Characterization and comparison of nicotine and cotinine metabolism in vitro and in vivo in DBA/2 and C57BL/6 mice

Electronic cigarettes alter cardiac rhythm and heart rate variability hyperacutely in mice

AIMS

There has been an unprecedented rise in electronic cigarette (EC) usage likely because of its perception of being safer than smoking. Recent studies show that EC exposures impact heart rate (HR) and heart rate variability (HRV), but how they are affected by the timing and frequency of exposures remain unclear. We examined the electrocardiographic (EKG) effects induced by brief EC exposures over time, their relation to EC aerosol particle and mass concentrations, and potential to promote prooxidative effects in the lungs.

METHODS & RESULTS

Six 10-week-old C57BL/6J mice, implanted with telemetry devices to monitor EKG activity continuously, were exposed once per week for three weeks to two EC exposures, each lasting 15-min followed by 45-min post-exposure periods. Filtered air (primary) and PBS aerosol (secondary) were used as controls. After combining weeks, EC aerosol induced bradycardia and increased time domain parameters during EC exposures with significant reductions in the post-exposure periods. Log-transformed frequency domain parameters were significantly elevated during and after exposures (p < 0.001). HRV changes occurred within minutes with similar trends observed in particle number and mass concentrations of EC aerosol. HR and HRV varied by week and parameter, with Week 2 and 3 effects overshadowing those in Week 1. ECs induced prooxidative effects in the lungs as evidenced by elevated potential for hydroxyl radical generation in bronchoalveolar lavage fluid of exposed mice (p = 0.003).

CONCLUSION

Short-term EC exposures altered murine HR and HRV within minutes during and after exposures, effects that were modulated by the timing and frequency of EC exposures.

Sex-dependent differences in the ability of nicotine to modulate discrimination learning and cognitive flexibility in mice

Nicotine, an addictive compound found in tobacco, functions as an agonist of nicotinic acetylcholine receptors (nAChRs) in the brain. Interestingly, nicotine has been reported to act as a cognitive enhancer in both human subjects and experimental animals. However, its effects in animal studies have not always been consistent, and sex differences have been identified in the effects of nicotine on several behaviors. Specifically, the role that sex plays in modulating the effects of nicotine on discrimination learning and cognitive flexibility in rodents is still unclear. Here, we evaluated sex-dependent differences in the effect of daily nicotine intraperitoneal (i.p.) administration at various doses (0.125, 0.25, and 0.5 mg/kg) on visual discrimination (VD) learning and reversal (VDR) learning in mice. In male mice, 0.5 mg/kg nicotine significantly improved performance in the VDR, but not the VD, task, while 0.5 mg/kg nicotine significantly worsened performance in the VD, but not VDR task in female mice. Furthermore, 0.25 mg/kg nicotine significantly worsened performance in the VD and VDR task only in female mice. Next, to investigate the cellular mechanisms that underlie the sex difference in the effects of nicotine on cognition, transcriptomic analyses were performed focusing on the medial prefrontal cortex tissue samples from male and female mice that had received continuous administration of nicotine for 3 or 18 days. As a result of pathway enrichment analysis and protein-protein interaction analysis using gene sets of differentially expressed genes, decreased expression of postsynaptic-related genes in males and increased expression of innate immunity-related genes in females were identified as possible molecular mechanisms related to sex differences in the effects of nicotine on cognition in discrimination learning and cognitive flexibility. Our result suggests that nicotine modulates cognitive function in a sex-dependent manner by alternating the expression of specific gene sets in the medial prefrontal cortex.

Voluntary nicotine consumption and reward in a subset of diversity outbred founder strains

BACKGROUND

Nicotine is largely responsible for the initiation and maintenance of tobacco dependence and contributes to a global health problem.

AIMS

This study characterizes nicotine oral consumption and preference in male and female mice of several Diversity Outbred (DO) founder strains: C57BL/6J, A/J, 129S1/SvImJ, PWK/PhJ, NOD/ShiLtJ, and CAST/EiJ. It assesses the impact of nicotine concentration on intake and preference, the potential interaction of strain with sex, and estimates the degree of heritable variation in nicotine consumption.

METHODS

Two-bottle choice oral self-administration paradigm was used to assess nicotine intake, nicotine preference, and total fluid intake in male and female mice of each strain in a concentration-response manner. A conditioned place preference (CPP) test was performed to evaluate the rewarding and aversive effects of nicotine in certain strains after systemic administration of the drug.

RESULTS

The highest nicotine-consuming strain was found to be 129S1/SvlmJ, and the lowest nicotine-consuming strain was A/J. Strain differences in nicotine intake were not due to differences in bitter and sweet tastes as shown in the saccharine and quinine two-bottle choice tests. A/J strain showed no significant CPP for nicotine while the 129S1/SvImJ strain showed a significant CPP for nicotine and a higher preference when compared to the C57BL/6J strain. Heritability estimates of nicotine intake were sex dependent and concentration dependent.

CONCLUSIONS

Data support that nicotine consumption patterns are heritable with an influence of genotype in a voluntary oral self-administration paradigm. Results pave the way for future studies with the highly recombinant DO mice that might lead to the identification of novel genetic loci and genes influencing nicotine consumption.

Brain Glucose Metabolism as a Readout of the Central Nervous System Impact of Cigarette Smoke Exposure and Withdrawal and the Effects of NFL-101, as an Immune-Based Drug Candidate for Smoking Cessation Therapy

Neuroimaging biomarkers are needed to investigate the impact of smoking withdrawal on brain function. NFL-101 is a denicotinized aqueous extract of tobacco leaves currently investigated as an immune-based smoking cessation therapy in humans. However, the immune response to NFL-101 and its ability to induce significant changes in brain function remain to be demonstrated. Brain glucose metabolism was investigated using [18F]fluoro-deoxy-glucose ([18F]FDG) PET imaging in a mouse model of cigarette smoke exposure (CSE, 4-week whole-body inhalation, twice daily). Compared with control animals, the relative uptake of [18F]FDG in CSE mice was decreased in the thalamus and brain stem (p < 0.001, n = 14 per group) and increased in the hippocampus, cortex, cerebellum, and olfactory bulb (p < 0.001). NFL-101 induced a humoral immune response (specific IgGs) in mice and activated human natural-killer lymphocytes in vitro. In CSE mice, but not in control mice, single-dose NFL-101 significantly increased [18F]FDG uptake in the thalamus (p < 0.01), thus restoring normal brain glucose metabolism after 2-day withdrawal in this nicotinic receptor-rich region. In tobacco research, [18F]FDG PET imaging provides a quantitative method to evaluate changes in the brain function associated with the withdrawal phase. This method also showed the CNS effects of NFL-101, with translational perspectives for future clinical evaluation in smokers.

Nicotine Formulation Influences the Autonomic and Arrhythmogenic Effects of Electronic Cigarettes

INTRODUCTION

Evidence is mounting that electronic cigarette (e-cig) use induces cardiac sympathetic dominance and electrical dysfunction conducive to arrhythmias and dependent upon nicotine. A variety of nicotine types and concentrations are available in e-cigs, but their relative cardiovascular effects remain unclear. Here we examine how different nicotine forms (racemic, free base, and salt) and concentrations influence e-cig-evoked cardiac dysfunction and arrhythmogenesis and provide a mechanism for nicotine-salt-induced autonomic imbalance.

METHODS

ECG-telemetered C57BL/6J mice were exposed to filtered air (FA) or e-cig aerosols from propylene glycol and vegetable glycerin solvents either without nicotine (vehicle) or with increasing nicotine concentrations (1%, 2.5%, and 5%) for three 9-minute puff sessions per concentration. Spontaneous ventricular premature beat (VPB) incidence rates, heart rate, and heart rate variability (HRV) were compared between treatments. Subsequently, to test the role of β1-adrenergic activation in e-cig-induced cardiac effects, mice were pretreated with atenolol and exposed to either FA or 2.5% nicotine salt.

RESULTS

During puffing and washout phases, ≥2.5% racemic nicotine reduced heart rate and increased HRV relative to FA and vehicle controls, indicating parasympathetic dominance. Relative to both controls, 5% nicotine salt elevated heart rate and decreased HRV during washout, suggesting sympathetic dominance, and also increased VPB frequency. Atenolol abolished e-cig-induced elevations in heart rate and declines in HRV during washout, indicating e-cig-evoked sympathetic dominance is mediated by β1-adrenergic stimulation.

CONCLUSIONS

Our findings suggest that inhalation of e-cig aerosols from nicotine-salt-containing e-liquids could increase the cardiovascular risks of vaping by inducing sympathetic dominance and cardiac arrhythmias.

IMPLICATIONS

Exposure to e-cig aerosols containing commercially relevant concentrations of nicotine salts may increase nicotine delivery and impair cardiac function by eliciting β1-adrenoceptor-mediated sympathoexcitation and provoking ventricular arrhythmias. If confirmed in humans, our work suggests that regulatory targeting of nicotine salts through minimum pH standards or limits on acid additives in e-liquids may mitigate the public health risks of vaping.

Chronic Exposure to E-Cigarettes Elevates CYP2A5 Activity, Protein Expression, and Cotinine-Induced Production of Reactive Oxygen Species in Mice

Coumarin 7'-hydroxylase activity, a specific marker of CYP2A5 activity, and the protein level were measured in liver microsomes of male mice after chronic exposure to e-cigarettes (e-cigs) (2.4% nicotine). After exposure for 240 minutes per day for 5 days, the activity and the protein level in preproenkephalin (ppENK)-heterozygous [ppENK (+/-)] mice were significantly elevated (P <0.05) compared with the untreated control. This elevation was not due to deletion of the ppENK gene because the activity did not differ among untreated ppENK (+/-), ppENK (-/-), and wild-type ppENK (+/+) controls. Hence, the elevation can reasonably be attributed to nicotine exposure. The production of reactive oxygen species (ROS) upon incubation of the hepatic microsomes of these mice with cotinine was higher in microsomes from the e-cig-treated mice compared with the untreated controls (P < 0.01). Liquid chromatography mass spectrometry assay showed three oxidation products of cotinine, viz trans 3'-hydroxycotinine (3'-HC), 5'-hydroxycotinine (5'-HC), and cotinine N-oxide (CNO) in the plasma of these mice. The result identifies these three oxidation reactions as the source of the observed ROS and also shows that, in nicotine-treated mice, the appropriate "nicotine metabolite ratio" is (3'-HC + 5'-HC + CNO)/cotinine. The results suggest intriguing possibilities that 1) this metabolite ratio may correlate with plasma nicotine clearance and hence impact nicotine's psychoactive effects and 2) chronic e-cig treatment causes ROS-induced oxidative stress, which may play a major role in the regulation of CYP2A5 expression. Our present results clearly show that both the activity and the protein level of CYP2A5 are elevated by repeated exposure to nicotine. SIGNIFICANCE STATEMENT: Nicotine, the psychoactive ingredient of tobacco, is eliminated as the oxidation products of cotinine in reactions catalyzed by the enzymes CYP2A5 in mice and CYP2A6 in humans. This study shows that repeated exposure to e-cigarettes elevates the level of CYP2A5 and the formation of reactive oxygen species. The results suggest an intriguing possibility that CYP2A5 may be upregulated by chronic nicotine exposure due to oxidative stress caused by the oxidation of cotinine in this preclinical model of human smokers.

Nicotine exacerbates diabetic nephropathy through upregulation of Grem1 expression

BACKGROUND

Diabetic nephropathy (DN) is a major complication of diabetes mellitus. Clinical reports indicate that smoking is a significant risk factor for chronic kidney disease, and the tobacco epidemic exacerbates kidney damage in patients with DN. However, the underlying molecular mechanisms remain unclear.

METHOD

In the present study, we used a diabetic mouse model to investigate the molecular mechanisms for nicotine-exacerbated DN. Twelve-week-old female mice were injected with streptozotocin (STZ) to establish a hyperglycemic diabetic model. After four months, the control and hyperglycemic diabetic mice were further divided into four groups (control, nicotine, diabetic mellitus, nicotine + diabetic mellitus) by intraperitoneal injection of nicotine or PBS. After two months, urine and blood were collected for kidney injury assay, and renal tissues were harvested for further molecular assays using RNA-seq analysis, real-time PCR, Western blot, and immunohistochemistry. In vitro studies, we used siRNA to suppress Grem1 expression in human podocytes. Then we treated them with nicotine and high glucose to compare podocyte injury.

RESULT

Nicotine administration alone did not cause apparent kidney injury, but it significantly increased hyperglycemia-induced albuminuria, BUN, plasma creatinine, and the kidney tissue mRNA expression of KIM-1 and NGAL. Results from RNA-seq analysis, real-time PCR, Western blot, and immunohistochemistry analysis revealed that, compared to hyperglycemia or nicotine alone, the combination of nicotine treatment and hyperglycemia significantly increased the expression of Grem1 and worsened DN. In vitro experiments, suppression of Grem1 expression attenuated nicotine-exacerbated podocyte injury.

CONCLUSION

Grem1 plays a vital role in nicotine-exacerbated DN. Grem1 may be a potential therapeutic target for chronic smokers with DN.

Strawberry Additive Increases Nicotine Vapor Sampling and Systemic Exposure But Does Not Enhance Pavlovian-Based Nicotine Reward in Mice

Nicotine is an addictive drug whose popularity has recently increased, particularly among adolescents, because of the availability of electronic nicotine devices (i.e., "vaping") and nicotine e-liquids containing additives with rich chemosensory properties. Some efforts to understand the role of these additives in nicotine reward suggest that they increase nicotine reward and reinforcement, but the sensory contributions of additives, especially in their vapor forms, are largely untested. Here, to better understand how a fruit-flavored (i.e., strawberry) additive influences nicotine reward and aversion, we used a conditioned place preference (CPP) procedure in which nicotine and a strawberry additive were delivered as a vapor to male and female adolescent mice. We found that nicotine vapor alone can lead to a dose-dependent CPP when using a biased design. The strawberry additive did not produce CPP on its own, and we did not observe an effect of the strawberry additive on nicotine vapor-induced reward. Nevertheless, mice exposed to nicotine plus strawberry additive vapor had higher plasma cotinine concentrations, which did not appear to reflect altered nicotine metabolism. Instead, by directly measuring vapor sampling through respiration monitoring, we uncovered an increase in the amount of sniffing toward strawberry-containing nicotine vapor compared with nicotine vapor alone. Together these data indicate that chemosensory-rich e-liquid additives may enhance the perceived sensory profile of nicotine vapors rather than the reward value per se, which leads to overall increased nicotine exposure.

Nicotinic acetylcholine receptors and learning and memory deficits in Neuroinflammatory diseases

Animal survival depends on cognitive abilities such as learning and memory to adapt to environmental changes. Memory functions require an enhanced activity and connectivity of a particular arrangement of engram neurons, supported by the concerted action of neurons, glia, and vascular cells. The deterioration of the cholinergic system is a common occurrence in neurological conditions exacerbated by aging such as traumatic brain injury (TBI), posttraumatic stress disorder (PTSD), Alzheimer's disease (AD), and Parkinson's disease (PD). Cotinine is a cholinergic modulator with neuroprotective, antidepressant, anti-inflammatory, antioxidant, and memory-enhancing effects. Current evidence suggests Cotinine's beneficial effects on cognition results from the positive modulation of the α7-nicotinic acetylcholine receptors (nAChRs) and the inhibition of the toll-like receptors (TLRs). The α7nAChR affects brain functions by modulating the function of neurons, glia, endothelial, immune, and dendritic cells and regulates inhibitory and excitatory neurotransmission throughout the GABA interneurons. In addition, Cotinine acting on the α7 nAChRs and TLR reduces neuroinflammation by inhibiting the release of pro-inflammatory cytokines by the immune cells. Also, α7nAChRs stimulate signaling pathways supporting structural, biochemical, electrochemical, and cellular changes in the Central nervous system during the cognitive processes, including Neurogenesis. Here, the mechanisms of memory formation as well as potential mechanisms of action of Cotinine on memory preservation in aging and neurological diseases are discussed.

Genetic differences in nicotine sensitivity and metabolism in C57BL/6J and NOD/ShiLtJ mouse strains

Genetic background impacts sensitivity to nicotine's rewarding and aversive effects and metabolism, which influences susceptibility to nicotine addiction. This is important because sensitivity to nicotine influences susceptibility to nicotine addiction. Thus, understanding genetic contribution to nicotine sensitivity can aid in identifying risk factors for nicotine addiction. Genetic variability in addiction phenotypes can be modeled in rodent systems, and comparisons of nicotine sensitivity in inbred mice can identify contributing genetic substrates. Our laboratory has identified differences in nicotine sensitivity in male mice from two inbred mouse strains, C57BL/6J and NOD/ShiLtJ. We found that the NOD/ShiLtJ strain experienced greater nicotine-induced locomotor depression and hypothermia than the C57BL/6J strain. To investigate possible differences in nicotine metabolism between strains, subjects were treated with acute nicotine and serum and urine samples were analyzed using LC-MS/MS to quantify nicotine and metabolites. This analysis revealed that NOD/ShiLtJ mice had similar serum nicotine but lower cotinine and 3'-hydroxycotinine levels after nicotine treatment when compared to C57BL/6J mice. Possible genetic factors mediating strain differences were identified by surveying nicotine sensitivity- and metabolism-related genes within the Mouse Phenome Database SNP retrieval tool. Polymorphisms were found in 15 of the 26 examined gene sequences. Liver expression levels of nicotine metabolism-related genes (Cyp2a5, Cyp2a4, and Aox1) were measured using qPCR. NOD/ShiLtJ mice showed lower expression of Cyp2a5 and Cyp2a4 and greater expression of Aox1 in liver tissue. These data demonstrate complex differences in nicotine sensitivity and metabolism driven by genetic differences between C57BL/6J and NOD/ShiLtJ inbred mouse strains.

Identification of 5-Hydroxycotinine in the Plasma of Nicotine-Treated Mice: Implications for Cotinine Metabolism and Disposition in Vivo

Two oxidation products of cotinine, 5-hydroxycotinine (5-HC) and cotinine N-oxide (CNO), were identified for the first time in vivo in the plasma of C57BL/6 mice after injection of nicotine (1 mg/kg) or exposure to an e-cigarette (e-cig) containing 2.4% nicotine. Liquid chromatography-mass spectrometry was used to separate 3-hydroxycotinine (3-HC), 5-HC, and CNO and to quantify each by the sensitive direct detection of their parent ion with m/z of 193.097. In nicotine-injected mice, 5-HC was as abundant as 3-HC 15 minutes postinjection, and CNO was readily detectable. In e-cig-exposed mice with plasma nicotine levels resembling that of human smokers, plasma 5-HC and CNO, as well as 3-HC, were readily quantifiable at the end of the 4-hour exposure time. In nicotine-injected mice, the combined concentration of 3-HC plus 5-HC plus CNO, all formed from cotinine by CYP2A5, was higher (P < 0.01) in females than in males, although the male-female difference in cotinine plasma level did not reach statistical significance. The result highlights the importance of considering these three oxidation products of cotinine in examining cotinine metabolism and disposition. Coumarin 7-hydroxylase activity, a specific marker of CYP2A5, measured in the hepatic microsomes of untreated mice showed that females have higher activity (P < 0.001) than males (N = 8 per sex). The abundance of plasma 5-hydroxycotinine in nicotine-treated mice raises intriguing questions about the site of its origin (hepatic or possibly kidney CYP2A5) and the routes of its disposition because urinary excretion of 5-HC has not been detected by liquid chromatography with tandem mass spectrometry in mice and is controversial in human smokers. SIGNIFICANCE STATEMENT: Nicotine is the active ingredient of tobacco, but its elimination route through its biomarker cotinine is not fully understood. By liquid chromatography-mass spectrometry, this study has identified and quantified for the first time 5-hydroxycotinine and cotinine N-oxide, which are oxidation products of cotinine, in the plasma of mice treated with nicotine or exposed to e-cigarettes. The results raise intriguing questions about nicotine disposition in vivo in this well established preclinical model of human smokers.

An inhibitory brainstem input to dopamine neurons encodes nicotine aversion

Nicotine stimulates the dopamine (DA) system, which is essential for its rewarding effect. Nicotine is also aversive at high doses; yet, our knowledge about nicotine's dose-dependent effects on DA circuits remains limited. Here, we demonstrate that high doses of nicotine, which induce aversion-related behavior in mice, cause biphasic inhibitory and excitatory responses in VTA DA neurons that can be dissociated by distinct projections to lateral and medial nucleus accumben subregions, respectively. Guided by computational modeling, we performed a pharmacological investigation to establish that inhibitory effects of aversive nicotine involve desensitization of α4β2 and activation of α7 nicotinic acetylcholine receptors. We identify α7-dependent activation of upstream GABA neurons in the laterodorsal tegmentum (LDT) as a key regulator of heterogeneous DA release following aversive nicotine. Finally, inhibition of LDT GABA terminals in VTA prevents nicotine aversion. Together, our findings provide a mechanistic circuit-level understanding of nicotine's dose-dependent effects on reward and aversion.

Genetic influences impacting nicotine use and abuse during adolescence: Insights from human and rodent studies

Nicotine use continues to be a major public health concern, with an alarming recent rise in electronic cigarette consumption. Heritability estimates of nicotine use and abuse range from 40% to 80%, providing strong evidence that genetic factors impact nicotine addiction-relevant phenotypes. Although nicotine use during adolescence is a key factor in the development of addiction, it remains unclear how genetic factors impact adolescent nicotine use and abuse. This review will discuss studies investigating genetic factors impacting nicotine use during adolescence. Evidence from both rodent and human studies will be summarized and integrated when possible. Human adolescent studies have largely included candidate gene studies for genes identified in adult populations, such as genes involved in nicotine metabolism, nicotinic acetylcholine receptor signaling, dopaminergic signaling, and other neurotransmitter signaling systems. Alternatively, rodent studies have largely taken a discovery-based approach identifying strain differences in adolescent nicotine addiction-relevant behaviors. Here, we aim to answer the following three questions by integrating human and rodent findings: (1) Are there genetic variants that uniquely impact nicotine use during adolescence? (2) Are there genetic variants that impact both adolescent and adult nicotine use? and (3) Do genetic factors in adolescence significantly impact long-term consequences of adolescent nicotine use? Determining answers for these three questions will be critical for the development of preventative measures and treatments for adolescent nicotine use and addiction.

Pharmacological and physiological response in Apoe-/- mice exposed to cigarette smoke or e-cigarette aerosols

OBJECTIVE

Electronic cigarettes (e-cigs) are popular nicotine delivery devices, yet the health effects remain unclear. To determine equivalent biomarkers, we characterized the immediate response in Apoe^-/- mice exposed to tank/box-mod e-cig (e-cig_tank), pod e-cig (e-cig_pod), or cig smoke.

MATERIALS AND METHODS

Reproducible puff profiles were generated for each aerosol and delivered to Apoe^-/- mice via a nose-only exposure system. Serum cotinine levels were quantified at various time points through ELISA and utilized to model cotinine pharmacokinetics. In addition, particle size measurements and mouse respiratory function were characterized to calculate particle dosimetry.

RESULTS AND DISCUSSION

Cig and e-cig_tank particles were lognormally distributed with similar count median diameters (cig: 178 ± 2, e-cig_tank: 200 ± 34nm), while e-cig_pod particles were bimodally distributed and smaller (116 ± 13 and 13.3 ± 0.4 nm). Minute volumes decreased with cig exposure (5.4 ± 2.7 mL/min) compared to baseline (90.8 ± 11.6 mL/min), and less so with e-cig_tank (45.2 ± 9.2 mL/min) and e-cig_pod exposures (58.6 ± 6.8 mL/min), due to periods of apnea in the cig exposed groups. Cotinine was absorbed and eliminated most rapidly in the e-cig_pod group (tmax = 14.5; t1/2' = 51.9 min), whereas cotinine was absorbed (cig: 50.4, e-cig_tank: 40.1 min) and eliminated (cig: 104.6, e-cig_tank: 94.1 min) similarly in the cig and e-cig_tank groups. For exposure times which equate the area under the cotinine-concentration curve, ∼6.4× (e-cig_tank) and 4.6× (e-cig_pod) more nicotine deposited in e-cig compared to cig exposed mice.

CONCLUSIONS

This study provides a basis for incorporating cotinine pharmacokinetics into preclinical exposure studies, allowing for longitudinal studies of structural and functional changes due to exposure.

Inbred mouse strain differences in alcohol and nicotine addiction-related phenotypes from adolescence to adulthood

Understanding the genetic basis of a predisposition for nicotine and alcohol use across the lifespan is important for public health efforts because genetic contributions may change with age. However, parsing apart subtle genetic contributions to complex human behaviors is a challenge. Animal models provide the opportunity to study the effects of genetic background and age on drug-related phenotypes, while controlling important experimental variables such as amount and timing of drug exposure. Addiction research in inbred, or isogenic, mouse lines has demonstrated genetic contributions to nicotine and alcohol abuse- and addiction-related behaviors. This review summarizes inbred mouse strain differences in alcohol and nicotine addiction-related phenotypes including voluntary consumption/self-administration, initial sensitivity to the drug as measured by sedative, hypothermic, and ataxic effects, locomotor effects, conditioned place preference or place aversion, drug metabolism, and severity of withdrawal symptoms. This review also discusses how these alcohol and nicotine addiction-related phenotypes change from adolescence to adulthood.

Genotypic Differences in the Effects of Menthol on Nicotine Intake and Preference in Mice

Menthol has been shown to exacerbate elements of nicotine addiction in humans and rodents; however, the mechanisms mediating its effects are not fully understood. This study examined the impact of genetic factors in menthol’s effects on oral nicotine consumption by comparing two inbred mouse strains with differing sensitivities to nicotine. C57BL/6J (B6J) mice are nicotine-preferring, while DBA/2J (D2J) mice are not. While the effects of menthol on oral nicotine consumption have been highlighted in B6J mice, it is unknown if they extend to the D2J strain as well. Consequently, adolescent (PND 21) and adult (PND 63), male and female D2J mice were subjected to the nicotine two-bottle choice (2BC) paradigm with orally and systemically administered menthol. Then, we evaluated its impact on nicotine pharmacological responses in conditioned reward and nociception after systemic administration and, lastly, investigated the potential involvement of the TAAR1 gene and α7 nAChRs in menthol’s effects. Menthol failed to enhance oral nicotine consumption in adult and adolescent female and male D2J mice. Moreover, this lack in effect was not due to nicotine concentration, oral aversion to menthol, or basal preference for nicotine. Menthol also failed to augment nicotine reward or enhance nicotine-induced antinociception in D2J mice, demonstrating that genetic background plays a significant role in sensitivity to menthol’s effects on nicotine. Furthermore, TAAR1 or α7 nAChRs did not seem to mediate menthol’s differential effects in D2J mice. These findings support the existence of genotype-specific mechanisms that may contribute to the variable effects of menthol in different populations.

Characterization of the Brain Functional Architecture of Psychostimulant Withdrawal Using Single-Cell Whole-Brain Imaging

Numerous brain regions have been identified as contributing to withdrawal behaviors, but it is unclear the way in which these brain regions as a whole lead to withdrawal. The search for a final common brain pathway that is involved in withdrawal remains elusive. To address this question, we implanted osmotic minipumps containing either saline, nicotine (24 mg/kg/d), cocaine (60 mg/kg/d), or methamphetamine (4 mg/kg/d) for one week in male C57BL/6J mice. After one week, the minipumps were removed and brains collected 8 h (saline, nicotine, and cocaine) or 12 h (methamphetamine) after removal. We then performed single-cell whole-brain imaging of neural activity during the withdrawal period when brains were collected. We used hierarchical clustering and graph theory to identify similarities and differences in brain functional architecture. Although methamphetamine and cocaine shared some network similarities, the main common neuroadaptation between these psychostimulant drugs was a dramatic decrease in modularity, with a shift from a cortical-driven to subcortical-driven network, including a decrease in total hub brain regions. These results demonstrate that psychostimulant withdrawal produces the drug-dependent remodeling of functional architecture of the brain and suggest that the decreased modularity of brain functional networks and not a specific set of brain regions may represent the final common pathway associated with withdrawal.

Nicotine formulations impact reinforcement-related behaviors in a mouse model of vapor self-administration

BACKGROUND

Electronic nicotine delivery systems (ENDS) differ from combustible cigarettes given that nicotine-salt or nicotine-freebase may be used depending on the product. We have investigated how nicotine-salt and freebase formulations alter e-Vape® self-administration (EVSA) behavior and plasma cotinine levels in male and female mice.

METHODS

Adult C57/BL6 J mice were used in EVSA and assigned vaping e-liquids (50:50 PGVG, 6 mg/mL nicotine-freebase, or 6 mg/mL nicotine-salt). Mice were escalated on a fixed ratio 1 (FR1) schedule in daily 2 h sessions and then transitioned to a FR3 to examine reinforcement-related behaviors.

RESULTS

Here we observed that mice assigned nicotine-salt exhibited increased EVSA on a FR3 schedule compared to nicotine-freebase. Additionally, mice assigned nicotine-salt exhibited higher plasma cotinine concentrations following delivery-controlled passive-inhalation sessions.

CONCLUSIONS

These data provide evidence nicotine-salt formulations may contribute to greater reinforcement-related behavior and highlight the need for further investigations regarding nicotine formulation in ENDS.

Multigenerational nicotine exposure affects offspring nicotine metabolism, nicotine-induced hypothermia, and basal corticosterone in a sex-dependent manner

Parental nicotine exposure can impact phenotypes in unexposed offspring. Our laboratory recently published data showing that nicotine reward and hippocampal gene expression involved in stress pathways were perturbed in F1 offspring of male C57BL/6J mice chronically exposed to nicotine. For the current study, we aimed to further test nicotine and stress-sensitivity phenotypes that may predict vulnerability to nicotine addiction in new cohorts of F1 offspring derived from nicotine-exposed males. We tested locomotor and body temperature sensitivity to acute nicotine administration, serum concentration of nicotine and nicotine metabolites after acute nicotine dosing, and serum corticosterone levels in male and female F1 offspring of nicotine- or saline-exposed males. Paternal nicotine exposure reduced sensitivity to nicotine-induced hypothermia in males, altered nicotine metabolite concentrations in males and females, and reduced serum basal corticosterone levels in females. These findings may point to reduced susceptibility to nicotine addiction-related phenotypes as a result of parental nicotine exposure.

In vitro and in vivo cardiac toxicity of flavored electronic nicotine delivery systems

The usage of flavored electronic nicotine delivery systems (ENDS) is popular, specifically in the teen and young adult age-groups. The possible cardiac toxicity of the flavoring aspect of ENDS is largely unknown. Vaping, a form of electronic nicotine delivery, uses "e-liquid" to generate "e-vapor," an aerosolized mixture of nicotine and/or flavors. We report our investigation into the cardiotoxic effects of flavored e-liquids. E-vapors containing flavoring aldehydes such as vanillin and cinnamaldehyde, as indicated by mass spectrometry, were more toxic in HL-1 cardiomyocytes than fruit-flavored e-vapor. Exposure of human induced pluripotent stem cell-derived cardiomyocytes to cinnamaldehyde or vanillin-flavored e-vapor affected the beating frequency and prolonged the field potential duration of these cells more than fruit-flavored e-vapor. In addition, vanillin aldehyde-flavored e-vapor reduced the human ether-à-go-go-related gene (hERG)-encoded potassium current in transfected human embryonic kidney cells. In mice, inhalation exposure to vanillin aldehyde-flavored e-vapor for 10 wk caused increased sympathetic predominance in heart rate variability measurements. In vivo inducible ventricular tachycardia was significantly longer, and in optical mapping, the magnitude of ventricular action potential duration alternans was significantly larger in the vanillin aldehyde-flavored e-vapor-exposed mice than in controls. We conclude that the widely popular flavored ENDS are not harm free, and they have a potential for cardiac harm. More studies are needed to further assess their cardiac safety profile and long-term health effects.NEW & NOTEWORTHY The use of electronic nicotine delivery systems (ENDS) is not harm free. It is not known whether ENDS negatively affect cardiac electrophysiological function. Our study in cell lines and in mice shows that ENDS can compromise cardiac electrophysiology, leading to action potential instability and inducible ventricular arrhythmias. Further investigations are necessary to assess the long-term cardiac safety profile of ENDS products in humans and to better understand how individual components of ENDS affect cardiac toxicity.

Postnatal Smoke Exposure Further Increases the Hepatic Nicotine Metabolism in Prenatally Smoke Exposed Male Offspring and Is Linked with Aberrant Cyp2a5 Methylation

Prenatal smoke exposure (PreSE) is a risk factor for nicotine dependence, which is further enhanced by postnatal smoke exposure (PostSE). One susceptibility gene to nicotine dependence is Cytochrome P450 (CYP) 2A6, an enzyme responsible for the conversion of nicotine to cotinine in the liver. Higher CYP2A6 activity is associated with nicotine dependence and could be regulated through DNA methylation. In this study we investigated whether PostSE further impaired PreSE-induced effects on nicotine metabolism, along with Cyp2a5, orthologue of CYP2A6, mRNA expression and DNA methylation. Using a mouse model where prenatally smoke-exposed adult offspring were exposed to cigarette smoke for 3 months, enzyme activity, mRNA levels, and promoter methylation of hepatic Cyp2a5 were evaluated. We found that in male offspring, PostSE increased PreSE-induced cotinine levels and Cyp2a5 mRNA expression. In addition, both PostSE and PreSE changed Cyp2a5 DNA methylation in male groups. PreSE however decreased cotinine levels whereas it had no effect on Cyp2a5 mRNA expression or methylation. These adverse outcomes of PreSE and PostSE were most prominent in males. When considered in the context of the human health aspects, the combined effect of prenatal and adolescent smoke exposure could lead to an accelerated risk for nicotine dependence later in life.

Prenatal smoke exposure induces persistent Cyp2a5 methylation and increases nicotine metabolism in the liver of neonatal and adult male offspring

Prenatal smoke exposure (PSE) is a risk factor for nicotine dependence. One susceptibility gene for nicotine dependence is Cytochrome P450 (CYP) 2A6, an enzyme responsible for the conversion of nicotine to cotinine and nicotine clearance in the liver. Higher activity of the CYP2A6 enzyme is associated with nicotine dependence, but no research has addressed the PSE effects on the CYP2A6 gene or its mouse homologue Cyp2a5. We hypothesized that PSE affects Cyp2a5 promoter methylation, Cyp2a5 mRNA levels, and nicotine metabolism in offspring. We used a smoke-exposed pregnant mouse model. RNA, DNA, and microsomal protein were isolated from liver tissue of foetal, neonatal, and adult offspring. Enzyme activity, Cyp2a5 mRNA levels, and Cyp2a5 methylation status of six CpG sites within the promoter region were analysed via HPLC, RT-PCR, and bisulphite pyrosequencing. Our data show that PSE induced higher cotinine levels in livers of male neonatal and adult offspring compared to controls. PSE-induced cotinine levels in neonates correlated with Cyp2a5 mRNA expression and promoter methylation at CpG-7 and CpG+45. PSE increased methylation in almost all CpG sites in foetal offspring, and this effect persisted at CpG-74 in male neonatal and adult offspring. Our results indicate that male offspring of mothers which were exposed to cigarette smoke during pregnancy have a higher hepatic nicotine metabolism, which could be regulated by DNA methylation. Given the detected persistence into adulthood, extrapolation to the human situation suggests that sons born from smoking mothers could be more susceptible to nicotine dependence later in life.

Whole body electronic cigarette exposure system for efficient evaluation of diverse inhalation conditions and products

Objectives: To develop and test a new system for whole body exposure of small animals to support investigation of the biological effects of aerosol generated by electronic cigarette (e-cig) products under diverse inhalation conditions with improved control and monitoring of the e-cig vape exposure and nicotine delivered to the animal's systemic circulation. Methods: A computer-controlled design, with built-in sensors for real time monitoring of O₂, CO₂, relative humidity, and temperature within the exposure chambers and port for measuring total particulate matter (TPM) was developed, constructed and tested. This design accommodates a variety of commercial vaping devices, offers software flexibility to adjust exposure protocols to mimic different users' puffing patterns, enables variable nicotine delivery to the animal's systemic circulation; minimizes travel time and alterations of aerosol quality or quantity by delivering aerosol directly to the exposure chamber, offers local or remote operation of up to six distinct exposure chambers from a single control unit, and can simultaneously test different exposure conditions or products in diverse animal groups, which reduces inter-run variability, saves time, and increases productivity. Results: The time course pattern of TPM concentration during different phases of the exposure cycle was measured. With increased puffing duration or number of exposure cycles, higher TPM exposure and plasma cotinine levels were observed with plasma cotinine levels in the range reported in light or heavy smokers. Conclusion: Overall, this novel, versatile, and durable exposure system facilitates high-throughput evaluation of the relative safety and potential toxicity of a variety of e-cig devices and liquids.

The Impact of Sex on Changes in Plasma Corticosterone and Cotinine Levels Induced by Nicotine in C57BL/6J Mice

We assessed if there were any sex-related differences in the ability of nicotine to increase plasma corticosterone secretion after single or repeated nicotine administration. For single-dose studies, male and female mice were habituated to the test room for 1 h and injected with saline or nicotine (0.25 or 1 mg/kg, subcutaneously (s.c.)). In repeated-dosing studies, mice were injected with saline or nicotine (1 mg/kg, s.c.) once daily for six days, and, on day 7, received nicotine (1 mg/kg, s.c.). Mice were then euthanized 15 min later, and trunk blood was collected for the measurement of corticosterone, nicotine, and cotinine. Our results showed that saline or nicotine each significantly increased plasma corticosterone levels in both males and females, with a greater response in female mice. Plasma corticosterone levels were increased in male but not female mice after being treated repeatedly compared to single nicotine administration. The level of cotinine, a biomarker of nicotine use, was significantly higher in female than in male mice. Taken together, these novel findings suggest that female mice respond to nicotine and the stress of handling more than male mice and provide for the first-time quantitative data on male–female differences in nicotine-induced elevations of corticosterone and cotinine plasma levels.

Electronic cigarette exposure disrupts blood-brain barrier integrity and promotes neuroinflammation

Electronic cigarette (e-cigarette) use has grown substantially since inception, particularly among adolescents and combustible tobacco users. Several cigarette smoke constituents with known neurovascular effect are present in e-cigarette liquids or formed during the vapor generation. The present study establishes inhaled models of cigarette and e-cigarette use with normalized nicotine delivery, then characterizes the impact on blood-brain barrier (BBB) function. Sequencing of microvessel RNA following exposure revealed downregulation of several genes with critical roles in BBB function. Reduced protein expression of Occludin and Glut1 is also observed at the tight junction in all groups following exposure. Pro-inflammatory changes in leukocyte-endothelial cell interaction are also noted, and mice exposed to nicotine-free e-cigarettes have impaired novel object recognition performance. On this basis, it is concluded that long term e-cigarette use may adversely impact neurovascular health. The observed effects are noted to be partly independent of nicotine content and nicotine may even serve to moderate the effects of non-nicotinic components on the blood-brain barrier.

Strain-Dependent Variability of Early Discovery Small Molecule Pharmacokinetics in Mice: Does Strain Matter?

Drug discovery programs routinely perform pharmacokinetic (PK) studies in mice to prioritize lead compounds based on anticipated exposure-efficacy and exposure-toxicity relationships. Because of logistical and/or technical issues, the strain of mouse in early discovery PK studies may not always match the strain in toxicity or efficacy studies. This elicits the question do appreciable strain-dependent differences in PK parameters exist to an extent that would warrant conducting PK studies in a strain that matches efficacy and toxicity models? To understand the impact that strain may have on PK parameters, we selected eight marketed drugs with well characterized absorption, distribution, metabolism, and excretion properties and diverse structures to perform PK studies in three common mouse strains (Bagg Albino c, C57BL/6, and CD-1). Some statistical strain-dependent differences were observed; however, we found good general agreement of PK parameters between strains: 88%, 100%, 75%, 76%, 94%, and 88% of compounds were within twofold across strains for clearance, volume of distribution at steady state, t 1/2, C max, T max, and oral bioavailability, respectively. Overall, we recommend that an approach using a single strain of mouse is appropriate for discovery screening PK studies, provided that proper caution is exercised. SIGNIFICANCE STATEMENT: The mouse strain in discovery pharmacokinetic (PK) studies may not match the strain in efficacy and toxicology studies. Currently, there is a gap in the literature addressing whether differences in PK parameters across mouse strains exist such that multiple PK studies are warranted. The results from this study indicated that the PK properties of clinically used drugs between mouse strains are within an acceptable range such that single strain PK is appropriate.

In vivo quantification and pharmacokinetic studies of cotinine in mice after smoke exposure by LC-MS/MS

A sensitive analytical method was developed and validated for the quantification of cotinine in mouse plasma after exposure to smoke of 0.5, 1.0, and 1.5 commercially available cigarettes, using liquid chromatography tandem mass spectrometry. The method was validated over a linear concentration range of 0.075-20.0 ng/mL with the R² value being higher than 0.99. Both the precision (coefficient of variation; %) and accuracy (relative error; %) were within acceptable criteria of <15%. The lower limit of quantification (LLOQ) for cotinine was 0.075 ng/mL with sufficient specificity, accuracy, and precision. Following exposure to 0.5, 1.0, and 1.5 cigarette smoke, it was observed that the AUC and the C_max increased linearly as the doses increased. The pharmacokinetics of cotinine was found linear for the range of 0.5-1.5 commercial cigarette smoke. The quantification of the concentration of cotinine in mouse plasma after smoke exposure will facilitate future behavioral and toxicological experiments in animals and may prove useful in predicting cotinine levels in humans during smoking.

Modeling drug exposure in rodents using e-cigarettes and other electronic nicotine delivery systems

Smoking tobacco products is the leading cause of preventable death worldwide. Coordinated efforts have successfully reduced tobacco cigarette smoking in the United States; however, electronic cigarettes (e-cigarette) and other electronic nicotine delivery systems (ENDS) recently have replaced traditional cigarettes for many users. While the clinical risks associated with long-term ENDS use remain unclear, advancements in preclinical rodent models will enhance our understanding of their overall health effects. This review examines the peripheral and central effects of ENDS-mediated exposure to nicotine and other drugs of abuse in rodents and evaluates current techniques for implementing ENDS in preclinical research.

C57BL/6 Substrain Differences in Pharmacological Effects after Acute and Repeated Nicotine Administration

Tobacco smoking is the major cause of disability and death in the United States and around the world. In addition, tobacco dependence and addiction express themselves as complex behaviors involving an interplay of genetics, environment, and psychological state. Mouse genetic studies could potentially elucidate the novel genes and/or gene networks regulating various aspects of nicotine dependence. Using the closely related C57BL/6 (B6) mice substrains, recent reports have noted phenotypic differences within C57BL/6J (B6J) and C57BL/6N (B6N) mice for some drugs of abuse: alcohol, opiates, and cocaine. However, the differences in nicotine’s effects have not yet been described in these substrains. We examined the phenotypic differences in these substrains following the acute and repeated administration of nicotine in several pharmacological measures, including locomotion (after acute and repeated exposure), body temperature, nociception, and anxiety-like behaviors. We report substrain differences in the pharmacological effects of acute and repeated nicotine administration in the B6 substrains. Overall, we show enhanced nicotine sensitivity to locomotion, hypothermia, antinociception, and anxiety-like behaviors in the B6J mouse substrain compared to B6N. In the repeated administration paradigm, both the B6N and B6J substrains showed no sensitized locomotor responses after repeated exposure to nicotine at the two doses tested. This study thus provides evidence that the B6 mouse substrains may be useful for genetic studies to elucidate some of the genetic variants involved in tobacco dependence and addiction.

Strain-specific altered nicotine metabolism in 3,3'-diindolylmethane (DIM) exposed mice

Two indole compounds, indole-3-carbinol (I3C) and its acid condensation product, 3,3'-diindolymethane (DIM), have been shown to suppress the expression of flavin-containing monooxygenases (FMO) and to induce some hepatic cytochrome P450s (CYPs) in rats. In liver microsomes prepared from rats fed I3C or DIM, FMO-mediated nicotine N-oxygenation was decreased, whereas CYP-mediated nicotine metabolism to nicotine iminium and subsequently to cotinine was unchanged. Therefore, it was hypothesized that in mice DIM would also suppress nicotine N-oxygenation without affecting CYP-mediated nicotine metabolism. Liver microsomes were produced from male and female C57BL/6 J and CD1 mice fed 2500 parts per million (ppm) DIM for 14 days. In liver microsomes from DIM-fed mice, FMO-mediated nicotine N-oxygenation did not differ from the controls, but CYP-mediated nicotine metabolism was significantly increased, with results varying by sex and strain. To confirm the effects of DIM in vivo, control and DIM-fed CD1 male mice were injected subcutaneously with nicotine, and the plasma concentrations of nicotine, cotinine and nicotine-N-oxide were measured over 30 minutes. The DIM-fed mice showed greater cotinine concentrations compared with the controls 10 minutes following injection. It is concluded that the effects of DIM on nicotine metabolism in vitro and in vivo differ between mice and rats and between mouse strains, and that DIM is an effective inducer of CYP-mediated nicotine metabolism in commonly studied mouse strains.

Dynamic changes in lung responses after single and repeated exposures to cigarette smoke in mice

Cigarette smoke is well recognized to cause injury to the airways and the alveolar walls over time. This injury usually requires many years of exposure, suggesting that the lungs may rapidly develop responses that initially protect it from this repetitive injury. Our studies tested the hypotheses that smoke induces an inflammatory response and changes in mRNA profiles that are dependent on sex and the health status of the lung, and that the response of the lungs to smoke differs after 1 day compared to 5 days of exposure. Male and female wildtype (WT) and Scnn1b-transgenic (βENaC) mice, which have chronic bronchitis and emphysematous changes due to dehydrated mucus, were exposed to cigarette smoke or sham air conditions for 1 or 5 days. The inflammatory response and gene expression profiles were analyzed in lung tissue. Overall, the inflammatory response to cigarette smoke was mild, and changes in mediators were more numerous after 1 than 5 days. βENaC mice had more airspace leukocytes than WT mice, and smoke exposure resulted in additional significant alterations. Many genes and gene sets responded similarly at 1 and 5 days: genes involved in oxidative stress responses were upregulated while immune response genes were downregulated. However, certain genes and biological processes were regulated differently after 1 compared to 5 days. Extracellular matrix biology genes and gene sets were upregulated after 1 day but downregulated by 5 days of smoke compared to sham exposure. There was no difference in the transcriptional response to smoke between WT and βENaC mice or between male and female mice at either 1 or 5 days. Taken together, these studies suggest that the lungs rapidly alter gene expression after only one exposure to cigarette smoke, with few additional changes after four additional days of repeated exposure. These changes may contribute to preventing lung damage.

Early-adolescent male C57BL/6J and DBA/2J mice display reduced sensitivity to acute nicotine administration

BACKGROUND

The initial response to nicotine is an important predictor of subsequent use. Multiple factors may alter this response including genetics and age of first use. Here we investigated the influence of age, genetics, and their interaction on nicotine sensitivity. We then examined whether these factors influence the relationship between initial behavioral responses and voluntary nicotine consumption in adulthood.

METHODS

We measured initial nicotine responses, including nicotine-induced locomotor depression and hypothermia following an acute intraperitoneal injection (0, 0.5, or 1 mg/kg), during early-adolescence, middle-adolescence, late-adolescence, or adulthood. Thirty-five days after the initial testing, mice were assessed for voluntary oral nicotine consumption.

RESULTS

Early-adolescent mice were more resistant to nicotine-induced hypothermia and locomotor depression than later ages, further hypothermia was influenced by genetics. In the DBA/2J  strain, early-adolescent mice were insensitive to nicotine-induced hypothermia, but this response developed at later ages. In contrast, C57BL/6J  mice were sensitive at all ages, but sensitivity increased across developmental age. There was little evidence of a relationship between initial behavioral response and choice nicotine consumption.

CONCLUSION

By understanding how age of exposure and genetics influence initial nicotine behavioral responses, we have a greater understanding of factors that make adolescents differentially sensitive to the effects of this drug.

Chrna5-Expressing Neurons in the Interpeduncular Nucleus Mediate Aversion Primed by Prior Stimulation or Nicotine Exposure

Genetic studies have shown an association between smoking and variation at the CHRNA5/A3/B4 gene locus encoding the α5, α3, and β4 nicotinic receptor subunits. The α5 receptor has been specifically implicated because smoking-associated haplotypes contain a coding variant in the CHRNA5 gene. The Chrna5/a3/b4 locus is conserved in rodents and the restricted expression of these subunits suggests neural pathways through which the reinforcing and aversive properties of nicotine may be mediated. Here, we show that, in the interpeduncular nucleus (IP), the site of the highest Chrna5 mRNA expression in rodents, electrophysiological responses to nicotinic acetylcholine receptor stimulation are markedly reduced in α5-null mice. IP neurons differ markedly from their upstream ventral medial habenula cholinergic partners, which appear unaltered by loss of α5. To probe the functional role of α5-containing IP neurons, we used BAC recombineering to generate transgenic mice expressing Cre-recombinase from the Chrna5 locus. Reporter expression driven by Chrna5Cre demonstrates that transcription of Chrna5 is regulated independently from the Chrna3/b4 genes transcribed on the opposite strand. Chrna5-expressing IP neurons are GABAergic and project to distant targets in the mesopontine raphe and tegmentum rather than forming local circuits. Optogenetic stimulation of Chrna5-expressing IP neurons failed to elicit physical manifestations of withdrawal. However, after recent prior stimulation or exposure to nicotine, IP stimulation becomes aversive. These results using mice of both sexes support the idea that the risk allele of CHRNA5 may increase the drive to smoke via loss of IP-mediated nicotine aversion.SIGNIFICANCE STATEMENT Understanding the receptors and neural pathways underlying the reinforcing and aversive effects of nicotine may suggest new treatments for tobacco addiction. Part of the individual variability in smoking is associated with specific forms of the α5 nicotinic receptor subunit gene. Here, we show that deletion of the α5 subunit in mice markedly reduces the cellular response to nicotine and acetylcholine in the interpeduncular nucleus (IP). Stimulation of α5-expressing IP neurons using optogenetics is aversive, but this effect requires priming by recent prior stimulation or exposure to nicotine. These results support the idea that the smoking-associated variant of the α5 gene may increase the drive to smoke via loss of IP-mediated nicotine aversion.

Influence of nicotine on orthodontic tooth movement: A systematic review of experimental studies in rats

OBJECTIVE

The objective of this systematic review was to assess the impact of nicotine administration on orthodontic tooth movement (OTM).

METHODS

A systematic search was conducted in PubMed, Scopus, EMBASE, MEDLINE (OVID) and Web of Knowledge databases and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. Studies evaluating the influence of nicotine on OTM, and with the presence of a control group (OTM without nicotine administration), were included. Quality assessment of the selected studies was performed following the Animal Research Reporting in Vivo Experiment (ARRIVE) guidelines.

RESULTS

Six of the initially identified 108 articles fulfilled the inclusion criteria and were selected. All included studies were performed in male rats, which underwent OTM with or without nicotine administration. Since there was a variation among the included studies regarding nicotine dosage and the duration and magnitude of force application during OTM only a qualitative analysis could be performed. The studies reported that nicotine administration accelerated OTM by inducing alveolar bone resorption around the moving teeth. It was also found that nicotine increased root resorption during experimental OTM. More standardized animal research or clinical studies are warranted to further evaluate the impact of nicotine on OTM.

CONCLUSIONS

On an experimental level, nicotine exposure in rats jeopardizes OTM by increasing alveolar bone loss and root resorption. From a clinical perspective, further studies are needed to assess the impact of habitual use of tobacco products on OTM.

Intrauterine smoke exposure deregulates lung function, pulmonary transcriptomes, and in particular insulin-like growth factor (IGF)-1 in a sex-specific manner

Prenatal exposure to tobacco smoke is a significant risk-factor for airway disease development. Furthermore, the high prevalence of pregnant smoking women requires the establishment of strategies for offspring lung protection. Therefore, we here aimed to understand the molecular mechanism of how prenatal smoke exposure affects fetal lung development. We used a mouse model recapitulating clinical findings of prenatally exposed children, where pregnant mice were exposed to smoke until c-section or spontaneous delivery, and offspring weight development and lung function was monitored. Additionally, we investigated pulmonary transcriptome changes in fetal lungs (GD18.5) by mRNA/miRNA arrays, network analyses and qPCR. The results demonstrated that prenatally exposed mice showed intrauterine and postnatal growth retardation, and impaired lung function. 1340 genes and 133 miRNAs were found to be significantly dysregulated by in utero smoke exposure, and we identified Insulin-like growth factor 1 (Igf1) as a top hierarchical node in a network analysis. Moreover, Igf1 mRNA was increased in female murine offspring and in prenatally exposed children. These findings suggest that prenatal smoking is associated with a dysregulation of several genes, including Igf1 in a sex-specific manner. Thus, our results could represent a novel link between smoke exposure, abberant lung development and impaired lung function.

Omega-3 Polyunsaturated Fatty Acids Protect Against Cigarette Smoke-Induced Oxidative Stress and Vascular Dysfunction

In cigarette smokers endothelial dysfunction, measured by flow-mediated dilation (FMD), precedes cardiovascular disease (CVD) and can be improved by supplementation with n - 3 polyunsaturated fatty acids (PUFAs). We developed a mouse model of cigarette smoke (CS)-induced endothelial dysfunction that resembles impaired FMD observed in human cigarette smokers and investigated the mechanism by which n - 3 PUFAs mediate vasoprotection. We hypothesized that loss of nitric oxide (NO)-dependent vasodilation in CS-exposed mice would be prevented by dietary n - 3 PUFAs via a decrease in oxidative stress. C57BL/6 mice were fed a chow or n - 3 PUFA diet for 8 weeks and then exposed to mainstream CS or filtered air for 5 days, 2 h/day. Mesenteric arterioles were preconstricted with U46619 and dilated by stepwise increases in pressure (0-40 mmHg), resulting in increases in flow, ± inhibitor of NO production or antioxidant, Tempol. Markers of oxidative stress were measured in lung and heart. CS-exposed mice on a chow diet had impaired FMD, resulting from loss of NO-dependent dilation, compared with air exposed mice. Tempol restored FMD by normalizing NO-dependent dilation and increasing NO-independent dilation. CS-exposed mice on the n - 3 PUFA diet had normal FMD, resulting from a significant increase in NO-independent dilation, compared with CS-exposed mice on a chow diet. Furthermore, n - 3 PUFAs decreased two CS-induced markers of oxidative stress, 8-epiprostaglandin-F2α levels and heme oxygenase-1 mRNA, and significantly attenuated CS-induced cytochrome P4501A1 mRNA expression. These data demonstrate that dietary n - 3 PUFAs can protect against CS-induced vascular dysfunction via multiple mechanisms, including increasing NO-independent vasodilation and decreasing oxidative stress.

A CYP2B6-humanized mouse model and its potential applications

CYP2B6 is a human microsomal cytochrome P450 enzyme with broad substrate selectivity. CYP2B6 is the only functional member of the human CYP2B gene subfamily, which differs from the situation in rodents, such as mouse, where multiple functional Cyp2b genes are expressed. Recent studies with Cyp2b knockout or knockdown mouse models have yielded insights into the in vivo roles of mouse CYP2B enzymes in drug disposition and xenobiotic toxicity. A CYP2B6-humanized mouse model (CYP2A13/2B6/2F1-transgenic/Cyp2abfgs-null), which expresses human CYP2B6 in the liver, and human CYP2A13 and CYP2F1 in the respiratory tract, but not any of the mouse Cyp2b genes, has also been established. In the CYP2B6-humanized mouse, the CYP2B6 transgene is expressed primarily in the liver, where it was found to be active toward prototype CYP2B6 substrate drugs. The regulatory elements of the CYP2B6 transgene appear to be compatible with mouse nuclear receptors that mediate CYP2B induction. Therefore, the CYP2B6-humanized mouse is a valuable animal model for studying the impact of CYP2B6 expression or induction on drug metabolism, drug efficacy, drug-drug interaction, and drug/xenobiotic toxicity. In this mini-review, we provide a brief background on CYP2B6 and the Cyp2b-knockout and CYP2B6-humanized mice, and discuss the potential applications and limitations of the current models.

Effects of Nicotine on Alcohol Drinking in Female Mice Selectively Bred for High or Low Alcohol Preference

BACKGROUND

Studies show that repeated nicotine use associates with high alcohol consumption in humans and that nicotine exposure sometimes increases alcohol consumption in animal models. However, the relative roles of genetic predisposition to high alcohol consumption, the alcohol drinking patterns, and the timing of nicotine exposure both with respect to alcohol drinking and developmental stage remain unclear. The studies here manipulated all these variables, using mice selectively bred for differences in free-choice (FC) alcohol consumption to elucidate the role of genetics and nicotine exposure in alcohol consumption behaviors.

METHODS

In Experiments 1 and 2, we assessed the effects of repeated nicotine (0, 0.5, or 1.5 mg/kg) injections immediately before binge-like (drinking-in-the-dark; Experiment 1) or during FC alcohol access (Experiment 2) on these alcohol drinking behaviors (immediately after injections and during re-exposure to alcohol access 14 days later) in adult high- (HAP2) and low-alcohol-preferring (LAP2) female mice (co-exposure model). In Experiments 3 and 4, we assessed the effects of repeated nicotine (0, 0.5, or 1.5 mg/kg) injections 14 days prior to binge-like and FC alcohol access on these alcohol drinking behaviors in adolescent HAP2 and LAP2 female mice (Experiment 3) or adult HAP2 female mice (Experiment 4).

RESULTS

In Experiment 1, we found that repeated nicotine (0.5 and 1.5 mg/kg) and alcohol co-exposure significantly increased binge-like drinking behavior in HAP2 but not LAP2 mice during the re-exposure phase after a 14-day abstinence period. In Experiment 2, 1.5 mg/kg nicotine injections significantly reduced FC alcohol intake and preference in the third hour postinjection in HAP2 but not LAP2 mice. No significant effects of nicotine treatment on binge-like or FC alcohol drinking were observed in Experiments 3 and 4.

CONCLUSIONS

These results show that the temporal parameters of nicotine and alcohol exposure, pattern of alcohol access, and genetic predisposition for alcohol preference influence nicotine's effects on alcohol consumption. These findings in selectively bred mice suggest that humans with a genetic history of alcohol use disorders may be more vulnerable to develop nicotine and alcohol co-use disorders.

Paternal nicotine exposure alters hepatic xenobiotic metabolism in offspring

Paternal environmental conditions can influence phenotypes in future generations, but it is unclear whether offspring phenotypes represent specific responses to particular aspects of the paternal exposure history, or a generic response to paternal ‘quality of life’. Here, we establish a paternal effect model based on nicotine exposure in mice, enabling pharmacological interrogation of the specificity of the offspring response. Paternal exposure to nicotine prior to reproduction induced a broad protective response to multiple xenobiotics in male offspring. This effect manifested as increased survival following injection of toxic levels of either nicotine or cocaine, accompanied by hepatic upregulation of xenobiotic processing genes, and enhanced drug clearance. Surprisingly, this protective effect could also be induced by a nicotinic receptor antagonist, suggesting that xenobiotic exposure, rather than nicotinic receptor signaling, is responsible for programming offspring drug resistance. Thus, paternal drug exposure induces a protective phenotype in offspring by enhancing metabolic tolerance to xenobiotics.

DOI:http://dx.doi.org/10.7554/eLife.24771.001

Until recently, it seemed impossible that the conditions a person or animal experiences during their lifetime might affect the health of their offspring and future generations. Research over the past decade, however, has shown that a parent’s environment can cause changes that can be passed to future generations. For example, studies in rodents have shown that a father’s diet influences the way their offspring metabolize food. Moreover, a male mouse exposed to stress or toxins fathers pups that often respond differently in stressful situations relative to other mice.

So, how do these traits get transferred to offspring via sperm and how specific is the next generation’s response to the environmental pressures faced by their fathers? Many studies so far have looked at environmental influences that may have broad biological effects, for example a high fat diet. Now, some scientists are trying to understand whether exposure to nicotine, which has a more targeted effect, causes drug-specific effects in offspring.

Vallaster et al. now show that mice whose fathers had been exposed to nicotine before mating are more able to withstand toxic levels of the chemical than mice whose fathers were never exposed to the drug. In the experiments, some male mice were given water with nicotine in it over the course of five weeks. Later, the offspring of these mice were exposed to nicotine to see whether they were more or less sensitive to it than offspring of unexposed males. It turns out the mice with nicotine-exposed fathers have a higher resistance to the toxic effects of nicotine and, unexpectedly, to toxic levels of cocaine as well. This suggests that the pups of nicotine-exposed fathers are not specifically programmed to respond to nicotine, but instead are more resistant to toxins in general.

Vallaster et al. found that the livers of the offspring of nicotine-exposed fathers appear to be better able to metabolize both drugs. Exposing the fathers to another drug called mecamylamine (which can prevent many of nicotine’s effects on the body) also made their offspring more resistant to nicotine, showing that multiple drugs may make offspring more toxin-resistant. Studies in humans will be needed to confirm whether a father’s nicotine use affects children the same way it does mice. Similar mice studies also may help scientists to study how other types of environmental exposure might affect a man’s future children.

DOI:http://dx.doi.org/10.7554/eLife.24771.002

Nicotine Enhances the Hypnotic and Hypothermic Effects of Alcohol in the Mouse

BACKGROUND

Ethanol (EtOH) and nicotine abuse are 2 leading causes of preventable mortality in the world, but little is known about the pharmacological mechanisms mediating co-abuse. Few studies have examined the interaction of the acute effects of EtOH and nicotine. Here, we examine the effects of nicotine administration on the duration of EtOH-induced loss of righting reflex (LORR) and characterize the nature of their pharmacological interactions in C57BL/6J mice.

METHODS

We assessed the effects of EtOH and nicotine and the nature of their interaction in the LORR test using isobolographic analysis after acute injection in C57BL/6J male mice. Next, we examined the importance of receptor efficacy using nicotinic partial agonists varenicline and sazetidine. We evaluated the involvement of major nicotinic acetylcholine receptor (nAChR) subtypes using nicotinic antagonist mecamylamine and nicotinic α4- and α7-knockout mice. The selectivity of nicotine's actions on EtOH-induced LORR was examined by testing nicotine's effects on the hypnotic properties of ketamine and pentobarbital. We also assessed the development of tolerance after repeated nicotine exposure. Last, we assessed whether the effects of nicotine on EtOH-induced LORR extend to hypothermia and EtOH intake in the drinking in the dark (DID) paradigm.

RESULTS

We found that acute nicotine injection enhances EtOH's hypnotic effects in a synergistic manner and that receptor efficacy plays an important role in this interaction. Furthermore, tolerance developed to the enhancement of EtOH's hypnotic effects by nicotine after repeated exposure of the drug. α4* and α7 nAChRs seem to play an important role in nicotine-EtOH interaction in the LORR test. In addition, the magnitude of EtOH-induced LORR enhancement by nicotine was more pronounced in C57BL/6J than DBA/2J mice. Furthermore, acute nicotine enhanced ketamine and pentobarbital hypnotic effects in the mouse. Finally, nicotine enhanced EtOH-induced hypothermia but decreased EtOH intake in the DID test.

CONCLUSIONS

Our results demonstrate that nicotine synergistically enhances EtOH-induced LORR in the mouse.

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

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

Developing a model of limited-access nicotine consumption in C57Bl/6J mice

Although United States smoking rates have been on the decline over the past few decades, cigarette smoking still poses a critical health and economic threat. Very few treatment options for smoking exist, and many of them do not lead to long-term abstinence. Preclinical models are necessary for understanding the effects of nicotine and developing treatments. Current self-administration models of nicotine intake may require surgical procedures and often result in low levels of intake. Further, they do not lend themselves to investigating treatments. The current study sought to develop a limited-access model of nicotine intake using the Drinking-in-the-Dark paradigm, which results in high levels of binge-like ethanol consumption that can be pharmacologically manipulated. The present study found that mice will consume nicotine under a range of parameters. Intakes under the preferred condition of 0.14mg/ml nicotine in 0.2% saccharin reached over 6mg/kg in two hours and were reduced by an injection of R(+)-baclofen. Mecamylamine did not significantly affect nicotine consumption. As nicotine and ethanol are often co-abused, nicotine intake was also tested in the presence of ethanol. When presented in the same bottle, mice altered nicotine intake under various concentrations to maintain consistent levels of ethanol intake. When nicotine and ethanol were presented in separate bottles, mice greatly reduced their nicotine intake while maintaining ethanol intake. In conclusion, these studies characterize a novel model of limited-access nicotine intake that can be pharmacologically manipulated.

Chronic treatment with varenicline changes expression of four nAChR binding sites in mice

INTRODUCTION

Chronic treatment with nicotine is known to increase the α4β2-nAChR sites in brain, to decrease α6β2-nAChR sites and to have minimal effect on α3β4-and α7-nAChR populations. Varenicline is now used as a smoking cessation treatment, with and without continued smoking or nicotine replacement therapy. Varenicline, like nicotine, upregulates the α4β2-nAChR sites; however, it is not known whether varenicline treatment changes expression of the other nAChR subtypes.

METHODS

Using a mouse model, chronic treatments (10 days) with varenicline (0.12  mg/kg/h) and/or nicotine (1 mg/kg/hr), alone or in combination, were compared for plasma and brain levels of drugs, tolerance to subsequent acute nicotine and expression of four subtypes of nAChR using autoradiography.

RESULTS

The upregulation of α4β2-nAChR sites elicited by chronic varenicline was very similar to that elicited by chronic nicotine. Treatment with both drugs somewhat increased up-regulation, indicating that these doses were not quite at maximum effect. Similar down-regulation was seen for α6β2-nAChR sites. Varenicline significantly increased both α3β4-and α7-nAChR sites while nicotine had less effect on these sites. The drug combination was similar to varenicline alone for α3β4-nAChR sites, while for α7 sites the drug combination was less effective than varenicline alone. Varenicline had small but significant effects on tolerance to acute nicotine.

CONCLUSIONS

Effects of varenicline in vivo may not be limited to the α4β2*-nAChR subtype. In addition, smoking cessation treatment with varenicline may not allow receptor numbers to be restored to baseline and may, in addition, change expression of other receptor subtypes.

Menthol attenuates respiratory irritation and elevates blood cotinine in cigarette smoke exposed mice

Addition of menthol to cigarettes may be associated with increased initiation of smoking. The potential mechanisms underlying this association are not known. Menthol, likely due to its effects on cold-sensing peripheral sensory neurons, is known to inhibit the sensation of irritation elicited by respiratory irritants. However, it remains unclear whether menthol modulates cigarette smoke irritancy and nicotine absorption during initial exposures to cigarettes, thereby facilitating smoking initiation. Using plethysmography in a C57Bl/6J mouse model, we examined the effects of L-menthol, the menthol isomer added to cigarettes, on the respiratory sensory irritation response to primary smoke irritants (acrolein and cyclohexanone) and smoke of Kentucky reference 2R4 cigarettes. We also studied L-menthol's effect on blood levels of the nicotine metabolite, cotinine, immediately after exposure to cigarette smoke. L-menthol suppressed the irritation response to acrolein with an apparent IC₅₀ of 4 ppm. Suppression was observed even at acrolein levels well above those necessary to produce a maximal response. Cigarette smoke, at exposure levels of 10 mg/m³ or higher, caused an immediate and marked sensory irritation response in mice. This response was significantly suppressed by L-menthol even at smoke concentrations as high as 300 mg/m³. Counterirritation by L-menthol was abolished by treatment with a selective inhibitor of Transient Receptor Potential Melastatin 8 (TRPM8), the neuronal cold/menthol receptor. Inclusion of menthol in the cigarette smoke resulted in roughly a 1.5-fold increase in plasma cotinine levels over those observed in mice exposed to smoke without added menthol. These findings document that, L-menthol, through TRPM8, is a strong suppressor of respiratory irritation responses, even during highly noxious exposures to cigarette smoke or smoke irritants, and increases blood cotinine. Therefore, L-menthol, as a cigarette additive, may promote smoking initiation and nicotine addiction.

Nicotine induces resistance to erlotinib via cross-talk between α 1 nAChR and EGFR in the non-small cell lung cancer xenograft model

OBJECTIVES

Given our previously published study, α 1 nicotinic acetylcholine receptor (nAChR) plays an essential role in nicotine-induced cell signaling and nicotine-induced resistance to epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) in non-small cell lung cancer (NSCLC) PC9 cells. The aim of this study was to investigate the potential mechanism between nAChR and EGFR for nicotine-induced resistance to EGFR-TKI erlotinib in the NSCLC xenograft model.

MATERIALS AND METHODS

We identified the role of nicotine to EGFR/AKT/ERK pathways and to erlotinib-resistance in NSCLC PC9 and HCC827 cells by MTS assay and western blot. Then, we established the PC9 xenograft model with nicotine exposure and treated mice with erlotinib combined with vehicle or nicotine.

RESULTS

We confirmed the effects of nicotine on EGFR/AKT/ERK pathways and determined nicotine's potential in preventing from the effect of erlotinib on NSCLC cells. Then, we showed that nicotine exposures can promote tumor growth and induce resistance to erlotinib in the PC9 xenograft model. Our results also indicated that chronic oral administration of nicotine can cause more significant erlotinib-resistance compared with acute i.v. injection of nicotine through activating α 1 nAChR and EGFR pathways.

CONCLUSIONS

These results suggest that nicotine contributes to the progression and erlotinib-resistance of the NSCLC xenograft model via the cooperation between nAChR and EGFR.

Rational design of novel CYP2A6 inhibitors

Inhibition of CYP2A6-mediated nicotine metabolism can reduce cigarette smoking. We sought potent and selective CYP2A6 inhibitors to be used as leads for drugs useful in smoking reduction therapy, by evaluating CYP2A6 inhibitory effect of novel formyl, alkyl amine or carbonitrile substituted aromatic core structures. The most potent CYP2A6 inhibitors were thienopyridine-2-carbaldehyde, benzothienophene-3-ylmethanamine, benzofuran-5-carbaldehyde and indole-5-carbaldehyde, with IC50 values below 0.5 μM for coumarin 7-hydroxylation. Nicotine oxidation was effectively inhibited in vitro by two alkyl amine compounds and benzofuran-5-carbonitrile. Some of these molecules could serve as potential lead molecules when designing CYP2A6 inhibitory drugs for smoking reduction therapy.

The effects of pre- and post-natal nicotine exposure and genetic background on the striatum and behavioral phenotypes in the mouse

Maternal tobacco use increases the risk of complications in pregnancy and also the risk of adverse fetal outcomes. Studies have established nicotine as the principal component of tobacco smoke that leads to the majority of negative reproductive outcomes associated with maternal tobacco use. It appears the neuroteratogenicity of nicotine is mediated by complex gene-environment interactions. Genetic background contributes to individual differences in nicotine-related phenotypes. The aim of the current study was to investigate the interaction between pre- and post-natal nicotine exposure and genetic background on the histology of the striatum and behavioral measures using DBA/2J (D2) and C57BL/6J (B6) inbred mice. Alterations in neuronal cell populations, striatal brain volume, and behavior - open field (OF) activity, novel object recognition (NOR), elevated plus maze (EPM), and passive avoidance (PA) - were evaluated on post-natal day (PN) 24 and PN75. Histological data showed that pre- and post-natal nicotine exposure resulted in decreased striatal volume among preadolescent B6 and reduced neuronal number within the striatum of preadolescent B6 mice. Behavioral data showed that pre- and post-natal nicotine exposure promoted hyperactivity in D2 female mice and disrupted NOR and PA memory. Specifically, NOR deficits were significant amongst adult male mice whereas PA deficits were seen across genetic background and sex. These data suggest that nicotine treatment, genetic background, developmental stage, and sex effect striatal morphology can lead to neurobehavioral alterations.

Cotinine concentration in serum correlates with tobacco smoke-induced emphysema in mice

Secondhand smoke (SHS) has been associated with a variety of adverse health outcomes in nonsmokers, including emphysema (a chronic obstructive pulmonary disease). One way to detect SHS exposure is to measure the concentration of cotinine, the primary metabolite of nicotine, in bodily fluids. We have developed a method for cotinine analysis by combining micellar electrokinetic chromatography with enrichment techniques. We employed the method to measure cotinine concentrations in serum samples of mice exposed to tobacco smoke for 12 or 24 weeks and found that it was 3.1-fold or 4.8-fold higher than those exposed to room air for the same period. Further, we investigated the morphological changes in lungs of mice and observed tobacco smoke induced emphysema. Our results indicate that the method can be used to measure cotinine and there is an association between the serum cotinine concentration and tobacco smoke-induced emphysema in mice.