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

Adult Consequences of Repeated Nicotine Vapor Inhalation in Adolescent Rats

INTRODUCTION

There has been a resurgence in nicotine inhalation in adolescents due to the popularity and availability of Electronic Nicotine Delivery Systems (ENDS). Almost five times as many US high-school seniors inhale nicotine vapor daily compared with those who smoke tobacco. This study was conducted to determine the impact of repeated adolescent vapor inhalation of nicotine on behavior in adulthood.

METHODS

Male and female Sprague-Dawley rats were exposed to 30-minute sessions of ENDS vapor inhalation, twice daily, from post-natal day (PND) 31-40. Conditions included vapor from the propylene glycol (PG) vehicle or nicotine (30 mg/mL in the PG). Animals were assessed for effects of nicotine on open field (PND 74-105) and wheel activity (PND 126-180) and for volitional exposure to nicotine vapor (PND 285-395). Plasma nicotine and cotinine were assessed in separate groups of male and female Wistar and Sprague-Dawley rats after a single nicotine inhalation session.

RESULTS

Group mean plasma nicotine ranged from 39 to 59 ng/mL post-session with minimal strain differences detected. Adolescent nicotine exposure enhanced sensitivity to the locomotor stimulating effects of nicotine (0.1-0.8 mg/kg, s.c.) in an open field in female rats, but didn't change the effects of nicotine on wheel activity. Female rats exposed to nicotine (30 mg/mL) vapor as adolescents responded more vigorously than PG-exposed females to nicotine vapor in a fixed ratio 5 challenge.

CONCLUSIONS

Repeated adolescent nicotine vapor inhalation leads to enhanced liability for volitional exposure to nicotine vapor in adulthood in female rats, but minimal change in spontaneous locomotor behavior.

IMPLICATIONS

These results show that adolescent vaping of nicotine can lead to lasting sensitization to the effects of nicotine in adulthood, including volitional responding for nicotine vapor. Demonstration of this in a controlled animal model establishes causality in a manner not possible from longitudinal evidence in human populations. These findings further highlight the importance of decreasing adolescent nicotine exposure to e-cigarettes to reduce consumption in adulthood.

Factors influencing JUUL e-cigarette nicotine vapour-induced reward, withdrawal, pharmacokinetics and brain connectivity in rats: sex matters

Though vaping likely represents a safer alternative to smoking, it is not without risks, many of which are not well understood, especially for vulnerable populations. Here we evaluate the sex- and age-dependent effects of JUUL nicotine vapour in rats. Following passive nicotine vapour exposures (from 59 mg/ml JUUL nicotine pods), rats were evaluated for reward-like behaviour, locomotion, and precipitated withdrawal. Pharmacokinetics of nicotine and its metabolites in brain and plasma and the long-term impact of nicotine vapour exposure on functional magnetic resonance imaging-based brain connectivity were assessed. Adult female rats acquired conditioned place preference (CPP) at a high dose (600 s of exposure) of nicotine vapour while female adolescents, as well as male adults and adolescents did not. Adult and adolescent male rats displayed nicotine vapour-induced precipitated withdrawal and hyperlocomotion, while both adult and adolescent female rats did not. Adult females showed higher venous and arterial plasma and brain nicotine and nicotine metabolite concentrations compared to adult males and adolescent females. Adolescent females showed higher brain nicotine concentration compared to adolescent males. Both network-based statistics and between-component group connectivity analyses uncovered reduced connectivity in nicotine-exposed rats, with a significant group by sex interaction observed in both analyses. The short- and long-term effects of nicotine vapour are affected by sex and age, with distinct behavioural, pharmacokinetic, and altered network connectivity outcomes dependent on these variables.

Adult consequences of repeated nicotine and Δ9-tetrahydrocannabinol (THC) vapor inhalation in adolescent rats

RATIONALE

Use of electronic drug delivery systems (EDDS, "e-cigarettes") to ingest nicotine and Δ9-tetrahydrocannabinol (THC) has surged in adolescents in the USA; five times as many high-school seniors vape nicotine daily using tobacco. At the same time, 19.5% of seniors use cannabis at least monthly, with 12% using EDDS to deliver it.

OBJECTIVES

This study was conducted to examine the impact of repeated adolescent vapor inhalation of nicotine and THC in rats.

METHODS

Female Sprague-Dawley rats were exposed to 30-min sessions of vapor inhalation, twice daily, from post-natal day (PND) 31 to PND 40. Conditions included vapor from the propylene glycol (PG) vehicle, nicotine (60 mg/mL in the PG), THC (100 mg/mL in the PG), or the combination of nicotine (60 mg/mL) and THC (100 mg/mL). Rats were assessed on wheel activity, heroin anti-nociception and nicotine and heroin vapor volitional exposure during adulthood.

RESULTS

Nicotine-exposed rats exhibited few differences as adults, but were less sensitive to anti-nociceptive effects of heroin (1 mg/kg, s.c.). THC- and THC + nicotine-exposed rats were less spontaneously active, and obtained fewer nicotine vapor deliveries as adults. In contrast, THC-exposed rats obtained volitional heroin vapor at rates indistinguishable from the non-THC-exposed groups. Repeated THC exposure also caused tolerance to temperature-disrupting effects of THC (5 mg/kg, i.p.).

CONCLUSIONS

These studies further confirm that the effects of repeated vapor exposure to THC in adolescence last into early to middle adulthood, including decreased volitional consumption of nicotine. Effects of repeated nicotine in adolescence were comparatively minor.

The Impact of High or Low Doses of Nicotine in a Mouse Model of Vapor Self-Administration

INTRODUCTION

A wide variety of nicotine concentrations and formulations are available to users of electronic nicotine delivery systems (ENDS). This is increasingly true when considering the many flavors available with ENDS products. To date, there have been few preclinical investigations into the impact of nicotine doses, with and without flavors, on vaping-related behaviors. This present study evaluated how nicotine concentrations relevant to tank-based and pod-based ENDS, with and without flavors, impact reinforcement-related behavior in a mouse model.

AIMS AND METHODS

Adult male and female C57/BL6J mice were used in vapor-inhalation self-administration assays. Mice were assigned e-liquids containing 6 mg/mL or 60 mg/mL nicotine. Additional mice were assigned these nicotine doses with green apple or menthol flavorants. Mice were trained on fixed-ratio 1 for 10, 2-hour sessions, then five sessions at FR3, three progressive ratio sessions, and two FR3 sessions.

RESULTS

We observed male mice exhibited higher reinforcement-related behavior to menthol-flavored 6 mg/mL nicotine when compared to female mice. Males were only observed to have a menthol-induced enhancement of self-administration at 6 mg/mL nicotine and not 60 mg/mL nicotine. However, female mice exhibited significant menthol-induced increases in reinforcement-related behaviors with 60 mg/mL nicotine.

CONCLUSIONS

These data provide evidence that males and females exhibit different dose sensitivities to nicotine. These sex-dependent differences in nicotine sensitivity also indicate that flavor-induced enhancement in nicotine intake is dependent on the different doses for each sex.

IMPLICATIONS

There has been much discussion recently regarding the impact of flavors on vaping-related behavior. Our current study may support prior investigations that suggest flavors enhance the palatability of nicotine-containing products. However, this current study provides evidence that males and females exhibit different sensitivities to nicotine.

Hyperactivity Induced By Vapor Inhalation of Nicotine in Male and Female Rats

Rationale

Preclinical models of electronic nicotine delivery system (ENDS; "e-cigarette") use have been rare, so there is an urgent need to develop experimental approaches to evaluate their effects.

Objective

To contrast the impact of inhaled nicotine across sex.

Methods

Male and female Wistar rats were exposed to vapor from a propylene glycol vehicle (PG), nicotine (NIC; 1-30 mg/mL in PG), or were injected with NIC (0.1-0.8 mg/kg, s.c.), and then assessed for changes in temperature and activity. The antagonist mecamylamine (2 mg/kg) was administered prior to NIC to verify pharmacological specificity. Plasma levels of nicotine and cotinine were determined after inhalation and injection.

Results

Activity increased in females for ~60 minutes after nicotine inhalation, and this was blocked by mecamylamine. A similar magnitude of hyperlocomotion was observed after s.c. administration. Body temperature was reduced after nicotine inhalation by female rats but mecamylamine increased this hypothermia. Increased locomotor activity was observed in male rats if inhalation was extended to 40 minutes or when multiple inhalation epochs were used per session. The temperature of male rats was not altered by nicotine. Plasma nicotine concentrations were slightly lower in male rats than in female rats after 30-minute nicotine vapor inhalation and slightly higher after nicotine injection (1.0 mg/kg, s.c.).

Conclusions

Nicotine inhalation increases locomotor activity in male and female rats to a similar or greater extent than by subcutaneous injection. Sex differences were observed, which may be related to lower nicotine plasma levels, lower baseline activity and/or a higher vehicle response in males.

Neuronal Excitability in the Medial Habenula and Ventral Tegmental Area Is Differentially Modulated by Nicotine Dosage and Menthol in a Sex-Specific Manner

The medial habenula (MHb) has been identified as the limiting factor for nicotine intake and facilitating nicotine withdrawal. However, few studies have assessed MHb neuronal excitability in response to nicotine, and, currently, a gap in knowledge is present for finding behavioral correlates to neuronal excitability in the region. Moreover, no study to date has evaluated sex or nicotine dosage as factors of excitability in the MHb. Here, we utilized an e-vape self-administration (EVSA) model to determine differences between sexes with different nicotine dosages ± menthol. Following this paradigm, we employed patch-clamp electrophysiology to assess key metrics of MHb neuronal excitability in relation to behavioral endpoints. We observed female mice self-administered significantly more than males, regardless of dosage. We also observed a direct correlation between self-administration behavior and MHb excitability with low-dose nicotine + menthol in males. Conversely, a high dose of nicotine ± menthol yields an inverse correlation between excitability and self-administration behavior in males only. In addition, intrinsic excitability in the ventral tegmental area (VTA) does not track with the amount of nicotine self-administered. Rather, they correlate to the active/inactive discrimination of mice. Using fast-scan cyclic voltammetry, we also observed that dopamine release dynamics are linked to reinforcement-related behavior in males and motivation-related behaviors in females. These results point to a sex-specific difference in the activity of the MHb and VTA leading to distinct differences in self-administration behavior. His could lend evidence to clinical observations of smoking and nicotine-use behavior differing between males and females.

Sex Differences in Response-Contingent Cannabis Vapor Administration During Adolescence Mediate Enduring Effects on Behavioral Flexibility and Prefrontal Microglia Activation in Rats

Introduction: Cannabis is the most used illicit drug in the United States. With many states passing legislation to permit its recreational use, there is concern that cannabis use among adolescents could increase dramatically in the coming years. Historically, it has been difficult to model real-world cannabis use to investigate the causal relationship between cannabis use in adolescence and behavioral and neurobiological effects in adulthood. Materials and Methods: We used a response-contingent vapor administration model to investigate long-term effects of cannabis use during adolescence on the medial prefrontal cortex (mPFC) and mPFC-dependent behaviors in male and female rats. Results: Adolescent (35- to 55-day-old) female rats had significantly higher rates of responding for vaporized Δ9-tetrahydrocannabinol (THC)-dominant cannabis extract (CANTHC) compared with adolescent males. In adulthood (70-110 days old), female, but not male, CANTHC rats also took more trials to reach criterion and made more regressive errors in an automated attentional set-shifting task compared with vehicle rats, thereby indicating sex differences in behavioral flexibility impairments. Notably, sex-treatment interactions were not observed when rats of each sex were exposed to a noncontingent CANTHC vapor dosing regimen that approximated CANTHC vapor deliveries earned by females. No differences were observed in effort-based decision making in either sex. In the mPFC, female (but not male) CANTHC rats displayed more reactive microglia with no changes in myelin basic protein expression or dendritic spine density. Conclusion: Altogether, these data reveal important sex differences in rates of responding for CANTHC vapor in adolescence that may confer enduring alterations to mPFC structure and function and suggest that there may be subtle differences in the effects of response-contingent versus noncontingent cannabis exposure that should be systematically examined in future studies.

Pharmacokinetic differences in nicotine and nicotine salts mediate reinforcement-related behavior: an animal model study

Since their introduction in the United States and Europe in 2007, electronic cigarettes (E-Cigs) have become increasingly popular among smokers. Nicotine, a key component in both tobacco and e-cigarettes, can exist in two forms: nicotine-freebase (FBN) and nicotine salts (NS). While nicotine salt is becoming more popular in e-cigarettes, the effect of nicotine salts on reinforcement-related behaviors remains poorly understood. This study aimed to compare the reinforcing effects of nicotine and nicotine salts in animal models of drug self-administration and explore potential mechanisms that may contribute to these differences. The results demonstrated that three nicotine salts (nicotine benzoate, nicotine lactate, and nicotine tartrate) resulted in greater reinforcement-related behaviors in rats compared to nicotine-freebase. Moreover, withdrawal-induced anxiety symptoms were lower in the three nicotine salt groups than in the nicotine-freebase group. The study suggested that differences in the pharmacokinetics of nicotine-freebase and nicotine salts in vivo may explain the observed behavioral differences. Overall, this study provides valuable insights into the reinforcing effects of nicotine as well as potential differences between nicotine-freebase and nicotine salts.

Adult consequences of repeated nicotine and Δ 9 -tetrahydrocannabinol (THC) vapor inhalation in adolescent rats

The use of Electronic Drug Delivery Systems (EDDS, "e-cigarettes") to ingest nicotine and Δ 9 -tetrahydrocannabinol (THC) has surged in adolescent populations in the United States, as five times as many high-school seniors vape nicotine daily as use tobacco. At the same time 19.5% of seniors use cannabis at least monthly, with 12% using EDDS to deliver it. This study was conducted to examine the impact of repeated adolescent vapor inhalation of nicotine and THC in rats. Female Sprague-Dawley rats were exposed to 30-minute sessions of vapor inhalation, twice daily, from Post-Natal Day (PND) 31 to PND 40. Conditions included vapor from the propylene glycol (PG) vehicle, Nicotine (60 mg/mL in the PG), THC (100 mg/mL in the PG) or the combination of Nicotine (60 mg/mL) and THC (100 mg/mL). Rats were assessed on wheel activity, heroin anti-nociception and nicotine and heroin vapor volitional exposure during adulthood. Nicotine exposed rats exhibited few differences as adults, but were less sensitive to anti-nociceptive effects of heroin (1 mg/kg, s.c.). THC- and THC+Nicotine-exposed rats were less spontaneously active, and obtained fewer nicotine vapor deliveries as adults. In contrast, THC exposed rats obtained volitional heroin vapor at rates indistinguishable from the non-THC-exposed groups. Repeated THC exposure also caused tolerance to temperature-disrupting effects of THC (5 mg/kg, i.p.). These studies further confirm that the effects of repeated vapor exposure to THC in adolescence last into early to middle adulthood, including decreased volitional consumption of nicotine. Effects of repeated nicotine in adolescence were comparatively minor.

Beyond the label: current evidence and future directions for the interrelationship between electronic cigarettes and mental health

Electronic cigarette use has dramatically increased over the last decade. With this recent technological development and wide range of constituents in various products, putative adverse effects on the brain and body have been largely unexplored. Here, we review current evidence linking electronic nicotine cigarette use with potential health consequences and provide evidence supporting an association between drug use and depression in humans. We also examine the biological effects of individual constituents in electronic cigarette aerosols, which include labeled ingredients, such as propylene glycol, vegetable glycerin, nicotine, and flavorants, as well as unlabeled ingredients found in the aerosols, such as carbonyls and heavy metals. Lastly, we examine the effects of electronic cigarette use on endogenous metabolism via changes in cytochrome P450 enzymes, which can thereby impact therapeutic outcomes. While the current evidence offers insight into the potential effects of electronic cigarette use on biological processes, further studies are necessary to determine the long-term clinical relevance of aerosol inhalation.

Quick nic: novel smokeless nicotine products and pediatric trends

PURPOSE OF REVIEW

The development and marketing of smokeless nicotine products in recent years have become increasingly popular among adolescents. In addition to well known conventional inhaled nicotine products, noninhaled products, such as nicotine toothpicks, orbs, lozenges, strips, and more, have dangerously captivated a new youth audience. Although smokeless nicotine products may seem to be less threatening than conventional inhaled nicotine products, there are significant risks associated with the use of these products, including addiction and severe health issues. The purpose of this review is to provide up-to-date information about alternative nicotine products currently on the market that may appeal to youth, and the dangers of nicotine use for pediatric populations.

RECENT FINDINGS

Smokeless nicotine products appeal to minors with their varying flavors and discrete packaging. These products may lead to nicotine toxicity as well as severe health problems, such as cancer, issues with reproduction, and heart attacks. Nicotine is extremely dangerous for young children; in fact, using nicotine products before the age of 18 years can lead to addiction and is linked to an increased likelihood of experimenting with stronger nicotine products or illicit drugs. The development of inconspicuous nicotine packaging has led to increasing concerns for accidental nicotine exposure and overdose in youth.

SUMMARY

Greater knowledge regarding current nicotine products on the market, specifically smokeless nicotine products, will help clinicians be more aware of the dangers associated with these products. Clinicians will be better able to provide their patients and families with proper guidance to avoid nicotine addiction, further drug use, and detrimental health issues. Caregivers and medical professionals must recognize novel and inconspicuous nicotine products commonly used among youth, understand the signs of nicotine abuse and dependence, and take measures to address possible nicotine-related health risks.

Development of Dependence in Smokers and Rodents With Voluntary Nicotine Intake: Similarities and Differences

INTRODUCTION

Smoking and vaping throughout adolescence and early adulthood lead to nicotine dependence. Nicotine withdrawal is associated with somatic and affective withdrawal symptoms that contribute to smoking and relapse. Affective nicotine withdrawal symptoms in humans include craving for cigarettes, depression, anxiety, trouble sleeping, and cognitive deficits.

METHODS

Herein, we review clinical studies that investigated nicotine dependence in people who smoke or vape. We also discuss studies that investigated the development of dependence in animals with oral nicotine intake, nicotine aerosol self-administration, and intravenous nicotine self-administration.

RESULTS

Clinical studies report that adolescents who smoke daily develop nicotine dependence before those who smoke infrequently, but ultimately all smokers become dependent in adulthood. Preclinical studies indicate that rats that self-administer nicotine also become dependent. Rats that self-administer nicotine display somatic withdrawal signs and affective withdrawal signs, including increased anxiety and depressive-like behavior, cognitive deficits, and allodynia. Most nicotine withdrawal signs were observed in rodents with daily (7 days/week) or intermittent long access (23-hour) to nicotine. Clinical smoking studies report symptoms of nicotine dependence in adolescents of both sexes, but virtually all preclinical nicotine self-administration studies have been done with adult male rats.

CONCLUSIONS

The role of sex and age in the development of dependence in nicotine self-administration studies remains under-investigated. However, the role of sex and age in nicotine withdrawal has been thoroughly evaluated in studies in which nicotine was administered noncontingently. We discuss the need for volitional nicotine self-administration studies that explore the gradual development of dependence during adolescence and adulthood in rodents of both sexes.

IMPLICATIONS

The reviewed clinical studies investigated the development of nicotine dependence in male and female adolescent and young adult smokers and vapers. These studies indicate that most adolescent smokers and vapers gradually become nicotine dependent. Preclinical studies with rodents show that nicotine intake in widely used self-administration models also leads to dependence. However, almost all animal studies that investigated the development of nicotine dependence have been conducted with adult male rats. To better model smoking and vaping, it is important that nicotine intake in rats or mice starts during adolescence and that both sexes are included.

Electronic Vaporization of Nicotine Salt or Freebase produces differential effects on metabolism, neuronal activity and behavior in male and female C57BL/6J mice

The use of Electronic Nicotine Delivery Systems (ENDS) is increasing in prevalence and popularity. ENDS are a rapidly evolving technology as devices and e-liquid formulations adapt to policy restrictions and market demand To identify the impacts of nicotine formulation and concentration, we exposed female and male C57BL/6J mice to passive electronic vaporization of different nicotine formulations (freebase or salt) and concentrations (1% or 3%) and measured serum nicotine metabolite levels, brain activity by cFos expression, and anxiety-like and motivated behavior using the novelty suppressed feeding test. We found that the 3% freebase nicotine vapor group displayed significantly higher serum nicotine levels than either 1% or 3% nicotine salt formulations, and female mice displayed higher serum nicotine and cotinine levels compared to males. Central amygdala (CeA) activity was significantly elevated in male mice following nicotine vapor exposure, but the increase was not significantly different between nicotine vapor groups. CeA activity in female mice was unaffected. In contrast increased activity in the ventral tegmental area (VTA) was only observed in female mice exposed to 3% nicotine freebase and specifically in the dopaminergic population. Anxiety-like behavior in female mice was relatively unaffected by nicotine vapor exposure, however male mice displayed increased anxiety-like behavior and reduced motivation to feed after vapor exposure, specifically in the 3% freebase group. These results identify important sex differences in the impact of nicotine formulation and concentration on nicotine metabolism, brain region-specific activity and anxiety-like behavior, which may have significant relevance for different consequences of vaping in men and women.

Development of a nicotine aerosol self-administration model in rats and the effects of e-liquid flavors

Electronic nicotine delivery system (ENDS) use is maintained by the positive reinforcement associated with preferred flavors. These flavors become conditioned reinforcers through pairings with primary reinforcers. This study sought to extend prior research with intravenous nicotine self-administration and develop a more ecologically valid preclinical model of aerosol self-administration in rats that incorporated flavors paired with sucrose. Rats were first trained to respond for oral sucrose with or without raspberry flavor to establish the flavor as a conditioned reinforcer for some groups. Rats were then exposed to aerosol self-administration. All groups responded for raspberry-flavored aerosol with or without nicotine. Rats responded more for raspberry flavored sucrose than unflavored sucrose. Despite raspberry increasing responding for sucrose, the flavor did not function as a conditioned reinforcer during aerosol self-administration and did not increase responding for nicotine. Throughout the aerosol self-administration phase, most groups responded more on the active than inactive lever, and some groups increased their response when the fixed ratio value was increased. At the end of the study, rats in nicotine groups earned similar or fewer aerosol deliveries than rats in vehicle groups. Aerosolized nicotine did not function as a reinforcer in this study, whereas aerosolized raspberry flavor may have maintained self-administration. Further preclinical investigation is needed to articulate the impact of flavors on ENDS use and whether they offset some aversive effects of nicotine or maintain responding on their own. If flavors reduce some aversive effects of self-administered nicotine, then policies to regulate flavors in e-liquids are prudent.

Chemical Flavorants in Vaping Products Alter Neurobiology in a Sex-Dependent Manner to Promote Vaping-Related Behaviors

Electronic nicotine delivery systems (ENDS) are distinctly different from combustible cigarettes because of the availability of flavor options. Subjective measures have been used to demonstrate that adults and adolescents prefer flavors for various reasons; (1) they are pleasing and (2) they mask the harshness of nicotine. Despite this, there have been few investigations into the molecular interactions that connect chemical flavorants to smoking or vaping-related behaviors. Here, we investigated the effects of three chemical flavorants (hexyl acetate, ethyl acetate, and methylbutyl acetate) that are found in green apple (GA) ENDS e-liquids but are also found in other flavor categories. We used a translationally relevant vapor self-administration mouse model and observed that adult male and female mice self-administered GA flavorants in the absence of nicotine. Using α4-mCherryα6-GFP nicotinic acetylcholine receptor (nAChR) mice, we observed that mice exposed to GA flavorants exhibited a sex-specific increase (upregulation) of nAChRs that was also brain-region specific. Electrophysiology revealed that mice exposed to GA flavorants exhibited enhanced firing of ventral tegmental area dopamine neurons. Fast-scan cyclic voltammetry revealed that electrically stimulated dopamine release in the nucleus accumbens core is increased in mice that are exposed to GA flavorants. These effects were similarly observed in the medial habenula. Overall, these findings demonstrate that ENDS flavors alone change neurobiology and may promote vaping-dependent behaviors in the absence of nicotine. Furthermore, the flavorant-induced changes in neurobiology parallel those caused by nicotine, which highlights the fact that nonmenthol flavorants may contribute to or enhance nicotine reward and reinforcement.SIGNIFICANCE STATEMENT The impact of flavors on vaping is a hotly debated topic; however, few investigations have examined this in a model that is relevant to vaping. Although a full understanding of the exact mechanism remains undetermined, our observations reveal that chemical flavorants in the absence of nicotine alter brain circuits relevant to vaping-related behavior. The fact that the flavorants investigated here exist in multiple flavor categories of vaping products highlights the fact that a multitude of flavored vaping products may pose a risk toward vaping-dependent behaviors even without the impact of nicotine. Furthermore, as the neurobiological changes have an impact on neurons of the reward system, there exists the possibility that nonmenthol flavorants may enhance nicotine reward and reinforcement.

A high-fat diet has sex-specific effects on nicotine vapor self-administration in mice

BACKGROUND

Previous investigations have shown that fat-rich diets increase vulnerability to drug dependence, including nicotine. Despite this knowledge, few investigations into the neurochemical mechanisms have been completed. Our objective here was to examine if high-fat diet (HFD) impacted nicotine intake and in parallel examine potential changes in dopamine signaling.

METHODS

Adult male and female C57/BL6J mice were used in nicotine e-vape® self-administration (EVSA) assays after being maintained on a standard diet or HFD for 6 weeks. In a separate cohort of mice, dopamine release in the nucleus accumbens core was examined with fast-scan cyclic voltammetry.

RESULTS

Female mice assigned to HFD exhibited increased nicotine EVSA during low-effort responding (FR1) when compared to standard-diet mice. HFD-assigned mice (male and female) also exhibited reduced active nose pokes in a progressive ratio task. Finally, HFD-mice exhibited reduced phasic dopamine release compared to standard-diet mice.

CONCLUSIONS

These show that fat-rich diets alter nicotine intake (females increase at low effort, males and females decrease at high effort) and this may occur due to HFD-induced decreases in NAc dopamine release.

The effects of electronic nicotine vapor on voluntary alcohol consumption in female and male C57BL/6 J mice

SIGNIFICANCE

Alcohol drinking and nicotine vaping often co-occur and dependence on both substances is common. However, the impact of nicotine vaping on alcohol consumption is not fully understood.

METHODS

We examined the effects of nicotine vaping on ethanol drinking in female and male C57BL/6 J mice using an electronic nicotine delivery system and intermittent access two-bottle choice (IA-2BC) drinking. Mice were exposed to electronic nicotine vapor (3%) or propylene glycol/vegetable glycerol (PG/VG) control for 3 h sessions daily for 4 weeks and voluntary alcohol consumption was monitored. Nicotine vapor exposure was stopped and voluntary alcohol drinking was measured for a 2 week abstinence period. We also examined the effects of alcohol and nicotine on locomotion, temperature, and nicotine metabolism.

RESULTS

Following acute nicotine vapor exposure, alcohol drinking was increased in males but not in females. Thermoregulation was disrupted following nicotine vapor exposure and voluntary drinking. Male and female mice displayed increased locomotor activity immediately following chronic nicotine vapor exposure, and an anxiolytic effect was seen in males. In nicotine vapor abstinence, female mice displayed increased alcohol consumption. Locomotor activity and anxiolytic effects remained elevated in male but not female mice. Female mice displayed higher levels of serum nicotine and hydroxycotinine, suggesting impaired metabolism following chronic drinking and nicotine vapor exposure.

CONCLUSION

Collectively, these results suggest that while both male and female ethanol-drinking mice experience the stimulatory effects of nicotine vapor, only in males is there a parallel increase in ethanol drinking and only females display impairments in nicotine metabolism after drinking.

The long-term effects of repeated heroin vapor inhalation during adolescence on measures of nociception and anxiety-like behavior in adult Wistar rats

RATIONALE

Adolescents represent a vulnerable group due to increased experimentation with illicit substances that is often associated with the adolescent period, and because adolescent drug use can result in long-term effects that differ from those caused by drug use initiated during adulthood.

OBJECTIVES

The purpose of the present study was to determine the effects of repeated heroin vapor inhalation during adolescence on measures of nociception, and anxiety-like behavior during adulthood in female and male Wistar rats.

METHODS

Rats were exposed twice daily to 30 min of heroin vapor from post-natal day (PND) 36 to PND 45. At 12 weeks of age, baseline thermal nociception was assessed across a range of temperatures with a warm-water tail-withdrawal assay. Anxiety-like behavior was assessed in an elevated plus-maze (EPM) and activity was measured in an open-field arena. Starting at 23 weeks of age, baseline thermal nociception was re-assessed, nociception was determined after acute heroin or naloxone injection, and anxiety-like behavior was redetermined in the EPM.

RESULTS

Adolescent heroin inhalation altered baseline thermal nociception in female rats at 12 weeks of age and in both female and male rats at ~ 23 weeks. Heroin-treated animals exhibited anxiety-like behavior when tested in the elevated plus-maze, showed blunted heroin-induced analgesia, but exhibited no effect on naloxone-induced hyperalgesia.

CONCLUSIONS

The present study demonstrates that heroin vapor inhalation during adolescence produces behavioral and physiological consequences in rats that persist well into adulthood.

Associations of Demographics, Dependence, and Biomarkers With Transitions in Tobacco Product Use in a Cohort of Cigarette Users and Dual Users of Cigarettes and E-cigarettes

INTRODUCTION

It is uncertain whether e-cigarettes facilitate smoking cessation in the real world. We aimed to understand whether and how transitions among cigarette, e-cigarette, and dual use are associated with sociodemographics, dependence measures, and biomarkers.

AIMS AND METHODS

We followed 380 adult daily cigarette users and dual users every 2 months for up to 2 years. We estimated transition rates between noncurrent, cigarette-only, e-cigarette-only, and dual use states using a multistate transition model. We estimated univariable hazard ratios (HR) for demographics, dependence measures for cigarettes and e-cigarettes, biomarkers, spousal or partner behaviors, and other measures.

RESULTS

We estimated that participants transitioned from cigarette-only to e-cigarette-only through a period of dual use. Dual users ceased smoking (transitioning to e-cigarette-only use) at a greater rate than cigarette-only users did (HR 2.44, 95% CI: 1.49, 4.02). However, of the 60% of dual users estimated to transition to single product use in 1 year, 83% would transition to cigarette-only use and only 17% to e-cigarette-only use. E-cigarette dependence measures were generally associated with reduced e-cigarette cessation rather than enhanced cigarette cessation. E-cigarette users motivated by harm or toxicity reduction or because of restrictions on where or when they could smoke had reduced rates of smoking relapse. Cigarette dependence and spousal smoking were barriers to cigarette cessation for dual users, while using e-cigarettes first in the morning, motivation to quit smoking, and sensory, social, and emotional enjoyment of e-cigarettes (secondary dependence motives) were facilitators of smoking cessation among dual users.

CONCLUSIONS

Tobacco control policy and interventions may be informed by the barriers and facilitators of product transitions.

IMPLICATIONS

Although e-cigarettes have the potential to promote smoking cessation, their real-world impact is uncertain. In this cohort, dual users were more likely to quit smoking than cigarette-only users, but the overall impact was small because most dual users returned to cigarette-only use. Moreover, e-cigarette dependence promoted continued dual use rather than smoking cessation. Yet, high motivation to quit smoking and the sensory, social, and emotional enjoyment of e-cigarettes facilitated smoking cessation in dual users. Better understanding the barriers and facilitators of transitions can help to develop regulations and interventions that lead to more effective use of e-cigarettes for smoking cessation.

Sex Differences in the Behavioural Outcomes of Prenatal Nicotine and Tobacco Exposure

Smoking remains the leading cause of preventable death worldwide. A combination of biological and environmental risk factors make women especially vulnerable to nicotine addiction, making it harder for them to quit smoking. Smoking during pregnancy, therefore, is still a major health concern, with epidemiological data suggesting a role for gestational nicotine exposure in the development of several behavioural disorders. Given there are significant sex-specific behavioural outcomes related to smoking in adolescence and adulthood, it is probable that the behavioural outcomes following gestational nicotine or tobacco exposure are similarly sex-dependent. This is an especially relevant topic as the current landscape of nicotine use shifts toward vaping, a mode of high doses of nicotine delivery that is largely believed to be a safer alternative to cigarettes among the public as well as among pregnant women. Here we review existing clinical and preclinical findings regarding the sex-dependent behavioural outcomes of prenatal nicotine exposure. We also highlight the challenges within this literature, particularly those areas in which further research is necessary to improve consistency within, and between, clinical and preclinical findings.

Increased Risky Choice and Reduced CHRNB2 Expression in Adult Male Rats Exposed to Nicotine Vapor

While the cognitive enhancing effects of nicotine use have been well documented, it has also been shown to impair decision making. The goal of this study was to determine if exposure to nicotine vapor increases risky decision making. The study also aims to investigate possible long-term effects of nicotine vapor exposure on the expression of genes coding for cholinergic and dopaminergic receptors in brain. Thirty-two adult male Sprague Dawley rats were exposed to 24 mg/mL nicotine vapor or vehicle control, immediately followed by testing in the probability discounting task for 10 consecutive days. Fifty-four days after the 10-day vapor exposure, animals were sacrificed and expression of genes coding for the α4 and β2 cholinergic receptor subunits, and dopamine D1 and D2 receptors, were analyzed using RT-PCR. Exposure to nicotine vapor caused an immediate and transient increase in risky choice. Analyses of gene expression identified significant reductions in CHRNB2 and DRD1 in the nucleus accumbens core and CHRNB2 and DRD2 in the medial prefrontal cortex of rats previously exposed to nicotine vapor, relative to vehicle controls. Results provide data on the negative cognitive effects of nicotine vapor exposure and identify cholinergic and dopaminergic mechanisms that may affected with repeated use.

Morphine Exposure Reduces Nicotine-Induced Upregulation of Nicotinic Receptors and Decreases Volitional Nicotine Intake in a Mouse Model

INTRODUCTION

Nicotine addiction remains a primary health concern as tobacco smoking remains the number one cause of preventable death in America. At the same time, America is still facing the threat of the opioid epidemic. While the prevalence of smoking combustible cigarettes or electronic nicotine delivery systems in the United States varies between 12% and 35%, the smoking rates among the opioid use dependent (OUD) population is 74%-97%. We examined changes in brain reward mechanisms in which co-use of nicotine and opioids may result in enhanced reward and reinforcement.

AIMS AND METHODS

Adult male and female α4-mCherryα6-GFP mice (C57BL/6J) were used in conditioned place preference (CPP) and microscopy assays to examine reward-related behavior and nicotinic acetylcholine receptor (nAChR) upregulation following treatments with saline, nicotine, morphine, or nicotine plus morphine. Following this, separate mice were trained in e-Vape self-administration assays to examine morphine's impact on nicotine reinforcement.

RESULTS

We observed that nicotine and morphine coexposure in a CPP assay did not produce enhanced reward-related behavior when compared with nicotine or morphine alone. In parallel we observed coexposure reduced nicotine-induced upregulation of nAChRs on ventral tegmental area dopamine and GABA neurons. Additionally, we observed that concurrent morphine exposure reduced nicotine (plus menthol) vapor self-administration in male and female mice.

CONCLUSIONS

While nicotine use is high among OUD individuals, our CPP assays suggest coexposure not only fails to enhance reward-related behavior but also reduces nicotine-induced changes in ventral tegmental area neurobiology. Our self-administration assays suggest that morphine exposure during nicotine acquisition reduces nicotine reinforcement-related behavior.

IMPLICATIONS

While some may postulate that the co-use of opioids and nicotine may be driven by reward-related mechanisms, our data indicate that opioid exposure may hinder nicotine intake due to reduced upregulation of nAChRs critical for nicotine reward and reinforcement. Thus, the high co-use in OUD individuals may be a result of other mechanisms and this warrants further investigations into nicotine and opioid co-use.

Animal Models to Investigate the Impact of Flavors on Nicotine Addiction and Dependence

BACKGROUND

Tobacco use in humans is a long-standing public health concern. Flavors are common additives in tobacco and alternative tobacco products, added to mask nicotine's harsh orosensory effects and increase the appeal of these products. Animal models are integral for investigating nicotine use and addiction and are helpful for understanding the effects of flavor additives on the use of nicotine delivery products.

OBJECTIVE

This review focuses on preclinical models to evaluate the contribution of flavor additives to nicotine addiction.

MATERIALS AND METHODS

An electronic literature search was conducted by authors up to May 2022. Original articles were selected.

RESULTS

The behavioral models of rodents described here capture multiple dimensions of human flavored nicotine use behaviors, including advantages and disadvantages.

CONCLUSION

The consensus of the literature search was that human research on nicotine use behavior has not caught up with fast-changing product innovations, marketing practices, and federal regulations. Animal models are therefore needed to investigate mechanisms underlying nicotine use and addiction. This review provides a comprehensive overvie.