Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum

Abstract

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl−-free and [Formula: see text]-free solutions. The responses were almost completely blocked by TTX (10−6 M) but not by atropine (10−5 M) or hexamethonium (10−4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT3 and 5-HT4), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors ( Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl−/[Formula: see text] secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl−/[Formula: see text] secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl−/[Formula: see text] secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

Recent advances in vasoactive intestinal peptide physiology and pathophysiology: focus on the gastrointestinal system

“…VIP is mainly localized in the myenteric and submucosal neurons and nerve terminals in the GI tract 20, 21 . Endogenous VIP is released by numerous stimuli such as acetylcholine (ACh) 22 , ATP 23 , serotonin (5-HT) 24 , substance P (SP) 25 , GLP-2 26 , and xenin-25 27 from at least two populations of VIP-positive nerves: cholinergic and non-cholinergic VIP-releasing nerves. In guinea pig small intestine, most VIP-positive nerves in the mucosa and submucosa are non-cholinergic secretomotor neurons 28 and well colocalized with neuronal nitric oxide synthase (nNOS) in human colonic circular muscles 29 .…”

“…VPAC1 in mice and humans is predominantly expressed in the colon relative to the small intestine 44 and is predominantly expressed in the mucosa and submucosa compared to the muscle layers in rat ileum 27 , suggesting that VIP effects on epithelial functions, including ion transport, mucus secretion, tight junction protein expression, and cell proliferation, are mainly mediated via VPAC1 activation. VPAC1 localization in the epithelial cells is thought to be on the basolateral membranes, since serosally applied VIP increases electrogenic anion secretion in the small and large intestine 45, 46 .…”

“…In the duodenum, exogenous VIP increases protective HCO 3 – secretion via a CFTR-dependent pathway 46 . In the ileum and colon, VIP increases electrogenic Cl – and HCO 3 – secretion 27, 45, 59 . VIP also increases Cl – secretion in porcine gallbladder 60 and increases porcine pancreatic fluid and HCO 3 – secretion 61 .…”

The effect of Xenin25 on spontaneous circular muscle contractions of rat distal colon in vitro

“…Xenin acts on the neurotensin receptor (Fuerle, 1998). Furthermore, increases in Cl − and HCO 3 − secretion induced by Xenin25 are mediated by the NT receptors in the enteric nervous system (Kaji et al, 2017; Kuwahara et al, 2019). Xenin25 plasma concentrations increase in peripheral circulation after food intake (Fuerle et al, 2003).…”

“…In the GI tract, Xenin25 affects various physiological functions by modulating GI motility, inhibiting gastric acid secretion, and stimulating exocrine pancreatic secretion (Chowdhury et al, 2013; Fuerle, 1998; Fuerle et al, 1997). We recently reported that Xenin25 affects intestinal ion transport through the activation of ENS; Xenin25 induces anion secretion through the activation of IPANs and noncholinergic secretomotor neurons in rat duodenum and ileum, respectively (Kaji et al, 2017; Kuwahara et al, 2019). Based on an in vitro experiment, Xenin25 has excitatory and inhibitory effects on smooth muscle activity in the guinea pig GI tract; Xenin induces a biphasic motor response of the small intestine with involvement of the neurotensin receptor, cooperating with muscarinic, purinergic, and tachykinin‐related mechanisms (Fuerle et al, 1996, 2002).…”