Capsaicin-sensitive extrinsic afferents are involved in acid-induced activation of distinct myenteric neurons in the rat stomach

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Extrinsic Primary Afferent Neurons Link Visceral Pain to Colon Motility Through a Spinal Reflex in Mice

BACKGROUND & AIMS

Proper colon function requires signals from extrinsic primary afferent neurons (ExPANs) located in spinal ganglia. Most ExPANs express the vanilloid receptor TRPV1, and a dense plexus of TRPV1-positive fibers is found around myenteric neurons. Capsaicin, a TRPV1 agonist, can initiate activity in myenteric neurons and produce muscle contraction. ExPANs might therefore form motility-regulating synapses onto myenteric neurons. ExPANs mediate visceral pain, and myenteric neurons mediate colon motility, so we investigated communication between ExPANs and myenteric neurons and the circuits by which ExPANs modulate colon function.

METHODS

In live mice and colon tissues that express a transgene encoding the calcium indicator GCaMP, we visualized levels of activity in myenteric neurons during smooth muscle contractions induced by application of capsaicin, direct colon stimulation, stimulation of ExPANs, or stimulation of preganglionic parasympathetic neuron (PPN) axons. To localize central targets of ExPANs, we optogenetically activated TRPV1-expressing ExPANs in live mice and then quantified Fos immunoreactivity to identify activated spinal neurons.

RESULTS

Focal electrical stimulation of mouse colon produced phased-locked calcium signals in myenteric neurons and produced colon contractions. Stimulation of the L6 ventral root, which contains PPN axons, also produced myenteric activation and contractions that were comparable to those of direct colon stimulation. Surprisingly, capsaicin application to the isolated L6 dorsal root ganglia, which produced robust calcium signals in neurons throughout the ganglion, did not activate myenteric neurons. Electrical activation of the ganglia, which activated even more neurons than capsaicin, did not produce myenteric activation or contractions unless the spinal cord was intact, indicating that a complete afferent-to-efferent (PPN) circuit was necessary for ExPANs to regulate myenteric neurons. In TRPV1-channel rhodopsin-2 mice, light activation of ExPANs induced a pain-like visceromotor response and expression of Fos in spinal PPN neurons.

CONCLUSIONS

In mice, ExPANs regulate myenteric neuron activity and smooth muscle contraction via a parasympathetic spinal circuit, linking sensation and pain to motility.

The homeostatic role of neuropeptide Y in immune function and its impact on mood and behaviour

Neuropeptide Y (NPY), one of the most abundant peptides in the nervous system, exerts its effects via five receptor types, termed Y1, Y2, Y4, Y5 and Y6. NPY's pleiotropic functions comprise the regulation of brain activity, mood, stress coping, ingestion, digestion, metabolism, vascular and immune function. Nerve-derived NPY directly affects immune cells while NPY also acts as a paracrine and autocrine immune mediator, because immune cells themselves are capable of producing and releasing NPY. NPY is able to induce immune activation or suppression, depending on a myriad of factors such as the Y receptors activated and cell types involved. There is an intricate relationship between psychological stress, mood disorders and the immune system. While stress represents a risk factor for the development of mood disorders, it exhibits diverse actions on the immune system as well. Conversely, inflammation is regarded as an internal stressor and is increasingly recognized to contribute to the pathogenesis of mood and metabolic disorders. Intriguingly, the cerebral NPY system has been found to protect against distinct disturbances in response to immune challenge, attenuating the sickness response and preventing the development of depression. Thus, NPY plays an important homeostatic role in balancing disturbances of physiological systems caused by peripheral immune challenge. This implication is particularly evident in the brain in which NPY counteracts the negative impact of immune challenge on mood, emotional processing and stress resilience. NPY thus acts as a unique signalling molecule in the interaction of the immune system with the brain in health and disease.

Acid-sensing ion channels in gastrointestinal function

Gastric acid is of paramount importance for digestion and protection from pathogens but, at the same time, is a threat to the integrity of the mucosa in the upper gastrointestinal tract and may give rise to pain if inflammation or ulceration ensues. Luminal acidity in the colon is determined by lactate production and microbial transformation of carbohydrates to short chain fatty acids as well as formation of ammonia. The pH in the oesophagus, stomach and intestine is surveyed by a network of acid sensors among which acid-sensing ion channels (ASICs) and acid-sensitive members of transient receptor potential ion channels take a special place. In the gut, ASICs (ASIC1, ASIC2, ASIC3) are primarily expressed by the peripheral axons of vagal and spinal afferent neurons and are responsible for distinct proton-gated currents in these neurons. ASICs survey moderate decreases in extracellular pH and through these properties contribute to a protective blood flow increase in the face of mucosal acid challenge. Importantly, experimental studies provide increasing evidence that ASICs contribute to gastric acid hypersensitivity and pain under conditions of gastritis and peptic ulceration but also participate in colonic hypersensitivity to mechanical stimuli (distension) under conditions of irritation that are not necessarily associated with overt inflammation. These functional implications and their upregulation by inflammatory and non-inflammatory pathologies make ASICs potential targets to manage visceral hypersensitivity and pain associated with functional gastrointestinal disorders. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Reprint of: Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Lack of cholinergic innervation in gastric mucosa does not affect gastrin secretion or basal acid output in neurturin receptor GFRα2 deficient mice

Efferent signals from the vagus nerve are thought to mediate both basal and meal-induced gastric acid secretion, and provide trophic support of the mucosa. However, the underlying mechanisms are incompletely understood. Neurturin, signalling via glial cell line-derived neurotrophic factor (GDNF)-family receptor α2 (GFRα2), is essential for parasympathetic innervation of many target tissues but its role in gastric innervation is unknown. Here we show that most nerve fibres in wild-type mouse gastric mucosa, including all positive for gastrin-releasing peptide, are cholinergic. GFRα2-deficient (KO) mice lacked virtually all cholinergic nerve fibres and associated glial cells in the gastric (oxyntic and pyloric) mucosa but not in the smooth muscle, consistent with the selective expression of neurturin mRNA in the gastric mucosa. 2-Deoxyglucose and hexamethonium failed to affect acid secretion in the GFRα2-KO mice indicating the lack of functional innervation in gastric mucosa. Interestingly, basal and maximal histamine-induced acid secretion did not differ between wild-type and GFRα2-KO mice. Moreover, circulating gastrin levels in both fasted and fed animals, thickness of gastric mucosa, and density of parietal and different endocrine cells were similar. Carbachol-stimulated acid secretion was higher in GFRα2-KO mice, while atropine reduced basal secretion similarly in both genotypes. We conclude that cholinergic innervation of gastric mucosa depends on neurturin-GFRα2 signalling but is dispensable for gastrin secretion and for basal and maximal acid output. Basal acid secretion in the KO mice appears to be, at least partly, facilitated by constitutive activity of muscarinic receptors.

Neuropeptide Y, peptide YY and pancreatic polypeptide in the gut-brain axis

The gut-brain axis refers to the bidirectional communication between the gut and the brain. Four information carriers (vagal and spinal afferent neurons, immune mediators such as cytokines, gut hormones and gut microbiota-derived signalling molecules) transmit information from the gut to the brain, while autonomic neurons and neuroendocrine factors carry outputs from the brain to the gut. The members of the neuropeptide Y (NPY) family of biologically active peptides, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are expressed by cell systems at distinct levels of the gut-brain axis. PYY and PP are exclusively expressed by endocrine cells of the digestive system, whereas NPY is found at all levels of the gut-brain and brain-gut axis. The major systems expressing NPY comprise enteric neurons, primary afferent neurons, several neuronal pathways throughout the brain and sympathetic neurons. In the digestive tract, NPY and PYY inhibit gastrointestinal motility and electrolyte secretion and in this way modify the input to the brain. PYY is also influenced by the intestinal microbiota, and NPY exerts, via stimulation of Y1 receptors, a proinflammatory action. Furthermore, the NPY system protects against distinct behavioural disturbances caused by peripheral immune challenge, ameliorating the acute sickness response and preventing long-term depression. At the level of the afferent system, NPY inhibits nociceptive input from the periphery to the spinal cord and brainstem. In the brain, NPY and its receptors (Y1, Y2, Y4, Y5) play important roles in regulating food intake, energy homeostasis, anxiety, mood and stress resilience. In addition, PP and PYY signal to the brain to attenuate food intake, anxiety and depression-related behaviour. These findings underscore the important role of the NPY-Y receptor system at several levels of the gut-brain axis in which NPY, PYY and PP operate both as neural and endocrine messengers.

Differential changes in Substance P, VIP as well as neprilysin levels in patients with gastritis or ulcer

The protective effect of capsaicin-sensitive sensory nerve (CSSN) activation was recently demonstrated in human gastric mucosa. We here examined changes in neuropeptides, specifically Substance P (SP), calcitonin-gene related peptide (CGRP) and vasoactive intestinal peptide (VIP) in patients with chronic gastritis or ulcer. Furthermore changes in neprilysin levels, which hydrolyse these neuropeptides, were determined. Gastric biopsies were obtained from both lesion- and normal-appearing mucosa of 57 patients. The presence of H. pylori infection was verified with rapid urease assay. Neuronal and non-neuronal levels of SP, VIP, CGRP and neprilysin activity were determined in freshly frozen biopsies. Immunohistochemical localization of neprilysin was performed in 30 paraffin embedded specimens. We here found that neuronal SP levels decreased significantly in normally appearing mucosa of patients with gastritis while levels of non-neuronal SP increased in diseased areas of gastritis and ulcer. The presence of H. pylori led to further decreases of SP levels. The content of VIP in both disease-involved and uninvolved mucosa, and expression of neprilysin, markedly decreased in patients with gastritis or ulcer. Since VIP, as well as SP fragments, formed following hydrolysis with neprilysin is recognized to have gastroprotective effects, decreased levels of VIP, SP and neprilysin may predispose to cellular damage.

Acid sensing by visceral afferent neurones

Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics.

The chemical code of porcine enteric neurons and the number of enteric glial cells are altered by dietary probiotics

BACKGROUND

The enteric nervous system (ENS) contains chemically coded populations of neurons that serve specific functions for the control of the gastrointestinal tract. The ability of neurons to modify their chemical code in response to luminal changes has recently been discovered. It is possible that enteric neuronal plasticity may sustain the adaptability of the gut to changes in intestinal activity or injury, and that gut neurons may respond to an altered intestinal environment by changing their neuropeptide expression.

METHODS

We used immunohistochemical methods to investigate the presence and localization of several neuronal populations and enteric glia in both the small (ileum) and large (cecum) intestine of piglets. We assessed their abundance in submucosal and myenteric plexus from animals treated with the probiotic Pediococcus acidilactici compared with untreated controls.

KEY RESULTS

The treated piglets had a larger number of galanin- and calcitonin gene-related peptide (CGRP)-immunoreactive neurons than controls, but this was limited to the submucosal plexus ganglia of the ileum. Moreover, immunohistochemistry revealed that glial fibrillary acidic protein-positive enteric glial cells were significantly higher in the inner and outer submucosal plexuses of treated animals.

CONCLUSIONS & INFERENCES

The neuronal and glial changes described here illustrate plasticity of the ENS in response to an altered luminal environment in the gastrointestinal tract.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

Effect of acupuncturing at the acupoint ST 36 on substance P and proopiomelanocortin expression in hypothalamus and adrenal in rats with cold-restraint stress-induced ulcer

AIM: To study the protective effect of acupuncturing at the acupoint ST 36 against cold-restraint stress-induced ulcer and the expression of substance P (SP) and proopiomelanocortin (POMC) associated with stress in hypothalamus and adrenal in rats.

METHODS: Twenty-two rats were randomized into 3 groups: normal control group (n = 6), stress group (n = 8), acupuncturing group (n = 8). The rats in the acupuncutring group received acupuncture at the acupoint ST 36 before cold-restraint stress. Ulcer index and serum cortisol level were used to evaluate the protective effect of acupuncturing at the acupoint ST 36; reverse transcription-polymerase chain reaction (RT-PCR) was used to detect the expression of SP and POMC in hypothalamus and adrenal, and the images were analyzed with semi-quantitative method.

RESULTS: The ulcer index and serum cortisol level in the acupuncturing group were significantly lower than those in the stress group (9.75 ± 1.91 vs 26.25 ± 4.40, P < 0.01; 66.83 nmol/L ± 12.25 nmol/L vs 104.38 nmol/L ± 8.31 nmol/L, P < 0.01). The expression of SP was up-regulated in hypothalamus (1.02 ± 0.42 vs 0.45 ± 0.12, P < 0.05) but down-regulated in adrenal (1.88 ± 0.82 vs 2.93 ± 1.08, P < 0.05) in the acupuncturing group as compared with that in the stress group. Acupuncturing at the acupoint ST 36 inhibited the sress-induced POMC expression in hypothalamus (0.56 ± 0.14 vs 0.82 ± 0.19, P < 0.01). There was no POMC expression in adrenal in the rats with sress-induced ulcer.

CONCLUSION: Acupuncturing at acupoint ST 36 can protect gastric mucosa against cold-restraint stress-induced ulcer by up-regulating SP expression in hypothalamus and down-regulating POMC expression in hypothalamus and SP expression in adrenal.

Efferent-like roles of afferent neurons in the gut: Blood flow regulation and tissue protection

The maintenance of gastrointestinal mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. To this end, the gastric mucosa is innervated by intrinsic sensory neurons and two populations of extrinsic sensory neurons: vagal and spinal afferents. Extrinsic afferent neurons constitute an emergency system that is called into operation when the gastrointestinal mucosa is endangered by noxious chemicals. The function of these chemoceptive afferents can selectively be manipulated and explored with the use of capsaicin which acts via a cation channel termed TRPV1. Many of the homeostatic actions of spinal afferents are brought about by transmitter release from their peripheral endings. When stimulated by noxious chemicals, these afferents enhance gastrointestinal blood flow and activate hyperaemia-dependent and hyperaemia-independent mechanisms of protection and repair. In the rodent foregut these local regulatory roles of sensory neurons are mediated by calcitonin gene-related peptide and nitric oxide. The pathophysiological potential of the neural emergency system is best portrayed by the gastric hyperaemic response to acid back-diffusion, which is governed by spinal afferent nerve fibres. This mechanism limits damage to the surface of the mucosa and creates favourable conditions for rapid restitution and healing of the wounded mucosa. Other extrinsic afferent neurons, particularly in the vagus nerve, subserve gastrointestinal homeostasis by signalling noxious events in the foregut to the central nervous system and eliciting autonomic, emotional-affective and neuroendocrine reactions. Under conditions of inflammation and injury, chemoceptive afferents are sensitized to peripheral stimuli and in this functional state contribute to the hyperalgesia associated with functional dyspepsia and irritable bowel syndrome. Thus, if GI pain is to be treated by sensory neuron-directed drugs it needs to be considered that these drugs do not inhibit nociception at the expense of GI mucosal vulnerability.

Differential expression of Y receptors and signaling pathways in intestinal circular and longitudinal smooth muscle

The expression and mechanisms of action of Y receptors were examined in dispersed intestinal smooth muscle cells of the rabbit. The mixed Y1/Y2 agonists, NPY and PYY, and the Y2 agonist, NPY13-36, elicited concentration-dependent contraction of circular muscle cells that was inhibited by the selective Y2 antagonist, BIIE 0246. The Y4 agonist, PP, elicited similar, though weaker, contraction that was insensitive to Y1 and Y2 antagonists. The Y1 agonist, [Leu31, Pro34]NPY, did not elicit contraction of circular muscle cells. All Y receptor agonists inhibited cAMP formation in a PTx-sensitive fashion. In contrast, none of the agonists caused contraction of longitudinal muscle cells, and only the mixed Y1/Y2 agonists, NPY and PYY, and the Y1 agonist, [Leu31, Pro34]NPY, inhibited cAMP formation and VIP-induced muscle cell relaxation. 125I-PYY binding in longitudinal muscle cells was inhibited by NPY, PYY, [Leu31, Pro34]NPY and the Y1 antagonist, BIBP 3226. Contraction of circular but not longitudinal muscle cells by Y2 and Y4 agonists was observed also in cells isolated from human jejunum. The results indicate that Y2 and Y4 receptors are present only in intestinal circular muscle cells where they mediate contraction that is insensitive to PTx or Ca2+ channel blockers. Y1 receptors, negatively coupled to adenylyl cyclase, are present in cells from both layers.

Nociceptive transmitter release in the dorsal spinal cord by capsaicin-sensitive fibers after noxious gastric stimulation

Little is known about transmitters that encode noxious gastric stimuli in the spinal cord. The release of glutamate, substance P, and CGRP from the spinal cord was therefore investigated in response to acid injury of the gastric mucosa. Dorsal halves of the caudal thoracic spinal cord (T7-T13) were removed 6 h after oral application of 0.5 M HCl or saline, transferred to a superfusion chamber, and the basal and capsaicin-stimulated (3.3 microM) transmitter release was determined. After acid injury, basal glutamate release increased 134% as compared to saline-treated animals. Capsaicin-stimulated release of CGRP and SP was 48% and 58% lower in acid- than in saline-treated animals, indicating that capsaicin-sensitive fibers in the dorsal spinal cord were already partially depleted by acid treatment. Capsaicin denervation reduced basal glutamate release by 33% after acid injury as compared to non-denervated acid-treated animals. Gastric origin and capsaicin sensitivity of glutamatergic, CGRP- and SP-containing primary afferents in thoracic dorsal root ganglia were then determined by retrograde tracing with True Blue and immunohistochemical labeling with the vanilloid receptor TRPV1. About 65% of True Blue-labeled cells were glutamatergic and more than 73% of this population expressed the TRPV1 receptor. Nearly all True Blue/CGRP (85%)- and True Blue/SP-positive cells (97%) coexpressed TRPV1. We conclude that noxious gastric stimulation with acid induces release of glutamate, SP, and CGRP from capsaicin-sensitive sensory afferents in the dorsal horn of the spinal cord where they may play an important role in gastric nociception and hyperalgesia.

Expression of c-fos in gastric myenteric plexus and spinal cord of rats with cervical spondylosis

AIM: To determine the expression of c-fos in gastric myenteric plexus and spinal cord of rats with cervical spondylosis and its clinical significance.

METHODS: A cervical spondylosis model was established in rats by destroying the stability of cervical posterior column, and the cord segments C_4-6 and gastric antrum were collected 3, 4 and 5 mo after the operation. Rats with sham operation were used as controls. c-fos neuronal counter-staining was performed with an immunohistochemistry method. Every third sections from C_4-6 segments were drawn. The 10 most labeled c-fos-immunoreactive (Fos-IR) neurons were counted, and the average number was used for statistical analysis. The mean of Fos-IR neurons in myenteric plexus was calculated after counting Fos-IR neurons in 25 ganglia from each antral preparation, and expressed as a mean count per myenteric ganglion.

RESULTS: There were a few c-fos-positive neurons in the cervical cord and antrum in the control group. There was an increased c-fos expression in model group 3, 4 and 5 mo after operation, whereas there was no significant increase in c-fos expression in the control group at 3, 4 and 5 mo. More importantly, there was a significant difference in c-fos expression between rats followed up for 3 mo and those for 5 mo in the model group (11.20±2.26 vs 27.68±4.36, P<0.05, for the cervical cord; and 11.3±2.3 vs 29.3±4.6, P<0.05, for the gastric antrum). There was no significant difference between rats followed up for 3 mo and those for 4 mo and between rats followed up for 4 mo and those for 5 mo in the model group.

CONCLUSION: c-fos expression in gastric myenteric plexus was dramatically associated with that in the spinal cord in rats with cervical spondylosis, suggesting that the gastrointestinal function may be affected by cervical spondylosis. If this hypothesis is confirmed by further studies, functional gastrointestinal diseases such as functional dyspepsia and irritable bowel syndrome could be explained by neurogastroenterology.

Extracellular signal-regulated kinase-1 and -2 are activated by gastric luminal injury in dorsal root ganglion neurons via n-methyl-d-aspartate receptors

Mitogen activated protein kinases such as phosphorylated extracellular signal-regulated kinase-1 and -2 (pERK 1/2) have been recently demonstrated to play an important role in somatic nociception and hyperalgesia. In the present study we examined whether pERK 1/2 is involved in the response of sensory neurons to a noxious visceral stimulation, in particular, of the gastric mucosa. After induction of gastric injury by oral administration of 0.5M HCl pERK 1/2 expression was determined by Western blotting of caudal thoracic dorsal root ganglia and by immunohistochemistry in stomach-innervating dorsal root ganglion neurons which were retrogradely labeled with True Blue. The content of pERK 1/2 remained unchanged in dorsal root ganglia until 2 h post-HCl, however, was found elevated 4 (approximately 80%) and 6 h (approximately 100%) after HCl administration. True Blue-labeled pERK 1/2-immunoreactive neurons were likewise increased 6 h post-HCl (204%) and were mainly of small size (20-40 microm) and negative for neurofilament 200 (approximately 76%). The majority of these cells also expressed the nociceptive transient receptor potential vanilloid receptor 1 (approximately 70%). The gastric mucosa was simultaneously examined for lesion formation showing highest percentage of damage 6 h post-HCl. Application of a N-methyl-D-aspartate receptor antagonist (MK-801; 100 microg/kg s.c.) significantly reduced HCl-induced pERK 1/2 expression and mucosal lesions 6 h post-HCl. Activation of the extracellular signal-regulated kinase-1 and -2 signaling cascade indicates that visceral primary afferents may sensitize after gastric noxious stimulation involving N-methyl-D-aspartate receptors. The extracellular signal-regulated kinase-1 and -2 pathway therefore may not only be of importance for somatic but also for visceral nociception.

Vanilloid receptor 1 antagonists attenuate disease severity in dextran sulphate sodium-induced colitis in mice

Neurogenic mechanisms have been implicated in the induction of inflammatory bowel disease (IBD). Vanilloid receptor type 1 (TRPV1) has been visualized on nerve terminals of intrinsic and extrinsic afferent neurones innervating the gastrointestinal tract and local administration of a TRPV1 antagonist, capsazepine, reduces the severity of dextran sulphate sodium (DSS)-induced colitis in rats (Gut 2003; 52: 713-9(1)). Our aim was to test whether systemically or orally administered TRPV1 antagonists attenuate experimental colitis induced by 5% DSS in Balb/c mice. Intraperitoneal capsazepine (2.5 mg kg(-1), bid), significantly reduced the overall macroscopic damage severity compared with vehicle-treated animals (80% inhibition, P < 0.05); however, there was no effect on myeloperoxidase (MPO) levels. An experimental TRPV1 antagonist given orally was tested against DSS-induced colitis, and shown to reverse the macroscopic damage score at doses of 0.5 and 5.0 mg kg(-1). Epithelial damage assessed microscopically was significantly reduced. MPO levels were attenuated by approximately 50%, and diarrhoea scores were reduced by as much as 70%. These results suggest that pharmacological modulation of TRPV1 attenuates indices of experimental colitis in mice, and that development of orally active TRPV1 antagonists might have therapeutic potential for the treatment of IBD.

Gastric secretion

PURPOSE OF REVIEW

The purpose of this chapter is to summarize and place into perspective the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

To prevent acid and pepsin from overwhelming mucosal defense mechanisms and causing injury, the secretion of gastric acid is precisely regulated by a variety of central (eg, neuropeptide Y, corticotropin-releasing factor, and neuromedin U) and peripheral (eg, gastrin, histamine, acetylcholine, somatostatin, cholecystokinin, calcitonin gene-related peptide, leptin, and parietal cell) pathways. These pathways regulate the acid-producing parietal cell directly and/or indirectly by regulating the secretion of histamine from enterochromaffin-like cells, gastrin from G cells, and somatostatin from D cells. Recently, genetically engineered mouse models have been used to reevaluate the neural, hormonal, and paracrine pathways that physiologically regulate acid secretion.

SUMMARY

An improved understanding of the pathways and mechanisms regulating gastric acid secretion should lead to the development of novel therapies to prevent and treat acid-peptic disorders as well as circumvent the adverse effects of currently used antisecretory medications such as the acid rebound observed after discontinuation of proton pump inhibitors.

Coexpression of Y1, Y2, and Y4 receptors in smooth muscle coupled to distinct signaling pathways

Coexpression of Y1, Y2, and Y4 receptors on smooth muscle cells was determined by reverse transcription-polymerase chain reaction, and the receptors were characterized by radioligand binding, selective receptor protection, and functional analysis of signaling pathways. 125I-peptide YY (PYY) binding was completely inhibited by neuropeptide Y (NPY) and PYY, and partially inhibited by the Y1 agonist [Leu31, Pro34]NPY or the Y2 agonist NPY13-36. In cells where Y1 receptors were preserved by selective receptor protection, 125I-PYY binding was selectively inhibited by the Y1 agonist or antagonist BIBP 3226 [(R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-D-arginine-amide]. Conversely, in cells where Y2 receptors were preserved, 125I-PYY binding was selectively inhibited by the Y2 agonist or antagonist BIIE 0246 [(S)N2-[1-[2-[4-[(R,S)-5,11-dihydro-6(66H)-oxodibenz[b,e]azepin-11-y]-1piperazinyl]-2-oxoethyl]cyclopentyl]acetyl]-N-[2-[1,2-dihydro-35(4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide]. All Y receptors activated preferentially Gi2, but only Y2 and Y4 receptors activated Gq. Consequently, Y2 agonists (NPY, PYY, and NPY13-36) and the Y4 agonist (pancreatic polypeptide) induced concentration-dependent contraction, inositol 1,4,5-trisphosphate (IP3) formation, and increase in cytosolic free Ca2+. Contraction induced by Y2 and Y4 agonists was not affected by 0 Ca2+, Ca2+ channel blockers, or pertussis toxin (PTx), but it was abolished by thapsigargin, U73122 [1-(6-(17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-25-dione], or the myosin light chain kinase inhibitor ML-9 [1-(5-chloronaphthalene-1-sulfonyl)homopiperazine, HCl]. Y2-mediated contraction was inhibited by the selective Y2 antagonist BIIE 0246. Insensitivity to PTx implied that the coupling to Gi did not initiate (Y1) or contribute (Y2 and Y4) to contraction. All Y receptor agonists inhibited cAMP formation in a PTx-sensitive manner. The patterns of contraction and inhibition of cAMP by various Y receptors were corroborated by selective receptor protection. The study demonstrates coexpression of Y1, Y2, and Y4 receptors on smooth muscle negatively coupled to adenylyl cyclase via Gi2. Coupling of Y2 and Y4 receptors to Gq determines their ability to induce IP3-dependent Ca2+ release and initiate contraction.

Expression and localization of c-Fos and NOS in the central nerve system following esophageal acid stimulation in rats

AIM

To determine the distribution of neurons expressing c-Fos and nitric oxide synthase (NOS) in the central nerve system (CNS) following esophageal acid exposure, and to investigate the relationship between c-Fos and NOS.

METHODS

Twelve Wistar rats were randomly divided into two equal groups. Hydrochloric acid with pepsin was perfused in the lower part of the esophagus for 60 min. As a control, normal saline was used. Thirty minutes after the perfusion, the rats were killed and brains were removed and processed for c-Fos immunohistochemistry and NADPH-d histochemistry. Blood pressure (BP), heart rate (HR), and respiratory rate (RR) during the experimental procedures were recorded every 10 min.

RESULTS

There were no significant differences in BP, HR and RR between the two groups. c-Fos immunoreactivity was significantly increased in rats receiving acid plus pepsin perfusion in amygdala (AM), paraventricular nucleus (PVN), parabrachial nucleus (PBN), nucleus tractus solitarius and dorsal motor nucleus of vagus (NTS/DMV), nucleus ambiguous (NA), reticular nucleus of medulla (RNM) and area postrema (AP). NOS reactivity in this group was significantly increased in PVN, PBN, NTS/DMV, RNM and AP. c-Fos and NOS had significant correlation between PVN, PBN, NTS/DMV, RNM and AP.

CONCLUSION

Acid plus pepsin perfusion of the esophagus results in neural activation in areas of CNS, and NO is likely one of the neurotransmitters in some of these areas.

Increased expression of TRPV1 receptor in dorsal root ganglia by acid insult of the rat gastric mucosa

It is still unknown which receptors of peripheral sensory pathways encode and integrate an acid-induced nociceptive event in the gastric mucosa. The transient receptor potential vanilloid receptor 1 (TRPV1) and the acid-sensing ion channel 3 (ASIC3) are two nociception-related receptors. Here we investigated (i) to what extent these receptors are distributed in stomach-innervating neurons of dorsal root and nodose ganglia, using immunohistochemistry and retrograde tracing, and (ii) whether their expression is altered in response to a noxious acid challenge of the stomach. We also explored the presence of TRPV1 in the gastric enteric nervous system because of its possible expression by intrinsic sensory neurons. Most stomach-innervating neurons in nodose ganglia were immunoreactive for TRPV1 (80%) and ASIC3 (75%), these results being similar in the dorsal root ganglia (71 and 82%). RT-PCR and Western blotting were performed up to 6 h after oral application of 0.5 m HCl to conscious rats. TRPV1 protein was increased in dorsal root but not in nodose ganglia whereas TRPV1 and ASIC3 mRNAs remained unchanged. TRPV1 mRNA was detected in longitudinal muscle-myenteric plexus preparations of control stomachs and was not altered by the acid challenge. Combined vagotomy and ganglionectomy abolished expression of TRPV1, indicating that it may derive from an extrinsic source. In summary, noxious acid challenge of the stomach increased TRPV1 protein in spinal but not vagal or intrinsic sensory afferents. The TRPV1 receptor may be a key molecule in the transduction of acid-induced nociception of the gastric mucosa and a mediator of visceral hypersensitivity.

Gastric enteric neurones that respond to luminal injury

Challenge of the rat gastric mucosa with HCl stimulates intrinsic neurones in the myenteric plexus of the stomach as demonstrated by immunohistochemical detection of c-Fos. In multiple labelling experiments of whole-mounts and sections of the gastric corpus we determined the chemical code of the stimulated neurones and investigated further whether neural pathways involving capsaicin-sensitive afferents, cholinergic neurones or the vagal system contribute to the stimulation of these neurones. Intragastric (IG) administration of 0.5 m HCl caused c-Fos expression in 12% of myenteric neurones, whereas IG saline failed to induce c-Fos. All stimulated neurones stained for nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY), but not for choline acetyltransferase (ChAT). Fibres coexpressing NOS/VIP/NPY were found predominantly in the external muscle layer and the muscularis mucosae of the stomach wall. Pretreatment with capsaicin or hexamethonium, combination of both pretreatments or vagotomy reduced HCl-induced c-Fos expression by 54%, 66%, 63% and 68%, respectively. The data indicate that mucosal acid challenge of the stomach stimulates inhibitory motor neurones in the myenteric plexus and that capsaicin-sensitive afferents as well as cholinergic neurones participate in the neuronal stimulation probably via a vago-vagal reflex.

Gastric inflammation triggers hypersensitivity to acid in awake rats

BACKGROUND & AIMS

Changes in visceral sensation contribute to the development of dyspepsia. Nonhuman models have previously focused on responses to mechanical stimulation. We studied the response to acid stimulation in the normal and inflamed stomach in rats.

METHODS

A balloon and gastrostomy catheter were implanted into the stomach. Electromyographic responses to gastric balloon distention or acid administration through the gastrostomy were recorded from the acromiotrapezius muscle. To characterize chemonociceptive pathways, 0.75 mL HCl (0.05-0.3 N) or saline were given intragastrically in controls and animals after vagotomy, splanchnic nerve resection, or chemical denervation with capsaicin. The effect of inflammation was examined after induction of mild diffuse gastritis using iodoacetamide or creating gastric ulcers by injecting 60% acetic acid for 45 seconds into a clamped area of the stomach.

RESULTS

Visceromotor electromyographic responses increased within 2 minutes after HCl administration (0.15 and 0.3 mol/L) but not saline or lower acid concentrations. Vagotomy and pretreatment with capsaicin but not splanchnic nerve resection abolished this response. Prior acid administration did not acutely sensitize animals to subsequent gastric distention. Gastritis and gastric ulcers enhanced the visceromotor responses to intragastric acid.

CONCLUSIONS

In awake rats, visceromotor responses to intragastric acid are quantifiable, reliable, and reproducible. Aversive responses to acute noxious chemical stimuli primarily require vagal but not spinal sensory pathways. Injury-induced sensitization to intragastric acid administration is consistent with a potential role of chemical stimulation in triggering dyspeptic symptoms.

The future of GI and liver research: editorial perspectives. IV. Visceral afferents: an update

The number of articles published in American Journal of Physiology Gastrointestinal and Liver Physiology over the last 15 years on visceral afferents has increased dramatically. This reflects our growing ability to study the characteristics and function of visceral afferents and also the recognition of their importance in the maintenance of homeostasis and also in a number of pathophysiological conditions. However, there are several key unanswered questions concerning the function of visceral afferents that await further investigation.