Distribution of the vanilloid receptor (VR1) in the gastrointestinal tract

Retinoids activate the irritant receptor TRPV1 and produce sensory hypersensitivity

Retinoids are structurally related derivatives of vitamin A and are required for normal vision as well as cell proliferation and differentiation. Clinically, retinoids are effective in treating many skin disorders and cancers. Application of retinoids evokes substantial irritating side effects, including pain and inflammation; however, the precise mechanisms accounting for the sensory hypersensitivity are not understood. Here we show that both naturally occurring and synthetic retinoids activate recombinant or native transient receptor potential channel vanilloid subtype 1 (TRPV1), an irritant receptor for capsaicin, the pungent ingredient of chili peppers. In vivo, retinoids produced pain-related behaviors that were either eliminated or significantly reduced by genetic or pharmacological inhibition of TRPV1 function. These findings identify TRPV1 as an ionotropic receptor for retinoids and provide cellular and molecular insights into retinoid-evoked hypersensitivity. These findings also suggest that selective TRPV1 antagonists are potential therapeutic drugs for treating retinoid-induced sensory hypersensitivity.

Extrinsic primary afferent signalling in the gut

Visceral sensory neurons activate reflex pathways that control gut function and also give rise to important sensations, such as fullness, bloating, nausea, discomfort, urgency and pain. Sensory neurons are organised into three distinct anatomical pathways to the central nervous system (vagal, thoracolumbar and lumbosacral). Although remarkable progress has been made in characterizing the roles of many ion channels, receptors and second messengers in visceral sensory neurons, the basic aim of understanding how many classes there are, and how they differ, has proven difficult to achieve. We suggest that just five structurally distinct types of sensory endings are present in the gut wall that account for essentially all of the primary afferent neurons in the three pathways. Each of these five major structural types of endings seems to show distinctive combinations of physiological responses. These types are: 'intraganglionic laminar' endings in myenteric ganglia; 'mucosal' endings located in the subepithelial layer; 'muscular-mucosal' afferents, with mechanosensitive endings close to the muscularis mucosae; 'intramuscular' endings, with endings within the smooth muscle layers; and 'vascular' afferents, with sensitive endings primarily on blood vessels. 'Silent' afferents might be a subset of inexcitable 'vascular' afferents, which can be switched on by inflammatory mediators. Extrinsic sensory neurons comprise an attractive focus for targeted therapeutic intervention in a range of gastrointestinal disorders.

Alterations in mucosal neuropeptides in patients with irritable bowel syndrome and ulcerative colitis in remission: a role in pain symptom generation?

BACKGROUND

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by chronic abdominal pain. The transient receptor potential vanilloid 1 (TRPV1) channel, which is involved in visceral pain signalling, has been shown to be up-regulated in IBS. Activation of TRPV1 leads to the release of neuropeptides, such as somatostatin and substance P (SP). We hypothesized that increased pain perception in IBS could be explained by increased transcription in TRPV1 and/or altered levels of neuropeptides. We therefore assessed the transcription of TRPV1 and the mucosal concentration of somatostatin and SP in IBS in comparison to healthy volunteers and patients with ulcerative colitis (UC) in remission as disease controls, and to ascertain their relationship to pain symptoms.

METHOD

Sigmoid colonic mucosal samples were collected from 12 patients with IBS, 34 patients with UC in remission and 9 healthy volunteers, in which groups TRPV1 mRNA levels were determined using quantitative polymerase chain reaction and neuropeptide concentrations by radioimmunoassay. Pain symptom intensity was determined by questionnaires.

RESULTS

Transcription of TRPV1 as well as the concentration of neuropeptides were significantly higher in IBS, but only the former correlated with pain symptom severity.

CONCLUSION

Increased transcription of TRPV1 may provide a possible explanation for pain generation in IBS. While the neuropeptides SP and somatostatin were both found to be increased in IBS, these changes are not sufficient to explain pain generation. Pain generation in IBS is probably explained by a complex redundancy in the regulation of local nociceptive mechanisms, which remains a subject of intensive investigation.

Transient receptor potential vanilloid type 1 channel (TRPV1) immunolocalization in the murine enteric nervous system is affected by the targeted C-terminal epitope of the applied antibody

The expression of transient receptor potential vanilloid type 1 channel (TRPV1) in the enteric nervous system is still the subject of debate. Although a number of studies have reported that TRPV1 is limited to extrinsic afferent fibers, other studies argue for an intrinsic expression of TRPV1. In the present study, reverse transcriptase PCR was employed to establish the expression of TRPV1 mRNA throughout the gastrointestinal tract. Using two antibodies directed against different epitopes of TRPV1, we were able to show at the protein level that the observed distribution pattern of TRPV1 is dependent on the antibody used in the immunohistochemical staining. A first antibody indeed mainly stained neuronal fibers, whereas a second antibody exclusively stained perikarya of enteric neurons throughout the mouse gastrointestinal tract. We argue that these different distribution patterns are due to the antibodies discriminating between different modulated forms of TRPV1 that influence the recognition of the targeted immunogen and as such distinguish intracellular from plasmalemmal forms of TRPV1. Our study is the first to directly compare these two antibodies within the same species and in identical conditions. Our observations underline that detailed knowledge of the epitope that is recognized by the antibodies employed in immunohistochemical procedures is a prerequisite for correctly interpreting experimental results.

Colonic mucosal mediators from patients with irritable bowel syndrome excite enteric cholinergic motor neurons

BACKGROUND

Mediators released in the mucosal milieu have been suggested to be involved in visceral hypersensitivity and abdominal pain in patients with irritable bowel syndrome (IBS). However, their impact on myenteric neurons remains unsettled.

METHODS

Mucosal biopsies were obtained from the descending colon of patients with IBS and controls. Mucosal mast cells were identified immunohistochemically. The impact of spontaneously released mucosal mediators on guinea pig electrically stimulated longitudinal muscle myenteric plexus (LMMP) preparations was assessed in vitro by means of selective receptor antagonists and inhibitors.

KEY RESULTS

Patients with IBS showed an increased mast cell count compared with controls. Application of mucosal mediators of IBS to LMMPs potentiated cholinergic twitch contractions, an effect directly correlated with mast cell counts. Enhanced contractions were inhibited by 50.3% with the prostaglandin D2 antagonist BW A868C, by 31.3% and 39% with the TRPV1 antagonists capsazepine and HC-030031, respectively, and by 60.5% with purinergic P2X antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Conversely, the serotonin1-4, histamine1-3, tachykinin1-3 receptor blockade, and serine protease inhibition had no significant effect.

CONCLUSIONS & INFERENCES

Colonic mucosal mediators from patients with IBS excite myenteric cholinergic motor neurons. These effects were correlated with mast cell counts and mediated by activation of prostanoid receptors, TRPV1, and P2X receptors. These results support the role of mucosal inflammatory mediators and mast cell activation in altered motor function of IBS.

Capsaicin induces theta-band synchronization between gustatory and autonomic insular cortices

In the insular cortex, the primary gustatory area caudally adjoins the primary autonomic area that is involved in visceral sensory-motor integration. However, it has not been addressed whether neural activity in the gustatory insula (Gu-I) is coordinated with that in the autonomic insula (Au-I). We have demonstrated that TRPV1 activation in Gu-I induces theta-band synchronization between Gu-I and Au-I in rat slice preparations. Electron-microscopic immunohistochemistry revealed that TRPV1 immunoreactivity was much higher in Gu-I than in Au-I, and was mostly detected in dendritic spines receiving asymmetrical synapses. Whole-cell voltage-clamp recordings revealed that, in Gu-I, capsaicin-induced currents in layer 3 (L3) pyramidal cells (PCs) displayed no apparent desensitization, while those in layer 5 (L5) PCs displayed Ca(2+)-dependent desensitization, suggesting that L3 and L5 PCs respond differentially to TRPV1 activation. Voltage-sensitive dye imaging demonstrated that TRPV1 activation in Gu-I can alter an optical response with a monophasic and columnar temporospatial pattern evoked within Gu-I into an oscillatory one extending over Gu-I and Au-I. Power and cross-power spectral analyses of optical responses revealed theta-band synchronization between Gu-I and Au-I. Whole-cell current-clamp recordings demonstrated that such theta-band waves were mediated by sustained rhythmic firings at 4 and 8 Hz in L3 and L5 PCs, respectively. These results strongly suggested that theta-band oscillatory neural coordination between Gu-I and Au-I was induced by two distinct TRPV1-mediated theta-rhythm firings in L3 and L5 PCs in Gu-I. This network coordination induced by TRPV1 activation could be responsible for autonomic responses to tasting and ingesting spicy foods.

Plasticity of TRPV1-Expressing Sensory Neurons Mediating Autonomic Dysreflexia Following Spinal Cord Injury

Spinal cord injury (SCI) triggers profound changes in visceral and somatic targets of sensory neurons below the level of injury. Despite this, little is known about the influence of injury to the spinal cord on sensory ganglia. One of the defining characteristics of sensory neurons is the size of their cell body: for example, nociceptors are smaller in size than mechanoreceptors or proprioceptors. In these experiments, we first used a comprehensive immunohistochemical approach to characterize the size distribution of sensory neurons after high- and low-thoracic SCI. Male Wistar rats (300 g) received a spinal cord transection (T3 or T10) or sham-injury. At 30 days post-injury, dorsal root ganglia (DRGs) and spinal cords were harvested and analyzed immunohistochemically. In a wide survey of primary afferents, only those expressing the capsaicin receptor (TRPV1) exhibited somal hypertrophy after T3 SCI. Hypertrophy only occurred caudal to SCI and was pronounced in ganglia far distal to SCI (i.e., in L4-S1 DRGs). Injury-induced hypertrophy was accompanied by a small expansion of central territory in the lumbar spinal dorsal horn and by evidence of TRPV1 upregulation. Importantly, hypertrophy of TRPV1-positive neurons was modest after T10 SCI. Given the specific effects of T3 SCI on TRPV1-positive afferents, we hypothesized that these afferents contribute to autonomic dysreflexia (AD). Rats with T3 SCI received vehicle or capsaicin via intrathecal injection at 2 or 28 days post-SCI; at 30 days, AD was assessed by recording intra-arterial blood pressure during colo-rectal distension (CRD). In both groups of capsaicin-treated animals, the severity of AD was dramatically reduced. While AD is multi-factorial in origin, TRPV1-positive afferents are clearly involved in AD elicited by CRD. These findings implicate TRPV1-positive afferents in the initiation of AD and suggest that TRPV1 may be a therapeutic target for amelioration or prevention of AD after high SCI.

Lesioning of TRPV1 expressing primary afferent neurons prevents PAR-2 induced motility, but not mechanical hypersensitivity in the rat colon

BACKGROUND

Proteinase activated receptor 2 (PAR-2) is expressed by many neurons in the colon, including primary afferent neurons that co-express transient receptor potential vanilloid 1 (TRPV1). Activation of PAR-2 receptors was previously found to enhance colonic motility, increase secretion and produce hypersensitivity to mechanical stimuli. This study examined the functional role of TRPV1/PAR-2 expressing neurons that innervate the colon by lesioning TRPV1 bearing neurons with the highly selective and potent TRPV1 agonist resiniferatoxin.

METHODS

Colonic motility in response to PAR-2 activation was evaluated in vitro using isolated segments of descending colon and in vivo using manometry. Colonic mechanical nociceptive thresholds were measured using colorectal distension. Transient receptor potential vanilloid 1 expressing neurons were selectively lesioned with resiniferatoxin.

KEY RESULTS

In vitro, the PAR-2 agonists, trypsin and SLIGRL did not alter contractions of colon segments when applied alone, however, the agents enhanced acetylcholine stimulated contraction. In vivo, PAR-2 agonists administered intraluminally induced contractions of the colon and produced hypersensitivity to colorectal distention. The PAR-2 agonist enhancement of colonic contraction was eliminated when TRPV1 expressing neurons were lesioned with resiniferatoxin, but the PAR-2 agonist induced hypersensitivity remained in the lesioned animals.

CONCLUSIONS & INFERENCES

Our findings indicate that TRPV1/PAR-2 expressing primary afferent neurons mediate an extrinsic motor reflex pathway in the colon. These data, coupled with our previous studies, also indicate that the recently described colospinal afferent neurons are nociceptive, suggesting that these neurons may be useful targets for the pharmacological control of pain in diseases such as irritable bowel syndrome.

Oral ingestion of Capsaicin, the pungent component of chili pepper, enhances a discreet population of macrophages and confers protection from autoimmune diabetes

Vanilloid receptor 1 (VR1) is expressed on immune cells as well as on sensory neurons. Here we report that VR1 can regulate immunological events in the gut in response to its ligand Capsaicin (CP), a nutritional factor, the pungent component of chili peppers. Oral administration of CP attenuates the proliferation and activation of autoreactive T cells in pancreatic lymph nodes (PLNs) but not other lymph nodes, and protects mice from development of type 1 diabetes (T1D). This is a general phenomenon and not restricted to one particular strain of mice. Engagement of VR1 enhances a discreet population of CD11b(+)/F4/80(+) macrophages in PLN, which express anti-inflammatory factors interleukin (IL)-10 and PD-L1. This population is essential for CP-mediated attenuation of T-cell proliferation in an IL-10-dependent manner. Lack of VR1 expression fails to inhibit proliferation of autoreactive T cells, which is partially reversed in (VR1(+/+) → VR1(-/-)) bone marrow chimeric mice, implying the role of VR1 in crosstalk between neuronal and immunological responses in vivo. These findings imply that endogenous ligands of VR1 can have profound effect on gut-mediated immune tolerance and autoimmunity by influencing the nutrient-immune interactions.

TRP Channels as Sensors and Signal Integrators of Redox Status Changes

Proteins are capable of sensing the redox status of cells. Cysteine residues, which react with oxidants, reductants, and electrophiles, have been increasingly recognized as the mediators of this redox sensitivity. Cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that a class of TRP channels is sensitive to changes in redox status and is notably susceptible to modifications of cysteine residues, such as oxidation, electrophilic reaction, and S-nitrosylation of sulfhydryls. In this review, we focus on TRP channels, which directly sense redox status, and discuss the biological significance of cysteine modifications and the consequences of this chemical reaction for physiological responses.

TRPA1 and substance P mediate colitis in mice

BACKGROUND & AIMS

The neuropeptides calcitonin gene-related peptide (CGRP) and substance P, and calcium channels, which control their release from extrinsic sensory neurons, have important roles in experimental colitis. We investigated the mechanisms of colitis in 2 different models, the involvement of the irritant receptor transient receptor potential of the ankyrin type-1 (TRPA1), and the effects of CGRP and substance P.

METHODS

We used calcium-imaging, patch-clamp, and neuropeptide-release assays to evaluate the effects of 2,4,6-trinitrobenzene-sulfonic-acid (TNBS) and dextran-sulfate-sodium-salt on neurons. Colitis was induced in wild-type, knockout, and desensitized mice.

RESULTS

TNBS induced TRPA1-dependent release of colonic substance P and CGRP, influx of Ca2+, and sustained ionic inward currents in colonic sensory neurons and transfected HEK293t cells. Analysis of mutant forms of TRPA1 revealed that TNBS bound covalently to cysteine (and lysine) residues in the cytoplasmic N-terminus. A stable sulfinic acid transformation of the cysteine-SH group, shown by mass spectrometry, might contribute to sustained sensitization of TRPA1. Mice with colitis had increased colonic neuropeptide release, mediated by TRPA1. Endogenous products of inflammatory lipid peroxidation also induced TRPA1-dependent release of colonic neuropeptides; levels of 4-hydroxy-trans-2-nonenal increased in each model of colitis. Colitis induction by TNBS or dextran-sulfate-sodium-salt was inhibited or reduced in TRPA1-/- mice and by 2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopro-pylphenyl)-acetamide, a pharmacologic inhibitor of TRPA1. Substance P had a proinflammatory effect that was dominant over CGRP, based on studies of knockout mice. Ablation of extrinsic sensory neurons prevented or attenuated TNBS-induced release of neuropeptides and both forms of colitis.

CONCLUSIONS

Neuroimmune interactions control intestinal inflammation. Activation and sensitization of TRPA1 and release of substance P induce and maintain colitis in mice.

Intrathecal injection of TRPV1 shRNA leads to increases in blood pressure in rats

AIM

The transient receptor potential vanilloid type 1 (TRPV1) channels have been implicated to play a role in blood pressure regulation. However, contribution of tissue specific TRPV1 to blood pressure regulation is largely unknown. Here, we test the hypothesis that TRPV1 expressed in dorsal root ganglia (DRG) of lower thoracic and upper lumbar segments (T8-L3) of the spinal cord and their central and peripheral terminals constitutes a counter regulatory mechanism preventing the increases in blood pressure.

METHODS

The expression of TRPV1 was knocked down by intrathecal injection of TRPV1 short-hairpin RNA (shRNA) in rats. Systolic blood pressure and mean arterial pressure (MAP) were recorded. The level of TRPV1 and tyrosine hydroxylase (TH) was measured by Western blot.

RESULTS

Intrathecal injection of TRPV1 shRNA (6 μg kg(-1) day(-1) ) for 3 days increased systolic blood pressure and MAP when compared to rats that received control shRNA (control shRNA: 112 ± 2 vs. TRPV1 shRNA: 123 ± 2 mmHg). TRPV1 expression was suppressed in T8-L3 segments of dorsal horn and DRG as well as mesenteric arteries of rats given TRPV1 shRNA. Contents of TH, a marker of sympathetic nerves, were increased in mesenteric arteries of rats treated with TRPV1 shRNA. Pretreatment with the α1-adrenoceptor blocker, prazosin (1 mg kg(-1) day(-1) , p.o.), abolished the TRPV1 shRNA-induced pressor effects.

CONCLUSION

Our data show that selective knockdown of TRPV1 expressed in DRG of T8-L3 segments of the spinal cord and their central and peripheral terminals increases blood pressure, suggesting that neuronal TRPV1 in these segments possesses a tonic anti-hypertensive effect possibly via suppression of the sympathetic nerve activity.

TRPV1 expressing extrinsic primary sensory neurons play a protective role in mouse oxazolone-induced colitis

TRPV1 expressing sensory neurons which have been considered to be largely associated with neurogenic inflammation were chemically denervated by capsaicin treatment in neonatal mice. However, neonatal capsaicin treatment aggravated mouse oxazolone-induced colitis, and did not affect the expression of calcitonin gene-related peptide (CGRP)- or substance P-immunoreactive nerve fibers in the colon. Meanwhile, the capsaicin-induced contraction was absent in the colon of neonatal capsaicin treatment mouse. These results suggest a protective role of TRPV1 expressing extrinsic sensory neurons in oxazolone-induced colitis and the involvement of some neurotransmitter other than CGRP and substance P in the pathogenesis of the colitis.

Desalted deep-sea water improves cognitive function in mice by increasing the production of insulin-like growth factor-I in the hippocampus

The stimulation of sensory neurons in the gastrointestinal (GI) tract improves cognitive function by increasing the hippocampal production of insulin-like growth factor-I (IGF-I) in mice. In the current study, we examined whether oral administration of desalted deep-sea water (DSW) increases the hippocampal production of IGF-I by stimulating sensory neurons in the GI tract, thereby improving cognitive function in mice. Desalted DSW increased calcitonin gene-related peptide (CGRP) release from dorsal root ganglion (DRG) neurons isolated from wild-type (WT) mice by activating transient receptor potential vanilloid 1. The plasma levels of IGF-I and tissue levels of CGRP, IGF-I, and IGF-I mRNA in the hippocampus were increased by oral administration of desalted DSW in WT mice. In these animals, nociceptive information originating from the GI tract was transmitted to the hippocampus via the spinothalamic pathway. Improvement of spatial learning was observed in WT mice after administration of desalted DSW. Distilled DSW showed results similar to those of desalted DSW in vitro and in vivo. None of the effects of desalted DSW in WT mice were observed after the administration of desalted DSW in CGRP-knockout (CGRP-/-) mice. No volatile compounds were detected in distilled DSW on GC-MS analysis. These observations suggest that desalted DSW may increase the hippocampal IGF-I production via sensory neuron stimulation in the Gl tract, thereby improving cognitive function in mice. Such effects of desalted DSW might not be dependent on the minerals but are dependent on the function of the water molecule itself.

Role of the transient receptor potential vanilloid 1 in inflammation and sepsis

The transient receptor potential vanilloid 1 (TRPV1) is a thermoreceptor that responds to noxious temperatures, as well as to chemical agonists, such as vanilloids and protons. In addition, its channel activity is notably potentiated by proinflammatory mediators released upon tissue damage. The TRPV1 contribution to sensory neuron sensitization by proalgesic agents has signaled this receptor as a prime target for analgesic and anti-inflammatory drug intervention. However, TRPV1 antagonists have notably failed in clinical and preclinical studies because of their unwanted side effects. Recent reports have unveiled previously unrecognized anti-inflammatory and protective functions of TRPV1 in several diseases. For instance, this channel has been suggested to play an anti-inflammatory role in sepsis. Therefore, the use of potent TRPV1 antagonists as a general strategy to treat inflammation must be cautiously considered, given the deleterious effects that may arise from inhibiting the population of channels that have a protective function. The use of TRPV1 antagonists may be limited to treating those pathologies where enhanced receptor activity contributes to the inflamed state. Alternatively, therapeutic paradigms, such as reduction of inflammatory-mediated increase of receptor expression in the cell surface, may be a better strategy to prevent abrogation of the TRPV1 subpopulation involved in anti-inflammatory and protective processes.

Relevance of mast cell-nerve interactions in intestinal nociception

Cross-talk between the immune- and nervous-system is considered an important biological process in health and disease. Because mast cells are often strategically placed between nerves and surrounding (immune)-cells they may function as important intermediate cells. This review summarizes the current knowledge on bidirectional interaction between mast cells and nerves and its possible relevance in (inflammation-induced) increased nociception. Our main focus is on mast cell mediators involved in sensitization of TRP channels, thereby contributing to nociception, as well as neuron-released neuropeptides and their effects on mast cell activation. Furthermore we discuss mechanisms involved in physical mast cell-nerve interactions. This article is part of a Special Issue entitled: Mast cells in inflammation.

TRPV1 expression in acupuncture points: response to electroacupuncture stimulation

The present study was to examine the distribution of transient receptor potential vanilloid type-1 (TRPV1) receptor immunoreactivity in the acupuncture points (acupoint), and determine the influences of electroacupuncture (EA) stimulation on TRPV1 expression. EA stimulation of BL 40 was conducted in two sessions of 20 min separated by an 80 min interval in anesthetized rats. Sections of skin containing BL 40, and its non-meridian control were examined by immunolabeling with antibodies directed against TRPV1. Without EA, the number of subepidermal nerve fibers expressing TRPV1 was higher in the acupoint than in non-acupoint control skin (p<0.01). The subepidermal nerve fibers showed the co-localization of TRPV1 with peripherine, a marker for the C-fibers and A-δ fibers. The expression of TRPV1 in nerve fibers is significantly increased by EA stimulation in acupoints (p<0.01). However the upregulation in the non acupoint meridian and the non-meridian control skin was short of statistical significance. Double immunostaining of TRPV1 and neuronal nitric oxide synthase (nNOS) revealed their co-localization in both the subepidermal nerve fibers and in the dermal connective tissue cells. These results show that a high expression of TRPV1 endowed with nNOS in subepidermal nerve fibers exists in the acupoints and the expression is increased by EA. We conclude that the higher expression of TRPV1 in the subepidermal nerve fibers and its upregulation after EA stimulation may play a key role in mediating the transduction of EA signals to the CNS, and its expression in the subepidermal connective tissue cells may play a role in conducting the local effect of the EA.

Specific hydroxy fatty acids in royal jelly activate TRPA1

This is the first report of TRPA1 activation by fatty acids. Activation of TRPA1 and TRPV1 induces thermogenesis and energy expenditure enhancement. In this study, we searched for novel agonists of TRPA1 and TRPV1 from a nonpungent food, royal jelly (RJ). We measured the activation of human TRPA1 and TRPV1 by RJ extracts and found that the hexane extract contains TRPA1 agonists. The main functional compounds in the hexane extract were trans-10-hydroxy-2-decenoic acid (HDEA) and 10-hydroxydecanoic acid (HDAA). These are characteristic fatty acids of RJ. Their EC50 values were about 1,000 times larger than that of AITC, and their maximal responses were equal. They activated TRPA1 more strongly than TRPV1. Their EC50 values for TRPV1 were 2 times larger, and the maximal response was less than half of that for TRPA1. Next, we studied the potencies of other lipid components for both receptors. Most of them have higher affinity to TRPA1 than TRPV1. Among them, dicarboxylic acids showed equal efficacy for both receptors, but those are present in only small amounts in RJ. We concluded that the main function of RJ is TRPA1 activation by HDEA and HDAA, the major components of the RJ lipid fraction.

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.

Resveratrol improves cognitive function in mice by increasing production of insulin-like growth factor-I in the hippocampus

We examined whether resveratrol increases insulin-like growth factor-I (IGF-I) production in the hippocampus by stimulating sensory neurons in the gastrointestinal tract, thereby improving cognitive function in mice. Resveratrol increased calcitonin gene-related peptide (CGRP) release from dorsal root ganglion (DRG) neurons isolated from wild-type (WT) mice. Increases in tissue levels of CGRP, IGF-I, and IGF-I mRNA and immunohistochemical expression of IGF-I were observed in the hippocampus at 3 weeks after oral administration of resveratrol in WT mice. Significant enhancement of angiogenesis and neurogenesis was observed in the dentate gyrus of the hippocampus in these animals (P<.01). Improvement of spatial learning in the Morris water maze was observed in WT mice after administration of resveratrol. None of the effects of resveratrol observed in WT mice were seen after resveratrol administration in CGRP-knockout (CGRP(-/-)) mice. Although red wine containing 20 mg/L of resveratrol produced effects similar to those of resveratrol administrationl in WT mice, neither red wine containing 3.1 mg/L of resveratrol nor white wine exhibited such effects in WT mice. Resveratrol was undetectable in the hippocampus of WT mice administered resveratrol and red wine containing 20 mg/L of resveratrol. These observations strongly suggest that resveratrol increases hippocampal IGF-I production via sensory neuron stimulation in the gastrointestinal tract, thereby improving cognitive function in mice.

Involvement of TRPV2 activation in intestinal movement through nitric oxide production in mice

Transient receptor potential channel vanilloid 2 (TRPV2) can detect various stimuli such as temperature (>52 °C), stretch, and chemicals, including 2-aminoethoxydiphenyl borate, probenecid, and lysophospholipids. Although expressed in many tissues, including sensory and motor neurons, TRPV2 expression and function in the gastrointestinal tract is poorly understood. Here, we show TRPV2 expression in the murine intestine and its involvement in intestinal function. Almost all mouse intestinal intrinsic sensory and inhibitory motor neurons, both cell bodies and nerve fibers, showed TRPV2 immunoreactivity. Several known TRPV2 activators increased cytosolic Ca²+ concentrations and evoked TRPV2-like current responses in dissociated myenteric neurons. Interestingly, mechanical stimuli activated inward currents in a strength-dependent manner, which were inhibited by a TRPV2 inhibitor tranilast. TRPV2 activation in isolated intestine inhibited spontaneous circular muscle contraction, which did not occur in the presence of the TRPV2 antagonist, tetrodotoxin or nitro oxide (NO) synthase pathway inhibitors. Also, increased intestinal NO production was observed in response to a TRPV2 agonist, and gastrointestinal transit in vivo was accelerated by TRPV2 agonists or an NO donor. In conclusion, TRPV2 may contribute to intestinal motility through NO production, and TRPV2 is a promising target for controlling intestinal movement.

Natural product ligands of TRP channels

Natural product ligands have contributed significantly to the deorphanisation of TRP ion channels. Furthermore, natural product ligands continue to provide valuable leads for the identification of ligands acting at "orphan" TRP channels. Additional naturally occurring modulators at TRP channels can be expected to be discovered in future, aiding in our understanding of not only their pharmacology and physiology, but also the therapeutic potential of this fascinating family of ion channels.

TRP channels in neurogastroenterology: opportunities for therapeutic intervention

The members of the superfamily of transient receptor potential (TRP) cation channels are involved in a plethora of cellular functions. During the last decade, a vast amount of evidence is accumulating that attributes an important role to these cation channels in different regulatory aspects of the alimentary tract. In this review we discuss the expression patterns and roles of TRP channels in the regulation of gastrointestinal motility, enteric nervous system signalling and visceral sensation, and provide our perspectives on pharmacological targeting of TRPs as a strategy to treat various gastrointestinal disorders. We found that the current knowledge about the role of some members of the TRP superfamily in neurogastroenterology is rather limited, whereas the function of other TRP channels, especially of those implicated in smooth muscle cell contractility (TRPC4, TRPC6), visceral sensitivity and hypersensitivity (TRPV1, TRPV4, TRPA1), tends to be well established. Compared with expression data, mechanistic information about TRP channels in intestinal pacemaking (TRPC4, TRPC6, TRPM7), enteric nervous system signalling (TRPCs) and enteroendocrine cells (TRPM5) is lacking. It is clear that several different TRP channels play important roles in the cellular apparatus that controls gastrointestinal function. They are involved in the regulation of gastrointestinal motility and absorption, visceral sensation and visceral hypersensitivity. TRP channels can be considered as interesting targets to tackle digestive diseases, motility disorders and visceral pain. At present, TRPV1 antagonists are under development for the treatment of heartburn and visceral hypersensitivity, but interference with other TRP channels is also tempting. However, their role in gastrointestinal pathophysiology first needs to be further elucidated.

Possible involvement of the transient receptor potential vanilloid type 1 channel in postoperative adhesive obstruction and its prevention by a kampo (traditional Japanese) medicine, daikenchuto

This study focused on the localization of transient receptor potential vanilloid type 1 (TRPV1) in the intestines in postoperative adhesion model rats and investigated the underlying mechanism for the anti-adhesion action of daikenchuto (DKT), especially in relation to TRPV1. Postoperative intestinal adhesion was induced by sprinkling talc in the small intestine. The expression of TRPV1 mRNA was examined by in situ hybridization and real-time RT-PCR. The effects of DKT and its major ingredient, hydroxy sanshool, with or without ruthenium red, a TRP-channel antagonist, on talc-induced intestinal adhesions were evaluated. The level of TRPV1 mRNA was higher in the adhesion regions of talc-treated rats than in normal small intestine of sham-operated rats. Localization of TRPV1 mRNA expression was identified in the submucosal plexus of both sham-operated and talc-treated rats; and in talc-treated rats, it was observed also in the myenteric plexus and regions of adhesion. Capsaicin, DKT, and hydroxy sanshool significantly prevented formation of intestinal adhesions. The effects of DKT and hydroxy sanshool were abrogated by subcutaneous injection of ruthenium red. These results suggest that pharmacological modulation of TRPV1 might be a possible therapeutic option in postoperative intestinal adhesion, which might be relevant to the prevention of postoperative adhesive obstruction by DKT.

Intragastric administration of capsiate, a transient receptor potential channel agonist, triggers thermogenic sympathetic responses

The sympathetic thermoregulatory system controls the magnitude of adaptive thermogenesis in correspondence with the environmental temperature or the state of energy intake and plays a key role in determining the resultant energy storage. However, the nature of the trigger initiating this reflex arc remains to be determined. Here, using capsiate, a digestion-vulnerable capsaicin analog, we examined the involvement of specific activation of transient receptor potential (TRP) channels within the gastrointestinal tract in the thermogenic sympathetic system by measuring the efferent activity of the postganglionic sympathetic nerve innervating brown adipose tissue (BAT) in anesthetized rats. Intragastric administration of capsiate resulted in a time- and dose-dependent increase in integrated BAT sympathetic nerve activity (SNA) over 180 min, which was characterized by an emergence of sporadic high-activity phases composed of low-frequency bursts. This increase in BAT SNA was abolished by blockade of TRP channels as well as of sympathetic ganglionic transmission and was inhibited by ablation of the gastrointestinal vagus nerve. The activation of SNA was delimited to BAT and did not occur in the heart or pancreas. These results point to a neural pathway enabling the selective activation of the central network regulating the BAT SNA in response to a specific stimulation of gastrointestinal TRP channels and offer important implications for understanding the dietary-dependent regulation of energy metabolism and control of obesity.

The protective effect of capsaicin receptor-mediated genistein postconditioning on gastric ischemia-reperfusion injury in rats

BACKGROUND

No published study has addressed the effect of genistein postconditioning on gastric ischemia-reperfusion (GI-R) injury in rats.

AIM

To examine whether capsaicin receptor-mediated genistein postconditioning protects against gastric ischemia-reperfusion injury via the PI3K/Akt signal pathway.

METHODS AND RESULTS

Chloraldurat-anesthetized rats underwent occlusion of the celiac artery for 30 min, followed by 60 min of reperfusion. Based on this animal model of gastric ischemia-reperfusion injury, genistein at doses of 100, 500 or 1,000 μg/kg was administered via peripheral vein 5 min before reperfusion. The dose of 500 μg/kg was optimal for postconditioning, at which the severity of I-R-induced gastric injury significantly decreased. Immunohistochemistry also showed that gastric mucosal cell apoptosis decreased. Capsazepine (CPZ), a specific antagonist for the capsaicin receptor, was administered (1,000 μg/kg, i.v.) just before ischemia. Capsaicin (50 mg/kg, s.c.) once a day for 4 days reversed the protective effects of genistein. Reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting showed increased calcitonin gene-related peptide (CGRP) expression in genistein group but not in capsazepine or capsaicin group. CGRP inhibitor CGRP8-37 also prevented the effects of genistein in decreasing gastric mucosal injury index. In addition, PI3K inhibitor LY294002 (1.5 mg/kg) reversed the protective effect of genistein. Compared with genistein group, Western blots also demonstrated decreased Akt phosphorylation in LY294002 group.

CONCLUSION

Our data suggest that capsaicin receptors mediated the protective effects of genistein postconditioning. CGRP secreted by activated capsaicin-sensitive neurons played an important role in the protective effects of genistein. PI3K/Akt pathway was also involved in the protective effects of genistein.

Distribution of transient receptor potential vanilloid 1 channel-expressing nerve fibers in mouse rectal and colonic enteric nervous system: relationship to peptidergic and nitrergic neurons

In the gut, transient receptor potential vanilloid (TRPV) 1 activation leads to release of neurotransmitters such as neuropeptides and nitric oxide. However, the distribution of TRPV1 nerve fibers and neurotransmitters released form sensory nerve endings in the enteric nervous system are currently not well understood. The present study investigated the immunohistochemical distribution of TRPV1 channels, sensory neuropeptides, and nitric oxide and their co-localization in mouse large intestine. Numerous TRPV1 and calcitonin gene-related peptide (CGRP) immunoreactivities were detected, mainly in the mucosa, submucosal layer, and myenteric plexus. Abundant substance P (SP), neurokinin A (NKA), and neuronal nitric oxide synthase (nNOS)-immunoreactivity were revealed in muscle layers. Motor function studies of circular and longitudinal muscles found that contractile responses to capsaicin in the rectum were most sensitive among the rectum, and distal, transverse, and proximal colon. Double labeling studies were carried out in horizontal sections of mouse rectum. TRPV1/protein gene product (PGP)9.5 double labeled axons were observed, but PGP9.5 and neuronal nuclear protein immunopositive cell bodies did not express TRPV1 immunoreactivity in the myenteric plexus. In the mucosa, submucosal layer, deep muscular plexus, circular muscle, myenteric plexus and longitudinal muscle layer, TRPV1 nerve fibers were found to contain CGRP, SP and nNOS. SP and NKA were almost entirely colocalized at the axons and cell bodies in all layers. Double labeling with c-Kit revealed that TRPV1 nerve fibers localized adjacent to the interstitial cells of Cajal (ICC). These results suggest that the TRPV1-expressing nerve and its neurotransmitters regulate various functions of the large intestine.

Visceral hypersensitivity induced by activation of transient receptor potential vanilloid type 1 is mediated through the serotonin pathway in rat colon

This study aimed to clarify the relationship between TRPV1 activation-induced visceral pain and the serotonin pathway in the colon of rats. The effects of para-chlorophenylalanine (pCPA) on visceral pain threshold pressure were assessed in capsaicin -induced visceral pain of rats. The expression of TRPV1 in the colon was examined by immunohistochemistry and Western blot analysis, and TRPV1 excitability in dorsal root ganglion (DRG) neurons was examined by whole-cell patch-clamp recording in pCPA-treated rats. Calcineurin and Ca(2+)-calmodulin-dependent kinase II (CaMKII), the important proteins in maintaining TRPV1 function in the colon, were also tested by Western blot analysis and immunofluorescence staining. Results showed that pCPA significantly increased the capsaicin-induced visceral pain threshold by 2.3-fold, and the enhanced visceral pain threshold corresponded with decreased 5-HT content (58% depleted) and enterochromaffin cell number (80% reduced). The reduced excitability of TRPV1 in DRG neurons, instead of changed TRPV1 expression, is responsible for the enhanced visceral pain threshold in 5-HT-depleted rats, and the mechanism may be related to the decreased expression of pCaMKII. These results indicate that visceral hypersensitivity induced by TRPV1 activation is modulated through 5-HT pathways and the attenuated function of TRPV1 and decreased protein expression of pCaMKII may play an important role in capsaicin-induced TRPV1 desensitization under 5-HT-depleted condition. The important role of TRPV1 and 5-HT in generating and maintaining visceral hypersensitivity may provide insights for the treatment of visceral hypersensitivity.

Capsaicin inhibits the spontaneous pacemaker activity in interstitial cells of cajal from the small intestine of mouse

Background/Aims

Capsaicin (8-methyl-N-vanillyl-6-ninenamide), a compound found in hot peppers, has been reported to have different physiological actions on different cell types. Not much work has been done about the effect of capsaicin on the function of interstitial cells of Cajal (ICC). In the present study, we examined the action of external application of capsaicin on pacemaker activity in the cultured ICC from the small intestine of mouse.

Methods

We investigated the effect of capsaicin on pacemaker currents in cultured ICC from the small intestine of mouse using a whole cell patch-clamp technique and Ca²⁺-imaging analysis.

Results

When capsaicin was applied externally to the pacemaker generating ICC, it completely inhibited the pacemaker potential under current-clamp mode (I = 0) and the pacemaker current under voltage-clamp mode at a -70 mV of holding potentials. The effect of capsaicin on pacemaker activity in ICC was shown dose dependently. The effect of capsaicin was not through the transient receptor potential of the vanilloid type 1 (TRPV1) channel as capsazepine did not block the effect of capsaicin. L-NAME, an inhibitor of nitric oxide synthase, also did not block the capsaicin-induced effects. When the action of capsaicin was examined in the intracellular calcium oscillation, it completely abolished the calcium oscillation.

Conclusions

These results prove that the capsaicin has the inhibitory effects on the ICC which is carried out neither through TRPV channel nor the nitric oxide production. Intracellular Ca²⁺ was also an important target for actions of capsaicin on ICC.

TRPV1: a target for next generation analgesics

Transient Receptor Potential Vanilloid 1 (TRPV1) is a Ca²⁺ permeant non-selective cation channel expressed in a subpopulation of primary afferent neurons. TRPV1 is activated by physical and chemical stimuli. It is critical for the detection of nociceptive and thermal inflammatory pain as revealed by the deletion of the TRPV1 gene. TRPV1 is distributed in the peripheral and central terminals of the sensory neurons and plays a role in initiating action potentials at the nerve terminals and modulating neurotransmitter release at the first sensory synapse, respectively. Distribution of TRPV1 in the nerve terminals innervating blood vessels and in parts of the CNS that are not subjected to temperature range that is required to activate TRPV1 suggests a role beyond a noxious thermal sensor. Presently, TRPV1 is being considered as a target for analgesics through evaluation of different antagonists. Here, we will discuss the distribution and the functions of TRPV1, potential use of its agonists and antagonists as analgesics and highlight the functions that are not related to nociceptive transmission that might lead to adverse effects.

Physiology and pharmacology of the vanilloid receptor

The identification and cloning of the vanilloid receptor 1 (TRPV1) represented a significant step for the understanding of the molecular mechanisms underlying the transduction of noxious chemical and thermal stimuli by peripheral nociceptors. TRPV1 is a non-selective cation channel gated by noxious heat, vanilloids and extracellular protons. TRPV1 channel activity is remarkably potentiated by pro-inflammatory agents, a phenomenon that is thought to underlie the peripheral sensitisation of nociceptors that leads to thermal hyperalgesia. Cumulative evidence is building a strong case for the involvement of this receptor in the etiology of both peripheral and visceral inflammatory pain, such as inflammatory bowel disease, bladder inflammation and cancer pain. The validation of TRPV1 receptor as a key therapeutic target for pain management has thrust intensive drug discovery programs aimed at developing orally active antagonists of the receptor protein. Nonetheless, the real challenge of these drug discovery platforms is to develop antagonists that preserve the physiological activity of TRPV1 receptors while correcting over-active channels. This is a condition to ensure normal pro-prioceptive and nociceptive responses that represent a safety mechanism to prevent tissue injury. Recent and exciting advances in the function, dysfunction and modulation of this receptor will be the focus of this review.

Inflammatory pain: the cellular basis of heat hyperalgesia

Injury or inflammation release a range of inflammatory mediators that increase the sensitivity of sensory neurons to noxious thermal or mechanical stimuli. The heat- and capsaicin-gated channel TRPV1, which is an important detector of multiple noxious stimuli, plays a critical role in the development of thermal hyperalgesia induced by a wide range of inflammatory mediators. We review here recent findings on the molecular mechanisms of sensitisation of TRPV1 by inflammatory mediators, including bradykinin, ATP, NGF and prostaglandins. We describe the signalling pathways believed to be involved in the potentiation of TRPV1, and our current understanding of how inflammatory mediators couple to these pathways.

The role of transient receptor potential vanilloid 1 (TRPV1) receptors in dextran sulfate-induced colitis in mice

The aim of this study was to investigate the involvement of transient receptor potential vanilloid 1 (TRPV1) receptors in oral dextran sulfate sodium-induced (DSS) colitis using TRPV1 knockout mice and their wild-type C57BL/6 counterparts. DSS (2% or 5%) was administered orally ad libitum for 7 days; the controls received tap water. Animal weight, stool consistency, and blood content were scored every day to calculate the disease activity index (DAI). After sacrificing the mice on day 7, the colons were cut into three equal segments (proximal, intermediate, and distal) for histology, myeloperoxidase (MPO), and cytokine measurements. In the 2% DSS-treated group, the lack of TRPV1 receptors decreased the DAI. Each colon segment of wild-type animals showed more than two-fold increase of MPO activity and more severe histological changes compared to the knockouts. This difference was not observed in case of 5% DSS, when extremely severe inflammation occurred in both groups. IL-1beta production was not altered by the absence of TRPV1. In conclusion, activation of TRPV1 channels enhances the clinical symptoms, histopathological changes, and neutrophil accumulation induced by 2% DSS. Elucidating the modulator role of TRPV1 channels in inflammatory bowel diseases may contribute to the development of novel anti-inflammatory drugs for their therapy.

Role of chemical stimulation of the duodenum in dyspeptic symptom generation

The response to chemical stimuli such as acid, nutrients, and capsaicin at the level of the duodenum is increasingly recognized as important in the etiology of dyspeptic symptoms. Increased duodenal acid exposure has been reported for patients with dyspeptic symptoms. Duodenal hypersensitivity to acid and the enhancing effect of duodenal acid on gastroduodenal mechanosensitivity may also contribute to dyspeptic symptom generation. Serotonergic signaling pathways may be involved in acid-induced dyspeptic symptoms. As for nutrients, lipid has been unequivocally shown to have a function in the pathogenesis of dyspeptic symptoms. Cholecystokinin (CCK) is an important mediator of the effects of duodenal lipid on gastroduodenal sensorimotor activities. It is unclear whether CCK hypersecretion or hypersensitivity to CCK is responsible for symptoms in dyspeptic patients. The presence of capsaicin in the duodenum evokes symptoms and affects gastric sensorimotor function. In patients with dyspepsia, capsaicin-induced symptoms appeared to occur earlier and to be more severe, however the effects of duodenal infusion and putative consequent gastric sensorimotor abnormalities have not been examined. Capsaicin activates transient receptor potential ion channel of the vanilloid type I, which can also be activated and sensitized by acid. The interaction between the different chemical stimuli is complex and has not yet been studied in patients with dyspeptic symptoms. In conclusion, the mechanisms underlying an enhanced response to duodenal chemical stimulation in patients with dyspeptic symptoms are partially understood. At the level of the duodenum, abnormalities may exist in stimulus intensity, mucosal mRNA expression, biosynthesis, release, or inactivation of mucosal mediators, or receptor expression on afferent nerve endings. Elucidation of the abnormalities involved will provide a basis for rational treatment of dyspeptic symptoms.

A model to study the phenotypic changes of interstitial cells of Cajal in gastrointestinal diseases

BACKGROUND & AIMS

Interstitial cells of Cajal (ICC) express the receptor tyrosine kinase, KIT, the receptor for stem cell factor. In the gastrointestinal (GI) tract, ICC are pacemaker cells that generate spontaneous electrical slow waves, and mediate inputs from motor neurons. Absence or loss of ICC are associated with GI motility disorders, including those consequent of diabetes. Studies of ICC have been hampered by the low density of these cells and difficulties in recognizing these cells in cell dispersions.

METHODS

Kit(+/copGFP) mice harboring a copepod super green fluorescent protein (copGFP) complementary DNA, inserted at the Kit locus, were generated. copGFP(+) ICC from GI muscles were analyzed using confocal microscopy and flow cytometry. copGFP(+) ICC from the jejunum were purified by a fluorescence-activated cell sorter and validated by cell-specific markers. Kit(+/copGFP) mice were crossbred with diabetic Lep(+/ob) mice to generate compound Kit(+/copGFP);Lep(ob/ob) mutant mice. copGFP(+) ICC from compound transgenic mice were analyzed by confocal microscopy.

RESULTS

copGFP in Kit(+/copGFP) mice colocalized with KIT immunofluorescence and thus was predominantly found in ICC. In other smooth muscles, mast cells were also labeled, but these cells were relatively rare in the murine GI tract. copGFP(+) cells from jejunal muscles were Kit(+) and free of contaminating cell-specific markers. Kit(+/copGFP);Lep(ob/ob) mice displayed ICC networks that were dramatically disrupted during the development of diabetes.

CONCLUSIONS

Kit(+/copGFP) mice offer a powerful new model to study the function and genetic regulation of ICC phenotypes. Isolation of ICC from animal models will help determine the causes and responses of ICC to therapeutic agents.

Cannabinoids and the gut: new developments and emerging concepts

Cannabis has been used to treat gastrointestinal (GI) conditions that range from enteric infections and inflammatory conditions to disorders of motility, emesis and abdominal pain. The mechanistic basis of these treatments emerged after the discovery of Delta(9)-tetrahydrocannabinol as the major constituent of Cannabis. Further progress was made when the receptors for Delta(9)-tetrahydrocannabinol were identified as part of an endocannabinoid system, that consists of specific cannabinoid receptors, endogenous ligands and their biosynthetic and degradative enzymes. Anatomical, physiological and pharmacological studies have shown that the endocannabinoid system is widely distributed throughout the gut, with regional variation and organ-specific actions. It is involved in the regulation of food intake, nausea and emesis, gastric secretion and gastroprotection, GI motility, ion transport, visceral sensation, intestinal inflammation and cell proliferation in the gut. Cellular targets have been defined that include the enteric nervous system, epithelial and immune cells. Molecular targets of the endocannabinoid system include, in addition to the cannabinoid receptors, transient receptor potential vanilloid 1 receptors, peroxisome proliferator-activated receptor alpha receptors and the orphan G-protein coupled receptors, GPR55 and GPR119. Pharmacological agents that act on these targets have been shown in preclinical models to have therapeutic potential. Here, we discuss cannabinoid receptors and their localization in the gut, the proteins involved in endocannabinoid synthesis and degradation and the presence of endocannabinoids in the gut in health and disease. We focus on the pharmacological actions of cannabinoids in relation to GI disorders, highlighting recent data on genetic mutations in the endocannabinoid system in GI disease.

Intracolonic capsaicin stimulates colonic motility and defecation in conscious dogs

BACKGROUND

The aim of this study was to investigate the effects of intracolonic capsaicin on colonic motility and defecation.

METHODS

The effects of capsaicin (1, 2, 5, and 10 mg) administrated into the proximal colon on ileocolonic motility and defecation were studied in neurally intact dogs with or without various antagonists (atropine, hexamethonium, ondansetron, propranolol, and FK224), dogs with extrinsic denervation of an ileocolonic segment, and dogs with enterically isolated ileocolonic loops equipped with strain gauge force transducers.

RESULTS

Capsaicin at 5 and 10 mg evoked giant migrating contractions in a dose-independent manner, and it induced defecations with more than 90% probability in neurally intact dogs. These effects of capsaicin were abolished by atropine and hexamethonium. Ondansetron inhibited the capsaicin-induced increase in colonic motility but did not affect the induction of defecation. The other antagonists had no effect. In dogs with extrinsic denervation, capsaicin did not evoke giant migrating contractions in the colon but still induced defecation in 30-40% of experiments. In dogs with ileocolonic loops, capsaicin did not stimulate colonic motility nor induce defecation.

CONCLUSION

These results indicate that intracolonic capsaicin causes giant migrating contractions and defecation. Intact extrinsic innervation, continuity of the colon, and intraluminal contents were considered necessary for this effect.

Homozygous TRPV1 315C influences the susceptibility to functional dyspepsia

OBJECTIVES

Capsaicin/vanilloid (transient receptor potential vanilloid 1, (TRPV1) receptor has been shown to be expressed in gastrointestinal tract and play a role as a member of sensory ion channel superfamily. The G315C polymorphism affects the TRPV1 gene and alters its protein level. We aimed to investigate the effect of TRPV1 G315C polymorphism on functional dyspepsia (FD) in a Japanese population.

METHODS

TRPV1 G315C polymorphism was genotyped in 98 subjects with no upper abdominal symptoms and 109 patients with FD. Severity of 7 upper gastrointestinal symptoms was assessed during cold water, and cold carbonated water drinking for randomly selected 20 healthy subjects.

RESULTS

We found a significant inverse association between TRPV1 315CC genotype and FD [CC vs. others; odds ratio (OR)=0.40, 95% confidence interval (CI)=0.38-0.82]. We also found that the same genotype held a lower risk of both epigastric pain syndrome (OR=0.25, 95% CI=0.09-0.73), postprandial syndrome (OR=0.27, 95% CI=0.07-0.96) according to Rome III, and Helicobacter pylori positive FD (OR=0.28, 95% CI=0.10-0.79). The evolution of symptom severity scale of 7 total symptoms (P=0.004), and heavy feeling in stomach (P=0.02) during cold carbonated water drinking were significantly lower among 315CC genotypes compared with others.

CONCLUSIONS

Homozygous TRPV1 315C influences the susceptibility to FD through altering the upper gastrointestinal sensation.

Effects of capsaicin on gastric acid secretion and mechanisms involved

Although capsaicin has multiple pharmacological actions, its effects on gastric acid secretion attract the most attention. Most studies show that low-dose capsaicin can inhibit gastric acid secretion while high-dose may accelerate gastric acid secretion. However, some other studies show that capsaicin does not affect gastric acid secretion at all. The difference in the effects of capsaicin on gastric acid secretion may be related with the location of vanilloid receptor subtype 1, dose of capsaicin, route of administration, and the release of some substances such as calcitonin gene-related peptide, neurokinin A, vasoactive intestinal peptide and substance P. Capsaicin is a potentially promising drug used for modulation of gastric acid secretion.

Cilostazol improves cognitive function in mice by increasing the production of insulin-like growth factor-I in the hippocampus

Insulin-like growth factor I (IGF-I) exerts beneficial effects on cognitive function by inducing angiogenesis and neurogenesis in the hippocampus. We demonstrated that stimulation of sensory neurons in the gastrointestinal tract increased IGF-I production in the hippocampus, and thereby improved cognitive function in mice. Since cAMP plays a critical role in stimulation of sensory neurons, the type III phosphodiesterase (PDE3) inhibitor cilostazol might increase IGF-I production in the hippocampus by stimulating sensory neurons and thus improve cognitive function in mice. We tested this hypothesis in the present study. Cilostazol increased the release of calcitonin gene-related peptide (CGRP) and levels of cAMP in dorsal root ganglion (DRG) neurons isolated from wild-type (WT) mice. Tissue levels of cAMP in the DRG and hippocampus and those of CGRP, IGF-I, and IGF-I mRNA in the hippocampus were increased after 4-week oral administration of cilostazol to WT mice. Levels of expression of c-fos in the spinal dorsal horns, parabrachial nuclei, the solitary tract nucleus, and the hippocampus were also increased in these animals. Significant enhancement of angiogenesis and neurogenesis was observed in the dentate gyrus of the hippocampus after cilostazol administration in WT mice. Significant improvement of spatial learning was also observed in WT mice administered cilostazol. However, none of these effects in WT mice were observed in CGRP-knockout mice. These observations suggest that cilostazol may improve cognitive function in mice by increasing the hippocampal production of IGF-I through stimulation of sensory neurons.

Non-pungent capsaicin analogs (capsinoids) increase metabolic rate and enhance thermogenesis via gastrointestinal TRPV1 in mice

Capsinoids are non-pungent capsaicin analogs which increase energy expenditure like capsaicin. However, the mechanisms underlying the enhancement of their energy expenditure despite their non-pungency are poorly understood. We suggest here that capsinoids increase energy expenditure in wild-type mice, but not in transient receptor potential vanilloid 1 (TRPV1) knockout mice, implying that capsinoids increase energy expenditure via TRPV1. The jejunal administration of capsinoids to anesthetized mice raised the temperature of the colon and intrascapular brown adipose tissue. Denervation of the extrinsic nerves connected to the jejunum inhibited this temperature elevation. These findings suggest that capsinoids increase energy expenditure by activating the intestinal extrinsic nerves. Although the jejunal administration of capsinoids did not raise the tail skin temperature, an intravenous injection of capsinoids did, indicating that capsinoids could barely pass through the intestinal wall into the blood. Taken together, gastrointestinal TRPV1 may be a critical target for capsinoids to enhance energy expenditure.

The paradoxical role of the transient receptor potential vanilloid 1 receptor in inflammation

The transient potential receptor vanilloid 1 (TRPV1) receptor is a non-selective cation channel that is chemically activated by capsaicin, the pungent component of hot peppers. In addition, endogenous compounds, in particular the endogenous cannabinoid receptor activator, anandamide, have been demonstrated to activate TRPV1 in vivo. TRPV1 receptors are also activated by temperatures within the noxious range (>43 degrees C) and low pH (<pH 6.0). TRPV1 receptors are predominantly expressed in primary afferent fibres which are peptidergic sensory neurones, such as the thinly myelinated A-delta and unmyelinated C-fibres. TRPV1 receptors have also been demonstrated to be present in non-neuronal cells. Historically, TRPV1 has been considered as a pro-inflammatory receptor due to its key role in several conditions, including neuropathic pain, joint inflammation and inflammatory bowel disease, amongst others. However, the purpose of this review is to underline the emerging new evidence which demonstrate paradoxical, protective functions for this unique receptor in vivo. For example, in experimentally induced sepsis, TRPV1 null mice demonstrated elevated levels of pathological markers in comparison to wild-type mice. In addition to the pro-inflammatory and protective roles of TRPV1 in pathophysiological states, TRPV1 has also been shown to have important functions under normal physiological conditions, for example in urinary bladder function, thermoregulation and neurogenesis. The emerging functions of TRPV1 highlight the necessity for further research in light of increasing reports of potential TRPV1 antagonists undergoing pre-clinical experimentations.

The vanilloid receptor TRPV1: role in cardiovascular and gastrointestinal protection

It has been shown that the transient receptor potential channel vanilloid type 1 (TRPV1) is able to sense a vast range of stimuli and exerts multiple functions under physiological or pathophysiological conditions. TRPV1 not only plays a fundamental role in pain signaling but also involves in many other physiological or pathophysiological functions including the beneficial effects on cardiovascular and gastrointestinal function. It has been found that TRPV1 could be activated by endogenous ligands such as anandamide, N-arachidonoyl dopamine and N-oleoyldopamine or by exogenous agonists such as capsaicin and rutaecarpine. Since capsaicin-sensitive sensory nerves (rich in TRPV1) are densely distributed in the cardiovascular and gastrointestinal system, activation of TRPV1 either by endogenous ligands or by exogenous agonists has been repeatedly reported to exert hypotensive effects or protective effects against cardiac or gastrointestinal injury through stimulating the synthesis and release of multiple neurotransmitters such as calcitonin gene-related peptide and substance P. Therefore, TRPV1 is not only a prime target for the pharmacological control of pain but also a useful target for drug development to treat various diseases including cardiovascular and gastrointestinal diseases. However, considering the contribution of TRPV1 to the development of inflammation in the gastrointestinal tract, the potential side effects of TRPV1 agonist cannot be neglected while in seeking and developing the novel TRPV1 agonists.

Essential role for TRPV1 in stress-induced (mast cell-dependent) colonic hypersensitivity in maternally separated rats

Irritable bowel syndrome is in part characterized by an increased sensitivity to colonic distension. Stress is an important trigger factor for symptom generation. We hypothesized that stress induces visceral hypersensitivity via mast cell degranulation and transient receptor ion channel 1 (TRPV1) modulation. We used the rat model of neonatal maternal separation (MS) to investigate this hypothesis. The visceromotor response to colonic distention was assessed in adult MS and non-handled (NH) rats before and after acute water avoidance (WA) stress. We evaluated the effect of the mast cell stabilizer doxantrazole, neutralizing antiserum against the mast cell mediator nerve growth factor (NGF) and two different TRPV1 antagonists; capsazepine (non-specific) and SB-705498 (TRPV1-specific). Immunohistochemistry was used to assess post-WA TRPV1 expression in dorsal root ganglia and the presence of immunocytes in proximal and distal colon. Retrograde labelled and microdissected dorsal root ganglia sensory neurons were used to evaluate TRPV1 gene transcription. Results showed that acute stress induces colonic hypersensitivity in MS but not in NH rats. Hypersensitivity was prevented by prestress administration of doxantrazole and anti-NGF. Capsazepine inhibited and SB-705498 reversed poststress hypersensitivity. In MS rats, acute stress induced a slight increase in colonic mast cell numbers without further signs of inflammation. Post-WA TRPV1 transcription and expression was not higher in MS than NH rats. In conclusion, the present data on stress-induced visceral hypersensitivity confirm earlier reports on the essential role of mast cells and NGF. Moreover, the results also suggest that TRPV1 modulation (in the absence of overt inflammation) is involved in this response. Thus, mast cells and TRPV1 are potential targets to treat stress-induced visceral hypersensitivity.

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.