A polymorphism in the mouse neuronal alpha4 nicotinic receptor subunit results in an alteration in receptor function

Genetic Modifiers of Oral Nicotine Consumption in Chrna5 Null Mutant Mice

The gene CHRNA5 is strongly associated with the level of nicotine consumption in humans and manipulation of the expression or function of Chrna5 similarly alters nicotine consumption in rodents. In both humans and rodents, reduced or complete loss of function of Chrna5 leads to increased nicotine consumption. However, the mechanism through which decreased function of Chrna5 increases nicotine intake is not well-understood. Toward a better understanding of how loss of function of Chrna5 increases nicotine consumption, we have initiated efforts to identify genetic modifiers of Chrna5 deletion-dependent oral nicotine consumption in mice. For this, we introgressed the Chrna5 knockout (KO) mutation onto a panel of C57BL/6J-Chr#A/J/NAJ chromosome substitution strains (CSS) and measured oral nicotine consumption in 18 CSS and C57BL/6 (B6) mice homozygous for the Chrna5 KO allele as well as their Chrna5 wild type littermates. As expected, nicotine consumption was significantly increased in Chrna5 KO mice relative to Chrna5 wildtype mice on a B6 background. Among the CSS homozygous for the Chrna5 KO allele, several exhibited altered nicotine consumption relative to B6 Chrna5 KO mice. Sex-independent modifiers were detected in CSS possessing A/J chromosomes 5 and 11 and a male-specific modifier was found on chromosome 15. In all cases nicotine consumption was reduced in the CSS Chrna5 KO mice relative to B6 Chrna5 KO mice and consumption in the CSS KO mice was indistinguishable from their wild type littermates. Nicotine consumption was also reduced in both Chrna5 KO and wildtype CSS mice possessing A/J chromosome 1 and increased in both KO and wild type chromosome 17 CSS relative to KO and wild type B6 mice. These results demonstrate the presence of several genetic modifiers of nicotine consumption in Chrna5 KO mice as well as identify loci that may affect nicotine consumption independent of Chrna5 genotype. Identification of the genes that underlie the altered nicotine consumption may provide novel insight into the mechanism through which Chrna5 deletion increases nicotine consumption and, more generally, a better appreciation of the neurobiology of nicotine intake.

Early-adolescent male C57BL/6J and DBA/2J mice display reduced sensitivity to acute nicotine administration

BACKGROUND

The initial response to nicotine is an important predictor of subsequent use. Multiple factors may alter this response including genetics and age of first use. Here we investigated the influence of age, genetics, and their interaction on nicotine sensitivity. We then examined whether these factors influence the relationship between initial behavioral responses and voluntary nicotine consumption in adulthood.

METHODS

We measured initial nicotine responses, including nicotine-induced locomotor depression and hypothermia following an acute intraperitoneal injection (0, 0.5, or 1 mg/kg), during early-adolescence, middle-adolescence, late-adolescence, or adulthood. Thirty-five days after the initial testing, mice were assessed for voluntary oral nicotine consumption.

RESULTS

Early-adolescent mice were more resistant to nicotine-induced hypothermia and locomotor depression than later ages, further hypothermia was influenced by genetics. In the DBA/2J  strain, early-adolescent mice were insensitive to nicotine-induced hypothermia, but this response developed at later ages. In contrast, C57BL/6J  mice were sensitive at all ages, but sensitivity increased across developmental age. There was little evidence of a relationship between initial behavioral response and choice nicotine consumption.

CONCLUSION

By understanding how age of exposure and genetics influence initial nicotine behavioral responses, we have a greater understanding of factors that make adolescents differentially sensitive to the effects of this drug.

CHRNA4 and ANKK1 Polymorphisms Influence Smoking-Induced Nicotinic Acetylcholine Receptor Upregulation

INTRODUCTION

Tobacco smoking leads to increased numbers of β2*-containing nicotinic acetylcholine receptors (β2*-nAChRs) throughout the brain, which return to nonsmoker levels over extended abstinence. The goal of the current study was to determine whether the degree of tobacco smoking-induced changes in β2*-nAChR availability is genetically influenced.

METHODS

In this study, 113 European Americans participated in one or two [(123)I]5-IA-85380 single photon emission computed tomography (SPECT) brain scans. Smokers (n = 58) participated in one scan at 7-9 days of abstinence and those who remained abstinent (n = 27) were imaged again at 6-8 weeks of abstinence. Age- and sex-matched nonsmokers (n = 55) participated in one scan. Blood samples were collected for DNA analysis and genotyped for single nucleotide polymorphisms (SNPs) in the CHRNA4 and ANKK1 gene loci. β2*-nAChR availability was measured in the thalamus, striatum, cortical regions, and cerebellum.

RESULTS

The CHRNA4 SNP rs2236196 and ANKK1 SNP rs4938015 were associated with significantly higher cerebellar and cortical β2*-nAChR availability in smokers versus nonsmokers for specific genotypes. There were no significant differences by carrier status in the change in β2*-nAChR availability in smokers from 7-9 days to 6-8 weeks of abstinence.

CONCLUSION

This study provides evidence for genetic regulation of tobacco smoking-induced changes in β2*-nAChR availability and suggests that β2*-nAChR availability could be an endophenotype mediating influences of CHRNA4 variants on nicotine dependence. These results highlight individual differences in the neurochemistry of nicotine dependence and may suggest the need for individualized programs for smoking cessation.

IMPLICATIONS

This study demonstrates genetic regulation of smoking-induced changes in β2*-nAChRs throughout the brain and highlights the need for personalized programs for smoking cessation.

nAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

INTRODUCTION

The prevalence of tobacco use in the population with schizophrenia is enormously high. Moreover, nicotine dependence is found to be associated with symptom severity and poor outcome in patients with schizophrenia. The neurobiological mechanisms that explain schizophrenia-nicotine dependence comorbidity are not known. This study systematically reviews the evidence highlighting the contribution of nicotinic acetylcholine receptors (nAChRs) to nicotine abuse in schizophrenia.

METHODS

Electronic data bases (Medline, Google Scholar, and Web of Science) were searched using the selected key words that match the aims set forth for this review. A total of 276 articles were used for the qualitative synthesis of this review.

RESULTS

Substantial evidence from preclinical and clinical studies indicated that dysregulation of α7 and β2-subunit containing nAChRs account for the cognitive and affective symptoms of schizophrenia and nicotine use may represent a strategy to remediate these symptoms. Additionally, recent meta-analyses proposed that early tobacco use may itself increase the risk of developing schizophrenia. Genetic studies demonstrating that nAChR dysfunction that may act as a shared vulnerability factor for comorbid tobacco dependence and schizophrenia were found to support this view. The development of nAChR modulators was considered an effective therapeutic strategy to ameliorate psychiatric symptoms and to promote smoking cessation in schizophrenia patients.

CONCLUSIONS

The relationship between schizophrenia and smoking is complex. While the debate for the self-medication versus addiction vulnerability hypothesis continues, it is widely accepted that a dysfunction in the central nAChRs represent a common substrate for various symptoms of schizophrenia and comorbid nicotine dependence.

Natural genetic variability of the neuronal nicotinic acetylcholine receptor subunit genes in mice: Consequences and confounds

Recent human genetic studies have identified genetic variants in multiple nicotinic acetylcholine receptor (nAChR) subunit genes that are associated with risk for nicotine dependence and other smoking-related measures. Genetic variability also exists in the nAChR subunit genes in mice. Most studies on mouse nAChR subunit gene variability to date have focused on Chrna4, the gene that encodes the α4 nAChR subunit and Chrna7, the gene that encodes the α7 nAChR subunit. However, genetic variability exists for all nAChR genes in mice. In this review, we will describe what is known about nAChR subunit gene polymorphisms in mice and how it relates to variability in nAChR expression and function in brain. The relationship between nAChR genetic variability in mice and the effects of nicotine on several behavioral and physiological measures also will be discussed. In addition, an overview of the contribution of other genetic variation to nicotine sensitivity in mice will be provided. Finally, the potential for natural genetic variability to confound and/or modify the results of studies that utilize genetically engineered mice will be considered. As an example of the ability of a natural genetic variant to modify the effect of an engineered mutation, data will be presented that demonstrate that the effect of Chrna5 deletion on oral nicotine intake is dependent upon naturally occurring variant alleles of Chrna4. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

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.

The smoking cessation drug varenicline improves deficient P20-N40 inhibition in DBA/2 mice

Varenicline, an FDA approved smoking cessation pharmacotherapy, is an α4β2* nicotinic acetylcholine receptor (nAChR) partial agonist and an α7* nAChR full agonist. Both subtypes of nAChR are involved in modulating auditory evoked responses in rodents. In DBA/2 mice, an inbred strain, auditory evoked responses to paired auditory stimuli fail to inhibit to the second stimulus. This mouse strain replicates the auditory evoked response inhibition deficit experienced by the majority of schizophrenia patients. In this current study, we examined the effects of five different doses of varenicline (0.06, 0.3, 0.6, 3 and 6mg/kg) on auditory evoked responses in anesthetized DBA/2 mice. We also administered α4β2* and α7* nAChR selective antagonists prior to varenicline administration to determine which nAChR subtypes mediate the effects of varenicline. Four of the five doses of varenicline produced improvements in auditory evoked response inhibition deficits. Selective blockade of either the α4β2* or α7* nAChR in competition with 0.6mg/kg varenicline prevented varenicline induced improvements. In competition with a higher dose of varenicline (3mg/kg) only blockade of the α4β2* nAChR prevented varenicline induced improvement in auditory evoked response inhibition. These data indicate the importance of α4β2* nAChRs and the potential involvement of the α7* subtype in varenicline's effects on auditory evoked responses in DBA/2 mice.

Chrna4 A529 knock-in mice exhibit altered nicotine sensitivity

The reasons why people smoke are varied, but research has shown that genetic influences on various aspects of nicotine addiction are a major factor. There also is a strong genetic influence on measures of nicotine sensitivity in mice. Despite the established contribution of genetics to nicotine sensitivity in mice and humans, no naturally occurring genetic variation has been identified that demonstrably alters sensitivity to nicotine in either species. However, one genetic variant has been implicated in altering nicotine sensitivity in mice is a T529A polymorphism in Chrna4, the gene that encodes the nicotinic receptor (nAChR) alpha4 subunit. The Chrna4 T529A polymorphism leads to a threonine to alanine substitution at position 529 of the alpha4 subunit. To more definitively address whether the Chrna4 T529A polymorphism does, in fact, influence sensitivity to nicotine, knock-in mice were generated in which the threonine codon at position 529 was mutated to an alanine codon. Compared with Chrna4 T529 littermate controls, the Chrna4 A529 knock-in mice exhibited greater sensitivity to the hypothermic effects of nicotine, reduced oral nicotine consumption and did not develop conditioned place preference to nicotine. The Chrna4 A529 knock-in mice also differed from T529 littermates for two parameters of acetylcholine-stimulated Rb+ efflux in midbrain: maximal efflux and the percentage of alpha4beta2* receptors with high sensitivity to activation by agonists. Results indicate that the polymorphism affects the function of midbrain alpha4beta2* nAChRs and contributes to individual differences in several behavioral and physiological responses to nicotine thought to be modulated by midbrain alpha4beta2* nAChRs.

Activation and inhibition of mouse muscle and neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes

Transgenic mouse models with nicotinic acetylcholine receptor (nAChR) knockouts and knockins have provided important insights into the molecular substrates of addiction and disease. However, most studies of heterologously expressed neuronal nAChR have used clones obtained from other species, usually human or rat. In this work, we use mouse clones expressed in Xenopus oocytes to provide a relatively comprehensive characterization of the three primary classes of nAChR: muscle-type receptors, heteromeric neuronal receptors, and homomeric alpha7-type receptors. We evaluated the activation of these receptor subtypes with acetylcholine and cytisine-related compounds, including varenicline. We also characterized the activity of classic nAChR antagonists, confirming the utility of mecamylamine and dihydro-beta-erythroidine as selective antagonists in mouse models of alpha3beta4 and alpha4beta2 receptors, respectively. We also conducted an in-depth analysis of decamethonium and hexamethonium on muscle and neuronal receptor subtypes. Our data indicate that, as with receptors cloned from other species, pairwise expression of neuronal alpha and beta subunits in oocytes generates heterogeneous populations of receptors, most likely caused by variations in subunit stoichiometry. Coexpression of the mouse alpha5 subunit had varying effects, depending on the other subunits expressed. The properties of cytisine-related compounds are similar for mouse, rat, and human nAChR, except that varenicline produced greater residual inhibition of mouse alpha4beta2 receptors than with human receptors. We confirm that decamethonium is a partial agonist, selective for muscle-type receptors, but also note that it is a nondepolarizing antagonist for neuronal-type receptors. Hexamethonium was a relatively nonselective antagonist with mixed competitive and noncompetitive activity.

Differential contribution of genetic variation in multiple brain nicotinic cholinergic receptors to nicotine dependence: recent progress and emerging open questions

Nicotine dependence (ND), a major public health challenge, is a complex, multifactorial behavior, in which both genetic and environmental factors have a role. Brain nicotinic acetylcholine receptor (nAChR)-encoding genes are among the most prominent candidate genes studied in the context of ND, because of their biological relevance as binding sites for nicotine. Until recently, most research on the role of nAChRs in ND has focused on two of these genes (encoding the alpha4- and beta2-subunits) and not much attention has been paid to the possible contribution of the other nine brain nAChR subunit genes (alpha2-alpha3, alpha5-alpha7, alpha9-alpha10, beta3-beta4) to the pathophysiology and genetics of ND. This situation has changed dramatically in the last 2 years during which intensive research had addressed the issue, mainly from the genetics perspective, and has shown the importance of the CHRNA5-CHRNA3-CHRNB4 and CHRNA6-CHRNB3 loci in ND-related phenotypes. In this review, we highlight recent findings regarding the contribution of non-alpha4/beta2-subunit containing nAChRs to ND, based on several lines of evidence: (1) human genetics studies (including linkage analysis, candidate-gene association studies and whole-genome association studies) of several ND-related phenotypes; (2) differential pharmacological and biochemical properties of receptors containing these subunits; (3) evidence from genetically manipulated mice; and (4) the contribution of nAChR genes to ND-related personality traits and neurocognitive profiles. Combining neurobiological genetic and behavioral perspectives, we suggest that genetic susceptibility to ND is not linked to one or two specific nAChR subtype genes but to several. In particular, the alpha3, alpha5-6 and beta3-4 nAChR subunit-encoding genes may play a much more pivotal role in the neurobiology and genetics of ND than was appreciated earlier. At the functional level, variants in these subunit genes (most likely regulatory) may have independent as well as interactive contributions to the ND phenotype spectrum. We address methodological challenges in the field, highlight open questions and suggest possible pathways for future research.

Characterization of pharmacological and behavioral differences to nicotine in C57Bl/6 and DBA/2 mice

Approximately 50-70% of the risk for developing nicotine dependence is attributed to genetics; therefore, it is of great significance to characterize the genetic mechanisms involved in nicotine reinforcement and dependence in hopes of generating better smoking cessation therapies. The overall goal of these studies was to characterize behavioral and pharmacological responses to nicotine in C57Bl/6 (B6) and DBA/2 (D2) mice, two inbred strains commonly used for genetic studies on behavioral traits. B6 and D2 mice where subjected to a battery of behavioral tests to measure nicotine's acute effects, calcium-mediated antinociceptive responses, tolerance to chronic treatment with osmotic mini pumps, and following three days of nicotine withdrawal. In general, D2 mice were less sensitive than B6 mice to the acute effects of nicotine, but were more sensitive to blockade of nicotine-induced antinociceptive responses by a calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor. B6, but not D2 mice, developed tolerance to nicotine and nicotine conditioned place preference (CPP). While B6 and D2 mice both expressed some physical withdrawal signs, affective withdrawal signs were not evident in D2 mice. These results provide a thorough, simultaneous evaluation of the pharmacological and behavioral differences to experimenter-administered nicotine as measured in several behavioral tests of aspects that contribute to smoking behavior. The B6 and D2 strains show wide phenotypic differences in their responses to acute or chronic nicotine. These results suggest that these strains may be useful progenitors for future genetic studies on nicotine behaviors across batteries of mouse lines such as the BXD recombinant inbred panel.

CHRNA5 gene D398N polymorphism in Japanese lung adenocarcinoma

BACKGROUND

Recently, to identify genetic factors that modify lung cancer risk, CHRNA5 non-synonymous variant amino acid position 398 (D398N) was identified. The site was a highly conserved in the second cellular loop of the nicotinic acetylcholine receptor subunit protein.

MATERIALS AND METHODS

We have investigated CHRNA5 gene polymorphism status in 302 surgically treated lung adenocarcinoma cases from Nagoya City University Hospital. The presence or absence of CHRNA5 polymorphism was analyzed by direct sequences. EGFR mutations status was already investigated and reported.

RESULTS

We detected nine cases (2.98%) of CHRNA5 polymorphism (D398N) in our cohort. Total EGFR mutations were present in 129 patients (42.7%). The polymorphism statuses were not correlated with gender (women; 2.1% versus men; 3.7%, P = 0.5119), smoking status (never smoker; 2.0% versus smoker; 4.0%, P = 0.3339), pathological stages (stage I; 2.6% versus stage II-IV; 3.8%, P = 0.7246), and EGFR mutation status of the lung adenocarcinomas (mutation; 2.3% versus wild type; 3.7%, P = 0.7373). In this analysis, CHRNA5 polymorphism (D398N) patients had significantly worse prognosis (5/9 were dead; mean survival = 27.1 mo) than the patients with CHRNA5 wild type (74/293 were dead; mean survival = 113.9 mo) (log-rank test; P = 0.0146).

CONCLUSION

Although CHRNA5 polymorphism is rare from Japanese lung cancer, polymorphism status might be correlated with shorter survival.

Chromosome 15q25.1 genetic markers associated with level of response to alcohol in humans

As with other genetically complex common psychiatric and medical conditions, multiple genetic and environmental components contribute to alcohol use disorders (AUDs), which can confound attempts to identify genetic components. Intermediate phenotypes are often more closely correlated with underlying biology and have often proven invaluable in genetic studies. Level of response (LR) to alcohol is an intermediate phenotype for AUDs, and individuals with a low LR are at increased risk. A high rate of concurrent alcohol and nicotine use and dependence suggests that these conditions may share biochemical and genetic mechanisms. Genetic association studies indicate that a genetic locus, which includes the CHRNA5-CHRNA3-CHRNB4 gene cluster, plays a role in nicotine consumption and dependence. Genetic association with alcohol dependence was also recently shown. We show here that two of the markers from the nicotine studies also show an association (multiple testing corrected P < 0.025) with several LR phenotypes in a sample of 367 siblings. Additional markers in the region were analyzed and shown to be located in a 250-kb expanse of high linkage disequilibrium containing three additional genes. These findings indicate that LR intermediate phenotypes have utility in genetic approaches to AUDs and will prove valuable in the identification of other genetic loci conferring susceptibility to AUDs.

Mouse chromosome 11 harbors genetic determinants of hippocampal strain-specific nicotinic receptor expression

Differences between isogenic mouse strains in cellular expression of the neuronal nicotinic acetylcholine (ACh) receptor subunit alpha 4 (nAChR alpha 4) by the dorsal hippocampus are well known. To investigate further the genetic basis of these variations, expression of the nAChR alpha 4 subunit was measured in congenic mouse lines derived from two strains exhibiting notable divergence in the expression of this subunit: C3H and C57BL/6. Congenic lines carrying reciprocally introgressed regions (quantitative trait loci; QTL) from chromosomes 4, 5, and 12 each retained the phenotype most closely associated with the parental strain. However, in congenic lines harboring the reciprocal transfer of a chromosome 11 QTL, a characteristic difference in the ratio of interneurons versus astrocytes expressing nAChR alpha 4 in the CA1 region is reversed relative to the parental strain. These finding suggest that this chromosomal segment harbors genes that regulate strain distinct hippocampal morphology that is revealed by nAChR alpha 4 expression.

Nicotinic acetylcholine receptor expression in the hippocampus of 27 mouse strains reveals novel inhibitory circuitry

Mouse strains are well-characterized to exhibit differences in their physiological and behavioral responses to nicotine. This report examines the expression of the high-affinity nicotine binding receptor subunit, neuronal nicotinic receptor subunit alpha 4 (nAChR alpha 4), in the dorsal hippocampus of 27 inbred mouse strains. Multiple differences among mouse strains in the cellular expression of nAChR alpha 4 between subregions of the hippocampal field are evident. Differences that we describe in the expression of nAChR alpha 4 suggest mouse strains of diverse genetic origin could exhibit significant variation in how this receptor contributes to modulating intrahippocampal circuitry. These findings define a genetic frame-work in which the strain-specific responses to nicotine include underlying contributions by the varied anatomical context in which nAChRs are expressed.

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.

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.

Cellular nicotinic receptor desensitization correlates with nicotine-induced acute behavioral tolerance in rats

RATIONALE

Individuals vary in their susceptibility to nicotine addiction. However, there is little evidence that behavioral sensitivity to nicotine is dependent upon the functional state of nicotinic cholinergic receptors (nAChRs).

OBJECTIVE

This study aims to determine the relationship between in vivo behavioral desensitization and in vitro desensitization of nAChR function.

METHODS

Male Sprague-Dawley rats trained to discriminate nicotine were tested for development of acute behavioral tolerance. The rats were injected with nicotine (0.4 mg/kg free base, s.c.), tested for nicotine discrimination for 2 min, then injected with the same dose of nicotine 90, 180, and 270 min after the first injection and tested for nicotine discrimination after each injection. Susceptibility of nAChRs of individual rats to desensitization was assessed by use of the (86)Rb(+) efflux assay using synaptosomes prepared from the "thalamus," which included the hypothalamus and midbrain as well as the thalamic nuclei. To desensitize nAChRs, synaptsosomes were superfused with low concentrations of nicotine (5, 10, 20, and 30 nM) before stimulation of (86)Rb(+) efflux with nicotine (10 muM).

RESULTS

The slopes of the behavioral desensitization were plotted as a function of the decline of nicotine-stimulated (86)Rb(+) efflux after in vitro desensitization. A significant correlation was observed between the in vitro desensitization of thalamic (86)Rb(+) efflux and the extent of behavioral desensitization of individual rats.

CONCLUSIONS

These findings are consistent with the idea that production of acute behavioral tolerance by nicotine is related to its ability to induce nAChR desensitization at the cellular level.

Cholinergic nicotinic receptor genes implicated in a nicotine dependence association study targeting 348 candidate genes with 3713 SNPs

Nicotine dependence is one of the world's leading causes of preventable death. To discover genetic variants that influence risk for nicotine dependence, we targeted over 300 candidate genes and analyzed 3713 single nucleotide polymorphisms (SNPs) in 1050 cases and 879 controls. The Fagerström test for nicotine dependence (FTND) was used to assess dependence, in which cases were required to have an FTND of 4 or more. The control criterion was strict: control subjects must have smoked at least 100 cigarettes in their lifetimes and had an FTND of 0 during the heaviest period of smoking. After correcting for multiple testing by controlling the false discovery rate, several cholinergic nicotinic receptor genes dominated the top signals. The strongest association was from an SNP representing CHRNB3, the beta3 nicotinic receptor subunit gene (P = 9.4 x 10(-5)). Biologically, the most compelling evidence for a risk variant came from a non-synonymous SNP in the alpha5 nicotinic receptor subunit gene CHRNA5 (P = 6.4 x 10(-4)). This SNP exhibited evidence of a recessive mode of inheritance, resulting in individuals having a 2-fold increase in risk of developing nicotine dependence once exposed to cigarette smoking. Other genes among the top signals were KCNJ6 and GABRA4. This study represents one of the most powerful and extensive studies of nicotine dependence to date and has found novel risk loci that require confirmation by replication studies.

α4β2* Nicotinic Acetylcholine Receptors Modulate the Effects of Ethanol and Nicotine on the Acoustic Startle Response

BACKGROUND

Ethanol modulates the functional activity of alpha4beta2 neuronal nicotinic cholinergic receptors (nAChR) when measured in vitro, but the potential role of alpha4beta2 nAChRs in regulating behavioral effects of ethanol is unknown. Recently, Tritto et al. (Tritto T, Stitzel JA, Marks MJ, Romm E, Collins AC (2002) Variability in response to nicotine in the LSxSS RI strains: potential role of polymorphisms in alpha4 and alpha6 nicotinic receptor genes. Pharmacogenetics 12:197-208) reported that a polymorphism (A529T) in the alpha4 nAChR subunit gene is associated with variability in nicotine's effects on startle in the LSxSS recombinant inbred (RI) strains. Ethanol also alters the acoustic startle response. Thus, we evaluated the potential role of alpha4beta2 nAChRs in modulating ethanol's effects on acoustic startle.

METHODS

The effects of ethanol on acoustic startle were determined in the LSxSS RI strains. In addition, the effects of ethanol and nicotine were also measured in alpha4 gain of function and beta2 null mutant mice. The beta2 mutants do not express the major variant of alpha4 nAChRs, alpha4beta2.

RESULTS

An association between the alpha4 A529T polymorphism and ethanol's effects on startle was found in the LSxSS RI strains; those strains that express the A529 variant of alpha4 were more sensitive to ethanol-induced depression of startle. The alpha4 gain of function mutants were more sensitive to the effects of both nicotine and ethanol and the beta2 null mutants were less sensitive to both drugs.

CONCLUSIONS

alpha4beta2-containing nAChRs may play important roles in modulating the effects of both ethanol and nicotine on the acoustic startle response. We suggest that nAChR subunit genes should be evaluated as potential contributors to both alcoholism and tobacco abuse.

Evidence of cellular nicotinic receptor desensitization in rats exhibiting nicotine-induced acute tolerance

RATIONALE

Individuals vary in their susceptibility to nicotine addiction. However, there is little evidence that behavioral sensitivity to nicotine is dependent upon the functional state of nicotinic cholinergic receptors (nAChRs).

OBJECTIVE

To determine the relationship between in vivo pharmacological desensitization (in other words, acute tolerance) and brain regional nAChR function.

METHODS

Male Sprague-Dawley rats, trained to discriminate nicotine (0.4 mg/kg free base) from saline in a two-lever drug discrimination task, were tested for the development of acute tolerance. Rats were injected with 0.4 mg/kg nicotine, tested for nicotine discrimination for 2 min, then injected with the same dose of nicotine 90 min, 180 min, and 270 min after the first injection and tested for nicotine discrimination after each injection. These subjects were separated into two groups, desensitizers (DZ) and nondesensitizers (NDZ), based upon performance in the repetitive dosing drug discrimination paradigm. The sensitivity of nAChRs in specific brain regions of these two groups was assessed by the use of an 86Rb+ efflux assay using synaptosomes prepared from the frontal cortex, hippocampus, striatum, and "thalamus," which included the midbrain and hypothalamus as well as the thalamus.

RESULTS

The nicotine-induced increase in 86Rb+ efflux was significantly greater in NDZ as compared to DZ in the "thalamus." There was no statistically significant difference in the effects of nicotine in the frontal cortex, hippocampus, and striatum of these two groups. A significant correlation was observed between thalamic 86Rb+ efflux and the rate of behavioral desensitization of individual rats.

CONCLUSION

These findings are consistent with the concept that the production of acute tolerance by nicotine in vivo correlates directly with its ability to induce nAChR desensitization at the cellular level.

N-n-alkylnicotinium analogs, a novel class of antagonists at alpha 4 beta 2* nicotinic acetylcholine receptors: inhibition of S(-)-nicotine-evoked 86Rb+ efflux from rat thalamic synaptosomes

Pyridine N-n-alkylation of S(-)-nicotine (NIC) affords N-n-alkylnicotinium analogs, previously shown to competitively inhibit [(3)H]NIC binding and interact with alpha4beta2* nicotinic receptors (nAChRs). The present study determined the ability of the analogs to inhibit NIC-evoked (86)Rb(+) efflux from rat thalamic synaptosomes to assess functional interaction with alpha4beta2* nAChRs. In a concentration-dependent manner, NIC evoked (86)Rb(+) efflux (EC(50) = 170 nmol/L). Analog-induced inhibition of NIC-evoked (86)Rb(+) efflux varied over a approximately 450-fold range. Analogs with long n-alkyl chain lengths (C(9)-C(12)) inhibited efflux in the low nmol/L range (IC(50) = 9-20 nmol/L), similar to dihydro-beta-erythroidine (IC(50) = 19 nmol/L). Compounds with shorter n-alkyl chain lengths (C(1)-C(8)) produced inhibition in the low micromol/L range (IC(50) = 3-12 micromol/L). C(10) and C(12) analogs completely inhibited NIC-evoked efflux, whereas C(1-9) analogs produced maximal inhibition of only 10% to 60%. While the C(10) analog N-n-decylnicotinium iodide (NDNI) did not produce significant inhibition of NIC-evoked dopamine release in previously reported studies, NDNI possesses high affinity for [(3)H]NIC binding sites (K(i) = 90 nmol/L) and is a potent and efficacious inhibitor of NIC-evoked (86)Rb(+) efflux as demonstrated in the current studies. Thus, NDNI is a competitive, selective antagonist at alpha4beta2* nAChRs.

Modulation of nicotine but not ethanol preference by the mouse Chrna4 A529T polymorphism

Available evidence indicates that common genes influence alcohol and tobacco abuse in humans. The studies reported here used mouse models to evaluate the hypothesis that genetically determined variability in the alpha4beta2* nicotinic receptor modulates genetically determined variability in the intake of both nicotine and alcohol. Data obtained with inbred mouse strains suggested an association between a polymorphism in the mouse alpha4 nAChR subunit gene, Chrna4, and variability in nicotine and ethanol preference. These associations were assessed in F2 animals derived by crossing C57BL/6-super(beta2-/-) mice and A/J mice. The results obtained by the authors indicate that the polymorphism in Chrna4 plays an important role in modulating variability in oral nicotine intake but is linked to a gene that regulates alcohol intake.

Functional characterization of mouse alpha4beta2 nicotinic acetylcholine receptors stably expressed in HEK293T cells

Mouse alpha4beta2 nicotinic acetylcholine receptors (nAchRs) were stably expressed in HEK293T cells. The function of this stable cell line, termed mmalpha4beta2, was assessed using an aequorin-based luminescence method that measures agonist-evoked changes in intracellular calcium. Agonist-elicited changes in intracellular calcium were due primarily to direct entry of calcium through the alpha4beta2 channel, although release of calcium from intracellular stores contributed approximately 28% of the agonist-evoked response. Agonist pharmacologies were very similar between the mmalpha4beta2 cells and most cell lines that stably express human alpha4beta2 nAchRs. Based on agonist profiles and sensitivity to the antagonist dihydro-beta-erythroidine (DHbetaE), the predominant alpha4beta2 nAchR expressed in the mmalpha4beta2 cells exhibits a pharmacology that most resembles the DHbetaE-sensitive component of 86Rb+ efflux from mouse brain synaptosomes. However, when evaluated with the aequorin assay, the mmalpha4beta2 nAchR was found to be atypically sensitive to blockade by the presumed alpha7-selective antagonist methyllycaconitine (MLA), exhibiting an IC50 value of 31 +/- 0.1 nm. Similar IC50 values have been reported for the MLA inhibition of nicotine-stimulated dopamine release, a response that is mediated by beta2-subunit-containing nAchRs and not alpha7-subunit-containing nAchRs. Consequently, at low nanomolar concentrations, MLA may not be as selective for alpha7-containing nAchRs as previously thought.

Genetic correlation between the free-choice oral consumption of nicotine and alcohol in C57BL/6J × C3H/HeJ F2 intercross mice

Previous studies in humans have demonstrated a high co-morbidity between alcoholism and smoking. This co-morbidity between alcohol and nicotine dependence can be attributed, in part, to common genetic factors. In rodents, behavioral and physiological responses to alcohol and nicotine also appear to share common genetic influences. In this report, the genetic correlation between free-choice oral nicotine and oral alcohol consumption was evaluated using an ascending two-bottle choice paradigm in C57BL/6xC3H/HeJ F2 intercross mice. For all concentrations of nicotine (25, 50, and 100 microg/ml) and alcohol (3, 6, and 10%) tested, nicotine consumption was significantly correlated with alcohol consumption. Nicotine consumption at the highest nicotine concentration tested (100 microg/ml) showed low, but significant, correlations with the number of [3H]-cytisine binding sites in the hippocampus (r=0.307) and the number of [125I]-alpha-bungarotoxin binding sites in the cortex (r=-0.328). No significant correlations between alcohol consumption and the number of either [3H]-cytisine or [125I]-alpha-bungarotoxin binding sites was observed. A polymorphism in the nicotinic receptor alpha4 subunit gene, Chrna4, showed a trend with nicotine consumption and a significant association with alcohol consumption in female but not male mice. These results indicate that common genetic factors influence nicotine and alcohol consumption in mice. However, neither individual differences in the expression of [3H]-cytisine or [125I]-alpha-bungarotoxin binding nicotinic receptors nor the polymorphism in Chrna4 likely contribute to the genetic overlap that influences the consumption of both of these drugs of abuse in C57BL/6xC3H/HeJ F2 mice.

Association of polymorphisms in nicotinic acetylcholine receptor alpha 4 subunit gene (CHRNA4), mu-opioid receptor gene (OPRM1), and ethanol-metabolizing enzyme genes with alcoholism in Korean patients

Findings obtained from several studies indicate that ethanol enhances the activity of alpha4beta2 neuronal nicotinic acetylcholine receptor and support the possibility that a polymorphism of the nicotinic acetylcholine receptor alpha4 subunit gene (CHRNA4) modulates enhancement of nicotinic receptor function by ethanol. To identify the association between the CfoI polymorphism of the CHRNA4 and alcoholism, we examined distribution of genotypes and allele frequencies in Korean patients diagnosed with alcoholism (n = 127) and Korean control subjects without alcoholism (n = 185) with polymerase chain reaction-restriction fragment length polymorphism methods. We were able to detect the association between the CfoI polymorphism of the CHRNA4 and alcoholism in Korean patients (genotype P = .023; allele frequency P = .047). The genotypes and allele frequencies of known polymorphisms in other alcoholism candidate genes, such as alcohol metabolism-related genes [alcohol dehydrogenase 2 (ADH2), aldehyde dehydrogenase 2 (ALDH2), alcohol dehydrogenase 3 (ADH3), and cytochrome P450 2E1 (CYP2E1)] and mu-opioid receptor gene (OPRM1), were studied. The polymorphisms of ADH2, ALDH2, and CYP2E1 were significantly different in Korean patients with alcoholism and Korean control subjects without alcoholism, but ADH3 and OPRM1 did not differ between the two groups.

Subsets of acetylcholine-stimulated 86Rb+ efflux and [125I]-epibatidine binding sites in C57BL/6 mouse brain are differentially affected by chronic nicotine treatment

Nicotinic cholinergic receptor (nAChR) sites that bind nicotine with high affinity (likely alpha4beta2-nAChR) increase following chronic nicotine treatment. Effects of chronic treatment on other nAChR binding sites and functional responses of nAChRs are less well studied. Therefore, C57BL/6 mice were intravenously infused for 10 days with saline or nicotine (five doses, 0.25-4.0 mg/kg/h) and nAChR function and three different nicotinic binding sites in 12 brain regions were assessed. Plasma nicotine and cotinine increased linearly with dose. 86Rb+ efflux with higher sensitivity to acetylcholine tended to decrease with increasing dose, whereas efflux with lower sensitivity to acetylcholine tended to increase. As anticipated, likely alpha4beta2-nAChR [125I]-epibatidine binding sites increased with treatment (estimated dosage for one-half maximal increase was 0.44 mg/kg/h, plasma nicotine approximately 20 ng/ml). 86Rb+ efflux with higher sensitivity to acetylcholine and cytisine-sensitive [125I]-epibatidine binding are predominantly alpha4beta2-nAChR. A high correlation between these parameters was observed across brain regions and slopes of these regression lines decreased with treatment dose, suggesting a decrease in function per unit receptor. Likely alpha3beta4-nAChR binding sites were unaffected even at the highest dose (4.0 mg/kg/h, approximately 210 ng/ml). A third set of diverse nAChR binding sites increased in some brain regions, but only after high-dose treatment.

Mouse strain-specific nicotinic acetylcholine receptor expression by inhibitory interneurons and astrocytes in the dorsal hippocampus

The response by individuals to nicotine is likely to reflect the interaction of this compound with target nAChRs. However, resolving how different genetic backgrounds contribute to unique mouse strain-specific responses to this compound remains an important and unresolved issue. To examine this question in detail, expression of the nicotine acetylcholine receptor (nAChR) subunits alpha3, alpha4, alpha5, alpha7, beta2, and beta4 was measured in the dorsal hippocampus using immunohistochemistry in mouse strains or lines BALB/c, C3H/J, C57BL/6, CBA/J, DBA/2, Long Sleep (LS), Short Sleep (SS), and CF1. The nAChRs in all mice colocalized with glutamic acid decarboxylase (GAD)-positive interneurons that were subclassified into at least four groups based on nAChR subunit heterogeneity. A notable difference between mouse strains was the expression of nAChRs by astrocyte subpopulations in CA1 subregions whose numbers vary inversely with nAChR-immunostained neurons. This novel relationship also correlated with published parameters of strain sensitivity to nicotine. Attempts to identify the origin of this significant difference in nAChR expression among strains included comparison of the entire nAChRalpha4 gene sequence. Although multiple polymorphisms were identified, including two that changed nAChRalpha4 amino acid coding, none of these clearly correlate with strain-related differences in cell type-specific nAChR expression. These findings suggest that mouse strain-specific behavioral and physiological responses to nicotine are likely to be a reflection of a complex interplay between genetic factors that shape differences in expression and cellular architecture of this modulatory neurotransmitter system in the mammalian nervous system.

Autonomic Nervous System Pharmacogenomics: A Progress Report

Pharmacogenetics, the inherited basis for interindividual differences in drug response, has rapidly expanded with the advent of new molecular tools and the sequencing of the human genome, yielding pharmacogenomics. We review here recent ideas and findings regarding pharmacogenomics of components of the autonomic nervous system, in particular, neuronal nicotinic acetylcholine receptors, postsynaptic receptors with which the parasympathetic and sympathetic neurotransmitters, acetylcholine (ACh) and norepinephrine, respectively, interact. The receptor subtypes that mediate these responses, M(1-3) muscarinic cholinergic receptors (mAChRs), and alpha(1A,B,D)-, alpha(2A,B,C)-, and beta(1,2,3)-adrenergic receptors (AR), show highly variable expression of genetic variants; variants of mAChRs and alpha(1)-ARs are relatively rare, whereas alpha(2)-AR and beta-AR subtype variants are quite common. The largest amount of data is available regarding variants of the latter ARs and represents efforts to associate certain receptor genotypes, most commonly, single nucleotide polymorphisms, with particular phenotypes (e.g., cardiovascular and metabolic responses). In vitro and in vivo studies have yielded inconsistent results; definitive conclusions are limited. We identify several conceptual and methodological problems with available data: sample size, ethnicity, tissue differences, coding versus noncoding variants, limited studies of haplotypes, and interaction among variants. Thus, although progress has been made in identifying genetic variation that influences drug response fo autonomic nervous system components, we are still at the early stages of defining the most critical genetic determinants and their role in human physiology and pharmacology.

Interaction of the Nicotinic Cholinergic System with Ethanol Withdrawal

The observation that alcohol and nicotine are commonly abused together suggests that the two drugs have common sites of action. In vitro studies indicate that nicotinic acetylcholine receptor (nAChR) function is enhanced by ethanol. Furthermore, some ethanol-related behaviors are associated with a region of mouse chromosome 2 that contains the gene encoding the alpha4 subunit of the nAChR (Chrna4). We have identified a polymorphism in Chrna4 that results in an alanine (A) or threonine (T) residue at position 529 in the second intracellular loop of the protein. Nicotinic receptors expressing the A variant have greater responses to nicotine and ethanol than receptors with the T variant when measured in vitro, but the possible effects of the polymorphism on the severity of ethanol withdrawal have not been assessed. The handling-induced convulsion (HIC) assay is an established method for studying drug withdrawal in vivo. We monitored the HIC responses of mice for 8 h after an injection of ethanol (4 g/kg). A survey of 16 mouse strains, as well as previously published data, indicated an association of the A/T polymorphism with ethanol withdrawal. This association was also found in wild-type animals from an F2 intercross of the A/J (A529-genotype) strain with C57BL/6J (T529-genotype) mice that also lack expression of the beta2 nAChR subunit. Beta2 -/- animals, which do not express alpha4beta2 nAChRs in the brain, exhibited significantly lower HIC responses and no effect of the polymorphism. These results suggest that the nicotinic cholinergic system and the A/T polymorphism modulate ethanol withdrawal.

The mouse Chrna4 A529T polymorphism alters the ratio of high to low affinity alpha 4 beta 2 nAChRs

Previously, a missense polymorphism was identified in the mouse nicotinic receptor alpha4 subunit gene, Chrna4. This polymorphism leads to an Ala/Thr variation at amino acid position 529 of the alpha4 subunit. Chrna4 A529T is associated with several measures of acute sensitivity to nicotine as well as with mouse strain differences in nicotine-stimulated (86)Rb(+) efflux from synaptosomes. Here, we report that the variant forms of the mouse alpha4 subunit confer functional differences when expressed with the beta2 subunit in a heterologous system. alpha4beta2 receptors containing the T529 variant of the alpha4 subunit exhibited a higher EC(50) value for the high affinity receptor population and an apparent reduced sensitivity to blockade by DHbetaE relative to alpha4beta2 receptors containing the A529 variant of the alpha4 subunit. Moreover, the proportion of the total agonist-elicited current contributed by the high affinity alpha4beta2 receptor population was greater for alpha4beta2 receptors containing the alpha4(T529) variant (64%) than the alpha4beta2 receptors containing the alpha4(A529) variant (41%). These data suggest that the polymorphism in the mouse alpha4 subunit is located in a previously unidentified functional domain of the receptor subunit that influences receptor function, including regulation of the affinity population ratio of alpha4beta2 receptors.

A polymorphism in the alpha4 nicotinic receptor gene (Chrna4) modulates enhancement of nicotinic receptor function by ethanol

BACKGROUND

Several studies indicate that ethanol enhances the activity of alpha4beta2 nicotinic acetylcholine receptors (nAChR). Our laboratory has identified a polymorphism in the alpha4 gene that results in the substitution of an alanine (A) for threonine (T) at amino acid position 529 in the second intracellular loop of the alpha4 protein. Mouse strains expressing the A variant have, in general, greater nAChR-mediated 86Rb+ efflux in response to nicotine than strains with the T variant. However, the possibility of the polymorphism modulating the effects of ethanol on the 86Rb+ efflux response has not been investigated.

METHODS

We have used the 86Rb+ efflux method to study the acute effects of ethanol on the function of the alpha4beta2 nAChR in the thalamus in six different mouse strains. Experiments were also performed on tissue samples taken from F2 intercross animals. The F2 animals were derived from A/J mice crossed with a substrain of C57BL/6J mice that carried a null mutation for the gene encoding the beta2 nAChR subunit.

RESULTS

In strains carrying the A polymorphism (A/J, AKR/J, C3H/Ibg), coapplication of ethanol (10-100 mM) with nicotine (0.03-300 microM) increased maximal ion flux when compared with nicotine alone with no effect on agonist potency. In contrast, ethanol had little effect on the nicotine concentration-response curve in tissue prepared from strains carrying the T polymorphism (Balb/Ibg, C57BL/6J, C58/J). Experiments with the F2 hybrids demonstrated that one copy of the A polymorphism was sufficient to produce a significant enhancement of nAChR function by ethanol (50 mM) in animals that were also beta2 +/+. Ethanol had no effect on nicotine concentration-response curves in T/T beta2 +/+ animals.

CONCLUSIONS

The results suggest that the A/T polymorphism influences the initial sensitivity of the alpha4beta2 nAChR to ethanol.