Neuronal control of the pyloric sphincter of the guinea-pig

Vagus nerve stimulation in bursts can efficiently modulate gastric contractions and contraction frequency at varying gastric pressures

OBJECTIVE

There has been recent clinical interest in the use of vagus nerve stimulation (VNS) for treating gastrointestinal disorders as an alternative to drugs or gastric electrical stimulation. However, effectiveness of burst stimulation has not been demonstrated. We investigated the ability of bursting and continuous VNS to influence gastric and pyloric activity under a range of stimulation parameters and gastric pressures. The goals of this study were to determine which parameters could optimally excite or inhibit gastric activity.

MATERIALS AND METHODS

Data were collected from 21 Sprague-Dawley rats. Under urethane anesthesia, a rubber balloon was implanted into the stomach, connected to a pressure transducer and a saline infusion pump. A pressure catheter was inserted at the pyloric sphincter and a bipolar nerve cuff was implanted onto the left cervical vagus nerve. The balloon was filled to 15 cmH2O. Stimulation trials were conducted in a consistent order; the protocol was then repeated at 25 and 35 cmH2O. The nerve was then transected and stimulation repeated to investigate directionality of effects.

RESULTS

Bursting stimulation at the bradycardia threshold caused significant increases in gastric contraction amplitude with entrainment to the bursting frequency. Some continuous stimulation trials could also cause increased contractions but without frequency changes. Few significant changes were observed at the pylorus, except for frequency entrainment. These effects could not be uniquely attributed to afferent or efferent activity.

SIGNIFICANCE

Our findings further elucidate the effects of different VNS parameters on the stomach and pylorus and provide a basis for future studies of bursting stimulation for gastric neuromodulation.

Mfge8 is expressed by pericytes in gastric antrum submucosa from patients with obesity

The main function of the stomach is to digest ingested food. Gastric antrum muscular contractions mix ingested food with digestive enzymes and stomach acid and propel the chyme through the pyloric sphincter at a rate in which the small intestine can process the chyme for optimal nutrient absorption. Mfge8 binding to α8β1 integrins helps regulate gastric emptying by reducing the force of antral smooth muscle contractions. The source of Mfge8 within gastric muscles is unclear. Since Mfge8 is a secreted protein, Mfge8 could be delivered via the circulation, or be locally secreted by cells within the muscle layers. In this study, we identify a source of Mfge8 within human gastric antrum muscles using spatial transcriptomic analysis. We show that Mfge8 is expressed in subpopulations of Mef2c+ perivascular cells within the submucosa layer of the gastric antrum. Mef2c is expressed in subpopulations of NG2+ and PDGFRB+ pericytes. Mfge8 is expressed in NG2+/Mef2c+ pericytes, but not in NG2+/Mef2c-, PDGFRB+/Mef2c-, or PDGFRB+/Mef2c+ pericytes. Mfge8 is absent from CD34+ endothelial cells but is expressed in a small population of perivascular ACTA2+ cells. We also show that α8 integrin is not expressed by interstitial cells of Cajal (ICC), supporting the findings that Mfge8 attenuates gastric antrum smooth muscle contractions by binding to α8β1 integrins on enteric smooth muscle cells. These findings suggest a novel, supplementary mechanism of regulation of gastric antrum motility by cellular regulators of capillary blood flow, in addition to the regulation of gastric antrum motility by the enteric nervous system and the SMC, ICC, and PDGFRα+ cell (SIP) syncytium.

Generation of Spontaneous Tone by Gastrointestinal Sphincters

An important feature of the gastrointestinal (GI) muscularis externa is its ability to generate phasic contractile activity. However, in some GI regions, a more sustained contraction, referred to as "tone," also occurs. Sphincters are muscles oriented in an annular manner that raise intraluminal pressure, thereby reducing or blocking the movement of luminal contents from one compartment to another. Spontaneous tone generation is often a feature of these muscles. Four distinct smooth muscle sphincters are present in the GI tract: the lower esophageal sphincter (LES), the pyloric sphincter (PS), the ileocecal sphincter (ICS), and the internal anal sphincter (IAS). This chapter examines how tone generation contributes to the functional behavior of these sphincters. Historically, tone was attributed to contractile activity arising directly from the properties of the smooth muscle cells. However, there is increasing evidence that interstitial cells of Cajal (ICC) play a significant role in tone generation in GI muscles. Indeed, ICC are present in each of the sphincters listed above. In this chapter, we explore various mechanisms that may contribute to tone generation in sphincters including: (1) summation of asynchronous phasic activity, (2) partial tetanus, (3) window current, and (4) myofilament sensitization. Importantly, the first two mechanisms involve tone generation through summation of phasic events. Thus, the historical distinction between "phasic" versus "tonic" smooth muscles in the GI tract requires revision. As described in this chapter, it is clear that the unique functional role of each sphincter in the GI tract is accompanied by a unique combination of contractile mechanisms.

Advances in the physiology of gastric emptying

There have been many recent advances in the understanding of various aspects of the physiology of gastric motility and gastric emptying. Earlier studies had discovered the remarkable ability of the stomach to regulate the timing and rate of emptying of ingested food constituents and the underlying motor activity. Recent studies have shown that two parallel neural circuits, the gastric inhibitory vagal motor circuit (GIVMC) and the gastric excitatory vagal motor circuit (GEVMC), mediate gastric inhibition and excitation and therefore the rate of gastric emptying. The GIVMC includes preganglionic cholinergic neurons in the DMV and the postganglionic inhibitory neurons in the myenteric plexus that act by releasing nitric oxide, ATP, and peptide VIP. The GEVMC includes distinct gastric excitatory preganglionic cholinergic neurons in the DMV and postganglionic excitatory cholinergic neurons in the myenteric plexus. Smooth muscle is the final target of these circuits. The role of the intramuscular interstitial cells of Cajal in neuromuscular transmission remains debatable. The two motor circuits are differentially regulated by different sets of neurons in the NTS and vagal afferents. In the digestive period, many hormones including cholecystokinin and GLP-1 inhibit gastric emptying via the GIVMC, and in the inter-digestive period, hormones ghrelin and motilin hasten gastric emptying by stimulating the GEVMC. The GIVMC and GEVMC are also connected to anorexigenic and orexigenic neural pathways, respectively. Identification of the control circuits of gastric emptying may provide better delineation of the pathophysiology of abnormal gastric emptying and its relationship to satiety signals and food intake.

The Influence of Gastric Antral Ulcerations on the Expression of Galanin and GalR1, GalR2, GalR3 Receptors in the Pylorus with Regard to Gastric Intrinsic Innervation of the Pyloric Sphincter

Gastric antrum ulcerations are common disorders occurring in humans and animals. Such localization of ulcers disturbs the gastric emptying process, which is precisely controlled by the pylorus. Galanin (Gal) and its receptors are commonly accepted to participate in the regulation of inflammatory processes and neuronal plasticity. Their role in the regulation of gastrointestinal motility is also widely described. However, there is lack of data considering antral ulcerations in relation to changes in the expression of Gal and GalR1, GalR2, GalR3 receptors in the pyloric wall tissue and galaninergic intramural innervation of the pylorus. Two groups of pigs were used in the study: healthy gilts and gilts with experimentally induced antral ulcers. By double immunocytochemistry percentages of myenteric and submucosal neurons expressing Gal-immunoreactivity were determined in the pyloric wall tissue and in the population of gastric descending neurons supplying the pyloric sphincter (labelled by retrograde Fast Blue neuronal tracer). The percentage of Gal-immunoreactive neurons increased only in the myenteric plexus of the pyloric wall (from 16.14±2.06% in control to 25.5±2.07% in experimental animals), while no significant differences in other neuronal populations were observed between animals of both groups. Real-Time PCR revealed the increased expression of mRNA encoding Gal and GalR1 receptor in the pyloric wall tissue of the experimental animals, while the expression(s) of GalR2 and GalR3 were not significantly changed. The results obtained suggest the involvement of Gal, GalR1 and galaninergic pyloric myenteric neurons in the response of pyloric wall structures to antral ulcerations.

Modulation of individual components of gastric motor response to duodenal glucose

AIM

To evaluate individual components of the antro-pyloro-duodenal (APD) motor response to graded small intestinal glucose infusions in healthy humans.

METHODS

APD manometry was performed in 15 healthy subjects (12 male; 40 ± 5 years, body mass index 26.5 ± 1.6 kg/m(2)) during four 20-min intraduodenal infusions of glucose at 0, 0.5, 1.0 and 1.5 kcal/min, in a randomised double-blinded fashion. Glucose solutions were infused at a rate of 1 mL/min and separated by 40-min "wash-out" period. Data are mean ± SE. Inferential analyses are repeated measure analysis of variance with Bonferroni post-hoc testing.

RESULTS

At 0 kcal/min frequency of pressure waves were: antrum (7.5 ± 1.8 waves/20 min) and isolated pyloric pressure waves (IPPWs) (8.0 ± 2.3 waves/20 min) with pyloric tone (0.0 ± 0.9 mmHg). Intraduodenal glucose infusion acutely increased IPPW frequency (P < 0.001) and pyloric tone (P = 0.015), and decreased antral wave frequency (P = 0.007) in a dose-dependent fashion. A threshold for stimulation was observed at 1.0 kcal/min for pyloric phasic pressure waves (P = 0.002) and 1.5 kcal/min for pyloric tone and antral contractility.

CONCLUSION

There is hierarchy for the activation of gastrointestinal motor responses to duodenal glucose infusion. An increase in IPPWs is the first response observed.

Vagal projections to the pylorus in the domestic pig (Sus scrofa domestica)

The goal of the present study was to examine the precise localization of the brainstem motor and primary sensory (nodose ganglion) vagal perikarya supplying the pylorus in the domestic pig. Using the Fast Blue retrograde tracing technique it has been established that all the vagal motor neurons projecting to the pylorus (about 337 ± 59 cells per animal) were localized bilaterally in the dorsal motor nucleus of the vagus nerve (DMX, 171 - left; 167 - right) and all other regions of the porcine brainstem were devoid of labeled neurons. The vagal perikarya supplying the porcine pylorus were dispersed throughout the whole rostro-caudal extent of the DMX and no somatotopic organization of these neurons was observed. The labeled neurons occurred individually or in groups up to five cell bodies per nuclear transverse cross section area (in the middle part of the nucleus). An immunocytochemical staining procedure disclosed that all Fast Blue labeled motor neurons were choline acetyltransferase (ChAT) immunoreactive, however some differences in immunofluorescence intensity occurred. The primary sensory vagal neurons were observed within the left (215±37 cells/animal) and right (148±21 cells/animal) nodose ganglion. The traced neurons were dispersed throughout the ganglia and no characteristic arrangement of these neurons was observed. The present experiment precisely indicates the sources of origin of the vagal motor and primary sensory neurons supplying the pyloric region in the pig, the animal of an increasing significance in biomedical research.

Quantification of the effects of the volume and viscosity of gastric contents on antral and fundic activity in the rat stomach maintained ex vivo

AIMS

The aim of this study was to examine the effect of varying the rheological properties of perfusate on the volume and muscular activity of the various compartments of the rat stomach.

METHODS

Image analysis was used to quantify the activity of the ex vivo stomach preparations when perfused according to a ramp profile.

RESULTS

The area of the fundus increased to a greater extent than that of the body when watery or viscous material was perfused. However, initial distension of the corpus was greater and occurred more rapidly when viscous material was perfused. Only the fundus expanded when perfusion followed the administration of verapamil. The frequency of antrocorporal contractions decreased significantly and the amplitude of antrocorporal contractions increased significantly with increase in gastric volume. The velocity of antrocorporal contractions did not vary with gastric volume but varied regionally in some preparations being faster distally than proximally. Neither the frequency, amplitude or velocity of antrocorporal contractions differed when pseudoplastic rather than watery fluid was perfused. However, the characteristics of antrocorporal contractions changed significantly when the stomach was perfused with material with rheological characteristics that induce different patterns of wall tension to those normally encountered. Hence, the mean frequency and speed of propagation of antrocorporal contractions increased and their direction of propagation became inconstant.

Intrinsic neuronal control of the pyloric sphincter of the lamb

To better understand the local neuronal network of the gastro-duodenal junction in ruminants, we identified the components of the enteric nervous system (ENS) innervating the pyloric sphincter (PS) of the lamb abomasum. The neurons were labelled after injecting the tracer Fast Blue (FB) into the wall of the PS, and the phenotype of the FB-labelled neurons was immunohistochemically investigated using antibodies against nitric oxide synthase (NOS), choline acetyltransferase (ChAT), tachykinin (substance P) and tyrosine hydroxylase (TH). The FB-labelled abomasal myenteric plexus (MP) neurons, observed up to 14cm from the PS, were NOS-immunoreactive (IR) (82+/-12%), ChAT-IR (51+/-29%), SP-IR (61+/-33%), and also TH-IR (2%). The descending nitrergic neurons were also SP-IR (64%) and ChAT-IR (21%); the cholinergic descending neurons were SP-IR (3%). The FB-labelled duodenal neurons were located only in the MP, up to 8cm from the sphincter and were ChAT-IR (79+/-16%), SP-IR (32+/-18%), NOS-IR (from 0 to 2%), and also TH-IR (4+/-3%). The cholinergic ascending neurons were also SP-IR (60%) whereas no ChAT-IR cells were NOS-IR. The findings of this research indicate that the sheep PS is innervated by long-projecting neurons of the abomasal and duodenal ENS.

Imaging and modelling of digestion in the stomach and the duodenum

Gastroduodenal physiology is traditionally understood in terms of motor-secretory functions and their electrical, neural and hormonal controls. In contrast, the fluid-mechanical functions that retain and disperse particles, expose substrate to enzymes, or replenish the epithelial boundary with nutrients are little studied. Current ultrasound and magnetic resonance imaging allows to visualize processes critical to digestion like mixing, dilution, swelling, dispersion and elution. Methodological advances in fluid mechanics allow to numerically analyse the forces promoting digestion. Pressure and flow fields, the shear stresses dispersing particles or the effectiveness of bolus mixing can be computed using information on boundary movements and on the luminal contents. These technological advances promise many additional insights into the mechanical processes that promote digestion and absorption.

Lack of pyloric interstitial cells of Cajal explains distinct peristaltic motor patterns in stomach and small intestine

The frequency and propagation velocity of distension-induced peristaltic contractions in the antrum and duodenum are distinctly different and depend on activation of intrinsic excitatory motoneurons as well as pacemaker cells, the interstitial cells of Cajal associated with Auerbach's plexus (ICC-AP). Because ICC are critical for coordination of motor activities along the long axis of many regions in the gut, the role of ICC in antroduodenal coordination was investigated. We used immunohistochemistry, electron microscopy, simultaneous multiple electrical recordings in vitro, and videofluoroscopy in vivo in mice and rats. A strongly reduced number of ICC-AP with loss of network characteristics was observed in a 4-mm area in the rat and a 1-mm area in the mouse pyloric region. The pyloric region showed a slow wave-free gap of 4.1 mm in rats and 1.3 mm in mice. Between antrum and duodenum, there was no interaction of electrical activities and in the absence of gastric emptying, there was no coordination of motor activities. When the pyloric sphincter opened, 2.4 s before the front of the antral wave reached the pylorus, the duodenum distended after receiving gastric content and aboral duodenal peristalsis was initiated, often disrupting other motor patterns. The absence of ICC-AP and slow wave activity in the pyloric region allows the antrum and duodenum to have distinct uncoordinated motor activities. Coordination of aborally propagating peristaltic antral and duodenal activity is initiated by opening of the pylorus, which is followed by distention-induced duodenal peristalsis. Throughout this coordinated motor activity, the pacemaker systems in antrum and duodenum remain independent.

Mediators of non-adrenergic non-cholinergic inhibitory neurotransmission in porcine jejunum

The purpose of this study was to determine the non-adrenergic non-cholinergic inhibitory neurotransmitter in pig jejunum. Intracellular electrical activity was recorded from circular smooth muscle cells. Inhibitory junction potentials (IJPs) evoked by electrical field stimulation were inhibited by tetrodotoxin (1 micromol L(-1)), omega-conotoxin GVIA (0.1 micromol L(-1)) tetrodotoxin, apamin (1 micromol L(-1)), 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U-73122; 10 micromol L(-1)) but not by N omega-nitro-l-arginine (l-NNA; 100 micromol L(-1)), haemoglobin (10 micromol L(-1)), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 micromol L(-1)) or 9-(tetrahydro-2-furyl)adenine (SQ-22536; 10 micromol L(-1)). S-nitroso-N-acetylpenicillamine (SNAP) hyperpolarized the membrane potential. This was inhibited by ODQ (3 micromol L(-1)) and charybdotoxin (0.1 micromol L(-1)). Adenosine-5-triphosphate (ATP; 100 micromol L(-1)) and 2-methylthio ATP (2-MeS-ATP; 100 micromol L(-1)) did not hyperpolarize the membrane potential and 6-N-N-diethyl-beta- gamma -dibromomethylene-d-adenosine-5'-triphosphate (ARL67156; 100 micromol L(-1)) did not modify IJPs. Carbon monoxide (CO; 10%) and tricarbonyl dichlororuthenium dimer ([Ru(CO3Cl2)]2; 100 micromol L(-1)) hyperpolarized the membrane potential however zinc, copper and tin protoporphyrin IX (100 micromol L(-1)) did not alter IJPs. Vasoactive intestinal peptide (VIP) hyperpolarized the membrane potential but 4-Cl-d-Phe6-Leu17-VIP (1 micromol L(-1)) did not modify IJPs. Pituitary adenylate cyclase activating peptide (PACAP)38 (0.5 micromol L(-1)) hyperpolarized the membrane potential. This was inhibited by apamin (1 micromol L(-1)) but not by tetrodotoxin (1 micromol L(-1)). Pituitary adenylate cyclase activating peptide6-38 (1 micromol L(-1)) inhibited IJPs. These data suggest that inhibitory neurotransmission in pig jejunum is mediated partly by PACAP.

Acid challenge delays gastric pressure adaptation, blocks gastric emptying and stimulates gastric fluid secretion in the rat

Functional dyspepsia can be associated with impaired gastric relaxation in response to food intake and delayed gastric emptying. In this study, we investigated whether luminal hydrochloric acid (HCl) may reproduce these motor alterations in phenobarbital-anaesthetized rats via activation of extrinsic neural pathways. Intragastric pressure (IGP) changes induced by a 2-mL fluid bolus were recorded with an oesophageal catheter, and gastric emptying was determined via the fluid volume recovered from the stomach 30-min post-bolus. Experiments involving acute nerve transections or pharmacological blockade of nitric oxide synthesis revealed that the initial increase of IGP after a 0.35 mol L(-1) HCl bolus is dampened by duodenogastric and gastrogastric relaxation reflexes depending on vagal and splanchnic pathways as well as nitric oxide. Compared with saline, HCl (0.15-0.5 mol L(-1)) delayed the subsequent decrease (adaptation) of IGP, inhibited gastric emptying and stimulated gastric fluid secretion as seen in stomachs with ligated pylorus. The acid-evoked delay in IGP adaptation and inhibition of gastric emptying involved duodenogastric and duodenopyloric extrinsic nerve reflexes, whereas the gastric fluid secretion was independent of the extrinsic innervation. It is proposed that the gastropyloric motor changes induced by luminal acid challenge have a bearing on the motor disturbances underlying functional dyspepsia.

Effects of leptin on cat intestinal motility

In a previous study, we established that leptin controls food intake and immune responses by acting on intestinal vagal chemosensitive mechanoreceptors via a functional link with interleukin-1 beta (Il-1 beta). Since the control of intestinal motility is one of the main roles of the vagal afferent fibres, we investigated the effects of leptin on intestinal electromyographic (EMG) activity which reflects intestinal motility. For this purpose, the effects of locally injected leptin on small intestine spontaneous EMG activity were studied in 23 anaesthetised cats. The EMG activity was recorded using bipolar electrodes implanted in the proximal small intestine. Leptin and Il-1 beta (0.1, 1 and 10 microg), administered through the artery irrigating the upper part of the intestine 20 min after cholecystokinin (CCK, 10 microg, I.A.), had significant (P < 0.001) excitatory effects on intestinal EMG activity. The effects of both substances were blocked by the endogenous interleukin-1 beta receptor antagonist (Il-1ra, 250 microg, I.A.), by atropine (250 microg, I.A.) and by vagotomy. In the absence of CCK, leptin and Il-1 beta had no effect on intestinal electrical activity. It can therefore be concluded that: (1) leptin is effective only after the previous intervention of CCK, (2) the enhancement of the electrical activity induced by leptin involves Il-1 beta receptors and the cholinergic excitatory pathway, (3) the modes whereby the leptin-induced enhancement of EMG activity occurs strongly suggest that these effects are due to a long-loop reflex involving intestinal vagal afferent fibres and the parasympathetic nervous system.

Origins and projections of nerve fibres in rat pyloric sphincter

Nerve fibres play an important role in the regulation of gastric emptying. The aims of this study were to clarify the distribution, projections and origin of neuronal type nitric oxide synthase (NOS)-, tyrosine hydroxylase (TH)-, vesicular acetylcholine transporter (VAchT)- and peptide-containing nerve fibres of the rat pyloric sphincter. Extrinsic and local denervations of the sphincter were performed in order to reveal the origin and projections of the various nerve fibre populations. Pylorus from control and denervated animals were processed for the immunocytochemical demonstration of cholecystokinin (CCK), enkephalin, gastrin-releasing peptide (GRP), somatostatin, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), pituitary adenylate cyclase-activating peptide (PACAP), substance P (SP), vasoactive intestinal peptide (VIP), galanin, NOS, VAchT and TH. VAchT, TH, nNOS, and all of the peptides investigated were found in nerve fibres innervating the pyloric sphincter, and coexistence of several putative neurotransmitters were revealed. Extrinsic denervation caused a total loss of NPY/TH-, SP/CGRP- and SP/CGRP/VIP/NOS/PACAP-containing nerve fibres. Local denervation immediately proximal to the sphincter markedly reduced the numbers of VIP/NOS/galanin- and VIP/NOS/galanin/PACAP +/- NPY-containing fibres within the sphincter suggesting an origin of these fibres in myenteric ganglia in the antral region; denervation at the level of the oxyntic-pyloric border had no effect. Local denervation immediately distal to the sphincter caused a marked decrease in VAchT-, SP/enkephalin-, enkephalin-, somatostatin-, CCK- and GRP-containing fibres within the sphincter suggesting that these emanate from the duodenum. The latter procedure also reduced the number of SP/CGRP-containing fibres of extrinsic origin within the pyloric sphincter.