Stress‐Induced Gut Dysmotility is Associated with Altered Intestinal BDNF‐TrkB Signaling

While stress is associated with incidence of gut dysmotility disorders, mechanisms by which stress causes dysmotility are not well understood. In the central nervous system (CNS), stress causes behavioral deficits by increasing the levels of Glucocorticoid hormone to suppress molecular signaling between brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin receptor kinase B (TrkB). We hypothesize that stress causes a Glucocorticoid-mediated reduction in BDNF-TrkB signaling in the enteric nervous system (ENS), whose neurons, which modulate diverse gut functions, reside entirely within the gut wall.Gut motility was assayed by analyzing the whole gut transit time (WGTT) of an orally gavaged carmine red dye in adult mice. Effect of stress was tested by comparing WGTT of Stress-Induced Dysmotility (SID) mice, wherein age-matched adult male mice were restrained for one hour, with that of unrestrained control mice. The effect of Glucocorticoids was tested by comparing WGTT of age-matched adult C57/BL6 male mice dosed with Dexamethasone (DEX, a Glucocorticoid receptor agonist, intra-peritoneal 5mg/kg), with that of saline-dosed control mice. Effect of DEX on enteric BDNF levels (using R&D ELISA kit) was tested by using total proteins isolated from preparations of adult murine longitudinal muscle-myenteric plexus (LM-MP) tissues cultured with and without DEX (1 µM) for 4 hours. TrkB exists in full-length and truncated (TrkB.T1) isoforms, and the presence of TrkB.T1 in ENS neurons was observed by immunofluorescence analyses of ENS tissue of TrkB.T1-V5 transgenic mice. Involvement of TrkB.T1 isoform in SID was tested by comparing WGTT of male transgenic mice containing partial (T1-Het) or complete loss (T1 KO) of TrkB.T1. Differences were compared for statistical significance using Student's t-test and Mann-Whitney test.5 consecutive days of 1 hour restraint caused a significant delay in WGTT of adult C57/BL6 SID mice, when compared to controls (Table 1). DEX caused a significant delay in WGTT, when compared to control mice (Table 1). DEX treatment caused a significant reduction in BDNF levels (Table 1). TrkB.T1-V5 mice show presence of TrkB.T1 on adult enteric neurons (Fig 1). A single hour of restraint caused a significant delay in WGTT in adult T1 KO mice, when compared with littermate T1 Het mice (Table 1).Restraint stress and DEX caused similar delay in gut transit in wildtype mice, and since DEX decreased gut BDNF levels, we infer that stress-induced increase in glucocorticoid signaling delayed gut transit by suppressing enteric BDNF levels. Increased susceptibility of T1 KO mice to SID highlights the important role of the truncated TrkB.T1 receptor in maintenance of normal gut motility during acute and moderate stress. Thus, our data implicates altered BDNF-TrkB.T1 signaling in the etiology of Stress Induced Dysmotility.