Stimulation of neuronal receptors, neuropeptides and cytokines during experimental oil of mustard colitis

The Role of TRP Channels in Sepsis and Colitis

To date, several members of the transient receptor potential (TRP) channels which provide a wide array of roles have been found in the gastrointestinal tract (GI). The goal of earlier research was to comprehend the intricate signaling cascades that contribute to TRP channel activation as well as how these receptors' activity affects other systems. Moreover, there is a large volume of published studies describing the role of TRP channels in a number of pathological disorders, including inflammatory bowel disease (IBD) and sepsis. Nevertheless, the generalizability of these results is subject to certain limitations. For instance, the study of IBD relies on various animal models and experimental methods, which are unable to precisely imitate the multifactorial chronic disease. The diverse pathophysiological mechanisms and unique susceptibility of animals may account for the inconsistency of the experimental data collected. The main purpose of this study was to conduct a comprehensive review and analysis of existing studies on transient receptor potential (TRP) channels implicating specific models of colitis and sepsis, with particular emphasis on their involvement in pathological disorders such as IBD and sepsis. Furthermore, the text endeavors to evaluate the generalizability of experimental findings, taking into consideration the limitations posed by animal models and experimental methodologies. Finally, we also provide an updated schematic of the most important and possible molecular signaling pathways associated with TRP channels in IBD and sepsis.

The Prokineticin System in Inflammatory Bowel Diseases: A Clinical and Preclinical Overview

Inflammatory bowel disease (IBD) includes Crohn's disease (CD) and ulcerative colitis (UC), which are characterized by chronic inflammation of the gastrointestinal (GI) tract. IBDs clinical manifestations are heterogeneous and characterized by a chronic relapsing-remitting course. Typical gastrointestinal signs and symptoms include diarrhea, GI bleeding, weight loss, and abdominal pain. Moreover, the presence of pain often manifests in the remitting disease phase. As a result, patients report a further reduction in life quality. Despite the scientific advances implemented in the last two decades and the therapies aimed at inducing or maintaining IBDs in a remissive condition, to date, their pathophysiology still remains unknown. In this scenario, the importance of identifying a common and effective therapeutic target for both digestive symptoms and pain remains a priority. Recent clinical and preclinical studies have reported the prokineticin system (PKS) as an emerging therapeutic target for IBDs. PKS alterations are likely to play a role in IBDs at multiple levels, such as in intestinal motility, local inflammation, ulceration processes, localized abdominal and visceral pain, as well as central nervous system sensitization, leading to the development of chronic and widespread pain. This narrative review summarized the evidence about the involvement of the PKS in IBD and discussed its potential as a druggable target.

Characterization of prokineticin system in Crohn's disease pathophysiology and pain, and its modulation by alcohol abuse: A preclinical study

BACKGROUND

Crohn's disease-(CD) pathogenesis is still unknown and chronic pain is a frequent symptom in CD-patients. Identifying novel therapeutic targets and predisposing factors is a primary goal. In this regard, prokineticin system-(PKS) appears a promising target.

AIMS AND METHODS

TNBS-model was used. DAI, abdominal and visceral pain, and muscle strength were monitored. CD-mice were sacrificed at two times (day 7 and 14 after TNBS) in order to identify PKS involvement in CD pathophysiology and pain. PKS characterization was performed in mesenteric lymph nodes-(MLN), colon, myenteric plexus-(MP), dorsal root ganglia-(DRGs) and spinal cord-(SC). Inflammation/neuroinflammation was also assessed in the same tissues. In order to evaluate alcohol abuse as a possible trigger for CD and its effect on PKS activation, naïve mice were administered (oral-gavage) with ethanol for 10 consecutive days. PKS as well as inflammation/neuroinflammation were evaluated in MLN, colon and MP.

RESULTS

TNBS treated-mice showed a rapid increase in DAI, abdominal/visceral hypersensitivity and a progressive strength loss. In all tissue analysed of CD-mice, a quick and significant increase of mRNA of PKs and PKRs was observed, associated with an increase of pro-inflammatory cytokines (IL-1β, IL-6 and TNFα) and macrophage/glia markers (iba1, CD11b and GFAP) levels. In alcohol abuse model, ethanol induced in colon and MP a significant PKS activation accompanied by inflammation/neuroinflammation.

CONCLUSIONS

We can assume that PKS may be involved in CD development and pain. Furthermore, alcohol appears to activate PKS and may be a trigger factor for CD.

Modification of the TRP Channel TRPA1 as a Relevant Factor in Migraine-Related Intracranial Hypersensitivity

Recently, the transient receptor potential ankyrin 1 (TRPA1) has gained more attention in migraine-related research. The involvement of the TRPA1 receptor in migraine headaches is proposed by the fact that TRPA1 may be a target of some migraine-triggering factors. Although it is doubtful that activation of TRPA1 alone is sufficient to induce pain, behavioral studies have demonstrated that TRPA1 is involved in injury- and inflammation-induced hypersensitivity. Here, we review the functional relevance of TRPA1 in headaches and its therapeutic potential, mainly focusing on its role in the development of hypersensitivity, referring to its altered expression in pathological conditions, and its functional interaction with other TRP channels.

Current overview of opioids in progression of inflammatory bowel disease; pharmacological and clinical considerations

Inflammatory bowel diseases (IBD) belong to a subgroup of persistent, long-term, progressive, and relapsing inflammatory conditions. IBD may spontaneously develop in the colon, resulting in tumor lesions in inflamed regions of the intestine, such as invasive carcinoma. The benefit of opioids for IBD treatment is still questionable, thereby we investigated databases to provide an overview in this context. This review demonstrates the controversial role of opioids in IBD therapy, their physiological and pharmacological functions in attenuating the IBD symptoms, and in improving inflammatory, oxidative stress, and the quality of life factors in IBD subjects. Data were extracted from clinical, in vitro, and in vivo studies in English, between 1995 and 2019, from PubMed, Google Scholar, Scopus, and Cochrane library. Based on recent reports, there are promising opportunities to target the opioid system and control the IBD symptoms. This study suggests a novel approach for future treatment of functional and inflammatory disorders such as IBD.

Supraspinal Mechanisms of Intestinal Hypersensitivity

Gut inflammation or injury causes intestinal hypersensitivity (IHS) and hyperalgesia, which can persist after the initiating pathology resolves, are often referred to somatic regions and exacerbated by psychological stress, anxiety or depression, suggesting the involvement of both the spinal cord and the brain. The supraspinal mechanisms of IHS remain to be fully elucidated, however, over the last decades the series of intestinal pathology-associated neuroplastic changes in the brain has been revealed, being potentially responsible for the phenomenon. This paper reviews current clinical and experimental data, including the authors' own findings, on these functional, structural, and neurochemical/molecular changes within cortical, subcortical and brainstem regions processing and modulating sensory signals from the gut. As concluded in the review, IHS can develop and maintain due to the bowel inflammation/injury-induced persistent hyperexcitability of viscerosensory brainstem and thalamic nuclei and sensitization of hypothalamic, amygdala, hippocampal, anterior insular, and anterior cingulate cortical areas implicated in the neuroendocrine, emotional and cognitive modulation of visceral sensation and pain. An additional contribution may come from the pathology-triggered dysfunction of the brainstem structures inhibiting nociception. The mechanism underlying IHS-associated regional hyperexcitability is enhanced NMDA-, AMPA- and group I metabotropic receptor-mediated glutamatergic neurotransmission in association with altered neuropeptide Y, corticotropin-releasing factor, and cannabinoid 1 receptor signaling. These alterations are at least partially mediated by brain microglia and local production of cytokines, especially tumor necrosis factor α. Studying the IHS-related brain neuroplasticity in greater depth may enable the development of new therapeutic approaches against chronic abdominal pain in inflammatory bowel disease.

Role of TRP Channels in Shaping the Gut Microbiome

Transient receptor potential (TRP) channel family proteins are sensors for pain, which sense a variety of thermal and noxious chemicals. Sensory neurons innervating the gut abundantly express TRPA1 and TRPV1 channels and are in close proximity of gut microbes. Emerging evidence indicates a bi-directional gut–brain cross-talk in several entero-neuronal pathologies; however, the direct evidence of TRP channels interacting with gut microbial populations is lacking. Herein, we examine whether and how the knockout (KO) of TRPA1 and TRPV1 channels individually or combined TRPA1/V1 double-knockout (dKO) impacts the gut microbiome in mice. We detect distinct microbiome clusters among the three KO mouse models versus wild-type (WT) mice. All three TRP-KO models have reduced microbial diversity, harbor higher abundance of Bacteroidetes, and a reduced proportion of Firmicutes. Specifically distinct arrays in the KO models are determined mainly by S24-7, Bacteroidaceae, Clostridiales, Prevotellaceae, Helicobacteriaceae, Rikenellaceae, and Ruminococcaceae. A1KO mice have lower Prevotella, Desulfovibrio, Bacteroides, Helicobacter and higher Rikenellaceae and Tenericutes; V1KO mice demonstrate higher Ruminococcaceae, Lachnospiraceae, Ruminococcus, Desulfovibrio and Mucispirillum; and A1V1dKO mice exhibit higher Bacteroidetes, Bacteroides and S24-7 and lower Firmicutes, Ruminococcaceae, Oscillospira, Lactobacillus and Sutterella abundance. Furthermore, the abundance of taxa involved in biosynthesis of lipids and primary and secondary bile acids is higher while that of fatty acid biosynthesis-associated taxa is lower in all KO groups. To our knowledge, this is the first study demonstrating distinct gut microbiome signatures in TRPA1, V1 and dKO models and should facilitate prospective studies exploring novel diagnostic/ therapeutic modalities regarding the pathophysiology of TRP channel proteins.

Transient Receptor Potential Channels and Inflammatory Bowel Disease

The transient receptor potential (TRP) cation channels are present in abundance across the gastrointestinal (GI) tract, serving as detectors for a variety of stimuli and secondary transducers for G-protein coupled receptors. The activation of TRP channels triggers neurogenic inflammation with related neuropeptides and initiates immune reactions by extra-neuronally regulating immune cells, contributing to the GI homeostasis. However, under pathological conditions, such as inflammatory bowel disease (IBD), TRP channels are involved in intestinal inflammation. An increasing number of human and animal studies have indicated that TRP channels are correlated to the visceral hypersensitivity (VHS) and immune pathogenesis in IBD, leading to an exacerbation or amelioration of the VHS or intestinal inflammation. Thus, TRP channels are a promising target for novel therapeutic methods for IBD. In this review, we comprehensively summarize the functions of TRP channels, especially their potential roles in immunity and IBD. Additionally, we discuss the contradictory findings of prior studies and offer new insights with regard to future research.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

The Secretomes of Painful Versus Nonpainful Human Schwannomatosis Tumor Cells Differentially Influence Sensory Neuron Gene Expression and Sensitivity

Schwannomatosis is a multiple tumor syndrome in which patients develop benign tumors along peripheral nerves throughout the body. The first symptom with which schwannomatosis patients often present, prior to discovery of tumors, is pain. This pain can be debilitating and is often inadequately alleviated by pharmacological approaches. Schwannomatosis-associated pain can be localized to the area of a tumor, or widespread. Moreover, not all tumors are painful, and the occurrence of pain is often unrelated to tumor size or location. We speculate that some individual tumors, but not others, secrete factors that act on nearby nerves to augment nociception by producing neuronal sensitization or spontaneous neuronal firing. We created cell lines from human SWN tumors with varying degrees of pain. We have found that conditioned medium (CM) collected from painful SWN tumors, but not that from nonpainful SWN tumors, sensitized DRG neurons, causing increased sensitivity to depolarization by KCl, increased response to noxious TRPV1 and TRPA1 agonists and also upregulated the expression of pain-associated genes in DRG cultures. Multiple cytokines were also detected at higher levels in CM from painful tumors. Taken together our data demonstrate a differential ability of painful versus non-painful human schwannomatosis tumor cells to secrete factors that augment sensory neuron responsiveness, and thus identify a potential determinant of pain heterogeneity in schwannomatosis.

Expression and Function of Transient Receptor Potential Ankyrin 1 Ion Channels in the Caudal Nucleus of the Solitary Tract

The nucleus of the solitary tract (NTS) receives visceral information via the solitary tract (ST) that comprises the sensory components of the cranial nerves VII, IX and X. The Transient Receptor Potential Ankyrin 1 (TRPA1) ion channels are non-selective cation channels that are expressed primarily in pain-related sensory neurons and nerve fibers. Thus, TRPA1 expressed in the primary sensory afferents may modulate the function of second order NTS neurons. This hypothesis was tested and confirmed in the present study using acute brainstem slices and caudal NTS neurons by RT-PCR, immunostaining and patch-clamp electrophysiology. The expression of TRPA1 was detected in presynaptic locations, but not the somata of caudal NTS neurons that did not express TRPA1 mRNA or proteins. Moreover, caudal NTS neurons did not show somatodendritic responsiveness to TRPA1 agonists, while TRPA1 immunostaining was detected only in the afferent fibers. Electrophysiological recordings detected activation of presynaptic TRPA1 in glutamatergic terminals synapsing on caudal NTS neurons evidenced by the enhanced glutamatergic synaptic neurotransmission in the presence of TRPA1 agonists. The requirement of TRPA1 for modulation of spontaneous synaptic activity was confirmed using TRPA1 knockout mice where TRPA1 agonists failed to alter synaptic efficacy. Thus, this study provides the first evidence of the TRPA1-dependent modulation of the primary afferent inputs to the caudal NTS. These results suggest that the second order caudal NTS neurons act as a TRPA1-dependent interface for visceral noxious-innocuous integration at the level of the caudal brainstem.

TRP channels in cardiac and intestinal fibrosis

It is now widely accepted that advanced fibrosis underlies many chronic inflammatory disorders and is the main cause of morbidity and mortality of the modern world. The pathogenic mechanism of advanced fibrosis involves diverse and intricate interplays between numerous extracellular and intracellular signaling molecules, among which the non-trivial roles of a stress-responsive Ca²⁺/Na⁺-permeable cation channel superfamily, the transient receptor potential (TRP) protein, are receiving growing attention. Available evidence suggests that several TRP channels such as TRPC3, TRPC6, TRPV1, TRPV3, TRPV4, TRPA1, TRPM6 and TRPM7 may play central roles in the progression and/or prevention of fibroproliferative disorders in vital visceral organs such as lung, heart, liver, kidney, and bowel as well as brain, blood vessels and skin, and may contribute to both acute and chronic inflammatory processes involved therein. This short paper overviews the current knowledge accumulated in this rapidly growing field, with particular focus on cardiac and intestinal fibrosis, which are tightly associated with the pathogenesis of atrial fibrillation and inflammatory bowel diseases such as Crohn's disease.

Inflammation-associated changes in DOR expression and function in the mouse colon

Endogenous opioids activate opioid receptors (ORs) in the enteric nervous system to control intestinal motility and secretion. The μ-OR mediates the deleterious side effects of opioid analgesics, including constipation, respiratory depression, and addiction. Although the δ-OR (DOR) is a promising target for analgesia, the function and regulation of DOR in the colon are poorly understood. This study provides evidence that endogenous opioids activate DOR in myenteric neurons that may regulate colonic motility. The DOR agonists DADLE, deltorphin II, and SNC80 inhibited electrically evoked contractions and induced neurogenic contractions in the mouse colon. Electrical, chemical, and mechanical stimulation of the colon evoked the release of endogenous opioids, which stimulated endocytosis of DOR in the soma and proximal neurites of myenteric neurons of transgenic mice expressing DOR fused to enhanced green fluorescent protein. In contrast, DOR was not internalized in nerve fibers within the circular muscle. Administration of dextran sulfate sodium induced acute colitis, which was accompanied by DOR endocytosis and an increased density of DOR-positive nerve fibers within the circular muscle. The potency with which SNC80 inhibited neurogenic contractions was significantly enhanced in the inflamed colon. This study demonstrates that DOR-expressing neurons in the mouse colon can be activated by exogenous and endogenous opioids. Activated DOR traffics to endosomes and inhibits neurogenic motility of the colon. DOR signaling is enhanced during intestinal inflammation. This study demonstrates functional expression of DOR by myenteric neurons and supports the therapeutic targeting of DOR in the enteric nervous system. NEW & NOTEWORTHY DOR is activated during physiologically relevant reflex stimulation. Agonist-evoked DOR endocytosis is spatially and temporally regulated. A significant proportion of DOR is internalized in myenteric neurons during inflammation. The relative proportion of all myenteric neurons that expressed DOR and the overlap with the nNOS-positive population are increased in inflammation. DOR-specific innervation of the circular muscle is increased in inflammation, and this is consistent with enhanced responsiveness to the DOR agonist SNC80.

A historical perspective on the role of sensory nerves in neurogenic inflammation

The term ‘neurogenic inflammation’ is commonly used, especially with respect to the role of sensory nerves within inflammatory disease. However, despite over a century of research, we remain unclear about the role of these nerves in the vascular biology of inflammation, as compared with their interacting role in pain processing and of their potential for therapeutic manipulation. This chapter attempts to discuss the progress in understanding, from the initial discovery of sensory nerves until the present day. This covers pioneering findings that these nerves exist, are involved in vascular events and act as important sensors of environmental changes, including injury and infection. This is followed by discovery of the contents they release such as the established vasoactive neuropeptides substance P and CGRP as well as anti-inflammatory peptides such as the opioids and somatostatin. The more recent emergence of the importance of the transient receptor potential (TRP) channels has revealed some of the mechanisms by which these nerves sense environmental stimuli. This knowledge enables a platform from which to learn of the potential role of neurogenic inflammation in disease and in turn of novel therapeutic targets.

Regulation of Pain and Itch by TRP Channels

Nociception is an important physiological process that detects harmful signals and results in pain perception. In this review, we discuss important experimental evidence involving some TRP ion channels as molecular sensors of chemical, thermal, and mechanical noxious stimuli to evoke the pain and itch sensations. Among them are the TRPA1 channel, members of the vanilloid subfamily (TRPV1, TRPV3, and TRPV4), and finally members of the melastatin group (TRPM2, TRPM3, and TRPM8). Given that pain and itch are pro-survival, evolutionarily-honed protective mechanisms, care has to be exercised when developing inhibitory/modulatory compounds targeting specific pain/itch-TRPs so that physiological protective mechanisms are not disabled to a degree that stimulus-mediated injury can occur. Such events have impeded the development of safe and effective TRPV1-modulating compounds and have diverted substantial resources. A beneficial outcome can be readily accomplished via simple dosing strategies, and also by incorporating medicinal chemistry design features during compound design and synthesis. Beyond clinical use, where compounds that target more than one channel might have a place and possibly have advantageous features, highly specific and high-potency compounds will be helpful in mechanistic discovery at the structure-function level.

Transient receptor potential ion channel function in sensory transduction and cellular signaling cascades underlying visceral hypersensitivity

Visceral hypersensitivity is an important mechanism underlying increased abdominal pain perception in functional gastrointestinal disorders including functional dyspepsia, irritable bowel syndrome, and inflammatory bowel disease in remission. Although the exact pathophysiological mechanisms are poorly understood, recent studies described upregulation and altered functions of nociceptors and their signaling pathways in aberrant visceral nociception, in particular the transient receptor potential (TRP) channel family. A variety of TRP channels are present in the gastrointestinal tract (TRPV1, TRPV3, TRPV4, TRPA1, TRPM2, TRPM5, and TRPM8), and modulation of their function by increased activation or sensitization (decreased activation threshold) or altered expression in visceral afferents have been reported in visceral hypersensitivity. TRP channels directly detect or transduce osmotic, mechanical, thermal, and chemosensory stimuli. In addition, pro-inflammatory mediators released in tissue damage or inflammation can activate receptors of the G protein-coupled receptor superfamily leading to TRP channel sensitization and activation, which amplify pain and neurogenic inflammation. In this review, we highlight the present knowledge on the functional roles of neuronal TRP channels in visceral hypersensitivity and discuss the signaling pathways that underlie TRP channel modulation. We propose that a better understanding of TRP channels and their modulators may facilitate the development of more selective and effective therapies to treat visceral hypersensitivity.

Transient receptor potential ankyrin 1 agonists improve intestinal transit in a murine model of postoperative ileus

BACKGROUND

Stimulation of transient receptor potential ankyrin 1 (TRPA1), which abundantly expressed in enterochromaffin cells (ECC), has been reported to exert apparently contradictory results in in vitro contractility and in vivo gastrointestinal (GI) transit evaluations. The pharmaceutical-grade Japanese traditional medicine daikenchuto (TU-100) has been reported to be beneficial for postoperative ileus (POI) and accelerate GI transit in animals and humans. TU-100 was recently shown to increase intestinal blood flow via stimulation of TRPA1 in the epithelial cells of the small intestine (SI).

METHODS

The effects of various TRPA1 agonists on motility were examined in a manipulation-induced murine POI model, in vitro culture of SI segments and an ECC model cell line, RIN-14B.

KEY RESULTS

Orally administered TRPA1 agonists, aryl isothiocyanate (AITC) and cinnamaldehyde (CA), TU-100 ingredients, [6]-shogaol (6S) and γ-sanshool (GS), improved SI transit in a POI model. The effects of AITC, 6S and GS but not CA were abrogated in TRPA1-deficient mice. SI segments show periodic peristaltic motor activity whose periodicity disappeared in TRPA1-deficient mice. TU-100 augmented the motility. AITC, CA and 6S increased 5-HT release from isolated SI segments and the effects of all these compounds except for CA were lost in TRPA1-deficient mice. 6S and GS induced a release of 5-HT from RIN-14B cells in a dose- and TRPA1-dependent manner.

CONCLUSIONS & INFERENCES

Intraluminal TRPA1 stimulation is a potential therapeutic strategy for GI motility disorders. Further investigation is required to determine whether 5-HT and/or ECC are involved in the effect of TRPA1 on motility.

Mu and delta opioid receptor knockout mice show increased colonic sensitivity

BACKGROUND

Opiates act through opioid receptors to diminish pain. Here, we investigated whether mu (MOR) and delta (DOR) receptor endogenous activity assessed in the whole mouse body or in particular at peripheral receptors on primary nociceptive neurons, control colonic pain.

METHODS

We compared global MOR and DOR receptor knockout (KO) mice, mice with a conditional deletion of MOR and DOR in Nav1.8-positive nociceptive primary afferent neurons (cKO), and control floxed mice of both genders for visceral sensitivity. Visceromotor responses to colorectal distension (CRD) and macroscopic colon scores were recorded on naïve mice and mice with acute colitis induced by 3% dextran sodium sulphate (DSS) for 5 days. Transcript expression for opioid genes and cytokines was measured by quantitative RT-PCR.

RESULTS

Naïve MOR and DOR global KO mice show increased visceral sensitivity that was not observed in cKO mice. MOR and preproenkephalin (Penk) were the most expressed opioid genes in colon. MOR KO mice had augmented kappa opioid receptor and Tumour-Necrosis-Factor-α and diminished Penk transcript levels while DOR, preprodynorphin and Interleukin-1β were unchanged. Global MOR KO females had a thicker colon than floxed females. No alteration was detected in DOR mutant animals. A 5-day DSS treatment led to comparable hypersensitivity in the different mouse lines.

CONCLUSION

Our results suggest that mu and delta opioid receptor global endogenous activity but not activity at the peripheral Nav1.8 neurons contribute to visceral sensitivity in naïve mice, and that endogenous MOR and DOR tones were insufficient to elicit analgesia after 5-day DSS-induced colitis.

SIGNIFICANCE

Knockout mice for mu and delta opioid receptor have augmented colon sensitivity in the CRD assay. It shows endogenous mu and delta opioid analgesia that may be explored as potential targets for alleviating chronic intestinal pain.

Persistent alterations in colonic afferent innervation in a rat model of postinfectious gut dysfunction: Role for changes in peripheral neurotrophic factors

BACKGROUND

Visceral hypersensitivity in the inflamed gut is related partly to the effects of peripheral neurotrophic factors (NTFs) on local afferent neurons. However, alterations in sensory afferents of distant areas remain unexplored. Using the Trichinella spiralis infection model, which causes a jejunitis, we investigated the remodeling of colonic afferents and the potential role of NTFs.

METHODS

Rats were infected with T. spiralis. Inflammatory-like changes, mucosal mast cells (MMCs) dynamics, and expression of nerve growth factor and glial cell line-derived NTFs (glial cell-derived neurotrophic factor, artemin, and neurturin) were determined in the colon up to day 30 postinfection. Functional responses of colonic afferents were determined assessing changes in the expression of sensory-related markers in thoracolumbar (TL)/lumbosacral (LS) dorsal root ganglias (DRGs) following intracolonic capsaicin.

KEY RESULTS

Trichinella spiralis induced an inflammatory-like response within the colon, partly resolved at day 30 postinfection, except for a persistent MMC infiltrate. While the jejunum of infected animals showed an up-regulation in the expression of NTFs, a transitory down-regulation was observed in the colon. Overall, T. spiralis effects on DRGs gene expression were restricted to a transient down-regulation of TPRV1. Stimulation with intracolonic capsaicin induced a down-regulation of TRPV1 levels in TL and LS DRGs, an effect enhanced in LS DRGs of infected animals, regardless the postinfection time considered.

CONCLUSIONS & INFERENCES

During intestinal inflammation, spread morphological and functional alterations, including remodeling of visceral afferents, are observed outside the primary region affected by the insult. Similar mechanisms might be operating in states of widespread alterations of visceral sensitivity.

Preclinical Assessment of Inflammatory Pain

While acute inflammation is a natural physiological response to tissue injury or infection, chronic inflammation is maladaptive and engenders a considerable amount of adverse pain. The chemical mediators responsible for tissue inflammation act on nociceptive nerve endings to lower neuronal excitation threshold and sensitize afferent firing rate leading to the development of allodynia and hyperalgesia, respectively. Animal models have aided in our understanding of the pathophysiological mechanisms responsible for the generation of chronic inflammatory pain and allowed us to identify and validate numerous analgesic drug candidates. Here we review some of the commonly used models of skin, joint, and gut inflammatory pain along with their relative benefits and limitations. In addition, we describe and discuss several behavioral and electrophysiological approaches used to assess the inflammatory pain in these preclinical models. Despite significant advances having been made in this area, a gap still exists between fundamental research and the implementation of these findings into a clinical setting. As such we need to characterize inherent pathophysiological pathways and develop new endpoints in these animal models to improve their predictive value of human inflammatory diseases in order to design safer and more effective analgesics.

TRPM8 on mucosal sensory nerves regulates colitogenic responses by innate immune cells via CGRP

TRPM8 is the molecular sensor for cold; however, the physiological role of TRPM8+ neurons at mucosal surfaces is unclear. Here we evaluated the distribution and peptidergic properties of TRPM8+ fibers in naive and inflamed colons, as well as their role in mucosal inflammation. We found that Trpm8(-/-) mice were hypersusceptible to dextran sodium sulfate (DSS)-induced colitis, and that Trpm8(-/-) CD11c+ DCs (dendritic cells) showed hyperinflammatory responses to toll-like receptor (TLR) stimulation. This was phenocopied in calcitonin gene-related peptide (CGRP) receptor-deficient mice, but not in substance P receptor-deficient mice, suggesting a functional link between TRPM8 and CGRP. The DSS phenotype of CGRP receptor-deficient mice could be adoptively transferred to wild-type (WT) mice, suggesting that CGRP suppresses the colitogenic activity of bone marrow-derived cells. TRPM8+ mucosal fibers expressed CGRP in human and mouse colon. Furthermore, neuronal CGRP contents were increased in colons from naive and DSS-treated Trpm8(-/-) mice, suggesting deficient CGRP release in the absence of TRPM8 triggering. Finally, treatment of Trpm8(-/-) mice with CGRP reversed their hyperinflammatory phenotype. These results suggest that TRPM8 signaling in mucosal sensory neurons is indispensable for the regulation of innate inflammatory responses via the neuropeptide CGRP.

Suspended moxibustion at Tianshu (ST25) inhibits prokineticin 1 and prokineticin receptor 1 expression in the spinal cord of rats with chronic visceral hypersensitivity

Suspended moxibustion can decrease the expression of prokineticin 1 and its receptor in colonic tissue from rats modeling chronic visceral hyperalgesia. This study aimed to verify if rat spinal cord prokineticin 1 and its receptor contribute to the analgesic effect of suspended moxibustion in a rat model of irritable bowel syndrome where rats display chronic visceral hypersensitivity. Results showed that suspended moxibustion at Tianshu (ST25) point significantly decreased visceral sensitivity to colorectal distention in a chronic visceral hyperalgesia rat model; also protein and mRNA expression of prokineticin 1 and prokineticin receptor 1 in the spinal cord of rats was significantly decreased. Experimental findings indicate that prokineticin 1 and prokineticin receptor 1 are involved in the analgesia using suspended moxibustion in rats with chronic visceral hyperalgesia.

Role of transient receptor potential channels in intestinal inflammation and visceral pain: novel targets in inflammatory bowel diseases

Transient receptor potential (TRP) channels are a large group of ion channels that are prevalent in mammalian tissues. They are widely distributed in the central and peripheral nervous systems, and in nonneuronal cells, where they are implicated in sensing temperature, noxious substances, and pain. TRPs play an important role in immune response and nociception and, therefore, may be involved in the pathogenesis of inflammatory bowel diseases, whose major symptoms include chronic inflammatory state and abdominal pain. In this review, we summarize what is known on TRP channels in inflammatory bowel disease and visceral pain; we focus in particular on TRPV1, TRPV4, TRPA1, and TRPM. We also analyze scientific reports that evidence potential use of TRP regulators in future inflammatory bowel disease treatment.

Upregulation of the transient receptor potential ankyrin 1 ion channel in the inflamed human and mouse colon and its protective roles

Transient Receptor Potential Ankyrin 1 (TRPA1) channels are localized on sensory nerves and several non-neural cells, but data on their functional significance are contradictory. We analysed the presence and alterations of TRPA1 in comparison with TRP Vanilloid 1 (TRPV1) at mRNA and protein levels in human and mouse intact and inflamed colons. The role of TRPA1 in a colitis model was investigated using gene-deficient mice. TRPA1 and TRPV1 expressions were investigated in human colon biopsies of healthy subjects and patients with inflammatory bowel diseases (IBD: ulcerative colitis, Crohn's disease) with quantitative PCR and immunohistochemistry. Mouse colitis was induced by oral 2% dextran-sulphate (DSS) for 10 days. For investigating the functions of TRPA1, Disease Activity Index (weight loss, stool consistency, blood content) was determined in C57BL/6-based Trpa1-deficient (knockout: KO) and wildtype (WT) mice. Sensory neuropeptides, their receptors, and inflammatory cytokines/chemokines were determined with qPCR or Luminex. In human and mouse colons TRPA1 and TRPV1 are located on epithelial cells, macrophages, enteric ganglia. Significant upregulation of TRPA1 mRNA was detected in inflamed samples. In Trpa1 KO mice, Disease Activity Index was significantly higher compared to WTs. It could be explained by the greater levels of substance P, neurokinins A and B, neurokinin 1 receptor, pituitary adenylate-cyclase activating polypeptide, vasoactive intestinal polypeptide, and also interleukin-1beta, macrophage chemoattractant protein-1, monokine induced by gamma interferon-1, tumor necrosis factor-alpha and B-lymphocyte chemoattractant in the distal colon. TRPA1 is upregulated in colitis and its activation exerts protective roles by decreasing the expressions of several proinflammatory neuropeptides, cytokines and chemokines.

Localisation and activation of the neurokinin 1 receptor in the enteric nervous system of the mouse distal colon

The substance P neurokinin 1 receptor (NK1R) regulates motility, secretion, inflammation and pain in the intestine. The distribution of the NK1R is a key determinant of the functional effects of substance P in the gut. Information regarding the distribution of NK1R in subtypes of mouse enteric neurons is lacking and is the focus of the present study. NK1R immunoreactivity (NK1R-IR) is examined in whole-mount preparations of the mouse distal colon by indirect immunofluorescence and confocal microscopy. The distribution of NK1R-IR within key functional neuronal subclasses was determined by using established neurochemical markers. NK1R-IR was expressed by a subpopulation of myenteric and submucosal neurons; it was mainly detected in large multipolar myenteric neurons and was colocalized with calcitonin gene-related peptide, neurofilament M, choline acetyltransferase and calretinin. The remaining NK1R-immunoreactive neurons were positive for nitric oxide synthase. NK1R was expressed by most of the submucosal neurons and was exclusively co-expressed with vasoactive intestinal peptide, with no overlap with choline acetyltransferase. Treatment with substance P resulted in the concentration-dependent internalisation of NK1R from the cell surface into endosome-like structures. Myenteric NK1R was mainly expressed by intrinsic primary afferent neurons, with minor expression by descending interneurons and inhibitory motor neurons. Submucosal NK1R was restricted to non-cholinergic secretomotor neurons. These findings highlight key differences in the neuronal distribution of NK1R-IR between the mouse, rat and guinea-pig, with important implications for the functional role of NK1R in regulating intestinal motility and secretion.

Modulation of gastrointestinal function by MuDelta, a mixed µ opioid receptor agonist/ µ opioid receptor antagonist

BACKGROUND & PURPOSE

Loperamide is a selective µ opioid receptor agonist acting locally in the gastrointestinal (GI) tract as an effective anti-diarrhoeal but can cause constipation. We tested whether modulating µ opioid receptor agonism with δ opioid receptor antagonism, by combining reference compounds or using a novel compound ('MuDelta'), could normalize GI motility without constipation.

EXPERIMENTAL APPROACH

MuDelta was characterized in vitro as a potent µ opioid receptor agonist and high-affinity δ opioid receptor antagonist. Reference compounds, MuDelta and loperamide were assessed in the following ex vivo and in vivo experiments: guinea pig intestinal smooth muscle contractility, mouse intestinal epithelial ion transport and upper GI tract transit, entire GI transit or faecal output in novel environment stressed mice, or four weeks after intracolonic mustard oil (post-inflammatory). Colonic δ opioid receptor immunoreactivity was quantified.

KEY RESULTS

δ Opioid receptor antagonism opposed µ opioid receptor agonist inhibition of intestinal contractility and motility. MuDelta reduced intestinal contractility and inhibited neurogenically-mediated secretion. Very low plasma levels of MuDelta were detected after oral administration. Stress up-regulated δ opioid receptor expression in colonic epithelial cells. In stressed mice, MuDelta normalized GI transit and faecal output to control levels over a wide dose range, whereas loperamide had a narrow dose range. MuDelta and loperamide reduced upper GI transit in the post-inflammatory model.

CONCLUSIONS AND IMPLICATIONS

MuDelta normalizes, but does not prevent, perturbed GI transit over a wide dose-range in mice. These data support the subsequent assessment of MuDelta in a clinical phase II trial in patients with diarrhoea-predominant irritable bowel syndrome.

Increased prokineticin 2 expression in gut inflammation: role in visceral pain and intestinal ion transport

BACKGROUND

Prokineticin 2 (PROK2) is an inflammatory cytokine-like molecule expressed predominantly by macrophages and neutrophils infiltrating sites of tissue damage. Given the established role of prokineticin signaling on gastrointestinal function, we have explored Prok2 gene expression in inflammatory conditions of the gastrointestinal tract and assessed the possible consequences on gut physiology.

METHODS

Prokineticin expression was examined in normal and colitic tissues using qPCR and immunohistochemistry. Functional responses to PROK2 were studied using calcium imaging and a novel antagonist, Compound 3, used to determine the role of PROK2 and prokineticin receptors in inflammatory visceral pain and ion transport.

KEY RESULTS

Prok2 gene expression was up-regulated in biopsy samples from ulcerative colitis patients, and similar elevations were observed in rodent models of inflammatory colitis. Prokineticin receptor 1 (PKR1) was localized to the enteric neurons and extrinsic sensory neurons, whereas Pkr2 expression was restricted to sensory ganglia. In rats, PROK2-increased intracellular calcium levels in cultured enteric and dorsal root ganglia neurons, which was blocked by Compound 3. Moreover, PROK2 acting at prokineticin receptors stimulated intrinsic neuronally mediated ion transport in rat ileal mucosa. In vivo, Compound 3 reversed intracolonic mustard oil-induced referred allodynia and TNBS-induced visceral hypersensitivity, but not non-inflammatory, stress-induced visceral pain.

CONCLUSIONS & INFERENCES

Elevated Prok2 levels, as a consequence of gastrointestinal tract inflammation, induce visceral pain via prokineticin receptors. This observation, together with the finding that PROK2 can modulate intestinal ion transport, raises the possibility that inhibitors of PROK2 signaling may have clinical utility in gastrointestinal disorders, such as irritable bowel syndrome and inflammatory bowel disease.

The role of TRPA1 in visceral inflammation and pain

Despite significant progress in our understanding of the cellular and molecular mechanisms underlying sensory transduction and nociception, clinical pain management remains a considerable challenge in health care and basic research. The identification of the superfamily of transient receptor potential (TRP) cation channels, particularly TRPV1 and TRPA1, has shed light on the molecular basis of pain signaling during inflammatory conditions. TRPV1 and TRPA1 are considered as potential targets in the treatment of inflammatory pain because of their ability to be activated by nociceptive signals and sensitized by pro-inflammatory mediators. Notably, TRPA1 is expressed in visceral afferent neurons and is known to participate in inflammatory responses and the establishment of hypersensitivity. This review summarizes the current knowledge of the role of TRPA1 in sensory transduction, particularly in the context of visceral inflammation and pain in the gastrointestinal and urinary tracts.

Genetic evidence for an essential role of neuronally expressed IL-6 signal transducer gp130 in the induction and maintenance of experimentally induced mechanical hypersensitivity in vivo and in vitro

Tenderness and mechanical allodynia are key symptoms of malignant tumor, inflammation and neuropathy. The proinflammatory cytokine interleukin-6 (IL-6) is causally involved in all three pathologies. IL-6 not only regulates innate immunity and inflammation but also causes nociceptor sensitization and hyperalgesia. In general and in most cell types including immune cells and sensory neurons, IL-6 binds soluble μ receptor subunits which heteromerizes with membrane bound IL-6 signal transducer gp130. In the present study, we used a conditional knock-out strategy to investigate the importance of signal transducer gp130 expressed in C nociceptors for the generation and maintenance of mechanical hypersensitivity. Nociceptors were sensitized to mechanical stimuli by experimental tumor and this nociceptor sensitization was preserved at later stages of the pathology in control mice. However, in mice with a conditional deletion of gp130 in Nav1.8 expressing nociceptors mechanical hypersensitivity by experimental tumor, nerve injury or inflammation recovery was not preserved in the maintenance phase and nociceptors exhibited normal mechanical thresholds comparable to untreated mice. Together, the results argue for IL-6 signal transducer gp130 as an essential prerequisite in nociceptors for long-term mechanical hypersensitivity associated with cancer, inflammation and nerve injury.

Transient receptor potential ankyrin 1 is expressed by inhibitory motoneurons of the mouse intestine

BACKGROUND & AIMS

Transient receptor potential ankyrin (TRPA) 1, an excitatory ion channel expressed by sensory neurons, mediates somatic and visceral pain in response to direct activation or noxious mechanical stimulation. Although the intestine is routinely exposed to irritant alimentary compounds and inflammatory mediators that activate TRPA1, there is no direct evidence for functional TRPA1 receptors on enteric neurons, and the effects of TRPA1 activation on intestinal function have not been determined. We characterized expression of TRPA1 by enteric neurons and determined its involvement in the control of intestinal contractility and transit.

METHODS

TRPA1 expression was characterized by reverse-transcription polymerase chain reaction and immunofluorescence analyses. TRPA1 function was examined by Ca(2+) imaging and by assays of contractile activity and transit.

RESULTS

We detected TRPA1 messenger RNA in the mouse intestine and TRPA1 immunoreactivity in enteric neurons. The cecum and colon had immunoreactivity for neuronal TRPA1, but the duodenum did not. TRPA1 immunoreactivity was also detected in inhibitory motoneurons and descending interneurons, cholinergic neurons, and intrinsic primary afferent neurons. TRPA1 activators, including cinnamaldehyde, allyl isothiocyanate (AITC), and 4-hydroxynonenal, increased [Ca(2+)](i) in myenteric neurons. These were reduced by a TRPA1 antagonist (HC-030031) or deletion of Trpa1. TRPA1 activation inhibited contractility of the segments of colon but not stomach or small intestine of Trpa1(+/+) but not Trpa1(-/-) mice; this effect was reduced by tetrodotoxin or N(G)-nitro-l-arginine methyl ester. Administration of AITC by gavage did not alter gastric emptying or small intestinal transit, but luminal AITC inhibited colonic transit via TRPA1.

CONCLUSIONS

Functional TRPA1 is expressed by enteric neurons, and activation of neuronal TRPA1 inhibits spontaneous neurogenic contractions and transit of the colon.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Small intestinal cannabinoid receptor changes following a single colonic insult with oil of mustard in mice

Cannabinoids are known to be clinically beneficial for control of appetite disorders and nausea/vomiting, with emerging data that they can impact other GI disorders, such as inflammation. Post-inflammatory irritable bowel syndrome (PI-IBS) is a condition of perturbed intestinal function that occurs subsequent to earlier periods of intestinal inflammation. Cannabinoid 1 receptor (CB1R) and CB2R alterations in GI inflammation have been demonstrated in both animal models and clinically, but their continuing role in the post-inflammatory period has only been implicated to date. Therefore, to provide direct evidence for CBR involvement in altered GI functions in the absence of overt inflammation, we used a model of enhanced upper GI transit that persists for up to 4 weeks after a single insult by intracolonic 0.5% oil of mustard (OM) in mice. In mice administered OM, CB1R immunostaining in the myenteric plexus was reduced at day 7, when colonic inflammation is subsiding, and then increased at 28 days, compared to tissue from age-matched vehicle-treated mice. In the lamina propria CB2R immunostaining density was also increased at day 28. In mice tested 28 day after OM, either a CB1R-selective agonist, ACEA (1 and 3 mg/kg, s.c.) or a CB2R-selective agonist, JWH-133 (3 and 10 mg/kg, s.c.) reduced the enhanced small intestinal transit in a dose-related manner. Doses of ACEA and JWH-133 (1 mg/kg), alone or combined, reduced small intestinal transit of OM-treated mice to a greater extent than control mice. Thus, in this post-colonic inflammation model, both CBR subtypes are up-regulated and there is increased efficacy of both CB1R and CB2R agonists. We conclude that CBR remodeling occurs not only during GI inflammation but continues during the recovery phase. Thus, either CB1R- or CB2-selective agonists could be efficacious for modulating GI motility in individuals experiencing diarrhea-predominant PI-IBS.

Chemo-nociceptive signalling from the colon is enhanced by mild colitis and blocked by inhibition of transient receptor potential ankyrin 1 channels

BACKGROUND AND PURPOSE

Transient receptor potential ankyrin 1 (TRPA1) channels are expressed by primary afferent neurones and activated by irritant chemicals including allyl isothiocyanate (AITC). Here we investigated whether intracolonic AITC causes afferent input to the spinal cord and whether this response is modified by mild colitis, morphine or a TRPA1 channel blocker.

EXPERIMENTAL APPROACH

One hour after intracolonic administration of AITC to female mice, afferent signalling was visualized by expression of c-Fos in laminae I-II(o) of the spinal dorsal horn at sacral segment S1. Mild colitis was induced by dextran sulphate sodium (DSS) added to drinking water for 1 week.

KEY RESULTS

Relative to vehicle, AITC (2%) increased expression of c-Fos in the spinal cord. Following induction of mild colitis by DSS (2%), spinal c-Fos responses to AITC, but not vehicle, were augmented by 41%. Colonic inflammation was present (increased myeloperoxidase content and disease activity score), whereas colonic histology, locomotion, feeding and drinking remained unchanged. Morphine (10 mg.kg(-1)) or the TRPA1 channel blocker HC-030031 (300 mg.kg(-1)) inhibited the spinal c-Fos response to AITC, in control and DSS-pretreated animals, whereas the response to intracolonic capsaicin (5%) was blocked by morphine but not HC-030031.

CONCLUSIONS AND IMPLICATIONS

Activation of colonic TRPA1 channels is signalled to the spinal cord. Mild colitis enhanced this afferent input that, as it is sensitive to morphine, is most likely of a chemonociceptive nature. As several irritant chemicals can be present in chyme, TRPA1 channels may mediate several gastrointestinal pain conditions.

Identifying the Ion Channels Responsible for Signaling Gastro-Intestinal Based Pain

We are normally unaware of the complex signalling events which continuously occur within our internal organs. Most of us only become cognisant when sensations of hunger, fullness, urgency or gas arise. However, for patients with organic and functional bowel disorders pain is an unpleasant and often debilitating reminder. Furthermore, chronic pain still represents a large unmet need for clinical treatment. Consequently, chronic pain has a considerable economic impact on health care systems and the afflicted individuals. In order to address this need we must understand how symptoms are generated within the gut, the molecular pathways responsible for generating these signals and how this process changes in disease states.

Neuroinflammation in inflammatory bowel disease

Inflammatory bowel disease is a chronic intestinal inflammatory condition, the pathology of which is incompletely understood. Gut inflammation causes significant changes in neurally controlled gut functions including cramping, abdominal pain, fecal urgency, and explosive diarrhea. These symptoms are caused, at least in part, by prolonged hyperexcitability of enteric neurons that can occur following the resolution of colitis. Mast, enterochromaffin and other immune cells are increased in the colonic mucosa in inflammatory bowel disease and signal the presence of inflammation to the enteric nervous system. Inflammatory mediators include 5-hydroxytryptamine and cytokines, as well as reactive oxygen species and the production of oxidative stress. This review will discuss the effects of inflammation on enteric neural activity and potential therapeutic strategies that target neuroinflammation in the enteric nervous system.

Prokineticin-1 evokes secretory and contractile activity in rat small intestine

BACKGROUND

Prokineticins 1 and 2 (PROK1 and PROK2) are so named because they contract gastrointestinal smooth muscle, yet little else is known about their role in gastrointestinal function. Therefore, we used a combination of approaches to elucidate the mechanisms by which PROK1 alters ileal contractility and secretion in rats.

METHODS

RT-PCR and immunofluorescence were used to determine PROK and receptor (PK-R) mRNA levels and PK-R1 localization, respectively. Upper GI transit and fluid secretion were determined in vivo. Contractility and intestinal epithelial ion transport were assessed in isolated ileal segments.

KEY RESULTS

In the gastric fundus, PROK1 mRNA is highly expressed (70-fold >PROK2 mRNA) whereas the ileum has the highest mRNA expression of its receptor. PK-R1 immunoreactivity is visualized in ileal crypt cells, and in submucosal and myenteric neurons. In ileal segments, PROK1 evokes biphasic contractile responses consisting of an early, TTX-sensitive response (EC(50) = 87.8 nmol L(-1)) followed by a late, TTX-insensitive (EC(50) = 72.4 nmol L(-1)) component that is abolished in mucosa-free preparations. Oral administration of PROK1 enhances small bowel transit (111 +/- 3% of control) and fluid secretion (340 +/- 90% of control) and in muscle-stripped ileal preparations increases short-circuit current (EC(50) = 8.2 nmol L(-1)) in a TTX-insensitive manner. The PROK1-evoked Cl- secretion is reduced by piroxicam (non-selective cyclooxygenase inhibitor), and a prostaglandin EP(4) receptor antagonist (AH23848), but not a thromboxane receptor antagonist (GR32191B).

CONCLUSIONS & INFERENCES

These results demonstrate that PROK1 has oral prokinetic and secretogogue activity and that it acts on the intestinal mucosa via PK-R1 and prostaglandin receptors to mediate these effects.

The role of brain-derived neurotrophic factor in experimental inflammation of mouse gut

Previous studies suggested that brain-derived neurotrophic factor (BDNF) might act as an important modulator in chronic pain states. However, no systematic study has used knock-out mice to clarify its effect on visceral sensitivity. In the present study, 2,4,6-trinitrobenzene sulfonic acid (TNBS) was administered to heterozygous (BDNF(+/-)) knock-out and wild-type (BDNF(+/+)) mice to induce colitis. Visceral response to colorectal distension (CRD) and bladder reactivity were recorded. Results demonstrated that in normal state, BDNF(+/-) and BDNF(+/+) mice did not differ in the visceral response to CRD at <60 mm Hg pressure and the bladder reactivity; however, with 60 mm Hg pressure, BDNF(+/-) mice showed a weaker visceral response to CRD. In inflammatory state of colitis, TNBS induced upregulation of BDNF in dorsal root ganglia of both genotypes while BDNF(+/-) mice showing significantly lower sensitivity in the colon at 30 mm Hg and lower sensitivity in bladder than BDNF(+/+) mice. The two genotypes showed no significant difference in inflammatory severity. Thus, BDNF deficiency results in developmental changes in colonic nociception in both control and inflammatory states, which are more significant in inflammatory state. For bladder reactivity, BDNF deficiency leads to lower sensitization in inflammatory state but has no effect in control state.

PERSPECTIVE

This article highlights the role of BDNF in colonic and referred bladder hyperalgesia in mice. The findings might help in determining novel pharmaceutical interventions targeted at BDNF to relieve abdominal pain.

Urine cytokines suggest an inflammatory response in the overactive bladder: a pilot study

PURPOSE

To study the hypothesis of detecting bladder inflammation associated with overactive bladder (OAB) through altered urine levels of cytokines, chemokines, and growth factors.

METHODS

Midstream urine specimens were collected from a prospective study done on eight asymptomatic control subjects and 17 idiopathic OAB patients. The urine was analyzed by a multiplex panel screen for 12 chemokines, cytokines, growth factors, and soluble receptors using Luminex™ xMAP(®) technology. Protein concentration values were normalized to the levels of creatinine.

RESULTS

This analysis revealed a significant elevation of seven key proteins in the urine of OAB patients relative to controls (*P < 0.05). A greater than tenfold elevation was measured in OAB, relative to controls, in the levels of monocyte chemotactic protein-1 (MCP-1), soluble fraction of the CD40 ligand (sCD40L) in urine was obtained from OAB patients relative to controls. At least five fold elevations were detected in the levels of macrophage inflammatory protein (MIP-1β), IL-12p70/p40, IL-5, epidermal growth factor (EGF), and growth-related oncogene GRO-α compared to controls. Significant threefold elevation was also noticed in the urine levels of sIL-2Rα, and IL-10 in the OAB group. The levels of the remaining proteins tested were not statistically significantly different from control values.

CONCLUSIONS

The presence of elevated levels in urine of inflammatory biomarkers involved in inflammation and tissue repair suggests a role for inflammation in OAB, and may help in diagnosis and treatment of this disease.

Localization of TRPV1 and contractile effect of capsaicin in mouse large intestine: high abundance and sensitivity in rectum and distal colon

We investigated immunohistochemical differences in the distribution of TRPV1 channels and the contractile effects of capsaicin on smooth muscle in the mouse rectum and distal, transverse, and proximal colon. In the immunohistochemical study, TRPV1 immunoreactivity was found in the mucosa, submucosal, and muscle layers and myenteric plexus. Large numbers of TRPV1-immunoreactive axons were observed in the rectum and distal colon. In contrast, TRPV1-positive axons were sparsely distributed in the transverse and proximal colon. The density of TRPV1-immunoreactive axons in the rectum and distal colon was much higher than those in the transverse and proximal colon. Axons double labeled with TRPV1 and protein gene product (PGP) 9.5 were detected in the myenteric plexus, but PGP 9.5-immunoreactive cell bodies did not colocalize with TRPV1. In motor function studies, capsaicin induced a fast transient contraction, followed by a large long-lasting contraction in the rectum and distal colon, whereas in the transverse and proximal colon only the transient contraction was observed. The capsaicin-induced transient contraction from the proximal colon to the rectum was moderately inhibited by an NK1 or NK2 receptor antagonist. The capsaicin-induced long-lasting contraction in the rectum and distal colon was markedly inhibited by an NK2 antagonist, but not by an NK1 antagonist. The present results suggest that TRPV1 channels located on the rectum and distal colon play a major role in the motor function in the large intestine.

Plasticity of enteric nerve functions in the inflamed and postinflamed gut

Inflammation of the gut alters the properties of the intrinsic and extrinsic neurons that innervate it. While the mechanisms of neuroplasticity differ amongst the inflammatory models that have been used, amongst various regions of the gut, and between intrinsic vs extrinsic neurons, a number of consistent features have been observed. For example, intrinsic and extrinsic primary afferent neurons become hyperexcitable in response to inflammation, and interneuronal synaptic transmission is facilitated in the enteric circuitry. These changes contribute to alterations in gut function and sensation in the inflamed bowel as well as functional disorders, and these changes persist for weeks beyond the point at which detectable inflammation has subsided. Thus, gaining a more thorough understanding of the mechanisms responsible for inflammation-induced neuroplasticity, and strategies to reverse these changes are clinically relevant goals. The purpose of this review is to summarize our current knowledge regarding neurophysiological changes that occur during and following intestinal inflammation, and to identify and address gaps in our knowledge regarding the role of enteric neuroplasticity in inflammatory and functional gastrointestinal disorders.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.