Modeling adolescent nicotine exposure: effects on cholinergic systems in rat brain regions

Longstanding smoking associated with frontal brain lobe atrophy: a 32-year follow-up study in women

OBJECTIVE

To examine the association between midlife tobacco smoking and late-life brain atrophy and white matter lesions.

METHODS

The study includes 369 women from the Prospective Population Study of Women in Gothenburg, Sweden. Cigarette smoking was reported at baseline 1968 (mean age=44 years) and at follow-up in 1974-1975 and 1980-1981. CT of the brain was conducted 32 years after baseline examination (mean age=76 years) to evaluate cortical atrophy and white matter lesions. Multiple logistic regressions estimated associations between midlife smoking and late-life brain lesions. The final analyses were adjusted for alcohol consumption and several other covariates.

RESULTS

Smoking in 1968-1969 (adjusted OR 1.85; 95% CI 1.12 to 3.04), in 1974-1975 (OR 2.37; 95% CI 1.39 to 4.04) and in 1980-1981 (OR 2.47; 95% CI 1.41 to 4.33) were associated with late-life frontal lobe atrophy (2000-2001). The strongest association was observed in women who reported smoking at all three midlife examinations (OR 2.63; 95% CI 1.44 to 4.78) and in those with more frequent alcohol consumption (OR 6.02; 95% CI 1.74 to 20.84). Smoking in 1980-1981 was also associated with late-life parietal lobe atrophy (OR 1.99; 95% CI 1.10 to 3.58). There were no associations between smoking and atrophy in the temporal or occipital lobe, or with white matter lesions.

CONCLUSION

Longstanding tobacco smoking was mainly associated with atrophy in the frontal lobe cortex. A long-term stimulation of nicotine receptors in the frontal neural pathway might be harmful for targeted brain cell.

Biological determinants impact the neurovascular toxicity of nicotine and tobacco smoke: A pharmacokinetic and pharmacodynamics perspective

Accumulating evidence suggests that the detrimental effect of nicotine and tobacco smoke on the central nervous system (CNS) is caused by the neurotoxic role of nicotine on blood-brain barrier (BBB) permeability, nicotinic acetylcholine receptor expression, and the dopaminergic system. The ultimate consequence of these nicotine associated neurotoxicities can lead to cerebrovascular dysfunction, altered behavioral outcomes (hyperactivity and cognitive dysfunction) as well as future drug abuse and addiction. The severity of these detrimental effects can be associated with several biological determinants. Sex and age are two important biological determinants which can affect the pharmacokinetics and pharmacodynamics of several systemically available substances, including nicotine. With regard to sex, the availability of gonadal hormone is impacted by the pregnancy status and menstrual cycle resulting in altered metabolism rate of nicotine. Additionally, the observed lower smoking cessation rate in females compared to males is a consequence of differential effects of sex on pharmacokinetics and pharmacodynamics of nicotine. Similarly, age-dependent alterations in the pharmacokinetics and pharmacodynamics of nicotine have also been observed. One such example is related to severe vulnerability of adolescence towards addiction and long-term behavioral changes which may continue through adulthood. Considering the possible neurotoxic effects of nicotine on the central nervous system and the deterministic role of sex as well as age on these neurotoxic effects of smoking, it has become important to consider sex and age to study nicotine induced neurotoxicity and development of treatment strategies for combating possible harmful effects of nicotine. In the future, understanding the role of sex and age on the neurotoxic actions of nicotine can facilitate the individualization and optimization of treatment(s) to mitigate nicotine induced neurotoxicity as well as smoking cessation therapy. Unfortunately, however, no such comprehensive study is available which has considered both the sex- and age-dependent neurotoxicity of nicotine, as of today. Hence, the overreaching goal of this review article is to analyze and summarize the impact of sex and age on pharmacokinetics and pharmacodynamics of nicotine and possible neurotoxic consequences associated with nicotine in order to emphasize the importance of including these biological factors for such studies.

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

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

Effects of adolescent substance use disorders on central cholinergic function

Adolescence is a transitional period between childhood and adulthood, in which the individual undergoes significant cognitive, behavioral, physical, emotional, and social developmental changes. During this period, adolescents engage in experimentation and risky behaviors such as licit and illicit drug use. Adolescents' high vulnerability to abuse drugs and natural reinforcers leads to greater risk for developing substance use disorders (SUDs) during adulthood. Accumulating evidence indicates that the use and abuse of licit and illicit drugs during adolescence and emerging adulthood can disrupt the cholinergic system and its processes. This review will focus on the effects of peri-adolescent nicotine and/or alcohol use, or exposure, on the cholinergic system during adulthood from preclinical and clinical studies. This review further explores potential cholinergic agents and pharmacological manipulations to counteract peri-adolescent nicotine and/or alcohol abuse.

Effects of chronic nicotine exposure on Δ9-tetrahydrocannabinol-induced locomotor activity and neural activation in male and female adolescent and adult rats

RATIONALE

High rates of comorbid tobacco and cannabis use in adolescents and young adults may be related to functional interactions between the nicotinic cholinergic and cannabinoid systems in the brain during development. This study examined the effects of chronic exposure to nicotine (the psychoactive component in tobacco) on acute exposure to delta-9-tetrahydrocannabinol (THC) (the psychoactive component of cannabis).

METHODS

Male and female adolescent and adult Sprague-Dawley rats (N = 112) were injected daily with nicotine (1 mg/kg, i.p.) or vehicle for 14 days, followed by a 14-day drug-free period. On test day, rats were injected with THC (5 mg/kg, i.p.) or vehicle, locomotor activity was recorded for 2 h, and brains harvested for c-Fos immunoreactivity (IR).

RESULTS

Locomotor activity and c-Fos IR changes induced by THC challenge were altered by nicotine pre-exposure and modified by age and sex. THC-induced suppression of locomotor activity was attenuated by nicotine pre-exposure in adult but not adolescent males. THC-induced suppression of locomotor activity was potentiated by nicotine pre-exposure in female adolescents, with no effects of THC or nicotine observed in female adults. THC increased c-Fos IR in the caudate, nucleus accumbens, stria terminalis, septum, amygdala, hypothalamus, and thalamus. Nicotine pre-exposure potentiated this effect in all regions. Several brain regions showed age and sex differences in c-Fos IR such that expression was greater in adults than adolescents and in females than males.

CONCLUSIONS

Chronic nicotine pre-exposure produces lasting effects on cannabinoid-mediated signalling in the brain and on behaviour that are mediated by age and sex.

FUNDING SUPPORT

NSERC.

Nicotine Effects on White Matter Microstructure in Young Adults

OBJECTIVE

Nicotine use is widely prevalent among youth, and is associated with white matter microstructural changes as measured by diffusion tensor imaging (DTI). In adults, nicotine use is generally associated with lower fractional anisotropy (FA), but in adolescents/young adults (≤30 years), microstructure appears healthier, indicated by higher FA. This cross-sectional study examined associations between nicotine use and white matter microstructure using fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) in young adults.

METHODS

Fifty-three participants (18 nicotine users [10 female]/35 controls [17 female]) ages 18-25 underwent MRI scan, neuropsychological battery, toxicology screening, and drug use interview. Nicotine group associations with FA and MD were examined in various white matter tracts. In significant tracts, AD and RD were measured. Exploratory correlations were conducted between significant tracts and verbal memory and sustained attention/working memory performance.

RESULTS

Nicotine users exhibited significantly lower FA than controls in the left anterior thalamic radiation, left inferior longitudinal fasciculus, left superior longitudinal fasciculus-temporal, and left uncinate fasciculus. In these tracts, AD and RD did not differ, nor did MD differ in any tract. White matter quality was positively correlated with sustained attention/working memory performance.

CONCLUSIONS

Cigarette smoking may disrupt white matter microstructure. These results are consistent with adult studies, but inconsistent with adolescent/young adult studies, likely due to methodological and sample age differences. Further studies should examine longitudinal effects of nicotine use on white matter microstructure in a larger sample.

The long-term cognitive consequences of adolescent exposure to recreational drugs of abuse

During adolescence, the brain continues to undergo vital developmental processes. In turn, complex behavioral and cognitive skills emerge. Unfortunately, neurobiological development during adolescence can be influenced by environmental factors such as drug exposure. Engaging in drug use during adolescence has been a long-standing health concern, especially how it predicts or relates to drug using behavior later in life. However, recent findings suggest that other behavioral domains, such as learning and memory, are also vulnerable to adolescent drug use. Moreover, it is becoming increasingly apparent that deficits in learning and memory following adolescent drug use endure into adulthood, well after drug exposure has subsided. Although persistent effects suggest an interaction between drug exposure and ongoing development during adolescence, the exact acute and long-term consequences of adolescent drug exposure on substrates of learning and memory are not fully understood. Thus, this review will summarize human and animal findings on the enduring cognitive deficits due to adolescent drug exposure. Moreover, due to the fact that adolescents are more likely to consume drugs of abuse legally available to adults, this review will focus on alcohol, nicotine, and marijuana. Further, given the critical role of the frontal cortex and hippocampus in various learning and memory domains, the impact adolescent use of the previous listed drugs on the neurobiology within these regions will also be discussed.

Using Metabolomics to Investigate Biomarkers of Drug Addiction

Drug addiction has been associated with an increased risk for cancer, psychological complications, heart, liver, and lung disease, as well as infection. While genes have been identified that can mark individuals at risk for substance abuse, the initiation step of addiction is attributed to persistent metabolic disruptions occurring following the first instance of narcotic drug use. Advances in analytical technologies can enable the detection of thousands of signals in body fluids and excreta that can be used to define biochemical profiles of addiction. Today, these approaches hold promise for determining how exposure to drugs, in the absence or presence of other environmentally relevant factors, can impact human metabolism. We posit that these can lead to candidate biomarkers of drug dependence, treatment, withdrawal, or relapse.

Effect of Smoking on Cognitive Functioning in Young Saudi Adults

BACKGROUND Smoking is the predominant form of tobacco consumption and is growing worldwide, particularly in the younger generation in the Middle-East. We aimed to determine the effects of tobacco smoking on cognitive functions among young Saudi adults. MATERIAL AND METHODS We recruited a group of cigarette smokers (N=22) and a group of controls (non-smokers) (N=30) from apparently healthy male volunteers aged 18-29 years. Cognitive function was assessed by using the Cambridge Neuropsychological Automated Battery (CANTAB). The cognitive functions outcome variables were the response time (attention-switching task [AST]), and the percentage of correct response (pattern recognition memory [PRM] task). Clinical, demographic, blood markers (brain-derived neurotrophic factor (BDNF) and apolipoprotein E) were assessed between groups. RESULTS The 2 groups were matched for age and educational status. In comparison to the control group, smokers showed significant cognitive impairments in AST-Latency (p=0.001), AST-Congruent (p=0.001), and AST-Incongruent condition (p=0.001). There was not significant difference in BDNF APOE serum level between the 2 groups. CONCLUSIONS These results indicate that attention and alertness were significantly impaired in smokers compared to non-smokers.

Mechanisms and genetic factors underlying co-use of nicotine and alcohol or other drugs of abuse

Concurrent use of tobacco and alcohol or psychostimulants represents a major public health concern, with use of one substance influencing consumption of the other. Co-abuse of these drugs leads to substantial negative health outcomes, reduced cessation, and high economic costs, but the underlying mechanisms are poorly understood. Epidemiological data suggest that tobacco use during adolescence plays a particularly significant role. Adolescence is a sensitive period of development marked by major neurobiological maturation of brain regions critical for reward processing, learning and memory, and executive function. Nicotine exposure during this time produces a unique and long-lasting vulnerability to subsequent substance use, likely via actions at cholinergic, dopaminergic, and serotonergic systems. In this review, we discuss recent clinical and preclinical data examining the genetic factors and mechanisms underlying co-use of nicotine and alcohol or cocaine and amphetamines. We evaluate the critical role of nicotinic acetylcholine receptors throughout, and emphasize the dearth of preclinical studies assessing concurrent drug exposure. We stress important age and sex differences in drug responses, and highlight a brief, low-dose nicotine exposure paradigm that may better model early use of tobacco products. The escalating use of e-cigarettes among youth necessitates a closer look at the consequences of early adolescent nicotine exposure on subsequent alcohol and drug abuse.

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

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

Genetic and environmental contributions to the relationships between brain structure and average lifetime cigarette use

Chronic cigarette use has been consistently associated with differences in the neuroanatomy of smokers relative to nonsmokers in case-control studies. However, the etiology underlying the relationships between brain structure and cigarette use is unclear. A community-based sample of male twin pairs ages 51-59 (110 monozygotic pairs, 92 dizygotic pairs) was used to determine the extent to which there are common genetic and environmental influences between brain structure and average lifetime cigarette use. Brain structure was measured by high-resolution structural magnetic resonance imaging, from which subcortical volume and cortical volume, thickness and surface area were derived. Bivariate genetic models were fitted between these measures and average lifetime cigarette use measured as cigarette pack-years. Widespread, negative phenotypic correlations were detected between cigarette pack-years and several cortical as well as subcortical structures. Shared genetic and unique environmental factors contributed to the phenotypic correlations shared between cigarette pack-years and subcortical volume as well as cortical volume and surface area. Brain structures involved in many of the correlations were previously reported to play a role in specific aspects of networks of smoking-related behaviors. These results provide evidence for conducting future research on the etiology of smoking-related behaviors using measures of brain morphology.

Altered human brain anatomy in chronic smokers: a review of magnetic resonance imaging studies

Cigarette smoking is becoming more prevalent in developing countries, such as China, and is the largest single cause of preventable death worldwide. New emerging reports are highlighting how chronic cigarette smoking plays a role in neural dysfunctions, such as cognitive decline. Basic animal experimental studies have shown that rats undergo persistent pathological brain changes after being given chronic levels of nicotine. What is perhaps less appreciated is the fact that chronic cigarette smoking induces subtle anatomical changes in the human brain. Consequently, this chapter aims to summarize and integrate the existing magnetic resonance imaging studies on both gray- and white-matter marcostructural and microstructural changes. The reviewed studies demonstrate that chronic cigarette smoking results in discrete and localized alterations in brain region tissue (both the gray and white matter of different brain regions), which may, in part, be responsible for different neural dysfunctions. In addition, we further discuss the possible pathological and neurobiological mechanisms of these nicotinic effects on the brain tissue. We will also address the limitations of the current studies on this issue and identify opportunities for future research.

Paradoxical effects of injection stress and nicotine exposure experienced during adolescence on learning in a serial multiple choice (SMC) task in adult female rats

Nicotine exposure in adolescent rats has been shown to cause learning impairments that persist into adulthood long after nicotine exposure has ended. This study was designed to assess the extent to which the effects of adolescent nicotine exposure on learning in adulthood can be accounted for by adolescent injection stress experienced concurrently with adolescent nicotine exposure. Female rats received either 0.033 mg/h nicotine (expressed as the weight of the free base) or bacteriostatic water vehicle by osmotic pump infusion on postnatal days 25-53 (P25-53). Half of the nicotine-exposed rats and half of the vehicle rats also received twice-daily injection stress consisting of intraperitoneal saline injections on P26-53. Together these procedures produced 4 groups: No Nicotine/No Stress, Nicotine/No Stress, No Nicotine/Stress, and Nicotine/Stress. On P65-99, rats were trained to perform a structurally complex 24-element serial pattern of responses in the serial multiple choice (SMC) task. Four general results were obtained in the current study. First, learning for within-chunk elements was not affected by either adolescent nicotine exposure, consistent with past work (Pickens, Rowan, Bevins, and Fountain, 2013), or adolescent injection stress. Thus, there were no effects of adolescent nicotine exposure or injection stress on adult within-chunk learning typically attributed to rule learning in the SMC task. Second, adolescent injection stress alone (i.e., without concurrent nicotine exposure) caused transient but significant facilitation of adult learning restricted to a single element of the 24-element pattern, namely, the "violation element," that was the only element of the pattern that was inconsistent with pattern structure. Thus, adolescent injection stress alone facilitated violation element acquisition in adulthood. Third, also consistent with past work (Pickens et al., 2013), adolescent nicotine exposure, in this case both with and without adolescent injection stress, caused a learning impairment in adulthood for the violation element in female rats. Thus, adolescent nicotine impaired adult violation element learning typically attributed to multiple-item learning in the SMC task. Fourth, a paradoxical interaction of injection stress and nicotine exposure in acquisition was observed. In the same female rats in which violation-element learning was impaired by adolescent nicotine exposure, adolescent nicotine experienced without adolescent injection stress produced better learning for chunk-boundary elements in adulthood compared to all other conditions. Thus, adolescent nicotine without concurrent injection stress facilitated adult chunk-boundary element learning typically attributed to concurrent stimulus-response discrimination learning and serial-position learning in the SMC task. To the best of our knowledge, the current study is the first to demonstrate facilitation of adult learning caused by adolescent nicotine exposure.

Female rats display enhanced rewarding effects of ethanol that are hormone dependent

BACKGROUND

Ethanol (EtOH) abuse is a major health and economic concern, particularly for females who appear to be more sensitive to the rewarding effects of EtOH. This study compared sex differences to the rewarding and aversive effects of EtOH using place-conditioning procedures in rats.

METHODS

Separate groups of adult (male, female, ovariectomized [OVX] female) and adolescent (male and female) rats received EtOH (0, 0.5, 1.0, 2.0, or 2.5 g/kg, intraperitoneal) and were confined to their initially nonpreferred side of our conditioning apparatus for 30 minutes. On alternate days, they received saline and were confined to the other side. Following 5 drug pairings, the rats were retested for preference behavior. Separate cohorts of the same groups of rats were injected with a similar dose range of EtOH, and blood EtOH levels (BELs) were compared 30 minutes later.

RESULTS

EtOH produced rewarding or aversive effects in a dose-dependent manner. An intermediate dose of EtOH (1.0 g/kg) produced rewarding effects in adult female, but not in male or OVX female rats, suggesting that ovarian hormones facilitate the rewarding effects of EtOH. Similarly, this intermediate dose of EtOH produced rewarding effects in adolescent female, but not in male rats. The highest dose of EtOH (2.5 g/kg) produced aversive effects that were similar across all adult groups. However, the aversive effects of EtOH were lower in adolescents than adults, suggesting that adolescents are less sensitive to the aversive effects of EtOH. The aversive effects of EtOH did not vary across the estrous cycle in intact adult females. There were also no group differences in BELs, suggesting that our results are not related to EtOH metabolism.

CONCLUSION

Our results in rats suggest that human females may be more vulnerable to EtOH abuse due to enhanced rewarding effects of this drug that are mediated by the presence of ovarian hormones.

Association between tobacco smoking and cognitive functioning in young adults

BACKGROUND

Tobacco smoking represents a considerable public health burden globally. Smoking in older adults is associated with cognitive impairment and more rapid age-associated cognitive decline, but there is a paucity of studies in younger people.

METHOD

Adults aged 18-29 years were recruited from a longitudinal study investigating impulsivity in young people. Exclusion criteria were presence of any axis-I morbidity or cannabis use. Subjects undertook neurocognitive assessment using the Cambridge Neuropsychological Test Automated Battery (CANTAB). Demographic, clinical, and cognitive differences between smokers (N = 37) and nonsmokers (N = 177) were characterized.

RESULTS

Groups were well matched in terms of age, education, income, and gender. In comparison to nonsmokers, nicotine users showed significant cognitive impairments on sustained attention (target detection: p= .005), spatial working memory (errors: p= .023, strategy use: p= .004), executive planning (p= .002), and did not appropriately adjust behavior as a function of risk (Gamble task risk adjustment: p= .004). Smokers were intact on general response speeds and response inhibition.

CONCLUSIONS

These data, using objective translational paradigms, support an association between tobacco smoking and cognitive problems in young people, with implications for such individuals and for society. Future studies should extend these results longitudinally to explore causality, and evaluate effects of nicotinic agents (including anti-smoking medications) on cognition.

Sex differences in adult cognitive deficits after adolescent nicotine exposure in rats

This study was designed to determine whether deficits in adult serial pattern learning caused by adolescent nicotine exposure persist as impairments in asymptotic performance, whether adolescent nicotine exposure differentially retards learning about pattern elements that are inconsistent with "perfect" pattern structure, and whether there are sex differences in rats' response to adolescent nicotine exposure as assessed by a serial multiple choice task. The current study replicated the results of our initial report (Fountain et al., 2008) using this task by showing that adolescent nicotine exposure (1.0mg/kg/day nicotine for 35days) produced a specific cognitive impairment in male rats that persisted into adulthood at least a month after adolescent nicotine exposure ended. In addition, sex differences were observed even in controls, with additional evidence that adolescent nicotine exposure significantly impaired learning relative to same-sex controls for chunk boundary elements in males and for violation elements in females. All nicotine-induced impairments were overcome by additional training so that groups did not differ at asymptote. An examination of the types of errors rats made indicated that adolescent nicotine exposure slowed learning without affecting rats' cognitive strategy in the task. This data pattern suggests that exposure to nicotine in adolescence may have impaired different aspects of adult stimulus-response discrimination learning processes in males and females, but left abstract rule learning processes relatively spared in both sexes. These effects converge with other findings in the field and reinforce the concern that adolescent nicotine exposure poses an important threat to cognitive capacity in adulthood.

Correlation between pulmonary function and brain volume in healthy elderly subjects

INTRODUCTION

Cigarette smoking decreases brain regional gray matter volume and is related to chronic obstructive lung disease (COPD). COPD leads to decreased pulmonary function, which is represented by forced expiratory volume in one second percentage (FEV1.0 %); however, it is unclear if decreased pulmonary function is directly related to brain gray matter volume decline. Because there is a link between COPD and cognitive decline, revealing a direct relationship between pulmonary function and brain structure is important to better understand how pulmonary function affects brain structure and cognitive function. Therefore, the purpose of this study was to analyze whether there were significant correlations between FEV1.0 % and brain regional gray and white matter volumes using brain magnetic resonance (MR) image data from 109 community-dwelling healthy elderly individuals.

METHODS

Brain MR images were processed with voxel-based morphometry using a custom template by applying diffeomorphic anatomical registration using the exponentiated lie algebra procedure.

RESULTS

We found a significant positive correlation between the regional white matter volume of the cerebellum and FEV1.0 % after adjusting for age, sex, and intracranial volume.

CONCLUSION

Our results suggest that elderly individuals who have a lower FEV1.0 % have decreased regional white matter volume in the cerebellum. Therefore, preventing decreased pulmonary function is important for cerebellar white matter volume in the healthy elderly population.

Does smoking affect schooling? Evidence from teenagers in rural China

Youth smoking can biologically reduce learning productivity. It can also reduce youths' expected returns to education and lower their motivation to go to school, where smoking is forbidden. Using rich household survey data from rural China, this study investigates the effect of youth smoking on educational outcomes. Youth smoking is clearly an endogenous variable; to obtain consistent estimates of its impact, we use counts of registered alcohol vendors and a food price index as instrumental variables. Since the variable that measures smoking behavior is censored for non-smoking adolescents, we implement a two-step estimation strategy to account for the censored nature of this endogenous regressor. The estimates indicate that smoking one cigarette per day during adolescence can lower students' scores on mathematics tests by about 0.08 standard deviations. However, we find no significant effect of youth smoking on either Chinese test scores or total years of schooling.

Automated measurement of nerve fiber density using line intensity scan analysis

Quantification of nerve fibers in peripheral and central nervous systems is important for the understanding of neuronal function, organization and pathological changes. However, current methods to quantify nerve fibers are resource-intensive and often provide an indirect measurement of nerve fiber density. Here, we describe an automated and efficient method for nerve fiber quantification, which we developed by making use of widely available software and analytical techniques, including Hessian-based feature extraction in NIH ImageJ and line intensity scan analysis. The combined use of these analytical tools through an automated routine enables reliable detection and quantification of nerve fibers from low magnification, non-uniformly labeled epifluorescence images. This allows for time-efficient determination of nerve density and also comparative analysis in large brain structures, such as hippocampus or between various regions of neural circuitry. Using this method, we have obtained accurate measurements of cholinergic fiber density in hippocampus and a large area of cortex in mouse brain sections immunolabeled with an antibody against the vesicular acetylcholine transporter (VAChT). The density values are comparable among animals tested, showing a high degree of reproducibility. Because our method can be performed at relatively low cost and in large tissue sections where nerve fibers can be labeled by various antibodies or visualized by expression of reporter proteins, such as green fluorescent protein in transgenic mice, we expect our method to be broadly useful in both research and clinical investigation. To our knowledge, this is the first method to reliably quantify nerve fibers through a rapid and automated protocol.

Threshold of adulthood for the onset of nicotine self-administration in male and female rats

The great majority of tobacco addiction begins during adolescence. More heavily addicted smokers begin smoking earlier, but differentiating the neurobehavioral impact of nicotine self-administration during adolescence from self-selection bias (whereby people more prone to heavy addiction also begin earlier) cannot be ethically unconfounded in humans. The goals of this research were to determine the age threshold for the adult-like nicotine self-administration and determine sex differences. Male and female Sprague-Dawley rats were tested for nicotine self-administration starting at 4, 5, 6, 7, and 8 weeks of age in an operant FR1 schedule for IV nicotine (0.03 mg/kg/infusion) in 45-min sessions for 2 weeks, with 1 week of enforced abstinence and 1 week of resumed access. This study replicated our earlier work that nicotine self-administration was increased in adolescent vs. adult rats and that the effect was more pronounced in adolescent males, but the increased nicotine self-administration was more persistent in adolescent-onset females. The age threshold for adult-like behavior was 6-7 weeks of age. Adolescent-onset nicotine self-administration had persisting effects of eggaurated increases of nicotine self-administration when fixed-ratio requirements for self-administration were lowered. Female rats that had begun nicotine self-administration during adolescence showed exaggerated increases in nicotine self-administration after a switch back to FR1 from FR8, indicating a lessened control over their self-administration. Adolescent-onset nicotine self-administration was not found to potentiate cocaine self-administration. Adolescent-onset nicotine self-administration causes persistent increases in nicotine self-administration in female rats even after they reach adulthood and disrupts control over self-administration behavior.

If nicotine is a developmental neurotoxicant in animal studies, dare we recommend nicotine replacement therapy in pregnant women and adolescents?

Tobacco use in pregnancy is a leading cause of perinatal morbidity and contributes in major ways to attention deficit hyperactivity disorder, conduct disorders and learning disabilities that emerge in childhood and adolescence. Over the past two decades, animal models of prenatal nicotine exposure have demonstrated that nicotine is a neurobehavioral teratogen that disrupts brain development by preempting the natural, neurotrophic roles of acetylcholine. Through its actions on nicotinic cholinergic receptors, nicotine elicits abnormalities of neural cell proliferation and differentiation, promotes apoptosis and produces deficits in the number of neural cells and in synaptic function. The effects eventually compromise multiple neurotransmitter systems because of the widespread regulatory role of cholinergic neurotransmission. Importantly, the long-term alterations include effects on reward systems that reinforce the subsequent susceptibility to nicotine addiction in later life. These considerations strongly question the appropriateness of nicotine replacement therapy (NRT) for smoking cessation in pregnant women, especially as the pharmacokinetics of the transdermal patch may actually enhance fetal nicotine exposure. Further, because brain maturation continues into adolescence, the period when smoking typically commences, adolescence is also a vulnerable period in which nicotine can change the trajectory of neurodevelopment. There are also serious questions as to whether NRT is actually effective as an aid to smoking cessation in pregnant women and adolescents. This review considers the ramifications of the basic science findings of nicotine's effects on brain development for NRT in these populations.

Adolescent exposure to nicotine impairs adult serial pattern learning in rats

In the present study investigating the effects of adolescent nicotine exposure on adult serial pattern learning, adolescent rats received daily i.p. injections of either 1.0 mg/kg nicotine or saline for 5 days per week for 5 weeks beginning on postnatal day 25 (P25), then were allowed 35 days drug free. Rats then began training on P95 as adults on a 24-element serial pattern composed of eight 3-element chunks. Adolescent exposure to 1.0 mg/kg nicotine produced persistent retardation of learning for the first element of each 3-element chunk of the pattern, that is, for chunk boundary elements, and transient retardation of learning for elements 2 and 3 of each chunk of the pattern, that is, for the within-chunk elements. Deficits at chunk boundaries were interpreted as deficits of phrasing cue discrimination learning whereas deficits for learning responses for elements within-chunks (elements 2 and 3 of chunks) were interpreted as deficits of rule learning. These results indicate that the effects of adolescent nicotine exposure on adult learning and cognitive capacity deserve further scrutiny.

Adult and periadolescent rats differ in expression of nicotinic cholinergic receptor subtypes and in the response of these subtypes to chronic nicotine exposure

Adolescence is a time of significant brain development, and exposure to nicotine during this period is associated with higher subsequent rates of dependence. Chronic nicotine exposure alters expression of nicotinic acetylcholine receptors (nAChRs), changing the pattern of nicotine responsiveness. We used quantitative autoradiography to measure three major subtypes of nAChRs after chronic nicotine exposure by osmotic minipump in adult and periadolescent rats. Comparison of control animals at the two different ages revealed that periadolescents express consistently greater numbers of alpha4beta2* nAChRs compared to the same brain regions of adults. Similar but less pronounced increases in alpha7 nAChRs were found in control periadolescent rats compared to adults. Binding of [(125)I]alpha-conotoxin MII (largely to alpha6* nAChRs) did not systematically differ between adults and periadolescents. The response to chronic nicotine exposure also differed by age. Up-regulation of alpha4beta2* nAChRs was prominent and widespread in adult animals; in periadolescents, alpha4beta2* up-regulation also occurred, but in fewer regions and to a lesser extent. A similar pattern of response was seen with alpha7 receptors: adults were more responsive than periadolescents to nicotine-induced up-regulation. In adult animals, chronic nicotine exposure did not cause up-regulation of alpha6* nAChRs; binding was down-regulated in three regions. Unlike the other subtypes, the response of alpha6* nAChRs to chronic nicotine was greater in periadolescents, with more regions showing greater down-regulation compared to adults. These differences in receptor expression and regulation between age groups are likely to be important given the unique vulnerability of adolescents to nicotine-induced behavioral changes and susceptibility to drug abuse.

Combined exposure to nicotine and ethanol in adolescent mice differentially affects anxiety levels during exposure, short-term, and long-term withdrawal

Smoking and consumption of alcoholic beverages are frequently associated during adolescence. This association could be explained by the cumulative behavioral effects of nicotine and ethanol, particularly those related to anxiety levels. However, despite epidemiological findings, there have been few animal studies of the basic neurobiology of the combined exposure in the adolescent brain. In the present work we assessed, through the use of the elevated plus maze, the short- and long-term anxiety effects of nicotine (NIC) and/or ethanol (ETOH) exposure during adolescence (from the 30th to the 45th postnatal day) in four groups of male and female C57BL/6 mice: (1) Concomitant NIC (nicotine free-base solution (50 microg/ml) in 2% saccharin to drink) and ETOH (ethanol solution (25%, 2 g/kg) i.p. injected every other day) exposure; (2) NIC exposure; (3) ETOH exposure; (4) Vehicle. C57BL/6 mice were selected, in spite of the fact that they present slower ethanol metabolism, because they readily consume nicotine in the concentration used in the present study. During exposure (45th postnatal day: PN45), our results indicated that ethanol was anxiolytic in adolescent mice and that nicotine reverted this effect. Short-term drug withdrawal (PN50) elicited sex-dependent effects: exposure to nicotine and/or ethanol was anxiogenic only for females. Although neither nicotine nor ethanol effects persisted up to 1 month postexposure (PN75), the coadministration elicited an anxiogenic response. In spite of the fact that generalizations based on the results from a single strain of mice are prone to shortcomings, our results suggest that the deficient response to the anxiolytic effects of ethanol in adolescents co-exposed to nicotine may drive higher ethanol consumption. Additionally, increased anxiety during long-term smoking and drinking withdrawal may facilitate relapse to drug use.

Adolescent nicotine treatment changes the response of acetylcholine systems to subsequent nicotine administration in adulthood

Nicotine alters the developmental trajectory of acetylcholine (ACh) systems in the immature brain, with vulnerability extending from fetal stages through adolescence. We administered nicotine to adolescent rats (postnatal days PN30-47) and then examined the subsequent response to nicotine given in adulthood (PN90-107), simulating plasma levels in smokers, and performing evaluations during nicotine treatment (PN105) and withdrawal (PN110, PN120 and PN130), as well as assessing persistent changes at 6 months of age (PN180). We measured nicotinic acetylcholine receptor (nAChR) binding, choline acetyltransferase (ChAT) activity, a marker for ACh terminals, and hemicholinium-3 (HC3) binding to the choline transporter, an index of ACh presynaptic activity. By itself, adolescent nicotine exposure evoked sex-selective deficits in cerebrocortical HC3 binding while elevating ChAT in young adulthood in striatum and midbrain. Nicotine given in adulthood produced profound nAChR upregulation lasting 2 weeks after discontinuing treatment, and decrements in cerebrocortical and striatal HC3 binding emerged during withdrawal, indicative of reduced ACh synaptic activity. For all three parameters, adolescent nicotine altered the responses to nicotine given in adulthood, producing both sensitization and desensitization that depended on sex and brain region, effects that parallel the disparate behavioral outcomes reported for these treatments. The interaction seen here for the impact of adolescent nicotine exposure on adult nicotine responses was substantially greater than that found previously for the effects of prenatal nicotine exposure on adult responses. Our findings thus reinforce the importance of adolescence as a critical period in which the future responsiveness to nicotine is programmed.

Neurobiological processes in adolescent addictive disorders

The purpose of this review is to summarize the neurobiological factors involved in the etiology of adolescent addiction and present evidence implicating various mechanisms in its development. Adolescents are at heightened risk for experimentation with substances, and early experimentation is associated with higher rates of SUD in adulthood. Both normative (e.g., immature frontal-limbic connections, immature frontal lobe development) and non-normative (e.g., lowered serotonergic function, abnormal hypothalamic-pituitary-adrenal axis function) neurobiological developmental factors can predispose adolescents to a heightened risk for SUD. In addition, a normative imbalance in the adolescent neurobiological motivational system may be caused by the relative underdevelopment of suppressive mechanisms when compared to stimulatory systems. These neurobiological liabilities may correspond to neurobehavioral impairments in decision-making, affiliation with deviant peers and externalizing behavior; these and other cognitive and behavioral traits converge with neurobiological factors to increase SUD risk. The progression to SUD acts as an amplifying feedback loop, where the development of SUD results in reciprocal impairments in neurobehavioral and neurobiological processes. A clearer understanding of adolescent neurobiology is a necessary step in the development of prevention and treatment interventions for adolescent SUD.

Separate or sequential exposure to nicotine prenatally and in adulthood: Persistent effects on acetylcholine systems in rat brain regions

Nicotine is a developmental neurotoxicant but the proposed "sensitization-homeostasis" model postulates that even in adulthood nicotine permanently reprograms synaptic function. We administered nicotine to rats throughout gestation or in adulthood (postnatal days PN90-107), simulating plasma levels in smokers, with evaluations on PN105, PN110, PN120, PN130 and PN180. We assessed nicotinic acetylcholine receptor (nAChR) binding, choline acetyltransferase activity, a marker for acetylcholine (ACh) terminals, and hemicholinium-3 (HC3) binding to the choline transporter, an index of ACh presynaptic activity. Prenatal nicotine exposure elicited persistent deficits in HC3 binding in male cerebral cortex and female striatum, but little change in other parameters. Nicotine given in adulthood produced profound nAChR upregulation lasting 2 weeks after discontinuing treatment. Decrements in cerebrocortical and striatal HC3 binding emerged during withdrawal and persisted through PN180, indicative of reduced ACh synaptic activity. Prenatal nicotine did not evoke any major alterations in the response to nicotine given in adulthood. The effects seen here are substantially different from those found previously for nicotine given to adolescent rats, which showed more prolonged nAChR upregulation and profound, widespread and persistent deficits in markers of ACh synaptic function; for adolescents, prenatal nicotine exposure desensitized nAChR responses, exacerbated withdrawal-induced ACh functional deficits, and worsened the long-term outcome. Our results indicate that the effects of nicotine during prenatal or adolescent stages are indeed distinct from the effects in adults, but that even adults show persistent changes after nicotine exposure, commensurate with the sensitization-homeostasis model. These effects may contribute to lifelong vulnerability to readdiction.

Increased plasma brain-derived neurotrophic factor levels in chronic smokers following unaided smoking cessation

Recent animal studies have suggested an association between nicotine and alterations in brain-derived neurotrophic factor (BDNF) expression levels. However, the role of BDNF in humans with nicotine dependence has not yet been investigated. In this study, we explored the differences in the plasma BDNF levels of chronic smokers and healthy nonsmokers, and we investigated the changes in plasma BDNF levels in chronic smokers following unaided smoking cessation. Forty voluntary participants (20 smokers and 20 nonsmokers) were enrolled in this study. We measured the plasma BDNF levels at baseline (both groups) and at the end of the two-month study period (smoker group only) using an enzyme-linked immunosorbent assay. A total of 12 smokers (60.0%) completed the two-month study. ANCOVA with age and body mass index as covariates showed that the baseline plasma BDNF levels in smokers were significantly lower than those in nonsmokers (F=4.626, p=0.038). The plasma BDNF levels in the smokers significantly increased from baseline after the two-month smoking cessation period (Z=-3.059, p=0.002). These findings suggest that BDNF may play a role in the pathophysiology of smoking behavior.

Permanent, sex-selective effects of prenatal or adolescent nicotine exposure, separately or sequentially, in rat brain regions: indices of cholinergic and serotonergic synaptic function, cell signaling, and neural cell number and size at 6 months of age

Nicotine is a neuroteratogen that disrupts neurodevelopment and synaptic function, with vulnerability extending into adolescence. We assessed the permanence of effects in rats on indices of neural cell number and size, and on acetylcholine and serotonin (5HT) systems, conducting assessments at 6 months of age, after prenatal nicotine exposure, adolescent exposure, or sequential exposure in both periods. For prenatal nicotine, indices of cell number and size showed few abnormalities by 6 months, but there were persistent deficits in cerebrocortical choline acetyltransferase activity and hemicholinium-3 binding to the presynaptic choline transporter, a pattern consistent with cholinergic hypoactivity; these effects were more prominent in males than females. The expression of 5HT receptors also showed permanent effects in males, with suppression of the 5HT(1A) subtype and upregulation of 5HT(2) receptors. In addition, cell signaling through adenylyl cyclase showed heterologous uncoupling of neurotransmitter responses. Nicotine exposure in adolescence produced lasting effects that were similar to those of prenatal nicotine. However, when animals were exposed to prenatal nicotine and received nicotine subsequently in adolescence, the adverse effects then extended to females, whereas the net effect in males was similar to that of prenatal nicotine by itself. Our results indicate that prenatal or adolescent nicotine exposure evoke permanent changes in synaptic function that transcend the recovery of less-sensitive indices of structural damage; further, prenatal exposure sensitizes females to the subsequent adverse effects of adolescent nicotine, thus creating a population that may be especially vulnerable to the lasting behavioral consequences of nicotine intake in adolescence.

Differential effects of psychoactive drugs in adolescents and adults

It is well known that most people who use psychoactive drugs started as teenagers. In spite of this, there has been little preclinical research on the effects of psychostimulants during adolescence. Recently, however, a number of laboratories have begun to focus on drug effects in adolescents as compared with adults. The data show that there are unique responses to drugs during this period of development. This review will focus on our current understanding of neurochemical and behavioral drug effects during adolescence.

Adolescent vs. adult-onset nicotine self-administration in male rats: duration of effect and differential nicotinic receptor correlates

Adolescence is the life stage when tobacco addiction typically begins. Adolescent neurobehavioral development may be altered by nicotine self-administration in a way that persistently potentiates addiction. Previously, we showed that female adolescent rats self-administer more nicotine than do adults and that the increased nicotine intake then persists through the transition to adulthood [E.D. Levin, A. Rezvani, D. Montoya, J. Rose, H. Swartzwelder, Adolescent-onset nicotine self-administration modeled in female rats, Psychopharmacology 169 (2003) 141-149.]. In the current study, male Sprague-Dawley rats were given access to nicotine via the standard operant IV self-administration procedure (nicotine bitartrate dose of 0.03 mg/kg/infusion). One group of male rats started during adolescence the other group started in young adulthood. After the end of the four-week period of self-administration brain regions of the rats were assessed for alpha4beta2 nicotinic receptor binding. We found that male rats, like females, show higher nicotine self-administration when starting during adolescence as compared to starting in adulthood (p<0.001). Indeed, the effect in adolescent males was even greater than that in females, with more than triple the rate of nicotine self-administration vs. the adult-onset group during the first 2 weeks. The adolescent onset nicotine-self-administering rats also had significantly greater high affinity nicotinic receptor binding in the midbrain and the striatum, whereas hippocampal binding did not differ between the age groups. Striatal values significantly correlated with nicotine self-administration during the first 2 weeks in the adult-onset group but not the adolescent-onset rats, suggesting that the differences in self-administration may depend in part on underlying disparities in synaptic responses to nicotine. After the initial 2 weeks, nicotine self-administration in male rats declined toward adult-like levels, as the adolescent rats approached adulthood. This study showed that adolescent male rats self-administer significantly more nicotine than do male adult rats, but that adolescent-onset nicotine self-administration in male rats declines over weeks of continued use to approach adult-onset levels. In a previous study, we found that female rats also show greater nicotine self-administration with adolescent onset vs. adult onset, but that the females continued higher rates of self-administration into adulthood. Our results thus reinforce the concept that the adolescent brain is unusually receptive to the effects of nicotine in a manner that reinforces the potential for addiction.

Smoking and structural brain deficits: a volumetric MR investigation

Growing evidence from animal studies indicates brain-damaging properties of nicotine exposure. Investigations in humans found a wide range of functional cerebral effects of nicotine and cigarette smoking, but studies focusing on brain damage are sparse. In 22 smokers and 23 never-smokers possible differences of the cerebral structures were investigated using magnetic resonance imaging and voxel-based morphometry. Significantly smaller grey matter volume and lower grey matter density (P = 0.05, corrected) were observed in the frontal regions (anterior cingulate, prefrontal and orbitofrontal cortex), the occipital lobe and the temporal lobe including parahippocampal gyrus, in smokers than in never-smokers. Group differences of either grey matter volume or grey matter density were also found in the thalamus, cerebellum and substantia nigra, among other regions. Smokers did not show greater volumes than never-smokers in any cerebral region. Magnitude of lifetime exposure to tobacco smoke (pack-years) was inversely correlated with volume of frontal and temporal lobes and cerebellum (P = 0.001, uncorrected). The data indicate structural deficits of several cortical and subcortical regions in smokers relative to never-smokers. The topographic profile of the group differences show some similarities to brain networks known to mediate drug reinforcement, attention and working memory processing. The present findings may explain in part the frequently reported cognitive dysfunctions in chronic cigarette consumers.

Periadolescent nicotine exposure produces sensitization to reinforcement by diazepam in the rat

Epidemiologic studies establish a relationship between nicotine use by adolescents and a subsequent involvement with drugs of abuse in adulthood. Recent research implicates the periadolescent period as a crucial time in development, during which nicotine use produces persistent adaptations that serve to predispose an individual to substance use. The present investigation evaluated the effects of periadolescent nicotine priming on young adult sensitization to reinforcement by a drug of abuse. Nicotine (0.4 mg/kg, intraperitoneal), mecamylamine (2 mg/kg, subcutaneous), mecamylamine and nicotine, or saline was administered as a once-daily injection to periadolescent (postnatal days 35-44) Sprague-Dawley male rats. The effects of periadolescent nicotine priming on reinforcement parameters in the young adult animal (postnatal day 80) were measured by conditioning a place preference with diazepam (1 mg/kg, subcutaneous). Rats were tested for place conditioning in a drug-free state. In contrast to other periadolescent treatment groups, rats treated with only nicotine during periadolescence showed a heterologous sensitization to the subthreshold dose of diazepam utilized during conditioning. Pretreatment with mecamylamine before periadolescent nicotine priming prevented the enhanced response to diazepam observed in the young adult animal. Priming with nicotine during late adolescence (postnatal days 60-69) failed to sensitize the adult rats to diazepam. This study supports a relationship between periadolescent nicotine priming and the production of persistent, behavioral adaptations in the young adult animal.

Increased nicotine self-administration following prenatal exposure in female rats

There is a significant association between maternal cigarette smoking during pregnancy and greater subsequent risk of smoking in female offspring. In animal models, prenatal nicotine exposure causes persistent alterations in cholinergic and monoaminergic systems, both of which are important for nicotine actions underlying tobacco addiction. Accordingly, the current study was conducted to determine if there is a cause-and-effect relationship between prenatal nicotine exposure and nicotine self-administration starting in adolescence. Pregnant rats were administered nicotine (6 mg/kg/day) by osmotic minipump infusion throughout gestation and then, beginning in adolescence and continuing into adulthood, female offspring were given access to nicotine via a standard operant IV self-administration procedure (0.03 mg/kg/infusion). Gestational nicotine exposure did not alter the initial rate of nicotine self-administration. However, when animals underwent one week of forced abstinence and then had a second opportunity to self-administer nicotine, the prenatally-exposed animals showed a significantly greater rate of self-administration than did the controls. Prenatal nicotine exposure causes increased nicotine self-administration, which is revealed only when the animals are allowed to experience a period of nicotine abstinence. This supports a cause-and-effect relationship between the higher rates of smoking in the daughters of women who smoke cigarettes during pregnancy and implicates a role for nicotine in this effect. Our results further characterize the long-term liabilities of maternal smoking but also point to the potential liabilities of nicotine-based treatments for smoking cessation during pregnancy.

Withdrawal from chronic nicotine in adolescent and adult rats

The purpose of the present experiment is to assess potential differences in nicotine withdrawal in both adolescent and adult rats. Nicotine dependence was induced via osmotic minipump in adolescent rats (releasing 22.2 mg/kg/day on Postnatal Day 28) and adults (release rate of 18.4 mg/kg/day on Postnatal Day 60); differential initial release rates were used across age to compensate for the more rapid weight gain of adolescents. On Day 7 of nicotine exposure, withdrawal was induced via the administration of a nicotinic antagonist, mecamylamine (1.0 mg/kg i.p.), and withdrawal-induced anxiogenesis assessed on the elevated plus maze. On Days 1 and 4 after pump removal, animals were examined for startle responses and prepulse inhibition in an acoustic startle chamber. Adult animals exhibited a nicotine withdrawal-induced increase in anxiety, while adolescents did not. One day following the removal of minipumps, only nicotine dependent adolescent animals exhibited a disruption in prepulse inhibition. Nicotine withdrawal failed to produce an alteration in acoustic startle response in either group. Together these data suggest that ontogenic differences in nicotine withdrawal are dependent on the withdrawal measure examined, with adolescents being less sensitive than adults to anxiety-like symptoms, while being more sensitive to withdrawal-induced cognitive disruption.

Prenatal nicotine exposure alters the responses to subsequent nicotine administration and withdrawal in adolescence: Serotonin receptors and cell signaling

Offspring of women who smoke during pregnancy are themselves more likely to take up smoking in adolescence, effects that are associated with a high rate of depression and increased sensitivity to withdrawal symptoms. To evaluate the biological basis for this relationship, we assessed effects on serotonin (5-hydroxytryptamine, 5HT) receptors and 5HT-mediated cellular responses in rats exposed to nicotine throughout prenatal development and then given nicotine in adolescence (postnatal days PN30-47.5), using regimens that reproduce plasma nicotine levels found in smokers. Evaluations were then made during the period of adolescent nicotine treatment and for up to one month after the end of treatment. Prenatal nicotine exposure, which elicits damage to 5HT projections in the cerebral cortex and striatum, produced sex-selective changes in the expression of 5HT(1A) and 5HT2 receptors, along with induction of adenylyl cyclase (AC), leading to sensitization of heterologous inputs operating through this signaling pathway. Superimposed on these effects, the AC response to 5HT was shifted toward inhibition. By itself, adolescent nicotine administration, which damages the same pathways, produced similar effects on receptors and the 5HT-mediated response, but a smaller overall induction of AC. Animals exposed to prenatal nicotine showed a reduced response to nicotine administered in adolescence, results in keeping with earlier findings of persistent desensitization. Our results indicate that prenatal nicotine exposure alters parameters of 5HT synaptic communication lasting into adolescence and changes the response to nicotine administration and withdrawal in adolescence, actions which may contribute to a subpopulation especially vulnerable to nicotine dependence.

Adolescent reward system perseveration due to nicotine: studies with methylphenidate

Previous data suggests adolescent nicotine exposure permanently disrupts reward systems. Behavioral pharmacological methods were used to assess the effects of adolescent nicotine exposure on methylphenidate (MPD) sensitivity and reward from adolescence to adulthood. For experiment one, testing was performed on adult mice exposed to nicotine (0.3 and 3.0 mg/kg, SC, M-F, b.i.d.) or saline during adolescence (PND 25-57). After a 28-day drug-free, time-off period, the locomotor effects (30 min, 30 cm traveled) of MPD (5, 10, 20, and 40 mg/kg, IP) were measured. Thereafter, mice underwent condition-place-preference testing (CPP). MPD (20 mg/kg) was paired with the subject's non-preferred side and saline with the preferred side. Conditioning sessions were conducted for 8 days with a drug-free post-test on the day following the final conditioning session. A second experiment was conducted to determine if adolescent mice respond differently to MPD compared to adult mice. The study compared adolescent mice (PND 25-46) to adult mice (PND 77-98) using identical MPD testing procedures as in experiment one. Adult subjects exposed to nicotine during adolescence behaved remarkably similar to adolescent subjects receiving only MPD. Both nicotine exposed subjects and naïve adolescents exhibited increased response to MPD's motor activating effects and a decreased response to MPD's rewarding effects. Taken together it appears that adolescent nicotine exposure retards the development of reward systems, thus, maintaining an adolescent state indefinitely, which could result in increased vulnerability to substance abuse problems throughout adulthood.

A unique role for striatal serotonergic systems in the withdrawal from adolescent nicotine administration

Adolescent smokers experience more severe withdrawal symptoms upon smoking cessation than do adults, even when daily smoking has occurred for only a short period or with low levels of consumption. Animal models of nicotine withdrawal indicate involvement of striatal serotonin (5-HT) systems in nicotine reward, withdrawal and craving. We evaluated indices of striatal 5-HT and dopamine (DA) synaptic activity, neurotransmitter levels and turnover (metabolite/transmitter ratio), after continuous nicotine infusions to adolescent rats from postnatal days 30 to 47, using a dose rate (6 mg/kg/day) that produces plasma levels typical of smokers. Withdrawal was accompanied by a significant and persistent loss of striatal 5-HT synaptic activity, evidenced by an initial decline in turnover followed by a reduction in 5-HT content without a compensatory increase in turnover. Similar effects were seen for striatal DA activity. These effects were superimposed on a loss of stimulatory 5-HT cellular responses and promotion of inhibitory responses as identified in an earlier work with this model. None of these alterations was seen during withdrawal in adult rats given the same regimen. The unique adolescent withdrawal effects were not seen when the adolescent nicotine treatment period was shortened to early adolescence (days 30-37), even if the administration paradigm was changed to twice-daily injections to maximize withdrawal stress. Our results are consistent with unique effects of adolescent nicotine withdrawal on striatal 5-HT and DA systems, and point to a potential for serotonin-specific reuptake inhibitors as alternatives to nicotine replacement therapy for smoking cessation in adolescents.

Individual differences in novelty-seeking behavior but not in anxiety response to a new environment can predict nicotine consumption in adolescent C57BL/6 mice

Considering that adolescence is associated with an increased motivation to seek out new stimuli and with low anxiety levels in exploring novel environments, and that both behavioral traits may be associated with substance abuse, we investigated whether the behavioral response to a novel environment can predict subsequent oral nicotine self-administration in adolescent C57BL/6 mice. On postnatal day 30 (PN30), the novelty-seeking behavior and anxiety levels were assessed in a hole board activity box. The total number of head-dips (DIP) was used to classify animals either into the high novelty (HN; DIP above median) or low novelty (LN; DIP below median) groups. The percentage of center squares crossed (CEN) was used to classify animals either into the high anxiety (HA; CEN below median) or low anxiety (LA; CEN above median) groups. From PN31 to PN41, all animals were given a free choice between tap water or a nicotine solution (10 microg/ml). LN mice did not change nicotine intake throughout the free choice procedure, however, HN mice presented a marked increase in consumption. There were no differences in consumption between HA and LA mice. Our results indicated that mice that presented a more intense novelty-seeking behavior increased their preference for nicotine during the free choice experiment but that anxiety levels did not predict nicotine consumption. These results suggest that higher motivation to seek out new experiences is a significant contributor to drug use in adolescents and that anxiety is probably not a major factor that determines differential nicotine consumption during adolescence.

Periadolescent nicotine exposure causes heterologous sensitization to cocaine reinforcement

There is increasing concern that abuse of tobacco during periadolescence increases the potential for later abuse of other drugs. To test this hypothesis, Sprague-Dawley rats received once-daily injections of either water or 0.4 mg/kg nicotine from postnatal day 35 through 44. Beginning on postnatal day 80, animals were tested in a 12-day cocaine-induced conditioned place preference (CPP) paradigm. Prior nicotine treatment enhanced the dose-response to cocaine. CPP training with 3.0 mg/kg i.p. cocaine increased time in drug-paired chambers by 50% in control rats and 94% in nicotine-exposed animals. Thus, periadolescent nicotine exposure produced long-term sensitization to an indirect-acting dopamine agonist.

Effects of smoking and smoking abstinence on cognition in adolescent tobacco smokers

BACKGROUND

In adult animals and humans, nicotine can produce short-term cognitive enhancement and, in some cases, neuroprotection. Recent work in animals, however, suggests that exposure to nicotine during adolescence might be neurotoxic. We tested for evidence of acute and chronic effects of tobacco smoking on cognition in adolescents who smoked tobacco daily and were compared with adolescent nonsmokers.

METHODS

Verbal working memory, verbal learning and memory, selective, divided, sustained attention, mood, symptoms of nicotine withdrawal, and tobacco craving were examined in 41 adolescent daily smokers and 32 nonsmokers who were similar in age, gender, and education. Analyses were controlled for general intelligence, reading achievement, parental educational attainment, baseline affective symptoms, and lifetime exposure to alcohol and cannabis.

RESULTS

In adolescent smokers, cessation of tobacco use increased tobacco craving, symptoms of nicotine withdrawal, and depressed mood. Adolescent smokers were found to have impairments in accuracy of working memory performance irrespective of recency of smoking. Performance decrements were more severe with earlier age of onset of smoking. Adolescent smokers experienced further disruption of working memory and verbal memory during smoking cessation. As a group, male smokers initiated smoking at an earlier age than female smokers and were significantly more impaired during tests of selective and divided attention than female smokers and nonsmokers.

CONCLUSIONS

Adolescent daily tobacco smokers experience acute impairments of verbal memory and working memory after smoking cessation, along with chronic decrements in cognitive performance that are consistent with preclinical evidence that neurotoxic effects of nicotine are more severe when exposure to nicotine occurs at earlier periods in development.

Differential behavioral effects of nicotine exposure in adolescent and adult rats

RATIONALE

Although the detrimental effects of nicotine in early brain development and the addictive properties in adulthood are well known, little is known about the neurobiological effects of nicotine in adolescence. An important question is whether adolescents and adults differ in the development of nicotine sensitization and drug-cue conditioning.

OBJECTIVE

To examine the behavioral effects of multiple, repeated injections of nicotine on both sensitization and drug-cue conditioning in the adolescent rat, and to compare this profile with the adult rat.

METHODS

Sixteen male adolescent (28 day) and 16 young adult (70 day) rats were given injections of either saline or nicotine and tested for motor activity for 90 min for ten consecutive days. Following 4 days of no testing, animals were given a sham injection and placed in the testing apparatus for 90 min. A dose-response curve for nicotine was also generated using two additional groups of ten adolescent and ten adult male rats.

RESULTS

Adolescent rats, unlike adults, did not exhibit signs of nicotine-cue conditioning, and displayed less robust sensitization to the locomotor effects of nicotine than adults. Dose-response testing revealed differences in adolescent responsivity to nicotine in measures of rearing, but not ambulation. Initial exposure to nicotine resulted in increased sensitivity to the motor-activating effects of nicotine but less sensitivity to the depressant effects of nicotine in rearing in adolescents.

CONCLUSIONS

Adolescent animals display different long-term neuroadaptive responses to nicotine than adult animals, possibly related to immature or still-developing plasticity mechanisms in the prefrontal cortex.