Effects of acute or chronic ethanol exposure during adolescence on behavioral inhibition and efficiency in a modified water maze task

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.

Schedule-induced alcohol intake during adolescence sex dependently impairs hippocampal synaptic plasticity and spatial memory

In a previous study, we demonstrated that intermittent ethanol administration in male adolescent animals impaired hippocampus-dependent spatial memory, particularly under conditions of excessive ethanol administration. In this current study, we subjected adolescent male and female Wistar rats an alcohol schedule-induced drinking (SID) procedure to obtain an elevated rate of alcohol self-administration and assessed their hippocampus-dependent spatial memory. We also studied hippocampal synaptic transmission and plasticity, as well as the expression levels of several genes involved in these mechanisms. Both male and female rats exhibited similar drinking patterns throughout the sessions of the SID protocol reaching similar blood alcohol levels in all the groups. However, only male rats that consumed alcohol showed spatial memory deficits which correlated with inhibition of hippocampal synaptic plasticity as long-term potentiation. In contrast, alcohol did not modify hippocampal gene expression of AMPA and NMDA glutamate receptor subunits, although there are differences in the expression levels of several genes relevant to synaptic plasticity mechanisms underlying learning and memory processes, related to alcohol consumption as Ephb2, sex differences as Pi3k or the interaction of both factors such as Pten. In conclusion, elevated alcohol intake during adolescence seems to have a negative impact on spatial memory and hippocampal synaptic plasticity in a sex dependent manner, even both sexes exhibit similar blood alcohol concentrations and drinking patterns.

Adolescent binge ethanol impacts H3K36me3 regulation of synaptic genes

Adolescence is marked in part by the ongoing development of the prefrontal cortex (PFC). Binge ethanol use during this critical stage in neurodevelopment induces significant structural changes to the PFC, as well as cognitive and behavioral deficits that can last into adulthood. Previous studies showed that adolescent binge ethanol causes lasting deficits in working memory, decreases in the expression of chromatin remodeling genes responsible for the methylation of histone 3 lysine 36 (H3K36), and global decreases in H3K36 in the PFC. H3K36me3 is present within the coding region of actively-transcribed genes, and safeguards against aberrant, cryptic transcription by RNA Polymerase II. We hypothesize that altered methylation of H3K36 could play a role in adolescent binge ethanol-induced memory deficits. To investigate this at the molecular level, ethanol (4 g/kg, i.g.) or water was administered intermittently to adolescent mice. RNA-and ChIP-sequencing were then performed within the same tissue to determine gene expression changes and identify genes and loci where H3K36me3 was disrupted by ethanol. We further assessed ethanol-induced changes at the transcription level with differential exon-use and cryptic transcription analysis - a hallmark of decreased H3K36me3. Here, we found ethanol-induced changes to the gene expression and H3K36me3-regulation of synaptic-related genes in all our analyses. Notably, H3K36me3 was differentially trimethylated between ethanol and control conditions at synaptic-related genes, and Snap25 and Cplx1 showed evidence of cryptic transcription in males and females treated with ethanol during adolescence. Our results provide preliminary evidence that ethanol-induced changes to H3K36me3 during adolescent neurodevelopment may be linked to synaptic dysregulation at the transcriptional level, which may explain the reported ethanol-induced changes to PFC synaptic function.

A mechanistic perspective on the health promoting effects of alcohol - A focus on epigenetics modification

While the detrimental effects of binge drinking are well recognized, low-to-moderate alcohol consumption may be beneficial to health, although the underlying mechanism(s) remains elusive. In this opinion article, we will examine the effects of low dose alcohol consumption from the perspective of epigenetic modulation. Biochemically, alcohol is metabolized into acetate and subsequently to acetyl-coA, which can modulate histone acetylation levels. While elevated levels of acetyl-CoA are detrimental for longevity, we argue that diminished acetyl-CoA also negatively affects fatty acid biosynthesis and histone acetylation, which play a critical role in gene expression and, ultimately, health span. Since mitochondrial function and glucose metabolism, which provide the main source of nucleocytoplasmic acetyl-CoA, are compromised with age, alcohol-derived acetate could be an alternative source of acetyl-CoA to compensate. Hence, the health benefits of low ethanol consumption may be more pronounced after midlife, since mitochondrial function and/or glucose metabolism are diminished in this phase of the life course. Indeed, various clinical alcohol consumption studies concur with this notion, and have shown that a low dose of regular alcohol intake after midlife brings about various health and survival benefits. The requirement for regular alcohol intake may also reflect the transient nature of ethanol-induced histone acetylation. Conversely, ethanol may also stimulate carcinogenesis by inhibiting DNA methylation, as it was shown to reduce various pathways leading to DNA and histone methylation. However, unlike acetylation, where ethanol directly increases the substrate for acetylation, this effect was only observed in the high alcohol exposure cohort. While alcohol-derived acetate may be beneficial for health after midlife, various detrimental effects of alcohol consumption remain, and hence, we do not advocate excessive drinking to increase acetate. This opinion article establishes a possible role of ethanol-derived acetate in achieving homeostasis and sustaining an organism's health span.

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.

Adolescent alcohol exposure alters threat avoidance in adulthood

Adolescent binge-like alcohol exposure impairs cognitive function and decision making in adulthood and may be associated with dysfunction of threat avoidance, a critical mechanism of survival which relies upon executive function. The present study investigated the impact of binge-like alcohol exposure during adolescence on active avoidance in adulthood. Male and female rats were subjected to adolescent intermittent ethanol (AIE) exposure by vapor inhalation and then tested in adulthood using a platform-mediated avoidance task. After training to press a lever to receive a sucrose reward, the rats were conditioned to a tone that co-terminated with a foot-shock. A motivational conflict was introduced by the presence of an escape platform that isolated the rat from the shock, but also prevented access to the sucrose reward while the rat was on the platform. During the task training phase, both male and female rats exhibited progressive increases in active avoidance (platform escape) in response to the conditioned tone, whereas innate fear behavior (freezing) remained relatively constant over training days. A history of AIE exposure did not impact either active avoidance or freezing behavior during task acquisition. On the test day following platform acquisition training, female rats exhibited higher levels of both active avoidance and freezing compared to male rats, while AIE-exposed male but not female rats exhibited significantly greater levels of active avoidance compared to controls. In contrast, neither male nor female AIE-exposed rats exhibited alterations in freezing compared to controls. Following 5 days of extinction training, female rats continued to display higher levels of active avoidance and freezing during tone presentation compared to males. Male AIE-exposed rats also had higher levels of both active avoidance and freezing compared to the male control rats. Together, the results demonstrate that female rats exhibit elevated levels of active avoidance and freezing compared to males and further reveal a sex-specific impact of AIE on threat responding in adulthood.

Adolescent alcohol exposure reduces dopamine 1 receptor modulation of prelimbic neurons projecting to the nucleus accumbens and basolateral amygdala

Binge drinking during adolescence is highly prevalent despite increasing evidence of its long-term impact on behaviors associated with modulation of behavioral flexibility by the medial prefrontal cortex (mPFC). In the present study, male and female rats underwent adolescent intermittent ethanol (AIE) exposure by vapor inhalation. After aging to adulthood, retrograde bead labelling and viral tagging were used to identify populations of neurons in the prelimbic region (PrL) of the mPFC that project to specific subcortical targets. Electrophysiological recording from bead-labelled neurons in PrL slices revealed that AIE did not alter the intrinsic excitability of PrL neurons that projected to either the NAc or the BLA. Similarly, recordings of spontaneous inhibitory and excitatory post-synaptic currents revealed no AIE-induced changes in synaptic drive onto either population of projection neurons. In contrast, AIE exposure was associated with a loss of dopamine receptor 1 (D1), but no change in dopamine receptor 2 (D2), modulation of evoked firing of both populations of projection neurons. Lastly, confocal imaging of proximal and apical dendritic tufts of viral-labelled PrL neurons that projected to the nucleus accumbens (NAc) revealed AIE did not alter the density of dendritic spines. Together, these observations provide evidence that AIE exposure results in disruption of D1 receptor modulation of PrL inputs to at least two major subcortical target regions that have been implicated in AIE-induced long-term changes in behavioral control.

Sex differences in the effects of adolescent intermittent ethanol exposure on exploratory and anxiety-like behavior in adult rats

Adolescent intermittent ethanol (AIE) exposure in rodents has been shown to alter adult behavior in several domains, including learning and memory, social interaction, affective behavior, and ethanol self-administration. AIE has also been shown to produce non-specific behavioral changes that compromise behavioral efficiency. Many studies of these types rely on measuring behavior in mazes and other enclosures that can be influenced by animals' activity levels and exploratory behavior, and relatively few such studies have assessed sex as a biological variable. To address the effects of AIE and its interaction with sex on these types of behavioral assays, male and female adolescent rats (Sprague Dawley) were exposed to 10 doses of AIE (5 g/kg, intra-gastrically [i.g.]), or control vehicle, over 16 days (postnatal day [PND] 30-46), and then tested for exploratory and anxiety-like behaviors on the novelty-induced hypophagia (NIH) task in an open field, the elevated plus (EPM) maze, and the Figure 8 maze. AIE reduced activity/exploratory behaviors in males on the anxiety-producing NIH and EPM tasks, but reduced activity in both males and females in the Figure 8 maze, a task designed to create a safe environment and reduce anxiety. Independent of AIE, females engaged in more rearing behavior than males during the NIH task but less in the EPM, in which they were also less active than males. AIE also increased EPM open arm time in females but not in males. These findings demonstrate previously unrecognized sex differences in the effects of AIE on activity, exploratory behavior, and anxiety-like behavior; additionally, they underscore the need to design future behavioral studies of AIE using sex as a variable and with rigorous attention to how AIE alters these behaviors.

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.

The effects of peri-adolescent alcohol use on the developing hippocampus

Adolescence is a period of continued brain development. Regions of the brain, such as the hippocampus, continue to undergo refinement and maturation throughout adolescence and into early adulthood. Adolescence is also a time of heightened sensitivity to novelty and reward, which contribute to an increase in risk-taking behaviors including the use of drugs and alcohol. Importantly, binge drinking is highly prevalent among adolescents and emerging adults. The hippocampus which is important for the integration of emotion, reward, homeostasis, and memory is particularly vulnerable to the neurotoxic effects of alcohol. In this chapter, we cover the fundamentals of hippocampal neuroanatomy and the current state of knowledge of the acute and chronic effects of ethanol in adolescent humans and adolescent rodent models. We focus on the hippocampal-dependent behavioral, structural, and neurochemical changes and identify knowledge gaps in our understanding of age-dependent neurobiological effects of alcohol use.

Chronic alcohol exposure during critical developmental periods differentially impacts persistence of deficits in cognitive flexibility and related circuitry

Cognitive flexibility in decision making depends on prefrontal cortical function and is used by individuals to adapt to environmental changes in circumstances. Cognitive flexibility can be measured in the laboratory using a variety of discrete, translational tasks, including those that involve reversal learning and/or set-shifting ability. Distinct components of flexible behavior rely upon overlapping brain circuits, including different prefrontal substructures that have separable impacts on decision making. Cognitive flexibility is impaired after chronic alcohol exposure, particularly during development when the brain undergoes rapid maturation. This review examines how cognitive flexibility, as indexed by reversal and set-shifting tasks, is impacted by chronic alcohol exposure in adulthood, adolescent, and prenatal periods in humans and animal models. We also discuss areas for future study, including mechanisms that may contribute to the persistence of cognitive deficits after developmental alcohol exposure and the compacting consequences from exposure across multiple critical periods.

Adolescent intermittent ethanol exposure induces sex-dependent divergent changes in ethanol drinking and motor activity in adulthood in C57BL/6J mice

With alcohol readily accessible to adolescents, its consumption leads to many adverse effects, including impaired learning, attention, and behavior. Adolescents report higher rates of binge drinking compared to adults. They are also more prone to substance use disorder in adulthood due to physiological changes during the adolescent developmental period. We used C57BL/6J male and female mice to investigate the long-lasting impact of binge ethanol exposure during adolescence on voluntary ethanol intake and open field behavior during later adolescence (Experiment 1) and during emerging adulthood (Experiment 2). The present set of experiments were divided into four stages: (1) adolescent intermittent vapor inhalation exposure, (2) abstinence, (3) voluntary ethanol intake, and (4) open field behavioral testing. During adolescence, male and female mice were exposed to air or ethanol using intermittent vapor inhalation from postnatal day (PND) 28-42. Following this, mice underwent short-term abstinence from PND 43-49 (Experiment 1) or protracted abstinence from PND 43-69 (Experiment 2). Beginning on PND 50-76 or PND 70-97, mice were assessed for intermittent voluntary ethanol consumption using a two-bottle choice drinking procedure over 28 days. Male adolescent ethanol-exposed mice showed increased ethanol consumption following short-term abstinence and following protracted abstinence. In contrast, female mice showed no changes in ethanol consumption following short-term abstinence and decreased ethanol consumption following protracted abstinence. There were modest changes in open field behavior following voluntary ethanol consumption in both experiments. These data demonstrate a sexually divergent shift in ethanol consumption following binge ethanol exposure during adolescence and differences in open field behavior. These results highlight sex-dependent vulnerability to developing substance use disorders in adulthood.

Acute ethanol administration produces larger spatial and nonspatial memory impairments in 29-33 month old rats compared to adult and 18-24 month old rats

The average age of the population in many countries is continuing to increase. Older people continue to consume alcohol, often in a binge like fashion. Previous research has demonstrated that older human subjects and aged animal subjects have an increased sensitivity to the effects of ethanol on a variety of behaviors. However, it has yet to be determined if acute ethanol exposure impairs spatial and/or nonspatial memory to a greater extent in aged rats compared to adult rats. In the current studies we trained male rats ranging in age from young adult (2 months of age) to aged rats (29-33 months of age) in the standard nonspatial task followed by the standard spatial task in the Morris water maze. Only animals deemed "cognitively-spared", that is aged animals that learn as well as young animals, were administered one of two doses of moderate ethanol and had their memory tested 30 min later. Acute ethanol administration produced similar performance impairments in spatial and nonspatial memory in all cognitively-spared animals except for the 29-33 month old animals which showed a significantly greater cognitive impairment in both tasks. In addition, blood ethanol levels were similar across all ages. The present work adds to the growing literature on the selective effects of acute ethanol exposure in aged animals.

Effect of alcohol use on the adolescent brain and behavior

Adolescence is a particularly vulnerable neurodevelopmental period marked by high rates of engagement with risky alcohol use. This review summarizes the cognitive and neural consequences following alcohol use during adolescence from longitudinal design studies in humans and animals. Findings from human adolescent studies suggest that binge drinking and heavy alcohol use is associated with poorer cognitive functioning on a broad range of neuropsychological assessments, including learning, memory, visuospatial functioning, psychomotor speed, attention, executive functioning, and impulsivity. Alcohol use during adolescence is associated with accelerated decreases in gray matter and attenuated increases in white matter volume, and aberrant neural activity during executive functioning, attentional control, and reward sensitivity tasks, when compared to non-drinking adolescents. Animal studies in rodents and non-human primates have replicated human findings, and suggest cognitive and neural consequences of adolescent alcohol use may persist into adulthood. Novel rodent studies demonstrate that adolescent alcohol use may increase reward responsiveness of the dopamine system to alcohol later in life, as well as disrupt adolescent neurogenesis, potentially through neuroinflammation, with long-lasting neural and behavioral effects into adulthood. Larger longitudinal human cognitive and neuroimaging studies with more diverse samples are currently underway which will improve understanding of the impact of polysubstance use, as well as the interactive effects of substance use, physical and mental health, and demographic factors on cognition and neurodevelopment.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Effects of chronic intermittent ethanol exposure during early and late adolescence on anxiety-like behaviors and behavioral flexibility in adulthood

Although both humans and laboratory rodents demonstrate cognitive and affective alterations associated with adolescent alcohol exposure, it is still unknown whether the consequences of early initiation of alcohol use differ from those of later binge drinking within the adolescent developmental period. The present study was designed to assess the effects of early and late AIE on (1) anxiety-like behavior under social (modified social interaction test) and non-social test circumstances (modified light/dark box test, elevated plus maze), and (2) behavioral flexibility, indexed via set shifting in males and females. Early-mid adolescent intermittent exposure (early AIE) occurred between postnatal days (P) 25 and 45, whereas late adolescent intermittent exposure (late AIE) was conducted between P45 and P65, with behavioral testing initiated not earlier than 25 days after repeated exposure to ethanol (4.0 g/kg intragastrically, every other day for a total of 11 exposures). Anxiety-like behavior on the EPM was evident in males and females following early AIE, whereas only males demonstrated non-social anxiety on the EPM following late AIE. Social anxiety-like alterations and deficits in behavioral flexibility were evident only in males following early AIE. Taken together, the results of the present study demonstrate a particular vulnerability of young adolescent males to long-lasting detrimental effects of repeated ethanol and an insensitivity of older adolescent females to the intermittent ethanol exposure paradigm.

Acute alcohol and cognition: Remembering what it causes us to forget

Addiction has been conceptualized as a specific form of memory that appropriates typically adaptive neural mechanisms of learning to produce the progressive spiral of drug-seeking and drug-taking behavior, perpetuating the path to addiction through aberrant processes of drug-related learning and memory. From that perspective, to understand the development of alcohol use disorders, it is critical to identify how a single exposure to alcohol enters into or alters the processes of learning and memory, so that involvement of and changes in neuroplasticity processes responsible for learning and memory can be identified early. This review characterizes the effects produced by acute alcohol intoxication as a function of brain region and memory neurocircuitry. In general, exposure to ethanol doses that produce intoxicating effects causes consistent impairments in learning and memory processes mediated by specific brain circuitry, whereas lower doses either have no effect or produce a facilitation of memory under certain task conditions. Therefore, acute ethanol does not produce a global impairment of learning and memory, and can actually facilitate particular types of memory, perhaps particular types of memory that facilitate the development of excessive alcohol use. In addition, the effects on cognition are dependent on brain region, task demands, dose received, pharmacokinetics, and tolerance. Additionally, we explore the underlying alterations in neurophysiology produced by acute alcohol exposure that help to explain these changes in cognition and highlight future directions for research. Through understanding the impact that acute alcohol intoxication has on cognition, the preliminary changes potentially causing a problematic addiction memory can better be identified.

Changes in Neuroimmune and Neuronal Death Markers after Adolescent Alcohol Exposure in Rats are Reversed by Donepezil

Adolescent intermittent ethanol (AIE) exposure diminishes neurogenesis and dendritic spine density in the dentate gyrus. The cholinesterase inhibitor, donepezil (Aricept), reverses AIE effects on dendritic spines, possibly by interacting with inflammatory and/or epigenetic mediators after AIE exposure. This study tests the hypothesis that donepezil reverses AIE-induced neuroimmune, and epigenetic changes in the adult dentate gyrus. Adolescent Sprague-Dawley male rats (PD30-43) were given 10 intermittent, intragastric doses of ethanol (5.0 g/kg) or isovolumetric water (AIW). Twenty-one days later half of the animals from each group were treated with either donepezil or isovolumetric water (i.g.) once daily for four days. Two hours after the last donepezil or water dose animals were sacrificed and brains prepared for immunohistochemical analyses. AIE reduced immunoreactivity for doublecortin (DCX) and increased immunoreactivity for activated caspase-3 and death receptor-3 in adulthood, suggesting an enduring attenuation of neurogenesis and an increase in progenitor death. These effects were reversed by donepezil treatment in adulthood. AIE also increased immunoreactivity for the inflammatory signaling molecules HMGB1 and RAGE, as well as the activated phosphorylated transcription factor pNFκB p65, and the gene silencing marker dimethylated histone H3K9. All of these AIE effects were also reversed by donepezil, with the exception of HMGB1.

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.

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.

Binge Drinking's Effects on the Developing Brain-Animal Models

Adolescence typically is a time of experimentation, including alcohol use and, particularly, binge drinking. Because the brain is still developing during adolescence, such exposure could have long-lasting effects. Animal models and adolescent intermittent ethanol exposure (AIE) paradigms have been used to help elucidate the consequences of adolescent binge drinking. These studies have identified cognitive deficits, particularly in challenging cognitive tasks, and behavioral alterations such as greater risk preferences, impulsivity, and disinhibition. AIE also is associated with changes in affect when the animals reach adulthood, including increased social anxiety and, sometimes, general anxiety. Animal models have demonstrated that AIE can result in retention of certain alcohol-related adolescent phenotypes (i.e., reduced sensitivity to alcohol's aversive effects and increased sensitivity to alcohol's rewarding effects) into adulthood, which may motivate continued elevated alcohol use. The detrimental effects of adolescent alcohol exposure extend to a diversity of lasting alterations in the brain, including reduced neurogenesis, increased proinflammatory responses, changes in gene expression through epigenetic mechanisms, and alterations in the activities of various neurotransmitter systems. Further exploration of these mechanisms in animal models and humans may lead to improved prevention and intervention efforts.

Adolescent Ethanol Exposure Enhances NMDA Receptor-Mediated Currents in Hippocampal Neurons: Reversal by Gabapentin

Adolescent intermittent ethanol (AIE) exposure compromises neural function into adulthood. We have reported that astrocyte-secreted thrombospondins, and their target neuronal receptors (α2δ−1) are upregulated in the hippocampus in adulthood after AIE, suggesting aberrant excitatory synaptogenesis and hyperexcitability in memory-related circuits. Gabapentin antagonizes the interaction of thrombospondins (TSPs) with the α2δ−1 receptor, and thus may reverse or ameliorate the effects of AIE on hippocampal function. Adolescent rats were exposed to AIE or vehicle. In adulthood, hippocampal slices were prepared. Half of the slices from each animal were pre-incubated in normal artificial cerebrospinal fluid (aCSF) while half were pre-incubated in aCSF containing gabapentin. Whole-cell voltage clamp recordings were then made from CA1 pyramidal cells in normal aCSF. Evoked, N-methyl-D-aspartate (NMDA) receptor-mediated currents were recorded at baseline, and after application of the GluN2B antagonist, RO25–6981. Current amplitudes were higher in neurons from AIE-exposed animals. However, no amplitude increase was observed in neurons from slices that had been pre-incubation in gabapentin. GluN2B antagonism reduced NMDA receptor-mediated currents more efficaciously in cells from AIE-exposed animals, an effect that was also reversed by pre-incubation in gabapentin. These findings identify a mechanism underlying the enduring effects of AIE, and a clinically-utilized agent that may ameliorate those effects.

Behavioral Inefficiency on a Risky Decision-Making Task in Adulthood after Adolescent Intermittent Ethanol Exposure in Rats

Adolescence is a period of development in neural circuits that are critical for adult functioning. There is a relationship between alcohol exposure and risky decision-making, though the enduring effects of adolescent ethanol exposure on risky decision-making in adulthood have not been fully explored. Studies using positive reinforcement have shown that adolescent intermittent ethanol (AIE) exposure results in higher levels of risky decision-making in adulthood, but the effects of AIE on punishment-mediated decision-making have not been explored. Adolescent rats were exposed to AIE or saline vehicle across a 16-day period, and then allowed to mature into adulthood. They were then trained to lever press for food reward and were assessed for risky decision-making by pairing increased levels of food reward with the probability of footshock punishment. AIE did not alter punishment-mediated risky decision-making. However, it did result in a significant increase in the delay to lever pressing. This finding is consistent with previous reports, using other behavioral tasks, which show decreased behavioral efficiency in adulthood after AIE. These findings indicate that AIE increases behavioral inefficiency, but not punishment-mediated risk-taking, in adulthood. Thus they contribute to a more nuanced understanding of the long-term effects of AIE on adult behavior.

Adolescent Alcohol Exposure Persistently Impacts Adult Neurobiology and Behavior

Adolescence is a developmental period when physical and cognitive abilities are optimized, when social skills are consolidated, and when sexuality, adolescent behaviors, and frontal cortical functions mature to adult levels. Adolescents also have unique responses to alcohol compared with adults, being less sensitive to ethanol sedative-motor responses that most likely contribute to binge drinking and blackouts. Population studies find that an early age of drinking onset correlates with increased lifetime risks for the development of alcohol dependence, violence, and injuries. Brain synapses, myelination, and neural circuits mature in adolescence to adult levels in parallel with increased reflection on the consequence of actions and reduced impulsivity and thrill seeking. Alcohol binge drinking could alter human development, but variations in genetics, peer groups, family structure, early life experiences, and the emergence of psychopathology in humans confound studies. As adolescence is common to mammalian species, preclinical models of binge drinking provide insight into the direct impact of alcohol on adolescent development. This review relates human findings to basic science studies, particularly the preclinical studies of the Neurobiology of Adolescent Drinking in Adulthood (NADIA) Consortium. These studies focus on persistent adult changes in neurobiology and behavior following adolescent intermittent ethanol (AIE), a model of underage drinking. NADIA studies and others find that AIE results in the following: increases in adult alcohol drinking, disinhibition, and social anxiety; altered adult synapses, cognition, and sleep; reduced adult neurogenesis, cholinergic, and serotonergic neurons; and increased neuroimmune gene expression and epigenetic modifiers of gene expression. Many of these effects are specific to adolescents and not found in parallel adult studies. AIE can cause a persistence of adolescent-like synaptic physiology, behavior, and sensitivity to alcohol into adulthood. Together, these findings support the hypothesis that adolescent binge drinking leads to long-lasting changes in the adult brain that increase risks of adult psychopathology, particularly for alcohol dependence.

Consequences of adolescent use of alcohol and other drugs: Studies using rodent models

Studies using animal models of adolescent exposure to alcohol, nicotine, cannabinoids, and the stimulants cocaine, 3,4-methylenedioxymethampethamine and methamphetamine have revealed a variety of persisting neural and behavioral consequences. Affected brain regions often include mesolimbic and prefrontal regions undergoing notable ontogenetic change during adolescence, although it is unclear whether this represents areas of specific vulnerability or particular scrutiny to date. Persisting alterations in forebrain systems critical for modulating reward, socioemotional processing and cognition have emerged, including apparent induction of a hyper-dopaminergic state with some drugs and/or attenuations in neurons expressing cholinergic markers. Disruptions in cognitive functions such as working memory, alterations in affect including increases in social anxiety, and mixed evidence for increases in later drug self-administration has also been reported. When consequences of adolescent and adult exposure were compared, adolescents were generally found to be more vulnerable to alcohol, nicotine, and cannabinoids, but generally not to stimulants. More work is needed to determine how adolescent drug exposure influences sculpting of the adolescent brain, and provide approaches to prevent/reverse these effects.

Ethanol alters the expression of ion channel genes in Daphnia pulex

BACKGROUND

Heavy drinking can increase heart rate and blood glucose, induce hypoxic tolerance, impair brain cognitive functions, and alter gene expressions. These phenomena may occur even in response to small dose of ethanol exposure or during its withdrawal.

OBJECTIVES

To evaluate whether persistent low concentrations of ethanol exposure affect organism function and the gene expressions of ion channels.

METHODS

Daphnids were randomized to receive placebo 300 min, 2 mM ethanol 300 min, or 2 mM ethanol 240 min and then placebo 60 min. Heart rate, glucose levels, phototactic behavior, and hypoxic tolerance were recorded during experiment. At the end of the study, changes in the mRNA levels of ion channel genes were assessed in response to exposure to ethanol using quantitative polymerase chain reaction (PCR) techniques.

RESULTS

Heart rate was reversibly increased by ethanol withdrawal and returned to basal levels upon re-exposure to ethanol. Fifteen of 120 ion channel transcripts were affected by persistent ethanol exposure. Neither ethanol withdrawal nor persistent exposures showed an effect on blood glucose, phototactic behavior, or hypoxic tolerance.

CONCLUSIONS

Small doses of ethanol can increase heart rate and alter gene expression of multiple ion channels in Daphnia pulex. Affected ion channel genes may assist in understanding the mechanism of ethanol adaptation and tolerance.

Adolescent Intermittent Alcohol Exposure: Deficits in Object Recognition Memory and Forebrain Cholinergic Markers

The long-term effects of intermittent ethanol exposure during adolescence (AIE) are of intensive interest and investigation. The effects of AIE on learning and memory and the neural functions that drive them are of particular interest as clinical findings suggest enduring deficits in those cognitive domains in humans after ethanol abuse during adolescence. Although studies of such deficits after AIE hold much promise for identifying mechanisms and therapeutic interventions, the findings are sparse and inconclusive. The present results identify a specific deficit in memory function after AIE and establish a possible neural mechanism of that deficit that may be of translational significance. Male rats (starting at PND-30) received exposure to AIE (5g/kg, i.g.) or vehicle and were allowed to mature into adulthood. At PND-71, one group of animals was assessed using the spatial-temporal object recognition (stOR) test to evaluate memory function. A separate group of animals was used to assess the density of cholinergic neurons in forebrain areas Ch1-4 using immunohistochemistry. AIE exposed animals manifested deficits in the temporal component of the stOR task relative to controls, and a significant decrease in the number of ChAT labeled neurons in forebrain areas Ch1-4. These findings add to the growing literature indicating long-lasting neural and behavioral effects of AIE that persist into adulthood and indicate that memory-related deficits after AIE depend upon the tasks employed, and possibly their degree of complexity. Finally, the parallel finding of diminished cholinergic neuron density suggests a possible mechanism underlying the effects of AIE on memory and hippocampal function as well as possible therapeutic or preventive strategies for AIE.

Adolescent alcohol exposure alters GABAA receptor subunit expression in adult hippocampus

BACKGROUND

The long-term consequences of adolescent alcohol abuse that persist into adulthood are poorly understood and have not been widely investigated. We have shown that intermittent exposure to alcohol during adolescence decreased the amplitude of GABAA receptor (GABAA R)-mediated tonic currents in hippocampal dentate granule cells in adulthood. The aim of this study was to investigate the enduring effects of chronic intermittent alcohol exposure during adolescence or adulthood on the expression of hippocampal GABAA Rs.

METHODS

We used a previously characterized tissue fractionation method to isolate detergent resistant membranes and soluble fractions, followed by Western blots to measure GABAA R protein expression. We also measured mRNA levels of GABAA R subunits using quantitative real-time polymerase chain reaction.

RESULTS

Although the protein levels of α1-, α4-, and δ-GABAA R subunits remained stable between postnatal day (PD) 30 (early adolescence) and PD71 (adulthood), the α5-GABAA R subunit was reduced across that period. In rats that were subjected to adolescent intermittent ethanol (AIE) exposure between PD30 and PD46, there was a significant reduction in the protein levels of the δ-GABAA R, in the absence of any changes in mRNA levels, at 48 hours and 26 days after the last ethanol (EtOH) exposure. Protein levels of the α4-GABAA R subunit were significantly reduced, but mRNA levels were increased, 26 days (but not 48 hours) after the last AIE exposure. Protein levels of α5-GABAA R were not changed by AIE, but mRNA levels were reduced at 48 hours but normalized 26 days after AIE. In contrast to the effects of AIE, chronic intermittent ethanol (CIE) exposure during adulthood had no effect on expression of any of the GABAA R subunits examined.

CONCLUSIONS

AIE produced both short- and long-term alterations of GABAA R subunits mRNA and protein expression in the hippocampus, whereas CIE produced no long-lasting effects on those measures. The observed reduction of protein levels of the δ-GABAA R, specifically, is consistent with previously reported altered hippocampal GABAA R-mediated electrophysiological responses after AIE. The absence of effects of CIE underscores the emerging view of adolescence as a time of distinctive vulnerability to the enduring effects of repeated EtOH exposure.

Adolescent intermittent alcohol exposure: persistence of structural and functional hippocampal abnormalities into adulthood

BACKGROUND

Human adolescence is a crucial stage of neurological development during which ethanol (EtOH) consumption is often at its highest. Alcohol abuse during adolescence may render individuals at heightened risk for subsequent alcohol abuse disorders, cognitive dysfunction, or other neurological impairments by irreversibly altering long-term brain function. To test this possibility, we modeled adolescent alcohol abuse (i.e., intermittent EtOH exposure during adolescence [AIE]) in rats to determine whether adolescent exposure to alcohol leads to long-term structural and functional changes that are manifested in adult neuronal circuitry.

METHODS

We specifically focused on hippocampal area CA1, a brain region associated with learning and memory. Using electrophysiological, immunohistochemical, and neuroanatomical approaches, we measured post-AIE changes in synaptic plasticity, dendritic spine morphology, and synaptic structure in adulthood.

RESULTS

We found that AIE-pretreated adult rats manifest robust long-term potentiation, induced at stimulus intensities lower than those required in controls, suggesting a state of enhanced synaptic plasticity. Moreover, AIE resulted in an increased number of dendritic spines with characteristics typical of immaturity. Immunohistochemistry-based analysis of synaptic structures indicated a significant decrease in the number of co-localized pre- and postsynaptic puncta. This decrease is driven by an overall decrease in 2 postsynaptic density proteins, PSD-95 and SAP102.

CONCLUSIONS

Taken together, these findings reveal that repeated alcohol exposure during adolescence results in enduring structural and functional abnormalities in the hippocampus. These synaptic changes in the hippocampal circuits may help to explain learning-related behavioral changes in adult animals preexposed to AIE.

Alcohol use across the lifespan: An analysis of adolescent and aged rodents and humans

Adolescence and old age are unique periods of the lifespan characterized by differential sensitivity to the effects of alcohol. Adolescents and the elderly appear to be more vulnerable to many of alcohol's physiological and behavioral effects compared to adults. The current review explores the differential effects of acute alcohol, predominantly in terms of motor function and cognition, in adolescent and aged humans and rodents. Adolescents are less sensitive to the sedative-hypnotic, anxiolytic, and motor-impairing effects of acute alcohol, but research results are less consistent as it relates to alcohol's effects on cognition. Specifically, previous research has shown adolescents to be more, less, and similarly sensitive to alcohol-induced cognitive deficits compared to adults. These equivocal findings suggest that learning acquisition may be differentially affected by ethanol compared to memory, or that ethanol-induced cognitive deficits are task-dependent. Older rodents appear to be particularly vulnerable to the motor- and cognitive-impairing effects of acute alcohol relative to younger adults. Given that alcohol consumption and abuse is prevalent throughout the lifespan, it is important to recognize age-related differences in response to acute and long-term alcohol. Unfortunately, diagnostic measures and treatment options for alcohol dependence are rarely dedicated to adolescent and aging populations. As discussed, although much scientific advancement has been made regarding the differential effects of alcohol between adolescents and adults, research with the aged is underrepresented. Future researchers should be aware that adolescents and the aged are uniquely affected by alcohol and should continue to investigate alcohol's effects at different stages of maturation.