SGML-related neurotransmitters/neuromodulators in ENS
Similar to the CNS, there are many types of neurons in the ENS, which differ from each other not only in morphology and structure, but also in neurotransmitter diversity. Apart from acetylcholine (ACh) and norepinephrine (NE), numerous endogenous substances, such as substance P (SP), vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), ATP, 5-hydroxytryptamine (5-HT) and nitric oxide (NO), have recently been found to play important roles as neurotransmitters in the gastrointestinal neural network. These neurotransmitters are also termed the non-adrenergic non-cholinergic (NANC) nerves. Gastrointestinal excitatory motor neurons release excitatory transmitters, such as ACh and SP, thus promoting gastrointestinal smooth muscle contraction and glandular secretion. On the contrary, inhibitory motor neurons release inhibitory transmitters, such as VIP and NO, thus suppressing gastrointestinal smooth muscle contraction and glandular secretion. All these gastrointestinal excitatory and inhibitory motor neurons can interact with each other under a complex and delicate balance. If this balance is broken, gastrointestinal dysfunction may be induced.
NO: NO is an active and unstable inorganic gaseous molecule secreted by NANC nerves in the ENS, which serves as a major inhibitory neurotransmitter in the gastrointestinal tract. Nitric oxide synthase (NOS) is a key rate-limiting enzyme in NO production, and NOS assessment can indirectly determine the changes in NO, which can be classified into eNOS, nNOS and iNOS. eNOS is mainly distributed in vascular endothelial cells, and the synthesized NO is mainly used to regulate GMBF and promote gastric mucosal repair. nNOS is mainly distributed in cells within the intermuscular and submucosal plexuses, endocrine cells and vascular endothelial cells, and the synthesized NO interacts with gastrointestinal hormones (VIP and ATP) to jointly regulate gastrointestinal motility. iNOS is highly expressed upon the stimulation of endotoxins and cytokines, which produces a large amount of NO within a short period of time, and ultimately results in cell damage. Previous studies have demonstrated that NO can inhibit gastric acid secretion and neutrophil adhesion, improve gastric mucosal blood circulation and eliminate oxygen free radicals, thereby protecting the gastric mucosa from injury[30,31]. It was reported by Nishiad et al that the expression level of iNOS increased significantly in the gastric mucosa of RWIS rats, while that of eNOS reduced significantly, indicating that the changes in iNOS and eNOS activities in the gastric mucosa are closely related to the incidence of GML. In SGML, L-NAME (NOS inhibitor) can decrease the production of NO, thus exacerbating acute GML and inhibiting the healing process of chronic gastric ulcers, while L-Arg (NO precursor) can obviously prevent the injury[32-34]. Wei et al found that NO is involved in RWIS, and can promote the SGML healing process.
The mechanisms of NO in protecting gastric mucosa are as follows: (1) NO can reduce vascular permeability, inhibit platelet adhesion and aggregation in gastric mucosal vascular endothelium, and prevent thrombosis. (2) Under physiological conditions, gastric mucosal vascular endothelium synthesizes NO, which in turn regulates vascular smooth muscle tension and maintains GMBF. (3) In acute GML, NO increases GMBF by dilating the mucosal blood vessels, thus promoting gastric mucosal repair. In addition, the secretion of gastric acid can also be inhibited by NO. Upon the reaction of stimulus against gastric mucosa, enterochromaffin cells and mastocytes can release histamine to stimulate parietal cells for gastric acid production, thus aggravating the mucosal lesion. In addition, endogenous NO can inhibit the stimulation of histamine through parietal cells, thus reducing gastric acid secretion and protecting gastric mucosa. It has been found that, through in vivo and in vitro experiments, the NO donor FK409 and sodium nitroprusside can markedly suppress the gastrin-induced increase in histamine release and gastric acid secretion in rats, and L-NAME further increases gastric acid secretion. Gastric mucous cells promote NO synthesis by expressing high-level NOS, and enhance the mucous barrier through the NO effects of promoting mucin synthesis and secretion. Based on the findings of previous experiments, RWIS-induced GMLs can weaken the synthesis and secretion of gastric mucus by reducing nNOS activity, while the NO donor L-Arg can increase nNOS activity and mucus secretion.
5-HT: 5-HT is an important neurotransmitter that widely exists in the brain and digestive tract. It has been estimated that 90% of 5-HT is synthesized and secreted by enterochromaffin cells in the gastrointestinal tract. There are four types of 5-HT receptors, of which 5-HT3R and 5-HT4R are the two receptor subtypes most closely related to gastrointestinal function. 5-HT3R is mainly found within the neurons of myenteric plexuses in the stomach and colon, while 5-HT4R is mainly distributed within the neurons of myenteric plexuses in the ascending colon, duodenum and gastric smooth muscles, as well as the intestinal submucosa. Both 5-HT3R and 5-HT4R are involved in gastrointestinal motility, gastric acid and mucus secretion, and regulation of local mucosal blood flow. Due to the absence of internal and external nerve fibers in the mucous epithelium, it is a transepithelial phenomenon that ENS senses and reacts to chemical stimulus in the enteric cavity, which may be partly mediated by enterochromaffin cells. Upon response to stimulus, these cells may release 5-HT, and subsequently activate the submucosal primary afferent nerve fibers through 5-HT3R distributed on the vagal afferent nerve fibers, and transmit intestinal information to the center, thereby regulating local excitement and inhibition. Serotonin reuptake transporter (SERT) is a type of translocator responsible for re-uptake of 5-HT from the synaptic cleft, which can rapidly eliminate 5-HT from the synapse and reduce 5-HT concentration in the intestinal tract and tissue space. Thus, pharmacological blockage of SERT function can obviously decrease gastrointestinal motility. Chen et al reported that, in SERT-knockout mice, watery diarrhea is likely to be caused by the enhanced action of the 5-HT signaling pathway, and slow transit constipation may be caused by the insensitivity of 5-HT receptors due to excessive 5-HT production. In addition, Morita et al showed that 5-HT3R and 5-HT4R antagonists can inhibit the antral motility during the interdigestive period (phase III) and colonic motility in dogs. The above data indicate that the 5-HT signaling system is involved in the sensory and motor functions of the gastrointestinal tract. Apart from its motor and sensory functions in the gastrointestinal tract, 5-HT inhibits gastric acidity by increasing the synthesis of mucus. It has been reported that 5-HT simulates prostaglandin (PG) synthesis by enhancing the activity of the cyclooxygenase pathway, which in turn stimulates mucosal blood flow and contributes to the secretion of mucus along with bicarbonate. Therefore, the 5-HT signaling system plays a vital role in the regulation of gastrointestinal motility, secretion and visceral sensation. An abnormal 5-HT signaling pathway may trigger gastrointestinal endocrine disorders, leading to a variety of gastrointestinal diseases. In addition, indomethacin-induced intestinal lesions in mice can be prevented by pretreatment with p-chlorophenylalanine (a 5-HT synthesis inhibitor). The administration of the 5-HT4 receptor agonist (mosapride) or 5-HT3 receptor antagonists (e.g., ondansetron and ramosetron) can dose-dependently reduce the severity of indomethacin-induced intestinal lesions, whereas a high dose of GR113808 (a 5-HT4 receptor antagonist) significantly aggravated these lesions. These findings suggest that endogenous 5-HT exerts a dual role in the pathogenesis of indomethacin-induced intestinal lesions: Pro-ulcerogenic action via 5-HT3 receptors and anti-ulcerogenic action via 5-HT4 receptors. Additionally, the cold-restraint stress significantly increased mean ulcer index values in gastric tissue, while a decrease in enterochromaffin cell count was observed with a concomitant decrease in 5-HT content and adherent mucosal thickness. Pretreatment with Aegle marmelos reduced mean ulcer index values, and increased enterochromaffin cell count, 5-HT content and adherent mucosal thickness in ulcerated gastric tissue, suggesting that the high enterochromaffin cell count and 5-HT levels exert protective effects against cold-restraint stress-induced gastric mucosal injury.
Hydrogen sulfide: Hydrogen sulfide (H2S) is a new type of endogenous gaseous signaling molecule, and many tissues in the body can catalyze L-cysteine to H2S via cystathionine-synthase (CBS) and cystathionine-γ-lyase (CSE). In recent years, it was found that H2S is involved in various physiological and pathological processes in the body, which in turn regulates the motility, resists the inflammation, affects the visceral sensitivity, and promotes glandular secretion. Specifically, its regulation of digestive tract motility is mainly manifested as the suppression of intestinal motility. Exogenous administration of sodium hydrosulfide (NaHS; a H2S donor) can enhance the secretion of colon mucosal and submucosal chloride in guinea pigs in a concentration-dependent manner, and this secretion can be inhibited by H2S CSE and CBS blockers. Krueger et al found that H2S promotes intestinal secretion via activation of transient receptor potential vanilloid-1 (TRPV1) receptor and the release of SP, thus activating cholinergic neurons. In addition, Dhaese et al found that NaHS can relax the gastric fundus smooth muscles of mice in a concentration-dependent manner, which can be weakened by myosin light chain phosphatase (MLCP) inhibitors, instead of ATP-sensitive potassium channel (KATP) blockers. This indicates that H2S-induced dilation of gastric fundus smooth muscles is realized, at least partly, by activating MLCP, but is not related to KATP.
H2S is able to protect the gastrointestinal tract and promote the repair of GML. Nonsteroidal anti-inflammatory drugs (NSAIDs) can markedly down-regulate the expression of CSE in gastric mucosa and reduce the synthesis of endogenous H2S. However, NaHS can be used to decrease the synthesis of tumor necrosis factor-alpha (TNF-α), intercellular adhesion molecule-1 and lymphocyte function associated antigen-1, reduce the adhesion of white blood cells to mesenteric vessels, increase GMBF and suppress GML caused by NSAIDs. Both exogenous administration and endogenous release of H2S are gastroprotective against cold restraint stress-induced gastric injury. It has been reported that NaHS attenuates the ulcer index by reducing gastric acid output, mucosal carbonyl content, pepsin activity and ROS generation. Moreover, H2S also reduces TNF-α level and myeloperoidase activity. Magierowski et al demonstrated that treatment with NaHS plays an important physiological role in gastric mucosal protection against stress-induced lesions. The protective effect of NaHS is often accompanied by an enhancement in gastric microcirculation, possibly mediated by a significant local increase in the gastric mucosal production of H2S. Furthermore, the mechanism of H2S-induced gastroprotection may involve activation of the endogenous prostaglandin/cyclooxygenase (PG/COX) system, increase biosynthesis of prostaglandin E2 (PGE2), afferent sensory fibers release of CGRP via VR-1 receptors and an anti-inflammatory effect resulting in the inhibition of pro-inflammatory cytokines (e.g., TNF-α).
CGRP: CGRP is the main transmitter of capsaicin-sensitive sensory nerves, with a wide variety of functions, which is widely distributed throughout the cardiovascular, gastrointestinal and respiratory systems and commonly recognized as a protector of gastric mucosa. CGRP is the most effective vasodilator discovered to date, and it can improve blood flow through two mechanisms. (1) CGRP binds to receptors on endothelial cells and up-regulates NOS to produce NO, thereby relaxing the vascular smooth muscles. (2) CGRP directly binds to receptors on vascular smooth muscle cells and increases GMBF by activating KATP channels without involving the endothelium. In addition, CGRP also exerts anti-apoptotic, anti-platelet aggregation, anti-oxidation, anti-proliferation and anti-aging properties. Previous findings have shown that CGRP can significantly inhibit gastric acid secretion. Following craniocerebral injury and severe burn, the concentrations of hydrogen ions in gastric juice may be elevated. It has also been found that excess gastric acid production is related to neuroendocrine disorders, and a decrease in CGRP secretion is caused by nerve center and hypothalamic injury. The gastric acid secretion in mice can be facilitated by an intracerebroventricular injection of thyrotropin-releasing hormone, but inhibited by CGRP. A large amount of experimental data has proved that noxious stimuli, such as GML, can trigger the sensory neurons to release CGRP, activate intermediate neurons and motor neurons. Besides, it also directly acts on smooth muscles and inhibits gastrointestinal motility, whose mechanisms are as follows: (1) The longitudinal muscles and circular muscles of the intestine are directly dilated. And (2) The NANC inhibitory neurons are stimulated, and other inhibitory neurotransmitters are released, such as NO and VIP, thereby inhibiting gas-trointestinal motility.
CGRP can help to improve local gastric microcirculation and protect against stress- or drug-induced GML by increasing gastrointestinal blood flow and regulating gastrointestinal motility. A previous study demonstrated that CGRP down-regulation is related to the pathogenesis of gastric ulcers. After stimulation, capsaicin-sensitive sensory nerve fibers may release CGRP, and then CGRP increases the levels of prostacyclin and PGE2 in gastric mucosa, thereby inhibiting the activation of neutrophils and degranulation of mastocytes, reducing the secretion of inflammatory mediators (e.g., histamine), and alleviating gastrointestinal inflammation. In a mouse model of ischemia-reperfusion injury, intraperitoneal treatment with CGRP significantly reduced gastric mucosal edema, hemorrhage, apoptosis, mucosal separation and inflammatory cell infiltration. RWIS-induced gastric ulcers were inversely correlated with the decrease in CGRP-like immunoreactivity observed in the whole thickness of the stomach corpus. Systemic administration of CGRP exhibited a significant decrease in the lesion index of RWIS-induced ulcers, and such protection was inhibited by the functional ablation of afferent neurons induced by capsaicin pretreatment. These findings suggest that endogenous CGRP plays a defensive role in RWIS-induced ulcers.
VIP: As one of the most important peptide neurotransmitters, VIP is widely distributed in the circulatory, immune, reproductive and digestive systems, as well as the central and peripheral nervous systems. VIP possesses dual functions in the body, and acts as both a gastrointestinal hormone and a neuropeptide, and has been considered a type of brain-gut peptide. VIP is mainly produced by the central and peripheral nervous systems, released by the parasympathetic postganglionic fibers and coexists with ACh, which plays a regulatory role in local mucosal immu-noregulation. In the digestive system, VIP is predominantly distributed in the endocrine cells of gastrointestinal mucosa as well as the submucosal plexuses and smooth muscle layer. Moreover, VIP regulates gastrointestinal absorption, inhibits gastric acid secretion, protects the gastrointestinal mucosa from acid-induced damage, and promotes the secretion of water, electrolytes, pancreatic juice and intestinal juice in the intestine. Additionally, VIP is able to induce smooth muscle relaxation and exert a potent vasodilator effect. The regulatory effect of VIP on gastrointestinal smooth muscle motility is closely related to the main inhibitory neurotransmitter NO, in which VIP can promote NO synthesis, relax circular muscles, inhibit gastric motility and reduce gastric tightness.
In addition, VIP exerts an impact on the integrity of the gastrointestinal mucous membrane barrier. For instance, VIP can induce the secretion of water and bicarbonate in the pancreas, thus promoting the formation and repair of the gastrointestinal mucous membrane barrier. Moreover, VIP can stimulate the production of intestinal juice, and inhibit the secretion of gastric acid and gastrin, thereby protecting the gastric mucosa, suppressing the occurrence of gastric and duodenal ulcers, and promoting the repair of ulcers. In ethanol-induced GML, the content of VIP decreases obviously in the gastric mucosa, by reducing the release of NO and increasing the production of endothelin. It has been reported that cold-restraint stress induces gastric lesions and mast cell degranulation, and exacerbates lipid peroxidation in gastric tissue. VIP can prevent stress-induced ulcers and mast cell degranulation, and protect gastric tissue from lipid peroxidation. When VIP was administered after stress-induced ulcers, it was demonstrated to be therapeutically beneficial, suggesting that VIP is valuable for the prevention of gastric mucosal damage induced by cold-restraint stress. Recent studies have shown that VIP significantly suppressed cold-restraint stress gastric lesions and markedly decreased the content of histamine in the tissue. Histamine plays an important role in the development of gastric ulcers, and mast cell-derived histamine might be essential during this process. The mechanisms of the action of VIP on gastric tissue histamine levels can be explained by its inhibitory effect on the release of gastrin hormone, mast cells and enterochromaffin-like cells.