Visceral Pain

Abdominal Symptom Improvement During Clinical Trials of Tenapanor in Patients With Irritable Bowel Syndrome With Constipation: A Post Hoc Analysis

INTRODUCTION

This post hoc analysis evaluated the efficacy of tenapanor on abdominal symptoms in patients with irritable bowel syndrome with constipation. Abdominal symptoms assessed included pain, discomfort, bloating, cramping, and fullness.

METHODS

The abdominal symptom data were pooled from 3 randomized controlled trials (NCT01923428, T3MPO-1 [NCT02621892], and T3MPO-2 [NCT02686138]). Weekly scores were calculated for each abdominal symptom, and the Abdominal Score (AS) was derived as the average of weekly scores for abdominal pain, discomfort, and bloating. The overall change from baseline during the 12 weeks was assessed for each symptom weekly score and the AS. The AS 6/12-week and 9/12-week response rates (AS improvement of ≥2 points for ≥6/12- or ≥9/12-week) were also evaluated. The association of weekly AS response status (reduction of ≥30%) with weekly complete spontaneous bowel movement (CSBM) status (=0 and >0) was assessed.

RESULTS

Among 1,372 patients (684 tenapanor [50 mg twice a day] and 688 placebo), the least squares mean change from baseline in AS was -2.66 for tenapanor vs -2.09 for placebo ( P < 0.0001). The 6/12-week AS response rate was 44.4% for tenapanor vs 32.4% for placebo ( P < 0.0001), and for 9/12-week AS, 30.6% for tenapanor vs 20.5% for placebo ( P < 0.0001). A significant association between weekly CSBM status and weekly AS response status was observed each week ( P < 0.0001), with a greater proportion achieving an AS reduction in patients with >0 CSBMs in a week.

DISCUSSION

Tenapanor significantly reduced abdominal symptoms in patients with irritable bowel syndrome with constipation, particularly pain, discomfort, and bloating measured by AS, compared with placebo.

Novel endomorphin analogues CEMR-1 and CEMR-2 produce potent and long-lasting antinociception with a favourable side effect profile at the spinal level

BACKGROUND AND PURPOSE

Endomorphins have shown great promise as pharmaceutics for the treatment of pain. We have previously confirmed that novel endomorphin analogues CEMR-1 and CEMR-2 behaved as potent μ agonists and displayed potent antinociceptive activities at the supraspinal and peripheral levels. The present study was undertaken to evaluate the antinociceptive properties of CEMR-1 and CEMR-2 following intrathecal (i.t.) administration. Furthermore, their antinociceptive tolerance and opioid-like side effects were also determined.

EXPERIMENTAL APPROACH

The spinal antinociceptive effects of CEMR-1 and CEMR-2 were determined in a series of pain models, including acute radiant heat paw withdrawal test, spared nerve injury-induced neuropathic pain, complete Freund's adjuvant-induced inflammatory pain, visceral pain and formalin pain. Antinociceptive tolerance was evaluated in radiant heat paw withdrawal test.

KEY RESULTS

Spinal administration of CEMR-1 and CEMR-2 produced potent and prolonged antinociceptive effects in acute pain. CEMR-1 and CEMR-2 may produce their antinociception through distinct μ receptor subtypes. These two analogues also exhibited significant analgesic activities in neuropathic, inflammatory, visceral and formalin pain at the spinal level. It is noteworthy that CEMR-1 showed non-tolerance-forming analgesic properties, while CEMR-2 exhibited substantially reduced antinociceptive tolerance. Furthermore, both analogues displayed no or reduced side effects on conditioned place preference response, physical dependence, locomotor activity and gastrointestinal transit.

CONCLUSIONS AND IMPLICATIONS

The present investigation demonstrated that CEMR-1 and CEMR-2 displayed potent and long-lasting antinociception with a favourable side effect profile at the spinal level. Therefore, CEMR-1 and CEMR-2 might serve as promising analgesic compounds with minimal opioid-like side effects.

A mouse DRG genetic toolkit reveals morphological and physiological diversity of somatosensory neuron subtypes

Dorsal root ganglia (DRG) somatosensory neurons detect mechanical, thermal, and chemical stimuli acting on the body. Achieving a holistic view of how different DRG neuron subtypes relay neural signals from the periphery to the CNS has been challenging with existing tools. Here, we develop and curate a mouse genetic toolkit that allows for interrogating the properties and functions of distinct cutaneous targeting DRG neuron subtypes. These tools have enabled a broad morphological analysis, which revealed distinct cutaneous axon arborization areas and branching patterns of the transcriptionally distinct DRG neuron subtypes. Moreover, in vivo physiological analysis revealed that each subtype has a distinct threshold and range of responses to mechanical and/or thermal stimuli. These findings support a model in which morphologically and physiologically distinct cutaneous DRG sensory neuron subtypes tile mechanical and thermal stimulus space to collectively encode a wide range of natural stimuli.

Spinal Cord Stimulation for Intractable Visceral Pain Originating from the Pelvic and Abdominal Region: A Narrative Review on a Possible New Indication for Patients with Therapy-Resistant Pain

Aim

Visceral pain, characterized by pain that is diffuse and challenging to localize, occurs frequently and is difficult to treat. In cases where the pain becomes intractable despite optimal medical management, it can affect patients' Quality of Life (QoL). Spinal Cord Stimulation (SCS) has emerged as a potential solution for intractable visceral pain.

Purpose

In this narrative review, we collected all evidence regarding the efficacy of SCS for visceral pain across various underlying conditions.

Methods

A comprehensive literature search was conducted in PubMed, Embase, and Web of Science in which articles published from October 1st, 1963 up to March 7th, 2023 were identified.

Results

Seventy articles were included in this review of which most were retrospective cohort studies, case series and case reports. The studies, often with a small number of participants, reported on SCS for chronic pancreatitis, anorectal pain and bowel disorders, gynaecological diagnoses, visceral pelvic pain, urological disorders and finally general visceral pain. They found positive effects on pain and/or symptom relief, opioid consumption, anxiety and depression and QoL. Complications occurred frequently but were often minor and reversible.

Conclusion

Better screening and selection criteria need to be established to optimally evaluate eligible patients who might benefit from SCS. A positive outcome of a sympathetic nerve block appears to be a potential indicator of SCS effectiveness. Additionally, women receiving SCS for endometriosis had a better outcome compared to other indications. Finally, SCS could also relief functional symptoms such as voiding problems and gastroparesis. Complications could often be resolved with revision surgery. Since SCS is expensive and not always covered by standard health insurance, the incorporation of cost-analyses is recommended. In order to establish a comprehensive treatment plan, including selection criteria for SCS, rigorous prospective, possibly randomized and controlled studies that are diagnosis-oriented, with substantial follow-up and adequate sample sizes, are needed.

The Coding Logic of Interoception

Interoception, the ability to precisely and timely sense internal body signals, is critical for life. The interoceptive system monitors a large variety of mechanical, chemical, hormonal, and pathological cues using specialized organ cells, organ innervating neurons, and brain sensory neurons. It is important for maintaining body homeostasis, providing motivational drives, and regulating autonomic, cognitive, and behavioral functions. However, compared to external sensory systems, our knowledge about how diverse body signals are coded at a system level is quite limited. In this review, we focus on the unique features of interoceptive signals and the organization of the interoceptive system, with the goal of better understanding the coding logic of interoception.

Intravesical Botulinum Toxin Injection Plus Hydrodistention Is More Effective in Patients with Bladder Pain-Predominant Interstitial Cystitis/Bladder Pain Syndrome

Intravesical botulinum toxin A (BoNT-A) injections are included in the interstitial cystitis/bladder pain syndrome (IC/BPS) treatment guidelines. However, the IC phenotype suitable for treatment with BoNT-A has not been clarified. Therefore, we identified the factors influencing treatment outcomes for intravesical BoNT-A injections in patients with non-Hunner IC/BPS (NHIC). This retrospective study included patients with NHIC who underwent 100 U BoNT-A intravesical injections over the past two decades. Six months after treatment, treatment outcomes were assessed using the Global Response Assessment (GRA). Outcome endpoints included GRA, clinical symptoms, urodynamic parameters, urine biomarkers, and the identification of factors contributing to satisfactory treatment outcomes. The study included 220 patients with NHIC (42 men, 178 women). The satisfactory group (n = 96, 44%) had significantly higher pain severity scores and IC symptoms index, larger maximum bladder capacity (MBC), and lower 8-isoprostane levels at baseline. Logistic regression revealed that larger MBC (≥760 mL) and bladder pain predominance were associated with satisfactory outcomes after BoNT-A injection. Subjective parameters and pain severity scores improved significantly in patients with bladder pain-predominant IC/BPS after BoNT-A injection. Thus, NHIC patients with bladder or pelvic pain are more likely to experience satisfactory outcomes following intravesical BoNT-A injections.

Spatial factors influencing the pain-ameliorating effect of CT-optimal touch: a comparative study for modulating temporal summation of second pain

Recent studies show that CT-optimal touch, gentle slow stroking of the skin, can reduce pain. However, much is unknown regarding the factors influencing its pain-ameliorating effect, such as tactile attention and touch application site. The current study investigates in 36 healthy individuals, whether CT-optimal touch can reduce temporal summation of second pain (TSSP) compared to CT non-optimal touch and tapping the skin. TSSP refers to activation of the C-nociceptors; by stimulating these fibers a burning and/or tingling sensation can be elicited. All participants underwent three conditions on both the contralateral and ipsilateral side of pain induction. The results show that tapping the skin did not reduce TSSP, meaning that pain reduction through touch cannot be explained by tactile attention effects. CT non-optimal touch only reduced TSSP when applied on the ipsilateral side. Importantly, CT-optimal touch effectively reduced TSSP when applied on the contralateral or ipsilateral side. Furthermore, CT-optimal touch was more effective in reducing TSSP compared to CT non-optimal touch and Tapping. This study shows that that CT-optimal touch can reduce TSSP and this effect appears to be independent of touch application site, which is highly relevant for implementing CT-optimal touch as a treatment.

Overexpression of EphB2 in the basolateral amygdala is crucial for inducing visceral pain sensitization in rats subjected to water avoidance stress

AIMS

Basolateral amygdala (BLA), as a center for stress responses and emotional regulation, is involved in visceral hypersensitivity of irritable bowel syndrome (IBS) induced by stress. In the present study, we aimed to investigate the role of EphB2 receptor (EphB2) in BLA and explore the underlying mechanisms in this process.

METHODS

Visceral hypersensitivity was induced by water avoidance stress (WAS). Elevated plus maze test, forced swimming test, and sucrose preference test were applied to assess anxiety- and depression-like behaviors. Ibotenic acid or lentivirus was used to inactivate BLA in either the induction or maintenance stage of visceral hypersensitivity. The expression of protein was determined by quantitative PCR, immunofluorescence, and western blot.

RESULTS

EphB2 expression was increased in BLA in WAS rats. Inactivation of BLA or downregulation of EphB2 in BLA failed to induce visceral hypersensitivity as well as anxiety-like behaviors. However, during the maintenance stage of visceral pain, visceral hypersensitivity was only partially relieved but anxiety-like behaviors were abolished by inactivation of BLA or downregulation of EphB2 in BLA. Chronic WAS increased the expression of EphB2, N-methyl-D-aspartate receptors (NMDARs), and postsynaptic density protein (PSD95) in BLA. Downregulation of EphB2 in BLA reduced NMDARs and PSD95 expression in WAS rats. However, activation of NMDARs after the knockdown of EphB2 expression still triggered visceral hypersensitivity and anxiety-like behaviors.

CONCLUSIONS

Taken together, the results suggest that EphB2 in BLA plays an essential role in inducing visceral hypersensitivity. In the maintenance stage, the involvement of EphB2 is crucial but not sufficient. The increase in EphB2 induced by WAS may enhance synaptic plasticity in BLA through upregulating NMDARs, which results in IBS-like symptoms. These findings may give insight into the treatment of IBS and related psychological distress.

Myenteric Plexus Immune Cell Infiltrations and Neurotransmitter Expression in Crohn's Disease and Ulcerative Colitis

BACKGROUND AND AIMS

Pain is a cardinal symptom in inflammatory bowel disease [IBD]. An important structure in the transduction of pain signalling is the myenteric plexus [MP]. Nevertheless, IBD-associated infiltration of the MP by immune cells lacks in-depth characterisation. Herein, we decipher intra- and periganglionic immune cell infiltrations in Crohn´s disease [CD] and ulcerative colitis [UC] and provide a comparison with murine models of colitis.

METHODS

Full wall specimens of surgical colon resections served to examine immune cell populations by either conventional immuno-histochemistry or immunofluorescence followed by either bright field or confocal microscopy. Results were compared with equivalent examinations in various murine models of intestinal inflammation.

RESULTS

Whereas the MP morphology was not significantly altered in IBD, we identified intraganglionic IBD-specific B cell- and monocyte-dominant cell infiltrations in CD. In contrast, UC-MPs were infiltrated by CD8+ T cells and revealed a higher extent of ganglionic cell apoptosis. With regard to the murine models of intestinal inflammation, the chronic dextran sulphate sodium [DSS]-induced colitis model reflected CD [and to a lesser extent UC] best, as it also showed increased monocytic infiltration as well as a modest B cell and CD8+ T cell infiltration.

CONCLUSIONS

In CD, MPs were infiltrated by B cells and monocytes. In UC, mostly CD8+ cytotoxic T cells were found. The chronic DSS-induced colitis in the mouse model reflected best the MP-immune cell infiltrations representative for IBD.

An adeno-associated viral labeling approach to visualize the meso- and microanatomy of mechanosensory afferents and autonomic innervation of the rat urinary bladder

The urinary bladder is supplied by a rich network of sensory and autonomic axons, commonly visualized by immunolabeling for neural markers. This approach demonstrates overall network patterning but is less suited to understanding the structure of individual motor and sensory terminals within these complex plexuses. There is a further limitation visualizing the lightly myelinated (A-delta) class of sensory axons that provides the primary mechanosensory drive for initiation of voiding. Whereas most unmyelinated sensory axons can be revealed by immunolabeling for specific neuropeptides, to date no unique neural marker has been identified to immunohistochemically label myelinated visceral afferents. We aimed to establish a non-surgical method to visualize and map myelinated afferents in the bladder in rats. We found that in rats, the adeno-associated virus (AAV), AAV-PHP.S, which shows a high tropism for the peripheral nervous system, primarily transduced myelinated dorsal root ganglion neurons, enabling us to identify the structure and regional distribution of myelinated (mechanosensory) axon endings within the muscle and lamina propria of the bladder. We further identified the projection of myelinated afferents within the pelvic nerve and lumbosacral spinal cord. A minority of noradrenergic and cholinergic neurons in pelvic ganglia were transduced, enabling visualization and regional mapping of both autonomic and sensory axon endings within the bladder. Our study identified a sparse labeling approach for investigating myelinated sensory and autonomic axon endings within the bladder and provides new insights into the nerve-bladder interface.

Molecular mechanisms of pain in acute pancreatitis: recent basic research advances and therapeutic implications

Although severe abdominal pain is the main symptom of acute pancreatitis, its mechanisms are poorly understood. An emerging body of literature evidence indicates that neurogenic inflammation might play a major role in modulating the perception of pain from the pancreas. Neurogenic inflammation is the result of a crosstalk between injured pancreatic tissue and activated neurons, which leads to an auto-amplification loop between inflammation and pain during the progression of acute pancreatitis. In this review, we summarize recent findings on the role of neuropeptides, ion channels, and the endocannabinoid system in acute pancreatitis-related pain. We also highlight potential therapeutic strategies that could be applied for managing severe pain in this disease.

Enteric glia promote visceral hypersensitivity during inflammation through intercellular signaling with gut nociceptors

Inflammation in the intestines causes abdominal pain that is challenging to manage. The terminals of sensory neurons innervating the gut are surrounded by glia. Here, using a mouse model of acute colitis, we found that enteric glia contribute to visceral pain by secreting factors that sensitized sensory nerves innervating the gut in response to inflammation. Acute colitis induced a transient increase in the production of proinflammatory cytokines in the intestines of male and female mice. Of these, IL-1β was produced in part by glia and augmented the opening of the intercellular communication hemichannel connexin-43 in glia, which made normally innocuous stimuli painful in female mice. Chemogenetic glial activation paired with calcium imaging in nerve terminals demonstrated that glia sensitized gut-innervating nociceptors only under inflammatory conditions. This inflammatory, glial-driven visceral hypersensitivity involved an increased abundance of the enzyme COX-2 in glia, resulting in greater production and release of prostaglandin E2 that activated EP4 receptors on sensory nerve terminals. Blocking EP4 receptors reduced nociceptor sensitivity in response to glial stimulation in tissue samples from colitis-model mice, and impairing glial connexin-43 reduced visceral hypersensitivity induced by IL-1β in female mice. The findings suggest that therapies targeting enteric glial-neuron signaling might alleviate visceral pain caused by inflammatory disorders.

Paraventricular Mast Cell-Derived Histamine Activates CRH Neurons to Mediate Adult Visceral Hypersensitivity Induced by Neonatal Maternal Separation

Neonatal maternal separation (NMS) is an early-life stress (ELS) that can result in adult visceral hypersensitivity, which is usually manifested as chronic visceral pain. Although mast cells and corticotropin-releasing hormone (CRH) neurons are involved in stress response, whether there is an interaction between mast cells and CRH neurons in hypothalamic paraventricular nucleus (PVN) during the ELS-induced visceral hypersensitivity remains elusive. Herein, we established an NMS model by separating neonatal mice from their mothers, and observed that these mice presented visceral hypersensitivity in adulthood, as indicated by elevated abdominal withdrawal reflex and lowered visceral pain threshold. The NMS-induced adult visceral hypersensitivity was accompanied by activation of mast cells and CRH neurons in PVN. Also, NMS increased the histamine content (an inflammatory mediator mainly released by mast cells) and histamine H2 receptor (H2R) expression of CRH neurons in PVN. Remarkably, intra-PVN administration with mast cell stabilizer attenuated the NMS-induced CRH neuronal activation and adult visceral pain, while histamine administration showed the opposite effects. Moreover, intra-PVN injection with H2R antagonist alleviated the NMS-induced CRH neuronal activation, PKA and CREB phosphorylation, and importantly, adult visceral pain. Together, our findings revealed a role of an interaction between paraventricular mast cells and CRH neurons in NMS-induced adult visceral hypersensitivity, thereby providing a perspective for the management of visceral pain.

Modulation of visceral pain by brain nuclei and brain circuits and the role of acupuncture: a narrative review

Visceral pain is a complex and heterogeneous pain condition that is often associated with pain-related negative emotional states, including anxiety and depression, and can exert serious effects on a patient's physical and mental health. According to modeling stimulation protocols, the current animal models of visceral pain mainly include the mechanical dilatation model, the ischemic model, and the inflammatory model. Acupuncture can exert analgesic effects by integrating and interacting input signals from acupuncture points and the sites of pain in the central nervous system. The brain nuclei involved in regulating visceral pain mainly include the nucleus of the solitary tract, parabrachial nucleus (PBN), locus coeruleus (LC), rostral ventromedial medulla (RVM), anterior cingulate cortex (ACC), paraventricular nucleus (PVN), and the amygdala. The neural circuits involved are PBN-amygdala, LC-RVM, amygdala-insula, ACC-amygdala, claustrum-ACC, bed nucleus of the stria terminalis-PVN and the PVN-ventral lateral septum circuit. Signals generated by acupuncture can modulate the central structures and interconnected neural circuits of multiple brain regions, including the medulla oblongata, cerebral cortex, thalamus, and hypothalamus. This analgesic process also involves the participation of various neurotransmitters and/or receptors, such as 5-hydroxytryptamine, glutamate, and enkephalin. In addition, acupuncture can regulate visceral pain by influencing functional connections between different brain regions and regulating glucose metabolism. However, there are still some limitations in the research efforts focusing on the specific brain mechanisms associated with the effects of acupuncture on the alleviation of visceral pain. Further animal experiments and clinical studies are now needed to improve our understanding of this area.

Nocebo effects in visceral pain: concept and design of the experimental randomized-controlled pain study 'NoVis'

The role of psychological factors in the pathophysiology and treatment of chronic visceral pain in disorders of gut-brain interactions (DGBI) is increasingly appreciated. Placebo research has underscored that expectations arising from the psychosocial treatment context and from prior experiences shape treatment responses. However, effects of negative expectations, i.e., nocebo effects, as they are likely crucial elements of DGBI patients' clinical reality, have thus far only rarely been investigated in the context of visceral pain, with untapped potential for improved prevention and treatment. The experimental randomized-controlled pain study "NoVis," carried out within the Collaborative Research Center (CRC) 289 ("Treatment Expectation"), aims to close gaps regarding the generation and persistence of nocebo effects in healthy volunteers. It is designed to elucidate effects of negative expectations in a multiple-threat paradigm with intensity-matched rectal distensions and cutaneous thermal stimuli, allowing to test nocebo effects in the visceral and somatic pain modalities. Negative expectations are experimentally induced by elements of doctor-patient communication (i.e., instruction) and/or by surreptitious amplification of symptom intensity (i.e., experience/learning) within a treatment context. Accordingly, the repeated measures between-subject design contains the between-group factors "treatment instruction" (negative vs. control) and "treatment experience" (negative vs. control), with volunteers randomized into four experimental groups undergoing several pain stimulation phases (repeated factor). This allows to compare the efficacy of instruction vs. experience, and more importantly, their combined effects on the magnitude of negative expectations and their impact on pain responses, which we expect will be greatest for the visceral modality. After a Baseline, short-term effects are assessed during a test phase accomplished on study day 1 (Test-1 Phase). To explore the persistence of effects, a second test phase is accomplished 1 week later (Test-2 Phase). Effects of negative expectations within and across pain modalities are assessed at the subjective and objective levels, with a focus on psychophysiological and neuroendocrine measures related to stress, fear, and anxiety. Since nocebo effects can play a considerable role in the generation, maintenance, or worsening of chronic visceral pain, and may even constitute risk factors for treatment failure, knowledge from experimental nocebo research has potential to improve treatment outcomes in DGBI and other clinical conditions associated with chronic visceral pain.

Animal models of interstitial cystitis/bladder pain syndrome

Interstitial Cystitis/Bladder Pain Syndrome (IC/BPS) is a chronic disorder characterized by pelvic and/or bladder pain, along with lower urinary tract symptoms that have a significant impact on an individual's quality of life. The diverse range of symptoms and underlying causes in IC/BPS patients pose a significant challenge for effective disease management and the development of new and effective treatments. To facilitate the development of innovative therapies for IC/BPS, numerous preclinical animal models have been developed, each focusing on distinct pathophysiological components such as localized urothelial permeability or inflammation, psychological stress, autoimmunity, and central sensitization. However, since the precise etiopathophysiology of IC/BPS remains undefined, these animal models have primarily aimed to replicate the key clinical symptoms of bladder hypersensitivity and pain to enhance the translatability of potential therapeutics. Several animal models have now been characterized to mimic the major symptoms of IC/BPS, and significant progress has been made in refining these models to induce chronic symptomatology that more closely resembles the IC/BPS phenotype. Nevertheless, it's important to note that no single model can fully replicate all aspects of the human disease. When selecting an appropriate model for preclinical therapeutic evaluation, consideration must be given to the specific pathology believed to underlie the development of IC/BPS symptoms in a particular patient group, as well as the type and severity of the model, its duration, and the proposed intervention's mechanism of action. Therefore, it is likely that different models will continue to be necessary for preclinical drug development, depending on the unique etiology of IC/BPS being investigated.

Current optimal pharmacologic therapies for overactive bladder

INTRODUCTION

Overactive bladder (OAB) is a common syndrome in adults. Current pharmacologic treatment includes antimuscarinic agents and β-3 adrenoceptor agonists. For non-responders to oral medication, intravesical injection of botulinum toxin A (BoNT-A) is an effective option. However, these treatments have potential adverse events and should be cautiously selected for appropriate patients. This review presents the recently published results of clinical trials and studies for patients with OAB and the underlying pathophysiology of OAB. Appropriate medical therapy based on pathophysiology of OAB is also presented.

AREAS COVERED

Literature search from Pubmed from 2001 to 2023 including clinical background, pharmacology, and clinical studies for OAB medications.

EXPERT OPINION

Treatment of OAB syndrome with any antimuscarinic or β-3 adrenoceptor agonist is feasible as a first-line approach. For patients with suboptimal therapeutic effect to full-dose antimuscarinics or mirabegron, combination with both drugs can improve efficacy. Intravesical BoNT-A 100-U injection provides therapeutic effects for refractory OAB. Patients who are refractory to initial pharmacotherapies should be investigated for the underlying pathophysiology; then an appropriate medication can be added, such as an α1-blocker or anti-inflammatory agents. Patient education about behavioral modification and therapies should always be provided with oral medication or BoNT-A injection for OAB patients.

Dietary monosodium glutamate increases visceral hypersensitivity in a mouse model of visceral pain

BACKGROUND

Monosodium glutamate (MSG) has been identified as a trigger of abdominal pain in irritable bowel syndrome (IBS), but the mechanism is unknown. This study examined whether MSG causes visceral hypersensitivity using a water-avoidance stress (WAS) mouse model of visceral pain.

METHODS

Mice were divided into four groups receiving treatment for 6 days: WAS + MSG gavage, WAS + saline gavage, sham-WAS + MSG gavage, and sham-WAS + saline gavage. The acute effects of intraluminal administration of 10 μM MSG on jejunal extrinsic afferent nerve sensitivity to distension (0-60 mmHg) were examined using ex vivo extracellular recordings. MSG was also applied directly to jejunal afferents from untreated mice. Glutamate concentration was measured in serum, and in the serosal compartment of Ussing chambers following apical administration.

KEY RESULTS

Acute intraluminal MSG application increased distension responses of jejunal afferent nerves from mice exposed to WAS + MSG. This effect was mediated by wide dynamic range and high-threshold units at both physiologic and noxious pressures (10-60 mmHg, p < 0.05). No effect of MSG was observed in the other groups, or when applied directly to the jejunal afferent nerves. Serum glutamate was increased in mice exposed to WAS + MSG compared to sham-WAS + saline, and serosal glutamate increased using WAS tissue (p = 0.0433).

CONCLUSIONS AND INFERENCES

These findings demonstrate that repeated exposure to MSG in mice leads to sensitization of jejunal afferent nerves to acute ex vivo exposure to MSG. This may contribute to visceral hypersensitivity reported in response to MSG in patients with IBS.

Global Trends in Research of Pain-Gut-Microbiota Relationship and How Nutrition Can Modulate This Link

INTRODUCTION

The link between gut microbiota and chronic painful conditions has recently gained attention. Nutrition, as a common intervention in daily life and medical practice, is closely related to microbiota and pain. However, no published bibliometric reports have analyzed the scientific literature concerning the link.

METHODS AND RESULTS

We used bibliometrics to identify the characteristics of the global scientific output over the past 20 years. We also aimed to capture and describe how nutrition can modulate the abovementioned link. Relevant papers were searched in the Web of Science database. All necessary publication and citation data were acquired and exported to Bibliometrix for further analyses. The keywords mentioned were illustrated using visualization maps. In total, 1551 papers shed light on the relationship from 2003 to 2022. However, only 122 papers discussed how nutritional interventions can modulate this link. The citations and attention were concentrated on the gut microbiota, pain, and probiotics in terms of the pain-gut relationship. Nutritional status has gained attention in motor themes of a thematic map.

CONCLUSIONS

This bibliometric analysis was applied to identify the scientific literature linking gut microbiota, chronic painful conditions, and nutrition, revealing the popular research topics and authors, scientific institutions, countries, and journals in this field. This study enriches the evidence moving boundaries of microbiota medicine as a clinical medicine.

PIEZO2 in somatosensory neurons controls gastrointestinal transit

The gastrointestinal tract is in a state of constant motion. These movements are tightly regulated by the presence of food and help digestion by mechanically breaking down and propelling gut content. Mechanical sensing in the gut is thought to be essential for regulating motility; however, the identity of the neuronal populations, the molecules involved, and the functional consequences of this sensation are unknown. Here, we show that humans lacking PIEZO2 exhibit impaired bowel sensation and motility. Piezo2 in mouse dorsal root, but not nodose ganglia is required to sense gut content, and this activity slows down food transit rates in the stomach, small intestine, and colon. Indeed, Piezo2 is directly required to detect colon distension in vivo. Our study unveils the mechanosensory mechanisms that regulate the transit of luminal contents throughout the gut, which is a critical process to ensure proper digestion, nutrient absorption, and waste removal.

The Endogenous Opioid Met-Enkephalin Modulates Thalamo-Cortical Excitation Inhibition Balance in a Medial Thalamus-Anterior Cingulate Cortex Circuit

Activation of opioid receptors in the anterior cingulate cortex (ACC) mediates aspects of analgesia induced by both exogenous and endogenous opioids. We have previously shown that opioid signaling disrupts both afferent excitatory and indirect inhibitory synaptic transmission from the medial thalamus (MThal) to the ACC, but the effects of endogenous opioids within this circuit remain poorly understood. The goal of the current study was to understand how the endogenous opioid, [Met]5-enkephalin (ME), modulates thalamic-driven excitatory and inhibitory synaptic transmission onto layer V pyramidal neurons in the ACC. We used pharmacology, brain slice electrophysiology and optogenetic stimulation to study opioid-mediated modulation of optically evoked glutamatergic and GABAergic transmission. The results revealed that ME inhibited both AMPA-mediated excitatory and GABA-mediated inhibitory synaptic transmission in the ACC. However, inhibitory transmission was more potently inhibited than excitatory transmission by ME. This preferential reduction in GABAA-mediated synaptic transmission was primarily due to the activation of delta opioid receptors by ME and resulted in a net disinhibition of MThal-ACC excitatory pathway. These results suggest that moderate concentrations of ME can lead to net excitation of ACC circuitry and that analgesia may be associated with disinhibition rather than inhibition of ACC subcircuits.

Peripheral nerves directly mediate the transneuronal translocation of silver nanomaterials from the gut to central nervous system

The blood circulation is considered the only way for the orally administered nanoparticles to enter the central nervous systems (CNS), whereas non-blood route-mediated nanoparticle translocation between organs is poorly understood. Here, we show that peripheral nerve fibers act as direct conduits for silver nanomaterials (Ag NMs) translocation from the gut to the CNS in both mice and rhesus monkeys. After oral gavage, Ag NMs are significantly enriched in the brain and spinal cord of mice with particle state however do not efficiently enter the blood. Using truncal vagotomy and selective posterior rhizotomy, we unravel that the vagus and spinal nerves mediate the transneuronal translocation of Ag NMs from the gut to the brain and spinal cord, respectively. Single-cell mass cytometry analysis revealed that enterocytes and enteric nerve cells take up significant levels of Ag NMs for subsequent transfer to the connected peripheral nerves. Our findings demonstrate nanoparticle transfer along a previously undocumented gut-CNS axis mediated by peripheral nerves.

Common and unique alterations of functional connectivity in major depressive disorder and bipolar disorder

OBJECTIVE

Major depressive disorder (MDD) and bipolar disorder (BD) are considered whole-brain disorders with some common clinical and neurobiological features. It is important to investigate neural mechanisms to distinguish between the two disorders. However, few studies have explored the functional dysconnectivity between the two disorders from the whole brain level.

METHODS

In this study, 117 patients with MDD, 65 patients with BD, and 116 healthy controls completed resting-state functional magnetic resonance imaging (R-fMRI) scans. Both edge-based network construction and large-scale network analyses were applied.

RESULTS

Results found that both the BD and MDD groups showed decreased FC in the whole brain network. The shared aberrant network across patients involves the visual network (VN), sensorimotor network (SMN), dorsal attention network (DAN), and ventral attention network (VAN), which is related to the processing of external stimuli. The default mode network (DMN) and the limbic network (LN) abnormalities were only found in patients with MDD. Furthermore, results showed the highest decrease in edges of patients with MDD in between-network FC in SMN-VN, whereas in VAN-VN of patients with BD.

CONCLUSIONS

Our findings indicated that both MDD and BD are extensive abnormal brain network diseases, mainly aberrant in those brain networks correlated to the processing of external stimuli, especially the attention network. Specific altered functional connectivity also was found in MDD and BD groups, respectively. These results may provide possible trait markers to distinguish the two disorders.

One immune system plays many parts: The dynamic role of the immune system in chronic pain and opioid pharmacology

The transition from acute to chronic pain is an ongoing major problem for individuals, society and healthcare systems around the world. It is clear chronic pain is a complex multidimensional biological challenge plagued with difficulties in pain management, specifically opioid use. In recent years the role of the immune system in chronic pain and opioid pharmacology has come to the forefront. As a highly dynamic and versatile network of cells, tissues and organs, the immune system is perfectly positioned at the microscale level to alter nociception and drive structural adaptations that underpin chronic pain and opioid use. In this review, we highlight the need to understand the dynamic and adaptable characteristics of the immune system and their role in the transition, maintenance and resolution of chronic pain. The complex multidimensional interplay of the immune system with multiple physiological systems may provide new transformative insight for novel targets for clinical management and treatment of chronic pain. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

A DRG genetic toolkit reveals molecular, morphological, and functional diversity of somatosensory neuron subtypes

Mechanical and thermal stimuli acting on the skin are detected by morphologically and physiologically distinct sensory neurons of the dorsal root ganglia (DRG). Achieving a holistic view of how this diverse neuronal population relays sensory information from the skin to the central nervous system (CNS) has been challenging with existing tools. Here, we used transcriptomic datasets of the mouse DRG to guide development and curation of a genetic toolkit to interrogate transcriptionally defined DRG neuron subtypes. Morphological analysis revealed unique cutaneous axon arborization areas and branching patterns of each subtype. Physiological analysis showed that subtypes exhibit distinct thresholds and ranges of responses to mechanical and/or thermal stimuli. The somatosensory neuron toolbox thus enables comprehensive phenotyping of most principal sensory neuron subtypes. Moreover, our findings support a population coding scheme in which the activation thresholds of morphologically and physiologically distinct cutaneous DRG neuron subtypes tile multiple dimensions of stimulus space.

Endocannabinoids, anandamide and 2-AG, regulate mechanosensitivity of mucosal afferents in the Guinea pig bladder

Bladder afferents play a crucial role in urine storage and voiding, and conscious sensations from the bladder. Endocannabinoids, anandamide (AEA) and 2-arachidonolylglycerol (2-AG), are endogenous ligands of G-protein coupled cannabinoid receptors 1 and 2 (CB1 and CB2) found in the CNS and peripheral organs. They also have off-target effects on some ligand- and voltage-gated channels. The aim of this study is to determine the role of AEA and 2-AG in regulation of mechanosensitivity of probable nociceptive neurons innervating the bladder - capsaicin-sensitive mucosal afferents. The activity of these afferents was determined by ex vivo single unit extracellular recordings in the guinea pig bladder. A stable analogue of anandamide, methanandamide (mAEA) evoked initial excitatory response of mucosal afferents followed by potentiation of their responses to mechanical stimulation. In the presence of TRPV1 antagonist (AMG9810), mAEA's effect on mechanosensitivity switched from excitatory to inhibitory. The inhibitory effect of mAEA is due to activation of both CB1 and CB2 cannabinoid receptors since it was abolished by combined application of selective CB1 (NESS0327) and CB2 (SR144528) antagonists. 2-AG application evoked a brief excitation of mucosal afferents, without potentiation of their mechanosensitivity, followed by the inhibition of their responses to mechanical stimulation. CB2 receptor antagonist, SR144528 abolished the inhibitory effect of 2-AG. Our data indicated that anandamide and 2-AG have opposite effects on mechanosensitivity of mucosal capsaicin-sensitive afferents in the guinea pig bladder; mAEA potentiated while 2-AG inhibited responses of mucosal afferents to mechanical stimulation. These findings are important for understanding of the role of endocannabinoids in regulating bladder sensation and function.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Gut enterochromaffin cells drive visceral pain and anxiety

Gastrointestinal (GI) discomfort is a hallmark of most gut disorders and represents an important component of chronic visceral pain1. For the growing population afflicted by irritable bowel syndrome, GI hypersensitivity and pain persist long after tissue injury has resolved2. Irritable bowel syndrome also exhibits a strong sex bias, afflicting women three times more than men1. Here, we focus on enterochromaffin (EC) cells, which are rare excitable, serotonergic neuroendocrine cells in the gut epithelium3-5. EC cells detect and transduce noxious stimuli to nearby mucosal nerve endings3,6 but involvement of this signalling pathway in visceral pain and attendant sex differences has not been assessed. By enhancing or suppressing EC cell function in vivo, we show that these cells are sufficient to elicit hypersensitivity to gut distension and necessary for the sensitizing actions of isovalerate, a bacterial short-chain fatty acid associated with GI inflammation7,8. Remarkably, prolonged EC cell activation produced persistent visceral hypersensitivity, even in the absence of an instigating inflammatory episode. Furthermore, perturbing EC cell activity promoted anxiety-like behaviours which normalized after blockade of serotonergic signalling. Sex differences were noted across a range of paradigms, indicating that the EC cell-mucosal afferent circuit is tonically engaged in females. Our findings validate a critical role for EC cell-mucosal afferent signalling in acute and persistent GI pain, in addition to highlighting genetic models for studying visceral hypersensitivity and the sex bias of gut pain.

Analysis of the spinal and vagal afferent innervation of the mouse colon using neuronal retrograde tracers

The gut-brain axis has received increasing attention recently due to evidence that colonic microbes can affect brain function and behavior. However, little is known about the innervation of the colon by a major component of the gut-brain axis, vagal afferent neurons. Furthermore, it is currently unknown whether individual NG neurons or DRG neurons innervate both the proximal and distal colon. We aimed to quantify the number of vagal and spinal afferent neurons that innervate the colon; and determine whether these individual neurons simultaneously innervate the mouse proximal and distal colon. C57Bl/6 mice received injections of a combination of retrograde tracers that were either injected into the muscularis externa of the proximal or the distal colon: fast blue, DiI and DiO. Five to seven percent of lumbosacral and thoracolumbar spinal afferent neurons, and 25% of vagal afferent neurons were labelled by injections of DiI and DiO into the colon. We also found that approximately 8% of NG neurons innervate the distal colon. Ten percent of labeled thoracolumbar and 15% of labeled lumbosacral DRG neurons innervate both the distal and proximal colon. Eighteen percent of labeled NG neurons innervated both the distal and proximal colon. In conclusion, vagal afferent innervation of the distal colon is less extensive than the proximal colon, whereas a similar gradient was not observed for the spinal afferent innervation. Furthermore, overlap appears to exist between the receptive fields of vagal and spinal afferent neurons that innervate the proximal and distal colon.

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.

Role of circadian rhythms and melatonin in bladder function in heath and diseases

The circadian system modulates all visceral organ physiological processes including urine storage and voiding. The "master clock" of the circadian system lies within suprachiasmatic nucleus of the hypothalamus while "peripheral clocks" are found in most peripheral tissue and organs, including the urinary bladder. Disruptions of circadian rhythms can cause organ malfunction and disorder or exacerbate pre-existing ones. It has been suggested that nocturia, which develops mostly in the elderly, could be a circadian-related disorder of the bladder. In the bladder, many types of gap junctions and ion channels in the detrusor, urothelium and sensory nerves are likely under strict local peripheral circadian control. The pineal hormone, melatonin, is a circadian rhythm synchroniser capable of controlling a variety of physiological processes in the body. Melatonin predominantly acts via the melatonin 1 and melatonin 2 G-protein coupled receptors expressed in the central nervous system, and many peripheral organs and tissues. Melatonin could be beneficial in the treatment of nocturia and other common bladder disorders. The ameliorating action of melatonin on bladder function is likely due to multiple mechanisms which include central effects on voiding and peripheral effects on the detrusor and bladder afferents. More studies are warranted to determine the precise mechanisms of circadian rhythm coordination of the bladder function and melatonin influences on the bladder in health and diseases.

Spinal cord astrocyte P2X7Rs mediate the inhibitory effect of electroacupuncture on visceral hypersensitivity of rat with irritable bowel syndrome

This study explored the role of P2X7 receptors in spinal cord astrocytes in the electroacupuncture-induced inhibition of visceral hypersensitivity (VH) in rats with irritable bowel syndrome (IBS). Visceral hypersensitivity of IBS was intracolonically induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS). Visceromotor responses to colorectal distension (CRD-20,40,60,80 mmHg) and abdominal withdrawal reflex scoring (AWRs) were recorded after electroacupuncture at bilateral Zusanli (ST36) and Sanyinjiao (SP6) acupoints to evaluate the analgesic effect of electroacupuncture on visceral pain in rats with IBS. Fluorocitric acid (FCA), an astrocyte activity inhibitor, was injected intrathecally before electroacupuncture intervention and AWRs were recorded. Western blot and real-time qPCR were used to detect the expression of NMDA and P2X7 receptor to observe the regulation effect of electroacupuncture on NMDA receptor in the spinal cord of rats with visceral hypersensitivity. Intrathecal injection of P2X7 agonist or antagonist was administered before electroacupuncture treatment. To observe the effect of P2X7 receptor in spinal astrocytes on the inhibition of visceral hyperalgesia by electroacupuncture, the changes of AWR score, NMDA receptor in the spinal cord, and GFAP expression in astrocytes were detected. Inflammation of the colon had basically subsided at day 21 post-TNBS; persistent visceral hypersensitivity could be suppressed by electroacupuncture. This analgesic effect could be inhibited by FCA. The analgesic effect, downregulation of NMDA receptor NR1 subunit, and P2X7 protein of electroacupuncture were all reversed by FCA. P2X7 receptor antagonist A740003 can cooperate with EA to carry out analgesic effect in rats with visceral pain and downregulate the expression of NR1, NR2B, and GFAP in spinal dorsal horn. However, the P2X7 receptor agonist BzATP could partially reverse the analgesic effect of EA, inhibiting the downregulatory effect of EA on the expression of NR1, NR2B, and GFAP. These results indicate that EA may downregulate the expression of the NMDA receptor by inhibiting the P2X7 receptor in the spinal cord, thereby inhibiting spinal cord sensitization in IBS rats with visceral pain, in which astrocytes are an important medium.

Spinal VGLUT3 lineage neurons drive visceral mechanical allodynia but not sensitized visceromotor reflexes

Visceral pain is among the most prevalent and bothersome forms of chronic pain, but their transmission in the spinal cord is still poorly understood. Here, we conducted focal colorectal distention (fCRD) to drive both visceromotor responses (VMRs) and aversion. We first found that spinal CCK neurons were necessary for noxious fCRD to drive both VMRs and aversion under naive conditions. We next showed that spinal VGLUT3 neurons mediate visceral allodynia, whose ablation caused loss of aversion evoked by low-intensity fCRD in mice with gastrointestinal (GI) inflammation or spinal circuit disinhibition. Importantly, these neurons were dispensable for driving sensitized VMRs under both inflammatory and central disinhibition conditions. Anatomically, a subset of VGLUT3 neurons projected to parabrachial nuclei, whose photoactivation sufficiently generated aversion in mice with GI inflammation, without influencing VMRs. Our studies suggest the presence of different spinal substrates that transmit nociceptive versus affective dimensions of visceral sensory information.

Intestinal neuropod cell GUCY2C regulates visceral pain

Visceral pain (VP) is a global problem with complex etiologies and limited therapeutic options. Guanylyl cyclase C (GUCY2C), an intestinal receptor producing cyclic GMP(cGMP), which regulates luminal fluid secretion, has emerged as a therapeutic target for VP. Indeed, FDA-approved GUCY2C agonists ameliorate VP in patients with chronic constipation syndromes, although analgesic mechanisms remain obscure. Here, we revealed that intestinal GUCY2C was selectively enriched in neuropod cells, a type of enteroendocrine cell that synapses with submucosal neurons in mice and humans. GUCY2Chi neuropod cells associated with cocultured dorsal root ganglia neurons and induced hyperexcitability, reducing the rheobase and increasing the resulting number of evoked action potentials. Conversely, the GUCY2C agonist linaclotide eliminated neuronal hyperexcitability produced by GUCY2C-sufficient - but not GUCY2C-deficient - neuropod cells, an effect independent of bulk epithelial cells or extracellular cGMP. Genetic elimination of intestinal GUCY2C amplified nociceptive signaling in VP that was comparable with chemically induced VP but refractory to linaclotide. Importantly, eliminating GUCY2C selectively in neuropod cells also increased nociceptive signaling and VP that was refractory to linaclotide. In the context of loss of GUCY2C hormones in patients with VP, these observations suggest a specific role for neuropod GUCY2C signaling in the pathophysiology and treatment of these pain syndromes.

The neurobiology of irritable bowel syndrome

Irritable bowel syndrome (IBS) is the most prevalent disorder of brain-gut interactions that affects between 5 and 10% of the general population worldwide. The current symptom criteria restrict the diagnosis to recurrent abdominal pain associated with altered bowel habits, but the majority of patients also report non-painful abdominal discomfort, associated psychiatric conditions (anxiety and depression), as well as other visceral and somatic pain-related symptoms. For decades, IBS was considered an intestinal motility disorder, and more recently a gut disorder. However, based on an extensive body of reported information about central, peripheral mechanisms and genetic factors involved in the pathophysiology of IBS symptoms, a comprehensive disease model of brain-gut-microbiome interactions has emerged, which can explain altered bowel habits, chronic abdominal pain, and psychiatric comorbidities. In this review, we will first describe novel insights into several key components of brain-gut microbiome interactions, starting with reported alterations in the gut connectome and enteric nervous system, and a list of distinct functional and structural brain signatures, and comparing them to the proposed brain alterations in anxiety disorders. We will then point out the emerging correlations between the brain networks with the genomic, gastrointestinal, immune, and gut microbiome-related parameters. We will incorporate this new information into a systems-based disease model of IBS. Finally, we will discuss the implications of such a model for the improved understanding of the disorder and the development of more effective treatment approaches in the future.

Minocycline Prevents the Development of Key Features of Inflammation and Pain in DSS-induced Colitis in Mice

Abdominal pain is a common feature in inflammatory bowel disease (IBD) patients, and greatly compromises their quality of life. Therefore, the identification of new therapeutic tools to reduce visceral pain is one of the main goals for IBD therapy. Minocycline, a broad-spectrum tetracycline antibiotic, has gained attention in the scientific community because of its immunomodulatory and anti-inflammatory properties. The aim of this study was to evaluate the potential of this antibiotic as a therapy for the management of visceral pain in dextran sodium sulfate (DSS)-induced colitis in mice. Preemptive treatment with minocycline markedly reduced histological features of intestinal inflammation and the expression of inflammatory markers (Tlr4, Tnfα, Il1ß, Ptgs2, Inos, Cxcl2, and Icam1), and attenuated the decrease of markers of epithelial integrity (Tjp1, Ocln, Muc2, and Muc3). In fact, minocycline restored normal epithelial permeability in colitic mice. Treatment with the antibiotic also reversed the changes in the gut microbiota profile induced by colitis. All these ameliorative effects of minocycline on both inflammation and dysbiosis correlated with a decrease in ongoing pain and referred hyperalgesia, and with the improvement of physical activity induced by the antibiotic in colitic mice. Minocycline might constitute a new therapeutic approach for the treatment of IBD-induced pain. PERSPECTIVE: This study found that the intestinal anti-inflammatory effects of minocycline ameliorate DSS-associated pain in mice. Therefore, minocycline might constitute a novel therapeutic strategy for the treatment of IBD-induced pain.

Identification of Potential Visceral Pain Biomarkers in Colon Exudates from Mice with Experimental Colitis: An Exploratory In Vitro Study

Chronic visceral pain (CVP) is extremely difficult to diagnose, and available analgesic treatment options are quite limited. Identifying the proteins secreted from the colonic nociceptors, or their neighbor cells within the tube walls, in the context of disorders that course with visceral pain, might be useful to decipher the mechanism involved in the establishment of CVP. Addressing this question in human with gastrointestinal disorders entails multiple difficulties, as there is not a clear classification of disease severity, and colonic secretion is not easy to manage. We propose using of a murine model of colitis to identify new algesic molecules and pathways that could be explored as pain biomarkers or analgesia targets. Descending colons from naïve and colitis mice with visceral hyperalgesia were excised and maintained ex vivo. The proteins secreted in the perfusion fluid before and during acute noxious distension were evaluated using high-resolution mass spectrometry (MS). Haptoglobin (Hp), PZD and LIM domain protein 3 (Pdlim3), NADP-dependent malic enzyme (Me1), and Apolipoprotein A-I (Apoa1) were increased during visceral insult, whilst Triosephosphate isomerase (Tpi1), Glucose-6-phosphate isomerase (Gpi1), Alpha-enolase (Eno1), and Isoform 2 of Tropomyosin alpha-1 chain (Tpm1) were decreased. Most identified proteins have been described in the context of different chronic pain conditions and, according to gene ontology analysis, they are also involved in diverse biological processes of relevance. Thus, animal models that mimic human conditions in combination with unbiased omics approaches will ultimately help to identify new pathophysiological mechanisms underlying pain that might be useful in diagnosing and treating pain. PERSPECTIVE: Our study utilizes an unbiased proteomic approach to determine, first, the clinical relevance of a murine model of colitis and, second, to identify novel molecules/pathways involved in nociception that would be potential biomarkers or targets for chronic visceral pain.

Research hotspots and trends in visceral pain research: A global comprehensive bibliometric analysis

Background

Visceral pain is a complex and heterogeneous disorder that is considered more prominent compared to somatic pain, due to its multiple and complex causes and accompanying emotional and mood disorders. Research has become increasingly extensive over the years, but a bibliometric analysis of this field is lacking. The aim of this study was to analyze global research trends in visceral pain over the past 40 years through visual analysis.

Methods

We conducted a comprehensive search of the literature from January 1981 to December 2021 using the Web of Science core database. The medical subject term 'visceral pain' was searched. We used CiteSpace and VOSviewer for bibliometric analysis and network visualization, including top-ranked authors, keywords, research collaborations, and literature co-occurrence network analysis.

Results

A total of 5,047 articles were included in the analysis. The number of articles on visceral pain has continued to grow steadily over the past 40 years. The United States (1,716 articles), University of California (159 articles), and Neurogastroenterology and Motility (276 articles) were the country, institution, and journal with the most publications, respectively. Keyword analysis showed that inflammation, visceral hypersensitivity, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), anxiety, and quality of life were the research trends and priorities in this research field.

Conclusion

Visceral pain-related research has received increasing attention in recent decades. However, there are still many unresolved issues in the field of visceral pain, such as the specific molecular mechanisms and clinical treatments between visceral pain and inflammation, IBD, IBS, anxiety, and quality of life, which may require further exploration based on modern scientific and technological means and more basic research, especially for the therapeutic targets of visceral pain, which may become a hot spot for future research and provide guidance for the treatment of clinical diseases related to visceral pain.

Randomised clinical trial: effects of MD-7246 on irritable bowel syndrome with diarrhoea

BACKGROUND

MD-7246, a delayed-release formulation of linaclotide, is designed to target the ileocaecal junction and caecum with the aim of relieving abdominal pain independently of bowel function.

AIMS

To evaluate the efficacy, safety and dose-response of MD-7246 in patients with irritable bowel syndrome with diarrhoea (IBS-D).

METHODS

A randomised, double-blind, phase 2 clinical trial enrolled adult patients with IBS-D (Rome IV criteria). Patients were randomised to placebo or once-daily oral MD-7246 300, 600 or 1200 μg for 12 weeks. Abdominal and bowel symptoms were assessed daily. Key efficacy endpoints were change from baseline in abdominal pain and responder rates for a 30% reduction in abdominal pain in 6/12 weeks. Additional abdominal pain responder and exploratory bowel function endpoints were also assessed.

RESULTS

Among the 388 randomised patients, there was no significant difference in mean change from baseline in abdominal pain between the MD-7246 300 μg, 600 μg and 1200 μg groups and placebo (-1.93, -1.58, -1.95 and - 2.01, respectively; p > 0.05 for each group vs placebo). The abdominal pain responder rates in the MD-7246 groups were similar to or lower than those in the placebo group. All doses of MD-7246 had a minimal effect on bowel function and were generally well tolerated.

CONCLUSIONS

MD-7246 at the doses studied did not improve abdominal pain relative to placebo in an IBS-D patient population. Similarly, most additional efficacy endpoints showed no improvement with MD-7246 relative to placebo.

TRPV1 and TRPM8 antagonists reduce cystitis-induced bladder hypersensitivity via inhibition of different sensitised classes of bladder afferents in guinea pigs

BACKGROUND AND PURPOSE

Interstitial cystitis (=painful bladder syndrome) is a chronic bladder syndrome characterised by pelvic and bladder pain, urinary frequency and urgency, and nocturia. Transient receptor potential (TRP) channels are an attractive target in reducing the pain associated with interstitial cystitis. The current study aims to determine the efficacy of combination of TRP vanilloid 1 (TRPV1) and TRP melastatin 8 (TRPM8) channel inhibition in reducing the pain associated with experimental cystitis in guinea pigs.

EXPERIMENTAL APPROACH

A novel animal model of non-ulcerative interstitial cystitis has been developed using protamine sulfate/zymosan in female guinea pigs. Continuous voiding cystometry was performed in conscious guinea pigs. Ex vivo "close-to-target" single unit extracellular recordings were made from fine branches of pelvic nerves entering the guinea pig bladder. Visceromotor responses in vivo were used to determine the effects of TRP channel antagonists on cystitis-induced bladder hypersensitivity.

KEY RESULTS

Protamine sulfate/zymosan treatment evoked mild inflammation in the bladder and increased micturition frequency in conscious animals. In cystitis, high threshold muscular afferents were sensitised via up-regulation of TRPV1 channels, high threshold muscular-mucosal afferents were sensitised via TRPM8 channels, and mucosal afferents by both. Visceromotor responses evoked by noxious bladder distension were significantly enhanced in cystitis and were returned to control levels upon administration of combination of low doses of TRPV1 and TRPM8 antagonists.

CONCLUSIONS AND IMPLICATIONS

The data demonstrate the therapeutic promises of combination of TRPV1 and TRPM8 antagonists for the treatment of bladder hypersensitivity in cystitis.

How should we define a nociceptor in the gut-brain axis?

In the past few years, there has been extraordinary interest in how the gut communicates with the brain. This is because substantial and gathering data has emerged to suggest that sensory nerve pathways between the gut and brain may contribute much more widely in heath and disease, than was originally presumed. In the skin, the different types of sensory nerve endings have been thoroughly characterized, including the morphology of different nerve endings and the sensory modalities they encode. This knowledge is lacking for most types of visceral afferents, particularly spinal afferents that innervate abdominal organs, like the gut. In fact, only recently have the nerve endings of spinal afferents in any visceral organ been identified. What is clear is that spinal afferents play the major role in pain perception from the gut to the brain. Perhaps surprisingly, the majority of spinal afferent nerve endings in the gut express the ion channel TRPV1, which is often considered to be a marker of "nociceptive" neurons. And, a majority of gut-projecting spinal afferent neurons expressing TRPV1 are activated at low thresholds, in the "normal" physiological range, well below the normal threshold for detection of painful sensations. This introduces a major conundrum regarding visceral nociception. How should we define a "nociceptor" in the gut? We discuss the notion that nociception from the gut wall maybe a process encrypted into multiple different morphological types of spinal afferent nerve ending, rather than a single class of sensory ending, like free-endings, suggested to underlie nociception in skin.

Gut reactions: emerging mechanisms of abdominal pain from food intake

Abdominal pain, which is a form of visceral pain, is a highly prevalent symptom worldwide frequently occurring following food ingestion. Its pathophysiology is complex, and many factors, including intestinal environmental cues, the immune system, or the molecular composition of foods, can influence the development of postprandial abdominal pain. Because of the poor efficacy of drug treatments, current strategies are often limited to the exclusion of culprit food(s) from the diet. However, there are two important limitations to this approach. First, patients suffering from food-induced abdominal pain usually recognize several food items as the cause of their gastrointestinal symptoms. Second, not all offending foods can always be identified by these patients. Newly identified mechanisms involving neuroimmune interactions and their communication with the intestinal microbiota shed light on the development of new therapeutic strategies. In this Mini-Review, these novel mechanisms and relevance of such findings are highlighted.

Immune-mediated food reactions in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder characterized by recurrent abdominal pain and an altered defecation pattern. Depending on the criteria used, it affects between 5 and 10% of the general population and has a serious impact on quality of life. Most patients with IBS show an induction or exacerbation of their symptoms, particularly abdominal pain, after eating certain foods. This raises the question of the role played by food in IBS pathophysiology. In this review, we describe the multiple risk factors of IBS, and we give an overview of the role of food as a trigger of IBS, distinguishing between immune and non-immune reactions to food. We finally highlight recent findings identifying an immune-mediated mechanism underlying food-induced abdominal pain in IBS.

Unveil the pain of endometriosis: from the perspective of the nervous system

Endometriosis is a chronic inflammatory disease with pelvic pain and uncharacteristic accompanying symptoms. Endometriosis-associated pain often persists despite treatment of the disease, thus it brings a deleterious impact on their personal lives as well as imposing a substantial economic burden on them. At present, mechanisms underlie endometriosis-associated pain including inflammatory reaction, injury, aberrant blood vessels and the morphological and functional anomaly of the peripheral and central nervous systems. The nerve endings are influenced by the physical and chemical factors surrounding the lesion, via afferent nerve to the posterior root of the spinal nerve, then to the specific cerebral cortex involved in nociception. However, our understanding of the aetiology and mechanism of this complex pain process caused by endometriosis remains incomplete. Identifying the pathogenesis of endometriosis is crucial to disease management, offering proper treatment, and helping patients to seek novel targets for the maintenance and contributors of chronic pain. The main aim of this review is to focus on every possible mechanism of pain related to endometriosis in both peripheral and central nervous systems, and to present related mechanisms of action from the interaction between peripheral lesions and nerves to the changes in transmission of pain, resulting in hyperalgesia and the corresponding alterations in cerebral cortex and brain metabolism.

Greater interruption of visual processing and memory encoding by visceral than somatic pain in healthy volunteers - An fMRI study

Visceral pain is regarded as more salient than somatic pain. It has greater affective and emotional components, i.e., it elicits higher levels of pain-related fear and is perceived as more unpleasant than somatic pain. In this fMRI study, we examined the neural effects of painful visceral as compared to painful somatic stimulation on visual processing and memory encoding in a visual categorization and surprise recognition task in healthy volunteers. During the categorization task, participants received either rectal distensions or heat stimuli applied to the forearm, with stimuli being individually matched for unpleasantness. Behaviorally, visceral pain reduced memory encoding as compared to somatic pain (Kleine-Borgmann et al., 2021). Imaging analyses now revealed that visceral pain was associated with reduced activity (i.e., greater pain-related interruption) in neural areas typically involved in visual processing and memory encoding. These include the parahippocampal gyrus, fusiform gyrus, striatum, occipital cortex, insula, and the amygdala. Moreover, reduced engagement of the lateral occipital complex during visual categorization under visceral pain was associated with higher visceral pain-related fear. These findings obtained in healthy volunteers shed light on the neural circuitry underlying the interruptive effect of visceral pain and pave the way for future studies in patient samples.

A syngeneic inoculation mouse model of endometriosis that develops multiple comorbid visceral and cutaneous pain like behaviours

ABSTRACT

Endometriosis is a chronic and debilitating condition, commonly characterised by chronic pelvic pain (CPP) and infertility. Chronic pelvic pain can be experienced across multiple pelvic organs, with comorbidities commonly effecting the bowel, bladder, and vagina. Despite research efforts into endometriosis pathophysiology, little is known about how endometriosis induces CPP, and as such, therapeutic interventions are lacking. The aim of this study was to characterise a syngeneic mouse model of endometriosis that mimics naturally occurring retrograde menstruation, thought to precede endometriosis development in patients, and determine whether these mice exhibit signs of CPP and altered behaviour. We characterised the development of endometriosis over 10 weeks following uterine tissue inoculation, measured in vivo and ex vivo hypersensitivity to mechanical stimuli across multiple visceral organs, and assessed alterations in animal spontaneous behaviour. We confirmed that inoculated uterine horn tissue formed into endometriosis lesions throughout the peritoneal cavity, with significant growth by 8 to 10 weeks post inoculation. Additionally, we found that mice with fully developed endometriosis displayed hypersensitivity evoked by (1) vaginal distension, (2) colorectal distension, (3) bladder distension, and (4) cutaneous thermal stimulation, compared to their sham counterparts. Moreover, endometriosis mice displayed alterations in spontaneous behaviour indicative of (5) altered bladder function and (6) anxiety. This model creates a foundation for mechanistical studies into the diffuse CPP associated with endometriosis and the development of targeted therapeutic interventions to improve the quality of life of women with endometriosis.

Urinary Tract Infection in Overactive Bladder: An Update on Pathophysiological Mechanisms

Overactive bladder (OAB) is a clinical syndrome defined by urinary urgency, increased daytime urinary frequency and/or nocturia, with or without urinary incontinence, that affects approximately 11% of the western population. OAB is accepted as an idiopathic disorder, and is charactersied clinically in the absence of other organic diseases, including urinary tract infection. Despite this, a growing body of research provides evidence that a significant proportion of OAB patients have active bladder infection. This review discusses the key findings of recent laboratory and clinical studies, providing insight into the relationship between urinary tract infection, bladder inflammation, and the pathophysiology of OAB. We summarise an array of clinical studies that find OAB patients are significantly more likely than control patients to have pathogenic bacteria in their urine and increased bladder inflammation. This review reveals the complex nature of OAB, and highlights key laboratory studies that have begun to unravel how urinary tract infection and bladder inflammation can induce urinary urgency and urinary frequency. The evidence presented in this review supports the concept that urinary tract infection may be an underappreciated contributor to the pathophysiology of some OAB patients.

Circulating Pro-inflammatory Cytokines Do Not Explain Interindividual Variability in Visceral Sensitivity in Healthy Individuals

A role of the immune system in the pathophysiology of pain and hyperalgesia has received growing attention, especially in the context of visceral pain and the gut-brain axis. While acute experimental inflammation can induce visceral hyperalgesia as part of sickness behavior in healthy individuals, it remains unclear if normal plasma levels of circulating pro-inflammatory cytokines contribute to interindividual variability in visceral sensitivity. We herein compiled data from a tightly screened and well-characterized sample of healthy volunteers (N = 98) allowing us to assess associations between visceral sensitivity and gastrointestinal symptoms, and plasma concentrations of three selected pro-inflammatory cytokines (i.e., TNF-α, IL-6, and IL-8), along with cortisol and stress-related psychological variables. For analyses, we compared subgroups created to have distinct pro-inflammatory cytokine profiles, modelling healthy individuals at putative risk or resilience, respectively, for symptoms of the gut-brain axis, and compared them with respect to rectal sensory and pain thresholds and subclinical GI symptoms. Secondly, we computed multiple regression analyses to test if circulating pro-inflammatory markers predict visceral sensitivity in the whole sample. Despite pronounced subgroup differences in pro-inflammatory cytokine and cortisol concentrations, we observed no differences in measures of visceroception. In regression analyses, cytokines did not emerge as predictors. The pain threshold was predicted by emotional state and trait variables, especially state anxiety, together explaining 10.9% of the variance. These negative results do not support the hypothesis that systemic cytokine levels contribute to normal interindividual variability in visceroception in healthy individuals. Trajectories to visceral hyperalgesia as key marker in disorders of gut-brain interactions likely involve complex interactions of biological and psychological factors in keeping with a psychosocial model. Normal variations in systemic cytokines do not appear to constitute a vulnerability factor in otherwise healthy individuals, calling for prospective studies in at risk populations.

Sensitisation of colonic nociceptors by TNFα is dependent on TNFR1 expression and p38 MAPK activity

Abstract

Visceral pain is a leading cause of morbidity in gastrointestinal diseases, which is exacerbated by the gut‐related side‐effects of many analgesics. New treatments are needed and further understanding of the mediators and mechanisms underpinning visceral nociception in disease states is required to facilitate this. The pro‐inflammatory cytokine TNFα is linked to pain in both patients with inflammatory bowel disease and irritable bowel syndrome, and has been shown to sensitize colonic sensory neurons. Somatic, TNFα‐triggered thermal and mechanical hypersensitivity is mediated by TRPV1 signalling and p38 MAPK activity respectively, downstream of TNFR1 receptor activation. We therefore hypothesized that TNFR1‐evoked p38 MAPK activity may also be responsible for TNFα sensitization of colonic afferent responses to the TRPV1 agonist capsaicin, and noxious distension of the bowel. Using Ca²⁺ imaging of dorsal root ganglion sensory neurons, we observed TNFα‐mediated increases in intracellular [Ca²⁺] and sensitization of capsaicin responses. The sensitizing effects of TNFα were dependent on TNFR1 expression and attenuated by p38 MAPK inhibition. Consistent with these findings, ex vivo colonic afferent fibre recordings demonstrated an enhanced response to noxious ramp distention of the bowel and bath application of capsaicin following TNFα pre‐treatment. Responses were reversed by p38 MAPK inhibition and absent in tissue from TNFR1 knockout mice. Our findings demonstrate a contribution of TNFR1, p38 MAPK and TRPV1 to TNFα‐induced sensitization of colonic afferents, highlighting the potential utility of these drug targets for the treatment of visceral pain in gastrointestinal disease.

Key points

The pro‐inflammatory cytokine TNFα is elevated in gastrointestinal disease and sensitizes colonic afferents via modulation of TRPA1 and Na_V1.8 activity. We further develop this understanding by demonstrating a role for p38 MAPK and TRPV1 in TNFα‐mediated colonic afferent sensitization. Specifically, we show that:

TNFα sensitizes sensory neurons and colonic afferents to the TRPV1 agonist capsaicin.

TNFα‐mediated sensitization of sensory neurons and colonic nociceptors is dependent on TNFR1 expression.

TNFα sensitization of sensory neurons and colonic afferents to capsaicin and noxious ramp distension is abolished by inhibition of p38 MAPK.

Collectively these data support the utility of targeting TNFα, TNFR1 and their downstream signalling via p38 MAPK for the treatment of visceral pain in gastrointestinal disease.