Decreased signs of nicotine withdrawal in mice null for the beta4 nicotinic acetylcholine receptor subunit

Rodent models of nicotine withdrawal syndrome

Simple, rapid and inexpensive rodent models of nicotine physical dependence and withdrawal syndrome have proved useful for preliminary screening of smoking cessation treatments. They have led to an exponential increase of knowledge regarding the underlying neurobiological mechanisms of dependence and withdrawal syndrome. The human nicotine withdrawal syndrome in smoking cessation is variable and multidimensional, involving irritability, anxiety, depression, cognitive and attentional impairments, weight gain, sleep disturbances, and craving for nicotine. Aside from sleep disturbances, analogous phenomena have been seen in rodent models using different measures of withdrawal intensity. It appears likely that different withdrawal phenomena may involve some partially divergent mechanisms. For example, depression-like phenomena may involve alterations in mechanisms such as the mesolimbic dopamine pathway from the ventral tegmental area to the nucleus accumbens. Irritability and anxiety may involve alterations in endogenous opioid systems and other regions, such as the amygdala. This chapter reviews many additional anatomical, neurochemical, and developmental elements that impact nicotine physical dependence.

New insights into the genetics of addiction

Drug addiction is a common brain disorder that is extremely costly to the individual and to society. Genetics contributes significantly to vulnerability to this disorder, but identification of susceptibility genes has been slow. Recent genome-wide linkage and association studies have implicated several regions and genes in addiction to various substances, including alcohol and, more recently, tobacco. Current efforts aim not only to replicate these findings in independent samples but also to determine the functional mechanisms of these genes and variants.

Nicotine is a selective pharmacological chaperone of acetylcholine receptor number and stoichiometry. Implications for drug discovery

The acronym SePhaChARNS, for "selective pharmacological chaperoning of acetylcholine receptor number and stoichiometry," is introduced. We hypothesize that SePhaChARNS underlies classical observations that chronic exposure to nicotine causes "upregulation" of nicotinic receptors (nAChRs). If the hypothesis is proven, (1) SePhaChARNS is the molecular mechanism of the first step in neuroadaptation to chronic nicotine; and (2) nicotine addiction is partially a disease of excessive chaperoning. The chaperone is a pharmacological one, nicotine; and the chaperoned molecules are alpha4beta2* nAChRs. SePhaChARNS may also underlie two inadvertent therapeutic effects of tobacco use: (1) the inverse correlation between tobacco use and Parkinson's disease; and (2) the suppression of seizures by nicotine in autosomal dominant nocturnal frontal lobe epilepsy. SePhaChARNS arises from the thermodynamics of pharmacological chaperoning: ligand binding, especially at subunit interfaces, stabilizes AChRs during assembly and maturation, and this stabilization is most pronounced for the highest-affinity subunit compositions, stoichiometries, and functional states of receptors. Several chemical and pharmacokinetic characteristics render exogenous nicotine a more potent pharmacological chaperone than endogenous acetylcholine. SePhaChARNS is modified by desensitized states of nAChRs, by acid trapping of nicotine in organelles, and by other aspects of proteostasis. SePhaChARNS is selective at the cellular, and possibly subcellular, levels because of variations in the detailed nAChR subunit composition, as well as in expression of auxiliary proteins such as lynx. One important implication of the SePhaChARNS hypothesis is that therapeutically relevant nicotinic receptor drugs could be discovered by studying events in intracellular compartments rather than exclusively at the surface membrane.

Progesterone modulation of alpha5 nAChR subunits influences anxiety-related behavior during estrus cycle

Smokers often report an anxiolytic effect of cigarettes. In addition, stress-related disorders such as anxiety, post-traumatic stress syndrome and depression are often associated with chronic nicotine use. To study the role of the alpha5 nicotinic acetylcholine receptor subunit in anxiety-related responses, control and alpha5 subunit null mice (alpha5(-/-)) were subjected to the open field activity (OFA), light-dark box (LDB) and elevated plus maze (EPM) tests. In the OFA and LDB, alpha5(-/-) behaved like wild-type controls. In the EPM, female alpha5(-/-) mice displayed an anxiolytic-like phenotype, while male alpha5(-/-) mice were undistinguishable from littermate controls. We studied the hypothalamus-pituitary-adrenal axis by measuring plasma corticosterone and hypothalamic corticotropin-releasing factor. Consistent with an anxiolytic-like phenotype, female alpha5(-/-) mice displayed lower basal corticosterone levels. To test whether gonadal steroids regulate the expression of alpha5, we treated cultured NTera 2 cells with progesterone and found that alpha5 protein levels were upregulated. In addition, brain levels of alpha5 mRNA increased upon progesterone injection into ovariectomized wild-type females. Finally, we tested anxiety levels in the EPM during the estrous cycle. The estrus phase (when progesterone levels are low) is anxiolytic-like in wild-type mice, but no cycle-dependent fluctuations in anxiety levels were found in alpha5(-/-) females. Thus, alpha5-containing neuronal nicotinic acetylcholine receptors may be mediators of anxiogenic responses, and progesterone-dependent modulation of alpha5 expression may contribute to fluctuations in anxiety levels during the ovarian cycle.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Behavioural and pharmacological mechanisms of bupropion's anti-smoking effects: recent preclinical and clinical insights

Ongoing studies continue to explore the behavioural and pharmacological effects of bupropion in smoking cessation studies and animal models of nicotine dependence. In the present review, the components of nicotine dependence that form the most likely targets of bupropion are identified within the context of an expanding preclinical and clinical literature regarding the anti-addictive properties of bupropion. Second, preclinical and clinical data that implicate specific pharmacological modes of action of bupropion in mediating the anti-smoking effects of the compound are discussed. Third, it is suggested that the unique mixed pharmacological profile of bupropion provides (1) attenuation of the multiple negative consequences of withdrawal via blockade of dopamine and noradrenaline reuptake; (2) replacement of the reward-facilitating and subjective effects of nicotine via blockade of dopaminergic reuptake; (3) attenuation of the rewarding effects of acute nicotine by nicotinic acetylcholine receptor blockade. The importance of species differences in bupropion metabolism in the interpretation of preclinical studies is highlighted. Finally, future studies are suggested to address identified gaps in the knowledge: most importantly, to provide stronger evidence for the role of noradrenaline reuptake inhibition in bupropion-induced attenuation of nicotine withdrawal. Future studies aimed at providing more evidence for the three-fold nature of the anti-smoking effects of bupropion are also suggested, along with the possibility of utilizing adjunct therapies to improve smoking cessation rates.

Subtypes of nicotinic acetylcholine receptors in nicotine reward, dependence, and withdrawal: evidence from genetically modified mice

Neuronal nicotinic acetylcholine receptors (nAChRs) can regulate the activity of many neurotransmitter pathways throughout the central nervous system and are considered to be important modulators of cognition and emotion. nAChRs are also the primary site of action in the brain for nicotine, the major addictive component of tobacco smoke. nAChRs consist of five membrane-spanning subunits (alpha and beta isoforms) that can associate in various combinations to form functional nAChR ion channels. Owing to a dearth of nAChR subtype-selective ligands, the precise subunit composition of the nAChRs that regulate the rewarding effects of nicotine and the development of nicotine dependence are unknown. The advent of mice with genetic nAChR subunit modifications, however, has provided a useful experimental approach to assess the contribution of individual subunits in vivo. Here, we review data generated from nAChR subunit knockout and genetically modified mice supporting a role for discrete nAChR subunits in nicotine reinforcement and dependence processes. Importantly, the rates of tobacco dependence are far higher in patients suffering from comorbid psychiatric illnesses compared with the general population, which may at least partly reflect disease-associated alterations in nAChR signaling. An understanding of the role of nAChRs in psychiatric disorders associated with high rates of tobacco addiction, therefore, may reveal novel insights into mechanisms of nicotine dependence. Thus, we also briefly review data generated from genetically modified mice to support a role for discrete nAChR subunits in anxiety disorders, depression, and schizophrenia.

Demonstration of functional alpha4-containing nicotinic receptors in the medial habenula

The medial habenula (MHb) exhibits exceptionally high levels of nicotinic acetylcholine receptors (nAChRs), but it remains unclear whether all expressed nAChR subunit mRNAs are translated to form functional receptors. In particular alpha4 subunits have not been reported to have any functional role, despite strong alpha4 mRNA expression in the ventrolateral MHb. We studied a strain of knock-in mice expressing fluorescent alpha4* nAChRs (alpha4YFP), as well as a knock-in strain expressing hypersensitive alpha4* nAChRs (alpha4L9'A). In alpha4YFP mice, there was strong fluorescence in the ventrolateral MHb. In hypersensitive alpha4L9'A mice, injections of a low dose of nicotine (0.1 mg/kg) led to strong c-fos expression in only the ventrolateral region of the MHb, but not in the MHb of wild-type (WT) mice. In MHb slice recordings, ventrolateral neurons from alpha4L9'A mice, but not from WT mice, responded robustly to nicotine (1 microM). Neurons in the medial aspect of the MHb had >10-fold smaller responses. Thus alpha4* nAChRs contribute to the selective activation of a subset of MHb neurons. Subunit composition analysis based on gain-of-function knock-in mice provides a useful experimental paradigm.

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

Transcription factor assembly on the nicotinic receptor beta4 subunit gene promoter

Nicotinic acetylcholine receptors are involved in a plethora of fundamental biological processes ranging from muscle contraction to formation of memories. The receptors are pentameric proteins whose subunits are encoded by distinct genes. Subunit composition of a mature nicotinic receptor is governed in part by the transcriptional regulation of each subunit gene. Here, using chromatin immunoprecipitation assays, we report the interaction of the transcription factors Sp1, Sp3, c-Jun and Sox10 with the beta4 subunit gene promoter in neuronal-like cell lines and rodent brain tissue. Our results corroborate previous in-vitro data demonstrating that these transcription factors interact with the beta4 promoter. Taken together, these data suggest that Sp1, Sp3, c-Jun and Sox10 regulate expression of the beta4 subunit gene in the mammalian brain.

Genetic variability in nicotinic acetylcholine receptors and nicotine addiction: converging evidence from human and animal research

Tobacco smoking is a leading preventable cause of death in the United States and produces a major health and economic burden. Although the majority of smokers want to quit, few are successful. These data highlight the need for additional research into the neurobiology of tobacco dependence. Addiction to nicotine, the main psychoactive component of tobacco, is influenced by multiple factors that include individual differences in genetic makeup. Twin studies have demonstrated that genetic factors can influence vulnerability to nicotine addiction, and subsequent research has identified genes that may alter sensitivity to nicotine. In humans, genome-wide and candidate gene association studies have demonstrated that genes encoding nicotinic acetylcholine receptor (nAChR) proteins are associated with multiple smoking phenotypes. Similarly, research in mice has provided evidence that naturally occurring variability in nAChR genes is associated with changes in nicotine sensitivity. Furthermore, the use of genetic knockout mice has allowed researchers to determine the nAChR genes that mediate the effects of nicotine, whereas research with knockin mice has demonstrated that changes to nAChR genes can dramatically alter nicotine sensitivity. This review will examine the genetic factors that alter susceptibility to nicotine addiction, with an emphasis on the genes that encode nAChR proteins.

Opposing actions of chronic stress and chronic nicotine on striatal function in mice

Stress is a major risk factor in drug addiction development and relapse. Virtually all drugs of abuse act by increasing extracellular dopamine levels in the striatum. To gain an understanding of the interaction between stress and drug exposure, we studied the effects of concomitant chronic nicotine and chronic stress exposure on mouse striatal dopamine levels. C57Bl6/J mice were treated with nicotine in the drinking water or control solution for at least 6 weeks. Some mice were chronically stressed by daily exposure to cold, shaking or restrain. Nicotine-treated mice showed up-regulation of epibatidine binding in several brain regions. In mice treated with both chronic nicotine and stress, epibatidine binding was increased in all studied areas except the dorsal striatum. Therefore, microdialysis was used to measure extracellular dopamine levels in the dorsal striatum of mice chronically treated with nicotine, stress, or both. To have a measure of striatal response to different challenges, we performed microdialysis after acute injection of saline, nicotine, and cocaine. Chronic nicotine enhanced nicotine-dependent dopamine release, while chronic stress blunted the response to cocaine. When mice were subjected to both chronic nicotine and chronic stress, nicotine- and cocaine-dependent dopamine release was undistinguishable from that of control animals. In conclusion, our data suggest that chronic stress and chronic nicotine counteract each other's effect on dopamine release in the striatum. This effect might be mediated by changes in nicotinic acetylcholine receptor up-regulation. This "normalization" of striatal function when both nicotine and stress are present might help explain the comorbidity between stress-related disorders and drug abuse.

Affective and somatic aspects of spontaneous and precipitated nicotine withdrawal in C57BL/6J and BALB/cByJ mice

The aversive aspects of nicotine withdrawal are powerful motivational forces contributing to the tobacco smoking habit. We evaluated measures of affective and somatic aspects of nicotine withdrawal in C57BL/6J and BALB/cByJ mice. Nicotine withdrawal was induced by termination of chronic nicotine delivery through osmotic minipumps or precipitated with the nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine or dihydro-beta-erythroidine (DHbetaE). A rate-independent discrete-trial intracranial self-stimulation threshold procedure was used to assess brain reward function. Anxiety-like behavior and sensorimotor gating were assessed in the light-dark box and prepulse inhibition (PPI) tests, respectively. Acoustic startle response and somatic signs of withdrawal were also evaluated. Spontaneous nicotine withdrawal after 14-day exposure to 10-40 mg/kg/day nicotine induced no alterations in anxiety-like behavior, startle reactivity, PPI, or somatic signs in either strain, and no changes in thresholds in C57BL/6J mice. Extended 28-day exposure to 40 mg/kg/day nicotine induced threshold elevations, increased somatic signs, and anxiety-like behavior 24 h post-nicotine in C57BL/6J mice; thresholds returned to baseline levels by day 4 in nicotine-exposed mice. Mecamylamine or DHbetaE administration induced threshold elevations in nicotine-exposed C57BL/6J mice compared with saline-exposed mice. In conclusion, administration of relatively high nicotine doses over prolonged periods of time induces both the affective and somatic aspects of spontaneous nicotine withdrawal in the mouse, while exposure to nicotine for shorter periods of time is sufficient for nAChR antagonist-precipitated nicotine withdrawal. The current study is one of the first to demonstrate reward deficits associated with both spontaneous and nAChR antagonist-precipitated nicotine withdrawal in C57BL/6J mice.

Decreased brain reward function during nicotine withdrawal in C57BL6 mice: evidence from intracranial self-stimulation (ICSS) studies

Deficits in brain reward function during nicotine withdrawal may serve as an important substrate for negative reinforcement that contributes to the persistence of the tobacco habit in human smokers. The ability to assess withdrawal-associated reward deficits in genetically modified mice may facilitate understanding of the neurobiological mechanisms of nicotine dependence. Here, we assessed the effects of nicotine withdrawal on brain reward function in mice, as measured by intracranial self-stimulation (ICSS) thresholds. Male C57BL6 mice were trained in a discrete-trial current-threshold ICSS procedure until stable reward thresholds were obtained. Mice then received experimenter-administered saline or nicotine (2 mg/kg/injection salt; x4 daily) injections for 7 consecutive days, and ICSS thresholds assessed for 3 days after cessation of injections. Thresholds were unaltered in nicotine- and saline-treated mice after cessation of injections, indicating that this treatment regimen was not sufficient to induce withdrawal-associated reward deficits. Next, mice were implanted subcutaneously with osmotic minipumps delivering a constant daily amount of saline or nicotine (24 mg/kg/day; free-base), with pumps surgically removed 13 days later. The nicotinic receptor antagonist mecamylamine (2 mg/kg) elevated ICSS thresholds in nicotine- but not saline-treated mice when administered 8-10 days after pump implantation. Similarly, reward thresholds were elevated in nicotine-treated mice 12-72 h after minipump removal. These data demonstrate that antagonist-precipitated or spontaneous withdrawal from nicotine delivered via osmotic minipumps induced reward deficits in mice. Further, these findings highlight the potential utility of the ICSS procedure for assessing this important affective component of nicotine withdrawal in genetically modified mice.

Differential role of nicotinic acetylcholine receptor subunits in physical and affective nicotine withdrawal signs

It has been suggested that the negative effects associated with nicotine withdrawal promote continued tobacco use and contribute to the high relapse rate of smoking behaviors. Thus, it is important to understand the receptor-mediated mechanisms underlying nicotine withdrawal to aid in the development of more successful smoking cessation therapies. The effects of nicotine withdrawal are mediated through nicotinic acetylcholine receptors (nAChRs); however, the role of nAChRs in nicotine withdrawal remains unclear. Therefore, we used mecamylamine-precipitated, spontaneous, and conditioned place aversion (CPA) withdrawal models to measure physical and affective signs of nicotine withdrawal in various nAChR knockout (KO) mice. beta2, alpha7, and alpha5 nAChR KO mice were chronically exposed to nicotine through surgically implanted osmotic minipumps. Our results show a loss of anxiety-related behavior and a loss of aversion in the CPA model in beta2 KO mice, whereas alpha7 and alpha5 KO mice displayed a loss of nicotine withdrawal-induced hyperalgesia and a reduction in somatic signs, respectively. These results suggest that beta2-containing nAChRs are involved in the affective signs of nicotine withdrawal, whereas non-beta2-containing nAChRs are more closely associated with physical signs of nicotine withdrawal; thus, the nAChR subtype composition may play an important role in the involvement of specific subtypes in nicotine withdrawal.

Decreased withdrawal symptoms but normal tolerance to nicotine in mice null for the alpha7 nicotinic acetylcholine receptor subunit

Withdrawal symptoms are a major deterrent when people try to quit smoking. The alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) is highly expressed in the brain, and has been suspected to play a major role in nicotine addiction. We studied the influence of alpha7-containing nAChRs on nicotine withdrawal and tolerance, in wild type mice and mice null for the alpha7 nAChR subunit (alpha7 -/-). For withdrawal experiments, animals were implanted with osmotic minipumps delivering nicotine for 13 days. A single intraperitoneal injection of the nAChR antagonists mecamylamine (MEC) or methyllycaconitine (MLA) was used to precipitate withdrawal. In wild type mice, both MEC- and MLA-precipitated somatic signs of withdrawal such as increased grooming, scratching and shaking. In alpha7 -/- mice, the somatic effects of MEC-precipitated nicotine withdrawal were significantly reduced. Interestingly, the presumed alpha7-specific antagonist MLA also precipitated withdrawal. Tolerance, which was measured as a decrease in nicotine-induced hypolocomotion after subchronic nicotine treatment, was normal in alpha7 -/- mice. Finally, because anxiety and withdrawal symptoms are highly correlated in humans, we studied anxiety-like behaviors in alpha7 -/- mice using a battery of anxiety-related tests. The behavior of alpha7 -/- mice was indistinguishable from that of control mice. Our results point to the alpha7 subunit as one of the players in nicotine withdrawal, but not in nicotine tolerance or basal anxiety-like behavior.

Multiple brain pathways and receptors underlying tobacco addiction

Over the last 20 years much progress has been made in understanding the pharmacologic basis of tobacco addiction. In particular, the role of nicotine in reinforcing smoking behavior has been studied from a variety of perspectives. This article discusses two important aspects of this topic: (1) brain pathways underlying tobacco addiction; and (2) the actions of nicotine at nicotinic cholinergic receptors. Recent evidence will be reviewed indicating that nicotine reinforces smoking behavior by acting on more than one subtype of nicotinic receptor. Similarly, the role of several brain pathways in tobacco addiction will be considered. Tobacco addiction may thus be seen as a complex neuropsychopharmacological disorder; further progress in smoking cessation treatment may require that we address the multiple molecular and brain components of this addiction.

Beta2 subunit containing acetylcholine receptors mediate nicotine withdrawal deficits in the acquisition of contextual fear conditioning

Acute nicotine enhances contextual fear conditioning, whereas withdrawal from chronic nicotine produces impairments. However, the nicotinic acetylcholine receptors (nAChR) that are involved in nicotine withdrawal deficits in contextual fear conditioning are unknown. The present study used genetic and pharmacological techniques to investigate the nAChR subtype(s) involved in the effects of nicotine withdrawal on contextual fear conditioning. beta2 or alpha 7 nAChR subunit knockout (KO) and corresponding wild-type (WT) mice were withdrawn from 12 days of chronic nicotine treatment (6.3mg/kg/day), and trained with 2 conditioned stimulus (CS; 85 dB white noise)--unconditioned stimulus (US; 0.57 mA footshock) pairings on day 13. On day 14, mice were tested for contextual and cued freezing. beta2 KO mice did not show nicotine withdrawal-related deficits in contextual fear conditioning, in contrast to WT mice and alpha 7 KO mice. A follow-up study investigated if nicotine withdrawal disrupts acquisition or recall of contextual fear conditioning. The high affinity nAChR antagonist dihydro-beta-erythroidine (DH beta E; 3mg/kg) was administered prior to training or testing to precipitate withdrawal in chronic nicotine-treated C57BL/6 mice. Deficits in contextual fear conditioning were observed in chronic nicotine-treated mice when DH beta E was administered prior to training, but not when administered at testing. These results indicate that beta2-containing nAChRs, such as the alpha 4 beta 2 receptor, mediate nicotine withdrawal deficits in contextual fear conditioning. In addition, nicotine withdrawal selectively affects acquisition but not recall or expression of the learned response.

Genetics of nicotinic acetylcholine receptors: Relevance to nicotine addiction

Human twin studies have suggested that there is a substantial genetic component underlying nicotine dependence, ongoing smoking and ability to quit. Similarly, animal studies have identified a number of genes and gene products that are critical for behaviors related to nicotine addiction. Classical genetic approaches, gene association studies and genetic engineering techniques have been used to identify the gene products involved in nicotine dependence. One class of genes involved in nicotine-related behavior is the family of nicotinic acetylcholine receptors (nAChRs). These receptors are the primary targets for nicotine in the brain. Genetic engineering studies in mice have identified a number of subunits that are critical for the ability of nicotine to activate the reward system in the brain, consisting of the dopaminergic cell bodies in the ventral tegmental area and their terminals in the nucleus accumbens and other portions of the mesolimbic system. In this review we will discuss the various lines of evidence suggesting that nAChRs may be involved in smoking behavior, and will review the human and animal studies that have been performed to date examining the genetic basis for nicotine dependence and smoking.

Long-term effects of chronic nicotine exposure on brain nicotinic receptors

Chronic nicotine exposure results in long-term homeostatic regulation of nicotinic acetylcholine receptors (nAChRs) that play a key role in the adaptative cellular processes leading to addiction. However, the relative contribution of the different nAChR subunits in this process is unclear. Using genetically modified mice and pharmacological manipulations, we provide behavioral, electrophysiological, and pharmacological evidence for a long-term mechanism by which chronic nicotine triggers opposing processes differentially mediated by beta2*- vs. alpha7*nAChRs. These data offer previously undescribed insights into the understanding of nicotine addiction and the treatment of several human pathologies by nicotine-like agents chronically acting on beta2*- or alpha7*nAChRs.

What's wrong with my mouse model?

Stress plays a key role in pathogenesis of anxiety and depression. Animal models of these disorders are widely used in behavioral neuroscience to explore stress-evoked brain abnormalities, screen anxiolytic/antidepressant drugs and establish behavioral phenotypes of gene-targeted or transgenic animals. Here we discuss the current situation with these experimental models, and critically evaluate the state of the art in this field. Noting a deficit of fresh ideas and especially new paradigms for animal anxiety and depression models, we review existing challenges and outline important directions for further research in this field.

Evaluating the suitability of nicotinic acetylcholine receptor antibodies for standard immunodetection procedures

Nicotinic acetylcholine receptors play important roles in numerous cognitive processes as well as in several debilitating central nervous system (CNS) disorders. In order to fully elucidate the diverse roles of nicotinic acetylcholine receptors in CNS function and dysfunction, a detailed knowledge of their cellular and subcellular localizations is essential. To date, methods to precisely localize nicotinic acetylcholine receptors in the CNS have predominantly relied on the use of anti-receptor subunit antibodies. Although data obtained by immunohistology and immunoblotting are generally in accordance with ligand binding studies, some discrepancies remain, in particular with electrophysiological findings. In this context, nicotinic acetylcholine receptor subunit-deficient mice should be ideal tools for testing the specificity of subunit-directed antibodies. Here, we used standard protocols for immunohistochemistry and western blotting to examine the antibodies raised against the alpha3-, alpha4-, alpha7-, beta2-, and beta4-nicotinic acetylcholine receptor subunits on brain tissues of the respective knock-out mice. Unexpectedly, for each of the antibodies tested, immunoreactivity was the same in wild-type and knock-out mice. These data imply that, under commonly used conditions, these antibodies are not suited for immunolocalization. Thus, particular caution should be exerted with regards to the experimental approach used to visualize nicotinic acetylcholine receptors in the brain.

Differential brain transcriptome of beta4 nAChR subunit-deficient mice: is it the effect of the null mutation or the background strain?

Studies using mice with beta4 nicotinic acetylcholine receptor (nAChR) subunit deficiency (beta4-/- mice) helped reveal the roles of this subunit in bradycardiac response to vagal stimulation, nicotine-induced seizure activity and anxiety. To identify genes that might be related to beta4-containing nAChRs activity, we compared the mRNA expression profiles of brains from beta4-/- and wild-type mice using Affymetrix U74Av2 microarray. Seventy-seven genes significantly differentiated between these two experimental groups. Of them, the two most downregulated were spastic paraplegia 21 (human) homolog (Spg21) and 6-pyruvoyl-tetrahydropterin synthase (Pts) genes. Since the targeted mutagenesis of the beta4 nAChR subunit was done by using two mouse strains, 129SvEv and C57BL/6J, it is possible that the genes closely linked to the mutated beta4 gene represent the 129SvEv allele and not the control C57BL/6J-driven allele. We examined this possibility by using public database and quantitative RT-PCR. The expression levels of Spg21 and Pts genes that, like the beta4 gene, are localized on mouse chromosome 9, as well as the expression levels of other genes located on this chromosome, were dependent on the mouse background strain. The 67 differentially expressed genes that are not located on chromosome 9 were further analyzed for overrepresented functional annotations and transcription regulatory elements compared with the entire microarray. Genes encoding for proteins involved in tyrosine phosphatase activity, calcium ion binding, cell growth and/or maintenance, and chromosome organization were overrepresented. Our data enhance the understanding of the molecular interactions involved in the beta4 nAChR subunit function. They also emphasize the need for careful interpretation of expression microarray studies done on genetically manipulated animals.

Genetic dissociation of two behaviors associated with nicotine addiction: beta-2 containing nicotinic receptors are involved in nicotine reinforcement but not in withdrawal syndrome

RATIONALE

Nicotine addiction is characterized by two distinct behaviors, chronic compulsive self-administration and the induction of a withdrawal syndrome upon cessation of nicotine consumption.

OBJECTIVE

To examine if these two processes rely on beta2-containing nicotinic receptors--beta2*nAChRs--we analyzed the behavior of mice lacking these receptors in the two situations.

RESULTS

First, we showed that, in contrast to wild-type (WT) mice, beta2-knockout (beta2-/-) mice exhibit no intra-ventral tegmental area (VTA) nicotine self-administration, whereas their ability to self-administer morphine is intact. However, beta2-/- mice showed some sensitivity to locomotor effects of nicotine, implying an effect of the drug on other nicotinic subtypes. Then, we observed that beta2-/- mice exhibited a normal nicotine withdrawal syndrome, i.e., increased levels of rearing and jumping upon precipitated withdrawal. Thus, the beta2*nAChRs are not involved in the behaviors induced by cessation of nicotine consumption.

CONCLUSION

Taken together, the present data demonstrated a genetic dissociation of two distinct behavioral patterns associated with nicotine addiction. They further suggested that independent molecular mechanisms underlie these two aspects, offering the possibility of controlling them separately.

Sazetidine-A, a novel ligand that desensitizes alpha4beta2 nicotinic acetylcholine receptors without activating them

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels found throughout the central and peripheral nervous systems. They are crucial to normal physiology and have been clearly implicated in nicotine addiction. In addition, they are possible therapeutic targets in a wide range of pathological conditions, including cognitive disorders, Parkinson's disease, and neuropathic pain. Nicotinic ligands are usually classified as agonists (or partial agonists), competitive antagonists, or noncompetitive antagonists. Sazetidine-A is a new nicotinic ligand that shows a different pharmacological profile from any of these known classes of ligands. Sazetidine-A competes with very high binding affinity (Ki approximately 0.5 nM) and selectivity for the alpha4beta2 nAChR subtype (Ki ratio alpha3beta4/alpha4beta2 approximately 24,000). Despite its high affinity, sazetidine-A neither activates nAChR channel function nor prevents channel activation when it is applied simultaneously with nicotine. However, when it is pre-incubated for 10 min with the receptors, it potently blocks nicotine-stimulated alpha4beta2 nAChR function (IC50 approximately 30 nM). The action of sazetidine-A may be explained by its very low affinity for the resting conformation of the alpha4beta2 nAChRs, and its very high affinity for the desensitized state of the receptor. We propose that sazetidine-A is a "silent desensitizer" of nAChRs, meaning that it desensitizes the receptor without first activating it. Furthermore, comparison of the effects of sazetidine-A and nicotine at alpha4beta2 nAChRs suggests that the predominant effects of nicotine and other nicotinic agonists are related to desensitization of the receptors and that sazetidine-A potently mimics these effects.

Roles of nicotinic acetylcholine receptor beta subunits in function of human alpha4-containing nicotinic receptors

Naturally expressed nicotinic acetylcholine receptors (nAChR) containing alpha4 subunits (alpha4*-nAChR) in combination with beta2 subunits (alpha4beta2-nAChR) are among the most abundant, high-affinity nicotine binding sites in the mammalian brain. beta4 subunits are also richly expressed and colocalize with alpha4 subunits in several brain regions implicated in behavioural responses to nicotine and nicotine dependence. Thus, alpha4beta4-nAChR also may exist and play important functional roles. In this study, properties were determined of human alpha4beta2- and alpha4beta4-nAChR heterologously expressed de novo in human SH-EP1 epithelial cells. Whole-cell currents mediated via human alpha4beta4-nAChR have approximately 4-fold higher amplitude than those mediated via human alpha4beta2-nAChR and exhibit much slower acute desensitization and functional rundown. Nicotinic agonists induce peak whole-cell current responses typically with higher functional potency at alpha4beta4-nAChR than at alpha4beta2-nAChR. Cytisine and lobeline serve as full agonists at alpha4beta4-nAChR but are only partial agonists at alpha4beta2-nAChR. However, nicotinic antagonists, except hexamethonium, have comparable affinities for functional alpha4beta2- and alpha4beta4-nAChR. Whole-cell current responses show stronger inward rectification for alpha4beta2-nAChR than for alpha4beta4-nAChR at a positive holding potential. Collectively, these findings demonstrate that human nAChR beta2 or beta4 subunits can combine with alpha4 subunits to generate two forms of alpha4*-nAChR with distinctive physiological and pharmacological features. Diversity in alpha4*-nAChR is of potential relevance to nervous system function, disease, and nicotine dependence.

Drugs and genotypes: how pharmacogenetic information could improve smoking cessation treatment

Current smoking cessation treatments are not optimal as the long-term abstinence rates are low. Nicotine replacement therapy and bupropion are the only pharmacotherapies widely available to smokers and there is a need to improve current cessation treatments and to develop new drug therapies. Our goal is to use pharmacogenetic information to improve smoking cessation treatments. Candidate genes include those involved in central mechanisms (such as genes encoding the nicotinic acetylcholine receptors, dopamine receptors, dopamine transporters and opioid receptors) and peripheral mechanisms (such as genes encoding the drug-metabolizing enzymes CYP2A6 and CYP2B6). Pharmacogenetics could be used to improve current smoking cessation treatments through two general approaches. The first would be to identify novel drug targets based on genetic variants that are associated with a natural protection against nicotine dependence, decreased smoking behaviour and/or increased smoking cessation. This knowledge could be used to develop drugs that can mimic these advantages, reducing the risk for acquisition, block the rewarding effects of smoking, decreasing the amount smoked and increasing cessation. The second would be to identify smokers with particular genetic variants who are likely to respond better to one treatment (or dose) versus another and to individualize their treatment to maximize long-term abstinence. This review will highlight examples of how pharmacogenetic information from central and peripheral candidate genes could contribute to improving smoking cessation outcomes by these two approaches.

Deletion of the beta 2 nicotinic acetylcholine receptor subunit alters development of tolerance to nicotine and eliminates receptor upregulation

RATIONALE

Chronic nicotine exposure induces both tolerance and upregulation of [3H]nicotine binding sites in rodent and human brain. However, the mechanism for chronic tolerance is unclear because a direct relationship between tolerance and receptor upregulation is not consistently observed.

OBJECTIVES

In the present experiments, the role of beta2* nicotinic acetylcholine receptors (nAChRs) on tolerance development and nAChR upregulation was examined following chronic nicotine treatment of beta2 wild-type (+/+), heterozygous (+/-), and null mutant (-/-) mice.

METHODS

Saline or nicotine (1, 2, or 4 mg/kg/h) was infused intravenously for 10 days. Locomotor activity and body temperature responses were measured before and after nicotine challenge injection to observe changes in nicotine sensitivity. [3H]Epibatidine binding was then measured in ten brain regions.

RESULTS

Beta2+/+ mice developed dose-dependent tolerance and upregulation of [3H]epibatidine binding sites. In contrast, beta2-/- mice, initially less sensitive to acute nicotine's effects, became more sensitive following treatment with the lowest chronic dose (1 mg/kg/h). Beta2-/- mice treated with 4.0 mg/kg/h nicotine were no longer supersensitive, indicating that tolerance developed at this higher dose. However, these changes in nicotine sensitivity occurred in the absence of any nAChR changes in either low- or high-affinity [3H]epibatidine sites. Responses of beta2+/- mice were intermediate between wild-type and mutant mice.

CONCLUSIONS

Upregulation of nAChRs in vivo requires the presence of the beta2 subunit. Changes in nicotine sensitivity occurred both in the presence (beta2+/+) and absence (beta2-/-) of beta2* nAChRs and suggest that mechanisms involving both beta2* and non-beta2* nAChR subtypes modulate adaptation to chronic nicotine exposure.

Why do young women smoke? I. Direct and interactive effects of environment, psychological characteristics and nicotinic cholinergic receptor genes

Despite the health hazards, cigarette smoking is disproportionately frequent among young women. A significant contribution of genetic factors to smoking phenotypes is well established. Efforts to identify susceptibility genes do not generally take into account possible interaction with environment, life experience and psychological characteristics. We recruited 501 female Israeli students aged 20-30 years, obtained comprehensive background data and details of cigarette smoking and administered a battery of psychological instruments. Smoking initiators (n=242) were divided into subgroups with high (n=127) and low (n=115) levels of nicotine dependence based on their scores on the Fagerstrom Tolerance Questionnaire and genotyped with noninitiators (n=142) for single nucleotide polymorphisms (SNPs) in 11 nicotinic cholinergic receptor genes. We found nominally significant (P<0.05) allelic and genotypic association with smoking initiation of SNP rs2072660 and multilocus haplotypes (P<0.007-0.05) in CHRNB2 and nominal (P<0.05) allelic or genotypic association of SNPs in CHRNA7 (rs1909884), CHRNA9 (rs4861065) and CHRNB3 (rs9298629) with nicotine dependence. Employing logistic regression and controlling for known risk factors, the best-fitting model for smoking initiation encompassed a 5 SNP haplotype in CHRNB2, neuroticism and novelty seeking (P=5.9 x 10(-14), Nagelkerke r(2)=0.30). For severity of nicotine dependence, two SNPs in CHRNA7 (rs1909884 and rs883473), one SNP in CHRNA5 (rs680244) and the interaction of a SNP in CHRNA7 (rs2337980) with neuroticism, were included in the model (P=2.24 x 10(-7), Nagelkerke r(2)=0.40). These findings indicate that background factors, psychological characteristics and genetic variation in nicotinic cholinergic receptors contribute independently or interactively to smoking initiation and to severity of nicotine dependence in young women.

Nicotine addiction and comorbidity with alcohol abuse and mental illness

The World Health Organization estimates that one-third of the global adult population smokes. Because tobacco use is on the rise in developing countries, death resulting from tobacco use continues to rise. Nicotine, the main addictive component of tobacco, initiates synaptic and cellular changes that underlie the motivational and behavioral alterations that culminate in addiction. Nicotine addiction progresses rapidly in adolescents and is most highly expressed in vulnerable people who have psychiatric illness or other substance abuse problems.

Nicotinic Receptor Subtypes in Rat Hippocampal Slices Are Differentially Sensitive to Desensitization and Early in Vivo Functional Up-Regulation by Nicotine and to Block by Bupropion

To identify the brain nicotinic acetylcholine receptor (nAChR) subtypes that may be involved in nicotine addiction, we investigated the actions of bupropion, a drug used in cigarette smoking cessation programs, and nicotine on three pharmacologically identified nAChRs in rat hippocampal slices, namely, type IA, type II, and type III nAChRs, likely representing alpha7, alpha4beta2, and alpha3beta4 subunits, respectively. Using whole-cell patch-clamp recordings from interneurons of acute hippocampal slices prepared from male rat pups, we studied the effect of nicotine on in vivo up-regulation and in vitro desensitization of nAChRs. Two subcutaneous injections of nicotine (0.586 mg/kg free base, in less than a day) to rats at postnatal days 14 to 15 significantly enhanced the magnitude of functional responses arising from type III and type II, but not type IA nAChRs. This treatment did not increase the functional affinity for acetylcholine at type II nAChRs. A single injection of nicotine also produced a significant increase in type III nAChR response. In addition, type III and type II, but not type IA nAChRs, are desensitized by in vitro exposure to nicotine at concentrations found in the venous blood of cigarette smokers. Bupropion at 1 muM produced 56, 15, and 0% inhibition of type III, type II, and type IA nAChR responses, respectively, in the slices. Our results suggest that in vivo-nicotine-induced nAChR up-regulation observed in neurons of intact brain tissue is a physiologically relevant phenomenon and that early up-regulation of type III and type II nAChRs could be an important biological signal in nicotine addiction.