Organization of vagal afferents in pylorus: mechanoreceptors arrayed for high sensitivity and fine spatial resolution?

Electromechanical coupling across the gastroduodenal junction

The gastroduodenal junction is uniquely capable of regulating digestive functions in the gastrointestinal system. The pyloric sphincter, which demarcates the stomach from the small intestine, acts as a mechanical and electrical barrier, isolating each organ, thus enabling independent behaviors that are critical for proper digestion. Unique electrical patterns in the stomach, pylorus, and duodenum underpin the distinct contractile patterns of these regions, and improper organization of these mechanical behaviors leads to clinical conditions such as gastroparesis and dumping syndrome. For this reason, the gastroduodenal junction should be a focal point in investigations of novel biomarkers of gastrointestinal dysfunction. This review summarizes the current knowledge of bioelectrical and mechanical characteristics of the gastroduodenal junction, as well as the relevant underlying anatomy. As there is limited documentation of physiological recordings from the gastroduodenal junction of humans, inferences are made from animal studies and from measurements taken from other regions of the gastrointestinal tract, where appropriate. We suggest hypotheses on gastroduodenal electromechanical coupling and propose further studies to support or reject these ideas. Improved physiological understanding of this region, and the advent of novel diagnostic and therapeutic tools are crucial aspects for the future of clinical gastrointestinal medicine.

The parasympathetic and sensory innervation of the proximal and distal colon in male mice

Introduction

The distributions of extrinsic neurons innervating the colon show differences in experimental animals from humans, including the vagal and spinal parasympathetic innervation to the distal colon. The neuroanatomical tracing to the mouse proximal colon has not been studied in details. This study aimed to trace the locations of extrinsic neurons projecting to the mouse proximal colon compared to the distal colon using dual retrograde tracing.

Methods

The parasympathetic and sensory neurons projecting to colon were assessed using Cholera Toxin subunit B conjugated to Alexa-Fluor 488 or 555 injected in the proximal and distal colon of the same mice.

Results

Retrograde tracing from the proximal and distal colon labeled neurons in the dorsal motor nucleus of the vagus (DMV) and the nodose ganglia, while the tracing from the distal colon did not label the parasympathetic neurons in the lumbosacral spinal cord at L6-S1. Neurons in the pelvic ganglia which were cholinergic projected to the distal colon. There were more neurons in the DMV and nodose ganglia projecting to the proximal than distal colon. The right nodose ganglion had a higher number of neurons than the left ganglion innervating the proximal colon. In the dorsal root ganglia (DRG), the highest number of neurons traced from the distal colon were at L6, and those from the proximal colon at T12. DRG neurons projected closely to the cholinergic neurons in the intermediolateral column of L6 spinal cord. Small percentages of neurons with dual projections to both the proximal and distal colon existed in the DMV, nodose ganglia and DRG. We also observed long projecting neurons traced from the caudal distal colon to the transverse and proximal colon, some of which were calbindin immunoreactive, while there were no retrogradely labeled neurons traced from the proximal to distal colon.

Discussion

These data demonstrated that the vagal motor and motor and sensory neurons innervate both the proximal and distal colon in mice, and the autonomic neurons in the intermediate zone of the lumbosacral spinal cord do not project directly to the mouse colon, which differs from that in humans.

Novel Insights into Postoperative Surveillance in Resected Pancreatic Cystic Neoplasms-A Review

Pancreatic cystic lesions (PCL) are frequently encountered in clinical practice and some are referred to surgery due to their neoplastic risk or malignant transformation. The management of PCL involves complex decision-making, with postoperative surveillance being a key component for long-term outcomes, due to the potential for recurrence and postoperative morbidity. Unfortunately, the follow-up of resected patients is far from being optimal and there is a lack of consensus on recommendations with regard to timing and methods of surveillance. Here, we summarize the current knowledge on the postoperative surveillance of neoplastic pancreatic cysts, focusing on the mechanisms and risk factors for recurrence, the recurrence rates according to the initial indication for surgery, the final result of the surgical specimen and neoplastic risk in the remaining pancreas, as well as the postsurgical morbidity comprising pancreatic exocrine insufficiency, metabolic dysfunction and diabetes after resection, according to the type of surgery performed. We analyze postsurgical recurrence rates and morbidity profiles, as influenced by different surgical techniques, to better delineate at-risk patients, and highlight the need for tailored surveillance strategies adapted to preoperative and operative factors with an impact on outcomes.

Spinal afferent innervation in flat-mounts of the rat stomach: anterograde tracing

The dorsal root ganglia (DRG) project spinal afferent axons to the stomach. However, the distribution and morphology of spinal afferent axons in the stomach have not been well characterized. In this study, we used a combination of state-of-the-art techniques, including anterograde tracer injection into the left DRG T7-T11, avidin-biotin and Cuprolinic Blue labeling, Zeiss M2 Imager, and Neurolucida to characterize spinal afferent axons in flat-mounts of the whole rat stomach muscular wall. We found that spinal afferent axons innervated all regions with a variety of distinct terminal structures innervating different gastric targets: (1) The ganglionic type: some axons formed varicose contacts with individual neurons within myenteric ganglia. (2) The muscle type: most axons ran in parallel with the longitudinal and circular muscles and expressed spherical varicosities. Complex terminal structures were observed within the circular muscle layer. (3) The ganglia-muscle mixed type: some individual varicose axons innervated both myenteric neurons and the circular muscle, exhibiting polymorphic terminal structures. (4) The vascular type: individual varicose axons ran along the blood vessels and occasionally traversed the vessel wall. This work provides a foundation for future topographical anatomical and functional mapping of spinal afferent axon innervation of the stomach under normal and pathophysiological conditions.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Vagal innervation of the stomach reassessed: brain-gut connectome uses smart terminals

Brain-gut neural communications have long been considered limited because of conspicuous numerical mismatches. The vagus, the parasympathetic nerve connecting brain and gut, contains thousands of axons, whereas the gastrointestinal (GI) tract contains millions of intrinsic neurons in local plexuses. The numerical paradox was initially recognized in terms of efferent projections, but the number of afferents, which comprise the majority (≈ 80%) of neurites in the vagus, is also relatively small. The present survey of recent morphological observations suggests that vagal terminals, and more generally autonomic and visceral afferent arbors in the stomach as well as throughout the gut, elaborate arbors that are extensive, regionally specialized, polymorphic, polytopic, and polymodal, commonly with multiplicities of receptors and binding sites-smart terminals. The morphological specializations and dynamic tuning of one-to-many efferent projections and many-to-one convergences of contacts onto afferents create a complex architecture capable of extensive peripheral integration in the brain-gut connectome and offset many of the disparities between axon and target numbers. Appreciating this complex architecture can help in the design of therapies for GI disorders.

Distributions and relationships of chemically defined enteroendocrine cells in the rat gastric mucosa

This paper provides quantitative data on the distributions of enteroendocrine cells (EEC), defined by the hormones they contain, patterns of colocalisation between hormones and EEC relations to nerve fibres in the rat gastric mucosa. The rat stomach has three mucosal types: non-glandular stratified squamous epithelium of the fundus and esophageal groove, a region of oxyntic glands in the corpus, and pyloric glands of the antrum and pylorus. Ghrelin and histamine were both contained in closed cells, not contacting the lumen, and were most numerous in the corpus. Gastrin cells were confined to the antrum, and 5-hydroxytryptamine (5-HT) and somatostatin cells were more frequent in the antrum than the corpus. Most somatostatin cells had basal processes that in the antrum commonly contacted gastrin cells. Peptide YY (PYY) cells were rare and mainly in the antrum. The only numerous colocalisations were 5-HT and histamine, PYY and gastrin and gastrin and histamine in the antrum, but each of these populations was small. Peptide-containing nerve fibres were found in the mucosa. One of the most common types was vasoactive intestinal peptide (VIP) fibres. High-resolution analysis showed that ghrelin cells were closely and selectively approached by VIP fibres. In contrast, gastrin cells were not selectively innervated by VIP or CGRP fibres. The study indicates that there are distinct populations of gastric EEC and selective innervation of ghrelin cells. It also shows that, in contrast to EEC of the small intestine, the majority of EEC within the stomach contained only a single hormone.

Gastric stimulation drives fast BOLD responses of neural origin

Functional magnetic resonance imaging (fMRI) is commonly thought to be too slow to capture any neural dynamics faster than 0.1 Hz. However, recent findings demonstrate the feasibility of detecting fMRI activity at higher frequencies beyond 0.2 Hz. The origin, reliability, and generalizability of fast fMRI responses are still under debate and await confirmation through animal experiments with fMRI and invasive electrophysiology. Here, we acquired single-echo and multi-echo fMRI, as well as local field potentials, from anesthetized rat brains given gastric electrical stimulation modulated at 0.2, 0.4 and 0.8 Hz. Such gastric stimuli could drive widespread fMRI responses at corresponding frequencies from the somatosensory and cingulate cortices. Such fast fMRI responses were linearly dependent on echo times and thus indicative of blood oxygenation level dependent nature (BOLD). Local field potentials recorded during the same gastric stimuli revealed transient and phase-locked broadband neural responses, preceding the fMRI responses by as short as 0.5 s. Taken together, these results suggest that gastric stimulation can drive widespread and rapid fMRI responses of BOLD and neural origin, lending support to the feasibility of using fMRI to detect rapid changes in neural activity up to 0.8 Hz under visceral stimulation.

Computational Models for the Mechanical Investigation of Stomach Tissues and Structure

Bariatric surgery is performed on obese people aiming at reducing the capacity of the stomach and/or the absorbing capability of the gastrointestinal tract. A more reliable and effective approach to bariatric surgery may integrate different expertise, in the areas of surgery, physiology and biomechanics, availing of a strong cooperation between clinicians and engineers. This work aimed at developing a computational model of the stomach, as a computational tool for the physio-mechanical investigation of stomach functionality and the planning of bariatric procedures. In this sense, coupled experimental and numerical activities were developed. Experimental investigations on pig and piglet stomachs aimed at providing information about stomach geometrical configuration and structural behavior. The computational model was defined starting from the analysis of data from histo-morphometric investigations and mechanical tests. A fiber-reinforced visco-hyperelastic constitutive model was developed to interpret the mechanical response of stomach tissues; constitutive parameters were identified considering mechanical tests at both tissue and structure levels. Computational analyses were performed to investigate the pressure-volume behavior of the stomach. The developed model satisfactorily interpreted results from experimental activities, suggesting its reliability. Furthermore, the model was exploited to investigate stress and strain fields within gastric tissues, as the stimuli for mechanoreceptors that interact with the central nervous system leading to the feeling of satiety. In this respect, the developed computational model may be employed to evaluate the influence of bariatric intervention on the stimulation of mechanoreceptors, and the following meal induced satiety.

The metabolic role of vagal afferent innervation

The regulation of energy and glucose balance contributes to whole-body metabolic homeostasis, and such metabolic regulation is disrupted in obesity and diabetes. Metabolic homeostasis is orchestrated partly in response to nutrient and vagal-dependent gut-initiated functions. Specifically, the sensory and motor fibres of the vagus nerve transmit intestinal signals to the central nervous system and exert biological and physiological responses. In the past decade, the understanding of the regulation of vagal afferent signals and of the associated metabolic effect on whole-body energy and glucose balance has progressed. This Review highlights the contributions made to the understanding of the vagal afferent system and examines the integrative role of the vagal afferent in gastrointestinal regulation of appetite and glucose homeostasis. Investigating the integrative and metabolic role of vagal afferent signalling represents a potential strategy to discover novel therapeutic targets to restore energy and glucose balance in diabetes and obesity.

Morphology of P2X3-immunoreactive nerve endings in the rat tracheal mucosa

Nerve endings with immunoreactivity for the P2X3 purinoreceptor (P2X3) in the rat tracheal mucosa were examined by immunohistochemistry of whole-mount preparations with confocal scanning laser microscopy. P2X3 immunoreactivity was observed in ramified endings distributed in the whole length of the trachea. The myelinated parent axons of P2X3-immunoreactive nerve endings ramified into several branches that extended two-dimensionally in every direction at the interface between the epithelial layer and lamina propria. The axonal branches of P2X3-immunoreactive endings branched off many twigs located just beneath the epithelium, and continued to intraepithelial axon terminals. The axon terminals of P2X3-immunoreactive endings were beaded, rounded, or club-like in shape and terminated between tracheal epithelial cells. Flat axon terminals sometimes partly ensheathed neuroendocrine cells with immunoreactivity for SNAP25 or CGRP. Some axons and axon terminals with P2X3 immunoreactivity were immunoreactive for P2X2, while some terminals were immunoreactive for vGLUT2. Furthermore, a retrograde tracing method using fast blue (FB) revealed that 88.4% of FB-labeled cells with P2X3 immunoreactivity originated from the nodose ganglion. In conclusion, P2X3-immunoreactive nerve endings in the rat tracheal mucosa have unique morphological characteristics, and these endings may be rapidly adapting receptors and/or irritant receptors that are activated by mucosal irritant stimuli.

Plasticity of gastrointestinal vagal afferent satiety signals

The vagal link between the gastrointestinal tract and the central nervous system (CNS) has numerous vital functions for maintaining homeostasis. The regulation of energy balance is one which is attracting more and more attention due to the potential for exploiting peripheral hormonal targets as treatments for conditions such as obesity. While physiologically, this system is well tuned and demonstrated to be effective in the regulation of both local function and promoting/terminating food intake the neural connection represents a susceptible pathway for disruption in various disease states. Numerous studies have revealed that obesity in particularly is associated with an array of modifications in vagal afferent function from changes in expression of signaling molecules to altered activation mechanics. In general, these changes in vagal afferent function in obesity further promote food intake instead of the more desirable reduction in food intake. It is essential to gain a comprehensive understanding of the mechanisms responsible for these detrimental effects before we can establish more effective pharmacotherapies or lifestyle strategies for the treatment of obesity and the maintenance of weight loss.

Individual sympathetic postganglionic neurons coinnervate myenteric ganglia and smooth muscle layers in the gastrointestinal tract of the rat

A full description of the terminal architecture of sympathetic axons innervating the gastrointestinal (GI) tract has not been available. To label sympathetic fibers projecting to the gut muscle wall, dextran biotin was injected into the celiac and superior mesenteric ganglia (CSMG) of rats. Nine days postinjection, animals were euthanized and stomachs and small intestines were processed as whole mounts (submucosa and mucosa removed) to examine CSMG efferent terminals. Myenteric neurons were counterstained with Cuprolinic Blue; catecholaminergic axons were stained immunohistochemically for tyrosine hydroxylase. Essentially all dextran-labeled axons (135 of 136 sampled) were tyrosine hydroxylase-positive. Complete postganglionic arbors (n = 154) in the muscle wall were digitized and analyzed morphometrically. Individual sympathetic axons formed complex arbors of varicose neurites within myenteric ganglia/primary plexus and, concomitantly, long rectilinear arrays of neurites within circular muscle/secondary plexus or longitudinal muscle/tertiary plexus. Very few CSMG neurons projected exclusively (i.e., ∼100% of an arbor's varicose branches) to myenteric plexus (∼2%) or smooth muscle (∼14%). With less stringent inclusion criteria (i.e., ≥85% of an axon's varicose branches), larger minorities of neurons projected predominantly to either myenteric plexus (∼13%) or smooth muscle (∼27%). The majority (i.e., ∼60%) of all individual CSMG postganglionics formed mixed, heterotypic arbors that coinnervated extensively (>15% of their varicose branches per target) both myenteric ganglia and smooth muscle. The fact that ∼87% of all sympathetics projected either extensively or even predominantly to smooth muscle, while simultaneously contacting myenteric plexus, is consistent with the view that these neurons control GI muscle directly, if not exclusively. J. Comp. Neurol. 524:2577-2603, 2016. © 2016 Wiley Periodicals, Inc.

Vagal Intramuscular Arrays: The Specialized Mechanoreceptor Arbors That Innervate the Smooth Muscle Layers of the Stomach Examined in the Rat

The fundamental roles that the stomach plays in ingestion and digestion notwithstanding, little morphological information is available on vagal intramuscular arrays (IMAs), the afferents that innervate gastric smooth muscle. To characterize IMAs better, rats were given injections of dextran biotin in the nodose ganglia, and, after tracer transport, stomach whole mounts were collected. Specimens were processed for avidin-biotin permanent labeling, and subsets of the whole mounts were immunohistochemically processed for c-Kit or stained with cuprolinic blue. IMAs (n = 184) were digitized for morphometry and mapping. Throughout the gastric muscle wall, IMAs possessed common phenotypic features. Each IMA was generated by a parent neurite arborizing extensively, forming an array of multiple (mean = 212) branches averaging 193 µm in length. These branches paralleled, and coursed in apposition with, bundles of muscle fibers and interstitial cells of Cajal. Individual arrays averaged 4.3 mm in length and innervated volumes of muscle sheet, presumptive receptive fields, averaging 0.1 mm(3) . Evaluated by region and by muscle sheet, IMAs displayed architectural adaptations to the different loci. A subset (32%) of circular muscle IMAs issued specialized polymorphic collaterals to myenteric ganglia, and a subset (41%) of antral longitudinal muscle IMAs formed specialized net endings associated with the serosal boundary. IMAs were concentrated in regional patterns that correlated with the unique biomechanical adaptations of the stomach, specifically proximal stomach reservoir functions and antral emptying operations. Overall, the structural adaptations and distributions of the IMAs were consonant with the hypothesized stretch receptor roles of the afferents.