Enhanced vulnerability to the rewarding effects of nicotine during the adolescent period of development

Acute nicotine vapor normalizes sensorimotor gating and reduces locomotor activity deficits in HIV-1 transgenic rats

Rationale

Despite improved life expectancy of people with HIV (PWH), HIV-associated neurocognitive impairment (NCI) persists, alongside deficits in sensorimotor gating and neuroinflammation. PWH exhibit high smoking rates, possibly due to neuroprotective, anti-inflammatory, and cognitive-enhancing effects of nicotine, suggesting potential self-medication.

Objectives

Here, we tested the effects of acute nicotine vapor exposure on translatable measures of sensorimotor gating and exploratory behavior in the HIV-1 transgenic (HIV-1Tg) rat model of HIV.

Methods

Male and female HIV-1Tg and F344 control rats (n=57) were exposed to acute nicotine or vehicle vapor. Sensorimotor gating was assessed using prepulse inhibition (PPI) of the acoustic startle response, and exploratory behavior was evaluated using the behavioral pattern monitor (BPM).

Results

Vehicle-treated HIV-1Tg rats exhibited PPI deficits at low prepulse intensities compared to F344 controls, as seen previously. No PPI deficits were observed in nicotine-treated HIV1-Tg rats, however. HIV-1Tg rats were hypoactive in the BPM relative to controls, whilst nicotine vapor increased activity and exploratory behavior across genotypes. Cotinine analyses confirmed comparable levels of the primary metabolite of nicotine across genotypes.

Conclusions

Previous findings of PPI deficits in HIV-1Tg rats were replicated and, importantly, attenuated by acute nicotine vapor. Evidence for similar cotinine levels suggest a nicotine-specific effect in HIV-1Tg rats. HIV-1Tg rats had reduced exploratory behavior compared to controls, attenuated by acute nicotine vapor. Therefore, acute nicotine may be beneficial for remediating sensorimotor and locomotor activity deficits in PWH. Future studies should determine the long-term effects of nicotine vapor on similar HIV/NCI-relevant behaviors.

Nicotine and fluoxetine alter adolescent dopamine-mediated behaviors via 5-HT1A receptor activation

Introduction

Abuse or misuse of tobacco, e-cigarettes, or antidepressants may have serious clinical consequences during adolescence, a sensitive period during brain development when the distinct neurobiology of adolescent serotonin (5-HT) and dopamine (DA) systems create unique behavioral vulnerabilities to drugs of abuse.

Methods

Using a pharmacological approach, we modeled the behavioral and neurochemical effects of subchronic (4-day) nicotine (60µg/kg, i.v.) or fluoxetine (1mg/kg, i.v.) exposure in adolescent and adult male rats.

Results

Nicotine and fluoxetine significantly enhance quinpirole-induced locomotor activity and initial cocaine self-administration in adolescents, but not adults. These effects were blocked by serotonin 5-HT1A receptor antagonists, WAY-100,635 (100 µg/kg, i.v.) or S-15535 (300 µg/kg, i.v.). Neurochemical and anatomical autoradiographic analysis of 8-OH-DPAT-stimulated [35S]GTPγS reveal that prior exposure to nicotine and fluoxetine results in both overlapping and distinct effects on regional 5-HT1A receptor activity. Both fluoxetine and nicotine enhance adolescent 5-HT1A receptor activity in the primary motor cortex (M1), whereas fluoxetine alone targets prefrontal cortical neurocircuitry and nicotine alone targets the amygdala.

Discussion

Given their different pharmacological profiles, comparison between WAY-100,635 and S-15535 indicates that postsynaptic 5-HT1A receptors mediate the behavioral effects of prior nicotine and fluoxetine exposure. In addition, within the adolescent M1, maladaptive changes in 5-HT signaling and 5-HT1A activity after nicotine or fluoxetine exposure may potentiate hyper-responsiveness to dopaminergic drugs and prime adolescent vulnerability for future substance abuse.

Adolescent nicotine exposure promotes adulthood opioid consumption that persists despite adverse consequences and increases the density of insular perineuronal nets

Adolescence marks a sensitive period for neurodevelopment wherein exposure to drugs of abuse may disrupt maturation and induce persistent changes in neurophysiology which may exacerbate the risk for developing substance use disorders in adulthood. Adolescent nicotine exposure (ANE) enhances motivation to obtain drugs of abuse, particularly opioids, and increases vulnerability for the development of opioid use disorder (OUD). Here, we characterized ANE effects on learning about the adverse consequences of opioid consumption in adulthood in the absence of further nicotine administration. First, we show that ANE engenders punishment resistant fentanyl self-administration in a heterogenous seeking-taking chain schedule of reinforcement at least at the tested dose of fentanyl (0.75 μg/kg). We found that ANE rats consumed significantly more fentanyl and contingent foot shock punishment was less efficacious in limiting fentanyl seeking in ANE rats, relative to nicotine-naïve controls. Next, we demonstrated that ANE limits learning about the deleterious consequences of acute opioid intoxication in adulthood. In a combined conditioned taste avoidance and place preference paradigm we found that ANE resulted in significant reductions in the strength of morphine-induced CTA, and a simultaneous enhancement of CPP at a higher dose that was less capable of driving reinforcement in naïve controls. Finally, we examined the expression of perineuronal nets (PNNs) within insular cortex (IC) and found ANE rats to have increased density of PNNs across the anterior IC and significantly more parvalbumin-labeled IC cells relative to naïve controls. Together, these data lay the framework for a mechanistic explanation of the extreme comorbidity between nicotine use and development of OUDs.

Unlocking the age-old secrets of reward and substance use

Although substance use is widespread across the lifespan from early adolescence to older adulthood, the prevalence of substance use disorder (SUD) differs between age groups. These age differences in SUD rates necessitate an investigation into how age moderates reward sensitivity, and consequently influences the risks and consequences related to substance use. This theoretical review integrates evidence from the literature to address the dynamic interplay between age and reward in the context of substance use. Overall, increasing evidence demonstrates that age moderates reward sensitivity and underlying reward system neurobiology. Reward sensitivity undergoes a non-linear trajectory across the lifespan. Low levels of reward sensitivity are associated with childhood and late adulthood. In contrast, high levels are associated with early to late adolescence, followed by a decline in the twenties. These fluctuations in reward sensitivity across the lifespan contribute to complex associations with substance use. This lends support to adolescence and young adulthood as vulnerable periods for the risk of subsequent SUD. More empirical research is needed to investigate reward sensitivity during SUD maintenance and recovery. Future research should also involve larger sample sizes and encompass a broader range of age groups, including older adults.

Behavioral and Pharmacokinetic Assessment of Nicotine e-Cigarette Inhalation in Female Rats

INTRODUCTION

Nicotine and tobacco use remain high both globally and in the United States, contributing to large health care expenditures. With a rise in e-cigarette use, it is important to have clinically relevant models of inhaled nicotine exposure.

AIMS AND METHODS

This study aims to extend prior preclinical nicotine inhalation animal data to females and provide both behavior and serum pharmacokinetics. We tested two inhalation doses of nicotine (24 mg/mL and 59 mg/ mL) and compared these to injected doses (0.4 mg/kg and 1 mg/kg). In addition, we assessed locomotor behavior after the same doses. Blood was collected at 10- and 120-minutes post-administration. We assessed nicotine and cotinine serum concentrations by LC-MS/MS.

RESULTS

Showed that while nicotine serum concentrations for the respective high and low-dose administrations were similar between both routes of administration, the route had differential effects on locomotor behavior. Inhaled nicotine showed a dose-dependent decrease in locomotor activity while injected doses showed the opposite trend.

CONCLUSIONS

Our results indicate that the route of administration is an important factor when establishing preclinical models of nicotine exposures. Given that the overall use of e-cigarettes in vulnerable populations is on the rise, our study provides important behavioral and pharmacokinetic information to advance our currently limited understanding of the effects of nicotine vapor exposure.

IMPLICATIONS

This study highlights behavioral differences between different routes of administration of similar doses of nicotine. Using a low and high dose of nicotine, we found that nicotine serum concentrations were similar between the different routes of administration. Our results indicate that different routes of administration have opposing effects on locomotor activity. These findings provide important implications for future behavioral models.

Oral administration of coenzyme Q10 ameliorates memory impairment induced by nicotine-ethanol abstinence through restoration of biochemical changes in male rat hippocampal tissues

Substance abuse among adolescents has become a growing issue throughout the world. The significance of research on this life period is based on the occurrence of neurobiological changes in adolescent brain which makes the individual more susceptible for risk-taking and impulsive behaviors. Alcohol and nicotine are among the most available drugs of abuse in adolescents. Prolonged consumption of nicotine and alcohol leads to drug dependence and withdrawal which induce various dysfunctions such as memory loss. Coenzyme Q10 (CoQ10) is known to improve learning and memory deficits induced by various pathological conditions such as Diabetes mellitus and Alzheimer's disease. In the present study we investigated whether CoQ10 treatment ameliorates memory loss following a nicotine-ethanol abstinence. Morris water maze and novel object recognition tests were done in male Wistar rats undergone nicotine-ethanol abstinence and the effect of CoQ10 was assessed on at behavioral and biochemical levels. Results indicated that nicotine-ethanol abstinence induces memory dysfunction which is associated with increased oxidative and inflammatory response, reduced cholinergic and neurotrophic function plus elevated Amyloid-B levels in hippocampi. CoQ10 treatment prevented memory deficits and biochemical alterations. Interestingly, this ameliorative effect of CoQ10 was found to be dose-dependent in most experiments and almost equipotential to that of bupropion and naloxone co-administration. CoQ10 treatment could effectively improve memory defects induced by nicotine-ethanol consumption through attenuation of oxidative damage, inflammation, amyloid-B level and enhancement of cholinergic and neurotrophic drive. Further studies are required to assess the unknown side effects and high dose tolerability of the drug in human subjects.

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.

Omega-3 fatty acids prevent nicotine withdrawal-induced impairment of learning and memory via affecting oxidative status, inflammatory response, cholinergic activity, BDNF and amyloid-B in rat hippocampal tissues

In the present study, the main objective was to reveal whether treatment by Omega-3 fatty acids could prevent the adverse effects of adolescent nicotine withdrawal on spatial and avoidance memory in male rats. For this purpose, Morris water maze and passive avoidance tests were performed on male Wistar rats and the hippocampal levels of oxidative stress markers, inflammatory indices, brain-derived neurotrophic factor, nitrite, amyloid-B and acetylcholinesterase (AChE) were measured. Moreover, density of dark neurons were assessed in CA1 and CA3 regions. Results showed that adolescent nicotine exposure followed by a period of drug cessation exacerbates the behavioral indices of learning and memory through affecting a variety of biochemical markers within the hippocampal tissues. These changes lead to elevation of oxidative and inflammatory markers, reduction of neurotrophic capacity and increased AChE activity in hippocampal tissues. In addition, it was observed that co-administration of nicotine with Omega-3 fatty acids significantly prevents nicotine withdrawal-induced adverse effects through restoration of the mentioned biochemical disturbances. Therefore, we suggest administration of Omega-3 fatty acids as a safe, inexpensive and effective therapeutic strategy for prevention of memory dysfunctions associated with nicotine abstinence during adolescence.

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.

Age-dependent effects of tobacco smoke and nicotine on cognition and the brain: A systematic review of the human and animal literature comparing adolescents and adults

Cigarette smoking is often initiated during adolescence and an earlier age of onset is associated with worse health outcomes later in life. Paradoxically, the transition towards adulthood also marks the potential for recovery, as the majority of adolescents are able to quit smoking when adulthood emerges. This systematic review aimed to evaluate the evidence from both human and animal studies for the differential impact of adolescent versus adult repeated and long-term tobacco and nicotine exposure on cognitive and brain outcomes. The limited human studies and more extensive yet heterogeneous animal studies, provide preliminary evidence of heightened fear learning, anxiety-related behaviour, reward processing, nicotinic acetylcholinergic receptors expression, dopamine expression and serotonin functioning after adolescent compared to adult exposure. Effects of nicotine or tobacco use on impulsivity were comparable across age groups. These findings provide novel insights into the mechanisms underlying adolescents' vulnerability to tobacco and nicotine. Future research is needed to translate animal to human findings, with a focus on directly linking a broader spectrum of brain and behavioural outcomes.

Nicotine on the developing brain

Developmental periods such as gestation and adolescence have enhanced plasticity leaving the brain vulnerable to harmful effects from nicotine use. Proper brain maturation and circuit organization is critical for normal physiological and behavioral outcomes. Although cigarette smoking has declined in popularity, noncombustible nicotine products are readily used. The misperceived safety of these alternatives lead to widespread use among vulnerable populations such as pregnant women and adolescents. Nicotine exposure during these sensitive developmental windows is detrimental to cardiorespiratory function, learning and memory, executive function, and reward related circuitry. In this review, we will discuss clinical and preclinical evidence of the adverse alterations in the brain and behavior following nicotine exposure. Time-dependent nicotine-induced changes in reward related brain regions and drug reward behaviors will be discussed and highlight unique sensitivities within a developmental period. We will also review long lasting effects of developmental exposure persisting into adulthood, along with permanent epigenetic changes in the genome which can be passed to future generations. Taken together, it is critical to evaluate the consequences of nicotine exposure during these vulnerable developmental windows due to its direct impact on cognition, potential trajectories for other substance use, and implicated mechanisms for the neurobiology of substance use disorders.

Omega-3 fatty acids prevent nicotine withdrawal-induced exacerbation of anxiety and depression by affecting oxidative stress balance, inflammatory response, BDNF and serotonin metabolism in rats

Adolescents are known to be more vulnerable than adults to the adverse effects of nicotine dependence. In the present study, we aimed to investigate whether adolescent nicotine exposure, followed by a period of abstinence, could affect the anxiety- and depressive-like behaviors in rats. For this purpose, behavioral assessments were carried out using open field test, elevated plus maze and forced swimming test in male rats received chronic nicotine intake during adolescence followed by a period of abstinence in adulthood, compared to their control counterparts. In addition, O3 pre-treatment was done at three different doses to reveal whether it could prevent nicotine withdrawal effects. Then, animals were euthanized and the cortical concentrations of oxidative stress markers, inflammatory indices, brain-derived neurotrophic factor, serotonin and the enzymatic activity of monoamine oxidase-A were measured. Results indicated that nicotine withdrawal exacerbates the behavioral signs of anxiety through alteration of the brain oxidative stress balance, inflammatory response and serotonin metabolism. Moreover, we found that omega 3 pre-treatment significantly prevents the nicotine withdrawal-induced complications by restoration of changes in the mentioned biochemical indices. Moreover, the improving effects of O3 fatty acids were found to be dose-dependent in all experiments. Taken together, we would like to suggest the O3 fatty acids supplementation as a safe, inexpensive and effective strategy for prevention or amelioration of detrimental effects induced by nicotine withdrawal at cellular and behavioral levels.

Alcohol and Nicotine Use among Adolescents: An Observational Study in a Sicilian Cohort of High School Students

In recent years, the mode of alcoholic intake known as binge drinking (BD) has become a common practice, especially among adolescents who, due to socio-environmental motives, tend to reach a rapid state of drunkenness. This drunkeness leads to alterations in brain areas responsible for executive functions and cognitive processes, as well as to the genesis of factors that predispose to lasting addiction. Likewise, nicotine leads to a comparable degree of addiction. On this basis, the aim of this research was to evaluate, on a cohort of 349 high school students (15−17 years old) in the province of Palermo, the following: (I) the drinking model of alcoholic beverages; (II) the use of nicotine and the degree of dependence; (III) the correlation between the consumption of alcoholic beverages and the use of nicotine. We employed the AUDIT-C test and the Fagerström test, two valid and standard instruments, in order to assess alcohol and nicotine use, respectively. Statistical analysis of the data showed that male and female students consumed alcohol prominently in a BD mode (77.2%, audit score (AS) 3.497, confidence interval (CI) 3.206−3.788; 69.6%, AS 2.793, CI 2.412−3.274) and nicotine (41.5%, Fagerström score (FS) 3.882, CI 3.519−4.245; 28%, FS 3.286, CI 2.547−4.024). Furthermore, a positive correlation between alcohol consumption and nicotine use was found for male (r = 0.6798, p < 0.0001) and female (r = 0.6572, p < 0.0001) students. This study provided further insights into the use of legal substances of abuse in adolescents, evidencing the obvious need for the promotion of specific school educational programs aimed at the wellbeing of youth populations.

Sex and age differences in approach behavior toward a port that delivers nicotine vapor

The goal of our laboratory is to study the mechanisms that promote nicotine use, particularly in vulnerable populations. To more closely mimic human use patterns, the present study employed nicotine vapor methods involving passive exposure for 14 days in adolescent and adult female and male rats. Age and sex differences in approach behavior (nosepokes) were assessed in a port that delivered nicotine plumes on Day 1 and 14 of our exposure regimen. Controls received ambient air in exposure chambers. After the final session, rats received a nicotinic receptor antagonist to precipitate withdrawal. Then, physical signs, anxiety-like behavior, and plasma levels of cotinine (a nicotine metabolite) were assessed. Over time, females displayed a larger increase in approach behavior to the nicotine port than males, an effect that was larger in adolescents. Nosepoke responses in adolescent females were correlated with anxiety-like behavior, but not physical signs of withdrawal. Adolescents gained more weight than adults regardless of treatment, and the weight gain was larger in male adolescents. Female adolescents also displayed the highest levels of cotinine than all other groups. These findings suggest that nicotine vapor produces greater motivational effects in adolescent females as compared to their adult and male counterparts.

Unique effects of nicotine across the lifespan

Smoking remains the leading cause of preventable death in the United States. Although combustible cigarettes are largely being replaced by tobacco-free products, nicotine use continues to increase in vulnerable populations, including youth, adolescents, and pregnant women. Nicotine exerts unique effects on specific brain regions during distinct developmental periods due to the dynamic expression of nicotinic acetylcholine receptors (nAChRs) throughout the lifespan. Nicotine exposure is a health concern not only for adults but also has neurotoxic effects on the fetus, newborn, child, and adolescent. In this review, we aim to highlight the dynamic roles of nAChRs throughout gestation, adolescence, and adulthood. We also provide clinical and preclinical evidence of the neurodevelopmental, cognitive, and behavioral consequences of nicotine exposure at different developmental periods. This comprehensive review highlights unique effects of nicotine throughout the lifespan to help elucidate interventions and public health measures to protect sensitive populations from nicotine exposure.

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.

The impact of adolescent nicotine exposure on alcohol use during adulthood: The role of neuropeptides

Nicotine and alcohol abuse and co-dependence represent major public health crises. Indeed, previous research has shown that the prevalence of alcoholism is higher in smokers than in non-smokers. Adolescence is a susceptible period of life for the initiation of nicotine and alcohol use and the development of nicotine-alcohol codependence. However, there is a limited number of pharmacotherapeutic agents to treat addiction to nicotine or alcohol alone. Notably, there is no effective medication to treat this comorbid disorder. This chapter aims to review the early nicotine use and its impact on subsequent alcohol abuse during adolescence and adulthood as well as the role of neuropeptides in this comorbid disorder. The preclinical and clinical findings discussed in this chapter will advance our understanding of this comorbid disorder's neurobiology and lay a foundation for developing novel pharmacotherapies to treat nicotine and alcohol codependence.

Blocking serotonin 2A (5-HT2A) receptors attenuates the acquisition of methamphetamine-induced conditioned place preference in adult female rats

Methamphetamine withdrawal can induce intense cravings leading to relapse. Contexts/cues paired with chronic methamphetamine use develop incentive motivational properties, promoting future drug-seeking and taking behavior. Research has shown that, in adult male rats, the selective 5-HT_2A receptor antagonist M100907 attenuates the acquisition of methamphetamine-induced conditioned place preference (CPP), a measure that examines conditioned associations between the rewarding properties of drugs and contexts. However, these findings have not been extended to adult female rats. The present study investigated the effects of M100907 on the acquisition of methamphetamine-CPP in adult female rats. During conditioning, rats were administered M100907 (0, 0.025, 0.25 mg/kg, i.p.) 15 min before methamphetamine (1 mg/kg, i.p.) and then placed into their initially non-preferred chamber for 30 min, or administered saline and placed into their initially preferred chamber for 30 min. Conditioning sessions were separated by four hours. Following four days of conditioning, the effects of M100907 on the acquisition of methamphetamine-CPP were assessed during a 15 min drug-free test trial. Pretreatment with M100907 dose-dependently attenuated the acquisition of methamphetamine-induced CPP. Blocking 5-HT_2A receptors with a low dose of the selective antagonist M100907 attenuated the rewarding effects of methamphetamine in adult female rats. These data provide further evidence that the 5-HT_2A receptor subtype is involved in the behavioral effects of methamphetamine.

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.

Adolescent nicotine treatment causes robust locomotor sensitization during adolescence but impedes the spontaneous acquisition of nicotine intake in adult female Wistar rats

Very few people are able to quit smoking, and therefore it is essential to know which factors contribute to the development of compulsive nicotine use. These studies aimed to investigate if early-adolescent nicotine exposure causes locomotor sensitization and affects anxiety-like behavior and the spontaneous acquisition of intravenous nicotine self-administration. Early-adolescent male and female rats were treated with nicotine from postnatal (P) days 24 to 42, and anxiety-like behavior and locomotor activity were investigated one day after the cessation of nicotine treatment and in adulthood (>P75). The spontaneous acquisition of nicotine self-administration was also investigated in adulthood. The rats self-administered 0.03 mg/kg/infusion of nicotine for six days under a fixed-ratio (FR) 1 schedule and four days under an FR2 schedule (3-h sessions). Repeated nicotine administration increased locomotor activity, rearing, and stereotypies in a small open field in adolescent male and female rats. One day after the last nicotine injection, the percentage of open arm entries in the elevated plus-maze test was decreased in the males and increased in the females. However, locomotor activity in the small open field was unaffected. Adolescent nicotine treatment did not affect anxiety-like behavior and locomotor activity in adulthood. During the 10-day nicotine self-administration period, the females had a higher level of nicotine intake than the males. Adolescent nicotine treatment decreased nicotine intake in the females. In conclusion, these findings indicate that repeated nicotine administration during adolescence causes robust behavioral sensitization and leads to lower nicotine intake in females throughout the acquisition period in adulthood in rats.

Adolescent nicotine and footshock exposure augments adult nicotine self-administration and drug-seeking without affecting baseline anxiety-like behaviour or stress responsivity in male rats

RATIONALE

Over the past decade, adolescent cigarette smoking has been declining. However, adolescent nicotine consumption via electronic cigarettes is rapidly gaining popularity. Earlier onset nicotine use is associated with increased risk of dependence. A bidirectional relationship between nicotine and stress exists; perceived stress is a predictor for nicotine use, and stress reduction is a commonly reported reason for using nicotine.

OBJECTIVES

We assessed the prolonged impact of adolescent high-dose nicotine and/or footshock exposure on adult nicotine self-administration, anxiety-like behaviour, and hormonal responsivity.

METHODS

During adolescence (postnatal day [P]28-56) male Sprague-Dawley rats were assigned to one of five groups: saline (SALPRE: 1 ml/kg, SC, every day), nicotine (NICPRE: 1 mg/kg, SC, alternating daily with saline; 14 total nicotine injections), footshock (SHOCKPRE: 8 of 0.5 s, 0.8 mA alternating sessions; saline every day), or combination nicotine and footshock (NIC+SHOCK: concurrent and alternating daily with saline, or NIC-SHOCK: alternating with saline on shock sessions). On P70, one cohort underwent spontaneous intravenous nicotine self-administration (0.03 mg/kg/infusion); another cohort was assessed for open-field behaviour (P71), then corticosterone (CORT) response to nicotine or footshock in adulthood (P72-73).

RESULTS

Intermittent adolescent nicotine or footshock alone (NICPRE and SHOCKPRE) did not potentiate adult spontaneous nicotine intake compared to SALPRE. However, both combination groups (NIC+SHOCK, NIC-SHOCK) showed increased adult nicotine consumption without associated differences in baseline anxiety-like behaviour or CORT response.

CONCLUSIONS

Adolescent nicotine and footshock stressors have a synergistic effect on adult nicotine consumption, enhancing nicotine intake. Avenues toward reducing stress in adolescent nicotine users may provide opportunities to reduce vulnerability to adult nicotine consumption.

Potentiated perceptual neural responses to learned threat during Pavlovian fear acquisition and extinction in adolescents

Adolescents' experience of heightened anxiety and increased vulnerability to develop anxiety disorders is believed to partly result from blunted fear extinction processes. However, whether this anxiety is mediated by adolescent-specific differences in perceptual responses to learned threat is not known. To investigate this, we used EEG to examine reinforcement-dependent changes in early visual event-related potentials in adolescents (N = 28, 13-14 years) and adults (N = 23, 25-26 years old) during a differential Pavlovian fear conditioning task, with one conditioned stimulus (CS+) paired with an aversive sound (unconditioned stimulus [US]) on 50% of trials, and another (CS-) never paired with the US. An immediate extinction phase followed, where both CSs were presented alone. We found age-dependent dissociations between explicit and implicit measures of fear learning. Specifically, both adolescents and adults demonstrated successful fear conditioning and extinction according to their explicit awareness of changes in CS contingencies and their evaluative CS ratings, and their differential skin conductance responses. However, for the first time we show age differences at the neural level in perceptual areas. Only adolescents showed greater visual P1 and N1 responses to the CS+ compared to the CS- during acquisition, a dissociation that for the N1 was maintained during extinction. We suggest that the adolescent perceptual hyper-responsivity to learned threat and blunted extinction reported here could be an adaptive mechanism to protect adolescents from harm. However, this hyper-responsivity may also confer greater vulnerability to experience pathological levels of anxiety at this developmental stage.

E-Cigarettes as a Growing Threat for Children and Adolescents: Position Statement From the European Academy of Paediatrics

As the tobacco epidemic has waned, it has been followed by the advent of electronic nicotine delivery devices (ENDS) primarily manufactured by the tobacco industry to try to recruit replacements for deceased tobacco addicts. This document sets out the ten recommendations of the European Academy of Paediatrics (EAP) with regard to e-cigarettes and children and young people (CYP). The EAP notes that nicotine is itself a drug of addiction, with toxicity to the foetus, child and adult, and were ENDS only to contain nicotine, their use to create a new generation of addicts would be rigorously opposed. However, e-cigarettes include numerous unregulated chemicals, including known carcinogens, whose acute and long term toxicities are unknown. The EAP asserts that there is incontrovertible evidence that the acute toxicity of e-cigarettes is greater than that of "traditional" tobacco smoking, and a variety of acute pulmonary toxicities, including acute lung injuries, have been recorded due to e-cigarettes usage. The chronic toxicity of e-cigarettes is unknown, but given the greater acute toxicity compared to tobacco, the EAP cannot assume that e-cigarettes are safer in the long term. The high uptake of e-cigarettes by CYP, including under-age children, is partly fuelled by deceitful marketing and internet exposure, which is also unregulated. Although proposed as aids to smoking cessation, there is no evidence that e-cigarettes add anything to standard smoking cessation strategies. In summary, the EAP regards these devices and liquids as very dangerous, and ineluctably opposed to their use, and their direct or indirect marketing.

OpenVape: An Open-Source E-Cigarette Vapor Exposure Device for Rodents

The prevalence of “vaping” has recently seen significant increases in North America, especially in adolescents. However, the behavioral correlates of vaping are largely unexplored. The uptake of existing technologies meant for rodent vapor inhalation remains limited because of a lack of affordability and versatility (ability to be used with a variety of vaporizers). The OpenVape (OV) offers an open-source, low-cost solution that can be used in a variety of research contexts. Here, we present a specific use case, combining the OV apparatus with JUUL e-cigarettes. This apparatus consists of Arduino-operated vacuum pumps that deliver vapor directly from e-cigarettes to exposure chambers. The OV is easy to build and customize for any type of vaporizer (e.g., nicotine pod or tank; cannabis flower or concentrates). To test the OV, we performed biochemical verification and behavioral studies. The behavioral test (conditioned place preference, CPP) was conducted using adolescent and adult animals to assess developmental differences in the rewarding effects of nicotine vapor, as previously observed with injected nicotine. These findings demonstrate that even after brief exposures to nicotine vapor, pharmacologically relevant nicotine and cotinine levels could be detected in plasma, and significant CPP was observed, especially in adolescent rats which showed preference at shorter puff delivery durations (lower nicotine doses) compared with adults. Together, these findings suggest that OV provides an affordable, open-source option for preclinical behavioral research into the effects of vaping.

High-dose adolescent nicotine exposure permits spontaneous nicotine self-administration in adult male rats

INTRODUCTION

While cigarette smoking rates have been steadily decreasing over the past decade, there has been a dramatic increase in nicotine use via e-cigarettes, especially during adolescence. Adolescent e-cigarette use is associated with a greater risk of future cigarette smoking, and increased rates of cigarette smoking in individuals who may have otherwise never tried cigarettes. In humans and rodents, early initiation of nicotine use has been associated with greater consumption, dependence, and persistent nicotine use. The present study sought to investigate the long-lasting effect of daily high-dose nicotine exposure during adolescence on nicotine consumption in adulthood.

METHOD

Male Sprague-Dawley rats were exposed daily to nicotine (1.0 mg/kg, subcutaneous), or vehicle (1 mL/kg saline, subcutaneous) during adolescence (post-natal day [P] 28-41). Adult nicotine self-administration (0.02 mg/kg/infusion, intravenous) was assessed beginning on P75 on fixed-ratio 1 (FR1), fixed-interval 1 min (FI1), and progressive ratio (PR) schedules of reinforcement.

RESULTS

Adolescent nicotine pre-exposure did not affect adult nicotine self-administration on the simple FR1 schedule, however increased intake and responding for nicotine was observed when a short delay was implemented on an FI1 schedule of reinforcement.

CONCLUSIONS

Adolescence is a critical period when the brain is especially vulnerable to the effects of nicotine. Nicotine exposure in adolescence enhances susceptibility to increased nicotine intake in adulthood on a reinforcement schedule more reflective of human nicotine intake patterns, and this effect can extend into adulthood even after termination of nicotine exposure during adolescence.

Unique, long-term effects of nicotine on adolescent brain

Adolescence is a time of major plasticity of brain systems that regulate motivated behavior and cognition, and is also the age of peak onset of nicotine use. Although there has been a decline in teen use of cigarettes in recent years, there has been a huge increase in nicotine vaping. It is therefore critically important to understand the impact of nicotine on this critical phase of brain development. Animal studies have shown that nicotine has unique effects on adolescent brain. The goal of this review is therefore to systematically evaluate age- and sex-differences in the effects of nicotine on brain and behavior. Both acute and chronic effects of nicotine on brain biochemistry and behavior, particularly drug reward, aversion, cognition and emotion, are evaluated. Gaps in our current knowledge that need to be addressed are also highlighted. This review compares and integrates human and animals findings. Although there can be no experimental studies in humans to confirm similar behavioral effects of teen nicotine exposure, an emerging observational literature suggests similarities across species. Given the substantial evidence for long-term negative impact of adolescent nicotine exposure on brain and behavior, further longitudinal assessment of health outcomes in teen and young adult e-cigarette users is warranted.

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.

Early-life ketamine exposure attenuates the preference for ethanol in adolescent Sprague-Dawley rats

Ketamine, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, produces quick and effective antidepressant results in depressed juvenile and adult individuals. The long-term consequences of using ketamine in juvenile populations are not well known, particularly as it affects vulnerability to drugs of abuse later in life, given that ketamine is also a drug of abuse. Thus, the current study examined whether early-life ketamine administration produces long-term changes in the sensitivity to the rewarding effects of ethanol, as measured using the conditioned place preference (CPP) paradigm. On postnatal day (PD) 21, juvenile male and female rats were pretreated with ketamine (0.0 or 20 mg/kg) for 10 consecutive days (i.e., PD 21-30) and then evaluated for ethanol-induced CPP (0.0, 0.125, 0.5, or 2.0 g/kg) from PD 32-39. Results revealed that early-life ketamine administration attenuated the rewarding properties of ethanol in male rats, as ketamine pretreated rats failed to exhibit ethanol-induced CPP at any dose compared to saline pretreated rats, which showed an increased preference towards the ethanol-paired compartment in a dose-dependent manner. In females, ethanol-induced CPP was generally less robust compared to males, but ketamine pretreatment resulted in a rightward shift in the dose-response curve, given that ketamine pretreated rats needed a higher dose of ethanol compared to saline pretreated rats to exhibit ethanol-induced CPP. When considered together, the findings suggest that early use of ketamine does not appear to enhance the vulnerability to ethanol later in life, but in contrast, it may attenuate the rewarding effects of ethanol.

Effects of nicotine exposure on oral methamphetamine self-administration, extinction, and drug-primed reinstatement in adolescent male and female rats

BACKGROUND

Adolescent nicotine exposure increases methamphetamine (MA) intake in adult male rats; however, little is known about how nicotine affects MA self-administration during the adolescent period. Therefore, we assessed whether exposing rats to nicotine during early or late adolescence affects oral MA self-administration.

METHODS

146 male and female rats were treated with saline or nicotine (0.16 or 0.64 mg/kg) from postnatal day (PD) 25-PD 34 (the early exposure phase) and/or PD 35-PD 55 (the late exposure phase). Rats began an oral MA self-administration procedure on PD 35.

RESULTS

Only the sex variable, but not nicotine, affected sucrose and MA acquisition, as female rats had more nose pokes than males during training. On the test sessions, female rats exposed to nicotine (0.64 mg/kg) in the early exposure phase had more active nose pokes than saline-treated female rats or nicotine-treated male rats. Rats exposed to nicotine (0.16 mg/kg) in the late exposure phase had fewer active nose pokes during testing than rats exposed to saline. Nose poke responding during extinction was not altered by nicotine exposure, but administering nicotine (0.16 or 0.64 mg/kg) to male rats in the early exposure phase did decrease nose pokes during the drug-primed reinstatement session.

CONCLUSIONS

Our results show that adolescent female rats are more sensitive to the reinforcing effects of oral sucrose and MA than adolescent males, and that preadolescent nicotine exposure enhances oral MA self-administration in female rats. These findings suggest that preteen nicotine use may increase vulnerability to later MA abuse in teenage girls.

Adolescence versus adulthood: Differences in basal mesolimbic and nigrostriatal dopamine transmission and response to drugs of abuse

Epidemiological studies have shown that people who begin experimenting drugs of abuse during adolescence are more likely to develop substance use disorders, and the earliest is the beginning of their use, the greatest is the likelihood to become dependent. Understanding the neurobiological changes increasing adolescent vulnerability to drug use is becoming imperative. Although all neurotransmitter systems undergo relevant developmental changes, dopamine system is of particular interest, given its role in a variety of functions related to reward, motivation, and decision making. Thus, in the present study, we investigated differences in mesolimbic and nigrostriatal dopamine transmission between adolescent (5, 6, 7 weeks of age) and adult rats (10-12 weeks of age), in basal conditions and following drug challenge, by using in vivo brain microdialysis. Although no significant difference between adolescents and adults was observed in dopamine basal levels in the nucleus accumbens (NAc)shell and core, reduced DA levels were found in the dorsolateral striatum (DLS) of early and mid-adolescent rats. Adolescent rats showed greater increase of dopamine in the NAc shell following nicotine (0.4 mg/kg), THC (1.0 mg/kg), and morphine (1.0 mg/kg), in the NAc core following nicotine and morphine, and in the DLS following THC, morphine, and cocaine (10 mg/kg). These results, while adding new insight in the development and functionality of the dopamine system during different stages of adolescence, might provide a neurochemical basis for the greater vulnerability of adolescents to drugs of abuse and for the postulated gateway effect of nicotine and THC toward abuse of other illicit substances.

Non-nicotine constituents in e-cigarette aerosol extract attenuate nicotine's aversive effects in adolescent rats

BACKGROUND

Development of preclinical methodology for evaluating the abuse liability of electronic cigarettes (ECs) in adolescents is urgently needed to inform FDA regulation of these products. We previously reported reduced aversive effects of EC liquids containing nicotine and a range of non-nicotine constituents (e.g., propylene glycol, minor tobacco alkaloids) compared to nicotine alone in adult rats as measured using intracranial self-stimulation. The goal of this study was to compare the aversive effects of nicotine alone and EC aerosol extracts in adolescent rats as measured using conditioned taste aversion (CTA), which can be conducted during the brief adolescent period.

METHODS AND RESULTS

In Experiment 1, nicotine alone (1.0 or 1.5 mg/kg, s.c.) produced significant CTA in adolescent rats in a two-bottle procedure, thereby establishing a model to study the effects of EC extracts. At a nicotine dose of 1.0 mg/kg, CTA to Vuse Menthol EC extract, but not Aroma E-Juice EC extract, was attenuated compared to nicotine alone during repeated two-bottle CTA tests (Experiment 2a). At a nicotine dose of 0.5 mg/kg, CTA to Vuse Menthol EC extract did not differ from nicotine alone during the first two-bottle CTA test but extinguished more rapidly across repeated two-bottle tests (Experiment 2b).

CONCLUSIONS

Non-nicotine constituents in Vuse Menthol EC extracts attenuated CTA in a two-bottle procedure in adolescents. This model may be useful for anticipating the abuse liability of ECs in adolescents and for modeling FDA-mandated changes in product standards for nicotine or other constituents in ECs.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Adolescent selfie: an Italian Society of Paediatrics survey of the lifestyle of teenagers

Background

Epidemiological studies worldwide indicate that teenagers are at risk of internalizing and externalizing problems that persist into adulthood. In our country, there are few epidemiological studies on adolescents internalizing and externalizing problems. These studies, however, were not conducted in all of Italy. The aim of this study, promoted by the Italian Society of Paediatrician (SIP), was to investigate: a) the lifestyle and the prevalence of internalizing and externalizing problems in Italian teenagers and b) the risk and protective factors in family and social contexts. A further aim was to analyse gender differences in the above-mentioned variables.

Methods

11,527 adolescents aged 13 to 21 years were recruited among students of Italian high schools. Participants were contacted by school authorities inviting them to participate in an internet survey on youth health and lifestyle. If they agreed to participate, the adolescents filled out an on-line anonymous questionnaire. The questionnaire was composed of 60 multiple choice items to investigate nutrition, body perception and lifestyle, internet exposure and sexual behaviour, externalizing and internalizing problems, family context and social context. Participation in the study was completely voluntary. The statistical significance of gender differences was tested by means of Chi-square analyses. Results displayed that Italian female adolescents are at risk of internalizing problems while males are at higher risk of externalizing problems. Moreover, several risk factors emerged from the family context in terms of violence, physical and psychological abuse.

Conclusions

It is critical to make paediatricians and schools aware of the main points to improve prevention and healthcare in the teenage population. To our knowledge, this is the first study in the Italian context to enrol more than 11,000 adolescents.

Abuse potential of 2-(4-iodo-2, 5-dimethoxyphenyl)N-(2-methoxybenzyl)ethanamine (25INBOMe); in vivo and ex vivo approaches

25INBOMe ("25-I", "N-Bomb"), one of new psychoactive substances (NPSs), is being abused for recreational purpose. However, the liability for abuse or dependence has not been systematically studied yet. The objective of the present study was to evaluate rewarding and reinforcing effects of 25INBOMe using conditioned place preference (CPP) and self-administration (SA) paradigms. In addition, ultrasonic vocalizations (USVs) were measured to investigate relationships between USVs and emotional state regarding dependence on psychoactive substances. To understand molecular mechanism involved in its action, dopamine (DA) level changes were analyzed using synaptosomes extracted from the striatal region of the brain. Expression level changes of SGK1 (serum/glucocorticoid regulated kinase 1) and PER2 (period circadian protein homolog 2), two putative biomarkers for drug dependence, were also analyzed. Results showed that 25INBOMe increased both CPP (0.3 mg/kg) and SA (0.03 mg/kg/infusion) and produced higher frequencies in USVs analysis. It also increased DA levels in the striatal region and changed expression levels of SGK1 and PER2. Results of the present study suggest that 25INBOMe might produce rewarding and reinforcing effects, indicating its dependence liability. In addition, frequencies of USV might be associated with emotional state of mice induced by psychoactive substances regarding substance dependence. This is the first systemic preclinical report on the dependence liability of 25INBOMe and the first attempt to introduce a possible relationship between USVs and emotional state of mice regarding substance dependency. Further studies are needed to clarify the mechanism involved in 25INBOMe dependency and determine the usefulness of USV measurement as a method for evaluating dependence liability.

Adolescent Attentional Bias toward Real-world Flavored E-cigarette Marketing

OBJECTIVES

E-cigarettes are now the most commonly-used tobacco product among adolescents; yet, little work has examined how the appealing food and flavor cues used in their marketing might attract adolescents' attention, thereby increasing willingness to try these products. In the present study, we tested whether advertisements for fruit/sweet/savory-flavored ("flavored") e-cigarettes attracted adolescent attention in real-world scenes more than tobacco flavored ("unflavored") e-cigarettes. Additionally, we examined the relationship between adolescent attentional bias and willingness to try flavored e-cigarettes.

METHODS

Participants were 46 adolescents (age range: 16-18 years). All participants took part in an eye-tracking paradigm that examined attentional bias to flavored and unflavored e-cigarette advertisements embedded in pictures of real-world storefront scenes. Afterwards, participants' willingness to try flavored and unflavored e-cigarettes was assessed.

RESULTS

In support of our primary hypothesis, adolescents looked longer and fixated more frequently on flavored (vs unflavored) e-cigarette advertisements. Moreover, this attentional bias towards flavored e-cigarette advertisements predicted a greater willingness to try flavored vs unflavored e-cigarettes.

CONCLUSIONS

These findings suggest that flavored e-cigarette marketing attracts the attention of adolescents, in- creases their willingness to try flavored e-cigarette products, and could, therefore, put them at greater risk for tobacco initiation.

Adolescent and adult nicotine exposure differentially impacts oral nicotine and oral saccharin self-administration in mice

Smokers that begin during adolescence are more likely to develop nicotine dependence than those who begin as adults. However, the factors that contribute to this remain largely unknown. Here we utilized a novel operant oral nicotine self-administration procedure in mice to assess the consequences of adolescent nicotine exposure on nicotine and saccharin (non-drug) reinforcement in adults. Animals were given non-contingent exposure to either saline or nicotine using the osmotic minipumps during both adolescence and adulthood for 2 weeks. Reinforcing efficacy for oral nicotine and saccharin was assessed using the progressive ratio schedule 2-weeks following the washout period in adults. Non-contingent nicotine exposure in adolescence drastically increased operant responding for oral nicotine but reduced responding for oral saccharin in the group re-exposed to nicotine in adulthood. Interestingly, adolescent nicotine-exposed mice that received saline exposure as adults exhibited higher preference for oral saccharin. However, breakpoints for oral nicotine in these mice remained comparable to control animals. Surprisingly, both adolescent and adult nicotine exposure increased inactive lever responding during self-administration presumably reflecting impulsive responding. Our data suggest that adolescent nicotine exposure produces an increase in reinforcement sensitivity in adulthood as reflected by increased saccharin self-administration but this sensitivity becomes biased towards nicotine self-administration when re-exposed to nicotine in adulthood. Moreover, nicotine/saccharin reinforcement could be impacted by changes in cognitive control, such as increased impulsivity. These distinct behavioral mechanisms may act in concert to facilitate maladaptive nicotine taking in smokers that initiate nicotine use during adolescence.

Adolescent rats fail to demonstrate a LiCl-induced pre-exposure effect: Implications for the balance of drug reward and aversion in adolescence

Adolescents display weaker taste avoidance induced by both abused and non-abused drugs than adults. Drug history attenuates avoidance learning in adults (the drug pre-exposure effect), but little is known about this phenomenon in adolescents. Given that the weaker taste avoidance in adolescence is thought to be a function of their relative insensitivity to the drug's aversive effects, it might be expected that the drug pre-exposure effect would be weaker in adolescents given that for some drugs this effect is mediated by associative blocking that depends on the association of environmental cues with the drug's aversive effects. To address this, in the present studies male adolescent (Experiment 1) and adult (Experiment 2) rats were given five spaced injections of LiCl prior to subsequent taste avoidance conditioning with LiCl. Consistent with past reports, adolescents displayed weaker taste avoidance than adults. While adults displayed attenuated LiCl-induced taste avoidance following LiCl pre-exposure, adolescents showed no evidence of this pre-exposure. This work is consistent with the view that adolescents are relatively insensitive to the aversive effects of drugs, an insensitivity potentially important in subsequent intake of drugs of abuse given that such intake is a function of the balance of their rewarding and aversive effects.

Nicotine-induced molecular alterations are modulated by GABAB receptor activity

It has been demonstrated that GABAB receptors modulate nicotine (NIC) reward effect; nevertheless, the mechanism implicated is not well known. In this regard, we evaluated the involvement of GABAB receptors on the behavioral, neurochemical, biochemical and molecular alterations associated with the rewarding effects induced by NIC in mice, from a pharmacological and genetic approach. NIC-induced rewarding properties (0.5 mg/kg, subcutaneously, sc) were evaluated by conditioned place preference (CPP) paradigm. CPP has three phases: preconditioning, conditioning and postconditioning. GABAB receptor antagonist 2-hydroxysaclofen (0.25, 0.5 and 1 mg/kg; intraperitoneally, ip) or the GABAB receptor agonist baclofen (3 mg/kg; ip) was injected before NIC during the conditioning phase. GABAB1 knockout (GABAB1 KO) mice received NIC during the conditioning phase. Vehicle and wild-type controls were employed. Neurochemical (dopamine, serotonin and their metabolites), biochemical (nicotinic receptor α4β2, α4β2nAChRs) and molecular (c-Fos) alterations induced by NIC were analyzed after the postconditioning phase by high-performance liquid chromatography (HPLC), receptor-ligand binding assays and immunohistochemistry, respectively, in nucleus accumbens (Acb), prefrontal cortex (PFC) and ventral tegmental area (VTA). NIC induced rewarding effects in the CPP paradigm and increased dopamine levels in Acb and PFC, α4β2nAChRs density in VTA and c-Fos expression in Acb shell (AcbSh), VTA and PFC. We showed that behavioral, neurochemical, biochemical and molecular alterations induced by NIC were prevented by baclofen. However, in 2-hydroxysaclofen pretreated and GABAB1 KO mice, these alterations were potentiated, suggesting that GABAB receptor activity is necessary to control alterations induced by NIC-induced rewarding effects. Therefore, the present findings provided important contributions to the mechanisms implicated in NIC-induced rewarding effects.

Sex as a biological variable: Drug use and abuse

The study of sex as a biological variable is a necessary emphasis across a wide array of endpoints, including basic neuroscience, medicine, mental health, physiology and behavior. The present review summarizes work from clinical and preclinical populations on sex differences in drug use and abuse, ranging from initiation to escalation/dysregulation and from drug cessation/abstinence to relapse. These differences are analyzed in the context of the addiction cycle conceptualization of Koob and his colleagues and address patterns of drug use (binge/intoxication), motivation underlying its use (withdrawal/negative affect) and likelihood and causes of craving and relapse of drug taking (preoccupation/anticipation). Following this overview, an assessment of the basis for the reported sex differences is discussed in the context of the affective (rewarding and aversive) properties of drugs of abuse and how such properties and their balance vary with sex and contribute to drug intake. Finally, the interaction of sex with several experiential (drug history) and subject (age) factors and how these interactions affect reward and aversion are discussed to highlight the importance of understanding such interactions in predicting drug use and abuse. We note that sex as a biological variable remains one of critical evaluation and that such investigations of sex differences in drug use and abuse continue and be expanded to assess all facets of their mediation, including these affective properties, how their balance may be impacted by the multiple conditions under which drugs are taken and how this overall balance affects drug use and addiction vulnerability.

Increased sign-tracking behavior in adolescent rats

An autoshaping procedure was used to test the notion that conditioned stimuli (CSs) gain greater incentive salience during adolescence than young adulthood under conditions of social isolation rearing and food restriction. Rats were single-housed and placed on food restriction during 10 daily training sessions in which a lever (CS⁺ ) was presented then followed immediately by a food unconditioned stimulus (US). A second lever (CS^- ) was presented on intermixed trials and was not reinforced. Despite the fact that food delivery was not contingent on the rats' behavior, all rats exhibited behaviors directed towards the lever (i.e., sign-tracking). In the adolescent group, the rate of lever pressing and the percentage of trials with a lever press were higher than in young adults. Initially, group differences were observed when rats were retrained when the adolescents had reached young adulthood. These findings support the hypothesis that cues that come to predict reward become imbued with excessive motivational value in adolescents, perhaps contributing to the hyper-responsiveness to reward-related stimuli typically observed during this period of development.

Nicotine pretreatment reduced cocaine-induced CPP and its reinstatement in a sex- and dose-related manner in adult C57BL/6J mice

Previous preclinical studies have shown that nicotine pretreatment during adolescence increases the reinforcing actions of cocaine. However, little is known about the effect of prior nicotine administration on cocaine-induced conditioned place preference (CPP) and its reinstatement in adult mice. Besides, little information is available regarding the role of sex in this cross-talk between nicotine and cocaine. Thus, we examined if nicotine administration during adulthood would differentially alter cocaine-induced CPP, its extinction and reinstatement in male versus female mice and if the dose of nicotine was important in this regard. To this end, mice were pretreated with saline or nicotine (0.25 or 1mg/kg; twice daily for seven days) and then tested for place preference before and after single and repeated conditioning with cocaine (15mg/kg). Mice were then exposed to extinction training and tested for reinstatement of CPP. Our results showed that male and female mice pretreated with saline and conditioned with cocaine each exhibited a robust CPP after a single cocaine conditioning. However, this response was blunted in mice pretreated with the lower but not higher dose of nicotine. Female mice pretreated with the lower dose nicotine also failed to show CPP after repeated conditioning. Reinstatement of cocaine-induced CPP was also blunted in these mice compared to their respective controls. Together, these results suggest that nicotine administration during adulthood exerts differential effects on cocaine-induced CPP and its reinstatement in male and female mice and the dose of nicotine is important in this regard.

Both nicotine reward and withdrawal are enhanced in a rodent model of diabetes

RATIONALE

It is presently unclear whether diabetic rats experience greater rewarding effects of nicotine and/or negative affective states produced by nicotine withdrawal.

OBJECTIVE

The present study utilized a rodent model of diabetes to examine the rewarding effects of nicotine and negative affective states and physical signs produced by withdrawal.

METHODS

Separate groups of rats received systemic administration of either vehicle or streptozotocin (STZ), which destroys insulin-producing beta cells in the pancreas and elevates glucose levels. Place conditioning procedures were utilized to compare the rewarding effects of nicotine (conditioned place preference; CPP) and negative affective states produced by withdrawal (conditioned place aversion; CPA) in vehicle- and STZ-treated rats. CPA and physical signs of withdrawal were compared after administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal in nicotine-dependent rats. A subsequent study utilized elevated plus maze (EPM) procedures to compare anxiety-like behavior produced by nicotine withdrawal in vehicle- and STZ-treated rats.

RESULTS

STZ-treated rats displayed greater rewarding effects of nicotine and a larger magnitude of aversive effects and physical signs produced by withdrawal as compared to vehicle-treated controls. STZ-treated rats also displayed higher levels of anxiety-like behavior on the EPM during nicotine withdrawal as compared to controls.

CONCLUSION

The finding that both nicotine reward and withdrawal are enhanced in a rodent model of diabetes implies that the strong behavioral effects of nicotine promote tobacco use in persons with metabolic disorders, such as diabetes.

Translational relevance of rodent models of hypothalamic-pituitary-adrenal function and stressors in adolescence

Elevations in glucocorticoids that result from environmental stressors can have programming effects on brain structure and function when the exposure occurs during sensitive periods that involve heightened neural development. In recent years, adolescence has gained increasing attention as another sensitive period of development, a period in which pubertal transitions may increase the vulnerability to stressors. There are similarities in physical and behavioural development between humans and rats, and rats have been used effectively as an animal model of adolescence and the unique plasticity of this period of ontogeny. This review focuses on benefits and challenges of rats as a model for translational research on hypothalamic-pituitary-adrenal (HPA) function and stressors in adolescence, highlighting important parallels and contrasts between adolescent rats and humans, and we review the main stress procedures that are used in investigating HPA stress responses and their consequences in adolescence in rats. We conclude that a greater focus on timing of puberty as a factor in research in adolescent rats may increase the translational relevance of the findings.

Age differences in appetitive Pavlovian conditioning and extinction in rats

Mounting evidence indicates that adolescents exhibit heightened sensitivity to rewards and reward-related cues compared to adults, and that adolescents are often unable to exert behavioral control in the face of such cues. Moreover, differences in reward processing during adolescence have been linked to heightened risk taking and impulsivity. However, little is known about the processes by which adolescents learn about the appetitive properties of environmental stimuli that signal reward. To address this, Pavlovian conditioning procedures were used to test for differences in excitatory conditioning between adult and adolescent rats using various schedules of reinforcement. Specifically, separate cohorts of adult and adolescent rats were trained under conditions of consistent (continuous) or intermittent (partial) reinforcement. We found that the acquisition of anticipatory responding to a continuously-reinforced cue proceeded similarly in adolescents and adults. In contrast, responding increased at a greater rate in adolescents compared to adults during presentations of a partially-reinforced cue. We subsequently compared the ability of adolescent and adult rats to dynamically adjust the representation of a reward-predictive cue during extinction trials, in which a secondary inhibitory representation is acquired for the previously-reinforced stimulus. We observed significant age differences in the ability to flexibly update cue representations during extinction, in that the appetitive properties of cues with a history of either continuous or partial reinforcement persisted to a greater extent in adolescents relative to adults.

Reward-centricity and attenuated aversions: An adolescent phenotype emerging from studies in laboratory animals

Adolescence is an evolutionarily conserved developmental period, with neural circuits and behaviors contributing to the detection, procurement, and receipt of rewards bearing similarity across species. Studies with laboratory animals suggest that adolescence is typified by a "reward-centric" phenotype-an increased sensitivity to rewards relative to adults. In contrast, adolescent rodents are reportedly less sensitive to the aversive properties of many drugs and naturally aversive stimuli. Alterations within the mesocorticolimbic dopamine and endocannabinoid systems likely contribute to an adolescent reward-sensitive, yet aversion-resistant, phenotype. Although early hypotheses postulated that developmental changes in dopaminergic circuitry would result in a "reward deficiency" syndrome, evidence now suggests the opposite: that adolescents are uniquely poised to seek out hedonic stimuli, experience greater "pleasure" from rewards, and consume rewarding stimuli in excess. Future studies that more clearly define the role of specific brain regions and neurotransmitter systems in the expression of behaviors toward reward- and aversive-related cues and stimuli are necessary to more fully understand an adolescent-proclivity for and vulnerability to rewards and drugs of potential abuse.

Self-administration of nicotine and cigarette smoke extract in adolescent and adult rats

Although smoking initiation typically occurs during adolescence, most preclinical studies of tobacco use involve adult animals. Furthermore, their focus is largely on nicotine alone, even though cigarette smoke contains thousands of constituents. The present study therefore aimed to determine whether aqueous constituents in cigarette smoke affect acquisition of nicotine self-administration during adolescence in rats. Adolescent and adult male rats, aged postnatal day (P) 25 and 85, respectively, were food trained on a fixed ratio 1 (FR1) schedule, then allowed to self-administer one of 5 doses of nicotine (0, 3.75, 7.5, 15, or 30 μg/kg) or aqueous cigarette smoke extract (CSE) with equivalent nicotine content. Three progressively more difficult schedules of reinforcement, FR1, FR2, and FR5, were used. Both adolescent and adult rats acquired self-administration of nicotine and CSE. Nicotine and CSE similarly increased non-reinforced responding in adolescents, leading to enhanced overall drug intake as compared to adults. When data were corrected for age-dependent alterations in non-reinforced responding, adolescents responded more for low doses of nicotine and CSE than adults at the FR1 reinforcement schedule. No differences in adolescent responding for the two drugs were seen at this schedule, whereas adults had fewer responses for CSE than for nicotine. However, when the reinforcement schedule was increased to FR5, animals dose-dependently self-administered both nicotine and CSE, but no drug or age differences were observed. These data suggest that non-nicotine tobacco smoke constituents do not influence the reinforcing effect of nicotine in adolescents.

Advances in Behavioral Laboratory Methods that Inform Tobacco Regulatory Science: A TCORS Working Group Special Issue

Objective

The 2009 Family Smoking Prevention and Tobacco Control Act (TCA) created unprecented enabling conditions for establishing national regulatory policy that reduces the burden of public health and societal problems associated with tobacco product use. The Center for Tobacco Products (CTP), created by the FDA to implement the TCA, developed a first-of-its-kind FDA/National Institutes of Health (NIH) collaborative program to fund Tobacco Centers of Regulatory Science (TCORS).

Methods

To assist the TCORS with addressing research priorites, working groups (WGs) comprised of FDA-CTP liasions and TCORS investigators were formed. Under the direction of the Center for Evaluation and Coordination of Trainin and Research (CECTR), the TCORS WGs seek to develop tangible work products in their respective areas of focus.

Results

The focus of the behavioral pharmacology WG evolved from publishing a narrow paper on behavioral methods in electronic cigarette research to a collection of papers on advances in behavioral laboratory methods that may inform tobacco regulatory science.

Conclusion

This Special Issue contains articles that address all of the CTP research priorities and demonstrates how advances in behavioral laboratory methods made by TCORS investigators can inform FDA efforst to regulate tobacco products.

Nicotine produces long-term increases in cocaine reinforcement in adolescent but not adult rats

Studies have shown that many smokers begin using nicotine during adolescence, yet the influence of early nicotine use on the response to other drugs of abuse in adulthood is not fully understood. In the current study, nicotine was administered to adolescent and adult rats for seven days. Thirty days later, cocaine-induced locomotor activity and cocaine self-administration were examined when the rats pretreated as adolescents were adults. Rats exposed to nicotine during early adolescence were sensitized thirty days later to the locomotor-activating effects of cocaine and self-administered a greater number of cocaine infusions than adolescent rats pretreated with vehicle. As a result of this increased intake, the cocaine self-administration dose-response curve was shifted upward indicating an increase in cocaine reinforcement. Rats pretreated with nicotine as adults, however, did not show a difference in locomotor activity or cocaine self-administration thirty days later compared to adult rats pretreated with vehicle. These findings suggest that early exposure to nicotine has long-term consequences on cocaine use. These data further suggest that nicotine use may carry a greater risk during adolescence than adulthood and adolescents who smoke may be particularly vulnerable to stimulant use. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Immediate early gene expression reveals interactions between social and nicotine rewards on brain activity in adolescent male rats

Smoking initiation predominantly occurs during adolescence, often in the presence of peers. Therefore, understanding the neural mechanisms underlying the rewarding effects of nicotine and social stimuli is vital. Using the conditioned place preference (CPP) procedure, we measured immediate early gene (IEG) expression in animals following exposure either to a reward-conditioned environment or to the unconditioned stimuli (US). Adolescent, male rats were assigned to the following CPP US conditions: (1) Saline+Isolated, (2) Nicotine+Isolated, (3) Saline+Social, or (4) Nicotine+Social. For Experiment 1, brain tissue was collected 90min following the CPP expression test and processed for Fos immunohistochemistry. We found that rats conditioned with nicotine with or without a social partner exhibited CPP; however, we found no group differences in Fos expression in any brain region analyzed, with the exception of the nucleus accumbens core that exhibited a social-induced attenuation in Fos expression. For Experiment 2, brain tissue was collected 90min following US exposure during the last conditioning session. We found social reward-induced increases in IEG expression in striatal and amydalar subregions. In contrast, nicotine reduced IEG expression in prefrontal and striatal subregions. Reward interactions were also found in the dorsolateral striatum, basolateral amygdala, and ventral tegmental area where nicotine alone attenuated IEG expression and social reward reversed this effect. These results suggest that in general social rewards enhance, whereas nicotine attenuates, activation of mesocorticolimbic regions; however, the rewards given together interact to enhance activation in some regions. The findings contribute to knowledge of how a social environment influences nicotine effects.