Mechanisms of gastro-oesophageal reflux in the ferret

Metabotropic glutamate receptors as novel therapeutic targets on visceral sensory pathways

Metabotropic glutamate receptors (mGluR) have a diverse range of structures and molecular coupling mechanisms. There are eight mGluR subtypes divided into three major groups. Group I (mGluR1 and 5) is excitatory; groups II (mGluR2 and 3) and III (mGluR 4, 6, and 7) are inhibitory. All mGluR are found in the mammalian nervous system but some are absent from sensory neurons. The focus here is on mGluR in sensory pathways from the viscera, where they have been explored as therapeutic targets. Group I mGluR are activated by endogenous glutamate or constitutively active without agonist. Constitutive activity can be exploited by inverse agonists to reduce neuronal excitability without synaptic input. This is promising for reducing activation of nociceptive afferents and pain using mGluR5 negative allosteric modulators. Many inhibitory mGluR are also expressed in visceral afferents, many of which markedly reduce excitability. Their role in visceral pain remains to be determined, but they have shown promise in inhibition of the triggering of gastro-esophageal reflux, via an action on mechanosensory gastric afferents. The extent of reflux inhibition is limited, however, and may not reach a clinically useful level. On the other hand, negative modulation of mGluR5 has very potent actions on reflux inhibition, which has produced the most likely candidates so far as therapeutic drugs. These act probably outside the central nervous system, and may therefore provide a generous therapeutic window. There are many unanswered questions about mGluR along visceral afferent pathways, the answers to which may reveal many more therapeutic candidates.

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.

Landmarks in the understanding and treatment of reflux disease

The last 50 years have seen a transformation in the understanding and treatment of reflux disease. The development and wide use of flexible endoscopy and progressively more sophisticated approaches to measurement of pathophysiological factors have been major drivers of advances. The recognition and progressive elucidation of the mechanical events that comprise the transient lower esophageal sphincter relaxation and how they lead to reflux provide a novel and firm foundation for tailoring therapies that act directly to reduce occurrence of reflux episodes, either surgically or pharmacologically. Novel GABA(B) agonist drugs have been shown to inhibit transient relaxations and are currently being evaluated in clinical trials on patients with reflux disease. Better understanding has extended to recognition of the extraordinarily high prevalence of reflux disease and of the ability of proton pump inhibitor drugs to deliver major benefits to a high proportion of patients with reflux disease. The life of the Gastroenterological Society of Australia has spanned the period of these major advances. A large number of the members of the Society and their associates have contributed substantially to these advances.

Gastroesophageal reflux disease in baboons (Papio sp.): a new animal model

BACKGROUND

Gastroesophageal reflux disease (GERD) is increasingly prevalent in the human population. Current animal models require surgical or other manipulation to produce symptoms. An animal model that exhibits spontaneous GERD would provide the opportunity for much-needed research examining the susceptibility, diagnosis, and treatment of GERD.

METHODS

Eight baboons (Papio hamadryas sp.) were diagnosed with GERD histopathologically using biopsies or postmortem tissues.

RESULTS

The disease was characterized by a spectrum of symptoms comparable with that found in the human population. Some subjects had no gross signs of clinical disease, but were diagnosed by histopathological examination. Almost all subjects presented with at least one clinical sign of the disease. Regurgitation was the most common.

CONCLUSIONS

The baboon may be a superior animal model for GERD research because it is a naturally occurring model and is anatomically and physiologically similar to humans.

Ghrelin selectively reduces mechanosensitivity of upper gastrointestinal vagal afferents

Ghrelin is a peptide released from gastric endocrine cells that has an orexigenic effect via a vagal pathway. Here we determine the effect of ghrelin on mechanosensitivity of upper-intestinal vagal afferent fibers in ferret and mouse. The responses of gastroesophageal vagal afferents to graded mechanical stimulation were determined in vitro before and during application of ghrelin to their peripheral endings. Three types of vagal afferent were tested: tension receptors responding to circumferential tension, mucosal receptors responding only to mucosal stroking, and tension/mucosal (TM) receptors in ferret esophagus that responded to both stimuli. In the mouse, ghrelin did not significantly affect the response of mucosal receptors to mucosal stroking with calibrated von Frey hairs. However, it significantly reduced responses of tension receptors to circumferential tension (P < 0.005; two-way ANOVA) by up to 40%. This inhibition was reversed by the ghrelin receptor antagonist [d-Lys-3]-growth hormone-releasing peptide (GHRP)-6. In the ferret, ghrelin significantly reduced the response of mucosal and TM receptors to mucosal stroking with calibrated von Frey hairs. Surprisingly, ghrelin did not significantly alter the response to circumferential tension in either tension or TM receptors. RT-PCR analysis indicated that both ghrelin and its receptor are expressed in vagal afferent cell bodies in mouse nodose ganglia. In conclusion, ghrelin selectively inhibits subpopulations of mechanically sensitive gastroesophageal vagal afferents; there is also potential for ghrelin release from vagal afferents. However, the subpopulation of afferents inhibited differs between species. These data have broad implications for ghrelin's role in food intake regulation and reflex control of gastrointestinal function.

Inhibition of transient lower esophageal sphincter relaxation and gastroesophageal reflux by metabotropic glutamate receptor ligands

BACKGROUND & AIMS

Transient lower esophageal sphincter relaxation (TLESR) is the major mechanism of gastroesophageal acid reflux. TLESR is mediated via vagal pathways, which may be modulated by metabotropic glutamate receptors (mGluRs). Group I mGluRs (mGluR1 and 5) have excitatory effects on neurons, whereas group II (mGluR2 and 3) and group III (mGluR4, 6, 7, and 8) are inhibitory. This study determined the effect of mGluRs on triggering of TLESR and reflux in an established conscious ferret model.

METHODS

Esophageal manometric/pH studies were performed in ferrets with chronic esophagostomies. TLESR were induced by a gastric load of 25 mL glucose (pH 3.5) and 30 mL air.

RESULTS

In control treated animals, gastric load induced 3.52 +/- 0.46 TLESRs per 47-minute study, 89.7% of which were associated with reflux episodes (n = 16). The mGluR5 antagonist MPEP inhibited TLESR dose dependently, with maximal 71% +/- 7% inhibition at 35 micromol/kg (n = 9; P < .0001). MPEP also significantly reduced reflux episodes (P < .001) and increased basal lower esophageal sphincter pressure (P < .05). MPEP inhibited swallowing dose dependently, suggesting a common action on trigger mechanisms for swallowing and TLESR. The more selective analogue, MTEP, had more potent effects (90% +/- 6% inhibition TLESR at 40 micromol/kg; n = 8; P < .0001). In contrast, the group I agonist DHPG tended to increase TLESR. The group II agonist (2R, 4R)-APDC was ineffective, whereas the group III agonist L-(AP4 slightly reduced TLESR (33% at 11 micromol/kg; P < .05). The selective mGluR8 agonist (S)-3, 4-DCPG inhibited TLESR by 54% at 15 micromol/kg (P < .01).

CONCLUSIONS

mGluR5 antagonists potently inhibit TLESR and reflux in ferrets, implicating mGluR5 in the mechanism of TLESR. mGluR5 antagonists are therefore promising as therapy for patients with GERD.

Gastroesophageal reflux disease and baclofen: is there a light at the end of the tunnel?

Transient lower esophageal sphincter relaxations (TLESRs) are rapid and prolonged relaxations of the lower esophageal sphincter (LES) that are not associated with swallowing. They are the mechanism by which most gastroesophageal reflux episodes occur in normal people and in patients with esophagitis. Transient LES relaxations appear to be mediated by a vagovagal reflex initiated by gastric distention. Baclofen is a g-aminobutyric acid (GABA) derivative that inhibits the production of TLESRs by acting as a GABA(B) receptor agonist at one or more loci along the vagovagal reflex arc. Animal and human studies suggest that baclofen decreases the number of reflux events and amount of esophageal acid exposure. Baclofen or another GABA(B) receptor agonist may be clinically useful in treatment of gastroesophageal reflux disease.

Cannabinoid1 receptor in the dorsal vagal complex modulates lower oesophageal sphincter relaxation in ferrets

Delta9-tetrahydrocannabinol (delta9-THC) is an effective anti-emetic; however, other potential gastrointestinal therapeutic effects of delta9-THC are less well-known. Here, we report a role of delta9-THC in a vago-vagal reflex that can result in gastro-oesophageal reflux, that is, gastric distension-evoked lower oesophageal sphincter (LOS) relaxation. Oesophageal, LOS and gastric pressures were measured using a miniaturized, manometric assembly in decerebrate, unanaesthetized ferrets.Gastric distension (30 ml) evoked LOS relaxation (70 +/- 8% decrease from baseline). Delta9-THC administered systemically (0.2 mg kg-1, iv.) or directly to the dorsal hindbrain surface (0.002 mg),significantly attenuated the nadir of the gastric distention-evoked LOS relaxation, and time to reach maximal response. Similar increases to maximal effect were observed after treatment with the cannabinoid receptor agonist WIN 55,212-2 (0.2 mg kg-1 iv.). The effect of systemic delta9-THC on gastric distention-evoked LOS relaxation was reversed by a selective cannabinoid1 (CBI) receptor antagonist, SR141617A (1 mg kg-1 i.v.). Since this reflex is vagally mediated, we used a CB1 receptor antiserum and immunocytochemistry to determine its distribution in ferret vagal circuitry. CBI receptor staining was present in cell bodies within the area postrema, nucleus tractus solitarius (NTS) and nodose ganglion. Intense terminal-like staining was noted within the NTS and dorsal motor vagal nucleus (DMN). Neither nodose ganglionectomy nor vagotomy altered the CB1 receptor terminal-like staining in the dorsal vagal complex. Retrogradely labelled gastric- or LOS-projecting DMN neurones did not express CBI receptors within their soma. Therefore, CBI receptor staining in the NTS and DMN is not due to primary vagal afferents or preganglionic neurones. These novel findings suggest that delta9-THC can modulate reflex LOS function and that the most likely site of action is via the CBI receptor within the NTS. This effect of delta9-THC may have implications in treatment of gastro-oesophageal reflux and other upper gut disorders.

The histological appearances of Nissen-type fundoplication in the ferret

Nissen fundoplication is of proven effectiveness in the surgical control of gastro-oesophageal reflux. However, our understanding of the effects of fundoplication upon foregut physiology is incomplete and post-operative symptoms are often poorly understood. This experimental study aimed systematically to characterize the tissue response to fundoplication in an animal model, to improve understanding of the effects of anti-reflux surgery upon foregut physiology. Nissen-type fundoplication was performed in the ferret, and the tissue response at 3 months examined histologically. Sham-operated animals that underwent laparotomy but no dissection or wrap, acted as controls. In fundoplicated animals, serosal fibrosis was observed in the gut wall, with patchy replacement of muscle by fibrous tissue. The ventral and dorsal vagal nerve trunks were identified intact within the wrap. In cases where the wrap had spontaneously disrupted, fibrosis was more extensive and there was evidence of nerve damage. This is the first systematic description of the histopathological response to Nissen fundoplication. In the intact wrap, the vagal trunks appear spared, but there is fibrosis in the serosa, extending into the muscularis of the distal oesophagus and region of the cardia. These findings are discussed in relation to the effects of Nissen fundoplication upon gastric physiology and postoperative symptoms.

Lower oesophageal sphincter relaxation evoked by stimulation of the dorsal motor nucleus of the vagus in ferrets

An understanding of the neural control of lower oesophageal sphincter (LOS) relaxation is clinically relevant because transient LOS relaxations (TLOSRs) are a mechanism of acid reflux into the oesophagus. Preganglionic motor neurones innervating the LOS are localized in the dorsal motor nucleus of the vagus (DMV). Based on a single study in cats, it is now widely accepted that these neurones are functionally organized into two separate populations, such that stimulation of the caudal and rostral DMV evokes LOS relaxation and contraction, respectively. Our goal was to map the functional LOS responses to chemical stimulation in the DMV and nucleus tractus solitarius (NTS) of ferrets, an animal model commonly used for conscious studies on TLOSRs, and to test whether DMV-evoked LOS relaxation is mediated through hexamethonium-sensitive vagal-inhibitory pathways to the LOS. We used miniaturized manometry with Dentsleeve to monitor LOS and oesophageal pressures in decerebrate unanaesthetized ferrets. LOS relaxation was evoked readily in response to gastric insufflation, which shows that the vago-vagal reflex was intact in this preparation. Microinjections of l-glutamate (12.5 nmol L-1 in 25 nL) were made into the DMV from approximately - 1.5 to + 2.0 mm relative to the obex. Microinjections into the caudal (- 1.5 to + 0.0 mm behind obex) and intermediate (+ 0.1 to + 1.0 mm rostral to obex) DMV both significantly decreased LOS pressure, and complete LOS relaxation was noted in 28/32 and 11/18 cases, respectively. LOS relaxation responses to DMV microinjection were highly reproducible and abolished by bilateral vagotomy or hexamethonium (15 mg kg-1 intravenously). A nitric oxide synthase inhibitor (l-NAME 100 mg kg-1 intramuscularly) significantly increased the time taken to reach the maximal response. Increases in LOS pressure (24 +/- 4 mmHg; n = 3) were obtained only when stimulation sites were located equal to greater than 1.5 mm rostral to the obex. LOS relaxation (- 78 +/- 10%; n = 6) was evoked by stimulation of the NTS but not immediately outside of the NTS (11 +/- 27%; n = 5). We conclude that there is a very extensive population of 'inhibitory' motor neurones in the DMV that may account for the predominant vagal-inhibitory tone in ferrets. As NTS stimulation evokes LOS relaxation and the predominant response to DMV stimulation is also LOS relaxation, this vago-vagal reflex may involve an excitatory interneurone between the NTS and DMV vagal inhibitory output.

Transient lower oesophageal sphincter relaxations--a pharmacological target for gastro-oesophageal reflux disease?

The oesophago-gastric junction functions as an anti-reflux barrier preventing increased exposure of the oesophageal mucosa to gastric contents. Failure of this anti-reflux barrier results in gastro-oesophageal reflux disease, and may lead to complications such as oesophagitis, Barrett's oesophagus and eventually oesophageal carcinoma. Recent studies have suggested that transient lower oesophageal sphincter relaxation is the main mechanism underlying gastro-oesophageal reflux. It involves a prolonged relaxation of the lower oesophageal sphincter, mediated by a vago-vagal neural pathway, synapsing in the brainstem. Several drugs, such as atropine, baclofen and loxiglumide, have been shown to reduce the rate of transient lower oesophageal sphincter relaxations and concomitantly the number of reflux episodes. These findings illustrate that transient lower oesophageal sphincter relaxations may represent a potential new target for the pharmacological treatment of gastro-oesophageal reflux disease. It is possible that the reduction in the number of transient lower oesophageal sphincter relaxations may also contribute to the beneficial effect of fundoplication and new endoscopic anti-reflux procedures. It should be emphasized, however, that other factors, such as low lower oesophageal sphincter pressure, the presence of a hiatal hernia and impaired oesophageal peristalsis, are also of great importance. Therefore, whether the targeting of transient lower oesophageal sphincter relaxations is the 'golden bullet' in anti-reflux therapy remains to be proven, as evidence of an effective control of gastro-oesophageal reflux in reflux patients is still lacking.

Individual inactivation of the sphincteric component of the gastroesophageal barrier causes reflux esophagitis in piglets

BACKGROUND/PURPOSE

The lower esophageal sphincter and the diaphragmatic crural sling form the gastroesophageal barrier. This work shows that division of the sphincteric component alone suffices to induce reflux esophagitis in piglets.

METHODS

Male piglets underwent either sham operation (n = 7) or extramucosal myotomy of the gastroesophageal junction (n = 8). Before and 1 week after the operation, pull-through manometry was performed under sedation. Pressures taken on the 4 quadrants at 20 1-mm intervals on both time endpoints were compared by pairwise Wilcoxon tests. The distal esophagus was studied histologically after 8 weeks.

RESULTS

The pressure profiles did not change after sham operation. In contrast, they were significantly flattened in the distal half of the high-pressure zone after myotomy reflecting disappearance of the sphincteric component of the barrier. Esophagitis was seen in all myotomized piglets but in none from the sham group.

CONCLUSIONS

Lower esophageal sphincter myotomy alone with preservation of the crural sling induces reflux esophagitis in piglets. This animal, widely available and not too costly, is an excellent model for gastroesophageal research.

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

The motor control of the lower esophageal sphincter (LES) is critical for normal swallowing and emesis, as well as for the prevention of gastroesophageal reflux. However, there are surprisingly few data on the central organization and neurochemistry of LES-projecting preganglionic neurons. There are no such data in ferrets, which are increasingly being used to study LES relaxation. Therefore, we determined the location of preganglionic neurons innervating the ferret LES, with special attention to their relationship with gastric fundus-projecting neurons. The neurochemistry of LES-projecting neurons was also investigated using two markers of "nontraditional" neurotransmitters in vagal preganglionic neurons, nitric oxide synthase (NOS), and dopamine (tyrosine hydroxylase: TH). Injection of cholera toxin B subunit (CTB)-horseradish peroxidase (HRP) into the muscular wall of the LES-labeled profiles throughout the rostrocaudal extent of the dorsal motor nucleus of the vagus (DMN) The relative numbers of profiles in three regions of the DMN from caudal to rostral are, 43 +/- 5, 67 +/- 11, and 113 +/- 30). A similar rostrocaudal distribution occurred after injection into the gastric fundus. When CTB conjugated with different fluorescent tags was injected into the LES and fundus both labels were noted in 56 +/- 3% of LES-labeled profiles overall. This finding suggests an extensive coinnervation of both regions by vagal motor neurons. There were significantly fewer LES-labeled profiles that innervated the antrum (16 +/- 9%). In the rostral DMN, 15 +/- 4% of LES-projecting neurons also contained NADPH-diaphorase activity; however, TH immunoreactivity was never identified in LES-projecting neurons. This finding suggests that NO, but not catecholamine (probably dopamine), is synthesized by a population of LES-projecting neurons. We conclude that there are striking similarities between LES- and fundic-projecting preganglionic neurons in terms of their organization in the DMN, presence of NOS activity and absence of TH immunoreactivity. Coinnervation of the LES and gastric fundus is logical, because the LES has similar functions to the fundus, which relaxes to accommodate food during ingestion and preceding emesis, but has quite different functions from the antrum, which provides mixing and propulsion of contents for gastric emptying. The presence of NOS in some LES-projecting neurons may contribute to LES relaxation, as it does in the case of fundic relaxation. The neurologic linkage of vagal fundic and LES relaxation may have clinical relevance, because it helps explain why motor disorders of the LES and fundus frequently occur together.

The anti-reflux barrier and mechanisms of gastro-oesophageal reflux

The lower oesophageal sphincter (LOS) is the major component of the anti-reflux barrier. The majority of reflux episodes occur because of intermittent brief complete lower oesophageal sphincter relaxations, transient LOS relaxations, rather than from chronic absence of LOS pressure. Recent advances in the understanding of the neural mechanisms and the receptors involved in the triggering of transient LOS relaxations have provided new insights into their control and offer the potential for the development of pharmacological therapy of reflux based on control of these events. Extrinsic support by the crural diaphragm is also important and loss of this support through development of hiatus hernia significantly compromises LOS function. This chapter reviews the components of the anti-reflux barrier, the patterns and mechanisms of LOS dysfunction underlying reflux episodes, and the interplay between sphincteric and non-sphincteric factors.

Triggering of transient LES relaxations in ferrets: role of sympathetic pathways and effects of baclofen

Activation of gastric vagal mechanoreceptors by distention is thought to be the trigger for transient lower esophageal sphincter relaxations (TLESR), which lead to gastroesophageal reflux. The contribution of higher-threshold gastric splanchnic mechanoreceptors is uninvestigated. GABA(B) receptor agonists, including baclofen, potently reduce triggering of TLESR by low-level gastric distention. We aimed to determine first whether this effect of baclofen is maintained at high-level distention and second the role of splanchnic pathways in triggering TLESR. Micromanometric/pH studies in conscious ferrets showed that intragastric glucose infusion (25 ml) increased triggering of TLESR and reflux. Both were significantly reduced by baclofen (7 micromol/kg ip) (P < 0.05). When 40 ml of air was added to the glucose infusion, more TLESR occurred than with glucose alone (P < 0.01). These were also reduced by baclofen (P < 0.001). TLESR after glucose/air infusion were assessed before and after splanchnectomy (2-4, 9-11, and 23-25 days), which revealed no change. Baclofen inhibits TLESR after both low- and high-level gastric distention. Splanchnic pathways do not contribute to increased triggering of TLESR by high-level gastric distention.

GABA(B) receptor-mediated effects on vagal pathways to the lower oesophageal sphincter and heart

GABA(B) receptors influencing vagal pathways to the lower oesophageal sphincter and heart were investigated. In urethane-anaesthetized ferrets, the GABA(B) agonist baclofen (7 micromol kg(-1) i.v.) increased basal lower oesophageal sphincter (LOS) pressure. This was reversed by antagonism with CGP35348 (100 micromol kg(-1) i.v.). Baclofen's effect was abolished by vagotomy, suggesting a central action, yet it was ineffective when given centrally (3 - 6 nmol i.c.v.). Peripheral vagal stimulation (10 Hz, 5 s duration) caused LOS inhibition, followed by excitation, then prolonged inhibition. Bradycardia was also evoked during stimulation. Bradycardia and LOS responses were abolished after chronic supranodose vagotomy, indicating that they were due to stimulation of vagal pre-ganglionic neurones, not antidromic stimulation of afferents. Baclofen (1 - 10 micromol kg(-1)) reduced bradycardia and enhanced LOS excitation, which was also seen in animals pretreated with atropine (400 microgram kg(-1) i.v.) and guanethidine (5 mg kg(-1) i.v.), but not in those pretreated with L-NAME (100 mg kg(-1) i.v.). Effects of baclofen (7 micromol kg(-1) i.v.) on vagal stimulation-induced LOS and cardiac responses were unchanged by the GABA(B) antagonists CGP35348 or CGP36742 (up to 112 micromol kg(-1) i.v.), but were reversed by CGP62349 (ED(50) 37 nmol kg(-1) i.v.) or CGP54626 (ED(50) 100 nmol kg(-1) i.v.). Responses of isolated LOS strips to electrical stimulation, capsaicin, NK-1, NK-2 and nicotinic receptor agonists were all unaffected by baclofen (</=200 microM). We conclude that baclofen reduces vagal output at two peripheral sites: one presynaptically on pre-ganglionic neurones (CGP35348-insensitive), and another (CGP35348-sensitive) that could not be identified. This demonstrates heterogeneity of GABA(B) receptors through differential sensitivity to antagonists.

Central control of lower esophageal sphincter relaxation

The lower esophageal sphincter is innervated by both parasympathetic (vagus) and sympathetic (primarily splanchnic) nerves; however, the vagal pathways are the ones that are essential for reflex relaxation of the lower esophageal sphincter (LES), such as that which occurs during transient LES relaxations. Vagal afferent sensory endings from the distal esophagus and LES terminate in the hindbrain nucleus tractus solitarius. The preganglionic motor innervation of the LES arises from the dorsal motor nucleus of the vagus. Together these nuclei comprise the dorsal vagal complex within which there is a neural network coordinating reflex control of the sphincter. Vagal efferent preganglionic neurons to the gastrointestinal tract are organized viscerotopically in the dorsal motor nucleus of the vagus. Stimulation of the dorsal motor nucleus of the vagus caudal to the opening of the fourth ventricle results in relaxations, whereas stimulation in the rostral portion of the nucleus evokes contractions of the LES. Few details are known about the neural circuitry that links sensory information from the stomach and esophagus within the nucleus tractus solitarius to these separate populations of neurons within the dorsal motor nucleus of the vagus. The motor vagal preganglionic output is primarily cholinergic, which ultimately stimulates excitatory or inhibitory motor neurons that control the smooth muscle tone. Excitatory neurons evoke muscarinic receptor-mediated muscle contraction. Inhibitory neurons evoke nitric oxide or vasoactive intestinal polypeptide-mediated relaxation of the lower esophageal sphincter. However, other neurotransmitters are found in vagal preganglionic neurons, including norepinephrine/dopamine and nitric oxide. A subpopulation of nitric oxide synthase-containing vagal preganglionic neurons innervate the upper gastrointestinal tract and mediate relaxation. The neurotransmitters and circuitry controlling lower esophageal sphincter pressure are important to characterize, because part of the dorsal vagal complex is outside of the blood-brain barrier and is a potential target for pharmacologic intervention in the treatment of such disorders as gastroesophageal reflux disease.

Nissen-type fundoplication and its effects on the emetic reflex and gastric motility in the ferret

Recurrent vomiting with failure to thrive is a common problem in neurologically impaired children. Many undergo fundoplication to control the underlying gastro-oesophageal reflux. The results of surgery are not always satisfactory and post-operative retching may be a major problem - a symptom indicative of activation of the emetic reflex. An animal model of antireflux surgery has been developed and used to investigate the effects of such surgery upon the emetic reflex and vagal influences on gastric motility. Following surgery, animals responded to a previously subemetic dose of a centrally acting opiate receptor agonist (loperamide), suggesting that fundoplication may sensitize the emetic reflex. A gastric vago-vagal reflex (tonic inhibition of corpus tone) and responses to direct stimulation of vagal motor efferents (both cholinergic and nonadrenergic noncholinergic responses) were not significantly affected by antireflux surgery. Mechanisms by which neural damage may sensitize the emetic reflex are discussed, together with the possible clinical implications for the management of post-operative symptoms in neurologically impaired children.

Inhibition of transient LES relaxations and reflux in ferrets by GABA receptor agonists

Transient lower esophageal sphincter (LES) relaxation is the major mechanism of gastroesophageal reflux. This study uses an established ferret model to evaluate GABA(B) receptor agonists' ability to reduce triggering of transient LES relaxations. One hundred sixty manometric/pH studies were performed on 18 conscious ferrets. In untreated animals, intragastric infusion of 25 ml glucose (pH 3.5) led to 2.0 +/- 0.6 reflux episodes over the first 30 min. Twenty-nine of forty-seven reflux episodes occurred during transient LES relaxation, and 18 occurred after downward drifts (<1 mmHg/s) in basal LES pressure. The GABA(B) receptor agonists baclofen (7 micromol/kg ip), CGP-44532, and SKF-97541 (both ED(50) <0.3 micromol/kg) reduced reflux episodes and transient LES relaxations. The putative peripherally selective GABA(B) receptor agonist 3-aminopropylphosphinic acid (80-240 micromol/kg) was ineffective, as was the GABA(A) receptor agonist muscimol (5 micromol/kg). Baclofen's inhibition of transient LES relaxations and reflux was unaffected by low-affinity GABA(B) receptor antagonists CGP-35348 and CGP-36742 at 100 micromol/kg but was reversed by higher-affinity CGP-54626 and CGP-62349 (0.7 micromol/kg) or by CGP-36742 at 200 micromol/kg. Therefore, GABA(B) receptor inhibition of reflux shows complex pharmacology. Our and other data indicate the therapeutic potential for these drugs.

Influence of different intragastric stimuli on triggering of transient lower oesophageal sphincter relaxation in the dog

Gastro-oesophageal reflux in the dog is mainly caused by transient lower oesophageal sphincter relaxation (TLOSR), the major stimulus for which is distension of the stomach. The possibility that liquid and/or acid sensors in the proximal stomach reduce the incidence and/or shorten the duration of TLOSR was addressed in the present study. Manometric recordings of the pharynx, oesophagus, lower oesophageal sphincter and stomach were made in awake dogs equipped with an oesophagostomy. TLOSRs were induced by insufflation of air or infusion of liquid nutrients with varying pH. Intragastric distension with air provoked TLOSRs with a significantly shorter duration than those seen after distension with liquid (4.3 +/- 0.5 vs 9.6 +/- 0.3 sec; P < 0.05). There were fewer TLOSRs at high intragastric pH (pH 5.0: 3.1 +/- 0.5/90 min) than at low pH (pH 1.5: 5.5 +/- 0.9/90 min, P < 0.05). Successfully propagated peristalsis following a TLOSR was more common after stimulation with liquid than with air. It can be concluded that there are H(+)-sensing mechanisms in the stomach which stimulate triggering of TLOSR. In addition, the reduced duration of TLOSR during air insufflation shows that the physical state of the distending stimulus can affect the patterning of TLOSR.

An in vitro study of the properties of vagal afferent fibres innervating the ferret oesophagus and stomach

1. A novel preparation of the oesophagus with attached vagus nerve from the ferret maintained in vitro was used to study the properties of single vagal afferent nerve fibres with identified receptive fields. 2. Recordings were made from three types of gastro-oesophageal vagal afferent fibres that were classified on the basis of their sensitivity to mechanical stimulation. There were those responding to mucosal stroking (mucosal receptors), to circular tension (tension receptors) and those responding to mucosal stroking and circular tension, which we have termed tension/mucosal (TM) receptors. 3. The conduction velocities for mucosal, TM and tension receptor fibres were 6.38 +/- 1.22 m s-1 (n = 22), 6.20 +/- 1.49 m s-1 (n = 13) and 5.33 +/- 0.86 m s-1 (n = 22), respectively. 4. Receptive fields of afferents showed random topographical distribution by fibre type and conduction velocity. They were found mainly distal but also occasionally proximal to the point of vagal dissection. 5. Twenty-eight per cent of mucosal, 63% of TM and 43% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. 6. In conclusion, this experimental preparation provides evidence for the existence of three types of oesophageal vagal afferent fibre, namely mucosal, tension and the newly identified tension/mucosal receptors.

Neuronal pathways and transmission to the lower esophageal sphincter of the guinea Pig

BACKGROUND & AIMS

The lower esophageal sphincter (LES) normally controls the opening and closing of the gastroesophageal junction to resist gastric reflux but allow swallowing. Neuronal pathways controlling the guinea pig LES were investigated anatomically and physiologically in isolated preparations.

METHODS

Intracellular recording from the LES with focal electrical stimulation and retrograde and anterograde neuronal tracing were used.

RESULTS

Electrical stimulation on the LES evoked inhibitory junction potentials (IJPs), which were reduced by 60% by 100 micromol/L N-nitro-L-arginine and subsequently blocked by 0.5 micromol/L apamin, unmasking excitatory junction potentials, which were abolished by 1 micromol/L hyoscine. Esophageal or vagal stimulation evoked IJPs, which were blocked by 100 micromol/L hexamethonium. Focal stimulation of the upper stomach evoked IJPs at 5-8 of 20 stimulation sites, which were abolished by cutting between the stimulation site and sphincter. Application of 1,1'-didodecyl-3,3,3', 3'-tetramethyl indocarbocyanine perchlorate (DiI) to the gastric sling muscle anterogradely labeled many motor axons in the sling muscle but few in the LES, confirming that the two muscles are separately innervated. DiI on the esophagus labeled nerve fibers, but not cell bodies, in the upper stomach.

CONCLUSIONS

The inhibitory motor neurons of the LES receive inputs from the vagus nerve, esophagus, and upper stomach.