Urocortins and CRF receptor type 2 variants in the male rat colon: gene expression and regulation by endotoxin and anti-inflammatory effect

The ghrelin agonist, HM01 activates central vagal and enteric cholinergic neurons and reverses gastric inflammatory and ileus responses in rats

BACKGROUND

Electrical vagal stimulation alleviates abdominal surgery (AS)-induced intestinal inflammation. Ghrelin receptors (GHS-Rs) are expressed in the brain and peripheral tissues. We investigated the influence of HM01, an orally active ghrelin agonist crossing the blood-brain barrier, on AS-induced gastric inflammation and emptying (GE) in rats.

METHODS

HM01 (6 mg/kg) or saline pretreatment was administered per orally (po) or intraperitoneally (ip). We assessed GE, gastric cytokine mRNA, and Fos positive cells in the dorsal motor nucleus of the vagus (DMN) and gastric corpus myenteric plexus (MP) in sham (anesthesia alone) and AS groups. The transcripts of GHS-R1 variants were determined in the medulla oblongata and gastric corpus of naïve rats.

KEY RESULTS

In vehicle pretreated rats, HM01 (ip) significantly increased the number of Fos immunoreactive cells in the MP and DMN in 55% and 52% of cholinergic neurons respectively. Hexamethonium did not modify HM01-induced Fos expression in the DMN while reducing it in the MP by 2-fold with values still significantly higher than that in control groups. AS upregulated gastric IL-1β and TNFα expression and inhibited GE by 66.6%. HM01 (po) abolished AS-induced gastric ileus and increased cytokine expression and elevated IL-10 by 4.0-fold versus vehicle/sham. GHS-R1a mRNA level was 5.4-fold higher than the truncated GHS-R1b isoform in the brain medulla and 40-fold higher in the gastric submucosa/muscle layers than in the mucosa.

CONCLUSIONS AND INFERENCE

Peripheral HM0 activates central vagal and myenteric cholinergic pathways that may influence both central and peripheral targets to prevent AS-induced gastric inflammatory and ileus.

Activation of CRF1 receptors expressed in brainstem autonomic nuclei stimulates colonic enteric neurons and secreto-motor function in male rats

BACKGROUND

Hypothalamic corticotropin-releasing factor (CRF) receptor 1 (CRF1 ) plays a role in acute stress-related stimulation of colonic motor function. Less is known on CRF1 signaling in the brainstem.

METHODS

We investigate CRF1 expression in the brainstem and the colonic response to 4th ventricle (4V) injection of CRF and urocortin (Ucn) 2 (3 µg/rat) in chronically cannulated male rats.

KEY RESULTS

Transcripts of CRF1 wild-type 1a and splice variants 1c, 1e, 1f, 1o along with three novel variants 1a-2 (desK-110 in exon 5), 1p (-exon 7), and 1q (exon 5 extension) were identified in the pons and medulla. The area postrema, nucleus tractus solitarius, dorsal motor nucleus of the vagus, locus coeruleus, and Barrington's nucleus isolated by laser capture microdissection expressed 1a, 1a-2, and 1p but not 1q. Compared to 4V vehicle, 4V CRF induced fecal pellet output (FPO) and diarrhea that were blocked by the CRF antagonist, astressin-B. CRF2 agonist, Ucn2 had no effect on basal or CRF-induced FPO. CRF actions were correlated with the induction of c-Fos immunoreactivity in myenteric neurons of the proximal and distal colon (pC, dC) and submucosal neurons of dC. c-Fos immunoreactivity occurred in 39% and 37% of myenteric cholinergic and 7% and 58% of nitrergic neurons in the pC and dC, respectively.

CONCLUSIONS & INFERENCES

CRF1a and its splice variants are expressed in brainstem nuclei, and activation of CRF1 signaling at the level of the brainstem stimulates colonic secretory-motor function through activation of colonic enteric neurons.

Effects of stress-related peptides on chloride secretion in the mouse proximal colon

BACKGROUND

Stress increases intestinal secretion and exacerbates symptoms of irritable bowel syndrome (IBS). Peripherally derived corticotropin-releasing factor (CRF) is known to mediate stress-induced intestinal secretion, presumably by activation of CRF₁ receptors in the gut. The present study aimed to ascertain the role of CRF₂ activation in intestinal secretion by three other members of CRF peptide family, urocortin (UCN) 1-3, in wild type (WT) and CRF₂ knockout (Crhr2^-/- ) mice.

METHODS

Mucosal/submucosal preparations from proximal colon of WT and Crhr2^-/- mice of both sexes were mounted in Ussing chambers for measurement of short-circuit current (I_sc ) as an indicator of ion secretion.

KEY RESULTS

Male mice demonstrated a significantly higher baseline I_sc than female in both WT and Crhr2^-/- genotypes. CRF and UCN1-3 (1 μM) caused greater increases in colonic I_sc (ΔI_sc ) in male than female. Colonic I_sc response to the selective CRF₁ agonist, stressin1, was similar in both sexes. In male mice, the selective CRF₂ agonists (UCN2 and UCN3) caused significantly greater ΔI_sc than CRF and stressin1. UCN2- and UCN3-evoked ΔI_SC was significantly reduced in preparations pretreated with the selective CRF₂ antagonist antisauvagine-30 and in Crhr2^-/- mice. The prosecretory effects of urocortins were due to increases in Cl^- secretion and involved enteric neurons and mast cells.

CONCLUSIONS AND INFERENCE

The findings revealed sex differences in baseline colonic secretion and responses to stress-related peptides. CRF₂ receptors play a more prominent role in colonic secretion in male mice. The greater baseline secretion and responses to UCNs may contribute to the higher prevalence of diarrhea-predominant IBS in males.

Urocortin 1: A putative excitatory neurotransmitter in the enteric nervous system

BACKGROUND

Urocortin 1 (Ucn1), a stress-related peptide, is a member of the corticotropin-releasing factor (CRF) family and acts as a CRF1 receptor agonist. Ucn1 and CRF1 receptor immunoreactivity are present in the enteric nervous system (ENS), and Ucn1 elicits contraction of colonic muscle strips. Considering these findings, we have hypothesized that Ucn1 acts as an excitatory neurotransmitter in the ENS. The present study was conducted to determine whether exogenously applied Ucn1 causes contractions, whether it participates in neurally mediated contraction, and whether it is released from the ENS of the rat colon.

METHODS

Isometric tension of the rat colonic muscle strips (middle to distal colon) in a longitudinal direction was measured. The effects of Ucn1 on phasic contractions were examined in the absence and presence of antalarmin (CRF1 receptor antagonist), tetrodotoxin (TTX), and atropine. The effects of antalarmin on electrical field stimulation (EFS)-induced contractions were examined in the absence and presence of atropine. Ucn1 peptide in the bath solution was measured after EFS using an EIA kit.

KEY RESULTS

Ucn1 caused a significant and dose-dependent increase in phasic contractions. These effects were completely inhibited by antalarmin, TTX, and atropine. EFS-induced contractions were inhibited by antalarmin. Atropine markedly reduced EFS-induced contractions, and antalarmin did not decrease these contractions further. EFS elicited a significant increase in the concentration of Ucn1 in the bath solution, and this increase was completely inhibited by TTX.

CONCLUSIONS AND INFERENCES

These results suggest that Ucn1 acts as an excitatory neurotransmitter in the ENS enhancing the cholinergic neurotransmission.

From psychology to physicality: how nerve growth factor transduces early life stress into gastrointestinal motility disorders later in life

Environmental stressors in early childhood can have a detrimental impact later in life, manifesting in functional gastrointestinal disorders including irritable bowel syndrome (IBS). The phenomenon is also observed in rodents, where neonatal-maternal separation, a model of early life stress, induces phenotypes similar to IBS; however, the underlying mechanisms remain unelucidated. Our recent study provided a mechanism for the pathogenesis in the gut, demonstrating that increased visceral hyperalgesia resulted from the expansion of the intestinal stem cell compartment leading to increased differentiation and proliferation of serotonin (5-hydroxytryptamine/5-HT)-producing enterochromaffin cells. Moreover, it identified nerve growth factor (NGF) as a key mediator of the pathogenesis; surprisingly, it exerts its effect via cross talk with Wnt/β-catenin signaling. This article addresses the roles of NGF in driving IBS and its potential clinical implications, outstanding questions in how psychological stimuli are transduced into physical phenotypes, as well as future directions of our findings. Abbreviations: 5-HT: 5-hydroxytryptamine/serotonin; BDNF: brain-derived neurotrophic factor; CRF: corticotrophin-releasing factor; EC: enterochromaffin; ENS: enteric nervous system; GI: gastrointestinal; GPCR: G-protein-coupled receptor; IBS (-D): irritable bowel syndrome (diarrhea predominant); LRP5/6: low-density lipoprotein receptor-related protein 5/6; MAPK: mitogen-activated protein kinase; NGF: nerve growth factor; NMS: neonatal-maternal separation; PI3K: phosphoinositode3-kinase; PLCγ: phospholipase c, gamma subtype; TrkA: tropomyosin receptor kinase A.

Brain and Gut CRF Signaling: Biological Actions and Role in the Gastrointestinal Tract

BACKGROUND

Corticotropin-releasing factor (CRF) pathways coordinate behavioral, endocrine, autonomic and visceral responses to stress. Convergent anatomical, molecular, pharmacological and functional experimental evidence supports a key role of brain CRF receptor (CRF-R) signaling in stress-related alterations of gastrointestinal functions. These include the inhibition of gastric acid secretion and gastric-small intestinal transit, stimulation of colonic enteric nervous system and secretorymotor function, increase intestinal permeability, and visceral hypersensitivity. Brain sites of CRF actions to alter gut motility encompass the paraventricular nucleus of the hypothalamus, locus coeruleus complex and the dorsal motor nucleus while those modulating visceral pain are localized in the hippocampus and central amygdala. Brain CRF actions are mediated through the autonomic nervous system (decreased gastric vagal and increased sacral parasympathetic and sympathetic activities). The activation of brain CRF-R2 subtype inhibits gastric motor function while CRF-R1 stimulates colonic secretomotor function and induces visceral hypersensitivity. CRF signaling is also located within the gut where CRF-R1 activates colonic myenteric neurons, mucosal cells secreting serotonin, mucus, prostaglandin E2, induces mast cell degranulation, enhances mucosal permeability and propulsive motor functions and induces visceral hyperalgesia in animals and humans. CRF-R1 antagonists prevent CRF- and stressrelated gut alterations in rodents while not influencing basal state.

DISCUSSION

These preclinical studies contrast with the limited clinical positive outcome of CRF-R1 antagonists to alleviate stress-sensitive functional bowel diseases such as irritable bowel syndrome.

CONCLUSION

The translational potential of CRF-R1 antagonists in gut diseases will require additional studies directed to novel anti-CRF therapies and the neurobiology of brain-gut interactions under chronic stress.

Mast cell corticotropin-releasing factor subtype 2 suppresses mast cell degranulation and limits the severity of anaphylaxis and stress-induced intestinal permeability

BACKGROUND

Psychological stress and heightened mast cell (MC) activation are linked with important immunologic disorders, including allergy, anaphylaxis, asthma, and functional bowel diseases, but the mechanisms remain poorly defined. We have previously demonstrated that activation of the corticotropin-releasing factor (CRF) system potentiates MC degranulation responses during IgE-mediated anaphylaxis and psychological stress through corticotropin-releasing factor receptor subtype 1 (CRF₁) expressed on MCs.

OBJECTIVE

In this study we investigated the role of corticotropin-releasing factor receptor subtype 2 (CRF₂) as a modulator of stress-induced MC degranulation and associated disease pathophysiology.

METHODS

In vitro MC degranulation assays were performed with bone marrow-derived mast cells (BMMCs) derived from wild-type (WT) and CRF₂-deficient (CRF₂^-/-) mice and RBL-2H3 MCs transfected with CRF₂-overexpressing plasmid or CRF₂ small interfering RNA. In vivo MC responses and associated pathophysiology in IgE-mediated passive systemic anaphylaxis and acute psychological restraint stress were measured in WT, CRF₂^-/-, and MC-deficient Kit^W-sh/W-sh knock-in mice.

RESULTS

Compared with WT mice, CRF₂^-/- mice exhibited greater serum histamine levels and exacerbated IgE-mediated anaphylaxis and colonic permeability. In addition, CRF₂^-/- mice exhibited increased serum histamine levels and colonic permeability after acute restraint stress. Experiments with BMMCs and RBL-2H3 MCs demonstrated that CRF₂ expressed on MCs suppresses store-operated Ca²⁺ entry signaling and MC degranulation induced by diverse MC stimuli. Experiments with MC-deficient Kit^W-sh/W-sh mice systemically engrafted with WT and CRF₂^-/- BMMCs demonstrated the functional importance of MC CRF₂ in modulating stress-induced pathophysiology.

CONCLUSIONS

MC CRF₂ is a negative global modulator of stimuli-induced MC degranulation and limits the severity of IgE-mediated anaphylaxis and stress-related disease pathogenesis.

Altered colonic sensory and barrier functions by CRF: roles of TLR4 and IL-1

Visceral allodynia and increased colonic permeability are considered to be crucial pathophysiology of irritable bowel syndrome (IBS). Corticotropin-releasing factor (CRF) and immune-mediated mechanisms have been proposed to contribute to these changes in IBS, but the precise roles have not been determined. We explored these issues in rats in vivo. The threshold of visceromotor response, i.e., abdominal muscle contractions induced by colonic balloon distention was electrophysiologically measured. Colonic permeability was estimated by quantifying the absorbed Evans blue in colonic tissue. Intraperitoneal injection of CRF increased the permeability, which was blocked by astressin, a non-selective CRF receptor antagonist, but astressin2-B, a selective CRF receptor subtype 2 (CRF2) antagonist did not modify it. Urocortin 2, a selective CRF2 agonist inhibited the increased permeability by CRF. Eritoran, a toll-like receptor 4 (TLR4) antagonist or anakinra, an interleukin-1 receptor antagonist blocked the visceral allodynia and the increased gut permeability induced by CRF. Subcutaneous injection of lipopolysaccharide (immune stress) or repeated water avoidance stress (WAS, psychological stress), 1 h daily for 3 days induced visceral allodynia and increased gut permeability (animal IBS models), which were also blocked by astressin, eritoran or anakinra. In conclusion, stress-induced visceral allodynia and increased colonic permeability were mediated via peripheral CRF receptors. CRF induced these visceral changes via TLR4 and cytokine system, which were CRF1 dependent, and activation of CRF2 inhibited these CRF1-triggered responses. CRF may modulate immune system to alter visceral changes, which are considered to be pivotal pathophysiology of IBS.

Microbiota affects the expression of genes involved in HPA axis regulation and local metabolism of glucocorticoids in chronic psychosocial stress

The commensal microbiota affects brain functioning, emotional behavior and ACTH and corticosterone responses to acute stress. However, little is known about the role of the microbiota in shaping the chronic stress response in the peripheral components of the hypothalamus-pituitary-adrenocortical (HPA) axis and in the colon. Here, we studied the effects of the chronic stress-microbiota interaction on HPA axis activity and on the expression of colonic corticotropin-releasing hormone (CRH) system, cytokines and 11β-hydroxysteroid dehydrogenase type 1 (11HSD1), an enzyme that determines locally produced glucocorticoids. Using specific pathogen-free (SPF) and germ-free (GF) BALB/c mice, we showed that the microbiota modulates emotional behavior in social conflicts and the response of the HPA axis, colon and mesenteric lymph nodes (MLN) to chronic psychosocial stress. In the pituitary gland, microbiota attenuated the expression of Fkbp5, a gene regulating glucocorticoid receptor sensitivity, while in the adrenal gland, it attenuated the expression of genes encoding steroidogenesis (MC2R, StaR, Cyp11a1) and catecholamine synthesis (TH, PNMT). The pituitary expression of CRH receptor type 1 (CRHR1) and of proopiomelanocortin was not influenced by microbiota. In the colon, the microbiota attenuated the expression of 11HSD1, CRH, urocortin UCN2 and its receptor, CRHR2, but potentiated the expression of cytokines TNFα, IFNγ, IL-4, IL-5, IL-6, IL-10, IL-13 and IL-17, with the exception of IL-1β. Compared to GF mice, chronic stress upregulated in SPF animals the expression of pituitary Fkbp5 and colonic CRH and UCN2 and downregulated the expression of colonic cytokines. Differences in the stress responses of both GF and SPF animals were also observed when immunophenotype of MLN cells and their secretion of cytokines were analyzed. The data suggest that the presence of microbiota/intestinal commensals plays an important role in shaping the response of peripheral tissues to stress and indicates possible pathways by which the environment can interact with glucocorticoid signaling.

Gastric corticotropin-releasing factor influences mast cell infiltration in a rat model of functional dyspepsia

Functional gastrointestinal disorders (FGIDs) are characterized by dysregulated gut-brain interactions. Emerging evidence shows that low-grade mucosal inflammation and immune activation contribute to FGIDs, including functional dyspepsia (FD). Stress plays an important role in the onset of FD symptoms. In human subjects with FD, presence of gastric mast cells has been reported, but factors that influence mast cell infiltration remain uncharacterized. Corticotropin-releasing factor (CRF) initiates the body's stress response and is known to degranulate mast cells. In this study, we delineated the role of the CRF system in the pathogenesis of FD in a rat model. Gastric irritation in neonate rat pups with iodoacetamide (IA) was used to induce FD-like symptoms. RNA interference (RNAi) was used to silence gastric CRF expression. Mast cell infiltrate in the stomach increased by 54% in IA-treated rats compared to controls and CRF-RNAi tended to decrease gastric mast cell infiltrate. Sucrose intake decreased in IA-treated rats and mast cell numbers showed a negative association with sucrose intake. IA treatment and transient silencing of gastric CRF increased hypothalamic CRF levels. In IA-treated rats, gastric levels of CRF receptor 2 (CRF2) decreased by ~76%, whereas hypothalamic CRF receptor 1 (CRF1) levels increased. Plasma levels of TNF-α showed a positive correlation with plasma CRF levels. Levels of phosphorylated p38 and ERK1/2 in the stomach showed a positive correlation with gastric CRF levels. Thus, CRF may contribute to low grade inflammation via modulating mast cell infiltration, cytokine levels, MAPK signaling, and the gut-brain axis.

Gut-Brain Neuroendocrine Signaling Under Conditions of Stress-Focus on Food Intake-Regulatory Mediators

The gut-brain axis represents a bidirectional communication route between the gut and the central nervous system comprised of neuronal as well as humoral signaling. This system plays an important role in the regulation of gastrointestinal as well as homeostatic functions such as hunger and satiety. Recent years also witnessed an increased knowledge on the modulation of this axis under conditions of exogenous or endogenous stressors. The present review will discuss the alterations of neuroendocrine gut-brain signaling under conditions of stress and the respective implications for the regulation of food intake.

Activation of Brain Somatostatin Signaling Suppresses CRF Receptor-Mediated Stress Response

Corticotropin-releasing factor (CRF) is the hallmark brain peptide triggering the response to stress and mediates—in addition to the stimulation of the hypothalamus-pituitary-adrenal (HPA) axis—other hormonal, behavioral, autonomic and visceral components. Earlier reports indicate that somatostatin-28 injected intracerebroventricularly counteracts the acute stress-induced ACTH and catecholamine release. Mounting evidence now supports that activation of brain somatostatin signaling exerts a broader anti-stress effect by blunting the endocrine, autonomic, behavioral (with a focus on food intake) and visceral gastrointestinal motor responses through the involvement of distinct somatostatin receptor subtypes.

Lipopolysaccharide induces visceral hypersensitivity: role of interleukin-1, interleukin-6, and peripheral corticotropin-releasing factor in rats

BACKGROUND

Lipopolysaccharide (LPS) induces visceral hypersensitivity, and corticotropin-releasing factor (CRF) also modulates visceral sensation. Besides, LPS increases CRF immunoreactivity in rat colon, which raises the possibility of the existence of a link between LPS and the CRF system in modulating visceral sensation. The present study tried to clarify this possibility.

METHODS

Visceral sensation was assessed by abdominal muscle contractions induced by colonic balloon distention, i.e., visceromotor response, electrophysiologically in conscious rats. The threshold of visceromotor response was measured before and after administration of drugs.

RESULTS

LPS at a dose of 1 mg/kg subcutaneously (sc) decreased the threshold at 3 h after the administration. Intraperitoneal (ip) administration of anakinra (20 mg/kg), an interleukin-1 (IL-1) receptor antagonist, or interleukin-6 (IL-6) antibody (16.6 µg/kg) blocked this effect. Additionally, IL-1β (10 µg/kg, sc) or IL-6 (10 µg/kg, sc) induced visceral allodynia. Astressin (200 µg/kg, ip), a non-selective CRF receptor antagonist, abolished the effect of LPS, but astressin₂-B (200 µg/kg, ip), a CRF receptor type 2 (CRF2) antagonist, did not alter it. Peripheral CRF receptor type 1 (CRF1) stimulation by cortagine (60 µg/kg, ip) exaggerated the effect of LPS, but activation of CRF2 by urocortin 2 (60 µg/kg, ip) abolished it.

CONCLUSIONS

LPS induced visceral allodynia possibly through stimulating IL-1 and IL-6 release. In addition, this effect was mediated through peripheral CRF signaling. Since the LPS-cytokine system is thought to contribute to altered visceral sensation in the patients with irritable bowel syndrome, these results may further suggest that CRF plays a crucial role in the pathophysiology of this disease.

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

Stress, sex, and the enteric nervous system

Made up of millions of enteric neurons and glial cells, the enteric nervous system (ENS) is in a key position to modulate the secretomotor function and visceral pain of the gastrointestinal tract. The early life developmental period, through which most of the ENS development occurs, is highly susceptible to microenvironmental perturbation. Over the past decade, accumulating evidence has shown the impact of stress and early life adversity (ELA) on host gastrointestinal pathophysiology. While most of the focus has been on alterations in brain structure and function, limited experimental work in rodents suggest that the enteric nervous system can also be directly affected, as shown by changes in the number, phenotype, and reactivity of enteric nerves. The work of Medland et al. in the current issue of this journal demonstrates that such alterations also occur in pigs, a larger mammalian species with high translational value to human. This work also highlights a sex-differential susceptibility of the ENS to the effect of ELA, which could contribute to the higher prevalence of GI disorders in women. In this mini-review, we will discuss the development and composition of the ENS and related gastrointestinal sensory motor and secretory functions. We will then focus on the influence of stress on the enteric nervous system, with a particular emphasis on neurodevelopmental changes. Finally, we will discuss the influence of sex on those parameters.