed that nicotine intake is higher in females than in males when given daily long access to nicotine. Daily nicotine self-administration led to more precipitated and spontaneous somatic withdrawal signs compared to saline self-administration, with no sex differences observed. In addition, cessation of nicotine intake led to a similar increase in activity in both males and females in the small open field test. However, cessation of nicotine intake did not increase anxiety-like behavior or cause anhedonia in either males or females. A time course analysis revealed that the nicotinic acetylcholine receptor antagonist mecamylamine affected nicotine intake differently in males and females, increasing intake in males and decreasing intake in females. Three weeks of forced abstinence led to an increase in nicotine and saline-seeking behavior. The rats exhibited more nicotine than saline seeking, and the females displayed more nicotine seeking than the males.

Discussion

The present findings demonstrate that females self-administer more nicotine and display more nicotine-seeking behavior than males. Furthermore, there were no sex differences in somatic withdrawal signs or activity during abstinence from nicotine. This work underscores the importance of considering sex differences across various aspects of addiction, including intake and relapse, when developing novel treatments for tobacco use disorder.

AKT2 modulates astrocytic nicotine responses in vivo

A better understanding of nicotine neurobiology is needed to reduce or prevent chronic addiction, ameliorate the detrimental effects of nicotine withdrawal, and increase successful cessation of use. Nicotine binds and activates two astrocyte-expressed nicotinic acetylcholine receptors (nAChRs), α4β2 and α7. We recently found that Protein kinase B-β (Pkb-β or Akt2) expression is restricted to astrocytes in mice and humans. To determine if AKT2 plays a role in astrocytic nicotinic responses, we generated astrocyte-specific Akt2 conditional knockout (cKO) and full Akt2 KO mice for in vivo and in vitro experiments. For in vivo studies, we examined mice exposed to chronic nicotine for two weeks in drinking water (200 μg/mL) and following acute nicotine challenge (0.09, 0.2 mg/kg) after 24 hrs. Our in vitro studies used cultured mouse astrocytes to measure nicotine-dependent astrocytic responses. We validated our approaches using lipopolysaccharide (LPS) exposure inducing astrogliosis. Sholl analysis was used to measure glial fibrillary acidic protein responses in astrocytes. Our data show that wild-type (WT) mice exhibit increased astrocyte morphological complexity during acute nicotine exposure, with decreasing complexity during chronic nicotine use, whereas Akt2 cKO mice showed increased astrocyte morphology complexity. In culture, we found that 100μM nicotine was sufficient for morphological changes and blocking α7 or α4β2 nAChRs prevented observed morphologic changes. Finally, we performed conditioned place preference (CPP) in Akt2 cKO mice and found that astrocytic AKT2 deficiency reduced nicotine preference compared to controls. These findings show the importance of nAChRs and Akt2 signaling in the astrocytic response to nicotine.

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.

Chronic oral nicotine exposure decreases aversive taste of nicotine, increases nicotine withdrawal and reinstatement, but cherry flavor does not alter nicotine's effects in adolescent rats

Although e-cigarette use among youth is recognized as an epidemic, there is limited understanding regarding nicotine's orosensory and chronic use effects in youth, and how fruit e-cigarette flavorings may influence nicotine's effects. We aimed to characterize the orosensory and chronic use effects of nicotine in adolescent rats. We also determined the acute and chronic effects of benzaldehyde, a cherry/berry/almond flavoring, on nicotine's taste, consumption, withdrawal, and reinstatement. Rats were examined for their acute taste responses to the different nicotine concentrations. The effects of chronic exposure on nicotine's taste, withdrawal, and reinstatement were also determined. In addition, impact of benzaldehyde on these nicotine use behaviors was evaluated. While taste responses to low nicotine concentrations did not differ from water, high nicotine concentrations induced aversion. Aversive responses to nicotine that were observed in naïve animals vanished after chronic nicotine exposure, indicating the development of tolerance to nicotine's aversive taste. Additionally, nicotine abstinence after chronic exposure induced withdrawal. Following abstinence, animals reinstated nicotine use. Further, animals showed higher preference to nicotine after reinstatement, compared to preference values before nicotine withdrawal. Benzaldehyde did not alter nicotine's taste reactivity, withdrawal, and reinstatement experiments. Some sex differences were found in benzaldehyde's taste response and choice behavior experiments.

Consensus statement on smoking cessation in patients with pain

Smoking is closely associated with the development of various cancers and tobacco-related illnesses such as cardiovascular and respiratory disorders. However, data are scarce on the relationship between smoking and both acute and chronic pain. In addition to nicotine, tobacco smoke contains more than 4000 different compounds. Although nicotine is not the sole cause of smoking-induced diseases, it plays a critical role in pain-related pathophysiology. Despite the acute analgesic effects of nicotine, long-term exposure leads to tolerance and increased pain sensitivity due to nicotinic acetylcholine receptor desensitization and neuronal plastic changes. The purpose of smoking cessation interventions in smoking patients with pain is primarily not only to reduce their pain and associated limitations in activities of daily living, but also to improve the outcomes of underlying pain-causing conditions and reduce the risks of tobacco-related disorders. This statement aims to summarize the available evidence on the impact of smoking on pain and to inform medical professionals of the significance of smoking cessation in patients with pain.

Chronic oral nicotine administration and withdrawal regulate the expression of neuropeptide Y and its receptors in the mesocorticolimbic system

Neuropeptide Y (NPY) and its receptors are involved in the regulation of mood, stress, and anxiety. In parallel, NPY signaling may play a vital role in the negative affective state induced by drug withdrawal. This study examined the changes in the transcript levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system during chronic nicotine exposure and withdrawal. Rats were administered with nicotine (initial dose: 25 μg/ml, maintenance dose: 50 μg/ml, free base) in drinking water for 12 weeks. Control group received only tap water. In the final week of the study, some of the nicotine-treated animals continued to receive nicotine (0-W), whereas some were withdrawn for either 24 (24-W) or 48 (48-W) h. All animals were decapitated after the evaluation of somatic signs (frequency of gasps, eye blinks, ptosis, shakes, teeth chatter) and the duration of locomotor activity and immobility. mRNA levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system were measured by quantitative real-time PCR (qRT-PCR). Results showed that nicotine withdrawal increased overall somatic signs. Moreover, chronic nicotine treatment increased the duration of locomotor activity, whereas withdrawal increased the duration of immobility. qRT-PCR analysis revealed that chronic nicotine treatment increased NPY mRNA levels in the hippocampus. On the other hand, 24- and 48-h withdrawals increased NPY mRNA levels in the amygdala and medial prefrontal cortex (mPFC), Y1 and Y2 mRNA levels in the nucleus accumbens and mPFC, and Y5 mRNA levels in the mPFC. These findings suggest that nicotine withdrawal enhances NPY signaling in the mesocorticolimbic system, which could be an important mechanism involved in regulating the negative affective state triggered during nicotine withdrawal.

Rodent models for nicotine withdrawal

BACKGROUND

Animal models are critical to improve our understanding of the neuronal mechanisms underlying nicotine withdrawal. Nicotine dependence in rodents can be established by repeated nicotine injections, chronic nicotine infusion via osmotic minipumps, oral nicotine intake, tobacco smoke exposure, nicotine vapor exposure, and e-cigarette aerosol exposure. The time course of nicotine withdrawal symptoms associated with these methods has not been reviewed in the literature.

AIM

The goal of this review is to discuss nicotine withdrawal symptoms associated with the cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure in rats and mice. Furthermore, age and sex differences in nicotine withdrawal symptoms are reviewed.

RESULTS

Cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure leads to nicotine withdrawal symptoms such as somatic withdrawal signs, changes in locomotor activity, anxiety- and depressive-like behavior, learning and memory deficits, attention deficits, hyperalgesia, and dysphoria. These withdrawal symptoms are most pronounced within the first week after cessation of nicotine exposure. Anxiety- and depressive-like behavior, and deficits in learning and memory may persist for several months. Adolescent (4-6 weeks old) rats and mice display fewer nicotine withdrawal symptoms than adults (>8 weeks old). In adult rats and mice, females show fewer nicotine withdrawal symptoms than males. The smoking cessation drugs bupropion and varenicline reduce nicotine withdrawal symptoms in rodents.

CONCLUSION

The nicotine withdrawal symptoms that are observed in rodents are similar to those observed in humans. Tobacco smoke and e-cigarette aerosol contain chemicals and added flavors that enhance the reinforcing properties of nicotine. Therefore, more valid animal models of tobacco and e-cigarette use need to be developed by using tobacco smoke and e-cigarette aerosol exposure methods to induce dependence.

Partial and full deletion of nicotinic acetylcholine receptor α4 and β2 subunits reduces sensitivity to acute nicotine administration and development of tolerance following chronic nicotine administration

The diversity of nicotinic cholinergic receptor (nAChR) subunits underlies the complex responses to nicotine. Mice differing in the expression of α4 and β2 subunits, which are most widely expressed in brain, were evaluated for the responses to acute nicotine administration on Y-maze crossings and rears, open-field locomotion and body temperature following chronic treatment with nicotine (0, 0.25, 1.0 and 4.0 mg/kg/h). Deletion or partial deletion of the α4, β2 or both nAChR subunits reduced the sensitivity of mice to acute nicotine administration. This reduced sensitivity was gene dose-dependent. Modification of α4 subunit expression elicited a greater reduction in sensitivity than the modification of β2 subunit expression. No measurable tolerance was observed for mice of any genotype following chronic treatment with 0.25 mg/kg/h nicotine. Modest tolerance was noted following treatment with 1.0 mg/kg/h. Greater tolerance was observed following treatment with 4.0 mg/kg/h. The extent of tolerance differed among the mice depending on genotype: wild-type (α4 and β2) developed measurable tolerance for all four tests. Heterozygotes (α4, β2 and α4/β2) developed tolerance for only Y-maze crossings and body temperature. Null mutants (α4 and β2) did not become tolerant. However, following chronic treatment with 4.0 mg/kg/h nicotine, wild type, α4 and α4 mice displayed increased Y-maze crossings following acute administration of 0.5 mg/kg nicotine that may reflect the activity of α6β2*-nAChR. These results confirm the importance of the α4 and β2 nAChR subunits in mediating acute and chronic effects of nicotine on locomotion and body temperature in the mouse.

Oral Nicotine Induces Oxidative Stress and Inflammation but Does Not Subvert Tumor Suppressor and DNA Repair Responses in Mice

Nicotine, responsible for the addictive properties of tobacco, is widely used in nicotine replacement therapy for tobacco use cessation. We investigated the time-dependent effect of treatment with nicotine on the tumor suppressor, DNA repair and immune responses. Swiss Albino mice (laca strain) of both sexes received nicotine dissolved at a dose of 100 µg/ml in 2% sucrose for 24 weeks, by oral gavage, while age- and gender-matched controls received only 2% sucrose for the same period. Nicotine-treated and control mice were sacrificed 6, 16 and 24 weeks post-treatment, and their tissues evaluated for alterations in histology, oxidative stress, TNF-α levels, nitric oxide (NO) and myeloperoxidase (MPO) release, tumor suppressor response and DNA repair response. Statistical significance of results was determined using Students' t test. The tissues of nicotine treated mice exhibited a large number of multinucleated and binucleated cells, enlarged nuclei and non-uniform distribution of cells, significant increase in expression of TNF-α gene and serum TNF-α, and time-dependent significant increase in lipid peroxidation, protein carbonylation, NO and MPO release when compared to age-and gender-matched controls. The mRNA expression of the tumor suppressor gene p53, its primary regulator Mdm2, and the DNA repair genes Brca2 and Ape1 were significantly elevated, but the corresponding protein levels remained largely unaltered. In conclusion, treatment with nicotine caused oxidative stress and inflammation which can cause widespread cellular damage from the very onset of treatment, without subverting the tumor suppressor and DNA repair responses.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Translational Research in Nicotine Addiction

While commendable strides have been made in reducing smoking initiation and improving smoking cessation rates, current available smoking cessation treatment options are still only mildly efficacious and show substantial interindividual variability in their therapeutic responses. Therefore, the primary goal of preclinical research has been to further the understanding of the neural substrates and genetic influences involved in nicotine's effects and reassess potential drug targets. Pronounced advances have been made by investing in new translational approaches and placing more emphasis on bridging the gap between human and rodent models of dependence. Functional neuroimaging studies have identified key brain structures involved with nicotine-dependence phenotypes such as craving, impulsivity, withdrawal symptoms, and smoking cessation outcomes. Following up with these findings, rodent-modeling techniques have made it possible to dissect the neural circuits involved in these motivated behaviors and ascertain mechanisms underlying nicotine's interactive effects on brain structure and function. Likewise, translational studies investigating single-nucleotide polymorphisms (SNPs) within the cholinergic, dopaminergic, and opioid systems have found high levels of involvement of these neurotransmitter systems in regulating the reinforcing aspects of nicotine in both humans and mouse models. These findings and coordinated efforts between human and rodent studies pave the way for future work determining gene by drug interactions and tailoring treatment options to each individual smoker.

Serotonin neurons in the median raphe nucleus bidirectionally regulate somatic signs of nicotine withdrawal in mice

In chronic smokers, nicotine withdrawal symptoms during tobacco cessation can lead to smoking relapse. In rodent models, chronic exposure to nicotine elicited physical dependence, whereas acute antagonism of nicotinic acetylcholine receptors (nAChRs) immediately precipitated withdrawal symptoms. Although the central serotonergic system plays an important role in nicotine withdrawal, the exact serotonergic raphe nuclei regulating these symptoms remain unknown. We used transgenic mice expressing archaerhodopsinTP009 or channelrhodopsin-2[C128S] exclusively in the central serotonergic neurons to selectively manipulate serotonergic neurons in each raphe nucleus. Nicotine withdrawal symptoms were precipitated by an acute injection of mecamylamine, a nonspecific nAChR antagonist, following chronic nicotine consumption. Somatic signs were used as measures of nicotine withdrawal symptoms. Acute mecamylamine administration significantly increased ptosis occurrence in nicotine-drinking mice compared with that in control-drinking mice. Optogenetic inhibition of the serotonergic neurons in the median raphe nucleus (MRN), but not of those in the dorsal raphe nucleus (DRN), mimicked the symptoms observed during mecamylamine-precipitated nicotine withdrawal even in nicotine-naïve mice following the administration of acute mecamylamine injection. Optogenetic activation of the serotonergic neurons in the MRN nearly abolished the occurrence of ptosis in nicotine-drinking mice. The serotonergic neurons in the MRN, but not those in the DRN, are necessary for the occurrence of somatic signs, a nicotine withdrawal symptom, and the activation of these neurons may act as a potential therapeutic strategy for preventing the somatic manifestations of nicotine withdrawal.

Central histaminergic transmission modulates the expression of chronic nicotine withdrawal induced anxiety-like and somatic behavior in mice

The present study investigated the plausible modulatory role of central histaminergic transmission on the expression of nicotine withdrawal induced anxiety and somatic behavior in mice. Abrupt cessation of chronic nicotine (2 mg/kg, i.p. × 3/day) treatment for 12 days to mice, expressed increased anxiety in light & dark test and total abstinence (somatic) score at 24 h post nicotine withdrawal time. The somatic signs includes a composite score of all behaviors such as grooming, rearing, jumping, body shakes, forelimb tremors, head shakes, abdominal constrictions, scratching, empty mouth chewing or teeth chattering, genital licking, tail licking. Mice exhibited higher expression to nicotine withdrawal induced anxiety in light & dark test at 24 h post-nicotine withdrawal time on pre-treatment centrally (i.c.v) with histaminergic agents like histamine (0.1, 50 μg/mouse), histamine H₃ receptor inverse agonist, thioperamide (2, 10 μg/mouse), histamine H₁ receptor agonist, FMPH (2, 6.5 μg/mouse) or H₂ receptor agonist amthamine (0.1, 0.5 μg/mouse) or intraperitoneally (i.p.) with histamine precursor, l-histidine (250, 500 mg/kg) as compared to control nicotine withdrawn animals. Furthermore, mice pre-treated with all these histaminergic agents except histamine H₁ receptor agonist, FMPH shows exacerbated expression to post-nicotine withdrawal induced total abstinence (somatic) score in mice. On the other hand, central injection of selective histamine H₁ receptor antagonist, cetirizine (0.1 μg/mouse, i.c.v.) or H₂ receptor antagonist, ranitidine (50 μg/mouse, i.c.v) to mice 10 min before 24 h post-nicotine withdrawal time completely alleviated the expression of nicotine withdrawal induced anxiety and somatic behavior. Thus, it can be contemplated that the blockade of central histamine H₁ or H₂ receptor during the nicotine withdrawal phase could be a novel approach to mitigate the nicotine withdrawal associated anxiety-like manifestations. Contribution of endogenous histamine via H₁ or H₂ receptor stimulation in the nicotine withdrawal induced anxiety and somatic behavior is proposed.

The Role of Nicotinic Receptors in the Attenuation of Autism-Related Behaviors in a Murine BTBR T + tf/J Autistic Model

Nicotinic receptors are distributed throughout the central and peripheral nervous system. Postmortem studies have reported that some nicotinic receptor subtypes are altered in the brains of autistic people. Recent studies have demonstrated the importance of nicotinic acetylcholine receptors (nAChRs) in the autistic behavior of BTBR T + tf/J mouse model of autism. This study was undertaken to examine the behavioral effects of targeted nAChRs using pharmacological ligands, including nicotine and mecamylamine in BTBR T + tf/J and C57BL/6J mice in a panel of behavioral tests relating to autism. These behavioral tests included the three-chamber social interaction, self-grooming, marble burying, locomotor activity, and rotarod test. We examined the effect of various oral doses of nicotine (50, 100, and 400 mcg/mL; po) over a period of 2 weeks in BTBR T + tf/J mouse model. The results indicated that the chronic administration of nicotine modulated sociability and repetitive behavior in BTBR T + tf/J mice while no effects observed in C57BL/6J mice. Furthermore, the nonselective nAChR antagonist, mecamylamine, reversed nicotine effects on sociability and increased repetitive behaviors in BTBR T + tf/J mice. Overall, the findings indicate that the pharmacological modulation of nicotinic receptors is involved in modulating core behavioral phenotypes in the BTBR T + tf/J mouse model. LAY SUMMARY: The involvement of brain nicotinic neurotransmission system plays a crucial role in regulating autism-related behavioral features. In addition, the brain of the autistic-like mouse model has a low acetylcholine level. Here, we report that nicotine, at certain doses, improved sociability and reduced repetitive behaviors in a mouse model of autism, implicating the potential therapeutic values of a pharmacological intervention targeting nicotinic receptors for autism therapy. Autism Res 2020, 13: 1311-1334. © 2020 International Society for Autism Research, Wiley Periodicals, Inc.

Maternal Nicotine Exposure During Gestation and Lactation Period Affects Behavior and Hippocampal Neurogenesis in Mouse Offspring

Cigarette smoking or nicotine exposure during pregnancy is associated with numerous obstetrical, fetal, and developmental complications, as well as an increased risk of adverse health consequences in the adult offspring. In this study, we examined the effects of maternal nicotine exposure during perinatal and lactation stages on behavioral performance and hippocampal neurogenesis in the adolescent stage of offspring mice. Female C57BL/mice received nicotine in drinking water (200 μg/ml nicotine) or vehicle (1% saccharin) starting from 2 weeks premating until the offspring were weaned on postnatal day 20. Experiments started on postnatal day 35. Female offspring with maternal nicotine exposure presented an increase in anxiety-like behavior in an open-field test. BrdU assay revealed that nicotine offspring presented an increase in cell proliferation in hippocampal dentate gyrus, but the number of BrdU⁺ cells was decreased in one week and further decreased in three weeks. The occurrence of disarray of DCX⁺ cells increased in both male and female nicotine offspring. The density of microglial marker protein Iba1 was significantly increased in the nicotine offspring. Furthermore, the expression of microglia marker Iba1, the CX3CL1, CX3CR1, and downstream molecules PKA and p-ErK were significantly increased in the nicotine group. In summary, maternal nicotine exposure affects both hippocampal neurogenesis and microglial activity in the adolescent offspring.

The effects of oral nicotine administration and abstinence on sleep in male C57BL/6J mice

BACKGROUND

Sleep disturbances are common in smoking cessation attempts and are predictive of relapse. Despite this knowledge, there is no established animal model to study the effect of nicotine abstinence on sleep and EEG parameters.

OBJECTIVES

The present study was conducted to characterize sleep and wakefulness in male C57BL/6J mice during periods of oral nicotine administration and abstinence.

METHODS

Male C57BL/6J mice were implanted with EEG/EMG recording devices. EEG/EMG data were recorded continuously for a period of 4 weeks. At the beginning of week 2, 200 μg/ml of nicotine was added to the 0.2% saccharin vehicle drinking solution. Following a 2-week period of oral nicotine administration, abstinence was initiated by excluding the nicotine from the 0.2% saccharin vehicle drinking solution. EEG/EMG were analyzed at pre-nicotine baseline, during nicotine administration, and on days 1, 2, and 5 of abstinence from nicotine.

RESULTS

Oral nicotine administration decreased total sleep time during the active phase, consistent with the stimulant actions of nicotine. In contrast, NREM sleep quantity was increased during the active phase on nicotine abstinence day 1 and REM sleep was decreased during days 2 and 5 of abstinence. Further, sleep fragmentation was increased during the inactive phase on all days of abstinence. Oral nicotine administration and abstinence from nicotine also altered EEG relative power frequencies during the inactive and active phase.

CONCLUSIONS

Both oral nicotine administration and abstinence lead to sleep disturbances in mice. Similarities between this model and human reports on the effect of nicotine/nicotine withdrawal on sleep support its utility in examining the molecular mechanisms that modulate the relationship between sleep, nicotine, and nicotine abstinence/withdrawal.

Effects of nicotine deprivation on current pain intensity among daily cigarette smokers

Animal research has consistently demonstrated increased pain in the context of nicotine deprivation, and there is cross-sectional evidence that tobacco smokers may experience greater pain following periods of smoking abstinence. This study aimed to examine current pain intensity as a function of nicotine deprivation among 137 daily tobacco smokers who did not endorse chronic pain and were recruited to participate in a primary study of the effects of smoking abstinence on experimental pain reactivity. Participants were randomized to either deprivation (12-24 hr abstinence) or continued ad lib smoking conditions. Compliance with the manipulation was biochemically verified via expired carbon monoxide (CO). Current pain intensity was assessed at baseline (Session 1) and following the deprivation manipulation (Session 2) using a single item that asked participants to indicate their current level of pain on a scale ranging from 0 (no pain) to 10 (pain as bad as you can imagine). At baseline, the majority of participants (51.1%) reported no pain (M = 1.75). As hypothesized, participants randomized to nicotine deprivation (vs. continued smoking) reported greater current pain intensity following the manipulation. Among smokers who reported no pain at baseline, those who abstained from smoking were nearly 3.5 times more likely to endorse pain at Session 2. These results suggest that daily tobacco smokers may experience greater pain during the first 12-24 hr of smoking abstinence. Future research should examine the role of pain in nicotine withdrawal, and whether tailored interventions may be needed to account for nicotine deprivation-induced amplification of pain. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Insights Into Nicotinic Receptor Signaling in Nicotine Addiction: Implications for Prevention and Treatment

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) belong to the Cys-loop ligandgated ion-channel (LGIC) superfamily, which also includes the GABA, glycine, and serotonin receptors. Many nAChR subunits have been identified and shown to be involved in signal transduction on binding to them of either the neurotransmitter acetylcholine or exogenous ligands such as nicotine. The nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and they are expressed at neuromuscular junctions throughout the nervous system.

METHODS AND RESULTS

Because different nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes, and different nAChRs are implicated in various physiological functions and pathophysiological conditions, nAChRs represent potential molecular targets for drug addiction and medical therapeutic research. This review intends to provide insights into recent advances in nAChR signaling, considering the subtypes and subunits of nAChRs and their roles in nicotinic cholinergic systems, including structure, diversity, functional allosteric modulation, targeted knockout mutations, and rare variations of specific subunits, and the potency and functional effects of mutations by focusing on their effects on nicotine addiction (NA) and smoking cessation (SC). Furthermore, we review the possible mechanisms of action of nAChRs in NA and SC based on our current knowledge.

CONCLUSION

Understanding these cellular and molecular mechanisms will lead to better translational and therapeutic operations and outcomes for the prevention and treatment of NA and other drug addictions, as well as chronic diseases, such as Alzheimer's and Parkinson's. Finally, we put forward some suggestions and recommendations for therapy and treatment of NA and other chronic diseases.

Nicotine Modulates Cognitive Function in D-Galactose-Induced Senescence in Mice

Here, we tested the claim that nicotine attenuates the signs of brain dysfunction in the model of brain aging induced by D-galactose (DGal) in mice. We administered nicotine at doses of 0.1, 0.5 and 1 mg/kg by the subcutaneous (s.c.) or at 0.1 mg/kg by the intranasal (i.n.) routes in mice that had received DGal at the dose of 500 mg/kg subcutaneous (s.c.) for 6 weeks. We assessed animal withdrawal signs as the number of presented somatic signs, thermal hyperalgesia, elevated plus maze (EPM) and open field tests. We evaluated spatial memory and recognition with Barnes maze and novel object recognition (NOR) tests. We tested brain tissue for reactive oxygen species (ROS), mitochondrial membrane potential, caspase-3, Bax, Bcl-2, cytochrome C, brain-derived neurotrophic factor and nerve growth factor levels. Nicotine administration in model groups (0.5 mg/kg s.c. and 0.1 mg/kg i.n. doses) significantly attenuated impairment of spatial and episodic memories in comparison to normal saline-received model group. These doses also reduced mito-oxidative damage as well as apoptosis and raised neurotrophic factors level in model groups in comparison to normal saline-received model group. The 1 mg/kg s.c. dose nicotine revealed withdrawal signs compared with the other nicotine-received groups. Nicotine at specific doses and routes has the potential to attenuate age-related cognitive impairment, mito-oxidative damage, and apoptosis. The doses raise neurotrophic factors without producing withdrawal signs.

Nicotine deprivation increases pain intensity, neurogenic inflammation, and mechanical hyperalgesia among daily tobacco smokers

An evolving reciprocal model posits that pain and tobacco smoking behavior interact in the manner of a positive feedback loop, resulting in greater pain and the maintenance of nicotine dependence. There is also reason to believe that abstaining from smoking may increase pain during the early stages of smoking cessation. The goal of this study was to test the effects of nicotine deprivation on experimental pain reactivity. Daily tobacco cigarette smokers (N = 165; 43% female) were randomized to either extended nicotine deprivation (12-24 hr smoking abstinence), minimal deprivation (2 hr smoking abstinence), or continued smoking conditions, prior to undergoing pain induction via topical capsaicin. As hypothesized, results indicated that extended deprivation (relative to continued smoking) increased capsaicin-induced pain intensity ratings, neurogenic inflammation, and mechanical hyperalgesia, thus implicating both central and peripheral mechanisms of action in the effects of smoking abstinence on pain reactivity. Pain intensity ratings were also positively correlated with nicotine withdrawal symptoms, and exploratory analyses suggest that pain sensitivity may increase with duration of smoking abstinence. Collectively, these findings indicate that smokers may experience a variety of negative pain-related sequelae during the early stages of a quit attempt. Future research should examine pain as a consequence or correlate of the nicotine withdrawal syndrome, and determine whether smokers may benefit from tailored cessation interventions that account for nicotine deprivation-induced amplification of pain. (PsycINFO Database Record

Nicotinic acetylcholine receptors (nAChRs) are expressed in Trpm5 positive taste receptor cells (TRCs)

Nicotine evokes chorda tympani (CT) taste nerve responses and an aversive behavior in Trpm5 knockout (KO) mice. The agonists and antagonists of nicotinic acetylcholine receptors (nAChRs) modulate neural and behavioral responses to nicotine in wildtype (WT) mice, Trpm5 KO mice and rats. This indicates that nicotine evokes bitter taste by activating a Trpm5-dependent pathway and a Trpm5-independent but nAChR-dependent pathway. Rat CT responses to ethanol are also partially inhibited by nAChR blockers, mecamylamine and dihydro-β-erythroidine. This indicates that a component of the bitter taste of ethanol is also nAChR-dependent. However, at present the expression and localization of nAChR subunits has not been investigated in detail in taste receptor cells (TRCs). To this end, in situ hybridization, immunohistochemistry and q-RT-PCR techniques were utilized to localize nAChR subunits in fungiform and circumvallate TRCs in WT mice, Trpm5-GFP transgenic mice, nAChR KO mice, and rats. The expression of mRNAs for α7, β2 and β4 nAChR subunits was observed in a subset of rat and WT mouse circumvallate and fungiform TRCs. Specific α3, α4, α7, β2, and β4 antibodies localized to a subset of WT mouse circumvallate and fungiform TRCs. In Trpm5-GFP mice α3, α4, α7, and β4 antibody binding was observed in a subset of Trpm5-positive circumvallate TRCs. Giving nicotine (100 μg/ml) in drinking water to WT mice for 3 weeks differentially increased the expression of α3, α4, α5, α6, α7, β2 and β4 mRNAs in circumvallate TRCs to varying degrees. Giving ethanol (5%) in drinking water to WT mice induced an increase in the expression of α5 and β4 mRNAs in circumvallate TRCs with a significant decrease in the expression of α3, α6 and β2 mRNAs. We conclude that nAChR subunits are expressed in Trpm5-positive TRCs and their expression levels are differentially altered by chronic oral exposure to nicotine and ethanol.

Nicotine promotes cervical carcinoma cell line HeLa migration and invasion by activating PI3k/Akt/NF-κB pathway in vitro

Cigarette smoking is one of highly risk factors of cervical cancer. Recently nicotine has been reported to increase proliferation and invasion in some smoking related cancers, like non-small cell lung cancer and esophageal squamous cell cancer. However, the effects and mechanisms of nicotine stimulation on cervical cancer cells are not clear. Here, we investigated the effects and mechanisms of nicotine stimulation on HeLa cells in vitro. In our study, we found that nicotine could accelerate HeLa cells migration and invasion, activate PI3K/Akt and NF-κB pathways and increase the expression of Vimentin in vitro. Moreover, we demonstrated that the specific PI3K inhibitor LY294002 could reverse nicotine-induced cell migration and invasion, NF-κB activation and up-regulation of Vimentin. Inhibition of NF-κB by Pyrrolidine dithiocarbamate (PDTC) also antagonized nicotine-induced cell migration, invasion and up-regulation of Vimentin. Simply put, these findings suggest that nicotine promotes cervical carcinoma cell line HeLa migration and invasion by activating PI3k/Akt/NF-κB pathway in vitro.

Comprehensive review of epidemiological and animal studies on the potential carcinogenic effects of nicotine per se

The effects of long-term use of nicotine per se on cancer risk, in the absence of tobacco extract or smoke, are not clearly understood. This review evaluates the strength of published scientific evidence, in both epidemiological and animal studies, for the potential carcinogenic effects of nicotine per se; that is to act as a complete carcinogen or as a modulator of carcinogenesis. For human studies, there appears to be inadequate evidence for an association between nicotine exposure and the presence of or lack of a carcinogenic effect due to the limited information available. In animal studies, limited evidence suggests an association between long-term nicotine exposure and a lack of a complete carcinogenic effect. Conclusive studies using current bioassay guidelines, however, are missing. In studies using chemical/physical carcinogens or transgenic models, there appears to be inadequate evidence for an association between nicotine exposure and the presence of or lack of a modulating (stimulating) effect on carcinogenesis. This is primarily due to the large number of conflicting studies. In contrast, a majority of studies provides sufficient evidence for an association between nicotine exposure and enhanced carcinogenesis of cancer cells inoculated in mice. This modulating effect was especially prominent in immunocompromized mice. Overall, taking the human and animal studies into consideration, there appears to be inadequate evidence to conclude that nicotine per se does or does not cause or modulate carcinogenesis in humans. This conclusion is in agreement with the recent US Surgeon General's 2014 report on the health consequences of nicotine exposure.

The influence of chronic nicotine treatment on proteins expressed in the mouse hippocampus and cortex

Chronic treatment with nicotine, the primary psychoactive substance in tobacco smoke, affects central nervous system functions, such as synaptic plasticity. Here, to clarify the effects of chronic nicotine treatment on the higher brain functions, proteomic analysis of the hippocampus and cortex of mice treated for 6 months with nicotine was performed using two-dimensional gel electrophoresis (2-DE) followed by mass spectrometry. There was significant change in the expression of 16 proteins and one phosphoprotein in the hippocampus (increased tubulin β-5, atp5b, MDH1, cytochrome b-c1 complex subunit 1, Hsc70, dynamin, profilin-2, 4-aminobutyrate aminotransferase, mitochondrial isoform 1 precursor, calpain small subunit 1, and vacuolar adenosine triphosphatase subunit B and decreased γ-actin, α-tubulin isotype M-α-2, putative β-actin, tubulin β-2A, NDUFA10, and G6PD) and 24 proteins and two phosphoproteins in the cortex (increased spectrin α chain, non-erythrocytic 1 isoform 1, tubulin β-5, γ-actin, creatine kinase B-type, LDH-B, secernin-1, UCH-L1, 14-3-3 γ, type II peroxiredoxin 1, PEBP-1, and unnamed protein product and decreased tubulin α-1C, α-internexin, γ-enolase, PDHE1-B, DPYL2, vacuolar adenosine triphosphatase subunit A, vacuolar adenosine triphosphatase subunit B, TCTP, NADH dehydrogenase Fe-S protein 1, protein disulfide-isomerase A3, hnRNP H2, γ-actin, atp5b, and unnamed protein product). Additionally, Western blotting validated the changes in dynamin, Hsc70, MDH1, NDUFA10, α-internexin, tubulin β-5 chain, and secernin-1. Thus, these findings indicate that chronic nicotine treatment changes the expression of proteins and phosphoproteins in the hippocampus and cortex. We propose that effect of smoking on higher brain functions could be mediated by alterations in expression levels of these proteins.

An Immune-Inflammation Gene Expression Signature in Prostate Tumors of Smokers

Smokers develop metastatic prostate cancer more frequently than nonsmokers, suggesting that a tobacco-derived factor is driving metastatic progression. To identify smoking-induced alterations in human prostate cancer, we analyzed gene and protein expression patterns in tumors collected from current, past, and never smokers. By this route, we elucidated a distinct pattern of molecular alterations characterized by an immune and inflammation signature in tumors from current smokers that were either attenuated or absent in past and never smokers. Specifically, this signature included elevated immunoglobulin expression by tumor-infiltrating B cells, NF-κB activation, and increased chemokine expression. In an alternate approach to characterize smoking-induced oncogenic alterations, we also explored the effects of nicotine in human prostate cancer cells and prostate cancer-prone TRAMP mice. These investigations showed that nicotine increased glutamine consumption and invasiveness of cancer cells in vitro and accelerated metastatic progression in tumor-bearing TRAMP mice. Overall, our findings suggest that nicotine is sufficient to induce a phenotype resembling the epidemiology of smoking-associated prostate cancer progression, illuminating a novel candidate driver underlying metastatic prostate cancer in current smokers.

Conditioned cortical reactivity to cues predicting cigarette-related or pleasant images

Through Pavlovian conditioning, reward-associated neutral stimuli can acquire incentive salience and motivate complex behaviors. In smokers, cigarette-associated cues may induce cravings and trigger smoking. Understanding the brain mechanisms underlying conditioned responses to cigarette-associated relative to other inherently pleasant stimuli might contribute to the development of more effective smoking cessation treatments that emphasize the rehabilitation of reward circuitry. Here we measured brain responses to geometric patterns (the conditioned stimuli, CSs) predicting cigarette-related, intrinsically pleasant and neutral images (the unconditioned stimuli, USs) using event-related potentials (ERPs) in 29 never-smokers, 20 nicotine-deprived smokers, and 19 non-deprived smokers. Results showed that during US presentation, cigarette-related and pleasant images prompted higher cortical positivity than neutral images over centro-parietal sensors between 400 and 800ms post-US onset (late positive potential, LPP). The LPP evoked by pleasant images was significantly larger than the LPP evoked by cigarette images. During CS presentation, ERPs evoked by geometric patterns predicting pleasant and cigarette-related images had significantly larger amplitude than ERPs evoked by CSs predicting neutral images. These effects were maximal over right parietal sites between 220 and 240ms post-CS onset and over occipital and frontal sites between 308 and 344ms post-CS onset. Smoking status did not modulate these effects. Our results show that stimuli with no intrinsic reward value (e.g., geometric patterns) may acquire rewarding properties through repeated pairings with established reward cues (i.e., cigarette-related, intrinsically pleasant).

Nighttime Administration of Nicotine Improves Hepatic Glucose Metabolism via the Hypothalamic Orexin System in Mice

Nicotine is known to affect the metabolism of glucose; however, the underlying mechanism remains unclear. Therefore, we here investigated whether nicotine promoted the central regulation of glucose metabolism, which is closely linked to the circadian system. The oral intake of nicotine in drinking water, which mainly occurred during the nighttime active period, enhanced daily hypothalamic prepro-orexin gene expression and reduced hyperglycemia in type 2 diabetic db/db mice without affecting body weight, body fat content, and serum levels of insulin. Nicotine administered at the active period appears to be responsible for the effect on blood glucose, because nighttime but not daytime injections of nicotine lowered blood glucose levels in db/db mice. The chronic oral treatment with nicotine suppressed the mRNA levels of glucose-6-phosphatase, the rate-limiting enzyme of gluconeogenesis, in the liver of db/db and wild-type control mice. In the pyruvate tolerance test to evaluate hepatic gluconeogenic activity, the oral nicotine treatment moderately suppressed glucose elevations in normal mice and mice lacking dopamine receptors, whereas this effect was abolished in orexin-deficient mice and hepatic parasympathectomized mice. Under high-fat diet conditions, the oral intake of nicotine lowered blood glucose levels at the daytime resting period in wild-type, but not orexin-deficient, mice. These results indicated that the chronic daily administration of nicotine suppressed hepatic gluconeogenesis via the hypothalamic orexin-parasympathetic nervous system. Thus, the results of the present study may provide an insight into novel chronotherapy for type 2 diabetes that targets the central cholinergic and orexinergic systems.

Modulation of social deficits and repetitive behaviors in a mouse model of autism: the role of the nicotinic cholinergic system

RATIONALE

Accumulating evidence implicates the nicotinic cholinergic system in autism spectrum disorder (ASD) pathobiology. Neuropathologic studies suggest that nicotinic acetylcholine (ACh) receptor (nAChR) subtypes are altered in brain of autistic individuals. In addition, strategies that increase ACh, the neurotransmitter for nicotinic and muscarinic receptors, appear to improve cognitive deficits in neuropsychiatric disorders and ASD.

OBJECTIVE

The aim of this study is to examine the role of the nicotinic cholinergic system on social and repetitive behavior abnormalities and exploratory physical activity in a well-studied model of autism, the BTBR T(+) Itpr3 (tf) /J (BTBR) mouse.

METHODS

Using a protocol known to up-regulate expression of brain nAChR subtypes, we measured behavior outcomes before and after BTBR and C57BL/6J (B6) mice were treated (4 weeks) with vehicle or nicotine (50, 100, 200, or 400 μg/ml).

RESULTS

Increasing nicotine doses were associated with decreases in water intake, increases in plasma cotinine levels, and at the higher dose (400 μg/ml) with weight loss in BTBR mice. At lower (50, 100 μg/ml) but not higher (200, 400 μg/ml) doses, nicotine increased social interactions in BTBR and B6 mice and at higher, but not lower doses, it decreased repetitive behavior in BTBR. In the open-field test, nicotine at 200 and 400 μg/ml, but not 100 μg/ml compared with vehicle, decreased overall physical activity in BTBR mice.

CONCLUSIONS

These findings support the hypotheses that the nicotinic cholinergic system modulates social and repetitive behaviors and may be a therapeutic target to treat behavior deficits in ASD. Further, the BTBR mouse may be valuable for investigations of the role of nAChRs in social deficits and repetitive behavior.

An essential role of acetylcholine-glutamate synergy at habenular synapses in nicotine dependence

A great deal of interest has been focused recently on the habenula and its critical role in aversion, negative-reward and drug dependence. Using a conditional mouse model of the ACh-synthesizing enzyme choline acetyltransferase (Chat), we report that local elimination of acetylcholine (ACh) in medial habenula (MHb) neurons alters glutamate corelease and presynaptic facilitation. Electron microscopy and immuno-isolation analyses revealed colocalization of ACh and glutamate vesicular transporters in synaptic vesicles (SVs) in the central IPN. Glutamate reuptake in SVs prepared from the IPN was increased by ACh, indicating vesicular synergy. Mice lacking CHAT in habenular neurons were insensitive to nicotine-conditioned reward and withdrawal. These data demonstrate that ACh controls the quantal size and release frequency of glutamate at habenular synapses, and suggest that the synergistic functions of ACh and glutamate may be generally important for modulation of cholinergic circuit function and behavior.

Increased CRF signalling in a ventral tegmental area-interpeduncular nucleus-medial habenula circuit induces anxiety during nicotine withdrawal

Increased anxiety is a prominent withdrawal symptom in abstinent smokers, yet the neuroanatomical and molecular bases underlying it are unclear. Here we show that withdrawal-induced anxiety increases activity of neurons in the interpeduncular intermediate (IPI), a subregion of the interpeduncular nucleus (IPN). IPI activation during nicotine withdrawal was mediated by increased corticotropin releasing factor (CRF) receptor-1 expression and signalling, which modulated glutamatergic input from the medial habenula (MHb). Pharmacological blockade of IPN CRF1 receptors or optogenetic silencing of MHb input reduced IPI activation and alleviated withdrawal-induced anxiety; whereas IPN CRF infusion in mice increased anxiety. We identified a mesointerpeduncular circuit, consisting of ventral tegmental area (VTA) dopaminergic neurons projecting to the IPN, as a potential source of CRF. Knockdown of CRF synthesis in the VTA prevented IPI activation and anxiety during nicotine withdrawal. These data indicate that increased CRF receptor signalling within a VTA-IPN-MHb circuit triggers anxiety during nicotine withdrawal.

Role of α5-containing nicotinic receptors in neuropathic pain and response to nicotine

Nicotinic receptors in the central nervous system (nAChRs) are known to play important roles in pain processing and modulate behavioral responses to analgesic drugs, including nicotine. The presence of the α5-neuronal nicotinic accessory subunit in the nicotinic receptor complex is increasingly understood to modulate reward and aversive states, addiction, and possibly pathological pain. In the current study, using α5-knockout (KO) mice and subunit-specific antibodies, we assess the role of α5-containing neuronal nicotinic receptors in neuropathic pain and in the analgesic response to nicotine. After chronic constriction injury (CCI) or partial sciatic nerve ligation (PSNL), no differences in mechanical, heat, or cold hyperalgesia were found in wild-type (WT) versus α5-KO littermate mice. The number of α5-containing nAChRs was decreased (rather than increased) after CCI in the spinal cord and in the thalamus. Nevertheless, thermal analgesic response to nicotine was marginally reduced in CCI α5-KO mice at 4 days after CCI, but not at later timepoints or after PSNL. Interestingly, upon daily intermittent nicotine injections in unoperated mice, WT animals developed tolerance to nicotine-induced analgesia to a larger extent than α5-KO mice. Our results suggest that α5-containing nAChRs mediate analgesic tolerance to nicotine but do not play a major role in neuropathic pain.

Translational Research on Nicotine Dependence

Nicotine dependence is a chronic, relapsing disorder with complex biological mechanisms underlying the motivational basis for this behavior. Although more than 70 % of current smokers express a desire to quit, most relapse within one year, underscoring the need for novel treatments. A key focus of translational research models addressing nicotine dependence has been on cross-validation of human and animal models in order to improve the predictive value of medication screening paradigms. In this chapter, we review several lines of research highlighting the utility of cross-validation models in elucidating the biological underpinnings of nicotine reward and reinforcement, identifying factors which may influence individual response to treatment, and facilitating rapid translation of findings to practice.

Nicotine withdrawal

An aversive abstinence syndrome manifests 4-24 h following cessation of chronic use of nicotine-containing products. Symptoms peak on approximately the 3rd day and taper off over the course of the following 3-4 weeks. While the severity of withdrawal symptoms is largely determined by how nicotine is consumed, certain short nucleotide polymorphisms (SNPs) have been shown to predispose individuals to consume larger amounts of nicotine more frequently--as well as to more severe symptoms of withdrawal when trying to quit. Additionally, rodent behavioral models and transgenic mouse models have revealed that specific nicotinic acetylcholine receptor (nAChR) subunits, cellular components, and neuronal circuits are critical to the expression of withdrawal symptoms. Consequently, by continuing to map neuronal circuits and nAChR subpopulations that underlie the nicotine withdrawal syndrome--and by continuing to enumerate genes that predispose carriers to nicotine addiction and exacerbated withdrawal symptoms--it will be possible to pursue personalized therapeutics that more effectively treat nicotine addiction.

The habenulo-interpeduncular pathway in nicotine aversion and withdrawal

Progress has been made over the last decade in our understanding of the brain areas and circuits involved in nicotine reward and withdrawal, leading to models of addiction that assign different addictive behaviors to distinct, yet overlapping, neural circuits (Koob and Volkow, 2010; Lobo and Nestler, 2011; Tuesta et al., 2011; Volkow et al., 2011). Recently the habenulo-interpeduncular (Hb-IPN) midbrain pathway has re-emerged as a new critical crossroad that influences the brain response to nicotine. This brain area is particularly enriched in nicotinic acetylcholine receptor (nAChR) subunits α5, α3 and β4 encoded by the CHRNA5-A3-B4 gene cluster, which has been associated with vulnerability to tobacco dependence in human genetics studies. This finding, together with studies in mice involving deletion and replacement of nAChR subunits, and investigations of the circuitry, cell types and electrophysiological properties, have begun to identify the molecular mechanisms that take place in the MHb-IPN which underlie critical aspects of nicotine dependence. In the current review we describe the anatomical and functional connections of the MHb-IPN system, as well as the contribution of specific nAChRs subtypes in nicotine-mediated behaviors. Finally, we discuss the specific electrophysiological properties of MHb-IPN neuronal populations and how nicotine exposure alters their cellular physiology, highlighting the unique role of the MHb-IPN in the context of nicotine aversion and withdrawal. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Chronic nicotine pretreatment is sufficient to upregulate α4* nicotinic receptors and increase oral nicotine self-administration in mice

Background

Understanding the underlying causes of nicotine addiction will require a multidisciplinary approach examining the key molecular, cellular and neuronal circuit functional changes that drive escalating levels of nicotine self-administration. In this study, we examined whether mice pretreated with chronic nicotine, at a dosing regimen that results in maximal nicotinic acetylcholine receptor (nAChR) upregulation, would display evidence of nicotine-dependent behaviour during nicotine self-administration.

Results

We investigated oral self-administration of nicotine using a two-bottle choice paradigm in which one bottle contained the vehicle (saccharine-sweetened water), while the other contained nicotine (200 μg/ml) in vehicle. Knock-in mice with YFP-tagged α4 nAChR subunits (α4YFP) were implanted with osmotic pumps delivering either nicotine (2 mg/kg/hr) or saline for 10 days. After 10 days of pretreatment, mice were exposed to the nicotine self-administration paradigm, consisting of four days of choice followed by three days of nicotine abstinence repeated for five weeks. Mice pre-exposed to nicotine had upregulated α4YFP nAChR subunits in the hippocampal medial perforant path and on ventral tegmental area GABAergic neurons as compared to chronic saline mice. Compared to control saline-pretreated mice, in a two bottle-choice experiment, nicotine-primed mice ingested a significantly larger daily dose of nicotine and also exhibited post-abstinence binge drinking of nicotine.

Conclusions

Chronic forced pre-exposure of nicotine is sufficient to induce elevated oral nicotine intake and supports the postulate that nAChR upregulation may be a key factor influencing nicotine self-administration.

Impact of nicotine metabolism on nicotine's pharmacological effects and behavioral responses: insights from a Cyp2a(4/5)bgs-null mouse

Nicotine metabolism is believed to affect not only nicotine's pharmacological effects but also nicotine addiction. As a key step toward testing this hypothesis, we have studied nicotine metabolism and nicotine's pharmacological and behavioral effects in a novel knockout mouse model [named Cyp2a(4/5)bgs-null] lacking a number of cytochrome P450 genes known to be or possibly involved in nicotine metabolism, including two Cyp2a and all Cyp2b genes. We found that, compared with wild-type mice, the Cyp2a(4/5)bgs-null mice showed >90% decreases in hepatic microsomal nicotine oxidase activity in vitro, and in rates of systemic nicotine clearance in vivo. Further comparisons of nicotine metabolism between Cyp2a(4/5)bgs-null and Cyp2a5-null mice revealed significant roles of both CYP2A5 and CYP2B enzymes in nicotine clearance. Compared with the behavioral responses in wild-type mice, the decreases in nicotine metabolism in the Cyp2a(4/5)bgs-null mice led to prolonged nicotine-induced acute pharmacological effects, in that null mice showed enhanced nicotine hypothermia and antinociception. Furthermore, we found that the Cyp2a(4/5)bgs-null mice developed a preference for nicotine in a conditioned place preference test, a commonly used test of nicotine's rewarding effects, at a nicotine dose that was 4-fold lower than what was required by wild-type mice. Thus, CYP2A/2B-catalyzed nicotine clearance affects nicotine's behavioral response as well as its acute pharmacological effects in mice. This result provides direct experimental support of the findings of pharmacogenetic studies that suggest linkage between rates of nicotine metabolism and smoking behavior in humans.

Activation of GABAergic neurons in the interpeduncular nucleus triggers physical nicotine withdrawal symptoms

BACKGROUND

Chronic exposure to nicotine elicits physical dependence in smokers, yet the mechanism and neuroanatomical bases for withdrawal symptoms are unclear. As in humans, rodents undergo physical withdrawal symptoms after cessation from chronic nicotine characterized by increased scratching, head nods, and body shakes.

RESULTS

Here we show that induction of physical nicotine withdrawal symptoms activates GABAergic neurons within the interpeduncular nucleus (IPN). Optical activation of IPN GABAergic neurons via light stimulation of channelrhodopsin elicited physical withdrawal symptoms in both nicotine-naive and chronic-nicotine-exposed mice. Dampening excitability of GABAergic neurons during nicotine withdrawal through IPN-selective infusion of an NMDA receptor antagonist or through blockade of IPN neurotransmission from the medial habenula reduced IPN neuronal activation and alleviated withdrawal symptoms. During chronic nicotine exposure, nicotinic acetylcholine receptors containing the β4 subunit were upregulated in somatostatin interneurons clustered in the dorsal region of the IPN. Blockade of these receptors induced withdrawal signs more dramatically in nicotine-dependent compared to nicotine-naive mice and activated nonsomatostatin neurons in the IPN.

CONCLUSIONS

Together, our data indicate that therapeutic strategies to reduce IPN GABAergic neuron excitability during nicotine withdrawal, for example, by activating nicotinic receptors on somatostatin interneurons, may be beneficial for alleviating withdrawal symptoms and facilitating smoking cessation.

Nicotine dependence produces hyperalgesia: role of corticotropin-releasing factor-1 receptors (CRF1Rs) in the central amygdala (CeA)

Because tobacco use has a large negative health and financial impact on society, it is critical to identify the factors that drive excessive use. These factors include the aversive withdrawal symptoms that manifest upon cessation of tobacco use, and may include increases in nociceptive processing. Corticotropin-releasing factor (CRF) signalling in the central amygdala (CeA) has been attributed an important role in: (1) central processing of pain, (2) excessive nicotine use that results in nicotine dependence, and (3) in mediating the aversive symptoms that manifest following cessation of tobacco exposure. Here, we describe three experiments in which the main hypothesis was that CRF/CRF1 receptor (CRF1R) signalling in the CeA mediates nicotine withdrawal-induced increases in nociceptive sensitivity in rats that are dependent on nicotine. In Experiment 1, nicotine-dependent rats withdrawn from chronic intermittent (14-h/day) nicotine vapor exhibited decreased hind paw withdrawal latencies in response to a painful thermal stimulus in the Hargreaves test, and this effect was attenuated by systemic administration of the CRF1R antagonist, R121919. In Experiment 2, nicotine-dependent rats withdrawn from nicotine vapor exhibited robust increases in mRNA for CRF and CRF1Rs in CeA. In Experiment 3, intra-CeA administration of R121919 reduced thermal nociception only in nicotine-dependent rats. Collectively, these results suggest that nicotine dependence increases CRF/CRF1R signalling in the CeA that mediates withdrawal-induced increases in sensitivity to a painful stimulus. Future studies will build on these findings by exploring the hypothesis that nicotine withdrawal-induced reduction in pain thresholds drive excessive nicotine use via CRF/CRF1R signalling pathways.

Nicotinic receptors regulate the dynamic range of dopamine release in vivo

Nicotinic acetylcholine receptors (nAChRs) are expressed presynaptically on dopamine axon terminals, and their activation by endogenous acetylcholine from striatal cholinergic interneurons enhances dopamine release both independently of and in concert with dopamine neuron activity. Acute nAChR inactivation is believed to enhance the contrast between low- and high-frequency dopamine cell activity. Although these studies reveal a key role for acute activation and inactivation of nAChRs in striatal microcircuitry, it remains unknown if chronic inactivation/desensitization of nAChRs can alter dopamine release dynamics. Using in vivo cyclic voltammetry in anaesthetized mice, we examined whether chronic inactivation of nAChRs modulates dopamine release across a parametric range of stimulation, varying both frequency and pulse number. Deletion of β2*nAChRs and chronic nicotine exposure greatly diminished dopamine release across the entire range of stimulation parameters. In addition, we observed a facilitation of dopamine release at low frequency and pulse number in wild-type mice that is absent in the β2* knockout and chronic nicotine mice. These data suggest that deletion or chronic desensitization of nAChRs reduces the dynamic range of dopamine release in response to dopamine cell activity, decreasing rather than increasing contrast between high and low dopamine activity.

Reexposure to nicotine during withdrawal increases the pacemaking activity of cholinergic habenular neurons

The discovery of genetic variants in the cholinergic receptor nicotinic CHRNA5-CHRNA3-CHRNB4 gene cluster associated with heavy smoking and higher relapse risk has led to the identification of the midbrain habenula-interpeduncular axis as a critical relay circuit in the control of nicotine dependence. Although clear roles for α3, β4, and α5 receptors in nicotine aversion and withdrawal have been established, the cellular and molecular mechanisms that participate in signaling nicotine use and contribute to relapse have not been identified. Here, using translating ribosome affinity purification (TRAP) profiling, electrophysiology, and behavior, we demonstrate that cholinergic neurons, but not peptidergic neurons, of the medial habenula (MHb) display spontaneous tonic firing of 2-10 Hz generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) pacemaker channels and that infusion of the HCN pacemaker antagonist ZD7288 in the habenula precipitates somatic and affective signs of withdrawal. Further, we show that a strong, α3β4-dependent increase in firing frequency is observed in these pacemaker neurons upon acute exposure to nicotine. No change in the basal or nicotine-induced firing was observed in cholinergic MHb neurons from mice chronically treated with nicotine. We observe, however, that, during withdrawal, reexposure to nicotine doubles the frequency of pacemaking activity in these neurons. These findings demonstrate that the pacemaking mechanism of cholinergic MHb neurons controls withdrawal, suggesting that the heightened nicotine sensitivity of these neurons during withdrawal may contribute to smoking relapse.

Pharmacokinetic and pharmacodynamics studies of nicotine after oral administration in mice: effects of methoxsalen, a CYP2A5/6 inhibitor

INTRODUCTION

The use of novel oral nicotine delivery devices and compositions for human consumption and for animal research studies has been increasing in the last several years.

METHODS

Studies were undertaken to examine whether the systemic administration of methoxsalen, an inhibitor of human CYP2A6 and mouse CYP2A5, would modulate nicotine pharmacokinetics and pharmacological effects (antinociception in the tail-flick, and hot-plate tests, and hypothermia) in male ICR mouse after acute oral nicotine administration.

RESULTS

Administration of intra peritoneal (ip) methoxsalen significantly increased nicotine's Cmax, prolonged the plasma half-life (fourfold decrease) of nicotine, and increased its area under the curve (AUC) compared with ip vehicle treatment. Methoxsalen pretreatment prolonged the duration of nicotine-induced antinociception and hypothermia (15mg/kg, po) for periods up to 6- and 24-hr postnicotine administration, respectively. Additionally, methoxsalen potentiated nicotine-induced antinociception and hypothermia as evidenced by leftward shifts in nicotine's dose-response curve. Furthermore, this prolongation of nicotine's effects after methoxsalen was associated with a parallel prolongation of nicotine plasma levels in mice. These data strongly suggest that variation in the rates of nicotine metabolic inactivation substantially alter pharmacological effects of nicotine given orally.

CONCLUSION

We have shown that the pharmacological effects of inhibiting nicotine's metabolism after oral administration in mice are profound. Our results suggest that inhibiting nicotine metabolism can be used to dramatically enhance nicotine's bioavailability and its resulting pharmacology, which further supports this inhibitory approach for clinical development of an oral nicotine replacement therapy.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.