Intra-amygdala muscimol decreases operant ethanol self-administration in dependent rats

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) that are thought to facilitate maladaptive behaviors that interfere 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 functionally altered by chronic ethanol exposure. Our recent work identified 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 exposure significantly increased intrinsic excitability as well as spontaneous excitatory and inhibitory postsynaptic currents (sE/IPSCs) 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 exposure also increased the frequency of sEPSCs 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 exposure. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

A dual-pathway architecture enables chronic stress to disrupt agency and promote habit formation

Chronic stress can change how we learn and, thus, how we make decisions. Here we investigated the neuronal circuit mechanisms that enable this. Using a multifaceted systems neuroscience approach in male and female mice, we reveal a dual pathway, amygdala-striatal neuronal circuit architecture by which a recent history of chronic stress disrupts the action-outcome learning underlying adaptive agency and promotes the formation of inflexible habits. We found that the basolateral amygdala projection to the dorsomedial striatum is activated by rewarding events to support the action-outcome learning needed for flexible, goal-directed decision making. Chronic stress attenuates this to disrupt action-outcome learning and, therefore, agency. Conversely, the central amygdala projection to the dorsomedial striatum mediates habit formation. Following stress this pathway is progressively recruited to learning to promote the premature formation of inflexible habits. Thus, stress exerts opposing effects on two amygdala-striatal pathways to disrupt agency and promote habit. These data provide neuronal circuit insights into how chronic stress shapes learning and decision making, and help understand how stress can lead to the disrupted decision making and pathological habits that characterize substance use disorders and mental health conditions.

Encoding and context-dependent control of reward consumption within the central nucleus of the amygdala

Dysregulation of the central amygdala is thought to underlie aberrant choice in alcohol use disorder, but the role of central amygdala neural activity during reward choice and consumption is unclear. We recorded central amygdala neurons in male rats as they consumed alcohol or sucrose. We observed activity changes at the time of reward approach, as well as lick-entrained activity during ongoing consumption of both rewards. In choice scenarios where rats could drink sucrose, alcohol, or quinine-adulterated alcohol with or without central amygdala optogenetic stimulation, rats drank more of stimulation-paired options when the two bottles contained identical options. Given a choice among different options, central amygdala stimulation usually enhanced consumption of stimulation-paired rewards. However, optogenetic stimulation during consumption of the less-preferred option, alcohol, was unable to enhance alcohol intake while sucrose was available. These findings indicate that the central amygdala contributes to refining motivated pursuit toward the preferred available option.

Neuronal nicotinic acetylcholine receptor of the central amygdala modulates the ethanol-induced tolerance to anxiolysis and withdrawal-induced anxiety in male rats

The nicotine acetylcholinergic receptor (nAchR) in the central nucleus of the amygdala (CeA) is known to modulate anxiety traits as well as ethanol-induced behavioral effects. Therefore, the present study investigated the role of CeA nAChR in the tolerance to ethanol anxiolysis and withdrawal-induced anxiety-related effects in rats on elevated plus maze (EPM). To develop ethanol dependence, rats were given free access to an ethanol-containing liquid diet for 10 days. To assess the development of tolerance, separate groups of rats were challenged with ethanol (2 g/kg, i.p.) on days 1, 3, 5, 7 and 10 during the period of ethanol exposure, followed by an EPM assessment. Moreover, expression of ethanol withdrawal was induced after switching ethanol-dependent rats to a liquid diet on day 11, and withdrawal-induced anxiety-like behavior was noted at different post-withdrawal time points using the EPM test. The ethanol-dependent rats were pretreated with intra-CeA (i.CeA) (bilateral) injections of nicotine (0.25 µg/rat) or mecamylamine (MEC) (5 ng/rat) before the challenge dose of ethanol on subthreshold tolerance on the 5th day or on peak tolerance day, that is, 7th or 10th, and before assessment of postwithdrawal anxiety on the 11th day on EPM. Bilateral i.CeA preadministration of nicotine before the challenge dose of ethanol on days 5, 7 and 10 exhibited enhanced tolerance, while injection of MEC, completely mitigated the tolerance to the ethanol-induced antianxiety effect. On the other hand, ethanol-withdrawn rats pretreated i.CeA with nicotine exacerbated while pretreatment with MEC, alleviated the ethanol withdrawal-induced anxiety on all time points. Thus, the present investigation indicates that stimulation of nAChR in CeA negatively modulates the ethanol-induced chronic behavioral effects on anxiety in rats. It is proposed that nAChR antagonists might be useful in the treatment of alcohol use disorder and ethanol withdrawal-related anxiety-like behavior.

LY2444296, a κ-opioid receptor antagonist, selectively reduces alcohol drinking in male and female Wistar rats with a history of alcohol dependence

Alcohol use disorder (AUD) remains a major public health concern. The dynorphin (DYN)/κ-opioid receptor (KOP) system is involved in actions of alcohol, particularly its withdrawal-associated negative affective states. This study tested the ability of LY2444296, a selective, short-acting, KOP antagonist, to decrease alcohol self-administration in dependent male and female Wistar rats at 8 h abstinence. Animals were trained to orally self-administer 10% alcohol (30 min/day for 21 sessions) and were made dependent via chronic intermittent alcohol vapor exposure for 6 weeks or exposed to air (nondependent). After 6 weeks, the effect of LY2444296 (0, 3, and 10 mg/kg, p.o.) was tested on alcohol self-administration at 8 h of abstinence. A separate cohort of rats was prepared in parallel, and their somatic withdrawal signs and alcohol self-administration were measured after LY2444296 administration at 8 h, 2 weeks, and 4 weeks abstinence. LY2444296 at 3 and 10 mg/kg significantly reduced physical signs of withdrawal in dependent rats at 8 h abstinence, only. Furthermore, 3 and 10 mg/kg selectively decreased alcohol self-administration in dependent rats at only 8 h abstinence. These results highlight the DYN/KOP system in actions of alcohol during acute abstinence, suggesting KOP antagonism could be beneficial for mitigating acute withdrawal signs and, in turn, significantly reduce excessive alcohol consumption associated with AUD.

Animal Models of Excessive Alcohol Consumption in Rodents

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

Astrocytic GABAergic Regulation in Alcohol Use and Major Depressive Disorders

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system (CNS). Most GABAergic neurons synthesize GABA from glutamate and release it in the synaptic cleft in the CNS. However, astrocytes can also synthesize and release GABA, activating GABA receptors in the neighboring neurons in physiological and pathological conditions. As the primary homeostatic glial cells in the brain, astrocytes play a crucial role in regulating GABA homeostasis and synaptic neurotransmission. Accumulating evidence demonstrates that astrocytic GABA dysregulation is implicated in psychiatric disorders, including alcohol use disorder (AUD) and major depressive disorder (MDD), the most prevalent co-occurring psychiatric disorders. Several current medications and emerging pharmacological agents targeting GABA levels are in clinical trials for treating AUD and MDD. This review offers a concise summary of the role of astrocytic GABA regulation in AUD and MDD. We also provide an overview of the current understanding and areas of debate regarding the mechanisms by which astrocytes regulate GABA in the CNS and their potential significance in the molecular basis of AUD and MDD, paving the way toward future research directions and potential therapeutic target areas within this field.

Brain proteomic atlas of alcohol use disorder in adult males

Alcohol use disorder (AUD) affects transcriptomic, epigenetic and proteomic expression in several organs, including the brain. There has not been a comprehensive analysis of altered protein abundance focusing on the multiple brain regions that undergo neuroadaptations occurring in AUD. We performed a quantitative proteomic analysis using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of human postmortem tissue from brain regions that play key roles in the development and maintenance of AUD, the amygdala (AMG), hippocampus (HIPP), hypothalamus (HYP), nucleus accumbens (NAc), prefrontal cortex (PFC) and ventral tegmental area (VTA). Brain tissues were from adult males with AUD (n = 11) and matched controls (n = 16). Across the two groups, there were >6000 proteins quantified with differential protein abundance in AUD compared to controls in each of the six brain regions. The region with the greatest number of differentially expressed proteins was the AMG, followed by the HYP. Pathways associated with differentially expressed proteins between groups (fold change > 1.5 and LIMMA p < 0.01) were analyzed by Ingenuity Pathway Analysis (IPA). In the AMG, adrenergic, opioid, oxytocin, GABA receptor and cytokine pathways were among the most enriched. In the HYP, dopaminergic signaling pathways were the most enriched. Proteins with differential abundance in AUD highlight potential therapeutic targets such as oxytocin, CSNK1D (PF-670462), GABAB receptor and opioid receptors and may lead to the identification of other potential targets. These results improve our understanding of the molecular alterations of AUD across brain regions that are associated with the development and maintenance of AUD. Proteomic data from this study is publicly available at www.lmdomics.org/AUDBrainProteomeAtlas/ .

Neurotoxicology of alcohol: a bibliometric and science mapping analysis

Alcohol consumption is common in many societies and has increased considerably, resulting in many socioeconomic and public health problems. In this sense, studies have been carried out in order to understand the mechanisms involved in alcohol consumption and related harmful effects. This study aimed to identify and map the knowledge and to perform bibliometric analysis of the neurotoxicology of alcohol based on the 100 most cited articles. A search was carried out in the Web of Science Core Collection database and information was extracted regarding the journal, authors, keywords, year of publication, number of citations, country and continent of the corresponding author. For each selected manuscript, the study design, alcohol exposure model, dose, period of exposure, and effect on the central nervous system and research hotspots were mapped. The journal with the highest number of publications was Alcoholism: Clinical and Experimental Research (n = 11 papers), the author who contributed the most was Crews FT (n = 8 papers), the studies had a total of 288 keywords and 75% of the publications were from the United States of America. The experimental studies evaluated the effects of prenatal and postnatal exposure and were conducted in rats and mice using doses ranging from 2.5 to 14 g/kg/day, with administration by subcutaneous, intraperitoneal, intragastric, or inhalation route or with free access through drinking bottles. Among the studies mapped, the oldest one (1989) aimed to understand the systemic damage and mechanisms of action involved, while the most recent focused on understanding the receptors and mechanisms involved in addiction, as well as genetic factors. Our results show the panorama of the most widespread scientific production in the scientific community on the neurotoxicology of ethanol, a high prevalence was observed in studies that addressed fetal alcohol syndrome and/or the effects of ethanol on neurodevelopment.

Theoretical Frameworks and Mechanistic Aspects of Alcohol Addiction: Alcohol Addiction as a Reward Deficit/Stress Surfeit Disorder

Alcohol use disorder (AUD) can be defined by a compulsion to seek and take alcohol, the loss of control in limiting intake, and the emergence of a negative emotional state when access to alcohol is prevented. Alcohol use disorder impacts multiple motivational mechanisms and can be conceptualized as a disorder that includes a progression from impulsivity (positive reinforcement) to compulsivity (negative reinforcement). Compulsive drug seeking that is associated with AUD can be derived from multiple neuroadaptations, but the thesis argued herein is that a key component involves the construct of negative reinforcement. Negative reinforcement is defined as drug taking that alleviates a negative emotional state. The negative emotional state that drives such negative reinforcement is hypothesized to derive from the dysregulation of specific neurochemical elements that are involved in reward and stress within basal forebrain structures that involve the ventral striatum and extended amygdala, respectively. Specific neurochemical elements in these structures include decreases in reward neurotransmission (e.g., decreases in dopamine and opioid peptide function in the ventral striatum) and the recruitment of brain stress systems (e.g., corticotropin-releasing factor [CRF]) in the extended amygdala, which contributes to hyperkatifeia and greater alcohol intake that is associated with dependence. Glucocorticoids and mineralocorticoids may play a role in sensitizing the extended amygdala CRF system. Other components of brain stress systems in the extended amygdala that may contribute to the negative motivational state of withdrawal include norepinephrine in the bed nucleus of the stria terminalis, dynorphin in the nucleus accumbens, hypocretin and vasopressin in the central nucleus of the amygdala, and neuroimmune modulation. Decreases in the activity of neuropeptide Y, nociception, endocannabinoids, and oxytocin in the extended amygdala may also contribute to hyperkatifeia that is associated with alcohol withdrawal. Such dysregulation of emotional processing may also significantly contribute to pain that is associated with alcohol withdrawal and negative urgency (i.e., impulsivity that is associated with hyperkatifeia during hyperkatifeia). Thus, an overactive brain stress response system is hypothesized to be activated by acute excessive drug intake, to be sensitized during repeated withdrawal, to persist into protracted abstinence, and to contribute to the compulsivity of AUD. The combination of the loss of reward function and recruitment of brain stress systems provides a powerful neurochemical basis for a negative emotional state that is responsible for the negative reinforcement that at least partially drives the compulsivity of AUD.

Encoding and context-dependent control of reward consumption within the central nucleus of the amygdala

The ability to evaluate and select a preferred option among a variety of available offers is an essential aspect of goal-directed behavior. Dysregulation of this valuation process is characteristic of alcohol use disorder, with the central amygdala being implicated in persistent alcohol pursuit. However, the mechanism by which the central amygdala encodes and promotes the motivation to seek and consume alcohol remains unclear. We recorded single-unit activity in male Long-Evans rats as they consumed 10% ethanol or 14.2% sucrose. We observed significant activity at the time of approach to alcohol or sucrose, as well as lick-entrained activity during the ongoing consumption of both alcohol and sucrose. We then evaluated the ability of central amygdala optogenetic manipulation time-locked to consumption to alter ongoing intake of alcohol or sucrose, a preferred non-drug reward. In closed two-choice scenarios where rats could drink only sucrose, alcohol, or quinine-adulterated alcohol with or without central amygdala stimulation, rats drank more of stimulation-paired options. Microstructural analysis of licking patterns suggests these effects were mediated by changes in motivation, not palatability. Given a choice among different options, central amygdala stimulation enhanced consumption if the stimulation was associated with the preferred reward while closed-loop inhibition only decreased consumption if the options were equally valued. However, optogenetic stimulation during consumption of the less-preferred option, alcohol, was unable to enhance overall alcohol intake while sucrose was available. Collectively, these findings indicate that the central amygdala processes the motivational value of available offers to promote pursuit of the most preferred available option.

Negative modulation of AMPA receptors bound to transmembrane AMPA receptor regulatory protein γ-8 blunts the positive reinforcing properties of alcohol and sucrose in a brain region-dependent manner in male mice

RATIONALE

The development and progression of alcohol use disorder (AUD) are widely viewed as maladaptive neuroplasticity. The transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) regulatory protein γ8 (TARP γ-8) is a molecular mechanism of neuroplasticity that has not been evaluated in AUD or other addictions.

OBJECTIVE

To address this gap in knowledge, we evaluated the mechanistic role of TARP γ-8 bound AMPAR activity in the basolateral amygdala (BLA) and ventral hippocampus (vHPC) in the positive reinforcing effects of alcohol, which drive repetitive alcohol use throughout the course of AUD, in male C57BL/6 J mice. These brain regions were selected because they exhibit high levels of TARP γ-8 expression and send glutamate projections to the nucleus accumbens (NAc), which is a key nucleus in the brain reward pathway.

METHODS AND RESULTS

Site-specific pharmacological inhibition of AMPARs bound to TARP γ-8 in the BLA via bilateral infusion of the selective negative modulator JNJ-55511118 (0-2 µg/µl/side) significantly decreased operant alcohol self-administration with no effect on sucrose self-administration in behavior-matched controls. Temporal analysis showed that reductions in alcohol-reinforced response rate occurred > 25 min after the onset of responding, consistent with a blunting of the positive reinforcing effects of alcohol in the absence of nonspecific behavioral effects. In contrast, inhibition of TARP γ-8 bound AMPARs in the vHPC selectively decreased sucrose self-administration with no effect on alcohol.

CONCLUSIONS

This study reveals a novel brain region-specific role of TARP γ-8 bound AMPARs as a molecular mechanism of the positive reinforcing effects of alcohol and non-drug rewards.

Blockade of orexin receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats

BACKGROUND AND PURPOSE

A major problem managing alcohol use disorder is the high vulnerability to relapse, even after long periods of abstinence. Chronic alcohol use dysregulates stress responsivity, rendering this system hyporesponsive and making individuals vulnerable to relapse. Orexin (hypocretin) plays a role in diverse physiological processes, including stress. Orexin neurons in the hypothalamus, project to the infralimbic cortex. This study asked does infralimbic cortex orexin transmission play a significant role in stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats.

EXPERIMENTAL APPROACH

Male and female rats were trained to self-administer 10% alcohol (3 weeks) and then made dependent via chronic intermittent alcohol vapour exposure. Following extinction (5 days·week-1 at 8 h abstinence for 10 sessions), rats received an intra- infralimbic cortex microinfusion of the OX1/2 antagonist TCS 1102 (15 μg/0.5 μl per side) and then tested for footshock stress-induced reinstatement of alcohol seeking. In a separate cohort, orexin regulation of infralimbic cortex neuronal activity at the time of reinstatement was investigated using ex vivo electrophysiology.

KEY RESULTS

TCS 1102 prevented reinstatement in dependent animals only. Moreover, Hcrtr mRNA expression in the hypothalamus and Hcrtr1/2 in the infralimbic cortex increased in alcohol-dependent animals at the time of testing. Dependence dampened basal orexin/OX receptor influence over infralimbic cortex GABAergic synapses (using TCS 1102) allow for greater stimulated orexin effects.

CONCLUSION AND IMPLICATIONS

Infralimbic cortex transmission is implicate in stress-induced reinstatement of alcohol-seeking behaviour in subjects with a history of alcohol dependence and show maladaptive recruitment of infralimbic cortex transmission by alcohol dependence.

Alcohol self-administration and nicotine withdrawal alter biomarkers of stress and inflammation and prefrontal cortex changes in Gβ subunits

Background: Although alcohol and nicotine are often used together, the biological consequences of these substances are not well understood. Identifying shared targets will inform cessation pharmacotherapies and provide a deeper understanding of how co-use of alcohol and nicotine impacts health, including biomarkers of stress and inflammation.Objective: We examined the effects of nicotine exposure and withdrawal on alcohol self-administration (SA), stress and inflammatory biomarkers, and a G-protein coupled receptor subunit (Gβ) in brain areas associated with drug use.Methods: Male rats were trained to SA alcohol and then received a nicotine pump (n = 7-8 per group). We assessed alcohol intake for 12 days during nicotine exposure and then following pump removal to elicit withdrawal. After the behavioral studies, we assessed plasma leptin, corticosterone, and interleukin-1β (IL-1β), and Gβ protein expression in the amygdala, nucleus accumbens (NAc), and prefrontal cortex (PFC).Results: Nicotine exposure or withdrawal did not alter alcohol intake (p > .05). Alcohol and nicotine withdrawal elevated corticosterone levels (p = .015) and decreased Gβ levels in the PFC (p = .004). In the absence of nicotine, alcohol SA suppressed IL-1β levels (p = .039). Chronic exposure to nicotine or withdrawal during alcohol SA did not alter leptin levels or Gβ expression in the amygdala or NAc (p's > .05).Conclusions: The combination of alcohol SA and nicotine withdrawal produced a persistent increase in stress biomarkers and a suppression in Gβ expression in the PFC, providing an important first step toward understanding the common biological mechanisms of alcohol/nicotine misuse.

Alcohol dependence and the ventral hippocampal influence on alcohol drinking in male mice

Examining neural circuits underlying persistent, heavy drinking provides insight into the neurobiological mechanisms driving alcohol use disorder. Facilitated by its connectivity with other parts of the brain such as the nucleus accumbens (NAc), the ventral hippocampus (vHC) supports many behaviors, including those related to reward seeking and addiction. These studies used a well-established mouse model of alcohol (ethanol) dependence. After surgery to infuse DREADD-expressing viruses (hM4Di, hM3Dq, or mCherry-only) into the vHC and position guide cannula above the NAc, male C57BL/6J mice were treated in the CIE drinking model that involved repeated cycles of chronic intermittent alcohol (CIE) vapor or air (CTL) exposure alternating with weekly test drinking cycles in which mice were offered alcohol (15% v/v) 2 h/day. Additionally, smaller groups of mice were evaluated for either cFos expression or glutamate release using microdialysis procedures. In CIE mice expressing inhibitory (hM4Di) DREADDs in the vHC, drinking increased as expected, but CNO (3 mg/kg intraperitoneally [i.p.]) given 30 min before testing did not alter alcohol intake. However, in CTL mice expressing hM4Di, CNO significantly increased alcohol drinking (∼30%; p < 0.05) to levels similar to the CIE mice. The vHC-NAc pathway was targeted by infusing CNO into the NAc (3 or 10 μM/side) 30 min before testing. CNO activation of the pathway in mice expressing excitatory (hM3Dq) DREADDs selectively reduced consumption in CIE mice back to CTL levels (∼35-45%; p < 0.05) without affecting CTL alcohol intake. Lastly, activating the vHC-NAc pathway increased cFos expression and evoked significant glutamate release from the vHC terminals in the NAc. These data indicate that reduced activity of the vHC increases alcohol consumption and that targeted, increased activity of the vHC-NAc pathway attenuates excessive drinking associated with alcohol dependence. Thus, these findings indicate that the vHC and its glutamatergic projections to the NAc are involved in excessive alcohol drinking.

Epigenetic Dysregulation in Alcohol-Associated Behaviors: Preclinical and Clinical Evidence

Alcohol use disorder (AUD) is characterized by loss of control over intake and drinking despite harmful consequences. At a molecular level, AUD is associated with long-term neuroadaptations in key brain regions that are involved in reward processing and decision-making. Over the last decades, a great effort has been made to understand the neurobiological basis underlying AUD. Epigenetic mechanisms have emerged as an important mechanism in the regulation of long-term alcohol-induced gene expression changes. Here, we review the literature supporting a role for epigenetic processes in AUD. We particularly focused on the three most studied epigenetic mechanisms: DNA methylation, Histone modification and non-coding RNAs. Clinical studies indicate an association between AUD and DNA methylation both at the gene and global levels. Using behavioral paradigms that mimic some of the characteristics of AUD, preclinical studies demonstrate that changes in epigenetic mechanisms can functionally impact alcohol-associated behaviors. While many studies support a therapeutic potential for targeting epigenetic enzymes, more research is needed to fully understand their role in AUD. Identification of brain circuits underlying alcohol-associated behaviors has made major advances in recent years. However, there are very few studies that investigate how epigenetic mechanisms can affect these circuits or impact the neuronal ensembles that promote alcohol-associated behaviors. Studies that focus on the role of circuit-specific and cell-specific epigenetic changes for clinically relevant alcohol behaviors may provide new insights on the functional role of epigenetic processes in AUD.

Voluntary and forced exposure to ethanol vapor produces similar escalation of alcohol drinking but differential recruitment of brain regions related to stress, habit, and reward in male rats

A major limitation of the most widely used current animal models of alcohol dependence is that they use forced exposure to ethanol including ethanol-containing liquid diet and chronic intermittent ethanol (CIE) vapor to produce clinically relevant blood alcohol levels (BAL) and addiction-like behaviors. We recently developed a novel animal model of voluntary induction of alcohol dependence using ethanol vapor self-administration (EVSA). However, it is unknown whether EVSA leads to an escalation of alcohol drinking per se, and whether such escalation is associated with neuroadaptations in brain regions related to stress, reward, and habit. To address these issues, we compared the levels of alcohol drinking during withdrawal between rats passively exposed to alcohol (CIE) or voluntarily exposed to EVSA and measured the number of Fos+ neurons during acute withdrawal (16 h) in key brain regions important for stress, reward, and habit-related processes. CIE and EVSA rats exhibited similar BAL and similar escalation of alcohol drinking and motivation for alcohol during withdrawal. Acute withdrawal from EVSA and CIE recruited a similar number of Fos+ neurons in the Central Amygdala (CeA), however, acute withdrawal from EVSA recruited a higher number of Fos+ neurons in every other brain region analyzed compared to acute withdrawal from CIE. In summary, while the behavioral measures of alcohol dependence between the voluntary (EVSA) and passive (CIE) model were similar, the recruitment of neuronal ensembles during acute withdrawal was very different. The EVSA model may be particularly useful to unveil the neuronal networks and pharmacology responsible for the voluntary induction and maintenance of alcohol dependence and may improve translational studies by providing preclinical researchers with an animal model that highlights the volitional aspects of alcohol use disorder.

Alternative use of suvorexant (Belsomra®) for the prevention of alcohol drinking and seeking in rats with a history of alcohol dependence

Alcohol use disorder (AUD) is one of the most treatment-resistant medical conditions globally. The orexin (Orx) system regulates diverse physiological processes, including stress, and is a system of interest for the development of pharmaceuticals to treat substance use disorders, particularly AUD. The present study tested the ability of the dual orexin receptor antagonist suvorexant (SUV), marketed by Merck as Belsomra®, for the treatment of insomnia, to decrease alcohol self-administration and the stress-induced reinstatement of alcohol-seeking behavior in male Wistar rats with a history of alcohol dependence. Rats were trained to orally self-administer 10% alcohol (30 min/day for 3 weeks) and were either made dependent via chronic intermittent alcohol vapor exposure (14 h ON, 10 h OFF) for 6 weeks or exposed to air (non-dependent). Starting on week 7, the effect of SUV (0-20 mg/kg, p.o.) was tested on alcohol self-administration at acute abstinence (8 h after vapor was turned OFF) twice weekly. A separate cohort of rats that were prepared in parallel was removed from alcohol vapor exposure and then subjected to extinction training for 14 sessions. Once extinction was achieved, the rats received SUV (0 and 5 mg/kg, p.o.) and were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. Suvorexant at 5, 10, and 20 mg/kg selectively decreased alcohol intake in dependent rats. Furthermore, 5 mg/kg SUV prevented the stress-induced reinstatement of alcohol-seeking behavior in dependent rats only. These results underscore the significance of targeting the Orx system for the treatment of substance use disorders generally and suggest that repurposing SUV could be an alternative approach for the treatment of AUD.

The lateral habenula is not required for ethanol dependence-induced escalation of drinking

The lateral habenula (LHb) is an epithalamic nuclei that has been shown to signal the aversive properties of ethanol. The present study tested the hypothesis that activity of the LHb is required for the acquisition and/or expression of dependence-induced escalation of ethanol drinking and somatic withdrawal symptoms. Male Sprague-Dawley rats completed 4 weeks of baseline drinking under a standard intermittent access two-bottle choice (2BC) paradigm before undergoing 2 weeks of daily chronic intermittent ethanol (CIE) via vapor inhalation. Following this CIE exposure period, rats resumed 2BC drinking to assess dependence-induced changes in voluntary ethanol consumption. CIE exposed rats exhibited a significant increase in ethanol drinking that was associated with high levels of blood alcohol and a reduction in somatic symptoms of ethanol withdrawal. However, despite robust cFos activation in the LHb during ethanol withdrawal, chemogenetic inhibition of the LHb did not alter either ethanol consumption or somatic signs of ethanol withdrawal. Consistent with this observation, ablating LHb outputs via electrolytic lesions of the fasciculus retroflexus (FR) did not alter the acquisition of somatic withdrawal symptoms or escalation of ethanol drinking in CIE-exposed rats. The LHb controls activity of the rostromedial tegmental nucleus (RMTg), a midbrain nucleus activated by aversive experiences including ethanol withdrawal. During ethanol withdrawal, both FR lesioned and sham control rats exhibited similar cFos activation in the RMTg, suggesting that RMTg activation during ethanol withdrawal does not require LHb input. These data suggest that, at least in male rats, the LHb is not necessary for the acquisition or expression of escalation of ethanol consumption or expression of somatic symptoms of ethanol withdrawal. Overall, our findings provide evidence that the LHb is dispensable for some of the negative consequences of ethanol withdrawal.

Effects of central amygdala chemogenetic manipulation and prior chronic alcohol exposure on Pavlovian-to-instrumental transfer

BACKGROUND

Recent work suggests that a history of chronic alcohol exposure can enhance the influence of nondrug reward cues on ongoing actions. This is often modeled in Pavlovian-to-instrumental transfer (PIT) tasks that examine the interaction between Pavlovian and instrumental learning processes, usually reflected as an increase in action vigor during the presentation of a reward-associated cue. Though prior chronic alcohol exposure strengthens this type of cue-guided behavior, the neural mechanisms underlying such enhancements are not known.

METHODS

In the present work, we examined the contribution of the central amygdala (CeA), a region strongly implicated in PIT behaviors and functionally altered by chronic alcohol exposure. We utilized Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to examine the impact of inhibitory and excitatory CeA manipulation on PIT behaviors in alcohol-naïve mice and mice with a history of chronic intermittent ethano vapor exposure and withdrawal (CIE).

RESULTS

Replicating previous work, we found that a history of CIE strengthened baseline PIT, in the absence of any CeA manipulation. We also found that activation of both inhibitory and excitatory DREADDs expressed in CeA enhanced PIT in alcohol-naïve mice, though the latter markedly reduced response rates. However, in mice exposed to CIE, activation of excitatory DREADD receptors expressed in CeA appeared to weaken PIT.

CONCLUSIONS

These results suggest that alcohol-induced disruptions in amygdala function may contribute to changes in appetitive behaviors, such as cue-guided responding, following chronic exposure to alcohol. Better elucidating the neural mechanisms that underlie disrupted cue-guided behavior following chronic alcohol exposure may help to understand and treat deficits in adaptive behavior associated with chronic alcohol use in humans.

Baclofen decreases compulsive alcohol drinking in rats characterized by reduced levels of GAT-3 in the central amygdala

While most individuals with access to alcohol drink it recreationally, some vulnerable individuals eventually lose control over their intake and progressively develop compulsive alcohol drinking and decreased interest in alternative sources of reinforcement, two key features of addiction. The neural and molecular mechanisms underlying this vulnerability to switch from controlled to compulsive alcohol intake have not been fully elucidated. It has been shown that rats having reduced levels of expression of the gamma-aminobutyric acid (GABA) transporter, GAT-3, in the amygdala tend to persist in seeking and drinking alcohol even when adulterated with quinine, suggesting that pharmacological interventions aimed at restoring GABA homeostasis in these individuals may provide a targeted treatment to limit compulsive alcohol drinking. Here, we tested the hypothesis that the GABA_B receptor agonist baclofen, which decreases GABA release, specifically reduces compulsive alcohol drinking in vulnerable individuals. In a large cohort of Sprague-Dawley rats allowed to drink alcohol under an intermittent two-bottle choice procedure, a cluster of individuals was identified that persisted in drinking alcohol despite adulteration with quinine or when an alternative ingestive reinforcer, saccharin, was available. In these rats, which were characterized by decreased GAT-3 mRNA levels in the central amygdala, acute baclofen administration (1.5 mg/kg, intraperitoneal) resulted in a decrease in compulsive drinking. These results indicate that low GAT-3 mRNA levels in the central amygdala may represent an endophenotype of vulnerability to develop a compulsive drinking of alcohol that is shown here to be mitigated by baclofen.

Effects of ceftriaxone on ethanol drinking and GLT-1 expression in ethanol dependence and relapse drinking

Repeated cycles of chronic intermittent ethanol (CIE) exposure increase voluntary consumption of alcohol (ethanol) in mice. Previous reports from our laboratory show that CIE increases extracellular glutamate in the nucleus accumbens (NAc) and that manipulating accumbal glutamate concentrations will alter ethanol drinking, indicating that glutamate homeostasis plays a crucial role in ethanol drinking in this model. A number of studies have shown that ceftriaxone increases GLT-1 expression, the major glutamate transporter, and that treatment with this antibiotic reduces ethanol drinking. The present studies examined the effects of ceftriaxone on ethanol drinking and GLT-1 in a mouse model of ethanol dependence and relapse drinking. The results show that ceftriaxone did not influence drinking at any dose in either ethanol-dependent or non-dependent mice. Further, ceftriaxone did not increase GLT-1 expression in the accumbens core or shell, with the exception of the ethanol-dependent mice receiving the highest dose of ceftriaxone. Interestingly, ethanol-dependent mice treated with only vehicle displayed reduced expression of GLT-1 in the accumbens shell and of the presynaptic mGlu2 receptor in the accumbens core. The reduced expression of the major glutamate transporter (GLT-1), as well as a receptor that regulates glutamate release (mGlu2), may help explain, at least in part, increased glutamatergic transmission in this model of ethanol dependence and relapse drinking.

Chronic ethanol vapor exposure potentiates cardiovascular responses to acute stress in male but not in female rats

BACKGROUND

Ethanol use is related to a wide variety of negative health outcomes, including cardiovascular diseases. Stress is also involved in numerous pathologies, such as cardiovascular diseases and psychiatric disorders. Sexual dimorphism is an important factor affecting cardiovascular response and has been proposed as a potential risk factor for sex-specific health problems in humans. Here, we evaluated the effect of prolonged ethanol vapor inhalation on arterial pressure, heart rate, and tail skin temperature responses to acute restraint stress, investigating differences between male and female rats.

METHODS

We exposed male and female Long-Evans rats to ethanol vapor for 14 h, followed by ethanol withdrawal for 10 h, for 30 consecutive days, or to room air (control groups). The animals underwent surgical implantation of a cannula into the femoral artery for assessment of arterial pressure and heart rate values. The tail skin temperature was measured as an indirect measurement of sympathetic vasomotor response.

RESULTS

Chronic ethanol vapor inhalation reduced basal heart rate in both female and male rats. Sex-related difference was observed in the decrease of tail cutaneous temperature evoked by stress, but not in the pressor and tachycardiac responses. Furthermore, prolonged ethanol inhalation enhanced the blood pressure and heart rate increase caused by acute restraint stress in male, but not in female rats. However, no effect of chronic ethanol vapor was observed in the tail cutaneous temperature response to restraint in either sex.

CONCLUSION

Chronic ethanol vapor exposure increased the cardiovascular reactivity to stress in male, but not in female rats.

Pituitary adenylate cyclase-activating polypeptide (PACAP) modulates dependence-induced alcohol drinking and anxiety-like behavior in male rats

Alcohol use disorder (AUD) is a devastating illness defined by periods of heavy drinking and withdrawal, often leading to a chronic relapsing course. Initially, alcohol is consumed for its positive reinforcing effects, but later stages of AUD are characterized by drinking to alleviate withdrawal-induced negative emotional states. Brain stress response systems in the extended amygdala are recruited by excessive alcohol intake, sensitized by repeated withdrawal, and contribute to the development of addiction. In this study, we investigated one such brain stress response system, pituitary adenylate cyclase-activating polypeptide (PACAP), and its cognate receptor, PAC1R, in alcohol withdrawal-induced behaviors. During acute withdrawal, rats exposed to chronic intermittent ethanol vapor (ethanol-dependent) displayed a significant increase in PACAP levels in the bed nucleus of the stria terminalis (BNST), a brain area within the extended amygdala critically involved in both stress and withdrawal. No changes in PACAP levels were observed in the central nucleus of the amygdala. Site-specific microinfusion of the PAC1R antagonist PACAP(6-38) into the BNST dose-dependently blocked excessive alcohol intake in ethanol-dependent rats without affecting water intake overall or basal ethanol intake in control, nondependent rats. Intra-BNST PACAP(6-38) also reversed ethanol withdrawal-induced anxiety-like behavior in ethanol-dependent rats, but did not affect this measure in control rats. Our findings show that chronic intermittent exposure to ethanol recruits the PACAP/PAC1R system of the BNST and that these neuroadaptations mediate the heightened alcohol drinking and anxiety-like behavior observed during withdrawal, suggesting that this system represents a major brain stress element responsible for the negative reinforcement associated with the "dark side" of alcohol addiction.

The Role of the Central Amygdala in Alcohol Dependence

Alcohol dependence is a chronically relapsing disorder characterized by compulsive drug-seeking and drug-taking, loss of control in limiting intake, and the emergence of a withdrawal syndrome in the absence of the drug. Accumulating evidence suggests an important role for synaptic transmission in the central nucleus of the amygdala (CeA) in mediating alcohol-related behaviors and neuroadaptive mechanisms associated with alcohol dependence. Acute alcohol facilitates γ-aminobutyric acid (GABA)ergic transmission in the CeA via both pre- and postsynaptic mechanisms, and chronic alcohol increases baseline GABAergic transmission. Acute alcohol inhibits glutamatergic transmission via effects at N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the CeA, whereas chronic alcohol up-regulates NMDA receptor (NMDAR)-mediated transmission. Pro- (e.g., corticotropin-releasing factor [CRF]) and antistress (e.g., nociceptin/orphanin FQ, oxytocin) neuropeptides affect alcohol- and anxiety-related behaviors, and also alter the alcohol-induced effects on CeA neurotransmission. Alcohol dependence produces plasticity in these neuropeptide systems, reflecting a recruitment of those systems during the transition to alcohol dependence.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

Toll-Like Receptor 4 Signaling and Drug Addiction

The emphasis of neuronal alterations and adaptations have long been the main focus of the studies of the mechanistic underpinnings of drug addiction. Recent studies have begun to appreciate the role of innate immune system, especially toll-like receptor 4 (TLR4) signaling in drug reward-associated behaviors and physiology. Drugs like opioids, alcohol and psychostimulants activate TLR4 signaling and subsequently induce proinflammatory responses, which in turn contributes to the development of drug addiction. Inhibition of TLR4 or its downstream effectors attenuated the reinforcing effects of opioids, alcohol and psychostimulants, and this effect is also involved in the withdrawal and relapse-like behaviors of different drug classes. However, conflicting results also argue that TLR4-related immune response may play a minimal part in drug addiction. This review discussed the preclinical evidence that whether TLR4 signaling is involved in multiple drug classes action and the possible mechanisms underlying this effect. Moreover, clinical studies which examined the potential efficacy of immune-base pharmacotherapies in treating drug addiction are also discussed.

Impact of sex, strain, and age on blood ethanol concentration and behavioral signs of intoxication during ethanol vapor exposure

Animal models of alcohol drinking and dependence are a critical resource for understanding the neurobiological mechanisms and development of more effective treatments for alcohol use disorder (AUD). Because most rat strains do not voluntarily consume large enough quantities of alcohol to adequately model heavy drinking, dependence, and withdrawal-related symptoms, researchers frequently turn to experimenter administered methods to investigate how prolonged and repeated exposure to large quantities of alcohol impacts brain and behavior. Vaporized ethanol is a common method used for chronically subjecting rodents to alcohol and has been widely used to model both binge and dependence-inducing heavy drinking patterns observed in humans. Rodent strain, sex, and age during exposure are all well-known to influence outcomes in experiments utilizing intraperitoneal or intragastric methods of repeated ethanol exposure. Yet, despite its frequent use, the impact of these variables on outcomes associated with ethanol vapor exposure has not been widely investigated. The present study analyzed data generated from over 700 rats across an eight-year period to provide a population-level assessment of variables influencing level of intoxication using vapor exposure. Our findings reveal important differences with respect to strain, sex, and age during ethanol exposure in the relationship between blood ethanol concentration and behavioral signs of intoxication. These data provide valuable scientific and practical insight for laboratories utilizing ethanol vapor exposure paradigms to model AUD in rats.

Blockade of Orexin Receptors in the Posterior Paraventricular Nucleus of the Thalamus Prevents Stress-Induced Reinstatement of Reward-Seeking Behavior in Rats With a History of Ethanol Dependence

Neural systems involved in processing natural rewards and drugs of abuse overlap and exposure to drugs of abuse induce neuroadaptations that can cause compulsive-like behavior. For example, the recruitment of the orexin (Orx) system by drugs of abuse has been proposed to induce neuroadaptations that in turn alter its function, reflected by maladaptive, compulsive, and addictive behavior. Orexin neurons project to the paraventricular nucleus of the thalamus (PVT)—particularly the posterior part (pPVT), a structure that plays a key role in stress regulation. This study investigated whether Orx transmission in the pPVT plays a role in stress-induced reinstatement of reward-seeking behavior toward ethanol (EtOH) and a highly palatable food reward [sweetened condensed milk (SCM)] in rats and whether this role changes with EtOH dependence. After being trained to orally self-administer EtOH or SCM, the rats were made dependent (EtOH_D and SCM_D) by chronic intermittent EtOH vapor exposure. The control nondependent groups (EtOH_ND and SCM_ND) were exposed to air. Following extinction, the rats were tested for stress-induced reinstatement of EtOH- and SCM-seeking behavior. Stress reinstated EtOH- and SCM-seeking behavior in all groups (EtOH_D/ND and SCM_D/ND). Administration of the dual Orx receptor (OrxR) antagonist TCS1102 (15 μg) in the pPVT prevented stress-induced reinstatement only in dependent rats (EtOH_D and SCM_D). In parallel, the qPCR analysis showed that Orx mRNA expression in the hypothalamus and OrxR1/R2 mRNA expression in the pPVT were increased at the time of testing in the EtOH_D and SCM_D groups. These results are the first to implicate Orx transmission in the pPVT in the stress-induced reinstatement of reward-seeking behavior in EtOH dependent rats and indicate the maladaptive recruitment of Orx transmission in the pPVT by EtOH dependence.

Sex differences in corticotropin releasing factor peptide regulation of inhibitory control and excitability in central amygdala corticotropin releasing factor receptor 1-neurons

The central amygdala (CeA) is a critical regulator of emotional behavior that has been implicated in psychiatric illnesses, including anxiety disorders and addiction. The CeA corticotropin releasing factor receptor 1 (CRF1) system has been implicated in alcohol use disorder (AUD) and mood disorders, and has been shown to regulate anxiety-like behavior and alcohol consumption in rodents. However, the effects of CRF signaling within the CRF receptor 1-containing (CRF1+) population of the CeA remain unclear, and the effects of ethanol and CRF1 manipulations in female rodents have not been assessed. Here, we characterized inhibitory control and CRF1 signaling in male and female CRF1-GFP reporter mice. Male and female CRF1+ CeA neurons exhibited similar baseline GABAergic signaling and excitability and were comparably sensitive to CRF-induced increases in presynaptic GABA release. CRF1 antagonism reduced GABA release onto CRF1-containing neurons comparably in both males and females. Acute ethanol application reduced GABA release onto CRF1+ neurons from males, but female CRF1+ neurons were insensitive to ethanol. Exogenous CRF increased the firing rate of CRF1-containing neurons to a greater extent in male cells versus female cells, and CRF1 antagonism reduced firing in females but not males. Together, these findings indicate a critical sex-specific role for the CRF system in regulating inhibitory control and excitability of CRF1-containing neurons in the central amygdala. Sex differences in sensitivity of CRF/CRF1 signaling provide useful context for the sex differences in psychiatric illness reported in human patients, particularly AUD.

Leveraging Neural Networks in Preclinical Alcohol Research

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

Cannabinoid 1 Receptor Antagonists Play a Neuroprotective Role in Chronic Alcoholic Hippocampal Injury Related to Pyroptosis Pathway

BACKGROUND

Alcohol use disorders affect millions of people worldwide, and there is growing evidence that excessive alcohol intake causes severe damage to the brain of both humans and animals. Numerous studies on chronic alcohol exposure in animal models have identified that many functional impairments are associated with the hippocampus, which is a structure exhibiting substantial vulnerability to alcohol exposure. However, the precise mechanisms that lead to structural and functional impairments of the hippocampus are poorly understood. Herein, we report a novel cell death type, namely pyroptosis, which accounts for alcohol neurotoxicity in mice.

METHODS

For this study, we used an in vivo model to induce alcohol-related neurotoxicity in the hippocampus. Adult male C57BL/6 mice were treated with 95% alcohol vapor either alone or in combination with selective cannabinoid receptor antagonists or agonists, and VX765 (Belnacasan), which is a selective caspase-1 inhibitor.

RESULTS

Alcohol-induced in vivo pyroptosis occurs because of an increase in the levels of pyroptotic proteins such as nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3), caspase-1, gasdermin D (GSDMD), and amplified inflammatory response. Our results indicated that VX765 suppressed the expression of caspase-1 and inhibited the maturation of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. Additionally, chronic alcohol intake created an imbalance in the endocannabinoid system and regulated 2 cannabinoid receptors (CB1R and CB2R) in the hippocampus. Specific antagonists of CB1R (AM251 and AM281) significantly ameliorated alcohol-induced pyroptosis signaling and inactivated the inflammatory response.

CONCLUSIONS

Alcohol induces hippocampal pyroptosis, which leads to neurotoxicity, thereby indicating that pyroptosis may be an essential pathway involved in chronic alcohol-induced hippocampal neurotoxicity. Furthermore, cannabinoid receptors are regulated during this process, which suggests promising therapeutic strategies against alcohol-induced neurotoxicity through pharmacologic inhibition of CB1R.

Multi-modal imaging reveals differential brain volumetric, biochemical, and white matter fiber responsivity to repeated intermittent ethanol vapor exposure in male and female rats

A generally accepted framework derived predominately from animal models asserts that repeated cycles of chronic intermittent ethanol (EtOH; CIE) exposure cause progressive brain adaptations associated with anxiety and stress that promote voluntary drinking, alcohol dependence, and further brain changes that contribute to the pathogenesis of alcoholism. The current study used CIE exposure via vapor chambers to test the hypothesis that repeated episodes of withdrawals from chronic EtOH would be associated with accrual of brain damage as quantified using in vivo magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), and MR spectroscopy (MRS). The initial study group included 16 male (~325g) and 16 female (~215g) wild-type Wistar rats exposed to 3 cycles of 1-month in vapor chambers + 1 week of abstinence. Half of each group (n = 8) was given vaporized EtOH to blood alcohol levels approaching 250 mg/dL. Blood and behavior markers were also quantified. There was no evidence for dependence (i.e., increased voluntary EtOH consumption), increased anxiety, or an accumulation of pathology. Neuroimaging brain responses to exposure included increased cerebrospinal fluid (CSF) and decreased gray matter volumes, increased Choline/Creatine, and reduced fimbria-fornix fractional anisotropy (FA) with recovery seen after one or more cycles and effects in female more prominent than in male rats. These results show transient brain integrity changes in response to CIE sufficient to induce acute withdrawal but without evidence for cumulative or escalating damage. Together, the current study suggests that nutrition, age, and sex should be considered when modeling human alcoholism.

Evidence for incentive salience sensitization as a pathway to alcohol use disorder

The incentive salience sensitization (ISS) theory of addiction holds that addictive behavior stems from the ability of drugs to progressively sensitize the brain circuitry that mediates attribution of incentive salience (IS) to reward-predictive cues and its behavioral manifestations. In this article, we establish the plausibility of ISS as an etiological pathway to alcohol use disorder (AUD). We provide a comprehensive and critical review of evidence for: (1) the ability of alcohol to sensitize the brain circuitry of IS attribution and expression; and (2) attribution of IS to alcohol-predictive cues and its sensitization in humans and non-human animals. We point out gaps in the literature and how these might be addressed. We also highlight how individuals with different alcohol subjective response phenotypes may differ in susceptibility to ISS as a pathway to AUD. Finally, we discuss important implications of this neuropsychological mechanism in AUD for psychological and pharmacological interventions attempting to attenuate alcohol craving and cue reactivity.

Inhibition of the rostromedial tegmental nucleus reverses alcohol withdrawal-induced anxiety-like behavior

Acute withdrawal from alcohol is associated with a number of unpleasant symptoms that play an important role in preventing recovery and long-term abstinence. Considerable research has focused on the role that neuropeptide systems and the amygdala play in mediating affective symptoms of acute withdrawal, but promising preclinical findings have not translated successfully into the clinic. The rostromedial tegmental nucleus (RMTg) has been implicated in both fear and anxiety. In addition, RMTg neurons exert inhibitory control over midbrain dopamine neurons, the activity of which are suppressed during acute withdrawal. Thus, we hypothesized that the RMTg may play a role in mediating symptoms of acute withdrawal. Using a chronic ethanol vapor exposure paradigm that renders rats physically dependent on ethanol, we observed significant withdrawal-induced enhancement of cFos expression in the RMTg. This was accompanied by a significant increase in somatic symptoms and a decrease in reward sensitivity as measured by intracranial self-stimulation (ICSS). Both measures followed a similar time course to RMTg cFos expression with peak symptom severity occurring 12 h following cessation of ethanol exposure. Heightened anxiety-like behavior was also observed in withdrawn rats at this same time point. RMTg inhibition had no effect on somatic signs of withdrawal or withdrawal-induced changes in reward sensitivity, but significantly attenuated withdrawal-induced anxiety-like behavior. Together, these data demonstrate that the RMTg plays a distinct role in the negative affective state associated with acute withdrawal and may therefore be critically involved in the neurobiological mechanisms that promote relapse during early stages of recovery.

Anti-relapse neurons in the infralimbic cortex of rats drive relapse-suppression by drug omission cues

Drug addiction is a chronic relapsing disorder of compulsive drug use. Studies of the neurobehavioral factors that promote drug relapse have yet to produce an effective treatment. Here we take a different approach and examine the factors that suppress-rather than promote-relapse. Adapting Pavlovian procedures to suppress operant drug response, we determined the anti-relapse action of environmental cues that signal drug omission (unavailability) in rats. Under laboratory conditions linked to compulsive drug use and heightened relapse risk, drug omission cues suppressed three major modes of relapse-promotion (drug-predictive cues, stress, and drug exposure) for cocaine and alcohol. This relapse-suppression is, in part, driven by omission cue-reactive neurons, which constitute small subsets of glutamatergic and GABAergic cells, in the infralimbic cortex. Future studies of such neural activity-based cellular units (neuronal ensembles/memory engram cells) for relapse-suppression can be used to identify alternate targets for addiction medicine through functional characterization of anti-relapse mechanisms.

Effects of Ethanol Exposure and Withdrawal on Neuronal Morphology in the Agranular Insular and Prelimbic Cortices: Relationship with Withdrawal-Related Structural Plasticity in the Nucleus Accumbens

The present study investigated the effects of chronic intermittent ethanol exposure and withdrawal on dendritic morphology and spine density in the agranular insular and prelimbic cortices. Adult male Sprague–Dawley rats were passively exposed to vaporized ethanol (~37 mg/L; 12 h/day) or air (control) for ten consecutive days. Dendritic length, branching, and spine density were quantified in layer II/III pyramidal neurons 24 hours or seven days following the final ethanol exposure. Compared to unexposed control animals there were structural alterations on neurons in the prelimbic cortex, and to a lesser extent the agranular insular cortex. The most prominent ethanol-related differences were the transient increases in dendritic length and branching in prelimbic neurons at 24 h post-cessation, and increased mushroom-shaped spines at seven days post-cessation. The results obtained in the prelimbic cortex are the opposite of those previously reported in the nucleus accumbens core (Peterson, et al. 2015), suggesting that these regions undergo distinct functional adaptations following ethanol exposure and withdrawal.

Ethanol and Cytokines in the Central Nervous System

The innate immune system plays a critical role in the ethanol-induced neuroimmune response in the brain. Ethanol initiates the innate immune response via activation of the innate immune receptors Toll-like receptors (TLRs, e.g., TLR4, TLR3, TLR7) and NOD-like receptors (inflammasome NLRs) leading to a release of a plethora of chemokines and cytokines and development of the innate immune response. Cytokines and chemokines can have pro- or anti-inflammatory properties through which they regulate the immune response. In this chapter, we will focus on key cytokines (e.g., IL-1, IL-6, TNF-α) and chemokines (e.g., MCP-1/CCL2) that mediate the ethanol-induced neuroimmune responses. In this regard, we will use IL-1β, as an example cytokine, to discuss the neuromodulatory properties of cytokines on cellular properties and synaptic transmission. We will discuss their involvement through a set of evidence: (1) changes in gene and protein expression following ethanol exposure, (2) association of gene polymorphisms (humans) and alterations in gene expression (animal models) with increased alcohol intake, and (3) modulation of alcohol-related behaviors by transgenic or pharmacological manipulations of chemokine and cytokine systems. Over the last years, our understanding of the molecular mechanisms mediating cytokine- and chemokine-dependent regulation of immune responses has advanced tremendously, and we review evidence pointing to cytokines and chemokines serving as neuromodulators and regulators of neurotransmission.

Prolonged Exposure to Alcohol Vapor Causes Change in Cardiovascular Function in Female but not in Male Rats

BACKGROUND

Alcohol abuse is a health concern worldwide. Studies have associated alcohol abuse with cardiovascular impairments. In this study, we investigated differences in the effects of chronic alcohol vapor exposure on cardiovascular function between male and female rats by using the alcohol vapor chamber method to induce alcohol addiction-like behaviors in rats.

METHODS

We exposed male and female Long-Evans rats to alcohol vapor for 14 hours, followed by ethanol withdrawal for 10 hours, for 30 consecutive days or room air (control groups). The animals underwent preparation for the surgical implantation of cannulas into femoral vessels, for allowing the assessment of the basal arterial pressure and heart rate values, baroreflex function, and autonomic activity.

RESULTS

Female control rats showed higher basal heart rate compared to male control rats. Chronic alcohol vapor inhalation reduced basal heart rate in females, but not in males; this effect was followed by an increase in the parasympathetic tone of the heart. Further, female rats subjected to alcohol vapor showed an increase in the baroreflex activity.

CONCLUSIONS

These findings suggest that females are more sensitive to chronic alcohol vapor exposure than males because they had a reduction in basal heart rate and changes in the baroreflex activity.

Oxytocin blocks enhanced motivation for alcohol in alcohol dependence and blocks alcohol effects on GABAergic transmission in the central amygdala

Oxytocin administration has been reported to decrease consumption, withdrawal, and drug-seeking associated with several drugs of abuse and thus represents a promising pharmacological approach to treat drug addiction. We used an established rat model of alcohol dependence to investigate oxytocin's effects on dependence-induced alcohol drinking, enhanced motivation for alcohol, and altered GABAergic transmission in the central nucleus of the amygdala (CeA). Intraperitoneal oxytocin administration blocked escalated alcohol drinking and the enhanced motivation for alcohol in alcohol-dependent but not nondependent rats. Intranasal oxytocin delivery fully replicated these effects. Intraperitoneal administration had minor but significant effects of reducing locomotion and intake of non-alcoholic palatable solutions, whereas intranasal oxytocin administration did not. In dependent rats, intracerebroventricular administration of oxytocin or the oxytocin receptor agonist PF-06655075, which does not cross the blood-brain barrier (i.e., it would not diffuse to the periphery), but not systemic administration of PF-06655075 (i.e., it would not reach the brain), decreased alcohol drinking. Administration of a peripherally restricted oxytocin receptor antagonist did not reverse the effect of intranasal oxytocin on alcohol drinking. Ex vivo electrophysiological recordings from CeA neurons indicated that oxytocin decreases evoked GABA transmission in nondependent but not in dependent rats, whereas oxytocin decreased the amplitude of spontaneous GABAergic responses in both groups. Oxytocin blocked the facilitatory effects of acute alcohol on GABA release in the CeA of dependent but not nondependent rats. Together, these results provide converging evidence that oxytocin specifically and selectively blocks the enhanced motivation for alcohol drinking that develops in alcohol dependence likely via a central mechanism that may result from altered oxytocin effects on CeA GABA transmission in alcohol dependence. Neuroadaptations in endogenous oxytocin signaling may provide a mechanism to further our understanding of alcohol use disorder.

Adenylyl cyclase 7 and neuropsychiatric disorders: A new target for depression?

Adenylyl cyclases (ACs) are enzymes that catalyze the production of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). Humans express nine isoforms of membranous ACs and a soluble AC. Studies with genetic knockout or overexpression rodent models have indicated that AC isoforms may be targeted to achieve specific therapeutic outcomes. AC1, for instance, has been suggested and pursued as a target for relieving pain. Notably, previous studies examining genetically modified mice as well as human genetic polymorphisms have suggested a link between AC7 activity and depressive disorders. In the present review we present an overview on AC function and discuss the most recent developments to target AC isoforms for drug therapies. We next focus on discussing the available literature on the molecular and animal pharmacology of AC7 highlighting the available studies on the role of AC7 in depressive disorders. In addition, we discuss other possible physiological functions of AC7 relating to ethanol effects and the immune system and conclude with considerations about pharmacological modulation of AC7.

Inactivation of a CRF-dependent amygdalofugal pathway reverses addiction-like behaviors in alcohol-dependent rats

The activation of a neuronal ensemble in the central nucleus of the amygdala (CeA) during alcohol withdrawal has been hypothesized to induce high levels of alcohol drinking in dependent rats. In the present study we describe that the CeA neuronal ensemble that is activated by withdrawal from chronic alcohol exposure contains ~80% corticotropin-releasing factor (CRF) neurons and that the optogenetic inactivation of these CeA CRF+ neurons prevents recruitment of the neuronal ensemble, decreases the escalation of alcohol drinking, and decreases the intensity of somatic signs of withdrawal. Optogenetic dissection of the downstream neuronal pathways demonstrates that the reversal of addiction-like behaviors is observed after the inhibition of CeA CRF projections to the bed nucleus of the stria terminalis (BNST) and that inhibition of the CRF^CeA-BNST pathway is mediated by inhibition of the CRF-CRF₁ system and inhibition of BNST cell firing. These results suggest that the CRF^CeA-BNST pathway could be targeted for the treatment of excessive drinking in alcohol use disorder.

Inducing Alcohol Dependence in Rats Using Chronic Intermittent Exposure to Alcohol Vapor

Alcohol use disorder (AUD) is a significant public health and economic burden and is often characterized by repeated bouts of alcohol intoxication and withdrawal. Virtually all organ systems are impacted by chronic alcohol exposure. These effects can be investigated using the rat as a model organism; however, rats typically will not self-administer alcohol to levels necessary to achieve physiological and behavioral aspects of dependence. The protocol described herein can be utilized to induce alcohol dependence in rats by administering alcohol vapor to the homecage for an extended period of time. This method allows the researcher to control the level, duration, and pattern of intoxication, and it reliably induces physiological and behavioral aspects of alcohol dependence, allowing for the study of biology and behavior with relevance for AUD in humans.

The offspring of rats selected for high or low ethanol intake at adolescence exhibit differential ethanol-induced Fos immunoreactivity in the central amygdala and in nucleus accumbens core

Adolescents exhibit, when compared to adults, altered responsivity to the unconditional effects of ethanol. It is unclear if this has a role in the excessive ethanol intake of adolescents. Wistar rats from the third filial generation (F₃) of a short-term breeding program which were selected for high (STDRHI) vs. low (STDRLO) ethanol intake during adolescence, were assessed for ethanol-induced (0.0, 1.25 or 2.5 g/kg) Fos immunoreactivity (Fos-ir) in the central (Ce), basolateral (BLA) and medial (Me) amygdaloid nuclei; nucleus accumbens core and shell (AcbC, AcbSh), ventral tegmental area (VTA), as well as prelimbic and infralimbic (PrL, IL) prefrontal cortices. Following i.p. administration of saline, and across the structures measured, Fos-ir was significantly greater in STDRHI than in STDRLO rats. Across both lines, baseline Fos-ir was significantly lower in BLA than in any other structure, whereas PrL, IL and Shell did not differ between each other and exhibited significantly greater level of baseline neural activation than Ce, Me, AcbC and VTA. STDRLO, but not STDRHI, rats exhibited ethanol-induced Fos-ir in Ce. STRDHI, but not STDRLO, rats exhibited an ethanol-induced Fos-ir depression in AcbC. Key maternal care behaviors (i.e., grooming of the pups, latency to retrieve the pups, time spent in the nest and time adopting a kiphotic posture) were fairly similar across lines. There were significant intergenerational variations in the amount self-licking behaviors in STDRHI dams as well as an increased amount of exploration of the cage in these animals, when compared to STDRLO counterparts. These results indicate that short term selection for differential alcohol intake during adolescence yields heightened neural activity at baseline (i.e., after vehicle) in STRDHI vs. STDRLO adolescent rats, and differential sensitivity to ethanol-induced Fos immunoreactivity in Ce and in AcbC. It is unlikely that rearing patterns explained the neural differences reported, between STDRHI and STDRLO rats.

The center of the emotional universe: Alcohol, stress, and CRF1 amygdala circuitry

The commonalities between different phases of stress and alcohol use as well as the high comorbidity between alcohol use disorders (AUDs) and anxiety disorders suggest common underlying cellular mechanisms governing the rewarding and aversive aspects of these related conditions. As an integrative center that assigns emotional salience to a wide variety of internal and external stimuli, the amygdala complex plays a major role in how alcohol and stress influence cellular physiology to produce disordered behavior. Previous work has illustrated the broad role of the amygdala in alcohol, stress, and anxiety. However, the challenge of current and future studies is to identify the specific dysregulations that occur within distinct amygdala circuits and subpopulations and the commonalities between these alterations in each disorder, with the long-term goal of identifying potential targets for therapeutic intervention. Specific intra-amygdala circuits and cell type-specific subpopulations are emerging as critical targets for stress- and alcohol-induced plasticity, chief among them the corticotropin releasing factor (CRF) and CRF receptor 1 (CRF1) system. CRF and CRF1 have been implicated in the effects of alcohol in several amygdala nuclei, including the basolateral (BLA) and central amygdala (CeA); however, the precise circuitry involved in these effects and the role of these circuits in stress and anxiety are only beginning to be understood.

Exploring Sex Differences in the Attenuation of Ethanol Drinking by Naltrexone in Dependent Rats During Early and Protracted Abstinence

BACKGROUND

Despite considerable efforts, few drugs are available for the treatment of alcohol (ethanol [EtOH]) use disorder (AUD). EtOH directly or indirectly modulates several aspects of the central nervous system, including neurotransmitter/neuromodulator systems. Relapse vulnerability is a challenge for the treatment of EtOH addiction. EtOH withdrawal symptoms create motivational states that lead to compulsive EtOH drinking and relapse even after long periods of abstinence. Among the therapeutics to treat AUD, naltrexone (NTX) is a pharmacological treatment for relapse. The present study evaluated the effect of NTX on EtOH drinking in male and female EtOH-dependent rats during abstinence.

METHODS

Wistar rats (males and females) were first trained to orally self-administer 10% EtOH. Half of the rats were then made dependent by chronic intermittent EtOH (CIE) vapor exposure, and the other half were exposed to air. Using this model, rats exhibit somatic and motivational signs of withdrawal. At the end of EtOH vapor (or air) exposure, the rats were tested for the effects of NTX (10 mg/kg, oral) on EtOH self-administration at 3 abstinence time points: acute abstinence (A-Abst, 8 hours), late abstinence (L-Abst, 2 weeks), and protracted abstinence (P-Abst, 6 weeks).

RESULTS

NTX decreased EtOH intake in nondependent rats, regardless of sex and abstinence time point. In postdependent rats, NTX decreased EtOH intake only at a delayed abstinence time point (P-Abst) in males, whereas it similarly reduced EtOH drinking in females at all abstinence time points.

CONCLUSIONS

The therapeutic efficacy of NTX depends on the time of intervention during abstinence and is different between males and females. The data further suggest that EtOH dependence causes different neuroadaptations in male and female rats, reflected by differential effects of NTX. The results underscore the significance of considering the duration of EtOH abstinence and sex as a biological variable as important factors when developing pharmacotherapies for AUD.

Inhibition of Glyoxalase 1 reduces alcohol self-administration in dependent and nondependent rats

Previous studies showed that the glyoxalase 1 (Glo1) gene modulates anxiety-like behavior, seizure susceptibility, depression-like behavior, and alcohol drinking in the drinking-in-the-dark paradigm in nondependent mice. Administration of the small-molecule GLO1 inhibitor S-bromobenzylglutathione cyclopentyl diester (pBBG) decreased alcohol drinking in nondependent mice, suggesting a possible therapeutic strategy. However, the preclinical therapeutic efficacy of pBBG in animal models of alcohol dependence remains to be demonstrated. We tested the effect of pBBG (7.5 and 25 mg/kg) on operant alcohol self-administration in alcohol-dependent and nondependent rats. Wistar rats were trained to self-administer 10% alcohol (v/v) and made dependent by chronic intermittent passive exposure to alcohol vapor for 5 weeks. Pretreatment with pBBG dose-dependently reduced alcohol self-administration in both nondependent and dependent animals, without affecting water self-administration. pBBG treatment was more effective in dependent rats than in nondependent rats. These data extend previous findings that implicated Glo1 in alcohol drinking in nondependent mice by showing even more profound effects in alcohol-dependent rats. These results suggest that the pharmacological inhibition of GLO1 is a relevant therapeutic target for the treatment of alcohol use disorders.

CRF1 Receptor-Dependent Increases in Irritability-Like Behavior During Abstinence from Chronic Intermittent Ethanol Vapor Exposure

BACKGROUND

In humans, emotional and physical signs of withdrawal from ethanol are commonly seen. Many of these symptoms, including anxiety-like and depression-like behavior, have been characterized in animal models of ethanol dependence. One issue with several current behavioral tests that measure withdrawal in animal models is that they are often not repeatable within subjects over time. Additionally, irritability, one of the most common symptoms of ethanol withdrawal in humans, has not been well characterized in animal models. The corticotropin-releasing factor (CRF)-CRF₁ receptor system has been suggested to be critical for the emergence of anxiety-like behavior in ethanol dependence, but the role of this system in irritability-like behavior has not been characterized.

METHODS

The present study compared the effects of chronic intermittent ethanol (CIE) vapor exposure-induced ethanol dependence on irritability-like behavior in rats using the bottle-brush test during acute withdrawal and protracted abstinence. Rats were trained to self-administer ethanol in operant chambers and then either left in a nondependent state or made dependent via CIE. Naïve, nondependent, and dependent rats were tested for irritability-like behavior in the bottle-brush test 8 hours and 2 weeks into abstinence from ethanol. Separate cohorts of dependent and nondependent rats were used to examine the effect of the specific CRF₁ receptor antagonist R121919 on irritability-like behavior.

RESULTS

Dependent rats exhibited escalated ethanol intake compared with their own pre-CIE baseline and nondependent rats. At both time points of abstinence, ethanol-dependent rats exhibited increased aggressive-like responses compared with naïve and nondependent rats. R121919 reduced irritability-like behavior in both dependent and nondependent rats, but dependent rats were more sensitive to R121919.

CONCLUSIONS

Irritability-like behavior is a clinically relevant and reliable measure of negative emotional states that is partially mediated by activation of the CRF-CRF₁ system and remains elevated during protracted abstinence in ethanol-dependent rats.