Differential expression of arc mRNA and other plasticity-related genes induced by nicotine in adolescent rat forebrain

Activity-regulated cytoskeletal-associated protein (Arc) in presynaptic terminals and extracellular vesicles in hippocampal synapses

The activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is a neuron-specific immediate early gene (IEG) product. The protein regulates synaptic strength through modulation of spine density and morphology, AMPA receptor endocytosis, and as being part of a retrovirus-like inter-cellular communication mechanism. However, little is known about the detailed subsynaptic localization of the protein, and especially its possible presynaptic localization. In the present study, we provide novel electron microscopical data of Arc localization at hippocampal Schaffer collateral synapses in the CA1 region. The protein was found in both pre-and postsynaptic cytoplasm in a majority of synapses, associated with small vesicles. We also observed multivesicular body-like structures positive for Arc. Furthermore, the protein was located over the presynaptic active zone and the postsynaptic density. The relative concentration of Arc was 25% higher in the postsynaptic spine than in the presynaptic terminal. Notably, small extracellular vesicles labeled for Arc were detected in the synaptic cleft or close to the synapse, supporting a possible transsynaptic transmission of the protein in the brain.

Tobacco and electronic cigarette smoking among in-school adolescents in Vietnam between 2013 and 2019: prevalence and associated factors

Background

Smoking among adolescents in schools is a major global public health concern. There is limited evidence regarding prevalence and associated factors in Vietnam.

Objective

To compare the prevalence of smoking and associated factors among in-school adolescents aged 13–17 years in Vietnam between 2013 and 2019.

Methods

Data were collected from two rounds of the national representative Vietnam Global School-based Student Health Survey (GSHS) conducted in 2013 (n = 3,331) and 2019 (n = 7,690). Logistic regression was used to identify the factors associated with tobacco and electronic cigarette smoking among in-school adolescents.

Results

There was a significant reduction in the prevalence of current smoking (water pipes and cigarettes) from 5.4% (95% CI: 4.0–7.2) in 2013 to 2.8% (95% CI: 2.2–3.6) in 2019. In 2019, 2.6% of the in-school adolescents reported having used electronic cigarette products 30 days prior to the survey. Factors associated with a significantly higher likelihood of current smoking status included gender, loneliness, suicidal ideation, sexual activity, truancy, and alcohol consumption. Similar patterns were observed for e-cigarettes.

Conclusion

Smoking among in-school adolescents in Vietnam decreased between 2013 and 2019. Follow-up studies are needed to further investigate causal factors so that future policies and communication programmes can be more effectively targeted to reduce smoking in adolescents.

The prevalence, mechanism of action, and toxicity of Nigerian psychoactive plants

Cannabis sativa, Datura stramonium, Nicotiana tabacum, and Carica papaya are plants that naturally grow in Nigeria. They are reportedly rich in neuroactive compounds that are capable of reacting with the nervous system to elicit psychoactive and/or toxic effects that deter predators. However, despite the toxicological potential of these plants, their recreational use is on the rise due to the psychoactivity they proffer and prevalence in Nigeria. The aim of the present study is to evaluate the plants' recreational use, mechanism of actions and toxicities. Relevant published documents on psychoactive plants in Nigeria were obtained from Web of Science between 2002 and 2020. Non-English documents, documents not in Science Citation Index Expanded and Google Scholar were removed while 1186 documents were reviewed. Results showed that the plants are recreationally used in Nigeria with a higher prevalence than the global frequency. They are very addictive and lead to dependence. The plants were also observed to elicit different mechanism of action, though the activation of monoaminergic neurotransmission system was common to all. Regrettably, the plants could be toxic when ingested under non-medical conditions. Conclusively, these plants are addictive with potential toxic effects. Therefore, control of the recreational use of these plants should be revamped and overhauled.

Racial and Ethnic Differences in E-Cigarette and Cigarette Use Among Adolescents

This study examined whether e-cigarette/cigarette use status would differ by student race/ethnicity. Using the 2017 Youth risk behavior survey (YRBS), weighted Chi-square tests with Rao-Scott adjustments and adjusted weighted multinomial logistic regression analysis were conducted to examine the relationship. Weighted Chi-square tests showed that American Indian/Alaska Native (AI/AN) students had the highest prevalence rates among dual users (16.2%) and e-cigarette only users (11.3%), while White peers had the highest prevalence rates among cigarette only users (3.5%). The results of weighted multinomial logistic regression indicated that AI/AN students had higher odds than White peers of being dual users (Relative risk ratio (RRR), 2.10, 95% CI, 1.01, 4.39), while Black, Hispanic, Asian and multi-racial groups had lower odds than White peers of being dual users. Additionally, Asian students had lower odds than White students of being e-cigarette only users, whereas Black and Asian students had lower odds than their White peers of being cigarette only users. Given that AI/AN students are most vulnerable to e-cigarette/cigarette use, there should be comprehensive tobacco prevention and intervention approaches that could narrow racial/ethnic differences among both youths and adults at population level.

AddictGene: An integrated knowledge base for differentially expressed genes associated with addictive substance

Addiction, a disorder of maladaptive brain plasticity, is associated with changes in numerous gene expressions. Nowadays, high-throughput sequencing data on addictive substance-induced gene expression have become widely available. A resource for comprehensive annotation of genes that show differential expression in response to commonly abused substances is necessary. So, we developed AddictGene by integrating gene expression, gene-gene interaction, gene-drug interaction and epigenetic regulatory annotation for over 70,156 items of differentially expressed genes associated with 7 commonly abused substances, including alcohol, nicotine, cocaine, morphine, heroin, methamphetamine, and amphetamine, across three species (human, mouse, rat). We also collected 1,141 addiction-related experimentally validated genes by techniques such as RT-PCR, northern blot and in situ hybridization. The easy-to-use web interface of AddictGene (http://159.226.67.237/sun/addictgedb/) allows users to search and browse multidimensional data on DEGs of their interest: 1) detailed gene-specific information extracted from the original studies; 2) basic information about the specific gene extracted from NCBI; 3) SNP associated with substance dependence and other psychiatry disorders; 4) expression alteration of specific gene in other psychiatric disorders; 5) expression patterns of interested gene across 31 primary and 54 secondary human tissues; 6) functional annotation of interested gene; 7) epigenetic regulators involved in the alteration of specific genes, including histone modifications and DNA methylation; 8) protein-protein interaction for functional linkage with interested gene; 9) drug-gene interaction for potential druggability. AddictGene offers a valuable repository for researchers to study the molecular mechanisms underlying addiction, and might provide valuable insights into potential therapies for drug abuse and relapse.

Multidimensional Intersection of Nicotine, Gene Expression, and Behavior

The cholinergic system plays a crucial role in nervous system function with important effects on developmental processes, cognition, attention, motivation, reward, learning, and memory. Nicotine, the reinforcing component of tobacco and e-cigarettes, directly acts on the cholinergic system by targeting nicotinic acetylcholine receptors (nAChRs) in the brain. Activation of nAChRs leads to a multitude of immediate and long-lasting effects in specific cellular populations, thereby affecting the addictive properties of the drug. In addition to the direct actions of nicotine in binding to and opening nAChRs, the subsequent activation of circuits and downstream signaling cascades leads to a wide range of changes in gene expression, which can subsequently alter further behavioral expression. In this review, we provide an overview of the actions of nicotine that lead to changes in gene expression and further highlight evidence supporting how these changes can often be bidirectional, thereby inducing subsequent changes in behaviors associated with further drug intake.

E-cigarette aerosols containing nicotine modulate nicotinic acetylcholine receptors and astroglial glutamate transporters in mesocorticolimbic brain regions of chronically exposed mice

Nicotine exposure increases the release of glutamate in part through stimulatory effects on pre-synaptic nicotinic acetylcholine receptors (nAChRs). To assess the impact of chronic electronic (e)-cigarette use on these drug dependence pathways, we exposed C57BL/6 mice to three types of inhalant exposures for 3 months; 1) e-cigarette aerosol generated from liquids containing nicotine (ECN), 2) e-cigarette aerosol generated from liquids containing vehicle chemicals without nicotine (Veh), and 3) air only (AC). We investigated the effects of daily e-cigarette exposure on protein levels of α7 nAChR and α4/β2 nAChR, gene expression and protein levels of astroglial glutamate transporters, including glutamate transporter-1 (GLT-1) and cystine/glutamate antiporter (xCT), in the frontal cortex (FC), striatum (STR) and hippocampus (HIP). We found that chronic inhalation of ECN increased α4/β2 nAChR in all brain regions, and increased α7 nAChR expression in the FC and STR. The total GLT-1 relative mRNA and protein expression were decreased in the STR. Moreover, GLT-1 isoforms (GLT-1a and GLT-1b) were downregulated in the STR in ECN group. However, inhalation of e-cigarette aerosol downregulated xCT expression in STR and HIP compared to AC and Veh groups. ECN group had increased brain-derived neurotrophic factor in the STR compared to control groups. Finally, mass spectrometry detected high concentrations of the nicotine metabolite, cotinine, in the FC and STR in ECN group. This work demonstrates that chronic inhalation of nicotine within e-cigarette aerosols significantly alters the expression of nAChRs and astroglial glutamate transporters in specific mesocorticolimbic brain regions.

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.

The relationship between nicotine and psychosis

Cigarette smoking is strongly associated with psychotic disorders such as schizophrenia. For several decades it was assumed that the relationship could be explained by reverse causation; that smoking was secondary to the illness itself, either through self-medication or a process of institutionalization, or was entirely explained by confounding by cannabis use or social factors. However, studies have exposed that such hypotheses cannot fully explain the association, and more recently a bidirectional relationship has been proposed wherein cigarette smoking may be causally related to risk of psychosis, possibly via a shared genetic liability to smoking and psychosis. We review the evidence for these candidate explanations, using findings from the latest epidemiological, neuroimaging, genetic and preclinical work.

ARC and BDNF expression after cocaine self-administration or cue-induced reinstatement of cocaine seeking in adolescent and adult male rats

Recreational drug use peaks during adolescence. Our research with adolescent vs adult male rats, however, shows that rats taking cocaine as adolescents have lower levels of cue-induced reinstatement of drug-seeking than adults, despite similar levels of intravenous (i.v.) cocaine self-administration. Lower rates of reinstatement in younger rats could be explained by higher levels of brain plasticity. Two neuroplasticity-related genes, activity-regulated cytoskeletal-associated gene (Arc) and brain-derived neurotrophic factor (Bdnf), influence cocaine self-administration and cue-induced reinstatement. We tested whether reinstatement of cocaine seeking correlates with expression of these genes in reinforcement-related brain regions. Adolescent and adult male rats (postnatal day 35 or 83-95 at start) were allowed to acquire lever-pressing maintained by i.v. infusions of cocaine in daily 2-h sessions over 13 days. At one of three experimental time points, rats were sacrificed and tissue collected to analyze Arc and Bdnf mRNA by in situ hybridization in the entire medial prefrontal cortex and entire nucleus accumbens, as well as relevant subregions: prelimbic cortex, infralimbic cortex, and nucleus accumbens core and shell. Despite taking similar amounts of cocaine, adolescents reinstated less than adults. Gene expression was most notable in the prelimbic cortex, was generally higher in adolescent-onset groups, and was higher with longer abstinence. These data partially support the hypothesis that higher levels of Arc and/or Bdnf gene expression in reinforcement-related brain regions of younger animals contribute to lower rates of extinction responding and/or reinstatement. Future studies should include mechanistic analysis of Arc, Bdnf, and their signaling pathways in age-dependent effects of cocaine.

The influence of adolescent nicotine exposure on ethanol intake and brain gene expression

Nicotine and alcohol are often co-abused. Adolescence is a vulnerable period for the initiation of both nicotine and alcohol use, which can lead to subsequent neurodevelopmental and behavioral alterations. It is possible that during this vulnerable period, use of one drug leads to neurobiological alterations that affect subsequent consumption of the other drug. The aim of the present study was to determine the effect of nicotine exposure during adolescence on ethanol intake, and the effect of these substances on brain gene expression. Forty-three adolescent female C57BL/6J mice were assigned to four groups. In the first phase of the experiment, adolescent mice (PND 36-41 days) were exposed to three bottles filled with water or nicotine (200 μg/ml) for 22 h a day and a single bottle of water 2 h a day for six days. In the second phase (PND 42-45 days), the 4-day Drinking-in-the-Dark paradigm consisting of access to 20% v/v ethanol or water for 2h or 4h (the last day) was overlaid during the time when the mice did not have nicotine available. Ethanol consumption (g/kg) and blood ethanol concentrations (BEC, mg %) were measured on the final day and whole brains including the cerebellum, were dissected for RNA sequencing. Differentially expressed genes (DEG) were detected with CuffDiff and gene networks were built using WGCNA. Prior nicotine exposure increased ethanol consumption and resulting BEC. Significant DEG and biological pathways found in the group exposed to both nicotine and ethanol included genes important in stress-related neuropeptide signaling, hypothalamic-pituitary-adrenal (HPA) axis activity, glutamate release, GABA signaling, and dopamine release. These results replicate our earlier findings that nicotine exposure during adolescence increases ethanol consumption and extends this work by examining gene expression differences which could mediate these behavioral effects.

Brain-wide maps of Fos expression during fear learning and recall

Fos induction during learning labels neuronal ensembles in the hippocampus that encode a specific physical environment, revealing a memory trace. In the cortex and other regions, the extent to which Fos induction during learning reveals specific sensory representations is unknown. Here we generate high-quality brain-wide maps of Fos mRNA expression during auditory fear conditioning and recall in the setting of the home cage. These maps reveal a brain-wide pattern of Fos induction that is remarkably similar among fear conditioning, shock-only, tone-only, and fear recall conditions, casting doubt on the idea that Fos reveals auditory-specific sensory representations. Indeed, novel auditory tones lead to as much gene induction in visual as in auditory cortex, while familiar (nonconditioned) tones do not appreciably induce Fos anywhere in the brain. Fos expression levels do not correlate with physical activity, suggesting that they are not determined by behavioral activity-driven alterations in sensory experience. In the thalamus, Fos is induced more prominently in limbic than in sensory relay nuclei, suggesting that Fos may be most sensitive to emotional state. Thus, our data suggest that Fos expression during simple associative learning labels ensembles activated generally by arousal rather than specifically by a particular sensory cue.

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.

Epigenetic and gene expression changes in the adolescent brain: What have we learned from animal models?

Adolescence is defined as the gradual period of transition between childhood and adulthood that is characterized by significant brain maturation, growth spurts, sexual maturation, and heightened social interaction. Although originally believed to be a uniquely human aspect of development, rodent and non-human primates demonstrate maturational patterns that distinctly support an adolescent stage. As epigenetic processes are essential for development and differentiation, but also transpire in mature cells in response to environmental influences, they are an important aspect of adolescent brain maturation. The purpose of this review article was to examine epigenetic programming in animal models of brain maturation during adolescence. The discussion focuses on animal models to examine three main concepts; epigenetic processes involved in normal adolescent brain maturation, the influence of fetal programming on adolescent brain development and the epigenome, and finally, postnatal experiences such as exercise and drugs that modify epigenetic processes important for adolescent brain maturation. This corollary emphasizes the utility of animal models to further our understanding of complex processes such as epigenetic regulation and brain development.

Nicotine administration enhances negative occasion setting in adolescent rats

Substantial research has established that exposure to nicotine during adolescence can lead to long-term changes in neural circuitry and behavior. However, relatively few studies have considered the effects of nicotine use on cognition during this critical stage of brain development. This is significant because the influence of nicotine on cognitive performance during adolescence may contribute to the development of regular nicotine use. For example, improvements in cognitive functioning may increase the perceived value of smoking and facilitate impulses to smoke. To address this, the present research tested the effects of nicotine on a form of inhibitory learning during adolescence. Specifically, adolescent rats were exposed to nicotine as they were trained in a negative occasion setting paradigm, in which successful performance depends on learning the conditions under which it is, or is not, appropriate to respond to a target stimulus. Here, we found that nicotine administration enhances negative occasion setting in adolescents. In addition, nicotine increased the amount of orienting behavior directed toward the inhibitory stimulus, suggesting that improvements in this form of behavioral inhibition may be attributed to nicotine-induced increases in attentional processing. These results may help elucidate the factors that contribute to the onset as well as continued use of products containing nicotine during adolescence and provide insight to increase the effectiveness of interventions targeted at reducing the prevalence of adolescent smoking.

Nicotine and the adolescent brain

Adolescence encompasses a sensitive developmental period of enhanced clinical vulnerability to nicotine, tobacco, and e-cigarettes. While there are sociocultural influences, data at preclinical and clinical levels indicate that this adolescent sensitivity has strong neurobiological underpinnings. Although definitions of adolescence vary, the hallmark of this period is a profound reorganization of brain regions necessary for mature cognitive and executive function, working memory, reward processing, emotional regulation, and motivated behavior. Regulating critical facets of brain maturation are nicotinic acetylcholine receptors (nAChRs). However, perturbations of cholinergic systems during this time with nicotine, via tobacco or e-cigarettes, have unique consequences on adolescent development. In this review, we highlight recent clinical and preclinical data examining the adolescent brain's distinct neurobiology and unique sensitivity to nicotine. First, we discuss what defines adolescence before reviewing normative structural and neurochemical alterations that persist until early adulthood, with an emphasis on dopaminergic systems. We review how acute exposure to nicotine impacts brain development and how drug responses differ from those seen in adults. Finally, we discuss the persistent alterations in neuronal signaling and cognitive function that result from chronic nicotine exposure, while highlighting a low dose, semi-chronic exposure paradigm that may better model adolescent tobacco use. We argue that nicotine exposure, increasingly occurring as a result of e-cigarette use, may induce epigenetic changes that sensitize the brain to other drugs and prime it for future substance abuse.

Social conditioned place preference in the captive ground squirrel (Ictidomys tridecemlineatus): Social reward as a natural phenotype

Social behaviors of wild animals are often considered within an ultimate framework of adaptive benefits versus survival risks. By contrast, studies of laboratory animals more typically focus on affective aspects of behavioral decisions, whether a rodent derives a rewarding experience from social encounter, and how this experience might be initiated and maintained by neural circuits. Artificial selection and inbreeding have rendered laboratory animals more affiliative and less aggressive than their wild conspecifics, leaving open the possibility that social reward is an artifact of domestication. We compared social behaviors of wild and captive population of juvenile 13-lined ground squirrels (Ictidomys tridecemlineatus), the latter being 2nd- and 3rd-generation descendants of wild individuals. At an age corresponding to emergence from the burrow, postnatal day (PD) 38, captive squirrels engaged in vigorous social approach and play and these juvenile behaviors declined significantly by PD 56. Similarly, young wild squirrels expressed social proximity and play; affiliative interactions declined with summer's progression and were replaced by agonistic chasing behaviors. Social conditioned place preference testing (conditioned PDs 40-50) indicated that adolescent squirrels derived a rewarding experience from social reunion. Our results support the contention that undomesticated rodents have the capacity for social reward and more generally suggest the possibility that positive affective experiences may support group cohesion, social cooperation, and altruism in the wild.

Adolescent nicotine induces persisting changes in development of neural connectivity

Adolescent nicotine induces persisting changes in development of neural connectivity. A large number of brain changes occur during adolescence as the CNS matures. These changes suggest that the adolescent brain may still be susceptible to developmental alterations by substances which impact its growth. Here we review recent studies on adolescent nicotine which show that the adolescent brain is differentially sensitive to nicotine-induced alterations in dendritic elaboration, in several brain areas associated with processing reinforcement and emotion, specifically including nucleus accumbens, medial prefrontal cortex, basolateral amygdala, bed nucleus of the stria terminalis, and dentate gyrus. Both sensitivity to nicotine, and specific areas responding to nicotine, differ between adolescent and adult rats, and dendritic changes in response to adolescent nicotine persist into adulthood. Areas sensitive to, and not sensitive to, structural remodeling induced by adolescent nicotine suggest that the remodeling generally corresponds to the extended amygdala. Evidence suggests that dendritic remodeling is accompanied by persisting changes in synaptic connectivity. Modeling, electrophysiological, neurochemical, and behavioral data are consistent with the implication of our anatomical studies showing that adolescent nicotine induces persisting changes in neural connectivity. Emerging data thus suggest that early adolescence is a period when nicotine consumption, presumably mediated by nicotine-elicited changes in patterns of synaptic activity, can sculpt late brain development, with consequent effects on synaptic interconnection patterns and behavior regulation. Adolescent nicotine may induce a more addiction-prone phenotype, and the structures altered by nicotine also subserve some emotional and cognitive functions, which may also be altered. We suggest that dendritic elaboration and associated changes are mediated by activity-dependent synaptogenesis, acting in part through D1DR receptors, in a network activated by nicotine. The adolescent nicotine effects reviewed here suggest that modification of late CNS development constitutes a hazard of adolescent nicotine use.

Cholinergic systems are essential for late-stage maturation and refinement of motor cortical circuits

Previous studies reported that early postnatal cholinergic lesions severely perturb early cortical development, impairing neuronal cortical migration and the formation of cortical dendrites and synapses. These severe effects of early postnatal cholinergic lesions preclude our ability to understand the contribution of cholinergic systems to the later-stage maturation of topographic cortical representations. To study cholinergic mechanisms contributing to the later maturation of motor cortical circuits, we first characterized the temporal course of cortical motor map development and maturation in rats. In this study, we focused our attention on the maturation of cortical motor representations after postnatal day 25 (PND 25), a time after neuronal migration has been accomplished and cortical volume has reached adult size. We found significant maturation of cortical motor representations after this time, including both an expansion of forelimb representations in motor cortex and a shift from proximal to distal forelimb representations to an extent unexplainable by simple volume enlargement of the neocortex. Specific cholinergic lesions placed at PND 24 impaired enlargement of distal forelimb representations in particular and markedly reduced the ability to learn skilled motor tasks as adults. These results identify a novel and essential role for cholinergic systems in the late refinement and maturation of cortical circuits. Dysfunctions in this system may constitute a mechanism of late-onset neurodevelopmental disorders such as Rett syndrome and schizophrenia.

Neural processing of reward in adolescent rodents

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The adolescent brain processes rewards differently than in adults.

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These differences occur even when behavior is similar between age groups.

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DS was the locus of substantial developmental differences in reward activity.

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Surprisingly, differences were not as pronounced in VS.

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These differences may have implications for adolescent psychiatric vulnerability.

Immaturities in adolescent reward processing are thought to contribute to poor decision making and increased susceptibility to develop addictive and psychiatric disorders. Very little is known; however, about how the adolescent brain processes reward. The current mechanistic theories of reward processing are derived from adult models. Here we review recent research focused on understanding of how the adolescent brain responds to rewards and reward-associated events. A critical aspect of this work is that age-related differences are evident in neuronal processing of reward-related events across multiple brain regions even when adolescent rats demonstrate behavior similar to adults. These include differences in reward processing between adolescent and adult rats in orbitofrontal cortex and dorsal striatum. Surprisingly, minimal age related differences are observed in ventral striatum, which has been a focal point of developmental studies. We go on to discuss the implications of these differences for behavioral traits affected in adolescence, such as impulsivity, risk-taking, and behavioral flexibility. Collectively, this work suggests that reward-evoked neural activity differs as a function of age and that regions such as the dorsal striatum that are not traditionally associated with affective processing in adults may be critical for reward processing and psychiatric vulnerability in adolescents.

Adolescent nicotine-induced dendrite remodeling in the nucleus accumbens is rapid, persistent, and D1-dopamine receptor dependent

Chronic nicotine exposure during adolescence induces dendritic remodeling of medium spiny neurons (MSNs) in the nucleus accumbens (NAcc) shell. While nicotine-induced dendritic remodeling has frequently been described as persistent, the trajectory of dendrite remodeling is unknown. Specifically, no study to date has characterized the structural plasticity of dendrites in the NAcc immediately following chronic nicotine, leaving open the possibility that dendrite remodeling emerges gradually over time. Further, the neuropharmacological mechanisms through which nicotine induces dendrite remodeling are not well understood. To address these questions, rats were co-administered chronic nicotine (0.5 mg/kg) and the D1-dopamine receptor (D1DR) antagonist SCH-23390 (0.05 mg/kg) subcutaneously every other day during adolescence. Brains were then processed for Golgi-Cox staining either 1 day or 21 days following drug exposure and dendrites from MSNs in the NAcc shell digitally reconstructed in 3D. Spine density was also measured at both time points. Our morphometric results show (1) the formation of new dendritic branches and spines 1 day following nicotine exposure, (2) new dendritic branches, but not spine density, remains relatively stable for at least 21 days, (3) the co-administration of SCH-23390 completely blocked nicotine-induced dendritic remodeling of MSNs at both early and late time points, suggesting the formation of new dendritic branches in response to nicotine is D1DR-dependent, and (4) SCH-23390 failed to block nicotine-induced increases in spine density. Overall this study provides new insight into how nicotine influences the normal trajectory of adolescent brain development and demonstrates a persistent form of nicotine-induced neuroplasticity in the NAcc shell that develops rapidly and is D1DR dependent.

An exploratory pilot study of the relationship between neural correlates of cognitive control and reduction in cigarette use among treatment-seeking adolescent smokers

Despite high rates of tobacco use during adolescence, few empirically validated smoking cessation strategies exist for adolescent smokers. Developing an understanding of the neural underpinnings of cognitive control processes in adolescent smokers, and their relationship to quit behaviors, may help advance the development of enhanced behavioral and pharmacological therapies. The current pilot study explored the relationship between brain responses during performance of the Stroop color-word interference task and reduction in tobacco use (as measured by changes in cotinine levels) in treatment-seeking adolescent smokers participating in a high school-based smoking-cessation program. Eleven adolescent daily smokers participated in a prequit session during which neural activity in response to congruent and incongruent events in a Stroop task was examined using functional MRI (fMRI). Changes in urine cotinine levels from prequit baseline to end of treatment were calculated and correlated with brain activity. Adolescents with greater activation in the inferior frontal gyrus, insula, thalamus, and anterior cingulate had greater reductions in cotinine levels. The preliminary observation of a relationship between treatment outcome and neural correlates of cognitive control prior to treatment onset provides insight into individual differences in adolescent brain function that might relate importantly to treatment outcome.

Early smoking onset and risk for subsequent nicotine dependence: a monozygotic co-twin control study

OBJECTIVE

Early onset of regular smoking is associated with an elevated risk for later nicotine dependence. Whether or not this association is causal is unknown and has substantial public policy implications.

METHOD

The authors used a monozygotic co-twin control study design. Pairs were selected from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders for discordance in age at onset of regular smoking. Nicotine dependence was measured by the Fagerström test for nicotine dependence and level of craving.

RESULTS

The authors identified 175 male-male and 69 female-female monozygotic twin pairs who differed by at least 2 years in age at onset of regular smoking. During their period of heaviest smoking, the twin who began smoking earlier had significantly higher Fagerström test scores in both the male-male (Cohen's d=0.20) and female-female twin pairs (d=0.26). Craving for cigarettes when unable to smoke was also higher in the early-onset member in both groups (male pairs, d=0.38; female pairs, d=0.25). The early-onset smoking twin did not differ from the later-onset twin in symptoms of alcohol or cannabis abuse or dependence, current alcohol use, or maximal level of cannabis, sedative, stimulant, or cocaine use.

CONCLUSIONS

Controlling for genetic and familial-environmental effects, age at onset of regular smoking predicted level of nicotine dependence. Consistent with the animal literature, these findings suggest that in humans, early nicotine exposure directly increases level of later nicotine dependence. These results should be interpreted in the context of the methodological strengths and limitations of the monozygotic co-twin design.

A role for the prefrontal cortex in heroin-seeking after forced abstinence by adult male rats but not adolescents

Adolescent drug abuse is hypothesized to increase the risk of drug addiction. Yet male rats that self-administer heroin as adolescents show attenuated drug-seeking after abstinence, compared with adults. Here we explore a role for neural activity in the medial prefrontal cortex (mPFC) in age-dependent heroin-seeking. Adolescent (35-day-old at start; adolescent-onset) and adult (86-day-old at start) male rats acquired lever-pressing maintained by heroin using a fixed ratio one reinforcement schedule (0.05 and 0.025 mg/kg per infusion). Following 12 days of forced abstinence, rats were tested for heroin-seeking over 1 h by measuring the number of lever presses on the active lever. Unbiased stereology was then used to estimate the number of Fos-ir(+) and Fos-ir(-) neurons in prelimbic and infralimbic mPFC. As before, adolescents and adults self-administered similar amounts of heroin, but subsequent heroin-seeking was attenuated in the younger rats. Similarly, the adolescent-onset group failed to show significant neural activation in the prelimbic or infralimbic mPFC during the heroin-seeking test, whereas the adult-onset heroin self-administration group showed two to six times more Fos-ir(+) neurons than their saline counterparts in both mPFC subregions. Finally, the overall number of neurons in the infralimbic cortex was greater in rats from the adolescent-onset groups than adults. The mPFC may thus have a key role in some age-dependent effects of heroin self-administration.

Short- and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function

More than 70% of adolescents report to have smoked a cigarette at least once. At the adolescent stage the brain has not completed its maturation. The prefrontal cortex (PFC), the brain area responsible for executive functions and attention performance, is one of the last brain areas to mature and is still developing during adolescence. Smoking during adolescence increases the risk of developing psychiatric disorders and cognitive impairment in later life. In addition, adolescent smokers suffer from attention deficits, which aggravate with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and that underlie the lasting effects on cognitive function. Here we provide an overview of these recent findings.

Nicotine exposure during adolescence alters the rules for prefrontal cortical synaptic plasticity during adulthood

The majority of adolescents report to have smoked a cigarette at least once. Adolescence is a critical period of brain development during which maturation of areas involved in cognitive functioning, such as the medial prefrontal cortex (mPFC), is still ongoing. Tobacco smoking during this age may compromise the normal course of prefrontal development and lead to cognitive impairments in later life. In addition, adolescent smokers suffer from attention deficits, which progress with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and underlie the lasting effects on cognitive function. In particular, the expression and function of metabotropic glutamate receptors (mGluRs) are changed and this has an impact on short- and long-term plasticity of glutamatergic synapses in the PFC and ultimately on the attention performance. Here, we review and discuss these recent findings.

Acute nicotine-induced tachyphylaxis is differentially manifest in the limbic system

Rapid tolerance develops to many of nicotine's behavioral and autonomic effects. A better understanding of the spatiotemporal patterns in neuronal activity as a consequence of acute nicotine tolerance (tachyphylaxis) may help explain its commonly found inverted 'U'-shaped biphasic dose-effect relationship on various behaviors. To this end, we employed high-resolution functional magnetic resonance imaging and relative cerebral blood volume (rCBV) as a marker of neuronal activity, to characterize the regional development of acute tolerance as a function of nicotine dose in naïve, anesthetized rats. A single intravenous nicotine injection at 0.1 and 0.3, but not 0.03 mg/kg, significantly increased neuronal activity in many neocortical areas. In contrast, dose-dependent increases in rCBV were most pronounced in limbic regions, such that responses seen at 0.1 mg/kg nicotine in accumbens, hippocampus, amygdala, and several other limbic areas were not seen following 0.3 mg/kg nicotine. Finally, whereas profound tolerance was observed in many cortical regions after the second of two paired nicotine injections at either 0.1 or 0.3 mg/kg, subcortical limbic structures showed only a weak trend for tolerance. Lack of rCBV changes in animals receiving nicotine methiodide, a quaternary nicotine analog that does not cross the blood-brain barrier, supports a direct neuronal effect of nicotine rather than an action on the vasculature. These data provide pharmacodynamic insight into the regional heterogeneity of nicotine tachyphylaxis development, which may be relevant to behavioral and neurobiological mechanisms associated with repeated tobacco consumption.

Neural correlates of response inhibition and cigarette smoking in late adolescence

Smoking is usually initiated in adolescence, and is the leading preventable cause of death in the United States. Little is known, however, about the links between smoking and neurobiological function in adolescent smokers. This study aimed to probe prefrontal cortical function in late adolescent smokers, using a response inhibition task, and to assess possible relationships between inhibition-related brain activity, clinical features of smoking behavior, and exposure to cigarette smoking. Participants in this study were otherwise healthy late adolescent smokers (15-21 years of age; n=25), who reported daily smoking for at least the 6 months before testing, and age- and education-matched nonsmokers (16-21 years of age; n=25), who each reported smoking fewer than five cigarettes in their lifetimes. The subjects performed the Stop-signal Task, while undergoing functional magnetic resonance imaging. There were no significant group differences in prefrontal cortical activity during response inhibition, but the Heaviness of Smoking Index, a measure of smoking behavior and dependence, was negatively related to neural function in cortical regions of the smokers. These findings suggest that smoking can modulate prefrontal cortical function. Given the late development of the prefrontal cortex, which continues through adolescence, it is possible that smoking may influence the trajectory of brain development during this critical developmental period.

Genome-wide expression analysis reveals diverse effects of acute nicotine exposure on neuronal function-related genes and pathways

Previous human and animal studies demonstrate that acute nicotine exposure has complicated influences on the function of the nervous system, which may lead to long-lasting effects on the behavior and physiology of the subject. To determine the genes and pathways that might account for long-term changes after acute nicotine exposure, a pathway-focused oligoarray specifically designed for drug addiction research was used to assess acute nicotine effect on gene expression in the neuron-like SH-SY5Y cells. Our results showed that 295 genes involved in various biological functions were differentially regulated by 1 h of nicotine treatment. Among these genes, the expression changes of 221 were blocked by mecamylamine, indicating that the majority of nicotine-modulated genes were altered through the nicotinic acetylcholine receptors (nAChRs)-mediated signaling process. We further identified 14 biochemical pathways enriched among the nicotine-modulated genes, among which were those involved in neural development/synaptic plasticity, neuronal survival/death, immune response, or cellular metabolism. In the genes significantly regulated by nicotine but blocked by mecamylamine, 13 enriched pathways were detected. Nine of these pathways were shared with those enriched in the genes regulated by nicotine, including neuronal function-related pathways such as glucocorticoid receptor signaling, p38 MAPK signaling, PI3K/AKT signaling, and PTEN signaling, implying that nAChRs play important roles in the regulation of these biological processes. Together, our results not only provide insights into the mechanism underlying the acute response of neuronal cells to nicotine but also provide clues to how acute nicotine exposure exerts long-term effects on the nervous system.

Chronic nicotine exposure produces lateralized, age-dependent dendritic remodeling in the rodent basolateral amygdala

This study investigated the dendritic morphology of neurons located in the right and left basolateral amygdala (BLA) and infralimbic (IL) cortex following chronic nicotine exposure during adolescence or adulthood. Sprague-Dawley rats were administered subcutaneous injections of nicotine (0.5 mg/kg; free base) or saline three times per week for 2 weeks (six total injections). The dose period began on either postnatal day (P) 32 (adolescent) or P61 (adult). Twenty days following the end of dosing, brains were processed for Golgi-Cox staining, and dendrites from principal neurons in the BLA and pyramidal neurons in the IL were digitally reconstructed in three dimensions. Morphometric analysis revealed a contrasting pattern of BLA dendritic morphology between the adolescent and adult pretreatment groups. In the adult control group, basilar dendritic length did not differ with respect to hemisphere. Nicotine induced robust hemispheric asymmetry by increasing dendritic length in the right hemisphere only. In contrast, adolescent nicotine exposure did not produce significant alteration of basilar dendritic morphology. There was, however, an indication that nicotine eliminated a naturally existing hemispheric asymmetry in the younger cohort. At both ages, nicotine produced a reduction in complexity of the apical tree of principal neurons. Chronic nicotine did not affect the dendritic morphology of pyramidal neurons from the IL in either age group, indicating another dimension of anatomical specificity. Collectively, these data implicate the BLA as a target for lasting neuroplasticity associated with chronic nicotine exposure. Further, hemispheric differences in dendritic morphology were uncovered that depended on the age of nicotine exposure, a finding that underscores the importance of considering laterality when investigating neurodevelopmental effects of drug exposure.

c-Fos expression during temporal order judgment in mice

The neuronal mechanisms for ordering sensory signals in time still need to be clarified despite a long history of research. To address this issue, we recently developed a behavioral task of temporal order judgment in mice. In the present study, we examined the expression of c-Fos, a marker of neural activation, in mice just after they carried out the temporal order judgment task. The expression of c-Fos was examined in C57BL/6N mice (male, n = 5) that were trained to judge the order of two air-puff stimuli delivered bilaterally to the right and left whiskers with stimulation intervals of 50–750 ms. The mice were rewarded with a food pellet when they responded by orienting their head toward the first stimulus (n = 2) or toward the second stimulus (n = 3) after a visual “go” signal. c-Fos-stained cell densities of these mice (test group) were compared with those of two control groups in coronal brain sections prepared at bregma −2, −1, 0, +1, and +2 mm by applying statistical parametric mapping to the c-Fos immuno-stained sections. The expression of c-Fos was significantly higher in the test group than in the other groups in the bilateral barrel fields of the primary somatosensory cortex, the left secondary somatosensory cortex, the dorsal part of the right secondary auditory cortex. Laminar analyses in the primary somatosensory cortex revealed that c-Fos expression in the test group was most evident in layers II and III, where callosal fibers project. The results suggest that temporal order judgment involves processing bilateral somatosensory signals through the supragranular layers of the primary sensory cortex and in the multimodal sensory areas, including marginal zone between the primary somatosensory cortex and the secondary sensory cortex.

Is the relationship between smoking and mental health influenced by other unhealthy lifestyle factors? Results from a 3-year follow-up study among adolescents in Oslo, Norway

PURPOSE

Several studies have confirmed that smoking is a risk factor for depression in adolescence. These studies have not controlled for other lifestyle factors. The aim of this longitudinal study was to assess the relationship between smoking and depressive symptoms, controlling for other lifestyle factors.

METHODS

This school-based longitudinal self-report survey was conducted among 15- and 18-year-old students in Oslo, Norway. From a baseline cohort of 3811 students, 2489 (65%) participants were followed up after 3 years and completed questionnaires at baseline and follow-up. Mental distress was assessed with Hopkins Symptom Checklist, version 10.

RESULTS

There was a statistically significant association between daily smoking at age 15 and mental distress at age 18 for girls, but not for boys (odds ratio [OR]=2.0 [1.5-2.8] and 1.3 [0.7-2.4], respectively). In girls, the association remained statistically significant even after adjustment for sociodemographic variables and several lifestyle factors. In an analysis of 15-years-old "never smokers," a statistically significant association was found between smoking and mental distress for both genders at age 18. Mentally distressed adolescents at age 15 did not show a higher proportion of smoking at age 18 compared with those not distressed.

CONCLUSIONS

In addition to supporting earlier findings that smoking seems to be causally related to depressive symptoms among adolescent girls, the contribution of this study is that this association only to some extent is confounded by other unhealthy lifestyle habits.

Opposite effect of phencyclidine on activity-regulated cytoskeleton-associated protein (Arc) in juvenile and adult limbic rat brain regions

The psychotomimetic effect of NMDA antagonists such as phencyclidine (PCP) in humans spurred the hypoglutamatergic theory of schizophrenia. This theory is supported by animal studies demonstrating schizophrenia-like behavioral and molecular changes following PCP administration to adult or neonatal animals. However, schizophrenia is believed to develop in part due to neurodevelopmental dysfunction during adolescence. Therefore, the effects of PCP in juvenile animals may better reflect the pathophysiology of schizophrenia. Here, we compare the effect of PCP (5mg/kg/day for 5 days) on activity-regulated cytoskeleton-associated protein (Arc) and parvalbumin mRNA expression in juvenile and adult rats. Arc is a marker for excitatory neurotransmission. Parvalbumin is a marker for GABAergic neurotransmission, known to be reduced in postmortem brains of schizophrenics. PCP reduced parvalbumin mRNA expression in the medial prefrontal cortex (mPFC), ventrolateral orbitofrontal cortex (VLO) and shell of the nucleus accumbens (ACCshell) in both juvenile and adult rats. Contrarily, PCP produced opposite effects on Arc mRNA expression in the mPFC, VLO and ACCshell, leading to decreased expression in juvenile and increased expression in adult rats. The differential effect of PCP in juvenile and adult rats may be caused by the immature functional state of the prefrontal cortex in juvenile rats. These results demonstrate differences between the effects of PCP in juvenile and adult rats. The decrease in Arc mRNA in juvenile rats corresponds best with the proposed "hypofrontality" in schizophrenia, suggesting the merits of using PCP in juvenile animals as a model for schizophrenia, as this would relate better to the typical onset and clinical features of schizophrenia.

Gene expression changes in the nucleus accumbens of alcohol-preferring rats following chronic ethanol consumption

The objective of this study was to determine the effects of binge-like alcohol drinking on gene expression changes in the nucleus accumbens (ACB) of alcohol-preferring (P) rats. Adult male P rats were given ethanol under multiple scheduled access (MSA; three 1-h dark cycle sessions/day) conditions for 8 weeks. For comparison purposes, a second ethanol drinking group was given continuous/daily alcohol access (CA; 24h/day). A third group was ethanol-naïve (W group). Average ethanol intakes for the CA and MSA groups were approximately 9.5 and 6.5 g/kg/day, respectively. Fifteen hours after the last drinking episode, rats were euthanized, the brains extracted, and the ACB dissected. RNA was extracted and purified for microarray analysis. The only significant differences were between the CA and W groups (p<0.01; Storey false discovery rate=0.15); there were 374 differences in named genes between these 2 groups. There were 20 significant Gene Ontology (GO) categories, which included negative regulation of protein kinase activity, anti-apoptosis, and regulation of G-protein coupled receptor signaling. Ingenuity analysis indicated a network of transcription factors, involving oncogenes (Fos, Jun, Junb had higher expression in the ACB of the CA group), suggesting increased neuronal activity. There were 43 genes located within rat QTLs for alcohol consumption and preference; 4 of these genes (Tgfa, Hspa5, Mtus1 and Creb3l2) are involved in anti-apoptosis and increased transcription, suggesting that they may be contributing to cellular protection and maintaining high alcohol intakes. Overall, these findings suggest that chronic CA drinking results in genomic changes that can be observed during the early acute phase of ethanol withdrawal. Conversely, chronic MSA drinking, with its associated protracted withdrawal periods, results in genomic changes that may be masked by tight regulation of these genes following repeated experiences of ethanol withdrawal.

Age dependent nicotinic influences over dopamine neuron synaptic plasticity

The dopamine (DA) system of the ventral midbrain plays a critical role as mammals learn adaptive behaviors driven by environmental salience and reward. Addictive drugs, including nicotine, exert powerful influences over the mesolimbic DA system by activating and desensitizing nicotinic acetylcholine receptors (nAChRs) in a subtype-dependent manner. Nicotine induces synaptic plasticity at excitatory synapses onto DA neurons, thereby sending elevated DA signals that participate during the reinforcement of addictive behaviors. While humans and animals of any developmental age are potentially vulnerable to these drug-induced effects, evidence from clinical and epidemiological studies indicates that adolescents have an increased risk of addiction. Although this risk arises from a complex set of variables including societal and psychosocial influences, a contributing factor involves age dependent sensitivity to addictive drugs. One aspect of that sensitivity is drug-induced synaptic plasticity at excitatory synapses onto the dopamine neurons in the ventral midbrain. A single, acute exposure to addictive drugs, including nicotine, produces long-term potentiation (LTP) that can be quantified by measuring the shift in the subtypes of ionotropic glutamate receptors mediating evoked synaptic transmission. This change in glutamatergic transmission is expressed as an increased ratio of AMPA receptors to NMDA receptors at glutamatergic synapses. Age-related differences in the excitability and the nicotine sensitivity within the midbrain dopamine system may contribute to the greater risk of nicotine addiction in adolescent animals and humans.

Nicotinic actions on neuronal networks for cognition: general principles and long-term consequences

Nicotine enhances cognitive performance in humans and laboratory animals. The immediate positive actions of nicotine on learning, memory and attention are well-documented. Several brain areas involved in cognition, such as the prefrontal cortex, have been implicated. Besides acute effects on these brain areas and on brain function, a picture is emerging showing that long-term consequences of nicotine exposure during adolescence can be detrimental for cognitive performance. The majority of adult smokers started the habit during adolescence. Our knowledge on the types of nicotinic receptors in the brain areas that are candidates for mediating nicotine's effects is increasing. However, much less is known about the underlying cellular mechanisms. A series of recent studies have uncovered exciting features of the mechanisms by which nicotine alters prefrontal cortex neuronal activity, synaptic plasticity, gene expression and cognitive function, and how these changes may have a lasting effect on the developing brain. In this review, we discuss these exciting findings and identify several common principles by which nicotinic receptor activation modulates cortical circuits involved in cognition. Understanding how nicotine induces long-term changes in neuronal circuits and alters plasticity in the prefrontal cortex is essential to determining how these mechanisms interact to alter cognition.

The Neurobiology of Nicotine Addiction: Clinical and Public Policy Implications

Clinicians, social scientists, researchers, and policy makers appreciate the need to understand the neurobiology of nicotine addiction and how this information can lead to new treatments and provide support for public policy debates on parity and preventing adolescent tobacco use. In a “bench-to-bedside” manner, this review covers both clinical and basic science perspectives. Both the reward and sensitization-homeostasis theories of nicotine addiction are supported by new understanding of clinical issues of rapid tolerance, withdrawal, sensitization, and craving when examined by functional brain imaging, genetics, and basic science studies of nicotinic acetylcholine receptors. This review provides information to help shape public policy, fight stigma, and improve clinical treatment and research. The fight for parity in health care requires education about the neurobiological basis of addiction versus the stigmatized bad habit or simple socialization. Parity must support reimbursement for nicotine replacement medications or other FDA approved medications and psychosocial treatments.

The dendritically targeted protein Dendrin is induced by acute nicotine in cortical regions of adolescent rat brain

The response of the brain to addictive substances such as nicotine includes the rapid induction of genes that influence synaptic events. This response is different in adolescent brain, which continues to undergo synaptic remodeling in regions that include reward-associated corticolimbic areas. We report here that acute nicotine (0.4 mg/kg), but not cocaine or exposure to a novel environment, induces the expression of the dendritically targeted, corticolimbic mRNA Dendrin in specific regions of adolescent brain. Acute nicotine resulted in an increase in Dendrin mRNA levels in the adolescent prefrontal cortex that was not evident in adult animals. The induction in Dendrin mRNA was a rapid, short-lived transcriptional event that resulted in changes in Dendrin protein. For example, an increase in Dendrin protein levels following nicotine treatment paralleled enhanced Dendrin immunoreactivity in the dendrites of pyramidal neurons of somatosensory cortex. As Dendrin is an important component of cytoskeletal modifications at the synapse, these results suggest that nicotine influences unique plasticity-related changes in the adolescent forebrain that differ from the adult.

The dynamic effects of nicotine on the developing brain

Nicotinic acetylcholine receptors (nAChRs) regulate critical aspects of brain maturation during the prenatal, early postnatal, and adolescent periods. During these developmental windows, nAChRs are often transiently upregulated or change subunit composition in those neural structures that are undergoing major phases of differentiation and synaptogenesis, and are sensitive to environmental stimuli. Nicotine exposure, most often via tobacco smoke, but increasingly via nicotine replacement therapy, has been shown to have unique effects on the developing human brain. Consistent with a dynamic developmental role for acetylcholine, exogenous nicotine produces effects that are unique to the period of exposure and that impact the developing structures regulated by acetylcholine at that time. Here we present a review of the evidence, available from both the clinical literature and preclinical animal models, which suggests that the diverse effects of nicotine exposure are best evaluated in the context of regional and temporal expression patterns of nAChRs during sensitive maturational periods, and disruption of the normal developmental influences of acetylcholine. We present evidence that nicotine interferes with catecholamine and brainstem autonomic nuclei development during the prenatal period of the rodent (equivalent to first and second trimester of the human), alters the neocortex, hippocampus, and cerebellum during the early postnatal period (third trimester of the human), and influences limbic system and late monoamine maturation during adolescence.

Age-dependent effects of low-dose nicotine treatment on cocaine-induced behavioral plasticity in rats

RATIONALE

Epidemiological evidence of early adolescent tobacco use, prior to that of marijuana and other illicit drugs, has led to the hypothesis that nicotine is a "gateway" drug that sensitizes reward pathways to the addictive effects of other psychostimulants.

OBJECTIVE

To test this hypothesis, we have compared the effect of a brief, low-dose nicotine pretreatment of adolescent and adult rats on subsequent locomotor response to acute and chronic cocaine.

MATERIALS AND METHODS

Adolescents, aged postnatal day (P) 28, and adults, aged P86, were given four daily injections of saline or nicotine (0.06 mg/kg, i.v.). At P32 and P90, rats were given acute injections of cocaine (0, 0.4 or 1.0 mg/kg, i.v.) and monitored for locomotor activity in either a habituated or novel test environment. To examine cocaine sensitization, rats were treated for 3 days with saline or cocaine (0.4 mg/kg, i.v.), and, after 1 day of withdrawal, were given a challenge dose of cocaine (0.4 mg/kg, i.v.).

RESULTS

Nicotine pretreatment did not affect acute, drug-induced locomotor activity at either age. However, age differences in cocaine response were observed, with adolescent animals showing enhanced locomotor activity in the novel environment. Adolescent controls did not exhibit cocaine-induced locomotor sensitization, whereas adults did. Nicotine pretreatment during adolescence promoted the development and expression of a sensitized response to repeated cocaine exposure similar to that observed in saline-pretreated adult controls.

CONCLUSIONS

These findings show that brief pretreatment with nicotine, in a low dose comparable to that inhaled in 2-4 cigarettes, enhances cocaine-induced behavioral plasticity in adolescent rats.

Long-lasting cognitive deficits resulting from adolescent nicotine exposure in rats

Adolescence is a developmental period, during which the brain and particularly medial prefrontal cortical (mPFC) regions thereof have not fully matured. Because epidemiological data have suggested that adolescent nicotine use may result in disturbances in cognitive function in adulthood, we investigated the long-term effects of adolescent nicotine exposure in rats. Male Wistar rats were exposed to either nicotine (three times daily, 0.4 mg/kg s.c.) or saline for 10 days during (postnatal day (PND) 34-43) or following (PND 60-69) adolescence. After 5 weeks during adulthood, separate groups of animals were tested in operant paradigms taxing attention and distinct measures of impulsivity. Visuospatial attention and impulsive action were tested in the five-choice serial reaction time task, whereas impulsive choice was assessed in the delayed reward task. Our data show that adolescent, but not postadolescent, nicotine exposure affects cognitive performance in adulthood and results in diminished attentional performance and increments in impulsive action, while leaving impulsive choice intact. This altered cognitive performance appeared to be associated with enhanced releasability of dopamine in the mPFC. Together, these data suggest that adolescence is a time window during which the brain is vulnerable to long-lasting cognitive disturbances resulting from nicotine exposure.

Acute nicotine activates c-fos and activity-regulated cytoskeletal associated protein mRNA expression in limbic brain areas involved in the central stress-response in rat pups during a period of hypo-responsiveness to stress

In adult rats, acute nicotine, the major psychoactive ingredient in tobacco smoke, stimulates the hypothalamic-pituitary-adrenal axis (HPA), resulting in activation of brain areas involved in stress and anxiety-linked behavior. However, in rat pups the first two postnatal weeks are characterized by hypo-responsiveness to stress, also called the 'stress non-responsive period' (SNRP). Therefore, we wanted to address the question if acute nicotine stimulates areas involved in the stress response during SNRP. To determine neuronal activation, the expression of the immediate-early genes c-fos and activity-regulated cytoskeletal associated protein (Arc) was studied in the central nucleus of the amygdala (CeA), bed nucleus stria terminalis (BST) and paraventricular hypothalamic nucleus (PVN), which are areas involved in the neuroendocrine and central stress response. Rat pups received nicotine tartrate (2 mg/kg) or saline by i.p. injection at postnatal days (P) 5, 7 and 10 and their brains were removed after 30 min. We used semi-quantitative radioactive in situ hybridization with gene specific antisense cRNA probes in coronal sections. In control pups, c-fos expression was low in most brain regions, but robust Arc hybridization was found in several areas including cingulate cortex, hippocampus and caudate. Acute nicotine resulted in significant induction of c-fos expression in the PVN and CeA at P5, P7 and P10, and in the BST at P7 and P10. Acute nicotine significantly induced expression of Arc in CeA at P5, P7 and P10, and in the BST at P10. In conclusion, acute nicotine age dependently activated different brain areas of the HPA axis during the SNRP. After P7, the response was more pronounced and included the BST, suggesting differential maturation of the HPA axis in response to nicotine.

A candidate gene approach identifies the CHRNA5-A3-B4 region as a risk factor for age-dependent nicotine addiction

People who begin daily smoking at an early age are at greater risk of long-term nicotine addiction. We tested the hypothesis that associations between nicotinic acetylcholine receptor (nAChR) genetic variants and nicotine dependence assessed in adulthood will be stronger among smokers who began daily nicotine exposure during adolescence. We compared nicotine addiction—measured by the Fagerstrom Test of Nicotine Dependence—in three cohorts of long-term smokers recruited in Utah, Wisconsin, and by the NHLBI Lung Health Study, using a candidate-gene approach with the neuronal nAChR subunit genes. This SNP panel included common coding variants and haplotypes detected in eight α and three β nAChR subunit genes found in European American populations. In the 2,827 long-term smokers examined, common susceptibility and protective haplotypes at the CHRNA5-A3-B4 locus were associated with nicotine dependence severity (p = 2.0×10⁻⁵; odds ratio = 1.82; 95% confidence interval 1.39–2.39) in subjects who began daily smoking at or before the age of 16, an exposure period that results in a more severe form of adult nicotine dependence. A substantial shift in susceptibility versus protective diplotype frequency (AA versus BC = 17%, AA versus CC = 27%) was observed in the group that began smoking by age 16. This genetic effect was not observed in subjects who began daily nicotine use after the age of 16. These results establish a strong mechanistic link among early nicotine exposure, common CHRNA5-A3-B4 haplotypes, and adult nicotine addiction in three independent populations of European origins. The identification of an age-dependent susceptibility haplotype reinforces the importance of preventing early exposure to tobacco through public health policies.

Tobacco use is a global health care problem, and persistent smoking takes an enormous toll on individual health. The onset of daily smoking in adolescence is related to chronic use and severe nicotine dependence in adulthood. Since nicotine is the key addictive chemical in tobacco, we tested the hypothesis that genetic variants within nicotinic acetylcholine receptors will influence the severity of addiction measured in adulthood. Using genomic resequencing to define the patterns of variation found in these candidate genes, we observed that common haplotypes in the CHRNA5-A3-B4 gene cluster are associated with adult nicotine addiction, specifically among those who began daily smoking before age 17. We show that in populations of European origins, one haplotype is a risk factor for dependence, one is protective, and one is neutral. These observations suggest that genetic determinants expressed during human adolescence contribute to the risk of lifetime addiction severity produced from early onset of cigarette use. Because disease risk from the adverse health effects of tobacco smoke is related to lifetime tobacco exposure, the finding that an age-dependent effect of these haplotypes has a strong influence on lifetime smoking behavior reinforces the public health significance of delaying smoking onset.