Negative Affect and Excessive Alcohol Intake Incubate during Protracted Withdrawal from Binge-Drinking in Adolescent, But Not Adult, Mice

Home cage voluntary alcohol consumption increases binge drinking without affecting abstinence-related depressive-like behaviors or operant responding in crossed high alcohol-preferring mice (cHAPs)

Chronic alcohol consumption can lead to tolerance and escalation of drinking in humans and animals, but mechanisms underlying these changes are not fully characterized. Preclinical models can delineate which mechanisms are involved. The chronic intermittent ethanol exposure (CIE) procedure uses forced exposure to vaporized alcohol that elicits withdrawal and increased responding for alcohol in operant tasks in C57BL/6J inbred mice. Chronic two-bottle choice (2BC) drinking in the same strain elicits abstinent-related depression-like behavior, suggestive of allostatic changes. Selected lines such as crossed High Alcohol Preferring (cHAP) mice voluntarily drink to blood alcohol concentrations comparable to those attained in CIE and could be used to assess how alcohol affects these same endpoints without the confounds of involuntary vapor inhalation. In three experiments, we assess how 2BC drinking in cHAP mice affects abstinence-related depressive- and anxiety-like behavior, operant responding for alcohol, and binge consumption using drinking-in-the-dark (DID). We hypothesized that cHAPs with home-cage drinking experience would exhibit more depressive behavior after abstinence, increased responding for alcohol in the operant box, and increased DID intake. Of these, a drinking history increased DID intake in female cHAPs only and increased sucrose preference and intake following abstinence, but had no effects on operant responding or NSFT latency and FST immobility following forced abstinence. These results are consistent with recent findings using slice electrophysiology showing tolerance to alcohol's actions on the dorsolateral striatum following 2BC drinking in female, but not male cHAP mice. Overall, these data suggest that cHAPs may require procedures allowing rapid intoxication, such as DID, to demonstrate changes in alcohol's rewarding effects.

Voluntary adolescent alcohol exposure does not increase adulthood consumption of alcohol in multiple mouse and rat models

Adolescence is a period of increased risk taking, including increased alcohol and drug use. Multiple clinical studies report a positive relationship between adolescent alcohol consumption and risk of developing an alcohol use disorder (AUD) in adulthood. However, few preclinical studies have attempted to tease apart the biological contributions of adolescent alcohol exposure, independent of other social, environmental, and stress factors, and studies that have been conducted show mixed results. Here we use several adolescent voluntary consumption of alcohol models, conducted across three institutes and with two rodent species, to investigate the ramifications of adolescent alcohol consumption on adulthood alcohol consumption in controlled, pre-clinical environments. We consistently demonstrate a lack of increase in adulthood alcohol consumption. This work highlights that risks seen in both human datasets and other murine drinking models may be due to unique social and environmental factors - some of which may be unique to humans.

Epigenetic regulation of microglia and neurons by proinflammatory signaling following adolescent intermittent ethanol (AIE) exposure and in human AUD

Adolescent alcohol drinking is linked to high rates of adult alcohol problems and alcohol use disorder (AUD). The Neurobiology of Alcohol Drinking in Adulthood (NADIA) consortium adolescent intermittent ethanol (AIE) models adolescent binge drinking, followed by abstinent maturation to adulthood to determine the persistent AIE changes in neurobiology and behavior. AIE increases adult alcohol drinking and preference, increases anxiety and reward seeking, and disrupts sleep and cognition, all risks for AUD. In addition, AIE induces changes in neuroimmune gene expression in neurons and glia that alter neurocircuitry and behavior. HMGB1 is a unique neuroimmune signal released from neurons and glia by ethanol that activates multiple proinflammatory receptors, including Toll-like receptors (TLRs), that spread proinflammatory gene induction. HMGB1 expression is increased by AIE in rat brain and in post-mortem human AUD brain, where it correlates with lifetime alcohol consumption. HMGB1 activation of TLR increase TLR expression. Human AUD brain and rat brain following AIE show increases in multiple TLRs. Brain regional differences in neurotransmitters and cell types impact ethanol responses and neuroimmune gene induction. Microglia are monocyte-like cells that provide trophic and synaptic functions, that ethanol proinflammatory signals sensitize or "prime" during repeated drinking cycles, impacting neurocircuitry. Neurocircuits are differently impacted dependent upon neuronal-glial signaling. Acetylcholine is an anti-inflammatory neurotransmitter. AIE increases HMGB1-TLR4 signaling in forebrain, reducing cholinergic neurons by silencing multiple cholinergic defining genes through upregulation of RE-1 silencing factor (REST), a transcription inhibitor known to regulate neuronal differentiation. HMGB1 REST induction reduces cholinergic neurons in basal forebrain and cholinergic innervation of hippocampus. Adult brain hippocampal neurogenesis is regulated by a neurogenic niche formed from multiple cells. In vivo AIE and in vitro studies find ethanol increases HMGB1-TLR4 signaling and other proinflammatory signaling as well as reducing trophic factors, NGF, and BDNF, coincident with loss of the cholinergic synapse marker vChAT. These changes in gene expression-transcriptomes result in reduced adult neurogenesis. Excitingly, HMGB1 antagonists, anti-inflammatories, and epigenetic modifiers like histone deacetylase inhibitors restore trophic the neurogenesis. These findings suggest anti-inflammatory and epigenetic drugs should be considered for AUD therapy and may provide long-lasting reversal of psychopathology.

Nutritional ketosis as treatment for alcohol withdrawal symptoms in female C57BL/6J mice

Upon both acute and prolonged alcohol intake, the brain undergoes a metabolic shift associated with increased acetate metabolism and reduced glucose metabolism, which persists during abstinence, putatively leading to energy depletion in the brain. This study evaluates the efficacy of ketogenic treatments to rescue psychiatric and neurochemical alterations during long-term alcohol withdrawal. Female mice were intermittently exposed to alcohol vapor or air for three weeks, during which mice were introduced to either a ketogenic diet (KD), control diet supplemented with ketone ester (KE) or remained on control diet (CD). Withdrawal symptoms were assessed over a period of four weeks followed by re-exposure using several behavioral and biochemical tests. Alcohol-exposed mice fed CD displayed long-lasting depressive-like symptoms measured by saccharin preference and tail suspension, as well as decreased norepinephrine levels and serotonin turnover in the hippocampus. Both KD and KE rescued anhedonia for up to three weeks of abstinence. KD mice showed higher latency to first immobility in the tail suspension test, as well as lower plasma cholesterol levels. Our findings show promising effects of nutritional ketosis in ameliorating alcohol withdrawal symptoms in mice. KD seemed to better rescue these symptoms compared to KE.

Animal Models of Excessive Alcohol Consumption in Rodents

The development of animal models that demonstrate excessive levels of alcohol consumption has played an important role in advancing our knowledge about neurobiological underpinnings and environmental circumstances that engender such maladaptive behavior. The use of these preclinical models has also provided valuable opportunities for discovering new and novel therapeutic targets that may be useful in the treatment of alcohol use disorder (AUD). While no single model can fully capture the complexities of AUD, the goal is to develop animal models that closely approximate characteristics of heavy alcohol drinking in humans to enhance their translational value and utility. A variety of experimental approaches have been employed to produce the desired phenotype of interest-robust and reliable excessive levels of alcohol drinking. Here we provide an updated review of five animal models that are commonly used. The models entail procedural manipulations of scheduled access to alcohol (time of day, duration, frequency), periods of time when access to alcohol is withheld, and history of alcohol exposure. Specially, the models involve (a) scheduled access to alcohol, (b) scheduled periods of alcohol deprivation, (c) scheduled intermittent access to alcohol, (d) scheduled-induced polydipsia, and (e) chronic alcohol (dependence) and withdrawal experience. Each of the animal models possesses unique experimental features that engender excessive levels of alcohol consumption. Both advantages and disadvantages of each model are described along with discussion of future work to be considered in developing more optimal models. Ultimately, the validity and utility of these models will lie in their ability to aid in the discovery of new and novel potential therapeutic targets as well as serve as a platform to evaluate treatment strategies that effectively reduce excessive levels of alcohol consumption associated with AUD.

Neuropharmacological Evidence Implicating Drug-Induced Glutamate Receptor Dysfunction in Affective and Cognitive Sequelae of Subchronic Methamphetamine Self-Administration in Mice

Methamphetamine (MA) is a highly addictive drug, and MA use disorder is often comorbid with anxiety and cognitive impairment. These comorbid conditions are theorized to reflect glutamate-related neurotoxicity within the frontal cortical regions. However, our prior studies of MA-sensitized mice indicate that subchronic, behaviorally non-contingent MA treatment is sufficient to dysregulate glutamate transmission in mouse brain. Here, we extend this prior work to a mouse model of high-dose oral MA self-administration (0.8, 1.6, or 3.2 g/L; 1 h sessions × 7 days) and show that while female C57BL/6J mice consumed more MA than males, MA-experienced mice of both sexes exhibited some signs of anxiety-like behavior in a behavioral test battery, although not all effects were concentration-dependent. No MA effects were detected for our measures of visually cued spatial navigation, spatial learning, or memory in the Morris water maze; however, females with a history of 3.2 g/L MA exhibited reversal-learning deficits in this task, and mice with a history of 1.6 g/L MA committed more working-memory incorrect errors and relied upon a non-spatial navigation strategy during the radial-arm maze testing. Relative to naïve controls, MA-experienced mice exhibited several changes in the expression of certain glutamate receptor-related proteins and their downstream effectors within the ventral and dorsal areas of the prefrontal cortex, the hippocampus, and the amygdala, many of which were sex-selective. Systemic pretreatment with the mGlu1-negative allosteric modulator JNJ 162596858 reversed the anxiety-like behavior expressed by MA-experienced mice in the marble-burying test, while systemic pretreatment with NMDA or the NMDA antagonist MK-801 bi-directionally affected the MA-induced reversal-learning deficit. Taken together, these data indicate that a relatively brief history of oral MA is sufficient to induce some signs of anxiety-like behavior and cognitive dysfunction during early withdrawal that reflect, at least in part, MA-induced changes in the corticolimbic expression of certain glutamate receptor subtypes of potential relevance to treating symptoms of MA use disorder.

Adolescent social housing protects against adult emotional and cognitive deficits and alters the PFC and NAc transcriptome in male and female C57BL/6J mice

Introduction

Adolescence is a critical period in cognitive and emotional development, characterized by high levels of social interaction and increases in risk-taking behavior including binge drinking. Adolescent exposure to social stress and binge ethanol have individually been associated with the development of social, emotional, and cognitive deficits, as well as increased risk for alcohol use disorder. Disruption of cortical development by early life social stress and/or binge drinking may partly underlie these enduring emotional, cognitive, and behavioral effects. The study goal is to implement a novel neighbor housing environment to identify the effects of adolescent neighbor housing and/or binge ethanol drinking on (1) a battery of emotional and cognitive tasks (2) adult ethanol drinking behavior, and (3) the nucleus accumbens and prefrontal cortex transcriptome.

Methods

Adolescent male and female C57BL/6J mice were single or neighbor housed with or without access to intermittent ethanol. One cohort underwent behavioral testing during adulthood to determine social preference, expression of anxiety-like behavior, cognitive performance, and patterns of ethanol intake. The second cohort was sacrificed in late adolescence and brain tissue was used for transcriptomics analysis.

Results

As adults, single housed mice displayed decreased social interaction, deficits in the novel object recognition task, and increased anxiety-like behavior, relative to neighbor-housed mice. There was no effect of housing condition on adolescent or adult ethanol consumption. Adolescent ethanol exposure did not alter adult ethanol intake. Transcriptomics analysis revealed that adolescent housing condition and ethanol exposure resulted in differential expression of genes related to synaptic plasticity in the nucleus accumbens and genes related to methylation, the extracellular matrix and inflammation in the prefrontal cortex.

Discussion

The behavioral results indicate that social interaction during adolescence via the neighbor housing model may protect against emotional, social, and cognitive deficits. In addition, the transcriptomics results suggest that these behavioral alterations may be mediated in part by dysregulation of transcription in the frontal cortex or the nucleus accumbens.

The effects of alcohol on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking

The aim of the present study was to determine the effects of binge drinking on anxiety-like, depression-like, and social behavior. The participation of the corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) in these effects was also investigated. Therefore, male C57BL/6 mice were exposed to drinking in the dark, a classical animal model for binge drinking, and treated intracerebroventricularly (icv) with selective CRF1 antagonist antalarmin or selective CRF2 antagonist astressin2B, immediately or 24 h after binge drinking. After 30 min, the animals were investigated in an elevated plus-maze test and a forced swim test for anxiety-like and depression-like signs, respectively. In addition, mice were tested in a three-chamber social interaction arena for sociability and preference for social novelty. Immediately after binge drinking, mice exposed to alcohol expressed anxiolytic and antidepressant effects, which were reduced by astressin2B, but not antalarmin. Moreover, mice exposed to alcohol showed increased sociability and preference for social novelty immediately after binge drinking. In contrast, 24 h after binge drinking mice exposed to alcohol presented anxiety-like and depression-like signs, which were reversed by antalarmin, but not astressin2B. However, mice exposed to alcohol did not show any significant change in social interaction after 24 h. The present study demonstrates that alcohol exerts different effects on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking, and that the anxiolytic and antidepressant effects produced by binge drinking are mediated by CRF2, whereas the anxiety-like and depression-like signs observed the next day are promoted by CRF1.

Consequences of adolescent drug use

Substance use in adolescence is a known risk factor for the development of neuropsychiatric and substance use disorders in adulthood. This is in part due to the fact that critical aspects of brain development occur during adolescence, which can be altered by drug use. Despite concerted efforts to educate youth about the potential negative consequences of substance use, initiation remains common amongst adolescents world-wide. Additionally, though there has been substantial research on the topic, many questions remain about the predictors and the consequences of adolescent drug use. In the following review, we will highlight some of the most recent literature on the neurobiological and behavioral effects of adolescent drug use in rodents, non-human primates, and humans, with a specific focus on alcohol, cannabis, nicotine, and the interactions between these substances. Overall, consumption of these substances during adolescence can produce long-lasting changes across a variety of structures and networks which can have enduring effects on behavior, emotion, and cognition.

A subchronic history of binge-drinking elicits mild, age- and sex-selective, affective, and cognitive anomalies in C57BL/6J mice

Introduction

Alcohol abuse is a risk factor for affective and cognitive disorders, with evidence indicating that adolescent-onset excessive drinking can result in long-term deficiencies in emotional regulation and cognition, with females more susceptible to the negative emotional and cognitive consequences of excessive alcohol consumption. However, our prior examination of the interactions between sex and the age of drinking-onset indicated minimal signs of anxiety-like behavior during alcohol withdrawal, which may have related to the concurrent anxiety testing of male and female subjects.

Methods

The present study addressed this potential confound by assaying for alcohol withdrawal-induced negative affect separately in males and females and expanded our investigation to include measures of spatial and working memory.

Results

Following 14 days of drinking under modified Drinking-in-the-Dark procedures (10, 20, and 40% alcohol v/v; 2 h/day), adolescent and adult binge-drinking mice of both sexes exhibited, respectively, fewer and more signs of negative affect in the light-dark shuttle-box and forced swim tests than their water-drinking counterparts. Adolescent-onset binge-drinking mice also exhibited signs of impaired working memory early during radial arm maze training during early alcohol withdrawal. When tested in late (30 days) withdrawal, only adult female binge-drinking mice buried more marbles than their water-drinking counterparts. However, adolescent-onset binge-drinking mice exhibited poorer spatial memory recall in a Morris water maze.

Discussion

These findings indicate that a subchronic (14-day) binge-drinking history induces mild, age- and sex-selective, changes in negative affect and cognition of potential relevance to understanding individual variability in the etiology and treatment of alcohol abuse and alcohol use disorder.

Chronic intermittent ethanol exposure during adolescence produces sex- and age-dependent changes in anxiety and cognition without changes in microglia reactivity late in life

Binge-like ethanol exposure during adolescence has been shown to produce long lasting effects in animal models including anxiety-like behavior that can last into young adulthood and impairments in cognition that can last throughout most of the lifespan. However, little research has investigated if binge-like ethanol exposure during adolescence produces persistent anxiety-like behavior and concomitantly impairs cognition late in life. Furthermore, few studies have investigated such behavioral effects in both female and male rats over the lifespan. Finally, it is yet to be determined if binge-like ethanol exposure during adolescence alters microglia activation in relevant brain regions late in life. In the present study female and male adolescent rats were exposed to either 3.0 or 5.0 g/kg ethanol, or water control, in a chronic intermittent pattern before being tested in the elevated plus maze and open field task over the next ∼18 months. Animals were then trained in a spatial reference task via the Morris water maze before having their behavioral flexibility tested. Finally, brains were removed, sectioned and presumptive microglia activation determined using autoradiography for [3H]PK11195 binding. Males, but not females, displayed an anxiety-like phenotype initially following the chronic intermittent ethanol exposure paradigm which resolved in adulthood. Further, males but not females had altered spatial reference learning and impaired behavioral flexibility late in life. Conversely, [3H]PK11195 binding was significantly elevated in females compared to males late in life and the level of microglia activation interacted as a function of sex and brain regions, but there was no long-term outcome related to adolescent alcohol exposure. These data further confirm that binge-like ethanol exposure during adolescence produces alterations in behavior that can last throughout the lifespan. In addition, the data suggest that microglia activation late in life is not exacerbated by prior binge-like ethanol exposure during adolescence but the expression is sex- and brain region-dependent across the lifespan.

Adolescent binge drinking leads to long-lasting changes in cortical microcircuits in mice

Adolescent drug consumption has increased risks to the individual compared to consumption in adulthood, due to the likelihood of long-term and permanent behavioral and neurological adaptations. However, little is known about how adolescent alcohol consumption influences the maturation and trajectory of cortical circuit development. Here, we explore the consequences of adolescent binge drinking on somatostatin (SST) neuronal function in superficial layers of the prelimbic (PL) cortex in male and female SST-Ai9 mice. We find that adolescent drinking-in-the-dark (DID) produces sex-dependent increases in intrinsic excitability of SST neurons, with no change in overall SST cell number, persisting well into adulthood. While we did not find evidence of altered GABA release from SST neurons onto other neurons within the circuit, we found a complementary reduction in layer II/III pyramidal neuron excitability immediately after binge drinking; however, this hypoexcitability rebounded towards increased pyramidal neuron activity in adulthood in females, suggesting long-term homeostatic adaptations in this circuit. Together, this suggests that binge drinking during key developmental timepoints leads to permanent changes in PL microcircuitry function, which may have broad behavioral implications.

Targeting Persistent Changes in Neuroimmune and Epigenetic Signaling in Adolescent Drinking to Treat Alcohol Use Disorder in Adulthood

Studies universally find early age of drinking onset is linked to lifelong risks of alcohol problems and alcohol use disorder (AUD). Assessment of the lasting effect of drinking during adolescent development in humans is confounded by the diversity of environmental and genetic factors that affect adolescent development, including emerging personality disorders and progressive increases in drinking trajectories into adulthood. Preclinical studies using an adolescent intermittent ethanol (AIE) exposure rat model of underage binge drinking avoid the human confounds and support lifelong changes that increase risks. AIE increases adult alcohol drinking, risky decision-making, reward-seeking, and anxiety as well as reductions in executive function that all increase risks for the development of an AUD. AIE causes persistent increases in brain neuroimmune signaling high-mobility group box 1 (HMGB1), Toll-like receptor, receptor for advanced glycation end products, and innate immune genes that are also found to be increased in human AUD brain. HMGB1 is released from cells by ethanol, both free and within extracellular vesicles, that act on neurons and glia, shifting transcription and cellular phenotype. AIE-induced decreases in adult hippocampal neurogenesis and loss of basal forebrain cholinergic neurons are reviewed as examples of persistent AIE-induced pathology. Both are prevented and reversed by anti-inflammatory and epigenetic drugs. Findings suggest AIE-increased HMGB1 signaling induces the RE-1 silencing transcript blunting cholinergic gene expression, shifting neuronal phenotype. Inhibition of HMGB1 neuroimmune signaling, histone methylation enzymes, and galantamine, the cholinesterase inhibitor, both prevent and reverse AIE pathology. These findings provide new targets that may reverse AUD neuropathology as well as other brain diseases linked to neuroimmune signaling. SIGNIFICANCE STATEMENT: Adolescent underage binge drinking studies find that earlier adolescent drinking is associated with lifelong alcohol problems including high levels of lifetime alcohol use disorder (AUD). Preclinical studies find the underage binge drinking adolescent intermittent ethanol (AIE) model causes lasting changes in adults that increase risks of developing adult alcohol problems. Loss of hippocampal neurogenesis and loss of basal forebrain cholinergic neurons provide examples of how AIE-induced epigenetic and neuroimmune signaling provide novel therapeutic targets for adult AUD.

Alcohol-Drinking Under Limited-Access Procedures During Mature Adulthood Accelerates the Onset of Cognitive Impairment in Mice

A history of heavy drinking increases vulnerability to, and the severity of, Alzheimer’s disease (AD) and related dementias, with alcohol use disorder identified as the strongest modifiable risk factor for early-onset dementia. Heavy drinking has increased markedly in women over the past 10 years, particularly in mature adult women during the coronavirus (COVID-19) pandemic. This is concerning as women are more sensitive to many alcohol-related disease states, including AD and related dementias. Herein, we conducted two studies to determine if a 1-month period of binge drinking during mature adulthood (i.e., 5–9 months of age) impairs spatial and working memory to a greater extent in female vs. male C57BL/6J (B6J) mice. The anxiogenic and cognitive-impairing effects of binge drinking were also compared between mature adult and old B6J mice (18 months of age) in a third study. Throughout, females consumed more alcohol than males, indicating that a sex difference in binge drinking persists into old age. Despite the sex difference in intake, we detected no consistent sex difference in our measures of alcohol withdrawal-induced anxiety during a behavioral test battery. Although mature adult females exhibited more cognitive deficits than males, the precise outcome exhibiting a female-selective effect varied across studies. Old mice drank lower amounts of alcohol than mature adult mice, yet their blood ethanol concentrations (BECs) were within error of the 80 mg/dl criterion for binge drinking, indicative of an age-related slowing of alcohol metabolism. As expected, 18-month-old controls exhibited more signs of cognitive impairment than their 6-month-old counterparts, and binge drinking history impaired the Morris water maze performance of mice of both ages. In contrast, binge drinking history impaired the radial arm maze performance of 6-month-old mice only, and the extent of the impairment was comparable to the behavior exhibited by the older mice. We conclude from our studies that: (1) both biological sex and the age of drinking onset are subject factors that impact voluntary alcohol consumption by mice into old age; (2) binge drinking during later life elicits a negative affective state that is relatively sex-independent; (3) binge drinking during both mature adulthood and old age impairs spatial learning and memory; (4) binge drinking during mature adulthood accelerates deficits in working memory; and (5) mature adult females tend to exhibit more alcohol-induced cognitive impairments than males. If relevant to humans, these findings suggest that binge-like drinking by older adult men and women induces a negative affective state and cognitive decline, but that mature adult women, in particular, may be more sensitive to both the immediate and persistent cognitive-impairing effects of heavy drinking.

Chronic Intermittent Ethanol Administration during Adolescence Produces Sex Dependent Impairments in Behavioral Flexibility and Survivability

Chronic intermittent ethanol exposure during adolescence produces behavioral impairments and neurobiological changes that can last into young adulthood. One such behavioral impairment is reduced behavioral flexibility, a behavioral impairment that has been correlated with the risk for increased ethanol intake. In the current study, we investigated if chronic intermittent ethanol exposure during adolescence alters cognition, including behavioral flexibility, over a 22-month testing period. Female and male rats were treated with either 3.0 g/kg or 5.0 g/kg ethanol via gavage in a chronic intermittent fashion during adolescence and then tested every 4 to 5 months on a series of cognitive measures in the Morris water maze. Chronic intermittent ethanol selectively impaired behavioral flexibility in both female and male rats, although the pattern of results was different as a function of sex. In addition, female, but not male, rats were impaired in a short-term relearning test. Finally, male rats administered ethanol during adolescence were significantly more likely to not survive the 22-month experiment compared to female rats administered ethanol during adolescence. The current results demonstrate that adolescence is a unique period of development where chronic intermittent ethanol exposure produces long-lasting, selective cognitive impairments across the lifespan.

Assessing negative affect in mice during abstinence from alcohol drinking: Limitations and future challenges

Alcohol use disorder (AUD) is frequently comorbid with mood disorders, and these co-occurring neuropsychiatric disorders contribute to the development and maintenance of alcohol dependence and relapse. In preclinical models, mice chronically exposed to alcohol display anxiety-like and depressive-like behaviors during acute withdrawal and protracted abstinence. However, in total, results from studies using voluntary alcohol-drinking paradigms show variable behavioral outcomes in assays measuring negative affective behaviors. Thus, the main objective of this review is to summarize the literature on the variability of negative affective behaviors in mice after chronic alcohol exposure. We compare the behavioral phenotypes that emerge during abstinence across different exposure models, including models of alcohol and stress interactions. The complicated outcomes from these studies highlight the difficulties of assessing negative affective behaviors in mouse models designed for the study of AUD. We discuss new behavioral assays, comprehensive platforms, and unbiased machine-learning algorithms as promising approaches to better understand the interaction between alcohol and negative affect in mice. New data-driven approaches in the understanding of mouse behavior hold promise for improving the identification of mechanisms, cell subtypes, and neurocircuits that mediate negative affect. In turn, improving our understanding of the neurobehavioral basis of alcohol-associated negative affect will provide a platform to test hypotheses in mouse models that aim to improve the development of more effective strategies for treating individuals with AUD and co-occurring mood disorders.

Areas of Convergence and Divergence in Adolescent Social Isolation and Binge Drinking: A Review

Adolescence is a critical developmental period characterized by enhanced social interactions, ongoing development of the frontal cortex and maturation of synaptic connections throughout the brain. Adolescents spend more time interacting with peers than any other age group and display heightened reward sensitivity, impulsivity and diminished inhibitory self-control, which contribute to increased risky behaviors, including the initiation and progression of alcohol use. Compared to adults, adolescents are less susceptible to the negative effects of ethanol, but are more susceptible to the negative effects of stress, particularly social stress. Juvenile exposure to social isolation or binge ethanol disrupts synaptic connections, dendritic spine morphology, and myelin remodeling in the frontal cortex. These structural effects may underlie the behavioral and cognitive deficits seen later in life, including social and memory deficits, increased anxiety-like behavior and risk for alcohol use disorders (AUD). Although the alcohol and social stress fields are actively investigating the mechanisms through which these effects occur, significant gaps in our understanding exist, particularly in the intersection of the two fields. This review will highlight the areas of convergence and divergence in the fields of adolescent social stress and ethanol exposure. We will focus on how ethanol exposure or social isolation stress can impact the development of the frontal cortex and lead to lasting behavioral changes in adulthood. We call attention to the need for more mechanistic studies and the inclusion of the evaluation of sex differences in these molecular, structural, and behavioral responses.

Effects of adolescent alcohol exposure via oral gavage on adult alcohol drinking and co-use of alcohol and nicotine in Sprague Dawley rats

BACKGROUND

Preclinical models simulating adolescent substance use leading to increased vulnerability for substance use disorders in adulthood are needed. Here, we utilized a model of alcohol and nicotine co-use to assess adult addiction vulnerability following adolescent alcohol exposure.

METHODS

In Experiment 1, adolescent (PND30) male and female Sprague-Dawley rats received 25% ethanol (EtOH) or a control solution via oral gavage every 8 h, for 2 days. In young adulthood, animals were tested with a 2-bottle choice between H20% and 15% EtOH or 0.2% saccharin/15% EtOH, followed by co-use of oral Sacc/EtOH and operant-based i.v. nicotine (0.03 mg/kg/infusion) self-administration. In Experiment 2, adolescents received control gavage, EtOH gavage, or no-gavage, and were tested in young adulthood in a 2-bottle choice between H20% and 15% EtOH, Sacc/EtOH, or 0.2% saccharin.

RESULTS

In Experiment 1, the adolescent EtOH gavage reduced adult EtOH consumption in the 2-bottle choice, but not during the co-use phase. During co-use, Sacc/EtOH served as an economic substitute for nicotine. In Experiment 2, the control gavage increased adult EtOH drinking relative to the no-gavage control group, an effect that was mitigated in the EtOH gavage group. In both experiments, treatment group differences in EtOH consumption were largely driven by males.

CONCLUSIONS

EtOH administration via oral gavage in adolescence decreased EtOH consumption in adulthood without affecting EtOH and nicotine co-use. Inclusion of a no-gavage control in Experiment 2 revealed that the gavage procedure increased adult EtOH intake and that including EtOH in the gavage buffered against the effect.

Voluntary alcohol binge-drinking in adolescent C57Bl6 mice induces delayed appearance of behavioural defects in both males and females

Adolescence is a developmental period characterized by significant changes in brain architecture and behaviour. The immaturity of the adolescent brain is associated with heightened vulnerability to exogenous agents, including alcohol. Alcohol is the most consumed drug among teenagers, and binge‐drinking during adolescence is a major public health concern. Studies have suggested that adolescent alcohol exposure may interfere with the maturation of frontal brain regions and lead to long‐lasting behavioural consequences. In this study, by using a slightly modified version of the Drinking in the Dark paradigm, adolescent C57Bl6 mice reach high blood alcohol concentration after voluntary binge‐drinking. In order to assess short‐ and long‐term consequences of adolescent alcohol exposure (AAE), a battery of behavioural tests was performed during late adolescence and during adulthood. We showed that AAE had no short‐term effect on young mice behaviour but rather increased anxiety‐ and depressive‐like behaviours, as well as alcohol consumption during adulthood. Moreover, alcohol binge‐drinking during adolescence dramatically decreased recognition memory performances and behavioural flexibility in both adult males and females. Furthermore, we showed that voluntary consumption of alcohol during adolescence did not trigger any major activation of the innate immune system in the prefrontal cortex. Together, our data suggest that voluntary alcohol binge‐drinking in adolescent mice induces a delayed appearance of behavioural impairments in adulthood.

Age modifies the effect of ethanol on behavior: Investigations in adolescent, adult and aged rats

The number of older people is increasing in most if not all countries in the world. In addition, the amount of alcohol consumption in the aged population is increasing and the consumption pattern is often in a binge fashion. However, little is known if the effects of alcohol, either acute or chronic exposure, vary in the older population compared to younger populations. The current mini-review will provide an overview of the effects of acute and chronic ethanol exposure at three different periods of development: adolescent, adult and aged on multiple different commonly studied behaviors. The overall conclusion is that biological age of the subject is a critical factor in understanding the effects of ethanol across the lifespan.

Regulation of glutamate signaling in the extended amygdala by adolescent alcohol exposure

Adolescence is a critical period for brain development and behavioral maturation, marked by increased risk-taking behavior and the initiation of drug use. There are significant changes in gray matter volume and pruning of synapses along with a shift in excitatory to inhibitory balance which marks the maturation of cognition and decision-making. Because of ongoing brain development, adolescents are particularly sensitive to the detrimental effects of drugs, including alcohol, which can cause long-lasting consequences into adulthood. The extended amygdala is a region critically implicated in withdrawal and negative affect such as anxiety and depression. As negative affective disorders develop during adolescence, the effects of adolescent alcohol exposure on extended amygdala circuitry needs further inquiry. Here we aim to provide a framework to discuss the existing literature on the extended amygdala, the neuroadaptations which result from alcohol use, and the intersection of factors which contribute to the long-lasting effects of this exposure.

The persistent impact of adolescent binge alcohol on adult brain structural, cellular, and behavioral pathology: A role for the neuroimmune system and epigenetics

Adolescence is a critical neurodevelopmental window for maturation of brain structure, neurocircuitry, and glia. This development is sculpted by an individual's unique experiences and genetic background to establish adult level cognitive function and behavioral makeup. Alcohol abuse during adolescence is associated with an increased lifetime risk for developing an alcohol use disorder (AUD). Adolescents participate in heavy, episodic binge drinking that causes persistent changes in neurocircuitry and behavior. These changes may underlie the increased risk for AUD and might also promote cognitive deficits later in life. In this chapter, we have examined research on the persistent effects of adolescent binge-drinking both in humans and in rodent models. These studies implicate roles for neuroimmune signaling as well as epigenetic reprogramming of neurons and glia, which create a vulnerable neuroenvironment. Some of these changes are reversible, giving hope for future treatments to prevent many of the long-term consequences of adolescent alcohol abuse.

BNST specific mGlu5 receptor knockdown regulates sex-dependent expression of negative affect produced by adolescent ethanol exposure and adult stress

Adolescent alcohol use is one of the strongest predictors for the development of an alcohol use disorder (AUD). Notably, this period of risk coincides with the development of affective disorders, which disproportionately impact and drive problematic drinking behavior in women. Stress is a particularly salient factor that drives relapse during periods of abstinence. Previous work in our lab has shown that adolescent intermittent ethanol vapor (AIE) produces sex-dependent changes in glutamatergic activity in the bed nucleus of the stria terminalis (BNST) and behavioral outcomes following acute restraint stress in adulthood. In females, AIE disrupts group 1 metabotropic glutamate (mGlu1/5) receptor activity and enhances anhedonia-like behavior. The current study site-specifically knocked down mGlu5 receptors in the BNST of male and female Grm5loxp mice, exposed them to AIE, and observed the interaction of AIE and stress on negative affect-like behaviors in adulthood. These negative affect-like behaviors included the novelty-induced hypophagia task following acute restraint stress, open field activity, and contextual fear conditioning. Overall, we replicated our previous findings that AIE enhanced anhedonia-like activity in the novelty-induced hypophagia task in females and fear acquisition in males. The primary effect of BNST-mGlu5 receptor knockdown was that it independently enhanced anhedonia-like activity in females. Correlation analyses revealed that behavior in these paradigms showed poor interdependence. These results indicate that preclinical models of negative affective-like states encompass distinct features that may have independent, clinically relevant mechanisms. Further, modulating mGlu5 receptors is a prospective treatment target for females experiencing anhedonic-like states that make them susceptible to alcohol relapse.

Hnrnph1 is a novel regulator of alcohol reward

BACKGROUND

Hnrnph1 is a validated quantitative trait gene for methamphetamine behavioral sensitivity that encodes for heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1). This RNA-binding protein is involved in all stages of RNA metabolism that impacts mesocorticolimbic dopamine neurotransmission to influence addiction-related behavior.

METHODS

We characterized the alcohol behavioral phenotypes of mice heterozygous for a deletion in the first coding exon of Hnrnph1 (Hnrnph1+/-). We examined alcohol intake under both continuous- and limited-access procedures, as well as alcohol-induced place-conditioning. Follow-up studies examined genotypic differences in the psychomotor-activating and sedative-hypnotic effects of acute and repeated alcohol, and a behavioral test battery was employed to determine the effects of Hnrnph1 deletion on the manifestation of negative affect during alcohol withdrawal.

RESULTS

Relative to wild-type (WT) controls, Hnrnph1+/- males exhibited blunted intake of high alcohol concentrations under both drinking procedures. Hnrnph1 deletion did not impact the conditioned rewarding properties of low-dose alcohol, but reversed the conditioned place-aversion elicited by higher alcohol doses (2 and 4 g/kg), with more robust effects in male versus female mice. No genotypic differences were observed for alcohol-induced locomotor activity. Hnrnph1+/- mice exhibited a modest increase in sensitivity to alcohol's sedative-hypnotic effects, but did not differ from WT mice with regard to tolerance to alcohol's sedative-hypnotic effects or alcohol metabolism, Inconsistent effects of Hnrnph1 deletion were observed in models for withdrawal-induced negative affect.

CONCLUSIONS

These data identify Hnrnph1 as a novel, male-selective, driver of alcohol consumption and high-dose alcohol aversion that is potentially relevant to the neurobiology of alcohol abuse and alcoholism.

Adolescent low-dose ethanol drinking in the dark increases ethanol intake later in life in C57BL/6J, but not DBA/2J mice

Alcohol is the most widely used and abused drug among youth in the United States. Youths aged 12-20 years old drink almost 11% of all alcohol consumed in the United States, and typically these young people are consuming alcohol in the form of binge drinking. Particularly concerning is that the risk of developing an alcohol use disorder over their lifetime increases the younger one begins to drink. Here we investigated the impact of ethanol drinking in early adolescence on adult ethanol intake using C57BL/6J and DBA/2J mice. We modeled low-dose drinking in adolescent mice using a modified Drinking in the Dark (DID) model where the total ethanol intake during adolescence was similar between the strains to specifically ask whether low-dose ethanol exposure in the high-alcohol preferring C57BL/6J strain will also lead to increased ethanol intake in adulthood. Our results show that low-dose ethanol drinking in early adolescence dramatically increases adult intake, but only in the alcohol-preferring C57BL/6J strain. Early adolescent ethanol exposure had no effect on ethanol intake in the alcohol-nonpreferring DBA/2J mice. These data add to the growing evidence that low-dose ethanol exposures, below the pharmacologically relevant dose, can also contribute to increased drinking in adulthood, but the effect may be influenced by genetic background.

Combined Effects of Repetitive Mild Traumatic Brain Injury and Alcohol Drinking on the Neuroinflammatory Cytokine Response and Cognitive Behavioral Outcomes

The relationship between alcohol consumption and traumatic brain injury (TBI) often focuses on alcohol consumption increasing the likelihood of incurring a TBI, rather than alcohol use outcomes after TBI. However, patients without a history of an alcohol use disorder can also show increased problem drinking after single or multiple TBIs. Alcohol and mild TBI share diffuse deleterious neurological impacts and cognitive impairments; therefore, the purpose of these studies was to determine if an interaction on brain and behavior outcomes occurs when alcohol is consumed longitudinally after TBI. To examine the impact of mild repetitive TBI (rmTBI) on voluntary alcohol consumption, mice were subjected to four mild TBI or sham procedures over a 2 week period, then offered alcohol (20% v/v) for 2 weeks using the two-bottle choice, drinking in the dark protocol. Following the drinking period, mice were evaluated for neuroinflammatory cytokine response or tested for cognitive and behavioral deficits. Results indicate no difference in alcohol consumption or preference following rmTBI as compared to sham; however, increases in the neuroinflammatory cytokine response due to alcohol consumption and some mild cognitive behavioral deficits after rmTBI and alcohol consumption were observed. These data suggest that the cytokine response to alcohol drinking and rmTBI + alcohol drinking is not necessarily aggregate, but the combination does result in an exacerbation of cognitive behavioral outcomes.

Non-selective orexin-receptor antagonist attenuates stress-re-stress-induced core PTSD-like symptoms in rats: Behavioural and neurochemical analyses

Post-traumatic stress disorder (PTSD) is a psychological disorder affecting many around the world. Growing evidence suggests that orexin-A is involved in the pathophysiology of depression and panic anxiety disorder. However, the role of orexin-A in PTSD remains unclear. Therefore, pharmacological manipulation of orexin-A can be a potential approach for the treatment of PTSD. Male Wistar rats were subjected to stress re-stress (SRS) by restraining them for 2 h followed by foot shock (FS) and halothane exposure on day-2 (D-2). Then the rats were weekly exposed to FS as re-stress cue . Suvorexant, an orexin antagonist (10, 20 and 30 mg/kg p.o.) and paroxetine (10 mg/kg p.o.) were administered from D-8 to D-32. Plasma and cerebrospinal fluid (CSF) were collected for corticosterone and orexin-A measurement. The analysis of serotonin and corticotropin-releasing factor receptor-1 (CRF-R1) were performed in the amygdalar tissue. SRS-induced PTSD-like symptoms like fear response, anxiety-like behaviour and hypocorticosteronism were attenuated by suvorexant and paroxetine. Interestingly, SRS exposed rats showed activation of orexin-A and serotonergic systems, which were also attenuated by suvorexant. Additionally, suvorexant ameliorated the extrahypothalamic induced upregulation of CRH-R1 in SRS-exposed rats. Therefore, orexin-A may be considered as a neurochemical-marker for PTSD and suvorexant alleviated PTSD-like symptoms through modulating orexinergic, serotonergic and neuroendocrine systems.

Absence of effects of intermittent access to alcohol on negative affective and anxiety-like behaviors in male and female C57BL/6J mice

Alcohol use disorder is highly comorbid with other neuropsychiatric disorders such as depression and anxiety. Importantly, women and men are affected differentially by heavy drinking, with women experiencing longer negative affective states after intoxication and increased likelihood to present with comorbid mood or anxiety disorders. In rodents, several studies using different alcohol administration models have shown the development of depressive-like or anxiety-like phenotypes that emerge during abstinence. In this study, we compared the emergence of negative affective behaviors during abstinence from 7 weeks of two-bottle choice intermittent access to 20% alcohol in male and female C57BL/6J mice, a drinking paradigm little studied in this context. Half of the mice were tested 24 hours into abstinence on the elevated zero maze and 19-20 days into abstinence in a novel object in the home cage encounter test. The other half of the mice were tested 27-28 days into abstinence with the novelty-suppressed feeding test. As expected, females drank more than males across the 7 weeks of access to alcohol. Drinking history did not affect performance on these tasks, with the exception of increasing the number of open arm entries on the elevated zero maze. Interestingly, in alcohol-naïve mice, females showed fewer anxiety-like behaviors than males in the elevated zero maze and the novelty-suppressed feeding test. Our results suggest that the intermittent access model does not reliably induce negative affective behaviors on these tasks, and that behavior in female and male mice differs across these tests. Rather, intermittent alcohol drinking may induce a mild form of behavioral disinhibition. Thus, the model of alcohol access is a critical factor in determining the appearance of behavioral disturbances that emerge during abstinence.

Sex Differences in the Neurobiology of Alcohol Use Disorder

Sex differences may play a critical role in modulating how chronic or heavy alcohol use impacts the brain to cause the development of alcohol use disorder (AUD). AUD is a multifaceted and complex disorder driven by changes in key neurobiological structures that regulate executive function, memory, and stress. A three-stage framework of addiction (binge/intoxication; withdrawal/negative affect; preoccupation/anticipation) has been useful for conceptualizing the complexities of AUD and other addictions. Initially, alcohol drinking causes short-term effects that involve signaling mediated by several neurotransmitter systems such as dopamine, corticotropin releasing factor, and glutamate. With continued intoxication, alcohol leads to dysfunctional behaviors that are thought to be due in part to alterations of these and other neurotransmitter systems, along with alterations in neural pathways connecting prefrontal and limbic structures. Using the three-stage framework, this review highlights examples of research examining sex differences in drinking and differential modulation of neural systems contributing to the development of AUD. New insights addressing the role of sex differences in AUD are advancing the field forward by uncovering the complex interactions that mediate vulnerability.

Affective Disruption During Forced Ethanol Abstinence in C57BL/6J and C57BL/6NJ Mice

BACKGROUND

In alcohol-dependent individuals, acute alcohol withdrawal results in severe physiological disruption, including potentially lethal central nervous system hyperexcitability. Although benzodiazepines successfully mitigate such symptoms, this treatment does not significantly reduce recidivism rates in postdependent individuals. Instead, persistent affective disturbances that often emerge weeks to months after initial detoxification appear to play a significant role in relapse risk; however, it remains unclear whether genetic predispositions contribute to their emergence, severity, and/or duration. Interestingly, significant genotypic and phenotypic differences have been observed among distinct C57BL/6 (B6) substrains, and, in particular, C57BL/6J (B6J) mice have been found to reliably exhibit higher voluntary ethanol (EtOH) intake and EtOH preference compared to several C57BL/6N (B6N)-derived substrains. To date, however, B6 substrains have not been directly compared on measures of acute withdrawal severity or affective-behavioral disruption during extended abstinence.

METHODS

Male and female B6J and B6NJ mice were exposed to either a 7-day chronic intermittent EtOH vapor (CIE) protocol or to ordinary room air in inhalation chambers. Subsequently, blood EtOH concentrations and handling-induced convulsions were evaluated during acute withdrawal, and mice were then tested weekly for affective behavior on the sucrose preference test, light-dark box test, and forced swim test throughout 4 weeks of (forced) abstinence.

RESULTS

Despite documented differences in voluntary EtOH intake between these substrains, we found little evidence for substrain differences in either acute withdrawal or long-term abstinence between B6J and B6NJ mice.

CONCLUSIONS

In B6J and B6NJ mice, both the acute and long-term sequelae of EtOH withdrawal are dependent on largely nonoverlapping gene networks relative to those underlying voluntary EtOH drinking.

Leveraging Neural Networks in Preclinical Alcohol Research

Alcohol use disorder is a pervasive healthcare issue with significant socioeconomic consequences. There is a plethora of neural imaging techniques available at the clinical and preclinical level, including magnetic resonance imaging and three-dimensional (3D) tissue imaging techniques. Network-based approaches can be applied to imaging data to create neural networks that model the functional and structural connectivity of the brain. These networks can be used to changes to brain-wide neural signaling caused by brain states associated with alcohol use. Neural networks can be further used to identify key brain regions or neural "hubs" involved in alcohol drinking. Here, we briefly review the current imaging and neurocircuit manipulation methods. Then, we discuss clinical and preclinical studies using network-based approaches related to substance use disorders and alcohol drinking. Finally, we discuss how preclinical 3D imaging in combination with network approaches can be applied alone and in combination with other approaches to better understand alcohol drinking.

Incubation of Negative Affect during Protracted Alcohol Withdrawal Is Age-, but Not Sex-Selective

A prior history of excessive drinking induces a negative affective state in both humans and laboratory rodents, the manifestation of which varies with the age of drinking-onset. In adolescent male mice, negative affect incubates over the course of a 30-day alcohol withdrawal period. In contrast, the negative affect exhibited by adult male mice is robust at 1 day withdrawal, but dissipates with the passage of time. As females tend to consume more alcohol than males, we aimed to explore the affective disturbances exhibited by adolescent and adult C57BL/6J mice of both sexes during more protracted alcohol withdrawal and to relate any behavioral changes observed to plasma corticosterone levels as a biochemical index of stress. Male and female, adolescent and adult, mice were subjected to 14 consecutive days of binge alcohol-drinking using a multi-bottle-choice Drinking-in-the-Dark (DID) procedure (5, 10, 20 and 40% v/v). Age- and sex-matched control mice consumed water only. On either withdrawal day 1 or 70, subgroups of animals were subjected a to 1-day behavioral test battery that included the light–dark box shuttle test, marble-burying test, and Porsolt forced swim test. As expected, adolescent mice consumed more alcohol than adults and females consumed more alcohol than males. However, despite binge-like levels of alcohol consumption, we detected relatively few signs of alcohol withdrawal-induced negative affect and there was no correlation between affective behavior and circulating corticosterone levels. We discuss these findings within the context of our published work, highlighting procedural differences that might account for the relatively weak effect of binge-drinking history upon anxiety and depressive-like behavior observed herein.

Adolescent Ethanol Exposure: Anxiety-Like Behavioral Alterations, Ethanol Intake, and Sensitivity

Adolescence is a developmental period associated with rapid age-specific physiological, neural, and hormonal changes. Behaviorally, human adolescents are characterized by age-typical increases in novelty-seeking and risk-taking, including the frequent initiation of alcohol and drug use. Alcohol use typically begins during early adolescence, and older adolescents often report high levels of alcohol consumption, commonly referred to as high-intensity drinking. Early-onset and heavy drinking during adolescence are associated with an increased risk of developing alcohol use disorders later in life. Yet, long-term behavioral consequences of adolescent alcohol use that might contribute to excessive drinking in adulthood are still not well understood. Recent animal research, however, using different exposure regimens and routes of ethanol administration, has made substantial progress in identifying the consequences of adolescent ethanol exposure that last into adulthood. Alterations associated with adolescent ethanol exposure include increases in anxiety-like behavior, impulsivity, risk-taking, and ethanol intake, although the observed alterations differ as a function of exposure regimens and routes of ethanol administration. Rodent studies have also shown that adolescent ethanol exposure produces alterations in sensitivity to ethanol, with these alterations reminiscent of adolescent-typical ethanol responsiveness. The goal of this mini-review article is to summarize the current state of animal research, focusing on the long-term consequences related to adolescent ethanol exposure, with a special emphasis on the behavioral alterations and changes to ethanol sensitivity that can foster high levels of drinking in adulthood.

Allosteric modulation of metabotropic glutamate receptors in alcohol use disorder: Insights from preclinical investigations

Metabotropic glutamate (mGlu) receptors are family C G protein-coupled receptors (GPCRs) that modulate neuronal excitability and synaptic transmission throughout the nervous system. Owing to recent advances in development of subtype-selective allosteric modulators of mGlu receptors, individual members of the mGlu receptor family have been proposed as targets for treating a variety of neurological and psychiatric disorders, including substance use disorders. In this chapter, we highlight preclinical evidence that allosteric modulators of mGlu receptors could be useful for reducing alcohol consumption and preventing relapse in alcohol use disorder (AUD). We begin with an overview of the preclinical models that are used to study mGlu receptor involvement in alcohol-related behaviors. Alcohol exposure causes adaptations in both expression and function of various mGlu receptor subtypes, and pharmacotherapies aimed at reversing these adaptations have the potential to reduce alcohol consumption and seeking. Positive allosteric modulators (PAMs) of mGlu₂ and negative allosteric modulators of mGlu₅ show particular promise for reducing alcohol intake and/or preventing relapse. Finally, this chapter discusses important considerations for translating preclinical findings toward the development of clinically useful drugs, including the potential for PAMs to avoid tolerance issues that are frequently observed with repeated administration of GPCR agonists.

Nucleus accumbens shell small conductance potassium channels underlie adolescent ethanol exposure-induced anxiety

Alcohol use typically begins in adolescence, increasing the likelihood of adult mental disorders such as anxiety. However, the cellular mechanisms underlying the consequences of adolescent alcohol exposure as well as the behavioral consequences remain poorly understood. We examined the effects of adolescent or adult chronic intermittent ethanol (CIE) exposure on intrinsic excitability of striatal medium-sized spiny neurons (MSNs) and anxiety levels. Rats underwent one of the following procedures: (1) light-dark transition (LDT) and open-field (OF) tests to evaluate anxiety levels and general locomotion; (2) whole-cell patch clamp recordings and biocytin labeling to assess excitability of striatal MSNs, as well as morphological properties; and (3) western blot immunostaining to determine small conductance (SK) calcium-activated potassium channel protein levels. Three weeks, but not 2 days, after CIE treatment, adolescent CIE-treated rats showed shorter crossover latency from the light to dark side in the LDT test and higher MSN excitability in the nucleus accumbens shell (NAcS). Furthermore, the amplitude of the medium afterhyperpolarization (mAHP), mediated by SK channels, and SK3 protein levels in the NAcS decreased concomitantly. Finally, increased anxiety levels, increased excitability, and decreased amplitude of mAHP of NAcS MSNs were reversed by SK channel activator 1-EBIO and mimicked by the SK channel blocker apamin. Thus, adolescent ethanol exposure increases adult anxiety-like behavior by downregulating SK channel function and protein expression, which leads to an increase of intrinsic excitability in NAcS MSNs. SK channels in the NAcS may serve as a target to treat adolescent alcohol binge exposure-induced mental disorders, such as anxiety in adulthood.

Mechanisms of Persistent Neurobiological Changes Following Adolescent Alcohol Exposure: NADIA Consortium Findings

The Neurobiology of Adolescent Drinking in Adulthood (NADIA) Consortium has focused on the impact of adolescent binge drinking on brain development, particularly on effects that persist into adulthood. Adolescent binge drinking is common, and while many factors contribute to human brain development and alcohol use during adolescence, animal models are critical for understanding the specific consequences of alcohol exposure during this developmental period and the underlying mechanisms. Using adolescent intermittent ethanol (AIE) exposure models, NADIA investigators identified long-lasting AIE-induced changes in adult behavior that are consistent with observations in humans, such as increased alcohol drinking, increased anxiety (particularly social anxiety), increased impulsivity, reduced behavioral flexibility, impaired memory, disrupted sleep, and altered responses to alcohol. These behavioral changes are associated with multiple molecular, cellular, and physiological alterations in the brain that persist long after AIE exposure. At the molecular level, AIE results in long-lasting changes in neuroimmune/trophic factor balance and epigenetic-microRNA (miRNA) signaling across glia and neurons. At the cellular level, AIE history is associated in adulthood with reduced expression of cholinergic, serotonergic, and dopaminergic neuron markers, attenuated cortical thickness, decreased neurogenesis, and altered dendritic spine and glial morphology. This constellation of molecular and cellular adaptations to AIE likely contributes to observed alterations in neurophysiology, measured by synaptic physiology, EEG patterns, and functional connectivity. Many of these AIE-induced brain changes replicate findings seen in postmortem brains of humans with alcohol use disorder (AUD). NADIA researchers are now elucidating mechanisms of these adaptations. Emerging data demonstrate that exercise, antiinflammatory drugs, anticholinesterases, histone deacetylase inhibitors, and other pharmacological compounds are able to prevent (administered during AIE) and/or reverse (given after AIE) AIE-induced pathology in adulthood. These studies support hypotheses that adolescent binge drinking increases risk of adult hazardous drinking and influences brain development, and may provide insight into novel therapeutic targets for AIE-induced neuropathology and AUDs.

Metabotropic Glutamate Receptor Subtype 5 in Alcohol-Induced Negative Affect

Allosteric modulators of metabotropic glutamate 5 receptors (mGlu5 receptors) have been identified as a promising treatment to independently alleviate both negative affective states and ethanol-seeking and intake. However, these conditions are often comorbid and might precipitate one another. Acute and protracted ethanol withdrawal can lead to negative affective states. In turn, these states are primary drivers of alcohol relapse, particularly among women. The current review synthesizes preclinical studies that have observed the role of mGlu5 receptor modulation in negative affective states following ethanol exposure. The primary behavioral assays discussed are ethanol-seeking and intake, development and extinction of ethanol-associated cues and contexts, behavioral despair, and anxiety-like activity. The work done to-date supports mGlu5 receptor modulation as a promising target for mediating negative affective states to reduce ethanol intake or prevent relapse. Limitations in interpreting these data include the lack of models that use alcohol-dependent animals, limited use of adolescent and female subjects, and a lack of comprehensive evaluations of negative affective-like behavior.

DID it or DIDn't it? Exploration of a failure to replicate binge-like alcohol-drinking in C57BL/6J mice

We previously reported that commercially-sourced C57BL/6J (B6) male mice with a history of adult-onset binge-drinking exhibit anxiety-like behavior in early withdrawal, while the negative affective state incubates during protracted withdrawal in adolescent-onset binge-drinking males. As the results of such studies are potentially confounded by age-related differences in reactivity to environmental stress, we employed a 2-bottle-choice DID procedure (20 and 40% alcohol; 20 min habituation to the drinking cage) to examine the effects of binge-drinking on negative affect in male and female, adult and adolescent, B6 mice from our university colony. Unexpectedly, the mice in the initial experiment exhibited very low alcohol intake, with little sign of withdrawal-induced negative affect. This failure to replicate prompted us to examine how the duration of drinking cage habituation, the number of alcohol concentrations presented and the animal source might influence the propensity to binge-drink. Herein, we show that both male and female adult mice from our colony will binge-drink when allowed 45 min to habituate to the drinking cages, irrespective of whether mice are offered a choice between 2, 3 or 4 alcohol concentrations. Further, when drinking under 4-bottle-choice procedures (5, 10, 20 and 40% alcohol), adult-onset binge-drinking females exhibit robust negative affect in early withdrawal akin to that reported previously for adult males; however, the negative affective state persists for at least 30 days into withdrawal. Also unlike males, adolescent-onset binge-drinking females exhibit some signs of negative affect, as well as potentiated alcohol intake, in early withdrawal, which persist into later withdrawal. These latter data suggest that the age-related differences in the temporal patterning of the negative affective state produced by alcohol withdrawal may vary as a function of sex, which may have implications for understanding sex differences in the etiology of affective disorders and alcoholism co-morbidity.

mGlu5 Receptor Blockade Within the Nucleus Accumbens Shell Reduces Behavioral Indices of Alcohol Withdrawal-Induced Anxiety in Mice

Withdrawal from binge-drinking increases negative affect, coinciding with increased expression of the metabotropic glutamate receptor 5 (mGlu5) within the shell of the nucleus accumbens (AcbSh). Supporting a causal-effect relationship, systemic treatment with the mGlu5 receptor antagonist MTEP [3-((2-Methyl-4-thiazolyl)ethynyl)pyridine] is anxiolytic in binge-drinking adult and adolescent mice. Here, we employed neuropharmacological approaches to examine the functional relevance of AcbSh mGlu5 for behavioral indices of alcohol withdrawal-induced hyper-anxiety. Adult (PND 56) and adolescent (PND 28) male C57BL/6J mice consumed alcohol under modified Drinking-in-the-Dark procedures (10, 20, and 40% alcohol v/v) for 14 days. At an alcohol withdrawal time-point when mice manifest robust behavioral signs of hyper-anxiety (1 and 28 days withdrawal for adults and adolescents, respectively), mice were infused intra-AcbSh with 0, 1 or 10 μg MTEP and then affect was assayed in the light-dark shuttle box, marble-burying and forced swim tests. Brain tissue was collected to evaluate changes in Egr1 (early growth response protein 1) induction to index AcbSh neuronal activity. As expected, alcohol-experienced mice exhibited behavioral signs of hyper-emotionality. The anxiolytic effects of intra-AchSh MTEP were modest, but dose-dependent, and varied with age of drinking-onset. In adult-onset mice, only the 1 μg MTEP dose reduced withdrawal-induced hyper-anxiety, whereas only the higher dose was effective in adolescent-onset animals. MTEP reduced Egr1 expression within the AcbSh, irrespective of alcohol drinking history or age of drinking-onset. However, only the high MTEP dose reduced Egr1 expression in adolescent-onset binging mice. These results implicate AcbSh mGlu5 in modulating alcohol withdrawal-induced negative affect and suggest age differences in the neurobiological effects of alcohol withdrawal and behavioral responsiveness to mGlu5 blockade within the AcbSh.

Ketamine administration during a critical period after forced ethanol abstinence inhibits the development of time-dependent affective disturbances

Forced abstinence from chronic two bottle-choice ethanol drinking produces the development of negative affective states in female C57BL/6J mice. We previously reported that this disrupted behavior is acutely reversed by administration of ketamine 30 min-prior to testing. Here we assessed whether ketamine can be used as an inoculant against the development of abstinence- dependent affective disturbances. In parallel, we examined the impact of ketamine administration on long-term potentiation (LTP) in the bed nucleus of the stria terminalis (BNST), a region implicated in affective disturbances. We administered ketamine (3 mg/kg i.p.) to female C57BL/6J mice with a history of chronic ethanol drinking at either the onset, two, or 6 days- post-abstinence and observed its impact on affective behavior in the elevated plus maze (EPM), the Novelty Suppressed Feeding Test (NSFT), and the Forced Swim Test (FST). In addition, we assessed BNST synaptic plasticity with field potential electrophysiology two to 3 weeks into abstinence. We found that early abstinence was associated with disrupted behavior on the EPM. Ketamine administered at the onset of forced abstinence prevented both the deficit in early EPM behavior, and the delayed deficits in NSFT and FST. However, ketamine administered either two or 6 days post-abstinence failed to prevent the abstinence-induced affective disturbances. To begin to explore potential alterations in neural circuit activity that accompanies these actions of ketamine, we assessed the impact of ketamine administration at the onset of forced abstinence and measured LTP induction in the BNST. We find that early ketamine administration persistently increased the capacity for LTP within the BNST. These findings suggest a critical period at the onset of forced abstinence in which ketamine inoculation can prevent the development of affective disturbances, in part by enhancing plasticity within the BNST.

mGlu5-dependent modulation of anxiety during early withdrawal from binge-drinking in adult and adolescent male mice

Binge alcohol-drinking elicits symptoms of negative affect such as anxiety upon cessation, which is a source of negative reinforcement for perpetuating this pattern of alcohol abuse. Binge-induced anxiety during early (24 h) withdrawal is associated with increased expression of metabotropic glutamate receptor 5 (mGlu5) within the nucleus accumbens shell (AcbSh) of adult male mice, but was unchanged in anxiety-resilient adolescents. Herein, we determined the role of mGlu5 signaling in withdrawal-induced anxiety via pharmacological manipulation using the mGlu5 negative allosteric modulator MTEP and the positive allosteric modulator CDPPB. Adult (PND 56) and adolescent (PND 28) male C57BL/6J mice binge-drank for 14 days under 3-bottle-choice procedures for 2 h/day; control animals drank water only. Approximately 24 h following the final alcohol presentation, animals were treated with 30 mg/kg IP MTEP, CDPPB, or vehicle and then tested, thirty minutes later, for behavioral signs of anxiety. Vehicle-treated binge-drinking adults exhibited hyperanxiety in all paradigms, while vehicle-treated binge-drinking adolescents did not exhibit withdrawal-induced anxiety. In adults, 30 mg/kg MTEP decreased alcohol-induced anxiety across paradigms, while 3 mg/kg MTEP was anxiolytic in adult water controls. CDPPB was modestly anxiogenic in both alcohol- and water-drinking mice. Adolescent animals showed minimal response to either CDPPB or MTEP, suggesting that anxiety in adolescence may be mGlu5-independent. These results demonstrate a causal role for mGlu5 in withdrawal-induced anxiety in adults and suggest age-related differences in the behavioral pharmacology of the negative reinforcing properties of alcohol.

Effects of adolescent alcohol consumption on the brain and behaviour

Per occasion, alcohol consumption is higher in adolescents than in adults in both humans and laboratory animals, with changes in the adolescent brain probably contributing to this elevated drinking. This Review examines the contributors to and consequences of the use of alcohol in adolescents. Human adolescents with a history of alcohol use differ neurally and cognitively from other adolescents; some of these differences predate the commencement of alcohol consumption and serve as potential risk factors for later alcohol use, whereas others emerge from its use. The consequences of alcohol use in human adolescents include alterations in attention, verbal learning, visuospatial processing and memory, along with altered development of grey and white matter volumes and disrupted white matter integrity. The functional consequences of adolescent alcohol use emerging from studies of rodent models of adolescence include decreased cognitive flexibility, behavioural inefficiencies and elevations in anxiety, disinhibition, impulsivity and risk-taking. Rodent studies have also showed that adolescent alcohol use can impair neurogenesis, induce neuroinflammation and epigenetic alterations, and lead to the persistence of adolescent-like neurobehavioural phenotypes into adulthood. Although only a limited number of studies have examined comparable measures in humans and laboratory animals, the available data provide evidence for notable across-species similarities in the neural consequences of adolescent alcohol exposure, providing support for further translational efforts in this context.

Prior binge-drinking history promotes the positive affective valence of methamphetamine in mice

An alcohol use disorder is a major predisposing factor for methamphetamine (MA) abuse. Further, MA-alcohol co-abuse is a risk factor for treatment discontinuation and non-compliance in MA-dependent individuals. No effective treatment exists for MA addiction, let alone treatments directed at those suffering from MA-alcohol addiction co-morbidity. Thus, it is imperative that we develop high-throughput animal models to study the biobehavioral interactions between MA and alcohol of relevance to the etiology and treatment of co-abuse. To this end, we reported that a history of binge alcohol-drinking [5,10, 20 and 40% (v/v); 2 h/day for 10-14 days] reduces MA reinforcement and intake, but it augments MA-preference and intake when drug availability is behaviorally non-contingent. To reconcile this apparent discrepancy in findings, we employed a comparable 2-week binge-drinking paradigm as that employed in our previous studies followed by place-conditioning procedures (4 pairings of 0.25, 0.5, 1, 2 or 4 mg/kg MA, i.p.). This was meant to determine how a prior binge-drinking history impacts the affective valence of MA and sensitivity to MA-induced psychomotor-activation/sensitization. Prior binge-drinking history blunted spontaneous locomotor activity and shifted the MA dose-place-preference function upwards of water drinking controls. The potentiation of MA-conditioned reward by prior binge-drinking history was independent of any alcohol effects upon the locomotor-activating or -sensitizing effects of MA. Based on these results we propose that the reduced MA reinforcement reported previously by our group likely reflects a compensatory response to an increased sensitivity to MA's positive subjective effects rather than increased sensitivity to the drug's psychomotor-activating effects.

Homer2 within the central nucleus of the amygdala modulates withdrawal-induced anxiety in a mouse model of binge-drinking

A history of binge-drinking decreases protein expression of the glutamate-related scaffolding protein Homer2 within the central nucleus of the amygdala (CEA), coinciding with behavioral signs of negative affect. To assess the functional relevance of this protein change for withdrawal-induced hyper-anxiety, adult (PND 56) and adolescent (PND 28) male C57BL/6J mice were administered an intra-CEA infusion of an adeno-associated viral vector (AAV) carrying either cDNA to express Homer2 (H2-cDNA) or GFP as control. Mice underwent 14 days of binge-drinking under multi-bottle, limited-access conditions and were assayed for behavioral signs of negative affect during withdrawal using the light-dark box, marble burying, and forced swim tests (FST). Following behavioral testing, all animals experienced 5 days of drinking to evaluate the effects of prior alcohol experience and Homer2 manipulation on subsequent alcohol consumption. During protracted (4 weeks) withdrawal, adolescent alcohol-experienced GFP controls showed increased signs of negative affect across all 3 assays, compared to water-drinking GFP animals, and also showed elevated alcohol consumption during the subsequent drinking period. Homer2-cDNA infusion in adolescent-onset alcohol-drinking animals was anxiolytic and reduced subsequent alcohol consumption. Conversely, Homer2-cDNA was anxiogenic and increased drinking in water-drinking adolescents. Unfortunately, the data from adult-onset alcohol-drinking animals were confounded by low alcohol consumption and negligible behavioral signs of anxiety. Nevertheless, the present results provide novel cause-effect evidence supporting a role for CEA Homer2 in the regulation of both basal anxiety and the time-dependent intensification of negative affective states in individuals with a history of binge-drinking during adolescence.

Homer2 and Alcohol: A Mutual Interaction

The past two decades of data derived from addicted individuals and preclinical animal models of addiction implicate a role for the excitatory glutamatergic transmission within the mesolimbic structures in alcoholism. The cellular localization of the glutamatergic receptor subtypes, as well as their signaling efficiency and function, are highly dependent upon discrete functional constituents of the postsynaptic density, including the Homer family of scaffolding proteins. The consequences of repeated alcohol administration on the expression of the Homer family proteins demonstrate a crucial and active role, particularly for the expression of Homer2 isoform, in regulating alcohol-induced behavioral and cellular neuroplasticity. The interaction between Homer2 and alcohol can be defined as a mutual relation: alcohol consumption enhances the expression of Homer2 protein isoform within the nucleus accumbens and the extended amygdala, cerebral areas where, in turn, Homer2 is able to mediate the development of the "pro-alcoholic" behavioral phenotype, as a consequence of the morpho-functional synaptic adaptations. Such findings are relevant for the detection of the strategic molecular components that prompt alcohol-induced functional and behavioral disarrangement as targets for future innovative treatment options.