Is an alpha-conotoxin MII-sensitive mechanism involved in the neurochemical, stimulatory, and rewarding effects of ethanol?

Activation of alpha4* nAChRs is necessary and sufficient for varenicline-induced reduction of alcohol consumption

Recently, the smoking cessation therapeutic varenicline, a nicotinic acetylcholine receptor (nAChR) partial agonist, has been shown to reduce alcohol consumption. However, the mechanism and nAChR subtype(s) involved are unknown. Here we demonstrate that varenicline and alcohol exposure, either alone or in combination, selectively activates dopaminergic (DAergic) neurons within the posterior, but not the anterior, ventral tegmental area (VTA). To gain insight into which nAChR subtypes may be involved in the response to alcohol, we analyzed nAChR subunit gene expression in posterior VTA DAergic neurons. Ethanol-activated DAergic neurons expressed higher levels of alpha4, alpha6, and beta3 subunit genes compared with nonactivated neurons. To examine the role of nicotinic receptors containing the alpha4 subunit (alpha4* nAChRs) in varenicline-induced reduction of alcohol consumption, we examined the effect of the drug in two complementary mouse models, a knock-out line that does not express the alpha4 subunit (alpha4 KO) and another line that expresses alpha4* nAChRs hypersensitive to agonist (Leu9'Ala). While varenicline (0.1-0.3 mg/kg, i.p.) reduced 2% and 20% alcohol consumption in wild-type (WT) mice, the drug did not significantly reduce consumption in alpha4 KO animals. Conversely, low doses of varenicline (0.0125-0.05 mg/kg, i.p.) that had little effect in WT mice dramatically reduced ethanol intake in Leu9'Ala mice. Infusion of varenicline into the posterior, but not the anterior VTA was sufficient to reduce alcohol consumption. Together, our data indicate that activation of alpha4* nAChRs is necessary and sufficient for varenicline reduction of alcohol consumption.

Neuronal nicotinic acetylcholine receptors as pharmacotherapeutic targets for the treatment of alcohol use disorders

Alcohol use disorders (AUDs) are complex, and developing effective treatments will require the combination of novel medications and cognitive behavioral therapy approaches. Epidemiological studies have shown there is a high correlation between alcohol consumption and tobacco use, and the prevalence of smoking in alcoholics is as high as 80% compared to about 30% for the general population. Both preclinical and clinical data provide evidence that nicotine administration increases alcohol intake and non-specific nicotinic receptor antagonists reduce alcohol-mediated behaviors. As nicotine interacts specifically with the neuronal nicotinic acetylcholine receptor (nAChR) system, this suggests that nAChRs play an important role in the behavioral effects of alcohol. In this review, we discuss the importance of nAChRs for the treatment of AUDs and argue that the use of FDA approved nAChR ligands, such as varenicline and mecamylamine, approved as smoking cessation aids may prove to be valuable treatments for AUDs. We also address the importance of combining effective medications with behavioral therapy for the treatment of alcohol dependent individuals.

Inhibitory influence of mecamylamine on the development and the expression of ethanol-induced locomotor sensitization in mice

Several evidences have indicated the involvement of neuronal nicotinic acetylcholine receptors (nAChR) in behavioral effects of drugs of abuse, including ethanol. nAChRs are implicated in ethanol-induced behaviors as well as neurochemical responses to ethanol. Recently, it is demonstrated that mecamylamine, a nAChR antagonist blocks cocaine-, d-amphetamine-, ephedrine-, nicotine-, and methylphenidate-induced psychomotor sensitization. However, no reports are available on its role in ethanol-induced psychomotor sensitization. Therefore, an attempt was made to evaluate its effect on ethanol-induced locomotor sensitization using a model previously described by us. The results revealed that acute administration of mecamylamine (1 and 2mg/kg, i.p.) blocked the acute stimulant effect of ethanol (2.0g/kg, i.p.). In addition, treatment with mecamylamine (0.5-2.0mg/kg, i.p.), 30min prior to the challenge dose of ethanol (2.0g/kg, i.p.) dose dependently attenuated expression of sensitization to locomotor stimulant effect of ethanol. Moreover, administration of mecamylamine (1 and 2mg/kg, i.p.) during development (prior to each ethanol injection on days 1, 4, 7, and 10) blocked acquisition as well as expression (day 15) of sensitization to locomotor stimulant effect of ethanol. Mecamylamine per se did not affect locomotor activity. Further, it also did not influence blood ethanol levels and rotarod performance in mice. These results support the hypothesis that neuroadaptive changes in nAChRs may participate in the development and the expression of ethanol-induced locomotor sensitization.

Modulation of ethanol consumption by genetic and pharmacological manipulation of nicotinic acetylcholine receptors in mice

RATIONALE

Alcohol and nicotine are commonly co-abused. Genetic correlations between responses to these drugs have been reported, providing evidence that common genes underlie the response to alcohol and nicotine. Nicotinic acetylcholine receptors (nAChRs) in the mesolimbic dopamine system are important in mediating nicotine response, and several studies suggest that alcohol may also interact with these nAChRs.

OBJECTIVE

The aim of this study was to examine the role of nAChRs containing α7 or β2 subunits in ethanol consumption.

METHODS

A two-bottle choice paradigm was used to determine ethanol consumption in wild-type and nAChR subunit knockout mice. Challenge studies were performed using the α4β2 nAChR partial agonist varenicline.

RESULTS

Mice lacking the β2 subunit consumed a similar amount of ethanol compared to their wild-type siblings in an ethanol-drinking paradigm. In contrast, mice lacking the α7 nAChR receptor subunit consumed significantly less ethanol than wild-type mice but consumed comparable amounts of water, saccharin, and quinine. In C57BL/6J mice, varenicline dose-dependently decreased ethanol consumption with a significant effect of 2 mg/kg, without affecting water or saccharin consumption. This effect of varenicline was not reversed in mice lacking either the α7 or β2 subunit, providing evidence that nAChRs containing one of these subunits are not required for this effect of varenicline.

CONCLUSIONS

This study provides evidence that α7 nAChRs are involved in ethanol consumption and supports the idea that pharmacological manipulation of nAChRs reduces ethanol intake. Additional nAChRs may also be involved in ethanol intake, and there may be functional redundancy in the nicotinic control of alcohol drinking.

Nicotinic receptor ligands reduce ethanol intake by high alcohol-drinking HAD-2 rats

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of many drugs of abuse, including ethanol. The present study examined the efficacy of cytisine, a nAChR partial agonist, and lobeline, a putative nAChR antagonist, on the maintenance of ethanol drinking by HAD-2 rats. Adult male HAD-2 rats were given access to ethanol (15 and 30%, with ad libitum access to water and food) 22 h/day for 12 weeks, beginning at 60 days of age, after which cytisine (0.0, 0.5, and 1.5 mg/kg) was tested for 3 consecutive days. The rats were given an 18-day washout period and were then tested with lobeline (0.0, 1.0, and 5.0 mg/kg) for 3 consecutive days. Ethanol intake was measured at 1, 4, and 22 h postinjection. Rats were injected intraperitoneally just before lights out (1200 h). There was a significant main effect of cytisine treatment on the second test day, with the 1.5 mg/kg dose significantly reducing ethanol intake at the 1- and 4-h time-points, relative to saline, and the 0.5 mg/kg dose inducing a significant reduction at the 4-h time-point. Conversely, lobeline treatment resulted in significant main effects of treatment for all three time-points within each test day, with the 5.0 mg/kg dose significantly reducing ethanol intake, relative to saline, at each time-point within each test day. These findings provide further evidence that activity at the nAChR influences ethanol intake and is a promising target for pharmacotherapy development for the treatment of alcohol dependence and relapse.

The genetic components of alcohol and nicotine co-addiction: from genes to behavior

Co-occurrence of alcohol and nicotine addiction in humans is well documented and there is good evidence that common genes may contribute to both disorders. Although genetic factors contributing to tobacco and alcohol problem use have been well established through adoption, twin and family studies, specific genes remain to be identified and their mode of action elucidated. Recent work from human genetics studies has provided evidence that neuronal nicotinic acetylcholine receptors (nAChR) genes may have a role in mediating early behaviors that are risk factors for alcohol and nicotine dependence, such as age of initiation and early subjective responses to the drugs. Converging evidence suggests that the dopaminergic system is likely to be important in mediating the pleasurable feelings of reward when activated by nicotine and/or alcohol consumption. The nAChRs are important components of the dopaminergic reward system because some of the receptors have been shown to activate the release of dopamine, and mice lacking genes for specific nAChR gene subunits show altered behavioral responses to nicotine and alcohol. Furthermore, complex interactions between other neurotransmitter circuits including GABA, glutamate and serotonin may be modulated by nAChRs, leading researchers to study genes involved in neurobiology shared by different drugs. Future studies aimed at understanding the variation among these genes, and their corresponding functional implications, will help elucidate how natural variants in nicotinic receptor genes contribute to these common co-morbid disorders.

Modulation of ethanol drinking-in-the-dark by mecamylamine and nicotinic acetylcholine receptor agonists in C57BL/6J mice

RATIONALE

Recent reports describe a restricted access ethanol consumption paradigm where C57Bl/6J mice drink until intoxicated. Termed "drinking in the dark" (DID), this paradigm has been used as a model of binge drinking. Although neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in alcohol drinking in rats pre-trained to self-administer ethanol, their role in binge-like ethanol consumption is unknown.

OBJECTIVES

To determine if nAChRs are involved in binge drinking as measured by the DID assay in C57Bl/6J mice.

MATERIALS AND METHODS

Adult male C57Bl/6J mice were injected i.p. with nicotinic receptor antagonists including mecamylamine, hexamethonium, dihydro-beta-erythroidine, and methyllycaconitine. Immediately following injection, mice were presented with 20% ethanol for 2 h in the DID assay to measure ethanol consumption. Nicotinic agonists including cytisine and nicotine were also evaluated. The effects of mecamylamine and nicotine on ethanol-induced dopaminergic neuronal activation in the VTA were evaluated via immunohistochemistry.

RESULTS

Mecamylamine dose dependently reduced ethanol consumption; whereas, the peripheral antagonist hexamethonium had no significant effect. Nicotinic agonists, cytisine and nicotine, reduced ethanol consumption. None of the effective nicotinic receptor drugs reduced sucrose drinking. Mecamylamine blocked ethanol activation of dopaminergic neurons while nicotine alone activated them without additional activation by ethanol.

CONCLUSIONS

Neuronal nAChRs are involved in ethanol consumption in the DID paradigm. The effects of mecamylamine, nicotine, and cytisine on ethanol intake appear to be specific because they do not reduce sucrose drinking. Mecamylamine reduces alcohol consumption by blocking activation of dopaminergic neurons; whereas, nicotinic agonists may activate the same reward pathway as alcohol.

The influence of mecamylamine on ethanol and sucrose self-administration

Neuronal nicotinic acetylcholine receptors (nAChRs) are believed to be critically involved in ethanol-related behaviors as well as in neurochemical responses to ethanol. However, discernment of nAChR contribution to ethanol reinforcement and consumption remains incomplete. The current studies examined the influence of the nAChR antagonist mecamylamine (MEC) on operant ethanol self-administration using a procedure that independently assessed appetitive and consumptive processes, and compared these findings to effects of MEC on sucrose self-administration. Male C57BL/6J (B6) mice were trained to respond for 30-min access to a retractable drinking tube containing either 10% v/v ethanol (10E) or 5% w/v sucrose (5S). Once trained, mice were habituated to saline injection and then treated with a series of MEC doses (0-8 mg/kg; i.p.) in a within-subject design. In a separate cohort, MEC was evaluated for its influence on locomotor activity. MEC dose-dependently reduced 10E and 5S self-administration. The suppression in ethanol intake was attributable to a reduction in bout frequency, whereas the attenuation in sucrose intake was due to a decrease in bout size. Doses of MEC (6-8 mg/kg) that altered drinking patterns were also found to impair locomotor activity. Although MEC non-selectively reduced 10E and 5S intakes in mice, there was some specificity in alterations of the underlying drinking pattern for each reinforcer. Assessment of drinking topography within an operant self-administration procedure may provide useful insights regarding the role of nAChR function in the regulation of ethanol consumption.

Alpha-conotoxins as pharmacological probes of nicotinic acetylcholine receptors

Cysteine-rich peptides from the venom of cone snails (Conus) target a wide variety of different ion channels. One family of conopeptides, the alpha-conotoxins, specifically target different isoforms of nicotinic acetylcholine receptors (nAChRs) found both in the neuromuscular junction and central nervous system. This family is further divided into subfamilies based on the number of amino acids between cysteine residues. The exquisite subtype selectivity of certain alpha-conotoxins has been key to the characterization of native nAChR isoforms involved in modulation of neurotransmitter release, the pathophysiology of Parkinson's disease and nociception. Structure/function characterization of alpha-conotoxins has led to the development of analogs with improved potency and/or subtype selectivity. Cyclization of the backbone structure and addition of lipophilic moieties has led to improved stability and bioavailability of alpha-conotoxins, thus paving the way for orally available therapeutics. The recent advances in phylogeny, exogenomics and molecular modeling promises the discovery of an even greater number of alpha-conotoxins and analogs with improved selectivity for specific subtypes of nAChRs.

Mysterious alpha6-containing nAChRs: function, pharmacology, and pathophysiology

Neuronal nicotinic acetylcholine receptors (nAChRs) are the superfamily of ligand-gated ion channels and widely expressed throughout the central and peripheral nervous systems. nAChRs play crucial roles in modulating a wide range of higher cognitive functions by mediating presynaptic, postsynaptic, and extrasynaptic signaling. Thus far, nine alpha (alpha2-alpha10) and three beta (beta2, beta3, and beta4) subunits have been identified in the CNS, and these subunits assemble to form a diversity of functional nAChRs. Although alpha4beta2- and alpha7-nAChRs are the two major functional nAChR types in the CNS, alpha6*-nAChRs are abundantly expressed in the midbrain dopaminergic (DAergic) system, including mesocorticolimbic and nigrostriatal pathways, and particularly present in presynaptic nerve terminals. Recently, functional and pharmacological profiles of alpha6*-nAChRs have been assessed with the use of alpha6 subunit blockers such as alpha-conotoxin MII and PIA, and also by using alpha6 subunit knockout mice. By modulating DA release in the nucleus accumbens (NAc) and modulating GABA release onto DAergic neurons in the ventral tegmental area (VTA), alpha6*-nAChRs may play important roles in the mediation of nicotine reward and addiction. Furthermore, alpha6*-nAChRs in the nigrostriatal DAergic system may be promising targets for selective preventative treatment of Parkinson's disease (PD). Thus, alpha6*-nAChRs may hold promise for future clinical treatment of human disorders, such as nicotine addiction and PD. In this review, we mainly focus on the recent advances in the understanding of alpha6*-nAChR function, pharmacology and pathophysiology.

Alcoholism: A Systems Approach From Molecular Physiology to Addictive Behavior

Alcohol consumption is an integral part of daily life in many societies. The benefits associated with the production, sale, and use of alcoholic beverages come at an enormous cost to these societies. The World Health Organization ranks alcohol as one of the primary causes of the global burden of disease in industrialized countries. Alcohol-related diseases, especially alcoholism, are the result of cumulative responses to alcohol exposure, the genetic make-up of an individual, and the environmental perturbations over time. This complex gene × environment interaction, which has to be seen in a life-span perspective, leads to a large heterogeneity among alcohol-dependent patients, in terms of both the symptom dimensions and the severity of this disorder. Therefore, a reductionistic approach is not very practical if a better understanding of the pathological processes leading to an addictive behavior is to be achieved. Instead, a systems-oriented perspective in which the interactions and dynamics of all endogenous and environmental factors involved are centrally integrated, will lead to further progress in alcohol research. This review adheres to a systems biology perspective such that the interaction of alcohol with primary and secondary targets within the brain is described in relation to the behavioral consequences. As a result of the interaction of alcohol with these targets, alterations in gene expression and synaptic plasticity take place that lead to long-lasting alteration in neuronal network activity. As a subsequent consequence, alcohol-seeking responses ensue that can finally lead via complex environmental interactions to an addictive behavior.

Effects of subunit selective nACh receptors on operant ethanol self-administration and relapse-like ethanol-drinking behavior

RATIONALE AND OBJECTIVES

The sensitivity to ethanol central effects is partially determined by the subunit composition of brain nicotinic acetylcholine receptors (nAChRs). Thus, the effects of intraventral tegmental area (VTA) administration of the nicotinic subunit-specific antagonist, alpha-conotoxin MII (alphaCtxMII, alpha(3)beta(2)*, beta(3)*, alpha(6)*), were compared to those of systemic mecamylamine (MEC, an allosteric negative modulator of the nAChR), dihydro-beta-erythroidine (DHbetaE, alpha(4)beta(2)*), and methyllycaconitine (MLA, alpha(7)*) to elucidate involvement of different subunits of nAChRs in operant ethanol self-administration and relapse-like activation of ethanol consumption after ethanol deprivation in rats.

METHODS

The effects of drugs were studied in rats trained for operant oral self-administration of ethanol (FR = 1). For ethanol deprivation, trained animals were subjected to a period of alcohol deprivation for 10 days. alphaCtxMII was given directly into the VTA through implanted permanent intracranial cannulae, whereas MEC, DHbetaE, and MLA were administered systemically.

RESULTS

alphaCtxMII reduced operant ethanol self-administration and blocked the deprivation-induced relapse-like ethanol consumption. MEC reduced operant ethanol self-administration and inhibited the deprivation-induced increase in alcohol consumption. DHbetaE did not alter ethanol self-administration in the lower-dose range but inhibited ethanol intake at a higher dose (4 mg/kg), although this effect might have been nonspecific. MLA failed to block self-administration of ethanol and relapse-like drinking after deprivation.

CONCLUSIONS

Our results indicate that nAChRs are involved in the modulation of operant alcohol self-administration and relapse-like alcohol drinking behavior in rats. Our observations support the working hypothesis that systemically active selective ligands for nAChR alpha(3)beta(2)*, beta(3), and/or alpha(6)* receptor subunits might be of therapeutic value for the treatment of alcoholism.

The smoking cessation medication varenicline attenuates alcohol and nicotine interactions in the rat mesolimbic dopamine system

Varenicline was recently approved as an aid for smoking cessation. Patients treated with varenicline have reported a concomitant reduction in their alcohol consumption. This compound has also been demonstrated to reduce alcohol seeking and consumption in alcohol high-preferring rats. Based on the extensive coabuse of nicotine and alcohol, the aim of the present study was to explore whether interactions among varenicline, nicotine, and ethanol in the brain reward system could indicate the use of varenicline also for alcohol dependence. Using the in vivo microdialysis method, we investigated the effects of systemic injections of varenicline on the extracellular accumbal dopamine levels in response to a systemic challenge of ethanol, nicotine, or the combination of nicotine and ethanol in the experimental rat. Acute systemic coadministration of varenicline and ethanol counteracted each others' respective enhancing effect on dopamine levels in the nucleus accumbens. However, after 5 days of varenicline pretreatment, acute combined varenicline and ethanol administration raised dopamine levels to the same extent as either drug alone. Furthermore, after varenicline pretreatment an acute injection of varenicline antagonized the dopamine stimulatory effect of acute nicotine as well as that of systemic coadministration of ethanol and nicotine. In contrast, a pronounced additive dopamine increase was observed when nicotine and ethanol were coadministered in vehicle-pretreated rats. The antismoking agent varenicline exhibits properties with respect to its interaction with ethanol and nicotine in the brain reward system that may be beneficial for treating patients with alcohol dependence with (and possibly also without) concomitant nicotine dependence.

The alpha 3 subunit gene of the nicotinic acetylcholine receptor is a candidate gene for ethanol stimulation

Alcohol and nicotine are coabused, and preclinical and clinical data suggest that common genes may influence responses to both drugs. A gene in a region of mouse chromosome 9 that includes a cluster of three nicotinic acetylcholine receptor (nAChR) subunit genes influences the locomotor stimulant response to ethanol. The current studies first used congenic mice to confirm the influential gene on chromosome 9. Congenic F(2) mice were then used to more finely map the location. Gene expression of the three subunit genes was quantified in strains of mice that differ in response to ethanol. Finally, the locomotor response to ethanol was examined in mice heterozygous for a null mutation of the alpha 3 nAChR subunit gene (Chrna3). Congenic data indicate that a gene on chromosome 9, within a 46 cM region that contains the cluster of nAChR subunit genes, accounts for 41% of the genetic variation in the stimulant response to ethanol. Greater expression of Chrna3 was found in whole brain and dissected brain regions relevant to locomotor behavior in mice that were less sensitive to ethanol-induced stimulation compared to mice that were robustly stimulated; the other two nAChR subunit genes in the gene cluster (alpha 5 and beta 4) were not differentially expressed. Locomotor stimulation was not expressed on the genetic background of Chrna3 heterozygous (+/-) and wild-type (+/+) mice; +/- mice were more sensitive than +/+ mice to the locomotor depressant effects of ethanol. Chrna3 is a candidate gene for the acute locomotor stimulant response to ethanol that deserves further examination.

Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors are involved in mediating the ghrelin-induced locomotor stimulation and dopamine overflow in nucleus accumbens

Previously, we have reported that the orexigenic peptide ghrelin activates the cholinergic-dopaminergic reward link, involving nicotinic acetylcholine receptors (nAChR). The alpha(3)-alpha(7) and beta(2)-beta(4) subunits of the nAChR can be combined into pentameric nAChRs, with different functional roles. The present experiments show that the locomotor stimulatory effects of ghrelin, either into laterodorsal tegmental area (LDTg) or ventral tegmental area (VTA), are mediated via ventral tegmental nAChR, but neither the alpha(4)beta(2) (using dihydro-beta-erythroidine) nor the alpha(7) (using methyllycaconitine) subtypes appears to be involved. On the other hand, the alpha(3)beta(2), beta(3) and/or alpha(6) (using alpha-conotoxin MII) subtypes in the VTA mediate the stimulatory and DA-enhancing effects of ghrelin, a pattern that ghrelin shares with ethanol (n=5-8). Radioligand-binding experiments shown that ghrelin does not interfere directly with nAChRs (n=26). We therefore suggest that the alpha(3)beta(2), beta(3) and/or alpha(6) subtypes might be pharmacological targets for treatment of addictive behaviours including compulsive overeating and alcoholism.

Neuropharmacology of alcohol addiction

Despite the generally held view that alcohol is an unspecific pharmacological agent, recent molecular pharmacology studies demonstrated that alcohol has only a few known primary targets. These are the NMDA, GABA(A), glycine, 5-hydroxytryptamine 3 (serotonin) and nicotinic ACh receptors as well as L-type Ca(2+) channels and G-protein-activated inwardly rectifying K(+) channels. Following this first hit of alcohol on specific targets in the brain, a second wave of indirect effects on a variety of neurotransmitter/neuropeptide systems is initiated that leads subsequently to the typical acute behavioural effects of alcohol, ranging from disinhibition to sedation and even hypnosis, with increasing concentrations of alcohol. Besides these acute pharmacodynamic aspects of alcohol, we discuss the neurochemical substrates that are involved in the initiation and maintenance phase of an alcohol drinking behaviour. Finally, addictive behaviour towards alcohol as measured by alcohol-seeking and relapse behaviour is reviewed in the context of specific neurotransmitter/neuropeptide systems and their signalling pathways. The activity of the mesolimbic dopaminergic system plays a crucial role during the initiation phase of alcohol consumption. Following long-term, chronic alcohol consumption virtually all brain neurotransmission seems to be affected, making it difficult to define which of the systems contributes the most to the transition from controlled to compulsive alcohol use. However, compulsive alcohol drinking is characterized by a decrease in the function of the reward neurocircuitry and a recruitment of antireward/stress mechanisms comes into place, with a hypertrophic corticotropin-releasing factor system and a hyperfunctional glutamatergic system being the most important ones.

A role for neuronal nicotinic acetylcholine receptors in ethanol-induced stimulation, but not cocaine- or methamphetamine-induced stimulation

RATIONALE

Cocaine (COC), ethanol (EtOH), and methamphetamine (MA) are widely abused substances and share the ability to induce behavioral stimulation in mice and humans. Understanding the biological basis of behavioral stimulation to COC, EtOH, and MA may provide a greater understanding of drug and alcohol abuse.

OBJECTIVES

In these studies we set out to determine if neuronal nicotinic acetylcholine receptors were involved in the acute locomotor responses to these drugs, our measure of behavioral stimulation.

METHODS

A panel of acetylcholine receptor antagonists was used to determine if nicotinic receptors were involved in EtOH- and psychostimulant-induced stimulation. We tested the effect of these drugs in genotypes of mice (FAST and DBA/2J) that are extremely sensitive to this drug effect. To determine which acetylcholine receptor subunits may be involved in this response, relative expression of the alpha3, alpha6, beta2, and beta4 subunit genes was examined in mice selectively bred for high and low response to EtOH.

RESULTS

Mecamylamine, but not hexamethonium, attenuated the acute locomotor response to EtOH. The acetylcholine receptor antagonist dihydro-beta-erythroidine and methyllycaconitine had no effect on this response. The alpha6 and beta4, but not alpha3 or beta2, subunits of the acetylcholine receptor were differentially expressed between mice bred for extreme differences in EtOH stimulation. Mecamylamine had no effect on psychostimulant-induced locomotor activity.

CONCLUSIONS

Neuronal nicotinic receptors are involved in EtOH, but not psychostimulant, stimulation. These studies suggest a lack of involvement of some nicotinic receptor subtypes, but more work is needed to determine the specific receptor subtypes involved in this behavior.

Nicotinic acetylcholine receptors in the ventral tegmental area mediate the dopamine activating and reinforcing properties of ethanol cues

RATIONALE

Cues associated with alcohol can elicit craving, support drug-seeking and precipitate relapse.

OBJECTIVES

We investigated the possible involvement of nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) in the conditioned reinforcing properties of ethanol-associated stimuli in the rat.

MATERIALS AND METHODS

First, using in vivo microdialysis, we analyzed the effect of VTA perfusion of the nonselective nAChR antagonist mecamylamine (MEC) or the selective alpha4beta2* nAChR antagonist dihydro-beta-erythroidine (DHbetaE) on the nucleus accumbens (nAc) dopaminergic response to the presentation of an ethanol-associated conditioned stimulus (CS). Second, rats were trained to associate a tone+light CS with the presentation of 10% ethanol and were subsequently tested on the acquisition of a new instrumental response with conditioned reinforcement (CR) after local VTA infusion of MEC, DHbetaE, or alpha-Conotoxin MII (alpha-CtxMII, a selective alpha3beta2* and alpha6* nAChR antagonist).

RESULTS

The ethanol-associated CS elevated nAc dopamine, an effect that was blocked by VTA perfusion of MEC but not DHbetaE. Systemic administration of MEC or local VTA infusion of MEC or alpha-CtxMII selectively blocked ethanol-associated CR, whereas systemic DHbetaE had no effect.

CONCLUSIONS

We hypothesize a novel mechanism by which alcohol-associated cues promote drug-seeking behavior via activation of dopamine-stimulating alpha-CtxMII-sensitive nAChRs in the VTA. Pharmacological manipulations of selective nAChRs may thus be possible treatment strategies to prevent cue-induced relapse.

Combined exposure to nicotine and ethanol throughout full gestation results in enhanced acquisition of nicotine self-administration in young adult rat offspring

RATIONALE

Epidemiological evidence shows positive correlation between either maternal cigarette smoking or alcohol consumption on subsequent drug-taking behavior in offspring. However, the consequences of full gestational exposure to both drugs have not been studied experimentally despite concurrent use frequently reported among women of childbearing age. Such comorbid gestational drug exposure may increase susceptibility to acquiring cigarette smoking (i.e., nicotine self-administration), a major gateway drug.

OBJECTIVES

We developed a noninvasive rat model for exposure to both nicotine (2-6 mg kg(-1) day(-1)) and EtOH (4 g/kg gavage) that continued throughout pregnancy and postnatal (P) days 2-12, the rodent equivalent of the human third trimester, a critical brain developmental period. Offspring with this full gestational exposure to both drugs (Nic+EtOH) were compared to controls: nicotine alone, EtOH alone, pair-fed (comparable nutrition and handling), and ad libitum chow-fed. At P60-90, offspring had unlimited chronic access to acquire i.v. nicotine self-administration.

RESULTS

There were no differences in gender ratio, stillbirths, birth weights, righting reflex, eye opening age, or weight gain. However, Nic+EtOH offspring of both genders acquired nicotine self-administration (15 or 30 microg kg(-1) injection(-1)) more rapidly, at a higher percentage, and at a higher level than offspring in the other cohorts.

CONCLUSION

Full gestational Nic+EtOH exposure produced no overt alterations in standard postnatal measures but resulted in an enhanced acquisition of nicotine self-administration in young adult offspring.

Characterization of ethanol-induced dopamine elevation in the rat nucleus accumbens

Ethanol-induced accumbal dopamine elevations have been linked to ethanol consumption. It is unclear, however, where along the mesolimbic dopamine system this effect is initiated and why the ethanol-induced dopamine elevations are transient, returning to pre-drug baseline before brain and blood ethanol levels decline. Using in vivo microdialysis, Experiment 1 investigated the effect of local ethanol application in the nucleus accumbens, the ventral tegmental area and the nucleus accumbens+the ventral tegmental area, on accumbal dopamine. Experiment 2 examined whether the rapid withdrawal of dopamine response to ethanol involves activation of GABA(A)-receptors, by analyzing the effect of accumbal co-perfusion of picrotoxin and ethanol. In Experiment 1, ethanol perfusion into the ventral tegmental area alone did not affect accumbal dopamine. Ethanol co-perfusion of one of the tested doses into the ventral tegmental+the nucleus accumbens produced higher dopamine levels than ethanol perfusion into the nucleus accumbens alone during 120-160 min following perfusion onset. In Experiment 2, accumbal ethanol perfusion caused a transient increase in nucleus accumbens dopamine. Co-perfusion of ethanol and picrotoxin produced a sustained dopamine elevation. These data support the hypothesis that the primary effect of ethanol on accumbal dopamine is in the nucleus accumbens, but that a secondary effect of nucleus accumbens ethanol perfusion, such as release of acetylcholine in the ventral tegmental area, enables ethanol to act as a nicotinic acetylcholine receptor co-agonist in this area. Moreover, recruitment of GABA(A)-receptor activity appears responsible for the second, declining phase with respect to dopamine levels following ethanol administration.

Smoking comorbidity in alcoholism: neurobiological and neurocognitive consequences

Considerable research attests to the adverse effects of chronic smoking on cardiac, pulmonary, and vascular function as well as on increased risk for various cancers. However, comparatively little is known about the effects of chronic smoking on brain function. Although smoking rates have decreased in the developed world (they have increased in the developing world), smoking rates have been at a persistently high level in individuals with alcohol use disorders. Despite the high prevalence of comorbid chronic smoking and alcohol dependence, very few studies have addressed the separate and interactive effects that smoking and alcoholic drinking may have on neurobiology and brain function. This symposium, which took place at the annual meeting of the Research Society on Alcoholism in Santa Barbara, California, on June 29, 2005, postulates that the neurobiologic and neurocognitive abnormalities commonly described in studies of alcohol-dependent individuals are modulated by concurrent abuse of tobacco products and that brain recovery in abstinent alcoholic individuals is affected by chronic smoking. Four expert speakers and a discussant from different research disciplines focus in this symposium on the description of neurobiological and neurobehavioral effects because of concomitant drinking and smoking. Understanding the potential separate effects and interactions of chronic nicotine/smoking and alcohol consumption promotes a better understanding of specific mechanisms and neurocognitive consequences of brain injury and brain recovery with abstinence. The material presented contributes useful information to ongoing discussions about treatment strategies for these comorbid disorders and valuable educational material that can be used to affect public perception about smoking and perhaps health policy.

Role of the subunit composition of central nicotinic acetylcholine receptors for the stimulatory and dopamine-enhancing effects of ethanol

AIMS

The stimulatory, rewarding, and dopamine (DA)-enhancing effects of ethanol may involve central nicotinic acetylcholine receptors (nAChR), especially those located in the ventral tegmental area (VTA). Identifying the subunit composition that mediates these effects of ethanol would increase the understanding of the neurochemical basis underlying the addictive properties of ethanol. In the present series of experiments, the role of the alpha(3)beta(2)(*) and/or beta(3)(*) and/or alpha(6)(*) subunits of the nAChR for the stimulatory and DA-enhancing effects of ethanol was investigated by using alpha-conotoxin MII (alphaCtxMII), selective to the alpha(3)beta(2)(*) and/or beta(3)(*) and/or the alpha(6)(*) subunits of the nAChR, and the alpha-conotoxin PIA-analogue (alphaCtxPIA-analogue), suggested to be selective to the alpha(6)(*) subunits.

METHODS

alphaCtxMII and the alphaCtxPIA-analogue were synthesized using a modified literature procedure. The purity and identity of the peptides were confirmed with HPLC and FAB-MS analyses, respectively. Locomotor activity and in vivo microdialysis in freely moving mice were used.

RESULTS

alphaCtxMII and the alphaCtxPIA-analogue were synthesized in good yields (>95%; >90%). In addition, we found that synthesized alphaCtxMII antagonized ethanol-induced locomotor stimulation, which confirms our previous results with the commercially available alphaCtxMII. Furthermore, the synthesized alphaCtxPIA-analogue, assumably also selective for alpha(6)(*) subunits of the nAChR, did neither antagonize the stimulatory nor the accumbal DA-enhancing effects of ethanol.

CONCLUSION

These results indicate that alphaCtxMII- but not alphaCtxPIA-analogue-sensitive receptors, i.e. the alpha(3)beta(2)(*) and/or beta(3)(*) rather than the alpha(6)(*) subunits of the nAChR, appear to be of greater importance for these effects of ethanol and that these subunits could constitute neurochemical targets for developing new drugs for the treatment of alcohol dependence.

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.