Deficits in the extinction of ethanol-seeking behavior following chronic intermittent ethanol exposure are attenuated with positive allosteric modulation of mGlu5

Enhanced Fear Extinction Through Infralimbic Perineuronal Net Digestion: The Modulatory Role of Adolescent Alcohol Exposure

Perineuronal nets (PNNs) are specialized components of the extracellular matrix that play a critical role in learning and memory. In a Pavlovian fear conditioning paradigm, degradation of PNNs affects the formation and storage of fear memories. This study examined the impact of adolescent intermittent ethanol (AIE) exposure by vapor inhalation on the expression of PNNs in the adult rat prelimbic (PrL) and infralimbic (IfL) subregions of the medial prefrontal cortex. Results indicated that following AIE, the total number of PNN positive cells in the PrL cortex increased in layer II/III but did not change in layer V. Conversely, in the IfL cortex, the number of PNN positive cells decreased in layer V, with no change in layer II/III. In addition, the intensity of PNN staining was significantly altered by AIE exposure, which narrowed the distribution of signal intensity, reducing the number of high and low intensity PNNs. Given these changes in PNNs, the next experiment assessed the effects of AIE and PNN digestion on extinction of a conditioned fear memory. In Air control rats, digestion of PNNs by bilateral infusion of Chondroitinase ABC (ChABC) into the IfL cortex enhanced fear extinction and reduced contextual fear renewal. In contrast, both fear extinction learning and contextual fear renewal remained unchanged following PNN digestion in AIE exposed rats. These results highlight the sensitivity of prefrontal PNNs to adolescent alcohol exposure and suggest that ChABC-induced plasticity is reduced in the IfL cortex following AIE exposure.

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.

Estrous cycle and hormone regulation of stress-induced reinstatement of reward seeking in female mice

Women are more vulnerable to stress-induced craving, which may be associated with increased vulnerability to relapse. Susceptibility to stress-induced craving also appears to be modulated by the menstrual cycle and is negatively correlated with circulating progesterone levels in women. However, the factors that contribute to relapse vulnerability are poorly characterized in female animals. In this study, we assessed whether chronic ethanol exposure, estrous cycle, or exogenous progesterone administration modulated vulnerability to stress-induced reinstatement. To model ethanol dependence, adult female C57Bl/6J mice underwent chronic intermittent ethanol (CIE) exposure via vapor inhalation. Seventy-two hours after the final ethanol exposure, food-restricted mice began training in a conditioned place preference paradigm (CPP) for a food reward, followed by extinction training. Mice were then subjected to forced swim stress and assessed for reinstatement of their preference for the reward-paired chamber. CIE did not affect stress-induced reinstatement. However, stress-induced reinstatement was attenuated during the diestrus phase, when endogenous levels of progesterone peak in female mice. Further, administration of exogenous progesterone mimicked the attenuated reinstatement observed in diestrus. These findings indicate that circulating hormone levels modulate susceptibility to relapse-like behaviors and implicate progesterone as a potential target for treating stress-induced relapse in women.

Therapeutic Interventions of Gut-Brain Axis as Novel Strategies for Treatment of Alcohol Use Disorder Associated Cognitive and Mood Dysfunction

Alcohol use disorders (AUD) is characterized by persistent or intermittent alcohol cravings and compulsive drinking. The functional changes in the central nervous system (CNS) after alcohol consumption are alcohol-associated cognitive impairment and mood disorders, which are major health issues reported in AUDs. Studies have shown that transferring the intestinal microbiota from AUDs patients to germ-free animals causes learning and memory dysfunction, depression and anxiety-like behavior, indicating the vital role of intestinal microbiota in development of neuropsychiatric disorders in AUD. Intestinal flora composition of AUD patients are significantly different from normal people, suggesting that intestinal flora imbalance orchestrate the development of neuropsychiatric disorders in AUD. Studies suggests that gut microbiome links bidirectional signaling network of the enteric nervous system (ENS) to central nervous system (CNS), forming gut-microbe-brain axis (brain-gut axis). In this review, we discussed pathogenesis and possible treatment of AUD-induced cognitive deficits, anxiety, and depression disorders. Further, we described the mechanism of intestinal flora imbalance and dysfunction of hippocampus-amygdala-frontal cortex (gut-limbic circuit system dysfunction). Therefore, we postulate therapeutic interventions of gut-brain axis as novel strategies for treatment of AUD-induced neuropsychiatric disorders.

Modulation of mGlu5 improves sensorimotor gating deficits in rats neonatally treated with quinpirole through changes in dopamine D2 signaling

This study analyzed whether the positive allosteric modulator of metabotropic glutamate receptor type 5 (mGlu5) 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) would alleviate deficits in prepulse inhibition (PPI) and affect dopamine (DA) D2 signaling in the dorsal striatum and prefrontal cortex (PFC) in the neonatal quinpirole (NQ) model of schizophrenia (SZ). Male and female Sprague-Dawley rats were neonatally treated with either saline (NS) or quinpirole HCL (1 mg/kg; NQ), a DAD2 receptor agonist, from postnatal days (P) 1-21. Rats were raised to P44 and behaviorally tested on PPI from P44-P48. Before each trial, rats were subcutaneous (sc) administered saline or CDPPB (10 mg/kg or 30 mg/kg). On P50, rats were given a spontaneous locomotor activity test after CDPPB or saline administration. On P51, the dorsal striatum and PFC were evaluated for both arrestin-2 (βA-2) and phospho-AKT protein levels. NQ-treated rats demonstrated a significant deficit in PPI, which was alleviated to control levels by the 30 mg/kg dose of CDPPB. There were no significant effects of CDPPB on locomotor activity. NQ treatment increased βA-2 and decreased phospho-AKT in both the dorsal striatum and PFC, consistent with an increase DAD2 signaling. The 30 mg/kg dose of CDPPB significantly reversed changes in βA-2 in the dorsal striatum and PFC and phospho-AKT in the PFC equivalent to controls. Both doses of CDPPB produced a decrease of phospho-AKT in the PFC compared to controls. This study revealed that a mGlu5 positive allosteric modulator was effective to alleviate PPI deficits and striatal DAD2 signaling in the NQ model of SZ.

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.

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.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

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.

The infralimbic cortex and mGlu5 mediate the effects of chronic intermittent ethanol exposure on fear learning and memory

RATIONALE AND OBJECTIVES

Alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD) often occur comorbidly. While the incidence of these disorders is increasing, there is little investigation into the interacting neural mechanisms between these disorders. These studies aim to identify cognitive deficits that occur as a consequence of fear and ethanol exposure, implement a novel pharmaceutical intervention, and determine relevant underlying neurocircuitry. Additionally, due to clinical sex differences in PTSD prevalence and alcohol abuse, these studies examine the nature of this relationship in rodent models.

METHODS

Animals were exposed to a model of PTSD+AUD using auditory fear conditioning followed by chronic intermittent ethanol exposure (CIE). Then, rats received extinction training consisting of multiple conditioned stimulus presentations in absence of the shock. Extinction recall and context-induced freezing were measured in subsequent tests. CDPPB, a metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulator, was used to treat these deficits, and region-specific effects were determined using microinjections.

RESULTS

These studies determined that CIE exposure led to deficits in fear extinction learning and heightened context-induced freezing while sex differences emerged in fear conditioning and extinction cue recall tests. Furthermore, using CDPPB, these studies found that enhancement of infralimbic (IfL) mGlu5 activity was able to recover CIE-induced deficits in both males and females.

CONCLUSIONS

These studies show that CIE induces deficits in fear-related behaviors and that enhancement of IfL glutamatergic activity can facilitate learning during extinction. Additionally, we identify novel pharmacological targets for the treatment of individuals who suffer from PTSD and AUD.

Chronic activation of fear engrams induces extinction-like behavior in ethanol-exposed mice

Alcohol withdrawal directly impacts the brain's stress and memory systems, which may underlie individual susceptibility to persistent drug and alcohol-seeking behaviors. Numerous studies demonstrate that forced alcohol abstinence, which may lead to withdrawal, can impair fear-related memory processes in rodents such as extinction learning; however, the underlying neural circuits mediating these impairments remain elusive. Here, we tested an optogenetic strategy aimed at mitigating fear extinction retrieval impairments in male c57BL/6 mice following exposure to alcohol (i.e., ethanol) and forced abstinence. In the first experiment, extensive behavioral extinction training in a fear-conditioned context was impaired in ethanol-exposed mice compared to controls. In the second experiment, neuronal ensembles processing a contextual fear memory in the dorsal hippocampus were tagged and optogenetically reactivated repeatedly in a distinct context in ethanol-exposed and control mice. Chronic activation of these cells resulted in a context-specific, extinction-like reduction in fear responses in both control and ethanol-exposed mice. These findings suggest that while ethanol can impair the retrieval an extinction memory, optogenetic manipulation of a fear engram is sufficient to induce an extinction-like reduction in fear responses.

Effects of the Positive Allosteric Modulator of Metabotropic Glutamate Receptor 5, VU-29, on Maintenance Association between Environmental Cues and Rewarding Properties of Ethanol in Rats

: Metabotropic glutamate subtype 5 (mGlu5) receptors are implicated in various forms of synaptic plasticity, including drugs of abuse. In drug-addicted individuals, associative memories can drive relapse to drug use. The present study investigated the potential of the mGlu5 receptor positive allosteric modulator (PAM), VU-29 (30 mg/kg, i.p.), to inhibit the maintenance of a learned association between ethanol and environmental context by using conditioned place preference (CPP) in rats. The ethanol-CPP was established by the administration of ethanol (1.0 g/kg, i.p. × 10 days) using an unbiased procedure. Following ethanol conditioning, VU-29 was administered at various post-conditioning times (ethanol free state at the home cage) to ascertain if there was a temporal window during which VU-29 would be effective. Our experiments indicated that VU-29 did not affect the expression of ethanol-induced CPP when it was given over two post-conditioning days. However, the expression of ethanol-CPP was inhibited by 10-day home cage administration of VU-29, but not by first 2-day or last 2-day injection of VU-29 during the 10-day period. These findings reveal that VU-29 can inhibit the maintenance of ethanol-induced CPP, and that treatment duration contributes to this effect of VU-29. Furthermore, VU-29 effect was reversed by pretreatment with either MTEP (the mGlu5 receptor antagonist), or MK-801 (the N-methyl-D-aspartate-NMDA receptor antagonist). Thus, the inhibitory effect of VU-29 is dependent on the functional interaction between mGlu5 and NMDA receptors. Because a reduction in ethanol-associated cues can reduce relapse, mGlu5 receptor PAM would be useful for therapy of alcoholism. Future research is required to confirm the current findings.

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

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

Autophagy alleviates ethanol-induced memory impairment in association with anti-apoptotic and anti-inflammatory pathways

Chronic excessive drinking leads to a wide spectrum of neurological disorders, including cognitive deficits, such as learning and memory impairment. However, the neurobiological mechanisms underlying these deleterious changes are still poorly understood. We conducted a comprehensive study to investigate the role and mechanism of autophagy in alcohol-induced memory impairment. To establish an ethanol-induced memory impairment mouse model, we allowed C57BL/6J mice intermittent access to 20% ethanol (four-bottle choice) to escalate ethanol drinking levels. Memory impairment was confirmed by a Morris water maze test. We found that mice exposed to EtOH (ethanol) and EtOH combined with the autophagy inhibitor 3-methyladenine (3-MA) showed high alcohol intake and blood alcohol concentration. We confirmed that the EtOH group exhibited notable memory impairment. Inhibition of autophagy by 3-MA worsened ethanol-induced memory impairment. Ethanol induced autophagy in the hippocampus of mice as indicated by western blotting, electron microscopy, RT-qPCR, and fluorescence confocal microscopy. We determined that the mTOR/BECN1 (S14) pathway is involved in ethanol-induced autophagy in vivo. Further, ethanol-induced autophagy suppressed the NLRP3 inflammatory and apoptosis pathways in the hippocampus in mice and in vitro. These findings suggest that autophagy activation in hippocampal cells alleviates ethanol-induced memory impairment in association with anti-apoptotic and anti-inflammatory pathways.

Compulsivity in Alcohol Use Disorder and Obsessive Compulsive Disorder: Implications for Neuromodulation

Alcohol use Disorder (AUD) is one of the leading causes of morbidity and mortality worldwide. The progression of the disorder is associated with the development of compulsive alcohol use, which in turn contributes to the high relapse rate and poor longer term functioning reported in most patients, even with treatment. While the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines AUD by a cluster of symptoms, parsing its heterogeneous phenotype by domains of behavior such as compulsivity may be a critical step to improve outcomes of this condition. Still, neurobiological underpinnings of compulsivity need to be fully elucidated in AUD in order to better design targeted treatment strategies. In this manuscript, we review and discuss findings supporting common mechanisms between AUD and OCD, dissecting the construct of compulsivity and focusing specifically on characteristic disruptions in habit learning and cognitive control in the two disorders. Finally, neuromodulatory interventions are proposed as a probe to test compulsivity as key pathophysiologic feature of AUD, and as a potential therapy for the subgroup of individuals with compulsive alcohol use, i.e., the more resistant stage of the disorder. This transdiagnostic approach may help to destigmatize the disorder, and suggest potential treatment targets across different conditions.

Metabotropic Glutamate Receptors in Alcohol Use Disorder: Physiology, Plasticity, and Promising Pharmacotherapies

Developing efficacious treatments for alcohol use disorder (AUD) has proven difficult. The insidious nature of the disease necessitates a deep understanding of its underlying biology as well as innovative approaches to ameliorate ethanol-related pathophysiology. Excessive ethanol seeking and relapse are generated by long-term changes to membrane properties, synaptic physiology, and plasticity throughout the limbic system and associated brain structures. Each of these factors can be modulated by metabotropic glutamate (mGlu) receptors, a diverse set of G protein-coupled receptors highly expressed throughout the central nervous system. Here, we discuss how different components of the mGlu receptor family modulate neurotransmission in the limbic system and other brain regions involved in AUD etiology. We then describe how these processes are dysregulated following ethanol exposure and speculate about how mGlu receptor modulation might restore such pathophysiological changes. To that end, we detail the current understanding of the behavioral pharmacology of mGlu receptor-directed drug-like molecules in animal models of AUD. Together, this review highlights the prominent position of the mGlu receptor system in the pathophysiology of AUD and provides encouragement that several classes of mGlu receptor modulators may be translated as viable treatment options.

Allosteric Modulation of GPCRs: New Insights and Potential Utility for Treatment of Schizophrenia and Other CNS Disorders

G-protein-coupled receptors (GPCRs) play critical roles in regulating brain function. Recent advances have greatly expanded our understanding of these receptors as complex signaling machines that can adopt numerous conformations and modulate multiple downstream signaling pathways. While agonists and antagonists have traditionally been pursued to target GPCRs, allosteric modulators provide several mechanistic advantages, including the ability to distinguish between closely related receptor subtypes. Recently, the discovery of allosteric ligands that confer bias and modulate some, but not all, of a given receptor's downstream signaling pathways can provide pharmacological modulation of brain circuitry with remarkable precision. In addition, allosteric modulators with unprecedented specificity have been developed that can differentiate between subpopulations of a given receptor subtype based on the receptor's dimerization state. These advances are not only providing insight into the biological roles of specific receptor populations, but hold great promise for treating numerous CNS disorders.