Organization and neurochemistry of vagal preganglionic neurons innervating the lower esophageal sphincter in ferrets

Pathophysiology and Therapy of Associated Features of Migraine

Migraine is a complex and debilitating disorder that is broadly recognised by its characteristic headache. However, given the wide array of clinical presentations in migraineurs, the headache might not represent the main troublesome symptom and it can even go unnoticed. Understanding migraines exclusively as a pain process is simplistic and certainly hinders management. We describe the mechanisms behind some of the most disabling associated symptoms of migraine, including the relationship between the central and peripheral processes that take part in nausea, osmophobia, phonophobia, vertigo and allodynia. The rationale for the efficacy of the current therapeutic arsenal is also depicted in this article. The associated symptoms to migraine, apart from the painful component, are frequent, under-recognised and can be more deleterious than the headache itself. The clinical anamnesis of a headache patient should enquire about the associated symptoms, and treatment should be considered and individualised. Acknowledging the associated symptoms as a fundamental part of migraine has permitted a deeper and more coherent comprehension of the pathophysiology of migraine.

Alpha-synuclein alters the faecal viromes of rats in a gut-initiated model of Parkinson's disease

Parkinson's disease (PD) is a chronic neurological disorder associated with the misfolding of alpha-synuclein (α-syn) into aggregates within nerve cells that contribute to their neurodegeneration. Recent evidence suggests α-syn aggregation may begin in the gut and travel to the brain along the vagus nerve, with microbes potentially a trigger initiating α-syn misfolding. However, the effects α-syn alterations on the gut virome have not been investigated. In this study, we show longitudinal faecal virome changes in rats administered either monomeric or preformed fibrils (PFF) of α-syn directly into their enteric nervous system. Differential changes in rat viromes were observed when comparing monomeric and PFF α-syn, with alterations compounded by the addition of LPS. Changes in rat faecal viromes were observed after one month and did not resolve within the study's five-month observational period. These results suggest that virome alterations may be reactive to host α-syn changes that are associated with PD development.

Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

At the most fundamental level, the bowel facilitates absorption of small molecules, regulates fluid and electrolyte flux, and eliminates waste. To successfully coordinate this complex array of functions, the bowel relies on the enteric nervous system (ENS), an intricate network of more than 500 million neurons and supporting glia that are organized into distinct layers or plexi within the bowel wall. Neuron and glial diversity, as well as neurotransmitter and receptor expression in the ENS, resembles that of the central nervous system. The most carefully studied ENS functions include control of bowel motility, epithelial secretion, and blood flow, but the ENS also interacts with enteroendocrine cells, influences epithelial proliferation and repair, modulates the intestinal immune system, and mediates extrinsic nerve input. Here, we review the many different cell types that communicate with the ENS, integrating data about ENS function into a broader view of human health and disease. In particular, we focus on exciting new literature highlighting relationships between the ENS and its lesser-known interacting partners.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

The origin of nausea in migraine-a PET study

Background

Nausea is a common and disabling symptom of migraine. The origin of nausea is not well understood although functional connections between trigeminal neurons and the nucleus tractus solitarius may explain occurrence of nausea with pain. However, nausea occurs as a premonitory symptom in about a quarter of patients, suggesting that a primary brain alteration unrelated to the experience of pain may be the reason for nausea.

Methods

We performed positron emission tomography scans with H₂¹⁵O PET in premonitory phase of nitroglycerin-induced migraine and compared patients with and without nausea.

Results

The results showed activation in rostral dorsal medulla and periaqueductal grey (PAG) in the nausea group, which was absent in the no nausea group. The rostral dorsal medullary area included the nucleus tractus solitarius, dorsal motor nucleus of the vagus nerve and the nucleus ambiguus, all of which are thought to be involved in brain circuits mediating nausea.

Conclusions

The results demonstrate that nausea can occur as a premonitory symptom in migraine, independent of pain and trigeminal activation. This is associated with activation of brain structures known to be involved in nausea. We conclude that nausea is a centrally driven symptom in migraine.

Gastric and lower esophageal sphincter pressures during nausea: a study using visual motion-induced nausea and high-resolution manometry

Nausea is the subjective unpleasant sensation that immediately precedes vomiting. Studies using barostats suggest that gastric fundus and lower esophageal sphincter (LES) relaxation precede vomiting. Unlike barostat, high-resolution manometry allows less invasive, detailed measurements of fundus pressure (FP) and axial movement of the gastroesophageal junction (GEJ). Nausea was induced in 12 healthy volunteers by a motion video and rated on a visual analog scale. FP was measured as the mean value of the five pressure channels that were clearly positioned below the LES. After intubation, a baseline (BL) recording of 15 min was obtained. This was followed by presentation of the motion video (at least 10 min, maximum 20 min) followed by 30 min recovery recording. Throughout the experiment we recorded autonomic nervous system (ANS) parameters [blood pressure, heart rate (HR), and cardiac vagal tone (CVT), which reflects efferent vagal activity]. Ten out of 12 subjects showed a drop in FP during peak nausea compared with BL (-4.0 ± 0.8 mmHg; P = 0.005), and 8/10 subjects showed a drop in LES pressure (-8.8 ± 2.5 mmHg; P = 0.04). Peak nausea preceded peak fundus and LES pressure drop. Nausea was associated with configuration changes at the GEJ such as LES shortening and esophageal lengthening. During nausea we observed a significantly increased HR and decreased CVT. In conclusion, nausea is associated with a drop in fundus and LES pressure, configuration changes at the GEJ as well as changes in the ANS activity such as an increased sympathetic tone (increased HR) and decreased parasympathetic tone (decreased CVT).

Review article: the clinical relevance of transient lower oesophageal sphincter relaxations in gastro-oesophageal reflux disease

BACKGROUND

Transient lower oesophageal sphincter relaxations (TLOSR) are considered the physiological mechanism that enables venting of gas from the stomach and appear as sphincter relaxations that are not induced by swallowing. It has become increasingly clear that most reflux episodes occur during TLOSRs and therefore play a key role in gastro-oesophageal reflux disease (GERD).

AIM

To describe the current knowledge about TLOSRs and its clinical implications.

METHODS

Search of the literature published in English using the PubMed database and relevant abstracts presented at international conventions.

RESULTS

Several factors influence the rate of TLOSRs including anti-reflux surgery, meal, body position, nutrition, lifestyle and a wide array of neurotransmitters. Ongoing insights in the neurotransmitters responsible for the modulation of TLOSRs, as well as the neural pathways involved in TLOSR induction, have lead to novel therapeutic targets. These therapeutic targets can serve as an add-on therapy in patients with an unsatisfactory response to proton pump inhibitor by inhibiting TLOSRs and its associated reflux events. However, the TLOSR-inhibiting drugs that are currently available still have significant side effects.

CONCLUSION

It is likely that in the future, selected GERD patients may benefit from transient lower oesophageal sphincter relaxation inhibition when compounds are found without significant side effects.

Sensory and motor innervation of the crural diaphragm by the vagus nerves

BACKGROUND & AIMS

During gastroesophageal reflux, transient lower esophageal sphincter relaxation and crural diaphragm (CD) inhibition occur concomitantly. Modifying vagus nerve control of transient lower esophageal sphincter relaxation is a major focus of development of therapeutics for gastroesophageal reflux disease, but neural mechanisms that coordinate the CD are poorly understood.

METHODS

Nerve tracing and immunolabeling were used to assess innervation of the diaphragm and lower esophageal sphincter in ferrets. Mechanosensory responses of vagal afferents in the CD and electromyography responses of the CD were recorded in novel in vitro preparations and in vivo.

RESULTS

Retrograde tracing revealed a unique population of vagal CD sensory neurons in nodose ganglia and CD motor neurons in brainstem vagal nuclei. Anterograde tracing revealed specialized vagal endings in the CD and phrenoesophageal ligament-sites of vagal afferent mechanosensitivity recorded in vitro. Spontaneous electromyography activity persisted in the CD following bilateral phrenicotomy in vivo, while vagus nerve stimulation evoked electromyography responses in the CD in vitro and in vivo.

CONCLUSIONS

We conclude that vagal sensory and motor neurons functionally innervate the CD and phrenoesophageal ligament. CD vagal afferents show mechanosensitivity to distortion of the gastroesophageal junction, while vagal motor neurons innervate both CD and distal esophagus and may represent a common substrate for motor control of the reflux barrier.

Spatial organization of neurons in the dorsal motor nucleus of the vagus synapsing with intragastric cholinergic and nitric oxide/VIP neurons in the rat

The dorsal motor nucleus of the vagus (DMV) contains preganglionic neurons that control gastric motility and secretion. Stimulation of different parts of the DMV results in a decrease or an increase in gastric motor activities, suggesting a spatial organization of vagal preganglionic neurons in the DMV. Little is known about how these preganglionic neurons in the DMV synapse with different groups of intragastric motor neurons to mediate contraction or relaxation of the stomach. We used pharmacological and immunohistochemical methods to characterize intragastric neural pathways involved in mediating gastric contraction and relaxation in rats. Microinjections of L-glutamate (L-Glu) into the rostral or caudal DMV produced gastric contraction and relaxation, respectively, in a dose-related manner. Intravenous infusion of hexamethonium blocked these actions, suggesting mediation via preganglionic cholinergic pathways. Atropine inhibited gastric contraction by 85.5 +/- 4.5%. Gastric relaxation was reduced by intravenous administration of N(G)-nitro-L-arginine methyl ester (L-NAME; 52.5 +/- 11.9%) or VIP antagonist (56.3 +/- 14.9%). Combined administration of L-NAME and VIP antagonist inhibited gastric relaxation evoked by L-Glu (87.8 +/- 4.3%). Immunohistochemical studies demonstrated choline acetyltransferase immunoreactivity in response to L-Glu microinjection into the rostral DMV in 88% of c-Fos-positive intragastric myenteric neurons. Microinjection of L-Glu into the caudal DMV evoked expression of nitric oxide (NO) synthase and VIP immunoreactivity in 81 and 39%, respectively, of all c-Fos-positive intragastric myenteric neurons. These data indicate spatial organization of the DMV. Depending on the location, microinjection of L-Glu into the DMV may stimulate intragastric myenteric cholinergic neurons or NO/VIP neurons to mediate gastric contraction and relaxation.

Hormones of the gut-brain axis as targets for the treatment of upper gastrointestinal disorders

The concept of the gut forming the centre of an integrated gut-brain-energy axis - modulating appetite, metabolism and digestion - opens up new paradigms for drugs that can tackle multiple symptoms in complex upper gastrointestinal disorders. These include eating disorders, nausea and vomiting, gastroesophageal reflux disease, gastroparesis, dyspepsia and irritable bowel syndrome. The hormones that modulate gastric motility represent targets for gastric prokinetic drugs, and peptides that modify eating behaviours may be targeted to develop drugs that reduce nausea, a currently poorly treated condition. The gut-brain axis may therefore provide a range of therapeutic opportunities that deliver a more holistic treatment of upper gastrointestinal disorders.

Anatomy and function of group III metabotropic glutamate receptors in gastric vagal pathways

Metabotropic glutamate receptors (mGluR) are classified into groups I (excitatory), II and III (inhibitory) mGluR. Activation of peripheral group III mGluR (mGluR4, mGluR6, mGluR7, mGluR8), particularly mGluR8, inhibits vagal afferent mechanosensitivity in vitro which translates into reduced triggering of transient lower oesophageal sphincter relaxations and gastroesophageal reflux in vivo. However, the expression and function of group III mGluR in central gastrointestinal vagal reflex pathways is not known. Here we assessed the expression of group III mGluR in identified gastric vagal afferents in the nodose ganglion (NG) and in the dorsal medulla. We also determined the central action of the mGluR8a agonist S-3,4-DCPG (DCPG) on nucleus tractus solitarius (NTS) neurons with gastric mechanosensory input in vivo. Labelling for mGluR4 and mGluR8 was abundant in gastric vagal afferents in the NG, at their termination site in the NTS (subnucleus gelatinosus) and in gastric vagal motorneurons, while labelling for mGluR6 and mGluR7 was weaker in these regions. DCPG (0.1 nmol or 0.001-10 nmol i.c.v.) inhibited or markedly attenuated responses of 8/10 NTS neurons excited by isobaric gastric distension with no effect on blood pressure or respiration; 2 NTS neurons were unaffected. The effects of DCPG were significantly reversed by the group III mGluR antagonist MAP4 (10 nmol, i.c.v.). In contrast, 4/4 NTS neurons inhibited by gastric distension were unaffected by DCPG. We conclude that group III mGluR are expressed in peripheral and central vagal pathways, and that mGluR8 within the NTS selectively reduce excitatory transmission along gastric vagal pathways.

Dorsal motor nucleus of the vagus: a site for evoking simultaneous changes in crural diaphragm activity, lower esophageal sphincter pressure, and fundus tone

The sphincter mechanism at the esophagogastric junction includes smooth muscle of the lower esophagus and skeletal muscle of the crural diaphragm (CD). Smooth muscle is known to be under the control of the dorsal motor nucleus of the vagus (DMV), while central nervous system (CNS) control of the CD is unknown. The main purposes of our study were to determine the CNS site that controls the CD and whether simultaneous changes in lower esophageal sphincter (LES) pressure and CD activity occur when this site is activated. Experiments were performed on anesthetized male ferrets whose LES pressure, CD activity, and fundus tone were monitored. To activate DMV neurons, L-glutamate was microinjected unilaterally into the DMV at three areas: intermediate, rostral, and caudal. Stimulation of the intermediate DMV decreased CD activity (-4.8 +/- 0.1 bursts/min and -0.3 +/- 0.01 mV) and LES pressure (-13.2 +/- 2.0 mmHg; n = 9). Stimulation of this brain site also produced an increase in fundus tone. Stimulation of the rostral DMV elicited increases in the activity of all three target organs (n = 5). Stimulation of the caudal DMV had no effect on the CD but did decrease both LES pressure and fundus tone (n = 5). All changes in LES pressure, fundus tone, and some DMV-induced changes in CD activity (i.e., bursts/min) were prevented by ipsilateral vagotomy. Our data indicate that simultaneous changes in activity of esophagogastric sphincters and fundus tone occur from rostral and intermediate areas of the DMV and that these changes are largely mediated by efferent vagus nerves.

Gastric Vagal Efferent Inhibition Evoked by Intravenous CRF Is Unrelated to Simultaneously Recorded Vagal Afferent Activity in Urethane-Anesthetized Rats

Corticotropin-releasing factor (CRF) injected peripherally or released in response to stressful challenges to the organism reduces gastric tone and contractility, in part by vagal pathways. However, information on the changes in gastric vagal impulse activity evoked by peripheral CRF administration is entirely lacking. Using a novel “dual recording” method in urethane-anesthetized rats, vagal efferent (VE) and afferent (VA) impulse activities were recorded simultaneously from separate, fine bundles dissected from the ventral gastric vagus nerve branch innervating the glandular stomach. Activity records for 38 VA single units (SUs) and 33 VE SUs were sorted from multiunit records obtained from 13 preparations. Intravenous (iv) administration of saline had no effect on multiunit VE activity, whereas CRF (1 μg/kg, iv) immediately inhibited VE activity, reaching a nadir of 54 ± 8.0% of preinjection levels at 3.0 min postinjection. CRF (1 μg/kg, iv) inhibited 25/33 (75.8%) VE SUs and excited three of 33 (9.1%) VE SUs. In contrast to potent effects on VE activity, iv CRF did not alter multiunit VA activity. Single-unit analysis, however, revealed five of 38 (13.1%) VA SUs excited by iv CRF at widely varying latencies (suggesting an indirect mode of action) and one inhibited VA SU. VA SUs excited after iv CRF did not respond during gastric distention and vice versa. These experiments are the first to use simultaneous recording of gastric VA and VE units. The data demonstrate a predominantly inhibitory influence of iv CRF on VE outflow to the hindstomach, not driven by gastric vagovagal reflex activity.

Taste-evoked Fos expression in nitrergic neurons in the nucleus of the solitary tract and reticular formation of the rat

The current investigation used double labeling for NADPHd and Fos-like immunoreactivity to define the relationship between nitric oxide synthase-containing neural elements and taste-activated neurons in the nucleus of the solitary tract (NST) and subjacent reticular formation (RF). Stimulation of awake rats with citric acid and quinine resulted in significant increases in the numbers of double-labeled neurons in both the NST and RF, suggesting that some medullary gustatory neurons utilize nitric oxide (NO) as a transmitter. Overall, double-labeled neurons were most numerous in the caudal reaches of the gustatory zone of the NST, where taste neurons receive inputs from the IXth nerve, suggesting a preferential role for NO neurons in processing gustatory inputs from the posterior oral cavity. However, double-labeled neurons also exhibited a preferential distribution depending on the gustatory stimulus. In the NST, double-labeled neurons were most numerous in the rostral central subnucleus after either stimulus but had a medial bias after quinine stimulation. In the RF, after citric acid stimulation, there was a cluster of double-labeled neurons with distinctive large soma in the parvicellular division of the lateral RF, subjacent to the rostral tip of NST. In contrast, in response to quinine, there was a cluster of double-labeled neurons with much smaller soma in the intermediate zone of the medial RF, a few hundred micrometers caudal to the citric acid cluster. These differential distributions of double-labeled neurons in the NST and RF suggest a role for NO in stimulus-specific gustatory autonomic and oromotor reflex circuits.

Peripheral versus central modulation of gastric vagal pathways by metabotropic glutamate receptor 5

Metabotropic glutamate receptors (mGluR) are classified into group I, II, and III mGluR. Group I (mGluR1, mGluR5) are excitatory, whereas group II and III are inhibitory. mGluR5 antagonism potently reduces triggering of transient lower esophageal sphincter relaxations and gastroesophageal reflux. Transient lower esophageal sphincter relaxations are mediated via a vagal pathway and initiated by distension of the proximal stomach. Here, we determined the site of action of mGluR5 in gastric vagal pathways by investigating peripheral responses of ferret gastroesophageal vagal afferents to graded mechanical stimuli in vitro and central responses of nucleus tractus solitarius (NTS) neurons with gastric input in vivo in the presence or absence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). mGluR5 were also identified immunohistochemically in the nodose ganglia and NTS after extrinsic vagal inputs had been traced from the proximal stomach. Gastroesophageal vagal afferents were classified as mucosal, tension, or tension-mucosal (TM) receptors. MPEP (1-10 microM) inhibited responses to circumferential tension of tension and TM receptors. Responses to mucosal stroking of mucosal and TM receptors were unaffected. MPEP (0.001-10 nmol icv) had no major effect on the majority of NTS neurons excited by gastric distension or on NTS neurons inhibited by distension. mGluR5 labeling was abundant in gastric vagal afferent neurons and sparse in fibers within NTS vagal subnuclei. We conclude that mGluR5 play a prominent role at gastroesophageal vagal afferent endings but a minor role in central gastric vagal pathways. Peripheral mGluR5 may prove a suitable target for reducing mechanosensory input from the periphery, for therapeutic benefit.

Functional effects and characteristics of cecum-projecting neurons in the dorsal motor nucleus of the vagus of rats

Preganglionic neurons in the dorsal motor nucleus of the vagus (DMV) innervate most of the gastrointestinal tract; with the stomach and the cecum/proximal colon having a greater proportion of vagal input. Cecum-projecting neurons have been thought to be distinct from other preganglionic neurons due to their location within the DMV, but it is unknown whether these neurons innervate the cecum exclusively or what effect their activation has on cecal motor activity. Therefore, we investigated the extent of coinnervation of cecum and stomach by vagal neurons, their neurochemistry, and the effect of DMV stimulation on intracecal and intragastric volumes. Fluorescent retrograde tracers injected into the serosa of the cecum and stomach revealed that in the DMV 49+/-5% CTB-labeled cecum-projecting neurons also innervated the stomach. Immunocytochemical staining for nitric oxide (NO) synthase and tyrosine hydroxylase indicated that only 3+/-1% and 4+/-1% of cecum-projecting neurons contained these markers, respectively. In anesthetized rats gastric and cecal volumes were measured by prototypic miniaturized dual barostats that were developed for use in rodents. Microinjection of l-glutamate into the DMV increased gastric contractile activity and tone, and reduced on-going cecum contractile activity (2.6+/-0.7 contractions/2 min after injection versus 8.2+/-0.4 contractions/2 min before injection, N = 5). The barostat was able to detect decreases (-0.88+/-0.13 ml) and increases (0.25+/-0.05 ml) in cecum volume in response to carbachol and sodium nitroprusside, respectively. In summary, cecum-projecting neurons are not an entirely exclusive population within the DMV because a percentage of these also innervate the stomach. Central vagal stimulation can modulate both gastric and cecum contractile activity. Together, these data support a role of the vagus in neural reflexes involving gastric and large bowel motor function, such as the immediate phase of the gastrocolonic reflex.

Pharmacological MRI in awake rats reveals neural activity in area postrema and nucleus tractus solitarius: relevance as a potential biomarker for detecting drug-induced emesis

Drug-induced vomiting (emesis) is a major concern in patient care and a significant hurdle in the development of novel therapeutics. With respect to the latter, rodents, such as the rat and mouse, are typically used in efficacy and safety studies; however, drug-induced emesis cannot be readily observed in these species due to the lack of an emetic reflex. It is known that emesis can be triggered by neural activity in brain regions including area postrema (AP) and nucleus tractus solitarius (NTS). In this study, using pharmacological magnetic resonance imaging (phMRI) and a blood-pool contrast agent, we imaged the hemodynamic consequences of brain activity in awake rats initiated by the administration of compounds (apomorphine 0.1, 0.3 micromol/kg i.v. and ABT-594 0.03, 0.1, 0.3 micromol/kg i.v.) that elicit emesis in other species. Regional drug-induced relative cerebral blood volume (rCBV) changes and percent activated area within the AP and NTS were calculated, in which a dose-dependent relationship was evident for both apomorphine and ABT-594. Additionally, to correlate with behavioral readouts, it was found that the activation of AP and NTS was observed at plasma concentrations consistent with those that induced emesis in ferrets for both drugs. Our data thus suggest that phMRI in awake rats may be a useful tool for predicting emetic liability of CNS-acting drugs and may provide insights into depicting the underlying emetic neural pathways in vivo.

Achalasia: physiology and etiopathogenesis

Achalasia is a disorder of esophageal motility that has been well documented for over 300 years. Despite this, the initiating factor or factors and the underlying mechanisms leading to the characteristic features of achalasia, the absence of distal esophageal peristalsis and abnormal lower esophageal sphincter relaxation, are still not well understood. Recent work has shed light on changes in neurotransmission and cell signaling in the lower esophagus and lower esophageal sphincter that lead to achalasia. A number of recent reviews have thoroughly discussed diagnostic and therapeutic modalities and the reader is referred to these for in-depth review of these topics. The focus of this review will be on our current understanding of the physiology of esophageal peristalsis and lower esophageal sphincter function as it relates to achalasia and on available evidence for etiology and proposed pathophysiologic mechanisms.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

Projections to the alimentary canal from the dopaminergic neurons in the dorsal motor nucleus of the vagus of the rat

The motility of the alimentary canal is regulated not only by neurons that contain acetylcholine or adrenaline, but also by nonadrenergic noncholinergic neurons. There are many neurons containing dopamine in the dorsal motor nucleus of the vagus (DMV). We examined the projections of these dopaminergic neurons to the alimentary canal with double-labeling immunohistochemistry for tyrosine hydroxylase (TH) and the retrograde tracer cholera toxin subunit b following its injection into the subdiaphragmatic esophagus, the cardia, the pylorus, the duodenum, the jejunum, and the ascending colon. Almost all double-labeled neurons were found in the half of the DMV caudal to the area postrema. In the caudal half of the DMV, about 58% of the TH-immunoreactive neurons projected to the cardia, about 36% projected to the pylorus, and about 28% projected to the subdiaphragmatic esophagus. Only a few TH-immunoreactive neurons projected to the duodenum, the jejunum, or the ascending colon. As a whole, less than 10% of the neurons in the DMV that projected to the alimentary canal showed TH-like immunoreactivity. These results suggest that some of the dopaminergic neurons in the DMV might regulate the activities of the stomach and the subdiaphragmatic esophagus.

Roles of substance P and NK(1) receptor in the brainstem in the development of emesis

The emetic response is primarily a protective reflex occurring in a wide variety of vertebrates in response to the ingestion of toxic compounds. The role of the nuclei in the brainstem, including the area postrema, nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the central pattern generator for vomiting, as well as the involvement of the abdominal visceral innervation relevant to the emetic reflex, have all been discussed by many researchers. The introduction of serotonin 5-HT(3)-receptor antagonists into clinical practice allowed for a dramatic improvement in the management of vomiting. However, vomiting still remains a significant problem. The mechanism of the emetic response is even more complicated than was first thought. This review attempts to bring together some of the evidence suggesting the roles of substance P and its receptor, neurokinin NK(1) receptor, in the brainstem nuclei in the development of emesis. Accordingly, NK(1)-receptor antagonists might represent novel drugs for the management of major types of emesis.

Central mechanisms of lower esophageal sphincter control

Lower esophageal sphincter (LES) tone is decreased during swallowing, during transient LES relaxations (TLESRs), and before emesis, and this decrease is due primarily to increasing inhibitory vagal output to the LES. Reflex-evoked relaxation of the LES is mediated by long-loop vagovagal reflexes that are coordinated by the dorsal vagal complex in the hindbrain medulla. A sequence of events occurs. Central control of TLESRs has not been studied directly; the information on how drugs may work centrally to reduce TLESRs is extrapolated from knowledge of how the brain evokes LES relaxation. Reduction of the frequency of TLESRs by a GABAB agonist, baclofen, is due to inhibition of vagal afferents, information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus, and vagal efferent outflow. Preliminary data show that cannabinoid receptor activation reduces information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus. The potential therapeutic usefulness of these types of agents that reduce TLESRs by acting centrally is promising.

Central neurocircuitry associated with emesis

Ingestion of toxin, traumatic events, adverse drug reactions, and motion can all result in nausea and emesis. In addition, cyclic vomiting syndrome is quite prevalent in the pediatric population. Coordination of the various autonomic changes associated with emesis occurs at the level of the medulla oblongata of the hindbrain. Chemosensitive receptors detect emetic agents in the blood and relay this information by means of neurons in the area postrema to the adjacent nucleus tractus solitarius (NTS). Abdominal vagal afferents that detect intestinal luminal contents and gastric tone also terminate in the NTS (gelatinosus, commissural, and medial subnuclei). The NTS is viscerotopically organized into subnuclei that subserve diverse functions related to swallowing (subnucleus centralis), gastric sensation (subnucleus gelatinosus), laryngeal and pharyngeal sensation (intermediate and interstitial NTS), baroreceptor function (medial NTS), and respiration (ventrolateral NTS). Neurons from the NTS project to a central pattern generator (CPG), which coordinates the sequence of behaviors during emesis, as well as directly to diverse populations of neurons in the ventral medulla and hypothalamus. Thus, it is critical to realize that there is not an isolated "vomiting center," but rather groups of loosely organized neurons throughout the medulla that may be activated in sequence by a CPG. The newer antiemetic agents appear to block receptors in the peripheral endings of vagal afferents to reduce "perception" of emetic stimuli and/or act in the dorsal vagal complex. A primary site of action of 5-HT(3)-receptor antagonists is by means of the vagal afferents. Neurokinin-1 receptor (NK(1)R) antagonists are antiemetics, because they act at a site in the dorsal vagal complex. Part of their effectiveness may be the result of inhibition of the NK(1)R on vagal motor neurons to prevent fundic relaxation, which is a prodromal event essential for emesis. Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the major psychoactive component of marijuana, can be therapeutically useful as an antiemetic. The site of action of Delta(9)-THC is on cannabinoid CB1 receptors in the dorsal vagal complex. However, it decreases fundic tone and antral motility. It is not easy to predict the potential antiemetic effects of drugs that alter motility. Although antiemetic drugs are available for management of acute chemotherapeutic-induced emesis, few treatments are effective for delayed emesis or cyclic vomiting syndrome.

Neural circuits in swallowing and abdominal vagal afferent-mediated lower esophageal sphincter relaxation

The purpose of this review is to identify the medullary subnuclei that house neural circuits for lower esophageal sphincter (LES) relaxation. LES relaxation may occur as a component of primary peristalsis elicited by superior laryngeal nerve (SLN) afferent stimulation, secondary peristalsis elicited by esophageal distention or as a component of belch reflex, and transient LES relaxation elicited by gastric vagal afferent stimulation. In mice, SLN stimulation at 10 Hz elicited complete swallowing reflex, including pharyngeal and esophageal peristalsis, and LES relaxation. SLN stimulation at 5 Hz elicited pharyngeal contractions and isolated LES relaxation, which is not accompanied by esophageal peristalsis. Electric stimulation of afferents in the ventral branch of the subdiaphragmatic vagus (vSDV) at 10 Hz also elicited isolated LES relaxation. Using these defined stimuli, c-fos expression was examined in the entire craniocaudal extent of the medullary nuclei. SLN stimulation at 10 Hz induced c-fos expression in neurons in: (1) interstitial (SolI), intermediate (SolIM), central (SolCe), occasional medial (SolM), and dorsomedial (SolDM) solitary subnuclei; (2) motor neurons in the nucleus ambiguus, including its semicompact (NAsc), loose (NAl), and compact (NAc) formations; and (3) dorsal motor nucleus of vagus, including its rostral (DMVr) and caudal (DMVc) parts. The activated neurons represent neurons involved with afferent SLN-mediated reflexes, including swallowing. SLN stimulation at 5 Hz evoked c-fos expression in neurons in SolI, SolIM, SolM, and SolDM but not in SolCe; and motor neurons in NAsc, NAl, and DMVc but not in NAc or DMVr. Stimulation of vSDV induced c-fos expression in neurons in SolM and SolDM and in motoneurons in DMVc. When considered with published reports in other animal species, these data support the speculation that (1) swallow-evoked primary peristalsis involves the following neural circuits: SolI/SolIM --> NAsc/NAl for pharyngeal and SolCe --> NAc for esophageal (striated muscle) peristalsis, SolM/SolDM --> preganglionic neurons in DMVc and DMVr and nitrergic and cholinergic neurons in myenteric plexus for esophageal (smooth muscle) peristalsis, and SolM/SolDM --> preganglionic neurons in DMVc --> postganglionic nitrergic neurons in the myenteric plexus for LES relaxation; and (2) abdominal vagus-stimulated isolated LES relaxation may involve neurons in SolM and SolDM --> preganglionic motor neurons in DMVc --> postganglionic nitrergic neurons in the myenteric plexus.

GABA(B)R expressed on vagal afferent neurones inhibit gastric mechanosensitivity in ferret proximal stomach

GABA(B)-receptor (GABA(B)R) agonists reduce transient lower esophageal sphincter relaxation (TLESR) and reflux episodes through an action on vagal pathways. In this study, we determined whether GABA(B)R are expressed on vagal afferent neurones and whether they modulate distension-evoked discharge of vagal afferents in the isolated stomach. Vagal mehanoreceptor activity was recorded following distensions of the isolated ferret proximal stomach before and after perfusion with the GABA(B)R-selective agonists baclofen and 3-aminopropylphosphinic acid (3-APPiA). Retrograde labeling and immunohistochemistry were used to identify GABA(B)R located on vagal afferent neurones in the nodose ganglia. Vagal afferent fibers responded to isovolumetric gastric distension with an increase in discharge. The GABA(B)-receptor agonists baclofen (5 x 10(-5) M) and 3-APPiA (10(-6) to 10(-5) M) but not muscimol (GABA(A)-selective agonist: 1.3 x 10(-5) M) significantly decreased afferent distension-response curves. The effect of baclofen (5 x 10(-5) M) was reversed by the GABA(B)-receptor antagonist CGP 62349 (10(-5) M). Over 93% of retrogradely labeled gastric vagal afferents in the nodose ganglia expressed immunoreactivity for the GABA(B)R. GABA(B)R expressed on vagal afferent fibers directly inhibit gastric mechanosensory activity. This is likely a contributing mechanism to the efficacy of GABA(B)-receptor agonists in reducing TLESR and reflux episodes in vivo.

Site of action of GABA(B) receptor for vagal motor control of the lower esophageal sphincter in ferrets and rats

BACKGROUND & AIMS

Stimulation of gamma-aminobutyric acid B metabotropic receptors (GBRs) by baclofen reduces the incidence of transient lower esophageal sphincter (LES) relaxations. The GBR effect may be a result of a central site of action in the dorsal vagal complex, where upper gastrointestinal vagal reflexes are integrated. Therefore, we first localized GBR immunostaining in the dorsal vagal complex. Next, we tested the hypothesis that baclofen modulates LES motor tone via GBR expressed by vagal efferent neurons.

METHODS

An antibody against the human GBR1b isoform was characterized and used for immunocytochemistry in rats and ferrets. Functional studies involved microinjection of L-glutamate into the caudal dorsal motor nucleus of the vagus to evoke an LES relaxation in decerebrate unanesthetized ferrets.

RESULTS

In both species, GBR1b was expressed in preganglionic motor neurons and, in ferrets, the receptor was highly expressed in identified LES-projecting preganglionic neurons. GBR1b immunostaining was also pronounced in the subnucleus centralis of the nucleus tractus solitarius. This distribution implicates GBR in control of the esophageal phase of swallowing at the level of the central program generator. In functional studies, centrally evoked LES relaxation (-73% +/- 8% mm Hg) was significantly attenuated after 7 micromol/kg intravenous baclofen (-37% +/- 10%; N = 5).

CONCLUSIONS

These data all suggest that GBR agonists inhibit LES relaxation via a site of action associated with vagal motor outflow to the LES.