Corticotropin releasing factor receptors and stress-related gut diseases: physiology and therapeutic potential

Corticotropin releasing factor receptors and stress-related gut diseases: physiology and therapeutic potential

By Yvette Taché, Center for Neurobiology of Stress and Digestive Diseases Research Center, Digestive Diseases Division, UCLA and VA Greater Los Angeles Health Care System, Los Angeles, CA, USA

Gastrointestinal (GI) motility disorders such as irritable bowel syndrome (IBS), functional dyspepsia, and gastroesophageal reflux diseases represent conditions commonly seen by gastroenterologists.

Research efforts are needed on the numerous systems and processes that may be impaired in functional and motility disorders, including the brain-gut interactions, the enteric nervous system (ENS), and sensory mechanisms.

In animal models and human diseases, gut inflammation, altered gut mucosal immunity, psychiatric conditions or psychosocial factors all may modulate the reciprocal pathways between the brain and the gut, inducing state of hypersensitivity.

Advances have been made in the degree to which emotional processes participate in the generation and/or exacerbation of symptoms in chronic functional disorders. In particular dysregulation of stress circuit can affect sensory and motor function of the gut. Clinical investigations also support the notion that stress contributes to visceral hypersensitivity of the gut observed in patients with irritable bowel syndrome (IBS) as established by their lowered threshold of pain to colorectal distension (CRD).

Prospective studies also established that there is a 4%-31% incidence of post infectious IBS following bacterial gastroenteritis and stressful life events increase the risk to develop such post-infection IBS.

The characterization and distribution of corticotropin-releasing factor (CRF) family of peptides, CRF, urocortin 1 (ucn 1), urocortin 2 (ucn 2) and urocortin 3 (ucn 3), and the two G-protein coupled receptors, CRF1 and CRF2, as well as the development of selective CRF1 and CRF2 receptor antagonists provided novel means to understand mechanisms involve in the stress response. CRF has preferential affinity toward the CRF1 receptor while Ucn 1 has equal affinity to both receptors and Ucn 2 and Ucn 3 are selective ligands for CRF2.

The activation of brain CRF1 signaling pathway plays a primary role in the endocrine (activation of pituitary adrenal axis), behavioral (anxiety, depression), autonomic (sympathetic and sacral parasympathetic activation, vagal inhibition), and decrease immune responses to stress. Combined anatomical, pharmacologic and molecular approaches support a role of CRF receptor activation in the brain as part of key mechanisms involved in stress-related alterations of gut propulsive function. Inhibition of gastric emptying and stimulation of colonic motor function are the commonly encountered patterns resulting from exposure to various stressors in animals and humans.

Activation of brain CRF2 receptors mediates stress-related inhibition gastric motor function while that of CRF1 receptors are involved in the stimulation colonic secretory and motor functions.

The brain and the gut overlap in their peptidergic content and recent studies provide anatomical support for the existence that the CRF signaling system is also part of the brain-gut peptidergic axis. The CRF system in the gut is regulated under stress conditions particularly in response to immune challenge.

In our studies we established that CRF2b receptor is a predominant subtype isoform expressed in the gastric corpus with a70-fold higher mRNA level than CRF1 receptors.

Lipopolysaccharide injected intraperitoneally at a low dose induces a time dependent increases in mRNA levels of ucn 1, ucn 2 or ucn 3 in both mucosa and submucosa plus muscle layers at 2, 6, 9 h, with peak levels at 6h, and returned to the control level at 24h while there was a downregulation of CRF2bR mRNA in the gastric mucosa and submucosa/muscle layers 2, 6, 9 h respectively after LPS injection, and recovery at 24h.

LPS injection inhibits gastric emptying that was prevented by the peripheral injection of CRF2 receptor selective antagonist, astressin2-B while the CRF1 antagonist, CP-154,526 had no effect.

We also established that the exogenous administration of Ucn 2 decreased dose-dependently gastric phasic activity and peptide action was blocked astressin2-B. These data indicate that CRF2 signaling pathway is prominently expressed in the rat stomach and regulated by LPS and plays a role in the associated delayed gastric emptying.

By contrast, in the colon CRF1 receptor signaling is a key component of the local arm of the colonic response to stress. We showed that CRF mRNA level, detected by reverse transcription-polymerase chain reaction (RT-PCR) was 1.3-fold higher in the distal than proximal colon and 3.4-fold higher in the proximal colonic submucosa plus muscle layers than in mucosa.

CRF immunoreactivity is located in the epithelia, lamina propria and crypts, and co-localized with tryptophan hydroxylase, a marker for enterochromaffin (EC) cells, and in enteric neurons.

Lipopolysaccharide increased defecation by 2.9-fold and upregulated CRF mRNA by 2.5-fold in the proximal and 1.1-fold in the distal colon and increase circulating levels of CRF as detected using a novel RAPID method of blood processing.

LPS-induced increased CRF mRNA expression occurred in the submucosa plus muscle layers (1.5-fold) and the mucosa of proximal colon (0.9-fold) and at the protein levels CRF immunoreactivity was elevated in the submucosal and myenteric plexuses of proximal and distal colon compared to saline groups.

Laser capture microdissection combined with RT-PCR and immunohistochemistry in longitudinal muscle myenteric plexus whole-mount colonic preparations revealed CRF1 receptor expression in myenteric neurons.

When CRF is injected peripherally, the peptide activate selectively myenteric plexus of proximal and distal colon as shown by Fos immunoreactivity, a marker of neuronal activation while no Fos was found in gastric corpus, antrum, duodenum, jejunum and ileum myenteric neurons.

Fos immmunoreaticity induced by CRF was located in 55 +/- 1.9% and 53 +/- 5.1% of CRF1 receptor-IR myenteric neurons and in 44 +/- 2.8% and 40 +/- 3.9% of cholinergic neurons with Dogiel type I morphology, and in 20 +/- 1.6% and 80 +/- 3.3% of nitrergic neurons in proximal and distal colon respectively.

Further functional studies established that intraperitoneal (ip) injection of the selective CRF1 agonist cortagine (10 ug/kg ip) results in a significant decreased of distal colonic transit time by 45% without affecting gastric transit, increased distal and transverse colonic contractility by 35.6 and 66.2%, respectively, and induced a 7.1-fold increase in defecation and watery diarrhea in 50% of rats during the 1st hour postinjection. Cortagine ip also increased colonic permeability, activated proximal and distal colonic myenteric neurons, and induced visceral hypersensitivity to a second set of phasic colorectal distention (CRD).

The CRF antagonist astressin abolished ip cortagine-induced hyperalgesia whereas injected intracerebroventricularly, it had no effect indicative of receptor interaction occurring in the periphery

In addition, CRF1 antagonists studies showed that peripheral CRF1 receptor activation contributes to the development and maintenance of hyperalgesia to CRD under conditions of acute or chronic stress.

Likewise in mice, cortagine stimulated defecation by 7.8-fold, induced 60% incidence of diarrhea, and increased visceral pain to CRD. The peptide also induces a pro-inflammatory profile selectively in terminal ileum unlike the proximal colon as shown by the up-regulation of TNFα and interleukin (IL)-1ß and down-regulation IL-10 and IFN gamma mRNA expression 2 h after injection.

Reduced mRNA levels of the tight junction molecules claudin-1 and -8 in the ileum, were associated with increased bacterial load in mesenteric lymph node and liver and apoptosis of ileal epithelial and lamina propria cells 8 h after cortagine.

Therefore stress-like ileal and colonic alterations are induced by ip cortagine in rats and mice through restricted activation of peripheral CRF1 receptors. These pre-clinical studies support a role for peripheral CRF1 signaling as peripheral effectors of the stress response in the gut. In addition of clinical phase I data support that targeting of CRF1 receptors may open new therapeutic venues for stress-related functional gastrointestinal disorders.

In summary, CRF and related peptide, ucn 1 acting on CRF1 receptor signaling pathways have been identified and characterized to coordinate the various facets of the endocrine and behavioral responses to stress.

The role of CRF signaling system at both central and peripheral levels is gaining also recognition as part of the neurobiological common denominator of IBS symptoms susceptible to stress and anxiety/depression.

There is evidence for elevated levels of CRF in patients with major depression, anxiety and vulnerability to stress as well as those suffering from obsessive compulsive disorders, posttraumatic stress disorders or childhood trauma. Investigations in IBS patients indicate also that there is an overactivity of the hypothalamic pituitary axis and enhanced plasma CRF response to mental stress.

In experimental animals, CRF receptors are expressed in hypothalamic, limbic nuclei and pontine circuitries regulating anxiety and autonomic outflow to the gut as well as in colonic myenteric neurons, mast cells and enterochromaffin cells.

Recently we developed an experimental model that recapture cardinal features of IBS-diarrhea predominant patients with regard to stress-related hyperalgesia to CRD, increased colonic permeability, mast cell degranulation, activation of the enteric nervous system, propulsive motor function (motility, transit, defecation and development of diarrhea using the administration of selective CRF1 receptor agonists, cortagine or stressin1 in rodents. CRF1 receptor antagonists blunt acute and chronic stress-induced visceral hyperalgesia and colonic propulsive events.

By contrast, activation of CRF2 receptors attenuated the CRF1 mediated colonic and hyperalgesic responses to either administration of CRF1 agonists or stress and contributed to gastric stasis induced by immune stressors.

These data suggest that dysregulation of CRF signaling pathways including an overactivity of CRF1 signaling and/or defective mounting of the CRF2 inhibitory mechanisms may play a role in the manifestations of symptoms in diarrhea predominant IBS.

©Typologos.com 2011