Importance of stress receptor‐mediated mechanisms in the amygdala on visceral pain perception in an intrinsically anxious rat

Stress-induced anxiety is a major health concern and can facilitate a multitude of disorders including irritable bowel syndrome (IBS), which is a functional gastrointestinal (GI) disorder characterized by episodes of abdominal pain with diarrhea and/or constipation (1). Although the etiology of IBS is unknown and the impact of stress and anxiety on the GI tract is poorly understood, recent clinical evidence suggests that there may be hypothalamic-pituitary-adrenal (HPA) axis dysregulation involved in the pathophysiology of IBS (2, 3). To substantiate the HPA dysregulation theory of IBS, exaggerated adrenocorticotropic hormone (ACTH), cortisol, and colonic motility responses to exogenous corticotrophin releasing factor (CRF) have all been observed in IBS patients compared to healthy controls (2, 4). A prominent feature of IBS symptomatology is altered visceral perception characterized by hypersensitivity of the colon to luminal distension (5, 6). In preclinical studies the Wistar-Kyoto (WKY) rat strain has been reported to be an effective model for studying anxiety and IBS (7-9). The WKY strain expresses high anxiety-like behavior compared to other strains (10, 11) in addition to colonic hypersensitivity to luminal distension demonstrated as an increased visceromotor response (VMR) to colorectal distension (CRD) (7). In WKY rats, there is also a strong positive correlation between increased central CRF and CRF type 1 receptor (CRF-1R) expression, with elevated anxiety–like behavior and colonic hypersensitivity (7, 12-14). WKY rats treated with a peripherally administered CRF-1R antagonist displayed colonic sensitivity that was similar to basal levels observed in the Fischer-344 (F344) rat strain that exhibit low-levels of anxiety-like behavior (11-13). Conversely, when F344 animals were administered CRF centrally via intracerebroventricular (i.c.v.) injection, colonic sensitivity resembled that seen in the WKY rats (12). Cortisol is also altered in states of anxiety and in IBS patients who exhibit elevated release in response to stress and have greater basal systemic cortisol levels compared to healthy volunteers (2, 3). Cortisol acts upon key limbic regions, such as the amygdala, to facilitate behavioral and psychological stress responses including GI motility (15). Further, preclinical studies show that implants of corticosterone (CORT) micropellets in F344 rats localized to the dorsal margin of the central nucleus of the amygdala (CeA) increases anxiety-like behavior as well as visceral hypersensitivity to luminal distension, similar to levels found in the WKY strain (12, 16, 17). CORT acts upon glucocorticoid and mineralocorticoid receptors (GR/MR) (18), which are highly expressed in the amygdala (19, 20). Specific activation of both GR and MR in the CeA has been implicated in the development of anxiety and visceral hypersensitivity (17, 21-23), suggesting that corticosteroid receptor activation in the CeA is involved in the induction of anxiety and visceral hypersensitivity. Although activation of GR/MR in the CeA induces anxiety-like behavior and visceral hypersensitivity in F344 rats, it is unknown whether the same mechanisms are present in WKY rats. Furthermore, although there is an increase in CRF activation involved in visceral hypersensitivity in WKY animals, the specific localization of CRF-mediated mechanisms remains unknown. Here we test the hypothesis that CRF and GR/MR activation of the CeA facilitates increased visceral hypersensitivity in the WKY strain.