Neuroimmune pathways in alcohol consumption: evidence from behavioral and genetic studies in rodents and humans

Platelet Endothelial Cell Adhesion Molecule-1 and Oligodendrogenesis: Significance in Alcohol Use Disorders

Alcoholism is a chronic relapsing disorder with few therapeutic strategies that address the core pathophysiology. Brain tissue loss and oxidative damage are key components of alcoholism, such that reversal of these phenomena may help break the addictive cycle in alcohol use disorder (AUD). The current review focuses on platelet endothelial cell adhesion molecule 1 (PECAM-1), a key modulator of the cerebral endothelial integrity and neuroinflammation, and a targetable transmembrane protein whose interaction within AUD has not been well explored. The current review will elaborate on the function of PECAM-1 in physiology and pathology and infer its contribution in AUD neuropathology. Recent research reveals that oligodendrocytes, whose primary function is myelination of neurons in the brain, are a key component in new learning and adaptation to environmental challenges. The current review briefly introduces the role of oligodendrocytes in healthy physiology and neuropathology. Importantly, we will highlight the recent evidence of dysregulation of oligodendrocytes in the context of AUD and then discuss their potential interaction with PECAM-1 on the cerebral endothelium.

Increased expression of M1 and M2 phenotypic markers in isolated microglia after four-day binge alcohol exposure in male rats

Microglia activation and neuroinflammation are common features of neurodegenerative conditions, including alcohol use disorders (AUDs). When activated, microglia span a continuum of diverse phenotypes ranging from classically activated, pro-inflammatory (M1) microglia/macrophages to alternatively activated, growth-promoting (M2) microglia/macrophages. Identifying microglia phenotypes is critical for understanding the role of microglia in the pathogenesis of AUDs. Therefore, male rats were gavaged with 25% (w/v) ethanol or isocaloric control diet every 8 h for 4 days and sacrificed at 0, 2, 4, and 7 days after alcohol exposure (e.g., T0, T2, etc.). Microglia were isolated from hippocampus and entorhinal cortices by Percoll density gradient centrifugation. Cells were labeled with microglia surface antigens and analyzed by flow cytometry. Consistent with prior studies, isolated cells yielded a highly enriched population of brain macrophages/microglia (>95% pure), evidenced by staining for the macrophage/microglia antigen CD11b. Polarization states of CD11b⁺CD45^low microglia were evaluated by expression of M1 surface markers, major histocompatibility complex (MHC) II, CD32, CD86, and M2 surface marker, CD206 (mannose receptor). Ethanol-treated animals begin to show increased expression of M1 and M2 markers at T0 (p = n.s.), with significant changes at the T2 time point. At T2, expression of M1 markers, MHC-II, CD86, and CD32 were increased (p < 0.05) in hippocampus and entorhinal cortices, while M2 marker, CD206, was increased significantly only in entorhinal cortices (p < 0.05). All effects resolved to control levels by T4. In summary, four-day binge alcohol exposure produces a transient increase in both M1 (MHC-II, CD32, and CD86) and M2 (CD206) populations of microglia isolated from the entorhinal cortex and hippocampus. Thus, these findings that both pro-inflammatory and potentially beneficial, recovery-promoting microglia phenotypes can be observed after a damaging exposure of alcohol are critically important to our understanding of the role of microglia in the pathogenesis of AUDs.

Protection against alcohol-induced neuronal and cognitive damage by the PPARγ receptor agonist pioglitazone

Binge alcohol drinking has emerged as a typical phenomenon in young people. This pattern of drinking, repeatedly leading to extremely high blood and brain alcohol levels and intoxication is associated with severe risks of neurodegeneration and cognitive damage. Mechanisms involved in excitotoxicity and neuroinflammation are pivotal elements in alcohol-induced neurotoxicity. Evidence has demonstrated that PPARγ receptor activation shows anti-inflammatory and neuroprotective properties. Here we examine whether treatment with the PPARγ agonist pioglitazone is beneficial in counteracting neurodegeneration, neuroinflammation and cognitive damage produced by binge alcohol intoxication. Adult Wistar rats were subjected to a 4-day binge intoxication procedure, which is commonly used to model excessive alcohol consumption in humans. Across the 4-day period, pioglitazone (0, 30, 60mg/kg) was administered orally twice daily at 12-h intervals. Degenerative cells were detected by fluoro-jade B (FJ-B) immunostaining in brain regions where expression of pro-inflammatory cytokines was also determined. The effects of pioglitazone on cognitive function were assessed in an operant reversal learning task and the Morris water maze task. Binge alcohol exposure produced selective neuronal degeneration in the hippocampal dentate gyrus and the adjacent entorhinal cortex. Pioglitazone reduced FJ-B positive cells in both regions and prevented alcohol-induced expression of pro-inflammatory cytokines. Pioglitazone also rescued alcohol-impaired reversal learning in the operant task and spatial learning deficits in the Morris water maze. These findings demonstrate that activation of PPARγ protects against neuronal and cognitive degeneration elicited by binge alcohol exposure. The protective effect of PPARγ agonist appears to be linked to inhibition of pro-inflammatory cytokines.

Midkine in the mouse ventral tegmental area limits ethanol intake and Ccl2 gene expression

Midkine (MDK) is a cytokine and neurotrophic factor that is more highly expressed in the brains of alcoholics and in mice predisposed to drink large amounts of ethanol, suggesting that MDK may regulate ethanol consumption. Here we measured ethanol consumption in male and female Mdk knockout (-/-) mice using the two-bottle choice and the drinking in the dark (DID) tests. We found that Mdk -/- mice consumed significantly more ethanol than wild-type controls in both tests. To determine if MDK acts in the ventral tegmental area (VTA) to regulate ethanol consumption, we delivered lentivirus expressing a Mdk shRNA into the VTA of male C57BL/6J mice to locally knockdown Mdk and performed the DID test. Mice expressing a Mdk shRNA in the VTA consumed more ethanol than mice expressing a control non-targeting shRNA, demonstrating that the VTA is one site in the brain through which MDK acts to regulate ethanol consumption. Since MDK also controls the expression of inflammatory cytokines in other organs, we examined gene expression of interleukin-1 beta (Il1b), tumor necrosis factor alpha (Tnfα) and the chemokine (C-C motif) ligand 2 (Ccl2) in the VTA of Mdk -/- mice and in mice expressing Mdk shRNA in the VTA. Expression of Ccl2 was elevated in the VTA of Mdk -/- mice and in mice expressing Mdk shRNA in the VTA. These results demonstrate that MDK functions in the VTA to limit ethanol consumption and levels of CCL2, a chemokine known to increase ethanol consumption.

Emerging roles for ncRNAs in alcohol use disorders

Chronic alcohol exposure produces widespread neuroadaptations and alterations in gene expression in human alcoholics and animal models. Technological advances in the past decade have increasingly highlighted the role of non-protein-coding RNAs (ncRNAs) in the regulation of gene expression and function. These recently characterized molecules were discovered to mediate diverse processes in the central nervous system, from normal development and physiology to regulation of disease, including alcoholism and other psychiatric disorders. This review will investigate the recent studies in human alcoholics and rodent models that have profiled different classes of ncRNAs and their dynamic alcohol-dependent regulation in brain.

Medications development for the treatment of alcohol use disorder: insights into the predictive value of animal and human laboratory models

Development of effective treatments for alcohol use disorder (AUD) represents an important public health goal. This review provides a summary of completed preclinical and clinical studies testing pharmacotherapies for the treatment of AUD. We discuss opportunities for improving the translation from preclinical findings to clinical trial outcomes, focusing on the validity and predictive value of animal and human laboratory models of AUD. Specifically, while preclinical studies of medications development have offered important insights into the neurobiology of the disorder and alcohol's molecular targets, limitations include the lack of standardized methods and streamlined processes whereby animal studies can readily inform human studies. Behavioral pharmacology studies provide a less expensive and valuable opportunity to assess the feasibility of a pharmacotherapy prior to initiating larger scale clinical trials by providing insights into the mechanism of the drug, which can then inform recruitment, analyses, and assessments. Summary tables are provided to illustrate the wide range of preclinical, human laboratory, and clinical studies of medications development for alcoholism. Taken together, this review highlights the challenges associated with animal paradigms, human laboratory studies, and clinical trials with the overarching goal of advancing treatment development and highlighting opportunities to bridge the gap between preclinical and clinical research.

Gene expression profiling in the human alcoholic brain

Long-term alcohol use causes widespread changes in gene expression in the human brain. Aberrant gene expression changes likely contribute to the progression from occasional alcohol use to alcohol use disorder (including alcohol dependence). Transcriptome studies have identified individual gene candidates that are linked to alcohol-dependence phenotypes. The use of bioinformatics techniques to examine expression datasets has provided novel systems-level approaches to transcriptome profiling in human postmortem brain. These analytical advances, along with recent developments in next-generation sequencing technology, have been instrumental in detecting both known and novel coding and non-coding RNAs, alternative splicing events, and cell-type specific changes that may contribute to alcohol-related pathologies. This review offers an integrated perspective on alcohol-responsive transcriptional changes in the human brain underlying the regulatory gene networks that contribute to alcohol dependence. This article is part of the Special Issue entitled "Alcoholism".

Sedative and Motor Incoordination Effects of Ethanol in Mice Lacking CD14, TLR2, TLR4, or MyD88

BACKGROUND

In our companion article, we examined the role of MyD88-dependent signaling in ethanol (EtOH) consumption in mice lacking key components of this inflammatory pathway and observed differential effects on drinking. Here, we studied the role of these same signaling components in the acute sedative, intoxicating, and physiological effects of EtOH. Toll-like receptor 4 (TLR4) has been reported to strongly reduce the duration of EtOH-induced sedation, although most studies do not support its direct involvement in EtOH consumption. We examined TLR4 and other MyD88 pathway molecules to determine signaling specificity in acute EtOH-related behaviors. We also studied other GABAergic sedatives to gauge the EtOH specificity and potential role for GABA in EtOH's sedative and intoxicating effects in the mutant mice.

METHODS

Loss of righting reflex (LORR) and recovery from motor incoordination were studied following acute injection of EtOH or other sedative drugs in male and female control C57BL/6J mice versus mice lacking CD14, TLR2, TLR4 (C57BL/10ScN), or MyD88. We also examined EtOH-induced hypothermia and blood EtOH clearance in these mice.

RESULTS

Male and female mice lacking TLR4 or MyD88 showed reduced duration of EtOH-induced LORR and faster recovery from EtOH-induced motor incoordination in the rotarod test. MyD88 knockout mice had slightly faster recovery from EtOH-induced hypothermia compared to control mice. None of the mutants differed from control mice in the rate of blood EtOH clearance. All of the mutants showed similar decreases in the duration of gaboxadol-induced LORR, but only mice lacking TLR4 were less sensitive to the sedative effects of pentobarbital. Faster recovery from diazepam-induced motor impairment was observed in CD14, TLR4, and MyD88 null mice of both sexes.

CONCLUSIONS

TLR4 and MyD88 were key mediators of the sedative and intoxicating effects of EtOH and GABAergic sedatives, indicating a strong influence of TLR4-MyD88 signaling on GABAergic function. Despite the involvement of TLR4 in EtOH's acute behaviors, it did not regulate EtOH consumption in any drinking model as shown in our companion article. Collectively, our studies demonstrate differential effects of TLR-MyD88 components in the acute versus chronic actions of EtOH.

Ethanol Consumption in Mice Lacking CD14, TLR2, TLR4, or MyD88

BACKGROUND

Molecular and behavioral studies support a role for innate immune proinflammatory pathways in mediating the effects of alcohol. Increased levels of Toll-like receptors (TLRs) have been observed in animal models of alcohol consumption and in human alcoholics, and many of these TLRs signal via the MyD88-dependent pathway. We hypothesized that this pathway is involved in alcohol drinking and examined some of its key signaling components.

METHODS

Different ethanol (EtOH)-drinking paradigms were studied in male and female control C57BL/6J mice versus mice lacking CD14, TLR2, TLR4 (C57BL/10ScN), or MyD88. We studied continuous and intermittent access 2-bottle choice (2BC) and 1-bottle and 2BC drinking-in-the-dark (DID) tests as well as preference for saccharin, quinine, and NaCl.

RESULTS

In the 2BC continuous access test, EtOH intake decreased in male TLR2 knockout (KO) mice, and we previously reported reduced 2BC drinking in male and female CD14 KO mice. In the intermittent access 2BC test, EtOH intake decreased in CD14 KO male and female mice, whereas drinking increased in MyD88 KO male mice. In the 2BC-DID test, EtOH drinking decreased in male and female mice lacking TLR2, whereas drinking increased in MyD88 KO male mice. In the 1-bottle DID test, EtOH intake decreased in female TLR2 KO mice. TLR2 KO and CD14 KO mice did not differ in saccharin preference but showed reduced preference for NaCl. MyD88 KO mice showed a slight reduction in preference for saccharin.

CONCLUSIONS

Deletion of key components of the MyD88-dependent pathway produced differential effects on EtOH intake by decreasing (TLR2 KO and CD14 KO) or increasing (MyD88 KO) drinking, while deletion of TLR4 had no effect. Some of the drinking effects depended on the sex of the mice and/or the EtOH-drinking model.

The link between inflammation, bugs, the intestine and the brain in alcohol dependence

In recent years, some new processes have been proposed to explain how alcohol may influence behavior, psychological symptoms and alcohol seeking in alcohol-dependent subjects. In addition to its important effect on brain and neurotransmitters equilibrium, alcohol abuse also affects peripheral organs including the gut. By yet incompletely understood mechanisms, chronic alcohol abuse increases intestinal permeability and alters the composition of the gut microbiota, allowing bacterial components from the gut lumen to reach the systemic circulation. These gut-derived bacterial products are recognized by immune cells circulating in the blood or residing in target organs, which consequently synthesize and release pro-inflammatory cytokines. Circulating cytokines are considered important mediators of the gut-brain communication, as they can reach the central nervous system and induce neuroinflammation that is associated with change in mood, cognition and drinking behavior. These observations support the possibility that targeting the gut microbiota, by the use of probiotics or prebiotics, could restore the gut barrier function, reduce systemic inflammation and may have beneficial effect in treating alcohol dependence and in reducing alcohol relapse.

Genetic and Pharmacologic Manipulation of TLR4 Has Minimal Impact on Ethanol Consumption in Rodents

Toll-like receptor 4 (TLR4) is a critical component of innate immune signaling and has been implicated in alcohol responses in preclinical and clinical models. Members of the Integrative Neuroscience Initiative on Alcoholism (INIA-Neuroimmune) consortium tested the hypothesis that TLR4 mediates excessive ethanol drinking using the following models: (1) Tlr4 knock-out (KO) rats, (2) selective knockdown of Tlr4 mRNA in mouse nucleus accumbens (NAc), and (3) injection of the TLR4 antagonist (+)-naloxone in mice. Lipopolysaccharide (LPS) decreased food/water intake and body weight in ethanol-naive and ethanol-trained wild-type (WT), but not Tlr4 KO rats. There were no consistent genotypic differences in two-bottle choice chronic ethanol intake or operant self-administration in rats before or after dependence. In mice, (+)-naloxone did not decrease drinking-in-the-dark and only modestly inhibited dependence-driven consumption at the highest dose. Tlr4 knockdown in mouse NAc did not decrease drinking in the two-bottle choice continuous or intermittent access tests. However, the latency to ethanol-induced loss of righting reflex increased and the duration decreased in KO versus WT rats. In rat central amygdala neurons, deletion of Tlr4 altered GABA_A receptor function, but not GABA release. Although there were no genotype differences in acute ethanol effects before or after chronic intermittent ethanol exposure, genotype differences were observed after LPS exposure. Using different species and sexes, different methods to inhibit TLR4 signaling, and different ethanol consumption tests, our comprehensive studies indicate that TLR4 may play a role in ethanol-induced sedation and GABA_A receptor function, but does not regulate excessive drinking directly and would not be an effective therapeutic target.

SIGNIFICANCE STATEMENT

Toll-like receptor 4 (TLR4) is a key mediator of innate immune signaling and has been implicated in alcohol responses in animal models and human alcoholics. Members of the Integrative Neuroscience Initiative on Alcoholism (INIA-Neuroimmune) consortium participated in the first comprehensive study across multiple laboratories to test the hypothesis that TLR4 regulates excessive alcohol consumption in different species and different models of chronic, dependence-driven, and binge-like drinking. Although TLR4 was not a critical determinant of excessive drinking, it was important in the acute sedative effects of alcohol. Current research efforts are directed at determining which neuroimmune pathways mediate excessive alcohol drinking and these findings will help to prioritize relevant pathways and potential therapeutic targets.

Effective Reduction in High Ethanol Drinking by Semisynthetic Tetracycline Derivatives

BACKGROUND

New pharmacotherapies to treat alcohol use disorders (AUD) are needed. Given the complex nature of AUD, there likely exist multiple novel drug targets. We, and others, have shown that the tetracycline drugs, minocycline and doxycycline, reduced ethanol (EtOH) drinking in mice. To test the hypothesis that suppression of high EtOH consumption is a general property of tetracyclines, we screened several derivatives for antidrinking activity using the Drinking-In-the-Dark (DID) paradigm. Active drugs were studied further using the dose-response relationship.

METHODS

Adult female and male C57BL/6J mice were singly housed and the DID paradigm was performed using 20% EtOH over a 4-day period. Mice were administered a tetracycline or its vehicle 20 hours prior to drinking. Water and EtOH consumption was measured daily. Body weight was measured at the start of drug injections and after the final day of the experiment. Blood was collected for EtOH content measurement immediately following the final bout of drinking.

RESULTS

Seven tetracyclines were tested at a 50 mg/kg dose. Only minocycline and tigecycline significantly reduced EtOH drinking, and doxycycline showed a strong effect size trend toward reduced drinking. Subsequent studies with these 3 drugs revealed a dose-dependent decrease in EtOH consumption for both female and male mice, with sex differences in efficacy. Minocycline and doxycycline reduced water intake at higher doses, although to a lesser degree than their effects on EtOH drinking. Tigecycline did not negatively affect water intake. The rank order of potency for reduction in EtOH consumption was minocycline > doxycycline > tigecycline, indicating efficacy was not strictly related to their partition coefficients or distribution constants.

CONCLUSIONS

Due to its effectiveness in reducing high EtOH consumption coupled without an effect on water intake, tigecycline was found to be the most promising lead tetracycline compound for further study toward the development of a new pharmacotherapy for the treatment of AUD.

Effective Reduction of Acute Ethanol Withdrawal by the Tetracycline Derivative, Tigecycline, in Female and Male DBA/2J Mice

BACKGROUND

Alcohol use disorder (AUD) is a spectrum disorder characterized by mild to severe symptoms, including potential withdrawal signs upon cessation of consumption. Approximately five hundred thousand patients with AUD undergo clinically relevant episodes of withdrawal annually (New Engl J Med, 2003, 348, 1786). Recent evidence indicates potential for drugs that alter neuroimmune pathways as new AUD therapies. We have previously shown the immunomodulatory drugs, minocycline and tigecycline, were effective in reducing ethanol (EtOH) consumption in both the 2-bottle choice and drinking-in-the-dark paradigms. Here, we test the hypothesis that tigecycline, a tetracycline derivative, will reduce the severity of EtOH withdrawal symptoms in a common acute model of alcohol withdrawal (AWD) using a single anesthetic dose of EtOH in seizure sensitive DBA/2J (DBA) mice.

METHODS

Naïve adult female and male DBA mice were given separate injections of 4 g/kg i.p. EtOH with vehicle or tigecycline (0, 20, 40, or 80 mg/kg i.p.). The 80 mg/kg dose was tested at 3 time points (0, 4, and 7 hours) post EtOH treatment. Handling-induced convulsions (HICs) were measured before and then over 12 hours following EtOH injection. HIC scores and areas under the curve were tabulated. In separate mice, blood EtOH concentrations (BECs) were measured at 2, 4, and 7 hours postinjection of 4 g/kg i.p. EtOH in mice treated with 0 and 80 mg/kg i.p. tigecycline.

RESULTS

AWD symptom onset, peak magnitude, and overall HIC severity were reduced by tigecycline drug treatment compared to controls. Tigecycline treatment was effective regardless of timing throughout AWD, with earlier treatment showing greater efficacy. Tigecycline showed a dose-responsive reduction in acute AWD convulsions, with no sex differences in efficacy. Importantly, tigecycline did not affect BECs over a time course of elimination.

CONCLUSIONS

Tigecycline effectively reduced AWD symptoms in DBA mice at all times and dosages tested, making it a promising lead compound for development of a novel pharmacotherapy for AWD. Further studies are needed to determine the mechanism of tigecycline action.

Inhibition of IKKβ Reduces Ethanol Consumption in C57BL/6J Mice

Proinflammatory pathways in neuronal and non-neuronal cells are implicated in the acute and chronic effects of alcohol exposure in animal models and humans. The nuclear factor-κB (NF-κB) family of DNA transcription factors plays important roles in inflammatory diseases. The kinase IKKβ mediates the phosphorylation and subsequent proteasomal degradation of cytosolic protein inhibitors of NF-κB, leading to activation of NF-κB. The role of IKKβ as a potential regulator of excessive alcohol drinking had not previously been investigated. Based on previous findings that the overactivation of innate immune/inflammatory signaling promotes ethanol consumption, we hypothesized that inhibiting IKKβ would limit/decrease drinking by preventing the activation of NF-κB. We studied the systemic effects of two pharmacological inhibitors of IKKβ, TPCA-1 and sulfasalazine, on ethanol intake using continuous- and limited-access, two-bottle choice drinking tests in C57BL/6J mice. In both tests, TPCA-1 and sulfasalazine reduced ethanol intake and preference without changing total fluid intake or sweet taste preference. A virus expressing Cre recombinase was injected into the nucleus accumbens and central amygdala to selectively knock down IKKβ in mice genetically engineered with a conditional Ikkb deletion (Ikkb^F/F). Although IKKβ was inhibited to some extent in astrocytes and microglia, neurons were a primary cellular target. Deletion of IKKβ in either brain region reduced ethanol intake and preference in the continuous access two-bottle choice test without altering the preference for sucrose. Pharmacological and genetic inhibition of IKKβ decreased voluntary ethanol consumption, providing initial support for IKKβ as a potential therapeutic target for alcohol abuse.

The Neurotrophic Factor Receptor p75 in the Rat Dorsolateral Striatum Drives Excessive Alcohol Drinking

Brain-derived neurotrophic factor (BDNF) signaling in the dorsolateral striatum (DLS) keeps alcohol intake in moderation. For example, activation of the BDNF receptor tropomyosin receptor kinase B (TrkB) in the DLS reduces intake in rats that consume moderate amounts of alcohol. Here, we tested whether long-term excessive consumption of alcohol produces neuroadaptations in BDNF signaling in the rat DLS. We found that BDNF was no longer able to gate alcohol self-administration after a history of repeated cycles of binge alcohol drinking and withdrawal. We then elucidated the possible neuroadaptations that could block the ability of BDNF to keep consumption of alcohol in moderation. We report that intermittent access to 20% alcohol in a two-bottle choice paradigm that models excessive alcohol drinking produces a mobilization of DLS p75 neurotrophin receptor (p75NTR), whose activities oppose those of the Trk receptors, including TrkB. These neuroadaptations were not observed in the DLS of rats exposed to continuous access to 10% alcohol or in rats consuming sucrose. Furthermore, short hairpin RNA (shRNA)-mediated knockdown of the p75NTR gene in the DLS, as well as intra-DLS infusion or systemic administration of the p75NTR modulator, LM11A-31, significantly reduced binge drinking of alcohol. Together, our results suggest that excessive alcohol consumption produces a change in BDNF signaling in the DLS, which is mediated by the recruitment of p75NTR. Our data also imply that modulators of p75NTR signaling could be developed as medications for alcohol abuse disorders.

SIGNIFICANCE STATEMENT

Neuroadaptations gate or drive excessive, compulsive alcohol drinking. We previously showed that brain-derived neurotrophic factor and its receptor, TrkB, in the dorsolateral striatum (DLS), are part of an endogenous system that keeps alcohol drinking in moderation. Here, we show that a history of excessive alcohol intake produces neuroadaptations in the DLS that preclude BDNF's ability to gate alcohol self-administration in rats by the recruitment of the low-affinity neurotrophin receptor, p75NTR, whose activities opposes those of the Trk receptors. Finally, we show that the administration of the p75NTR modulator, LM11A-31, significantly reduces excessive alcohol intake suggesting that the drug may be developed as a new treatment for alcohol abuse disorders.

Impact of the Innate Immune Response in the Actions of Ethanol on the Central Nervous System

The innate immune response in the central nervous system (CNS) participates in both synaptic plasticity and neural damage. Emerging evidence from human and animal studies supports the role of the neuroimmune system response in many actions of ethanol (EtOH) on the CNS. Research studies have shown that alcohol stimulates brain immune cells, microglia, and astrocytes, by activating innate immune receptors Toll-like receptors (TLRs) and NOD-like receptors (inflammasome NLRs) triggering signaling pathways, which culminate in the production of pro-inflammatory cytokines and chemokines that lead to neuroinflammation. This review focuses on evidence that indicates the participation of TLRs and the inflammasome NLRs signaling response in many effects of EtOH on the CNS, such as neuroinflammation associated with brain damage, cognitive and behavioral dysfunction, and adolescent brain development alterations. It also reviews findings that indicate the role of TLR4-dependent signaling immune molecules in alcohol consumption, reward, and addiction. The research data suggest that overactivation of TLR4 or NLRs increases pro-inflammatory cytokines and mediators to cause neural damage in the cerebral cortex and hippocampus, while modest TLR4 activation, along with the generation of certain cytokines and chemokines in specific brain areas (e.g., amygdala, ventral tegmental area), modulate neurotransmission, alcohol drinking, and alcohol rewards. Elimination of TLR4 and NLRP3 abolishes many neuroimmune effects of EtOH. Despite much progress being made in this area, there are some research gaps and unanswered questions that this review discusses. Finally, potential therapies that target neuroimmune pathways to treat neuropathological and behavioral consequences of alcohol abuse are also evaluated.

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

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

Stress and Withdrawal from Chronic Ethanol Induce Selective Changes in Neuroimmune mRNAs in Differing Brain Sites

Stress is a strong risk factor in alcoholic relapse and may exert effects that mimic aspects of chronic alcohol exposure on neurobiological systems. With the neuroimmune system becoming a prominent focus in the study of the neurobiological consequences of stress, as well as chronic alcohol exposure proving to be a valuable focus in this regard, the present study sought to compare the effects of stress and chronic ethanol exposure on induction of components of the neuroimmune system. Rats were exposed to either 1 h exposure to a mild stressor (restraint) or exposure to withdrawal from 15 days of chronic alcohol exposure (i.e., withdrawal from chronic ethanol, WCE) and assessed for neuroimmune mRNAs in brain. Restraint stress alone elevated chemokine (C–C motif) ligand 2 (CCL2), interleukin-1-beta (IL-1β), tumor necrosis factor alpha (TNFα) and toll-like receptor 4 (TLR4) mRNAs in the cerebral cortex within 4 h with a return to a control level by 24 h. These increases were not accompanied by an increase in corresponding proteins. Withdrawal from WCE also elevated cytokines, but did so to varying degrees across different cytokines and brain regions. In the cortex, stress and WCE induced CCL2, TNFα, IL-1β, and TLR4 mRNAs. In the hypothalamus, only WCE induced cytokines (CCL2 and IL-1β) while in the hippocampus, WCE strongly induced CCL2 while stress and WCE induced IL-1β. In the amygdala, only WCE induced CCL2. Finally—based on the previously demonstrated role of corticotropin-releasing factor 1 (CRF1) receptor inhibition in blocking WCE-induced cytokine mRNAs—the CRF1 receptor antagonist CP154,526 was administered to a subgroup of stressed rats and found to be inactive against induction of CCL2, TNFα, or IL-1β mRNAs. These differential results suggest that stress and WCE manifest broad neuroimmune effects in brain depending on the cytokine and brain region, and that CRF inhibition may not be a relevant mechanism in non-alcohol exposed animals. Overall, these effects are complex in terms of their neuroimmune targets and neuroanatomical specificity. Further investigation of the differential distribution of cytokine induction across neuroanatomical regions, individual cell types (e.g., neuronal phenotypes and glia), severity of chronic alcohol exposure, as well as across differing stress types may prove useful in understanding differential mechanisms of induction and for targeting select systems for pharmacotherapeutic intervention in alcoholism.

Evaluation of TLR4 Inhibitor, T5342126, in Modulation of Ethanol-Drinking Behavior in Alcohol-Dependent Mice

AIMS

Several lines of evidence support a critical role of TLR4 in the neuroimmune responses associated with alcohol disorders and propose inhibitors of TLR4 signaling as potential treatments for alcoholism. In this work, we investigated the effect of T5342126 compound, a selective TLR4 inhibitor, on excessive drinking and microglial activation associated with ethanol dependence.

METHODS

We used 2BC-CIE (two-bottle choice-chronic ethanol intermittent vapor exposure) paradigm to induce ethanol dependence in mice. After induction of the ethanol dependence, we injected T5342126 (i.p., 57 mg/kg) for 14 days while monitoring ethanol intake by 2BC (limited access to ethanol) method.

RESULTS

T5342126 decreased ethanol drinking in both ethanol-dependent and non-dependent mice but T5342126 showed also dose-dependent non-specific effects represented by decreased animal locomotor activity, saccharine intake, and body core temperature. Six days after the last ethanol-drinking session, we examined the immunohistochemical staining of Iba-1 (ionized calcium-binding adapter molecule 1), a microglial activation marker, in the central nucleus of the amygdala (CeA) and dentate gyrus (DG) of the hippocampus. Notably, T5342126 reduced Iba-1 density in the CeA of both ethanol-dependent and non-dependent mice injected with T5342126. There were no significant differences in the DG Iba-1 density among the treatment groups.

CONCLUSIONS

Collectively, our data suggest that T5342126, via blocking TLR4 activation, contributes to the reduction of ethanol drinking and ethanol-induced neuroimmune responses. However, the non-specific effects of T5342126 may play a significant role in the T5342126 effects on ethanol drinking and thus, may limit its therapeutic potential for treatment of alcohol dependence.

SHORT SUMMARY

T5342126, an experimental TLR4 inhibitor, is effective in reducing ethanol drinking and inhibiting the activation and proliferation of microglia in both ethanol-dependent and non-dependent mice. However, T5342126's use as a potential candidate for the treatment of alcohol addiction may be limited due to its non-specific effects.

Alcohol resistance in Drosophila is modulated by the Toll innate immune pathway

A growing body of evidence has shown that alcohol alters the activity of the innate immune system and that changes in innate immune system activity can influence alcohol-related behaviors. Here, we show that the Toll innate immune signaling pathway modulates the level of alcohol resistance in Drosophila. In humans, a low level of response to alcohol is correlated with increased risk of developing an alcohol use disorder. The Toll signaling pathway was originally discovered in, and has been extensively studied in Drosophila. The Toll pathway is a major regulator of innate immunity in Drosophila, and mammalian Toll-like receptor signaling has been implicated in alcohol responses. Here, we use Drosophila-specific genetic tools to test eight genes in the Toll signaling pathway for effects on the level of response to ethanol. We show that increasing the activity of the pathway increases ethanol resistance whereas decreasing the pathway activity reduces ethanol resistance. Furthermore, we show that gene products known to be outputs of innate immune signaling are rapidly induced following ethanol exposure. The interaction between the Toll signaling pathway and ethanol is rooted in the natural history of Drosophila melanogaster.

A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

Involvement of TLR4 in the long-term epigenetic changes, rewarding and anxiety effects induced by intermittent ethanol treatment in adolescence

Studies in humans and experimental animals have demonstrated the vulnerability of the adolescent brain to actions of ethanol and the long-term consequences of binge drinking, including the behavioral and cognitive deficits that result from alcohol neurotoxicity, and increased risk to alcohol abuse and dependence. Although the mechanisms that participate in these effects are largely unknown, we have shown that ethanol by activating innate immune receptors, toll-like receptor 4 (TLR4), induces neuroinflammation, impairs myelin proteins and causes cognitive dysfunctions in adolescent mice. Since neuroimmune signaling is also involved in alcohol abuse, the aim of this study was to assess whether ethanol treatment in adolescence promotes the long-term synaptic and molecular events associated with alcohol abuse and addiction. Using wild-type (WT) and TLR4-deficient (TLR4-KO) adolescent mice treated intermittently with ethanol (3g/kg) for 2 weeks, we showed that binge-like ethanol treatment in adolescent mice promotes short- and long-term alterations in synaptic plasticity and epigenetic changes in the promoter region of bdnf and fosb, which increased their expression in the mPFC of young adult animals. These molecular events were associated with long-term rewarding and anxiogenic-related behavioral effects, along with increased alcohol preference. Our results further showed the participation of neuroimmune system activation and the TLR4 signaling response since deficient mice in TLR4 (TLR4-KO) are protected against molecular and behavioral alterations of ethanol in the adolescent brain. Our results highlight a new role of the neuroimmune function and open up new avenues to develop pharmacological treatments that can normalize the immune signaling responsible for long-term effects in adolescence, including alcohol abuse and related disorders.

Time-Course Analysis of Brain Regional Expression Network Responses to Chronic Intermittent Ethanol and Withdrawal: Implications for Mechanisms Underlying Excessive Ethanol Consumption

Long lasting abusive consumption, dependence, and withdrawal are characteristic features of alcohol use disorders (AUD). Mechanistically, persistent changes in gene expression are hypothesized to contribute to brain adaptations leading to ethanol toxicity and AUD. We employed repeated chronic intermittent ethanol (CIE) exposure by vapor chamber as a mouse model to simulate the cycles of ethanol exposure and withdrawal commonly seen with AUD. This model has been shown to induce progressive ethanol consumption in rodents. Brain CIE-responsive expression networks were identified by microarray analysis across five regions of the mesolimbic dopamine system and extended amygdala with tissue harvested from 0-hours to 7-days following CIE. Weighted Gene Correlated Network Analysis (WGCNA) was used to identify gene networks over-represented for CIE-induced temporal expression changes across brain regions. Differential gene expression analysis showed that long-lasting gene regulation occurred 7-days after the final cycle of ethanol exposure only in prefrontal cortex (PFC) and hippocampus. Across all brain regions, however, ethanol-responsive expression changes occurred mainly within the first 8-hours after removal from ethanol. Bioinformatics analysis showed that neuroinflammatory responses were seen across multiple brain regions at early time-points, whereas co-expression modules related to neuroplasticity, chromatin remodeling, and neurodevelopment were seen at later time-points and in specific brain regions (PFC or HPC). In PFC a module containing Bdnf was identified as highly CIE responsive in a biphasic manner, with peak changes at 0 hours and 5 days following CIE, suggesting a possible role in mechanisms underlying long-term molecular and behavioral response to CIE. Bioinformatics analysis of this network and several other modules identified Let-7 family microRNAs as potential regulators of gene expression changes induced by CIE. Our results suggest a complex temporal and regional pattern of widespread gene network responses involving neuroinflammatory and neuroplasticity related genes as contributing to physiological and behavioral responses to chronic ethanol.

A dysbiotic subpopulation of alcohol-dependent subjects

The vast majority of studies that assessed the importance of biological factors for the development of psychiatric disorders focused on processes occurring at the brain level. Alcohol-dependence is a very frequent psychiatric disorder where psycho-pharmacological interventions are only of moderate efficacy. Our laboratory has recently described that a subpopulation of alcohol-dependent subjects, that accounted for approximately 40% of individuals tested, presented with an increased intestinal permeability, with a dysbiosis, with alterations in the metabolomic content of faeces--that could play a role in the increased permeability--and finally with a more severe profile of alcohol-dependence than the other non-dysbiotic subpopulation. In this addendum, we discuss the implications of our observations for the pathophysiology of alcohol dependence where we try to discriminate which addiction dimensions are likely related to the gut microbiota alterations and whether these alterations are the cause or the consequence of drinking habits.

Genes and Alcohol Consumption: Studies with Mutant Mice

In this chapter, we review the effects of global null mutant and overexpressing transgenic mouse lines on voluntary self-administration of alcohol. We examine approximately 200 publications pertaining to the effects of 155 mouse genes on alcohol consumption in different drinking models. The targeted genes vary in function and include neurotransmitter, ion channel, neuroimmune, and neuropeptide signaling systems. The alcohol self-administration models include operant conditioning, two- and four-bottle choice continuous and intermittent access, drinking in the dark limited access, chronic intermittent ethanol, and scheduled high alcohol consumption tests. Comparisons of different drinking models using the same mutant mice are potentially the most informative, and we will highlight those examples. More mutants have been tested for continuous two-bottle choice consumption than any other test; of the 137 mouse genes examined using this model, 97 (72%) altered drinking in at least one sex. Overall, the effects of genetic manipulations on alcohol drinking often depend on the sex of the mice, alcohol concentration and time of access, genetic background, as well as the drinking test.

Commonalities and Distinctions Among Mechanisms of Addiction to Alcohol and Other Drugs

BACKGROUND

Alcohol abuse is comorbid with abuse of many other drugs, some with similar pharmacology and others quite different. This leads to the hypothesis of an underlying, unitary dysfunctional neurobiological basis for substance abuse risk and consequences.

METHODS

In this review, we discuss commonalities and distinctions of addiction to alcohol and other drugs. We focus on recent advances in preclinical studies using rodent models of drug self-administration.

RESULTS

While there are specific behavioral and molecular manifestations common to alcohol, psychostimulant, opioid, and nicotine dependence, attempts to propose a unifying theory of the addictions inevitably face details where distinctions are found among classes of drugs.

CONCLUSIONS

For alcohol, versus other drugs of abuse, we discuss and compare advances in: (i) neurocircuitry important for the different stages of drug dependence; (ii) transcriptomics and genetical genomics; and (iii) enduring effects, noting in particular the contributions of behavioral genetics and animal models.

Commentary on: Are we overpathologizing everyday life? A tenable blueprint for behavioral addiction research. Can the emerging domain of behavioral addictions bring a new reflection for the field of addictions, by stressing the issue of the context of addiction development?

BACKGROUND

This paper is a commentary to the article entitled: "Are we overpathologizing everyday life? A tenable blueprint for behavioral addiction research", by Billieux, Schimmenti, Khazaal, Maurage and Heeren (2015).

METHODS AND AIMS

In this manuscript, we commented on two aspects developed by the authors. Billieux et al. (2015) propose that the recent development of propositions of behavioral addiction is driven by an unwise application of an addiction model to excessive behaviors and rests on a confirmatory research strategy that does not question the psychological processes underlying the development of the conduct. They also show that applying a process driven strategy leads to a more appropriate description of the reality of the behavior and conduct, in particular by describing a variety of motivations for the excessive behavior, which is central to understanding the nature of the conduct. We believe that this new approach, which is fruitful to the emerging domain of behavioral addictions, could also apply to the domain of addictions in general. The latter is characterized by the application of a generic biological model, largely influenced by animal models, focusing on neurophysiological determinants of addiction. This approach may have decreased the attention paid to dimensions of addictions that are more specifically human. We will firstly briefly argue on the limitation of this neurophysiological addiction model for the field of excessive behavioral conducts. Secondly, we will argue for an approach centered on the differentiation of motivations and on the adaptive dimension of the behavior when it first developed and on the evocation of a transition where the conduct became independent of its original function.

CONCLUSIONS

The emerging domain of behavioral addictions, where no animal model has been developed so far, may bring a new reflection that may apply to the domain of addictions in general, with a specific attention to human questions.