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.