It is not "either/or": activation and desensitization of nicotinic acetylcholine receptors both contribute to behaviors related to nicotine addiction and mood

Complex relationships of nicotinic receptor actions and cognitive functions

Nicotine has been shown in a variety of studies to improve cognitive function including learning, memory and attention. Nicotine both stimulates and desensitizes nicotinic receptors, thus acting both as an agonist and a net antagonist. The relative roles of these two actions for nicotine-induced cognitive improvement have not yet been fully determined. We and others have found that acute nicotinic antagonist treatment can improve learning and attention. Nicotine acts on a variety of nicotinic receptor subtypes. The relative role and interactions of neuronal nicotinic receptor subtypes for cognition also needs to be better characterized. Nicotine acts on nicotinic receptors in a wide variety of brain areas. The role of some of these areas such as the hippocampus has been relatively well studied but other areas like the thalamus, which has the densest nicotinic receptor concentration are still only partially characterized. In a series of studies we characterized nicotinic receptor actions, anatomic localization and circuit interactions, which are critical to nicotine effects on the cognitive functions of learning, memory and attention. The relative role of increases and decreases in nicotinic receptor activation by nicotine were determined in regionally specific studies of the hippocampus, the amygdala, the frontal cortex and the mediodorsal thalamic nucleus with local infusions of antagonists of nicotinic receptor subtypes (α7 and α4β2). The understanding of the functional neural bases of cognitive function is fundamental to the more effective development of nicotinic drugs for treating cognitive dysfunction.

Nicotinic acetylcholine receptors controlling attention: behavior, circuits and sensitivity to disruption by nicotine

Attention is a central cognitive function that enables long-term engagement in a task and suppression of irrelevant information to obtain future goals. The prefrontal cortex (PFC) is the main link in integrating emotional and motivational state of an animal to regulate top-down attentional processes. Acetylcholine modulates PFC neuronal networks by activating nicotinic acetylcholine receptors (nAChRs) to support attention. However, how neuronal activity changes in the PFC during attention and which nAChR subtypes mediate this is only rudimentarily understood, but progress is being made. Recently, exciting new insights were obtained in the dynamics of cholinergic signaling in the PFC and modes of acetylcholine transmission via nAChRs in the cortex. In addition, mechanisms are uncovered on how the PFC circuitry is regulated by nAChRs. Novel studies show that endogenous activation of nAChRs in the PFC plays a central role in controlling attention. Here, we review current insights into how different subtypes of nAChRs expressed by distinct types of neurons in the PFC circuitry shape attention. In addition we discuss the impact of nicotine on the cholinergic system and prefrontal cortical circuits. Low concentrations of nicotine, as experienced by smokers, interfere with cholinergic signaling. In the long-term exposure to nicotine during adolescence leads to maladaptive adaptations of the PFC circuitry, which ultimately leads to a decrement in attention performance, again emphasizing the importance of nAChRs in attention.

Multiple CNS nicotinic receptors mediate L-dopa-induced dyskinesias: studies with parkinsonian nicotinic receptor knockout mice

Accumulating evidence supports the idea that drugs acting at nicotinic acetylcholine receptors (nAChRs) may be beneficial for Parkinson's disease, a neurodegenerative movement disorder characterized by a loss of nigrostriatal dopaminergic neurons. Nicotine administration to parkinsonian animals protects against nigrostriatal damage. In addition, nicotine and nAChR drugs improve L-dopa-induced dyskinesias, a debilitating side effect of L-dopa therapy which remains the gold-standard treatment for Parkinson's disease. Nicotine exerts its antidyskinetic effect by interacting with multiple nAChRs. One approach to identify the subtypes specifically involved in L-dopa-induced dyskinesias is through the use of nAChR subunit null mutant mice. Previous work with β2 and α6 nAChR knockout mice has shown that α6β2* nAChRs were necessary for the development/maintenance of L-dopa-induced abnormal involuntary movements (AIMs). The present results in parkinsonian α4 nAChR knockout mice indicate that α4β2* nAChRs also play an essential role since nicotine did not reduce L-dopa-induced AIMs in such mice. Combined analyses of the data from α4 and α6 knockout mice suggest that the α6α4β2β3 subtype may be critical. In contrast to the studies with α4 and α6 knockout mice, nicotine treatment did reduce L-dopa-induced AIMs in parkinsonian α7 nAChR knockout mice. However, α7 nAChR subunit deletion alone increased baseline AIMs, suggesting that α7 receptors exert an inhibitory influence on L-dopa-induced AIMs. In conclusion, α6β2*, α4β2* and α7 nAChRs all modulate L-dopa-induced AIMs, although their mode of regulation varies. Thus drugs targeting one or multiple nAChRs may be optimal for reducing L-dopa-induced dyskinesias in Parkinson's disease.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

α4α6β2* nicotinic acetylcholine receptor activation on ventral tegmental area dopamine neurons is sufficient to stimulate a depolarizing conductance and enhance surface AMPA receptor function

Tobacco addiction is a serious threat to public health in the United States and abroad, and development of new therapeutic approaches is a major priority. Nicotine activates and/or desensitizes nicotinic acetylcholine receptors (nAChRs) throughout the brain. nAChRs in ventral tegmental area (VTA) dopamine (DA) neurons are crucial for the rewarding and reinforcing properties of nicotine in rodents, suggesting that they may be key mediators of nicotine's action in humans. However, it is unknown which nAChR subtypes are sufficient to activate these neurons. To test the hypothesis that nAChRs containing α6 subunits are sufficient to activate VTA DA neurons, we studied mice expressing hypersensitive, gain-of-function α6 nAChRs (α6L9'S mice). In voltage-clamp recordings in brain slices from adult mice, 100 nM nicotine was sufficient to elicit inward currents in VTA DA neurons via α6β2* nAChRs. In addition, we found that low concentrations of nicotine could act selectively through α6β2* nAChRs to enhance the function of 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) receptors on the surface of these cells. In contrast, α6β2* activation did not enhance N-methyl-D-aspartic acid receptor function. Finally, AMPA receptor (AMPAR) function was not similarly enhanced in brain slices from α6L9'S mice lacking α4 nAChR subunits, suggesting that α4α6β2* nAChRs are important for enhancing AMPAR function in VTA DA neurons. Together, these data suggest that activation of α4α6β2* nAChRs in VTA DA neurons is sufficient to support the initiation of cellular changes that play a role in addiction to nicotine. α4α6β2* nAChRs may be a promising target for future smoking cessation pharmacotherapy.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Two decades of smoking cessation treatment research on smokers with depression: 1990-2010

INTRODUCTION

Adults with depression smoke at higher rates than other adults leaving a large segment of this population, who already incur increased health-related risks, vulnerable to the enormous harmful consequences of smoking. Yet, the impact that depression has on smoking cessation is not clear due to the mixed results of past research. The primary aims of this review were to synthesize the research examining the relationship of depression to smoking cessation outcomes over a 20-year period, to examine the gender and racial composition of these studies, and to identify directions for future research.

METHODS

Potential articles published between January 1, 1990 and December 31, 2010 were identified through a MEDLINE search of the terms "clinical trial," "depression," and "smoking cessation." 68 studies used all three terms and met the inclusion criteria.

RESULTS

The majority of studies examined either a past diagnosis of major depression or current depression symptoms. Within the few studies that examined the interaction of gender and depression on smoking cessation, depression had a greater impact on treatment outcomes for women than men. No study reported examining the interactive impact of race and depression on treatment outcomes.

CONCLUSIONS

Although attention to the relationship of depression and smoking cessation outcomes has increased over the past 20 years, little information exists to inform a treatment approach for smokers with Current Major Depressive Disorder, Dysthymia, and Minor Depression and few studies report gender and racial differences in the relationship of depression and smoking cessation outcomes, thus suggesting major areas for targeted research.

Reinstatement of nicotine seeking is mediated by glutamatergic plasticity

Nicotine abuse and addiction is a major health liability. Nicotine, an active alkaloid in tobacco, is self-administered by animals and produces cellular adaptations in brain regions associated with drug reward, such as the nucleus accumbens. However, it is unknown whether, akin to illicit drugs of abuse such as cocaine or heroin, the adaptations endure and contribute to the propensity to relapse after discontinuing nicotine use. Using a rat model of cue-induced relapse, we made morphological and electrophysiological measures of synaptic plasticity, as well as quantified glutamate overflow, in the accumbens after 2 wk of withdrawal with extinction training. We found an enduring basal increase in dendritic spine head diameter and in the ratio of AMPA to NMDA currents in accumbens spiny neurons compared with yoked saline animals at 2 wk after the last nicotine self-administration session. This synaptic potentiation was associated with an increase in both AMPA (GluA1) and NMDA (GluN2A and GluN2B) receptor subunits, and a reduction in the glutamate transporter-1 (GLT-1). When nicotine seeking was reinstated by presentation of conditioned cues, there were parallel increases in behavioral responding, extracellular glutamate, and further increases in dendritic spine head diameter and ratio of AMPA to NMDA currents within 15 min. These findings suggest that targeting glutamate transmission might inhibit cue-induced nicotine seeking. In support of this hypothesis, we found that pharmacological inhibition of GluN2A with 3-Chloro-4-fluoro-N-[4-[[2-(phenylcarbonyl)hydrazino]carbonyl]benzyl]benzenesulfonamide (TCN-201) or GluN2B with ifenprodil abolished reinstated nicotine seeking. These results indicate that up-regulated GluN2A, GluN2B, and rapid synaptic potentiation in the accumbens contribute to cue-induced relapse to nicotine use.

Layer-specific interference with cholinergic signaling in the prefrontal cortex by smoking concentrations of nicotine

Adolescence is a period in which the developing prefrontal cortex (PFC) is sensitive to maladaptive changes when exposed to nicotine. Nicotine affects PFC function and repeated exposure to nicotine during adolescence impairs attention performance and impulse control during adulthood. Nicotine concentrations experienced by smokers are known to desensitize nicotinic acetylcholine receptors (nAChRs), but the impact thereof on PFC circuits is poorly understood. Here, we investigated how smoking concentrations of nicotine (100-300 nm) interfere with cholinergic signaling in the mouse PFC. nAChR desensitization depends on subunit composition. Since nAChR subunits are differentially expressed across layers of the PFC neuronal network, we hypothesized that cholinergic signaling through nAChRs across layers would suffer differentially from exposure to nicotine. Throughout the PFC, nicotine strongly desensitized responses to ACh in neurons expressing β2* nAChRs, whereas ACh responses mediated by α7 nAChRs were not hampered. The amount of desensitization of β2* nAChR currents depended on neuron type and cortical layer. β2*-mediated responses of interneurons in LII-III and LVI completely desensitized, while cholinergic responses in LV interneurons and LVI pyramidal cells showed less desensitization. This discrepancy depended on α5 subunit expression. Two-photon imaging of neuronal population activity showed that prolonged exposure to nicotine limited cholinergic signaling through β2* nAChRs to deep PFC layers where α5 subunits were expressed. Together, our results demonstrate a layer-dependent decrease in cholinergic activation of the PFC through nAChRs by nicotine. These mechanisms may be one of the first steps leading up to the pathophysiological changes associated with nicotine exposure during adolescence.

How cigarette smoking may increase the risk of anxiety symptoms and anxiety disorders: a critical review of biological pathways

Multiple studies have demonstrated an association between cigarette smoking and increased anxiety symptoms or disorders, with early life exposures potentially predisposing to enhanced anxiety responses in later life. Explanatory models support a potential role for neurotransmitter systems, inflammation, oxidative and nitrosative stress, mitochondrial dysfunction, neurotrophins and neurogenesis, and epigenetic effects, in anxiety pathogenesis. All of these pathways are affected by exposure to cigarette smoke components, including nicotine and free radicals. This review critically examines and summarizes the literature exploring the role of these systems in increased anxiety and how exposure to cigarette smoke may contribute to this pathology at a biological level. Further, this review explores the effects of cigarette smoke on normal neurodevelopment and anxiety control, suggesting how exposure in early life (prenatal, infancy, and adolescence) may predispose to higher anxiety in later life. A large heterogenous literature was reviewed that detailed the association between cigarette smoking and anxiety symptoms and disorders with structural brain changes, inflammation, and cell-mediated immune markers, markers of oxidative and nitrosative stress, mitochondrial function, neurotransmitter systems, neurotrophins and neurogenesis. Some preliminary data were found for potential epigenetic effects. The literature provides some support for a potential interaction between cigarette smoking, anxiety symptoms and disorders, and the above pathways; however, limitations exist particularly in delineating causative effects. The literature also provides insight into potential effects of cigarette smoke, in particular nicotine, on neurodevelopment. The potential treatment implications of these findings are discussed in regards to future therapeutic targets for anxiety. The aforementioned pathways may help mediate increased anxiety seen in people who smoke. Further research into the specific actions of nicotine and other cigarette components on these pathways, and how these pathways interact, may provide insights that lead to new treatment for anxiety and a greater understanding of anxiety pathogenesis.

Cognitive function during nicotine withdrawal: Implications for nicotine dependence treatment

Nicotine withdrawal is associated with deficits in neurocognitive function including sustained attention, working memory, and response inhibition. Several convergent lines of evidence suggest that these deficits may represent a core dependence phenotype and a target for treatment development efforts. A better understanding of the mechanisms underlying withdrawal-related cognitive deficits may lead to improve nicotine dependence treatment. We begin with an overview of the neurocognitive effects of withdrawal in rodent and human models, followed by discussion of the neurobehavioral mechanisms that are thought to underlie these effects. We then review individual differences in withdrawal-related neurocognitive effects including genetics, gender, and psychiatric comorbidity. We conclude with a discussion of the implications of this research for developing improved therapies, both pharmacotherapy and behavioral treatments, that target cognitive symptoms of nicotine withdrawal. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Molecules and circuits involved in nicotine addiction: The many faces of smoking

Tobacco smoking in humans is one of the most persistent and widespread addictions and is driven by nicotine in tobacco smoke. Over the last several decades, understanding of the molecular and cellular basis for nicotine addiction has increased tremendously as a result of pharmacological, molecular genetic, electrophysiological and behavioral studies of nicotine reinforcement. Studies of the biological basis for nicotine reinforcement has helped in the design of new treatments for smoking cessation such as varenicline; however, smokers report that they smoke for many reasons, including the ability to control symptoms of anxiety and depression or the desire to control appetite. Further, developmental exposure to tobacco smoke increases the likelihood of adult smoking. Here we review what is known about the molecular and circuit basis for a number of behaviors related to tobacco smoking. Leveraging the knowledge from studies of different behaviors mediated by nicotine receptors in multiple brain circuits could provide points of convergence that will inform future therapeutic development for smoking cessation. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

α4β2 Nicotinic receptors play a role in the nAChR-mediated decline in L-dopa-induced dyskinesias in parkinsonian rats

L-Dopa-induced dyskinesias are a serious long-term side effect of dopamine replacement therapy for Parkinson's disease for which there are few treatment options. Our previous studies showed that nicotine decreased l-dopa-induced abnormal involuntary movements (AIMs). Subsequent work with knockout mice demonstrated that α6β2* nicotinic receptors (nAChRs) play a key role. The present experiments were done to determine if α4β2* nAChRs are also involved in l-dopa-induced dyskinesias. To approach this, we took advantage of the finding that α6β2* nAChRs are predominantly present on striatal dopaminergic nerve terminals, while a significant population of α4β2* nAChRs are located on other neurons. Thus, a severe dopaminergic lesion would cause a major loss in α6β2*, but not α4β2* nAChRs. Experiments were therefore done in which rats were unilaterally lesioned with 6-hydroxydopamine, at a dose that led to severe nigrostriatal damage. The dopamine transporter, a dopamine nerve terminal marker, was decreased by >99%. This lesion also decreased striatal α6β2* nAChRs by 97%, while α4β2* nAChRs were reduced by only 12% compared to control. A series of β2* nAChR compounds, including TC-2696, TI-10165, TC-8831, TC-10600 and sazetidine reduced l-dopa-induced AIMs in these rats by 23-32%. TC-2696, TI-10165, TC-8831 were also tested for parkinsonism, with no effect on this behavior. Tolerance did not develop with up to 3 months of treatment. Since α4α5β2 nAChRs are also predominantly on striatal dopamine terminals, these data suggest that drugs targeting α4β2 nAChRs may reduce l-dopa-induced dyskinesias in late stage Parkinson's disease.

Chemistry and pharmacological studies of 3-alkoxy-2,5-disubstituted-pyridinyl compounds as novel selective α4β2 nicotinic acetylcholine receptor ligands that reduce alcohol intake in rats

Neuronal acetylcholine receptors mediate the addictive effects of nicotine and may also be involved in alcohol addiction. Varenicline, an approved smoking cessation medication, showed clear efficacy in reducing alcohol consumption in heavy-drinking smokers. More recently, sazetidine-A, which selectively desensitizes α4β2 nicotinic receptors, was shown to significantly reduce alcohol intake in a rat model. To develop novel therapeutics for treating alcohol use disorder, we designed and synthesized novel sazetidine-A analogues containing a methyl group at the 2-position of the pyridine ring. In vitro pharmacological studies revealed that some of the novel compounds showed overall pharmacological property profiles similar to that of sazetidine-A but exhibited reduced agonist activity across all nicotinic receptor subtypes tested. In rat studies, compound (S)-9 significantly reduced alcohol uptake. More importantly, preliminary results from studies in a ferret model indicate that these novel nAChR ligands have an improved adverse side-effect profile in comparison with that of varenicline.

Effects of COMT genotype on sensory gating and its modulation by nicotine: Differences in low and high P50 suppressors

Elevated smoking rates seen in schizophrenia populations may be an attempt to correct neuropathologies associated with deficient nicotinic acetylcholine receptors and/or dopaminergic systems using exogenous nicotine. However, nicotine's effects on cognitive processing and sensory gating have been shown to be baseline-dependent. Evidence of a restorative effect on sensory gating deficits by nicotine-like agonists has been demonstrated, however, its underlying mechanisms in the context of dopamine dysregulation are unclear. Catechol-O-methyltransferase (COMT), a key dopamine regulator in the brain, contains a co-dominant allele in which a valine-to-methionine substitution causes variations in enzymatic activity leading to reduced synaptic dopamine levels in the Val/Val genotype. Using a randomized, double-blind, placebo-controlled design with 57 non-smokers, this study examined the effects of COMT genotype on sensory gating and its modulation by nicotine in low vs. high suppressors. The results were consistent with the hypothesis that increased dopamine resulting from nicotine stimulation or Met allelic activity would benefit gating in low suppressors and impair gating in high suppressors, and that this gating improvement with nicotine would be more evident in Val carriers who were low suppressors, while the gating impairment would be more evident in Met carriers who were high suppressors. These findings reaffirm the importance of baseline-dependency and suggest a subtle relationship between COMT genotype and baseline-stratified levels of sensory gating, which may help to explain the variability of cognitive abilities in schizophrenia populations.

The nicotine-mediated decline in l-dopa-induced dyskinesias is associated with a decrease in striatal dopamine release

l-dopa-induced dyskinesias (LIDs) are a side effect of Parkinson's disease therapy that is thought to arise, at least in part, because of excessive dopaminergic activity. Thus, drugs that regulate dopaminergic tone may provide an approach to manage LIDs. Our previous studies showed that nicotine treatment reduced LIDs in Parkinsonian animal models. This study investigates whether nicotine may exert its beneficial effects by modulating pre-synaptic dopaminergic function. Rats were unilaterally lesioned by injection of 6-hydroxydopamine (6-OHDA) (2 × 3 ug per site) into the medial forebrain bundle to yield moderate Parkinsonism. They were then implanted with minipumps containing vehicle or nicotine (2.0 mg/kg/d) and rendered dyskinetic with l-dopa (8 mg/kg plus 15 mg/kg benserazide). Lesioning alone decreased the striatal dopamine transporter, nicotinic receptor (nAChR) levels, and nAChR-mediated (3)H-dopamine release, consistent with previous results. Nicotine administration reduced l-dopa-induced abnormal involuntary movements throughout the course of the study (4 months). Nicotine treatment led to declines in the striatal dopamine transporter, α6β2* nAChRs and various components of α6β2* and α4β2* nAChR-mediated release. l-dopa treatment had no effect. These data suggest that nicotine may improve LIDs in Parkinsonian animal models by dampening striatal dopaminergic activity.

Effects of chronic sazetidine-A, a selective α4β2 neuronal nicotinic acetylcholine receptors desensitizing agent on pharmacologically-induced impaired attention in rats

RATIONALE

Nicotine and nicotinic agonists have been shown to improve attentional function. Nicotinic receptors are easily desensitized, and all nicotinic agonists are also desensitizing agents. Although both receptor activation and desensitization are components of the mechanism that mediates the overall effects of nicotinic agonists, it is not clear how each of the two opposed actions contributes to attentional improvements. Sazetidine-A has high binding affinity at α4β2 nicotinic receptors and causes a relatively brief activation followed by a long-lasting desensitization of the receptors. Acute administration of sazetidine-A has been shown to significantly improve attention by reversing impairments caused by the muscarinic cholinergic antagonist scopolamine and the NMDA glutamate antagonist dizocilpine.

METHODS

In the current study, we tested the effects of chronic subcutaneous infusion of sazetidine-A (0, 2, or 6 mg/kg/day) on attention in Sprague-Dawley rats. Furthermore, we investigated the effects of chronic sazetidine-A treatment on attentional impairment induced by an acute administration of 0.02 mg/kg scopolamine.

RESULTS

During the first week period, the 6-mg/kg/day sazetidine-A dose significantly reversed the attentional impairment induced by scopolamine. During weeks 3 and 4, the scopolamine-induced impairment was no longer seen, but sazetidine-A (6 mg/kg/day) significantly improved attentional performance on its own. Chronic sazetidine-A also reduced response latency and response omissions.

CONCLUSIONS

This study demonstrated that similar to its acute effects, chronic infusions of sazetidine-A improve attentional performance. The results indicate that the desensitization of α4β2 nicotinic receptors with some activation of these receptors may play an important role in improving effects of sazetidine-A on attention.

Translational research in nicotine dependence

Nicotine addiction accounts for 4.9 million deaths each year. Furthermore, although smoking represents a significant health burden in the United States, at present there are only three FDA-approved pharmacotherapies currently on the market: (1) nicotine replacement therapy, (2) bupropion, and (3) varenicline. Despite this obvious gap in the market, the complexity of nicotine addiction in addition to the increasing cost of drug development makes targeted drug development prohibitive. Furthermore, using combinations of mouse and human studies, additional treatments could be developed from off-the-shelf, currently approved medication lists. This article reviews translational studies targeting manipulations of the cholinergic system as a viable therapeutic target for nicotine addiction.

Improvement of attentional function with antagonism of nicotinic receptors in female rats

Nicotinic agonists have been shown in a variety of studies to improve cognitive function. Since nicotinic receptors are easily desensitized by agonists, it is not completely clear to what degree receptor desensitization or receptor activation are responsible for nicotinic agonist-induced cognitive improvement. In the current study, the effect of the neuronal nicotinic cholinergic α4β2 receptor antagonist dihydro-β-erythroidine (DHβE) and the α7 nicotinic receptor antagonist methyllycaconitine (MLA) on attentional function was determined. Adult female Sprague-Dawley rats were trained on the visual signal detection task. They were required to discriminate whether or not a light signal occurred on a trial and respond with a lever press on one side after a signal and the opposite side after the absence of a signal in order to receive a food pellet reinforcer. Acute administration of the α4β2 antagonist DHβE improved attentional function either alone or in reversing the attentional impairment caused by the NMDA glutamate antagonist dizocilpine (MK-801). Acute administration of MLA also significantly attenuated the dizocilpine-induced attentional impairment. In previous research we have shown that the α4β2 nicotinic desensitizing agent and partial agonist sazetidine-A also was effective in reversing dizocilpine-induced attentional impairments on the signal detection task and that low doses of the general nicotinic antagonist mecamylamine improved learning and memory. The current studies indicate that blockade of nicotinic receptors can effectively attenuate attentional impairments. Development of drugs that provide a net decrease in nicotinic receptor activity either through antagonism or desensitization could be worth exploring for beneficial effects for treating cognitive impairments.

Nicotine content and abstinence state have different effects on subjective ratings of positive versus negative reinforcement from smoking

Despite the well-known adverse health consequences of smoking, approximately 20% of US adults smoke tobacco cigarettes. Much of the research on smoking reinforcement and the maintenance of tobacco smoking behavior has focused on nicotine; however, a number of other non-nicotine factors are likely to influence the reinforcing effects of smoked tobacco. A growing number of studies suggest that non-nicotine factors, through many pairings with nicotine, are partially responsible for the reinforcing effect of smoking. Additionally, both clinical studies and preclinical advances in our understanding of nicotinic receptor regulation suggest that abstinence from smoking may influence smoking reinforcement. These experiments were conducted for 2 reasons: to validate a MRI-compatible cigarette smoking device; and to simultaneously investigate the impact of nicotine, smoking-associated conditioned reinforcers, and smoking abstinence state on subjective ratings of smoking reinforcement. Participants smoked nicotine and placebo cigarettes through an fMRI compatible device in an overnight-abstinent state or in a nonabstinent state, after having smoked a cigarette 25minutes prior. Outcome measures were within-subject changes in physiology and subjective ratings of craving and drug effect during the smoking of nicotine or placebo cigarettes on different days in both abstinence states. Cigarette type (nicotine vs. placebo) had a significant effect on positive subjective ratings of smoking reinforcement ("High", "Like Drug", "Feel Drug"; nicotine>placebo). In contrast, abstinence state was found to have significant effects on both positive and negative ratings of smoking reinforcement ("Crave", "Anxiety", "Irritability"; abstinence>nonabstinence). Interaction effects between abstinence and nicotine provide clues about the importance of neuroadaptive mechanisms operating in dependence, as well as the impact of conditioned reinforcement on subjective ratings of smoking-induced high.

The role of estrogen receptor β and nicotinic cholinergic receptors in postpartum depression

Postpartum depression (PPD) is a devastating disease occurring in approximately 20% of women. Women who suffer from PPD appear to be more sensitive to postpartum hormonal changes than women who do not experience this form of depression. Furthermore, women who quit smoking prior to or during pregnancy, and who develop PPD, are at an increased risk of smoking relapse. Unfortunately, the mechanistic relationship between the pathophysiology of PPD and smoking relapse is unknown. Here we review the roles of both estrogen receptor beta (ERβ) and cholinergic nicotinic receptors (nAChRs) in the pathogenesis of depression and propose a mechanistic rationale to explain the high rate of smoking relapse exhibited by women who develop PPD.

The stereotypy-inducing and OCD-like effects of chronic 'binge' cocaine are modulated by distinct subtypes of nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

High rates of cigarette smoking occur in cocaine-dependent individuals, reflecting an involvement of nicotinic acetylcholine receptors (nAChRs) in cocaine-elicited behaviour. This study was designed to assess the contribution of different nAChR subtypes to the behavioural and neurochemical effects of chronic cocaine treatment.

EXPERIMENTAL APPROACH

Cocaine (15 mg·kg(-1) , i.p.) was administered to male C57BL/6J mice in a chronic 'binge' paradigm, with and without the coadministration of the α7 preferring nAChR antagonist methyllycaconitine (MLA; 5 mg·kg(-1) , i.p.) or the β2* nAChR antagonist dihydro-β-erythroidine (DHβE; 2 mg·kg(-1) , i.p.). Quantitative autoradiography was used to examine the effect of cocaine exposure on α7 and α4β2* nAChRs, and on the high-affinity choline transporter.

KEY RESULTS

MLA+cocaine administration induced an intense self-grooming behaviour, indicating a likely role for α7 nAChRs in modulating this anxiogenic, compulsive-like effect of cocaine. In the major island of Calleja, a key area of action for neuroleptics, MLA+cocaine reduced choline transporter binding compared with cocaine (with or without DHβE) administration. DHβE treatment prevented the induction of stereotypy sensitisation to cocaine but prolonged locomotor sensitisation, implicating heteromeric β2* nAChRs in the neuroadaptations mediating cocaine-induced behavioural sensitisation. 'Binge' cocaine treatment region-specifically increased α4β2* nAChR binding in the midbrain dopaminergic regions: ventral tegmental area and substantia nigra pars compacta.

CONCLUSIONS AND IMPLICATIONS

We have shown a differential, subtype-selective, contribution of nAChRs to the behavioural and neurochemical sequelae of chronic cocaine administration. These data support the clinical utility of targeting specific nAChR subtypes for the alleviation of cocaine-abuse symptomatology.

Optimizing cholinergic tone through lynx modulators of nicotinic receptors: implications for plasticity and nicotine addiction

The cholinergic system underlies both adaptive (learning and memory) and nonadaptive (addiction and dependency) behavioral changes through its ability to shape and regulate plasticity. Protein modulators such as lynx family members can fine tune the activity of the cholinergic system and contribute to the graded response of the cholinergic system, stabilizing neural circuitry through direct interaction with nicotinic receptors. Release of this molecular brake can unmask cholinergic-dependent mechanisms in the brain. Lynx proteins have the potential to provide top-down control over plasticity mechanisms, including addictive propensity. If this is indeed the case, then, what regulates the regulator? Transcriptional changes of lynx genes in response to pharmacological, physiological, and pathological alterations are explored in this review.

Effects of chronic low- and high-dose nicotine on cognitive flexibility in C57BL/6J mice

The addictive nature of nicotine remains a global health problem. Despite the availability of treatments for smoking cessation, relapse to smoking after quit attempts still remains very high. Here, we evaluated the effects of chronic nicotine in male C57BL/6J mice in an operant cognitive flexibility task that required the animals to progress sequentially through multiple phases including visual discrimination, strategy shifting and response reversal. As frontostriatal circuits involving discrete regions of dorsal striatum contribute directly to decision-making processes, and BDNF modulates synaptic plasticity and learning, we also assessed the effects of nicotine on striatal BDNF expression. Osmotic minipumps containing either of the two doses of nicotine (low: 6.3 mg/kg/day; high: 18 mg/kg/day) or saline (control) were implanted for chronic delivery that lasted 4 weeks. Nicotine-treated mice exhibited greater response accuracy during visual discrimination. Neither dose of nicotine affected learning of new egocentric response strategy during set-shifting. However, higher but not lower dose of nicotine impaired reversal learning by increasing perseverative responding to the previously non-reinforced stimulus. Furthermore, this effect was associated with reduced BDNF levels in the dorsal striatum. Collectively, these findings suggest that higher relapse rates often observed in high nicotine-dependent smokers may be attributed to impairments in inhibitory control processes. Moreover, striatal BDNF may play a critical role in nicotine-induced alterations in cognitive flexibility.

Neuroadaptation in nicotine addiction: update on the sensitization-homeostasis model

The role of neuronal plasticity in supporting the addictive state has generated much research and some conceptual theories. One such theory, the sensitization-homeostasis (SH) model, postulates that nicotine suppresses craving circuits, and this triggers the development of homeostatic adaptations that autonomously support craving. Based on clinical studies, the SH model predicts the existence of three distinct forms of neuroplasticity that are responsible for withdrawal, tolerance and the resolution of withdrawal. Over the past decade, many controversial aspects of the SH model have become well established by the literature, while some details have been disproven. Here we update the model based on new studies showing that nicotine dependence develops through a set sequence of symptoms in all smokers, and that the latency to withdrawal, the time it takes for withdrawal symptoms to appear during abstinence, is initially very long but shortens by several orders of magnitude over time. We conclude by outlining directions for future research based on the updated model, and commenting on how new experimental studies can gain from the framework put forth in the SH model.

The relationship of major depressive disorder and gender to changes in smoking for current and former smokers: longitudinal evaluation in the US population

AIMS

Although depression and smoking are correlated highly, the relationship of major depressive disorder (MDD) to smoking cessation and relapse remains unclear. This study compared changes in smoking for current and former smokers with and without current and life-time MDD over a 3-year period.

DESIGN

Analysis of two waves of longitudinal data from the National Institute on Alcohol Abuse and Alcoholism's National Epidemiologic Survey on Alcohol and Related Conditions (wave 1, 2001-02; wave 2, 2004-05).

SETTING

Data were collected through face-to-face interviews from non-institutionalized United States civilians, aged 18 years and older, in 50 states and the District of Columbia.

PARTICIPANTS

A total of 11 973 adults (54% male) classified as current or former daily smokers at wave 1 and completed wave 2.

MEASUREMENTS

Classification as current or former smokers at wave 1 and wave 2.

FINDINGS

Smoking status remained stable for most participants. Wave 1 current daily smokers with current MDD [odds ratio (OR) = 1.38, 95% confidence interval (CI): 1.03, 1.85] and life-time MDD (OR = 1.52, 95% CI: 1.15, 2.01) were more likely than those without the respective diagnosis to report continued smoking at wave 2. Wave 1 former daily smokers with current MDD (OR = 0.44, 95% CI: 0.26, 0.76) were less likely to report continued abstinence at wave 2. None of the gender × MDD diagnosis interactions were significant. Patterns of results remained similar when analyses were limited to smokers with nicotine dependence.

CONCLUSIONS

Current and life-time major depressive disorders are associated with a lower likelihood of quitting smoking and current major depressive disorder is associated with greater likelihood of smoking relapse.

Nicotine-mediated improvement in L-dopa-induced dyskinesias in MPTP-lesioned monkeys is dependent on dopamine nerve terminal function

L-dopa-induced dyskinesias (LIDs) are abnormal involuntary movements that develop with long term L-dopa therapy for Parkinson's disease. Studies show that nicotine administration reduced LIDs in several parkinsonian animal models. The present work was done to understand the factors that regulate the nicotine-mediated reduction in LIDs in MPTP-lesioned nonhuman primates. To approach this, we used two groups of monkeys, one with mild-moderate and the other with more severe parkinsonism rendered dyskinetic using L-dopa. In mild-moderately parkinsonian monkeys, nicotine pretreatment (300 μg/ml via drinking water) prevented the development of LIDs by ~75%. This improvement was maintained when the nicotine dose was lowered to 50 μg/ml but was lost with nicotine removal. Nicotine re-exposure again decreased LIDs. By contrast, nicotine treatment did not reduce LIDs in monkeys with more severe parkinsonism. We next determined how nicotine's ability to reduce LIDs correlated with lesion-induced changes in the striatal dopamine transporter and (3)H-dopamine release in these two groups of monkeys. The striatal dopamine transporter was reduced to 54% and 28% of control in mild-moderately and more severely parkinsonian monkeys, respectively. However, basal, K(+), α4β2* and α6β2* nAChR-evoked (3)H-dopamine release were near control levels in striatum of mild-moderately parkinsonian monkeys. By contrast, these same release measures were reduced to a significantly greater extent in striatum of more severely parkinsonian monkeys. Thus, nicotine best improves LIDs in lesioned monkeys in which striatal dopamine transmission is still relatively intact. These data suggest that nicotine treatment would most effectively reduce LIDs in patients with mild to moderate Parkinson's disease.

Effects of the novel α7 nicotinic acetylcholine receptor agonist ABT-107 on sensory gating in DBA/2 mice: pharmacodynamic characterization

Nicotinic acetylcholine receptor (nAChR) agonists improve sensory gating deficits in animal models and schizophrenic patients. The aim of this study was to determine whether the novel and selective α7 nAChR full agonist 5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy]pyridazin-3-yl)-1H-indole (ABT-107) improves sensory gating deficits in DBA/2 mice. Sensory gating was measured by recording hippocampal-evoked potential P20-N40 waves and determining gating test/conditioning (T/C) ratios in a paired auditory stimulus paradigm. ABT-107 at 0.1 μmol/kg (average plasma concentration of 1.1 ng/ml) significantly improved sensory gating by lowering T/C ratios during a 30-min period after administration in unanesthetized DBA/2 mice. ABT-107 at 1.0 μmol/kg was ineffective at 30 min after administration when average plasma levels were 13.5 ng/ml. However, the 1.0 μmol/kg dose was effective 180 min after administration when plasma concentration had fallen to 1.9 ng/ml. ABT-107 (0.1 μmol/kg) also improved sensory gating in anesthetized DBA/2 mice pretreated with α7 nAChR-desensitizing doses of nicotine (6.2 μmol/kg) or ABT-107 (0.1 μmol/kg) itself. Moreover, repeated b.i.d. dosing of ABT-107 (0.1 μmol/kg) was as efficacious as a single dose. The acute efficacy of ABT-107 (0.1 μmol/kg) was blocked by the nAChR antagonist methyllycaconitine, but not by the α4β2 nAChR antagonist dihydro-β-erythroidine. These studies demonstrate that ABT-107 improves sensory gating through the activation of nAChRs, and efficacy is sustained under conditions of repeated dosing or with prior nAChR activation with nicotine.

Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice

RATIONALE

Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4β2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment.

OBJECTIVES

The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4β2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration.

METHODS

Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague-Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day).

RESULTS

4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration.

CONCLUSIONS

These results support the hypothesis that neuronal α4β2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4β2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.

Chronic sazetidine-A at behaviorally active doses does not increase nicotinic cholinergic receptors in rodent brain

Chronic nicotine administration increases α4β2 neuronal nicotinic acetylcholine receptor (nAChR) density in brain. This up-regulation probably contributes to the development and/or maintenance of nicotine dependence. nAChR up-regulation is believed to be triggered at the ligand binding site, so it is not surprising that other nicotinic ligands also up-regulate nAChRs in the brain. These other ligands include varenicline, which is currently used for smoking cessation therapy. Sazetidine-A (saz-A) is a newer nicotinic ligand that binds with high affinity and selectivity at α4β2* nAChRs. In behavioral studies, saz-A decreases nicotine self-administration and increases performance on tasks of attention. We report here that, unlike nicotine and varenicline, chronic administration of saz-A at behaviorally active and even higher doses does not up-regulate nAChRs in rodent brains. We used a newly developed method involving radioligand binding to measure the concentrations and nAChR occupancy of saz-A, nicotine, and varenicline in brains from chronically treated rats. Our results indicate that saz-A reached concentrations in the brain that were ∼150 times its affinity for α4β2* nAChRs and occupied at least 75% of nAChRs. Thus, chronic administration of saz-A did not up-regulate nAChRs despite it reaching brain concentrations that are known to bind and desensitize virtually all α4β2* nAChRs in brain. These findings reinforce a model of nicotine addiction based on desensitization of up-regulated nAChRs and introduce a potential new strategy for smoking cessation therapy in which drugs such as saz-A can promote smoking cessation without maintaining nAChR up-regulation, thereby potentially increasing the rate of long-term abstinence from nicotine.

Insights into the neurobiology of the nicotinic cholinergic system and nicotine addiction from mice expressing nicotinic receptors harboring gain-of-function mutations

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated, cation-selective ion channels expressed throughout the brain. Although these channels have been investigated for several decades, it is still challenging 1) to identify the important nAChR subunits in cholinergic transmission and nicotine dependence and 2) to develop nAChR subtype-specific ligands. To overcome these challenges, we and others have studied mice expressing mutant, gain-of-function nAChR subunits. In this review, we discuss this research approach and the results it has yielded to date. Gain-of-function mutations, including those in nAChR subunits, provide an approach that is complementary to loss-of-function studies such as gene knockouts; the former allows one to answer questions of sufficiency and the latter addresses questions of necessity. Mutant mice expressing gain-of-function nAChR subunits are commonly produced using traditional gene targeting in embryonic stem cells, but novel approaches such as bacterial artificial chromosome transgenesis have yielded important insights as well. α7 nAChRs were the first nAChRs to be targeted with a gain-of-function mutation, followed by a pair of α4 nAChR gain-of-function mutant mice. These α4 nAChR gain-of-function mice (α4 L9'S mice, followed by α4 L9'A mice) provided an important system to probe α4 nAChR function in vivo, particularly in the dopamine reward system. α6 nAChR gain-of-function mice provided the first robust system allowing specific manipulation of this receptor subtype. Other targeted mutations in various nAChR subunits have also been produced and have yielded important insights into nicotinic cholinergic biology. As nAChR research advances and more details associated with nAChR expression and function emerge, we expect that existing and new mouse lines expressing gain-of-function nAChR subunits will continue to provide new insights.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Neural expression of nicotine's antidepressant properties during tryptophan depletion: an EEG study in healthy volunteers at risk for depression

Nicotine amelioration of serotonergically mediated mood dysregulation may contribute to the comorbidity between cigarette smoking and depression, a disorder which is associated with aberrant activation and hemispheric asymmetry in frontal and posterior cortical regions. This randomized, double-blind study in 20 healthy volunteers with a positive family history of depression examined the effects of transdermal nicotine on mood and EEG changes accompanying transient reductions in serotonin induced by acute tryptophan depletion (ATD). Increased self-ratings of depressed mood and elevation in left frontal high alpha power (decreased activation) were evidenced with ATD (vs. balanced mixture) in participants treated with the placebo but not the nicotine treated group. Nicotine alone increased vigor and posterior high alpha bilaterally, and during ATD it prevented the reduction in left frontal high alpha that was evident in the placebo patch group. These findings indicate that in depression prone individuals, nicotine acts to stabilize the mood lowering and associated frontal functional asymmetry elicited by an acute decrease in brain serotonin.

A single administration of low-dose varenicline saturates α4β2* nicotinic acetylcholine receptors in the human brain

The primary objective of this project was to determine the α4β2(*) nicotinic acetylcholine receptor (nAChR) occupancy in human brain of a single low dose of varenicline (0.5 mg), and to explore the relationship between receptor occupancy by varenicline and tobacco withdrawal symptoms ((*)denoting other putative nAChR subunits). Otherwise healthy smokers (n=9) underwent two positron emission tomography (PET) sessions with the selective α4β2(*) radioligand 2-FA. For the PET sessions, participants received either a low dose of varenicline (0.5 mg) or matching placebo pill (double-blind, random order) before imaging. For both sessions, participants received bolus plus continuous infusions of 2-FA, were scanned for 1 h after allowing the radiotracer to reach a steady state, smoked to satiety, and were scanned for 2 more hours. We estimated the fractional receptor occupancy by a single dose of varenicline (0.5 mg) and the corresponding varenicline dissociation constant (K(V)), along with the effect of low-dose varenicline, pill placebo, and smoking-to-satiety on withdrawal rating scales. The data are compatible with 100% occupancy of α4β2(*) nAChRs by a single dose of varenicline, with a 90% lower confidence limit of 89% occupancy for the thalamus and brainstem. The corresponding 90% upper limit on effective K(V) with respect to plasma varenicline was 0.49 nM. Smoking to satiety, but not low-dose varenicline, significantly reduced withdrawal symptoms. Our findings demonstrate that low-dose varenicline results in saturation of α4β2(*) nAChRs in the thalamus and brainstem without reducing withdrawal symptoms.

Neuroplasticity in cigarette smokers is altered under withdrawal and partially restituted by nicotine exposition

Nicotine improves cognitive functions by modulating neuroplasticity and cortical excitability in nonsmoking subjects. As shown recently, the positive effect of nicotine on cognition might at least partially be caused by a focusing effect of nicotine on neuroplasticity in these subjects. Concordant to this, smokers under nicotine withdrawal show reduced cognitive abilities, which are at least partially restituted by nicotine consumption. We aimed to explore the neurophysiological foundation of these effects by exploring nonfocal and focal plasticity-inducing protocols in human smokers under nicotine withdrawal and exposition. Focal, synapse-specific plasticity was induced by paired associative stimulation (PAS), while nonfocal plasticity was induced by transcranial direct current stimulation (tDCS). Each subject (12) received placebo and nicotine patches combined with one of the stimulation protocols to the primary motor cortex. Corticospinal excitability was monitored by transcranial magnetic stimulation-induced motor-evoked potential amplitudes. In smokers during nicotine withdrawal, facilitatory plasticity induced by tDCS and PAS was abolished, but restituted by nicotine. In contrast, excitability-diminishing plasticity was not affected by nicotine withdrawal. Under nicotine, the inhibitory aftereffects of PAS were delayed and prolonged, while the tDCS-generated excitability reduction was abolished. Thus, absent facilitatory plasticity in smokers during nicotine withdrawal is restituted by nicotine, favoring the deficit-compensating hypothesis of nicotine consumption. These results might shed further light on the proposed mechanism of nicotine on cognition and attention, which might be connected to nicotine addiction and probability of relapse in smokers.

Nicotine vaccines to assist with smoking cessation: current status of research

Tobacco smoking causes cardiovascular, respiratory and malignant disease, and stopping smoking is among the key medical interventions to lower the worldwide burden of these disorders. However, the addictive properties of cigarette smoking, including nicotine inhalation, render most quit attempts unsuccessful. Recommended therapies, including combinations of counselling and medication, produce long-term continuous abstinence rates of no more than 30%. Thus, more effective treatment options are needed.

An intriguing novel therapeutic concept is vaccination against nicotine. The basic principle of this approach is that, after entering the systemic circulation, a substantial proportion of nicotine can be bound by antibodies. Once bound to antibodies, nicotine is no longer able to cross the blood-brain barrier. As a consequence, the rewarding effects of nicotine are diminished, and relapse to smoking is less likely to occur. Animal studies indicate that antibodies profoundly change the pharmacokinetics of the drug and can interfere with nicotine self-administration and impact on the severity of withdrawal symptoms. To date, five phase I/II clinical trials using vaccines against nicotine have been published. Results have been disappointing in that an increase in quit rates was only observed in small groups of smokers displaying particularly high antibody titres.

The failure of encouraging preclinical data to completely translate to clinical studies may be partially explained by shortcomings of animal models of addiction and an incomplete understanding of the complex physiological and behavioural processes contributing to tobacco addiction. This review summarizes the current status of research and suggests some directions for the future development of vaccines against nicotine. Ideally, these vaccines could one day become part of a multifaceted approach to treating tobacco addiction that includes counselling and pharmacotherapy.

Partial agonists for α4β2 nicotinic receptors stimulate dopaminergic neuron firing with relatively enhanced maximal effects

BACKGROUND AND PURPOSE

Partial agonists selective for α4β2 nicotinic ACh receptors have been developed for smoking cessation as they induce weak activation of native α4β2* receptors and inhibit effect of nicotine. However, it is unclear whether at brain functions there is an existence of receptor reserve that allows weak receptor activation to induce maximum physiological effects. We assessed the extent of α4β2 partial agonist-induced increase of firing rate in dopaminergic neurons and evaluated the influence of receptor reserve.

EXPERIMENTAL APPROACH

The relative maximal effects and potencies of six nicotinic agonists were assessed on recombinant human α4β2 and α7 receptors expressed in mammalian cell lines by measuring calcium influx. Agonist-induced increase of the spontaneous firing rate of dopaminergic neurons was recorded using microelectrodes in the ventral tegmental area of rat brain slices.

KEY RESULTS

All α4β2 partial and full agonists increased the firing rate concentration-dependently. Their sensitivity to subtype-selective antagonists showed predominant activation of native α4β2* receptors. However, partial agonists with relative maximal effects as low as 33% on α4β2 receptors maximally increased the firing rate and induced additional depolarization block of firing, demonstrating that partial activation of receptors caused the maximum increase in firing rate in the presence of a receptor reserve.

CONCLUSIONS AND IMPLICATIONS

Partial α4β2 agonists induced relatively enhanced effects on the firing rate of dopaminergic neurons, and the effect was mainly attributed to the existence of native α4β2* receptor reserve. The results have implications in the understanding of physiological effects and therapeutic efficacies of α4β2 partial agonists.

The nicotinic acetylcholine receptors of zebrafish and an evaluation of pharmacological tools used for their study

Zebrafish (Danio rerio) have been used to study multiple effects of nicotine, for example on cognition, locomotion, and stress responses, relying on the assumption that pharmacological tools will operate similarly upon molecular substrates in the fish and mammalian systems. We have cloned the zebrafish nicotinic acetylcholine receptor (nAChR) subunits and expressed key nAChR subtypes in Xenopus oocytes including neuronal (α4β2, α2β2, α3β4, and α7) and muscle (α1β1(b)ɛδ) nAChR. Consistent with studies of mammalian nAChR, nicotine was relatively inactive on muscle-type receptors, having both low potency and efficacy. It had high efficacy but low potency for α7 receptors, and the best potency and good efficacy for α4β2 receptors. Cytisine, a key lead compound for the development of smoking cessation agents, is a full agonist for both mammalian α7 and α3β4 receptors, but a full agonist only for the fish α7, with surprisingly low efficacy for α3β4. The efficacy of cytisine for α4β2 was somewhat greater than typically reported for mammalian α4β2. The ganglionic blocker mecamylamine was most potent for blocking α3β4 receptors, least potent for α7, and roughly equipotent for the muscle receptors and the β2-containing nAChR. However, the block of β2-containing receptors was slowly reversible, consistent with effective targeting of these CNS-type receptors in vivo. Three prototypical α7-selective agonists, choline, tropane, and 4OH-GTS-21, were tested, and these agents were observed to activate both fish α7 and α4β2 nAChR. Our data therefore indicate that while some pharmacological tools used in zebrafish may function as expected, others will not.

Long-term nicotine exposure depresses dopamine release in nonhuman primate nucleus accumbens

Tobacco use is a leading cause of preventable deaths worldwide. However, current smoking cessation therapies have very limited long-term success rates. Considerable research effort is therefore focused on identification of central nervous system changes with nicotine exposure because this may lead to more successful treatment options. Although recent work suggests that α6β2* nicotinic acetylcholine receptors (nAChRs) play a dominant role in dopaminergic function in rodent nucleus accumbens, the effects of long-term nicotine exposure remain to be determined. Here, we used cyclic voltammetry to investigate α6β2* nAChR-mediated release with long-term nicotine treatment in nonhuman primate nucleus accumbens shell. Control studies showed that nAChR-mediated dopamine release occurs predominantly through the α6β2* receptor subtype. Unexpectedly, there was a complete loss of α6β2* nAChR-mediated activity after several months of nicotine treatment. This decline in function was observed with both single- and multiple-pulse-stimulated dopamine release. Paired-pulse studies showed that the facilitation of dopamine release with multiple pulsing observed in controls in the presence of nAChR antagonist was lost with long-term nicotine treatment. Nicotine-evoked [(3)H]dopamine release from nucleus accumbens synaptosomes was similar in nicotine- and vehicle-treated monkeys, indicating that long-term nicotine administration does not directly modify α6β2* nAChR-mediated dopamine release. Dopamine uptake rates, as well as dopamine transporter and α6β2* nAChRs levels, were also not changed with nicotine administration. These data indicate that nicotine exposure, as occurs with smoking, has major effects on cellular mechanisms linked to α6β2* nAChR-mediated dopamine release and that this receptor subtype may represent a novel therapeutic target for smoking cessation.

Role for α6 nicotinic receptors in l-dopa-induced dyskinesias in parkinsonian mice

L-Dopa-induced dyskinesias are a serious side effect that develops in most Parkinson's disease patients on dopamine replacement therapy. Few treatment options are available to manage dyskinesias; however,recent studies show that nicotine reduces these abnormal involuntary movements (AIMs) in parkinsonian animals by acting at nicotinic acetylcholine receptors (nAChRs). Identification of the nAChR subtypes that mediate this reduction in AIMs is important as it will help in the development of nAChR subtype selective drugs for their treatment. Here we investigate the role of α6β2* nAChRs, a subtype selectively present in the nigrostriatal pathway, using a6 nAChR subunit null mutant (α6⁻/⁻) mice.Wildtype and α6⁻/⁻ mice were lesioned by unilateral injection of 6-hydroxydopamine (3 mg/ml) into the medial forebrain bundle. They were then given L-dopa (3 mg/kg) plus benserazide (15 mg/kg) 2e3 wk later. L-dopa-induced AIMs developed to a similar extent in α6⁻/⁻ and wildtype mice.However, AIMs in α6⁻/⁻ mice declined to ~50% of that in wildtype mice with continued L-dopa treatment. Nicotine treatment also decreased AIMs by ~50% in wildtype mice, although not in α6⁻/⁻ mice. There were no effects on parkinsonism under any experimental condition. To conclude, the similar declines in L-dopa-induced AIMs in nicotine-treated wildtype mice and in α6⁻/⁻ mice treated with and without nicotine indicate an essential role for α6β2* nAChRs in the maintenance of L-dopa-induced AIMs.These findings suggest that α6β2* nAChR drugs have potential for reducing L-dopa-induced dyskinesias in Parkinson's disease.

Varenicline is a potent partial agonist at α6β2* nicotinic acetylcholine receptors in rat and monkey striatum

Extensive evidence indicates that varenicline reduces nicotine craving and withdrawal symptoms by modulating dopaminergic function at α4β2* nicotinic acetylcholine receptors (nAChRs) (the asterisk indicates the possible presence of other nicotinic subunits in the receptor complex). More recent data suggest that α6β2* nAChRs also regulate dopamine release and mediate nicotine reinforcement. The present experiments were therefore done to test the effect of varenicline on α6β2* nAChRs and their function, because its interaction with this subtype is currently unclear. Receptor competition studies showed that varenicline inhibited α6β2* nAChR binding (K(i) = 0.12 nM) as potently as α4β2* nAChR binding (K(i) = 0.14 nM) in rat striatal sections and with ∼20-fold greater affinity than nicotine. Functionally, varenicline was more potent in stimulating α6β2* versus α4β2* nAChR-mediated [(3)H]dopamine release from rat striatal synaptosomes with EC(50) values of 0.007 and 0.086 μM, respectively. However, it acted as a partial agonist on α6β2* and α4β2* nAChR-mediated [(3)H]dopamine release with maximal efficacies of 49 and 24%, respectively, compared with nicotine. We also evaluated varenicline's action in striatum of monkeys, a useful animal model for comparison with humans. Varenicline again potently inhibited monkey striatal α6β2* (K(i) = 0.13 nM) and α4β2* (K(i) = 0.19 nM) nAChRs in competition studies. Functionally, it potently stimulated both α6β2* (EC(50) = 0.014 μM) and α4β2* (EC(50) = 0.029 μM) nAChR-mediated [(3)H]dopamine release from monkey striatal synaptosomes, again acting as a partial agonist relative to nicotine at both subtypes. These data suggest that the ability of varenicline to interact at α6β2* nAChRs may contribute to its efficacy as a smoking cessation aid.

The nicotinic acetylcholine receptor as a target for antidepressant drug development

An important new area of antidepressant drug development involves targeting the nicotinic acetylcholine receptor (nAChR). This receptor, which is distributed widely in regions of the brain associated with depression, is also implicated in other important processes that are relevant to depression, such as stress and inflammation. The two classes of drugs that target nAChRs can be broadly divided into mecamylamine- and cytisine-based compounds. These drugs probably exert their effects via antagonism at α4β2 nAChRs, and strong preclinical data support the antidepressant efficacy of both classes when used in conjunction with other primary antidepressants (e.g., monoamine reuptake inhibitors). Although clinical data remain limited, preliminary results in this area constitute a compelling argument for further evaluation of the nAChR as a target for future antidepressant drug development.

Enhanced attenuation of nicotine discrimination in rats by combining nicotine-specific antibodies with a nicotinic receptor antagonist

Tobacco addiction requires activation by nicotine of a variety of central nicotinic acetylcholine receptors (nAChRs). In animals, both nAChR antagonists and immunization against nicotine can reduce nAChR activation by nicotine and block a variety of addiction-relevant behaviors. However, clinical use of nAChR antagonists for smoking cessation is limited by dose-related side effects, and immunization does not reliably produce sufficient antibody levels in smokers to enhance smoking cessation rates. Combining these approaches may be one way of addressing the limitations of each while enhancing overall efficacy. This study examined the individual and combined effects of passive immunization with the monoclonal nicotine-specific antibody Nic311 and the nicotinic receptor antagonist mecamylamine (MEC) on nicotine's discriminative stimulus effects. Rats were trained to discriminate 0.4 mg/kg of nicotine from saline using a two-lever operant discrimination procedure. Antagonism of nicotine discrimination by Nic311 (160 mg/kg i.v.) and ascending doses of MEC (0.03, 0.1, 0.3, and 1.0 mg/kg s.c.) was assessed across four consecutive daily 2-min extinction test sessions using a 2×2 design. Nic311 alone produced a 24-48% reduction in % nicotine-lever responding (%NLR) across all four test sessions. MEC produced a dose-dependent decrease in %NLR, with no effect at the two lowest doses and 80-93% attenuation at the two highest doses. Nic311 combined with MEC significantly suppressed %NLR at every MEC dose (85-92% reduction across all four test sessions). Very low doses of MEC that were ineffective alone completely blocked nicotine discrimination when combined with Nic311. These data demonstrate that nicotine-specific antibodies and MEC can work synergistically to suppress the subjective effects of nicotine and suggest that low doses of MEC may significantly enhance the efficacy of immunotherapy.

Neuronal nicotinic receptor ligands modulate chronic nicotine-induced ethanol consumption in C57BL/6J mice

Alcohol and nicotine are commonly abused drugs in humans and evidence suggests that neuronal nicotinic acetylcholine receptors (nAChRs) in the midbrain dopamine system are common targets for the neurobehavioral interactions between alcohol (ethanol) and nicotine. The present study examined the efficacy of nAChR ligands with different pharmacological profiles such as cytisine, lobeline and dihydro-β-erythroidine (DHβE) to modulate chronic nicotine-induced increase in ethanol intake by C57BL/6J mice, using a two-bottle choice procedure. After establishment of baseline ethanol preference (10%, v/v), animals received daily subcutaneous injections of saline, nicotine (0.4 mg/kg) or different doses of cytisine, lobeline or DHβE 15 min prior to nicotine, for 10 days. Ethanol and water were presented immediately after the last (saline or nicotine) injection and fluid levels were monitored for post 1 h and 2 h treatment. Compared to control, nicotine injection significantly increased mean ethanol intake over 10 days, at both post 1 h and 2 h. Pretreatment with cytisine (0.5, 1.5 or 3.0 mg/kg) or lobeline (4.0 or 10.0 mg/kg) significantly reduced nicotine-induced increase in ethanol intake post 1 h and 2 h, without affecting water consumption. DHβE (0.5 or 2.0 mg/kg) failed to suppress nicotine-induced ethanol intake across 2 h post injection. These results indicate that nAChRmediated signaling is critical in regulating nicotine-induced ethanol drinking behaviors.

Stress degradation studies on varenicline tartrate and development of a validated stability-indicating HPLC method

A simple, rapid and stability-indicating reversed-phase liquid chromatographic method was developed for the assay of varenicline tartrate (VRT) in the presence of its degradation products generated from forced decomposition studies. The HPLC separation was achieved on a C18 Inertsil column (250 mm × 4.6 mm i.d. particle size is 5 μm) employing a mobile phase consisting of ammonium acetate buffer containing trifluoroacetic acid (0.02M; pH 4) and acetonitrile in gradient program mode with a flow rate of 1.0 mL min(-1). The UV detector was operated at 237 nm while column temperature was maintained at 40 °C. The developed method was validated as per ICH guidelines with respect to specificity, linearity, precision, accuracy, robustness and limit of quantification. The method was found to be simple, specific, precise and accurate. Selectivity of the proposed method was validated by subjecting the stock solution of VRT to acidic, basic, photolysis, oxidative and thermal degradation. The calibration curve was found to be linear in the concentration range of 0.1-192 μg mL(-1) (R(2) = 0.9994). The peaks of degradation products did not interfere with that of pure VRT. The utility of the developed method was examined by analyzing the tablets containing VRT. The results of analysis were subjected to statistical analysis.

Nicotinic neuromodulation in auditory cortex requires MAPK activation in thalamocortical and intracortical circuits

Activation of nicotinic acetylcholine receptors (nAChRs) by systemic nicotine enhances sensory-cognitive function and sensory-evoked cortical responses. Although nAChRs mediate fast neurotransmission at many synapses in the nervous system, nicotinic regulation of cortical processing is neuromodulatory. To explore potential mechanisms of nicotinic neuromodulation, we examined whether intracellular signal transduction involving mitogen-activated protein kinase (MAPK) contributes to regulation of tone-evoked responses in primary auditory cortex (A1) in the mouse. Systemic nicotine enhanced characteristic frequency (CF) tone-evoked current-source density (CSD) profiles in A1, including the shortest-latency (presumed thalamocortical) current sink in layer 4 and longer-latency (presumed intracortical) sinks in layers 2-4, by increasing response amplitudes and decreasing response latencies. Microinjection of the MAPK kinase (MEK) inhibitor U0126 into the thalamus, targeting the auditory thalamocortical pathway, blocked the effect of nicotine on the initial (thalamocortical) CSD component but did not block enhancement of longer-latency (intracortical) responses. Conversely, microinjection of U0126 into supragranular layers of A1 blocked nicotine's effect on intracortical, but not thalamocortical, CSD components. Simultaneously with enhancement of CF-evoked responses, responses to spectrally distant (nonCF) stimuli were reduced, implying nicotinic "sharpening" of frequency receptive fields, an effect also blocked by MEK inhibition. Consistent with these physiological results, acoustic stimulation with nicotine produced immunolabel for activated MAPK in A1, primarily in layer 2/3 cell bodies. Immunolabel was blocked by intracortical microinjection of the nAChR antagonist dihydro-β-erythroidine, but not methyllycaconitine, implicating α4β2*, but not α7, nAChRs. Thus activation of MAPK in functionally distinct forebrain circuits--thalamocortical, local intracortical, and long-range intracortical--underlies nicotinic neuromodulation of A1.

Withdrawal from chronic nicotine exposure alters dopamine signaling dynamics in the nucleus accumbens

BACKGROUND

Unaided attempts to quit smoking commonly fail during the first 2 weeks of the nicotine withdrawal syndrome. Alterations in dopamine (DA) signaling correlate with withdrawal from chronic nicotine exposure, but those changes have not been well-characterized.

METHODS

Mice were administered nicotine in their drinking water for 4 or 12 weeks. Then nicotine was withheld for 1 to 10 days while DA signaling was characterized with in vivo microdialysis or fast-scan cyclic voltammetry.

RESULTS

Upon withdrawal of nicotine, the basal DA concentration in the nucleus accumbens decreased as measured by microdialysis. The length of time that the low basal DA state lasted depended on the length of the chronic nicotine treatment. Microdialysis indicated that acute re-exposure to nicotine during withdrawal temporarily reversed this hypodopaminergic state. Voltammetry measurements supported the microdialysis results by showing that nicotine withdrawal decreased tonic and phasic DA release. The basal DA concentration and tonic DA signals, however, were disproportionately lower than the phasic DA signals. Therefore, the phasic/tonic DA signaling ratio was increased during the withdrawal period.

CONCLUSIONS

The relative increase in the sensitivity of DA release to phasic stimulation suggests an increase in the signal-to-noise relationship of DA signaling during the withdrawal period. Therefore, the DA signal produced by acute nicotine re-exposure produces a DA response that might reinforce relapse to drug use (i.e., smoking). Because the basal DA concentration is low during withdrawal, therapies aimed at elevating the background DA signal represent a reasonable treatment strategy for nicotine-dependent individuals attempting to quit.