Novel Insights Into the Mechanisms of Abdominal Pain in Obstructive Bowel Disorders

Role of mechanoregulation in mast cell-mediated immune inflammation of the smooth muscle in the pathophysiology of esophageal motility disorders

Major esophageal disorders involve obstructive transport of bolus to the stomach, causing symptoms of dysphagia and impaired clearing of the refluxed gastric contents. These may occur due to mechanical constriction of the esophageal lumen or loss of relaxation associated with deglutitive inhibition, as in achalasia-like disorders. Recently, immune inflammation has been identified as an important cause of esophageal strictures and the loss of inhibitory neurotransmission. These disorders are also associated with smooth muscle hypertrophy and hypercontractility, whose cause is unknown. This review investigated immune inflammation in the causation of smooth muscle changes in obstructive esophageal bolus transport. Findings suggest that smooth muscle hypertrophy occurs above the obstruction and is due to mechanical stress on the smooth muscles. The mechanostressed smooth muscles release cytokines and other molecules that may recruit and microlocalize mast cells to smooth muscle bundles, so that their products may have a close bidirectional effect on each other. Acting in a paracrine fashion, the inflammatory cytokines induce genetic and epigenetic changes in the smooth muscles, leading to smooth muscle hypercontractility, hypertrophy, and impaired relaxation. These changes may worsen difficulty in the esophageal transport. Immune processes differ in the first phase of obstructive bolus transport, and the second phase of muscle hypertrophy and hypercontractility. Moreover, changes in the type of mechanical stress may change immune response and effect on smooth muscles. Understanding immune signaling in causes of obstructive bolus transport, type of mechanical stress, and associated smooth muscle changes may help pathophysiology-based prevention and targeted treatment of esophageal motility disorders.NEW & NOTEWORTHY Esophageal disorders such as esophageal stricture or achalasia, and diffuse esophageal spasm are associated with smooth muscle hypertrophy and hypercontractility, above the obstruction, yet the cause of such changes is unknown. This review suggests that smooth muscle obstructive disorders may cause mechanical stress on smooth muscle, which then secretes chemicals that recruit, microlocalize, and activate mast cells to initiate immune inflammation, producing functional and structural changes in smooth muscles. Understanding the immune signaling in these changes may help pathophysiology-based prevention and targeted treatment of esophageal motility disorders.

Piezo2 channels expressed by colon-innervating TRPV1-lineage neurons mediate visceral mechanical hypersensitivity

Inflammatory and functional gastrointestinal disorders such as irritable bowel syndrome (IBS) and obstructive bowel disorder (OBD) underlie the most prevalent forms of visceral pain. Although visceral pain can be generally provoked by mechanical distension/stretch, the mechanisms that underlie visceral mechanosensitivity in colon-innervating visceral afferents remain elusive. Here, we show that virally mediated ablation of colon-innervating TRPV1-expressing nociceptors markedly reduces colorectal distention (CRD)-evoked visceromotor response (VMR) in mice. Selective ablation of the stretch-activated Piezo2 channels from TRPV1 lineage neurons substantially reduces mechanically evoked visceral afferent action potential firing and CRD-induced VMR under physiological conditions, as well as in mouse models of zymosan-induced IBS and partial colon obstruction (PCO). Collectively, our results demonstrate that mechanosensitive Piezo2 channels expressed by TRPV1-lineage nociceptors powerfully contribute to visceral mechanosensitivity and nociception under physiological conditions and visceral hypersensitivity under pathological conditions in mice, uncovering potential therapeutic targets for the treatment of visceral pain.

Mechanisms of lymphoid depletion in bowel obstruction

Background and aims: Bowel obstruction (BO) causes not only gastrointestinal dysfunctions but also systemic responses such as sepsis, infections, and immune impairments. The mechanisms involved are not well understood. In this study, we tested the hypothesis that BO leads to lymphoid depletion in primary and peripheral lymphoid organs, which may contribute to systemic responses. We also sought to uncover mechanisms of lymphoid depletion in BO. Methods: Partial colon obstruction was induced with a band in the distal colon of Sprague-Dawley rats, and wild-type and osteopontin knockout (OPN^-/-) mice. Obstruction was maintained for 7 days in rats and 4 days in mice. Thymus, bone marrow, spleen, and mesenteric lymph node (MLN) were taken for flow cytometry analysis. Results: The weight of thymus, spleen, and MLN was significantly decreased in BO rats, compared to sham. B and T lymphopoiesis in the bone marrow and thymus was suppressed, and numbers of lymphocytes, CD4⁺, and CD8⁺ T cells in the spleen and MLN were all decreased in BO. Depletion of gut microbiota blocked BO-associated lymphopenia in the MLN. Corticosterone antagonism partially attenuated BO-associated reduction of lymphocytes in the thymus and bone marrow. Plasma OPN levels and OPN expression in the distended colon were increased in BO. Deletion of the OPN gene did not affect splenic lymphopenia, but attenuated suppression of lymphopoiesis in the bone marrow and thymus in BO. Conclusions: BO suppresses lymphocyte generation and maintenance in lymphoid organs. Mechanical distention-induced OPN, corticosterone, and gut microbiota are involved in the immune phenotype in BO.

Evidence for the presence and release of BDNF in the neuronal and non-neuronal structures of the internal anal sphincter

BACKGROUND

Data on the neuromodulatory effects of brain-derived neurotrophic factor (BDNF) in the gastrointestinal tract were recently reported, but there are still no data on the presence, distribution, and release of BDNF in the gastrointestinal tract, including the internal anal sphincter (IAS).

METHODS

We examined the presence and distribution of BDNF and its receptor TrkB in the different IAS structures (neuronal and non-neuronal) via immunohistochemical and immunocytochemical analyses. We also monitored the release of BDNF in an IAS muscle bath (consisting of smooth muscle cells [SMCs], myenteric plexus, and submucosal plexus) before and after different agonists, and electrical field stimulation in the absence and presence of neurotoxin tetrodotoxin.

KEY RESULTS

BDNF/TrkB was found to be present in all layers of the IAS, especially the smooth muscle, mucosa, myenteric plexus, and submucosal plexus. Detailed analyses revealed a significant colocalization between BDNF and TrkB in different structures, especially in the smooth muscle, the SMCs, and both plexuses. Data further showed higher levels of BDNF in the cytosol and that of TrkB toward the periphery of the SMCs.

CONCLUSIONS & INFERENCES

These studies showed that BDNF/TrkB was present not only in the enteric nervous system (ENS), but also in the SMCs. For the neuromodulatory effects, BDNF is released locally from the ENS ((myenteric (10.01 ± 0.23 pg/ml) and submucosal plexus (9.05 ± 0.51 pg/ml)) and the SMCs (18.63 ± 1.63 pg/ml). Collectively, these findings have pathophysiological and therapeutic implications regarding the role of BDNF/TrkB in the IAS-associated rectoanal motility disorders.

Targeting Mechano-Transcription Process as Therapeutic Intervention in Gastrointestinal Disorders

Mechano-transcription is a process whereby mechanical stress alters gene expression. The gastrointestinal (GI) tract is composed of a series of hollow organs, often encountered by transient or persistent mechanical stress. Recent studies have revealed that persistent mechanical stress is present in obstructive, functional, and inflammatory disorders and alters gene transcription in these conditions. Mechano-transcription of inflammatory molecules, pain mediators, pro-fibrotic and growth factors has been shown to play a key role in the development of motility dysfunction, visceral hypersensitivity, inflammation, and fibrosis in the gut. In particular, mechanical stress-induced cyclooxygenase-2 (COX-2) and certain pro-inflammatory mediators in gut smooth muscle cells are responsible for motility dysfunction and inflammatory process. Mechano-transcription of pain mediators such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) may lead to visceral hypersensitivity. Emerging evidence suggests that mechanical stress in the gut also leads to up-regulation of certain proliferative and pro-fibrotic mediators such as connective tissue growth factor (CTGF) and osteopontin (OPN), which may contribute to fibrostenotic Crohn's disease. In this review, we will discuss the pathophysiological significance of mechanical stress-induced expression of pro-inflammatory molecules, pain mediators, pro-fibrotic and growth factors in obstructive, inflammatory, and functional bowel disorders. We will also evaluate potential therapeutic targets of mechano-transcription process for the management of these disorders.

Electroacupuncture and Moxibustion Modulate the BDNF and TrkB Expression in the Colon and Dorsal Root Ganglia of IBS Rats with Visceral Hypersensitivity

Objective

To evaluate the effects of electroacupuncture and moxibustion on brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase receptor B (TrkB) protein and mRNA expressions in the colon and dorsal root ganglia of IBS rats with visceral hypersensitivity and to explore their underlying therapeutic mechanisms.

Method

Forty Sprague Dawley rats were randomly divided into normal, model, model + mild moxibustion (MM), model + electroacupuncture (EA), and model + pinaverium bromide (PB) groups, with eight rats in each group. Chronic visceral hypersensitive IBS rat models were established by colorectal distension (CRD) with mustard oil clyster. Rats in the MM and EA groups, respectively, received moxibustion and electroacupuncture treatments on the Tianshu (ST25) and Shangjuxu (ST37) acupoints once daily for 7 days, and rats in the PB group received pinaverium bromide by oral gavage once daily for 7 consecutive days. After treatment, rats underwent abdominal withdrawal reflex (AWR) scoring under CRD and colon histopathological examination. Immunohistochemistry and real-time quantitative PCR (RT-qPCR) were used to study the protein and mRNA expressions of BDNF and TrkB in the rat colon and dorsal root ganglia.

Results

Compared with the normal group, AWR scores and body weight were clearly increased in the model group rats (both P < 0.01). The body weights were significantly elevated (P < 0.01, P < 0.05), but the AWR scores were reduced (P < 0.05, P < 0.01), after electroacupuncture and mild moxibustion treatment. Compared with levels in normal rats, BDNF and TrkB protein and mRNA expressions were significantly elevated in the IBS model rats (P < 0.01) but were downregulated after mild moxibustion, electroacupuncture, and Western medicine treatment (P < 0.01).

Conclusion

Electroacupuncture and moxibustion improved visceral hypersensitivity of IBS rats possibly by reducing BDNF and TrkB protein and mRNA expressions in the colon and dorsal root ganglia.

An opioid receptor-independent mechanism underlies motility dysfunction and visceral hyperalgesia in opioid-induced bowel dysfunction

Constipation and abdominal pain are commonly encountered in opioid-induced bowel dysfunction (OBD). The underlying mechanisms are incompletely understood, and treatments are not satisfactory. As patients with OBD often have fecal retention, we aimed to determine whether fecal retention plays a pathogenic role in the development of constipation and abdominal pain in OBD, and if so to investigate the mechanisms. A rodent model of OBD was established by daily morphine treatment at 10 mg/kg for 7 days. Bowel movements, colonic muscle contractility, visceromotor response to colorectal distention, and cell excitability of colon-projecting dorsal root ganglion neurons were determined in rats fed with normal pellet food, or with clear liquid diet. Morphine treatment (Mor) reduced fecal outputs starting on day 1, and caused fecal retention afterward. Compared with controls, Mor rats demonstrated suppressed muscle contractility, increased neuronal excitability, and visceral hypersensitivity. Expression of cyclooxygenase-2 (COX-2) and nerve growth factor (NGF) was upregulated in the smooth muscle of the distended colon in Mor rats. However, prevention of fecal retention by feeding rats with clear liquid diet blocked upregulation of COX-2 and NGF, restored muscle contractility, and attenuated visceral hypersensitivity in Mor rats. Moreover, inhibition of COX-2 improved smooth muscle function and fecal outputs, whereas anti-NGF antibody administration attenuated visceral hypersensitivity in Mor rats. Morphine-induced fecal retention is an independent pathogenic factor for motility dysfunction and visceral hypersensitivity in rats with OBD. Liquid diet may have therapeutic potential for OBD by preventing fecal retention-induced mechanotranscription of COX-2 and NGF.NEW & NOTEWORTHY Our preclinical study shows that fecal retention is a pathogenic factor in opioid-induced bowel dysfunction, as prevention of fecal retention with liquid diet improved motility and attenuated visceral hyperalgesia in morphine-treated animals by blocking expression of cyclooxygenase-2 and nerve growth factor in the colon.

Precision Lactobacillus reuteri therapy attenuates luminal distension-associated visceral hypersensitivity by inducing peripheral opioid receptors in the colon

Luminal distension and abdominal pain are major clinical hallmarks of obstructive bowel disorders and functional bowel disorders linked to gut dysbiosis. Our recent studies found that chronic lumen distension increased visceral sensitivity and decreased abundance of gut commensal Lactobacillus reuteri in a rodent model of partial colon obstruction (OB). To establish causation, we performed precision microbial therapy to assess whether recolonization of L. reuteri prevents visceral hypersensitivity in lumen distension, and if so, to identify the gut-microbiota mechanism. Lumen distension was induced in Sprague-Dawley rats by implanting an OB band in the distal colon for up to 7 days. L. reuteri strains or vehicle were gavage ingested 1 × 10 colony-forming units/g daily starting 2 days before OB. L. reuteri rat strains that were able to recolonize obstructed colon significantly improved food intake and body weight in OB rats, and attenuated referred visceral hyperalgesia measured by the withdrawal response to von Frey filament applications to the abdomen. Mechanistically, L. reuteri treatment attenuated hyperexcitability of the dorsal root ganglia neurons projecting to the distended colon by promoting opioid receptor function in affected tissues. The expression of µ, δ, and κ opioid receptors was significantly downregulated in colonic muscularis externae and sensory neurons in OB rats. However, L. reuteri treatment prevented the loss of opioid receptors. Furthermore, administration of peripheral opioid receptor antagonist naloxone methiodide abolished the analgesic effect of L. reuteri in OB. In conclusion, precision L. reuteri therapy prevents lumen distension-associated visceral hypersensitivity by local bacterial induction of opioid receptors.

Adhesive small bowel obstruction - an update

Small bowel obstruction (SBO) accounts for 12-16% of emergency surgical admissions and 20% of emergency surgical procedures. Even with the advent of laparoscopic surgery, intra-abdominal adhesions remain a significant cause of SBO, accounting for 65% of cases. History and physical examination are essential to identify signs of bowel ischemia as this indicates a need for urgent surgical exploration. Another critical aspect of evaluation includes establishing the underlying cause for obstruction and distinguishing between adhesive and non-adhesive etiologies as adhesive SBO (ASBO) can be managed non-operatively in 70-90% of patients. A patient with a history of abdominopelvic surgery along with one or more cardinal features of obstruction should be suspected to have ASBO until proven otherwise. Triad of severe pain, pain out of proportion to the clinical findings, and presence of an abdominal scar suggest possible closed-loop obstruction. Computed tomography has higher sensitivity and specificity compared to plain films and is recommended by the Bologna guidelines. Correcting fluid and electrolyte imbalance is an initial crucial step to mitigate severe hypovolemia. Patients should proceed with surgery if symptoms of bowel compromise are present, or if symptoms do not resolve or have worsened. Surgery is indicated in patients with ischemia, strangulation, perforation, peritonitis, or failure of non-operative treatment. With advances in minimal access technology and increasing experience, laparoscopic adhesiolysis is recommended. Mechanical adhesion barriers are an effective measure to prevent adhesion formation.

Neurotrophic factors in enteric physiology and pathophysiology

Neurotrophic factors are traditionally recognized for their roles in differentiation, growth, and survival of specific neurons in the central and peripheral nervous system. Some neurotrophic factors are essential for the development and migration of the enteric nervous system along the fetal and post-natal gut. Over the last two decades, several non-developmental functions of neurotrophic factors have been characterized. In the adult gastrointestinal tract, neurotrophic factors regulate gut sensation, motility, epithelial barrier function, and protect enteric neurons and glial cells from damaging insults in the microenvironment of the gut. In this issue of Neurogastroenterology and Motility, Fu et al demonstrate that brain-derived neurotrophic factor plays a role in the pathogenesis of distention-induced abdominal pain in bowel obstruction. In light of this interesting finding, this mini-review highlights some of the recent advances in understanding of the physiological and pathophysiological roles of neurotrophic factors in the adult gut.