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 ␣4, ␣6, and ␤3 subunit genes compared with nonactivated neurons. To examine the role of nicotinic receptors containing the ␣4 subunit (␣4* 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 ␣4 subunit (␣4 KO) and another line that expresses ␣4* nAChRs hypersensitive to agonist (Leu9ЈAla). 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 ␣4 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 Leu9ЈAla 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 ␣4* nAChRs is necessary and sufficient for varenicline reduction of alcohol consumption.

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.Methods-Adult male C57Bl/6J mice were injected i.p. with nicotinic receptor antagonists including mecamylamine, hexamethonium, dihydro-β erythroidine, and methyllycaconitine. Immediately following injection mice were presented with 20 % ethanol for 2 hours 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.

Tolerance, described as the loss of drug effectiveness over time, is an important component of addiction. The degree of acute behavioral tolerance to alcohol exhibited by a naïve subject can predict the likelihood of alcohol abuse. Thus, the determinants of acute tolerance are important to understand. Calcium-and voltage-gated (BK) potassium channels, consisting of pore forming ␣ and modulatory ␤ subunits, are targets of ethanol (EtOH) action. Here, we examine the role, at the molecular, cellular, and behavioral levels, of the BK ␤4 subunit in acute tolerance. Single channel recordings in HEK-293 cells show that, in the absence of ␤4, EtOH potentiation of activity exhibits acute tolerance, which is blocked by coexpressing the ␤4 subunit. BK channels in acutely isolated medium spiny neurons from WT mice (in which the ␤4 subunit is well-represented) exhibit little tolerance. In contrast, neuronal BK channels from ␤4 knockout (KO) mice do display acute tolerance. Brain slice recordings showed tolerance to EtOH's effects on spike patterning in KO but not in WT mice. In addition, ␤4 KO mice develop rapid tolerance to EtOH's locomotor effects, whereas WT mice do not. Finally, in a restricted access ethanol self-administration assay, ␤4 KO mice drink more than their WT counterparts. Taken together, these data indicate that the ␤4 subunit controls ethanol tolerance at the molecular, cellular, and behavioral levels, and could determine individual differences in alcohol abuse and alcoholism, as well as represent a therapeutic target for alcoholism.electrophysiology ͉ knockout mice ͉ striatum ͉ addiction ͉ plasticity A lcohol abuse is the third largest cause of preventable mortality in the world. Tolerance, described as the gradual loss of drug effectiveness over time, is a hallmark of abused drugs. This phenomenon is particularly important in the response to acute alcohol because the degree of tolerance exhibited by a naïve subject can predict the likelihood to develop alcohol abuse (1-4). Thus, identifying the mechanistic and molecular underpinnings of tolerance is essential for understanding the pathophysiology of alcoholism, as well as determining potential therapeutic targets for alcohol abuse. The neurobiology of tolerance is thought to involve several types of adaptation, ranging from alteration in membrane lipid composition (5) to neuroadaptative changes in target proteins (6, 7).In recent years, large conductance calcium-and voltage-gated potassium (BK) channels have emerged as one of the key targets of ethanol action, yet their role in the physiological and behavioral response to alcohol are unknown. Invertebrate studies suggest that BK channels may be important for the development of tolerance to ethanol (8, 9). In mammals, BK channels exist as a complex formed by the association of the pore-forming ␣ subunit with the auxiliary ␤ subunit. The ␣ subunit is encoded by only one gene (slo) with several splice variants (STREX, P27, insertless, etc.), whereas the ␤ subunit is the product of four distinct genes (␤1-␤4)...

Alcohol and nicotine are often co-abused. As many as 80–95% of alcoholics are also smokers, suggesting that ethanol and nicotine, the primary addictive component of tobacco smoke, may functionally interact in the central nervous system and/or share a common mechanism of action. While nicotine initiates dependence by binding to and activating neuronal nicotinic acetylcholine receptors (nAChRs), ligand-gated cation channels normally activated by endogenous acetylcholine (ACh), ethanol is much less specific with the ability to modulate multiple gene products including those encoding voltage-gated ion channels, and excitatory/inhibitory neurotransmitter receptors. However, emerging data indicate that ethanol interacts with nAChRs, both directly and indirectly, in the mesocorticolimbic dopaminergic (DAergic) reward circuitry to affect brain reward systems. Like nicotine, ethanol activates DAergic neurons of the ventral tegmental area (VTA) which project to the nucleus accumbens (NAc). Blockade of VTA nAChRs reduces ethanol-mediated activation of DAergic neurons, NAc DA release, consumption, and operant responding for ethanol in rodents. Thus, ethanol may increase ACh release into the VTA driving activation of DAergic neurons through nAChRs. In addition, ethanol potentiates distinct nAChR subtype responses to ACh and nicotine in vitro and in DAergic neurons. The smoking cessation therapeutic and nAChR partial agonist, varenicline, reduces alcohol consumption in heavy drinking smokers and rodent models of alcohol consumption. Finally, single nucleotide polymorphisms in nAChR subunit genes are associated with alcohol dependence phenotypes and smoking behaviors in human populations. Together, results from pre-clinical, clinical, and genetic studies indicate that nAChRs may have an inherent role in the abusive properties of ethanol, as well as in nicotine and alcohol co-dependence.

Background-Nicotine and alcohol are the two most co-abused drugs in the world suggesting a common mechanism of action may underlie their rewarding properties. While nicotine elicits reward by activating ventral tegmental area (VTA) dopaminergic (DAergic) neurons via high affinity neuronal nicotinic acetylcholine receptors (nAChRs), the mechanism by which alcohol activates these neurons is unclear.

Recently, we investigated the molecular mechanisms of the smoking cessation drug varenicline, a nicotinic acetylcholine receptor (nAChR) partial agonist, in its ability to decrease voluntary ethanol intake in mice. Previous to our study, other labs had shown that this drug can decrease ethanol consumption and seeking in rat models of ethanol intake. Although varenicline was designed to be a high affinity partial agonist of nAChRs containing the α4 and β2 subunits (designated as α4β2*), at higher concentrations it can also act upon α3β2*, α6*, α3β4* and α7 nAChRs. Therefore, to further elucidate the nAChR subtype responsible for varenicline-induced reduction of ethanol consumption, we utilized a pharmacological approach in combination with two complimentary nAChR genetic mouse models, a knock-out line that does not express the α4 subunit (α4 KO) and another line that expresses α4* nAChRs hypersensitive to agonist (the Leu9'Ala line). We found that activation of α4* nAChRs was necessary and sufficient for varenicline-induced reduction of alcohol consumption. Consistent with this result, here we show that a more efficacious nAChR agonist, nicotine, also decreased voluntary ethanol intake, and that α4* nAChRs are critical for this reduction.

Similar to drugs of abuse, the hedonic value of food is mediated, at least in part, by the mesostriatal dopamine (DA) system. Prolonged intake of either high calorie diets or drugs of abuse both lead to a blunting of the DA system. Most studies have focused on DAergic alterations in the striatum, but little is known about the effects of high calorie diets on ventral tegmental area (VTA) DA neurons. Since high calorie diets produce addictive-like DAergic adaptations, it is possible these diets may increase addiction susceptibility. However, high calorie diets consistently reduce psychostimulant intake and conditioned place preference in rodents. In contrast, high calorie diets can increase or decrease ethanol drinking, but it is not known how a junk food diet (cafeteria diet) affects ethanol drinking. In the current study, we administered a cafeteria diet consisting of bacon, potato chips, cheesecake, cookies, breakfast cereals, marshmallows, and chocolate candies to male Wistar rats for 3–4 weeks, producing an obese phenotype. Prior cafeteria diet feeding reduced homecage ethanol drinking over 2 weeks of testing, and transiently reduced sucrose and chow intake. Importantly, cafeteria diet had no effect on ethanol metabolism rate or blood ethanol concentrations following 2g/kg ethanol administration. In midbrain slices, we showed that cafeteria diet feeding enhances DA D2 receptor (D2R) autoinhibition in VTA DA neurons. These results show that junk food diet-induced obesity reduces ethanol drinking, and suggest that increased D2R autoinhibition in the VTA may contribute to deficits in DAergic signaling and reward hypofunction observed with obesity.

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