α6β2 nicotinic acetylcholine receptors influence locomotor activity and ethanol consumption

The role of nicotinic receptors in alcohol consumption

The use of alcohol causes significant morbidity and mortality across the globe. Alcohol use disorder (AUD) is defined by the excessive use of this drug despite a negative impact on the individual's life. While there are currently medications available to treat AUD, they have limited efficacy and several side effects. As such, it is essential to continue to look for novel therapeutics. One target for novel therapeutics is nicotinic acetylcholine receptors (nAChRs). Here we systematically review the literature on the involvement of nAChRs in alcohol consumption. Data from both genetic and pharmacology studies provide evidence that nAChRs modulate alcohol intake. Interestingly, pharmacological modulation of all nAChR subtypes examined can decrease alcohol consumption. The reviewed literature demonstrates that nAChRs should continue to be investigated as novel therapeutics for AUD.

α-Conotoxin TxIB Improved Behavioral Abnormality and Changed Gene Expression in Zebrafish (Danio rerio) Induced by Alcohol Withdrawal

Background and Purpose: Alcohol use disorder (AUD) is a serious public health issue and affects the lives of numerous people. Previous studies have shown a link between nicotinic acetylcholine receptors (nAChR) and alcohol addiction. However, the role of α6β2* nAChR in alcohol addiction remains obscure, and whether α6β2* nAChR can be used as a potential drug target for alcohol withdrawal need to be studied.

Methods: Zebrafish (Danio rerio) were exposed to 0.2% alcohol for 14 days followed by 7 days of repeated withdrawal and then retro-orbitally injected with α-conotoxin TxIB (a selective α6β2* nAChR antagonist). Open Field Test was applied to characterize zebrafish behavior parameters. The monoamine neurotransmitter amounts and their mRNA expression in the zebrafish brain were identified using ELISA and quantitative real-time PCR (RT-PCR). RNA-sequencing (RNA-seq) and subsequent bioinformatics analysis were employed to explore the potential network regulation of TxIB after alcohol withdrawal.

Results: The max speed in the center area of the Open Field Test was significantly higher in the withdrawal group whereas TxIB injection corrected this abnormality. The amount and mRNA expression of monoamine neurotransmitters did not change significantly after alcohol withdrawal and TxIB administration. RNA sequencing of zebrafish brain indicated a total of 657 genes showed aberrant expression and among which 225 were reversed after TxIB injection. These reversed genes were significantly enriched in the calcium ion binding pathway and the gene expression profile was further validated by RT-PCR.

Conclusion: Our finding suggests α-conotoxin TxIB improved behavioral abnormality induced by alcohol-withdrawal, and changed gene expression mainly in the calcium signaling pathway. Therefore, α-conotoxin TxIB is expected to become a potential therapeutic agent for alcohol withdrawal.

α-Conotoxin TxIB Inhibits Development of Morphine-Induced Conditioned Place Preference in Mice via Blocking α6β2* Nicotinic Acetylcholine Receptors

Morphine, the main component of opium, is a commonly used analgesic in clinical practice, but its abuse potential limits its clinical application. Nicotinic acetylcholine receptors (nAChRs) in the mesolimbic circuitry play an important role in the rewarding effects of abused drugs. Previous studies have showed that α6β2* (* designated other subunits) nAChRs are mainly distributed in dopaminergic neurons in the midbrain area, which regulates the release of dopamine. So α6β2* nAChRs are regarded as a new target to treat drug abuse. α-Conotoxin TxIB was discovered in our lab, which is the most selective ligand to inhibit α6β2* nAChRs only. Antagonists of α6β2* nAChRs decreased nicotine, cocaine, and ethanol rewarding effects previously. However, their role in morphine addiction has not been reported so far. Thus, it is worth evaluating the effect of α-conotoxin TxIB on the morphine-induced conditioned place preference (CPP) and its behavioral changes in mice. Our results showed that TxIB inhibited expression and acquisition of morphine-induced CPP and did not produce a rewarding effect by itself. Moreover, repeated injections of TxIB have no effect on learning, memory, locomotor activity, and anxiety-like behavior. Therefore, blocking α6/α3β2β3 nAChRs inhibits the development of morphine-induced CPP. α-Conotoxin TxIB may be a potentially useful compound to mitigate the acquisition and/or retention of drug-context associations.

Sex Differences in the Nicotinic Acetylcholine Receptor System of Rodents: Impacts on Nicotine and Alcohol Reward Behaviors

Alcohol and nicotine are the two most widely used and misused drugs around the world, and co-consumption of both substances is highly prevalent. Multiple lines of evidence show a profound effect of sex in many aspects of alcohol and nicotine reward, with women having more difficulty quitting smoking and showing a faster progression toward developing alcohol use disorder compared with men. Both alcohol and nicotine require neuronal nicotinic acetylcholine receptors (nAChRs) to elicit rewarding effects within the mesolimbic system, representing a shared molecular pathway that likely contributes to the frequent comorbidity of alcohol and nicotine dependence. However, the majority of preclinical studies on the mechanisms of alcohol and nicotine reward behaviors utilize only male rodents, and thus our understanding of alcohol and nicotine neuropharmacology relies heavily on male data. As preclinical research informs the development and refinement of therapies to help patients reduce drug consumption, it is critical to understand the way biological sex and sex hormones influence the rewarding properties of alcohol and nicotine. In this review, we summarize what is known about sex differences in rodent models of alcohol and nicotine reward behaviors with a focus on neuronal nAChRs, highlighting exciting areas for future research. Additionally, we discuss the way circulating sex hormones may interact with neuronal nAChRs to influence reward-related behavior.

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.

Cholinergic System and Its Therapeutic Importance in Inflammation and Autoimmunity

Neurological and immunological signals constitute an extensive regulatory network in our body that maintains physiology and homeostasis. The cholinergic system plays a significant role in neuroimmune communication, transmitting information regarding the peripheral immune status to the central nervous system (CNS) and vice versa. The cholinergic system includes the neurotransmitter\ molecule, acetylcholine (ACh), cholinergic receptors (AChRs), choline acetyltransferase (ChAT) enzyme, and acetylcholinesterase (AChE) enzyme. These molecules are involved in regulating immune response and playing a crucial role in maintaining homeostasis. Most innate and adaptive immune cells respond to neuronal inputs by releasing or expressing these molecules on their surfaces. Dysregulation of this neuroimmune communication may lead to several inflammatory and autoimmune diseases. Several agonists, antagonists, and inhibitors have been developed to target the cholinergic system to control inflammation in different tissues. This review discusses how various molecules of the neuronal and non-neuronal cholinergic system (NNCS) interact with the immune cells. What are the agonists and antagonists that alter the cholinergic system, and how are these molecules modulate inflammation and immunity. Understanding the various functions of pharmacological molecules could help in designing better strategies to control inflammation and autoimmunity.

The role of nicotinic acetylcholine receptors in alcohol-related behaviors

Alcohol use disorder (AUD) causes an alarming economic and health burden in the United States. Unfortunately, this disease does not exist in isolation; AUD is highly comorbid with nicotine use. Results from both human and animal models demonstrate a genetic correlation between alcohol and nicotine behaviors. These data support the idea of shared genetic and neural mechanisms underlying these behaviors. Nicotine acts directly at nicotinic acetylcholine receptors (nAChR) to have its pharmacological effect. Interestingly, alcohol also acts both directly and indirectly at these receptors. Research utilizing genetically engineered rodents and pharmacological manipulations suggest a role for nAChR in several ethanol behaviors. The current manuscript collates this literature and discusses findings that implicate specific nAChR subunits in ethanol phenotypes. These data suggest future directions for targeting nAChR as novel therapeutics for AUD.

The α3β4 nicotinic acetylcholine receptor antagonist 18-Methoxycoronaridine decreases binge-like ethanol consumption in adult C57BL/6J mice

Binge alcohol drinking is a health burden in the United States, which has an alarming economic impact. Unfortunately, medications available for alcohol abuse have low efficacy or adverse side effects, creating a need to evaluate novel therapies. Growing research suggests that 18-Methoxycoronaridine (18-MC), an α3β4 nicotinic acetylcholine receptor (nAChR) antagonist, may be effective at reducing ethanol consumption. However, its effects on binge-like ethanol consumption and other ethanol behaviors have not been examined. The present study examined the effect of α3β4 nAChRs antagonism on basal locomotor activity in male and female C57BL/6J mice. Next we tested the effect of 18-MC on binge-like ethanol consumption, ethanol-induced sedation, and ethanol metabolism. Finally, we tested the effect of α3β4 nAChRs on saccharin consumption to ensure effects were specific for ethanol. We observed that 18-MC decreased binge-like ethanol consumption without altering saccharin consumption, the sedative effects of ethanol, or ethanol metabolism. High doses of 18-MC caused locomotor sedation in C57BL/6J mice, but the effects were brief and likely did not contribute to differences in ethanol consumption. Our results support the involvement of the α3β4 nAChRs in binge-like ethanol intake, and further work should explore the use of 18-MC for treatment of alcohol use disorders.

Adolescent social stress increases anxiety-like behavior and ethanol consumption in adult male and female C57BL/6J mice

Exposure to social stress is an important risk factor for comorbid affective disorders and problem alcohol use. To better understand mechanisms involved in social stress-induced affective disorder and alcohol use co-morbidity, we studied the effects of adolescent social stress on anxiety- and depression-like behaviors and binge-like ethanol consumption. Male and female C57BL/6J mice were exposed to chronic variable social stress (CVSS) or control conditions throughout adolescence (postnatal days, PND, 25-59) and then tested for anxiety-like behavior in the elevated plus maze and a novel open field environment, or depression-like behavior using the forced swim test on PND 64-66. Mice were then tested for binge-like ethanol consumption using the Drinking-in-the-Dark model. Male and female mice exposed to adolescent CVSS had increased adult anxiety-like behavior and increased locomotor adaptation to a novel environment. Further, CVSS mice consumed significantly more ethanol, but not saccharin, than controls. Despite group differences in both anxiety-like behavior and ethanol consumption, there was no relationship between these outcomes within individual mice. These data suggest that exposure to adolescent social stress is an important risk factor for later alcohol use and affective behaviors, but that social stress does not necessarily dictate co-morbidity of these outcomes.

Varenicline modulates ethanol and saccharin consumption in adolescent male and female C57BL/6J mice

Adolescence is a critical period in brain development that coincides with the initiation of alcohol use. Nicotinic acetylcholine receptors (nAChR) have been shown to modulate ethanol behaviors in adult humans and in animal models; however, the role of these receptors in adolescent ethanol behaviors has not been explored. Throughout adolescence, nAChR expression undergoes large-scale developmental changes which may alter behavioral responses to ethanol. Here we examined the effect of varenicline, a nAChR partial agonist, on ethanol consumption, ataxia, sedation, and metabolism in adolescent male and female C57BL/6J mice. The effect of varenicline on ethanol consumption was tested through the Drinking-in-the-Dark (DID) paradigm that models binge-like ethanol consumption. To ensure that results were specific for ethanol, we also tested the effect of varenicline on saccharin consumption. Additionally, varenicline was administered 30min prior to an acute injection of ethanol before being tested for ataxia on the balance beam, sedation using the loss of righting reflex, or ethanol metabolism. Varenicline dose dependently decreased ethanol consumption, but also influenced saccharin intake. Varenicline showed no significant effect on ethanol metabolism, ataxia, or sedation. Unlike its effects in adult animals, varenicline is able to reduce ethanol consumption without increasing the ataxic and sedative effects of ethanol. This work suggests that the neurobiological mechanisms of ethanol behaviors may change across the lifespan and highlights the need for more research on the role of nAChRs in ethanol behaviors throughout development.