Intrarectal injection of glycerol induces hypersensitivity to rectal distension in healthy subjects without modifying rectal compliance

Visceral pain from colon and rectum: the mechanotransduction and biomechanics

Visceral pain is the cardinal symptom of functional gastrointestinal (GI) disorders such as the irritable bowel syndrome (IBS) and the leading cause of patients' visit to gastroenterologists. IBS-related visceral pain usually arises from the distal colon and rectum (colorectum), an intraluminal environment that differs greatly from environment outside the body in chemical, biological, thermal, and mechanical conditions. Accordingly, visceral pain is different from cutaneous pain in several key psychophysical characteristics, which likely underlies the unsatisfactory management of visceral pain by drugs developed for other types of pain. Colorectal visceral pain is usually elicited from mechanical distension/stretch, rather than from heating, cutting, pinching, or piercing that usually evoke pain from the skin. Thus, mechanotransduction, i.e., the encoding of colorectal mechanical stimuli by sensory afferents, is crucial to the underlying mechanisms of GI-related visceral pain. This review will focus on colorectal mechanotransduction, the process of converting colorectal mechanical stimuli into trains of action potentials by the sensory afferents to inform the central nervous system (CNS). We will summarize neurophysiological studies on afferent encoding of colorectal mechanical stimuli, highlight recent advances in our understanding of colorectal biomechanics that plays critical roles in mechanotransduction, and review studies on mechano-sensitive ion channels in colorectal afferents. This review calls for focused attention on targeting colorectal mechanotransduction as a new strategy for managing visceral pain, which can also have an added benefit of limited CNS side effects, because mechanotransduction arises from peripheral organs.

Plausibility criteria for putative pathophysiological mechanisms in functional gastrointestinal disorders: a consensus of experts

BACKGROUND AND AIMS

The functional gastrointestinal disorders (FGIDs) are extremely common conditions associated with a considerable personal, social and health economic burden. Managing FGIDs in clinical practice is challenging because of the uncertainty of symptom-based diagnosis, the high frequency of overlap between these conditions and the limited efficacy of available therapies. It has often been argued that successful drug development and management of FGIDs requires knowledge of the underlying pathophysiology. Numerous and highly variable candidate pathophysiological mechanisms have been implicated in the generation of FGID symptoms, but there is no current consensus on how to best define the relevance of these disturbances.

METHODS

A group of international experts on FGIDs developed plausibility criteria that should be fulfilled by relevant pathophysiological mechanisms in FGIDs.

RESULTS

Five criteria are proposed: (1) the presence of the abnormality in a subset of patients, (2) temporal association between proposed mechanism and symptom(s), (3) correlation between the level of impairment of the mechanism and symptom(s), (4) induction of the symptom(s) by provoking the pathophysiological abnormality in healthy subjects and (5) treatment response by a therapy specifically correcting the underlying disorder or congruent natural history of symptoms and dysfunction in the absence of specific therapy. Based on strength of evidence for these five criteria according to the Grading of Recommendations Assessment, Development and Evaluation system, a plausibility score can be calculated for each mechanism.

CONCLUSION

Evaluation of the strength of evidence for candidate pathophysiological abnormalities fulfilling these five plausibility criteria will help to identify the most relevant mechanisms to target for novel diagnostic approaches and for the development of new therapies.

Pharmacological, Pharmacokinetic, and Pharmacogenomic Aspects of Functional Gastrointestinal Disorders

This article reviews medications commonly used for the treatment of patients with functional gastrointestinal disorders. Specifically, we review the animal models that have been validated for the study of drug effects on sensation and motility; the preclinical pharmacology, pharmacokinetics, and toxicology usually required for introduction of new drugs; the biomarkers that are validated for studies of sensation and motility endpoints with experimental medications in humans; the pharmacogenomics applied to these medications and their relevance to the FGIDs; and the pharmacology of agents that are applied or have potential for the treatment of FGIDs, including psychopharmacologic drugs.

A randomized, double-blind, placebo-controlled trial of polyethylene glycol effects on fasting and postprandial rectal sensitivity and symptoms in hypersensitive constipation-predominant irritable bowel syndrome

AIM

To assess the effect of polyethylene glycol 3350 (PEG) on fasting and postprandial (PP) perception of rectal distension and symptoms in hypersensitive constipation-predominant irritable bowel syndrome (IBS-C).

METHODS

Forty-two patients meeting Rome II criteria for IBS-C and with a pain threshold of < 32 mmHg were included in a randomized, double-blind, placebo-controlled trial. Patients received either oral PEG, 3.45 g t.i.d. orally for 30 days or placebo. Rectal sensitivity was assessed before and after treatment with a barostat using the ascending method of limits, during basal and PP periods.

RESULTS

No changes in fasting and PP rectal tone and thresholds for first sensation, gas sensation, urge to defecate, and pain was observed with PEG in relation to placebo. In both groups, pressure at which patients crossed the thresholds for noxious (PEG: from 28 ± 8.8 to 22 ± 6.9 mmHg) and non noxious (PEG: from 16 ± 4.9 to 12 ± 3.6 mmHg) stimuli decreased compared with pretreatment values. PEG improved consistency of faeces and showed a trend to diminish blood in faeces. PEG and placebo increased bowel movements per week (P < 0.001), and relieved symptoms without significant side-effects.

CONCLUSIONS

Both PEG 3350 and placebo were clinically useful in patients with IBS-C, an effect that cannot be explained by changes in rectal tone and sensation. The results support the concept that visceral sensitivity is not stable and has a heterogeneous response to drugs, and suggest the existence of a post healing hypersensitivity state.

Central processing of noxious somatic stimuli in patients with irritable bowel syndrome compared with healthy controls

OBJECTIVE

To compare a central analgesic mechanism known as diffuse noxious inhibitory controls (DNIC) using somatic test stimuli and somatic conditioning stimuli, (CS) in irritable bowel syndrome (IBS) patients and healthy controls.

METHODS

Participants were 48 premenopausal females (27 with IBS), mean age of 29 years. The phasic heat test stimulus (peak temperature, 50 degrees C) was applied to the left palm. The DNIC effect, which measured reductions in average pain ratings (APR) during counter irritation (submersion of the participant's right hand in painful 12 degrees C circulating water) compared with baseline, was compared between groups. In addition, a second, counterbalanced, CS protocol (right hand submerged in nonpainful 32 degrees C circulating water) was performed. Differences in APR between the 2 counterirritation protocols were compared between groups to control for nonspecific effects known to influence DNIC. Psychologic measures and cardiovascular reactivity were also assessed.

RESULTS

IBS patients demonstrated smaller DNIC than controls (P=0.011, repeated measures analysis of variance), and greater state-anxiety, depression, catastrophizing, and anger-out expression (P<0.05). Group differences in DNIC were enhanced after controlling for nonspecific effects occurring during the nonpainful CS, and for psychologic measures (P=0.001, repeated measures analysis of covariance). There were no group differences in age, cardiovascular reactivity, APR, or pain ratings for the 12 degrees C CS.

DISCUSSION

These data demonstrate deficient DNIC in IBS. This is the first study to adequately control for alternative explanations of pain reduction during counterirritation. Only by controlling for nonspecific effects can evidence of deficient DNIC be attributed to dysregulation in endogenous analgesic mechanisms.

Novel techniques to study visceral hypersensitivity in irritable bowel syndrome

Visceral hypersensitivity has emerged as a key hypothesis in explaining the painful symptoms of irritable bowel syndrome (IBS), and it has been proposed as a "biologic marker" for the condition. Visceral hypersensitivity can be influenced by peripheral and central mechanisms affecting pain perception. The optimal method for its assessment in humans has not been determined. Current techniques include stimulation via the computerized barostat and electrical stimulation, response measures including the lower limb reflex, and brain imaging modalities such as functional MRI and positron emission tomography. It has been shown that IBS patients have decreased sensory thresholds to colonic and rectal balloon distention by barostat. Studies using electrical stimulation and the RIII lower limb reflex have further confirmed enhanced visceral perception in IBS. Evidence from more recent neuro-imaging studies suggests that IBS patients have abnormal activation of brain circuits involved in emotional and cognitive modulation of sensory information, resulting in ineffective pain modulation; these circuits may have a pathophysiologic role in enhancing visceral perception. There are few effective pharmacologic treatments that relieve IBS symptoms, and improved understanding of brain-gut interactions and factors relating to enhanced visceral perception may guide us in developing more efficacious treatments.

Mechanisms of hypersensitivity in IBS and functional disorders

General introduction The concept of visceral hypersensitivity is accepted as being germane to several functional gastrointestinal disorders (FGIDs). The causes or risk factors associated with this hypersensitivity are unclear. This article addresses the proposed mechanisms leading to hypersensitivity: from genetic to inflammatory disorders, from central to peripheral alterations of function. However, in order to place visceral hypersensitivity in a more global perspective as an aetiological factor for FGIDs, it also provides a review of recent evidence regarding the role of other peripheral mechanisms (the intraluminal milieu), as also genetic factors in the pathophysiology of these disorders. The article has been divided into five independent sections. The first three sections summarize the evidence of visceral hypersensitivity as a biological marker of functional gut disorders, the peripheral and central mechanisms involved, and the role of inflammation on hypersensitivity. In opposition to visceral hypersensitivity as an isolated phenomenon in functional gut disorders, the last two sections focus on the importance of peripheral mechanisms, like motor disturbances, specifically those resulting on altered transport of intestinal gas, and alterations of the intraluminal milieu and genetics.

Serotonergic and non-serotonergic targets in the pharmacotherapy of visceral hypersensitivity

Visceral hypersensitivity is considered a key mechanism in the pathogenesis of functional gastrointestinal (GI) disorders. Targeting visceral hypersensitivity seems an attractive approach to the development of drugs for functional GI disorders. This review summarizes current knowledge on targets for the treatment of visceral hypersensitivity, and the status of current and future drug and probiotic treatment development, and the role of pharmacogenomic factors.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

Pain and mechanical properties of the rectum in patients with active ulcerative colitis

BACKGROUND

The pain, urgency, and incontinence in ulcerative colitis may be related to changes in viscoelastic properties of the gut wall or to alterations of the sensory pathways. In the present study, we used an advanced rectal probe to study the mechanosensory and smooth muscle properties in patients with active disease.

METHODS

Nine patients with ulcerative colitis (mean age 39.5 years) with exacerbation limited to the rectum and sigmoid colon and 17 age-matched healthy subjects were included. The rectum was distended before and after pharmacological relaxation of the smooth muscle until moderate pain was reported, and the cross-sectional area, volume, pressure, tension, and strain were computed. To investigate central integration of a tonic stimulus, the bag was finally distended to the pain threshold; then, the cross-sectional area was held constant for 2 min.

RESULTS

The patients were hypersensitive to mechanical stimuli as assessed by the cross-sectional area (F = 21.7; P < 0.001). There were no differences in compliance or stiffness between the 2 groups, but the hypersensitivity was abolished after muscle relaxation. Together with the muscle analysis, this finding demonstrated that the smooth muscles were tonically contracted in the inflamed rectum, resulting in a decreased rectal circumference. The tonic distensions did not evoke central integration of the pain response, indicating that hyperalgesia is more likely related to peripheral factors.

CONCLUSIONS

Patients with active ulcerative colitis have hypersensitivity and increased tone of the smooth muscles, which may explain the symptoms. Drugs that affect smooth muscle contraction may be helpful in difficult cases.

Alterations of sensori-motor functions of the digestive tract in the pathophysiology of irritable bowel syndrome

Pathophysiology of irritable bowel syndrome (IBS) is based upon multiple factors that have been organised in a comprehensive model centred around the brain-gut axis. The brain-gut axis encompasses nerve pathways linking the enteric and the central nervous systems and contains a large proportion of afferent fibres. Functionally and anatomically, visceral nerves are divided in to two categories: the parasympathetic pathways distributing to the upper gut through the vagi and to the hindgut, through the pelvic and pudendal nerves, and the sympathetic pathways, arising form the spinal cord and distributing to the midgut via the paravertebral ganglia. Several abnormalities of gut sensori-motor function have been described in patients with IBS. Abnormal motility patterns have been described at the intestinal and colonic levels. Changes in colonic motility are mainly related to bowel disturbances linked to IBS but do not correlate with pain. More recently, visceral hypersensitivity has been recognised as a main characteristic of patients with IBS. It is defined by an exaggerated perception of luminal distension of various segments of the gut and related to peripheral changes in the processing of visceral sensations as well as modulation of perception by centrally acting factors including mood and stress. Viscero-visceral reflexes link the two edges of the brain-gut axis and may account for the origin of symptoms in some pathological conditions. Recent advances in the understanding of the role of myenteric plexus allowed recognition of several neurotransmitters involved at the level of both the afferent and efferent pathways. Targeting the receptors of these neurotransmitters is a promising way for development of new treatments for IBS.

Tachykinin NK2 receptor antagonists for the treatment of irritable bowel syndrome

Tachykinin NK2 receptors are expressed in the gastrointestinal tract of both laboratory animals and humans. Experimental data indicate a role for these receptors in the regulation of intestinal motor functions (both excitatory and inhibitory), secretions, inflammation and visceral sensitivity. In particular, NK2 receptor stimulation inhibits intestinal motility by activating sympathetic extrinsic pathways or NANC intramural inhibitory components, whereas a modulatory effect on cholinergic nerves or a direct effect on smooth muscle account for the NK2 receptor-mediated increase in intestinal motility. Accordingly, selective NK2 receptor antagonists can reactivate inhibited motility or decrease inflammation- or stress-associated hypermotility. Intraluminal secretion of water is increased by NK2 receptor agonists via a direct effect on epithelial cells, and this mechanism is active in models of diarrhoea since selective antagonists reverse the increase in faecal water content in these models. Hyperalgesia in response to intraluminal volume signals is possibly mediated through the stimulation of NK2 receptors located on peripheral branches of primary afferent neurones. NK2 receptor antagonists reduce the hyper-responsiveness that occurs following intestinal inflammation or application of stressful stimuli to animals. Likewise, NK2 receptor antagonists reduce intestinal tissue damage induced by chemical irritation of the intestinal wall or lumen. In healthy volunteers, the selective NK2 antagonist nepadutant reduced the motility-stimulating effects and irritable bowel syndrome-like symptoms triggered by intravenous infusion of neurokinin A, and displayed other characteristics that could support its use in patients. It is concluded that blockade of peripheral tachykinin NK2 receptors should be considered as a viable mechanism for decreasing the painful symptoms and altered bowel habits of irritable bowel syndrome patients.

Alterations of enteric mucosal mast cells in patients with irritable bowel syndrome

AIM: To study alterations of mast cells in colon mucosa of patients with irritable bowel syndrome.

METHODS: Ten healthy volunteers and 20 patients with irritable bowel syndrome entered in this study. By colonoscopy biopsies, the mast cells (MC) in mucosa of cecal colon, transverse colon and rectum were stained by immunohistochemistry and the number of mast cells and percentage of degranulated mast cells were counted.

RESULTS: MC number increased significantly in the cecum and transverse colon of diarrhea IBS patients, compared with controls (P <0.01 or P<0.05). MC number increased significantly in the cecum of constipation IBS patients compared with controls (P<0.05). MC number was significantly increased in the caecum of diarrhea IBS patients than constipation IBS patients (P<0.05). The percentages of degranulated mast cells in cecal colon, transverse colon and rectum mucosa of diarrhea IBS patients were significantly higher than those in constipation IBS patients and controls (P<0.01or P<0.05).

CONCLUSION: The mast cells play an important role in the pathogenesis of irritable bowel syndrome.

Gut pain and hyperalgesia induced by capsaicin: a human experimental model

Human experimental visceral pain models using chemical stimulation are needed for the study of visceral hyperexcitability. Our aim was to stimulate the human gut with chemical activators (capsaicin, glycerol) and measure quantitatively the induced hyperexcitability to painful mechanical gut distension. Ten otherwise healthy subjects with an ileostoma participated. Increasing volumes of capsaicin 50 microg/ml (0.25, 0.5, 0.75, 1.0, 1.5, 2.0, and 3 ml), glycerol (2.5, 5, and 10 ml) or saline (2.5, 5, and 10 ml) intermingled with sham stimuli were randomly applied to the ileum via the stomal opening at three occasions separated by a week. After each application, pain intensity, qualities, and referred pain area were assessed together with the pain threshold to distension of the proximal gut. 'Boring' and 'hot' pain were evoked in all subjects by low doses (median 0.5 ml) of capsaicin. The median pain onset, peak pain, and pain duration were 55, 85, and 420 s, respectively. Referred somatic pain developed around the stomal opening with a correlation between the pain area and pain intensity. After application of capsaicin, significant hyperalgesia was found to distension of the gut (a 28% reduction pressure in pain threshold). No significant manifestations were found after application of glycerol and saline. Application of capsaicin to the human ileum induces pain and mechanical hyperalgesia. Specific activation of nociceptors in the gut mucosa provides new possibilities to study clinical relevant visceral pain mechanisms.

Induction of non-painful and painful intestinal sensations by hypertonic saline: a new human experimental model

BACKGROUND AND AIMS

To develop a pain model for chemical stimulation of the human gut.

METHODS

In a double-blind experimental study 10 subjects with a previously performed sigmoidostomy were randomised to receive injections with either isotonic or hypertonic saline in the colonic mucosa. In the hypertonic experimental arm, 0.1 ml of 0.9%, 2%, 4%, and 6% and 0.2 ml of 2% and 4% saline were given. In the placebo arm, six 0.9% saline injections of the same quantities were given. In a separate experiment 0.8 ml 4% saline was infused into the mucosa by a pump over a period of 2min. The pain intensity was rated on a 0-10 visual analogue scale with 5 as the pain threshold.

RESULTS

The hypertonic saline injections resulted in local as well as referred non-painful and painful sensations in 9 out of the 10 subjects. The evoked sensations were mostly described as a smarting sensation with an intensity of median 1 (range 0-5.6) for 0.1 ml 2% saline to median 2.9 (range 0-6.2) for 0.2 ml 4% saline. Seven subjects reported referred sensations to the abdominal skin. Continuous infusion of 4% saline resulted in a consistent sensory response in all subjects, with a median intensity of 4.1 (range 2.1-8.1). This sensory intensity was reproducible in 70% in a retest experiment after median 7 weeks. In the placebo arm a total of 70 isotonic saline injections only resulted in inconsistent, short-lasting non-painful sensations in three subjects.

CONCLUSION

The model represents a safe method for direct chemical activation of the sensory endings in the human gut. The model may be used for pharmacological screening of analgesics and for basic investigations in patients suffering from gastrointestinal diseases.