Effect of menthol on nicotine pharmacokinetics in rats after cigarette smoke inhalation

Pharmacokinetic analysis of nicotine and its metabolites (cotinine and trans-3'-hydroxycotinine) in male Sprague-Dawley rats following nose-only inhalation, oral gavage, and intravenous infusion of nicotine

Nicotine is an alkaloid found in tobacco. Human exposure to nicotine primarily occurs through the use of tobacco products. To date, limited nicotine pharmacokinetic data in animals have been reported. This study exposed male Sprague-Dawley rats to vehicle (and/or air) or 4 doses of nicotine via nose-only inhalation (INH), oral gavage (PO), and intravenous (IV) infusion. Plasma, 6 tissues (brain, heart, lung, liver, kidney, and muscle), and urine were collected at multiple timepoints from 5 min to 48 h post-dose. The concentrations of nicotine, cotinine, and trans-3'-hydroxycotinine (3-OH-cotinine) were determined, and the pharmacokinetic profiles were compared among the 4 doses for each route. The results indicated that after single nicotine dose, nicotine bioavailability was 53% via PO. Across all the administration routes and doses, nicotine was quickly distributed to all 6 tissues; kidney had the highest nicotine and cotinine levels, and the lung had the highest 3-OH-cotinine levels; nicotine was metabolized extensively to cotinine and cotinine was metabolized to a lesser extent to 3-OH-cotinine; the elimination of plasma nicotine, cotinine, and 3-OH-cotinine followed first-order kinetics; plasma nicotine had a shorter half-life than cotinine or 3-OH-cotinine; the half-lives of plasma nicotine, cotinine, and 3-OH-cotinine were dose- and route-independent; and nicotine and cotinine were major urinary excretions followed by 3-OH-cotinine. Nicotine, cotinine, and 3-OH-cotinine levels in plasma, tissues, and urine exhibited dose-dependent increases. These study findings improve our understanding of the pharmacokinetics of nicotine, cotinine, and 3-OH-cotinine across different routes of exposure.

Pharmacokinetics of freebase nicotine and nicotine salts following subcutaneous administration in male rats

Nicotine lactate, nicotine tartrate, nicotine benzoate, and freebase nicotine (FBN) are four forms of nicotine salt systems that are present in tobacco products. However, few in vivo studies have compared their pharmacological (pK) efficacies, which are important for understanding their roles in the addiction and abuse of tobacco and nicotine products. In this work, the pK of the above nicotine salt systems was studied by subcutaneously injecting their aqueous solutions in rats and obtaining blood samples from the jugular vein. Nicotine levels in the blood were analyzed by LC-MS/MS. The results demonstrated that rapid nicotine absorption occurred in all nicotine systems. Of them, NB had the smallest Tmax , while FBN had the largest Tmax . The nicotine metabolic rate and clearance decreased for FBN, indicating that nicotine retention in the body was higher than for the other three salt-based systems. Compared with nicotine salts, FBN could reach and maintain a higher concentration in the animal model. Additionally, as the benzoic acid ratios increased, the Cmax of the nicotine benzoate (NB) in the plasma decreased. This indicates that the lower the pH, the lower the Cmax . When different concentrations of NB were used, the higher the NB concentration, the greater the Cmax and AUC(0-t) . These results demonstrate that nicotine adsorption by NB in the animal model depended on both pH and concentration. This baseline information could be used to explain different clinical pharmacological observations in humans, though this study only considered the effects of nicotine on pharmacokinetics in vivo.

Effects of excessive alcohol drinking on nicotine biotransformation in rats

Alcohol and nicotine (tobacco smoke) are often used together, and taking both addictive substances is associated with an increased risk of certain diseases. It is extremely important to understand the pharmacodynamic and pharmacokinetic mechanisms of the interaction between nicotine and ethanol, which are still not fully understood. The study aimed to evaluate the influence of chronic alcohol consumption on nicotine biotransformation in ethanol-preferring and non-preferring male and female rats. Rats were divided into four groups depending on their alcohol preferences and gender. Nicotine, nornicotine, nicotine N-oxide, cotinine, trans-3'-hydroxycotinine, and cotinine N-oxide in rats plasma were determined by LC-MS/MS after five days of exposure to tobacco smoke. A non-compartmental analysis of nicotine and its metabolites was used for pharmacokinetic parameters calculation. Our experimental results showed that the rate of nicotine elimination depends on gender, regardless of alcohol preferences (significantly slower in females than in males). Mean residence timeof nornicotine, cotinine, and trans-3'-hydroxycotinine were significantly higher in alcohol-preferring male rats than in alcohol preferring female rats. In non-alcohol preferring female rats compared to ethanol-preferring female rats, significantly more nicotine N-oxide (fivefold) and trans-3'-hydroxycotinine (twofold) reached the general circulation unchanged. Drinking ethanol influenced the elimination of nornicotine and cotinine in male rats. Ethanol consumption was identified as a modifier of nicotine pharmacokinetics and this was gender-dependent.

Rodent models for nicotine withdrawal

BACKGROUND

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

AIM

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

RESULTS

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

CONCLUSION

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

The Biological Impact of Menthol on Tobacco Dependence

In the 1920s, tobacco companies created a marketing campaign for what would one day be their most profitable series of products: mentholated tobacco cigarettes. Menthol provides the smoker with a pleasant mint flavor in addition to a cooling sensation of the mouth, throat, and lungs, giving relief from the painful irritation caused by tobacco smoke. Promising a healthier cigarette using pictures of doctors in white coats and even cartoon penguins, tobacco companies promoted these cigarettes to young, beginner smokers and those with respiratory health concerns. Today, smoking tobacco cigarettes causes one in five US Americans to die prematurely, crowning it as the leading cause of preventable death. In contrast to the dubious health claims by tobacco companies, mentholated cigarettes are in fact more addictive. Smokers of mentholated cigarettes have lower successful quit rates and in some cases are resistant to both behavioral and pharmacological treatment strategies. There is now considerable evidence, especially in the last 5 years, that suggest menthol might influence the addictive potential of nicotine-containing tobacco products via biological mechanisms. First, menthol alters the expression, stoichiometry, and function of nicotinic receptors. Second, menthol's chemosensory properties operate to mask aversive properties of using tobacco products. Third, menthol's chemosensory properties aid in serving as a conditioned cue that can both enhance nicotine intake and drive relapse. Fourth, menthol alters nicotine metabolism, increasing its bioavailability. This review discusses emerging evidence for these mechanisms, with an emphasis on preclinical findings that may shed light on why menthol smokers exhibit greater dependence.

IMPLICATIONS

Mentholated cigarettes have been shown to have greater addictive potential than their nonmentholated counterparts. Evidence is pointing toward multiple mechanisms of action by which menthol may alter tobacco dependence. Understanding menthol's biological functions as it pertains to nicotine dependence will be helpful in crafting novel pharmacotherapies that might better serve menthol smokers. In addition, a better understanding of menthol's pharmacology as it relates to tobacco dependence will be valuable for informing policy decisions on the regulation of mentholated cigarettes.

Menthol Cigarettes, Tobacco Dependence, and Smoking Persistence: The Need to Examine Enhanced Cognitive Functioning as a Neuropsychological Mechanism

INTRODUCTION

Despite the overall decline in the prevalence of cigarette use in the United States, menthol cigarette use among smokers is rising, and evidence shows that it may lead to more detrimental effects on public health than regular cigarette use. One of the mechanisms by which nicotine sustains tobacco use and dependence is due to its cognitive enhancing properties, and basic science literature suggests that menthol may also enhance nicotine's acute effect on cognition.

AIMS AND METHODS

The purpose of this review is to suggest that the cognitive enhancing effects of menthol may be a potentially important neuropsychological mechanism that has yet to be examined. In this narrative review, we provide an overview of basic science studies examining neurobiological and cognitive effects of menthol and menthol cigarette smoking. We also review studies examining menthol essential oils among humans that indicate menthol alone has acute cognitive enhancing properties. Finally, we present factors influencing the rising prevalence of menthol cigarette use among smokers and the importance of this gap in the literature to improve public health and smoking cessation treatment.

CONCLUSIONS

Despite the compelling evidence for menthol's acute cognitive enhancing and reinforcing effects, this mechanism for sustaining tobacco dependence and cigarette use has yet to be examined and validated among humans. On the basis of the basic science evidence for menthol's neurobiological effects on nicotinic receptors and neurotransmitters, perhaps clarifying menthol's effect on cognitive performance can help to elucidate the complicated literature examining menthol and tobacco dependence.

IMPLICATIONS

Menthol cigarette use has continued to be a topic of debate among researchers and policy makers, because of its implications for understanding menthol's contribution to nicotine dependence and smoking persistence, as well as its continued use as a prevalent flavoring in tobacco and nicotine products in the United States and internationally. As international tobacco regulation policies have begun to target menthol cigarettes, research studies need to examine how flavoring additives, specifically menthol, may acutely influence neurobiological and cognitive functioning as a potential mechanism of sustained smoking behavior to develop more effective treatments.

Impact of Menthol on Oral Nicotine Consumption in Female and Male Sprague Dawley Rats

INTRODUCTION

One of the preferable flavors in oral nicotine delivery systems is menthol which masks the harshness of tobacco. However, possible interactions between oral menthol and nicotine on intake and preference remain unclear. Therefore, we aimed to determine the impact of menthol on oral nicotine consumption.

METHODS

Adult Sprague Dawley female and male rats (n = 8 per group) were given a choice of water or drug solution by using two-bottle free choice paradigm for 2 weeks: vehicle (5% ethanol), nicotine (20 mg/L), menthol (1 g/L) and mentholated nicotine groups. At the end of the study, plasma nicotine levels were determined.

RESULTS

When rats were given a choice of nicotine or water, nicotine intake was similar between female and male rats. Menthol addition to nicotine solution significantly increased nicotine intake and preference in male but not female rats without a considerable effect on total fluid intake and body weight change in either sex. The average nicotine intake in male rats was 0.5 ± 0.05 and 1.4 ± 0.12 mg/kg/day for nicotine and menthol-nicotine combination (p < .05), respectively. The average nicotine intake in female rats was 0.6 ± 0.05 and 0.6 ± 0.03 mg/kg/day for nicotine and menthol-nicotine combination (p > .05), respectively. Plasma nicotine levels were not significantly different between the groups in either male (nicotine group: 20.8 ± 4.9, mentholated nicotine group: 31.9 ± 3.2 ng/mL) or female (nicotine group: 24.0 ± 3.3, mentholated nicotine group: 17.8 ± 2.9 ng/mL) rats (p > .05).

CONCLUSIONS

Menthol increases oral nicotine consumption in male, but not female, rats.

IMPLICATIONS

This study may provide data on the co-use of menthol and nicotine in smokeless tobacco, particularly oral dissolvable tobacco products.

Menthol blunts the interoceptive discriminative stimulus effects of nicotine in female but not male rats

RATIONALE

Menthol is a widely used tobacco constituent that has shown to enhance nicotine's reinforcing effects.

OBJECTIVE

To determine whether injected menthol also alters nicotine's stimulus effects, we used a drug discrimination task.

METHODS

A total of 57 adult Sprague-Dawley rats (28M, 29F) received 20 positive and 20 negative days (intermixed) of discrimination training. On positive days, rats received a group-specific menthol and nicotine injection (VEH + 0.1 NIC, 1 M + 0.1 NIC, 5 M + 0.1 NIC, VEH + 0.4 NIC, 1 M + 0.4 NIC, 5 M + 0.4 NIC; mg/kg) before eight 15-s cue light presentations (conditioned stimulus (CS)), each followed by 4-s sucrose access. On negative days, all rats were injected with vehicle and saline before eight non-reinforced CS presentations. Next, rats underwent generalization testing with 30 dose combinations of menthol and nicotine. The change in drug-mediated anticipatory goal tracking during the CS was calculated as a difference score (CS minus pre-CS responding).

RESULTS

All groups readily acquired drug discrimination. However, difference scores for the 5M + 0.1 NIC group were lower for females. Additionally, females had lower scores for 0.05, 0.1, and 0.4 mg/kg nicotine generalization tests. The lowest nicotine dose discriminable from saline was 0.05 mg/kg for females but 0.025 mg/kg for males. Co-administration with 5 or 10 mg/kg menthol weakened discrimination performance between 0.1 and 0.4 mg/kg and between 0.1 and 0.05 mg/kg nicotine for 0.1 mg/kg nicotine training groups.

CONCLUSIONS

Female rats that were trained with 0.1 mg/kg nicotine were more sensitive to menthol's modulatory effects on nicotine's stimulus effects. This highlights the importance of taking sex and training dose into account when evaluating the interoceptive stimulus effects of nicotine and menthol.

Adolescent nicotine and tobacco smoke exposure enhances nicotine self-administration in female rats

Most people start experimenting with tobacco products or e-cigarettes in early adolescence and become habitual smokers in late adolescence or adulthood. These studies investigated if exposure to tobacco smoke or nicotine during early and mid-adolescence affects nicotine intake in late adolescence and early adulthood. Male and female rats were exposed to tobacco smoke from low- and high-nicotine SPECTRUM cigarettes or nicotine (0.3 mg/kg, twice a day) from postnatal day (P) 24-42. The self-administration sessions started at P55. The rats self-administered nicotine for 14-15 days under a fixed-ratio 1 schedule, and on the first day, the maximum number of infusions was twenty. Exposure to smoke from high, but not low, nicotine cigarettes during adolescence increased nicotine self-administration in female but not male rats. Adolescent treatment with nicotine facilitated nicotine self-administration. On the first day of nicotine self-administration, nicotine-treated rats reached the maximum number of infusions before the saline-treated control rats. Nicotine intake was also higher in the nicotine-treated female rats than in the saline-treated females. There was no sex difference in nicotine intake in controls when the data from the studies were combined. Smoke exposure led to a dose-dependent increase in plasma nicotine and cotinine levels in adolescent rats. Exposure to smoke from high-nicotine cigarettes and 0.3 mg/kg of nicotine led to plasma nicotine and cotinine levels that are similar to those in tobacco users. These findings indicate that in females, but not males, exposure to nicotine during adolescence may facilitate smoking and e-cigarette use later in life.

Route of administration effects on nicotine discrimination in female and male mice

BACKGROUND

Use of electronic cigarettes (e-cigarettes) has increased exponentially since their appearance on the U.S. market around 2007. To provide preclinical models of vaping that incorporate olfactory cues and chemosensory effects (including flavors) that play a role in human vaping behavior, the feasibility of using a modified e-cigarette device for delivery of aerosolized nicotine was examined in a nicotine discrimination procedure in mice.

METHODS

Adult female and male C57BL/6 mice were trained to discriminate 0.75 mg/kg subcutaneous (s.c.) nicotine from saline. After determination of a s.c. nicotine dose-effect curve, aerosolized freebase nicotine and nicotine-containing tobacco products (i.e., non-flavored and Arctic Blast e-liquids) were evaluated.

RESULTS

Nicotine (s.c.) dose-dependently substituted in mice of both sexes, although females showed less sensitivity and greater variability. By contrast, aerosolized nicotine, regardless of formulation, produced concentration-dependent increases up to maximum of 46-62% nicotine-associated responding. Brain nicotine concentrations for each sex were similar for s.c. 0.75 mg/kg nicotine and 30 mg/ml freebase nicotine.

CONCLUSIONS

Mice of both sexes readily acquired s.c. nicotine discrimination, but females showed less sensitivity. Further, all three formulations of aerosolized nicotine produced increases in nicotine-like responding in mice of each sex. However, the maximum magnitude of these increases did not engender a similar degree of substitution as s.c. 0.75 mg/kg nicotine, despite similar brain concentrations of nicotine at 30 mg/ml aerosolized nicotine. Additional research is needed for determination of the reason(s); however, results here demonstrate initial feasibility for examination of the discriminative stimulus effects of vaped drugs such as nicotine.

Menthol Cigarette Smoke Induces More Severe Lung Inflammation Than Non-menthol Cigarette Smoke Does in Mice With Subchronic Exposure - Role of TRPM8

In smokers with chronic obstructive pulmonary disease, more severe lung inflammation is associated with menthol cigarette smoking compared to non-menthol cigarette smoking. However, the mechanisms remain unclear. Menthol is an activator of transient receptor potential melastatin-8 (TRPM8), which is also sensitive to reactive oxygen species (ROS). Our recent in vitro study demonstrated that the extracts of menthol cigarette smoke (M-CS) can induce greater ROS-sensitive, TRPM8-mediated, mitogen-activated protein kinase (MAPK)-dependent inflammatory responses in lung epithelial cells than the extracts of non-menthol cigarette smoke (Non-M-CS) can. In this study, we tested the hypothesis that M-CS can induce more severe lung inflammation than Non-M-CS can via the additional action of menthol in M-CS on epithelial and lung TRPM8 in mice. Compared with Non-M-CS exposure, subchronic M-CS exposure for 7 days up-regulated the epithelial and lung expression of TRPM8, induced more vigorous activation of epithelial and lung MAPKs, and caused more severe lung inflammation. The MAPK activation was evidenced by the increased expression of phosphor-extracellular signal-regulated and phosphor-c-Jun N-terminal kinases. The lung inflammation was evidenced by pathohistological findings and increases in several inflammatory indices. Moreover, treatment with a TRPM8 antagonist (N-(3-aminopropyl)-2-{[(3-methylphenyl)methyl]oxy}-N-(2-thienylmethyl)benzamide; AMTB) greatly suppressed the MAPK activation and lung inflammation induced by Non-M-CS and M-CS, and the residual responses to these two types of CS did not differ. Conversely, the levels of biomarkers of acute CS exposure (20 min), including carboxyhemoglobin and cotinine (a nicotine metabolite) in blood plasma, and superoxide and hydrogen peroxide (two major types of ROS) in bronchoalveolar lavage fluid, did not show significant differences in the mice with Non-M-CS and M-CS exposure. We concluded that M-CS could induce greater TRPM8-mediated activation of MAPKs and lung inflammation than Non-M-CS could in mice with subchronic exposure. The augmented inflammatory effects of M-CS are unlikely due to a larger total amount of CS inhaled, but may be caused by an additional stimulation of epithelial and lung TRPM8 by menthol in M-CS. A common stimulant (presumably ROS) generated by both CS types may also stimulate TRPM8, activate MAPKs, and induce lung inflammation because treatment with AMTB could reduce these responses to Non-M-CS.

Effect of overnight smoking abstinence on a marker for microglial activation: a [11C]DAA1106 positron emission tomography study

RATIONALE

Microglia are the main immune cells in the central nervous system and participate in neuroinflammation. When activated, microglia express increased levels of the translocator protein 18 kDa (TSPO), thereby making TSPO availability a marker for neuroinflammation. Using positron emission tomography (PET) scanning, our group recently demonstrated that smokers in the satiated state had 16.8% less binding of the radiotracer [¹¹C]DAA1106 (a radioligand for TSPO) in the brain than nonsmokers.

OBJECTIVES

We sought to determine the effect of overnight smoking abstinence on [¹¹C]DAA1106 binding in the brain.

METHODS

Forty participants (22 smokers and 18 nonsmokers) completed the study (at one of two sites) and had usable data, which included images from a dynamic [¹¹C]DAA1106 PET scanning session (with smokers having been abstinent for 17.9 ± 2.3 h) and a blood sample for TSPO genotyping. Whole brain standardized uptake values (SUVs) were determined, and analysis of variance was performed, with group (overnight abstinent smoker vs. nonsmoker), site, and TSPO genotype as factors, thereby controlling for site and genotype.

RESULTS

Overnight abstinent smokers had lower whole brain SUVs (by 15.5 and 17.0% for the two study sites) than nonsmokers (ANCOVA, P = 0.004). The groups did not significantly differ in injected radiotracer dose or body weight, which were used to calculate SUV.

CONCLUSIONS

These results in overnight abstinent smokers are similar to those in satiated smokers, indicating that chronic cigarette smoking leads to global impairment of microglial activation which persists into early abstinence. Other explanations for study results, such as smoking leading to reduced numbers of microglia or smokers having more rapid metabolism of the radiotracer than nonsmokers, are also possible.

Status and Future Directions of Preclinical Behavioral Pharmacology in Tobacco Regulatory Science

Behavioral pharmacology is a branch of the experimental analysis of behavior that has had great influence in drug addiction research and policy. This paper provides an overview of recent behavioral pharmacology research in the field of tobacco regulatory science, which provides the scientific foundation for the Food and Drug Administration Center for Tobacco Products (FDA CTP) to set tobacco control policies. The rationale and aims of tobacco regulatory science are provided, including the types of preclinical operant behavioral models it deems important for assessing the abuse liability of tobacco products and their constituents. We then review literature relevant to key regulatory actions being considered by the FDA CTP, including regulations over nicotine and menthol content of cigarettes, and conclude with suggesting some directions for future research. The current era of tobacco regulatory science provides great opportunities for behavioral pharmacologists to address the leading cause of preventable death and disease worldwide.

Effect of Cigarette Smoking on a Marker for Neuroinflammation: A [11C]DAA1106 Positron Emission Tomography Study

In the brain, microglia continuously scan the surrounding extracellular space in order to respond to damage or infection by becoming activated and participating in neuroinflammation. When activated, microglia increase the expression of translocator protein (TSPO) 18 kDa, thereby making the TSPO expression a marker for neuroinflammation. We used the radiotracer [¹¹C]DAA1106 (a ligand for TSPO) and positron emission tomography (PET) to determine the effect of smoking on availability of this marker for neuroinflammation. Forty-five participants (30 smokers and 15 non-smokers) completed the study and had usable data. Participants underwent a dynamic PET scanning session with bolus injection of [¹¹C]DAA1106 (with smokers in the satiated state) and blood draws during PET scanning to determine TSPO affinity genotype and plasma nicotine levels. Whole-brain standardized uptake values (SUVs) were determined, and analysis of variance was performed, with group (smoker vs non-smoker) and genotype as factors, thereby controlling for genotype. Smokers and non-smokers differed in whole-brain SUVs (P=0.006) owing to smokers having 16.8% lower values than non-smokers. The groups did not differ in injected radiotracer dose or body weight, which were used to calculate SUV. An inverse association was found between whole-brain SUV and reported cigarettes per day (P<0.05), but no significant relationship was found for plasma nicotine. Thus, smokers have less [¹¹C]DAA1106 binding globally than non-smokers, indicating less microglial activation. Study findings are consistent with much prior research demonstrating that smokers have impaired inflammatory functioning compared with non-smokers and that constituents of tobacco smoke other than nicotine affect inflammatory processes.

Abuse liability assessment of an e-cigarette refill liquid using intracranial self-stimulation and self-administration models in rats

BACKGROUND

The popularity of electronic cigarettes (ECs) has increased dramatically despite their unknown health consequences. Because the abuse liability of ECs is one of the leading concerns of the Food and Drug Administration (FDA), models to assess it are urgently needed to inform FDA regulatory decisions regarding these products. The purpose of this study was to assess the relative abuse liability of an EC liquid compared to nicotine alone in rats. Because this EC liquid contains non-nicotine constituents that may enhance its abuse liability, we hypothesized that it would have greater abuse liability than nicotine alone.

METHODS

Nicotine alone and nicotine dose-equivalent concentrations of EC liquid were compared in terms of their acute effects on intracranial self-stimulation (ICSS) thresholds, acquisition of self-administration, reinforcing efficacy (i.e., elasticity of demand), blockade of these behavioral effects by mecamylamine, nicotine pharmacokinetics and nicotinic acetylcholine receptor binding and activation.

RESULTS

There were no significant differences between formulations on any measure, except that EC liquid produced less of an elevation in ICSS thresholds at high nicotine doses.

CONCLUSIONS

Collectively, these findings suggest that the relative abuse liability of this EC liquid is similar to that of nicotine alone in terms of its reinforcing and reinforcement-enhancing effects, but that it may have less aversive/anhedonic effects at high doses. The present methods may be useful for assessing the abuse liability of other ECs to inform potential FDA regulation of those products.

Effect of Menthol-preferring Status on Response to Intravenous Nicotine

OBJECTIVES

Menthol, a flavoring agent, is found in approximately 90% of cigarettes, but at much higher levels in menthol than non-menthol cigarettes. Menthol is reportedly included in cigarettes for its cooling and soothing effects, but also additional actions that affect smokers' receipt and processing of nicotine. In this study we investigated the response to short-term abstinence and acute nicotine delivery in menthol-preferring and non-menthol-preferring smokers.

METHODS

Nicotine dependent participants (N = 134) participated in an intravenous nicotine delivery session following overnight smoking abstinence. Participants were intravenously administered a placebo and 2 escalating nicotine doses. We compare subjective and physiological responses to nicotine and smoking urges, withdrawal, and cognitive performance following overnight abstinence and post-nicotine between regular 'menthol' smokers and 'non-menthol' cigarette smokers.

RESULTS

Relative to non-menthol-preferring smokers, menthol-preferring smokers re a smaller reduction in smoking urges from overnight abstinence baseline to post-nicotine end-of-session and rated less subjective differences between nicotine doses.

CONCLUSIONS

Differences between menthol-preferring and non-menthol-preferring smokers' responses to abstinence or acute nicotine could reflect pre-existing individual differences that may have in initial development of menthol preferences, or could have arisen secondarily to pro use of menthol versus non-menthol cigarettes.

Enhancing effect of menthol on nicotine self-administration in rats

RATIONALE

Tobacco smoking is a leading preventable cause of premature death in the USA. Menthol is a significant flavoring additive in tobacco products. Clinical evidence suggests that menthol may promote tobacco smoking and nicotine dependence. However, it is unclear whether menthol enhances the reinforcing actions of nicotine and thus facilitates nicotine consumption. This study employed a rat model of nicotine self-administration to examine the effects of menthol on nicotine-taking behavior.

METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to press a lever for intravenous nicotine self-administration under a fixed-ratio 5 schedule of reinforcement. In separate groups, rats self-administered nicotine at four different doses (0.0075, 0.015, 0.03, and 0.06 mg/kg/infusion). Five minutes prior to the two test sessions, menthol (5 mg/kg) or its vehicle was administered intraperitoneally in all rats in a counterbalanced design within each group. In separate rats that self-administered 0.015 mg/kg/infusion nicotine, menthol dose-response function was determined. Menthol was also tested on food self-administration.

RESULTS

An inverted U-shaped nicotine dose-response curve was observed. Menthol pretreatment shifted the nicotine dose-response curve to the left. The facilitating effect of menthol on the self-administration of 0.015 mg/kg/infusion nicotine was dose-dependent, whereas it produced similar effects at doses above the threshold of 2.5 mg/kg. Menthol tended to suppress the self-administration of food pellets.

CONCLUSIONS

These data demonstrate that menthol enhances the reinforcing effects of nicotine, and the effect of menthol was specific to nicotine. The findings suggest that menthol directly facilitates nicotine consumption, thereby contributing to tobacco smoking.

Effects of different routes of nicotine administration on gastric morphology and hormonal secretion in rats

What is the central question of this study? Does chronic administration of nicotine by different routes affect gastric hormonal secretions and morphology in rats? What is the main finding and its importance? Chronic nicotine administration increased levels of gastrin, ghrelin and histamine but decreased prostaglandin E2 . Nicotine administered orally and by inhalation had a marked negative impact on the histological structure of the gastric mucosa compared with intraperitoneal administration. The negative impact of nicotine administration on gastric structure was associated with an increased concentration of gastrin and decreased prostaglandin E2 , which might be the cause of gastric/peptic ulcers in heavy smokers. The increase in ghrelin concentration and its effect following chronic nicotine administration needs further investigation. The aim was to assess the effects of different routes of chronic nicotine administration on gastric morphology and hormonal secretion; mainly gastrin, ghrelin, histamine and prostaglandin E2 (PGE2 ). Forty adult male albino rats were randomly assigned into four groups (10 rats per group), treated for 21 days as follows: control group (given standard rat pellets and water only); oral nicotine-treated group [50 μg (ml drinking water)(-1) ]; intraperitoneal nicotine-treated group [0.5 mg (kg body weight)(-1) ]; and inhaled nicotine-treated group [0.5 mg (kg body weight)(-1) ]. Concentrations of gastrin, ghrelin, PGE2 and histamine in serum and gastric tissue homogenates were assessed using ELISA kits. Stomach fundus was processed for histopathology and immunohistochemistry using light and electron microscopy. Different routes of chronic nicotine administration resulted in a significant increase in serum and gastric homogenate gastrin and ghrelin concentrations and a significant decrease in serum and homogenate PGE2 concentrations compared with the control group. Moreover, nicotine administration via oral and inhalation routes caused gastric erosion, transformation of peptic cells into the mucous variety, a significant increase in parietal cell numbers and an increase in expression of gastrin. In conclusion, the negative impact of nicotine administration on gastric structure that is associated with an increased concentration of gastrin and decreased concentration PGE2 might be the leading cause of gastric/peptic ulcers in heavy smokers. The increased ghrelin concentration and its effect following nicotine chronic administration needs further investigation. Based on these findings, we suggest that the alteration in gastric structure following chronic administration of nicotine can be prevented by reducing gastrin secretion and/or targeting its receptors.

Nicotine-Cadmium Interaction Alters Exploratory Motor Function and Increased Anxiety in Adult Male Mice

In this study we evaluated the time dependence in cadmium-nicotine interaction and its effect on motor function, anxiety linked behavioural changes, serum electrolytes, and weight after acute and chronic treatment in adult male mice. Animals were separated randomly into four groups of n = 6 animals each. Treatment was done with nicotine, cadmium, or nicotine-cadmium for 21 days. A fourth group received normal saline for the same duration (control). Average weight was determined at 7-day interval for the acute (D1-D7) and chronic (D7-D21) treatment phases. Similarly, the behavioural tests for exploratory motor function (open field test) and anxiety were evaluated. Serum electrolytes were measured after the chronic phase. Nicotine, cadmium, and nicotine-cadmium treatments caused no significant change in body weight after the acute phase while cadmium-nicotine and cadmium caused a decline in weight after the chronic phase. This suggests the role of cadmium in the weight loss observed in tobacco smoke users. Both nicotine and cadmium raised serum Ca²⁺ concentration and had no significant effect on K⁺ ion when compared with the control. In addition, nicotine-cadmium treatment increased bioaccumulation of Cd²⁺ in the serum which corresponded to a decrease in body weight, motor function, and an increase in anxiety.

Not so Cool? Menthol's discovered actions on the nicotinic receptor and its implications for nicotine addiction

Nicotine cigarette smoke is a large public health burden worldwide, contributing to various types of disease. Anti-tobacco media campaigns and control programs have significantly reduced smoking in the United States, yet trends for menthol cigarette smoking have not been as promising. Menthol cigarette smoking is particularly prevalent among young adults and African Americans, with implications for long-term impacts on health care. Continuing high rates of menthol cigarette addiction call into question the role of menthol in nicotine addiction. To date, a biological basis for the high rate of addiction and relapse among menthol cigarette smokers has not been defined. Studies have demonstrated a role for menthol in the metabolism of nicotine in the body. More recent findings now reveal an interaction between menthol and the nicotinic acetylcholine (nACh) receptor in cells. This receptor is central to the actions of nicotine in the brain, and plays an important role in nicotine addiction. The newly discovered effect of menthol on nACh receptors may begin to explain the unique addictive properties of menthol cigarettes.

Up-regulation of nicotinic acetylcholine receptors in menthol cigarette smokers

One-third of smokers primarily use menthol cigarettes and usage of these cigarettes leads to elevated serum nicotine levels and more difficulty quitting in standard treatment programmes. Previous brain imaging studies demonstrate that smoking (without regard to cigarette type) leads to up-regulation of β(2)*-containing nicotinic acetylcholine receptors (nAChRs). We sought to determine if menthol cigarette usage results in greater nAChR up-regulation than non-menthol cigarette usage. Altogether, 114 participants (22 menthol cigarette smokers, 41 non-menthol cigarette smokers and 51 non-smokers) underwent positron emission tomography scanning using the α(4)β(2)* nAChR radioligand 2-[(18)F]fluoro-A-85380 (2-FA). In comparing menthol to non-menthol cigarette smokers, an overall test of 2-FA total volume of distribution values revealed a significant between-group difference, resulting from menthol smokers having 9-28% higher α(4)β(2)* nAChR densities than non-menthol smokers across regions. In comparing the entire group of smokers to non-smokers, an overall test revealed a significant between-group difference, resulting from smokers having higher α(4)β(2)* nAChR levels in all regions studied (36-42%) other than thalamus (3%). Study results demonstrate that menthol smokers have greater up-regulation of nAChRs than non-menthol smokers. This difference is presumably related to higher nicotine exposure in menthol smokers, although other mechanisms for menthol influencing receptor density are possible. These results provide additional information about the severity of menthol cigarette use and may help explain why these smokers have more trouble quitting in standard treatment programmes.