Prenatal exposure to maternal cigarette smoking interacts with a polymorphism in the alpha6 nicotinic acetylcholine receptor gene to influence drug use and striatum volume in adolescence

Dopamine and Norepinephrine Tissue Levels in the Developing Limbic Brain Are Impacted by the Human CHRNA6 3'-UTR Single-Nucleotide Polymorphism (rs2304297) in Rats

We previously demonstrated that a genetic single-nucleotide polymorphism (SNP, rs2304297) in the 3' untranslated region (UTR) of the human CHRNA6 gene has sex- and genotype-dependent effects on nicotine-induced locomotion, anxiety, and nicotine + cue-induced reinstatement in adolescent rats. This study aims to investigate how the CHRNA6 3'-UTR SNP influences dopaminergic and noradrenergic tissue levels in brain reward regions during baseline and after the reinstatement of drug-seeking behavior. Naïve adolescent and adult rats, along with those undergoing nicotine + cue reinstatement and carrying the CHRNA6 3'-UTR SNP, were assessed for dopamine (DA), norepinephrine (NE), and metabolites in reward pathway regions. The results reveal age-, sex-, and genotype-dependent baseline DA, NE, and DA turnover levels. Post-reinstatement, male α6GG rats show suppressed DA levels in the Nucleus Accumbens (NAc) Shell compared to the baseline, while nicotine+ cue-induced reinstatement behavior correlates with neurotransmitter levels in specific brain regions. This study emphasizes the role of CHRNA6 3'-UTR SNP in the developmental maturation of the dopaminergic and noradrenergic system in the adolescent rat brain, with tissue levels acting as predictors of nicotine + cue-induced reinstatement.

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.

Sex- and Genotype-Dependent Nicotine-Induced Behaviors in Adolescent Rats with a Human Polymorphism (rs2304297) in the 3'-UTR of the CHRNA6 Gene

In human adolescents, a single nucleotide polymorphism (SNP), rs2304297, in the 3′-UTR of the nicotinic receptor subunit gene, CHRNA6, has been associated with increased smoking. To study the effects of the human CHRNA6 3′-UTR SNP, our lab generated knock-in rodent lines with either C or G SNP alleles. The objective of this study was to determine if the CHRNA6 3′-UTR SNP is functional in the knock-in rat lines. We hypothesized that the human CHRNA6 3′-UTR SNP knock-in does not impact baseline but enhances nicotine-induced behaviors. For baseline behaviors, rats underwent food self-administration at escalating schedules of reinforcement followed by a locomotor assay and a series of anxiety tests (postnatal day (PN) 25-39). In separate cohorts, adolescent rats underwent 1- or 4-day nicotine pretreatment (2×, 30 μg/kg/0.1 mL, i.v.). After the last nicotine injection (PN 31), animals were assessed behaviorally in an open-field chamber, and brain tissue was collected. We show the human CHRNA6 3′-UTR SNP knock-in does not affect food reinforcement, locomotor activity, or anxiety. Further, 4-day, but not 1-day, nicotine exposure enhances locomotion and anxiolytic behavior in a genotype- and sex-specific manner. These findings demonstrate that the human CHRNA6 3′-UTR SNP is functional in our in vivo model.

Prenatal alcohol and tobacco exposures and the risk of cannabis use in offspring: Findings from a population-based cohort study

BACKGROUND

There is a paucity of prospective longitudinal studies examining the associations between maternal use of alcohol and tobacco during pregnancy and the risk of cannabis use in offspring. The aim of this study was to examine the association between prenatal alcohol and tobacco exposures and offspring cannabis use.

METHODS

Data were from the Raine Study, a longitudinal prospective birth cohort based in Western Australia. Cannabis use at 17 years of age was measured with a self-reported questionnaire developed to capture risky behaviors in adolescents. Associations between prenatal alcohol and tobacco exposures and the risk of cannabis use in offspring were examined using log-binomial regression models, computing relative risk (RR). We also computed the E-values (E) to estimate the extent of unmeasured confounding.

RESULTS

After adjusting for potential confounders, we observed increased risks of cannabis use in offspring exposed to first trimester prenatal alcohol use (RR = 1.38, 95% CI: 1.09-1.75; E = 2.10, CI:1.40) and tobacco use (RR = 1.42, 95% CI: 1.08-1.86; E = 2.19, CI:1.37) as well as third trimester prenatal alcohol use (RR = 1.39, 95% CI: 1.09-1.79; E = 2.13, CI:1.40) and tobacco use (RR = 1.39, 95% CI: 1.09-1.79; E = 2.21, CI:1.34]. We also noted dose-response associations in which risk estimates in offspring increased with the level of exposures to prenatal alcohol and tobacco use.

CONCLUSION

These findings provide epidemiological evidence for effects of prenatal alcohol and tobacco exposures on offspring cannabis use. Although these results should be confirmed by other studies, the present study adds to the mounting evidence suggesting that women should be encouraged to abstain from alcohol and tobacco during pregnancy.

Sex- and genotype-dependent nicotine plus cue-primed reinstatement is enhanced in adolescent Sprague Dawley rats containing the human CHRNA6 3'-UTR polymorphism (rs2304297)

RATIONALE

Large-scale human candidate gene studies have indicated that a genetic variant (rs2304297) in the alpha(α)6 nicotinic acetylcholine receptor (nAChR) subunit, encoded by the CHRNA6 gene, may play a key role in adolescent nicotine addictive behavior. We hypothesized that the polymorphism selectively enhances nicotine + cue-primed reinstatement, but not nicotine- or cue-reinstatement in α6 ^GG (risk) vs. α6 ^CC (non-risk) allele carriers, without having baseline effects on natural rewards.

METHODS

Using CRISPR-Cas9 genomic engineering, we developed a humanized rat line with the human gene variant of the CHRNA6 3'-UTR ^{C 123 G} polymorphism in Sprague-Dawley rats. Genetically modified adolescent male and female rats were food trained under a fixed-ratio (FR)1 schedule of reinforcement and progressively increased to FR5. Animals were implanted with catheters and began nicotine self-administration (15 μg/kg/infusion) at FR5. Upon reaching stable responding, reinforced behavior was extinguished by removal of drug and cues. Reinstatement testing began for cue only, nicotine only, and nicotine + cue in a Latin Square Design. Animals were returned to extinction conditions for 2 days minimum between testing.

RESULTS

For natural food rewards, nicotine self-administration, progressive ratio, and extinction, adolescent male and female (α6 ^GG and α6 ^CC ) rats exhibited equivalent behaviors. Male α6 ^GG rats show enhanced nicotine + cue-primed reinstatement when compared with male α6 ^CC rats. This genotype effect on reinstatement was not seen in female rats.

CONCLUSION

Our findings support the in vivo functional role of the human CHRNA6 3'-UTR SNP genetic variant in sex-dependently enhancing nicotine seeking behavior in adolescent rats. Overall, the findings support clinical and preclinical data highlighting a role of α6 nAChRs mediating sex heterogeneity in substance use and related phenotypes.

Emerging Roles for 3' UTRs in Neurons

The 3′ untranslated regions (3′ UTRs) of mRNAs serve as hubs for post-transcriptional control as the targets of microRNAs (miRNAs) and RNA-binding proteins (RBPs). Sequences in 3′ UTRs confer alterations in mRNA stability, direct mRNA localization to subcellular regions, and impart translational control. Thousands of mRNAs are localized to subcellular compartments in neurons—including axons, dendrites, and synapses—where they are thought to undergo local translation. Despite an established role for 3′ UTR sequences in imparting mRNA localization in neurons, the specific RNA sequences and structural features at play remain poorly understood. The nervous system selectively expresses longer 3′ UTR isoforms via alternative polyadenylation (APA). The regulation of APA in neurons and the neuronal functions of longer 3′ UTR mRNA isoforms are starting to be uncovered. Surprising roles for 3′ UTRs are emerging beyond the regulation of protein synthesis and include roles as RBP delivery scaffolds and regulators of alternative splicing. Evidence is also emerging that 3′ UTRs can be cleaved, leading to stable, isolated 3′ UTR fragments which are of unknown function. Mutations in 3′ UTRs are implicated in several neurological disorders—more studies are needed to uncover how these mutations impact gene regulation and what is their relationship to disease severity.

Daughters of Mothers Who Smoke: A Population-based Cohort Study of Maternal Prenatal Tobacco use and Subsequent Prenatal Smoking in Offspring

BACKGROUND

Prenatal exposure to tobacco is associated with adverse health outcomes for the mother and child, and has been associated with an increased risk of tobacco smoking and nicotine dependence in offspring. The objective of this study was to examine the risk of prenatal smoking, among daughters, associated with maternal prenatal smoking.

METHODS

We used a population-based cohort study design, with linked vital records data of mothers and daughters delivering 1984-96 and 1996-2013, respectively, in Washington State. The exposure of interest was mothers' prenatal smoking (any vs. no smoking at any time during pregnancy), while the outcome was daughters' prenatal smoking (similarly assessed). We used multivariable log-binomial regression to obtain estimates of the relative risk (RR) and 95% confidence interval (CI).

RESULTS

Daughters exposed to maternal prenatal smoking were more likely to smoke during their pregnancy, compared to unexposed daughters (RR 1.78, 95% CI 1.72, 1.84, adjusted for the year the daughter delivered, her marital status and educational attainment, and the mothers' race/ethnicity).

CONCLUSIONS

In this relatively young population, we found that daughters exposed to maternal prenatal smoking have an increased risk of smoking later on during their own pregnancy, emphasizing the importance of exposures during the prenatal period. The mechanisms leading to prenatal smoking are multifactorial and likely include behavioural, genetic, epigenetic and environmental factors. An understanding of this risk factor for prenatal smoking may guide health care providers to better target smoking cessation interventions to at-risk populations.

Saguenay Youth Study: a multi-generational approach to studying virtual trajectories of the brain and cardio-metabolic health

This paper provides an overview of the Saguenay Youth Study (SYS) and its parental arm. The overarching goal of this effort is to develop trans-generational models of developmental cascades contributing to the emergence of common chronic disorders, such as depression, addictions, dementia and cardio-metabolic diseases. Over the past 10 years, we have acquired detailed brain and cardio-metabolic phenotypes, and genome-wide genotypes, in 1029 adolescents recruited in a population with a known genetic founder effect. At present, we are extending this dataset to acquire comparable phenotypes and genotypes in the biological parents of these individuals. After providing conceptual background for this work (transactions across time, systems and organs), we describe briefly the tools employed in the adolescent arm of this cohort and highlight some of the initial accomplishments. We then outline in detail the phenotyping protocol used to acquire comparable data in the parents.

Developmental consequences of fetal exposure to drugs: what we know and what we still must learn

Most drugs of abuse easily cross the placenta and can affect fetal brain development. In utero exposures to drugs thus can have long-lasting implications for brain structure and function. These effects on the developing nervous system, before homeostatic regulatory mechanisms are properly calibrated, often differ from their effects on mature systems. In this review, we describe current knowledge on how alcohol, nicotine, cocaine, amphetamine, Ecstasy, and opiates (among other drugs) produce alterations in neurodevelopmental trajectory. We focus both on animal models and available clinical and imaging data from cross-sectional and longitudinal human studies. Early studies of fetal exposures focused on classic teratological methods that are insufficient for revealing more subtle effects that are nevertheless very behaviorally relevant. Modern mechanistic approaches have informed us greatly as to how to potentially ameliorate the induced deficits in brain formation and function, but conclude that better delineation of sensitive periods, dose-response relationships, and long-term longitudinal studies assessing future risk of offspring to exhibit learning disabilities, mental health disorders, and limited neural adaptations are crucial to limit the societal impact of these exposures.

Maternal cigarette smoking during pregnancy predicts drug use via externalizing behavior in two community-based samples of adolescents

BACKGROUND AND AIMS

Prenatal exposure to maternal cigarette smoking (PEMCS) is associated with a higher probability of substance use in adolescence. We explore if externalizing behavior mediates this relationship, while controlling for a number of potential covariates of this mediation process.

METHODS

We used data obtained in two geographically distinct community samples of adolescents. The first (cross-sectional) sample consisted of 996 adolescents (12-18 years of age) recruited from the Saguenay Youth Study (SYS) in Canada (47% with PEMCS). The second (longitudinal) sample consisted of 1141 adolescents (49% with PEMCS) from the Northern Finland Birth Cohort (NFBC1986). In both samples, externalizing behavior and substance use were assessed during adolescence. In the NFBC1986 cohort, externalizing behavior was also assessed in childhood.

RESULTS

In both populations, PEMCS is associated with a higher likelihood of adolescent drug experimentation. In the NFBC1986 cohort, exposed (versus non-exposed) adolescents experiment with an extra 1.27 [B = 0.24, 95% confidence intervals (CI) = 0.15, 0.33 P < 0.001] drugs. In the SYS cohort, a clear protective effect of not being exposed is shown: non-exposed (versus exposed) adolescents are 1.5 times [B = -0.42, 95% CI = -0.75, -0.09, P = 0.013] less likely to take drugs. These associations between PEMCS and drug experimentation remain in the multivariate and mediational analyses.

CONCLUSIONS

Prenatal exposure to maternal cigarette smoking appears to be associated with a higher probability of experimenting with drugs during adolescence, both directly and indirectly via externalizing behavior and the number of peers reported as using drugs.

Prenatal cigarette smoking: Long-term effects on young adult behavior problems and smoking behavior

We examined the long-term effects of prenatal cigarette smoke exposure (PCSE) on the behavior problems and smoking behavior of 22-year-old offspring. The mothers of these offspring were interviewed about their tobacco and other drug use during pregnancy at the fourth and seventh gestational months, and at delivery. Data on the offspring are from interviews at age 22 (n=608). Behavior problems were measured by the Adult Self-Report (ASR) with the following outcome scales: total behavior problems, externalizing, internalizing, attention, anxiety/depression, withdrawn, thought, intrusive, aggression, somatic and rule breaking behavioral problems. Young adult smoking behavior was measured using self-reported average daily cigarettes, and was validated with urine cotinine. Nicotine dependence was measured with the Fagerström Tobacco and Nicotine Dependence (FTND) scale. Regression analyses tested the relations between trimester-specific PCSE and young adult's behavioral problems and smoking behavior, adjusting for demographic and maternal psychological characteristics, and other prenatal substance exposures. Exposed young adults had significantly higher scores on the externalizing, internalizing, aggression, and somatic scales of the ASR. These young adults were also more likely to have a history of arrests. Young adults with PCSE also had a higher rate of smoking and nicotine dependence. Our previous findings of the relations between PCSE and aggressive behavior in early childhood and PCSE and smoking behavior in early adolescence extend into young adulthood.

A single administration of low-dose varenicline saturates α4β2* nicotinic acetylcholine receptors in the human brain

The primary objective of this project was to determine the α4β2(*) nicotinic acetylcholine receptor (nAChR) occupancy in human brain of a single low dose of varenicline (0.5 mg), and to explore the relationship between receptor occupancy by varenicline and tobacco withdrawal symptoms ((*)denoting other putative nAChR subunits). Otherwise healthy smokers (n=9) underwent two positron emission tomography (PET) sessions with the selective α4β2(*) radioligand 2-FA. For the PET sessions, participants received either a low dose of varenicline (0.5 mg) or matching placebo pill (double-blind, random order) before imaging. For both sessions, participants received bolus plus continuous infusions of 2-FA, were scanned for 1 h after allowing the radiotracer to reach a steady state, smoked to satiety, and were scanned for 2 more hours. We estimated the fractional receptor occupancy by a single dose of varenicline (0.5 mg) and the corresponding varenicline dissociation constant (K(V)), along with the effect of low-dose varenicline, pill placebo, and smoking-to-satiety on withdrawal rating scales. The data are compatible with 100% occupancy of α4β2(*) nAChRs by a single dose of varenicline, with a 90% lower confidence limit of 89% occupancy for the thalamus and brainstem. The corresponding 90% upper limit on effective K(V) with respect to plasma varenicline was 0.49 nM. Smoking to satiety, but not low-dose varenicline, significantly reduced withdrawal symptoms. Our findings demonstrate that low-dose varenicline results in saturation of α4β2(*) nAChRs in the thalamus and brainstem without reducing withdrawal symptoms.

Prenatal cigarette smoke exposure and early initiation of multiple substance use

INTRODUCTION

Earlier studies have shown a relation between prenatal cigarette smoke exposure (PCSE) and offspring initiation of tobacco use. No prior study has examined the association between PCSE and early initiation of multiple substances (EIMS) including marijuana and alcohol in addition to tobacco. We investigated the association between PCSE and multiple substance use during adolescence.

METHODS

Pregnant women attending an urban prenatal clinic were selected to participate in the prospective longitudinal study based on their substance use. This study is based on the 16-year follow-up phase and consists of 579 mother-offspring dyads. The women were of lower socioeconomic status, 54% were Black, and 53% reported smoking cigarettes. 52% of the offspring were female. EIMS is a measure of the number of substances initiated prior to age 16 by the adolescents; it ranged from 0 (no initiation, N = 166) to 3 (all, N = 162).

RESULTS

Adolescents exposed to tobacco during first trimester of gestation were 1.4 times more likely to initiate multiple substances by age 16 than the nonexposed group. PCSE was a significant predictor of EIMS after controlling for other prenatal exposures, home environment, and demographic characteristics, using ordinal polytomous logistic regression. Other risk factors of EIMS were maternal and adolescent depression, less strict and less involved parenting, offspring attention problems, and lack of participation in a youth club.

CONCLUSIONS

There is a significant relation between PCSE and adolescent's EIMS.

Quantitative Molecular Imaging of Neuronal Nicotinic Acetylcholine Receptors in the Human Brain with A-85380 Radiotracers

Neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in a spectrum of cognitive functions as well as psychiatric and neurodegenerative disorders, including tobacco addiction and Alzheimer's Disease. The examination of neuronal nAChRs in living humans is a relatively new field. Researchers have developed brain-imaging radiotracers for nAChRs, with radiolabeled A-85380 compounds having the most widespread use. We provide a brief background on nAChRs, followed by a discussion of the development and application of A-85380 radiotracers in human imaging studies. We describe potential future studies using nicotinic receptor radioligands for the study of tobacco addiction, including the mechanism of action of the smoking-cessation therapy varenicline. Throughout this review, we focus on the significant potential that resides in the identification and quantification of nAChRs in the living human brain.

Maternal smoking during pregnancy is associated with epigenetic modifications of the brain-derived neurotrophic factor-6 exon in adolescent offspring

Prenatal exposure to maternal cigarette smoking (PEMCS) is associated with variations in brain and behavior in adolescence. Epigenetic mechanisms may mediate some of the consequences of PEMCS through methylation of deoxyribonucleic acid (DNA) in genes important for brain development, such as the brain-derived neurotrophic factor (BDNF). In the current study, we used bisulfite sequencing to assess DNA methylation of the BDNF promoter in the blood of adolescents whose mothers smoked during pregnancy. We demonstrate that PEMCS is associated with higher rates of DNA methylation in the BDNF-6 exon. These results suggest that PEMCS may lead to long-term down-regulation of BDNF expression via the increase of DNA methylation in its promoter region. Such mechanisms could, in turn, lead to modifications in both development and plasticity of the brain exposed in utero to maternal cigarette smoking.