Novel insights in the role of peripheral corticotropin-releasing factor and mast cells in stress-induced visceral hypersensitivity

Authors

  • M. Larauche

    1. Division of Digestive Diseases, Department of Medicine, Oppenheimer Family Center for Neurobiology of Stress, David Geffen School of Medicine, University of California Los Angeles, and CURE: Digestive Diseases Research Center, VA Greater Los Angeles Healthcare System, Los Angeles, CA, USA
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Address for Correspondence
M. Larauche, Division of Digestive Diseases, Department of Medicine, Oppenheimer Family Center for Neurobiology of Stress, David Geffen School of Medicine, University of California, and CURE: Digestive Diseases Research Center, VA Greater Los Angeles Healthcare System, Los Angeles, CA 90073, USA.
Tel: (310) 478-3711/Ext 41827; fax: (310) 268-4963;
e-mail: mlarauche@mednet.ucla.edu

Abstract

Visceral hypersensitivity is one of the hallmarks in irritable bowel syndrome (IBS) pathophysiology. Stress is well known to affect visceral sensitivity in humans and rodents, an effect which is associated in part with alterations of intestinal epithelial permeability in rodents. Although the pathophysiology of visceral hypersensitivity is still unclear, two key factors have been identified as playing a major role in its modulation, namely peripheral corticotropin-releasing factor (CRF) and mast cells. In a recent study in Neurogastroenterology and Motility, van den Wijngaard et al. demonstrate that the mast-cell dependent visceral hypersensitivity observed in maternally separated rats after an acute exposure to a psychological stress can be prevented but not reversed by the peripherally restricted CRF receptor antagonist, α-helical CRF9–41. They further show that the preventive effect of the CRF receptor antagonist is linked to a stabilization of mast cells and maintenance of the epithelial barrier at the colonic level. These data suggest that post stress mast cell activation and subsequent visceral hypersensitivity are not targeted by peripheral CRF receptor antagonists. These novel insights in the role of peripheral CRF in the modulation of stress-induced visceral hypersensitivity add to our growing understanding of the mechanisms that may lie at the origin of visceral pain disturbances following stress and will contribute to enhance the development of drugs that may have potential therapeutic benefits for IBS patients.

Visceral hypersensitivity, the enhanced perception of visceral stimuli and pain, is one of the hallmarks of irritable bowel syndrome (IBS) pathophysiology. Stress (psychosocial and physical) is shown to trigger or exacerbate IBS symptoms.1,2 Acute exposure to cold or noise and chronic life stressful events increase visceral sensitivity to rectosigmoid distension in IBS patients.3,4 Similarly, in rodents several studies support a strong association between stress (acute or chronic) and increased visceral sensitivity to colorectal distension (CRD).5–9 Besides, stress in rodents is associated with increased intestinal permeability,10,11 a phenomenon which appears as a prerequisite for visceral hypersensitivity.12 Likewise, alterations of permeability have been shown in vitro in colonic biopsies of both post-infectious and non post-infectious diarrhea-predominant IBS patients, with the degree of barrier dysfunction being related to the onset and severity of abdominal pain and visceral hypersensitivity.13 However, a direct causal relationship between increased permeability and stress is not yet established in humans.

Corticotropin-releasing factor, CRF and CRF-related peptides, urocortin 1 (Ucn 1), Ucn 2 and Ucn 3, are key mediators of the bodily responses to stress. They exert their biological actions by interacting with CRF1 and CRF2 receptors.14 Over the years, convergent preclinical evidence has accumulated suggesting that stress-related alterations of visceral sensitivity are primarily mediated by the activation of brain CRF/CRF1 signaling.14 However, recent clinical and preclinical studies point to an equally important contribution of the peripheral CRF signaling in visceral hypersensitivity.15–21 Visceral hyperalgesia induced by colonic electrical stimulation in diarrhea-predominant IBS patients was reduced by preventive intravenous administration of the non-selective and peripherally restricted CRF receptors antagonists, α-helical CRF9–41 or astressin,17,18 although a systemic injection of the preferential CRF1 agonist, ovine CRF lowered pain thresholds to repetitive rectal distensions in healthy humans.15,16 In rats, we found that sustained visceral hyperalgesia induced by repeated exposure to water avoidance stress (WAS) for 10 days, could be prevented by peripheral administration of a non selective CRF1/CRF2 receptor antagonist, astressin, before each stress session supporting the participation of the peripheral CRF signaling to the development of visceral hypersensitivity.22 Furthermore, peripheral injection of CRF and of the selective CRF1 agonist cortagine were shown to induce visceral hypersensitivity to CRD in rodents,19,20 an effect abolished by peripheral, but not central, administration of astressin at equivalent dose in the case of cortagine.20

In rodents, both acute (restraint, WAS, cold) and chronic stress (WAS 5–10 days, maternal separation) increase the paracellular and transcellular permeability in the colon.23–27 At the colonic level, this alteration is abolished by pretreatment of rats with the peripheral administration of the non selective CRF antagonists astressin or α-helical CRF9–4111,23,24,28 or the selective CRF1 antagonist, SSR-125543,23 highlighting the participation of CRF1 receptors in the modulation of colonic permeability. Further support to a role for peripheral CRF signaling in the modulation of colonic permeability comes from studies showing an increase in both paracellular and transcellular permeability in rat colonic tissue after exposure to CRF, sauvagine or Ucn 3 in vitro.11,28–30 In human colonic biopsies mounted in Ussing chambers, CRF administered on the serosal side induced an increased uptake of HRP by endocytosis, a sign of an increased transcellular permeability, but did not affect paracellular permeability as assessed by permeation of 51Cr-EDTA and transepithelial resistance,31 raising the possibility of species differences in the epithelial response to peripheral CRF signaling activation.

Gastrointestinal mast cells, which occupy a strategic position in the brain-gut axis due to their location at the interface between the intestinal immune and enteric nervous system are a well established target of stress and peripheral CRF as demonstrated by the increase in mast cell mediator release in animals and humans exposed to stress.32,33 Mast cells have also both CRF1 and CRF2 receptor subtypes at their surface.31,34 There is growing evidence that mast cells may bear a key role in visceral hypersensitivity. Not only is their number significantly increased in the small and large intestine in both animal models of visceral hypersensitivity and in IBS patients,35–40 but their proximity to sensory nerves in colonic tissue is also positively correlated to abdominal pain.35,41 Mast cells contain and release a large variety of preformed (proteases, serotonin, histamine) and neoformed (prostaglandins, cytokines) mediators in response to a variety of stimuli. These mediators have been shown to be released in higher quantity and/or quality in gut tissues of IBS patients compared to controls.35,41 Exposure to the supernatant obtained from colonic biopsies of patients with IBS but not from healthy controls increases the excitability of rat mesenteric sensory nerves,41 induces visceral hypersensitivity in mice,42,43 and evokes abnormally high activation of human enteric neurons.44 Moreover, mast cell mediators released by stress can contribute to visceral hypersensitivity by altering the epithelial barrier function.29,31,45 Stabilization of mast cell with ketotifen and doxantrazole was shown to be effective in preventing stress-induced visceral hypersensitivity in rodents.7,23 In the same line, recent reports suggest that mast cell stabilization with ketotifen increases the threshold for discomfort in IBS patients with visceral hypersensitivity, reduces IBS symptoms and improves health-related quality of life.46

Over the past 15 years, various animal models have been developed to get an insight into the underlying mechanisms of visceral hypersensitivity and the influence of stress on visceral pain pathways.20,47,48 Among those, neonatal maternal separation stress is considered one of the most relevant rodent model of IBS.47 In humans, early traumatic experiences (abuse, neglect, and life-threatening situation during childhood) have indeed been found to increase the risk of IBS development later in life.47 This has been confirmed in rodents, where rat pups separated from their mothers for several hours each day, from day 2 to day 14 after birth, have been shown to develop hypersensitivity to rectal distension when exposed to an acute stressor.5,8,49

In this issue of Neurogastroenterology and Motility, van den Wijngaard et al.50 assessed both the preventive and reversal effect of a peripherally restricted CRF antagonist, α-helical CRF9–41, and a mast cell blocker, doxantrazole, against visceral hypersensitivity in the Long Evans rats maternal separation model. In agreement with their previous studies, the authors showed that maternally separated Long Evans rats develop visceral hypersensitivity to CRD, compared to non handled rats, only when they are exposed to an acute psychological stress. They also demonstrate that the visceral hypersensitivity observed in maternally separated rats is sustained and remains strong up to at least 31 days after exposure to stress. Using the mast cell stabilizer doxantrazole, they are able to prevent and reverse the stress-induced visceral hypersensitivity, suggesting the involvement of mast cells in the early and later stages of stress-induced visceral hypersensitivity induced by acute WAS in maternally separated rats. Most interestingly, their data provide evidence that this sustained visceral hypersensitivity can be prevented but not reversed by administration of the peripherally restricted CRF antagonist, α-helical CRF9–41. The prevention of stress-induced visceral hypersensitivity by the CRF antagonist is linked to stabilization of mast cells in the early stages of the stress as seen by a higher level of rat mast cell protease 2 (RMCP2) in the colon of rats pretreated with α-helical CRF9–41 compared to vehicle and is associated with a maintenance of the epithelial barrier as shown by higher levels of occludin expression in the distal colon of treated rats compared to controls. Their data therefore suggest that persistent post stress mast cell activation and subsequent visceral hypersensitivity are not targeted by CRF receptor antagonists.

Future Research

In view of the recent disappointment of clinical trials for IBS with the CRF1 antagonists, the work by van den Wijngaard et al.50 provides an attractive explanation as to why clinical trials may have failed this far. As detailed earlier, the majority of clinical and preclinical studies which assessed the effect of stress, CRF and mast cells on visceral sensitivity were performed using preventive administration of CRF antagonists or mast cell stabilizers. This strategy may have been inadequate and may have prevented the translation of these researches to IBS patients who for the most part have been experiencing symptoms for many years.

However, without lessening the merit of the authors and the importance of their findings, one must remain cautious before applying the data obtained in animal models directly to human. Indeed, although based on the results of this study, we can exclude a participation of peripheral CRF in the sustained visceral hypersensitivity induced by acute WAS in maternally separated rats, there is no evidence that administration of centrally acting CRF antagonists would not be able to reverse the long-term sensitization induced by stress in this model. Central CRF signaling has been found to be profoundly altered in maternally separated rats and may well contribute to sustained alterations of visceral sensitivity.51 In support of this, in a different rat model of visceral hypersensitivity induced by chronic WAS, a single systemic administration of centrally acting CRF1 antagonists is shown to have a potent effect in reversing the sustained visceral hypersensitivity.52 Therefore, it would be important to test the effect of a centrally acting CRF antagonist in this MS model to exclude any role of CRF signaling in late stages of established visceral hypersensitivity. Furthermore, assessing in maternally separated rats the levels of expression of CRF receptors both centrally and peripherally in the early and late stages of the sustained visceral hypersensitivity could also give additional information on the reasons behind the lack of effect of the peripheral CRF antagonist. As CRF does not seem to play a role in the maintenance of visceral hyperalgesia in maternally separated rats exposed to WAS, it would be interesting to assess, as recently reported in a murine model of post-infectious IBS,53 whether those rats present changes in their colonic DRG neurons that may render them more sensitized to subthreshold levels of inflammatory mediators such as proteases, thereby contributing to the perpetuation of visceral hypersensitivity. In addition, as there is increasing evidence of strain differences in the stress response of rodents, and this study was performed with Long Evans rats, a strain which is less sensitive to the effects of stress, it would be important to assert the reproducibility of the phenomenon observed in other strains of rats with different stress responsiveness to have stronger translational support for the lack of effect of CRF in chronic visceral pain. Lastly, all IBS patients do not present with a history of early life trauma. Whether or not the observations of van den Wijngaard et al.50 are also valid in other models of visceral hypersensitivity would strengthen the translational aspect of their results.

Conclusion

In summary, this article by van den Wijngaard et al.50 provides novel crucial data on the pivotal role of peripheral CRF and mast cells in stress-induced visceral hypersensitivity, which will help pave the way to a better understanding of the mechanisms involved in chronic visceral hypersensitivity and may lead to better treatments of visceral pain symptoms in IBS patients, in particular as relates to the use of CRF antagonists.

Disclosures

No competing interests declared.

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