Nicotinic regulation of microglia: potential contributions to addiction

Clinical and preclinical studies have identified immunosuppressive effects of nicotine, with potential implications for treating nicotine addiction. Here we review how nicotine can regulate microglia, the resident macrophages in the brain, and corresponding effects of nicotine on neuroimmune signaling. There is significant evidence that activation of α7 nicotinic acetylcholine receptors (nAChRs) on microglia can trigger an anti-inflammatory cascade that alters microglial polarization and activity, cytokine release, and intracellular calcium concentrations, leading to neuroprotection. These anti-inflammatory effects of nicotine-dependent α7 nAChR signaling are lost during withdrawal, suggesting that neuroimmune signaling is potentiated during abstinence, and thus, heightened microglial activity may drive circuit disruption that contributes to withdrawal symptoms and hyperkatifeia. In sum, the clinical literature has highlighted immunomodulatory effects of nicotine and the potential for anti-inflammatory compounds to treat addiction. The preclinical literature investigating the underlying mechanisms points to a role of microglial engagement in the circuit dysregulation and behavioral changes that occur during nicotine addiction and withdrawal, driven, at least in part, by activation of α7 nAChRs on microglia. Specifically targeting microglial signaling may help alleviate withdrawal symptoms in people with nicotine dependence and help to promote abstinence.

Nicotine use disorder and Neuregulin 3: Opportunities for precision medicine

Nicotine use disorder remains a major public health emergency despite years of trumpeting the consequences of smoking. This is likely due to the complex interplay of genetics and nicotine exposure across the lifespan of these individuals. Genetics influence all aspects of life, including complex disorders such as nicotine use disorder. This review first highlights the critical neurocircuitry underlying nicotine dependence and withdrawal, and then describes the cellular signaling mechanisms involved. Finally, current genetic, genomic, and transcriptomic evidence for new drug development of smoking cessation aids is discussed, with a focus on the Neuregulin 3 Signaling Pathway.

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.

Evaluation of an Image-Derived Input Function for Kinetic Modeling of Nicotinic Acetylcholine Receptor-Binding PET Ligands in Mice

Positron emission tomography (PET) radioligands that bind with high-affinity to α4β2-type nicotinic receptors (α4β2Rs) allow for in vivo investigations of the mechanisms underlying nicotine addiction and smoking cessation. Here, we investigate the use of an image-derived arterial input function and the cerebellum for kinetic analysis of radioligand binding in mice. Two radioligands were explored: 2-[18F]FA85380 (2-FA), displaying similar pKa and binding affinity to the smoking cessation drug varenicline (Chantix), and [18F]Nifene, displaying similar pKa and binding affinity to nicotine. Time-activity curves of the left ventricle of the heart displayed similar distribution across wild type mice, mice lacking the β2-subunit for ligand binding, and acute nicotine-treated mice, whereas reference tissue binding displayed high variation between groups. Binding potential estimated from a two-tissue compartment model fit of the data with the image-derived input function were higher than estimates from reference tissue-based estimations. Rate constants of radioligand dissociation were very slow for 2-FA and very fast for Nifene. We conclude that using an image-derived input function for kinetic modeling of nicotinic PET ligands provides suitable results compared to reference tissue-based methods and that the chemical properties of 2-FA and Nifene are suitable to study receptor response to nicotine addiction and smoking cessation therapies.

Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by In Vitro Cellular Studies and In Vivo PET Imaging

A question relevant to nicotine addiction is how nicotine and other nicotinic receptor membrane-permeant ligands, such as the anti-smoking drug varenicline (Chantix), distribute in brain. Ligands, like varenicline, with high pKa and high affinity for α4β2-type nicotinic receptors (α4β2Rs) are trapped in intracellular acidic vesicles containing α4β2Rs in vitro Nicotine, with lower pKa and α4β2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligands in vivo in male and female mouse brain and identifying the trapping brain organelle in vitro as Golgi satellites (GSats). Two PET 18F-labeled imaging ligands were chosen: [18F]2-FA85380 (2-FA) with varenicline-like pKa and affinity and [18F]Nifene with nicotine-like pKa and affinity. [18F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in α4β2R-containing GSats. In contrast, [18F]Nifene kinetics were rapid, consistent with its binding to α4β2Rs but no trapping. Specific [18F]2-FA and [18F]Nifene signals were eliminated in β2 subunit knock-out (KO) mice or by acute nicotine (AN) injections demonstrating binding to sites on β2-containing receptors. Chloroquine (CQ), which dissipates GSat pH gradients, reduced [18F]2-FA distributions while having little effect on [18F]Nifene distributions in vivo consistent with only [18F]2-FA trapping in GSats. These results are further supported by in vitro findings where dissipation of GSat pH gradients blocks 2-FA trapping in GSats without affecting Nifene. By combining in vitro and in vivo imaging, we mapped both the brain-wide and subcellular distributions of weak-base nicotinic receptor ligands. We conclude that ligands, such as varenicline, are trapped in neurons in α4β2R-containing GSats, which results in very slow release long after nicotine is gone after smoking.SIGNIFICANCE STATEMENT Mechanisms of nicotine addiction remain poorly understood. An earlier study using in vitro methods found that the anti-smoking nicotinic ligand, varenicline (Chantix) was trapped in α4β2R-containing acidic vesicles. Using a fluorescent-labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were α4β2R-containing Golgi satellites (GSats). In vivo PET imaging with F-18-labeled nicotinic ligands provided additional evidence that differences in PET ligand trapping in acidic vesicles were the cause of differences in PET ligand kinetics and subcellular distributions. These findings combining in vitro and in vivo imaging revealed new mechanistic insights into the kinetics of weak base PET imaging ligands and the subcellular mechanisms underlying nicotine addiction.

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

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

New Insights in the Involvement of the Endocannabinoid System and Natural Cannabinoids in Nicotine Dependence

Nicotine, the main psychoactive component in tobacco smoke, plays a major role in tobacco addiction, producing a high morbidity and mortality in the world. A great amount of research has been developed to elucidate the neural pathways and neurotransmitter systems involved in such a complex addictive behavior. The endocannabinoid system, which has been reported to participate in the addictive properties of most of the prototypical drugs of abuse, is also implicated in nicotine dependence. This review summarizes and updates the main behavioral and biochemical data involving the endocannabinoid system in the rewarding properties of nicotine as well as in nicotine withdrawal and relapse to nicotine-seeking behavior. Promising results from preclinical studies suggest that manipulation of the endocannabinoid system could be a potential therapeutic strategy for treating nicotine addiction.

Attenuating nicotine's effects with high affinity human anti-nicotine monoclonal antibodies

Use of nicotine-specific monoclonal antibodies (mAbs) to sequester and reduce nicotine distribution to brain has been proposed as a therapeutic approach to treat nicotine addiction (the basis of tobacco use disorder). A series of monoclonal antibodies with high affinity for nicotine (nic•mAbs) was isolated from B-cells of vaccinated smokers. Genes encoding 32 unique nicotine binding antibodies were cloned, and the mAbs expressed and tested by surface plasmon resonance to determine their affinity for S-(–)-nicotine. The highest affinity nic•mAbs had binding affinity constants (K_D) between 5 and 67 nM. The 4 highest affinity nic•mAbs were selected to undergo additional secondary screening for antigen-specificity, protein properties (including aggregation and stability), and functional in vivo studies to evaluate their capacity for reducing nicotine distribution to brain in rats. The 2 most potent nic•mAbs in single-dose nicotine pharmacokinetic experiments were further tested in a dose-response in vivo study. The most potent lead, ATI-1013, was selected as the lead candidate based on the results of these studies. Pretreatment with 40 and 80 mg/kg ATI-1013 reduced brain nicotine levels by 56 and 95%, respectively, in a repeated nicotine dosing experiment simulating very heavy smoking. Nicotine self-administration was also significantly reduced in rats treated with ATI-1013. A pilot rat 30-day repeat-dose toxicology study (4x200mg/kg ATI-1013) in the presence of nicotine indicated no drug-related safety concerns. These data provide evidence that ATI-1013 could be a potential therapy for the treatment of nicotine addiction.

Association of NRG3 and ERBB4 gene polymorphism with nicotine dependence in Turkish population

BACKGROUND

Nicotine dependence (ND) is characterized by regular smoking, anxiety, irritation, difficulty concentrating, impatience, restlessness, tremor, dizziness, hunger, nicotine demand, and the individual's reluctance to quit despite knowing the health risks of smoking. Recently, it has been reported that the Neuregulin 3 (NRG3)/Erb-B2 receptor tyrosine kinase 4 (ERBB4) signaling pathway plays a role in ND. NRG3, which is activated after nicotine intake, binds to ERBB4 and causes GABA release. GABA reduces anxiety and tension, which are one of the nicotine withdrawal symptoms. Therefore we aimed to investigate the relationship between NRG3 and ERBB4 gene polymorphisms and ND.

MATERIALS AND METHODS

The study population was comprised of patients with ND (n = 200) and healthy non-smoker control subjects (n = 200) who were matched for age, sex, and compared for comorbidity factors such as alcohol, smoking, duration, and education (age range 18-60). Genotypes were detected by Real-Time PCR using TaqMan technology. The Fagerström Nicotine Dependence Test (FTND) score was 5 and above for the patient group and 0 for the control group. DNA was obtained from whole peripheral blood and six polymorphisms of Neuregulin 3 (NRG3) (rs1836724, rs7562566, and rs10048757) and Erb-B2 Receptor Tyrosine Kinase 4 (ERBB4) (rs1764072, rs6584400, and rs10883934) genes were analyzed by real-time PCR method.

RESULTS

Our findings show that the six selected SNPs are not significantly associated with ND in the Turkish population and no correlation with dependence levels (p > 0.05).

CONCLUSION

Although our findings do not show a relationship between ND and these polymorphisms, it is the first study to investigate these single nucleotide polymorphisms (SNPs) for the first time in ND and to find some genotypes in the Turkish population when compared to other populations. Also, our findings are important in terms of their contribution to the literature and forensic genetics.

Effects of Nicotine Metabolic Rate on Cigarette Reinforcement

INTRODUCTION

The rate of nicotine metabolism, estimated by the nicotine metabolite ratio (NMR), is an important determinant of tobacco dependence. This study investigated the effect of NMR on smoking behavior due to nicotine reinforcement during ad libitum smoking.

AIMS AND METHODS

As part of a larger study, participants were stratified based on saliva NMR as fast and slow metabolizers. After smoking a cigarette and measuring nicotine blood concentrations, participants smoked as desired over a 90-minute period. Analysis included time to first cigarette, total number of cigarettes, total number of puffs, and weight of tobacco consumed.

RESULTS

Sixty-one (48%) participants were fast metabolizers and 66 (52%) slow metabolizers by NMR. No significant differences were found regarding the smoking topography variables by NMR. Normal metabolizers by genotype (n = 79) had a shorter time to first cigarette than reduced metabolizers (n = 39; p = .032). Blacks smoked fewer cigarettes (p = .008) and took fewer total puffs (p = .002) compared with Whites. Among Whites, fast metabolizers by NMR had a shorter time to first cigarette compared with slow metabolizers (p = .014). Among fast metabolizers, Whites had, compared with Blacks, shorter latency to first cigarette (p = .003) and higher number of total puffs (p = .014) and cigarettes smoked (p = .014). Baseline cigarettes per day and nicotine elimination half-life significantly predicted topography outcomes.

CONCLUSIONS

Saliva NMR did not predict cigarette reinforcement during a relatively brief period of ad libitum smoking. Differences were seen by race, with White fast metabolizers by NMR having shorter time to first cigarettes compared with slow metabolizers.

IMPLICATIONS

After a 90-minute period of nicotine abstinence, NMR was not significantly associated with smoking reinforcement. Slow and fast metabolizers had similar time to first cigarette, number of cigarettes smoked, total number of puffs, and tobacco consumed; however, within-race differences show that within Whites, fast metabolizers had a faster time to first cigarette than slow metabolizers.

The Impact of Electronic Nicotine Delivery System (ENDS) Flavors on Nicotinic Acetylcholine Receptors and Nicotine Addiction-Related Behaviors

Over the past two decades, combustible cigarette smoking has slowly declined by nearly 11% in America; however, the use of electronic cigarettes has increased tremendously, including among adolescents. While nicotine is the main addictive component of tobacco products and a primary concern in electronic cigarettes, this is not the only constituent of concern. There is a growing market of flavored products and a growing use of zero-nicotine e-liquids among electronic cigarette users. Accordingly, there are few studies that examine the impact of flavors on health and behavior. Menthol has been studied most extensively due to its lone exception in combustible cigarettes. Thus, there is a broad understanding of the neurobiological effects that menthol plus nicotine has on the brain including enhancing nicotine reward, altering nicotinic acetylcholine receptor number and function, and altering midbrain neuron excitability. Although flavors other than menthol were banned from combustible cigarettes, over 15,000 flavorants are available for use in electronic cigarettes. This review seeks to summarize the current knowledge on nicotine addiction and the various brain regions and nicotinic acetylcholine receptor subtypes involved, as well as describe the most recent findings regarding menthol and green apple flavorants, and their roles in nicotine addiction and vaping-related behaviors.

Outcomes and predictive factors for successful smoking cessation therapy in COPD patients with nicotine dependence

BACKGROUND

The data on smoking cessation treatment outcomes for smokers with chronic obstructive pulmonary disease (COPD) are limited. The present study assessed the effectiveness of smoking cessation interventions at our clinic.

METHODS

Data from a prospective registry of a 3-month smoking cessation program were evaluated. The primary outcome, smoking cessation, was defined as the complete abstinence from smoking between the 8-week and 12-week clinic visits. Pulmonary function and health-related quality of life using St. George's Respiratory Questionnaire (SGRQ) were assessed at baseline and at the end of the program.

RESULTS

Out of the 155 COPD patients with nicotine dependence (female/male = 39/116; mean age, 67.2 ± 9.8 years; mean forced expiratory volume in 1 s (FEV1), 59.7 ± 21.1% predicted), 107 participants completed the program. Among the completers, 74 achieved smoking cessation. In the multivariate analysis, mental disorders (odds ratio [OR] 3.678, 95% confidence interval [CI]: 1.182, 11.445), higher exhaled carbon monoxide (CO) level (OR 1.080, 95% CI: 1.013, 1.151) and lower FEV1/forced vital capacity (FVC) (OR 0.958, 95% CI: 0.923, 0.995) were negatively associated with successful smoking termination. Significant changes in pulmonary function were found in quitters but not in continuous smokers (increases in FEV1 by 0.09 L/s [95% CI: 0.03, 0.15] and peak expiratory flow by 0.23 L/s [95% CI: 0.01, 0.44]). SGRQ total scores improved significantly in both quitters (-5.4 [95% CI: -8.4, -2.5]) and continuous smokers (-7.0 [95% CI: -11.6, -2.5]).

CONCLUSION

In the program completers, the exhaled CO levels, FEV1/FVC ratio, and presence of mental disorders were significantly associated with program success or failure in COPD patients with nicotine dependence.

Δ8 -Tetrahydrocannabivarin has potent anti-nicotine effects in several rodent models of nicotine dependence

BACKGROUND AND PURPOSE

Both types of cannabinoid receptors-CB1 and CB2 -regulate brain functions relating to addictive drug-induced reward and relapse. CB1 receptor antagonists and CB2 receptor agonists have anti-addiction efficacy, in animal models, against a broad range of addictive drugs. Δ9 -Tetrahydrocannabivarin (Δ9 -THCV)-a cannabis constituent-acts as a CB1 antagonist and a CB2 agonist. Δ8 -Tetrahydrocannabivarin (Δ8 -THCV) is a Δ9 -THCV analogue with similar combined CB1 antagonist/CB2 agonist properties.

EXPERIMENTAL APPROACH

We tested Δ8 -THCV in seven different rodent models relevant to nicotine dependence-nicotine self-administration, cue-triggered nicotine-seeking behaviour following forced abstinence, nicotine-triggered reinstatement of nicotine-seeking behaviour, acquisition of nicotine-induced conditioned place preference, anxiety-like behaviour induced by nicotine withdrawal, somatic withdrawal signs induced by nicotine withdrawal, and hyperalgesia induced by nicotine withdrawal.

KEY RESULTS

Δ8 -THCV significantly attenuated intravenous nicotine self-administration and both cue-induced and nicotine-induced relapse to nicotine-seeking behaviour in rats. Δ8 -THCV also significantly attenuated nicotine-induced conditioned place preference and nicotine withdrawal in mice.

CONCLUSIONS AND IMPLICATIONS

We conclude that Δ8 -THCV may have therapeutic potential for the treatment of nicotine dependence. We also suggest that tetrahydrocannabivarins should be tested for possible anti-addiction efficacy in a broader range of preclinical animal models, against other addictive drugs, and eventually in humans.

A Review on the Receptor-ligand Molecular Interactions in the Nicotinic Receptor Signaling Systems

Nicotine is regarded as the main active addictive ingredient in tobacco products driving continued tobacco abuse behavior (smoking) to the addiction behavior, whereas nicotinic acetylcholine receptors (nAChR) is the crucial effective apparatus or molecular effector of nicotine and acetylcholine and other similar ligands. Many nAChR subunits have been revealed to bind to either neurotransmitters or exogenous ligands, such as nicotine and acetylcholine, being involved in the nicotinic receptor signal transduction. Therefore, the nicotinic receptor signalling molecules and the receptor-ligand molecular interactions between nAChRs and their ligands are universally regarded as crucial mediators of cellular functions and drug targets in medical treatment and clinical diagnosis. Given numerous endeavours have been made in defining the roles of nAChRs in response to nicotine and other addictive drugs, this review focuses on studies and reports in recent years on the receptor-ligand interactions between nAChR receptors and ligands, including lipid-nAChR and protein-nAChR molecular interactions, relevant signal transduction pathways and their molecular mechanisms in the nicotinic receptor signalling systems. All the references were carefully retrieved from the PubMed database by searching key words "nicotine", "acetylcholine", "nicotinic acetylcholine receptor(s)", "nAChR*", "protein and nAChR", "lipid and nAChR", "smok*" and "tobacco". All the relevant referred papers and reports retrieved were fully reviewed for manual inspection. This effort intend to get a quick insight and understanding of the nicotinic receptor signalling and their molecular interactions mechanisms. Understanding the cellular receptor-ligand interactions and molecular mechanisms between nAChRs and ligands will lead to a better translational and therapeutic operations and outcomes for the prevention and treatment of nicotine addiction and other chronic drug addictions in the brain's reward circuitry.

Optimization of a nicotine degrading enzyme for potential use in treatment of nicotine addiction

BACKGROUND

Smoking and tobacco use continue to be the largest preventable causes of death globally. A novel therapeutic approach has recently been proposed: administration of an enzyme that degrades nicotine, the main addictive component of tobacco, minimizing brain exposure and reducing its reinforcing effects. Pre-clinical proof of concept has been previously established through dosing the amine oxidase NicA2 from Pseudomonas putida in rat nicotine self-administration models of addiction.

RESULTS

This paper describes efforts towards optimizing NicA2 for potential therapeutic use: enhancing potency, improving its pharmacokinetic profile, and attenuating immunogenicity. Libraries randomizing residues located in all 22 active site positions of NicA2 were screened. 58 single mutations with 2- to 19-fold enhanced catalytic activity compared to wt at 10 μM nicotine were identified. A novel nicotine biosensor assay allowed efficient screening of the many primary hits for activity at nicotine concentrations typically found in smokers. 10 mutants with improved activity in rat serum at or below 250 nM were identified. These catalytic improvements translated to increased potency in vivo in the form of further lowering of nicotine blood levels and nicotine accumulation in the brains of Sprague-Dawley rats. Examination of the X-ray crystal structure suggests that these mutants may accelerate the rate limiting re-oxidation of the flavin adenine dinucleotide cofactor by enhancing molecular oxygen's access. PEGylation of NicA2 led to prolonged serum half-life and lowered immunogenicity observed in a human HLA DR4 transgenic mouse model, without impacting nicotine degrading activity.

CONCLUSIONS

Systematic mutational analysis of the active site of the nicotine-degrading enzyme NicA2 has yielded 10 variants that increase the catalytic activity and its effects on nicotine distribution in vivo at nicotine plasma concentrations found in smokers. In addition, PEGylation substantially increases circulating half-life and reduces the enzyme's immunogenic potential. Taken together, these results provide a viable path towards generation of a drug candidate suitable for human therapeutic use in treating nicotine addiction.

Nicotine absorption during electronic cigarette use among regular users

BACKGROUND

The capability of electronic cigarette devices (e-cigs) to deliver nicotine is key to their potential to replace combustible cigarettes. We compared nicotine delivery and subjective effects associated with the use of two classes of e-cigarettes and cigarettes.

METHODS

14 e-cigarette users were instructed to vape their own e-cigarette device every 20 seconds for 10 minutes while blood was drawn at 1, 2, 4, 6, 8, 10,12, and 15 minutes after initiating vaping. Users rated withdrawal symptoms and side effects before and after vaping. E-cigarette devices were classified as first-generation (same size as cigarette, no activation button) or advanced (larger than cigarette with an activation button). Separately, 10 cigarette smokers completed a similar protocol. Fisher's Exact Test and two-sided t-tests were used as appropriate to determine differences in outcomes between first-generation e-cigarette users, advanced e-cigarette users, and smokers.

RESULTS

Compared to first-generation devices, advanced devices were associated with greater serum nicotine Cmax (ng/ml) (11.5 v. 2.8, p = 0.0231) and greater nicotine boost (ng/ml) (10.8 v. 1.8, p = 0.0177). Overall, e-cigarettes users experienced a significant reduction in withdrawal and craving, although there were no significant differences between users of first-generation and advanced devices. Comparing e-cigarettes overall to cigarettes, cigarettes were associated with greater Cmax (25.9 v. 9.0, p = 0.0043) and greater nicotine boost (21.0 v. 8.2, p = 0.0128).

CONCLUSIONS

Advanced e-cigarettes delivered significantly more nicotine than first-generation devices but less than combustible cigarettes. Overall, e-cigarette use was associated with a reduction in withdrawal and craving with no reported side effects. The wide variation in nicotine absorption from different e-cigarette devices should be considered in studies of e-cigarettes for smoking cessation.

Chronic Nicotine Exposure Alters the Neurophysiology of Habenulo-Interpeduncular Circuitry

Antagonism of nicotinic acetylcholine receptors (nAChRs) in the medial habenula (MHb) or interpeduncular nucleus (IPN) triggers withdrawal-like behaviors in mice chronically exposed to nicotine, implying that nicotine dependence involves the sensitization of nicotinic signaling. Identification of receptor and/or neurophysiological mechanisms underlying this sensitization is important, as it could promote novel therapeutic strategies to reduce tobacco use. Using an approach involving photoactivatable nicotine, we previously demonstrated that chronic nicotine (cNIC) potently enhances nAChR function in dendrites of MHb neurons. However, whether cNIC modulates downstream components of the habenulo-interpeduncular (Hb-IP) circuit is unknown. In this study, cNIC-mediated changes to Hb-IP nAChR function were examined in mouse (male and female) brain slices using molecular, electrophysiological, and optical techniques. cNIC enhanced action potential firing and modified spike waveform characteristics in MHb neurons. Nicotine uncaging revealed nAChR functional enhancement by cNIC on proximal axonal membranes. Similarly, nAChR-driven glutamate release from MHb axons was enhanced by cNIC. In IPN, the target structure of MHb axons, neuronal morphology, and nAChR expression is complex, with stronger nAChR function in the rostral subnucleus [rostral IPN (IPR)]. As in MHb, cNIC induced strong upregulation of nAChR function in IPN neurons. This, coupled with cNIC-enhanced nicotine-stimulated glutamate release, was associated with stronger depolarization responses to brief (1 ms) nicotine uncaging adjacent to IPR neurons. Together, these results indicate that chronic exposure to nicotine dramatically alters nicotinic cholinergic signaling and cell excitability in Hb-IP circuits, a key pathway involved in nicotine dependence.SIGNIFICANCE STATEMENT This study uncovers several neuropharmacological alterations following chronic exposure to nicotine in a key brain circuit involved in nicotine dependence. These results suggest that smokers or regular users of electronic nicotine delivery systems (i.e., "e-cigarettes") likely undergo sensitization of cholinergic circuitry in the Hb-IP system. Reducing the activity of Hb-IP nAChRs, either volitionally during smoking cessation or inadvertently via receptor desensitization during nicotine intake, may be a key trigger of withdrawal in nicotine dependence. Escalation of nicotine intake in smokers, or tolerance, may involve stimulation of these sensitized cholinergic pathways. Smoking cessation therapeutics are only marginally effective, and by identifying cellular/receptor mechanisms of nicotine dependence, our results take a step toward improved therapeutic approaches for this disorder.

Functional Neurocircuits and Neuroimaging Biomarkers of Tobacco Use Disorder

Drug abuse and addiction remain major public health issues, exemplified by the opioid epidemic currently devastating the United States. Treatment outcomes across substance use disorders remain unacceptably poor, wherein drug discovery/development for this multifaceted neuropsychiatric disorder focuses on single molecular-level targets. Rather, our opinion is that a systems-level neuroimaging perspective is crucial for identifying novel therapeutic targets, biomarkers to stratify patients, and individualized treatment strategies. Focusing on tobacco use disorder, we advocate a brain systems-level perspective linking two abuse-related facets (i.e., statelike withdrawal and traitlike addiction severity) with specific neurocircuitry (insula- and striatum-centered networks). To the extent that precise neurocircuits mediate distinct facets of abuse, treatment development must adopt not only a systems-level perspective, but also multi-intervention rather than mono-intervention practices.

Comparing Smoking Topography and Subjective Measures of Usual Brand Cigarettes Between Pregnant and Non-Pregnant Smokers

Introduction

Most pregnant smokers report abruptly reducing their cigarettes per day (CPD) by ~50% after learning of pregnancy and making further smaller reductions over the remainder of their pregnancy. Laboratory and naturalistic studies with non-pregnant smokers have found that these types of reductions often lead to changes in smoking topography (i.e., changes in smoking intensity to maintain a desired blood-nicotine level). If pregnant women smoke more intensely, they may expose themselves and their offspring to similar levels of toxicants despite reporting reductions in CPD.

Methods

Pregnant and non-pregnant female smokers (n = 20 and 89, respectively) participated. At the experimental session, after biochemical confirmation of acute abstinence, participants smoked one usual brand cigarette ad lib through a Borgwaldt CReSS Desktop Smoking Topography device. Carbon monoxide (CO) and measures of nicotine withdrawal, craving, and reinforcement derived from smoking were also collected.

Results

The two groups did not differ on demographic or smoking characteristics at screening, except nicotine metabolism rate, which as expected, was faster in pregnant smokers. Analyses suggest that none of the smoking topography parameters differed between pregnant and non-pregnant smokers, although pregnant smokers had a significantly smaller CO boost. Both groups reported similar levels of relief of withdrawal and craving after smoking, but other subjective effects suggest that pregnant smokers find smoking less reinforcing than non-pregnant smokers.

Conclusions

Pregnant smokers do not smoke cigarettes differently than non-pregnant women, but appear to find smoking comparatively less pleasurable.

Implications

This is the first study to assess smoking topography in pregnant women. Pregnant women appear to be at increased risk for smoking cigarettes with more intensity because of (1) their tendency to make significant abrupt reductions in the number of cigarettes they smoke each day after learning of pregnancy and (2) an increase in nicotine metabolism induced by pregnancy. Despite these changes, the present results suggest that pregnant women do not smoke cigarettes more intensely or in a way that causes more toxicant exposure, perhaps due to a reportedly less pleasurable smoking experience.

Selective breeding for high alcohol consumption and response to nicotine: locomotor activity, dopaminergic in the mesolimbic system, and innate genetic differences in male and female alcohol-preferring, non-preferring, and replicate lines of high-alcohol drinking and low-alcohol drinking rats

RATIONALE

There is evidence for a common genetic link between alcohol and nicotine dependence. Rodents selectively bred for high alcohol consumption/responsivity are also more likely to self-administer nicotine than controls.

OBJECTIVES

The experiments examined the response to systemic nicotine, the effects of nicotine within the drug reward pathway, and innate expression of nicotine-related genes in a brain region regulating drug reward/self-administration in multiple lines of rats selectively bred for high and low alcohol consumption.

METHODS

The experiments examined the effects of systemic administration of nicotine on locomotor activity, the effects of nicotine administered directly into the (posterior ventral tegmental area; pVTA) on dopamine (DA) release in the nucleus accumbens shell (AcbSh), and innate mRNA levels of acetylcholine receptor genes in the pVTA were determined in 6 selectively bred high/low alcohol consuming and Wistar rat lines.

RESULTS

The high alcohol-consuming rat lines had greater nicotine-induced locomotor activity compared to low alcohol-consuming rat lines. Microinjections of nicotine into the pVTA resulted in DA release in the AcbSh with the dose response curves for high alcohol-consuming rats shifted leftward and upward. Genetic analysis of the pVTA indicated P rats expressed higher levels of α2 and β4.

CONCLUSION

Selective breeding for high alcohol preference resulted in a genetically divergent behavioral and neurobiological sensitivity to nicotine. The observed behavioral and neurochemical differences between the rat lines would predict an increased likelihood of nicotine reinforcement. The data support the hypothesis of a common genetic basis for drug addiction and identifies potential receptor targets.

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Metabotropic Glutamate Receptors 2 and 3 as Targets for Treating Nicotine Addiction

Tobacco smoking, driven by the addictive properties of nicotine, continues to be a worldwide health problem. Based on the well-established role of glutamatergic neurotransmission in drug addiction, novel medication development strategies seek to halt nicotine consumption and prevent relapse to tobacco smoking by modulating glutamate transmission. The presynaptic inhibitory metabotropic glutamate receptors 2 and 3 (mGluR2/3) are key autoreceptors on glutamatergic terminals that maintain glutamate homeostasis. Accumulating evidence suggests the critical role of mGluR2/3 in different aspects of nicotine addiction, including acquisition and maintenance of nicotine taking, nicotine withdrawal, and persistent nicotine seeking even after prolonged abstinence. The involvement of mGluR2/3 in other neuropsychiatric conditions, such as anxiety, depression, schizophrenia, Alzheimer's disease, Parkinson's disease, and pain, provides convincing evidence suggesting that mGluR2/3 may provide an effective therapeutic approach for comorbidity of smoking and these conditions. This focused review article highlights that mGluR2/3 provide a promising target in the search for smoking cessation medication with novel mechanisms of actions that differ from those of currently U.S. Food and Drug Administration-approved pharmacotherapies.

Pathways to precision medicine in smoking cessation treatments

Cigarette smoking is highly addictive and modern genetic research has identified robust genetic influences on nicotine dependence. An important step in translating these genetic findings to clinical practice is identifying the genetic factors affecting smoking cessation in order to enhance current smoking cessation treatments. We reviewed the significant genetic variants that predict nicotine dependence, smoking cessation, and response to cessation pharmacotherapy. These data suggest that genetic risks can predict smoking cessation outcomes and moderate the effect of pharmacological treatments. Some pharmacogenetic findings have been replicated in meta-analyses or in multiple smoking cessation trials. The variation in efficacy between smokers with different genetic markers supports the notion that personalized smoking cessation intervention based upon genotype could maximize the efficiency of such treatment while minimizing side effects, thus influencing the number needed to treat (NNT) and the number needed to harm. In summary, as precision medicine is revolutionizing healthcare, smoking cessation may be one of the first areas where genetic variants may identify individuals at increased risk. Current evidence strongly suggests that genetic variants predict cessation failure and that cessation pharmacotherapy effectiveness is modulated by biomarkers such as nicotinic cholinergic receptor α5 subunit (CHRNA5) genotypes or nicotine metabolism ratio (NMR). These findings strengthen the case for the development and rigorous testing of treatments that target patients with different biological risk profiles.

Use of Electronic Cigarettes Leads to Significant Beta2-Nicotinic Acetylcholine Receptor Occupancy: Evidence From a PET Imaging Study

Background

Electronic cigarettes (ECs) can influence nicotine addiction by delivering aerosolized nicotine. We investigated if nicotine from ECs is delivered to the brain β2*-nicotinic acetylcholine receptors (β2*-nAChR) and how this relates to the behavioral effects and nicotine delivery from cigarettes.

Methods

Seven nicotine users participated in positron emission tomography (PET) studies with (-)-[18F]Flubatine before and after nicotine challenge with 0, 8, and 36 mg/ml nicotine in a 3.3 Volt, 1.5 Ohm EC or a standard tobacco cigarette. Craving was evaluated before and after product use.

Results

Average β2*-nAChR occupancy was higher after 36 mg/ml EC challenge compared to 8 mg/ml EC at trend level. Average β2*-nAChR occupancy after tobacco cigarette smoking was 68 ± 18% and was not different compared with 8 mg/ml (64 ± 17%,) or 36 mg/ml (84 ± 3%) nicotine in EC users. Area under the curve (AUC) of blood nicotine level was higher in the cigarette smoking group compared with the 8mg/ml group (p = 0.03), but similar compared with the 36 mg/ml EC (p = 0.29). Drug craving was reduced after use of the tobacco cigarette, 8 mg/ml EC, and 36 mg/ml EC.

Conclusions

In this novel investigation of EC effects at β2*-nAChRs, we show that average β2*-nAChR occupancy was higher after 36 mg/ml EC challenge compared with 8 mg/ml EC. Receptor occupancy and arterial blood nicotine levels after cigarette smoking were similar to 36 mg/ml EC use under controlled conditions. These findings suggest that the ECs studied here have abuse liability and may provide an adequate alternative nicotine delivery system for cigarette smokers.

Implications

This is the first study to directly determine the neurologic effects of electronic cigarettes on human brain beta-2 nicotinic acetylcholine receptors using PET neuroimaging with (-)-[18F]Flubatine, a novel radiotracer. Our findings suggest that the e-cigarettes studied here have abuse liability and may provide an adequate alternative nicotine delivery system for cigarette smokers.

Salience network coupling is linked to both tobacco smoking and symptoms of attention deficit hyperactivity disorder (ADHD)

INTRODUCTION

Attention deficit hyperactivity disorder (ADHD) symptoms, even those below diagnostic threshold, enhance the likelihood of nicotine dependence, suggesting a neurobiological link between disorders. Of particular interest is the salience network (SN), which mediates attention to salient internal/external stimuli to guide behavior and is anchored by the dorsal anterior cingulate cortex (dACC) and bilateral anterior insula (AI). Disrupted interactions between the SN and the default mode (DMN) and central executive networks (CEN) have been noted in both ADHD and nicotine dependence. Further, enhanced intra-SN coupling between the dACC-AI influences aspects of nicotine dependence such as reactivity to smoking cues.

METHODS

To identify links between SN functional connectivity and ADHD symptoms in nicotine dependence, we compared 21 nicotine dependent individuals with 17 non-smokers on ADHD symptoms as measured by the ADHD self-report scale (ASRS) and resting state intra and inter-SN functional connectivity.

RESULTS

Relative to healthy controls, nicotine dependent individuals had significantly higher ASRS scores and greater dACC-AI coupling. No group differences were noted on inter-SN network coupling. A significant association was found between ASRS and dACC-AI coupling both in the entire cohort and specifically when evaluating nicotine dependent individuals alone.

CONCLUSIONS

The greater ASRS scores in nicotine dependent individuals is in line with existent literature and the stronger dACC-AI coupling in smokers further supports the role of this network in nicotine dependence. The significant association between dACC-AI coupling and ASRS suggests that intra-SN coupling strength may impact neurocognitive functioning associated with both ADHD symptoms and nicotine dependence.

Cognitive Effects of Nicotine: Recent Progress

BACKGROUND

Cigarette smoking is the main cause of preventable death in developed countries. While the direct positive behavioral reinforcing effect of nicotine has historically been considered the primary mechanism driving the development of TUD, accumulating contemporary research suggests that the cognitive-enhancing effects of nicotine may also significantly contribute to the initiation and maintenance of TUD, especially in individuals with pre-existing cognitive deficits.

METHODS

We provide a selective overview of recent advances in understanding nicotine's effects on cognitive function, a discussion of the role of cognitive function in vulnerability to TUD, followed by an overview of the neurobiological mechanisms underlying the cognitive effects of nicotine.

RESULTS

Preclinical models and human studies have demonstrated that nicotine has cognitiveenhancing effects. Attention, working memory, fine motor skills and episodic memory functions are particularly sensitive to nicotine's effects. Recent studies have demonstrated that the α4, β2, and α7 subunits of the nicotinic acetylcholine receptor (nAChR) participate in the cognitive-enhancing effects of nicotine. Imaging studies have been instrumental in identifying brain regions where nicotine is active, and research on the dynamics of large-scale networks after activation by, or withdrawal from, nicotine hold promise for improved understanding of the complex actions of nicotine on human cognition.

CONCLUSION

Because poor cognitive performance at baseline predicts relapse among smokers who are attempting to quit smoking, studies examining the potential efficacy of cognitive-enhancement as strategy for the treatment of TUD may lead to the development of more efficacious interventions.

Nicotine Metabolite Ratio (NMR) Prospectively Predicts Smoking Relapse: Longitudinal Findings From ITC Surveys in Five Countries

INTRODUCTION

The ratio of trans 3'-hydroxycotinine (3HC) to cotinine (nicotine metabolite ratio [NMR]) is a biomarker of the rate of nicotine metabolism, with higher NMR indicating faster metabolism. Higher NMR has been found to be associated with higher daily cigarette consumption and less success stopping smoking in cessation trials. This study examines differences in NMR among population-based samples of smokers in the five countries and explores the relationship between NMR and smoking abstinence.

METHODS

Participants (N = 874) provided saliva samples during International Tobacco Control (ITC) surveys in the United States, United Kingdom, Mauritius, Mexico, and Thailand conducted in 2010/2011 with follow-up surveys in 2012/2013. When all samples were received, they were sent to a common laboratory for analysis using liquid chromatography and tandem mass spectroscopy.

RESULTS

There was significant variation in NMR across countries (F = 15.49, p < .001). Those who reported smoking at follow-up had a mean NMR of 0.32, compared to a mean NMR of 0.42 in participants who reported that they had stopped (F = 8.93; p = .003). Higher mean NMR values were also associated with longer quit duration (p = .007). There was no substantial difference in NMR between current smokers who made a failed quit attempt and those who made no attempt-both had significantly lower NMR compared to those who quit and remained abstinent. Smokers with a higher NMR were more likely to report that they stopped smoking compared to those with a lower NMR (odds ratio = 2.67; 95% confidence interval: 1.25 to 5.68).

CONCLUSIONS

These results suggest faster nicotine metabolizers may be less likely to relapse following a quit attempt. This finding differs from results of clinical trials testing stop smoking medications, where slower metabolizers have been found to be more likely to maintain abstinence from smoking.

IMPLICATIONS

Results of this study suggest faster nicotine metabolizers may be less likely to relapse following a quit attempt. This finding differs from results of clinical trials testing stop smoking medications, where slower metabolizers have been found to be more likely to maintain abstinence from smoking.

Expression profile of nicotinic acetylcholine receptor subunits in the brain of HIV-1 transgenic rats given chronic nicotine treatment

Abuse of addictive substances, including cigarettes, is much greater in HIV-1-infected individuals than in the general population and challenges the efficiency of highly active anti-retroviral therapy (HAART). The HIV-1 transgenic (HIV-1Tg) rat, an animal model used to study drug addiction in HIV-1-infected patients on HAART, displays abnormal neurobehavioral responses to addictive substances. Given that the cholinergic system plays an essential part in the central reward circuitry, we evaluated the expression profile of nine nicotinic acetylcholine receptor (nAChR) subunit genes in the central nervous system (CNS) of HIV-1Tg rats. We found that nAChR subunits were differentially expressed in various brain regions in HIV-1Tg rats compared to F344 control rats, with more subunits altered in the ventral tegmental area (VTA) and nucleus accumbens (NAc) of the HIV-1Tg rats than in other brain regions. We also found that chronic nicotine treatment (0.4 mg/kg/day) decreased the mRNA expression of nAChR subunits α6, β3, and β4 in the VTA of HIV-1Tg rats, whereas expression of α4 and α6 subunits in the NAc increased. No such changes were observed in F344 rats. Together, our data suggest that HIV-1 proteins alter the expression of nAChRs, which may contribute to the vulnerability to cigarette smoking addiction in HIV-1 patients.

Selective CRF2 receptor agonists ameliorate the anxiety- and depression-like state developed during chronic nicotine treatment and consequent acute withdrawal in mice

The aim of the present study was to investigate the effects of the selective agonists of the corticotropin-releasing factor (CRF) 2 receptor, urocortin 2 (UCN 2) and urocortin 3 (UCN 3), on the anxiety- and depression-like signs induced by acute nicotine withdrawal in mice. In order to do so, male CFLP mice were exposed for 7 days to repeated intraperitoneal (IP) injection with nicotine or saline solution and 1day of acute withdrawal and then a single intracerebroventricular (ICV) injection with UCN 2, UCN 3 or saline solution. After 30min the mice were observed in an elevated plus-maze test or a forced swim test, for anxiety- and depression-like behavior. After 5min of testing, the plasma corticosterone concentration reflecting the activity of the hypothalamic-pituitary-adrenal (HPA) axis was also determined by a chemo-fluorescent method. Half of the animals were treated ICV and evaluated on the 8th day, the other half on the 9th day. On the 8th day, nicotine-treated mice presented signs of anxiolysis and depression, but no significant elevation of the plasma corticosterone concentration. On the 9th day, nicotine-treated mice exhibited signs of anxiety and depression and a significant increase of the plasma corticosterone levels. Central administration of UCN 2 or UCN 3 ameliorated the anxiety- and depression-like state including the hyperactivity of the HPA axis, developed during acute withdrawal following chronic nicotine treatment. The present study suggests that selective CRF2 receptor agonists could be used as a therapy in nicotine addiction.

Cigarette Use and Striatal Dopamine D2/3 Receptors: Possible Role in the Link between Smoking and Nicotine Dependence

BACKGROUND

Cigarette smoking induces dopamine release in the striatum, and smoking- or nicotine-induced ventral striatal dopamine release is correlated with nicotine dependence. Smokers also exhibit lower dopamine D2/3 receptor availability in the dorsal striatum than nonsmokers. Negative correlations of striatal dopamine D2/3 receptor availability with smoking exposure and nicotine dependence, therefore, might be expected but have not been tested.

METHODS

Twenty smokers had positron emission tomography scans with [¹⁸F]fallypride to measure dopamine D2/3 receptor availability in ventral and dorsal regions of the striatum and provided self-report measures of recent and lifetime smoking and of nicotine dependence.

RESULTS

As reported before, lifetime smoking was correlated with nicotine dependence. New findings were that ventral striatal dopamine D2/3 receptor availability was negatively correlated with recent and lifetime smoking and also with nicotine dependence.

CONCLUSION

The results suggest an effect of smoking on ventral striatal D2/3 dopamine receptors that may contribute to nicotine dependence.

Precision Medicine for Tobacco Dependence: Development and Validation of the Nicotine Metabolite Ratio

Quitting smoking significantly reduces the risk of tobacco-related morbidity and mortality, yet there is a high rate of relapse amongst smokers who try to quit. Phenotypic biomarkers have the potential to improve smoking cessation outcomes by identifying the best available treatment for an individual smoker. In this review, we introduce the nicotine metabolite ratio (NMR) as a reliable and stable phenotypic measure of nicotine metabolism that can guide smoking cessation treatment among smokers who wish to quit. We address how the NMR accounts for sources of variation in nicotine metabolism including genotype and other biological and environmental factors such as estrogen levels, alcohol use, body mass index, or menthol exposure. Then, we highlight clinical trials that validate the NMR as a biomarker to predict therapeutic response to different pharmacotherapies for smoking cessation. Current evidence supports the use of nicotine replacement therapy for slow metabolizers, and non-nicotine treatments such as varenicline for normal metabolizers. Finally, we discuss future research directions to elucidate mechanisms underlying NMR associations with treatment response, and facilitate the implementation of the NMR as biomarker in clinical practice to guide smoking cessation.

Overexpression of CRF in the BNST diminishes dysphoria but not anxiety-like behavior in nicotine withdrawing rats

Smoking cessation leads to dysphoria and anxiety, which both increase the risk for relapse. This negative affective state is partly mediated by an increase in activity in brain stress systems. Recent studies indicate that prolonged viral vector-mediated overexpression of stress peptides diminishes stress sensitivity. Here we investigated whether the overexpression of corticotropin-releasing factor (CRF) in the bed nucleus of the stria terminalis (BNST) diminishes nicotine withdrawal symptoms in rats. The effect of nicotine withdrawal on brain reward function was investigated with an intracranial self-stimulation (ICSS) procedure. Anxiety-like behavior was investigated in the elevated plus maze test and a large open field. An adeno-associated virus (AAV) pseudotype 2/5 vector was used to overexpress CRF in the lateral BNST and nicotine dependence was induced using minipumps. Administration of the nicotinic receptor antagonist mecamylamine and cessation of nicotine administration led to a dysphoria-like state, which was prevented by the overexpression of CRF in the BNST. Nicotine withdrawal also increased anxiety-like behavior in the elevated plus maze test and large open field test and slightly decreased locomotor activity in the open field. The overexpression of CRF in the BNST did not prevent the increase in anxiety-like behavior or decrease in locomotor activity. The overexpression of CRF increased CRF1 and CRF2 receptor gene expression and increased the CRF2/CRF1 receptor ratio. In conclusion, the overexpression of CRF in the BNST prevents the dysphoria-like state associated with nicotine withdrawal and increases the CRF2/CRF1 receptor ratio, which may diminish the negative effects of CRF on mood.

Interactions between the endocannabinoid and nicotinic cholinergic systems: preclinical evidence and therapeutic perspectives

RATIONALE

Several lines of evidence suggest that endocannabinoid and nicotinic cholinergic systems are implicated in the regulation of different physiological processes, including reward, and in the neuropathological mechanisms of psychiatric diseases, such as addiction. A crosstalk between these two systems is substantiated by the overlapping distribution of cannabinoid and nicotinic acetylcholine receptors in many brain structures.

OBJECTIVE

We will review recent preclinical data showing how the endocannabinoid and nicotinic cholinergic systems interact bidirectionally at the level of the brain reward pathways, and how this interaction plays a key role in modulating nicotine and cannabinoid intake and dependence.

RESULTS

Many behavioral and neurochemical effects of nicotine that are related to its addictive potential are reduced by pharmacological blockade or genetic deletion of type-1 cannabinoid receptors, inhibition of endocannabinoid uptake or metabolic degradation, and activation of peroxisome proliferator-activated-receptor-α. On the other hand, cholinergic antagonists at α7 nicotinic acetylcholine receptors as well as endogenous negative allosteric modulators of these receptors are effective in blocking dependence-related effects of cannabinoids.

CONCLUSIONS

Pharmacological manipulation of the endocannabinoid system and endocannabinoid-like neuromodulators shows promise in the treatment of nicotine dependence and in relapse prevention. Likewise, drugs acting at nicotinic acetylcholine receptors might prove useful in the therapy of cannabinoid dependence. Research by Steven R. Goldberg has significantly contributed to the progress in this research field.

Effects of ceftriaxone on ethanol, nicotine or sucrose intake by alcohol-preferring (P) rats and its association with GLT-1 expression

Increased glutamatergic neurotransmission appears to mediate the reinforcing properties of drugs of abuse, including ethanol (EtOH). We have shown that administration of ceftriaxone (CEF), a β-lactam antibiotic, reduced EtOH intake and increased glutamate transporter 1 (GLT-1) expression in mesocorticolimbic regions of male and female alcohol-preferring (P) rats. In the present study, we tested whether CEF administration would reduce nicotine (NIC) and/or EtOH intake by adult female P rats. P rats were randomly assigned to 4 groups: (a) 5% sucrose (SUC) or 10% SUC [SUC], (b) 5% SUC+0.07mg/ml NIC and 10% SUC+0.14mg/ml NIC [NIC-SUC], 15% EtOH and 30% EtOH [EtOH] and (d) 15% EtOH+0.07mg/ml NIC and 30% EtOH+0.14mg/ml NIC [NIC-EtOH]. After achieving stable intakes (4weeks), the rats were administered 7 consecutive, daily i.p. injections of either saline or 200mg/kg CEF. The effects of CEF on intake were significant but differed across the reinforcers; such that ml/kg/day SUC was reduced by ∼30%, mg/kg/day NIC was reduced by ∼70% in the NIC-SUC group and ∼40% in the EtOH-NIC group, whereas g/kg/day EtOH was reduced by ∼40% in both the EtOH and EtOH-NIC group. The effects of CEF on GLT-1 expression were also studied. We found that CEF significantly increased GLT-1 expression in the prefrontal cortex and the nucleus accumbens of the NIC and NIC-EtOH rats as compared to NIC and NIC-EtOH saline-treated rats. These findings provide further support for GLT-1-associated mechanisms in EtOH and/or NIC abuse. The present results along with previous reports of CEF's efficacy in reducing cocaine self-administration in rats suggest that modulation of GLT-1 expression and/or activity is an important pharmacological target for treating polysubstance abuse and dependence.

Association of Long-Term Nicotine Abstinence With Normal Metabotropic Glutamate Receptor-5 Binding

BACKGROUND

Nicotine addiction is a major public health problem and is associated with primary glutamatergic dysfunction. We recently showed marked global reductions in metabotropic glutamate receptor type 5 (mGluR5) binding in smokers and recent ex-smokers (average abstinence duration of 25 weeks). The goal of this study was to examine the role of mGluR5 downregulation in nicotine addiction by investigating a group of long-term ex-smokers (abstinence >1.5 years), and to explore associations between mGluR5 binding and relapse in recent ex-smokers.

METHODS

Images of mGluR5 receptor binding were acquired in 14 long-term ex-smokers, using positron emission tomography with radiolabeled [11C]ABP688, which binds to an allosteric site with high specificity.

RESULTS

Long-term ex-smokers and individuals who had never smoked showed no differences in mGluR5 binding in any of the brain regions examined. Long-term ex-smokers showed significantly higher mGluR5 binding than recent ex-smokers, most prominently in the frontal cortex (42%) and thalamus (57%).

CONCLUSIONS

Our findings suggest that downregulation of mGluR5 is a pathogenetic mechanism underlying nicotine dependence and the high relapse rate in individuals previously exposed to nicotine. Therefore, mGluR5 receptor binding appears to be an effective biomarker in smoking and a promising target for the discovery of novel medication for nicotine dependence and other substance-related disorders.

nAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

INTRODUCTION

The prevalence of tobacco use in the population with schizophrenia is enormously high. Moreover, nicotine dependence is found to be associated with symptom severity and poor outcome in patients with schizophrenia. The neurobiological mechanisms that explain schizophrenia-nicotine dependence comorbidity are not known. This study systematically reviews the evidence highlighting the contribution of nicotinic acetylcholine receptors (nAChRs) to nicotine abuse in schizophrenia.

METHODS

Electronic data bases (Medline, Google Scholar, and Web of Science) were searched using the selected key words that match the aims set forth for this review. A total of 276 articles were used for the qualitative synthesis of this review.

RESULTS

Substantial evidence from preclinical and clinical studies indicated that dysregulation of α7 and β2-subunit containing nAChRs account for the cognitive and affective symptoms of schizophrenia and nicotine use may represent a strategy to remediate these symptoms. Additionally, recent meta-analyses proposed that early tobacco use may itself increase the risk of developing schizophrenia. Genetic studies demonstrating that nAChR dysfunction that may act as a shared vulnerability factor for comorbid tobacco dependence and schizophrenia were found to support this view. The development of nAChR modulators was considered an effective therapeutic strategy to ameliorate psychiatric symptoms and to promote smoking cessation in schizophrenia patients.

CONCLUSIONS

The relationship between schizophrenia and smoking is complex. While the debate for the self-medication versus addiction vulnerability hypothesis continues, it is widely accepted that a dysfunction in the central nAChRs represent a common substrate for various symptoms of schizophrenia and comorbid nicotine dependence.

Targeting Dopaminergic System for Treating Nicotine Dependence

BACKGROUND

Smoking is the world's leading cause of preventable death among populations. Cigarette smoking increases the risk of numerous health problems, including heart diseases, stroke, atherosclerosis and many types of cancer, including lung, stomach and bladder cancers.

OUTCOMES

Many individuals find it difficult to stop smoking because of the addictive effects of nicotine and the presence of several monoamine oxidase (MAO) inhibitors in the tobacco smoke extract.

OBJECTIVE

The development of novel, safe and effective medications for smoking cessation is a high public health priority.

RESULTS

The role of mesocorticolimbic dopaminergic pathways in withdrawal symptoms and general reinforcement processes clearly recommends dopaminergic system as a potential target for the treatment of nicotine addiction.

CONCLUSION

This review article discusses the new pharmacological treatments of nicotine dependence, which are targeting dopaminergic neurotransmission. This includes blockade of dopamine transporter and inhibition of MAO as pharmacotherapy for the treatment of nicotine dependence.

Nicotinic acetylcholine receptors: upregulation, age-related effects and associations with drug use

Nicotinic acetylcholine receptors are ligand-gated ion channels that exogenously bind nicotine. Nicotine produces rewarding effects by interacting with these receptors in the brain's reward system. Unlike other receptors, chronic stimulation by an agonist induces an upregulation of receptor number that is not due to increased gene expression in adults; while upregulation also occurs during development and adolescence there have been some opposing findings regarding a change in corresponding gene expression. These receptors have also been well studied with regard to human genetic associations and, based on evidence suggesting shared genetic liabilities between substance use disorders, numerous studies have pointed to a role for this system in comorbid drug use. This review will focus on upregulation of these receptors in adulthood, adolescence and development, as well as the findings from human genetic association studies which point to different roles for these receptors in risk for initiation and continuation of drug use.

Thyroid hormone signaling: Contribution to neural function, cognition, and relationship to nicotine

Cigarette smoking is common despite its adverse effects on health, such as cardiovascular disease and stroke. Understanding the mechanisms that contribute to the addictive properties of nicotine makes it possible to target them to prevent the initiation of smoking behavior and/or increase the chance of successful quit attempts. While highly addictive, nicotine is not generally considered to be as reinforcing as other drugs of abuse. There are likely other mechanisms at work that contribute to the addictive liability of nicotine. Nicotine modulates aspects of the endocrine system, including the thyroid, which is critical for normal cognitive functioning. It is possible that nicotine's effects on thyroid function may alter learning and memory, and this may underlie some of its addictive potential. Here, we review the literature on thyroid function and cognition, with a focus on how nicotine alters thyroid hormone signaling and the potential impact on cognition. Changes in cognition are a major symptom of nicotine addiction. Current anti-smoking therapies have modest success at best. If some of the cognitive effects of nicotine are mediated through the thyroid hormone system, then thyroid hormone agonists may be novel treatments for smoking cessation therapies. The content of this review is important because it clarifies the relationship between smoking and thyroid function, which has been ill-defined in the past. This review is timely because the reduction in smoking rates we have seen in recent decades, due to public awareness campaigns and public smoking bans, has leveled off in recent years. Therefore, novel treatment approaches are needed to help reduce smoking rates further.

Resting-state functional connectivity and nicotine addiction: prospects for biomarker development

Given conceptual frameworks of addiction as a disease of intercommunicating brain networks, examinations of network interactions may provide a holistic characterization of addiction-related dysfunction. One such methodological approach is the examination of resting-state functional connectivity, which quantifies correlations in low-frequency fluctuations of the blood oxygen level-dependent magnetic resonance imaging signal between disparate brain regions in the absence of task performance. Here, evidence of differentiated effects of chronic nicotine exposure, which reduces the efficiency of network communication across the brain, and acute nicotine exposure, which increases connectivity within specific limbic circuits, is discussed. Several large-scale resting networks, including the salience, default, and executive control networks, have also been implicated in nicotine addiction. The dynamics of connectivity changes among and between these large-scale networks during nicotine withdrawal and satiety provide a heuristic framework with which to characterize the neurobiological mechanism of addiction. The ability to simultaneously quantify effects of both chronic (trait) and acute (state) nicotine exposure provides a platform to develop a neuroimaging-based addiction biomarker. While such development remains in its early stages, evidence of coherent modulations in resting-state functional connectivity at various stages of nicotine addiction suggests potential network interactions on which to focus future addiction biomarker development.

N-acetylcysteine for therapy-resistant tobacco use disorder: a pilot study

INTRODUCTION

N-Acetylcysteine (NAC) may have efficacy in treating tobacco use disorder (TUD) by reducing craving and smoking reward. This study examines whether treatment with NAC may have a clinical efficacy in the treatment of TUD.

METHODS

A 12-week double blind randomized controlled trial was conducted to compare the clinical efficacy of NAC 3 g/day versus placebo. We recruited 34 outpatients with therapy resistant TUD concurrently treated with smoking-focused group behavioral therapy. Participants had assessments of daily cigarette use (primary outcome), exhaled carbon monoxide (CO(EXH)) (secondary outcome), and quit rates as defined by CO(EXH) < 6 ppm. Depression was measured with the Hamilton Depression Rating Scale (HDRS). Data were analyzed using conventional and modified intention-to-treat endpoint analyses.

RESULTS

NAC treatment significantly reduced the daily number of cigarettes used (Δ mean ± SD = -10.9 ± 7.9 in the NAC-treated versus -3.2 ± 6.1 in the placebo group) and CO(EXH) (Δ mean ± SD = -10.4 ± 8.6 ppm in the NAC-treated versus -1.5 ± 4.5 ppm in the placebo group); 47.1% of those treated with NAC versus 21.4% of placebo-treated patients were able to quit smoking as defined by CO(EXH) < 6 ppm. NAC treatment significantly reduced the HDRS score in patients with tobacco use disorder.

CONCLUSIONS

These data show that treatment with NAC may have a clinical efficacy in TUD. NAC combined with appropriate psychotherapy appears to be an efficient treatment option for TUD.

Inside-out neuropharmacology of nicotinic drugs

Upregulation of neuronal nicotinic acetylcholine receptors (AChRs) is a venerable result of chronic exposure to nicotine; but it is one of several consequences of pharmacological chaperoning by nicotine and by some other nicotinic ligands, especially agonists. Nicotinic ligands permeate through cell membranes, bind to immature AChR oligomers, elicit incompletely understood conformational reorganizations, increase the interaction between adjacent AChR subunits, and enhance the maturation process toward stable AChR pentamers. These changes and stabilizations in turn lead to increases in both anterograde and retrograde traffic within the early secretory pathway. In addition to the eventual upregulation of AChRs at the plasma membrane, other effects of pharmacological chaperoning include modifications to endoplasmic reticulum stress and to the unfolded protein response. Because these processes depend on pharmacological chaperoning within intracellular organelles, we group them as "inside-out pharmacology". This term contrasts with the better-known, acute, "outside-in" effects of activating and desensitizing plasma membrane AChRs. We review current knowledge concerning the mechanisms and consequences of inside-out pharmacology. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Time-course of extracellular nicotine and cotinine levels in rat brain following administration of nicotine: effects of route and ethanol coadministration

RATIONALE

Nicotine and ethanol are commonly coabused drugs, and nicotine-laced ethanol products are growing in popularity. However, little is known about time-course changes in extracellular nicotine and cotinine levels in rat models of ethanol and nicotine coabuse.

OBJECTIVES

The objective of the present study was to determine the time-course changes in brain levels of nicotine and cotinine following subcutaneous (SC) and intragastric (IG) nicotine administration in alcohol-preferring (P) and Wistar rats.

METHODS

In vivo microdialysis was used to collect dialysate samples from the nucleus accumbens shell (NACsh) for nicotine and cotinine determinations, following SC administration of (-)-nicotine (0.18, 0.35, and 0.70 mg/kg) in female P and Wistar rats or IG administration of (-)-nicotine (0.35 and 0.70 mg/kg) in 15 % (v/v) ethanol or water in female P rats.

RESULTS

SC nicotine produced nicotine and cotinine dialysate levels as high as 51 and 14 ng/ml, respectively. IG administration of 15 % EtOH + 0.70 mg/kg nicotine in P rats resulted in maximal nicotine and cotinine dialysate levels of 19 and 14 ng/ml, respectively, whereas administration of 0.70 mg/kg nicotine in water resulted in maximal nicotine and cotinine levels of 21 and 25 ng/ml, respectively. Nicotine and cotinine levels were detectable within the first 15 and 45 min, respectively, after IG administration.

CONCLUSIONS

Overall, the results of this study suggest that nicotine is rapidly adsorbed and produces relevant extracellular brain concentrations of nicotine and its pharmacologically active metabolite, cotinine. The persisting high brain concentrations of cotinine may contribute to nicotine addiction.

Mu Opioid Receptor Binding Correlates with Nicotine Dependence and Reward in Smokers

The rewarding effects of nicotine are associated with activation of nicotine receptors. However, there is increasing evidence that the endogenous opioid system is involved in nicotine's rewarding effects. We employed PET imaging with [¹¹C]carfentanil to test the hypotheses that acute cigarette smoking increases release of endogenous opioids in the human brain and that smokers have an upregulation of mu opioid receptors (MORs) when compared to nonsmokers. We found no significant changes in binding potential (BP_ND) of [¹¹C]carfentanil between the placebo and the active cigarette sessions, nor did we observe differences in MOR binding between smokers and nonsmokers. Interestingly, we showed that in smokers MOR availability in bilateral superior temporal cortices during the placebo condition was negatively correlated with scores on the Fagerström Test for Nicotine Dependence (FTND). Also in smokers, smoking-induced decreases in [¹¹C]carfentanil binding in frontal cortical regions were associated with self-reports of cigarette liking and wanting. Although we did not show differences between smokers and nonsmokers, the negative correlation with FTND corroborates the role of MORs in superior temporal cortices in nicotine addiction and provides preliminary evidence of a role of endogenous opioid signaling in frontal cortex in nicotine reward.

Sensory reinforcement-enhancing effects of nicotine via smoking

As has been found in nicotine research on animals, research on humans has shown that acute nicotine enhances reinforcement from rewards unrelated to nicotine intake, but this effect may be specific to rewards from stimuli that are "sensory" in nature. We assessed acute effects of nicotine via smoking on responding for music or video rewards (sensory), for monetary reward (nonsensory), or for no reward (control), to gauge the generalizability of nicotine's reinforcement-enhancing effects. Using a fully within-subjects design, dependent smokers (N = 20) participated in 3 similar experimental sessions, each following overnight abstinence (verified by carbon monoxide <10 ppm) and varying only in the smoking condition. Sessions involved no smoking or smoking "denicotinized" ("denic;" 0.05 mg) or nicotine (0.6 mg) Quest brand cigarettes in controlled fashion prior to responding on a simple operant computer task for each reward separately using a progressive ratio schedule. The reinforcing effects of music and video rewards, but not money, were significantly greater due to the nicotine versus denic cigarette (i.e., nicotine per se), whereas there were no differences between denic cigarette smoking and no smoking (i.e., smoking behavior per se), except for no reward. These effects were not influenced by withdrawal relief from either cigarette. Results that generalize from an auditory to a visual reward confirm that acute nicotine intake per se enhances the reinforcing value of sensory rewards, but its effects on the value of other (perhaps nonsensory) types of rewards may be more modest.

Involvement of glutamatergic and GABAergic systems in nicotine dependence: Implications for novel pharmacotherapies for smoking cessation

Tobacco smoking continues to be a major global health hazard despite significant public awareness of its harmful consequences. Although several treatment options are currently available for smoking cessation, these medications are effective in only a small subset of smokers, and relapse rates continue to be high. Therefore, a better understanding of the neurobiological mechanisms that mediate tobacco dependence is essential for the development of effective smoking cessation medications. Nicotine is the primary psychoactive component of tobacco that drives the harmful tobacco smoking habit. Nicotine binds to nicotinic acetylcholine receptors (nAChRs) in the brain, resulting in the release of a wide range of neurotransmitters, including glutamate and γ-aminobutyric acid (GABA). This review article focuses on the role of the excitatory glutamate system and inhibitory GABA system in nicotine dependence. Accumulating evidence suggests that blockade of glutamatergic transmission or facilitation of GABAergic transmission attenuates the positive reinforcing and incentive motivational aspects of nicotine, inhibits the reward-enhancing and conditioned rewarding effects of nicotine, and blocks nicotine-seeking behavior. Chronic nicotine exposure produced long-term neuroadaptations that contribute to nicotine withdrawal, but the role of GABA and glutamate transmission in nicotine withdrawal is less understood. Overall, the findings presented in this review provide strong converging evidence for the potential effectiveness of glutamatergic and GABAergic medications in nicotine dependence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

[Reconsideration of nicotine and other substance dependence: a clue from dependence-related mentation including reward, motivation, learning, delusion and hallucination toward understanding the concept of non-substance-related addiction]

Nicotine produces core symptoms of substance dependence (craving and withdrawal) without any psychotic symptoms. The psychopharmacological structure of craving is hypothesized to be constituted by three components: the primary reinforcing property of a substance, the secondary reinforcing property of that substance (conditioned aspects of the environment, such as contextual or specific cues associated with substance taking), and the negative affective motivational property during withdrawal (i.e. the desire to avoid the dysphoric withdrawal symptoms elicits craving). Among the three components, the primary reinforcing property of a substance forms the most fundamental factor for establishing substance dependence. Sensitization or reverse tolerance observed in locomotor activity of animals, which had been believed to be a methamphetamine psychosis model, is demonstrated to reflect the establishment of conditioned reinforcement. Finally, non-substance-related addiction such as gambling, internet, and sex is discussed. From the aspect of the above hypothetical psychopharmacological structure of craving, the most significant difference between substance dependence and non-substance-related addiction is that the primary reinforcing property of non-substance reward is relatively intangible in comparison with that of a substance of abuse.

The cannabinoid CB2 receptor is necessary for nicotine-conditioned place preference, but not other behavioral effects of nicotine in mice

RATIONALE

Whereas cannabinoid CB1 receptors have long been known to contribute to the rewarding effects and dependence liability of many drugs of abuse, recent studies have implicated the involvement of cannabinoid CB2 receptors.

OBJECTIVE

Here, we evaluated the role of CB2 receptors in the rewarding properties of nicotine, as assessed in the conditioned place preference (CPP) paradigm and mecamylamine-precipitated withdrawal in nicotine dependent mice.

METHODS

Using complementary pharmacological and genetic approaches, we investigated the involvement of CB2 receptors in nicotine- and cocaine-induced CPP in mice and mecamylamine-precipitated withdrawal in nicotine-dependent mice. We also determined whether deletion of CB2 receptors affects nicotine-induced hypothermia and hypoalgesia.

RESULTS

Nicotine-induced (0.5 mg/kg) CPP was completely blocked by selective CB2 antagonist, SR144528 (3 mg/kg) in wild-type mice, and was absent in CB2 (-/-) mice. Conversely, the CB2 receptor agonist, O-1966 (1, 3, 5, 10, 20 mg/kg) given in combination with a subthreshold dose of nicotine (0.1 mg/kg) elicited a place preference. In contrast, O-1966 (20 mg/kg) blocked cocaine (10 mg/kg)-induced CPP in wild type mice, while CB2 (-/-) mice showed unaltered cocaine CPP. CB2 (+/+) and (-/-) nicotine-dependent mice showed almost identical precipitated withdrawal responses and deletion of CB2 receptor did not alter acute somatic effects of nicotine.

CONCLUSIONS

Collectively, these results indicate that CB2 receptors are required for nicotine-induced CPP in the mouse, while it is not involved in nicotine withdrawal or acute effects of nicotine. Moreover, these results suggest that CB2 receptors play opposing roles in nicotine- and cocaine-induced CPP.