Casey O. Ligon

Background: Psychological stress is a risk factor for irritable bowel syndrome, a functional gastrointestinal pain disorder featuring abnormal brain‐gut connectivity. The guanylate cyclase‐C (GC‐C) agonist linaclotide has been shown to relieve abdominal pain in IBS‐C and exhibits antinociceptive effects in rodent models of post‐inflammatory visceral hypersensitivity. However, the role GC‐C signaling plays in psychological stress‐induced visceral hypersensitivity is unknown. Here, we test the hypothesis that GC‐C agonism reverses stress‐induced colonic hypersensitivity via inhibition of nociceptive afferent signaling resulting in normalization of stress‐altered corticotropin‐releasing factor (CRF) expression in brain regions involved in pain perception and modulation.Methods: Adult female rats were exposed to water avoidance stress or sham stress for 10 days, and the effects of linaclotide on stress‐induced changes in colonic sensitivity, corticolimbic phospho‐extracellular signal‐regulated kinase (pERK), and CRF expression were measured using a combination of behavioral assessments, immunohistochemistry, and qRT‐PCR.Key Results: Stressed rats exhibited colonic hypersensitivity and elevated corticolimbic pERK on day 11, which was inhibited by linaclotide. qRT‐PCR analysis revealed dysregulated CRF expression in the medial prefrontal cortex, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala on day 28. Dysregulated CRF expression was not affected by linaclotide treatment.Conclusions and Inferences: Our results demonstrate that exposure to repeated stress induces chronic colonic hypersensitivity in conjunction with altered corticolimbic activation and CRF expression. GC‐C agonism attenuated stress‐induced colonic hypersensitivity and ERK phosphorylation, but had no effect on CRF expression, suggesting the analgesic effects of linaclotide occur independent of stress‐driven CRF gene expression in corticolimbic circuitry.

Abdominal pain represents a significant complaint in patients with irritable bowel syndrome (IBS). While the etiology of IBS is incompletely understood, prior exposure to gastrointestinal inflammation or psychologic stress is frequently associated with the development of symptoms. Inflammation or stress-induced expression of growth factors or cytokines may contribute to the pathophysiology of IBS. Here, we aimed to investigate the therapeutic potential of inhibiting the receptor of glial cell line-derived neurotrophic factor, rearranged during transfection (RET), in experimental models of inflammation and stress-induced visceral hypersensitivity resembling IBS sequelae. In RET-cyan fluorescent protein [(CFP) Ret^CFP/+] mice, thoracic and lumbosacral dorsal root ganglia were shown to express RET, which colocalized with calcitonin gene-related peptide. To understand the role of RET in visceral nociception, we employed GSK3179106 as a potent, selective, and gut-restricted RET kinase inhibitor. Colonic hyperalgesia, quantified as exaggerated visceromotor response to graded pressures (0–60 mm Hg) of isobaric colorectal distension (CRD), was produced in multiple rat models induced 1) by colonic irritation, 2) following acute colonic inflammation, 3) by adulthood stress, and 4) by early life stress. In all the rat models, RET inhibition with GSK3179106 attenuated the number of abdominal contractions induced by CRD. Our findings identify a role for RET in visceral nociception. Inhibition of RET kinase with a potent, selective, and gut-restricted small molecule may represent a novel therapeutic strategy for the treatment of IBS through the attenuation of post-inflammatory and stress-induced visceral hypersensitivity.

Background: Chronic visceral pain is persistent pain emanating from thoracic, pelvic, or abdominal origin that is poorly localized with regard to the specific organ affected. The prevalence can range up to 25% in the adult population as chronic visceral pain is a common feature of many visceral disorders, which may or may not be accompanied by distinct structural or histological abnormalities within the visceral organs. Mounting evidence suggests that changes in epigenetic mechanisms are involved in the top‐down or bottom‐up sensitization of pain pathways and the development of chronic pain. Epigenetic changes can lead to long‐term alterations in gene expression profiles of neurons and consequently alter functionality of peripheral neurons, dorsal root ganglia, spinal cord, and brain neurons. However, epigenetic modifications are dynamic, and thus, detrimental changes may be reversible. Hence, external factors/therapeutic interventions may be capable of modulating the epigenome and restore normal gene expression for extended periods of time.Purpose: The goal of this review is to highlight the latest discoveries made toward understanding the epigenetic mechanisms that are involved in the development or maintenance of chronic visceral pain. Furthermore, this review will provide evidence supporting that targeting these epigenetic mechanisms may represent a novel approach to treat chronic visceral pain.

Background: The expression of RET in the developing enteric nervous system (ENS) suggests that RET may contribute to adult intestinal function. ENS cholinergic nerves play a critical role in the control of colonic function through the release of acetylcholine (ACh). In the current study, we hypothesized that a RET‐mediated mechanism may regulate colonic ion transport and motility through modulation of cholinergic nerves.Methods: The effect of RET inhibition on active ion transport was assessed electrophysiologically in rat colonic tissue mounted in Ussing chambers via measurements of short circuit current (Isc) upon electrical field stimulation (EFS) or pharmacologically with cholinergic agonists utilizing a gastrointestinal (GI)‐restricted RET inhibitor. We assessed the effect of the RET inhibitor on propulsive motility via quantification of fecal pellet output (FPO) induced by the acetylcholinesterase inhibitor neostigmine.Key Results: We found that enteric ganglia co‐expressed RET and choline acetyltransferase (ChAT) transcripts. In vitro, the RET kinase inhibitor GSK3179106 attenuated the mean increase in Isc induced by either EFS or carbachol but not bethanechol. In vivo, GSK3179106 significantly reduced the prokinetic effect of neostigmine.Conclusion and Inferences: Our findings provide evidence that RET‐mediated mechanisms regulate colonic function by maintaining cholinergic neuronal function and enabling ACh‐evoked chloride secretion and motility. We suggest that modulating the cholinergic control of the colon via a RET inhibitor may represent a novel target for the treatment of intestinal disorders associated with increased secretion and accelerated GI transit such as irritable bowel syndrome with diarrhea (IBS‐D).

Background: Irritable bowel syndrome (IBS) and bladder pain syndrome (BPS) are female‐predominant, chronic functional pain disorders that are associated with early life stress (ELS) and therapeutic options for such patients remain limited. Linaclotide, a guanylate cyclase‐C (GC‐C) agonist, relieves abdominal pain and bowel symptoms in adult patients suffering from IBS with constipation. Here, we test the hypothesis that linaclotide will reverse colon and bladder hyperalgesia in a female‐specific rodent model of adverse early life experience.Methods: Neonatal rats were exposed to an odor‐attachment learning paradigm of early life stress (ELS). In adulthood, the effect of linaclotide (3 μg kg⁻¹ d⁻¹, p.o.) on colonic and bladder sensitivity was assessed via quantification of the visceromotor response to colorectal distension and the frequency of withdrawal responses to the application of von Frey hairs to the suprapubic region. In another cohort of rats, the effect of linaclotide on ELS‐induced colonic and bladder permeability was investigated via measurements of transepithelial electrical resistance (TEER).Key Results: Rats exposed to unpredictable ELS exhibited colonic and bladder hypersensitivity that was significantly reduced by linaclotide compared to vehicle‐treated controls. Colonic and bladder tissue isolated from adult rats exposed to unpredictable ELS exhibited a decrease in colonic and bladder TEER that was reversed by linaclotide.Conclusions and Inferences: Our results demonstrate that neonatal rats exposed to unpredictable ELS develop increased sensitivity and permeability of the colon and bladder in adulthood through a mechanism involving activation of peripheral GC‐C signaling.