Tobacco smoke containing high or low levels of nicotine during adolescence: effects on novelty-seeking and anxiety-like behaviors in mice

The impact of adolescent stress on nicotine use and affective disorders in rodent models

Recent findings indicate that stress exposure during adolescence contributes to the development of both nicotine use and affective disorders, suggesting a potential shared biological pathway. One key system that may mediate the association between adolescent stress and nicotine or affective outcomes is the hypothalamic-pituitary-adrenal (HPA) axis. Here we reviewed evidence regarding the effects of adolescent stress on nicotine responses and affective phenotypes and the role of the HPA-axis in these relationships. Literature indicates that stress, possibly via HPA-axis dysfunction, is a risk factor for both nicotine use and affective disorders. In rodent models, adolescent stress modulates behavioural responses to nicotine and increases the likelihood of affective disorders. The exact role that the HPA-axis plays in altering nicotine sensitivity and affective disorder development after adolescent stress remains unclear. However, it appears likely that adolescent stress-induced nicotine use and affective disorders are precipitated by repetitive activation of a hyperactive HPA-axis. Together, these preclinical studies indicate that adolescent stress is a risk factor for nicotine use and anxiety/depression phenotypes. The findings summarized here suggest that the HPA-axis mediates this relationship. Future studies that pharmacologically manipulate the HPA-axis during and after adolescent stress are critical to elucidate the exact role that the HPA-axis plays in the development of nicotine use and affective disorders following adolescent stress.

Sex- and age-dependent differences in nicotine susceptibility evoked by developmental exposure to tobacco smoke and/or ethanol in mice

Either tobacco smoking or alcohol consumption during pregnancy sex-selectively increases susceptibility to drugs of abuse later in life. Considering that pregnant smoking women are frequently intermittent consumers of alcoholic beverages, here, we investigated whether a short-term ethanol exposure restricted to the brain growth spurt period when combined with chronic developmental exposure to tobacco smoke aggravates susceptibility to nicotine in adolescent and adult mice. Swiss male and female mice were exposed to tobacco smoke (SMK; research cigarettes 3R4F, whole-body exposure, 8 h/daily) or ambient air during the gestational period and until the tenth postnatal day (PN). Ethanol (ETOH, 2 g/Kg, 25%, i.p.) or saline was injected in the pups every other day from PN2 to PN10. There were no significant differences in cotinine (nicotine metabolite) and ethanol serum levels among SMK, ETOH and SMK + ETOH groups. During adolescence (PN30) and adulthood (PN90), nicotine (NIC, 0.5 mg/Kg) susceptibility was evaluated in the conditioned place preference and open field tests. NIC impact was more evident in females: SMK, ETOH and SMK + ETOH adolescent females were equally more susceptible to nicotine-induced place preference than control animals. At adulthood, SMK and SMK + ETOH adult females exhibited a nicotine-evoked hyperlocomotor profile in the open field, with a stronger effect in the SMK + ETOH group. Our results indicate that ethanol exposure during the brain growth spurt, when combined to developmental exposure to tobacco smoke, increases nicotine susceptibility with stronger effects in adult females. This result represents a worsened outcome from the early developmental dual exposure and may predispose nicotine use/abuse later in life.

Mood-related behavioral and neurochemical alterations in mice exposed to low chlorpyrifos levels during the brain growth spurt

Organophosphates are among the most used pesticides. Particularly, chlorpyrifos (CPF) is responsible for a number of deleterious effects on brain development, which may program behavioral changes later in life. Here, we investigated whether a regimen of early low level CPF exposure that did not result in a significant inhibition of acetylcholinesterase (AChE) had deleterious effects on mood-related behaviors, as well as on cholinergic and serotonergic biomarkers in the mice brain. From the 3^rd to 9^th postnatal day (PN), male and female Swiss mice were subcutaneously injected with CPF. Mice were submitted to a battery of behavioral tests from PN60 to PN63: open field, elevated plus maze and forced swimming tests. The cholinergic and serotonergic biomarkers were assessed at PN10 and PN63. Our data indicated that early CPF exposure increased anxiety-like behavior in females and altered decision-making behavior in both sexes. Most biochemical alterations were sex-dependent and restricted to females. At PN10, CPF female mice showed increased serotonin and choline transporter binding in cerebral cortex. Distinctively, in adult females, the effects indicated a hypoactive state: CPF exposure reduced 5-HT_1a receptor binding in cerebral cortex, as well as serotonin transporter binding and choline acetyltransferase activity in brainstem. Our results indicate that CPF exposure during the brain growth spurt deregulates serotonergic and cholinergic biomarkers. The effects are consistent with impaired synaptic function, may be related to long-term mood disorders and point out to higher female susceptibility.

Nicotine Reduction in Cigarettes: Literature Review and Gap Analysis

BACKGROUND

The US Food and Drug Administration (FDA) is considering reducing nicotine levels in cigarettes to "minimally or non-addictive levels." However, important research gaps remain, and the FDA must determine when the available research is sufficient to support moving forward.

METHODS

The authors conducted a systematic review of research articles in PubMed relating to nicotine reduction. Building on a review of risk assessment best practices, the authors also developed a risk assessment framework for tobacco regulation and used it to guide a gap analysis of nicotine reduction research.

RESULTS

The final sample consisted of 78 articles. The majority examined either nicotine dependence on very low nicotine cigarettes (VLNCs) or markers of potential health effects of using VLNCs. One-third of the identified articles reported results from four large randomized controlled trials (RCTs). While these studies report promising results and suggest that a nicotine reduction rule would be a powerful tool to reduce cigarette smoking, our gap analysis suggests that there is a need for studies that better reflect the use and availability of a wide range of tobacco/nicotine products and the potential for dual- or multi-product use.

CONCLUSION

The current body of research on nicotine reduction is weighted towards RCTs, which is appropriate for a policy that has not yet been implemented anywhere in the world. The FDA must consider a wide range of factors that may impact a product standard's public health impact, including those difficult to assess in RCTs, such as a nicotine reduction rule's impact on smoking initiation and relapse.

IMPLICATIONS

This systematic review presents a gap analysis based on a risk assessment framework to help identify remaining research priorities to inform FDA's potential product standard to reduce nicotine levels in cigarettes. Quickly addressing those gaps would support the FDA's effort to develop a nicotine reduction product standard that will be effective and withstand legal challenges.

Environmental Tobacco Smoke During the Early Postnatal Period of Mice Interferes With Brain 18 F-FDG Uptake From Infancy to Early Adulthood - A Longitudinal Study

Exposure to environmental tobacco smoke (ETS) is associated with high morbidity and mortality, mainly in childhood. Our aim was to evaluate the effects of postnatal ETS exposure in the brain 2-deoxy-2-[¹⁸F]-fluoro-D-glucose (¹⁸F-FDG) uptake of mice by positron emission tomography (PET) neuroimaging in a longitudinal study. C57BL/6J mice were exposed to ETS that was generated from 3R4F cigarettes from postnatal day 3 (P3) to P14. PET analyses were performed in male and female mice during infancy (P15), adolescence (P35), and adulthood (P65). We observed that ETS exposure decreased ¹⁸F-FDG uptake in the whole brain, both left and right hemispheres, and frontal cortex in both male and female infant mice, while female infant mice exposed to ETS showed decreased ¹⁸F-FDG uptake in the cerebellum. In addition, all mice showed reduced ¹⁸F-FDG uptake in infancy, compared to adulthood in all analyzed VOIs. In adulthood, ETS exposure during the early postnatal period decreased brain ¹⁸F-FDG uptake in adult male mice in the cortex, striatum, hippocampus, cingulate cortex, and thalamus when compared to control group. ETS induced an increase in ¹⁸F-FDG uptake in adult female mice when compared to control group in the brainstem and cingulate cortex. Moreover, male ETS-exposed animals showed decreased ¹⁸F-FDG uptake when compared to female ETS-exposed in the whole brain, brainstem, cortex, left amygdala, striatum, hippocampus, cingulate cortex, basal forebrain and septum, thalamus, hypothalamus, and midbrain. The present study shows that several brain regions are vulnerable to ETS exposure during the early postnatal period and these effects on ¹⁸F-FDG uptake are observed even a long time after the last exposure. This study corroborates our previous findings, strengthening the idea that exposure to tobacco smoke in a critical period interferes with brain development of mice from late infancy to early adulthood.

Adolescent drug exposure: A review of evidence for the development of persistent changes in brain function

Over the past decade, many studies have indicated that adolescence is a critical period of brain development and maturation. The refinement and maturation of the central nervous system over this prolonged period, however, makes the adolescent brain highly susceptible to perturbations from acute and chronic drug exposure. Here we review the preclinical literature addressing the long-term consequences of adolescent exposure to common recreational drugs and drugs-of-abuse. These studies on adolescent exposure to alcohol, nicotine, opioids, cannabinoids and psychostimulant drugs, such as cocaine and amphetamine, reveal a variety of long-lasting behavioral and neurobiological consequences. These agents can affect development of the prefrontal cortex and mesolimbic dopamine pathways and modify the reward systems, socio-emotional processing and cognition. Other consequences include disruption in working memory, anxiety disorders and an increased risk of subsequent drug abuse in adult life. Although preventive and control policies are a valuable approach to reduce the detrimental effects of drugs-of-abuse on the adolescent brain, a more profound understanding of their neurobiological impact can lead to improved strategies for the treatment and attenuation of the detrimental neuropsychiatric sequelae.

Lifelong exposure to caffeine increases anxiety-like behavior in adult mice exposed to tobacco smoke during adolescence

Caffeine and tobacco smoke are among the most frequently self-administered licit psychoactive drugs in the world. Both drugs affect anxiety levels, however, little is known on the impact of the dual exposure in the adolescent brain, the period during which smoking begins. Considering that anxiety is a relevant factor for smoking maintenance and relapse, we investigated the effects of lifelong exposure to caffeine on anxiety levels of Swiss mice exposed to tobacco smoke during adolescence. Caffeine was administrated during all prenatal and postnatal life (CAF, 0.1 g/l to drink). From postnatal day 30-45, animals were exposed to tobacco smoke (SMK, whole body exposure, 8 h/day) generated from research cigarettes type 3R4F (nicotine = 0.73 mg/per cigarette). Four groups were analyzed: (1) CAF + SMK exposure; (2) SMK exposure; (3) CAF exposure; (4) Control. Anxiety levels were assessed in the elevated plus maze at the end of smoke exposure (PN45), at short- (PN55) and long-term (PN75) withdrawal. Caffeine exposure reduced decision making time (time in center of maze) during adolescence (PN45 and PN55). In addition, caffeine increased anxiety-like behavior during long-term tobacco smoke withdrawal. The present study provides experimental evidence that caffeine and tobacco smoke during adolescence interact resulting in emotional dysregulation during tobacco smoke withdrawal. Particularly, increased anxiety-like behavior during long-term withdrawal in CAF + SMK animals demonstrates late-emergent effects. In this sense, our data suggest that lifelong caffeine exposure may be an important factor in tobacco relapse.

Tobacco smoke and ethanol during adolescence: Both combined- and single-drug exposures lead to short- and long-term disruption of the serotonergic system in the mouse brain

The impairment of the serotonergic system contributes to nicotine and ethanol effects on mood, suggesting that this system is targeted by each of these drugs and that co-exposure possibly worsens the disruption. Here, we tested this hypothesis in an adolescent mice model of tobacco smoke and/or ethanol exposure. From postnatal day (PN) 30-45, Swiss mice were exposed to one of the following: 1) tobacco smoke (SMK; research cigarettes 2R1F, whole-body exposure, 8 h/daily); 2) ethanol (ETOH; 2 g/kg i.p., every other day); 3) SMK + ETOH; 4) Control (VEH). At PN45 (end-of-exposure), hippocampal serotonin transporter (5 H TT) binding was increased in SMK and decreased in ETOH male mice. At PN50 (short-term deprivation), cortical 5 H TT was reduced in all drug-exposed mice. In the hippocampus, similar deficits were identified in females. In both brain regions, the effects of SMK + ETOH deprivation on 5 H TT were equivalent to the damage caused by either drug. At PN50, hippocampal 5 H T1A receptor binding was reduced in ETOH and SMK + ETOH mice. Similar results were observed in the male cortex. In females, deficits were identified in SMK mice. In both brain regions, SMK + ETOH 5 H T1A deficits reflected the summation of SMK and ETOH outcomes. At PN75 (long-term deprivation), there was a late-emergent increase in cortical 5 H T1A binding in SMK mice, while cortical 5 H T2 receptor binding was similarly increased in SMK and SMK + ETOH groups. Adolescent SMK and/or ETOH serotonergic impairment is sex-dependent and most evident during short-term deprivation. SMK + ETOH deprivation evokes serotonergic disruption that is at least equivalent to that caused by either drug alone.

Tobacco-Free Cigarette Smoke Exposure Induces Anxiety and Panic-Related Behaviours in Male Wistar Rats

Smokers, who generally present with lung damage, are more anxious than non-smokers and have an associated augmented risk of panic. Considering that lung damage signals specific neural pathways that are related to affective responses, the aim of the present study was to evaluate the influence of pulmonary injury on anxiety and panic-like behaviours in animals exposed to cigarette smoke with and without tobacco. Male Wistar rats were divided into the following groups: a control group (CG); a regular cigarette group (RC); and a tobacco-free cigarette (TFC) group. Animals were exposed to twelve cigarettes per day for eight consecutive days. The animals were then exposed to an elevated T-maze and an open field. The RC and TFC groups presented increases in inflammatory cell inflow, antioxidant enzyme activity, and TBARS levels, and a decrease in the GSH/GSSG ratio was observed in the TFC group. Exposure to RC smoke reduced anxiety and panic-related behaviours. On the other hand, TFC induced anxiety and panic-related behaviours. Thus, our results contradict the concept that nicotine is solely accountable for shifted behavioural patterns caused by smoking, in that exposure to TFC smoke causes anxiety and panic-related behaviours.

Energy drink enhances the behavioral effects of alcohol in adolescent mice

Mixing alcohol with energy drinks has become increasingly popular among teenagers and young adults due to the prevailing view that the stimulant properties of energy drinks decrease the depressant effects of alcohol. Surprisingly, in spite of energy drinks being heavily marketed to and consumed by adolescents, there is scarcely available preclinical data on the neurobehavioral effects of energy drinks mixed with alcohol during adolescence. Thus, here we examine the effects of the combined exposure to alcohol and energy drink on adolescent mice using a variety of behavioral tasks to assess locomotor activity, righting reflex and motor coordination. At postnatal day 40, male and female Swiss mice were assigned to the following experimental groups: alcohol diluted in energy drink (Ed+Etoh), alcohol diluted in water (Etoh) or controls (Ctrl: energy drink or water). Alcohol and energy drink (Red Bull) concentrations were 4g/kg and 8ml/kg, respectively, and all solutions were administered via oral gavage. When compared to Etoh mice, Ed+Etoh animals displayed greater locomotor activity and increased anxiety-like behaviors in the open-field, lost their righting reflexes sooner and displayed poorer motor coordination in the rotarod. Collectively, our findings indicate that alcohol-induced deficits in adolescent mice are worsened by energy drink and go against the view that the stimulant properties of energy drinks can antagonize the adverse effects of alcohol.

Use of Electronic Nicotine Delivery Systems Among Adolescents: Status of the Evidence and Public Health Recommendations

Although the prevalence of tobacco smoking has been declining in recent years, the use of electronic nicotine delivery systems (ENDS) such as of electronic cigarettes, vaporizers, and hookahs has been steadily rising, especially among adolescents. ENDS are not only advertised to children, but their sale via the Internet has made them easily accessible to youth. In general, children perceive ENDS as safe, or at least safer than smoking traditional combustible tobacco products; however, exposure to nicotine may have deleterious effects on the developing brain. Concern also persists that ENDS may be a "starter" drug that may lead to further tobacco, drug, and/or alcohol use. In contrast to this precautionary stance that is associated with calls for legislative oversight of ENDS marketing and sales, harm reductionists claim that the risks posed by ENDS are minor in comparison with those of combustible tobacco products and that ENDS may be used as a means of nicotine replacement for smoking cessation, despite no concrete evidence to support this assertion. Many medical and health-related organizations have produced position statements concerning ENDS use, including among adolescents. This article summarizes the advantages and disadvantages of using ENDS espoused in these position statements, especially as they relate to use among adolescents. [Pediatr Ann. 2017;46(2):e69-e77.].

A ten fold reduction of nicotine yield in tobacco smoke does not spare the central cholinergic system in adolescent mice

The tobacco industry has gradually decreased nicotine content in cigarette smoke but the impact of this reduction on health is still controversial. Since the central cholinergic system is the primary site of action of nicotine, here, we investigated the effects of exposure of adolescent mice to tobacco smoke containing either high or low levels of nicotine on the central cholinergic system and the effects associated with cessation of exposure. From postnatal day (PN) 30 to 45, male and female Swiss mice were exposed to tobacco smoke (whole body exposure, 8h/day, 7 days/week) generated from 2R1F (HighNic group: 1.74mg nicotine/cigarette) or 4A1 (LowNic group: 0.14mg nicotine/cigarette) research cigarettes, whereas control mice were exposed to ambient air. Cholinergic biomarkers were assessed in the cerebral cortex and midbrain by the end of exposure (PN45), at short- (PN50) and long-term (PN75) deprivation. In the cortex, nicotinic cholinergic receptor upregulation was observed with either type of cigarette. In the midbrain, upregulation was detected only in HighNic mice and remained significant in females at short-term deprivation. The high-affinity choline transporter was reduced in the cortex: of HighNic mice by the end of exposure; of both HighNic and LowNic females at short-term deprivation; of LowNic mice at long-term deprivation. These decrements were separable from effects on choline acetyltransferase and acetylcholinesterase activities, suggesting cholinergic synaptic impairment. Here, we demonstrated central cholinergic alterations in an animal model of tobacco smoke exposure during adolescence. This system was sensitive even to tobacco smoke with very low nicotine content.

In utero and Lactational Exposure to Acetamiprid Induces Abnormalities in Socio-Sexual and Anxiety-Related Behaviors of Male Mice

Neonicotinoids, a widely used group of pesticides designed to selectively bind to insect nicotinic acetylcholine receptors, were considered relatively safe for mammalian species. However, they have been found to activate vertebrate nicotinic acetylcholine receptors and could be toxic to the mammalian brain. In the present study, we evaluated the developmental neurotoxicity of acetamiprid (ACE), one of the most widely used neonicotinoids, in C57BL/6J mice whose mothers were administered ACE via gavage at doses of either 0 mg/kg (control group), 1.0 mg/kg (low-dose group), or 10.0 mg/kg (high-dose group) from gestational day 6 to lactation day 21. The results of a battery of behavior tests for socio-sexual and anxiety-related behaviors, the numbers of vasopressin-immunoreactive cells in the paraventricular nucleus of the hypothalamus, and testosterone levels were used as endpoints. In addition, behavioral flexibility in mice was assessed in a group-housed environment using the IntelliCage, a fully automated mouse behavioral analysis system. In adult male mice exposed to ACE at both low and high doses, a significant reduction of anxiety level was found in the light-dark transition test. Males in the low-dose group also showed a significant increase in sexual and aggressive behaviors. In contrast, neither the anxiety levels nor the sexual behaviors of females were altered. No reductions in the testosterone level, the number of vasopressin-immunoreactive cells, or behavioral flexibility were detected in either sex. These results suggest the possibility that in utero and lactational ACE exposure interferes with the development of the neural circuits required for executing socio-sexual and anxiety-related behaviors in male mice specifically.