Nicotinic cholinergic synaptic mechanisms in the ventral tegmental area contribute to nicotine addiction

Expression of sensitized β2 nAChR subunits in VTA neurons enhances intravenous nicotine self-administration in male rats

Ventral tegmental area (VTA) nicotinic acetylcholine receptors (nAChRs) are important for nicotine reinforcement. To determine whether and to what extent these receptors are sufficient for nicotine reinforcement, we expressed β2Leu9'Ser (i.e. sensitized) nAChR subunits in the VTA of adult male rats and assessed the nicotine dose-response relationship in intravenous self-administration (SA). β2Leu9'Ser rats self-administered nicotine doses 50-100 fold lower than the lowest doses that control rats would respond for. Expression of WT β2 subunits confirmed that this enhanced sensitivity to nicotine was due to the Leu9'Ser mutation in β2. Higher unit doses were associated with strong escalation in β2Leu9'Ser rats over 17 fixed ratio sessions. Escalation was minimal or absent in control rats at the same unit doses. In progressive ratio SA, β2Leu9'Ser rats exhibited higher breakpoints than control rats when the nicotine unit dose was 1.5 μg/kg/inf or higher. In intermittent access SA, β2Leu9'Ser rats exhibited response patterns very similar to control rats. By adding nicotine dose-response data, progressive ratio assays, and intermittent access results that rule out stereotypy, these data significantly extend our previous finding that nicotine activation of the mesolimbic dopamine pathway is sufficient for nicotine reinforcement.

Ghrelin Amplifies the Nicotine-Induced Release of Dopamine in the Bed Nucleus of Stria Terminalis (BNST)

Ghrelin is an orexigenic neuropeptide that is known for stimulating the release of growth hormone (GH) and appetite. In addition, ghrelin has been implicated in addiction to drugs such as nicotine. Nicotine is the principal psychoactive component in tobacco and is responsible for the reward sensation produced by smoking. In our previous in vitro superfusion studies, it was demonstrated that ghrelin and nicotine stimulate equally the dopamine release in the rat amygdala, and ghrelin amplifies the nicotine-induced dopamine release in the rat striatum. However, less attention was paid to the actions of ghrelin and nicotine in the bed nucleus of the stria terminalis (BNST). Therefore, in the present study, nicotine and ghrelin were superfused to the BNST of male Wistar rats, and the dopamine release from the BNST was measured in vitro. In order to determine which receptors mediate these effects, mecamylamine, a non-selective nicotinic acetylcholine receptor (nAchR) antagonist, and GHRP-6, a selective growth hormone secretagogue receptor (GHS-R1A) antagonist, were also superfused to the rat BNST. Nicotine significantly increased the release of dopamine, and this effect was significantly inhibited by mecamylamine. Ghrelin increased dopamine release even more significantly than nicotine did, and this effect was significantly inhibited by GHRP-6. Moreover, when administered together, ghrelin significantly amplified the nicotine-induced release of dopamine in the BNST, and this additive effect was reversed partly by mecamylamine and partly by GHRP-6. Therefore, the present study provides a new base of evidence for the involvement of ghrelin in dopamine signaling implicated in nicotine addiction.

CHRNA5 gene variation affects the response of VTA dopaminergic neurons during chronic nicotine exposure and withdrawal

Nicotine is the principal psychoactive component in tobacco that drives addiction through its action on neuronal nicotinic acetylcholine receptors (nAChR). The nicotinic receptor gene CHRNA5, which encodes the α5 subunit, is associated with nicotine use and dependence. In humans, the CHRNA5 missense variant rs16969968 (G > A) is associated with increased risk for nicotine dependence and other smoking-related phenotypes. In rodents, α5-containing nAChRs in dopamine (DA) neurons within the ventral tegmental area (VTA) powerfully modulate nicotine reward and reinforcement. Although the neuroadaptations caused by long-term nicotine exposure are being actively delineated at both the synaptic and behavioral levels, the contribution of α5-containing nAChRs to the cellular adaptations associated with long-term nicotine exposure remain largely unknown. To gain insight into the mechanisms behind the influence of α5-containing nAChRs and the rs16969968 polymorphism on nicotine use and dependence, we used electrophysiological approaches to examine changes in nAChR function arising in VTA neurons during chronic nicotine exposure and multiple stages of nicotine withdrawal. Our results demonstrate that CHRNA5 mutation leads to profound changes in VTA nAChR function at baseline, during chronic nicotine exposure, and during short-term and prolonged withdrawal. Whereas nAChR function was suppressed in DA neurons from WT mice undergoing withdrawal relative to drug-naïve or nicotine-drinking mice, α5-null mice exhibited an increase in nAChR function during nicotine exposure that persisted throughout 5-10 weeks of withdrawal. Re-expressing the hypofunctional rs16969968 CHRNA5 variant in α5-null VTA DA neurons did not rescue the phenotype, with α5-SNP neurons displaying a similar increased response to ACh during nicotine exposure and early stages of withdrawal. These results demonstrate the importance of VTA α5-nAChRs in the response to nicotine and implicate them in the time course of withdrawal.

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

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

Effects of Chronic Inhalation of Electronic Cigarette Vapor Containing Nicotine on Neurobehaviors and Pre/Postsynaptic Neuron Markers

Nicotine-exposed animal models exhibit neurobehavioral changes linked to impaired synaptic plasticity. Previous studies highlighted alterations in neurotransmitter levels following nicotine exposure. Vesicular glutamate transporter (VGLUT1) and vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) are essential for the transport and release of glutamate and GABA, respectively, from presynaptic neurons into synapses. In our work, an e-cigarette device was used to deliver vapor containing nicotine to C57BL/6J mice for four weeks. Novel object recognition, locomotion, and Y-maze tests were performed to investigate the behavioral parameters. Protein studies were conducted to study the hippocampal expression of VGLUT1, VGAT, and postsynaptic density protein 95 (PSD95) as well as brain cytokine markers. Long-term memory and locomotion tests revealed that e-cigarette aerosols containing nicotine modulated recognition memory and motor behaviors. We found that vapor exposure increased VGLUT1 expression and decreased VGAT expression in the hippocampus. No alterations were found in PSD95 expression. We observed that vapor-containing nicotine exposure altered certain brain cytokines such as IFNβ-1 and MCP-5. Our work provides evidence of an association between neurobehavioral changes and altered hippocampal VGLUT1 and VGAT expression in mice exposed to e-cigarette vapors containing nicotine. Such exposure was also associated with altered neurobehaviors, which might affect neurodegenerative diseases.

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.

Smoke signals: A study of the neurophysiological reaction of smokers and non-smokers to smoking cues inserted into antismoking public service announcements

Tobacco addiction is one of the biggest health emergencies in the world, Antismoking Public Service Announcements (PSAs) represent the main public tool against smoking; however, smoking-related cues (SCs) often included in PSAs can trigger ambiguous cerebral reactions that could impact the persuasiveness and efficacy of the antismoking message. This study aimed to investigate the electroencephalographic (EEG) response in adult smokers and non-smokers during the exposure to SCs presented in antismoking PSAs video, in order to identify eventual neurophysiological features of SCs' 'boomerang effect' elicited in smokers. EEG frontal Alpha asymmetry and frontal Theta were analyzed in 92 adults (30 no smokers, 31 low smokers, 31 high smokers) from EEG recorded during the vision of 3 antismoking PSAs, statistical analysis was conducted using ANOVA. Main results showed a significant interaction between smoking cue condition (Pre and Post) and smoking habit (in particular for female heavy smokers) for the frontal Alpha asymmetry. Since the relative higher right frontal Alpha activity is associated with approach towards a stimulus, it is suggested that the relative left frontal Alpha increase in response to SCs might reflect an appetitive approach in response to it. In the light of the Incentive Sensitization Theory, this pattern can be interpreted as a neurophysiological signal in response to SCs that could undermine the message's effectiveness contributing to the maintenance of the addiction.

Epigenetic Mechanisms Mediate Nicotine-Induced Reward and Behaviour in Zebrafish

Nicotine induces long-term changes in the neural activity of the mesocorticolimbic reward pathway structures. The mechanisms involved in this process have not been fully characterized. The hypothesis discussed here proposed that epigenetic regulation participates in the installation of persistent adaptations and long-lasting synaptic plasticity generated by nicotine action on the mesolimbic dopamine neurons of zebrafish. The epigenetic mechanisms induced by nicotine entail histone and DNA chemical modifications, which have been described to lead to changes in gene expression. Among the enzymes that catalyze epigenetic chemical modifications, histone deacetylases (HDACs) remove acetyl groups from histones, thereby facilitating DNA relaxation and making DNA more accessible to gene transcription. DNA methylation, which is dependent on DNA methyltransferase (DNMTs) activity, inhibits gene expression by recruiting several methyl binding proteins that prevent RNA polymerase binding to DNA. In zebrafish, phenylbutyrate (PhB), an HDAC inhibitor, abolishes nicotine rewarding properties together with a series of typical reward-associated behaviors. Furthermore, PhB and nicotine alter long- and short-term object recognition memory in zebrafish, respectively. Regarding DNA methylation effects, a methyl group donor L-methionine (L-met) was found to dramatically reduce nicotine-induced conditioned place preference (CPP) in zebrafish. Simultaneous treatment with DNMT inhibitor 5-aza-2’-deoxycytidine (AZA) was found to reverse the L-met effect on nicotine-induced CPP as well as nicotine reward-specific effects on genetic expression in zebrafish. Therefore, pharmacological interventions that modulate epigenetic regulation of gene expression should be considered as a potential therapeutic method to treat nicotine addiction.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Roles of the Functional Interaction between Brain Cholinergic and Dopaminergic Systems in the Pathogenesis and Treatment of Schizophrenia and Parkinson's Disease

Most physiologic processes in the brain and related diseases involve more than one neurotransmitter system. Thus, elucidation of the interaction between different neurotransmitter systems could allow for better therapeutic approaches to the treatments of related diseases. Dopaminergic (DAergic) and cholinergic neurotransmitter system regulate various brain functions that include cognition, movement, emotion, etc. This review focuses on the interaction between the brain DAergic and cholinergic systems with respect to the pathogenesis and treatment of schizophrenia and Parkinson’s disease (PD). We first discussed the selection of motor plans at the level of basal ganglia, the major DAergic and cholinergic pathways in the brain, and the receptor subtypes involved in the interaction between the two signaling systems. Next, the roles of each signaling system were discussed in the context of the negative symptoms of schizophrenia, with a focus on the α7 nicotinic cholinergic receptor and the dopamine D₁ receptor in the prefrontal cortex. In addition, the roles of the nicotinic and dopamine receptors were discussed in the context of regulation of striatal cholinergic interneurons, which play crucial roles in the degeneration of nigrostriatal DAergic neurons and the development of L-DOPA-induced dyskinesia in PD patients. Finally, we discussed the general mechanisms of nicotine-induced protection of DAergic neurons.

Nicotinic acetylcholine receptors and nicotine addiction: A brief introduction

Nicotine is a highly addictive drug found in tobacco that drives its continued use despite the harmful consequences. The initiation of nicotine abuse involves the mesolimbic dopamine system, which contributes to the rewarding sensory stimuli and associative learning processes in the beginning stages of addiction. Nicotine binds to neuronal nicotinic acetylcholine receptors (nAChRs), which come in a diverse collection of subtypes. The nAChRs that contain the α4 and β2 subunits, often in combination with the α6 subunit, are particularly important for nicotine's ability to increase midbrain dopamine neuron firing rates and phasic burst firing. Chronic nicotine exposure results in numerous neuroadaptations, including the upregulation of particular nAChR subtypes associated with long-term desensitization of the receptors. When nicotine is no longer present, for example during attempts to quit smoking, a withdrawal syndrome develops. The expression of physical withdrawal symptoms depends mainly on the α2, α3, α5, and β4 nicotinic subunits in the epithalamic habenular complex and its target regions. Thus, nicotine affects diverse neural systems and an array of nAChR subtypes to mediate the overall addiction process. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

E-cigarettes, nicotine, the lung and the brain: multi-level cascading pathophysiology

Tobacco smoking is highly addictive and causes respiratory disease, cardiovascular disease and multiple types of cancer. Electronic-cigarettes (e-cigarettes) are non-combustible tobacco alternatives that aerosolize nicotine and flavouring agents in a propylene glycol-vegetable glycerine vehicle. They were originally envisaged as a tobacco cessation aid, but whether or not they help people to quit tobacco use is controversial. In this review, we have compared and contrasted what is known regarding the effects of nicotine on the lungs vs. the effects of nicotine in the brain in the context of addiction. Critically, both combustible tobacco products and e-cigarettes contain nicotine, a highly addictive, plant-derived alkaloid that binds to nicotinic acetylcholine receptors (nAChRs). Nicotine's reinforcing properties are primarily mediated by activation of the brain's mesolimbic reward circuitry and release of the neurotransmitter dopamine that contribute to the development of addiction. Moreover, nicotine addiction drives repeated intake that results in chronic pulmonary exposure to either tobacco smoke or e-cigarettes despite negative respiratory symptoms. Beyond the brain, nAChRs are also highly expressed in peripheral neurons, epithelia and immune cells, where their activation may cause harmful effects. Thus, nicotine, a key ingredient of both conventional and electronic cigarettes, produces neurological effects that drive addiction and may damage the lungs in the process, producing a complex, multilevel pathological state. We conclude that vaping needs to be studied by multi-disciplinary teams that include pulmonary and neurophysiologists as well as behaviourists and addiction specialists to fully understand their impact on human physiology.

Cocaine- and amphetamine-regulated transcript promoter regulated by nicotine in nerve growth factor-treated PC12 cells

Nicotine and cocaine- and amphetamine-regulated transcripts (CART) have several overlapping functions, such as the regulation of reward, feeding behavior, stress response, and anxiety. Previous studies showed that nicotine regulates CART expression in various brain regions. However, the molecular mechanisms underlying this regulation are not known. This study investigated the regulatory effect of nicotine on promoter activity of the CART gene in PC12 cells, which were differentiated into a neuronal phenotype by nerve growth factor (NGF) treatment. Two vectors containing reporter genes (Gaussia luciferase or mCherry) and the 1,140-bp upstream of the transcriptional start site of the mouse CART gene are used to analyze the CART promoter activity. Transient transfection of PC12 cells with either vector displayed strong promoter activity in both undifferentiated and differentiated PC12 cells. CART promoter activity in the PC12 cell line is increased by forskolin or NGF treatment. In differentiated PC12 cells, exposure to 50 nM nicotine for 6 h increased CART promoter activity. However, treatment with higher nicotine doses for 6 h and treatment with all nicotine doses for 24 h showed no effect. A nicotine concentration of 50 nM is comparable to brain nicotine levels experienced by chronic smokers over long periods of time. Taken together, these data indicate that nicotine may exert some of its actions through the regulation of CART transcription in the brain.

Effects of Chronic Inhalation of Electronic Cigarette Vapor Containing Nicotine on Neurotransmitters in the Frontal Cortex and Striatum of C57BL/6 Mice

Electronic (E)-cigarettes are the latest form of nicotine delivery device and are highly popular in the general population. It is currently unknown whether vaping E-cigarettes (E-CIGs) leads to nicotine addiction. Alterations in the levels of the neurotransmitters in the mesocorticolimbic areas have been reported to mediate the initiation and development of nicotine addiction. Therefore, to determine whether E-CIGs activate the same addiction pathways as conventional cigarettes, we investigated for the effects of daily inhalation of nicotine (24 mg/ml)-containing E-CIG vapor for 6 months on the concentrations of these neurotransmitters in the frontal cortex (FC) and striatum (STR) of male C57BL/6 mice as compared to control group that was exposed to air only. We reported here that 6-month E-CIG vapor containing nicotine inhalation decreased dopamine concentration only in the STR. There were no changes in serotonin concentrations in the FC or STR. Chronic E-CIG exposure also increased glutamate concentration in the STR alone, while glutamine concentrations were increased in both the FC and STR. We found that E-CIG exposure also decreased GABA concentration only in the FC. These data suggest that chronic E-CIG use alters homeostasis of several neurotransmitters in the mesocorticolimbic areas, which may result in the development of nicotine dependence in E-CIG users.

Functional crosstalk of nucleus accumbens CB1 and OX2 receptors in response to nicotine-induced place preference

In the present study, we have evaluated the existence of functional interaction between orexin-2 receptor (OX2R) and cannabinoid-1 receptor (CB1R) in the nucleus accumbens core (NAcc), in nicotine-induced conditioned place preference (CPP) of Wistar male rat. Nicotine (0.5 mg/kg; s.c.) in the course of conditioning, produced a significant place preference, without any effect on the locomotor activity. Intra-NAcc administration of ineffective and effective doses of TCS-OX2-29 (2 and 6 ng/rat), a selective OX2R antagonist and AM251 (10 and 50 ng/rat), a selective CB1R antagonist, showed a significant interaction between OX2R and CB1R in the acquisition of nicotine-induced CPP (p < 0.01), and the locomotor activity (p < 0.05). No significant interaction was observed between these two receptors in the expression of nicotine-induced CPP. Our findings provide insight into the possible interaction of OX2R and CB1R of the NAcc in nicotine addiction. We propose a potential interaction between cannabinoid and orexinergic systems within the NAcc, in producing the rewarding effects.

Orexin type-2 receptor blockade prevents the nicotine-induced excitation of nucleus accumbens core neurons in rats: An electrophysiological perspective

BACKGROUND

The nucleus accumbens core (NAcc) expresses both orexin and nicotinic acetylcholine receptors (nAChRs). Orexin is among important neurotransmitters, which regulates addictive properties of drugs of abuse including nicotine. The role of orexin-2 receptor (OX2R) in the regulation of NAcc neural activity in response to nicotine has not yet been studied. Hence, in this study, we examined whether the OX2R antagonist (TCS-OX2-29) can adjust the effects of nicotine on electrical activity of NAcc neurons, in urethane-anesthetized rats, using the single unit recording.

METHODS

Neuronal firing of NAcc was recorded for 15 min, then TCS-OX2-29 (OX2R-antagonist; 1, 3 and 10 ng/rat) or DMSO were microinjected into NAcc, just 5 min before subcutaneous (sc) administration of nicotine (0.5 mg/kg) or saline. The spontaneous firing activity was recorded for 70 min, after nicotine injection.

RESULTS

The results demonstrated that nicotine significantly excites the NAcc neurons and interestingly, the administration of TCS-OX2-29 (3 and 10 ng/rat) into the NAcc, inhibited nicotine-induced increases of NAcc neuronal responses. Furthermore, administration of TCS-OX2-29 (10 ng/rat), just 5 min before sc administration of saline instead of nicotine, did not significantly alter the neuronal responses, compared to the saline-control group.

CONCLUSION

Our results showed that, although OX2R blockade alone did not affect neuronal activity in the NAcc, it was able to prevent the exciting effects of nicotine on NAcc neuronal activity. Therefore, we proposed that orexin has a potential modulator effect, in response to nicotine.

α4β2 Nicotinic receptor desensitizing compounds can decrease self-administration of cocaine and methamphetamine in rats

Sazetidine-A [6-(5(((S)-azetidine-2-yl)methoxy)pyridine-3-yl)hex-5-yn-1-ol] is a selective α4β2 nicotinic receptor desensitizing agent and partial agonist. Sazetidine-A has been shown in our previous studies to significantly reduce nicotine and alcohol self-administration in rats. The question arises whether sazetidine-A would reduce self-administration of other addictive drugs as well. Nicotinic receptors on the dopaminergic neurons in the ventral tegmental area play an important role in controlling the activity of these neurons and release of dopamine in the nucleus accumbens, which is critical mechanism for reinforcing value of drugs of abuse. Previously, we showed that the nonspecific nicotinic antagonist mecamylamine significantly reduces cocaine self-administration in rats. In this study, we acutely administered systemically sazetidine-A and two other selective α4β2 nicotinic receptor-desensitizing agents, VMY-2-95 and YL-2-203, to young adult female Sprague-Dawley rats and determined their effects on IV self-administration of cocaine and methamphetamine. Cocaine self-administration was significantly reduced by 0.3 mg/kg of sazetidine-A. In another set of rats, sazetidine-A (3 mg/kg) significantly reduced methamphetamine self-administration. VMY-2-95 significantly reduced both cocaine and methamphetamine self-administration with threshold effective doses of 3 and 0.3 mg/kg, respectively. In contrast, YL-2-203 did not significantly reduce cocaine self-administration at the same dose range and actually significantly increased cocaine self-administration at the 1 mg/kg dose. YL-2-203 (3 mg/kg) did significantly decrease methamphetamine self-administration. Sazetidine-A and VMY-2-95 are promising candidates to develop as new treatments to help addicts successfully overcome a variety of addictions including tobacco, alcohol as well as the stimulant drugs cocaine and methamphetamine.

Circulating small non-coding RNAs associated with age, sex, smoking, body mass and physical activity

Small non-coding RNAs (sncRNA) are regulators of cell functions and circulating sncRNAs from the majority of RNA classes are potential non-invasive biomarkers. Understanding how common traits influence ncRNA expression is essential for assessing their biomarker potential. In this study, we identify associations between sncRNA expression and common traits (sex, age, self-reported smoking, body mass, self-reported physical activity). We used RNAseq data from 526 serum samples from the Janus Serum Bank and traits from health examination surveys. Ageing showed the strongest association with sncRNA expression, both in terms of statistical significance and number of RNAs, regardless of RNA class. piRNAs were abundant in the serum samples and they were associated to sex. Interestingly, smoking cessation generally restored RNA expression to non-smoking levels, although for some sncRNAs smoking-related expression levels persisted. Pathway analysis suggests that smoking-related sncRNAs target the cholinergic synapses and may therefore potentially play a role in smoking addiction. Our results show that common traits influence circulating sncRNA expression. It is clear that sncRNA biomarker analyses should be adjusted for age and sex. In addition, for specific sncRNAs, analyses should also be adjusted for body mass, smoking, physical activity and technical factors.

Progressive modulation of accumbal neurotransmission and anxiety-like behavior following protracted nicotine withdrawal

Due to the highly addictive properties of nicotine, a low percentage of users successfully maintain cessation for longer periods of time. This might be linked to neuroadaptations elicited by the drug, and understanding progressive changes in neuronal function might provide critical insight into nicotine addiction. We have previously shown that neurotransmission in the nucleus accumbens (nAc), a key brain region with respect to drug reinforcement and relapse, is suppressed for as long as seven months after a brief period of nicotine treatment. Studies were therefore performed to define the temporal properties of these effects, and to assess behavioral correlates to altered neurotransmission. Ex vivo electrophysiology revealed progressive depression of synaptic efficacy in the nAc of rats previously receiving nicotine. In addition, following three months of nicotine withdrawal, the responses to GABA_A receptor modulating drugs were blunted together with downregulation of several GABA_A receptor subunits. In correlation to reduced accumbal neurotransmission, a reduced anxiety-like behavior; assessed in the elevated plus-maze and marble burying tests, were identified in animals pre-treated with nicotine. Lastly, to test the causal relationship between suppressed excitability in the nAc and reduced anxiety-like behavior, rats received local administration of diazepam in the nAc while monitoring behavioral effects on the elevated plus-maze. These results show that nicotine produces long-lasting changes in the GABAergic system, which are observed first after extended withdrawal. Our data also suggest that nicotine produces a progressive suppression of accumbal excitability, which could result in behavioral alterations that may have implications for further drug intake.

Chronic nicotine differentially affects murine transcriptome profiling in isolated cortical interneurons and pyramidal neurons

Background

Nicotine is known to differentially regulate cortical interneuron and pyramidal neuron activities in the neocortex, while the underlying molecular mechanisms have not been well studied. In this study, RNA-sequencing was performed in acutely isolated cortical somatostatin (Sst)- positive interneurons and pyramidal neurons (Thy1) from mice treated with systemic nicotine for 14 days. We assessed the differentially expressed genes (DEGs) by nicotine in Sst- or Thy1- neurons, respectively, and then compared DEGs between Sst- and Thy1- neurons in the absence and presence of nicotine.

Results

In Sst-neurons, the DEGs by nicotine were associated with glycerophospholipid and nicotinate and nicotinamide metabolism; while in Thy1-neurons those related to immune response and purine and pyrimidine metabolisms were affected. Under basal condition, the DEGs between Sst- and Thy1- neurons were frequently associated with signal transduction, phosphorylation and potassium channel regulation. However, some new DEGs between Sst- and Thy1- neurons were found after nicotine, the majority of which belong to mitochondrial respiratory chain complex.

Conclusions

Nicotine differentially affected subset of genes in Sst- and Thy1- neurons, which might contribute to the distinct effect of nicotine on interneuron and pyramidal neuron activities. Meanwhile, the altered transcripts associated with mitochondrial activity were found between interneurons and pyramidal neurons after chronic nicotine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3593-x) contains supplementary material, which is available to authorized users.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Alcohol and nicotine interactions: pre-clinical models of dependence

While the co-morbidity of alcohol (ethanol) and tobacco (nicotine) dependence is well described, the processes that underpin this strong connection are still under debate. With the increasing popularity of electronic cigarettes (e-cigarettes), it is now becoming more important to look to the neurobiological mechanisms involving alcohol and nicotine interactions to effectively treat a new generation of co-dependent individuals. Researchers have already recognized that the neuropathology produced by the combination of nicotine and ethanol is likely to produce an addictive nature very different to that of either one alone, and are employing a mixture of pre-clinical techniques to establish and investigate every stage in the development of both nicotine and ethanol-seeking behaviors. While it is agreed that multiple pathways orchestrate the complex reward profile of alcohol and nicotine co-addiction, several lines of evidence suggest the convergent site of action is within the mesolimbic dopaminergic system, at neuronal nicotinic acetylcholine receptors (nAChRs). A whole host of strategies are currently being employed to discover and unravel previously unknown or ill understood neurobiological processes in the brain, contributing greatly toward the development of novel pharmacotherapies with the aim of improving patient outcomes. This review intends to shed some light on the most influential and most recent pre-clinical work that is leading the charge in modeling this complicated relationship.

Tobacco nitrosamines as culprits in disease: mechanisms reviewed

The link between tobacco abuse and cancer is well-established. However, emerging data indicate that toxins in tobacco smoke cause cellular injury due to enhanced toxic/metabolic effects of metabolites, disruption of intracellular signaling mechanisms, and formation of DNA, protein, and lipid adducts that impair function and promote oxidative stress and inflammation. These effects of smoking, which are largely non-carcinogenic, can be produced by tobacco-specific nitrosamines and their metabolites. These factors could account for the increased rates of neurodegeneration and insulin resistance diseases among smokers. Herein, we review nicotine and tobacco-specific nitrosamine metabolism, mechanisms of adduct formation, DNA damage, mutagenesis, and potential mechanisms of disease.

Nicotine produces chronic behavioral sensitization with changes in accumbal neurotransmission and increased sensitivity to re-exposure

Tobacco use is often associated with long-term addiction as well as high risk of relapse following cessation. This is suggestive of persistent neural adaptations, but little is known about the long-lasting effects of nicotine on neural circuits. In order to investigate the long-term effects of nicotine exposure, Wistar rats were treated for 3 weeks with nicotine (0.36 mg/kg), and the duration of behavioral and neurophysiological adaptations was evaluated 7 months later. We found that increased drug-induced locomotion persisted 7 months after the initial behavioral sensitization. In vitro analysis of synaptic activity in the core and shell of the nucleus accumbens (nAc) revealed a decrease in input/output function in both regions of nicotine-treated rats as compared to vehicle-treated control rats. In addition, administration of the dopamine D2 receptor agonist quinpirole (5 μM) significantly increased evoked population spike amplitude in the nAc shell of nicotine-treated rats as compared to vehicle-treated control rats. To test whether nicotine exposure creates long-lasting malleable circuits, animals were re-exposed to nicotine 7 months after the initial exposure. This treatment revealed an increased sensitivity to nicotine among animals previously exposed to nicotine, with higher nicotine-induced locomotion responses than observed initially. In vitro electrophysiological recordings in re-exposed rats detected an increased sensitivity to dopamine D2 receptor activation. These results suggest that nicotine produces persistent neural adaptations that make the system sensitive and receptive to future nicotine re-exposure.

Critical needs in drug discovery for cessation of alcohol and nicotine polysubstance abuse

Polysubstance abuse of alcohol and nicotine has been overlooked in our understanding of the neurobiology of addiction and especially in the development of novel therapeutics for its treatment. Estimates show that as many as 92% of people with alcohol use disorders also smoke tobacco. The health risks associated with both excessive alcohol consumption and tobacco smoking create an urgent biomedical need for the discovery of effective cessation treatments, as opposed to current approaches that attempt to independently treat each abused agent. The lack of treatment approaches for alcohol and nicotine abuse/dependence mirrors a similar lack of research in the neurobiology of polysubstance abuse. This review discusses three critical needs in medications development for alcohol and nicotine co-abuse: (1) the need for a better understanding of the clinical condition (i.e. alcohol and nicotine polysubstance abuse), (2) the need to better understand how these drugs interact in order to identify new targets for therapeutic development and (3) the need for animal models that better mimic this human condition. Current and emerging treatments available for the cessation of each drug and their mechanisms of action are discussed within this context followed by what is known about the pharmacological interactions of alcohol and nicotine. Much has been and will continue to be gained from studying comorbid alcohol and nicotine exposure.

Exposure to nicotine increases nicotinic acetylcholine receptor density in the reward pathway and binge ethanol consumption in C57BL/6J adolescent female mice

Nearly 80% of adult smokers begin smoking during adolescence. Binge alcohol consumption is also common during adolescence. Past studies report that nicotine and ethanol activate dopamine neurons in the reward pathway and may increase synaptic levels of dopamine in the nucleus accumbens through nicotinic acetylcholine receptor (nAChR) stimulation. Activation of the reward pathway during adolescence through drug use may produce neural alterations affecting subsequent drug consumption. Consequently, the effect of nicotine exposure on binge alcohol consumption was examined along with an assessment of the neurobiological underpinnings that drive adolescent use of these drugs. Adolescent C57BL/6J mice (postnatal days 35-44) were exposed to either water or nicotine (200μg/ml) for ten days. On the final four days, ethanol intake was examined using the drinking-in-the-dark paradigm. Nicotine-exposed mice consumed significantly more ethanol and displayed higher blood ethanol concentrations than did control mice. Autoradiographic analysis of nAChR density revealed higher epibatidine binding in frontal cortical regions in mice exposed to nicotine and ethanol compared to mice exposed to ethanol only. These data show that nicotine exposure during adolescence increases subsequent binge ethanol consumption, and may affect the number of nAChRs in regions of the brain reward pathway, specifically the frontal cortex.

Dopaminergic and cholinergic learning mechanisms in nicotine addiction

Nicotine addiction drives tobacco use by one billion people worldwide, causing nearly six million deaths a year. Nicotine binds to nicotinic acetylcholine receptors that are normally activated by the endogenous neurotransmitter acetylcholine. The widespread expression of nicotinic receptors throughout the nervous system accounts for the diverse physiological effects triggered by nicotine. A crucial influence of nicotine is on the synaptic mechanisms underlying learning that contribute to the addiction process. Here, we focus on the acquisition phase of smoking addiction and review animal model studies on how nicotine modifies dopaminergic and cholinergic signaling in key nodes of the reinforcement circuitry: ventral tegmental area, nucleus accumbens (NAc), amygdala, and hippocampus. Capitalizing on mechanisms that subserve natural rewards, nicotine activates midbrain dopamine neurons directly and indirectly, and nicotine causes dopamine release in very broad target areas throughout the brain, including the NAc, amygdala, and hippocampus. In addition, nicotine orchestrates local changes within those target structures, alters the release of virtually all major neurotransmitters, and primes the nervous system to the influence of other addictive drugs. Hence, understanding how nicotine affects the circuitry for synaptic plasticity and learning may aid in developing reasoned therapies to treat nicotine addiction.

Neuronal Nicotinic Acetylcholine Receptor Structure and Function and Response to Nicotine

Nicotinic acetylcholine receptors (nAChRs) belong to the "Cys-loop" superfamily of ligand-gated ion channels that includes GABAA, glycine, and serotonin (5-HT3) receptors. There are 16 homologous mammalian nAChR subunits encoded by a multigene family. These subunits combine to form many different nAChR subtypes with various expression patterns, diverse functional properties, and differing pharmacological characteristics. Because cholinergic innervation is pervasive and nAChR expression is extremely broad, practically every area of the brain is impinged upon by nicotinic mechanisms. This review briefly examines the structural and functional properties of the receptor/channel complex itself. The review also summarizes activation and desensitization of nAChRs by the low nicotine concentrations obtained from tobacco. Knowledge of the three-dimensional structure and the structural characteristics of channel gating has reached an advanced stage. Likewise, the basic functional properties of the channel also are reasonably well understood. It is these receptor/channel properties that underlie the participation of nAChRs in nearly every anatomical region of the mammalian brain.

Cholinergic modulation of dopamine pathways through nicotinic acetylcholine receptors

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.

Ethanol and nicotine interaction within the posterior ventral tegmental area in male and female alcohol-preferring rats: evidence of synergy and differential gene activation in the nucleus accumbens shell

RATIONALE

Ethanol and nicotine are frequently co-abused. The biological basis for the high co-morbidity rate is not known. Alcohol-preferring (P) rats will self-administer EtOH or nicotine directly into the posterior ventral tegmental area (pVTA).

OBJECTIVE

The current experiments examined whether sub-threshold concentrations of EtOH and nicotine would support the development of self-administration behaviors if the drugs were combined.

METHODS

Rats were implanted with a guide cannula aimed at the pVTA. Rats were randomly assigned to groups that self-administered sub-threshold concentrations of EtOH (50 mg%) or nicotine (1 μM) or combinations of ethanol (25 or 50 mg%) and nicotine (0.5 or 1.0 μM). Alterations in gene expression downstream projections areas (nucleus accumbens shell, AcbSh) were assessed following a single, acute exposure to EtOH (50 mg%), nicotine (1 μM), or ethanol and nicotine (50 mg% + 1 μM) directly into the pVTA.

RESULTS

The results indicated that P rats would co-administer EtOH and nicotine directly into the pVTA at concentrations that did not support individual self-administration. EtOH and nicotine directly administered into the pVTA resulted in alterations in gene expression in the AcbSh (50.8-fold increase in brain-derived neurotrophic factor (BDNF), 2.4-fold decrease in glial cell line-derived neurotrophic factor (GDNF), 10.3-fold increase in vesicular glutamate transporter 1 (Vglut1)) that were not observed following microinjections of equivalent concentrations/doses of ethanol or nicotine.

CONCLUSION

The data indicate that ethanol and nicotine act synergistically to produce reinforcement and alter gene expression within the mesolimbic dopamine system. The high rate of co-morbidity of alcoholism and nicotine dependence could be the result of the interactions of EtOH and nicotine within the mesolimbic dopamine system.

Selective breeding for high alcohol preference increases the sensitivity of the posterior VTA to the reinforcing effects of nicotine

The rate of codependency for alcohol and nicotine is extremely high. Numerous studies have indicated that there is a common genetic association for alcoholism and nicotine dependency. The current experiments examined whether selective breeding for high alcohol preference in rats may be associated with increased sensitivity of the posterior ventral tegmental area (pVTA) to the reinforcing properties of nicotine. In addition, nicotine can directly bind to the serotonin-3 (5-HT3 ) receptor, which has been shown to mediate the reinforcing properties of other drugs of abuse within the pVTA Wistar rats were assigned to groups that were allowed to self-infuse 0, 10, 50, 100, 200, 400 or 800 μM nicotine in two-lever (active and inactive) operant chambers. P rats were allowed to self-infuse 0, 1, 10, 50 or 100 μM nicotine. Co-infusion of 5-HT3 receptor antagonists with nicotine into the pVTA was also determined. P rats self-infused nicotine at lower concentrations than required to support self-administration in Wistar rats. In addition, P rats received more self-infusions of 50 and 100 μM nicotine than Wistar rats; including a 5HT3 receptor antagonist (LY-278,584 or zacopride) with nicotine reduced responding on the active lever. Overall, the data support an association between selective breeding for high alcohol preference and increased sensitivity of the pVTA to the reinforcing properties of nicotine. In addition, the data suggest that activation of 5HT3 receptors may be required to maintain the local reinforcing actions of nicotine within the pVTA.

Enhanced alcohol-seeking behavior by nicotine in the posterior ventral tegmental area of female alcohol-preferring (P) rats: modulation by serotonin-3 and nicotinic cholinergic receptors

RATIONALE

Alcohol and nicotine co-use can reciprocally promote self-administration and drug-craving/drug-seeking behaviors. To date, the neurocircuitry in which nicotine influences ethanol (EtOH) seeking has not been elucidated. Clinical and preclinical research has suggested that the activation of the mesolimbic dopamine system is involved in the promotion of drug seeking. Alcohol, nicotine, and serotonin-3 (5-HT3) receptors interact within the posterior ventral tegmental area (pVTA) to regulate drug reward. Recently, our laboratory has reported that systemic administration of nicotine can promote context-induced EtOH seeking.

OBJECTIVES

The goals of the current study were to (1) determine if microinjections of pharmacologically relevant levels of nicotine into the pVTA would enhance EtOH seeking, (2) determine if coadministration of nicotinic cholinergic receptor antagonist (nACh) or 5-HT3 receptor antagonists would block the ability of nicotine microinjected into the pVTA to promote EtOH seeking, and (3) determine if 5-HT3 receptors in the pVTA can modulate EtOH seeking.

RESULTS

Nicotine (100 and 200 μM) microinjected into the pVTA enhanced EtOH seeking. Coinfusion with 200 μM mecamylamine (nACh antagonist) or 100 and 200 μM zacopride (5-HT3 receptor antagonist) blocked the observed nicotine enhancement of EtOH seeking. The data also indicated that microinjection of 1 μM CPBG (5-HT3 receptor agonist) promotes context-induced EtOH seeking; conversely, microinjection of 100 and 200 μM zacopride alone reduced context-induced EtOH seeking.

CONCLUSIONS

Overall, the results show that nicotine-enhanced EtOH-seeking behavior is modulated by 5-HT3 and nACh receptors within the pVTA and that the 5-HT3 receptor system within pVTA may be a potential pharmacological target to inhibit EtOH-seeking behaviors.

Basolateral amygdala CB1 cannabinoid receptors mediate nicotine-induced place preference

In the present study, the effects of bilateral microinjections of cannabinoid CB1 receptor agonist and antagonist into the basolateral amygdala (intra-BLA) on nicotine-induced place preference were examined in rats. A conditioned place preference (CPP) apparatus was used for the assessment of rewarding effects of the drugs in adult male Wistar rats. Subcutaneous (s.c.) administration of nicotine (0.2mg/kg) induced a significant CPP, without any effect on the locomotor activity during the testing phase. Intra-BLA microinjection of a non-selective cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.1-0.5 μg/rat) with an ineffective dose of nicotine (0.1mg/kg, s.c.) induced a significant place preference. On the other hand, intra-BLA administration of AM251 (20-60 ng/rat), a selective cannabinoid CB1 receptor antagonist inhibited the acquisition of nicotine-induced place preference. It should be considered that the microinjection of the same doses of WIN 55,212-2 or AM251 into the BLA, by itself had no effect on the CPP score. The administration of a higher dose of AM251 (60 ng/rat) during the acquisition decreased the locomotor activity of animals on the testing phase. Interestingly, the microinjection of AM251 (20 and 40 ng/rat), but not WIN55,212-2 (0.1-0.5 μg/rat), into the BLA inhibited the expression of nicotine-induced place preference without any effect on the locomotor activity. Taken together, these findings support the possible role of endogenous cannabinoid system of the BLA in the acquisition and the expression of nicotine-induced place preference. Furthermore, it seems that there is a functional interaction between the BLA cannabinoid receptors and nicotine in producing the rewarding effects.

Age-related changes in nicotine response of cholinergic and non-cholinergic laterodorsal tegmental neurons: implications for the heightened adolescent susceptibility to nicotine addiction

The younger an individual starts smoking, the greater the likelihood that addiction to nicotine will develop, suggesting that neurobiological responses vary across age to the addictive component of cigarettes. Cholinergic neurons of the laterodorsal tegmental nucleus (LDT) are importantly involved in the development of addiction, however, the effects of nicotine on LDT neuronal excitability across ontogeny are unknown. Nicotinic effects on LDT cells across different age groups were examined using calcium imaging and whole-cell patch clamping. Within the youngest age group (P7-P15), nicotine induced larger intracellular calcium transients and inward currents. Nicotine induced a greater number of excitatory synaptic currents in the youngest animals, whereas larger amplitude inhibitory synaptic events were induced in cells from the oldest animals (P15-P34). Nicotine increased neuronal firing of cholinergic cells to a greater degree in younger animals, possibly linked to development associated differences found in nicotinic effects on action potential shape and afterhyperpolarization. We conclude that in addition to age-associated alterations of several properties expected to affect resting cell excitability, parameters affecting cell excitability are altered by nicotine differentially across ontogeny. Taken together, our data suggest that nicotine induces a larger excitatory response in cholinergic LDT neurons from the youngest animals, which could result in a greater excitatory output from these cells to target regions involved in development of addiction. Such output would be expected to be promotive of addiction; therefore, ontogenetic differences in nicotine-mediated increases in the excitability of the LDT could contribute to the differential susceptibility to nicotine addiction seen across age.

Varenicline for smoking cessation: A review of the literature

BACKGROUND

Smoking is the leading preventable risk to human health. Various agents have been used to promote smoking cessation, but none has had long-term efficacy. Varenicline, a new nicotinic ligand based on the structure of cytosine, was approved by the US Food amd Drug Administration for use as a smoking cessation aid.

OBJECTIVES

The aims of this review were to provide an overview on the mechanism of action and preclinical and clinical data of the new drug, varenicline, and to discuss the current and future impact of varenicline as a treatment for smoking cessation.

METHODS

MEDLINE, BIOSIS, and Google scholar databases were searched (March 1, 2007-July 1, 2008) using the terms varenicline, smoking cessation, and nicotinic receptors. Full-text articles in English were selected for reference, and articles presenting the mechanism of action, pharmacokinetics, and data from preclinical and clinical trials were included.

RESULTS

The initial literature search yielded 70 papers. A total of 20 articles fulfilled the inclusion criteria. Varenicline, an α4β2 nicotinic acetylcholine receptor partial agonist, inhibits dopaminergic activation produced by smoking and decreases the craving and withdrawal syndrome that accompanies cessation attempts. In Phase III clinical trials, the carbon monoxide-confirmed 4-week continuous abstinence rates were significantly higher with varenicline than with buproprion sustained release or placebo for weeks 9 through 12. Varenicline has been found to be well tolerated, with nausea being the most commonly reported (28.1%) adverse event.

CONCLUSIONS

Varenicline is the first drug for smoking cessation that has been found to have significant effectiveness in long-term relapse prevention (up to 52 weeks). Varenicline, with its unique profile of agonist and antagonist properties, increased cessation rates (both short- and long-term) compared with both placebo and bupropion sustained release.

Mechanisms of metabonomic for a gateway drug: nicotine priming enhances behavioral response to cocaine with modification in energy metabolism and neurotransmitter level

Nicotine, one of the most commonly used drugs, has become a major concern because tobacco serves as a gateway drug and is linked to illicit drug abuse, such as cocaine and marijuana. However, previous studies mainly focused on certain genes or neurotransmitters which have already been known to participate in drug addiction, lacking endogenous metabolic profiling in a global view. To further explore the mechanism by which nicotine modifies the response to cocaine, we developed two conditioned place preference (CPP) models in mice. In threshold dose model, mice were pretreated with nicotine, followed by cocaine treatment at the dose of 2 mg/kg, a threshold dose of cocaine to induce CPP in mice. In high-dose model, mice were only treated with 20 mg/kg cocaine, which induced a significant CPP. ¹H nuclear magnetic resonance based on metabonomics was used to investigate metabolic profiles of the nucleus accumbens (NAc) and striatum. We found that nicotine pretreatment dramatically increased CPP induced by 2 mg/kg cocaine, which was similar to 20 mg/kg cocaine-induced CPP. Interestingly, metabolic profiles showed considerable overlap between these two models. These overlapped metabolites mainly included neurotransmitters as well as the molecules participating in energy homeostasis and cellular metabolism. Our results show that the reinforcing effect of nicotine on behavioral response to cocaine may attribute to the modification of some specific metabolites in NAc and striatum, thus creating a favorable metabolic environment for enhancing conditioned rewarding effect of cocaine. Our findings provide an insight into the effect of cigarette smoking on cocaine dependence and the underlying mechanism.

Nicotine reinforcement is reduced by cannabinoid CB1 receptor blockade in the ventral tegmental area

Cannabinoid type 1 (CB1) receptors control the motivational properties and reinforcing effects of nicotine. Indeed, peripheral administration of a CB1 receptor antagonist dramatically decreases both nicotine taking and seeking. However, the neural substrates through which the cannabinoid CB1 receptors regulate the voluntary intake of nicotine remain to be elucidated. In the present study, we sought to determine whether central injections of a CB1 receptor antagonist delivered either into the ventral tegmental area (VTA) or the nucleus accumbens (NAC) may alter nicotine intravenous self-administration (IVSA). Rats were first trained to self-administer nicotine (30 μg/kg/0.1 ml). The effect of central infusions of the CB1 antagonist AM 251 (0, 1 and 10 μg/0.5 μl/side) on nicotine-taking behavior was then tested. Intra-VTA infusions of AM 251 dose dependently reduced IVSA with a significant decrease for the dose 10 μg/0.5 μl/side. Moreover, operant responding for water was unaltered by intra-VTA AM 251 at the same dose. Surprisingly, intra-NAC delivery of AM 251 did not alter nicotine behavior at all. These data suggest that in rats chronically exposed to nicotine IVSA, the cannabinoid CB1 receptors located in the VTA rather than in the NAC specifically control nicotine reinforcement and, subsequently, nicotine-taking behavior.

Cortical control of VTA function and influence on nicotine reward

Tobacco use is a major public health problem. Nicotine acts on widely distributed nicotinic acetylcholine receptors (nAChRs) in the brain and excites dopamine (DA) neurons in the ventral tegmental area (VTA). The elicited increase of DA neuronal activity is thought to be an important mechanism for nicotine reward and subsequently the transition to addiction. However, the current understanding of nicotine reward is based predominantly on the data accumulated from in vitro studies, often from VTA slices. Isolated VTA slices artificially terminate communications between neurons in the VTA and other brain regions that may significantly alter nicotinic effects. Consequently, the mechanisms of nicotinic excitation of VTA DA neurons under in vivo conditions have received only limited attention. Building upon the existing knowledge acquired in vitro, it is now time to elucidate the integrated mechanisms of nicotinic reward on intact systems that are more relevant to understanding the action of nicotine or other addictive drugs. In this review, we summarize recent studies that demonstrate the impact of prefrontal cortex (PFC) on the modulation of VTA DA neuronal function and nicotine reward. Based on existing evidence, we propose a new hypothesis that PFC-VTA functional coupling serves as an integration mechanism for nicotine reward. Moreover, addiction may develop due to nicotine perturbing the PFC-VTA coupling and thereby eliminating the PFC-dependent cognitive control over behavior.

Potential substrates for nicotine and alcohol interactions: a focus on the mesocorticolimbic dopamine system

Epidemiological studies consistently find correlations between nicotine and alcohol use, yet the neural mechanisms underlying their interaction remain largely unknown. Nicotine and alcohol (i.e., ethanol) share many common molecular and cellular targets that provide potential substrates for nicotine-alcohol interactions. These targets for interaction often converge upon the mesocorticolimbic dopamine system, where the link to drug self-administration and reinforcement is well documented. Both nicotine and alcohol activate the mesocorticolimbic dopamine system, producing downstream dopamine signals that promote the drug reinforcement process. While nicotine primarily acts via nicotinic acetylcholine receptors, alcohol acts upon a wider range of receptors and molecular substrates. The complex pharmacological profile of these two drugs generates overlapping responses that ultimately intersect within the mesocorticolimbic dopamine system to promote drug use. Here we will examine overlapping targets between nicotine and alcohol and provide evidence for their interaction. Based on the existing literature, we will also propose some potential targets that have yet to be directly tested. Mechanistic studies that examine nicotine-alcohol interactions would ultimately improve our understanding of the factors that contribute to the associations between nicotine and alcohol use.

Impacts of chronic low-level nicotine exposure on Caenorhabditis elegans reproduction: identification of novel gene targets

Effects and mechanisms of chronic exposure to low levels of nicotine is an area fundamentally important however less investigated. We employed the model organism Caenorhabditis elegans to investigate potential impacts of chronic (24h) and low nicotine exposure (6.17-194.5 μM) on stimulus-response, reproduction, and gene expressions. Nicotine significantly affects the organism's response to touch stimulus (p=0.031), which follows a dose-dependent pattern. Chronic nicotine exposure promotes early egg-laying events and slightly increased egg productions during the first 72 h of adulthood. The expressions of 10 (egl-10, egl-44, hlh-14, ric-3, unc-103, unc-50, unc-68, sod-1, oxi-1, and old-1) out of 18 selected genes were affected significantly. Other tested genes were cat-4, egl-19, egl-47, egl-5, lin-39, unc-43, pink-1, and age-1. Changes in gene expression were more evident at low dosages than at relatively high levels. Genes implicated in reproduction, cholinergic signaling, and stress response were regulated by nicotine, suggesting widespread physiological impacts of nicotine.

Mesolimbic dopamine and habenulo-interpeduncular pathways in nicotine withdrawal

The majority of people who attempt to quit smoking without some assistance relapse within the first couple of weeks, indicating the increased vulnerability during the early withdrawal period. The habenula, which projects via the fasciculus retroflexus to the interpeduncular nucleus, plays an important role in the withdrawal syndrome. Particularly the α2, α5, and β4 subunits of the nicotinic acetylcholine receptor have critical roles in mediating the somatic manifestations of withdrawal. Furthermore, withdrawal from nicotine induces a hypodopaminergic state, but there is a relative increase in the sensitivity to phasic dopamine release that is caused by nicotine. Therefore, acute nicotine re-exposure causes a phasic DA response that more potently reinforces relapse to smoking during the withdrawal period.

Nicotine modulates the long-lasting storage of fear memory

Late post-training activation of the ventral tegmental area (VTA)-hippocampus dopaminergic loop controls the entry of information into long-term memory (LTM). Nicotinic acetylcholine receptors (nAChR) modulate VTA function, but their involvement in LTM storage is unknown. Using pharmacological and behavioral tools, we found that α7-nAChR-mediated cholinergic interactions between the pedunculopontine tegmental nucleus and the medial prefrontal cortex modulate the duration of fear-motivated memories, maybe by regulating the activation state of VTA-hippocampus dopamine connections.