Toll-like receptor 4 contributes to the inhibitory effect of morphine on colonic motility in vitro and in vivo

Opioids and the Gastrointestinal Tract: The Role of Peripherally Active µ-Opioid Receptor Antagonists in Modulating Intestinal Permeability

Opioid receptors are found throughout the gastrointestinal tract, including the large intestine. Many patients treated with opioids experience opioid-induced constipation (OIC). Laxatives are not effective in most patients, and in those who do initially respond, the efficacy of laxatives generally diminishes over time. In addition, OIC does not spontaneously resolve for most patients. However, complications of opioids extend far beyond simply slowing gastrointestinal transit. Opioid use can affect intestinal permeability through a variety of mechanisms. Toll-like receptors are a crucial component of innate immunity and are tightly regulated within the gut epithelium. Pathologic µ-opioid receptor (MOR) and toll-like receptor signaling, resulting from chronic opioid exposure, disrupts intestinal permeability leading to potentially harmful bacterial translocation, elevated levels of bacterial toxins, immune activation, and increased cytokine production. Peripherally active MOR antagonists, including methylnaltrexone, are effective at treating OIC. Benefits extend beyond simply blocking the MOR; these agents also act to ameliorate opioid-induced disrupted intestinal permeability. In this review, we briefly describe the physiology of the gastrointestinal epithelial border and discuss the impact of opioids on gastrointestinal function. Finally, we consider the use of peripherally active MOR antagonists to treat disrupted intestinal permeability resulting from opioid use and discuss the potential for improved morbidity and mortality in patients treated with methylnaltrexone for opioid-induced bowel disorders.

Lipopolysaccharide accelerates peristalsis by stimulating glucagon-like peptide-1 release from L cells in the rat proximal colon

Upon epithelial barrier dysfunction, lipopolysaccharide (LPS) stimulates glucagon-like peptide-1 (GLP-1) secretion from enteroendocrine L cells by activating Toll-like receptor 4 (TLR4). Because GLP-1 accelerates peristalsis in the proximal colon, the present study aimed to explore whether LPS facilitates colonic peristalsis by stimulating L cell-derived GLP-1 release. In isolated segments of rat proximal colon that were serosally perfused with physiological salt solution and luminally perfused with 0.9% saline, peristaltic wall motion was video recorded and converted into spatio-temporal maps. Fluorescence immunohistochemistry was also carried out. Intraluminal administration of LPS (100 or 1 µg mL-1 but not 100 ng mL-1) increased the frequency of oro-aboral propagating peristaltic contractions. The LPS-induced acceleration of colonic peristalsis was blocked by TAK-242 (the TLR4 antagonist), exendin-3 (the GLP-1 receptor antagonist) or BIBN4096 (the calcitonin gene-related peptide receptor antagonist). GLP-1-positive epithelial cells co-expressed TLR4 immunoreactivity. In aspirin-pretreated preparations where epithelial barrier function had been impaired, a lower dose of LPS (100 ng mL-1) became capable of accelerating peristalsis. By contrast, luminally applied dimethyl sulphoxide, a reactive oxygen species scavenger that protects epithelial integrity, attenuated the prokinetic effects of a higher dose of LPS (100 µg mL-1). In colonic segments of a stress rat model leading to a leaky gut, LPS induced more pronounced prokinetic effects. Colonic L cells may well sense luminal LPS via TLR4 triggering the release of GLP-1 that stimulates calcitonin gene-related peptide-containing neurons. The resultant acceleration of peristalsis would facilitate excretion of Gram-negative bacteria from the intestine, and thus L cells may have a protective role against intestinal bacterial infections. KEY POINTS: Colonic epithelial cells form a barrier against bacterial invasion but also may contribute more actively to the exclusion of luminal pathogen by stimulating colonic motility. Luminal lipopolysaccharide (LPS) accelerated colonic peristalsis by stimulating calcitonin gene-related peptide-containing neurons. The prokinetic effect of LPS was mediated by the secretion of glucagon-like peptide-1 from enteroendocrine L cells in which Toll-like receptor 4 was expressed. The LPS-mediated acceleration of peristalsis depended on epithelial barrier integrity. L cells have a defensive role against Gram-negative bacterial infections by facilitating faecal excretion, and could be a potential therapeutic target for gastrointestinal infections.

Modified Zhizhu Pill improves the loperamide-induced slow transit constipation via gut microbiota and neurotransmitters in microbiota-gut-brain axis

BACKGROUND

Slow-transmission constipation is a type of intractable constipation with unknown etiology and unclear pathogenesis.

OBJECTIVE

The intention of this study was to evaluate the therapeutic effect and possible mechanism of Modified Zhizhu Pills on loperamide-induced slow transit constipation.

METHODS

The effects of the Modified Zhizhu Pill were evaluated in a rat model of constipation induced by subcutaneous administration of loperamide. Fecal parameters (fecal count, fecal water content, and fecal hardness) were measured in constipated rats. The substance, target, and pathway basis of the Modified Zhizhu Pill on constipation was investigated using network pharmacology. The microflora in rats was determined. Serum neurotransmitters (acetylcholine and 5-hydroxytryptamine) were measured in rats and their relationship with the gut microbiota was assessed.

RESULTS

Modified Zhizhu Pill increased the number of bowel movements and fecal water content, and decreased fecal hardness and transit time. Network pharmacological analysis showed that Modified Zhizhu Pill can target multiple constipation-related targets and pathways through multiple potential active ingredients. Modified Zhizhu Pill alleviated loperamide-induced microbiota dysbiosis. Modified Zhizhu Pill increased serum 5-hydroxytryptamine and acetylcholine. The increase in serum 5-hydroxytryptamine and acetylcholine was associated with rat gut microbiota.

CONCLUSION

These results suggest that Modified Zhizhu Pill may increase intestinal motility and ultimately relieve constipation by improving microecological dysbiosis and neurotransmission.

Comparison of inflammatory and behavioral responses to chronic stress in female and male mice

Major depressive disorder (MDD) is a debilitating disease with a high worldwide prevalence. Despite its greater prevalence in women, male animals are used in most preclinical studies of depression even though there are many sex differences in key components of depression, such as stress responses and immune system functions. In the present study, we found that chronic restraint stress-induced depressive-like behaviors are quite similar in male and female mice, with both sexes displaying increased immobility time in the tail suspension test and reduced social interactions, and both sexes exhibited deficits in working and spatial memories. However, in contrast to the similar depressive-like behaviors developed by male and female mice in response to stress, they displayed different patterns of pro-inflammatory cytokine increases in the periphery and the brain, different changes in microglia, and different changes in the expression of Toll-like receptor 4 in response to stress. Treatment with (+)-naloxone, a Toll-like receptor 4 antagonist that previously demonstrated anti-depressant-like effects in male mice, was more efficacious in male than female mice in reducing the deleterious effects of stress, and its effects were not microbiome-mediated. Altogether, these results suggest differential mechanisms to consider in potential sex-specific treatments of depression.

The potential interplay between Opioid and the Toll-Like Receptor 4 (TLR-4)

CONTEXT

Opioids are available for the management of severe and chronic pain. However, long-term use of high-dose opioids could lead to physiologic tolerance, hyperalgesia, gastrointestinal immobility, addiction, respiratory depression, tumor progression, and inhibition of the immune system. It seems some of these adverse effects of opioids might be induced by TLR-4 signaling.

OBJECTIVE

The review aims to investigate the potential interplay between opioids and TLR-4 in CNS, gastrointestinal, cancer, and immune system.

METHODS

The search of PubMed, Embase, Scopus, web of sciences, and Google scholar was performed for all relevant studies published. From a total of 513 papers obtained at the initial database search, publications including in silico, in vitro, and in vivo studies were selected for the review.

RESULTS

A comprehensive review of studies indicated that using opioids for the reduction of pain might induce adverse effects such as analgesic tolerance, hyperalgesia, cancer progression, and suppression of the immune system. Some studies have indicated these effects may be due to a change in the level of expression and signaling pathway of TLR-4. The generalizability of the results was limited due to the inconsistency of findings.

CONCLUSIONS

More studies are needed to clarify TLR-4-mediated opioid effects on the biology or stages of the disease as well as the role of different types of opioids, appropriate dosage, and exposure in various contexts. Designing the drug candidate and doing many formulation studies for different diseases and various stages of disease could be associated with effective treatment and pain management.

Interaction of Opioids with TLR4-Mechanisms and Ramifications

Simple Summary

Recent evidence indicates that opioids can be active at a receptor that is abundantly expressed on innate immune cells as well as cancer cells: the receptor is termed toll-like receptor 4 (TLR4). TLR4 is increasingly recognised as playing key roles in tumour biology and anticancer defences. However, the issue of whether TLR4 mediates some of the effects of opioids on tumour growth and metastasis is entirely unknown. We review existing evidence, mechanisms, and functional consequences of the action of opioids at TLR4. This opens new avenues of research on the role of opioids in cancer.

Abstract

The innate immune receptor toll-like receptor 4 (TLR4) is known as a sensor for the gram-negative bacterial cell wall component lipopolysaccharide (LPS). TLR4 activation leads to a strong pro-inflammatory response in macrophages; however, it is also recognised to play a key role in cancer. Recent studies of the opioid receptor (OR)-independent actions of opioids have identified that TLR4 can respond to opioids. Opioids are reported to weakly activate TLR4, but to significantly inhibit LPS-induced TLR4 activation. The action of opioids at TLR4 is suggested to be non-stereoselective, this is because OR-inactive (+)-isomers of opioids have been shown to activate or to inhibit TLR4 signalling, although there is some controversy in the literature. While some opioids can bind to the lipopolysaccharide (LPS)-binding cleft of the Myeloid Differentiation factor 2 (MD-2) co-receptor, pharmacological characterisation of the inhibition of opioids on LPS activation of TLR4 indicates a noncompetitive mechanism. In addition to a direct interaction at the receptor, opioids affect NF-κB activation downstream of both TLR4 and opioid receptors and modulate TLR4 expression, leading to a range of in vivo outcomes. Here, we review the literature reporting the activity of opioids at TLR4, its proposed mechanism(s), and the complex functional consequences of this interaction.

A contemporary review of Clostridioides difficile infections in patients with haematologic diseases

Clostridioides (Clostridium) difficile (C. difficile) infection is one of the most common causes of increased morbidity and mortality. Approximately 500 000 C. difficile infections (CDIs) occur each year in the United States, and they result in more than 29 000 deaths. Patients with haematologic diseases are at a higher risk for this infection due to frequent hospitalization and exposure to treatment-associated risk factors. Whilst several currently available antimicrobial agents offer resolution, recurrence of infection remains a major concern. Recent advancement in deciphering C. difficile virulence mechanisms and identification of its allies in contributing to the infection has led to the development of alternative treatment strategies. Here, we will provide a contemporary discussion of how major risk factors in haematologic diseases, such as immunosuppression, chemoradiation, use of antibiotic, proton pump inhibitor and opioid, and deficiency in butyrate and antimicrobial peptides contribute to C. difficile infection. Next, we will highlight different approaches to control and mitigate this infection such as antibiotic stewardship and faecal microbiota transplantation. Finally, we will explore several emerging treatments such as use of pre- and probiotics, immunotherapy and microbiome-sparing agents.

Glial and neuroimmune cell choreography in sexually dimorphic pain signaling

Chronic pain is a major global health issue that affects all populations regardless of sex, age, ethnicity/race, or country of origin, leading to persistent physical and emotional distress and to the loss of patients' autonomy and quality of life. Despite tremendous efforts in the elucidation of the mechanisms contributing to the pathogenesis of chronic pain, the identification of new potential pain targets, and the development of novel analgesics, the pharmacological treatment options available for pain management remain limited, and most novel pain medications have failed to achieve advanced clinical development, leaving many patients with unbearable and undermanaged pain. Sex-specific susceptibility to chronic pain conditions as well as sex differences in pain sensitivity, pain tolerance and analgesic efficacy are increasingly recognized in the literature and have thus prompted scientists to seek mechanistic explanations. Hence, recent findings have highlighted that the signaling mechanisms underlying pain hypersensitivity are sexually dimorphic, which sheds light on the importance of conducting preclinical and clinical pain research on both sexes and of developing sex-specific pain medications. This review thus focuses on the clinical and preclinical evidence supporting the existence of sex differences in pain neurobiology. Attention is drawn to the sexually dimorphic role of glial and immune cells, which are both recognized as key players in neuroglial maladaptive plasticity at the origin of the transition from acute pain to chronic pathological pain. Growing evidence notably attributes to microglial cells a pivotal role in the sexually dimorphic pain phenotype and in the sexually dimorphic analgesic efficacy of opioids. This review also summarizes the recent advances in understanding the pathobiology underpinning the development of pain hypersensitivity in both males and females in different types of pain conditions, with particular emphasis on the mechanistic signaling pathways driving sexually dimorphic pain responses.

Necrotizing enterocolitis and its association with the neonatal abstinence syndrome

OBJECTIVE:

The purpose of this study was to describe an identified association between necrotizing enterocolitis (NEC) and prenatal opioid exposure with neonatal abstinence syndrome (NAS) in late preterm and full-term neonates.

STUDY DESIGN:

In this single-center retrospective cohort study, we analyzed inborn neonates with the diagnosis of NEC discharged from 2012 through 2017. We compared infants with NEC > 35 weeks’ gestation to those with NEC<35 weeks’ gestation. We compared gestational age, birth weight, age of onset of symptoms, and incidence of prenatal drug exposure between groups. Significance was determined using Mann-Whitney and Fisher’s exact tests.

RESULTS:

Over the study period, 23 infants were identified with NEC, 9 (39%) were babies > 35 weeks at birth and 14 (61%) < 35 weeks. Those > 35 weeks had a higher birth weight, earlier onset of symptoms, and a higher percentage of prenatal exposure to opioids compared to those < 35 weeks’ gestation. We further described seven infants with late gestational age onset NEC associated with prenatal opioid exposure.

CONCLUSIONS:

In this cohort of infants with NEC discharged over a 6 year period we found a higher than expected percentage of infants born at a later gestational age. We speculate that prenatal opioid exposure might be a risk factor for NEC in neonates born at > 35 weeks.

Differential alteration in gut microbiome profiles during acquisition, extinction and reinstatement of morphine-induced CPP

Substance addiction is a chronic and complicated disease involving genetic and environmental factors. Coregulated by the above factors, perturbations of the gut microbiome have been shown to have an essential role in the development of many neuropsychiatric disorders, including addiction. However, shifts in the gut microbiome during different stages of morphine addiction remain uncharacterized. In the present study, we harvested fecal samples from mice at the acquisition (both the control and morphine groups), extinction and reinstatement stages of morphine-induced conditioned place preference (CPP). Gut microbiome profiles were detected with 16S ribosomal RNA gene sequencing. We observed an increase in community richness following morphine conditioning, and it decreased after 4 weeks of abstinence. The abundance of Verrucomicrobia increased and Bacteroides decreased at the acquisition of morphine-induced CPP, while a recovery trend was found at the extinction stage. Several discriminative genera were identified for the characterization of different stages of morphine CPP. Functional analysis of taxa with differential abundance between CPP stages was mainly enriched in the pathways of amino acid metabolism. Taken together, our findings will extend the association between dysbiosis of the gut microbiome and the opioid-induced rewarding or reinforcing behaviors.

Toll-Like Receptor 4 (TLR4)/Opioid Receptor Pathway Crosstalk and Impact on Opioid Analgesia, Immune Function, and Gastrointestinal Motility

Toll-like receptor 4 (TLR4) recognizes exogenous pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs) and initiates the innate immune response. Opioid receptors (μ, δ, and κ) activate inhibitory G-proteins and relieve pain. This review summarizes the following types of TLR4/opioid receptor pathway crosstalk: (a) Opioid receptor agonists non-stereoselectively activate the TLR4 signaling pathway in the central nervous system (CNS), in the absence of lipopolysaccharide (LPS). Opioids bind to TLR4, in a manner parallel to LPS, activating TLR4 signaling, which leads to nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) expression and the production of the pro-inflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6. (b) Opioid receptor agonists inhibit the LPS-induced TLR4 signaling pathway in peripheral immune cells. Opioids operate as pro-inflammatory cytokines, resulting in neuroinflammation in the CNS, but they mediate immunosuppressive effects in the peripheral immune system. It is apparent that TLR4/opioid receptor pathway crosstalk varies dependent on the cell type and activating stimulus. (c) Both the TLR4 and opioid receptor pathways activate the mitogen-activated protein kinase (MAPK) pathway. This crosstalk is located downstream of the TLR4 and opioid receptor signaling pathways. Furthermore, the classic opioid receptor can also produce pro-inflammatory effects in the CNS via MAPK signaling and induce neuroinflammation. (d) Opioid receptor agonists induce the production of high mobility group box 1 (HMGB1), an endogenous TLR4 agonist, supporting intercellular (neuron-to-glia or glia-to-neuron) interactions. This review also summarizes the potential effects of TLR4/opioid receptor pathway crosstalk on opioid analgesia, immune function, and gastrointestinal motility. Opioids non-stereoselectively activate the TLR4 pathway, and together with the subsequent release of pro-inflammatory cytokines such as IL-1 by glia, this TLR4 signaling initiates the central immune signaling response and modifies opioid pharmacodynamics. The DAMP HMGB1 is associated with the development of neuropathic pain. To explain morphine-induced persistent sensitization, a positive feedback loop has been proposed; this involves an initial morphine-induced amplified release of IL-1β and a subsequent exacerbated release of DAMPs, which increases the activation of TLR4 and the purinergic receptor P2X7R. Opioid receptor (μ, δ, and κ) agonists are involved in many aspects of immunosuppression. The intracellular TLR4/opioid receptor signaling pathway crosstalk induces the formation of the β-arrestin-2/TNF receptor-associated factor 6 (TRAF6) complex, which contributes to morphine-induced inhibition of LPS-induced TNF-α secretion in mast cells. A possible molecular mechanism is that the TLR4 pathway initially triggers the formation of the β-arrestin-2/TRAF6 complex, which is amplified by opioid receptor signaling, suggesting that β-arrestin-2 acts as a functional component of the TLR4 pathway.

Pharmacological inhibition of TLR4 ameliorates muscle and liver ceramide content after disuse in previously physically active mice

Toll-like receptor 4 (TLR4) is a proposed mediator of ceramide accumulation, muscle atrophy, and insulin resistance in skeletal muscle. It is currently unknown whether pharmacological inhibition of TLR4, using the TLR4-specific inhibitor TAK-242 during muscle disuse, is able to prevent changes in intracellular ceramide species and consequently preserve muscle size and insulin sensitivity in physically active mice. To address this question, we subjected running wheel-conditioned C57BL/6 male mice (13 wk old; ∼10/group) to 7 days of hindlimb suspension (HS), 7 days of continued wheel running (WR), or daily injections of TAK-242 during HS (HS + TAK242) for 7 days. We measured hindlimb muscle morphology, intramuscular and liver ceramide content, HOMA-IR, mRNA proxies of ceramide turnover and lipid trafficking, and muscle fatty acid and glycerolipid content. As a result, soleus and liver ceramide abundance was greater (P < 0.05) in HS vs. WR but was reduced with TLR4 inhibition (HS + TAK-242 vs. HS). Muscle mass declined (P < 0.01) with HS (vs. WR), but TLR4 inhibition did not prevent this loss (soleus: P = 0.08; HS vs. HS + TAK-242). HOMA-IR was impaired (P < 0.01) in HS versus WR mice, but only fasting blood glucose was reduced with TLR4 inhibition (HS + TAK-242 vs HS, P < 0.05). Robust decreases in muscle Spt2 and Cd36 mRNA and muscle lipidomic trafficking may partially explain reductions in ceramides with TLR4 inhibition. In conclusion, pharmacological TLR4 inhibition in wheel-conditioned mice prevented ceramide accumulation during the early phase of hindlimb suspension (7 days) but had little effect on muscle size and insulin sensitivity.

Morphine Exacerbates Postfracture Nociceptive Sensitization, Functional Impairment, and Microglial Activation in Mice

BACKGROUND

Emerging evidence suggests that opioid use immediately after surgery and trauma may worsen outcomes. In these studies, the authors aimed to determine whether morphine administered for a clinically relevant time period (7 days) in a tibia fracture orthopedic surgery model had adverse effects on postoperative recovery.

METHODS

Mice were given morphine twice daily for 7 days after unilateral tibial fracture and intramedullary pin fixation to model orthopedic surgery and limb trauma. Mechanical allodynia, limb-specific weight bearing, gait changes, memory, and anxiety were measured after injury. In addition, spinal cord gene expression changes as well as glial activation were measured. Finally, the authors assessed the effects of a selective Toll-like receptor 4 antagonist, TAK-242, on nociceptive and functional changes after injury.

RESULTS

Tibial fracture caused several weeks of mechanical nociceptive sensitization (F(1, 216) = 573.38, P < 0.001, fracture + vehicle vs. sham + vehicle, n = 10 per group), and this change was exacerbated by the perioperative administration of morphine (F(1, 216) = 71.61, P < 0.001, fracture + morphine vs. fracture + vehicle, n = 10 per group). In additional testing, injured limb weight bearing, gait, and object location memory were worse in morphine-treated fracture mice than in untreated fracture mice. Postfracture expression levels of several genes previously associated with opioid-induced hyperalgesia, including brain-derived neurotrophic factor and prodynorphin, were unchanged, but neuroinflammation involving Toll-like receptor 4 receptor-expressing microglia was observed (6.8 ± 1.5 [mean ± SD] cells per high-power field for fracture + vehicle vs. 12 ± 2.8 fracture + morphine, P < 0.001, n = 8 per /group). Treatment with a Toll-like receptor 4 antagonist TAK242 improved nociceptive sensitization for about 2 weeks in morphine-treated fracture mice (F(1, 198) = 73.36, P < 0.001, fracture + morphine + TAK242 vs. fracture + morphine, n = 10 per group).

CONCLUSIONS

Morphine treatment beginning at the time of injury impairs nociceptive recovery and other outcomes. Measures preventing glial activation through Toll-like receptor 4 signaling may reduce the adverse consequences of postoperative opioid administration.

From dysmotility to virulent pathogens: implications of opioid use in the ICU

PURPOSE OF REVIEW

Gastrointestinal dysmotility occurs frequently in the critically ill. Although the causes underlying dysmotility are multifactorial, both pain and its treatment with exogenous opioids are likely causative factors. The purpose of this review is to describe the effects of pain and opioids on gastrointestinal motility; outline the rationale for and evidence supporting the administration of opioid antagonists to improve dysmotility; and describe the potential influence opioids drugs have on the intestinal microbiome and infectious complications.

RECENT FINDINGS

Opioid drugs are frequently prescribed in the critically ill to alleviate pain. In health, opioids cause gastric dysmotility, yet the evidence for this in critical illness is inconsistent and limited to observational studies. Administration of opioid antagonists may improve gastrointestinal motility, but data are sparse, and these agents cannot be recommended outside of clinical trials. Although critical illness is associated with alterations in the microbiome, the extent to which opioid administration influences these changes, and the subsequent development of infection, remains uncertain.

SUMMARY

Replication of clinical studies from ambulant populations in critical care is required to ascertain the independent influence of opioid administration on gastrointestinal motility and infectious complications.

Fusobacterium nucleatum promotes M2 polarization of macrophages in the microenvironment of colorectal tumours via a TLR4-dependent mechanism

Fusobacterium nucleatum (Fn) has been shown to promote colorectal cancer (CRC) development by inhibiting host anti-tumour immunity. However, the impact of Fn infection on macrophage polarization and subsequent intestinal tumour formation as well as the underlying molecular pathways has not been investigated. We investigated the impact of Fn infection on macrophage polarization in human CRCs and cultured macrophages as well as the effects on macrophage phenotype and intestinal tumour formation in ApcMin/+ mice. We also examined whether macrophage-polarized activation challenged by Fn infection via a TLR4-dependent mechanism involved the IL-6/STAT3/c-MYC signalling cascade. Our data showed that macrophages are a major tumour-infiltrating immune cell type in human CRCs with Fn infection (P < 0.001). Fn infection increased M2 polarization of macrophages in vitro and in vivo, leading to intestinal tumour growth in ApcMin/+ mice. Moreover, Fn infection induced high expression of TLR4, IL-6, STAT3, p-STAT3, and c-MYC in cultured macrophages challenged with Fn, which was blocked by TAK-242 pre-treatment (P < 0.05). Interestingly, c-MYC protein was mainly co-localized with CD206+ M2 macrophages with Fn infection. In conclusion, we show that Fn infection increased M2 polarization of macrophages in vitro and in vivo. Furthermore, Fn infection enhanced colorectal tumour growth in a TLR4-dependent manner involving activation of the IL-6/p-STAT3/c-MYC signalling pathway. For the first time, our results indicate an immunosuppressive effect of Fn by promoting M2 polarization of macrophages through a TLR4-dependent mechanism, which may serve as a promising target for immunotherapy of Fn-related CRC.

Fusobacterium nucleatum Potentiates Intestinal Tumorigenesis in Mice via a Toll-Like Receptor 4/p21-Activated Kinase 1 Cascade

BACKGROUND

The underlying pathogenic mechanism of Fusobacterium nucleatum in the carcinogenesis of colorectal cancer has been poorly understood.

METHODS

Using C57BL/6-ApcMin/+ mice, we investigated gut microbial structures with F. nucleatum, antibiotics, and Toll-like receptor 4 (TLR4) antagonist TAK-242 treatment. In addition, we measured intestinal tumor formation and the expression of TLR4, p21-activated kinase 1 (PAK1), phosphorylated-PAK1 (p-PAK1), phosphorylated-β-catenin S675 (p-β-catenin S675), and cyclin D1 in mice with different treatments.

RESULTS

Fusobacterium nucleatum and antibiotics treatment altered gut microbial structures in mice. In addition, F. nucleatum invaded into the intestinal mucosa in large amounts but were less abundant in the feces of F. nucleatum-fed mice. The average number and size of intestinal tumors in F. nucleatum groups was significantly increased compared to control groups in ApcMin/+ mice (P < 0.05). The expression of TLR4, PAK1, p-PAK1, p-β-catenin S675, and cyclin D1 was significantly increased in F. nucleatum groups compared to the control groups (P < 0.05). Moreover, TAK-242 significantly decreased the average number and size of intestinal tumors compared to F. nucleatum groups (P < 0.05). The expression of p-PAK1, p-β-catenin S675, and cyclin D1 was also significantly decreased in the TAK-242-treated group compared to F. nucleatum groups (P < 0.05).

CONCLUSIONS

Fusobacterium nucleatum potentiates intestinal tumorigenesis in ApcMin/+ mice via a TLR4/p-PAK1/p-β-catenin S675 cascade. Fusobacterium nucleatum-induced intestinal tumorigenesis can be inhibited by TAK-242, implicating TLR4 as a potential target for the prevention and therapy of F. nucleatum-related colorectal cancer.

Differential effect of morphine on gastrointestinal transit, colonic contractions and nerve-evoked relaxations in Toll-Like Receptor deficient mice

Toll-like receptors (TLRs) are expressed in enteric neurons, glia, gastrointestinal (GI) smooth muscle and mucosa, yet their functional roles in the GI tract are not fully understood. TLRs have been linked to many of the undesirable central effects of chronic opioid administration including hyperalgesia and dependence via activation of central microglia. Opioid-induced bowel dysfunction (OIBD) remains a primary reason for the reduction or withdrawal of opioid analgesics. Morphine-induced inhibition of colonic motility was assessed in vivo by GI transit studies and in vitro using isolated colons from wildtype (WT) and TLR deficient mice. Morphine slowed movement of ingested content in WT but this retardation effect was attenuated in TLR4^−/− and TLR2/4^−/−. In isolated colons, morphine reduced amplitude and frequency colonic migrating motor contractions in both WT and TLR2/4^−/−. Electrical field stimulation elicited distal colon relaxation that was potentiated by morphine in WT but not in TLR2/4^−/−. Inhibitory junction potentials were of similar amplitude and kinetics in WT and TLR2/4^−/− distal colon and not altered by morphine. Enteric nerve density and proportion of nitrergic nerves were similar in WT and TLR2/4^−/− distal colon. These data suggest an involvement of TLRs in opioid pharmacodynamics and thus a potential interventional target for OIBD.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Activation of μ-opioid receptor and Toll-like receptor 4 by plasma from morphine-treated mice

In this study, we quantified the ability of opioids present in biological samples to activate the μ-opioid receptor and TLR4 using cell-based assays. Each assay was standardised, in the presence of plasma, using morphine, its μ receptor-active metabolite morphine-6 glucuronide (M6G) and its μ receptor-inactive, but TLR4-active metabolite morphine-3 glucuronide (M3G). Specificity was verified using antagonists. Morphine- and M6G-spiked plasma samples exhibited μ receptor activation, which M3G-spiked plasma lacked. In contrast, M3G showed moderate but consistent activation of TLR-4. Plasma samples were collected at a number of time points from mice administered morphine (1 or 10mg/kg every 12h for 3days) or saline. Morphine administration led to intermittent μ receptor activation, reversed by μ receptor antagonists, and to TRL4 activation at time points where M3G is measured in plasma. Interestingly, this protocol of morphine administration also led to TLR4-independent NF-κB activation, at time points where M3G was not detected, presumably via elevation of circulating cytokines including, but not limited to, TNFα. Circulating TNFα was increased after three days of morphine administration, and TNFα mRNA elevated in the spleen of morphine-treated mice.

Diverse action of lipoteichoic acid and lipopolysaccharide on neuroinflammation, blood-brain barrier disruption, and anxiety in mice

Microbial metabolites are known to affect immune system, brain, and behavior via activation of pattern recognition receptors such as Toll-like receptor 4 (TLR4). Unlike the effect of the TLR4 agonist lipopolysaccharide (LPS), the role of other TLR agonists in immune-brain communication is insufficiently understood. We therefore hypothesized that the TLR2 agonist lipoteichoic acid (LTA) causes immune activation in the periphery and brain, stimulates the hypothalamic-pituitary-adrenal (HPA) axis and has an adverse effect on blood-brain barrier (BBB) and emotional behavior. Since LTA preparations may be contaminated by LPS, an extract of LTA (LTAextract), purified LTA (LTApure), and pure LPS (LPSultrapure) were compared with each other in their effects on molecular and behavioral parameters 3h after intraperitoneal (i.p.) injection to male C57BL/6N mice. The LTAextract (20mg/kg) induced anxiety-related behavior in the open field test, enhanced the circulating levels of particular cytokines and the cerebral expression of cytokine mRNA, and blunted the cerebral expression of tight junction protein mRNA. A dose of LPSultrapure matching the amount of endotoxin/LPS contaminating the LTAextract reproduced several of the molecular and behavioral effects of LTAextract. LTApure (20mg/kg) increased plasma levels of tumor necrosis factor-α (TNF-α), interleukin-6 and interferon-γ, and enhanced the transcription of TNF-α, interleukin-1β and other cytokines in the amygdala and prefrontal cortex. These neuroinflammatory effects of LTApure were associated with transcriptional down-regulation of tight junction-associated proteins (claudin 5, occludin) in the brain. LTApure also enhanced circulating corticosterone, but failed to alter locomotor and anxiety-related behavior in the open field test. These data disclose that TLR2 agonism by LTA causes peripheral immune activation and initiates neuroinflammatory processes in the brain that are associated with down-regulation of BBB components and activation of the HPA axis, although emotional behavior (anxiety) is not affected. The results obtained with an LTA preparation contaminated with LPS hint at a facilitatory interaction between TLR2 and TLR4, the adverse impact of which on long-term neuroinflammation, disruption of the BBB and mental health warrants further analysis.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Gut Homeostasis, Microbial Dysbiosis, and Opioids

Gut homeostasis plays an important role in maintaining animal and human health. The disruption of gut homeostasis has been shown to be associated with multiple diseases. The mutually beneficial relationship between the gut microbiota and the host has been demonstrated to maintain homeostasis of the mucosal immunity and preserve the integrity of the gut epithelial barrier. Currently, rapid progress in the understanding of the host-microbial interaction has redefined toxicological pathology of opioids and their pharmacokinetics. However, it is unclear how opioids modulate the gut microbiome and metabolome. Our study, showing opioid modulation of gut homeostasis in mice, suggests that medical interventions to ameliorate the consequences of drug use/abuse will provide potential therapeutic and diagnostic strategies for opioid-modulated intestinal infections. The study of morphine's modulation of the gut microbiome and metabolome will shed light on the toxicological pathology of opioids and its role in the susceptibility to infectious diseases.

Irinotecan-Induced Gastrointestinal Dysfunction and Pain Are Mediated by Common TLR4-Dependent Mechanisms

Strong epidemiological data indicate that chemotherapy-induced gut toxicity and pain occur in parallel, indicating common underlying mechanisms. We have recently outlined evidence suggesting that TLR4 signaling may contribute to both side effects. We therefore aimed to determine if genetic deletion of TLR4 improves chemotherapy-induced gut toxicity and pain. Forty-two female wild-type (WT) and 42 Tlr4 null (-/-) BALB/c mice weighing between 18 and 25 g (10-13 weeks) received a single 270 mg/kg (i.p.) dose of irinotecan hydrochloride or vehicle control and were killed at 6, 24, 48, 72, and 96 hours. Bacterial sequencing was conducted on cecal samples of control animals to determine the gut microbiome profile. Gut toxicity was assessed using validated clinical and histopathologic markers, permeability assays, and inflammatory markers. Chemotherapy-induced pain was assessed using the validated rodent facial grimace criteria, as well as immunologic markers of glial activation in the lumbar spinal cord. TLR4 deletion attenuated irinotecan-induced gut toxicity, with improvements in weight loss (P = 0.0003) and diarrhea (P < 0.0001). Crypt apoptosis was significantly decreased in BALB/c-Tlr4(-/-billy) mice (P < 0.0001), correlating with lower mucosal injury scores (P < 0.005). Intestinal permeability to FITC-dextran (4 kDa) and LPS translocation was greater in WT mice than in BALB/c-Tlr4(-/-billy) (P = 0.01 and P < 0.0001, respectively). GFAP staining in the lumbar spinal cord, indicative of astrocytic activation, was increased at 6 and 72 hours in WT mice compared with BALB/c-Tlr4(-/-billy) mice (P = 0.008, P = 0.01). These data indicate that TLR4 is uniquely positioned to mediate irinotecan-induced gut toxicity and pain, highlighting the possibility of a targetable gut/CNS axis for improved toxicity outcomes. Mol Cancer Ther; 15(6); 1376-86. ©2016 AACR.

Expression and physiology of opioid receptors in the gastrointestinal tract

PURPOSE OF REVIEW

Stimulation of opioid receptors elicits analgesic effect not only in the central nervous system, but also in the gastrointestinal tract, where a high concentration of opioid receptors can be found within the enteric nervous system as well as muscular and immune cells. Along with antinociception, opioid receptors in the stomach and intestine relay signals crucial for secretory and motor gastrointestinal function.

RECENT FINDINGS

The review focuses on the utility of opioid receptor antagonists, which is generally contributing to the management of postoperative ileus and opioid bowel dysfunction in chronic pain patients nonetheless, opioid receptor antagonists can also be useful in the treatment of irritable bowel syndrome and chronic idiopathic constipation. The study also discusses several antidiarrheal opioid agonists, as well as opioids and opioid mimetics encompassing the concept of ligand-biased agonism and truncated opioid receptor splice variants.

SUMMARY

Good understanding of the localization and the role of opioid receptors is vital for regulation of various pathophysiological processes in the gastrointestinal tract and may simultaneously provide a tempting approach in eliminating adverse effects related to centrally acting opioids.

Neuroimmune pharmacological approaches

Intestinal inflammation is a major health problem which impairs the quality of life, impacts mental health and is exacerbated by stress and psychiatric disturbances which, in turn, can affect disease prognosis and response to treatment. Accumulating evidence indicates that the immune system is an important interface between intestinal inflammation and the enteric, sensory, central and autonomic nervous systems. In addition, the neuroimmune interactions originating from the gastrointestinal tract are orchestrated by the gut microbiota. This article reviews some major insights into this complex homeostatic network that have been achieved during the past two years and attempts to put these advances into perspective with novel opportunities of pharmacological intervention.