Nicotine withdrawal produces a decrease in extracellular levels of dopamine in the nucleus accumbens that is lower in adolescent versus adult male rats

Divergent gene expression patterns in alcohol and opioid use disorders lead to consistent alterations in functional networks within the Dorsolateral Prefrontal Cortex

Substance Use Disorders (SUDs) manifest as persistent drug-seeking behavior despite adverse consequences, with Alcohol Use Disorder (AUD) and Opioid Use Disorder (OUD) representing prevalent forms associated with significant mortality rates and economic burdens. The co-occurrence of AUD and OUD is common, necessitating a deeper comprehension of their intricate interactions. While the causal link between these disorders remains elusive, shared genetic factors are hypothesized. Leveraging public datasets, we employed genomic and transcriptomic analyses to explore conserved and distinct molecular pathways within the dorsolateral prefrontal cortex associated with AUD and OUD. Our findings unveil modest transcriptomic overlap at the gene level between the two disorders but substantial convergence on shared biological pathways. Notably, these pathways predominantly involve inflammatory processes, synaptic plasticity, and key intracellular signaling regulators. Integration of transcriptomic data with the latest genome-wide association studies (GWAS) for problematic alcohol use (PAU) and OUD not only corroborated our transcriptomic findings but also confirmed the limited shared heritability between the disorders. Overall, our study indicates that while alcohol and opioids induce diverse transcriptional alterations at the gene level, they converge on select biological pathways, offering promising avenues for novel therapeutic targets aimed at addressing both disorders simultaneously.

Chronic Nicotine Consumption and Withdrawal Regulate Melanocortin Receptor, CRF, and CRF Receptor mRNA Levels in the Rat Brain

Alterations in the various neuropeptide systems in the mesocorticolimbic circuitry have been implicated in negative effects associated with drug withdrawal. The corticotropin-releasing factor (CRF) and α-melanocyte-stimulating hormone are two peptides that may be involved. This study investigated the regulatory effects of chronic nicotine exposure and withdrawal on the mRNA levels of melanocortin receptors (MC3R, MC4R), CRF, and CRF receptors (CRFR1 and CRFR2) expressed in the mesocorticolimbic system. Rats were given drinking water with nicotine or without nicotine (control group) for 12 weeks, after which they continued receiving nicotine (chronic exposure) or were withdrawn from nicotine for 24 or 48 h. The animals were decapitated following behavioral testing for withdrawal signs. Quantitative real-time PCR analysis demonstrated that nicotine exposure (with or without withdrawal) increased levels of CRF and CRFR1 mRNA in the amygdala, CRF mRNA in the medial prefrontal cortex, and CRFR1 mRNA in the septum. Nicotine withdrawal also enhanced MC3R and MC4R mRNA levels in different brain regions, while chronic nicotine exposure was associated with increased MC4R mRNA levels in the nucleus accumbens. These results suggest that chronic nicotine exposure and withdrawal regulate CRF and melanocortin signaling in the mesocorticolimbic system, possibly contributing to negative affective state and nicotine addiction.

Chronic exposure to cigarette smoke extract increases nicotine withdrawal symptoms in adult and adolescent male rats

The aim of the current study was to determine whether non-nicotine constituents of cigarette smoke contribute to nicotine dependence in adolescent and adult male Sprague Dawley rats. For 10 days animals were given three times daily intravenous injections of nicotine (1.5 mg/kg/day) or cigarette smoke extract (CSE) containing an equivalent dose of nicotine. Both spontaneous and mecamylamine-precipitated withdrawal were then measured. Chronic treatment with CSE induced significantly greater somatic and affective withdrawal signs than nicotine in both adolescents and adults. Mecamylamine-precipitated somatic signs were similar at both ages. In contrast, animals spontaneously withdrawn from chronic drug treatment exhibited significant age differences: whereas adolescents chronically treated with nicotine did not show somatic signs, those treated with CSE showed similar physical withdrawal to those of adults. Mecamylamine did not precipitate anxiety-like behavior at either age. However, both adolescents and adults showed significant anxiety in a light-dark box test 18 h after spontaneous withdrawal. Anxiety-like behavior was still evident in an open field test 1 month after termination of drug treatment, with adolescents showing significantly greater affective symptoms than adults. Our findings indicate that non-nicotine constituents of cigarette smoke do contribute to dependence in both adolescents and adults and emphasize the importance of including smoke constituents with nicotine in animal models of tobacco dependence.

Role of noradrenergic transmission within the ventral bed nucleus of the stria terminalis in nicotine withdrawal-induced aversive behavior

AIM

Cessation of smoking induces nicotine withdrawal symptoms such as anxiety, depression, and dysphoria, which could lead to smoking relapse. In the present study, we examined the role of noradrenergic transmission within the ventral bed nucleus of the stria terminalis (vBNST) on nicotine withdrawal-induced aversive behavior.

METHODS

Nicotine dependence in rats was established by subcutaneous implantation with a nicotine-filled osmotic minipump on day 1. Nicotine withdrawal was precipitated by administration of the nicotine receptor antagonist, mecamylamine (3.0 mg/kg, s.c.), on day 15. Nicotine withdrawal-induced intra-vBNST noradrenaline release and aversive behavior were examined by in vivo microdialysis and a conditioned place aversion (CPA) test, respectively.

RESULTS

Intra-vBNST noradrenaline release was significantly increased during nicotine withdrawal. Nicotine withdrawal induced aversive behavior, which was attenuated by intra-vBNST injection of the β-adrenoceptor antagonist, timolol.

CONCLUSIONS

These results suggest that enhanced noradrenergic transmission via β-adrenoceptors in the vBNST plays a crucial role in nicotine withdrawal-induced aversive behavior.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

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.

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Selective Inhibition of Phosphodiesterase 7 Enzymes Reduces Motivation for Nicotine Use through Modulation of Mesolimbic Dopaminergic Transmission

Approximately 5 million people die from diseases related to nicotine addiction and tobacco use each year. The nicotine-induced increase of corticomesolimbic dopaminergic (DAergic) transmission and hypodopaminergic conditions occurring during abstinence are important for maintaining drug-use habits. We examined the notion of reequilibrating DAergic transmission by inhibiting phosphodiesterase 7 (PDE7), an intracellular enzyme highly expressed in the corticomesolimbic circuitry and responsible for the degradation of cyclic adenosine monophosphate (cAMP), the main second messenger modulated by DA receptor activation. Using selective PDE7 inhibitors, we demonstrated in male rats that systemic PDE7 enzyme inhibition reduced nicotine self-administration and prevented reinstatement to nicotine seeking evoked by cues or by the pharmacological stressor yohimbine. The effect was also observed by direct application of the PDE7 inhibitors into the nucleus accumbens (NAc) shell but not into the core. Inhibition of PDE7 resulted in increased DA- and cAMP-regulated neuronal phosphoprotein and cAMP response element-binding protein and their phosphorylated forms in the NAc. It also enhanced the DA D1 receptor agonism-mediated effects, indicating potentiation of protein kinase A-dependent transmission downstream of D1 receptor activation. In electrophysiological recordings from DA neurons in the lateral posterior ventral tegmental area, the PDE7 inhibitors attenuated the spontaneous activity of DA neurons. This effect was exerted through the potentiation of D1 receptor signaling and the subsequent facilitation of γ-aminobutyric acid transmission. The PDE7 inhibitors did not elicit conditioned place preference and did not induce intravenous self-administration, indicating lack of reinforcing properties. Thus, PDE7 inhibitors have the potential to treat nicotine abuse.SIGNIFICANCE STATEMENT The World Health Organization estimates that there are 1.25 billion smokers worldwide, representing one-third of the global population over the age of 15. Nicotine-induced increase of corticomesolimbic DAergic transmission and hypodopaminergic conditions occurring during abstinence are critical for maintaining drug-use habits. Here, we demonstrate that nicotine consumption and relapse to nicotine seeking are attenuated by reequilibrating DAergic transmission through inhibition of PDE7, an intracellular enzyme responsible for the degradation of cAMP, the main second messenger modulated by DA receptor activation. PDE7 inhibition may represent a novel treatment approach to aid smoking cessation.

Alterations of Amphetamine Reward by Prior Nicotine and Alcohol Treatment: The Role of Age and Dopamine

Evidence suggests that nicotine and alcohol can each serve as a gateway drug. We determined whether prior nicotine and alcohol treatment would alter amphetamine reward. Also, we examined whether age and dopaminergic neurotransmission are important in this regard. Male and female adolescent and adult C57BL/6J mice were tested for baseline place preference. Mice then received six conditioning with saline/nicotine (0.25 mg/kg) twice daily, followed by six conditioning with saline/ethanol (2 g/kg). Control mice were conditioned with saline/saline throughout. Finally, mice were conditioned with amphetamine (3 mg/kg), once in the nicotine-alcohol-paired chamber, and tested for place preference 24 h later. The following day, mice were challenged with amphetamine (1 mg/kg) and tested for place preference under a drugged state. Mice were then immediately euthanized, their brain removed, and nucleus accumbens isolated and processed for the level of dopamine receptors and transporter and glutamate receptors. We observed a greater amphetamine-induced place preference in naïve adolescents than adult mice with no change in state-dependent place preference between the two age groups. In contrast, amphetamine induced a significant place preference in adult but not adolescent mice with prior nicotine-alcohol exposure under the drug-free state. The preference was significantly greater in adults than adolescents under the drugged state. The enhanced response was associated with higher dopamine-transporter and D₁ but reduced D₂ receptors’ expression in adult rather than adolescent mice, with no changes in glutamate receptors levels. These results suggest that prior nicotine and alcohol treatment differentially alters amphetamine reward in adult and adolescent mice. Alterations in dopaminergic neurotransmission may be involved in this phenotype.

Neurochemical and Behavioral Characterization after Acute and Repeated Exposure to Novel Synthetic Cannabinoid Agonist 5-MDMB-PICA

Since the early 2000s, herbal mixtures containing synthetic cannabinoids (SCs), broadly known as Spice/K2, have been marketed as a legal marijuana surrogate and have become very popular among adolescents. Adolescence is a critical period of development, which is associated with an increased vulnerability to the central effects of drugs. Despite growing concerns about the negative effects of the use of SCs, newly synthetized compounds are increasingly detected in drugs seized by the authorities, posing a serious threat to public health. 5F-MDMB-PICA has been recently detected and classified as a highly potent agonist of CB1 and CB2 cannabinoid receptors. Here, we first investigated the rewarding properties of 5F-MDMB-PICA in C57BL/6 adolescent and adult mice by in vivo brain microdialysis. Data showed that acute administration of a selected dose of 5F-MDMB-PICA (0.01 mg/kg i.p.) stimulates the release of dopamine in the nucleus accumbens shell of adolescent, but not of adult, mice. To further investigate the consequences of repeated exposure to this dose of 5F-MDMB-PICA, a separate group of adolescent mice was treated for 14 consecutive days and evaluated for behavioral abnormalities at adulthood, starting from 7 days after drug discontinuation. Data showed that this group of adult mice displayed an anxiety-like and compulsive-like state as revealed by an altered performance in the marble burying test. Our study suggests an alarming vulnerability of adolescent mice to the effects of 5F-MDMB-PICA. These findings provide a useful basis for understanding and evaluating both early and late detrimental effects that may derive from the use of SCs during adolescence.

Chronic exposure to cigarette smoke extract upregulates nicotinic receptor binding in adult and adolescent rats

Heavy smokers display increased radioligand binding of nicotinic acetylcholine receptors (nAChRs). This "upregulation" is thought to be a contributing factor to tobacco dependence. Although cigarette smoke contains thousands of constituents that can contribute to nicotine dependence, it is not well understood whether non-nicotine constituents contribute to nAChR upregulation. In this study, we used an aqueous cigarette smoke extract (CSE), which contains nicotine and soluble constituents of cigarette smoke, to induce nAChR upregulation in adult and adolescent rats. To do this, male rats were exposed to nicotine or CSE (1.5 mg/kg/day nicotine equivalent, intravenously) daily for ten days. This experimental procedure produces equivalent levels of brain and plasma nicotine in nicotine- and CSE-treated animals. We then assessed nAChR upregulation using quantitative autoradiography to measure changes in three nAChR types. Adolescents were found to have consistently greater α4β2 nAChR binding than adults in many brain regions. Chronic nicotine exposure did not significantly increase nAChR binding in any brain region at either age. Chronic CSE exposure selectively increased α4β2 nAChR binding in adolescent medial amygdala and α7 binding in adolescent central amygdala and lateral hypothalamus. CSE also increased α3β4 nAChR binding in the medial habenula and interpeduncular nucleus, and α7 binding in the medial amygdala, independent of age. Overall, this work provides evidence that cigarette smoke constituents influence nAChR upregulation in an age-, nAChR type- and region-dependent manner.

Molecular and neuronal mechanisms underlying the effects of adolescent nicotine exposure on anxiety and mood disorders

Tobacco addiction is highly co-morbid with a variety of mental health conditions, including schizophrenia, mood and anxiety disorders. Nicotine, the primary psychoactive compound in tobacco-related products is known to functionally modulate brain circuits that are disturbed in these disorders. Nicotine can potently regulate the transmission of various neurochemicals, including dopamine (DA), γ-amino-butyric acid (GABA) and glutamate, within various mesocorticolimbic structures, such as the ventral tegmental area (VTA), nucleus accumbens (NAc) and prefrontal cortex (PFC), all of which show pathologies in these disorders. Many neuropsychiatric diseases have etiological origins during neurodevelopment, typically occurring during vulnerable periods of adolescent or pre-natal brain development. During these neurodevelopmental periods, exposure to extrinsic drug insults can induce enduring and long-term pathophysiological sequelae that ultimately increase the risk of developing chronic mental health disorders in later life. These vulnerability factors are of growing concern given rising rates of adolescent nicotine exposure via traditional tobacco use and the increasing use of alternative nicotine delivery formats such as vaping and e-cigarettes. A large body of clinical and pre-clinical evidence points to an important role for adolescent exposure to nicotine and increased vulnerability to developing mood and anxiety disorders in later life. This review will examine current clinical and pre-clinical evidence that pinpoints specific mechanisms within the mesocorticolimbic circuitry and molecular biomarkers linked to the association between adolescent nicotine exposure and increased risk of developing mood and anxiety-related disorders. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

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.

Adolescence versus adulthood: Differences in basal mesolimbic and nigrostriatal dopamine transmission and response to drugs of abuse

Epidemiological studies have shown that people who begin experimenting drugs of abuse during adolescence are more likely to develop substance use disorders, and the earliest is the beginning of their use, the greatest is the likelihood to become dependent. Understanding the neurobiological changes increasing adolescent vulnerability to drug use is becoming imperative. Although all neurotransmitter systems undergo relevant developmental changes, dopamine system is of particular interest, given its role in a variety of functions related to reward, motivation, and decision making. Thus, in the present study, we investigated differences in mesolimbic and nigrostriatal dopamine transmission between adolescent (5, 6, 7 weeks of age) and adult rats (10-12 weeks of age), in basal conditions and following drug challenge, by using in vivo brain microdialysis. Although no significant difference between adolescents and adults was observed in dopamine basal levels in the nucleus accumbens (NAc)shell and core, reduced DA levels were found in the dorsolateral striatum (DLS) of early and mid-adolescent rats. Adolescent rats showed greater increase of dopamine in the NAc shell following nicotine (0.4 mg/kg), THC (1.0 mg/kg), and morphine (1.0 mg/kg), in the NAc core following nicotine and morphine, and in the DLS following THC, morphine, and cocaine (10 mg/kg). These results, while adding new insight in the development and functionality of the dopamine system during different stages of adolescence, might provide a neurochemical basis for the greater vulnerability of adolescents to drugs of abuse and for the postulated gateway effect of nicotine and THC toward abuse of other illicit substances.

Neurobiological Considerations for Tobacco Use Disorder

PURPOSE OF REVIEW

Neurobiological studies of tobacco/nicotine use examining genetic, molecular, functional, and behavioral correlates have improved our understanding of nicotine/tobacco dependence and have informed treatment. Recent work extending previously established findings and reporting novel methodologies and discoveries in preclinical and human studies are reviewed.

RECENT FINDINGS

Recent work in preclinical models has focused on the differential roles of nicotinic receptor subtypes and nicotine's effects on neural systems beyond cortico-striatal dopaminergic pathways, and utilizing advanced methodologies such as pharmacogenetics, optogenetics and rodent fMRI to identify targets for treatment. Likewise, human neuroimaging studies have identified molecular and functional dynamic shifts associated with tobacco/nicotine use that further inform treatment.

SUMMARY

Nicotine/tobacco use is associated with widespread neural adaptations that are persistent and function to maintain addiction. The continued identification of genetic, molecular, neural, and behavioral endophenotypes related to nicotine/tobacco use, dependence, and addiction will facilitate the development and delivery of personalized treatment.

Amino acid modulation of dopamine in the nucleus accumbens mediates sex differences in nicotine withdrawal

The aversive effect of nicotine withdrawal is greater in female versus male rats, and we postulate that this sex difference is mediated in the nucleus accumbens (NAc). Nicotine withdrawal induces decreases in NAc dopamine and increases in acetylcholine (ACh) levels in male rats. To our knowledge, these neurochemical markers of nicotine withdrawal have not been compared in female versus male rats. Given the role of amino acids in modulating NAc dopaminergic and cholinergic transmission, concomitant measures of gamma-aminobutyric acid (GABA) and glutamate levels were also compared across sex. Rats received continuous nicotine exposure for 14 days, and then NAc dialysate was collected during baseline and following administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal. Chronic nicotine exposure was associated with larger increases in baseline dopamine, GABA and glutamate levels in the NAc of female versus male rats, whereas baseline ACh was only increased in male rats. During withdrawal, both sexes displayed equivalent increases in NAc ACh levels. As expected, male rats displayed decreases in dopamine, coupled with increases in GABA and decreases in glutamate levels, suggesting the possibility of increased inhibitory tone in the NAc during withdrawal. Relative to males, female rats displayed larger decreases in NAc dopamine and related increases in GABAergic transmission. As female rats also showed elevated glutamate levels that persist during withdrawal, it is suggested that sex differences may arise from increased glutamatergic drive of inhibitory tone in the NAc. The findings provide a potential mechanism whereby the aversive effects of nicotine withdrawal are enhanced in female rats.

Brain substrates of early (4h) cigarette abstinence: Identification of treatment targets

INTRODUCTION

Research indicates that overnight nicotine abstinence disrupts neural activity in the mesocorticolimbic reward network; however, less is known about the time course of abstinence-induced brain changes. To examine the potential neural effects of early abstinence, we used arterial spin labeling perfusion fMRI, to measure regional cerebral blood flow (rCBF) changes in the resting brain induced by 4h of nicotine abstinence.

METHODS

In a repeated measures design, 5min of resting perfusion fMRI data were acquired in awake nicotine-dependent individuals (eyes open) during 'smoking as usual' (SMK) and following 4h of monitored nicotine abstinence (ABS) conditions (N=20). Conditions were compared using a paired t test in SPM8. Craving was assessed prior to each condition.

RESULTS

Compared to SMK, ABS significantly increased craving and reduced rCBF in select regions, including the hippocampus and ventral striatum (cluster corr, α=0.01, 943 contiguous voxels). The magnitude of the abstinence-induced change in rCBF correlated with the magnitude of the change in craving across conditions in select regions, including the medial and lateral orbitofrontal cortices and the anterior ventral insula (r values ranging from 0.59-0.74).

CONCLUSIONS

Results show that as few as 4h of abstinence can reduce resting rCBF in multiple nodes of the brain's mesocorticolimbic network, disrupting neural processing. Identifying early withdrawal treatment targets has far-reaching implications, which include thwarting relapse proclivities. Results parallel those of the extant human literature and are in agreement with an extensive preclinical literature showing compromised mesolimbic dopaminergic function and impairments in reward function during nicotine withdrawal.

Nicotine pretreatment reduced cocaine-induced CPP and its reinstatement in a sex- and dose-related manner in adult C57BL/6J mice

Previous preclinical studies have shown that nicotine pretreatment during adolescence increases the reinforcing actions of cocaine. However, little is known about the effect of prior nicotine administration on cocaine-induced conditioned place preference (CPP) and its reinstatement in adult mice. Besides, little information is available regarding the role of sex in this cross-talk between nicotine and cocaine. Thus, we examined if nicotine administration during adulthood would differentially alter cocaine-induced CPP, its extinction and reinstatement in male versus female mice and if the dose of nicotine was important in this regard. To this end, mice were pretreated with saline or nicotine (0.25 or 1mg/kg; twice daily for seven days) and then tested for place preference before and after single and repeated conditioning with cocaine (15mg/kg). Mice were then exposed to extinction training and tested for reinstatement of CPP. Our results showed that male and female mice pretreated with saline and conditioned with cocaine each exhibited a robust CPP after a single cocaine conditioning. However, this response was blunted in mice pretreated with the lower but not higher dose of nicotine. Female mice pretreated with the lower dose nicotine also failed to show CPP after repeated conditioning. Reinstatement of cocaine-induced CPP was also blunted in these mice compared to their respective controls. Together, these results suggest that nicotine administration during adulthood exerts differential effects on cocaine-induced CPP and its reinstatement in male and female mice and the dose of nicotine is important in this regard.

Both nicotine reward and withdrawal are enhanced in a rodent model of diabetes

RATIONALE

It is presently unclear whether diabetic rats experience greater rewarding effects of nicotine and/or negative affective states produced by nicotine withdrawal.

OBJECTIVE

The present study utilized a rodent model of diabetes to examine the rewarding effects of nicotine and negative affective states and physical signs produced by withdrawal.

METHODS

Separate groups of rats received systemic administration of either vehicle or streptozotocin (STZ), which destroys insulin-producing beta cells in the pancreas and elevates glucose levels. Place conditioning procedures were utilized to compare the rewarding effects of nicotine (conditioned place preference; CPP) and negative affective states produced by withdrawal (conditioned place aversion; CPA) in vehicle- and STZ-treated rats. CPA and physical signs of withdrawal were compared after administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal in nicotine-dependent rats. A subsequent study utilized elevated plus maze (EPM) procedures to compare anxiety-like behavior produced by nicotine withdrawal in vehicle- and STZ-treated rats.

RESULTS

STZ-treated rats displayed greater rewarding effects of nicotine and a larger magnitude of aversive effects and physical signs produced by withdrawal as compared to vehicle-treated controls. STZ-treated rats also displayed higher levels of anxiety-like behavior on the EPM during nicotine withdrawal as compared to controls.

CONCLUSION

The finding that both nicotine reward and withdrawal are enhanced in a rodent model of diabetes implies that the strong behavioral effects of nicotine promote tobacco use in persons with metabolic disorders, such as diabetes.

Nicotine Increases Alcohol Intake in Adolescent Male Rats

Background: Use of alcohol and tobacco, the two most concurrently abused drugs, typically first occurs during adolescence. Yet, there have been no systematic analyses of ethanol (EtOH) and nicotine (Nic) interactions during adolescence. Recent animal studies report that kappa-opioid (KOR) receptor activation mediates age differences in drug reinforcement. Our hypothesis is that concurrent self-administration of EtOH and Nic will be greater in adolescent rats because of age differences in KOR function. Furthermore, exposure to alcohol and nicotine during adolescence has been reported to increase EtOH intake in adulthood. We performed a longitudinal animal study and hypothesized adolescent rats allowed to self-administer nicotine would drink more alcohol as adults. Methods: Adolescent, postnatal day (P)32, and adult (P90) male and female Sprague-Dawley rats were allowed to self-administer EtOH, Nic, or a combination of both, EtOH+Nic, in an intravenous self-administration paradigm. The role of KOR was pharmacologically evaluated with the KOR antagonist, norbinaltorphamine (norBNI) and with the KOR agonist, U50,488H. Alcohol drinking was subsequently evaluated with male rats in a drinking in the dark (DID), 2-bottle choice test. Results: Concurrent Nic increased EtOH intake in adolescent males, but not in adults or females. Pharmacological blockade of KOR with norBNI robustly increased EtOH+Nic self-administration in adult male rats, but had no effect with female rats. Lastly, in our longitudinal study with male rats, we found prior self-administration of Nic or EtOH+Nic during adolescence increased subsequent oral EtOH intake, whereas prior self-administration of EtOH alone in adults increased subsequent EtOH drinking. Conclusions: There are major age- and sex-differences in the reinforcing effects of EtOH+Nic. Adolescent males are sensitive to the reinforcing interactions of the two drugs, whereas this effect is inhibited by KOR activation in male adults. Nicotine self-administration in adolescent males also increased subsequent oral EtOH intake. These findings suggest that brain mechanisms underlying the reinforcing effects of EtOH and nicotine are both age- and sex-dependent, and that tobacco or e-cigarette use may increase the vulnerability of teenage boys to alcohol abuse.

Nicotinic receptor blockade decreases fos immunoreactivity within orexin/hypocretin-expressing neurons of nicotine-exposed rats

Tobacco smoking is the leading cause of preventable death in the United States. Nicotine is the principal psychoactive ingredient in tobacco that causes addiction. The structures governing nicotine addiction, including those underlying withdrawal, are still being explored. Nicotine withdrawal is characterized by negative affective and cognitive symptoms that enhance relapse susceptibility, and suppressed dopaminergic transmission from ventral tegmental area (VTA) to target structures underlies behavioral symptoms of nicotine withdrawal. Agonist and partial agonist therapies help 1 in 4 treatment-seeking smokers at one-year post-cessation, and new targets are needed to more effectively aid smokers attempting to quit. Hypothalamic orexin/hypocretin neurons send excitatory projections to dopamine (DA)-producing neurons of VTA and modulate mesoaccumbal DA release. The effects of nicotinic receptor blockade, which is commonly used to precipitate withdrawal, on orexin neurons remain poorly investigated and present an attractive target for intervention. The present study sought to investigate the effects of nicotinic receptor blockade on hypothalamic orexin neurons using mecamylamine to precipitate withdrawal in rats. Separate groups of rats were treated with either chronic nicotine or saline for 7-days at which point effects of mecamylamine or saline on somatic signs and anxiety-like behavior were assessed. Finally, tissue from rats was harvested for immunofluorescent analysis of Fos within orexin neurons. Results demonstrate that nicotinic receptor blockade leads to reduced orexin cell activity, as indicated by lowered Fos-immunoreactivity, and suggest that this underlying cellular activity may be associated with symptoms of nicotine withdrawal as effects were most prominently observed in rats given chronic nicotine. We conclude from this study that orexin transmission becomes suppressed in rats upon nicotinic receptor blockade, and that behavioral symptoms associated with nicotine withdrawal may be aided by intervention upon orexinergic transmission.

Effects of Smoking Cessation on Presynaptic Dopamine Function of Addicted Male Smokers

BACKGROUND

There is evidence of abnormal cerebral dopamine transmission in nicotine-dependent smokers, but it is unclear whether dopaminergic abnormalities are due to acute nicotine abuse or whether they persist with abstinence. We addressed this question by conducting longitudinal positron emission tomography (PET) examination of smokers before and after 3 months of abstinence.

METHODS

We obtained baseline 6-[(18)F]fluoro-L-DOPA (FDOPA)-PET scans in 15 nonsmokers and 30 nicotine-dependent smokers, who either smoked as per their usual habit or were in acute withdrawal. All smokers then underwent cessation treatment, and successful abstainers were re-examined by FDOPA-PET after 3 months of abstinence (n = 15). Uptake of FDOPA was analyzed using a steady-state model yielding estimates of the dopamine synthesis capacity (K); the turnover of tracer dopamine formed in living brain (kloss); and the tracer distribution volume (Vd), which is an index of dopamine storage capacity.

RESULTS

Compared with nonsmokers, K was 15% to 20% lower in the caudate nuclei of consuming smokers. Intraindividual comparisons of consumption and long-term abstinence revealed significant increases in K in the right dorsal and left ventral caudate nuclei. Relative to acute withdrawal, Vd significantly decreased in the right ventral and dorsal caudate after prolonged abstinence. Severity of nicotine dependence significantly correlated with dopamine synthesis capacity and dopamine turnover in the bilateral ventral putamen of consuming smokers.

CONCLUSIONS

The results suggest a lower dopamine synthesis capacity in nicotine-dependent smokers that appears to normalize with abstinence. Further investigations are needed to clarify the role of dopamine in nicotine addiction to help develop smoking prevention and cessation treatments.

Neuroscience of nicotine for addiction medicine: novel targets for smoking cessation medications

Morbidity and mortality associated with tobacco smoking constitutes a significant burden on healthcare budgets all over the world. Therefore, promoting smoking cessation is an important goal of health professionals and policy makers throughout the world. Nicotine is a major psychoactive component in tobacco that is largely responsible for the widespread addiction to tobacco. A majority of the currently available FDA-approved smoking cessation medications act via neuronal nicotinic receptors. These medications are effective in approximately half of all the smokers, who want to quit and relapse among abstinent smokers continues to be high. In addition to relapse among abstinent smokers, unpleasant effects associated with nicotine withdrawal are a major motivational factor in continued tobacco smoking. Over the last two decades, animal studies have helped in identifying several neural substrates that are involved in nicotine-dependent behaviors including those associated with nicotine withdrawal and relapse to tobacco smoking. In this review, first the role of specific brain regions/circuits that are involved in nicotine dependence will be discussed. Next, the review will describe the role of specific nicotinic receptor subunits in nicotine dependence. Finally, the review will discuss the role of classical neurotransmitters (dopamine, serotonin, noradrenaline, glutamate, and γ-aminobutyric acid) as well as endogenous opioid and endocannabinoid signaling in nicotine dependence. The nicotinic and nonnicotinic neural substrates involved in nicotine-dependent behaviors can serve as possible targets for future smoking cessation medications.

Nicotine withdrawal increases stress-associated genes in the nucleus accumbens of female rats in a hormone-dependent manner

INTRODUCTION

Previous work led to our hypothesis that sex differences produced by nicotine withdrawal are modulated by stress and dopamine systems in the nucleus accumbens (NAcc). We investigated our hypothesis by studying intact females to determine whether the mechanisms that promote withdrawal are ovarian-hormone mediated.

METHODS

Female rats were ovariectomized (OVX) or received sham surgery (intact) on postnatal day (PND 45-46). On PND 60, they received sham surgery (controls) or were prepared with nicotine pumps. Fourteen days later, half of the rats had their pumps removed (nicotine withdrawal) and the other half received sham surgery (nicotine exposure). Twenty-four hours later, the rats were tested for anxiety-like behavior using the elevated plus maze and light/dark transfer procedures. The NAcc was then dissected for analysis of several genes related to stress (CRF, UCN, CRF-R1, CRF-R2, CRF-BP, and Arrb2) or receptors for dopamine (Drd1 and Drd2) and estradiol (Esr2).

RESULTS

During withdrawal, intact females displayed an increase in anxiety-like behavior in both tests and CRF, UCN, and Drd1 gene expression. During nicotine exposure, intact females displayed a decrease in CRF-R1, CRF-R2, Drd3, and Esr2 gene expression and an increase in CRF-BP. This pattern of results was absent in OVX females.

CONCLUSIONS

Nicotine withdrawal produced an increase in anxiety-like behavior and stress-associated genes in intact females that is distinct from changes produced by nicotine exposure. The latter effects were absent in OVX females, suggesting that stress produced by withdrawal is ovarian-hormone mediated. These findings have important implications towards understanding tobacco use liability among females.

Reduced dopamine release in the nucleus accumbens core of adult rats following adolescent binge alcohol exposure: age and dose-dependent analysis

RATIONALE

Alcohol use in adolescence is one of the most significant predictors of alcohol dependence in adulthood, yet the neurochemical mechanisms underlying this heightened vulnerability remain unknown. Whereas research has focused on characterizing adaptations in the mesolimbic dopamine (DA) system following ethanol exposure in adolescence, whether these changes persist into adulthood has yet to be determined.

OBJECTIVES

The objective of this study is to investigate the effects of binge-intermittent ethanol administration in adolescence (P30-50) or early adulthood (P60-80) on DA in the nucleus accumbens (NAc) core after an ethanol challenge in adulthood following a period of abstinence.

METHODS

Male Sprague Dawley rats (n = 160) were administered intermittent ethanol injections, 1 or 3 g/kg, intraperitoneally (i.p.) every other day for 20 days starting on either P30 or 60. Following an ethanol-free period of either 7, 14, or 28 days, we measured DA efflux following an ethanol challenge (3 g/kg, i.p.) using electrochemical recording electrodes bilaterally implanted into the NAc core.

RESULTS

Moderate-dose ethanol administration (1 g/kg, i.p.) during adolescence significantly decreased ethanol-evoked DA release in adulthood at 7 and 14 days, but not 28 days, following pretreatment exposure compared to saline controls. Relative to rats pretreated with ethanol in adulthood, moderate-dose ethanol in adolescence significantly reduced DA efflux at all time points measured. Additionally, adult rats pretreated with high dose ethanol administration (3 g/kg, i.p.) displayed significantly decreased DA compared to adolescents after 28 days of withdrawal.

CONCLUSIONS

Binge-intermittent ethanol administration during adolescence may induce age-dependent neuroadaptations in the mesolimbic DA system compared to ethanol-treated adults during protracted ethanol withdrawal.

Varenicline enhances dopamine release facilitation more than nicotine after long-term nicotine treatment and withdrawal

An important factor contributing to the high relapse rates among smokers is nicotine withdrawal symptoms. Multiple studies suggest that decreased dopamine release in nucleus accumbens plays a key role in withdrawal. However, recent reports showed that long-term nicotine exposure itself also decreases accumbal dopamine release, suggesting that additional mechanisms are involved in withdrawal. Here, we used real-time cyclic voltammetry in brain slices containing the nucleus accumbens to further elucidate the changes in dopamine release linked to nicotine withdrawal. Rats received vehicle or nicotine via the drinking water for 2-3 months. Studies assessing the expression of somatic signs in vehicle-treated, nicotine-treated, and 24-h nicotine withdrawn rats showed that nicotine withdrawal led to a significant increase in somatic signs. Subsequent voltammetry studies showed that long-term nicotine decreased single-pulse-stimulated dopamine release via an interaction at α6β2* receptors. Nicotine withdrawal led to a partial recovery in α6β2* receptor-mediated release. In addition, long-term nicotine treatment alone increased dopamine release paired-pulse ratios and this was partially reversed with nicotine removal. We then evaluated the effect of bath-applied nicotine and varenicline on dopamine release. Nicotine and varenicline both decreased single-pulse-stimulated release in vehicle-treated, nicotine-treated, and nicotine withdrawn rats. However, bath-applied varenicline increased paired-pulse ratios to a greater extent than nicotine during long-term nicotine treatment and after its withdrawal. Altogether these data suggest that nicotine withdrawal is associated with a partial restoration of dopamine release measures to control levels and that varenicline's differential modulation of dopamine release may contribute to its mechanism of action.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Attenuation by baclofen of nicotine rewarding properties and nicotine withdrawal manifestations

RATIONALE

Nicotine is a major active ingredient in tobacco and plays a major role in tobacco addiction. In rodents, repeated nicotine administration produces behavioral responses related to its addictive properties, such as reinforcing effects and physical dependence.

OBJECTIVES

The aim of the present study was to evaluate the possible role of GABAB receptor in responses induced by repeated nicotine administration in Swiss Webster mice.

RESULTS

Nicotine hydrogen tartrate salt (0.5 mg/kg, s.c.) administration induced rewarding properties in the conditioning place preference test. The GABAB receptor agonist, baclofen (3 mg/kg, i.p.) abolished the rewarding properties induced by nicotine hydrogen tartrate salt (0.5 mg/kg, s.c.). In addition, naloxone-precipitated nicotine withdrawal induced somatic manifestations, anxiety-like effects in the elevated plus maze test and dysphoric manifestations in the conditioned place aversion paradigm. Baclofen (2 and 3 mg/kg, i.p.) prevented the somatic manifestations and the anxiety-like effects associated with naloxone-precipitated nicotine withdrawal but not the dysphoric manifestations.

CONCLUSIONS

These results showed that nicotine rewarding properties and negative aspects of nicotine withdrawal, such as anxiety-like effects and somatic manifestations, can be modulated by the GABAB receptor activity. This study now reveals a novel possible application of baclofen to develop new therapeutic strategies to achieve smoking cessation.

Cholinergic transmission during nicotine withdrawal is influenced by age and pre-exposure to nicotine: implications for teenage smoking

Adolescence is a unique period of development characterized by enhanced tobacco use and long-term vulnerability to neurochemical changes produced by adolescent nicotine exposure. In order to understand the underlying mechanisms that contribute to developmental differences in tobacco use, this study compared changes in cholinergic transmission during nicotine exposure and withdrawal in naïve adult rats compared to (1) adolescent rats and (2) adult rats that were pre-exposed to nicotine during adolescence. The first study compared extracellular levels of acetylcholine (ACh) in the nucleus accumbens (NAc) during nicotine exposure and precipitated withdrawal using microdialysis procedures. Adolescent (postnatal day, PND, 28-42) and adult rats (PND60-74) were prepared with osmotic pumps that delivered nicotine for 14 days (adolescents 4.7 mg/kg/day; adults 3.2 mg/kg/day; expressed as base). Another group of adults was exposed to nicotine during adolescence and then again in adulthood (pre-exposed adults) using similar methods. Control rats received a sham surgery. Following 13 days of nicotine exposure, the rats were implanted with microdialysis probes in the NAc. The following day, dialysis samples were collected during baseline and following systemic administration of the nicotinic receptor antagonist mecamylamine (1.5 and 3.0 mg/kg, i.p.) to precipitate withdrawal. A second study compared various metabolic differences in cholinergic transmission using the same treatment procedures as the first study. Following 14 days of nicotine exposure, the NAc was dissected and acetylcholinesterase (AChE) activity was compared across groups. In order to examine potential group differences in nicotine metabolism, blood plasma levels of cotinine (a nicotine metabolite) were also compared following 14 days of nicotine exposure. The results from the first study revealed that nicotine exposure increased baseline ACh levels to a greater extent in adolescent versus adult rats. During nicotine withdrawal, ACh levels in the NAc were increased in a similar manner in adolescent versus adult rats. However, the increase in ACh that was observed in adult rats experiencing nicotine withdrawal was blunted in pre-exposed adults. These neurochemical effects do not appear to be related to nicotine metabolism, as plasma cotinine levels were similar across all groups. The second study revealed that nicotine exposure increased AChE activity in the NAc to a greater extent in adolescent versus adult rats. There was no difference in AChE activity in pre-exposed versus naïve adult rats. In conclusion, our results suggest that nicotine exposure during adolescence enhances baseline ACh in the NAc. However, the finding that ACh levels were similar during withdrawal in adolescent and adult rats suggests that the enhanced vulnerability to tobacco use during adolescence is not related to age differences in withdrawal-induced increases in cholinergic transmission. Our results also suggest that exposure to nicotine during adolescence suppresses withdrawal-induced increases in cholinergic responses during withdrawal. Taken together, this report illustrates important short- and long-term changes within cholinergic systems that may contribute to the enhanced susceptibility to tobacco use during adolescence.

Addicted to palatable foods: comparing the neurobiology of Bulimia Nervosa to that of drug addiction

RATIONALE

Bulimia nervosa (BN) is highly comorbid with substance abuse and shares common phenotypic and genetic predispositions with drug addiction. Although treatments for the two disorders are similar, controversy remains about whether BN should be classified as addiction.

OBJECTIVES

Here, we review the animal and human literature with the goal of assessing whether BN and drug addiction share a common neurobiology.

RESULTS

Similar neurobiological features are present following administration of drugs and bingeing on palatable food, especially sugar. Specifically, both disorders involve increases in extracellular dopamine (DA), D1 binding, D3 messenger RNA (mRNA), and ΔFosB in the nucleus accumbens (NAc). Animal models of BN reveal increases in ventral tegmental area (VTA) DA and enzymes involved in DA synthesis that resemble changes observed after exposure to addictive drugs. Additionally, alterations in the expression of glutamate receptors and prefrontal cortex activity present in human BN or following sugar bingeing in animals are comparable to the effects of addictive drugs. The two disorders differ in regards to alterations in NAc D2 binding, VTA DAT mRNA expression, and the efficacy of drugs targeting glutamate to treat these disorders.

CONCLUSIONS

Although additional empirical studies are necessary, the synthesis of the two bodies of research presented here suggests that BN shares many neurobiological features with drug addiction. While few Food and Drug Administration-approved options currently exist for the treatment of drug addiction, pharmacotherapies developed in the future, which target the glutamate, DA, and opioid systems, may be beneficial for the treatment of both BN and drug addiction.

Stress in adolescence and drugs of abuse in rodent models: role of dopamine, CRF, and HPA axis

RATIONALE

Research on adolescence and drug abuse increased substantially in the past decade. However, drug-addiction-related behaviors following stressful experiences during adolescence are less studied. We focus on rodent models of adolescent stress cross-sensitization to drugs of abuse.

OBJECTIVES

Review the ontogeny of behavior, dopamine, corticotropin-releasing factor (CRF), and the hypothalamic-pituitary-adrenal (HPA) axis in adolescent rodents. We evaluate evidence that stressful experiences during adolescence engender hypersensitivity to drugs of abuse and offer potential neural mechanisms.

RESULTS AND CONCLUSIONS

Much evidence suggests that final maturation of behavior, dopamine systems, and HPA axis occurs during adolescence. Stress during adolescence increases amphetamine- and ethanol-stimulated locomotion, preference, and self-administration under many conditions. The influence of adolescent stress on subsequent cocaine- and nicotine-stimulated locomotion and preference is less clear. The type of adolescent stress, temporal interval between stress and testing, species, sex, and the drug tested are key methodological determinants for successful cross-sensitization procedures. The sensitization of the mesolimbic dopamine system is proposed to underlie stress cross-sensitization to drugs of abuse in both adolescents and adults through modulation by CRF. Reduced levels of mesocortical dopamine appear to be a unique consequence of social stress during adolescence. Adolescent stress may reduce the final maturation of cortical dopamine through D2 dopamine receptor regulation of dopamine synthesis or glucocorticoid-facilitated pruning of cortical dopamine fibers. Certain rodent models of adolescent adversity are useful for determining neural mechanisms underlying the cross-sensitization to drugs of abuse.

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the β subunit of cholera toxin (Ctβ) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8-9-week-old) rats. After intra-VTA injections of Ctβ, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA-projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA-projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age-dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities.

Sex differences in the effect of wheel running on subsequent nicotine-seeking in a rat adolescent-onset self-administration model

RATIONALE

Wheel running attenuates nicotine-seeking in male adolescent rats; however, it is not known if this effect extends to females.

OBJECTIVE

To determine if wheel running during abstinence would differentially attenuate subsequent nicotine-seeking in male and female rats that had extended access to nicotine self-administration during adolescence.

METHODS

Male (n = 49) and female (n = 43) adolescent rats self-administered saline or nicotine (5 μg/kg) under an extended access (23-h) paradigm. Following the last self-administration session, rats were moved to polycarbonate cages for an abstinence period where they either had access to a locked or unlocked running wheel for 2 h/day. Subsequently, nicotine-seeking was examined under a within-session extinction/cue-induced reinstatement paradigm. Due to low levels of nicotine-seeking in females in both wheel groups, additional groups were included that were housed without access to a running wheel during abstinence.

RESULTS

Females self-administered more nicotine as compared to males; however, within males and females, intake did not differ between groups prior to wheel assignment. Compared to saline controls, males and females that self-administered nicotine showed a significant increase in drug-seeking during extinction. Wheel running during abstinence attenuated nicotine-seeking during extinction in males. In females, access to either locked or unlocked wheels attenuated nicotine-seeking during extinction. While responding was reinstated by cues in both males and females, levels were modest and not significantly affected by exercise in this adolescent-onset model.

CONCLUSIONS

While wheel running reduced subsequent nicotine-seeking in males, access to a wheel, either locked or unlocked, was sufficient to suppress nicotine-seeking in females.

Neuropsychiatric events with varenicline: a modified prescription-event monitoring study in general practice in England

BACKGROUND

Varenicline (Champix(®)), launched in the UK in December 2006, is indicated for the treatment of smoking cessation in adults (≥18 years of age). In 2008, the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK issued a warning suggesting that varenicline was associated with disparate neuropsychiatric symptoms, including depression, suicidal thoughts and behaviour. In response to this regulatory warning, the Drug Safety Research Unit conducted a modified prescription-event monitoring (M-PEM) study to monitor the safety of varenicline.

OBJECTIVE

The aim of this study was to estimate the incidence and examine the pattern of neuropsychiatric events reported to general practitioners (GPs) in England during the immediate postmarketing period for varenicline.

METHODS

A postmarketing surveillance study was conducted using the observational cohort technique of M-PEM. Patients were identified from dispensed prescriptions issued by primary care physicians between December 2006 and March 2007. Data on exposure, previous history of psychiatric illness and events reported during and after treatment were collected from questionnaires. In order to determine whether hazards for neuropsychiatric events of interest (depression, anxiety, aggression, suicidal ideation, non-fatal self-harm) were non-constant over time (which could indicate a possible association with the drug), the pattern of events was examined by plotting the smoothed hazard function estimate and then fitting a Weibull model. The Weibull model shape parameter (β) and 95 % confidence interval were used as a test for a non-constant hazard function (where a value of 1 indicates a constant hazard over time). In addition to this analysis, the difference in incidence densities (IDs) between month 1 and months 2-3 were calculated and compared.

RESULTS

The cohort comprised of 12,159 patients (median age 47 years [interquartile range 19]; 56.9 % [n = 6924 female]). The number of events reported during treatment, reason for stopping, adverse drug reactions (ADRs), and the p-value for the Weibull shape parameter were as follows: depression (n = 94; 42; 19; p = 0.144); anxiety (n = 94; 49; 9; p = 0.009); aggression (n = 7; 4; 2; p = 0.465); suicidal ideation (n = 8; 4; 1; p = 0.989) and non-fatal self-harm (n = 5; 1; 0; p = 0.771). No differences in the IDs between months 1 and months 2-3 were found for any of the events.

CONCLUSION

Whilst between 7 and 17 % of neuropsychiatric events were attributed to the drug by GPs and approximately 20-50 % were given as reasons for stopping, no signal was raised using the ID differences approach, and only anxiety was flagged as a potential signal for an ADR using the Weibull model. The signal for anxiety requires further evaluation to determine whether the drug plays a part in the development of anxiety or whether it is a withdrawal symptom caused by smoking cessation. Analysis methods will lack power when the numbers of events are low even when a large number of participants are included in the study.

Adolescence is a period of development characterized by short- and long-term vulnerability to the rewarding effects of nicotine and reduced sensitivity to the anorectic effects of this drug

This study compared nicotine intake and changes in food intake and weight gain in naïve adolescent, naïve adult, and adult rats that were exposed to nicotine during adolescence. An extended intravenous self-administration (IVSA) model was used whereby rats had 23-hour access to saline or increasing doses of nicotine (0.03, 0.06, and 0.09 mg/kg/0.1 mL infusion) for 4-day intervals separated by 3-day periods of abstinence. Rats began IVSA as adolescents (PND 32-34) or adults (PND 75). A separate group of rats was exposed to nicotine via osmotic pumps (4.7 mg/kg) for 14 days during adolescence and then began nicotine IVSA as adults (PND 75). The rats that completed the nicotine IVSA regimen were also tested for nicotine-seeking behavior during extinction. The results revealed that nicotine intake was highest in adolescents followed by adults that were pre-exposed to nicotine during adolescence as compared to naïve adults. A similar pattern of nicotine-seeking behavior was observed during extinction. In contrast to nicotine intake, naïve adults displayed robust appetite and weight suppressant effects of nicotine, an effect that was absent in adolescents and adults that were pre-exposed to nicotine during adolescence. Our findings suggest that adolescence is a unique period of enhanced vulnerability to the reinforcing effects of nicotine. Although adolescents gain weight faster than adults, the food intake and weight suppressant effects of nicotine are reduced during adolescence. Importantly, our findings suggest that adolescent nicotine exposure produces long-lasting consequences that enhance nicotine reward and promote tolerance to the anorectic effects of this drug.

Olfactory bulb monoamine concentrations vary with time of day

The olfactory bulb (OB) has been recently identified as a circadian oscillator capable of operating independently of the master circadian pacemaker, the suprachiasmatic nuclei of the hypothalamus. OB oscillations manifest as rhythms in clock genes, electrical activity, and odor sensitivity. Dopamine, norepinephrine, and serotonin have been shown to modulate olfactory information processing by the OB and may be part of the mechanism that underlies diurnal changes in olfactory sensitivity. Rhythmic release of these neurotransmitters could generate OB rhythms in electrical activity and olfactory sensitivity. We hypothesized that these monoamines were rhythmically released in the OB. To test our hypotheses, we examined monoamine levels in the OB, over the course of a day, by high-performance liquid chromatography coupled to electrochemical detection. We observed that dopamine and its metabolite, 3-4-dihydroxyphenylacetic acid, rhythmically fluctuate over the day. In contrast, norepinephrine is arrhythmic. Serotonin and its metabolite hydroxyindoleacetic acid appear to rhythmically fluctuate. Each of these monoamines has been shown to alter OB circuit behavior and influence odor processing. Rhythmic release of serotonin may be a mechanism by which the suprachiasmatic nuclei communicate, indirectly, with the OB.

The effects of abused drugs on adolescent development of corticolimbic circuitry and behavior

Adolescence is a period of significant neurobiological change that occurs as individuals transition from childhood to adulthood. Because the nervous system is in a relatively labile state during this stage of development, it may be especially sensitive to experience-induced plasticity. One such experience that is relatively common to adolescents is the exposure to drugs of abuse, particularly alcohol and psychostimulants. In this review, we highlight recent findings on the long-lasting effects of exposure to these drugs during adolescence in humans as well as in animal models. Whenever possible, our focus is on studies that use comparison groups of adolescent- and adult-exposed subjects as this is a more direct test of the hypothesis that adolescence represents a period of enhanced vulnerability to the effects of drug-induced plasticity. Lastly, we suggest areas of future investigation that are needed and methodological concerns that should be addressed.

A mechanistic hypothesis of the factors that enhance vulnerability to nicotine use in females

Women are particularly more vulnerable to tobacco use than men. This review proposes a unifying hypothesis that females experience greater rewarding effects of nicotine and more intense stress produced by withdrawal than males. We also provide a neural framework whereby estrogen promotes greater rewarding effects of nicotine in females via enhanced dopamine release in the nucleus accumbens (NAcc). During withdrawal, we suggest that corticotropin-releasing factor (CRF) stress systems are sensitized and promote a greater suppression of dopamine release in the NAcc of females versus males. Taken together, females display enhanced nicotine reward via estrogen and amplified effects of withdrawal via stress systems. Although this framework focuses on sex differences in adult rats, it is also applied to adolescent females who display enhanced rewarding effects of nicotine, but reduced effects of withdrawal from this drug. Since females experience strong rewarding effects of nicotine, a clinical implication of our hypothesis is that specific strategies to prevent smoking initiation among females are critical. Also, anxiolytic medications may be more effective in females that experience intense stress during withdrawal. Furthermore, medications that target withdrawal should not be applied in a unilateral manner across age and sex, given that nicotine withdrawal is lower during adolescence. This review highlights key factors that promote nicotine use in females, and future studies on sex-dependent interactions of stress and reward systems are needed to test our mechanistic hypotheses. Future studies in this area will have important translational value toward reducing health disparities produced by nicotine use in females. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Attenuated effects of experimenter-administered heroin in adolescent vs. adult male rats: physical withdrawal and locomotor sensitization

OBJECTIVES

Early onset of heroin use during adolescence might increase chances of later drug addiction. Prior work from our laboratory suggests, however, that adolescent male rats are actually less sensitive than adults to some enduring effects of heroin self-administration. In the present study, we tested two likely correlates of sensitivity to behavioral reinforcement in rats: physical withdrawal and locomotor sensitization.

METHODS

Adolescent (35 days old at start) and adult (79 days old) male Sprague-Dawley rats were administered escalating doses of heroin, increasing from 1.0 to 8.0 mg/kg (i.p.) every 12 h, across 13 days. Somatic signs of spontaneous withdrawal were scored 12 and 24 h after the last injection, and then every 24 h for 5 days; locomotion was recorded concurrently. Challenge injections of heroin (1 mg/kg i.p.) were given at four points: as the first of the escalating doses (day 1), at days 7 and 13 during the escalating regimen, and after 12 days of forced abstinence. Body mass and food intake were measured throughout experimentation.

RESULTS

A heroin withdrawal syndrome was not observed among adolescents as it was among adults, including somatic signs as well as reduced locomotion, body mass, and food intake. On the other hand, heroin-induced locomotor sensitization did not differ across ages.

CONCLUSION

Reduced withdrawal is consistent with the attenuated reinforcing effects of heroin among adolescent male rats that we reported previously. Thus, it is possible that adolescent rats could reveal important neuroprotective factors for use in treatment of heroin dependence.

Dysregulation of kappa-opioid receptor systems by chronic nicotine modulate the nicotine withdrawal syndrome in an age-dependent manner

RATIONALE

Mechanisms that mediate age differences during nicotine withdrawal are unclear.

OBJECTIVE

This study compared kappa-opioid receptor (KOR) activation in naïve and nicotine-treated adolescent and adult rats using behavioral and neurochemical approaches to study withdrawal.

METHODS

The behavioral models used to assess withdrawal included conditioned place and elevated plus maze procedures. Deficits in dopamine transmission in the nucleus accumbens (NAcc) were examined using microdialysis procedures. Lastly, the effects of KOR stimulation and blockade on physical signs produced upon removal of nicotine were examined in adults.

RESULTS

Nicotine-treated adults displayed a robust aversion to an environment paired with a KOR agonist versus naïve adults. Neither of the adolescent groups displayed a place aversion. KOR activation produced an increase in anxiety-like behavior that was highest in nicotine-treated adults versus all other groups. KOR activation produced a decrease in NAcc dopamine that was largest in nicotine-treated adults versus all other groups. Lastly, KOR activation facilitated physical signs of withdrawal upon removal of nicotine and KOR blockade reduced this effect.

CONCLUSION

Chronic nicotine enhanced the affective, anxiogenic, and neurochemical effects produced by KOR activation in adult rats. Our data suggest that chronic nicotine elicits an increase in KOR function, and this may contribute to nicotine withdrawal since KOR activation facilitated and KOR blockade prevented withdrawal signs upon removal of nicotine. Given that chronic nicotine facilitated the neurochemical effects of KOR agonists in adults but not in adolescents, it is suggested that KOR regulation of mesolimbic dopamine may contribute to age differences in nicotine withdrawal.

Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age-related effects, as adolescent rats experience lower withdrawal-related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma-aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal-related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

Heroin self-administration and reinstatement of heroin-seeking in adolescent vs. adult male rats

RATIONALE

Heroin abuse is prevalent among teenagers, and early onset drug use might predict long-term drug dependence. However, adolescent sensitivity to drug reinforcement has not been explored thoroughly in animal models.

OBJECTIVES

This study aimed to compare intravenous (i.v.) self-administration of heroin, as well as extinction and reinstatement of heroin-seeking, in adolescent vs. adult male rats.

METHODS

Adolescent (35 days old at start) and adult (86 days old at start) male Sprague-Dawley rats spontaneously acquired lever pressing maintained by i.v. heroin infusions. In experiment 1, self-administration was tested on a fixed ratio 1 schedule of reinforcement (0.05 and 0.025 mg/kg per infusion), followed by within-session extinction and reinstatement tests after 1 or 12 days of abstinence. In experiment 2, self-administration was tested on a progressive ratio schedule (0.0125-0.1 mg/kg per infusion), followed 12 days later by a single test of extinction responding in the presence of cues.

RESULTS

In experiment 1, adolescent rats self-administered more heroin than adults. After 1 or 12 days of abstinence, adolescents exhibited less heroin-seeking than adults, although levels of heroin-seeking increased over abstinence period for both age groups. In experiment 2, adolescents and adults reached the same maximal response ratio (breakpoint), although adolescents earned more infusions when response requirements were low. For extinction responding in the presence of cues, heroin-seeking was similar across ages.

CONCLUSIONS

Lower levels of heroin-seeking suggest that younger rats are less sensitive than adults to some residual effects of heroin intake.

Phasic D1 and tonic D2 dopamine receptor signaling double dissociate the motivational effects of acute nicotine and chronic nicotine withdrawal

Nicotine, the main psychoactive ingredient of tobacco smoke, induces negative motivational symptoms during withdrawal that contribute to relapse in dependent individuals. The neurobiological mechanisms underlying how the brain signals nicotine withdrawal remain poorly understood. Using electrophysiological, genetic, pharmacological, and behavioral methods, we demonstrate that tonic but not phasic activity is reduced during nicotine withdrawal in ventral tegmental area dopamine (DA) neurons, and that this pattern of signaling acts through DA D2 and adenosine A2A, but not DA D1, receptors. Selective blockade of phasic DA activity prevents the expression of conditioned place aversions to a single injection of nicotine in nondependent mice, but not to withdrawal from chronic nicotine in dependent mice, suggesting a shift from phasic to tonic dopaminergic mediation of the conditioned motivational response in nicotine dependent and withdrawn animals. Either increasing or decreasing activity at D2 or A2A receptors prevents the aversive motivational response to withdrawal from chronic nicotine, but not to acute nicotine. Modification of D1 receptor activity prevents the aversive response to acute nicotine, but not to nicotine withdrawal. This double dissociation demonstrates that the specific pattern of tonic DA activity at D2 receptors is a key mechanism in signaling the motivational effects experienced during nicotine withdrawal, and may represent a unique target for therapeutic treatments for nicotine addiction.

Neuronal mechanisms underlying development of nicotine dependence: implications for novel smoking-cessation treatments

Tobacco smoking causes high rates of mortality and morbidity throughout the world. Despite the availability of smoking-cessation medications, maintenance of long-term abstinence is difficult, and most individuals who attempt to quit smoking relapse. Although tobacco smoke contains many substances, researchers and policymakers agree that nicotine is a major cause of tobacco dependence. Understanding the neural substrates of nicotine dependence is essential for the development of more effective antismoking medications than those currently available. This article focuses on the neural substrates, especially nicotinic acetylcholine receptors, that mediate the reinforcing effects of nicotine and the development of nicotine dependence. Neuroadaptations in the function of the neurotransmitters dopamine, glutamate, and gamma-aminobutyric acid (GABA), which have been shown to be critically involved in nicotine dependence, are also reviewed. Finally, the article discusses progress in the discovery and development of smoking-cessation medications.