A Mechanism Linking Two Known Vulnerability Factors for Alcohol Abuse: Heightened Alcohol Stimulation and Low Striatal Dopamine D2 Receptors

Anterior Cingulate Cortex Contributes to the Hyperlocomotion under Nitrogen Narcosis

Nitrogen narcosis is a neurological syndrome that manifests when humans or animals encounter hyperbaric nitrogen, resulting in a range of motor, emotional, and cognitive abnormalities. The anterior cingulate cortex (ACC) is known for its significant involvement in regulating motivation, cognition, and action. However, its specific contribution to nitrogen narcosis-induced hyperlocomotion and the underlying mechanisms remain poorly understood. Here we report that exposure to hyperbaric nitrogen notably increased the locomotor activity of mice in a pressure-dependent manner. Concurrently, this exposure induced heightened activation among neurons in both the ACC and dorsal medial striatum (DMS). Notably, chemogenetic inhibition of ACC neurons effectively suppressed hyperlocomotion. Conversely, chemogenetic excitation lowered the hyperbaric pressure threshold required to induce hyperlocomotion. Moreover, both chemogenetic inhibition and genetic ablation of activity-dependent neurons within the ACC reduced the hyperlocomotion. Further investigation revealed that ACC neurons project to the DMS, and chemogenetic inhibition of ACC-DMS projections resulted in a reduction in hyperlocomotion. Finally, nitrogen narcosis led to an increase in local field potentials in the theta frequency band and a decrease in the alpha frequency band in both the ACC and DMS. These results collectively suggest that excitatory neurons within the ACC, along with their projections to the DMS, play a pivotal role in regulating the hyperlocomotion induced by exposure to hyperbaric nitrogen.

Identification of Phosphodiesterase-7A (PDE7A) as a Novel Target for Reducing Ethanol Consumption in Mice

BACKGROUND

Ethanol elicits a rapid stimulatory effect and a subsequent, prolonged sedative response, which are potential predictors of EtOH consumption by decreasing adenosine signaling; this phenomenon also reflects the obvious sex difference. cAMP (cyclic Adenosine Monophosphate)-PKA (Protein Kinase A) signaling pathway modulation can influence the stimulatory and sedative effects induced by EtOH in mice. This study's objective is to clarify the role of phosphodiesterase (PDE) in mediating the observed sex differences in EtOH responsiveness between male and female animals.

METHODS

EtOH was administered i.p. for 7 days to identify the changes in PDE isoforms in response to EtOH treatment. Additionally, EtOH consumption and preference of male and female C57BL/6J mice were assessed using the drinking-in-the-dark and 2-bottle choice tests. Further, pharmacological inhibition of PDE7A heterozygote knockout mice was performed to investigate its effects on EtOH-induced stimulation and sedation in both male and female mice. Finally, Western blotting analysis was performed to evaluate the alterations in cAMP-PKA/Epac2 pathways.

RESULTS

EtOH administration resulted in an immediate upregulation in PDE7A expression in female mice, indicating a strong association between PDE7A and EtOH stimulation. Through the pharmacological inhibition of PDE7A KD mice, we have demonstrated for the first time, to our knowledge, that PDE7A selectively attenuates EtOH responsiveness and consumption exclusively in female mice, whichmay be associated with the cAMP-PKA/Epac2 pathway and downstream phosphorylation of CREB and ERK1/2.

CONCLUSIONS

Inhibition or knockdown of PDE7A attenuates EtOH responsivenessand consumption exclusively in female mice, which is associated with alterations in the cAMP-PKA/Epac2 signaling pathways, thereby highlighting its potential as a novel therapeutic target for alcohol use disorder.

Role of histaminergic regulation of astrocytes in alcohol use disorder

Alcohol use disorder (AUD) is a severe, yet not fully understood, mental health problem. It is associated with liver, pancreatic, and gastrointestinal diseases, thereby highly increasing the morbidity and mortality of these individuals. Currently, there is no effective and safe pharmacological therapy for AUD. Therefore, there is an urgent need to increase our knowledge about its neurophysiological etiology to develop new treatments specifically targeted at this health condition. Recent findings have shown an upregulation in the histaminergic system both in alcohol dependent individuals and in animals with high alcohol preference. The use of H3 histaminergic receptor antagonists has given promising therapeutic results in animal models of AUD. Interestingly, astrocytes, which are ubiquitously present in the brain, express the three main histamine receptors (H1, H2 and H3), and in the last few years, several studies have shown that astrocytes could play an important role in the development and maintenance of AUD. Accordingly, alterations in the density of astrocytes in brain areas such as the prefrontal cortex, ventral striatum, and hippocampus that are critical for AUD-related characteristics have been observed. These characteristics include addiction, impulsivity, motor function, and aggression. In this work, we review the current state of knowledge on the relationship between the histaminergic system and astrocytes in AUD and propose that histamine could increase alcohol tolerance by protecting astrocytes from ethanol-induced oxidative stress. This increased tolerance could lead to high levels of alcohol intake and therefore could be a key factor in the development of alcohol dependence.

Knockdown of Tlr3 in dorsal striatum reduces ethanol consumption and acute functional tolerance in male mice

Systemic activation of toll-like receptor 3 (TLR3) signaling using poly(I:C), a TLR3 agonist, drives ethanol consumption in several rodent models, while global knockout of Tlr3 reduces drinking in C57BL/6J male mice. To determine if brain TLR3 pathways are involved in drinking behavior, we used CRISPR/Cas9 genome editing to generate a Tlr3 floxed (Tlr3F/F) mouse line. After sequence confirmation and functional validation of Tlr3 brain transcripts, we injected Tlr3F/F male mice with an adeno-associated virus expressing Cre recombinase (AAV5-CMV-Cre-GFP) to knockdown Tlr3 in the medial prefrontal cortex, nucleus accumbens, or dorsal striatum (DS). Only Tlr3 knockdown in the DS decreased two-bottle choice, every-other-day (2BC-EOD) ethanol consumption. DS-specific deletion of Tlr3 also increased intoxication and prevented acute functional tolerance to ethanol. In contrast, poly(I:C)-induced activation of TLR3 signaling decreased intoxication in male C57BL/6J mice, consistent with its ability to increase 2BC-EOD ethanol consumption in these mice. We also found that TLR3 was highly colocalized with DS neurons. AAV5-Cre transfection occurred predominantly in neurons, but there was minimal transfection in astrocytes and microglia. Collectively, our previous and current studies show that activating or inhibiting TLR3 signaling produces opposite effects on acute responses to ethanol and on ethanol consumption. While previous studies, however, used global knockout or systemic TLR3 activation (which alter peripheral and brain innate immune responses), the current results provide new evidence that brain TLR3 signaling regulates ethanol drinking. We propose that activation of TLR3 signaling in DS neurons increases ethanol consumption and that a striatal TLR3 pathway is a potential target to reduce excessive drinking.

Leucine-rich repeat kinase 2 limits dopamine D1 receptor signaling in striatum and biases against heavy persistent alcohol drinking

The transition from hedonic alcohol drinking to problematic drinking is a hallmark of alcohol use disorder that occurs only in a subset of drinkers. This transition requires long-lasting changes in the synaptic drive and the activity of striatal neurons expressing dopamine D1 receptor (D1R). The molecular mechanisms that generate vulnerability in some individuals to undergo the transition are less understood. Here, we report that the Parkinson's-related protein leucine-rich repeat kinase 2 (LRRK2) modulates striatal D1R function to affect the behavioral response to alcohol and the likelihood that mice transition to heavy, persistent alcohol drinking. Constitutive deletion of the Lrrk2 gene specifically from D1R-expressing neurons potentiated D1R signaling at the cellular and synaptic level and enhanced alcohol-related behaviors and drinking. Mice with cell-specific deletion of Lrrk2 were more prone to heavy alcohol drinking, and consumption was insensitive to punishment. These findings identify a potential novel role for LRRK2 function in the striatum in promoting resilience against heavy and persistent alcohol drinking.

Dopamine D2 receptors in nucleus accumbens cholinergic interneurons increase impulsive choice

Impulsive choice, often characterized by excessive preference for small, short-term rewards over larger, long-term rewards, is a prominent feature of substance use and other neuropsychiatric disorders. The neural mechanisms underlying impulsive choice are not well understood, but growing evidence implicates nucleus accumbens (NAc) dopamine and its actions on dopamine D2 receptors (D2Rs). Because several NAc cell types and afferents express D2Rs, it has been difficult to determine the specific neural mechanisms linking NAc D2Rs to impulsive choice. Of these cell types, cholinergic interneurons (CINs) of the NAc, which express D2Rs, have emerged as key regulators of striatal output and local dopamine release. Despite these relevant functions, whether D2Rs expressed specifically in these neurons contribute to impulsive choice behavior is unknown. Here, we show that D2R upregulation in CINs of the mouse NAc increases impulsive choice as measured in a delay discounting task without affecting reward magnitude sensitivity or interval timing. Conversely, mice lacking D2Rs in CINs showed decreased delay discounting. Furthermore, CIN D2R manipulations did not affect probabilistic discounting, which measures a different form of impulsive choice. Together, these findings suggest that CIN D2Rs regulate impulsive decision-making involving delay costs, providing new insight into the mechanisms by which NAc dopamine influences impulsive behavior.

Dopamine D2 receptors in the bed nucleus of the stria terminalis modulate alcohol-related behaviors

Dysregulation of the dopamine (DA) system is a hallmark of substance abuse disorders, including alcohol use disorder (AUD). Of the DA receptor subtypes, the DA D2 receptors (D2Rs) play a key role in the reinforcing effects of alcohol. D2Rs are expressed in numerous brain regions associated with the regulation of appetitive behaviors. One such region is the bed nucleus of the stria terminalis (BNST), which has been linked to the development and maintenance of AUD. Recently, we identified alcohol withdrawal-related neuroadaptations in the periaqueductal gray/dorsal raphe to BNST DA circuit in male mice. However, the role of D2R-expressing BNST neurons in voluntary alcohol consumption is not well characterized. In this study, we used a CRISPR-Cas9-based viral approach, to selectively reduce the expression of D2Rs in BNST VGAT neurons and interrogated the impact of BNST D2Rs in alcohol-related behaviors. In male mice, reduced D2R expression potentiated the stimulatory effects of alcohol and increased voluntary consumption of 20% w/v alcohol in a two-bottle choice intermittent access paradigm. This effect was not specific to alcohol, as D2R deletion also increased sucrose intake in male mice. Interestingly, cell-specific deletion of BNST D2Rs in female mice did not alter alcohol-related behaviors but lowered the threshold for mechanical pain sensitivity. Collectively, our findings suggest a role for postsynaptic BNST D2Rs in the modulation of sex-specific behavioral responses to alcohol and sucrose.

Intoxicating effects of alcohol depend on acid-sensing ion channels

Persons at risk for developing alcohol use disorder (AUD) differ in their sensitivity to acute alcohol intoxication. Alcohol effects are complex and thought to depend on multiple mechanisms. Here, we explored whether acid-sensing ion channels (ASICs) might play a role. We tested ASIC function in transfected CHO cells and amygdala principal neurons, and found alcohol potentiated currents mediated by ASIC1A homomeric channels, but not ASIC1A/2 A heteromeric channels. Supporting a role for ASIC1A in the intoxicating effects of alcohol in vivo, we observed marked alcohol-induced changes on local field potentials in basolateral amygdala, which differed significantly in Asic1a-/- mice, particularly in the gamma, delta, and theta frequency ranges. Altered electrophysiological responses to alcohol in mice lacking ASIC1A, were accompanied by changes in multiple behavioral measures. Alcohol administration during amygdala-dependent fear conditioning dramatically diminished context and cue-evoked memory on subsequent days after the alcohol had cleared. There was a significant alcohol by genotype interaction. Context- and cue-evoked memory were notably worse in Asic1a-/- mice. We further examined acute stimulating and sedating effects of alcohol on locomotor activity, loss of righting reflex, and in an acute intoxication severity scale. We found loss of ASIC1A increased the stimulating effects of alcohol and reduced the sedating effects compared to wild-type mice, despite similar blood alcohol levels. Together these observations suggest a novel role for ASIC1A in the acute intoxicating effects of alcohol in mice. They further suggest that ASICs might contribute to intoxicating effects of alcohol and AUD in humans.

Animal model for high consumption and preference of ethanol and its interplay with high sugar and butter diet, behavior, and neuroimmune system

Introduction

Mechanisms that dictate the preference for ethanol and its addiction are not only restricted to the central nervous system (CNS). An increasing body of evidence has suggested that abusive ethanol consumption directly affects the immune system, which in turn interacts with the CNS, triggering neuronal responses and changes, resulting in dependence on the drug. It is known that neuroinflammation and greater immune system reactivity are observed in behavioral disorders and that these can regulate gene transcription. However, there is little information about these findings of the transcriptional profile of reward system genes in high consumption and alcohol preference. In this regard, there is a belief that, in the striatum, an integrating region of the brain reward system, the interaction of the immune response and the transcriptional profile of the Lrrk2 gene that is associated with loss of control and addiction to ethanol may influence the alcohol consumption and preference. Given this information, this study aimed to assess whether problematic alcohol consumption affects the transcriptional profile of the Lrrk2 gene, neuroinflammation, and behavior and whether these changes are interconnected.

Methods

An animal model developed by our research group has been used in which male C57BL/6 mice and knockouts for the Il6 and Nfat genes were subjected to a protocol of high fat and sugar diet intake and free choice of ethanol in the following stages: Stage 1 (T1)—Dietary treatment, for 8 weeks, in which the animals receive high-calorie diet, High Sugar and Butter (HSB group), or standard diet, American Institute of Nutrition 93-Growth (AIN93G group); and Stage 2 (T2)—Ethanol consumption, in which the animals are submitted, for 4 weeks, to alcohol within the free choice paradigm, being each of them divided into 10 groups, four groups continued with the same diet and in the other six the HSB diet is substituted by the AIN93G diet. Five groups had access to only water, while the five others had a free choice between water and a 10% ethanol solution. The weight of the animals was evaluated weekly and the consumption of water and ethanol daily. At the end of the 12-week experiment, anxiety-like behavior was evaluated by the light/dark box test; compulsive-like behavior by Marble burying, transcriptional regulation of genes Lrrk2, Tlr4, Nfat, Drd1, Drd2, Il6, Il1β, Il10, and iNOS by RT-qPCR; and inflammatory markers by flow cytometry. Animals that the diet was replaced had an ethanol high preference and consumption.

Results and discussion

We observed that high consumption and preference for ethanol resulted in (1) elevation of inflammatory cells in the brain, (2) upregulation of genes associated with cytokines (Il6 and Il1β) and pro-inflammatory signals (iNOS and Nfat), downregulation of anti-inflammatory cytokine (Il10), dopamine receptor (Drd2), and the Lrrk2 gene in the striatum, and (3) behavioral changes such as decreased anxiety-like behavior, and increased compulsive-like behavior. Our findings suggest that interactions between the immune system, behavior, and transcriptional profile of the Lrrk2 gene influence the ethanol preferential and abusive consumption.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Circuit dysfunctions of associative and sensorimotor basal ganglia loops in alcohol use disorder: insights from animal models

Persons that develop Alcohol Use Disorder (AUD) experience behavioral changes that include compulsion to seek and take alcohol despite its negative consequences on the person's psychosocial, health and economic spheres, inability to limit alcohol intake and a negative emotional/ motivational state that emerges during withdrawal. During all the stages of AUD executive functions, i.e. the person's ability to direct their behavior towards a goal, working memory and cognitive flexibility are eroded. Animal models of AUD recapitulate aspects of action selection impairment and offer the opportunity to benchmark the underlying circuit mechanisms. Here we propose a circuit-based approach to AUD research focusing on recent advances in behavioral analysis, neuroanatomy, genetics, and physiology to guide future research in the field.

The neurobiological markers of acute alcohol's subjective effects in humans

The ingestion of alcohol yields acute biphasic subjective effects: stimulation before sedation. Despite their predictive relevance to the development of alcohol use disorders (AUD), the neurobiological markers accounting for the biphasic effects of alcohol remain poorly understood in humans. Informed by converging lines of evidence, this study tested the hypothesis that alcohol ingestion acutely increases gamma-aminobutyric acid (GABA)-mediated inhibition, which would positively and negatively predict the feeling of stimulation and sedation, respectively. To do so, healthy participants (n = 20) ingested a single dose of 94% ABV alcohol (males: 1.0 ml/kg; females: 0.85 ml/kg) in a randomized placebo-controlled cross-over design. The alcohol's biphasic effects were assessed with the Brief-Biphasic Alcohol Effects Scale, and non-invasive neurobiological markers were measured with transcranial magnetic stimulation, before and every 30 min (up to 120 min) after the complete ingestion of the beverage. Results showed that acute alcohol ingestion selectively increased the duration of the cortical silent period (CSP) as compared to placebo, suggesting that alcohol increases non-specific GABAergic inhibition. Importantly, CSP duration positively and negatively predicted increases in the feeling of stimulation and sedation, respectively, suggesting that stimulation emerges as GABAergic inhibition increases and that sedation emerges as GABAergic inhibition returns to baseline values. Overall, these results suggest that modulations of GABAergic inhibition are central to the acute biphasic subjective effects of alcohol, providing a potential preventive target to curb the progression of at-risk individuals to AUD.

How changes in dopamine D2 receptor levels alter striatal circuit function and motivation

It was first posited, more than five decades ago, that the etiology of schizophrenia involves overstimulation of dopamine receptors. Since then, advanced clinical research methods, including brain imaging, have refined our understanding of the relationship between striatal dopamine and clinical phenotypes as well as disease trajectory. These studies point to striatal dopamine D2 receptors, the main target for all current antipsychotic medications, as being involved in both positive and negative symptoms. Simultaneously, animal models have been central to investigating causal relationships between striatal dopamine D2 receptors and behavioral phenotypes relevant to schizophrenia. We begin this article by reviewing the circuit, cell-type and subcellular locations of dopamine D2 receptors and their downstream signaling pathways. We then summarize results from several mouse models in which D2 receptor levels were altered in various brain regions, cell-types and developmental periods. Behavioral, electrophysiological and anatomical consequences of these D2 receptor perturbations are reviewed with a selective focus on striatal circuit function and alterations in motivated behavior, a core negative symptom of schizophrenia. These studies show that D2 receptors serve distinct physiological roles in different cell types and at different developmental time points, regulating motivated behaviors in sometimes opposing ways. We conclude by considering the clinical implications of this complex regulation of striatal circuit function by D2 receptors.

Cocaine shifts dopamine D2 receptor sensitivity to gate conditioned behaviors

Cocaine addiction is a chronic, relapsing disorder characterized by maladaptation in the brain mesolimbic and nigrostriatal dopamine system. Although changes in the properties of D2-receptor-expressing medium spiny neurons (D2-MSNs) and connected striatal circuits following cocaine treatment are known, the contributions of altered D2-receptor (D2R) function in mediating the rewarding properties of cocaine remain unclear. Here, we describe how a 7-day exposure to cocaine alters dopamine signaling by selectively reducing the sensitivity, but not the expression, of nucleus accumbens D2-MSN D2Rs via an alteration in the relative expression and coupling of G protein subunits. This cocaine-induced reduction of D2R sensitivity facilitated the development of the rewarding effects of cocaine as blocking the reduction in G protein expression was sufficient to prevent cocaine-induced behavioral adaptations. These findings identify an initial maladaptive change in sensitivity by which mesolimbic dopamine signals are encoded by D2Rs following cocaine exposure.

Alcohol and the brain: from genes to circuits

Alcohol use produces wide-ranging and diverse effects on the central nervous system. It influences intracellular signaling mechanisms, leading to changes in gene expression, chromatin remodeling, and translation. As a result of these molecular alterations, alcohol affects the activity of neuronal circuits. Together, these mechanisms produce long-lasting cellular adaptations in the brain that in turn can drive the development and maintenance of alcohol use disorder (AUD). We provide an update on alcohol research, focusing on multiple levels of alcohol-induced adaptations, from intracellular changes to changes in neural circuits. A better understanding of how alcohol affects these diverse and interlinked mechanisms may lead to the identification of novel therapeutic targets and to the development of much-needed novel and efficacious treatment options.

Astrocyte-neuron interaction in the dorsal striatum-pallidal circuits and alcohol-seeking behaviors

In the striatum, two main types of GABAergic medium spiny neurons (MSNs), denoted striatonigral (or direct-pathway MSNs, dMSNs) and striatopallidal neurons (indirect-pathway MSNs, iMSNs), form circuits with distinct pallidal nuclei, which sends "GO" or "NO-GO" signals through the thalamus. These striatopallidal circuits evaluate and execute reward-seeking and taking behaviors. Especially, the dorsal striatum can be further divided into the dorsomedial striatum (DMS, equivalent to caudate in primates and humans) and dorsolateral striatum (DLS, equivalent to putamen), which orchestrates goal-directed and habitual reward-seeking and taking behaviors, respectively. Using optogenetics, chemogenetics and in vivo calcium imaging technologies combined with electrophysiology and digitalized behavior phenotyping, recent studies have revealed cell-, circuit- and context-specific functions of these microcircuits in addictive behaviors. Also, region-specific astrocytes regulate the homeostatic activities of the dMSNs and iMSNs as well as the downstream circuits, which determine the net balance of cortico-striato-pallidal activities to the thalamic neurons. This review will summarize the recent progress of striatopallidal circuits focusing on astrocyte-neuron interaction and, reward- and alcohol-seeking behaviors. Our review will also discuss the translational and clinical implications of these microcircuit studies. This article is part of the special Issue on "Neurocircuitry Modulating Drug and Alcohol Abuse".

Increased alcohol consumption in sleep-restricted rats is mediated by delta FosB induction

The reduction of sleep hours is a public health problem in contemporary society. It is estimated that humans sleep between 1.5 and 2 h less, per night, than 100 years ago. The reduction of sleep hours is a risk factor for developing cardiovascular, metabolic, and psychiatric problems. Previous studies have shown that low sleep quality is a factor that favors relapse in addicted patients. In rodents, sleep deprivation increases the preference for methylphenidate and the self-administration of cocaine. However, it is unknown whether chronic sleep restriction induces voluntary alcohol consumption in rats and whether alcohol intake is associated with delta FosB expression in the brain reward circuit. Potentially, chronic sleep restriction could make the brain vulnerable and consequently promote addictive behavior. Therefore, the present study's objective was to evaluate alcohol consumption in a chronic sleep restriction model and determine the expression of delta FosB in brains of adult rats. For this purpose, male Wistar rats (300-350 g body weight) were divided into four experimental groups (n = 6 each group): control (without manipulation), sleep restriction (SR) for 7 days, SR and ethanol exposure (Ethanol + SR), and a group with just ethanol exposure (Ethanol). At the end of the management, rats were sacrificed, and the brains were dissected and processed for immunohistochemical detection of delta FosB. The results showed that SR stimulates alcohol consumption compared to unrestricted-sleep rats and induces a significant increase in the number of delta FosB-positive cells in brain nuclei within the motivation/brain reward circuit. These results suggest that chronic reduction of sleep hours is a risk factor for developing a preference for alcohol consumption.

Receptors and Channels Associated with Alcohol Use: Contributions from Drosophila

Alcohol Use Disorder (AUD) is a debilitating disorder that manifests as problematic patterns of alcohol use. At the core of AUD's behavioral manifestations are the profound structural, physiological, cellular, and molecular effects of alcohol on the brain. While the field has made considerable progress in understanding the neuromolecular targets of alcohol we still lack a comprehensive understanding of alcohol's actions and effective treatment strategies. Drosophila melanogaster is a powerful model for investigating the neuromolecular targets of alcohol because flies model many of the core behavioral elements of AUD and offer a rich genetic toolkit to precisely reveal the in vivo molecular actions of alcohol. In this review, we focus on receptors and channels that are often targeted by alcohol within the brain. We discuss the general roles of these proteins, their role in alcohol-associated behaviors across species, and propose ways in which Drosophila models can help advance the field.

Three Weeks of Binge Alcohol Drinking Generates Increased Alcohol Front-Loading and Robust Compulsive-Like Alcohol Drinking in Male and Female C57BL/6J Mice

BACKGROUND

Current models of compulsive-like quinine-adulterated alcohol (QuA) drinking in mice, if improved, could be more useful for uncovering the neural mechanisms of compulsive-like alcohol drinking. The purpose of these experiments was to further characterize and improve the validity of a model of compulsive-like QuA drinking in C57BL/6J mice. We sought to determine whether compulsive-like alcohol drinking could be achieved following 2 or 3 weeks of Drinking-in-the-Dark (DID), whether it provides evidence for a robust model of compulsive-like alcohol drinking by inclusion of a water control group and use of a highly concentrated QuA solution, whether repeated QuA exposures alter compulsive-like drinking, and whether there are sex differences in compulsive-like alcohol drinking.

METHODS

Male and Female C57BL/6J mice were allowed free access to either 20% alcohol or tap water for 2 hours each day for approximately 3 weeks. After 2 or 3 weeks, the mice were given QuA (500 μM) and the effect of repeated QuA drinking sessions on compulsive-like alcohol drinking was assessed. 3-minute front-loading, 2 hour binge-drinking, and blood alcohol concentrations were determined.

RESULTS

Compulsive-like QuA drinking was achieved after 3 weeks, but not 2 weeks, of daily alcohol access as determined by alcohol history mice consuming significantly more QuA than water history mice and drinking statistically nondifferent amounts of QuA than nonadulterated alcohol at baseline. Thirty-minute front-loading of QuA revealed that alcohol history mice front-loaded significantly more QuA than water history mice, but still found the QuA solution aversive. Repeated QuA exposures did not alter these patterns, compulsive-like drinking did not differ by sex, and BACs for QuA drinking were at the level of a binge.

CONCLUSIONS

These data suggest that compulsive-like QuA drinking can be robustly achieved following 3 weeks of DID and male and female C57BL/6J mice do not differ in compulsive-like alcohol drinking.

cAMP-Fyn signaling in the dorsomedial striatum direct pathway drives excessive alcohol use

Fyn kinase in the dorsomedial striatum (DMS) of rodents plays a central role in mechanisms underlying excessive alcohol intake. The DMS is comprised of medium spiny neurons (MSNs) that project directly (dMSNs) or indirectly (iMSNs) to the substantia nigra. Here, we examined the cell-type specificity of Fyn's actions in alcohol use. First, we knocked down Fyn selectively in DMS dMSNs or iMSNs of mice and measured the level of alcohol consumption. We found that downregulation of Fyn in dMSNs, but not in iMSNs, reduces excessive alcohol but not saccharin intake. D1Rs are coupled to Gαs/olf, which activate cAMP signaling. To examine whether Fyn's actions are mediated through cAMP signaling, DMS dMSNs were infected with GαsDREADD, and the activation of Fyn signaling was measured following CNO treatment. We found that remote stimulation of cAMP signaling in DMS dMSNs activates Fyn and promotes the phosphorylation of the Fyn substrate, GluN2B. In contract, remote activation of GαsDREADD in DLS dMSNs did not alter Fyn signaling. We then tested whether activation of GαsDREADD in DMS dMSNs or iMSNs alters alcohol intake and observed that CNO-dependent activation of GαsDREADD in DMS dMSNs but not iMSNs increases alcohol but not saccharin intake. Finally, we examined the contribution of Fyn to GαsDREADD-dependent increase in alcohol intake, and found that systemic administration of the Fyn inhibitor, AZD0503 blocks GαsDREADD-dependent increase in alcohol consumption. Our results suggest that the cAMP-Fyn axis in the DMS dMSNs is a molecular transducer of mechanisms underlying the development of excessive alcohol consumption.

Region-dependent regulation of acute ethanol on γ oscillation in the rat hippocampal slices

BACKGROUND

Ethanol use disorders are a serious medical and public health problem in the world today. Acute ethanol intoxication can lead to cognitive dysfunction such as learning and memory impairment. Gamma oscillations (γ, 30-80 Hz) are synchronized rhythmic activity generated by population of neurons within local network, and closely related to learning and memory function. The hippocampus is a critical anatomic structure that supports learning and memory. On the grounds of structure and function, hippocampus can be divided into the intermediate (IH), the dorsal (DH), and ventral hippocampus (VH). The current study is the first to investigate the effects of acute ethanol on γ oscillations in these sub-regions of rat hippocampal slices.

METHODS

The sustained γ oscillations were induced by 200 nM kainate (KA) in the CA3c of IH, DH, and VH. When KA-induced γ oscillation reached the steady state, ethanol (50 mM or 100 mM) was applied and the effects of ethanol on γ oscillation power was measured in the slices sequentially sectioned from ventral to dorsal hippocampus of adult rats.

RESULTS

In the intermediate hippocampal slices, compared with control (KA only), ethanol (50 mM) caused 36.1 ± 3.9% decrease in γ power (p < 0.05, n = 10), while ethanol (100 mM) caused 55.3 ± 5.5% decrease in γ power (p < 0.001, n = 14). In the dorsal hippocampus, only ethanol (100 mM) caused 18.1 ± 8.6% decrease in γ power (p < 0.05, n = 12). However, in the ventral hippocampus, neither 50 mM nor 100 mM ethanol affected γ oscillation.

CONCLUSIONS

Our results demonstrate that ethanol may produce the differential suppression of γ oscillations in a dose-dependent manner in different sub-regions of hippocampus, suggesting that the modulation of ethanol on hippocampal γ oscillation is region-dependent.