Gastrointestinal afferents as targets of novel drugs for the treatment of functional bowel disorders and visceral pain

Immunomodulatory actions of a Polynesian herb Noni (Morinda citrifolia) and its clinical applications

Morinda citrifolia (Noni) is a popular traditional medicinal plant consumed in various forms in several countries around the world as a complementary and alternative treatment due to its established health benefits. Noni is rich in bioactive substances and has significantly exhibited pro-oxidant and immunomodulatory effects. In this review, we highlight the pharmacological basis related to the phytochemicals and polysaccharides present in Noni and its potential therapeutic effects. We screened electronic databases such as PubMed, Google Scholar, Scopus for scientific literature. Our results indicate that Noni is beneficial for various diseases with its crude extracts showing therapeutic benefit for a wide range of pathological diseases. We believe that further pharmacological and toxicological studies in addition to well-designed controlled clinical trials can validate Noni to be an effective and novel natural product for prophylactic and therapeutic use of several diseases.

Ion channels, ion channel receptors, and visceral hypersensitivity in irritable bowel syndrome

Ion channels are expressed throughout the gastrointestinal system and regulate nearly every aspect of digestion, including fluid secretion and absorption, motility, and visceral sensitivity. It is therefore not surprising that in the setting of functional bowel disorders, such as irritable bowel syndrome (IBS), ion channels are often altered in terms of expression level and function and are a target of pharmacological intervention. This is particularly true of their role in driving abdominal pain through visceral hypersensitivity (VH), which is the main reason IBS patients seek medical care. In the study by Scanzi et al., in the current issue of this journal, they provide evidence that the T-type voltage-gated calcium channel (Ca_v ) Ca_v 3.2 is upregulated in human IBS patients, and is necessary for the induction of an IBS-like disease state in mice. In this mini-review, we will discuss the contribution of specific ion channels to VH in IBS, both in human patients and rodent models. We will also discuss how Ca_v 3.2 may play a role as an integrator of multiple environmental stimuli contributing toward VH.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Clinical Endocannabinoid Deficiency Reconsidered: Current Research Supports the Theory in Migraine, Fibromyalgia, Irritable Bowel, and Other Treatment-Resistant Syndromes

Medicine continues to struggle in its approaches to numerous common subjective pain syndromes that lack objective signs and remain treatment resistant. Foremost among these are migraine, fibromyalgia, and irritable bowel syndrome, disorders that may overlap in their affected populations and whose sufferers have all endured the stigma of a psychosomatic label, as well as the failure of endless pharmacotherapeutic interventions with substandard benefit. The commonality in symptomatology in these conditions displaying hyperalgesia and central sensitization with possible common underlying pathophysiology suggests that a clinical endocannabinoid deficiency might characterize their origin. Its base hypothesis is that all humans have an underlying endocannabinoid tone that is a reflection of levels of the endocannabinoids, anandamide (arachidonylethanolamide), and 2-arachidonoylglycerol, their production, metabolism, and the relative abundance and state of cannabinoid receptors. Its theory is that in certain conditions, whether congenital or acquired, endocannabinoid tone becomes deficient and productive of pathophysiological syndromes. When first proposed in 2001 and subsequently, this theory was based on genetic overlap and comorbidity, patterns of symptomatology that could be mediated by the endocannabinoid system (ECS), and the fact that exogenous cannabinoid treatment frequently provided symptomatic benefit. However, objective proof and formal clinical trial data were lacking. Currently, however, statistically significant differences in cerebrospinal fluid anandamide levels have been documented in migraineurs, and advanced imaging studies have demonstrated ECS hypofunction in post-traumatic stress disorder. Additional studies have provided a firmer foundation for the theory, while clinical data have also produced evidence for decreased pain, improved sleep, and other benefits to cannabinoid treatment and adjunctive lifestyle approaches affecting the ECS.

Surgical intestinal manipulation increases gene expression of TrkA, CGRP, and PAR-2 IN dorsal root ganglia in the rat

BACKGROUND

Surgical handling of the bowel evokes degranulation of peritoneal mast cells (PMC). Nonetheless, role of PMCs in postoperative ileus (POI) is somewhat controversial. We aimed to investigate if intestinal manipulation elicits changes in afferent mediators related to MC activation and alteration of gastrointestinal (GI) motility.

METHODS

Postoperative ileus was induced by intestinal manipulation in Sprague-Dawley rats. Additionally, compound 48/80 (C48/80) and ketotifen were used to modulate MC activity. Rat mast cell protease 6 (RMCP-6, ELISA) release was determined in peritoneal lavage 20 min after intestinal manipulation. At 24 h, GI transit was determined. Gene expression of calcitonin gene-related peptide (CGRP), protease-activated receptor-2 (PAR-2), nerve growth factor (NGF), and TrkA receptor was determined (PCR) in dorsal root ganglia (DRG). Ileal wall inflammation was assessed by myeloperoxidase (MPO) activity, interleukin-6 expression (IL-6).

KEY RESULTS

Intestinal manipulation and exposure to C48/80-induced degranulation of PMCs delayed GI transit and up-regulated IL-6 and MPO activity. Intestinal manipulation, but not C48/80, up-regulated CGRP, PAR-2, and NGF/TrkA in DRGs. Ketotifen only improved gastric emptying and fecal output. Up-regulation of CGRP and TrkA expression in DRG was not prevented by ketotifen.

CONCLUSIONS & INFERENCES

Postoperative ileus is accompanied by activation of CGRP, NGF-TrkA, and PAR-2 in DRGs. Our results suggest that these mediators could be a target in further POI studies in order to find new therapeutic targets for this medical condition.

Acute tryptophan depletion reduces kynurenine levels: implications for treatment of impaired visuospatial memory performance in irritable bowel syndrome

RATIONALE

A visuospatial episodic memory impairment has recently been identified in irritable bowel syndrome. Increased tryptophan metabolism along the kynurenine pathway has also been reported in irritable bowel syndrome, which may play a role in altered cognitive performance as peripheral kynurenine can cross the blood brain barrier and lead to the production of neuroactive metabolites, which modulate glutamatergic and cholinergic signalling, key neurotransmitter systems involved in cognitive function.

OBJECTIVES

Utilising the acute tryptophan depletion (ATD) protocol, the aim of this study was to examine if manipulating peripheral levels of tryptophan regulates cognitive performance in irritable bowel syndrome and also to determine for the first time if the ATD protocol alters kynurenine supply to the central nervous system.

METHODS

In this double-blind, placebo-controlled, crossover design study, nine female patients with irritable bowel syndrome and 14 matched female healthy controls participant completed a range of tests from the CANTAB(®) following ATD and placebo. Plasma tryptophan and kynurenine, self-report measures of gastrointestinal symptoms, mood and arousal were determined pre- and post-treatment on each study day.

RESULTS

Following placebo (p = 0.016) but not ATD (p > 0.05), patients with irritable bowel syndrome exhibited impaired visuospatial memory performance (Paired Associates Learning (PAL) test). In addition, ATD significantly decreased (p < 0.001) and placebo significantly increased (p < 0.001) plasma kynurenine levels in both groups.

CONCLUSIONS

Manipulating peripheral tryptophan and kynurenine levels using ATD modulates hippocampal-mediated cognitive performance in irritable bowel syndrome but not healthy controls. These data may have important implications for reducing cognitive impairment in irritable bowel syndrome.

Acupuncture for visceral pain: neural substrates and potential mechanisms

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. Despite much advances, the pathophysiological mechanism is still poorly understood comparing with its somatic counterpart and, as a result, the therapeutic efficacy is usually unsatisfactory. Acupuncture has long been used for the management of numerous disorders in particular pain and visceral pain, characterized by the high therapeutic benefits and low adverse effects. Previous findings suggest that acupuncture depresses pain via activation of a number of neurotransmitters or modulators including opioid peptides, serotonin, norepinephrine, and adenosine centrally and peripherally. It endows us, by advancing the understanding of the role of ion channels and gut microbiota in pain process, with novel perspectives to probe the mechanisms underlying acupuncture analgesia. In this review, after describing the visceral innervation and the relevant afferent pathways, in particular the ion channels in visceral nociception, we propose three principal mechanisms responsible for acupuncture induced benefits on visceral pain. Finally, potential topics are highlighted regarding the future studies in this field.

Visceral hypersensitivity in symptomatic diverticular disease and the role of neuropeptides and low grade inflammation

BACKGROUND

  Recurrent abdominal pain is reported by a third of patients with diverticulosis, particularly those with previous episodes of acute diverticulitis. The current understanding of the etiology of this pain is poor. Our aim was to assess visceral sensitivity in patients with diverticular disease and its association with markers of previous inflammation and neuropeptides.

METHODS

  Patients with asymptomatic and symptomatic diverticular disease underwent a flexible sigmoidoscopy and biopsy followed 5-10 days later by visceral sensitivity testing with barostat-mediated rectal distension. Inflammation was assessed by staining of serotonin (5HT) and CD3 positive cells. mRNA levels of tumor necrosis factor alpha (TNF α) and interleukin-6 (IL-6) were quantitated using RT-PCR. Neuropeptide expression was assessed from percentage area staining with substance P (SP) and mRNA levels of the neurokinin 1 & 2 receptors (NK1 & NK2), and galanin 1 receptor (GALR1).

KEY RESULTS

  Thirteen asymptomatic and 12 symptomatic patients were recruited. The symptomatic patients had a lower first reported threshold to pain (28.4 mmHg i.q.r 25.0-36.0) than the asymptomatic patients (47 mmHg i.q.r 36.0-52.5, P < 0.001). Symptomatic patients had a higher median overall pain rating for the stimuli than the asymptomatic patients (P < 0.02). Symptomatic patients had greater median relative expression of NK1 and TNF alpha mRNA compared with asymptomatic patients. There was a significant correlation between barostat VAS pain scores and NK 1 expression (Figure 4, r(2) 0.54, P < 0.02).

CONCLUSIONS & INFERENCES

  Patients with symptomatic diverticular disease exhibit visceral hypersensitivity, and this may be mediated by ongoing low grade inflammation and upregulation of tachykinins.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Stress-related alterations of visceral sensation: animal models for irritable bowel syndrome study

Stressors of different psychological, physical or immune origin play a critical role in the pathophysiology of irritable bowel syndrome participating in symptoms onset, clinical presentation as well as treatment outcome. Experimental stress models applying a variety of acute and chronic exteroceptive or interoceptive stressors have been developed to target different periods throughout the lifespan of animals to assess the vulnerability, the trigger and perpetuating factors determining stress influence on visceral sensitivity and interactions within the brain-gut axis. Recent evidence points towards adequate construct and face validity of experimental models developed with respect to animals' age, sex, strain differences and specific methodological aspects such as non-invasive monitoring of visceromotor response to colorectal distension as being essential in successful identification and evaluation of novel therapeutic targets aimed at reducing stress-related alterations in visceral sensitivity. Underlying mechanisms of stress-induced modulation of visceral pain involve a combination of peripheral, spinal and supraspinal sensitization based on the nature of the stressors and dysregulation of descending pathways that modulate nociceptive transmission or stress-related analgesic response.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Increased plasma d-lactic acid associated with impaired memory in rats

AIM

d-Lactic acidosis is associated with memory impairment in humans. Recent research indicates that d-lactic acid may inhibit the supply of energy from astrocytes to neurons involved with memory formation. However, little is known about the effects of increased hind-gut fermentation due to changes in diet on circulating lactic acid concentrations and memory.

METHOD

Thirty-six male Wistar rats were fed three dietary treatments: a commercial rat and mouse chow, a soluble carbohydrate based diet or a fermentable carbohydrate based diet. The parameters estimating memory were examined by employing the object recognition test. Physical parameters of fermentation including hind-gut and plasma lactic acid concentrations were examined after sacrifice, either 3 or 21h after feeding.

RESULTS

Increased fermentation in the hind-gut of rats, indicated by lower caecum pH, was associated with increased plasma l-lactic acid (r=-0.41, p=0.020) and d-lactic acid (r=-0.33, p=0.087). Memory, being able to discriminate between a familiar and a novel object during the object recognition test, was reduced with increasing plasma d-lactic acid (r=-0.51, p=0.021).

CONCLUSIONS

Memory impairment was associated with alterations in plasma d-lactic acid following the fermentation of carbohydrate in the hind-gut. Further work is still required to determine whether these effects are mediated centrally or via direct connections through the enteric nervous system.

The role of peripheral cannabinoid receptors type 1 in rats with visceral hypersensitivity induced by chronic restraint stress

Background/Aims

This study was designed to investigate the possibility that the enhanced nociceptive responsiveness associated with canabonoid type 1 receptors (CB1Rs) and identify its role in mediating visceral hypersensitivity induced by chronic restraint stress.

Methods

Rats were exposed to daily partial restraint stress or sham partial restraint stress with intraperitoneal injection of the vehicle, CB1R agonist or antagonist for 4 consecutive days. We tested the visceromotor reflex to colorectal distention at day 0 and 5. Reverse-transcription polymerase chain reaction and Western blot were used to assess the expression of CB1Rs.

Results

Intraperitoneal CB1 agonist (ACEA) injection significantly diminished (p < 0.05) the enhanced visceromotor reflex to colorectal distention at day 5 in stressed rats. Change in electromyogram response after ACEA over baseline, at pressure of 40 mmHg (+13.3 ± 2.2), 60 mmHg (+15.3 ± 2.8) and 80 mmHg (+17.0 ± 4.0) were much lower than in the control animals, which were +35.9 ± 5.1, +41.1 ± 6.3 and +54.1 ± 9.6, respectively. Whereas, CB1 antagonist (SR141716A) had an opposite effect. Compared with control group, the change in electromyogram response after SR141716A over baseline was significantly enhanced (p < 0.05) for the distending pressure of 40 mmHg (+56.0 ± 10.3), 60 mmHg (+74.6 ± 12.3) and 80 mmHg (+82.9 ± 11.0), respectively. Reverse-transcription polymerase chain reaction and Western blotting demonstrated the stress-induced up-regulation of colon CB1Rs (p < 0.05).

Conclusions

Our results suggest there is a key contribution of peripheral CB1Rs involved in the maintenance of visceral hyperalgesia after repeated restraint stress, providing a novel mechanism for development of peripheral visceral sensitization.

Altered expression of P2X3 in vagal and spinal afferents following esophagitis in rats

Purinergic P2X(3) receptors are predominantly expressed in small diameter primary afferent neurons and activation of these receptors by adenosine triphosphate is reported to play an important role in nociceptive signaling. The objective of this study was to investigate the expression of P2X(3) receptors in spinal and vagal sensory neurons and esophageal tissues following esophagitis in rats. Two groups of rats were used including 7 days fundus-ligated (7D-ligated) esophagitis and sham-operated controls. Esophagitis was produced by ligating the fundus and partial obstruction of pylorus that initiated reflux of gastric contents. The sham-operated rats underwent midline incision without surgical manipulation of the stomach. Expressions of P2X(3) receptors in thoracic dorsal root ganglia (DRGs), nodose ganglia (NGs), and esophageal tissues were evaluated by RT-PCR, western blot and immunohistochemistry. Esophageal neurons were identified by retrograde transport of Fast Blue from the esophagus. There were no significant differences in P2X(3) mRNA expressions in DRGs (T1-T3) and NGs between 7D-ligated and sham-operated rats. However, there was an upregulation of P2X(3) mRNA in DRGs (T6-T12) and in the esophageal muscle. At protein level, P2X(3) exhibited significant upregulation both in DRGs and in NGs of rats having chronic esophagitis. Immunohistochemical analysis exhibited a significant increase in P2X(3) and TRPV1 co-expression in DRGs and NGs in 7D-ligated rats compared to sham-operated rats. The present findings suggest that chronic esophagitis results in upregulation of P2X(3) and its co-localization with TRPV1 receptor in vagal and spinal afferents. Changes in P2X(3) expression in vagal and spinal sensory neurons may contribute to esophageal hypersensitivity following acid reflux-induced esophagitis.

Review article: visceral hypersensitivity in irritable bowel syndrome: molecular mechanisms and therapeutic agents

BACKGROUND

Although development of visceral pain is an important defensive mechanism, hypersensitivity results in a significant clinical problem and is likely to be one of the major factors involved in the pathogenesis of abdominal and chest pain in functional bowel disorders (FBDs). Understanding of the molecular mechanisms involved in peripheral sensitization of visceral nociceptors has advanced as a result of the experimental studies, especially in animal models, which have led to knowledge and identification of key mediators and receptors.

AIM

To provide a comprehensive review focused on the peripheral mechanisms believed to be responsible for sensitization and potential molecular targets for a disorder which is common, distressing and has sub-optimal treatment options.

METHODS

Literature review using Ovid and Pubmed from 1966.

RESULTS

There is substantial interest in the development of new drugs for treatment of FBDs in the background of advances in understanding the molecular and physiological mechanisms of visceral hypersensitivity. The potential drug targets include TPRV1, ASICs, voltage-gated sodium channels, ATP, PAR-2, cannabinoid, prostaglandin, tachykinin and 5HT(3) receptors.

CONCLUSION

It is anticipated that with advancing molecular understanding of the basis of visceral hypersensitivity, the next decade will see accelerated development of new molecules for treatment of functional bowel diseases.

A novel herbal preparation desensitizes mesenteric afferents to bradykinin in the rat small intestine

Herbal preparations are evolving as promising agents for the treatment of functional gastrointestinal disorders which are considered to be secondary to visceral hypersensitivity. We aimed to determine whether a new combination of six herbal extracts reduces the sensitivity of intestinal afferents in rat. Male Wistar rats (250-350 g, n = 6 per group) were gavaged with either vehicle or 2.5, 5 or 10 mL kg(-1) of STW 5-II, a herbal preparation which contains six extracts. Two hours later, animals were anaesthetized and extracellular multi-unit mesenteric afferent nerve recordings were obtained in the proximal jejunum in vivo. Afferent discharge to 5-hydroxy-tryptamine (5-HT) (5, 10, 20 and 40 microg kg(-1), i.v.), luminal distension (0-60 mmHg) and bradykinin (BK) (15, 30 and 60 microg kg(-1), i.v.) was recorded. At baseline, spontaneous afferent discharge was not different following pretreatment with the various doses of STW 5-II compared with vehicle. The pressure-dependent increase in afferent discharge to intraluminal ramp distension and the dose-dependent increase in afferent firing following 5-HT were also uninfluenced by STW 5-II pretreatment. In contrast, the afferent nerve responses to 15, 30 and 60 microg kg(-1) of BK were reduced following 10 mL kg(-1) STW 5-II with peaks at 106 +/- 19, 153 +/- 22 and 156 +/- 25 imp s(-1) compared with 160 +/- 15, 228 +/- 14 and 220 +/- 16 imp s(-1) following vehicle pretreatment (mean +/- SEM, P < 0.05). Intestinal afferent sensitivity to BK which plays a prime role in nociception was reduced following STW 5-II. Thus, STW 5-II may be of therapeutic use for conditions that involve neuronal hypersensitivity and the release of BK in the intestine.

Gastrointestinal endocannabinoid system: multifaceted roles in the healthy and inflamed intestine

1. The endogenous cannabinoid (endocannabinoid) system is emerging as a key modulator of intestinal physiology, influencing motility, secretion, epithelial integrity and immune function in the gut, in addition to influencing satiety and emesis. 2. Accumulating evidence suggests that the endocannabinoid system may play a pivotal role in the pathophysiology of gastrointestinal disease, particularly in the light of recent studies demonstrating an effect of endocannabinoids on the development of experimental inflammation and linkages with functional clinical disorders characterized by altered motility. 3. The predominant endocannabinoids, anandamide and 2-arachidonoylglycerol, not only mediate their effects via two recognized cannabinoid receptor subtypes, namely CB(1) and CB(2), but emerging evidence now shows they are also substrates for cyclo-oxygenase (COX)-2, generating a distinct and novel class of prostaglandin ethanolamides (prostamides) and prostaglandin glycerol esters. These compounds are bioactive and may mediate an array of biological effects distinct to those of conventional prostanoids. 4. The effects of prostamides on gastrointestinal motility, secretion, sensation and immune function have not been characterized extensively. Prostamides may play an important role in gastrointestinal inflammation, particularly given the enhanced expression of both COX-2 and endocannabinoids that occurs in the inflamed gut. 5. Further preclinical studies are needed to determine the therapeutic potential of drugs targeting the endocannabinoid system in functional and inflammatory gut disorders, to assist with the determination of feasibility for clinical translation.

Intraesophageal chemicals enhance responsiveness of upper thoracic spinal neurons to mechanical stimulation of esophagus in rats

Esophageal hypersensitivity is one of the most common causes of noncardiac chest pain in patients. In this study, we investigated whether exposure of the esophagus to acid and other chemical irritants affected activity of thoracic spinal neurons responding to esophageal distension (ED) in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital sodium-anesthetized, -paralyzed, and -ventilated male rats. ED (0.2 or 0.4 ml, 20 s) was produced by water inflation of a latex balloon placed orally into the middle thoracic region of the esophagus. The chemicals were administered via a tube that was passed through the stomach and placed in the thoracic esophagus. To irritate the esophagus, 0.2 ml of HCl (0.01 N), bradykinin (10 microg/ml), or capsaicin (10 microg/ml) were injected for 1-2 min. Only neurons excited by ED were included in this study. Results showed that intraesophageal instillation of HCl, bradykinin, and capsaicin increased activity in 3/20 (15%), 7/25 (28%), and 9/20 (45%) neurons but enhanced excitatory responses to ED in 9/17 (53%), 8/15 (53%), and 7/11 (64%) of the remaining spinal neurons, respectively. Furthermore, intraesophageal chemicals were more likely to enhance the responsiveness of low-threshold neurons than high-threshold neurons to the esophageal mechanical stimulus. Normal saline (pH 7.4, 0.2 ml) or vehicle instilled in the esophagus did not significantly affect activity or ED responses of neurons. We conclude that enhanced responses of thoracic spinal neurons to ED by the chemically challenged esophagus may provide a possible pathophysiological basis for visceral hypersensitivity in patients with gastroesophageal reflux and/or esophagitis.

Purinergic receptors in gastrointestinal inflammation

Purinergic receptors comprise a family of transmembrane receptors that are activated by extracellular nucleosides and nucleotides. The two major classes of purinergic receptors, P1 and P2, are expressed widely in the gastrointestinal tract as well as immune cells. The purinergic receptors serve a variety of functions from acting as neurotransmitters, to autocoid and paracrine signaling, to cell activation and immune response. Nucleosides and nucleotide agonist of purinergic receptors are released by many cell types in response to specific physiological signals, and their levels are increased during inflammation. In the past decade, the advent of genetic knockout mice and the development of highly potent and selective agonists and antagonists for the purinergic receptors have significantly advanced the understanding of purinergic receptor signaling in health and inflammation. In fact, agonist/antagonists of purinergic receptors are emerging as therapeutic modalities to treat intestinal inflammation. In this article, the distribution of the purinergic receptors in the gastrointestinal tract and their physiological and pathophysiological role in intestinal inflammation will be reviewed.

Low dose lipid formulations: effects on gastric emptying and biliary secretion

PURPOSE

Food stimulates changes to gastrointestinal secretion and motility patterns, however, the effect of smaller quantities of lipid, such as that contained in a lipid-based drug formulation, has not been detailed. This study aimed to examine the effects of small quantities of lipid on gastric emptying and biliary secretion.

METHODS

The influence of oral administration of three lipid-based formulations and a negative control formulation on gastric emptying and biliary secretion was evaluated in 16 healthy male subjects using gamma scintigraphy, ultrasonography and duodenal aspiration.

RESULTS

Low quantities (2 g) of long chain lipid stimulated gall bladder contraction and elevated intestinal bile salt, phospholipid and cholesterol levels. Changes in gastric emptying were also evident, although these did not reach statistical significance. Administration of a similar quantity of medium chain lipid, however, had little effect on gastric emptying and gallbladder contraction and did not stimulate appreciable increases in intestinal concentrations of biliary-derived lipids.

CONCLUSIONS

The quantities of long chain lipid that might be administered in a pharmaceutical formulation stimulate gallbladder contraction and elevate intestinal levels of bile salt and phospholipid. This effect is a likely contributor to the ability of lipid based formulations to enhance the absorption of poorly water-soluble drugs.

Cannabinoid 1 (CB1) receptors coupled to cholinergic motorneurones inhibit neurogenic circular muscle contractility in the human colon

The effects of cannabinoid subtype 1 (CB(1)) receptor activation were determined on smooth muscle, inhibitory and excitatory motorneuronal function in strips of human colonic longitudinal muscle (LM) and circular muscle (CM) in vitro. Electrical field stimulation (EFS; 0.5-20 Hz, 50 V) evoked a relaxation in LM and CM precontracted with a neurokinin-2 (NK-2) selective receptor agonist (beta-ala(8)-neurokinin A; 10(-6) M) in the presence of atropine (10(-6) M); this was unaltered following pretreatment with the CB(1)-receptor selective agonist arachidonyl-2-chloroethylamide (ACEA; 10(-6) M). In the presence of nitric oxide synthase blockade with N-nitro-L-arginine (10(-4) M), EFS evoked a frequency-dependent 'on-contraction' during stimulation and an 'off-contraction' following stimulus cessation. On-contractions were significantly inhibited in CM strips by pretreatment with ACEA (10(-6) M). These inhibitory effects were reversed in the presence of the CB(1) receptor-selective antagonist N-(piperidine-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (10(-7) M). ACEA did not alter LM or CM contractile responses to acetylcholine or NK-2 receptor-evoked contraction. Immunohistochemical studies revealed a colocalisation of CB(1) receptors to cholinergic neurones in the human colon based on colabelling with choline acetyltransferase, in addition to CB(1) receptor labelling in unidentified structures in the CM. In conclusion, activation of CB(1) receptors coupled to cholinergic motorneurones selectively and reversibly inhibits excitatory nerve transmission in colonic human colonic CM. These results provide evidence of a direct role for cannabinoids in the modulation of motor activity in the human colon by coupling to cholinergic motorneurones.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

Adenosine 5'-triphosphate and its relationship with other mediators that activate pelvic nerve afferent neurons in the rat colorectum

Evidence of a role for purinergic signalling in visceral afferents involving P2X₂, P2X₃ and P2Y₁ receptors exists, which appears to be important during inflammation. This study aimed to evaluate the degree of interaction between adenosine 5′-triphosphate (ATP) and other mediators that activate sensory nerves in the colorectum. Recordings from pelvic nerve afferents were made during application of agents to the in-vitro colorectal preparation. Analysis allowed calculation of single unit activity. When applied individually, bradykinin (78%) and 5-hydoxytryptamine (77%) activated the greatest number of neurons, followed by substance P, protons, ATP and capsaicin. Prostaglandin E₂ stimulated the least number (54%) and had a longer latency. Seventy-seven percent of all units studied either responded to both ATP and capsaicin or to neither, giving the greatest degree of activity correlation. Five percent of units were activated by all seven agents and no units were activated by a single agent alone. 5-hydroxytryptamine, capsaicin and protons, when co-applied with ATP, increased pelvic nerve activity to a greater degree than the sum of the individual responses. It is concluded that ATP activates pelvic nerve afferents and acts synergistically with protons, capsaicin and 5-hydroxytryptamine. The pattern of neuronal activation suggests that visceral afferents are polymodal but the receptor expression on their terminals is variable.

Review article: current and emerging therapies for functional dyspepsia

Functional dyspepsia represents a heterogeneous group of gastrointestinal disorders marked by the presence of upper abdominal pain or discomfort. Although its precise definition has evolved over the last several decades, this disorder remains shrouded in controversy. The symptoms of functional dyspepsia may overlap with those of other functional bowel disorders including irritable bowel syndrome and non-erosive reflux disease. There may be coexistent psychological distress or disease complicating its presentation and response to therapy. Given the prevalence and chronicity of functional dyspepsia, it remains a great burden to society. Suspected physiological mechanisms underlying functional dyspepsia include altered motility, altered visceral sensation, inflammation, nervous system dysregulation and psychological distress. Yet the exact pathophysiological mechanisms that cause symptoms in an individual patient remain difficult to delineate. Numerous treatment modalities have been employed including dietary modifications, pharmacological agents directed at various targets within the gastrointestinal tract and central nervous system, psychological therapies and more recently, complementary and alternative treatments. Unfortunately, to date, all of these therapies have yielded only marginal results. A variety of emerging therapies are being developed for functional dyspepsia. Most of these therapies are intended to normalize pain perception and gastrointestinal motor and reflex function in this group of patients.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Possible role of nitric oxide in visceral hypersensitivity in patients with irritable bowel syndrome

Visceral hypersensitivity is a consistent finding in a considerable proportion of patients with irritable bowel syndrome (IBS), and may provide a physiological basis for the development of IBS symptoms. In this study, we aimed to confirm the hypothesis that nitric oxide (NO) is involved in maintaining visceral hypersensitivity in IBS. Ten healthy volunteers (HV) and 12 IBS patients with documented hypersensitivity to rectal distension underwent a rectal barostat study. The effect of placebo and the specific NO synthase inhibitor NG -monomethyl-L-arginine (L-NMMA) on resting volume, rectal sensitivity to distension and rectal compliance was evaluated in a double-blind, randomized, cross-over fashion. NG -monomethyl-L-arginine did not alter resting volumes in HV or IBS patients. In HV, l-NMMA did not alter rectal sensory thresholds compared to placebo (45 +/- 3 and 46 +/- 3 mmHg, respectively). In contrast, L-NMMA significantly increased the threshold for discomfort/pain in IBS patients (placebo: 18 +/- 2, l-NMMA: 21 +/- 3 mmHg, P < 0.05). Rectal compliance was not affected by L-NMMA. Although NO does not seem to play a major role in normal rectal sensation or tone, we provide evidence that NO may be involved in the pathophysiology of visceral hypersensitivity in IBS.

Gastrointestinal pain in functional bowel disorders: sensory neurons as novel drug targets

Functional bowel disorders (FBDs) are defined by symptoms of gastrointestinal (GI) dysfunction, discomfort and pain in the absence of a demonstrable organic cause. Since the prevalence of FBDs, particularly functional dyspepsia and irritable bowel syndrome, can be as high as 20%, FBDs represent a significant burden in terms of direct healthcare and productivity costs. There is emerging evidence that the discomfort and pain experienced by many FBD patients is due to persistent hypersensitivity of primary afferent neurons, which may develop in response to infection, inflammation or other insults. This concept identifies vagal and spinal sensory neurons as important targets for novel therapies of GI hyperalgesia. Sensory neuron-specific targets can be grouped into three categories: receptors and sensors at the peripheral nerve terminals, ion channels relevant to nerve excitability and conduction and transmitter receptors. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons, such as the transient receptor potential channel TRPV1, acid-sensing ion channels and tetrodotoxin-resistant Na + channels.

Review article: endocannabinoids and their receptors in the enteric nervous system

The therapeutic actions of cannabinoids have been known for centuries. In the last 25 years this area of research has grown exponentially with the discovery of specific cannabinoid receptors and endogenous ligands. In the enteric nervous system of gastrointestinal tract, cannabinoid receptors are located on enteric nerve terminals where they exert inhibitory actions on neurotransmission to reduce motility and secretion. Endogenous cannabinoids are present in the enteric nervous system, as are the degradative enzymes necessary to inhibit their action. The cellular mechanism of action of endocannabinoids has not been established in the enteric nervous system. Endocannabinoids not only act at cannabinoid receptors, but potentially also at vanilloid and 5-HT3 receptors, both of which are expressed in the gastrointestinal tract. The interactions between endocannabinoids and these other important receptor systems have not been extensively investigated. A greater understanding of the endocannabinoid system in the enteric nervous system could lead to advances with important therapeutic potential in the treatment of gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, secretory diarrhoea and gastro-oesophageal reflux disease.

Functional abdominal pain

Functional abdominal pain or functional abdominal pain syndrome (FAPS) is an uncommon functional gut disorder characterised by chronic or recurrent abdominal pain attributed to the gut but poorly related to gut function. It is associated with abnormal illness behaviour and patients show psychological morbidity that is often minimised or denied in an attempt to discover an organic cause for symptoms. Thus the conventional biomedical approach to the management of such patients is unhelpful and a person's symptom experience is more usefully investigated using a biopsychosocial evaluation, which necessarily entails a multidisciplinary system of healthcare provision. Currently the pathophysiology of the disorder is poorly understood but is most likely to involve a dysfunction of central pain mechanisms either in terms of attentional bias, for example, hypervigilance or a failure of central pain modulation/inhibition. Although modern neurophysiological investigation of patients is promising and may provide important insights into the pathophysiology of FAPS, current clinical management relies on an effective physician-patient relationship in which limits on clinical investigation are set and achievable treatment goals tailored to the patient's needs are pursued.

The endocannabinoid system in the physiology and pathophysiology of the gastrointestinal tract

Numerous investigations have recently demonstrated the important roles of the endocannabinoid system in the gastrointestinal (GI) tract under physiological and pathophysiological conditions. In the GI tract, cannabinoid type 1 (CB1) receptors are present in neurons of the enteric nervous system and in sensory terminals of vagal and spinal neurons, while cannabinoid type 2 receptors are located in immune cells. Activation of CB1 receptors was shown to modulate several functions in the GI tract, including gastric secretion, gastric emptying and intestinal motility. Under pathophysiological conditions induced experimentally in rodents, the endocannabinoid system conveys protection to the GI tract (e.g. from inflammation and abnormally high gastric and enteric secretions). Such protective activities are largely in agreement with anecdotal reports from folk medicine on the use of Cannabis sativa extracts by subjects suffering from various GI disorders. Thus, the endocannabinoid system may serve as a potentially promising therapeutic target against different GI disorders, including frankly inflammatory bowel diseases (e.g. Crohn's disease), functional bowel diseases (e.g. irritable bowel syndrome) and secretion- and motility-related disorders. As stimulation of this modulatory system by CB1 receptor agonists can lead to unwanted psychotropic side effects, an alternative and promising avenue for therapeutic applications resides in the treatment with CB1 receptor agonists that are unable to cross the blood-brain barrier, or with compounds that inhibit the degradation of endogenous ligands (endocannabinoids) of CB1 receptors, hence prolonging the activity of the endocannabinoid system.

Frontiers in functional dyspepsia

Functional dyspepsia (FD) refers to unexplained pain or discomfort in the upper abdomen and is commonly seen in gastroenterology practice. The underlying pathophysiologic mechanisms associated with FD are unclear, although traditionally, delayed gastric emptying, visceral hypersensitivity to acid or mechanical distention, and impaired gastric accommodation have been implicated as putative physiologic disturbances. It also remains uncertain whether FD and irritable bowel syndrome are different presentations of the same disorder. Recent data on pathophysiologic mechanisms of FD have focused on postprandial motor disturbances (accelerated gastric emptying, antral-fundic incoordination, and abnormal phasic contractions), alterations of neurohormonal mechanisms in response to a meal, and previous acute infection. Pharmacologic therapies for FD may be guided by these novel mechanisms, as current available therapeutic options are limited. Novel prokinetics and gastric accommodation modulators, visceral analgesics, and agents targeting the neurohormonal response to food ingestion are the next therapeutic frontiers in FD. This review summarizes traditional knowledge and more recent advances in the pathophysiology of FD and potential therapeutic opportunities.

Review article: drugs interfering with visceral sensitivity for the treatment of functional gastrointestinal disorders--the clinical evidence

At present, the concept of visceral hypersensitivity provides the leading hypothesis regarding the generation of symptoms in functional gastrointestinal disorders. This paper discusses the current clinical evidence for drugs that have been proposed to interfere with visceral sensitivity in functional gastrointestinal disorders. Several possible pharmacological targets have been identified to reduce visceral pain and to reverse the processes underlying the persistence of visceral hypersensitivity. However, most of the available evidence comes from experimental animal models and cannot simply be extrapolated to patients with functional gastrointestinal disorders. In this review, we selected five drug classes that have been shown to exhibit visceral analgesic properties in experimental studies, and of which data were available regarding their clinical efficacy. These included opioid substances, serotonergic agents, antidepressants, somatostatin analogues and alpha(2)-adrenergic agonists. Although clinical trials show a limited benefit, in particular for serotonergic agents, the evidence illustrating that these effects result from normalization of visceral sensation is currently lacking. Therefore, we conclude that the concept of targeting visceral hypersensitivity as a treatment for functional gastrointestinal disorders is still controversial. Future evaluations require patient selection based on the presence of visceral hypersensitivity and application of compounds that exhibit 'true' viscerosensory effects.

Cannabinoids and the digestive tract

In the digestive tract there is evidence for the presence of high levels of endocannabinoids (anandamide and 2-arachidonoylglycerol) and enzymes involved in the synthesis and metabolism of endocannabinoids. Immunohistochemical studies have shown the presence of CB1 receptors on myenteric and submucosal nerve plexuses along the alimentary tract. Pharmacological studies have shown that activation of CB1 receptors produces relaxation of the lower oesophageal sphincter, inhibition of gastric motility and acid secretion, as well as intestinal motility and secretion. In general, CB1-induced inhibition of intestinal motility and secretion is due to reduced acetylcholine release from enteric nerves. Conversely, endocannabinoids stimulate intestinal primary sensory neurons via the vanilloid VR1 receptor, resulting in enteritis and enhanced motility. The endogenous cannabinoid system has been found to be involved in the physiological control of colonic motility and in some pathophysiological states, including paralytic ileus, intestinal inflammation and cholera toxin-induced diarrhoea. Cannabinoids also possess antiemetic effects mediated by activation of central and peripheral CB1 receptors. Pharmacological modulation of the endogenous cannabinoid system could provide a new therapeutic target for the treatment of a number of gastrointestinal diseases, including nausea and vomiting, gastric ulcers, secretory diarrhoea, paralytic ileus, inflammatory bowel disease, colon cancer and gastro-oesophageal reflux conditions.

Combined determination with functional and morphological studies of origin of nerve fibers expressing transient receptor potential vanilloid 1 in the myenteric plexus of the rat jejunum

The aim of this study was to determine the action of capsaicin in isolated rat intestine and the origin of nerve fibers expressing transient receptor potential vanilloid 1 (TRPV1: capsaicin receptor) in the rat jejunum by combination of functional and immunohistochemical experiments. Capsaicin (1 microM) produced a prolonged relaxation response (52. +/-15.3% of the relaxation response to papaverine, mean +/- S.D., n=27) of the isolated jejunum in the presence of atropine and guanethidine. Pretreatment with the TRPV1 antagonist, capsazepine (10 microM) and ruthenium red (3 microM) significantly reduced the relaxation response to capsaicin by 78% (P<0.01) and 38% (P<0.05), respectively. Tetrodotoxin and calcitonin gene-related peptide (CGRP)-desensitization significantly reduced the response to capsaicin by 72% (P<0.01) and 42% (P<0.01), respectively. Therefore, we investigated the distribution of TRPV1-immunoreactivity (IR) in the myenteric plexus of the rat jejunum. Using antisera raised against either the N-terminal or C-terminal domains of rat TRPV1, TRPV1-IR was present in the nerve fibers, but not in the cell bodies of myenteric neurons. These TRPV1-immunoreactive nerve fibers were running in myenteric ganglia and their interconnecting strands. Most TRPV1-immunoreactive nerve fibers showed CGRP-IR, whereas few VR1-immunoreactive nerve fibers showed substance P-IR. After chronic denervation of the extrinsic nerve supply to the jejunum, both the relaxation response to capsaicin and TRPV1-immunoreactive nerve fibers completely disappeared. These findings indicate that these TRPV1-immunoreactive nerve fibers in the rat jejunum derive from extrinsic neurons and that activation of TRPV1 produces the relaxation response in the rat jejunum, at least in part, through the release of CGRP from nerve fibers expressing TRPV1.

Functional dyspepsia: drugs for new (and old) therapeutic targets

The therapeutic management of functional dyspepsia remains a major challenge for the gastroenterologist. Current therapies available are based on putative underlying pathophysiologic mechanisms, including gastric acid sensitivity, slow gastric emptying and Helicobacter pylori infection, but only a small proportion of patients achieve symptomatic benefit from these therapeutic approaches. Relatively novel mechanistic concepts under testing include impaired gastric accomodation, visceral hypersensitivity, and central nervous system dysfunction. Serotonergic modulators (e.g. the 5-HT4 agonist tegaserod, the 5-HT3 antagonist alosetron and the 5-HT1P agonist sumatriptan), CCK-1 antagonists (e.g. dexloxiglumide), opioid agonists (e.g. asimadoline), N-methyl-D-aspartate (NMDA) receptor antagonists (e.g dextromethorphan), neurokinin antagonists (e.g. talnetant), capsaicin-like agents and antidepressants are among the agents currently under investigation. It seems unlikely, however, that targeting a single mechanism with an individual drug will result in complete symptom remission in most cases.

The endogenous cannabinoid system protects against colonic inflammation

Excessive inflammatory responses can emerge as a potential danger for organisms' health. Physiological balance between pro- and anti-inflammatory processes constitutes an important feature of responses against harmful events. Here, we show that cannabinoid receptors type 1 (CB1) mediate intrinsic protective signals that counteract proinflammatory responses. Both intrarectal infusion of 2,4-dinitrobenzene sulfonic acid (DNBS) and oral administration of dextrane sulfate sodium induced stronger inflammation in CB1-deficient mice (CB1(-/-)) than in wild-type littermates (CB1(+/+)). Treatment of wild-type mice with the specific CB1 antagonist N-(piperidino-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-pyrazole-3-carboxamide (SR141716A) mimicked the phenotype of CB1(-/-) mice, showing an acute requirement of CB1 receptors for protection from inflammation. Consistently, treatment with the cannabinoid receptor agonist R(-)-7-hydroxy-Delta(6)-tetra-hydrocannabinol-dimethylheptyl (HU210) or genetic ablation of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH) resulted in protection against DNBS-induced colitis. Electrophysiological recordings from circular smooth muscle cells, performed 8 hours after DNBS treatment, revealed spontaneous oscillatory action potentials in CB1(-/-) but not in CB1(+/+) colons, indicating an early CB1-mediated control of inflammation-induced irritation of smooth muscle cells. DNBS treatment increased the percentage of myenteric neurons expressing CB1 receptors, suggesting an enhancement of cannabinoid signaling during colitis. Our results indicate that the endogenous cannabinoid system represents a promising therapeutic target for the treatment of intestinal disease conditions characterized by excessive inflammatory responses.

Enteric P2X receptors as potential targets for drug treatment of the irritable bowel syndrome

The irritable bowel syndrome (IBS) is a gastrointestinal motility disorder affecting millions of patients. IBS symptoms include diarrhea, constipation and pain. The etiology of IBS is due partly to changes in the function of nerves supplying the gastrointestinal tract, immune system activation and to psychological factors. P2X receptors are multimeric ATP-gated cation channels expressed by neuronal and non-neuronal cells. Sensory nerve endings in the gastrointestinal tract express P2X receptors. ATP released from gastrointestinal cells activates P2X receptors on sensory nerve endings to stimulate motor reflexes and to transmit nociceptive signals. Antagonists acting at P2X receptors on sensory nerves could attenuate abdominal pain in IBS patients. Primary afferent neurons intrinsic to the gut, and enteric motor- and interneurons express P2X receptors. These neurons participate in motor reflexes. Agonists acting at enteric P2X receptors may enhance gastrointestinal propulsion and secretion, and these drugs could be useful for treating constipation-predominant IBS. Antagonists acting at enteric P2X receptors would decrease propulsion and secretion and they might be useful for treating diarrhea-predominant IBS. Current knowledge of P2X receptor distribution and function in the gut of laboratory animals provides a rational basis for further exploration of the therapeutic potential for drugs acting at P2X receptors in IBS patients. However, more information about P2X receptor distribution and function in the human gastrointestinal tract is needed. Data on the distribution and function of P2X receptors on gastrointestinal immune cells would also provide insights into the therapeutic potential of P2X receptor agents in IBS.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Purinergic mechanisms contribute to mechanosensory transduction in the rat colorectum

BACKGROUND & AIMS

Adenosine 5'-triphosphate plays a role in peripheral sensory mechanisms and, in particular, mechanosensory transduction in the urinary system. P2X(3) receptors are selectively expressed on small-diameter sensory neurons in the dorsal root ganglia; sensory neurons from dorsal root ganglia L1 and S1 supply the colorectum. This study investigated whether purinergic signaling contributes to mechanosensory transduction in the rat colorectum.

METHODS

A novel in vitro rat colorectal preparation was used to elucidate whether adenosine 5'-triphosphate is released from the mucosa in response to distention and to evaluate whether it contributes to sensory nerve discharge during distention.

RESULTS

P2X(3) receptor immunostaining was present on subpopulations of neurons in L1 and S1 dorsal root ganglia, which supply the rat colorectum. Distention of the colorectum led to pressure-dependent increases in adenosine 5'-triphosphate release from colorectal epithelial cells and also evoked pelvic nerve excitation, which was mimicked by application of adenosine 5'-triphosphate and alpha,beta-methylene adenosine 5'-triphosphate. The sensory nerve discharges evoked by distention were potentiated by alpha,beta-methylene adenosine 5'-triphosphate and ARL-67156, an adenosine triphosphatase inhibitor, and were attenuated by the selective P2X(1), P2X(3), and P2X(2/3) antagonist 2',3'-O-trinitrophenyl-adenosine 5'-triphosphate and by the nonselective P2 antagonists pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid and suramin. Adenosine, after ectoenzymatic breakdown of adenosine 5'-triphosphate, seems to be involved in the longer-lasting distention-evoked sensory discharge. Single-fiber analysis showed that high-threshold fibers were particularly affected by alpha,beta-methylene adenosine 5'-triphosphate, suggesting a correlation between purinergic activation and nociceptive stimuli.

CONCLUSIONS

Adenosine 5'-triphosphate contributes to mechanosensory transduction in the rat colorectum, and this is probably associated with pain.

Evolving concepts in functional gastrointestinal disorders: promising directions for novel pharmaceutical treatments

In recent years there has been an increasing appreciation of the complexity of functional gastrointestinal disorders. These represent a spectrum of conditions which may affect any part of the gastrointestinal tract in which there appears to be dysregulation of visceral function and afferent sensation and a strong association with emotional factors and stress. There is a clear psychological dimension, with up to 60% of irritable bowel syndrome (IBS) patients reported to have psychological co-morbidities and altered pain perception is also common in comparison with control populations. The role of the enteric nervous system, the sensory pathways and the brain as well as the influence of the latter on sympathetic and parasympathetic outflow have likewise attracted increasing interest and have led to exciting new methods to study their complex interactions. The concept of low-grade inflammation, such as might occur after infection, acting as a trigger for neuromuscular dysfunction has also led to the broad integrative hypotheses that help to explain the biopsychosocial dimensions seen in functional gastrointestinal disease. The multi-component model places a major emphasis on neurogastroenterology and enteric and neuro-immune interactions where new approaches to pharmacotherapy lie. Drugs may affect motility, visceral sensation and other aspects of gut function such as secretion or absorption. More particularly, however, has been the search for and attempts to influence important mediators of these primary gut functions. Such targets include serotonin and selected 5-HT receptors, which are involved in gut motility, visceral sensation and other aspects of gut function, CCK receptors which are involved in the mediation of pain in the gut and nociception in the CNS, opioid receptors involved in pain in the brain, spinal cord and periphery, muscarinic M3-receptors, substance P and neurokinin A and B receptors which are involved in motor adaptation and pain transmission in association with inflammation, gabba receptors involved in nociception and cannabinoid receptors which are involved in the control of acetyl choline release in the gut. With a better understanding of the structures and pathways involved in visceral perception and hyperalgesia, in the CNS, spinal cord and the gut and new pharmacological tools we will be better able to elucidate the neuropharmacology of visceral perception and its relationship to gut dysfunction. It is likely that there will be multiple therapeutic options based on the spectrum of abnormalities capable of causing the spectrum of symptoms of functional gastrointestinal disorders in any individual patient.

Acid-sensing pathways in rat gastrointestinal mucosa

The gastrointestinal mucosa serves as the interface between the luminal contents, including nutrients and injurious substances, and submucosal structures. Secreted gastric acid is one of the principal injurious components of the luminal contents. To be protected against harm from this acid, the epithelium has an "early warning" system that can activate potent defense mechanisms. We studied the mechanisms that defend the epithelium against luminal acid-induced injury, including the regulation of epithelial intracellular pH (pHi), blood flow, and mucus gel secretion in the perfused rat duodenum, and the pathways involved in the activation and regulation of these mechanisms. Physiological concentrations of luminal acid acidified the epithelial cells and increased blood flow (hyperemic response) and mucus gel thickness. The hyperemic response to acid was abolished by inhibitors of the Na+/H+ exchange, vanilloid receptors (VR), calcitonin gene-related peptide (CGRP) receptors, and nitric oxide (NO) synthase, and also by sensory afferent denervation, but not by pretreatment with a nonselective cyclooxygenase (COX) inhibitor. Mucus secretion in response to luminal acid was delayed by an interruption to the capsaicin pathway, which includes VR, capsaicin-sensitive afferent nerves, CGRP, and NO, and was abolished by COX inhibition. These observations support the hypothesis that the capsaicin pathway is an acid-sensing pathway that promotes hyperemia and mucus secretion in response to luminal acid. The COX pathway is a secondary regulatory system for mucus secretion. A similar acid-sensing capsaicin pathway is also present in the colon, suggesting that the gastrointestinal mucosa "tastes" luminal acidity through epithelial-VR communication.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.

Involvement of kappa-opioid receptors in peripheral response to nerve stimulation in kappa-opioid receptor knockout mice

1 The present study aimed to evaluate the role of kappa-opioid receptors at two peripheral sites, the vas deferens and the proximal colon, in kappa-opioid receptor knockout mice. We investigated the role of the kappa-opioid receptor in the vas deferens twitch response and in the colonic "off-contraction", a rebound contractile response which follows the inhibitory response to low frequencies stimulation (10, 20, 30 Hz) and which has been suggested to "locally" reproduce the contractile component of the peristaltic reflex. 2 Transmural stimulation of the vas deferens at lower frequencies (10 Hz, 10 V, 1 ms pulse trains lasting 0.5 s) evoked a contractile response that was significantly higher in the preparations from knockout mice because of lack of kappa-opioid receptors than in wild type mice. A selective kappa-opioid receptor agonist, U-50,488H, induced a dose-dependent inhibition of the electrically stimulated contraction in vas deferens. The percentages of reduction of the twitch response were significantly lower in knockout mice than in wild type mice after treatment with U-50,488H. The reduction of twitch response caused by U-50,488H was not reversed by administration of nor-binaltorphimine (nor-BNI) (5 x 10-6 m), a selective kappa-opioid receptor antagonist, in preparations from both knockout mice and wild type mice. U-50,488H has no effect on postsynaptic adrenergic receptors, as its administration did not affect the direct contractile response to noradrenaline. 3 Transmural stimulation (5 Hz, 20 V, 2 ms pulse trains lasting 30 s) induced inhibition of spontaneous activity of colonic strips during the period of stimulation, followed by an "off-contraction" after the cessation of stimulation. The statistical evaluation of the "off-contraction" responses between the two strains showed no significant difference. The off-contraction, measured in specimens from knockout mice, was inhibited concentration-dependently by U-50,488H (P < 0.01) and significantly less than from wild type mice. 4 The effect of U-50,488H was not reversed by administration of nor-BNI (5 x 10-6 m), either in preparations from knockout mice or from wild type mice. 5 Our data may suggest that kappa-opioid receptors are involved in some peripheral responses to the nerve stimulation, as indicated by the effect of U-50,488H, a selective kappa-opioid receptor agonist. However, the involvement of kappa-opioid receptor was also present, although less apparent, in kappa -opioid receptor knockout mice, suggesting either that this drug acts not only on kappa-opioid receptors but also on other receptor sites, such as kappa-like receptors. An alternative interpretation can be related to a sodium channel blocking action of U-50,488H, which could explain the inhibitory effects of twitch response still present but less evident in knockout strain and the lack of effect of the antagonist nor-BNI.