Genetics of nicotinic acetylcholine receptors: Relevance to nicotine addiction

The Prenatal Hypoxic Pathology Associated with Maternal Stress Predisposes to Dysregulated Expression of the chrna7 Gene and the Subsequent Development of Nicotine Addiction in Adult Offspring

INTRODUCTION

Previous studies have shown that fetal hypoxia predisposes individuals to develop addictive disorders in adulthood. However, the specific impact of maternal stress, mediated through glucocorticoids and often coexisting with fetal hypoxia, is not yet fully comprehended.

METHODS

To delineate the potential effects of these pathological factors, we designed models of prenatal severe hypoxia (PSH) in conjunction with maternal stress and prenatal intrauterine ischemia (PII). We assessed the suitability of these models for our research objectives by measuring HIF1α levels and evaluating the glucocorticoid neuroendocrine system. To ascertain nicotine dependence, we employed the conditioned place aversion test and the startle response test. To identify the key factor implicated in nicotine addiction associated with PSH, we employed techniques such as Western blot, immunohistochemistry, and correlational analysis between chrna7 and nr3c1 genes across different brain structures.

RESULTS

In adult rats exposed to PSH and PII, we observed increased levels of HIF1α in the hippocampus (HPC). However, the PSH group alone exhibited reduced glucocorticoid receptor levels and disturbed circadian glucocorticoid rhythms. Additionally, they displayed signs of nicotine addiction in the conditioned place aversion and startle response tests. We also observed elevated levels of phosphorylated DARPP-32 protein in the nucleus accumbens (NAc) indicated compromised glutamatergic efferent signaling. Furthermore, there was reduced expression of α7 nAChR, which modulates glutamate release, in the medial prefrontal cortex (PFC) and HPC. Correlation analysis revealed strong associations between chrna7 and nr3c1 expression in both brain structures.

CONCLUSION

Perturbations in the glucocorticoid neuroendocrine system and glucocorticoid-dependent gene expression of chrna7 associated with maternal stress response to hypoxia in prenatal period favor the development of nicotine addiction in adulthood.

Recent Studies on the Development of Nicotine Abuse and Behavioral Changes Induced by Chronic Stress Depending on Gender

Nowadays, stressful situations are an unavoidable element of everyday life. Stressors activate a number of complex mental and physiological reactions in the organism, thus affecting the state of health of an individual. Stress is the main risk factor in the development of mental disorders, such as depression and other disorders developing as a result of addiction. Studies indicate that women are twice as likely as men to develop anxiety, depression and therefore addiction, e.g., to nicotine. Even though the data presented is indicative of significant differences between the sexes in the prevalence of these disorders, the majority of preclinical animal models for investigating stress-induced disorders use predominantly male subjects. However, the recent data indicates that this type of studies has also been launched in female rodents. Therefore, conducting research on both sexes allows for a more accurate understanding and assessment of the impact of stress on stress-induced behavioral, peripheral and molecular changes in the body and brain. In this manuscript we have gathered the data from 41 years (from 1981-2022) on the influence of stress on the development of depression and nicotine addiction in both sexes.

Hedgehog pathway and its inhibitors in chronic obstructive pulmonary disease (COPD)

COPD affects millions of people and is now ranked as the third leading cause of death worldwide. This largely untreatable chronic airway disease results in irreversible destruction of lung architecture. The small lung hypothesis is now supported by epidemiological, physiological and clinical studies. Accordingly, the early and severe COPD phenotype carries the most dreadful prognosis and finds its roots during lung growth. Pathophysiological mechanisms remain poorly understood and implicate individual susceptibility (genetics), a large part of environmental factors (viral infections, tobacco consumption, air pollution) and the combined effects of those triggers on gene expression. Genetic susceptibility is most likely involved as the disease is severe and starts early in life. The latter observation led to the identification of Mendelian inheritance via disease-causing variants of SERPINA1 - known as the basis for alpha-1 anti-trypsin deficiency, and TERT. In the last two decades multiple genome wide association studies (GWAS) identified many single nucleotide polymorphisms (SNPs) associated with COPD. High significance SNPs are located in 4q31 near HHIP which encodes an evolutionarily highly conserved physiological inhibitor of the Hedgehog signaling pathway (HH). HHIP is critical to several in utero developmental lung processes. It is also implicated in homeostasis, injury response, epithelial-mesenchymal transition and tumor resistance to apoptosis. A few studies have reported decreased HHIP RNA and protein levels in human adult COPD lungs. HHIP^+/- murine models led to emphysema. HH pathway inhibitors, such as vismodegib and sonidegib, are already validated in oncology, whereas other drugs have evidenced in vitro effects. Targeting the Hedgehog pathway could lead to a new therapeutic avenue in COPD. In this review, we focused on the early and severe COPD phenotype and the small lung hypothesis by exploring genetic susceptibility traits that are potentially treatable, thus summarizing promising therapeutics for the future.

A systematic review of genetic variation within nicotinic acetylcholine receptor genes and cigarette smoking cessation

BACKGROUND

Nicotine produces its effects by binding to nicotinic acetylcholine receptors (nAChRs). Variants of genes encoding properties of nAChRs are candidates for affecting likelihood of smoking cessation.

METHODS

A systematic review was conducted summarizing evidence of associations between single nucleotide polymorphisms (SNPs) of nAChR genes and smoking cessation. From 24 articles meeting inclusion criteria, summary odds ratios (ORs) for associations between nine SNPs and smoking cessation were calculated from 26 studies (N = 233-29,072) stratified by gene, ancestry, study design, and pharmacotherapy; SNPs in linkage disequilibrium were pooled. Results for a tenth SNP from two GWAS were summarized.

RESULTS

People of European ancestry with minor alleles of CHRNA5 rs16969968 and CHRNA3 rs1051730 had longer time to cessation [HR = 0.90, 95 % CI 0.88 - 0.92 (n = 2 studies)] and lower odds of cessation [OR = 0.88, 95 % CI 0.80 - 0.97 (n = 5 cohort studies), OR = 0.64, 95 % CI 0.45 - 0.90 (n = 4 placebo arms)]. Risk of persistent smoking associated with these alleles was attenuated in smokers receiving nicotine replacement therapy (NRT). Recipients of bupropion alone or with NRT with these alleles had higher, though not statistically significant, odds of cessation. Results for CHRNA5 rs588765 and rs680244 were similar to rs16969968/rs1051730 findings. Evidence was limited for other SNPs.

CONCLUSION

Evidence consistently indicates the minor alleles of four SNPs within CHRNA3 or CHRNA5 are risk alleles for cessation failure. Analysis by pharmacotherapy revealed bupropion may be the most efficacious intervention for people with these alleles.

Biological determinants impact the neurovascular toxicity of nicotine and tobacco smoke: A pharmacokinetic and pharmacodynamics perspective

Accumulating evidence suggests that the detrimental effect of nicotine and tobacco smoke on the central nervous system (CNS) is caused by the neurotoxic role of nicotine on blood-brain barrier (BBB) permeability, nicotinic acetylcholine receptor expression, and the dopaminergic system. The ultimate consequence of these nicotine associated neurotoxicities can lead to cerebrovascular dysfunction, altered behavioral outcomes (hyperactivity and cognitive dysfunction) as well as future drug abuse and addiction. The severity of these detrimental effects can be associated with several biological determinants. Sex and age are two important biological determinants which can affect the pharmacokinetics and pharmacodynamics of several systemically available substances, including nicotine. With regard to sex, the availability of gonadal hormone is impacted by the pregnancy status and menstrual cycle resulting in altered metabolism rate of nicotine. Additionally, the observed lower smoking cessation rate in females compared to males is a consequence of differential effects of sex on pharmacokinetics and pharmacodynamics of nicotine. Similarly, age-dependent alterations in the pharmacokinetics and pharmacodynamics of nicotine have also been observed. One such example is related to severe vulnerability of adolescence towards addiction and long-term behavioral changes which may continue through adulthood. Considering the possible neurotoxic effects of nicotine on the central nervous system and the deterministic role of sex as well as age on these neurotoxic effects of smoking, it has become important to consider sex and age to study nicotine induced neurotoxicity and development of treatment strategies for combating possible harmful effects of nicotine. In the future, understanding the role of sex and age on the neurotoxic actions of nicotine can facilitate the individualization and optimization of treatment(s) to mitigate nicotine induced neurotoxicity as well as smoking cessation therapy. Unfortunately, however, no such comprehensive study is available which has considered both the sex- and age-dependent neurotoxicity of nicotine, as of today. Hence, the overreaching goal of this review article is to analyze and summarize the impact of sex and age on pharmacokinetics and pharmacodynamics of nicotine and possible neurotoxic consequences associated with nicotine in order to emphasize the importance of including these biological factors for such studies.

The Persistent Challenge of Developing Addiction Pharmacotherapies

There are currently effective Food and Drug Administration (FDA)-approved therapies for alcohol, nicotine, and opioid use disorders. This article will review the development of eight compounds used in the treatment of drug addiction with an emphasis on pharmacological mechanisms and the utility of preclinical animal models of addiction in therapeutic development. In contrast to these successes, animal research has identified a number of promising medications for the treatment of psychostimulant use disorder, none of which have proven to be clinically effective. A specific example of an apparently promising pharmacotherapeutic for cocaine that failed clinically will be examined to determine whether this truly represents a challenge to the predictive validity of current models of cocaine addiction. In addition, the development of promising cocaine use disorder therapeutics derived from animal research will be reviewed, with some discussion regarding how preclinical studies might be modified to better inform clinical outcomes.

Nicotine metabolism predicted by CYP2A6 genotypes in relation to smoking cessation: A systematic review

INTRODUCTION

Identifying genetic factors associated with smoking cessation could inform precision cessation interventions. Of major interest is genetic variation in nicotine metabolism, largely predicted by CYP2A6 variations.

METHODS

We conducted a systematic literature review to summarize the population-based evidence of the association between CYP2A6 and smoking cessation.In the 12 studies meeting the inclusion criteria, the known functional metabolic effect of CYP2A6 variants was used to classify nicotine metabolism as normal (>75% metabolic activity), intermediate (50.1 - 75% activity), slow (25 - 50% activity), and poor (<25% activity). Summary odds ratios of smoking cessation were calculated across metabolic groups, stratified by ancestry and whether participants received pharmacotherapy or placebo/no treatment.

RESULTS

Among untreated people of European ancestry (n = 4 studies), those with CYP2A6 reduced metabolism were more likely to quit smoking than those with normal metabolism [Summary OR = 2.05, 95% CI 1.23 - 3.42] and the likelihood of cessation increased as nicotine metabolism decreased. Nicotine replacement therapy attenuated the association at end-of-treatment, while bupropion modified the association such that intermediate/slow metabolizers were less likely to quit than normal metabolizers [Summary OR = 0.86, 95% CI 0.79 - 0.94]. Among untreated Asian people (n = 3 studies), results differed compared to those with European ancestry: those with slow metabolism were less likely to have quit smoking than normal metabolizers [Summary OR = 0.52, 95% CI 0.38 - 0.71]. Evidence for people of African ancestry (n = 1 study) suggested the CYP2A6 association with cessation may differ compared to those of European ancestry.

IMPLICATIONS

Most studies included in this review were of European ancestry populations; these showed slower nicotine metabolism was associated with increased likelihood of smoking cessation in a dose-related manner. Pharmacotherapy appeared to attenuate or modify this association among people of European ancestry, but it is unclear whether the change in the association remains consistent after treatment ceases. This finding has implications for precision medicine cessation interventions. Based on only a few studies of people of Asian or African ancestry, the association between CYP2A6 variants and cessation may differ from that observed among those of European ancestry, but more evidence is needed.

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.

The Role of CaMKII and ERK Signaling in Addiction

Nicotine is the predominant addictive compound of tobacco and causes the acquisition of dependence through its interactions with nicotinic acetylcholine receptors and various neurotransmitter releases in the central nervous system. The Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) play a pivotal role in synaptic plasticity in the hippocampus. CaMKII is involved in long-term potentiation induction, which underlies the consolidation of learning and memory; however, the roles of CaMKII in nicotine and other psychostimulant-induced addiction still require further investigation. This article reviews the molecular mechanisms and crucial roles of CaMKII and ERK in nicotine and other stimulant drug-induced addiction. We also discuss dopamine (DA) receptor signaling involved in nicotine-induced addiction in the brain reward circuitry. In the last section, we introduce the association of polyunsaturated fatty acids and cellular chaperones of fatty acid-binding protein 3 in the context of nicotine-induced addiction in the mouse nucleus accumbens and provide a novel target for the treatment of drug abuse affecting dopaminergic systems.

α-Conotoxin TxID and [S9K]TxID, α3β4 nAChR Antagonists, Attenuate Expression and Reinstatement of Nicotine-Induced Conditioned Place Preference in Mice

Tobacco smoking has become a prominent health problem faced around the world. The α3β4 nicotinic acetylcholine receptor (nAChR) is strongly associated with nicotine reward and withdrawal symptom. α-Conotoxin TxID, cloned from Conus textile, is a strong α3β4 nAChR antagonist, which has weak inhibition activity of α6/α3β4 nAChR. Meanwhile, its analogue [S9K]TxID only inhibits α3β4 nAChR (IC₅₀ = 6.9 nM), and has no inhibitory activity to other nAChRs. The present experiment investigates the effect of α3β4 nAChR antagonists (TxID and [S9K]TxID) on the expression and reinstatement of nicotine-induced conditioned place preference (CPP) and explores the behaviors of acute nicotine in mice. The animal experimental results showed that TxID and [S9K] TxID could inhibit the expression and reinstatement of CPP, respectively. Moreover, both had no effect in acute nicotine experiment and the locomotor activity in mice. Therefore, these findings reveal that the α3β4 nAChR may be a potential target for anti-nicotine addiction treatment. [S9K]TxID, α3β4 nAChR antagonist, exhibit a superior effect for anti-nicotine addiction, which is promising to develop a novel smoking cessation drug.

Approved and emerging smoking cessation treatments for people with schizophrenia spectrum disorders: A narrative review

This review focuses on smoking cessation treatments for people with schizophrenia spectrum disorders. It concludes with comments on the significance of the research and why it constitutes an original contribution. We searched PubMed (National Library of Medicine), and PsycINFO (Ovid) (2006-2020) for studies on schizophrenic disorder (schizophrenia or psychotic or psychosis or severe mental illness) and smoking cessation treatment (smoking cessation treatment or varenicline or tobacco cessation or reduction or bupropion or NRT or behavioral treatment or e-cigarette). Studies found evidence suggesting that pharmacotherapy combined with behavioural therapy for smoking cessation is effective amongst smokers with schizophrenia spectrum disorders, although more long-term research is required. This review summarised and critically reviewed also studies on vaping as a smoking cessation strategy for smokers with schizophrenia spectrum disorders and evidence suggests that they may effective as smoking cessation tool and may be less harmful alternatives to combustible cigarette smoking. Consequently, e-cigarettes could be considered as an applicable instrument for Tobacco Harm Reduction (THR) and smoking cessation. Overall, there are very few studies of e-cigarettes for smoking cessation in patients with schizophrenia and these studies are very small. They have promising results, but more research is needed.

Tobacco smoking, related harm and motivation to quit smoking in people with schizophrenia spectrum disorders

This narrative review focuses on the topic of tobacco smoking amongst people with schizophrenia spectrum disorders. We searched PubMed, PsycInfo and Scopus databases for schizophrenia spectrum disorders and smoking and included articles about the epidemiology of tobacco smoking in people with schizophrenia spectrum disorders, examining the relationship between smoking and mental health. This narrative review describes that a higher prevalence, frequency and impact of both high nicotine dependence and its harmful effects in patients with schizophrenia spectrum disorders compared with those in the general population. Despite several existent theories, the reasons for high smoking rates, the high dependence on nicotine and severity of nicotine withdrawal symptoms are not fully understood. The main aim of this paper is to inform mental health personnel and particularly clinical and health psychologists about the impact and role of tobacco smoking for smokers with schizophrenia spectrum disorders.

An Exome-Wide Association Study Identifies New Susceptibility Loci for Age of Smoking Initiation in African- and European-American Populations

INTRODUCTION

Cigarette smoking is one of the largest causes of preventable death worldwide. This study aimed to identify susceptibility loci for age at smoking initiation (ASI) by performing an exome-wide association analysis.

METHODS

A total of 2510 smokers of either African-American (AA) or European-American (EA) origin were genotyped and analyzed at both the single nucleotide polymorphism (SNP) and gene levels. After removal of those SNPs with a minor allele frequency (<0.01), 48091 and 34933 SNPs for AAs and EAs, respectively, were used to conduct a SNP-based association analysis. Gene-based analyses were then performed for all SNPs examined within each gene. Further, we estimated the proportion of variance explained by all common SNPs included in the analysis.

RESULTS

The strongest signals were detected for SNPs rs17849904 in the pitrilysin metallopeptidase 1 gene (PITRM1) in the AA sample (p = 9.02 × 10-7) and rs34722354 in the discoidin domain of the receptor tyrosine kinase 2 gene (DDR2) in the EA sample (p = 9.74 × 10-7). Both SNPs remained significant after Bonferroni correction for the number of SNPs tested. Subsequently, the gene-based association analysis revealed a significantly associated gene, DHRS7, in the AA sample (p = 5.00 × 10-6), a gene previously implicated in nicotine metabolism.

CONCLUSIONS

Our study revealed two susceptibility loci for age of smoking initiation in the two ethnic samples, with the first being PITRM1 for AA smokers and the second DDR2 for EA smokers. In addition, we found DHRS7 to be a plausible candidate for ASI in the AA sample from our gene-based association analysis.

IMPLICATIONS

PITRM1 and DHRS7 for African-American smokers and DDR2 for European-American smokers are new candidate genes for smoking initiation. These genes represent new additions to smoking initiation, an important but less studied phenotype in nicotine dependence research.

Neurotoxicity of e-cigarettes

It appears that electronic cigarettes (EC) are a less harmful alternative to conventional cigarette (CC) smoking, as they generate substantially lower levels of harmful carcinogens and other toxic compounds. Thus, switching from CC to EC may be beneficial for smokers. However, recent accounts of EC- or vaping-associated lung injury (EVALI) has raised concerns regarding their adverse health effects. Additionally, the increasing popularity of EC among vulnerable populations, such as adolescents and pregnant women, calls for further EC safety evaluation. In this state-of-the-art review, we provide an update on recent findings regarding the neurological effects induced by EC exposure. Moreover, we discuss possible neurotoxic effects of nicotine and numerous other chemicals which are inherent both to e-liquids and EC aerosols. We conclude that in recognizing pertinent issues associated with EC usage, both government and scientific researchers must address this public health issue with utmost urgency.

Randomized Clinical Trials Investigating Innovative Interventions for Smoking Cessation in the Last Decade

Every year, billions of dollars are spent treating smoking and related conditions, yet smoking-related morbidity and mortality continue to rise. There are currently only three FDA-approved medications for smoking cessation: nicotine replacement therapy, bupropion, and varenicline. Although these medications increase abstinence rates, most individuals relapse following treatment. This chapter reviews clinical trials published within the past 10 years investigating novel smoking cessation pharmacotherapies. Among these pharmacotherapies, some showed promising results, such as cytisine and endocannabinoid modulators, whereas others failed to produce significant effects. More research is needed to develop drugs that produce higher rates of long-term abstinence and to determine which subgroups of patients benefit from a given treatment.

Current advances in research in treatment and recovery: Nicotine addiction

The health harms of combusted tobacco use are undeniable. With market and regulatory pressures to reduce the harms of nicotine delivery by combustion, the tobacco product landscape has diversified to include smokeless, heated, and electronic nicotine vaping products. Products of tobacco combustion are the main cause of smoking-induced disease, and nicotine addiction sustains tobacco use. An understanding of the biology and clinical features of nicotine addiction and the conditioning of behavior that occurs via stimuli paired with frequent nicotine dosing, as with a smoked cigarette, is important for informing pharmacologic and behavioral treatment targets. We review current advances in research on nicotine addiction treatment and recovery, with a focus on conventional combustible cigarette use. Our review covers evidence-based methods to treat smoking in adults and policy approaches to prevent nicotine product initiation in youth. In closing, we discuss emerging areas of evidence and consider new directions for advancing the field.

Prevalência de ansiedade e fatores associados em adultos

RESUMO Objetivo Identificar a prevalência de transtornos de ansiedade em uma amostra de base populacional e fatores associados. Métodos Estudo transversal de base populacional realizado com indivíduos entre 18 e 35 anos. As variáveis sociodemográficas, índice de massa corporal, presença de doença crônica, abuso de álcool e tabagismo foram analisadas. Os transtornos de ansiedade foram verificados pela Mini Internacional Neuropsychiatric Interview 5.0. Foi utilizado o teste Qui-quadrado, considerando o intervalo de confiança de 95%. Resultados A amostra foi constituída por 1.953 pessoas. A prevalência de transtornos de ansiedade foi de 27,4%. Agorafobia (17,9%) e transtorno de ansiedade generalizada (14,3%) foram os quadros mais prevalentes. Mulheres apresentaram maior prevalência de ansiedade, com 32,5%, quando comparadas aos homens (21,3%) (p < 0,001). As variáveis sexo, anos de estudo, renda, doença crônica, tabagismo e álcool foram associadas a mais de três transtornos de ansiedade investigados (p < 0,001). Conclusão Os dados demonstram que os transtornos de ansiedade são muito frequentes em adultos, sendo mais prevalentes entre as mulheres. Estão associadas aos transtornos ansiosos, principalmente, as condições socioeconômicas e substâncias licitas. Conhecer as prevalências dos transtornos de ansiedade e fatores associados pode auxiliar profissionais de saúde a elaborarem melhores diagnósticos e tratamentos.

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

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

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.

The role of nicotinic receptor genes (CHRN) in the pathways of prenatal tobacco exposure on smoking behavior among young adult light smokers

BACKGROUND

Prenatal tobacco exposure (PTE) is associated with more frequent smoking among young, light smokers. Little is known about how nicotinic acetylcholine receptor (CHRN) genes may contribute to this relationship.

METHODS

Data were drawn from a longitudinal cohort of young light smokers of European ancestry (N = 511). Three single nucleotide polymorphisms (SNPs) among offspring, rs16969968 and rs6495308 in CHRNA5A3B4 and rs2304297 in CHRNB3A6, were analyzed with respect to whether they 1) predict PTE status; 2) confound the previously-reported effects of PTE on future smoking; 3) have effects on youth smoking frequency that are mediated through PTE; and 4) have effects that are moderated by PTE.

RESULTS

rs2304297 and rs6495308 were associated with increased likelihood and severity of PTE, respectively. In a path analysis, rs16969968 directly predicted more frequent smoking in young adulthood (B = 1.50, p = .044); this association was independent of, and not mediated by, PTE. The risk of rs16969968 (IRR = 1.07, p = .015) and the protective effect of rs2304297 (IRR = 0.84, p < .001) on smoking frequency were not moderated by PTE. PTE moderated the effect of rs6495308, such that these alleles were protective against later smoking frequency only among non-exposed youth (IRR = 0.85, p < .001).

CONCLUSIONS

The association between offspring CHRNB3A6 and PTE is a novel finding. The risk of rs16969968 on youth smoking is independent and unrelated to that of PTE among young, light smokers. PTE moderates the protective effect of rs6495308 on youth smoking frequency. However, PTE's pathway to youth smoking behavior was not explained by these genetic factors, leaving its mechanism(s) of action unclear.

Kynurenine 3-Monooxygenase Gene Associated With Nicotine Initiation and Addiction: Analysis of Novel Regulatory Features at 5' and 3'-Regions

Tobacco smoking is widespread behavior in Qatar and worldwide and is considered one of the major preventable causes of ill health and death. Nicotine is part of tobacco smoke that causes numerous health risks and is incredibly addictive; it binds to the α7 nicotinic acetylcholine receptor (α7nAChR) in the brain. Recent studies showed α7nAChR involvement in the initiation and addiction of smoking. Kynurenic acid (KA), a significant tryptophan metabolite, is an antagonist of α7nAChR. Inhibition of kynurenine 3-monooxygenase enzyme encoded by KMO enhances the KA levels. Modulating KMO gene expression could be a useful tactic for the treatment of tobacco initiation and dependence. Since KMO regulation is still poorly understood, we aimed to investigate the 5' and 3'-regulatory factors of KMO gene to advance our knowledge to modulate KMO gene expression. In this study, bioinformatics methods were used to identify the regulatory sequences associated with expression of KMO. The displayed differential expression of KMO mRNA in the same tissue and different tissues suggested the specific usage of the KMO multiple alternative promoters. Eleven KMO alternative promoters identified at 5'-regulatory region contain TATA-Box, lack CpG Island (CGI) and showed dinucleotide base-stacking energy values specific to transcription factor binding sites (TFBSs). The structural features of regulatory sequences can influence the transcription process and cell type-specific expression. The uncharacterized LOC105373233 locus coding for non-coding RNA (ncRNA) located on the reverse strand in a convergent manner at the 3'-side of KMO locus. The two genes likely expressed by a promoter that lacks TATA-Box harbor CGI and two TFBSs linked to the bidirectional transcription, the NRF1, and ZNF14 motifs. We identified two types of microRNA (miR) in the uncharacterized LOC105373233 ncRNA, which are like hsa-miR-5096 and hsa-miR-1285-3p and can target the miR recognition element (MRE) in the KMO mRNA. Pairwise sequence alignment identified 52 nucleotides sequence hosting MRE in the KMO 3' UTR untranslated region complementary to the ncRNA LOC105373233 sequence. We speculate that the identified miRs can modulate the KMO expression and together with alternative promoters at the 5'-regulatory region of KMO might contribute to the development of novel diagnostic and therapeutic algorithm for tobacco smoking.

Sex differences in the nicotinic acetylcholine and dopamine receptor systems underlying tobacco smoking addiction

Converging lines of evidence suggest that men generally smoke for nicotine-related reinforcement, whereas women smoke for non-nicotine factors. Women have more difficulty quitting smoking and are less responsive to nicotine replacement therapies than men, underscoring the importance of examining sex differences in the neurochemical mechanisms underlying nicotine-motivated behavior. We review the recent imaging literature on sex differences in the nicotinic acetylcholine receptor system and in the dopaminergic system in response to nicotine administration and tobacco smoking. We offer an explanation to unify imaging findings related to the dopamine system. We then propose a course of action for future medication development for tobacco smoking addiction.

Analyzing the genes related to nicotine addiction or schizophrenia via a pathway and network based approach

The prevalence of tobacco use in people with schizophrenia is much higher than in general population, which indicates a close relationship between nicotine addiction and schizophrenia. However, the molecular mechanism underlying the high comorbidity of tobacco smoking and schizophrenia remains largely unclear. In this study, we conducted a pathway and network analysis on the genes potentially associated with nicotine addiction or schizophrenia to reveal the functional feature of these genes and their interactions. Of the 276 genes associated with nicotine addiction and 331 genes associated with schizophrenia, 52 genes were shared. From these genes, 12 significantly enriched pathways associated with both diseases were identified. These pathways included those related to synapse function and signaling transduction, and drug addiction. Further, we constructed a nicotine addiction-specific and schizophrenia-specific sub-network, identifying 11 novel candidate genes potentially associated with the two diseases. Finally, we built a schematic molecular network for nicotine addiction and schizophrenia based on the results of pathway and network analysis, providing a systematic view to understand the relationship between these two disorders. Our results illustrated that the biological processes underlying the comorbidity of nicotine addiction and schizophrenia was complex, and was likely induced by the dysfunction of multiple molecules and pathways.

Nicotinic Receptor Abnormalities in the Cerebellar Cortex of Sudden Unexplained Fetal and Infant Death Victims-Possible Correlation With Maternal Smoking

Nicotinic acetylcholine receptors (nAChRs) are cationic channels of the neuronal cell membrane, differentially expressed in the central nervous system which, when activated by endogenous acetylcholine or exogenous nicotine, are able to enhance cholinergic transmission. The aim of this study was to investigate in human perinatal age the immunohistochemical expression of the α7-nAChR subtype, given its involvement in neuronal differentiation and its significant vulnerability to the toxic effects of nicotine. Thirty fetuses (with a gestational age between 25 and 40 weeks) and 35 infants (1-6 months old), suddenly died of known (controls) and unknown causes (unexplained deaths), with smoking and nonsmoking mothers, were included in this study. A negative or low immunoexpression of α7-nAChRs, indicative of their inactivation, was observed in the granular layers of the cerebellar cortex in 66% of the sudden unexplained perinatal deaths and 11% of the controls. A high correlation was also observed between these findings and maternal smoking. Apart from the well-known adverse effects of nicotine exposure during pregnancy, it may also cause significant alterations in cerebellar cholinergic transmission in areas of the brain involved in vital functions. These events may give us insights into the pathogenetic mechanisms leading to sudden unexplained fetal and infant death.

Classification Tree Analysis as a Method for Uncovering Relations Between CHRNA5A3B4 and CHRNB3A6 in Predicting Smoking Progression in Adolescent Smokers

INTRODUCTION

Prior research suggests the CHRNA5A3B4 and CHRNB3A6 gene clusters have independent effects on smoking progression in young smokers. Here classification tree analysis uncovers conditional relations between these genes.

METHODS

Conditional classification tree and random forest analyses were employed to predict daily smoking at 6-year follow-up in a longitudinal sample of young smokers (N = 480) who had smoked at least one puff at baseline and were of European ancestry. Potential predictors included gender, lifetime smoking, Nicotine Dependence Syndrome Scale (NDSS), and five single nucleotide polymorphisms (SNPs) tagging CHRNB3A6 and CHRNA5A3B4 Haplotypes A, B, and C. Conditional random forest analysis was used to calculate variable importance.

RESULTS

The classification tree identified NDSS, the CHRNB3A6 SNP rs2304297, and the CHRNA5A3B4 Haplotype C SNP rs6495308 as predictive of year 6 daily smoking with the baseline NDSS identified as the strongest predictor. The CHRNB3A6 protective effect was contingent on a lower level of baseline NDSS, whereas the CHRNA5A3B4 Haplotype C protective effect was seen at a higher level of baseline NDSS. A CHRNA5A3B4 Haplotype C protective effect also was observed in participants with low baseline NDSS who had no CHRNB3A6 rs2304297 minor allele.

CONCLUSIONS

The protective effects of CHRNA5A3B4 Haplotype C and CHRNB3A6 on smoking progression are conditional on different levels of baseline cigarette use. Also, duplicate dominant epistasis between SNPs indicated the minor allele of either SNP afforded comparable protective effects in the absence of a minor allele at the other locus. Possible mechanisms underlying these conditional relations are discussed.

IMPLICATIONS

The substantive contributions of this paper are the demonstration of a difference in the protective effects of CHRNB3A6 and CHRNA5A3B4 Haplotype C in young smokers attributable to level of cigarette use, as well as observation of duplicate dominant epistasis between the two markers. The methodological contribution is demonstrating that classification tree and random forest statistical methods can uncover conditional relations among genetic effects not detected with more common regression methods.

Different Hypothalamic Nicotinic α7 Receptor Expression and Response to Low Nicotine Dose in Alcohol-Preferring and Alcohol-Avoiding Rats

BACKGROUND

The aim of this study was to examine possible differences in nicotinic acetylcholine receptors and responses in rats with genetic preference or avoidance for alcohol. This was done by using 2 rat lines with high alcohol preference (Alko Alcohol [AA]) or alcohol avoidance (Alko Non-Alcohol [ANA]).

METHODS

Locomotor activity was measured following nicotine and histamine H3 receptor (H3R) antagonist treatment. In situ hybridization and receptor ligand binding experiments were used in drug-naïve animals to examine the expression of different α nicotinic receptor subunits.

RESULTS

The AA rats were found to be more sensitive to the stimulatory effect of a low dose of nicotine than ANA rats, which were not significantly activated. Combination of histamine H3R antagonist, JNJ-39220675, and nicotine resulted to similar locomotor activation as nicotine alone. To further understand the mechanism underlying the difference in nicotine response in AA and ANA rats, we studied the expression of α5, α6, and α7 nicotinic receptor subunits in specific brain areas of AA and ANA rats. We found no differences in the expression of α5 nicotinic receptor subunits in the medial habenula and hippocampus or in α6 subunit in the ventral tegmental area and substantia nigra. However, the level of α7 nicotinic receptor subunit mRNA was significantly lower in the tuberomamillary nucleus of posterior hypothalamus of alcohol-preferring AA rats than in alcohol-avoiding ANA rats. Also the hypothalamic [125I-α-bungarotoxin binding was lower in AA rats indicating lower levels of α7 nicotinic receptors.

CONCLUSIONS

The lower expression and receptor binding of α7 nicotinic receptors in the tuberomamillary nucleus of AA rats suggest a difference in the regulation of brain histamine neurons between the rat lines since the α7 nicotinic receptors are located in histaminergic neurons. Stronger nicotine-induced locomotor response, mediated partially via α7 receptors, and previously described high alcohol consumption in AA rats could be explained by the found difference in tuberomamillary α7 receptor levels.

Recent Advances in Nicotinic Receptor Signaling in Alcohol Abuse and Alcoholism

Alcohol is the most commonly abused legal substance and alcoholism is a serious public health problem. It is a leading cause of preventable death in the world. The cellular and molecular mechanisms of alcohol reward and addiction are still not well understood. Emerging evidence indicates that unlike other drugs of abuse, such as nicotine, cocaine, or opioids, alcohol targets numerous channel proteins, receptor molecules, and signaling pathways in the brain. Previously, research has identified brain nicotinic acetylcholine receptors (nAChRs), a heterogeneous family of pentameric ligand-gated cation channels expressed in the mammalian brain, as critical molecular targets for alcohol abuse and dependence. Genetic variations encoding nAChR subunits have been shown to increase the vulnerability to develop alcohol dependence. Here, we review recent insights into the rewarding effects of alcohol, as they pertain to different nAChR subtypes, associated signaling molecules, and pathways that contribute to the molecular mechanisms of alcoholism and/or comorbid brain disorders. Understanding these cellular changes and molecular underpinnings may be useful for the advancement of brain nicotinic-cholinergic mechanisms, and will lead to a better translational and therapeutic outcome for alcoholism and/or comorbid conditions.

Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function

Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

Nicotinic receptor modulation to treat alcohol and drug dependence

Alcohol and drug dependence are serious public health problems worldwide. The prevalence of alcohol and drug dependence in the United States and other parts of the world is significant. Given the limitations in the efficacy of current pharmacotherapies to treat these disorders, research in developing alternative pharmacotherapies continues. Preclinical and clinical evidence thus far has indicated that brain nicotinic acetylcholine receptors (nAChRs) are important pharmacological targets for the development of medications to treat alcohol and drug dependence. The nAChRs are a super family of ligand gated ion channels, and are expressed throughout the brain with twelve neuronal nAChR subunits (α2–α10 and β2–β4) identified. Here, we review preclinical and clinical evidence involving a number of nAChR ligands that target different nAChR subtypes in alcohol and nicotine addiction. The important ligands include cytisine, lobeline, mecamylamine, varenicline, sazetidine A and others that target α4β2^* nAChR subtypes as small molecule modulators of the brain nicotinic cholinergic system are also discussed. Taken together, both preclinical and clinical data exist that support nAChR–based ligands as promising therapeutic agents for the treatment of alcohol and drug dependence.

Drug-dependent behaviors and nicotinic acetylcholine receptor expressions in Caenorhabditis elegans following chronic nicotine exposure

Nicotine, the major psychoactive compound in tobacco, targets nicotinic acetylcholine receptors (nAChRs) and results in drug dependence. The nematode Caenorhabditis elegans' (C. elegans) genome encodes conserved and extensive nicotinic receptor subunits, representing a useful system to investigate nicotine-induced nAChR expressions in the context of drug dependence. However, the in vivo expression pattern of nAChR genes under chronic nicotine exposure has not been fully investigated. To define the role of nAChR genes involved in nicotine-induced locomotion changes and the development of tolerance to these effects, we characterized the locomotion behavior combining the use of two systems: the Worm Tracker hardware and the WormLab software. Our results indicate that the combined system is an advantageous alternative to define drug-dependent locomotion behavior in C. elegans. Chronic (24-h dosing) nicotine exposure at 6.17 and 61.7μM induced nicotine-dependent behaviors, including drug stimulation, tolerance/adaption, and withdrawal responses. Specifically, the movement speed of naïve worms on nicotine-containing environments was significantly higher than on nicotine-free environments, suggesting locomotion stimulation by nicotine. In contrast, the 24-h 6.17μM nicotine-treated worms exhibited significantly higher speeds on nicotine-free plates than on nicotine-containing plates. Furthermore significantly increased locomotion behavior during nicotine cessation was observed in worms treated with a higher nicotine concentration of 61.7μM. The relatively low locomotion speed of nicotine-treated worms on nicotine-containing environments also indicates adaption/tolerance of worms to nicotine following chronic nicotine exposure. In addition, this study provides useful information regarding the comprehensive in vivo expression profile of the 28 "core" nAChRs following different dosages of chronic nicotine treatments. Eleven genes (lev-1, acr-6, acr-7, acr-11, lev-8, acr-14, acr-16, acr-20, acr-21, ric-3, and unc-29) were significantly up-regulated following 61.7μM nicotine treatment, in which worms showed significantly increased locomotion behavior. This study provides insights into the linkage between nicotine-induced locomotion behavior and the regulation of nicotinic acetylcholine receptors.

The role of the M4 lipid-sensor in the folding, trafficking, and allosteric modulation of nicotinic acetylcholine receptors

With the availability of high resolution structural data, increasing attention has focused on the mechanisms by which drugs and endogenous compounds allosterically modulate nicotinic acetylcholine receptor (nAChR) function. Lipids are potent modulators of the nAChR from Torpedo. Membrane lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations, as well as influencing the transitions between these conformational states. Structural and functional data highlight a role for the lipid-exposed M4 transmembrane α-helix of each subunit in lipid sensing, and suggest that lipids influence gating by altering the binding of M4 to the adjacent transmembrane α-helices, M1 and M3. M4 has also been implicated in both the folding and trafficking of nAChRs to the cell surface, as well as in the potentiation of nAChR gating by neurosteroids. Here, we discuss the roles of M4 in the folding, trafficking, and allosteric modulation of nAChRs. We also consider the hypothesis that variable chemistry at the M4-M1/M3 transmembrane α-helical interface in different nAChR subunits governs the capacity for potentiation by activating lipids. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Genetic background influences nicotine-induced conditioned place preference and place aversion in mice

This study was designed to determine whether genetic differences influence the rewarding effects of nicotine in 4 inbred strains of mice (DBA/2, BALB/c, C3H, and C57BL/6). Nicotine (subcutaneous) induced a place preference in DBA/2 and BALB/c mice but a place aversion in C57BL/6 mice. A low dose of nicotine produced a significant place preference, whereas a high dose of nicotine produced place aversion in C3H mice. These effects were completely reversed by the nicotinic receptor antagonist mecamylamine. These results strongly suggest that a conditioned state, such as rewarding effects or aversive effects, can be influenced by genetic background.

Association of the CHRNA4 neuronal nicotinic receptor subunit gene with frequency of binge drinking in young adults

Background

Binge drinking is responsible for over half of all alcohol‐related deaths and results in significant health and economic costs to individuals and society. Knowledge of genetic aspects of this behavior, particularly as it emerges in young adulthood, could lead to improved treatment and prevention programs.

Methods

We have focused on the association of variation in neuronal nicotinic receptor subunit genes (CHRNs) in a cohort of 702 Hispanic and non‐Hispanic White young adults who are part of the Social and Emotional Contexts of Adolescent Smoking Patterns (SECASP) study. Fifty‐five single nucleotide polymorphisms (SNPs) covering the variation in 5 CHRNs (CHRNA4,CHRNB2,CHRNA2, CHRNB3A6, and CHRNA5A3B4) were studied.

Results

Frequency of binge drinking and other correlated alcohol consumption measures were significantly associated with SNPs in CHRNA4 (p‐values ranged from 0.0003 to 0.02), but not with SNPs in other CHRNs. This association was independent of smoking status in our cohort.

Conclusions

Variants in CHRNA4 may contribute to risk of binge drinking in young adults in this cohort. Results will need to be confirmed in independent samples.

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

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

The association between CHRN genetic variants and dizziness at first inhalation of cigarette smoke

Numerous single nucleotide polymorphisms (SNPs) in multiple nicotinic receptor genes (CHRN) are associated with smoking. However few studies have examined the association between CHRN SNPs and subjective responses to smoking in adolescents which may relate to sustained smoking, such as dizziness at first inhalation. The objective of this study was to investigate the association between 61 SNPs in eight CHRN genes (CHRNA3, CHRNA4, CHRNA5, CHRNA6, CHRNA7, CHRNB2, CHRNB3, CHRNB4) and dizziness at first inhalation. Data were available from a longitudinal cohort investigation of 1293 students 12-13year-old at baseline. Students completed self-report questionnaires at school every 3months for 5years during secondary school, and a mailed questionnaire three years later. DNA extracted from blood or saliva was genotyped for 61 CHRN SNPs selected using a gene tagging approach. Associations were modeled using logistic regression controlling for sex, race and age at first cigarette. Complete data were available for 356 of 475 participants (75%) who initiated smoking. The minor alleles of three SNPs in CHRNA6 (rs7812298, rs2304297, rs7828365) were associated with a decreased probability of dizziness (OR(95% CI)=0.54 (0.36, 0.81), 0.59 (0.40, 0.86) and 0.58 (0.36, 0.95), respectively), while one SNP in each of three other genes (rs3743077 (CHRNA3), rs755204 (CHRNA4), rs7178176 (CHRNA7)) was associated with an increased probability of dizziness (OR(95% CI)=1.40 (1.02, 1.90), 1.85 (1.05, 3.27) and 1.51 (1.06, 2.15), respectively). Thus, several SNPs located in CHRN genes are associated with dizziness at first inhalation, a smoking initiation phenotype that may relate to sustained smoking.

Positive allosteric modulation of α4β2 nicotinic acetylcholine receptors as a new approach to smoking reduction: evidence from a rat model of nicotine self-administration

RATIONALE

The α4β2 subtype of nicotinic acetylcholine receptors (nAChRs) plays a central role in the mediation of nicotine reinforcement. Positive allosteric modulators (PAMs) at α4β2 nAChRs facilitate the intrinsic efficiency of these receptors, although they do not directly activate the receptors. α4β2 PAMs are hypothesized to reduce nicotine self-administration in subjects engaged in routine nicotine consumption. The present study tested this hypothesis using a rat model of nicotine self-administration.

METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to intravenously self-administer nicotine (0.03 mg/kg per infusion, free base) on a fixed-ratio 5 schedule. The effects of the α4β2 PAM desformylflustrabromine (dFBr), α4β2 agonist 5-iodo-A-85380, and acetylcholinesterase inhibitor galantamine on nicotine intake were examined. The ability of dFBr and 5-iodo-A-85380 to substitute for nicotine was also assessed.

RESULTS

dFBr and 5-iodo-A-85380 dose-dependently reduced nicotine self-administration without changing lever responses for food. Galantamine decreased the self-administration of nicotine and food at high doses. Unlike 5-iodo-A-85380, dFBr failed to substitute for nicotine in supporting self-administration behavior.

CONCLUSIONS

These results demonstrated the effectiveness of dFBr in reducing nicotine intake and the inability of dFBr to support self-administration behavior. These findings suggest that positive allosteric modulation of α4β2 nAChRs may be a promising target for the treatment of nicotine addiction. Moreover, α4β2 PAMs, in contrast to agonist medications, may have clinical advantages because they may have little liability for abuse because of their lack of reinforcing actions on their own.

Effect of neuronal nicotinic acetylcholine receptor genes (CHRN) on longitudinal cigarettes per day in adolescents and young adults

INTRODUCTION

Few studies have sought to identify specific genetic markers associated with cigarettes per day (CPD) during adolescence and young adulthood, the period of greatest vulnerability for the development of nicotine dependence.

METHODS

We used a longitudinal design to investigate the effect of neuronal nicotinic acetylcholine receptor (CHRN) subunit genes on CPD from 15 to 21 years of age in young smokers of European descent (N = 439, 59% female). The number of CPD typically smoked during the previous 30 days was self-reported. Single nucleotide polymorphisms (SNPs) from CHRN genes were genotyped using DNA extracted from saliva samples collected at the 5-year assessment. Mixed-model analyses of SNP effects were computed across age at the time of assessment using log-transformed CPD as the phenotype. Data from the 1000 Genomes Project were used to clarify the architecture of CHRN genes to inform SNP selection and interpretation of results.

RESULTS

CPD was associated with a CHRNB3A6 region tagged by rs2304297, with CHRNA5A3B4 haplotype C (tagged by rs569207), and with the CHRNA2 SNP rs2271920, ps < .004. The reliability of single-SNP associations was supported by the correspondence between a more extensive set of SNP signals and the underlying genetic architecture. The 3 signals identified in this study appear to make independent contributions to CPD, and their combined effect accounts for 5.5% of the variance in log-transformed CPD.

CONCLUSIONS

Level of CPD during adolescence and young adulthood is associated with CHRNB3A6, CHRNA5A3B4, and CHRNA2.

Nicotinic acetylcholine receptors controlling attention: behavior, circuits and sensitivity to disruption by nicotine

Attention is a central cognitive function that enables long-term engagement in a task and suppression of irrelevant information to obtain future goals. The prefrontal cortex (PFC) is the main link in integrating emotional and motivational state of an animal to regulate top-down attentional processes. Acetylcholine modulates PFC neuronal networks by activating nicotinic acetylcholine receptors (nAChRs) to support attention. However, how neuronal activity changes in the PFC during attention and which nAChR subtypes mediate this is only rudimentarily understood, but progress is being made. Recently, exciting new insights were obtained in the dynamics of cholinergic signaling in the PFC and modes of acetylcholine transmission via nAChRs in the cortex. In addition, mechanisms are uncovered on how the PFC circuitry is regulated by nAChRs. Novel studies show that endogenous activation of nAChRs in the PFC plays a central role in controlling attention. Here, we review current insights into how different subtypes of nAChRs expressed by distinct types of neurons in the PFC circuitry shape attention. In addition we discuss the impact of nicotine on the cholinergic system and prefrontal cortical circuits. Low concentrations of nicotine, as experienced by smokers, interfere with cholinergic signaling. In the long-term exposure to nicotine during adolescence leads to maladaptive adaptations of the PFC circuitry, which ultimately leads to a decrement in attention performance, again emphasizing the importance of nAChRs in attention.

Rational development of addiction pharmacotherapies: successes, failures, and prospects

There are currently effective, U.S. Food and Drug Administration (FDA)-approved therapies for alcohol, nicotine, and opioid addiction. In some cases these therapeutics were rationally designed and tested using a combination of various animal models of addiction. In many cases, however, effective drug therapies for addiction were derived from the testing of compounds developed for other CNS disorders (e.g., analgesics and antidepressants), which were tested clinically in the absence of prior animal research using addiction models. This article will review the development of eight compounds that are currently most effective in the treatment of alcohol, opioid, and nicotine addiction with an emphasis on pharmacological mechanisms as well as the utility of animal models of addiction in the development of these therapeutics. In contrast to these successes, animal research has identified a number of promising medications for the treatment of psychostimulant addiction, none of which have proven to be effective clinically. This raises questions about the validity of current animal models of psychostimulant addiction. A specific example of an apparently promising pharmacotherapeutic for cocaine addiction (the D1 dopamine receptor antagonist ecopipam) that failed clinically will be examined to determine if this truly represents a challenge to the predictive validity of current models of cocaine addiction. In addition, the development of promising cocaine addiction therapeutics derived from animal research will be reviewed.

Effect of nicotine pretreatment on arsenic-induced oxidative stress in male Wistar rats

Humans are commonly exposed to nicotine, one of the most important lifestyle chemicals. The occurrence of high levels of arsenic in the groundwater of the southeast region of Asia has received much attention in the past decade and has become a global health concern. Predominant occurrence of both these chemicals and ease of their human exposure led us to investigate the effect of nicotine, a major tobacco alkaloid, on arsenic toxicity. Adult male rats were pre-exposed to two different doses of nicotine (0.75 and 3 mg/kg, intraperitoneally) for 7 days followed by 30 days of arsenic exposure (50 ppm sodium arsenite in drinking water). Nicotine pre-exposure resulted in an increased brain arsenic accumulation and a decreased liver arsenic concentration. Arsenic also caused a significant oxidative stress in the blood, brain and liver of the exposed rats. Glutathione- S-transferase, a phase II enzyme, was inhibited by both arsenic and nicotine but no such inhibition was noted in arsenic-treated animals pre-exposed to nicotine. Upon nicotine pre-exposure, brain acetylcholinesterase increased, while monoamine oxidase (MAO) decreased. The toxic effects of MAO significantly attenuated with nicotine pre-exposure. The present study suggests that nicotine may not be the major contributing factor for the previously reported synergistic toxic interaction between tobacco and arsenic. Nicotine pre-exposure in arsenic-exposed animals revealed interesting toxicokinetics and oxidative stress modulating interactions in the brain and liver of rats, which requires further exploration.

Why have tobacco control policies stalled? Using genetic moderation to examine policy impacts

BACKGROUND

Research has shown that tobacco control policies have helped produce the dramatic decline in use over the decades following the 1964 surgeon general's report. However, prevalence rates have stagnated during the past two decades in the US, even with large tobacco taxes and expansions of clean air laws. The observed differences in tobacco control policy effectiveness and why policies do not help all smokers are largely unexplained.

OBJECTIVE

The aim of this study was to determine the importance of genetics in explaining response to tobacco taxation policy by testing the potential of gene-policy interaction in determining adult tobacco use.

METHODS

A moderated regression analysis framework was used to test interactive effects between genotype and tobacco policy in predicting tobacco use. Cross sectional data of US adults from the National Health and Nutrition Examination Survey (NHANES) linked with genotype and geocodes were used to identify tobacco use phenotypes, state-level taxation rates, and variation in the nicotinic acetylcholine receptor (CHRNA6) genotype. Tobacco use phenotypes included current use, number of cigarettes smoked per day, and blood serum cotinine measurements.

RESULTS

Variation in the nicotinic acetylcholine receptor was found to moderate the influence of tobacco taxation on multiple measures of tobacco use. Individuals with the protective G/G polymorphism (51% of the sample) responded to taxation while others had no response. The estimated differences in response by genotype were C/C genotype: b = -0.016 se = 0.018; G/C genotype: b = 0.014 se = 0.017; G/G genotype: b = -0.071 se 0.029.

CONCLUSIONS

This study provides novel evidence of "gene-policy" interaction and suggests a genetic mechanism for the large differences in response to tobacco policies. The inability for these policies to reduce use for individuals with specific genotypes suggests alternative methods may be needed to further reduce use.

Short- and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function

More than 70% of adolescents report to have smoked a cigarette at least once. At the adolescent stage the brain has not completed its maturation. The prefrontal cortex (PFC), the brain area responsible for executive functions and attention performance, is one of the last brain areas to mature and is still developing during adolescence. Smoking during adolescence increases the risk of developing psychiatric disorders and cognitive impairment in later life. In addition, adolescent smokers suffer from attention deficits, which aggravate with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and that underlie the lasting effects on cognitive function. Here we provide an overview of these recent findings.

Insights into the neurobiology of the nicotinic cholinergic system and nicotine addiction from mice expressing nicotinic receptors harboring gain-of-function mutations

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated, cation-selective ion channels expressed throughout the brain. Although these channels have been investigated for several decades, it is still challenging 1) to identify the important nAChR subunits in cholinergic transmission and nicotine dependence and 2) to develop nAChR subtype-specific ligands. To overcome these challenges, we and others have studied mice expressing mutant, gain-of-function nAChR subunits. In this review, we discuss this research approach and the results it has yielded to date. Gain-of-function mutations, including those in nAChR subunits, provide an approach that is complementary to loss-of-function studies such as gene knockouts; the former allows one to answer questions of sufficiency and the latter addresses questions of necessity. Mutant mice expressing gain-of-function nAChR subunits are commonly produced using traditional gene targeting in embryonic stem cells, but novel approaches such as bacterial artificial chromosome transgenesis have yielded important insights as well. α7 nAChRs were the first nAChRs to be targeted with a gain-of-function mutation, followed by a pair of α4 nAChR gain-of-function mutant mice. These α4 nAChR gain-of-function mice (α4 L9'S mice, followed by α4 L9'A mice) provided an important system to probe α4 nAChR function in vivo, particularly in the dopamine reward system. α6 nAChR gain-of-function mice provided the first robust system allowing specific manipulation of this receptor subtype. Other targeted mutations in various nAChR subunits have also been produced and have yielded important insights into nicotinic cholinergic biology. As nAChR research advances and more details associated with nAChR expression and function emerge, we expect that existing and new mouse lines expressing gain-of-function nAChR subunits will continue to provide new insights.

Nicotine exposure during adolescence alters the rules for prefrontal cortical synaptic plasticity during adulthood

The majority of adolescents report to have smoked a cigarette at least once. Adolescence is a critical period of brain development during which maturation of areas involved in cognitive functioning, such as the medial prefrontal cortex (mPFC), is still ongoing. Tobacco smoking during this age may compromise the normal course of prefrontal development and lead to cognitive impairments in later life. In addition, adolescent smokers suffer from attention deficits, which progress with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and underlie the lasting effects on cognitive function. In particular, the expression and function of metabotropic glutamate receptors (mGluRs) are changed and this has an impact on short- and long-term plasticity of glutamatergic synapses in the PFC and ultimately on the attention performance. Here, we review and discuss these recent findings.

Mice lacking the β4 subunit of the nicotinic acetylcholine receptor show memory deficits, altered anxiety- and depression-like behavior, and diminished nicotine-induced analgesia

RATIONALE

The role of β4-containing nicotinic acetylcholine receptors (nAChRs) in cognition, anxiety, depression, and analgesia in the absence of nicotine is unclear.

METHODS

Wild-type (β4(+/+)) and knockout (β4(-/-)) mice for the nAChR β4 subunit were tested in behavioral tests assessing cognitive function, affective behaviors, and nociception.

RESULTS

There were no learning and memory deficits in β4(-/-) mice compared with β4(+/+) mice during the acquisition of the Barnes maze, contextual fear conditioning, and Y maze tasks. In the Barnes maze memory retention test, male β4(-/-) mice showed reduced use of the spatial search strategy, indicating small spatial memory deficits compared with β4(+/+) mice. In the cue-induced fear conditioning memory retention test, β4(-/-) mice exhibited reduced freezing time compared with β4(+/+) mice. Compared with β4(+/+) mice, β4(-/-) mice exhibited decreased anxiety-like behavior in the light-dark box. Depression-like behavior in β4(-/-) mice was decreased in the tail suspension test and increased in the forced swim test compared with β4(+/+) mice. β4(-/-) mice did not differ from β4(+/+) mice in basal nociception but were less sensitive to the antinociceptive effect of nicotine in 2 tests of acute thermal pain.

CONCLUSIONS

Lack of β4-containing nAChRs resulted in small deficits in hippocampus- and amygdala-dependent memory retention functions. β4-containing nAChRs are involved in anxiety- and depression-like behaviors and contribute to the analgesic effects of nicotine.