Dafne Balemans

Transient Receptor Potential Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades Underlying Visceral Hypersensitivity

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

245 Evidence for a New Mechanism Underlying Persistent Visceral Hypersensitivity and Increased Permeability in a Model of Post-Infectious IBS

and cortisol responses to stimulation with CRH and ACTH: 1) differ between IBS patients and healthy controls (HCs), 2) are affected by sex and EALs, 3) are associated with GR mRNA expression. Methods: Male and female Rome III+ IBS patients and HCs underwent CRH (1μg/kg ovine) and ACTH (250μg) stimulation tests with serial plasma ACTH and cortisol levels measured. GR mRNA levels were measured from peripheral blood mononuclear cells using real-time PCR. Presence of EALs <18 yrs of age was measured with the Trauma History questionnaire. Linear mixed effects models were used to compare ACTH and cortisol response measured across time between groups with a knot at the peak of the curve to model the rates of change from baseline to peak (rise) and from peak to the last time point (decline). Area under the curve (AUC) was also measured. Results: 60 IBS patients (65%F, mean age 33yrs) and 56 HCs (52%F, mean age 31yrs) underwent hormone stimulation studies. 74 IBS (65%F, mean age 33yrs) and 69 HCs (52%F, mean age 33yrs) had GR mRNA expression levels. A subset (37 IBS, 28 HCs) had both hormone stimulation and GR mRNA studies. 34 (60%) IBS patients and 27 (48%) HCs had +EAL. CRH stimulation test: IBS patients showed a slower rate of decline from peak ACTH levels than HCs (p<0.05). Women with IBS had a slower rate of decline of ACTH (p=0.01) and cortisol (p=0.001) than men with IBS. ACTH stimulation test: Cortisol responses were similar in IBS and HCs, likely due to a significant interaction between IBS and sex (p<0.001). Within women, IBS had a lower cortisol response (AUC) than HCs. In contrast, in men, IBS had a greater cortisol response than HCs. GR mRNA expression: Levels were lower in IBS vs HCs (p= 0.03), which was mainly seen within men. Lower GR mRNA levels were associated with faster rise in ACTH and cortisol responses (p=0.01-0.03). EAL did not have a significant effect on hormone responses. Conclusion: HPA axis response was dysregulated in IBS. The enhanced HPA axis response was associated with downregulation of the GR mediated negative feedback system. Sex differences in the stress response may play a role in the female predominance of IBS.

Evidence for long-term sensitization of the bowel in patients with post-infectious-IBS

Post-infectious irritable bowel syndrome (PI-IBS) is a common gastrointestinal disorder characterized by persistent abdominal pain despite recovery from acute gastroenteritis. The underlying mechanisms are unclear, although long-term changes in neuronal function, and low grade inflammation of the bowel have been hypothesized. We investigated the presence and mechanism of neuronal sensitization in a unique cohort of individuals who developed PI-IBS following exposure to contaminated drinking water 7 years ago. We provide direct evidence of ongoing sensitization of neuronal signaling in the bowel of patients with PI-IBS. These changes occur in the absence of any detectable tissue inflammation, and instead appear to be driven by pro-nociceptive changes in the gut micro-environment. This is evidenced by the activation of murine colonic afferents, and sensitization responses to capsaicin in dorsal root ganglia (DRGs) following application of supernatants generated from tissue biopsy of patients with PI-IBS. We demonstrate that neuronal signaling within the bowel of PI-IBS patients is sensitized 2 years after the initial infection has resolved. This sensitization appears to be mediated by a persistent pro-nociceptive change in the gut micro-environment, that has the capacity to stimulate visceral afferents and facilitate neuronal TRPV1 signaling.

Response to Keszthelyi and Masclee.

To the Editor: We thank Keszthelyi and Masclee (1) for their interest in our study and are pleased to note that they agree with our conclusion that transient receptor potential vanilloid 1 (TRPV1) may be an important player in the visceral hypersensitivity of irritable bowel syndrome (IBS) patients (2). The authors comment on the potential role of neuropeptides such as substance P, calictonine-gene-related peptide, and neurokinin A and regret that we did not measure these mediators to obtain a more mechanistic insight into the differential nociceptive responses observed in our study (2). The reasoning is that TRPV1 activation will lead to the release of these neuropeptides from nociceptive nerve fibers and presumably may further activate or sensitize TRPV1. Although this may theoretically be correct, no increase in mucosal substance P levels in IBS compared with healthy controls was measured by Keszthelyi et al. (3) in their recent study, making it less likely that substance P may be involved. To what extent CGRP or neurokinin A may sensitize or upregulate TRPV1 expression is definitely an interesting hypothesis to be further evaluated. We, however, hypothesized that the observed increased pain response to rectal application of capsaicin in the absence of increased expression of TRPV1 would result from TRPV1 sensitization by mast cell mediators rather than by neuropeptides. This hypothesis was based on the abundant evidence in the literature supporting a role for mast cells in IBS, and on our recent clinical study showing clinical improvement following treatment with the mast cell stabilizer and histamine 1 receptor antagonist ketotifen (4). To test this hypothesis, we pewrformed live calcium imaging recordings of submucosal neurons in rectal biopsies, and demonstrated that TRPV1 is sensitized in IBS patients compared with healthy controls, a finding that was mimicked by histamine and reversed by the histamine 1 receptor antagonist pyrilamine. Similar findings were obtained using murine dorsal root ganglia neurons, indicating that histamine, most likely released by mast cells, sensitizes TRPV1 through histamine 1 receptors (5). This was further corroborated by our recent proof-of-concept clinical trial showing improved global symptom relief and reduced abdominal pain in IBS patients during a 12-week treatment with the histamine 1 receptor antagonist ebastine (6). So, although neuropeptides released from afferent nerve fibers may potentially be involved in the sensitization of TRPV1, our recent evidence would rather indicate that mast cell mediators, in particular histamine, may be more likely.