Chronic alcohol exposure alters behavioral and synaptic plasticity of the rodent prefrontal cortex

Understanding How Acute Alcohol Impacts Neural Encoding in the Rodent Brain

Alcohol impacts neural circuitry throughout the brain and has wide-ranging effects on the biophysical properties of neurons in these circuits. Articulating how these wide-ranging effects might eventually result in altered computational properties has the potential to provide a tractable working model of how alcohol alters neural encoding. This chapter reviews what is currently known about how acute alcohol influences neural activity in cortical, hippocampal, and dopaminergic circuits as these have been the primary focus of understanding how alcohol alters neural computation. While other neural systems have been the focus of exhaustive work on this topic, these brain regions are the ones where in vivo neural recordings are available, thus optimally suited to make the link between changes in neural activity and behavior. Rodent models have been key in developing an understanding of how alcohol impacts the function of these circuits, and this chapter therefore focuses on work from mice and rats. While progress has been made, it is critical to understand the challenges and caveats associated with experimental procedures, especially when performed in vivo, which are designed to answer this question and if/how to translate these data to humans. The hypothesis is discussed that alcohol impairs the ability of neural circuits to acquire states of neural activity that are transiently elevated and characterized by increased complexity. It is hypothesized that these changes are distinct from the traditional view of alcohol being a depressant of neural activity in the forebrain.

Distinct roles of the left and right prelimbic cortices in the modulation of ethanol consumption in male mice under acute and chronic social defeat stress

RATIONALE

Chronic stress exposure disrupts the medial prefrontal cortex's (mPFC) ability to regulate impulses, leading to the loss of control over alcohol drinking in rodents, emphasizing the critical role of this forebrain area in regulating alcohol consumption. Moreover, chronic stress exposure causes lateralization of mPFC functions with volumetric and functional changes, resulting in hyperactivity in the right hemisphere and functional decrease in the left.

OBJECTIVES

This study investigated the inhibitory role of the left prelimbic cortex (LPrL) on ethanol consumption induced by chronic social defeat stress (SDS) in male mice and to examine if inactivation of the LPrL causes disinhibition of the right mPFC, leading to an increase in ethanol consumption. We also investigated the role of lateralization and neurochemical alterations in the mPFC related to ethanol consumption induced by chronic SDS. To this end, we examined the activation patterns of ΔFosB, VGLUT2, and GAD67 in the left and right mPFC.

RESULTS

Temporarily blocking the LPrL or right PrL (RPrL) cortices during acute SDS did not affect male mice's voluntary ethanol consumption in male mice. When each cortex was blocked in mice previously exposed to chronic SDS, ethanol consumption also remained unaffected. However, male mice with LPrL lesions during chronic SDS showed an increase in voluntary ethanol consumption, which was associated with enhanced ΔFosB/VGLUT2-positive neurons within the RPrL cortex.

CONCLUSIONS

The results suggest that the LPrL may play a role in inhibiting ethanol consumption induced by chronic SDS, while the RPrL may be involved in the disinhibition of ethanol consumption.

Differential regulation of G protein-coupled receptor-associated proteins in the caudate and the putamen of cynomolgus macaques following chronic ethanol drinking

The dorsal striatum is composed of the caudate nucleus and the putamen in human and non-human primates. These two regions receive different cortical projections and are functionally distinct. The caudate is involved in the control of goal-directed behaviors, while the putamen is implicated in habit learning and formation. Previous reports indicate that ethanol differentially influences neurotransmission in these two regions. Because neurotransmitters primarily signal through G protein-coupled receptors (GPCRs) to modulate neuronal activity, the present study aimed to determine whether ethanol had a region-dependent impact on the expression of proteins that are involved in the trafficking and function of GPCRs, including G protein subunits and their effectors, protein kinases, and elements of the cytoskeleton. Western blotting was performed to examine protein levels in the caudate and the putamen of male cynomolgus macaques that self-administered ethanol for 1 year under free access conditions, along with control animals that self-administered an isocaloric sweetened solution under identical operant conditions. Among the 18 proteins studied, we found that the levels of one protein (PKCβ) were increased, and 13 proteins (Gαi1/3, Gαi2, Gαo, Gβ1γ, PKCα, PKCε, CaMKII, GSK3β, β-actin, cofilin, α-tubulin, and tubulin polymerization promoting protein) were reduced in the caudate of alcohol-drinking macaques. However, ethanol did not alter the expression of any proteins examined in the putamen. These observations underscore the unique vulnerability of the caudate nucleus to changes in protein expression induced by chronic ethanol exposure. Whether these alterations are associated with ethanol-induced dysregulation of GPCR function and neurotransmission warrants future investigation.

Pre- and postsynaptic signatures in the prelimbic cortex associated with "alcohol use disorder" in the rat

The transition to alcohol use disorder (AUD) involves persistent neuroadaptations in executive control functions primarily regulated by the medial prefrontal cortex. However, the neurophysiological correlates to behavioral manifestations of AUD are not fully defined. The association between cortical neuroadaptations and behavioral manifestations of addiction was studied using a multi-symptomatic operant model based on the DSM-5 diagnostic criteria for AUD. This model aimed to characterize an AUD-vulnerable and AUD-resistant subpopulation of outbred male Wistar rats and was combined with electrophysiological measurements in the prelimbic cortex (PL). Mirroring clinical observations, rats exhibited individual variability in their vulnerability to develop AUD-like behavior, including motivation to seek for alcohol (crit 1), increased effort to obtain the substance (crit 2), and continued drinking despite negative consequences (crit 3). Only a small subset of rats met all the aforementioned AUD criteria (3 crit, AUD-vulnerable), while a larger fraction was considered AUD-resilient (0 crit). The development of AUD-like behavior was characterized by disruptions in glutamatergic synaptic activity, involving decreased frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and heightened intrinsic excitability in layers 2/3 PL pyramidal neurons. These alterations were concomitant with a significant impairment in the ability of mGlu2/3 receptors to negatively regulate glutamate release in the PL but not in downstream regions like the basolateral amygdala or nucleus accumbens core. In conclusion alterations in PL synaptic activity were strongly associated with individual addiction scores, indicating their role as potential markers of the behavioral manifestations linked to AUD psychopathology.

Chronic ethanol exposure produces long-lasting, subregion-specific physiological adaptations in RMTg-projecting mPFC neurons

Chronic ethanol exposure produces neuroadaptations in the medial prefrontal cortex (mPFC) which facilitate the maladaptive behaviors interfering with recovery from alcohol use disorder. Despite evidence that different cortico-subcortical projections play distinct roles in behavior, few studies have examined the physiological effects of chronic ethanol at the circuit level. The rostromedial tegmental nucleus (RMTg) is a GABAergic midbrain region involved in aversive signaling and is functionally altered by chronic ethanol exposure. Our recent work identified a dense input from the mPFC to the RMTg, yet the effects of chronic ethanol exposure on this circuitry is unknown. In the current study, we examined physiological changes after chronic ethanol exposure in prelimbic (PL) and infralimbic (IL) mPFC neurons projecting to the RMTg. Adult male Long-Evans rats were injected with fluorescent retrobeads into the RMTg and rendered dependent using a 14-day chronic intermittent ethanol (CIE) vapor exposure paradigm. Whole-cell patch-clamp electrophysiological recordings were performed in fluorescently-labeled (RMTg-projecting) and -unlabeled (projection-undefined) layer 5 pyramidal neurons 7-10 days following ethanol exposure. CIE significantly increased intrinsic excitability as well as excitatory and inhibitory synaptic drive in RMTg-projecting IL neurons. In contrast, no lasting changes in excitability were observed in RMTg-projecting PL neurons, although a CIE-induced reduction in excitability was observed in projection-undefined PL neurons. CIE also increased excitatory synaptic drive in RMTg-projecting PL neurons. These data uncover novel subregion- and circuit-specific neuroadaptations in the mPFC following chronic ethanol exposure and reveal that the IL mPFC-RMTg projection is uniquely vulnerable to long-lasting effects of chronic ethanol.

Abstinence and Fear Experienced during This Period Produce Distinct Cortical and Hippocampal Adaptations in Alcohol-Dependent Rats

Previous studies demonstrate that ethanol dependence induced by repeating cycles of chronic intermittent ethanol vapor exposure (CIE) followed by protracted abstinence produces significant gray matter damage via myelin dysfunction in the rodent medial prefrontal cortex (mPFC) and alterations in neuronal excitability in the mPFC and the dentate gyrus (DG) of the hippocampus. Specifically, abstinence-induced neuroadaptations have been associated with persistent elevated relapse to drinking. The current study evaluated the effects of forced abstinence for 1 day (d), 7 d, 21 d, and 42 d following seven weeks of CIE on synaptic plasticity proteins in the mPFC and DG. Immunoblotting revealed reduced expression of CaMKII in the mPFC and enhanced expression of GABAA and CaMKII in the DG at the 21 d time point, and the expression of the ratio of GluN2A/2B subunits did not change at any of the time points studied. Furthermore, cognitive performance via Pavlovian trace fear conditioning (TFC) was evaluated in 3 d abstinent rats, as this time point is associated with negative affect. In addition, the expression of the ratio of GluN2A/2B subunits and a 3D structural analysis of neurons in the mPFC and DG were evaluated in 3 d abstinent rats. Behavioral analysis revealed faster acquisition of fear responses and reduced retrieval of fear memories in CIE rats compared to controls. TFC produced hyperplasticity of pyramidal neurons in the mPFC under control conditions and this effect was not evident or blunted in abstinent rats. Neurons in the DG were unaltered. TFC enhanced the GluN2A/2B ratio in the mPFC and reduced the ratio in the DG and was not altered by abstinence. These findings indicate that forced abstinence from CIE produces distinct and divergent alterations in plasticity proteins in the mPFC and DG. Fear learning-induced changes in structural plasticity and proteins contributing to it were more profound in the mPFC during forced abstinence.

Nuclear GAPDH in cortical microglia mediates cellular stress-induced cognitive inflexibility

We report a mechanism that underlies stress-induced cognitive inflexibility at the molecular level. In a mouse model under subacute cellular stress in which deficits in rule shifting tasks were elicited, the nuclear glyceraldehyde dehydrogenase (N-GAPDH) cascade was activated specifically in microglia in the prelimbic cortex. The cognitive deficits were normalized with a pharmacological intervention with a compound (the RR compound) that selectively blocked the initiation of N-GAPDH cascade without affecting glycolytic activity. The normalization was also observed with a microglia-specific genetic intervention targeting the N-GAPDH cascade. At the mechanistic levels, the microglial secretion of High-Mobility Group Box (HMGB), which is known to bind with and regulate the NMDA-type glutamate receptors, was elevated. Consequently, the hyperactivation of the prelimbic layer 5 excitatory neurons, a neural substrate for cognitive inflexibility, was also observed. The upregulation of the microglial HMGB signaling and neuronal hyperactivation were normalized by the pharmacological and microglia-specific genetic interventions. Taken together, we show a pivotal role of cortical microglia and microglia-neuron interaction in stress-induced cognitive inflexibility. We underscore the N-GAPDH cascade in microglia, which causally mediates stress-induced cognitive alteration.

Chronic Ethanol Exposure Produces Persistent Impairment in Cognitive Flexibility and Decision Signals in the Striatum

Lack of cognitive flexibility is a hallmark of substance use disorders and has been associated with drug-induced synaptic plasticity in the dorsomedial striatum (DMS). Yet the possible impact of altered plasticity on real-time striatal neural dynamics during decision-making is unclear. Here, we identified persistent impairments induced by chronic ethanol (EtOH) exposure on cognitive flexibility and striatal decision signals. After a substantial withdrawal period from prior EtOH vapor exposure, male, but not female, rats exhibited reduced adaptability and exploratory behavior during a dynamic decision-making task. Reinforcement learning models showed that prior EtOH exposure enhanced learning from rewards over omissions. Notably, neural signals in the DMS related to the decision outcome were enhanced, while those related to choice and choice-outcome conjunction were reduced, in EtOH-treated rats compared to the controls. These findings highlight the profound impact of chronic EtOH exposure on adaptive decision-making, pinpointing specific changes in striatal representations of actions and outcomes as underlying mechanisms for cognitive deficits.

Chronic alcohol-induced long-lasting working memory deficits are associated with altered histone H3K9 dimethylation in the prefrontal cortex

Introduction

Epigenetic modifications have emerged as key contributors to the enduring behavioral, molecular and epigenetic neuroadaptations during withdrawal from chronic alcohol exposure. The present study investigated the long-term consequences of chronic alcohol exposure on spatial working memory (WM) and associated changes of transcriptionally repressive histone H3 lysine 9 dimethylation (H3K9me2) in the prefrontal cortex (PFC).

Methods

Male C57BL/6 mice were allowed free access to either 12% (v/v) ethanol for 5 months followed by a 3-week abstinence period or water. Spatial WM was assessed through the spontaneous alternation T-maze test. Alcoholic and water mice received daily injections of GABAB agonist baclofen or saline during alcohol fading and early withdrawal. Global levels of histone modifications were determined by immunohistochemistry.

Results

Withdrawal mice displayed WM impairments along with reduced prefrontal H3K9me2 levels, compared to water-drinking mice. The withdrawal-induced decrease of H3K9me2 occurred concomitantly with increased level of permissive H3K9 acetylation (H3K9ac) in the PFC. Baclofen treatment rescued withdrawal-related WM deficits and fully restored prefrontal H3K9me2 and H3K9ac. Alcohol withdrawal induced brain region-specific changes of H3K9me2 and H3K9ac after testing, with significant decreases of both histone marks in the dorsal hippocampus and no changes in the amygdala and dorsal striatum. Furthermore, the magnitude of H3K9me2 in the PFC, but not the hippocampus, significantly and positively correlated with individual WM performances. No correlation was observed between H3K9ac and behavioral performance. Results also indicate that pre-testing intraperitoneal injection of UNC0642, a selective inhibitor of histone methyltransferase G9a responsible for H3K9me2, led to WM impairments in water-drinking and withdrawal-baclofen mice. Collectively, our results demonstrate that alcohol withdrawal induced brain-region specific alterations of H3K9me2 and H3K9ac, an effect that persisted for at least three weeks after cessation of chronic alcohol intake.

Conclusion

The findings suggest a role for long-lasting decreased H3K9me2 specifically in the PFC in the persistent WM impairments related to alcohol withdrawal.

Prefrontal cortex glutamatergic adaptations in a mouse model of alcohol use disorder

Alcohol use disorder (AUD) produces cognitive deficits, indicating a shift in prefrontal cortex (PFC) function. PFC glutamate neurotransmission is mostly mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type ionotropic receptors (AMPARs); however preclinical studies have mostly focused on other receptor subtypes. Here we examined the impact of early withdrawal from chronic ethanol on AMPAR function in the mouse medial PFC (mPFC). Dependent male C57BL/6J mice were generated using the chronic intermittent ethanol vapor-two bottle choice (CIE-2BC) paradigm. Non-dependent mice had access to water and ethanol bottles but did not receive ethanol vapor. Naïve mice had no ethanol exposure. We used patch-clamp electrophysiology to measure glutamate neurotransmission in layer 2/3 prelimbic mPFC pyramidal neurons. Since AMPAR function can be impacted by subunit composition or plasticity-related proteins, we probed their mPFC expression levels. Dependent mice had higher spontaneous excitatory postsynaptic current (sEPSC) amplitude and kinetics compared to the Naïve/Non-dependent mice. These effects were seen during intoxication and after 3-8 days withdrawal, and were action potential-independent, suggesting direct enhancement of AMPAR function. Surprisingly, 3 days withdrawal decreased expression of genes encoding AMPAR subunits (Gria1/2) and synaptic plasticity proteins (Dlg4 and Grip1) in Dependent mice. Further analysis within the Dependent group revealed a negative correlation between Gria1 mRNA levels and ethanol intake. Collectively, these data establish a role for mPFC AMPAR adaptations in the glutamatergic dysfunction associated with ethanol dependence. Future studies on the underlying AMPAR plasticity mechanisms that promote alcohol reinforcement, seeking, drinking and relapse behavior may help identify new targets for AUD treatment.

Psilocybin-induced default mode network hypoconnectivity is blunted in alcohol-dependent rats

Alcohol Use Disorder (AUD) adversely affects the lives of millions of people, but still lacks effective treatment options. Recent advancements in psychedelic research suggest psilocybin to be potentially efficacious for AUD. However, major knowledge gaps remain regarding (1) psilocybin's general mode of action and (2) AUD-specific alterations of responsivity to psilocybin treatment in the brain that are crucial for treatment development. Here, we conducted a randomized, placebo-controlled crossover pharmaco-fMRI study on psilocybin effects using a translational approach with healthy rats and a rat model of alcohol relapse. Psilocybin effects were quantified with resting-state functional connectivity using data-driven whole-brain global brain connectivity, network-based statistics, graph theory, hypothesis-driven Default Mode Network (DMN)-specific connectivity, and entropy analyses. Results demonstrate that psilocybin induced an acute wide-spread decrease in different functional connectivity domains together with a distinct increase of connectivity between serotonergic core regions and cortical areas. We could further provide translational evidence for psilocybin-induced DMN hypoconnectivity reported in humans. Psilocybin showed an AUD-specific blunting of DMN hypoconnectivity, which strongly correlated to the alcohol relapse intensity and was mainly driven by medial prefrontal regions. In conclusion, our results provide translational validity for acute psilocybin-induced neural effects in the rodent brain. Furthermore, alcohol relapse severity was negatively correlated with neural responsivity to psilocybin treatment. Our data suggest that a clinical standard dose of psilocybin may not be sufficient to treat severe AUD cases; a finding that should be considered for future clinical trials.

Progenitor Cells Play a Role in Reinstatement of Ethanol Seeking in Adult Male and Female Ethanol Dependent Rats

Female and male glial fibrillary acidic protein-thymidine kinase (GFAP-TK) transgenic rats were made ethanol dependent via a six-week chronic intermittent ethanol vapor (CIE) and ethanol drinking (ED) procedure. During the last week of CIE, a subset of male and female TK rats was fed valcyte to ablate dividing progenitor cells and continued the diet until the end of this study. Following week six, all CIE rats experienced two weeks of forced abstinence from CIE-ED, after which they experienced relapse to drinking, extinction, and reinstatement of ethanol seeking sessions. CIE increased ED in female and male rats, with females having higher ethanol consumption during CIE and relapse sessions compared with males. In both sexes, valcyte reduced the levels of Ki-67-labeled progenitor cells in the subgranular zone of the dentate gyrus and did not alter the levels in the medial prefrontal cortex (mPFC). Valcyte increased ED during relapse, increased lever responses during extinction and, interestingly, enhanced latency to extinguish ethanol-seeking behaviors in males. Valcyte reduced the reinstatement of ethanol-seeking behaviors triggered by ethanol cues in females and males. Reduced seeking by valcyte was associated with the normalization of cytokines and chemokines in plasma isolated from trunk blood, indicating a role for progenitor cells in peripheral inflammatory responses. Reduced seeking by valcyte was associated with increases in tight junction protein claudin-5 and oligodendrogenesis in the dentate gyrus and reduction in microglial activity in the dentate gyrus and mPFC in females and males, demonstrating a role for progenitor cells in the dentate gyrus in dependence-induced endothelial and microglial dysfunction. These data suggest that progenitor cells born during withdrawal and abstinence from CIE in the dentate gyrus are aberrant and could play a role in strengthening ethanol memories triggered by ethanol cues via central and peripheral immune responses.

Sex-specific biobehavioral regulation of persistent inflammatory pain by alcohol

BACKGROUND

Although a large percentage of chronic pain patients consume alcohol to manage their pain, there is a significant gap in knowledge regarding the mechanisms underlying the antinociceptive effects of alcohol.

METHODS

To determine the longitudinal analgesic effects of alcohol, we utilized the complete Freund's adjuvant (CFA) model of inflammatory pain in adult female and male Wistar rats. Both somatic and negative motivational aspects of pain were measured using the electronic von Frey (mechanical nociception) system, thermal probe test (thermal nociception), and mechanical conflict avoidance task (pain avoidance-like behavior). Tests were conducted at baseline and 1 and 3 weeks following intraplantar CFA or saline administration. At both time points post-CFA, animals were treated with each of three doses of alcohol (intraperitoneal; 0, 0.5, and 1.0 g/kg) over separate days in a Latin square design.

RESULTS

Alcohol produced dose-dependent mechanical analgesia and antihyperalgesia in females but only antihyperalgesia in males. Although alcohol continued to attenuate CFA-induced decreases in both thermal and mechanical nociceptive thresholds between 1 and 3 weeks post-CFA, it appeared less effective at increasing thresholds 3 weeks after CFA induction.

CONCLUSIONS

These data suggest that individuals may develop tolerance to alcohol's ability to alleviate both somatic and negative motivational symptoms of chronic pain over time. We also discovered sex-specific neuroadaptations in protein kinase A-dependent phosphorylation of GluR1 subunits and extracellular signal-regulated kinase (ERK 1/2) phosphorylation in nociceptive brain centers of animals receiving an alcohol challenge 1 week post-CFA. Together, these findings illustrate a sex-specific regulation of behavioral and neurobiological indices of persistent pain by alcohol.

Cognitive Alterations in Addictive Disorders: A Translational Approach

The cognitive decline in people with substance use disorders is well known and can be found during both the dependence and drug abstinence phases. At the clinical level, cognitive decline impairs the response to addiction treatment and increases dropout rates. It can be irreversible, even after the end of drug abuse consumption. Improving our understanding of the molecular and cellular alterations associated with cognitive decline could be essential to developing specific therapeutic strategies for its treatment. Developing animal models to simulate drug abuse-induced learning and memory alterations is critical to continue exploring this clinical situation. The main aim of this review is to summarize the most recent evidence on cognitive impairment and the associated biological markers in patients addicted to some of the most consumed drugs of abuse and in animal models simulating this clinical situation. The available information suggests the need to develop more studies to further explore the molecular alterations associated with cognitive impairment, with the ultimate goal of developing new potential therapeutic strategies.

Acute ethanol exposure leads to long-term effects on memory, behavior, and transcriptional regulation in the zebrafish brain

Alcohol consumption is associated with alterations in memory and learning processes in humans and animals. In this context, research models such as the zebrafish (Danio rerio) arise as key organisms in behavioral and molecular studies that attempt to clarify alterations in the Central Nervous System (CNS), like those related to alcohol use. Accordingly, we used the zebrafish as a model to evaluate the effects of ethanol on the learning and memory process, as well as its relationship with behavior and transcriptional regulation of lrfn2, lrrk2, grin1a, and bdnf genes in the brain. To this end, for the memory and learning evaluation, we conducted the Novel Object Recognition test (NOR); for behavior, the Novel Tank test; and for gene transcription, qPCR, after 2 h, 24 h, and 8 days of ethanol exposure. As a result, we noticed in the NOR that after 8 days of ethanol exposure, the control group spent more time exploring the novel object than when compared to 2 h post-exposure, indicating that naturally zebrafish remember familiar objects. In animals in the Treatment group, however, no object recognition behavior was observed, suggesting that alcohol affected the learning and memory processes of the animals and stimulated an anxiolytic effect in them. Regarding transcriptional regulation, 24 h after alcohol exposure, we found hyper-regulation of bdnf and, after 8 days, a hypo-regulation of lrfn2 and lrrk2. To conclude, we demonstrated that ethanol exposure may have influenced learning ability and memory formation in zebrafish, as well as behavior and regulation of gene transcription. These data are relevant for further understanding the application of zebrafish in research associated with ethanol consumption and behavior.

Impact of alcohol exposure on neural development and network formation in human cortical organoids

Prenatal alcohol exposure is the foremost preventable etiology of intellectual disability and leads to a collection of diagnoses known as Fetal Alcohol Spectrum Disorders (FASD). Alcohol (EtOH) impacts diverse neural cell types and activity, but the precise functional pathophysiological effects on the human fetal cerebral cortex are unclear. Here, we used human cortical organoids to study the effects of EtOH on neurogenesis and validated our findings in primary human fetal neurons. EtOH exposure produced temporally dependent cellular effects on proliferation, cell cycle, and apoptosis. In addition, we identified EtOH-induced alterations in post-translational histone modifications and chromatin accessibility, leading to impairment of cAMP and calcium signaling, glutamatergic synaptic development, and astrocytic function. Proteomic spatial profiling of cortical organoids showed region-specific, EtOH-induced alterations linked to changes in cytoskeleton, gliogenesis, and impaired synaptogenesis. Finally, multi-electrode array electrophysiology recordings confirmed the deleterious impact of EtOH on neural network formation and activity in cortical organoids, which was validated in primary human fetal tissues. Our findings demonstrate progress in defining the human molecular and cellular phenotypic signatures of prenatal alcohol exposure on functional neurodevelopment, increasing our knowledge for potential therapeutic interventions targeting FASD symptoms.

Distinct cortico-striatal compartments drive competition between adaptive and automatized behavior

Cortical and basal ganglia circuits play a crucial role in the formation of goal-directed and habitual behaviors. In this study, we investigate the cortico-striatal circuitry involved in learning and the role of this circuitry in the emergence of inflexible behaviors such as those observed in addiction. Specifically, we develop a computational model of cortico-striatal interactions that performs concurrent goal-directed and habit learning. The model accomplishes this by distinguishing learning processes in the dorsomedial striatum (DMS) that rely on reward prediction error signals as distinct from the dorsolateral striatum (DLS) where learning is supported by salience signals. These striatal subregions each operate on unique cortical input: the DMS receives input from the prefrontal cortex (PFC) which represents outcomes, and the DLS receives input from the premotor cortex which determines action selection. Following an initial learning of a two-alternative forced choice task, we subjected the model to reversal learning, reward devaluation, and learning a punished outcome. Behavior driven by stimulus-response associations in the DLS resisted goal-directed learning of new reward feedback rules despite devaluation or punishment, indicating the expression of habit. We repeated these simulations after the impairment of executive control, which was implemented as poor outcome representation in the PFC. The degraded executive control reduced the efficacy of goal-directed learning, and stimulus-response associations in the DLS were even more resistant to the learning of new reward feedback rules. In summary, this model describes how circuits of the dorsal striatum are dynamically engaged to control behavior and how the impairment of executive control by the PFC enhances inflexible behavior.

Animal models for studies of alcohol effects on the trajectory of age-related cognitive decline

There is growing interest in understanding how ethanol use interacts with advancing age to influence the brain and cognition. Animal models are employed to investigate the cellular and molecular mechanisms of brain aging and age-related neurodegenerative diseases that underlie cognitive decline. However, all too often research on problems and diseases of the elderly are conducted in healthy young animals, providing little clinical relevance. The validity of animal models is discussed, and confounds due to age-related differences in anxiety, sensory-motor function, and procedural learning are highlighted in order to enhance the successful translation of preclinical results into clinical settings. The mechanism of action of ethanol on brain aging will depend on the dose, acute or chronic treatment, or withdrawal from treatment and the age examined. Due to the fact that humans experience alcohol use throughout life, important questions concern the effects of the dose and duration of ethanol treatment on the trajectory of cognitive function. Central to this research will be questions of the specificity of alcohol effects on cognitive functions and related brain regions that decline with age, as well as the interaction of alcohol with mechanisms or biomarkers of brain aging. Alternatively, moderate alcohol use may provide a source of reserve and resilience against brain aging. Longitudinal studies have the advantage of being sensitive to detecting the effects of treatment on the emergence of cognitive impairment in middle age and can minimize effects of stress/anxiety associated with the novelty of alcohol exposure and behavioral testing, which disproportionately influence aged animals. Finally, the effect of alcohol on senescent neurophysiology and biomarkers of brain aging are discussed. In particular, the interaction of age and effects of alcohol on inflammation, oxidative stress, N-methyl-d-aspartate receptor function, and the balance of excitatory and inhibitory synaptic transmission are highlighted.

Modulation of neuronal excitability by binge alcohol drinking

Drug use poses a serious threat to health systems throughout the world. The number of consumers rises every year being alcohol the drug of abuse most consumed causing 3 million deaths (5.3% of all deaths) worldwide and 132.6 million disability-adjusted life years. In this review, we present an up-to-date summary about what is known regarding the global impact of binge alcohol drinking on brains and how it affects the development of cognitive functions, as well as the various preclinical models used to probe its effects on the neurobiology of the brain. This will be followed by a detailed report on the state of our current knowledge of the molecular and cellular mechanisms underlying the effects of binge drinking on neuronal excitability and synaptic plasticity, with an emphasis on brain regions of the meso-cortico limbic neurocircuitry.

Significantly elevated phosphatidylethanol levels in recent suicide attempters, but not in depressed controls and healthy volunteers

INTRODUCTION

Suicide is a complex transdiagnostic phenomenon. It is strongly associated with, but not exclusive to major depressive disorder (MDD). Hazardous alcohol drinking has also been linked to an increased risk of suicidal behaviours, however, it is often underreported. The study aimed to evaluate whether an objective measure of chronic alcohol use, phosphatidylethanol (PEth) could be useful as a biomarker in clinical practice.

METHOD

ology. The present case-control multi-centric study recruited 156 participants into three study groups: 52 patients treated for major depressive disorder (MDD), 51 individuals immediately following a suicide attempt (SA), and 53 volunteers. Sociodemographic data, medical history, and laboratory data, including PEth concentrations and C-reactive protein levels, were collected from study participants.

RESULTS

PEth concentrations were the highest in suicide attempters (232,54 ± 394,01 ng/ml), followed by patients with MDD (58,39 ± 135,82 ng/ml), and the control group (24,45 ± 70,83 ng/ml) (Kruskall Wallis χ2 = 12.23, df = 2, p = .002). In a multinomial logistic regression model with adjustments, PEth concentration was able to predict belonging to suicide attempters' group, but not to depression group (p = .01). Suicide attempters were also more likely to underreport their recent alcohol consumption.

LIMITATIONS

We did not analyze SA methods, psychiatric comorbidity and several other factors that might be associated with PEth levels, such as body mass index, race, and haemoglobin levels. Sample recruited in hospital settings may not be representative of the whole population. The results of this adult-only study cannot be generalized to adolescents.

CONCLUSIONS

PEth levels in recent suicide attempters significantly exceeded those of patients with MDD and controls. Suicide attempters also were more likely to underreport their alcohol consumption when questioned about their consuption. PEth might be an interesting biomarker to evaluate individuals at risk of SA.

Early-life low-level lead exposure alters anxiety-like behavior, voluntary alcohol consumption and AC5 protein content in adult male and female C57BL/6 J mice

Despite efforts to eradicate sources of environmental lead (Pb), children, predominately in lower socioeconomic areas, are still frequently exposed to unsafe levels of Pb from soils, dust, and water. Human studies suggest that Pb exposure is associated with altered drug consumption in adults; however, there is limited research at comparable exposure levels (blood Pb levels <10 μg/dL). To model how early-life, low-level Pb exposure affects alcohol consumption in adulthood, we exposed postnatal day (PND) 21 C57Bl/6 J mice to either 30 ppm or 0 ppm Lead (IV) Acetate in distilled water until PND 42, and testing began in adulthood. We predicted that mice with early-life Pb exposure would exhibit greater anxiety-like behavior and consume more alcohol in a three-week Drinking-in-the-Dark procedure (20% v/v) and a 24-h two-bottle choice procedure (10% v/v). We also predicted that Pb exposure would decrease whole-brain content of Adenylate Cyclase-5 (AC5), a protein linked to anxiety-like behaviors and alcohol drinking. There was no difference in limited-access binge-like consumption between exposure groups; however, Pb-exposed mice displayed higher two-bottle choice alcohol intake and preference. Furthermore, Pb-exposed mice exhibited greater anxiety-like behaviors in experiments conducted before an alcohol drinking history but not after. Finally, Pb-exposed mice exhibited an upregulation of whole-brain AC5 protein content. However, this difference was not found in the nucleus accumbens, dorsomedial or dorsolateral striatum. These findings conclude that early-life Pb exposure alters voluntary alcohol consumption and whole-brain AC5 protein content in adulthood. Future studies are necessary to further understand the mechanism behind how Pb exposure alters alcohol intake.

Alcohol Exposure Induces Depressive and Anxiety-like Behaviors via Activating Ferroptosis in Mice

Alcohol use disorder (AUD) is a global public health problem and is frequently comorbid with mental disorders, including anxiety and depression. Ferroptosis is an iron-dependent cell death, which is involved in the pathological process of various diseases such as neurodegenerative diseases, but the role of ferroptosis in the mediation of AUD and its induced mental disorders is unclear. In this study, we aimed to investigate whether ferroptosis was involved in alcohol-induced depressive and anxiety-like behaviors in mice. Following an 8-week period of intermittent alcohol exposure, the alcohol group showed noticeable depressive and anxiety-like behaviors. In addition, nissl staining revealed that alcohol exposure induced neuron damage in the hippocampus (Hip) and prefrontal cortex (PFC) of mice. The levels of synapse-related proteins were significantly reduced in the alcohol group. Iron staining demonstrated that alcohol increased the number of iron-positive staining cells. The protein expression of the transferrin receptor (TFRC) was increased, and the expression of glutathione peroxidase 4 (GPX4) was decreased, respectively, in the alcohol group. Furthermore, the ferroptosis inhibitor ferrostatin-1 significantly prevented alcohol-induced neuron damage and enhanced the expression of N-methyl-d-aspartic acid (NMDA) receptor 2B (NR2B), α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor 1 (GluA1) and GPX4 in vitro. These results indicated that alcohol exposure could induce depressive and anxiety-like behaviors, and that this effect may occur via activating ferroptosis.

Ethanol withdrawal-induced adaptations in prefrontal corticotropin releasing factor receptor 1-expressing neurons regulate anxiety and conditioned rewarding effects of ethanol

Prefrontal circuits are thought to underlie aberrant emotion contributing to relapse in abstinence; however, the discrete cell-types and mechanisms remain largely unknown. Corticotropin-releasing factor and its cognate type-1 receptor, a prominent brain stress system, is implicated in anxiety and alcohol use disorder (AUD). Here, we tested the hypothesis that medial prefrontal cortex CRF1-expressing (mPFCCRF1+) neurons comprise a distinct population that exhibits neuroadaptations following withdrawal from chronic ethanol underlying AUD-related behavior. We found that mPFCCRF1+ neurons comprise a glutamatergic population with distinct electrophysiological properties and regulate anxiety and conditioned rewarding effects of ethanol. Notably, mPFCCRF1+ neurons undergo unique neuroadaptations compared to neighboring neurons including a remarkable decrease in excitability and glutamatergic signaling selectively in withdrawal, which is driven in part by the basolateral amygdala. To gain mechanistic insight into these electrophysiological adaptations, we sequenced the transcriptome of mPFCCRF1+ neurons and found that withdrawal leads to an increase in colony-stimulating factor 1 (CSF1) in this population. We found that selective overexpression of CSF1 in mPFCCRF1+ neurons is sufficient to decrease glutamate transmission, heighten anxiety, and abolish ethanol reinforcement, providing mechanistic insight into the observed mPFCCRF1+ synaptic adaptations in withdrawal that drive these behavioral phenotypes. Together, these findings highlight mPFCCRF1+ neurons as a critical site of enduring adaptations that may contribute to the persistent vulnerability to ethanol misuse in abstinence, and CSF1 as a novel target for therapeutic intervention for withdrawal-related negative affect.

The effects of acamprosate on prefrontal cortical function are mimicked by CaCl2 and they are influenced by the history of alcohol exposure

Alcohol use disorder is associated with functional changes in the medial prefrontal cortex (mPFC), which include altered glutamatergic transmission and deficits in executive functions that contribute to relapse. Acamprosate (calcium-bis N-acetylhomotaurinate) reduces alcohol craving and relapse, effects that are thought to be mediated by acamprosate's ability to ameliorate alcohol-induced dysregulation of glutamatergic signaling. Treatment with acamprosate and its active moiety calcium (CaCl2) both improve deficits in cognitive flexibility in postdependent mice following chronic intermittent ethanol (CIE) exposure. Here, we show that mice that self-administered alcohol under goal-directed conditions (i.e., operant responding on a fixed-ratio schedule) also display similar deficits in cognitive flexibility and altered glutamatergic signaling in the mPFC, both of which were improved with acamprosate or CaCl2. However, under conditions shown to bias behavior towards habitual responding (operant self-administration after CIE exposure, or on a variable interval schedule), alcohol-induced changes to glutamatergic transmission were unaffected by either acamprosate or CaCl2 treatment. Together, these findings suggest that the variable effects of acamprosate on synaptic signaling may reflect a shift in mPFC networks related to the loss of behavioral control in habitual alcohol-seeking.

Individual differences in behavioral flexibility predict future volitional ethanol consumption in mice

Cognitive control is key to regulating alcohol intake and preventing relapse. Behavioral inflexibility can prevent adaptive strategies such as mindfulness or other relapse-prevention behaviors. In a mouse model we investigated whether individual variability in behavioral flexibility (using attentional set-shifting task; ASST) predicts future alcohol intake. Adult male and female C57BL/6J mice were subjected to ASST using a bowl-digging paradigm where mice identify a baited bowl based on compound odor and textural cues. This was completed prior to any alcohol exposure. Individual performance across mice varied within the group. We integrated several metrics, specifically ASST stage completed, trials to completion, and errors performed to produce an individual performance index measure of behavioral flexibility. Afterward, ASST mice were trained to drink ethanol (15%, v/v, 1 h/day) for 3-4 weeks until intake stabilized. Using this prospective approach, we identified an inverse relationship between behavioral flexibility and drinking-less-flexible mice had a propensity to consume more alcohol. Similar relationships have been identified previously in non-human primates and rats. Our results show that the relationship between alcohol and behavioral flexibility is a robust trait that is conserved across species and can be used in mice to study neural substrates underlying these behaviors.

Dysregulated kappa-opioid receptors in the medial prefrontal cortex contribute to working memory deficits in alcohol dependence

Impaired working memory is one symptom contributing to compromised executive function in alcohol use disorder (AUD). Dysregulation of cortical dynorphin (DYN) and κ-opioid receptors (KORs) has been implicated in alcohol dependence-induced impairment in executive function. The present experiments test the hypothesis that dysregulated medial prefrontal cortex (mPFC) KORs contribute to impaired working memory in alcohol dependence. Alcohol dependence was induced in male Wistar rats via 4 months of intermittent ethanol vapor exposure prior to training/testing in an mPFC-dependent working memory task (delayed nonmatching-to-sample task; DNMST). mPFC KOR function in alcohol-naïve rats was compared with that of alcohol-dependent and nondependent rats using a DYN A-stimulated [35^S ]GTPγS coupling assay. A functional role for mPFC KORs in the regulation of working memory was assessed via intra-mPFC infusions of a KOR agonist prior to assessment in the DNMST, and the contribution of mPFC KORs to compromised working memory in dependence was assessed via mPFC infusions of the KOR antagonist norbinaltorphimine (nor-BNI). In alcohol-dependent rats, impaired performance in the DNMST confirmed compromised working memory. Furthermore, DYN A-stimulated mPFC KOR function was pathologically increased in alcohol-dependent rats compared with nondependent and alcohol-naïve rats. Additionally, mPFC KOR involvement in working memory was functionally confirmed by intra-mPFC KOR agonist-induced deficits in DNMST performance. Importantly, alcohol dependence-induced impairment in the DNMST was ameliorated by intra-mPFC KOR antagonism. Regulation of working memory by mPFC KORs and alcohol dependence-induced dysregulation of mPFC KOR function identify a novel therapeutic target to treat AUD-related symptoms of working memory impairment.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

Effects of Maternal Deprivation on the Prefrontal Cortex of Male Rats: Cellular, Neurochemical, and Behavioral Outcomes

Stressful events experienced during early life are associated with increased vulnerability of developing psychopathology in adulthood. In the present study, we exposed 9-day-old Wistar rats to 24 h maternal deprivation (MD) with the aim to investigate the impact of early life stress (ELS) on morphological, biochemical, and functional aspects of the prefrontal cortex (PFC), a brain region particularly sensitive to stress. We found that in the superficial medial orbital cortex (MO), young adult male rats had reduced density of GAD67 and CCK immunopositive cells, while the rostral part of the ventral lateral orbital cortex (roVLO) showed a decrease in the density of GAD67 immunopositive cells in both superficial and deep layers. In addition, the superficial rostral part of area 1 of the cingulate cortex (roCg1) and deep prelimbic cortex (PrL) was also affected by MD indicated by the reduction in PV immunopositive cellular density. Furthermore, MD induced upregulation of brain-derived neurotrophic factor (BDNF), while it did not affect the overall expression of Iba1 in neonatal or young adult PFC as measured by Western blot, however, microglial activation in young adult MD rats was detected immunohistochemically in deep layers of MO and infralimbic cortex (IL). Interestingly, when young adult male rats were subjected to a behavioral flexibility test in a T-maze, MD rats showed a subtle impairment in T-maze reversal learning indicating a mildly affected PFC function. Taken together, our findings demonstrated that MD reduced the density of interneurons and induced microglial activation, in particular, PFC areas at young adulthood, and could alter synaptic plasticity accompanied by PFC dysfunction.

Psilocybin targets a common molecular mechanism for cognitive impairment and increased craving in alcoholism

Alcohol-dependent patients commonly show impairments in executive functions that facilitate craving and can lead to relapse. However, the molecular mechanisms leading to executive dysfunction in alcoholism are poorly understood, and new effective pharmacological treatments are desired. Here, using a bidirectional neuromodulation approach, we demonstrate a causal link between reduced prefrontal mGluR2 function and both impaired executive control and alcohol craving. A neuron-specific prefrontal mGluR2 knockdown in rats generated a phenotype of reduced cognitive flexibility and excessive alcohol seeking. Conversely, virally restoring prefrontal mGluR2 levels in alcohol-dependent rats rescued these pathological behaviors. In the search for a pharmacological intervention with high translational potential, psilocybin was capable of restoring mGluR2 expression and reducing relapse behavior. Last, we propose a FDG-PET biomarker strategy to identify mGluR2 treatment-responsive individuals. In conclusion, we identified a common molecular pathological mechanism for both executive dysfunction and alcohol craving and provided a personalized mGluR2 mechanism-based intervention strategy for medication development for alcoholism.

Optogenetic manipulation of the prelimbic cortex during fear memory reconsolidation alters fear extinction in a preclinical model of comorbid PTSD/AUD

RATIONALE AND OBJECTIVE

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are disorders of learning and memory that often occur comorbidly. Exposure to trauma-related cues can increase alcohol intake in PTSD patients that are using alcohol to self-medicate. The recurrence of anxiety symptoms with subsequent alcohol use may initiate a destructive cycle where stress and alcohol exposure impair the function of the prefrontal cortex (PFC). While the incidence of these disorders has steadily increased, current therapies and treatments often lack efficacy. Thus, investigation into the underlying neurocircuitry responsible for the establishment and maintenance of these disorders is necessary to develop novel treatment targets.

METHODS

The present study examined the effects of ethanol exposure on the ability to create new learned associations around previously conditioned fear cues in a rat model. Animals were exposed to fear conditioning followed by chronic intermittent ethanol to translationally model trauma exposure followed by alcohol abuse. Optogenetics was used to inhibit the prelimbic (PrL) or infralimbic (IfL) cortex during fear memory reconsolidation, and fear behaviors were measured during subsequent extinction and spontaneous recovery tests. Results and conclusion Chronic ethanol exposure led to deficits in fear extinction learning and increased freezing during spontaneous recovery, both of which were prevented following inhibition of the PrL, but not the IfL, during memory reconsolidation. These results support the involvement of the PrL in fear learning and memory, and strongly suggest that the PrL could serve as a potential target for the treatment of the learning and memory deficits that occur following exposure to stress and alcohol.

Somatostatin neurons control an alcohol binge drinking prelimbic microcircuit in mice

Somatostatin (SST) neurons have been implicated in a variety of neuropsychiatric disorders such as depression and anxiety, but their role in substance use disorders, including alcohol use disorder (AUD), is not fully characterized. Here, we found that repeated cycles of alcohol binge drinking via the Drinking-in-the-Dark (DID) model led to hypoactivity of SST neurons in the prelimbic (PL) cortex by diminishing their action potential firing capacity and excitatory/inhibitory transmission dynamic. We examined their role in regulating alcohol consumption via bidirectional chemogenetic manipulation. Both hM3Dq-induced excitation and KORD-induced silencing of PL SST neurons reduced alcohol binge drinking in males and females, with no effect on sucrose consumption. Alcohol binge drinking disinhibited pyramidal neurons by augmenting SST neurons-mediated GABA release and synaptic strength onto other GABAergic populations and reducing spontaneous inhibitory transmission onto pyramidal neurons. Pyramidal neurons additionally displayed increased intrinsic excitability. Direct inhibition of PL pyramidal neurons via hM4Di was sufficient to reduce alcohol binge drinking. Together these data revealed an SST-mediated microcircuit in the PL that modulates the inhibitory dynamics of pyramidal neurons, a major source of output to subcortical targets to drive reward-seeking behaviors and emotional response.

Sexually dimorphic prelimbic cortex mechanisms play a role in alcohol dependence: protection by endostatin

Angiogenesis or proliferation of endothelial cells plays a role in brain microenvironment homeostasis. Previously we have shown enhanced expression of markers of angiogenesis in the medial prefrontal cortex during abstinence in an animal model of ethanol dependence induced by chronic intermittent ethanol vapor (CIE) and ethanol drinking (ED) procedure. Here we report that systemic injections of the angiogenesis inhibitor endostatin reduced relapse to drinking behavior in female CIE-ED rats without affecting relapse to drinking in male CIE-ED rats, and female and male nondependent ED rats. Endostatin did not alter relapse to sucrose drinking in both sexes. Endostatin reduced expression of platelet endothelial cell adhesion molecule-1 (PECAM-1) in all groups; however, rescued expression of tight junction protein claudin-5 in the prelimbic cortex (PLC) of female CIE-ED rats. In both sexes, CIE-ED enhanced microglial activation in the PLC and this was selectively prevented by endostatin in female CIE-ED rats. Endostatin prevented CIE-ED-induced enhanced NF-kB activity and expression and Fos expression in females and did not alter reduced Fos expression in males. Analysis of synaptic processes within the PLC revealed sexually dimorphic adaptations, with CIE-ED reducing synaptic transmission and altering synaptic plasticity in the PLC in females, and increasing synaptic transmission in males. Endostatin prevented the neuroadaptations in the PLC in females via enhancing phosphorylation of CaMKII, without affecting the neuroadaptations in males. Our multifaceted approach is the first to link PLC endothelial cell damage to the behavioral, neuroimmune, and synaptic changes associated with relapse to ethanol drinking in female subjects, and provides a new therapeutic strategy to reduce relapse in dependent subjects.

Drugs of Abuse Differentially Alter the Neuronal Excitability of Prefrontal Layer V Pyramidal Cell Subtypes

The medial prefrontal cortex (mPFC) plays an important role in regulating executive functions including reward seeking, task flexibility, and compulsivity. Studies in humans have demonstrated that drugs of abuse, including heroin, cocaine, methamphetamine, and alcohol, alter prefrontal function resulting in the consequential loss of inhibitory control and increased compulsive behaviors, including drug seeking. Within the mPFC, layer V pyramidal cells, which are delineated into two major subtypes (type I and type II, which project to subcortical or commissurally to other cortical regions, respectively), serve as the major output cells which integrate information from other cortical and subcortical regions and mediate executive control. Preclinical studies examining changes in cellular physiology in the mPFC in response to drugs of abuse, especially in regard to layer V pyramidal subtypes, are relatively sparse. In the present study, we aimed to explore how heroin, cocaine, methamphetamine, ethanol, and 3,4-methylenedioxypyrovalerone (MDPV) alter the baseline cellular physiology and excitability properties of layer V pyramidal cell subtypes. Specifically, animals were exposed to experimenter delivered [intraperitoneal (i.p.)] heroin, cocaine, the cocaine-like synthetic cathinone MDPV, methamphetamine, ethanol, or saline as a control once daily for five consecutive days. On the fifth day, whole-cell physiology recordings were conducted from type I and type II layer V pyramidal cells in the mPFC. Changes in cellular excitability, including rheobase (i.e., the amount of injected current required to elicit action potentials), changes in input/output curves, as well as spiking characteristics induced by each substance, were assessed. We found that heroin, cocaine, methamphetamine, and MDPV decreased the excitability of type II cells, whereas ethanol increased the excitability of type I pyramidal cells. Together, these results suggest that heroin, cocaine, MDPV, and methamphetamine reduce mPFC commissural output by reducing type II excitability, while ethanol increases the excitability of type I cells targeting subcortical structures. Thus, separate classes of abused drugs differentially affect layer V pyramidal subtypes in the mPFC, which may ultimately give rise to compulsivity and inappropriate synaptic plasticity underlying substance use disorders.

Drug addiction: from bench to bedside

Drug addiction is responsible for millions of deaths per year around the world. Still, its management as a chronic disease is shadowed by misconceptions from the general public. Indeed, drug consumers are often labelled as "weak", "immoral" or "depraved". Consequently, drug addiction is often perceived as an individual problem and not societal. In technical terms, drug addiction is defined as a chronic, relapsing disease resulting from sustained effects of drugs on the brain. Through a better characterisation of the cerebral circuits involved, and the long-term modifications of the brain induced by addictive drugs administrations, first, we might be able to change the way the general public see the patient who is suffering from drug addiction, and second, we might be able to find new treatments to normalise the altered brain homeostasis. In this review, we synthetise the contribution of fundamental research to the understanding drug addiction and its contribution to potential novel therapeutics. Mostly based on drug-induced modifications of synaptic plasticity and epigenetic mechanisms (and their behavioural correlates) and after demonstration of their reversibility, we tried to highlight promising therapeutics. We also underline the specific temporal dynamics and psychosocial aspects of this complex psychiatric disease adding parameters to be considered in clinical trials and paving the way to test new therapeutic venues.

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.

Ethanol modulation of cortico-basolateral amygdala circuits: Neurophysiology and behavior

This review highlights literature relating the anatomy, physiology, and behavioral contributions by projections between rodent prefrontal cortical areas and the basolateral amygdala. These projections are robustly modulated by both environmental experience and exposure to drugs of abuse including ethanol. Recent literature relating optogenetic and chemogenetic dissection of these circuits within behavior both compliments and occasionally challenges roles defined by more traditional pharmacological or lesion-based approaches. In particular, cortico-amygdala circuits help control both aversive and reward-seeking. Exposure to pathology-producing environments or abused drugs dysregulates the relative 'balance' of these outcomes. Modern circuit-based approaches have also shown that overlapping populations of neurons within a given brain region frequently govern both aversion and reward-seeking. In addition, these circuits often dramatically influence 'local' cortical or basolateral amygdala excitatory or inhibitory circuits. Our understanding of these neurobiological processes, particularly in relation to ethanol research, has just begun and represents a significant opportunity.

Chronic intermittent ethanol and lipopolysaccharide exposure differentially alter Iba1-derived microglia morphology in the prelimbic cortex and nucleus accumbens core of male Long-Evans rats

Accumulating evidence has linked pathological changes associated with chronic alcohol exposure to neuroimmune signaling mediated by microglia. Prior characterization of the microglial structure-function relationship demonstrates that alterations in activity states occur concomitantly with reorganization of cellular architecture. Accordingly, gaining a better understanding of microglial morphological changes associated with ethanol exposure will provide valuable insight into how neuroimmune signaling may contribute to ethanol-induced reshaping of neuronal function. Here we have used Iba1-staining combined with high-resolution confocal imaging and 3D reconstruction to examine microglial structure in the prelimbic (PL) cortex and nucleus accumbens (NAc) in male Long-Evans rats. Rats were either sacrificed at peak withdrawal following 15 days of exposure to chronic intermittent ethanol (CIE) or 24 hr after two consecutive injections of the immune activator lipopolysaccharide (LPS), each separated by 24 hr. LPS exposure resulted in dramatic structural reorganization of microglia in the PL cortex, including increased soma volume, overall cellular volume, and branching complexity. In comparison, CIE exposure was associated with a subtle increase in somatic volume and differential effects on microglia processes, which were largely absent in the NAc. These data reveal that microglial activation following a neuroimmune challenge with LPS or exposure to chronic alcohol exhibits distinct morphometric profiles and brain region-dependent specificity.

Alcohol Consumption during Adulthood Does Not Impair Later Go/No-Go Reversal Learning in Male Rats

Reversal learning tasks are used to model flexible decision-making in laboratory animals, and exposure to drugs of abuse can cause long-term impairments in reversal learning. However, the long-term effects of alcohol on reversal learning have varied. We evaluated whether six weeks of voluntary alcohol consumption through chronic intermittent alcohol access (elevated by food restriction) in adult male rats would impair rats in a go/no-go reversal learning task when tested at an interval beyond acute withdrawal. In our go/no-go task, rats were reinforced for pressing one lever or withholding from pressing another lever, and the identities of the two levers were switched twice (once rats reached an accuracy criterion). We found no evidence that prior alcohol consumption altered discrimination or reversal learning in our task. This replicates previous patterns from our laboratory that higher alcohol consumption in food-restricted rats did not impair discrimination or reversal learning in a different go/no-go task and that alcohol consumption in free-fed adolescent/early adult rats did not impair go/no-go discrimination or reversal learning in the same task. It is unclear whether this represents an insensitivity of this task to alcohol exposure generally or whether an alcohol exposure procedure that leads to higher blood ethanol concentration (BEC) levels would impair learning. More research is needed to investigate these possibilities.

Different Effects of Alcohol Exposure on Action and Outcome-Related Orbitofrontal Cortex Activity

Alcohol dependence can result in long-lasting deficits to decision-making and action control. Neurobiological investigations have identified orbitofrontal cortex (OFC) as important for outcome-related contributions to goal-directed actions during decision-making. Prior work has shown that alcohol dependence induces long-lasting changes to OFC function that persist into protracted withdrawal and disrupts goal-directed control over actions. However, it is unclear whether these changes in function alter representation of action and outcome-related neural activity in OFC. Here, we used the well-validated chronic intermittent ethanol (CIE) exposure and withdrawal procedure to model alcohol dependence in mice and performed in vivo extracellular recordings during an instrumental task in which lever-press actions made for a food outcome. We found alcohol dependence disrupted goal-directed action control and increased OFC activity associated with lever-pressing but decreased OFC activity during outcome-related epochs. The ability to decode outcome-related information, but not action information, from OFC activity following CIE exposure was reduced. Hence, chronic alcohol exposure induced a long-lasting disruption to OFC function such that activity associated with actions was enhanced, but OFC activity contributions to outcome-related information was diminished. This has important implications for hypotheses regarding compulsive and habitual phenotypes observed in addiction.

Neurobiology of alcohol seeking behavior

Alcohol addiction is a chronic relapsing brain disease characterized by an impaired ability to stop or control alcohol use despite adverse consequences. A main challenge of addiction treatment is to prevent relapse, which occurs in more than >50% of newly abstinent patients with alcohol disorder within 3 months. In people suffering from alcohol addiction, stressful events, drug-associated cues and contexts, or re-exposure to a small amount of alcohol trigger a chain of behaviors that frequently culminates in relapse. In this review, we first present the preclinical models that were developed for the study of alcohol seeking behavior, namely the reinstatement model of alcohol relapse and compulsive alcohol seeking under a chained schedule of reinforcement. We then provide an overview of the neurobiological findings obtained using these animal models, focusing on the role of opioids systems, corticotropin-release hormone and neurokinins, followed by dopaminergic, glutamatergic, and GABAergic neurotransmissions in alcohol seeking behavior.

Altered Activity of Lateral Orbitofrontal Cortex Neurons in Mice following Chronic Intermittent Ethanol Exposure

The lateral orbitofrontal cortex (LOFC) is thought to encode information associated with consumption of rewarding substances and is essential for flexible decision-making. Indeed, firing patterns of LOFC neurons are modulated following changes in reward value associated with an action outcome relationship. Damage to the LOFC impairs behavioral flexibility in humans and is associated with suboptimal performance in reward devaluation protocols in rodents. As chronic intermittent ethanol (CIE) exposure also impairs OFC-dependent behaviors, we hypothesized that CIE exposure would alter LOFC neuronal activity during alcohol drinking, especially under conditions when the reward value of ethanol was modulated by aversive or appetitive tastants. To test this hypothesis, we monitored LOFC activity using GCaMP6f fiber photometry in mice receiving acute injections of ethanol and in those trained in operant ethanol self-administration. In naive mice, an acute injection of ethanol caused a dose-dependent decrease in the frequency but not amplitude of GCaMP6f transients. In operant studies, mice were trained on a fixed ratio one schedule of reinforcement and were then separated into CIE or Air groups. Following four cycles of CIE exposure, GCaMP6f activity was recorded during self-administration of alcohol, alcohol+quinine (aversive), or alcohol+sucrose (appetitive) solutions. LOFC neurons showed discrete patterns of activity surrounding lever presses and surrounding drinking bouts. Responding for and consumption of ethanol was greatly enhanced by CIE exposure, was aversion resistant, and was associated with signs of LOFC hyperexcitability. CIE-exposed mice also showed altered patterns of LOFC activity that varied with the ethanol solution consumed.

Role of heat shock transcription factor 2 in the NMDA-dependent neuroplasticity induced by chronic ethanol intake in mouse hippocampus

Ethanol consumption impairs learning and memory through disturbances of NMDA-type glutamate receptor-dependent synaptic plasticity (long-term depression [LTD] and long-term potentiation [LTP]) in the hippocampus. Recently, we demonstrated that two ethanol binge-like episodes in young adult rats selectively blocked NMDA-LTD in hippocampal slices, increased NMDA receptor sensitivity to a GluN2B subunit antagonist, and induced cognitive deficits. Here, using knockout adult mice, we show that a stress-responsive transcription factor of the heat shock factor family, HSF2, which is involved in the perturbation of brain development induced by ethanol, participates in these processes. In the absence of ethanol, hsf2^-/- mice show a selective loss of LTD in the hippocampus, which is associated with an increased sensitivity of NMDA-field excitatory postsynaptic potentials (fEPSPs) to a GluN2B antagonist, compared with wild-type (WT) mice. These results suggest that HSF2 is required for proper glutamatergic synaptic transmission and LTD plasticity. After 1 month of chronic ethanol consumption in a two-bottle choice paradigm, WT mice showed an increase in hippocampal synaptic transmission, an enhanced sensitivity to GluN2B antagonist, and a blockade of LTD. In contrast, such modulation of synaptic transmission and plasticity were absent in hsf2^-/- mice. We conclude that HSF2 is an important mediator of both glutamatergic neurotransmission and synaptic plasticity in basal conditions and also mediates ethanol-induced neuroadaptations of the hippocampus network after chronic ethanol intake.

Increased Synaptic Strength and mGlu2/3 Receptor Plasticity on Mouse Prefrontal Cortex Intratelencephalic Pyramidal Cells Following Intermittent Access to Ethanol

BACKGROUND

The medial prefrontal cortex (PFC) is crucial for regulating craving and alcohol seeking in alcohol use disorder (AUD) patients and alcohol seeking in animal models. Maladaptive changes in volitional ethanol (EtOH) intake have been associated with PFC function, yet synaptic adaptations within PFC have not been consistently detected in voluntary drinking rodent models. At least 80% of the neurons in PFC are glutamatergic pyramidal cells. Pyramidal cells provide the predominant cortical output to several brain regions relevant to AUD, including structures within the telencephalon (IT: e.g., basal ganglia, amygdala, other neocortical regions) and outside the telencephalon (ET: e.g., lateral hypothalamus, midbrain monoaminergic structures, thalamus).

METHODS

In addition to their anatomical distinctions, studies from several laboratories have revealed that prefrontal cortical IT and ET pyramidal cells may be differentiated by specific electrophysiological parameters. These distinguishable parameters make it possible to readily classify pyramidal cells into separable subtypes. Here, we employed and validated the hyperpolarization sag ratio as a diagnostic proxy for separating ET (type A) and IT (type B) neurons. We recorded from deep-layer prelimbic PFC pyramidal cells of mice 1 day after 4 to 5 weeks of intermittent access (IA) EtOH exposure.

RESULTS

Membrane properties were not altered by IA EtOH, but excitatory postsynaptic strength onto IT type B neurons was selectively enhanced in slices from IA EtOH mice. The increased excitatory drive was accompanied by enhanced mGlu_2/3 receptor plasticity on IT type B neurons, providing a potential translational approach to mitigate cognitive and motivational changes to PFC function related to binge drinking.

CONCLUSIONS

Together, these studies provide insight into the specific PFC neurocircuits altered by voluntary drinking. In addition, the findings provide an additional rationale for developing compounds that potentiate mGlu₂ and/or mGlu₃ receptor function as potential treatments for AUD.

Adolescent intermittent ethanol exposure does not alter responsiveness to ifenprodil or expression of vesicular GABA and glutamate transporters

Adolescent intermittent ethanol (AIE) exposure in the rat results in a retention of adolescent-like responsiveness to ethanol into adulthood characterized by enhanced sensitivity to socially facilitating and decreased sensitivity to socially suppressing and aversive effects. Similar pattern of responsiveness to social and aversive effects of the selective glutamate NMDA NR2B receptor antagonist ifenprodil is evident in adolescent rats, suggesting that AIE would also retain this pattern of ifenprodil sensitivity into adulthood. Social (Experiment 1) and aversive (measured via conditioned taste aversion; Experiment 2) effects of ifenprodil were assessed in adult male and female rats following AIE exposure. Sensitivity to the social and aversive effects of ifenprodil was not affected by AIE exposure. Experiment 3 assessed protein expression of vesicular transporters of GABA (vGAT) and glutamate (vGlut2) within the prelimbic cortex and nucleus accumbens in adolescents versus adults and in AIE adults versus controls. vGlut2 expression was higher in adolescents relative to adults within the PrL, but lower in the NAc. AIE adults did not retain these adolescent-typical ratios. These findings suggest that AIE is not associated with the retention of adolescent-typical sensitivity to NR2B receptor antagonism, along with no AIE-induced shift in vGlut2 to vGAT ratios.

Interaction of chronic intermittent ethanol and repeated stress on structural and functional plasticity in the mouse medial prefrontal cortex

Stress is a risk factor that plays a considerable role in the development and maintenance of alcohol (ethanol) abuse and relapse. Preclinical studies examining ethanol-stress interactions have demonstrated elevated ethanol drinking, cognitive deficits, and negative affective behaviors in mice. However, the neural adaptations in prefrontal cortical regions that drive these aberrant behaviors produced by ethanol-stress interactions are unknown. In this study, male C57BL/6J mice were exposed to chronic intermittent ethanol (CIE) and repeated forced swim stress (FSS). After two cycles of CIE x FSS, brain slices containing the prelimbic (PrL) and infralimbic (IfL) cortex were prepared for analysis of adaptations in dendritic spines and synaptic plasticity. In the PrL cortex, total spine density was increased in mice exposed to CIE. Immediately following induction of long-term potentiation (LTP), the fEPSP slope was increased in the PrL of CIE x FSS treated mice, indicative of a presynaptic adaptation on post-tetanic potentiation (PTP). In the IfL cortex, CIE exposure regardless of FSS experience resulted in an increase in spine density. FSS alone or when combined with CIE exposure increased PTP following LTP induction. Repeated FSS episodes increased IfL cortical paired-pulse facilitation, a second measure of presynaptic plasticity. In summary, CIE exposure resulted in structural adaptations while repeated stress exposure drove metaplastic changes in presynaptic function, demonstrating distinct morphological and functional changes in PrL and IfL cortical neurons. Thus, the structural and functional adaptations may be one mechanism underlying the development of excessive drinking and cognitive deficits associated with ethanol-stress interactions.

Bidirectional causality between addiction and cognitive deficits

Cognitive deficits are highly comorbid with substance use disorders. Deficits span multiple cognitive domains, are associated with disease severity across substance classes, and persist long after cessation of substance use. Furthermore, recovery of cognitive function during protracted abstinence is highly predictive of treatment adherence, relapse, and overall substance use disorder prognosis, suggesting that addiction may be best characterized as a disease of executive dysfunction. While the association between cognitive deficits and substance use disorders is clear, determining causalities is made difficult by the complex interplay between these variables. Cognitive dysfunction present prior to first drug use can act as a risk factor for substance use initiation, likelihood of pathology, and disease trajectory. At the same time, substance use can directly cause cognitive impairments even in individuals without preexisting deficits. Thus, parsing preexisting risk factors from substance-induced adaptations, and how they may interact, poses significant challenges. Here, focusing on psychostimulants and alcohol, we review evidence from clinical literature implicating cognitive deficits as a risk factor for addiction, a consequence of substance use, and the role the prefrontal cortex plays in these phenomena. We then review corresponding preclinical literature, highlighting the high degree of congruency between animal and human studies, and emphasize the unique opportunity that animal models provide to test causality between cognitive phenotypes and substance use, and to investigate the underlying neurobiology at a cellular and molecular level. Together, we provide an accessible resource for assessing the validity and utility of forward- and reverse-translation between these clinical and preclinical literatures.

Synaptic Plasticity and its Modulation by Alcohol

Alcohol is one of the oldest pharmacological agents used for its sedative/hypnotic effects, and alcohol abuse and alcohol use disorder (AUD) continues to be major public health issue. AUD is strongly indicated to be a brain disorder, and the molecular and cellular mechanism/s by which alcohol produces its effects in the brain are only now beginning to be understood. In the brain, synaptic plasticity or strengthening or weakening of synapses, can be enhanced or reduced by a variety of stimulation paradigms. Synaptic plasticity is thought to be responsible for important processes involved in the cellular mechanisms of learning and memory. Long-term potentiation (LTP) is a form of synaptic plasticity, and occurs via N-methyl-D-aspartate type glutamate receptor (NMDAR or GluN) dependent and independent mechanisms. In particular, NMDARs are a major target of alcohol, and are implicated in different types of learning and memory. Therefore, understanding the effect of alcohol on synaptic plasticity and transmission mediated by glutamatergic signaling is becoming important, and this will help us understand the significant contribution of the glutamatergic system in AUD. In the first part of this review, we will briefly discuss the mechanisms underlying long term synaptic plasticity in the dorsal striatum, neocortex and the hippocampus. In the second part we will discuss how alcohol (ethanol, EtOH) can modulate long term synaptic plasticity in these three brain regions, mainly from neurophysiological and electrophysiological studies. Taken together, understanding the mechanism(s) underlying alcohol induced changes in brain function may lead to the development of more effective therapeutic agents to reduce AUDs.

Role of Hippocampal Neurogenesis in Alcohol Withdrawal Seizures

Chronic alcohol consumption results in alcohol use disorder (AUD). Interestingly, however, sudden alcohol withdrawal (AW) after chronic alcohol exposure also leads to a devastating series of symptoms, referred to as alcohol withdrawal syndromes. One key feature of AW syndromes is to produce phenotypes that are opposite to AUD. For example, while the brain is characterized by a hypoactive state in the presence of alcohol, AW induces a hyperactive state, which is manifested as seizure expression. In this review, we discuss the idea that hippocampal neurogenesis and neural circuits play a key role in neuroadaptation and establishment of allostatic states in response to alcohol exposure and AW. The intrinsic properties of dentate granule cells (DGCs), and their contribution to the formation of a potent feedback inhibitory loop, endow the dentate gyrus with a "gate" function, which can limit the entry of excessive excitatory signals from the cortex into the hippocampus. We discuss the possibility that alcohol exposure and withdrawal disrupts structural development and circuitry integration of hippocampal newborn neurons, and that this altered neurogenesis impairs the gate function of the hippocampus. Failure of this gate function is expected to alter the ratio of excitatory to inhibitory (E/I) signals in the hippocampus and to induce seizure expression during AW. Recent functional studies have shown that specific activation and inhibition of hippocampal newborn DGCs are both necessary and sufficient for the expression of AW-associated seizures, further supporting the concept that neurogenesis-induced neuroadaptation is a critical target to understand and treat AUD and AW-associated seizures.

Acute Ethanol Exposure Enhances Synaptic Plasticity in the Dorsal Striatum in Adult Male and Female Rats

Background:

Acute (ex vivo) and chronic (in vivo) alcohol exposure induces neuroplastic changes in the dorsal striatum, a critical region implicated in instrumental learning.

Objective:

Sex differences are evident in alcohol reward and reinforcement, with female rats consuming higher amount of alcohol in operant paradigms compared to male rats. However, sex differences in the neuroplastic changes produced by acute alcohol in the dorsal striatum have been unexplored.

Methods:

Using electrophysiological recordings from dorsal striatal slices obtained from adult male and female rats, we investigated the effects of ex vivo ethanol exposure on synaptic transmission and synaptic plasticity. Ethanol (44 mM) enhanced basal synaptic transmission in both sexes. Ethanol also enhanced long-term potentiation in both sexes. Other measures of synaptic plasticity including paired-pulse ratio were unaltered by ethanol in both sexes.

Results:

The results suggest that alterations in synaptic plasticity induced by acute ethanol, at a concentration associated with intoxication, could play an important role in alcohol-induced experience-dependent modification of corticostriatal circuits underlying the learning of goal-directed instrumental actions and formation of habits mediating alcohol seeking and taking.

Conclusions:

Taken together, understanding the mechanism(s) underlying alcohol induced changes in corticostriatal function may lead to the development of more effective therapeutic agents to reduce habitual drinking and seeking associated with alcohol use disorders.

Sex and Age Effects on Neurobehavioral Toxicity Induced by Binge Alcohol

Historically, most alcohol neurotoxicity studies were conducted in young adult males and focused on chronic intake. There has been a shift towards studying the effects of alcohol on the adolescent brain, due to alcohol consumption during this formative period disrupting the brain's developmental trajectory. Because the most typical pattern of adolescent alcohol intake is heavy episodic (binge) drinking, there has also been a shift towards the study of binge alcohol-induced neurobehavioral toxicity. It has thus become apparent that binge alcohol damages the adolescent brain and there is increasing attention to sex-dependent effects. Significant knowledge gaps remain in our understanding of the effects of binge alcohol on the female brain, however. Moreover, it is unsettling that population-level studies indicate that the prevalence of binge drinking is increasing among American women, particularly those in older age groups. Although study of adolescents has made it apparent that binge alcohol disrupts ongoing brain maturational processes, we know almost nothing about how it impacts the aging brain, as studies of its effects on the aged brain are relatively scarce, and the study of sex-dependent effects is just beginning. Given the rapidly increasing population of older Americans, it is crucial that studies address age-dependent effects of binge alcohol, and given the increase in binge drinking in older women who are at higher risk for cognitive decline relative to men, studies must encompass both sexes. Because adolescence and older age are both characterized by age-typical brain changes, and because binge drinking is the most common pattern of alcohol intake in both age groups, the knowledge that we have amassed on binge alcohol effects on the adolescent brain can inform our study of its effects on the aging brain. In this review, we therefore cover the current state of knowledge of sex and age-dependent effects of binge alcohol, as well as statistical and methodological considerations for studies aimed at addressing them.