Innately activated TLR4 signal in the nucleus accumbens is sustained by CRF amplification loop and regulates impulsivity

Neuroactive Steroids, Toll-like Receptors, and Neuroimmune Regulation: Insights into Their Impact on Neuropsychiatric Disorders

Pregnane neuroactive steroids, notably allopregnanolone and pregnenolone, exhibit efficacy in mitigating inflammatory signals triggered by toll-like receptor (TLR) activation, thus attenuating the production of inflammatory factors. Clinical studies highlight their therapeutic potential, particularly in conditions like postpartum depression (PPD), where the FDA-approved compound brexanolone, an intravenous formulation of allopregnanolone, effectively suppresses TLR-mediated inflammatory pathways, predicting symptom improvement. Additionally, pregnane neurosteroids exhibit trophic and anti-inflammatory properties, stimulating the production of vital trophic proteins and anti-inflammatory factors. Androstane neuroactive steroids, including estrogens and androgens, along with dehydroepiandrosterone (DHEA), display diverse effects on TLR expression and activation. Notably, androstenediol (ADIOL), an androstane neurosteroid, emerges as a potent anti-inflammatory agent, promising for therapeutic interventions. The dysregulation of immune responses via TLR signaling alongside reduced levels of endogenous neurosteroids significantly contributes to symptom severity across various neuropsychiatric disorders. Neuroactive steroids, such as allopregnanolone, demonstrate efficacy in alleviating symptoms of various neuropsychiatric disorders and modulating neuroimmune responses, offering potential intervention avenues. This review emphasizes the significant therapeutic potential of neuroactive steroids in modulating TLR signaling pathways, particularly in addressing inflammatory processes associated with neuropsychiatric disorders. It advances our understanding of the complex interplay between neuroactive steroids and immune responses, paving the way for personalized treatment strategies tailored to individual needs and providing insights for future research aimed at unraveling the intricacies of neuropsychiatric disorders.

Effect of rifampicin on TLR4-signaling pathways in the nucleus accumbens of the rat brain during abstinence of long-term alcohol treatment

AIMS

The treatment with the antibiotic rifampicin (Rif) led to a decrease in the frequency of neurodegenerative pathologies. There are suggestions that the mechanism of action of Rif may be mediated by its effect on toll-like receptor (TLR)4-dependent pathways. We evaluated the expression status of TLR4-dependent genes during abstinence from long-term alcohol treatments in the nucleus accumbens (NAc) of the rat brain, and also studied the effects of Rif to correct these changes.

METHODS

The long-term alcohol treatment was performed by intragastric delivery of ethanol solution. At the end of alcohol treatment intraperitoneal injections of Rif (100 mg/kg) or saline were made. Extraction of the brain structures was performed on the 10th day of abstinence from alcohol. We used the SYBR Green qPCR method to quantitatively analyze the relative expression levels of the studied genes.

RESULTS

The long-term alcohol treatment promotes an increase in the level of TLR4 mRNA and mRNA of its endogenous ligand high-mobility group protein B1 during abstinence drop alcohol in NAc of rats. The use of Rif in our study led to a decrease in the increased expression of high-mobility group protein B1, Tlr4, and proinflammatory cytokine genes (Il1β, Il6) in the NAc of the rat brain during abstinence of long-term alcohol treatment. In addition, Rif administration increased the decreased mRNA levels of anti-inflammatory cytokines (Il10, Il11).

CONCLUSION

The data obtained indicate the ability of Rif to correct the mechanisms of the TLR4 system genes in the NAc of the rat brain during alcohol abstinence.

Neurosteroid [3α,5α]-3-hydroxy-pregnan-20-one enhances IL-10 production via endosomal TRIF-dependent TLR4 signaling pathway

Background

Previous studies demonstrated the inhibitory effect of allopregnanolone (3α,5α-THP) on the activation of inflammatory toll-like receptor 4 (TLR4) signals in RAW264.7 macrophages and the brains of selectively bred alcohol-preferring (P) rats. In the current study, we investigated the impact of 3α,5α-THP on the levels of IL-10 and activation of the TRIF-dependent endosomal TLR4 pathway.

Methods

The amygdala and nucleus accumbens (NAc) of P rats, which exhibit innately activated TLR4 pathways as well as RAW264.7 cells, were used. Enzyme-linked immunosorbent assays (ELISA) and immunoblotting assays were used to ascertain the effects of 3α,5α-THP on the TRIF-dependent endosomal TLR4 pathway and endosomes were isolated to examine translocation of TLR4 and TRIF. Additionally, we investigated the effects of 3α,5α-THP and 3α,5α-THDOC (0.1, 0.3, and 1.0 µM) on the levels of IL-10 in RAW264.7 macrophages. Finally, we examined whether inhibiting TRIF (using TRIF siRNA) in RAW264.7 cells altered the levels of IL-10.

Results

3α,5α-THP administration facilitated activation of the endosomal TRIF-dependent TLR4 pathway in males, but not female P rats. 3α,5α-THP increased IL-10 levels (+13.2 ± 6.5%) and BDNF levels (+21.1 ± 11.5%) in the male amygdala. These effects were associated with increases in pTRAM (+86.4 ± 28.4%), SP1 (+122.2 ± 74.9%), and PI(3)K-p110δ (+61.6 ± 21.6%), and a reduction of TIRAP (-13.7 ± 6.0%), indicating the activation of the endosomal TRIF-dependent TLR4 signaling pathway. Comparable effects were observed in NAc of these animals. Furthermore, 3α,5α-THP enhanced the accumulation of TLR4 (+43.9 ± 11.3%) and TRIF (+64.8 ± 32.8%) in endosomes, with no significant effect on TLR3 accumulation. Additionally, 3α,5α-THP facilitated the transition from early endosomes to late endosomes (increasing Rab7 levels: +35.8 ± 18.4%). In RAW264.7 cells, imiquimod (30 µg/mL) reduced IL-10 while 3α,5α-THP and 3α,5α-THDOC (0.1, 0.3, and 1.0 µM) restored IL-10 levels. To determine the role of the TRIF-dependent TLR4 signaling pathway in IL-10 production, the downregulation of TRIF (-62.9 ± 28.2%) in RAW264.7 cells led to a reduction in IL-10 levels (-42.3 ± 8.4%). TRIF (-62.9 ± 28.2%) in RAW264.7 cells led to a reduction in IL-10 levels (-42.3 ± 8.4%) and 3α,5α-THP (1.0 µM) no longer restored the reduced IL-10 levels.

Conclusion

The results demonstrate 3α,5α-THP enhancement of the endosomal TLR4-TRIF anti-inflammatory signals and elevations of IL-10 in male P rat brain that were not detected in female P rat brain. These effects hold significant implications for controlling inflammatory responses in both the brain and peripheral immune cells.

Alcohol use disorder (AUD) is associated with enhanced sensitivity to cellular lipopolysaccharide challenge

BACKGROUND

Inflammation has been associated with alcohol use disorder (AUD). A novel method to characterize AUD-related immune signaling involves probing Toll-like receptor (TLR)-4 stimulated monocyte production of intracellular cytokines (ICCs) via lipopolysaccharide (LPS). We evaluated relationships between AUD and ICC production at rest and after LPS stimulation.

METHODS

We analyzed blood samples from 36 participants (AUD N = 14; Controls N = 22), collected across time, with ICC expression assessed at rest (i.e., unstimulated) and following stimulation with LPS (i.e., a total of 5 repeated unstimulated or stimulated measures/participant). Markers assessed included tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), TNF-α and IL-6 co-expression, and interferon (IFN). For each marker, we constructed linear mixed models with AUD, LPS, and timepoint as fixed effects (BMI as covariate), allowing for random slope and intercept. AUD × LPS was included as an interaction.

RESULTS

For TLR4-stimulated monocyte production of TNF-α, there were effects of AUD (p < 0.01), LPS (p < 0.001), and AUD × LPS interaction (p < 0.05), indicating that individuals with AUD showed greater unstimulated- and stimulated monocyte expression of TNF-α. Similarly, for TLR4-stimulated monocyte co-expression of TNF-α and IL-6, there were effects of AUD (p < 0.01), LPS (p < 0.001), and AUD × LPS interaction (p < 0.05). No AUD or LPS effects were found for IL-6. Timepoint effects were observed on IL-6 and TNF-α/IL-6 co-expression (p < 0.001). Finally, for IFN there were also effects of AUD (p < 0.05), LPS (p < 0.001), and AUD × LPS (p < 0.001).

CONCLUSIONS

Individuals with AUD showed greater resting or unstimulated levels of intracellular monocyte expression of TNF-α and IL-6/TNF-α co-expression than controls. AUD was associated with increases in TLR4-stimulated monocyte production of TNF-α and co-production of IL-6 and TNF-α. This is, to our knowledge, the first study to investigate relationships between AUD and monocyte production of proinflammatory cytokines, at rest and in response to TLR4 stimulation with LPS. The study extends previous findings on the roles of proinflammatory cytokines in AUD and serves as a critical proof of concept for the use of this method to probe neuroimmune mechanisms underlying AUD.

[The effect of rifampicin on the system of Toll-like receptors in the nucleus accumbens of the brain of long-term alcoholized rats during alcohol withdrawal]

Nucleus accumbens (NAc) is the ventral part of the striatum of the brain; it is an important part of the mesolimbic pathway involved in the reward system that mediates the formation of various forms of addiction, in particular alcohol addiction. Neuroimaging data and in vitro studies indicate the development of a pronounced neurodegenerative process in the NAc, with long-term alcohol use, but the key mechanisms mediating this process remain unknown. In recent years, the attention of researchers has been focused on studying the system of Toll-like receptors (TLRs), the increased activity of which is clearly shown in the cerebral cortex and hippocampus during prolonged alcohol exposure, but there is a need to study the role of this system in other brain structures. In this study, we have shown that prolonged alcohol exposure (2 months) with moderate doses of ethanol (2 g/kg) promotes a pronounced increase in the expression of the Tlr4 gene and its endogenous ligand Hmgb1 in NAc during the period of alcohol withdrawal in rats. Injections of rifampicin (100 mg/kg) reduced the elevated expression level of Hmgb1, Tlr4, as well as pro-inflammatory cytokine genes (IL1β, IL6), while the administration of the drug increased the reduced level of mRNA of anti-inflammatory cytokines (IL10, IL11).

Inhibition of human macrophage activation via pregnane neurosteroid interactions with toll-like receptors: Sex differences and structural requirements

We recently discovered that (3α,5α)3-hydroxypregnan-20-one (allopregnanolone) inhibits pro-inflammatory toll-like receptor (TLR) activation and cytokine/chemokine production in mouse macrophage RAW264.7 cells. The present studies evaluate neurosteroid actions upon TLR activation in human macrophages from male and female healthy donors. Buffy coat leukocytes were obtained from donors at the New York Blood Center (http://nybloodcenter.org/), and peripheral blood mononuclear cells were isolated and cultured to achieve macrophage differentiation. TLR4 and TLR7 were activated by lipopolysaccharide (LPS) or imiquimod in the presence/absence of allopregnanolone or related neurosteroids and pro-inflammatory markers were detected by ELISA or western blotting. Cultured human monocyte-derived-macrophages exhibited typical morphology, a mixed immune profile of both inflammatory and anti-inflammatory markers, with no sex difference at baseline. Allopregnanolone inhibited TLR4 activation in male and female donors, preventing LPS-induced elevations of TNF-α, MCP-1, pCREB and pSTAT1. In contrast, 3α,5α-THDOC and SGE-516 inhibited the TLR4 pathway activation in female, but not male donors. Allopregnanolone completely inhibited TLR7 activation by imiquimod, blocking IL-1-β, IL-6, pSTAT1 and pIRF7 elevations in females only. 3α,5α-THDOC and SGE-516 partially inhibited TLR7 activation, only in female donors. The results indicate that allopregnanolone inhibits TLR4 and TLR7 activation in cultured human macrophages resulting in diminished cytokine/chemokine production. Allopregnanolone inhibition of TLR4 activation was found in males and females, but inhibition of TLR7 signals exhibited specificity for female donors. 3α,5α-THDOC and SGE-516 inhibited TLR4 and TLR7 pathways only in females. These studies demonstrate anti-inflammatory effects of allopregnanolone in human macrophages for the first time and suggest that inhibition of pro-inflammatory cytokines/chemokines may contribute to its therapeutic actions.

Immune treatments for alcohol use disorder: A translational framework

While the immune system is essential for survival, an excessive or prolonged inflammatory response, such as that resulting from sustained heavy alcohol use, can damage the host and contribute to psychiatric disorders. A growing body of literature indicates that the immune system plays a critical role in the development and maintenance of alcohol use disorder (AUD). As such, there is enthusiasm for treatments that can restore healthy levels of inflammation as a mechanism to reduce drinking and promote recovery. In this qualitative literature review, we provide a conceptual rationale for immune therapies and discuss progress in medications development for AUD focused on the immune system as a treatment target. This review is organized into sections based on primary signaling pathways targeted by the candidate therapies, namely: (a) toll-like receptors, (b) phosphodiesterase inhibitors, (c) peroxisome proliferator-activated receptors, (d) microglia and astrocytes, (e) other immune pharmacotherapies, and (f) behavioral therapies. As relevant within each section, we examine the basic biological mechanisms of each class of therapy and evaluate preclinical research testing the role of the therapy on mitigating alcohol-related behaviors in animal models. To the extent available, translational findings are reviewed with discussion of completed and ongoing randomized clinical trials and their findings to date. An applied and clinically focused approach is taken to identify the potential clinical applications of the various treatments reviewed. We conclude by delineating the most promising candidate treatments and discussing future directions by considering opportunities for immune treatment development and personalized medicine for AUD.

Neurosteroid allopregnanolone (3α,5α-THP) inhibits inflammatory signals induced by activated MyD88-dependent toll-like receptors

We have shown that endogenous neurosteroids, including pregnenolone and 3α,5α-THP inhibit toll-like receptor 4 (TLR4) signal activation in mouse macrophages and the brain of alcohol-preferring (P) rat, which exhibits innate TLR4 signal activation. The current studies were designed to examine whether other activated TLR signals are similarly inhibited by 3α,5α-THP. We report that 3α,5α-THP inhibits selective agonist-mediated activation of TLR2 and TLR7, but not TLR3 signaling in the RAW246.7 macrophage cell line. The TLR4 and TLR7 signals are innately activated in the amygdala and NAc from P rat brains and inhibited by 3α,5α-THP. The TLR2 and TLR3 signals are not activated in P rat brain and they are not affected by 3α,5α-THP. Co-immunoprecipitation studies indicate that 3α,5α-THP inhibits the binding of MyD88 with TLR4 or TLR7 in P rat brain, but the levels of TLR4 co-precipitating with TRIF are not altered by 3α,5α-THP treatment. Collectively, the data indicate that 3α,5α-THP inhibits MyD88- but not TRIF-dependent TLR signal activation and the production of pro-inflammatory mediators through its ability to block TLR-MyD88 binding. These results have applicability to many conditions involving pro-inflammatory TLR activation of cytokines, chemokines, and interferons and support the use of 3α,5α-THP as a therapeutic for inflammatory disease.

(3α,5α)3-hydroxypregnan-20-one (3α,5α-THP) regulation of hypothalamic and extrahypothalamic corticotropin releasing factor (CRF): Sexual dimorphism and brain region specificity in Sprague Dawley rats

CRF is the main activator of the hypothalamic-pituitary-adrenal (HPA) axis in response to stress. CRF neurons are found mainly in the hypothalamus, but CRF positive cells and CRF1 receptors are also found in extrahypothalamic structures, including amygdala (CeA), hippocampus, NAc and VTA. CRF release in the hypothalamus is regulated by inhibitory GABAergic interneurons and extrahypothalamic glutamatergic inputs, and disruption of this balance is found in stress-related disorders and addiction. (3α,5α)3-hydroxypregnan-20-one (3α,5α-THP), the most potent positive modulator of GABA_A receptors, attenuates the stress response reducing hypothalamic CRF mRNA expression and ACTH and corticosterone serum levels. In this study, we explored 3α,5α-THP regulation of hypothalamic and extrahypothalamic CRF mRNA and peptide expression, in male and female Sprague Dawley rats, following vehicle or 3α,5α-THP administration (15 mg/kg). In the hypothalamus, we found sex differences in CRF mRNA expression (females +74%, p < 0.01) and CRF peptide levels (females -71%, p < 0.001). 3α,5α-THP administration reduced hypothalamic CRF mRNA expression only in males (-50%, p < 0.05) and did not alter CRF peptide expression in either sex. In hippocampus and CeA, 3α,5α-THP administration reduced CRF peptide concentrations only in the male (hippocampus -29%, p < 0.05; CeA -62%, p < 0.01). In contrast, 3α,5α-THP injection increased CRF peptide concentration in the VTA of both males (+32%, p < 0.01) and females (+26%, p < 0.01). The results show sex and region-specific regulation of CRF signals and the response to 3α,5α-THP administration. This data may be key to successful development of therapeutic approaches for stress-related disorders and addiction.

A quantitative and qualitative study on the neuropsychiatric sequelae of acutely ill COVID-19 inpatients in isolation facilities

This study examined the neuropsychiatric sequelae of acutely ill patients with coronavirus disease 2019 (COVID-19) infection who received treatment in hospital isolation wards during the COVID-19 pandemic. Ten COVID-19 patients who received treatment in various hospitals in Chongqing, China; 10 age- and gender-matched psychiatric patients; and 10 healthy control participants residing in the same city were recruited. All participants completed a survey that collected information on demographic data, physical symptoms in the past 14 days and psychological parameters. Face-to-face interviews with COVID-19 patients were also performed using semi-structured questions. Among the COVID-19 patients, 40% had abnormal findings on the chest computed topography scan, 20% had dysosmia, 10% had dysgeusia, and 80% had repeated positivity on COVID-19 reverse-transcription polymerase chain reaction testing. COVID-19 and psychiatric patients were significantly more worried about their health than healthy controls (p = 0.019). A greater proportion of COVID-19 patients experienced impulsivity (p = 0.016) and insomnia (p = 0.039) than psychiatric patients and healthy controls. COVID-19 patients reported a higher psychological impact of the outbreak than psychiatric patients and healthy controls, with half of them having clinically significant symptoms of posttraumatic stress disorder. COVID-19 and psychiatric patients had higher levels of depression, anxiety and stress than healthy controls. Three themes emerged from the interviews with COVID-19 patients: (i) The emotions experienced by patients after COVID-19 infection (i.e., shock, fear, despair, hope, and boredom); (ii) the external factors that affected patients' mood (i.e., discrimination, medical expenses, care by healthcare workers); and (iii) coping and self-help behavior (i.e., distraction, problem-solving and online support). The future direction in COVID-19 management involves the development of a holistic inpatient service to promote immune and psychological resilience.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Administration of a putative pro-dopamine regulator, a neuronutrient, mitigates alcohol intake in alcohol-preferring rats

BACKGROUND

Excessive alcohol intake is a serious but preventable public health problem in the United States and worldwide. Alcohol and other substance use disorders occur co-morbid with more generalized reward deficiency disorders, characterized by a reduction in dopamine (DA) signaling within the reward pathway, and classically associated with increased impulsivity, risk taking and subsequent drug seeking behavior. It is postulated that increasing dopamine availability and thus restoring DA homeostasis in the mesocorticolimbic system could reduce the motivation to seek and consume ethanol. Here, we treated animals with a neuro-nutrient, KB220Z also known as Synaptamine, designed to augment DA signaling.

METHOD

KB220Z was administered to genetically alcohol-preferring (P) adult male and female rats by oral gavage (PO), intraperioneally (IP), or subcutaneously (SQ) for 4 consecutive days at a 3.4 mL/Kg rat equivalent dose and compared to saline (SQ, IP) or water (PO) controls. Subsequent to treatment, lever pressing and consumption of 10 % ethanol or control 3% sucrose during operant responding was assessed using a drinking in the dark multiple scheduled access (DIDMSA) binge drinking protocol. Locomotor and elevated zero maze activity, and DRD2 mRNA expression via in situ hybridization (ISH) were assessed independently following 4 days of a SQ regimen of KB220Z.

RESULTS

KB220Z administered via IP and SQ markedly and immediately reduced binge drinking of 10 % ethanol in both male and female rats whereas PO administration took at least 3 days to decrease lever pressing for ethanol in both male and female rats. There was no effect of SQ KB220Z on 3% sucrose drinking. Elevated activity in the open field was significantly decreased, and time spent in the open arm of the EZM was moderately reduced. The regimen of SQ KB220Z did not impact the number of DRD2 punctae in neurons of the NAc, but the NAc shell expressed more DRD2 mRNA/cell than NAc core independent of KB220Z.

CONCLUSION

KB220Z attenuates ethanol drinking and other RDS behaviors in P rats possibly by acting on the dopaminergic system, but not by effecting an increase in NAc DRD2 mRNA expression.

A Rationale for Allopregnanolone Treatment of Alcohol Use Disorders: Basic and Clinical Studies

For many years, research from around the world has suggested that the neuroactive steroid (3α,5α)-3-hydroxypregnan-20-one (allopregnanolone or 3α,5α-THP) may have therapeutic potential for treatment of various symptoms of alcohol use disorders (AUDs). In this critical review, we systematically address all the evidence that supports such a suggestion, delineate the etiologies of AUDs that are addressed by treatment with allopregnanolone or its precursor pregnenolone, and the rationale for treatment of various components of the disease based on basic science and clinical evidence. This review presents a theoretical framework for understanding how endogenous steroids that regulate the effects of stress, alcohol, and the innate immune system could play a key role in both the prevention and the treatment of AUDs. We further discuss cautions and limitations of allopregnanolone or pregnenolone therapy with suggestions regarding the management of risk and the potential for helping millions who suffer from AUDs.

GABAAR α2-activated neuroimmune signal controls binge drinking and impulsivity through regulation of the CCL2/CX3CL1 balance

BACKGROUND AND PURPOSE

Toll-like receptors (TLRs) are a family of innate immune system receptors that respond to pathogen-derived and tissue damage-related ligands and are increasingly recognized for their impact on homeostasis and its dysregulation in the nervous system. TLR signaling participates in brain injury and addiction, but its role in the alcohol-seeking behavior, which initiates alcohol drinking, is still poorly understood. In this review, we discuss our findings designed to elucidate the potential contribution of the activated TLR4 signal located in neurons, on impulsivity and the predisposition to initiate alcohol drinking (binge drinking).

RESULTS

Our findings indicate that the TLR4 signal is innately activated in neurons from alcohol-preferring subjects, identifying a genetic contribution to the regulation of impulsivity and the alcohol-seeking propensity. Signal activation is through the non-canonical, previously unknown, binding of TLR4 to the α2 subunit of the γ-aminobutyric 2 acid A receptor (GABAAR α2). Activation is sustained by the stress hormone corticotrophin-releasing factor (CRF) and additional still poorly recognized ligand/scaffold proteins. Focus is on the effect of TLR4 signal activation on the balance between pro- and anti-inflammatory chemokines [chemokine (C-C motif) ligand 2 (CCL2)/chemokine (C-X3-C motif) ligand 1 (CX3CL1)] and its effect on binge drinking.

CONCLUSION

The results are discussed within the context of current findings on the distinct activation and functions of TLR signals located in neurons, as opposed to immune cells. They indicate that the balance between pro- and anti-inflammatory TLR4 signaling plays a major role in binge drinking. These findings have major impact on future basic and translational research, including the development of potential therapeutic and preventative strategies.

Endogenous Neurosteroid (3α,5α)3-Hydroxypregnan-20-one Inhibits Toll-like-4 Receptor Activation and Pro-inflammatory Signaling in Macrophages and Brain

The endogenous neurosteroid (3α,5α)3-hydroxypregnan-20-one (3α,5α-THP, allopregnanolone) has protective activity in animal models of alcoholism, depression, traumatic brain injury, schizophrenia, multiple sclerosis, and Alzheimer’s disease that is poorly understood. Because these conditions involve proinflammatory signaling through toll-like receptors (TLRs), we examined the effects of 3α,5α-THP, and pregnenolone on TLR4 activation in both the periphery and the central nervous system (CNS). We used monocytes/macrophages (RAW264.7) as a model of peripheral immune signaling and studied innately activated TLR4 in the ventral tegmental area (VTA) of selectively bred alcohol-preferring (P) rats. LPS activated the TLR4 pathway in RAW264.7 cells as evidenced by increased levels of p-TAK1, TRAF6, NF-κB p50, phospho-NF-κB- p65, pCREB, HMGB1, and inflammatory mediators, including MCP-1 and TNFα. Both 3α,5α-THP and pregnenolone (0.5–1.0μM) substantially (~80%) inhibited these effects, indicating pronounced inhibition of TLR4 signaling. The mechanism of inhibition appears to involve blockade of TLR4/MD-2 protein interactions in RAW246.7 cells. In VTA, 3α,5α-THP (15 mg/kg, IP) administration reduced TRAF6 (~20%), CRF (~30%), and MCP-1 (~20%) levels, as well as TLR4 binding to GABA_A receptor α2 subunits (~60%) and MyD88 (~40%). The data suggest that inhibition of proinflammatory neuroimmune signaling underlies protective effects of 3α,5α-THP in immune cells and brain, apparently involving blocking of protein-protein interactions that initiate TLR4-dependent signaling. Inhibition of pro-inflammatory TLR4 activation represents a new mechanism of 3α,5α-THP action in the periphery and the brain.

The GABAA Receptor α2 Subunit Activates a Neuronal TLR4 Signal in the Ventral Tegmental Area that Regulates Alcohol and Nicotine Abuse

Alcoholism initiates with episodes of excessive alcohol drinking, known as binge drinking, which is one form of excessive drinking (NIAAA Newsletter, 2004) that is related to impulsivity and anxiety (Ducci et al., 2007; Edenberg et al., 2004) and is also predictive of smoking status. The predisposition of non-alcohol exposed subjects to initiate binge drinking is controlled by neuroimmune signaling that includes an innately activated neuronal Toll-like receptor 4 (TLR4) signal. This signal also regulates cognitive impulsivity, a heritable trait that defines drug abuse initiation. However, the mechanism of signal activation, its function in dopaminergic (TH+) neurons within the reward circuitry implicated in drug-seeking behavior [viz. the ventral tegmental area (VTA)], and its contribution to nicotine co-abuse are still poorly understood. We report that the γ-aminobutyric acid_A receptor (GABA_AR) α2 subunit activates the TLR4 signal in neurons, culminating in the activation (phosphorylation/nuclear translocation) of cyclic AMP response element binding (CREB) but not NF-kB transcription factors and the upregulation of corticotropin-releasing factor (CRF) and tyrosine hydroxylase (TH). The signal is activated through α2/TLR4 interaction, as evidenced by co-immunoprecipitation, and it is present in the VTA from drug-untreated alcohol-preferring P rats. VTA infusion of neurotropic herpes simplex virus (HSV) vectors for α2 (pHSVsiLA2) or TLR4 (pHSVsiTLR4) but not scrambled (pHSVsiNC) siRNA inhibits signal activation and both binge alcohol drinking and nicotine sensitization, suggesting that the α2-activated TLR4 signal contributes to the regulation of both alcohol and nicotine abuse.