Abolishment of TNBS-induced visceral hypersensitivity in mast cell deficient rats
Introduction
The irritable bowel syndrome (IBS) is the most common functional gut disorder characterized by abdominal pain or discomfort (Foxx-Orenstein and Clarida, 2001, Olden, 2002). A growing number of studies have demonstrated that colorectal sensory threshold to mechanical distention stimuli is markedly decreased in IBS patients, indicating a phenomenon referred to as visceral hypersensitivity (Thompson et al., 1999, Camilleri and Prather, 1992). Although the mechanisms underlying the phenomenon remained to be fully investigated, recent studies have suggested that mucosal mast cells play a key role in the pathogenesis of visceral hypersensitivity associated with IBS (Wood, 2002, Barbara et al., 2007) Mucosal mast cells locate in close proximity to the enteric sensory nerves throughout the gut, and degranulate in response to various mechanical and chemical stimuli, releasing histamine, tryptase, eicosanoids and cytokines that can excite the sensory nerves (Stead et al., 1989, Bueno et al., 1997, Galli et al., 1999, Stevens and Austen, 1989). In addition, it has been demonstrated that some IBS patients show a prominent colonic mucosal mast cell infiltration associated with increased degranulation rate and spontaneous release of histamine and tryptase (Barbara et al., 2004, Barbara et al., 2007), suggesting that mediators released from activated mast cells are responsible for the increased sensitivity of enteric sensory nerves in patients with IBS.
The role of mucosal mast cells in the pathogenesis of visceral pain has also been investigated in rats. Coelho et al. reported that mast cell degranulation induced by lipopolysaccharide and BrX-537A resulted in a significant increase in the rectal sensitivity to mechanical distention in rats, and that the hypersensitivity was significantly suppressed by pretreatment with doxantrazole, an inhibitor of mast cell mediator release (Coelho et al., 2000). More recently, it was demonstrated that visceral hypersensitivity occurred in the rat acetic acid colitis model in association with the higher degranulation rate of colonic mucosal mast cells (La et al., 2004). These findings point to the key role played by intestinal mast cells in development of visceral hypersensitivity in rats.
Recently, it has been reported that an injection of 2,4,6-trinitrobenzene sulfonic acid (TNBS) into the rat proximal colon resulted in a significant decrease in the sensory threshold of the non-inflamed distal colon to mechanical distention stimuli (Diop et al., 2002). This experimental visceral hypersensitivity is unique compared to other animal models in that pain threshold to mechanical distention can be determined at the intact region of the gut, and that the hypersensitivity lasts for over 2 weeks, providing a useful tool for investigation of the pathophysiology and therapy of IBS (Diop et al., 2002; Ohashi et al., 2007a, Ohashi et al., 2007b). Earlier workers have demonstrated a significant suppression of TNBS-induced visceral hypersensitivity by pretreatment with antibodies against nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) that are known to be released from mast cells, and hypothesized that mast cells are playing an important role as a source of the neurotrophins in the development of the experimental visceral hypersensitivity in rats (Delafoy et al., 2003, Delafoy et al., 2006). In a previous study, we have found that a prominent mucosal mast cell infiltration with a concomitant increase in spontaneous mediator release occurred in the sensitized colon of the TNBS-treated rats, and that the induced visceral hypersensitivity was significantly suppressed by a mast cell stabilizer doxantrazole, indicating a possible involvement of mucosal mast cells (Ohashi et al., 2007a).
In the present study, in order to confirm and further extend the roles of mucosal mast cells in the pathogenesis of IBS, we investigated the impact of mast cell deficiency on development of TNBS-induced visceral hypersensitivity using mast cell deficient Ws/Ws rats.
Section snippets
Animals
Male Ws/Ws and W+/W+ rats were purchased from SLC Co. (Shizuoka, Japan). The origin of Ws/Ws and W+/W+ rats has been described in the previous paper (Niwa et al., 1991). They were kept under conditions of constant temperature (23 ± 2 °C) and humidity (55 ± 15%) with a 12-h light/dark conditions with free access to normal laboratory chow and tap water. All procedures employed in this study were approved by the institutional Animal Ethics Committees according to the Laboratory Animal Welfare
TNBS-induced visceral hypersensitivity
Fig. 1 illustrates changes in pain threshold of the distal colon to mechanical distention stimuli. An injection of TNBS into the proximal colon through laparotomy resulted in a significant decrease in pain threshold of the distal colon in W+/W+ rats. In contrast, TNBS, injected in a similar manner into the proximal colon, failed to produce a significant change in colonic pain threshold in mast cell deficient Ws/Ws rats. There was no difference between sham-operated Ws/Ws and W+/W+ rats in the
Discussion
Mucosal mast cells are implicated in the pathogenesis of visceral hypersensitivity in relation to the close proximity to the enteric sensory nerves and capability to release a wide variety of excitatory mediators, such as histamine and proteases (Bueno et al., 1997, Galli et al., 1999, Stevens and Austen, 1989). In the present study, in order to elucidate the roles of intestinal mast cells in development of visceral hypersensitivity, the change in colonic pain threshold after an intra-colonic
Acknowledgments
The authors' special thanks are to Drs. Nigel Bunnett (Department of Surgery and Physiology, University of California) and John Furness (Department of Anatomy and Cell Biology, University of Melbourne) for continuous encouragement and helpful suggestions on this study.
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