L A Blackshaw

Different contributions of ASIC channels 1a, 2, and 3 in gastrointestinal mechanosensory function

Aims: Members of the acid sensing ion channel (ASIC) family are strong candidates as mechanical transducers in sensory function. The authors have shown that ASIC1a has no role in skin but a clear influence in gastrointestinal mechanotransduction. Here they investigate further ASIC1a in gut mechanoreceptors, and compare its influence with ASIC2 and ASIC3. Methods and results: Expression of ASIC1a, 2, and 3 mRNA was found in vagal (nodose) and dorsal root ganglia (DRG), and was lost in mice lacking the respective genes. Recordings of different classes of splanchnic colonic afferents and vagal gastro-oesophageal afferents revealed that disruption of ASIC1a increased the mechanical sensitivity of all afferents in both locations. Disruption of ASIC2 had varied effects: increased mechanosensitivity in gastro-oesophageal mucosal endings, decreases in gastro-oesophageal tension receptors, increases in colonic serosal endings, and no change in colonic mesenteric endings. In ASIC3-/- mice, all afferent classes had markedly reduced mechanosensitivity except gastro-oesophageal mucosal receptors. Observations of gastric emptying and faecal output confirmed that increases in mechanosensitivity translate to changes in digestive function in conscious animals. Conclusions: These data show that ASIC3 makes a critical positive contribution to mechanosensitivity in three out of four classes of visceral afferents. The presence of ASIC1a appears to provide an inhibitory contribution to the ion channel complex, whereas the role of ASIC2 differs widely across subclasses of afferents. These findings contrast sharply with the effects of ASIC1, 2, and 3 in skin, suggesting that targeting these subunits with pharmacological agents may have different and more pronounced effects on mechanosensitivity in the viscera.

Expression of taste molecules in the upper gastrointestinal tract in humans with and without type 2 diabetes

Objective: Nutrient feedback from the small intestine modulates upper gastrointestinal function and energy intake; however, the molecular mechanism of nutrient detection is unknown. In the tongue, sugars are detected via taste T1R2 and T1R3 receptors and signalled via the taste G-protein α-gustducin (Gαgust) and the transient receptor potential ion channel, TRPM5. These taste molecules are also present in the rodent small intestine, and may regulate gastrointestinal function. Subjects and methods: Absolute transcript levels for T1R2, T1R3, Gαgust and TRPM5 were quantified in gastrointestinal mucosal biopsies from subjects with and without type 2 diabetes; immunohistochemistry was used to locate Gαgust. Effects of luminal glucose on jejunal expression of taste molecules were also quantified in mice. Results: T1R2, T1R3, Gαgust and TRPM5 were preferentially expressed in the proximal small intestine in humans, with immunolabelling for Gαgust localised to solitary cells dispersed throughout the duodenal villous epithelium. Expression of T1R2, T1R3, TRPM5 (all p<0.05) and Gαgust (p<0.001) inversely correlated with blood glucose concentration in type 2 diabetes subjects but, as a group, did not differ from control subjects. Transcript levels of T1R2 were reduced by 84% following jejunal glucose perfusion in mice (p<0.05). Conclusions: Taste molecules are expressed in nutrient detection regions of the proximal small intestine in humans, consistent with a role in “tasting”. This taste molecule expression is decreased in diabetic subjects with elevated blood glucose concentration, and decreased by luminal glucose in mice, indicating that intestinal “taste” signalling is under dynamic metabolic and luminal control.

Post-inflammatory colonic afferent sensitisation: different subtypes, different pathways and different time courses.

Objective: Intestinal infection evokes hypersensitivity in a subgroup of patients with irritable bowel syndrome (IBS) long after healing of the initial injury. Trinitrobenzene sulfonic acid (TNBS)-induced colitis in rodents likewise results in delayed maintained hypersensitivity, regarded as a model of some aspects of IBS. The colon and rectum have a complex sensory innervation, comprising five classes of mechanosensitive afferents in the splanchnic and pelvic nerves. Their plasticity may hold the key to underlying mechanisms in IBS. Our aim was therefore to determine the contribution of each afferent class in each pathway towards post-inflammatory visceral hypersensitivity. Design: TNBS was administered rectally and mice were studied after 7 (acute) or 28 (recovery) days. In vitro preparations of mouse colorectum with attached pelvic or splanchnic nerves were used to examine the mechanosensitivity of individual colonic afferents. Results: Mild inflammation of the colon was evident acutely which was absent at the recovery stage. TNBS treatment did not alter proportions of the five afferent classes between treatment groups. In pelvic afferents little or no difference in response to mechanical stimuli was apparent in any class between control and acute mice. However, major increases in mechanosensitivity were recorded from serosal afferents in mice after recovery, while responses from other subtypes were unchanged. Both serosal and mesenteric splanchnic afferents were hypersensitive at both acute and recovery stages. Conclusions: Colonic afferents with high mechanosensory thresholds contribute to inflammatory hypersensitivity, but not those with low thresholds. Pelvic afferents become involved mainly following recovery from inflammation, whereas splanchnic afferents are implicated during both inflammation and recovery.

Acute effects of capsaicin on gastrointestinal vagal afferents.

Capsaicin is an important tool for investigation of thin afferent fibres, but its acute effects on subtypes of vagal afferent endings are unknown. In the gastrointestinal tract, these subtypes are: muscle endings (thought to be purely tension sensitive), mucosal endings (sensitive to stroking and chemical stimuli) and endings in the oesophagus with both properties. Acute capsaicin sensitivity was investigated in ferrets using in vivo and in vitro methods. Single-fibre activity was recorded from 63 vagal afferents: 12 Adelta-fibres, 15 C-fibres and 36 unclassified fibres with endings in the oesophagus (n=42), stomach (n=19) and duodenum (n=2). Responses to capsaicin occurred independently of motility changes and were therefore due to direct activation of the receptor ending. In the oesophagus in vivo, two of 10 tension receptors and one of one mucosal receptor responded to intraluminal application of 3.25 mM capsaicin. In the stomach and duodenum, five of 14 tension receptors and two of four mucosal receptors responded to close-systemic (32-164 nmol) capsaicin. In an in vitro gastro-oesophageal preparation, three of five tension, four of 21 mucosal and two of eight tension/mucosal receptors responded to topical application of 1mM capsaicin. Occurrence of responses was therefore unrelated to location of endings and isolation of tissue. Responsiveness was also unrelated to conduction velocity. Capsaicin caused desensitization of responses to further capsaicin application in 37% of afferents. It additionally caused cross-desensitization to mechanical stimuli, which was also seen in afferents that did not respond directly to capsaicin. In conclusion, capsaicin acutely activates all subtypes of gut vagal afferents in vivo and in vitro, although responsiveness is restricted to 30% of fibres and follows no specific pattern. Acute desensitization may be induced with or without a response.

Immune Activation in Irritable Bowel Syndrome: Can Neuroimmune Interactions Explain Symptoms?

Irritable bowel syndrome (IBS) is a functional disorder of the gastrointestinal (GI) tract characterized by pain or discomfort from the lower abdominal region, which is associated with altered bowel habit. Despite its prevalence, there is currently a lack of effective treatment options for patients. IBS has long been considered as a neurological condition resulting from alterations in the brain gut axis, but immunological alterations are increasingly reported in IBS patients, consistent with the hypothesis that there is a chronic, but low-grade, immune activation. Mediators released by immune cells act to either dampen or amplify the activity of GI nerves. Release of a number of these mediators correlates with symptoms of IBS, highlighting the importance of interactions between the immune and the nervous systems. Investigation of the role of microbiota in these interactions is in its early stages, but may provide many answers regarding the mechanisms underlying activation of the immune system in IBS. Identifying what the key changes in the GI immune system are in IBS and how these changes modulate viscerosensory nervous function is essential for the development of novel therapies for the underlying disorder.

TRP channels: new targets for visceral pain

How often do gastroenterologists advise patients to “avoid spicy food” without really knowing why? The answer may lie in the family of transient receptor potential (TRP) channels, which includes receptors for compounds found in common herbs and spices. This aim of this review is not really to answer this question, but it will shed light on why spices may cause symptoms. Instead, its aim is to explore the TRP family for potential targets that may in fact reduce gut symptoms. Chronic pain and discomfort of unknown origin in functional gastrointestinal disorders represent a large unmet need for treatment and consequent economic impact. There are also features of other gut disease which generate symptoms with obscure origins. In order to understand how symptoms are generated in the gut and transmitted to the central nervous system, we need to know at least three principles of extrinsic sensory nerve function—first, what types are there and what do they signal?; secondly, what is the molecular basis of sensory transduction?; and thirdly, how does all of this change in disease? These questions are a key focus of this article, with a particular emphasis on the role of TRP channels in each case. Although there are no drugs yet available for clinical use that target TRP channels, some of the early pointers are identified that hold promise for their use in pharmacotherapy of gastrointestinal sensory dysfunction. ### Subtypes of sensory fibres Peripheral endings of sensory afferent fibres can be classified into five subtypes in the mouse gastrointestinal tract according to the location of their mechanoreceptive fields.1 2 These are: mucosal, muscular (or tension receptor), muscular–mucosal (or tension–mucosal), serosal and mesenteric afferents (fig 1).1 2 3 4 Mucosal afferents respond exclusively to fine tactile stimulation of the luminal surface. Anatomically, they appear as bare endings in the lamina propria …

TRPV1-expressing sensory fibres and IBS: links with immune function

In a recent issue of Gut , van Beurden et al showed the usefulness of ultrasound in the diagnosis of intestinal worms in a case of human taeniasis ( Gut 2008; 57 :515 and 524). The examination was even more sensitive than the stool analysis, which remained negative. However, and specifically in the case of human taeniasis, ultrasound techniques might have a double diagnostic advantage: the detection of the intestinal adult tapeworm as well as the search of the presence of extraintestinal Taenia larval stages in the same patient. Human taeniasis caused by pig ( Taenia solium ) or cattle ( T saginata …