PI 3‐kinase‐ and ERK‐MAPK‐dependent mechanisms underlie Glucagon‐Like Peptide‐1‐mediated activation of Sprague Dawley colonic myenteric neurons

Abstract

Background Glucagon‐like peptide (GLP‐1) can modify colonic function, with beneficial effects reported in the functional bowel disorder, irritable bowel syndrome (IBS). IBS pathophysiology is characterized by hyper‐activation of the hypothalamic‐pituitary‐adrenal stress axis and altered microbial profiles. This study aims to characterize the neuronal and functional effects of GLP‐1 in healthy rat colons to aid understanding of its beneficial effects in moderating bowel dysfunction. Methods Immunofluorescent and calcium imaging of myenteric neurons prepared from Sprague Dawley rat colons was carried out to elucidate the neuromodulatory actions of the GLP‐1 receptor agonist, exendin‐4 (Ex‐4). Colonic contractile activity was assessed using organ bath physiological recordings. Key results Ex‐4 induced an elevation of intracellular calcium arising from store release and influx via voltage‐gated calcium channels. Ex‐4 activated both ERK‐MAPK and PI 3‐kinase signaling cascades. Neuronal activation was found to underlie suppression of contractile activity in colonic circular muscle. Although the stress hormone, corticotropin‐releasing factor (CRF) potentiated the neuronal response to Ex‐4, and the functional effects of Ex‐4 on colonic circular muscle activity were not altered. Conclusions and inferences Ex‐4 evoked neurally regulated suppression of rat colonic circular muscle activity. In myenteric neurons, the neurostimulatory effects of Ex‐4 were dependent upon activation of PI 3‐kinase and ERK‐MAPK signaling cascades. No further change in circular muscle function was noted in the presence of CRF suggesting that stress does not impact on colonic function in health. Further studies in a model of IBS are needed to determine whether mechanisms are modified in the context of bowel dysfunction.

Glucagon-Like Peptide-1 Secreting L-Cells Coupled to Sensory Nerves Translate Microbial Signals to the Host Rat Nervous System

“…Forty-four percentage (n = 14 /34 neurons from three SMP preparations) of neurons responded to Ex-4 with a robust, but variable, increase in [Ca 2+ ] i . The calcium response was reproducible upon second application (n = 10 neurons from three SMP preparations, p = 0.489, Figure 3C) and, consistent with previous studies in myenteric neurons (O'Brien et al, 2019), Ex-3 (9-39) abolished this response (n = 22 neurons from three SMP preparations, p < 0.0001, Figure 3D). In hemisected colonic tissue, where half of the mucosa was retained (Figure 3E), the mucosal application of indole did not affect neuronal calcium levels per se (Figure 3F).…”

The Glucagon‐like peptide‐1 receptor agonist, exendin‐4, ameliorated gastrointestinal dysfunction in the Wistar Kyoto rat model of Irritable Bowel Syndrome

“…48 Therefore, in our studies Ex-4 was administered 1 hour prior to behavioral assessments. 49 The immunofluorescently visualized GLP-1 receptors may therefore be predominantly expressed on innervating fibers. [14][15][16]46 We have previously established the intricate relationship between hormones and immune molecules and demonstrated the efficacy of xIL-6R in reducing stress-induced defecation in WKY rats.…”

“…[14][15][16]46 We have previously established the intricate relationship between hormones and immune molecules and demonstrated the efficacy of xIL-6R in reducing stress-induced defecation in WKY rats. 31,49 The amplitude of the calcium response evoked by Ex-4 is suppressed when IL-6 is coapplied. in S100-labeled myenteric glial cells.…”

“…in S100-labeled myenteric glial cells. 49 The immunofluorescently visualized GLP-1 receptors may therefore be predominantly expressed on innervating fibers. Indeed, GLP-1Rs have been detected in human colonic nerve fibers and dorsal root ganglion cells.…”

“…51 It is noteworthy, given that GLP-1 and IL-6 receptors are not commonly expressed in the same cells, that cross talk is apparent in the neurostimulatory actions of the receptor agonists. 31,49 The amplitude of the calcium response evoked by Ex-4 is suppressed when IL-6 is coapplied. Although this relationship needs to be researched further, neuronal cross talk between these molecules could impact of colonic function.…”