Potent modulation of neuronal nicotinic acetylcholine receptor-channel by ethanol

Prevalence, correlates, and behavioral outcomes of alcohol gifting in China

Introduction

Alcohol gifting is a very common practice in China. However, little is known about the potentially adverse consequences of alcohol gifting. This study aimed to investigate the prevalence of, and factors associated with, alcohol gifting, and explore whether drinking and tobacco use were associated with alcohol gifting.

Methods

Using a cross-sectional multi-stage survey, a sample of 982 household heads from Guangdong Province and 530 household heads from Shaanxi Province was collected online from 30 April to 30 July 2020 in China. Participants completed questionnaires regarding socio-demographic characteristics, social capital, drinking status, and gifting alcohol behavior. Chi-square analysis and multiple logistic regression analysis were used to identify the factors associated with alcohol gifting, and to identify its relationship with alcohol and cigarette use status.

Results

Multiple logistic regression analysis showed that age, gender, household annual income, province, drinking status, and social participation were prominent correlates of both offering and receiving alcohol. Participants who were married, had an education level of junior high school, or had a large social network had higher odds of receiving alcohol. When both alcohol gifting behaviors were included in the models, participants who offered alcohol had 2.15 (95% CI: 1.63–2.85) times higher odds of current drinking than those who didn’t offer alcohol and participants who received alcohol had 1.87 (95% CI: 1.45–2.41) times higher odds of current drinking than those who did not receive alcohol. Those who received alcohol had significantly higher odds of current smoking (AOR = 1.64; 95% CI: 1.25–2.14), while those who offered alcohol had significantly lower odds of current smoking (AOR = 0.71;95% CI:0.53–0.95).

Conclusions

Social participation is an important correlate of alcohol gifting. Alcohol receiving behaviors were significantly associated with both current alcohol and tobacco use. These associations can be used to inform alcohol gifting interventions in China.

Behavioral genetics of alcohol's effects in three zebrafish (Danio rerio) populations

Alcohol abuse is one of the most dangerous and serious problems for patients and society. Interpopulation studies are important in understanding how genetic background contributes to the effects of alcohol. In this study, we applied a chronic alcohol exposure protocol in three zebrafish populations (Danio rerio; both sexes; AB, TU, and outbred fish - OB). We analyzed the behavioral responses and mRNA expression involved in neurotransmitter metabolism - th1, tph1, ache, ada1, gaba1, gad1b, and bdnf. Locomotion patterns were similar between populations (increased speed after acute alcohol and unaltered locomotion after chronic and withdrawal treatments). All populations exhibited increased expression of genes associated with locomotion (th1, gad1b, and gaba1) after acute alcohol exposure. Anxiety-like responses increased in AB and TU fish during withdrawal and decreased in AB fish after acute alcohol exposure. Genes related to anxiety-like behavior (tph1 and ada1) were overexpressed in AB and TU fish after acute and withdrawal treatments, while OB fish exhibited unaltered responses. Bdnf levels decreased during withdrawal in AB and OB fish, while TU showed upregulated levels in both chronic and withdrawal treatments. Our results suggest that zebrafish populations respond differently to alcohol exposure, which may contribute to understanding the mechanisms underlying alcohol use and dependence. Moreover, we found that a more diverse genetic background (OB) was related to higher variability in behavioral and mRNA expression, demonstrating that inbred populations (AB and TU) may be useful tools in identifying alcohol use and abuse mechanisms.

Alcohol-induced Aggression

Intraspecies aggression is commonly focused on securing reproductive resources such as food, territory, and mates, and it is often males who do the fighting. In humans, individual acts of overt physical aggression seem maladaptive and probably represent dysregulation of the pathways underlying aggression. Such acts are often associated with ethanol consumption. The Drosophila melanogaster model system, which has long been used to study how ethanol affects the nervous system and behavior, has also been used to study the molecular origins of aggression. In addition, ethanol-induced aggression has been demonstrated in flies. Recent publications show that ethanol stimulates Drosophila aggression in 2 ways: the odor of ethanol and the consumption of ethanol both make males more aggressive. These ethanol effects occur at concentrations that flies likely experience in the wild. A picture emerges of males arriving on their preferred reproductive site-fermenting plant matter-and being stimulated by ethanol to fight harder to secure the site for their own use. Fly fighting assays appear to be a suitable bioassay for studying how low doses of ethanol reshape neural signaling.

Bidirectional sex-dependent regulation of α6 and β3 nicotinic acetylcholine receptors by protein kinase Cε

Nicotine and alcohol are the most commonly abused substances worldwide and are frequently coabused. Nicotinic acetylcholine receptors (nAChRs) containing the α6 and β3 subunits are expressed in neural reward circuits and are critical for nicotine and alcohol reward. nAChRs are dynamically regulated by signaling molecules such as protein kinase C epsilon (PKCε), which impact transcription of α6 and β3 subunit mRNA (Chrna6 and Chrnb3, respectively). Previous work found decreased expression of Chrna6 and Chrnb3 transcripts in the ventral midbrain of male PKCε-/- mice, who also consume less nicotine and alcohol compared with wild-type (WT) littermates. Using RT-qPCR, we show that female PKCε-/- mice have higher expression of Chrna6 and Chrnb3 transcripts in the ventral midbrain, which functionally impacts nAChR-dependent behavior as female but not male PKCε-/- mice exhibit locomotor hypersensitivity to low-dose (0.25 mg/kg i.p.) nicotine. Female PKCε-/- mice show no differences in alcohol-induced sedation in the loss-of-righting reflex assay (4.0 g/kg i.p.) compared with WT littermates, whereas male PKCε-/- mice have enhanced sedation compared with WT mice. Female PKCε-/- mice also show reduced immobility time in response to varenicline (1.0 mg/kg i.p.) compared with WT littermates in the tail suspension test, and this effect was absent in male mice. Additionally, we found that female PKCε-/- mice show altered alcohol and nicotine consumption patterns in chronic voluntary two-bottle choice assays. Our data reveal a bidirectional effect of sex in the transcriptional regulation of nicotinic receptors by PKCε, highlighting the importance of studying both sexes in preclinical animal models.

Alpha6-containing nicotinic acetylcholine receptor is a highly sensitive target of alcohol

Alcohol use disorder (AUD) is a serious public health problem that results in tremendous social, legal and medical costs to society. Unlike other addictive drugs, there is no specific molecular target for ethanol (EtOH). Here, we report a novel molecular target that mediates EtOH effects at concentrations below those that cause legally-defined inebriation. Using patch-clamp recording of human α6*-nicotinic acetylcholine receptor (α6*-nAChR) function when heterologously expressed in SH-EP1 human epithelial cells, we found that 0.1-5 mM EtOH significantly enhances α6*-nAChR-mediated currents with effects that are dependent on both EtOH and nicotine concentrations. EtOH exposure increased both whole-cell current rising slope and decay constants. This EtOH modulation was selective for α6*-nAChRs since it did not affect α3β4-, α4β2-, or α7-nAChRs. In addition, 5 mM EtOH also increased the frequency and amplitude of dopaminergic neuron transients in mouse brain nucleus accumbens slices, that were blocked by the α6*-nAChR antagonist, α-conotoxin MII, suggesting a role for native α6*-nAChRs in low-dose EtOH effects. Collectively, our data suggest that α6*-nAChRs are sensitive targets mediating low-dose EtOH effects through a positive allosteric mechanism, which provides new insight into mechanisms involved in pharmacologically-relevant alcohol effects contributing to AUD.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

The nicotinic receptor drug sazetidine-A reduces alcohol consumption in mice without affecting concurrent nicotine consumption

Alcohol and nicotine addiction are frequently co-morbid. The nicotinic acetylcholine receptors (nAChRs) are critical for both alcohol and nicotine addiction mechanisms, since nAChR drugs that reduce nicotine consumption have been shown to also reduce alcohol consumption. Sazetidine-A, a pre-clinical nAChR drug with agonist and desensitizing effects at α4β2 and α7 nAChRs, has been reported to reduce alcohol consumption and nicotine self-administration in rats when administered at high doses. However, this effect has not been replicated in mice. In this study, we examined the effect of sazetidine-A on alcohol and nicotine consumption in male and female mice utilizing voluntary oral consumption procedures previously developed in our lab. We found that sazetidine-A (1 mg/kg, i.p) reduced overnight alcohol consumption, but did not affect nicotine consumption when presented either alone or concurrently with alcohol. Sazetidine-A did not reduce water or saccharin consumption at any dose tested. In a chronic co-consumption experiment in which either alcohol or nicotine was re-introduced after one week of forced abstinence, sazetidine-A attenuated post-abstinence consumption of alcohol but not nicotine. Sazetidine-A also significantly reduced alcohol consumption in an acute, binge drinking-in-the-dark procedure. Finally, we tested the effect of sazetidine-A on alcohol withdrawal, and found that sazetidine-A significantly reduced handling-induced convulsions during alcohol withdrawal. Collectively, these data suggest a novel role for the nAChR targets of sazetidine-A in specifically mediating alcohol consumption, separate from the involvement of nAChRs in mediating nicotine consumption. Delineation of this pathway may provide insight into novel therapies for the treatment of alcohol use disorders.

Titrating Tipsy Targets: The Neurobiology of Low-Dose Alcohol

Limited attention has been given to our understanding of how the brain responds to low-dose alcohol (ethanol) and what molecular and cellular targets mediate these effects. Even at concentrations lower than 10mM (0.046 g% blood alcohol concentration, BAC), below the legal driving limit in the USA (BAC 0.08 g%), alcohol impacts brain function and behavior. Understanding what molecular and cellular targets mediate the initial effects of alcohol and subsequent neuroplasticity could provide a better understanding of vulnerability or resilience to developing alcohol use disorders. We review here what is known about the neurobiology of low-dose alcohol, provide insights into potential molecular targets, and discuss future directions and challenges in further defining targets of low-dose alcohol at the molecular, cellular, and circuitry levels.

Structural Basis of Alcohol Inhibition of the Pentameric Ligand-Gated Ion Channel ELIC

The structural basis for alcohol modulation of neuronal pentameric ligand-gated ion channels (pLGICs) remains elusive. We determined an inhibitory mechanism of alcohol on the pLGIC Erwinia chrysanthemi (ELIC) through direct binding to the pore. X-ray structures of ELIC co-crystallized with 2-bromoethanol, in both the absence and presence of agonist, reveal 2-bromoethanol binding in the pore near T237(6') and the extracellular domain (ECD) of each subunit at three different locations. Binding to the ECD does not appear to contribute to the inhibitory action of 2-bromoethanol and ethanol as indicated by the same functional responses of wild-type ELIC and mutants. In contrast, the ELIC-α1β3GABA_AR chimera, replacing the ELIC transmembrane domain (TMD) with the TMD of α1β3GABA_AR, is potentiated by 2-bromoethanol and ethanol. The results suggest a dominant role of the TMD in modulating alcohol effects. The X-ray structures and functional measurements support a pore-blocking mechanism for inhibitory action of short-chain alcohols.

Voluntary co-consumption of alcohol and nicotine: Effects of abstinence, intermittency, and withdrawal in mice

Alcohol and nicotine are often used together, and there is a high rate of co-occurrence between alcohol and nicotine addiction. Most animal models studying alcohol and nicotine interactions have utilized passive drug administration, which may not be relevant to human co-addiction. In addition, the interactions between alcohol and nicotine in female animals have been understudied, as most studies have used male animals. To address these issues, we developed models of alcohol and nicotine co-consumption in male and female mice that utilized voluntary, oral consumption of unsweetened alcohol, nicotine and water. We first examined drug consumption and preference in single-drug, sequential alcohol and nicotine consumption tests in male and female C57BL/6 and DBA/2J mice. We then tested chronic continuous and intermittent access alcohol and nicotine co-consumption procedures. We found that male and female C57BL/6 mice readily co-consumed unsweetened alcohol and nicotine. In our continuous co-consumption procedures, we found that varying the available nicotine concentration during an alcohol abstinence period affected compensatory nicotine consumption during alcohol abstinence, and affected rebound alcohol consumption when alcohol was re-introduced. Consumption of alcohol and nicotine in an intermittent co-consumption procedure produced higher alcohol consumption levels, but not nicotine consumption levels, compared with the continuous co-consumption procedures. Finally, we found that intermittent alcohol and nicotine co-consumption resulted in physical dependence. Our data show that these voluntary co-consumption procedures can be easily performed in mice and can be used to study behavioral interactions between alcohol and nicotine consumption, which may better model human alcohol and nicotine co-addiction.

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

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

Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors

General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Nicotine dependence syndrome scale and craving: comparing nicotine-dependent individuals with and without comorbid alcohol dependence

INTRODUCTION

Although several studies have explored craving for certain drugs, there is limited data describing the relationship between alcohol and nicotine craving from a multidimensional perspective among individuals with comorbid nicotine dependence (ND) and alcohol dependence (AD).

METHODS

We compared a group of male patients diagnosed with ND and AD (n = 160) to a group of male patients diagnosed with ND only (n = 235). Smoking- and drinking-related clinical features were measured, including craving levels and the Nicotine Dependence Syndrome Scale (NDSS), which is a multidimensional questionnaire measuring ND. Subsequently, we studied factors that influenced smoking and alcohol craving in the ND and AD group.

RESULTS

Regarding the NDSS, the sum, priority and tolerance scores were significantly higher in the ND and AD group compared with the ND only group (P < 0.000, P < 0.000 and P = 0.001, respectively). In the comorbid group, regression analyses revealed that alcohol craving and Fagerstrom Test for Nicotine Dependence (FTND) scores contributed to nicotine craving (beta coefficient = 0.37, P = 0.005 and beta coefficient = 0.35, P = 0.026, respectively) and these two factors explained 36% of the variance). Nicotine craving appeared to be the only factor that contributed to alcohol craving (beta coefficient = 0.35, P = 0.002), and nicotine craving explained 16% of the variance.

DISCUSSION

This study may help clarify the clinical relationship between comorbid alcohol and nicotine dependence and help guide the development of effective treatment strategies for ND and AD patients.

Neuronal nicotinic acetylcholine receptors are important targets for alcohol reward and dependence

Neuronal nicotinic acetylcholine receptors are important targets for alcohol reward and dependence. Alcoholism is a serious public health problem and has been identified as the third major cause of preventable mortality in the world. Worldwide, about 2 billion people consume alcohol, with 76.3 million having diagnosable alcohol use disorders. Alcohol is currently responsible for the death of 4% of adults worldwide (about 2.5 million deaths each year), and this number will be significantly increased by 2020 unless effective action is taken. Alcohol is the most commonly abused substance by humans. Ethanol (EtOH) is the intoxicating agent in alcoholic drinks that can lead to abuse and dependence. Although it has been extensively studied, the mechanisms of alcohol reward and dependence are still poorly understood. The major reason is that, unlike other addictive drugs (eg, morphine, cocaine or nicotine) that have specific molecular targets, EtOH affects much wider neuronal functions. These functions include phospholipid membranes, various ion channels and receptors, synaptic and network functions, and intracellular signaling molecules. The major targets in the brain that mediate EtOH's effects remain unclear. This knowledge gap results in a therapeutic barrier in the treatment of alcoholism. Interestingly, alcohol and nicotine are often co-abused, which suggests that neuronal nicotinic acetylcholine receptors (nAChRs), the molecular targets for nicotine, may also contribute to alcohol's abusive properties. Here, we briefly summarize recent lines of evidence showing how EtOH modulates nAChRs in the mesolimbic pathway, which provides a perspective that nAChRs are important targets mediating alcohol abuse.

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

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

Nicotine modulates alcohol-seeking and relapse by alcohol-preferring (P) rats in a time-dependent manner

BACKGROUND

Alcohol is frequently co-abused with smoking. In humans, nicotine use can increase alcohol craving and consumption. The objectives of the current study were to assess the acute effects of nicotine on alcohol seeking and relapse at 2 different time points.

METHODS

Adult female alcohol-preferring (P) rats were trained in 2-lever operant chambers to self-administer 15% ethanol (EtOH) (v/v) and water on a concurrent fixed-ratio 5-fixed-ratio 1 (FR5-FR1) schedule of reinforcement in daily 1-hour sessions. Following 10 weeks of daily 1-hour sessions, rats underwent 7 extinction sessions, followed by 2 weeks in their home cages. Rats were then returned to the operant chambers without EtOH or water being present for 4 sessions (Pavlovian Spontaneous Recovery [PSR]). Rats were then given a week in their home cage before being returned to the operant chambers with access to EtOH and water (relapse). Nicotine (0, 0.1, 0.3, or 1.0 mg/kg) was injected subcutaneously immediately or 4 hours prior to PSR or relapse testing.

RESULTS

Injections of nicotine immediately prior to testing reduced (5 to 10 responses PSR; 50 to 60 responses relapse), whereas injections of nicotine 4 hours prior to testing increased (up to 150 responses for PSR; up to 400 responses for relapse with 1.0 mg/kg dose) responses on the EtOH lever during PSR and relapse tests.

CONCLUSIONS

The results of this study demonstrate that acute effects of nicotine on EtOH-seeking and relapse behaviors may be time dependent, with the immediate effects being a result of nicotine possibly acting as a substitute for EtOH, whereas with a delay of 4 hours, priming effects of nicotine alterations in nicotinic receptors, and/or the effects of nicotine's metabolites (i.e., cotinine and nornicotine) may enhance the expression of EtOH-seeking and relapse behaviors.

Acute alcohol action and desensitization of ligand-gated ion channels

Ethanol exerts its biological actions through multiple receptors, including ion channels. Ion channels that are sensitive to pharmacologically relevant ethanol concentrations constitute a heterogeneous set, including structurally unrelated proteins solely sharing the property that their gating is regulated by a ligand(s). Receptor desensitization is almost universal among these channels, and its modulation by ethanol may be a crucial aspect of alcohol pharmacology and effects in the body. We review the evidence documenting interactions between ethanol and ionotropic receptor desensitization, and the contribution of this interaction to overall ethanol action on channel function. In some cases, such as type 3 serotonin, nicotinic acetylcholine, GABA-A, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, ethanol actions on apparent desensitization play a significant role in acute drug action on receptor function. In a few cases, mutagenesis helped to identify different areas within a receptor protein that differentially sense n-alcohols, resulting in differential modulation of receptor desensitization. However, desensitization of a receptor is linked to a variety of biochemical processes that may alter protein conformation, such as the lipid microenvironment, post-translational channel modification, and channel subunit composition, the relative contribution of these processes to ethanol interactions with channel desensitization remains unclear. Understanding interactions between ethanol and ionotropic receptor desensitization may help to explain different ethanol actions 1) when ethanol is evaluated in vitro on cloned channel proteins, 2) under physiological or pathological conditions or in distinct cell domains with modified ligand concentration and/or receptor conformation. Finally, receptor desensitization is likely to participate in molecular and, possibly, behavioral tolerance to ethanol, which is thought to contribute to the risk of alcoholism.

Effects of moderate chronic ethanol consumption on hippocampal nicotinic receptors and associative learning

A number of studies have reported that ethanol exposure induces changes in different brain systems. The hippocampus is a brain region that is very vulnerable to ethanol exposition, which functionally results in impairment of learning and memory processes reported in heavy drinkers. Hippocampal nicotinic receptors are involved in learning and memory. In this study, we determined the effects of ethanol on the main hippocampal subtypes of neural nicotinic receptors (alpha7 and alpha4beta2) in rats non-selected for alcohol consumption, in order to check for possible changes on these receptors that could be linked with alterations in learning acquisition. Binding assays were carried out with [3H]methyllycaconitine ([3H]MLA) to study the alpha7 and [3H]nicotine to study alpha4beta2 receptors. Auto-shaping, continuous ratio and extinction procedures were used as behavioral tests. The results show that moderate chronic ethanol consumption for 10 weeks produces: (a) a decrease of both hippocampal nicotinic receptor subtypes without alterations in affinity; (b) no differences in behavioral performance between control rats and ethanol-drinking rats in auto-shaping and continuous ratio; (c) an improvement of performance of extinction paradigm. These results indicate that chronic ethanol consumption, at moderate levels, induces changes in hippocampal nicotinic receptors but does not impair acquisition and performance of new associative learning and even improves some kind of paradigms. These results may have implications in the biochemical basis of interactions between alcohol and nicotine and the effects of these drugs on behavior.

Ethanol blocks nicotine-induced seizures in mice: comparison with midazolam and baclofen

Low doses of ethanol may antagonize the pharmacological effects of nicotine. Recently, it has been shown that the effects of ethanol on nicotine discrimination are not correlated with blood ethanol levels. The aim of the present study was to evaluate whether ethanol (0.5-2g/kg, i.p.) could block nicotine-induced seizures in C57BL/6J mice and to correlate ethanol's actions with blood ethanol concentrations. For comparison, the effects of a gamma-aminobutyric acid A (GABAA)/benzodiazepine receptor positive modulator, midazolam (0.25-40 mg/kg, i.p.), and a gamma-aminobutyric acid B receptor agonist, baclofen (2.5-20 mg/kg, i.p.), were assessed in the same procedure. Nicotine (3-9 mg/kg, s.c.) induced clonic-tonic seizures in a dose-dependent manner. Ethanol, administered 5 or 50 min before nicotine, dose dependently antagonized seizures elicited by 6 mg/kg nicotine. The anticonvulsant effects of ethanol correlated with blood ethanol levels and were comparable to those exerted by midazolam. Baclofen antagonized only the tonic component of nicotine-induced convulsions. The anticonvulsant doses of ethanol (0.5-2 g/kg), midazolam (0.5-1 mg/kg), and baclofen (5-10 mg/kg) did not affect spontaneous locomotor activity in a control experiment. The present results indicate that (i) ethanol may block nicotine-induced seizures in mice at doses that do not alter locomotor activity and (ii) the anti-seizure effects of ethanol depend on blood ethanol levels and are comparable to those exerted by the GABAA positive modulator midazolam.

Effects of ethanol on nicotine-induced conditioned place preference in C57BL/6J mice

It has been shown that small doses of ethanol (<or= 1.0 g/kg) may antagonize the discriminative stimulus properties of nicotine. The aim of the present study was to evaluate whether ethanol could antagonize nicotine's rewarding effects in the conditioned place preference procedure. For comparison, effects of ethanol on nicotine-induced seizures were assessed. Male C57BL/6J mice were used in all experiments. Lower doses of nicotine (0.3 and 0.6 mg/kg, s.c.) induced significant conditioned place preference, while higher doses (0.9 and 1.2 mg/kg) induced neither conditioned place preference nor conditioned place aversion. In the following experiments, ethanol (0.5 or 1.0 g/kg, i.p.) was administered 5 min before 0.3 mg/kg nicotine. Ethanol did not antagonize nicotine-induced conditioned place preference. Contrary to our hypothesis, a non-significant (p = 0.07) enhancement of nicotine-induced place preference conditioning was observed in mice pre-treated with 1.0 g/kg ethanol. Both doses of ethanol (0.5 and 1.0 g/kg) suppressed seizures elicited by a high dose of nicotine (6.0 mg/kg). Ethanol totally eliminated clonic-tonic component of nicotine-induced seizures. Maximal blood ethanol levels after i.p. administration of 0.5 or 1.0 g/kg ethanol exceeded 60 and 115 mg%, respectively. The present results may indicate that the rewarding and seizure-inducing effects of nicotine are differentially modulated by clinically relevant concentrations of ethanol in mice.

The interactive effect of alcohol and nicotine on NGF-treated pheochromocytoma cells

Previous studies have reported that alcohol exposure reduces the number of neuronal-like pheochromocytoma (PC12) cells in culture. In this study, the interactive effect of coexposure of alcohol and nicotine on PC12 cell numbers was examined in comparison with the effect derived from alcohol or nicotine exposure individually. Moreover, the role of apoptosis in mediating changes in PC12 cell numbers was also investigated. It was hypothesized that alcohol would result in cell loss, and the presence of nicotine would attenuate the damaging effects of alcohol. PC12 cells were exposed to alcohol (100 mM), nicotine (10 microM), or both alcohol and nicotine for 24, 48, 72, or 96 h. Caspase-3 activity and DNA fragmentation, markers for apoptotic cell death, were measured to determine the role of apoptosis in mediating decreases in PC12 cell numbers. The findings indicated that both alcohol and nicotine exposure significantly decreased PC12 cell numbers when compared with the control treatment. Furthermore, the coexposure of these two drugs caused a significantly greater decrease in cell numbers when compared with cells exposed to either alcohol or nicotine alone. This additive effect was related to the duration of exposure with a marked reduction in cell numbers following 96 h of coexposure to alcohol and nicotine. Neither alcohol nor nicotine exposure appeared to alter caspase-3 activity or DNA fragmentation levels, suggesting that the reduction in PC12 cell numbers following alcohol and/or nicotine exposure may possibly be due to factors other than apoptosis, such as interference with proliferation rates.

Nicotine-induced conditioned taste aversion in the rat: Effects of ethanol

It has been shown that small doses of ethanol antagonise the discriminative stimulus properties of nicotine in the rat. The aim of the present study was to evaluate whether ethanol could antagonise the aversive stimulus effects of nicotine. Wistar rats were trained to associate nicotine injections with a novel tasting fluid (0.1% saccharin) in the conditioned taste aversion procedure. Nicotine (0.3 mg/kg, s.c.) was injected 5 min after the end of a 20-min exposure to the saccharin solution. Ethanol (0.25-0.5 g/kg, i.p.) was administered 5 or 50 min before nicotine. In general, ethanol did not inhibit nicotine-induced conditioned taste aversion. Contrary to the findings in drug discrimination studies, a slight but significant enhancement of nicotine-induced taste aversion conditioning was observed after ethanol pre-treatment. Blood ethanol levels were measured in a separate group of rats. Maximal blood ethanol levels after i.p. administration of 0.25 or 0.5 g/kg ethanol exceeded 20 and 80 mg%, respectively. Concluding, the present results may indicate that ethanol does not attenuate nicotine-induced conditioned taste aversion in the rat.

Extended blockade of the discriminative stimulus effects of nicotine with low doses of ethanol

The aim of the present study was to further evaluate effects of ethanol on nicotine discrimination and to correlate these effects with blood ethanol levels. Rats were trained to discriminate 0.3 mg/kg nicotine from its vehicle in the standard two-lever operant procedure. In antagonism tests, small doses of ethanol (0.25-0.5 g/kg) were injected either 5 or 50 min before nicotine. Both doses of ethanol partially antagonized the nicotine cue regardless of the pre-treatment time. Ethanol attenuated also inhibitory effects of nicotine on the rate of responding. Suppression of the cueing effects of nicotine was noted even 60 min after the injection of 0.25 g/kg ethanol, i.e. at the time point when the blood ethanol level was close to zero. Ethanol-induced antagonism of the nicotine cue disappeared when longer time (110 min) was allowed to elapse between the ethanol (0.5 g/kg) and nicotine injection. Concluding, the present results may indicate that the effects of ethanol on nicotine discrimination are not primarily related to blood ethanol levels.

Voluntary ethanol intake increases extracellular acetylcholine levels in the ventral tegmental area in the rat

AIMS

Concurrent use of ethanol and nicotine (tobacco) is often seen in human beings. In previous animal experiments, we have demonstrated that nicotinic acetylcholine receptors, especially alpha-conotoxin MII and mecamylamine sensitive receptors located in the ventral tegmental area may be involved in the stimulatory, dopamine enhancing, and rewarding effects of ethanol in rodents. Ethanol may exert these effects via direct interaction with nicotinic acetylcholine receptors and/or indirectly via enhancement of extracellular acetylcholine levels in the ventral tegmental area. The present experiments investigated a possible indirect effect of ethanol in stimulating the mesoaccumbal dopamine system.

METHODS

Neurochemical effects of voluntary ethanol intake on extracellular ventral tegmental acetylcholine and accumbal dopamine levels were measured by means of in vivo microdialysis with a two-probe approach in freely moving rats.

RESULTS

Obtained data indicate that voluntary ethanol intake ( approximately 0.7 g/kg/h) leads to an increase of extracellular acetylcholine levels in the ventral tegmental area, and an almost time-locked increase of dopamine levels in the nucleus accumbens. A positive correlation between the ventral tegmental acetylcholine levels and ethanol intake as well as preference was also observed.

CONCLUSION

The present results suggest that voluntary ethanol intake enhances extracellular ventral tegmental acetylcholine that may interact with nicotinic acetylcholine receptors, possibly alpha-conotoxin MII sensitive receptors, localized in the ventral tegmental area that subsequently may stimulate dopamine overflow in the nucleus accumbens.

Intrahippocampal nicotine in alcohol drinking rats--effects on lever-press response

We have previously shown differences on learning processes between alcohol drinking and non-alcohol drinking rats. Underlying these effects, functional differences in the septo-hippocampal pathway were hypothesized. We have performed a dose-response study for intrahippocampal nicotine (CA1) on acquisition and extinction of the lever-press response and antagonization test by co-administration of mecamylamine. Results show that the administration of nicotine in CAI region has a detrimental dose-dependent effect on acquisition in alcohol drinkers, with a dose of 10 nM being the most disruptive. In the controls, only doses of 10 and 20 nM had detrimental effect. The effect of nicotine (10 nM) was partially (alcoholics) or fully (controls) antagonized by mecamylamine co-administration (30 nM). Summarizing, the alcohol groups showed a dose-response curve for nicotine shifted leftwards, and a partial antagonism of these effects by mecamylamine; these effects may be consequence of the functional sensitization of the nicotinic responsivity in the CAI region which were produced by the chronic alcohol.

Chronic Ethanol Reduces Nicotine-Induced Dopamine Release in PC12 Cells

BACKGROUND

There is a high correlation between alcohol and nicotine use; that is, alcohol use is associated with high levels of smoking. One important aspect of nicotine addiction appears to be the activation of nicotinic acetylcholine receptors on dopaminergic neurons projecting from the ventral tegmental area to the nucleus accumbens. The release of dopamine from these neurons is thought to mediate, at least in part, the reward of nicotine consumption. If chronic alcohol consumption affects the amount of dopamine released in response to nicotine, it could contribute to the high level of smoking seen in alcoholics.

METHODS

We have used an in vitro model system to study the effects of chronic ethanol exposure on acute nicotine-induced dopamine release and the withdrawal from ethanol. A pheochromocytoma cell line (PC12 cells) was exposed to ethanol for periods of 3 to 96 hr, followed by a 5 min exposure to nicotine. Dopamine released in response to nicotinic stimulation was measured by high-pressure liquid chromatography.

RESULTS

Exposure of PC12 cells to chronic ethanol resulted in a time- and dose-dependent inhibition of nicotine-induced dopamine release. A moderate dose of ethanol (50 mM) resulted in a significant reduction in as little as 3 hr. The cells demonstrated a form of cross-tolerance in that they showed diminished response to nicotine even though they had never been exposed to nicotine. After ethanol was withdrawn from the cells after a chronic exposure (96 hr), dopamine release slowly returned to normal levels but demonstrated a significant period of "overshoot" or hyperresponsiveness between 24 and 48 hr after withdrawal.

CONCLUSIONS

These results show that chronic ethanol exposure decreases nicotine-induced dopamine release and demonstrate a period of hyperresponsiveness during withdrawal from ethanol. These studies suggest potential interactions between chronic ethanol and nicotine that may provide insight into such phenomena as cross-tolerance and increased use of nicotine by alcoholics.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

The role of neuronal nicotinic acetylcholine receptor subunits in autonomic ganglia: lessons from knockout mice

Neuronal nicotinic acetylcholine receptors (nAChR), composed of 12 subunits (alpha2-alpha10, beta2-beta4), are expressed in autonomic ganglia, playing a central role in autonomic transmission. The repertoire of nicotinic subunits in autonomic ganglia includes alpha3, alpha5, alpha7, beta2 and beta4 subunits. In the last 10 years, heterologous expression studies have revealed much about the nature of neuronal nAChRs. However, there is only limited understanding of subunit actions in autonomic system. Functional deletions of subunit by gene knockout in animals could overcome these limitations. We review recent studies on nAChRs on autonomic ganglia for physiological and pharmacological properties and potential locations of the subunits.

Ethanol inhibition of glycine-activated responses in neurons of ventral tegmental area of neonatal rats

The brain is particularly sensitive to alcohol during the period of its rapid growth. To better understand the mechanism(s) involved, we studied ethanol effects on glycine-activated responses of ventral tegmental area (VTA) neurons isolated from the newborn rat, using whole cell and gramicidin perforated patch-clamp techniques. Previously we reported that 0.1-40 mM ethanol enhances glycine-induced responses of 35% of VTA neurons. We now direct our attention to the inhibitory effects of ethanol observed in 45% (312 of 694) of neonatal VTA neurons. Under current-clamp conditions, 1 mM ethanol had no effect on the membrane potential of these cells, but it decreased glycine-induced membrane depolarization and the frequency of spontaneous action potentials. Under voltage-clamp conditions, 0.1-10 mM ethanol did not elicit a current but depressed the glycine-induced currents. The ethanol-induced inhibition of glycine current was independent of membrane potential (between -60 and +60 mV). Likewise, ethanol did not alter the reversal potential of the glycine-activated currents. Ethanol-mediated inhibition of glycine current depended on the glycine concentration. While ethanol strongly depressed currents activated by 30 microM glycine, it had no appreciable effect on maximal currents activated by 1 mM glycine. In the presence of ethanol (1 mM), the EC(50) for glycine increased from 32 +/- 5 to 60 +/- 3 microM. Thus ethanol may decrease the agonist affinity of glycine receptors. A kinetic analysis indicated that ethanol shortens the time constant of glycine current deactivation but has no effect on activation. In conclusion, by altering VTA neuronal function, ethanol-induced changes in glycine receptors may contribute to neurobehavioral manifestations of the fetal alcohol syndrome.

Neuronal nicotinic receptors in the human brain

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.

Saturable ethanol binding in rat liver mitochondria

The binding of ethanol to rat liver mitochondria is shown to be saturable at physiologically relevant ethanol concentrations. This effect is reversible and is not observed in extracted mitochondrial phospholipids. Brief exposure of the mitochondria to heat abolishes saturable ethanol binding. Previously, saturable ethanol binding was reported in rat liver microsomes. Taken together, the studies indicate that saturable ethanol binding motifs may be widespread in cellular membranes. The possibility is raised that incomplete expression of the hydrophobic effect in membrane assembly results in the expression of amphipathic packing defects which display an affinity for and a sensitivity to ethanol. The presence of saturable binding modalities is reconciled with the long-standing consensus on the biodistribution of ethanol - that ethanol's interactions with tissue are negligible - on the grounds that the affinities of ethanol and of water for membranes are similar; consequently, free ethanol concentrations are insensitive to the presence of tissue despite significant ethanol binding. A fraction of the binding sites possess submillimolar affinities for ethanol consistent with published functional studies, both in vitro and in vivo, that reported submillimolar efficacies for ethanol.

Neuronal nicotinic acetylcholine receptors: a new target site of ethanol

Whereas a variety of neuroreceptors and ion channels have been demonstrated to be affected by ethanol including GABAA receptors, NMDA receptors, non-NMDA glutamate receptors, 5-HT3 receptors and voltage-gated calcium channels, neuronal nicotinic acetylcholine receptors (nnAChRs) have recently emerged as a new target site of ethanol. The nnAChRs are different from the muscle type nicotinic AChRs with respect to their molecular architecture and pharmacology. This article briefly reviews the structure, distribution and function of nnAChRs for which a considerable amount of information has been rapidly accumulated during the past 5-10 years. The potent and unique action of ethanol on nnAChRs has been unveiled only during the past few years. Most recent developments along this line of ethanol action are discussed in this paper.

Action of ethanol on responses to nicotine from cerebellar Purkinje neurons: relationship to methyllycaconitine (MLA) inhibition of nicotine responses

The effect of ethanol on responses to nicotine from rat cerebellar Purkinje neurons was investigated using extracellular single-unit recording. Systemic administration of ethanol initially enhanced the nicotine-induced inhibition from 50% of the Purkinje neurons. However, irrespective of whether there was an initial enhancement, systemic administration of ethanol antagonized the response to nicotine from the majority of Purkinje neurons. When varying ethanol concentrations were electro-osmotically applied to this neuronal cell type, the responses to nicotine (6/8) were enhanced when a low concentration of ethanol (40 mM) was in the pipette, whereas the majority of nicotine responses (10/11) were antagonized when a higher concentration of ethanol (160 mM) was applied to Purkinje neurons. Thus, the concentration of ethanol presented to the neuron seemed to explain the biphasic consequence of systemically administered ethanol on responses to nicotine. In order to determine whether ethanol affected a specific nACh receptor subtype containing the alpha-7 subunit, it was initially established that the nicotinic antagonists, alpha-bungarotoxin (alpha-BTX) and methyllycaconitine (MLA), which are associated with this subunit, had identical actions on responses to nicotine from Purkinje neurons. When MLA was tested against responses to nicotine from this cell type, MLA antagonized the response to nicotine from 45% (9/20) of the neurons tested. In a direct comparison of the action of ethanol to inhibit responses to nicotine with the action of MLA on the same Purkinje neuron, ethanol inhibited responses to nicotine on all neurons sensitive to MLA. However, ethanol also affected nicotine-induced neural changes from some Purkinje neurons not sensitive to MLA antagonism of nicotine. These data support the supposition that ethanol affects a nACh receptor subtype which has an alpha-7 subunit as well as other nACh receptor subtypes without this specific subunit.

Modulation of the neuronal nicotinic acetylcholine receptor-channel by the nootropic drug nefiracetam

The effects of nefiracetam (DM-9384) on the neuronal nicotinic acetylcholine (ACh) receptor-channel were studied by the whole-cell patch clamp technique using PC12 cells. Nefiracetam had a dual effect on ACh-induced currents: it augmented the currents induced by low concentrations (10-30 microM) of ACh and suppressed those induced by high concentrations (100-1000 microM) of ACh. These effects were reversible after washing with drug-free solution. The stimulating effect of nefiracetam was clearly observed at a concentration of 10 microM, and slight increases in currents were detected even at 0.1 microM or 1 microM. Nefiracetam at 100 microM suppressed the currents induced by a low concentration (10 microM) of ACh. The rate of desensitization of ACh-induced current was greatly accelerated by nefiracetam, and this effect could not be reversed by washing with drug-free solution. When added to the internal pipette solution, the protein kinase A inhibitor KT 5720 (0. 6 microM), but not the protein kinase C inhibitor calphostin C (0.5 microM), abolished the nefiracetam stimulation of the ACh receptor. Pre-incubation of cells with 200 ng/ml pertussis toxin for 24 h also abolished the nefiracetam action. Thus, the nefiracetam modulation of the neuronal nicotinic ACh receptor-channel is exerted via G proteins and protein kinase A. The stimulation of the ACh receptor may be directly related to the cognitive enhancing action of nefiracetam.

Ion channel modulation as the basis for general anesthesia

(1) Modulation of the function of the GABA(A) and neuronal nicotinic acetylcholine receptor channels caused by general anesthetics and modulation of the GABA(A) receptor-channel by halothane, enflurane, isoflurane, and n-octanol was channel state-dependent. (3) Halothane modulation of the GABA(A) receptor was independent of subunits, but n-octanol modulation was subunit-dependent. (4) Ethanol at 30-100 microM was very potent in accelerating the desensitization of currents induced by acetylcholine. (5) The ethanol modulation was subunit- and state-dependent, occurring in the alpha3beta4 combination but only weakly in the alpha3beta2 combination. (6) In contrast, halothane at 430 microM (approximately 1 MAC) potently suppressed ACh-induced currents in the alpha3beta2 subunit combination.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Discrimination of ethanol in rats: effects of nicotine, diazepam, CGP 40116, and 1-(m-chlorophenyl)-biguanide

The drug discrimination paradigm was used to evaluate the role of certain ligand-gated ion channels in the discriminative stimulus properties of ethanol. Rats were trained to discriminate ethanol (1.0 g/kg) from saline vehicle under the FR10 schedule of sweetened milk reinforcement. The discrimination of lower ethanol doses was enhanced by either the GABA(A) receptor positive modulator, diazepam (0.5 mg/kg), or nicotinic acetylcholine receptor agonist, nicotine (0.3 mg/kg). Neither diazepam nor nicotine produced any effect on the rate of responding. Both the NMDA receptor competitive antagonist, CGP 40116 (0.5 mg/kg) and the 5-HT) receptor agonist, 1-(m-chlorophenyl)-biguanide (5.0 mg/kg) enhanced the cueing properties of lower ethanol doses, but these effects were associated with a significant reduction in the response rate. The ethanol-like stimulus effects produced by diazepam or CGP 40116 were not influenced by 0.3 mg/kg nicotine. In contrast, CGP 40116 moderately enhanced the ethanol-like stimulus effects of diazepam. The present results show that: 1) pretreatment with nicotine, diazepam, CGP 40116 or 1-(m-chlorophenyl)-biguanide enhance the ethanol discrimination; 2) neither the GABA(A) nor the NMDA receptor complex alone is critically involved in the nicotine-induced enhancement of the ethanol discrimination; 3) NMDA receptor competitive antagonist and GABAergic benzodiazepine derivative may produce moderate additive effects in rats trained to discriminate ethanol.

Differential modulation of rat neuronal nicotinic receptor subtypes by acute application of ethanol

1. We have studied the effects of acute ethanol (EtOH) exposure on the agonist responses of rat neuronal nicotinic receptors expressed in Xenopus oocytes by means of voltage clamp techniques. 2. In some cells, agonist-induced current responses with the alpha 3 beta 4 subunit combination could be either significantly potentiated or inhibited (range 25% to 237% of control response) by low ethanol concentrations (1-30 mM). At high ethanol concentrations (100-300 mM) robust potentiations were observed (range 135% to 305% of control). 3. The low EtOH concentration effects on the alpha 3 beta 4 subtype exhibited tolerance with repeated EtOH exposure. 4. In general, the alpha 3 beta 2, alpha 4-1 beta 2 and alpha 4-1 beta 4 subunit combinations were less sensitive to low concentrations of ethanol, but respectively showed potentiations of up to 178%, 226% and 154% at high EtOH concentrations. 5. The alpha 7 homomeric receptor was also relatively insensitive at low EtOH concentrations. At high EtOH concentrations, potentiations, inhibitions or no alteration of control agonist response were observed (range 88% to 141% of control). 6. We conclude that all the neuronal nicotinic receptor subunit combinations tested here can be modulated by high concentrations of EtOH in a rapidly reversible manner. This modulation may underlie some of the behavioural effects of ethanol. The alpha 3 beta 4 subunit combination may be especially sensitive to modulation by low EtOH concentrations. This remarkable sensitivity and plasticity of nicotinic receptors may contribute to a process of mutual reinforcement in nicotine and alcohol addiction.