Nicotine and the developing brain: Insights from preclinical models

Preclinical Models of Attention Deficit Hyperactivity Disorder: Neurobiology, Drug Discovery, and Beyond

OBJECTIVE

We offer an overview of ADHD research using mouse models of nicotine exposure.

METHOD

Nicotine exposure of C57BL/6 or Swiss Webster mice occurred during prenatal period only or during the prenatal and the pre-weaning periods. Behavioral, neuroanatomical and neurotransmitter assays were used to investigate neurobiological mechanisms of ADHD and discover candidate ADHD medications.

RESULTS

Our studies show that norbinaltorphimine, a selective kappa opioid receptor antagonist is a candidate novel non-stimulant ADHD treatment and that a combination of methylphenidate and naltrexone has abuse deterrent potential with therapeutic benefits for ADHD. Other studies showed transgenerational transmission of ADHD-associated behavioral traits and demonstrated that interactions between untreated ADHD and repeated mild traumatic brain injury produced behavioral traits not associated with either condition alone.

CONCLUSION

Preclinical models contribute to novel insights into ADHD neurobiology and are valuable tools for drug discovery and translation to benefit humans with ADHD.

Insights into Pharmacological Activities of Nicotine and 6-Hydroxy-L-nicotine, a Bacterial Nicotine Derivative: A Systematic Review

The purported cognitive benefits associated with nicotine and its metabolites in the brain are a matter of debate. In this review, the impact of the pharmacologically active metabolite of a nicotine derivative produced by bacteria named 6-hydroxy-L-nicotine (6HLN) on memory, oxidative stress, and the activity of the cholinergic system in the brain was examined. A search in the PubMed, Science Direct, Web of Science, and Google Scholar databases, limiting entries to those published between 1992 and 2023, was conducted. The search focused specifically on articles about nicotine metabolites, memory, oxidative stress, and cholinergic system activity, as well as enzymes or pathways related to nicotine degradation in bacteria. The preliminary search resulted in 696 articles, and following the application of exclusion criteria, 212 articles were deemed eligible for inclusion. This review focuses on experimental studies supporting nicotine catabolism in bacteria, and the chemical and pharmacological activities of nicotine and its metabolite 6HLN.

Cigarette Smoking Abstinence Among Pregnant Individuals Using E-Cigarettes or Nicotine Replacement Therapy

Importance

Smoking cigarettes during pregnancy can impair maternal and child health, and pregnant individuals have increasingly used electronic cigarettes (e-cigarettes) for various reasons, including quitting smoking.

Objective

To assess smoking abstinence rates among pregnant individuals who used e-cigarettes compared with those who used nicotine replacement therapy (NRT).

Design, Setting, and Participants

This cohort study is a secondary data analysis of phase 8 of the US Pregnancy Risk Assessment Monitoring System, conducted between 2016 and 2020. Eligible participants included pregnant individuals who smoked combustible cigarettes within the 3 months before pregnancy and either used e-cigarettes or NRT during pregnancy. Data analysis was conducted from March 2022 to April 2023.

Exposures

Combustible cigarette use within 3 months before pregnancy and use of either e-cigarettes or NRT during pregnancy.

Main Outcomes and Measures

The primary outcome was the individual's self-reported smoking abstinence status during the last 3 months of pregnancy. Weighted percentages were reported and weighted multivariable logistic regression models were used to examine the association of e-cigarette use vs NRT with smoking abstinence. A propensity score was used to control for confounding by sociodemographics, pregnancy characteristics, prepregnancy smoking intensity, depression, behavioral support, and hookah use.

Results

The cohort included 1329 pregnant individuals (759 ≥25 years [60.2%]; 766 non-Hispanic White individuals [79.8%]) of whom 781 had an education level of high school or lower (61.4%), and 952 had an annual household income of $48 000 or less (81.5%). Of the 1329 individuals, 890 (unweighted percentage, 67.0%) were existing e-cigarette users, 67 (unweighted percentage, 5.0%) were new e-cigarette users, and 372 (unweighted percentage, 28.0%) were NRT users. Compared with individuals who used NRT during pregnancy, individuals who used e-cigarettes had a higher rate of smoking abstinence in late pregnancy (456 individuals [50.8%] vs 67 individuals [19.4%]; propensity score adjusted odds ratio [OR], 2.47; 95% CI, 1.17-5.20; P = .02). In the secondary analysis stratified by the timing of e-cigarette use initiation, existing users of e-cigarettes who initiated before pregnancy had a higher smoking abstinence rate than NRT users (446 users [53.1%] vs 67 users [19.4%]; adjusted OR, 2.61; 95% CI, 1.23-5.51; P = .01). However, new e-cigarette users who initiated use during pregnancy had a similar smoking abstinence rate in late pregnancy when compared with NRT users (10 users [20.6%] vs 67 users [19.4%]; adjusted OR, 1.13; 95% CI, 0.22-5.87; P = .88).

Conclusions and Relevance

These findings suggest that individuals who used e-cigarettes during pregnancy had a higher smoking abstinence rate in late pregnancy than individuals who used NRT, especially for those who initiated e-cigarette use before pregnancy, indicating that replacement of cigarettes with e-cigarettes during pregnancy may be a viable strategy for harm reduction.

Learning and memory deficits produced by aspartame are heritable via the paternal lineage

Environmental exposures produce heritable traits that can linger in the population for one or two generations. Millions of individuals consume substances such as artificial sweeteners daily that are declared safe by regulatory agencies without evaluation of their potential heritable effects. We show that consumption of aspartame, an FDA-approved artificial sweetener, daily for up to 16-weeks at doses equivalent to only 7-15% of the FDA recommended maximum daily intake value (equivalent to 2-4 small, 8 oz diet soda drinks per day) produces significant spatial learning and memory deficits in mice. Moreover, the cognitive deficits are transmitted to male and female descendants along the paternal lineage suggesting that aspartame's adverse cognitive effects are heritable, and that they are more pervasive than current estimates, which consider effects in the directly exposed individuals only. Traditionally, deleterious environmental exposures of pregnant and nursing women are viewed as risk factors for the health of future generations. Environmental exposures of men are not considered to pose similar risks. Our findings suggest that environmental exposures of men can produce adverse impact on cognitive function in future generations and demonstrate the need for considering heritable effects via the paternal lineage as part of the regulatory evaluations of artificial sweeteners.

Transgenerational transmission of aspartame-induced anxiety and changes in glutamate-GABA signaling and gene expression in the amygdala

We report the effects of aspartame on anxiety-like behavior, neurotransmitter signaling and gene expression in the amygdala, a brain region associated with the regulation of anxiety and fear responses. C57BL/6 mice consumed drinking water containing 0.015% or 0.03% aspartame, a dose equivalent of 8 to 15% of the FDA recommended maximum human daily intake, or plain drinking water. Robust anxiety-like behavior (evaluated using open field test and elevated zero maze) was observed in male and female mice consuming the aspartame-containing water. Diazepam, an allosteric modulator of the GABA-A receptor, alleviated the anxiety-like behavior. RNA sequencing of the amygdala followed by KEGG biological pathway analysis of differentially expressed genes showed glutamatergic and GABAergic synapse pathways as significantly enriched. Quantitative PCR showed upregulation of mRNA for the glutamate NMDA receptor subunit 2D (Grin2d) and metabotropic receptor 4 (Grm4) and downregulation of the GABA-A receptor associated protein (Gabarap) mRNA. Thus, taken together, our diazepam and gene expression data show that aspartame consumption shifted the excitation-inhibition equilibrium in the amygdala toward excitation. Even more strikingly, the anxiety-like behavior, its response to diazepam, and changes in amygdala gene expression were transmitted to male and female offspring in two generations descending from the aspartame-exposed males. Extrapolation of the findings to humans suggests that aspartame consumption at doses below the FDA recommended maximum daily intake may produce neurobehavioral changes in aspartame-consuming individuals and their descendants. Thus, human population at risk of aspartame's potential mental health effects may be larger than current expectations, which only include aspartame-consuming individuals.