Transmitter mechanisms in vagal afferent-induced reduction of lower oesophageal sphincter (LOS) pressure in the rat

Physiology of the LES

The purpose of this review is to consider the neuromuscular mechanism of LES contractility both by itself and in relation to the esophagogastric junction (EGJ) complex in order to appreciate the intricacies of EGJ valvular function.

Anorectal sleeve micromanometry for the diagnosis of Hirschsprung's disease in newborns

BACKGROUND/PURPOSE

An accurate diagnosis is mandatory for surgery in newborns with Hirschsprung's disease (HD). Acetylcholinesterase staining of rectal suction biopsy specimens is widely performed in the diagnosis of HD, but results are sometimes incorrect or atypical in newborns. We report the usefulness of our method of anorectal manometry using a specially designed sleeve microassembly for the diagnosis of neonatal HD.

METHODS

Anorectal manometry was conducted without sedation in 41 newborns, aged 2 to 30 days (19 newborns were within the first week of life), with abdominal distension. A silastic assembly with a 2-cm-long sleeve sensor and 5 side holes arrayed along the sleeve was designed to reduce the effects of displacement of pressure sensors relative to the anal sphincter. Rectoanal inhibitory reflex (RAIR) was examined with rectal balloon distension.

RESULTS

Thirty-two subjects who showed falls of anal sphincter pressure fulfilling the criteria for RAIR were diagnosed to be without HD. Nine patients without an appropriate RAIR were subsequently confirmed to have HD based on operative pathologic findings. Parameters of anal sphincter function did not differ significantly between the subjects with and without RAIR.

CONCLUSIONS

An anorectal sleeve micromanometric technique is useful in the diagnostic workup of newborns suspected of having HD.

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.

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.

Neuronal control of the gastric sling muscle of the guinea pig

The gastric sling (oblique) muscle (GSM), located close to the lower esophageal sphincter (LES), is involved in gastric motor function and may cooperate with the LES in controlling propulsion between the esophagus and stomach. Neuronal pathways and transmission to the GSM were investigated in isolated esophagus-stomach preparations by using intracellular recording with the focal electrical stimulation and neuroanatomical tracing method. Focal stimulation on the GSM evoked inhibitory junction potentials (IJPs) that were reduced to 45% by 100 microM N-nitro-L-arginine and subsequently blocked by 0.5 microM apamin, thereby unmasking excitatory junction potentials (EJPs), which were abolished by 1 microM hyoscine. Vagal and esophageal stimulation evoked IJPs that were blocked by 100 microM hexamethonium. Vagal stimulation also evoked EJPs after blockade of IJPs. Application of 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate to the GSM labeled muscle motor neurons located in the stomach mainly close to the GSM, with a few neurons (2%) in the esophagus. The majority (79%) of labeled neurons were immunoreactive for choline acetyltransferase and, hence, excitatory motor neurons. Inhibitory motor neurons (nitric oxide synthase immunoreactive; 15%) were clustered in the midline near the gastroesophageal region. These results demonstrate that the GSM is innervated primarily by gastric excitatory and inhibitory motor neurons and some esophageal neurons. Both excitatory (acetylcholine) and inhibitory (nitric oxide and apamin-sensitive component) transmission can be activated via vagal-enteric pathways.

Mechanism of cholecystokinin-induced relaxation of the rat stomach

This study was designed to investigate the mechanism(s) and the site(s) of action of cholecystokinin (CCK) responsible for the smooth muscle relaxation of the rat stomach. Under xylazine and ketamine anesthesia, an extraluminal force transducer was implanted on the serosal surface of the gastric body to monitor the circular muscle motility. CCK8 (1-100 pmol, i.v.) caused predominantly inhibitory effects (65%) on gastric motility but sometimes no effect (20%), excitatory effects (10%) or biphasic effects (5%) were observed in 125 rats tested. CCK8 consistently caused relaxation in rats pretreated with yohimbine, while CCK8 caused contraction in rats pretreated with propranolol. CCK8-induced relaxation in the presence of yohimbine was abolished by pretreatment with guanethidine, 6-hydroxydopamine, celiac ganglionectomy and hexamethonium but not by VIP antiserum or a nitric oxide (NO) inhibitor. CCK8-induced relaxation was significantly reduced by perineural capsaicin treatment of the celiac ganglia or vagal trunk. Subsequent truncal vagotomy had no effect on CCK8-induced relaxation in rats with perivagal capsaicin treatment, but completely abolished CCK8-induced relaxation in rats with capsaicin treatment of the celiac ganglia. Our present study suggests that CCK8 predominantly stimulates vagal and splanchnic afferents, resulting in vago-splanchnic and splanchno-splanchnic reflexes. Released catecholamine from splanchnic efferents by CCK8 can induce both excitatory and inhibitory reflexes via alpha2 and beta adrenergic receptors located on gastric smooth muscle cells, respectively. These finding may explain some of the variable results reported for the actions of CCK8 on gastric motility.

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.

Gastroesophageal reflux disease during pregnancy

Pregnant patients with symptomatic GERD should be managed aggressively with lifestyle modification and dietary changes. Antacids and antacids/alginic acids combination or sucralfate should be considered first-line medical therapy. If symptoms are not adequately relieved or complications develop, treatment with cimetidine or ranitidine should be considered; these H2 receptor antagonists are preferred during pregnancy. Nizatidine cannot be recommended. Proton-pump inhibitors should be used with caution because little human experience is available. Despite this caveat, both proton-pump inhibitors are likely to be safe during pregnancy.

Mechanisms of gastro-oesophageal reflux in the ferret

Transient lower oesophageal sphincter (LOS) relaxation is the major mechanism of gastro-oesophageal reflux in humans--an event unassociated with swallowing. Mechanisms involved in triggering transient LOS relaxation are poorly understood, and their further study requires a small animal model. In this study we aimed to establish methods for prolonged ambulant oesophageal manometry in ferrets, and to determine motor events associated with reflux episodes and their triggering by different gastric nutrient loads. Forty-two studies were performed on nine ferrets with chronic cervical oesophagostomies, through which a manometric assembly was introduced and secured to a collar, which incorporated a microphone for detection of swallows. The assembly included a gastric feeding channel, one gastric and four oesophageal manometric sideholes, a 2.5-cm-long LOS sleeve sensor, and an oesophageal pH electrode. Intragastric infusions were given over 2 min, the first after a 30-min control recording period, and in 29/42 studies, a second infusion was given 60 min later. Infusions were either 25 mL 10% dextrose solution, pH 3.5 (22 studies), 25 mL triglyceride emulsion (Intralipid) pH 3.5 (11 studies), or 25 mL air (nine studies). Episodes of oesophageal acidification were absent before gastric infusions. After infusion, 2.1 +/- 0.2 episodes occurred over the first 30 min. After glucose infusion, 15/18 acidification episodes (83%) occurred during transient LOS relaxation, and 3/18 (17%) occurred after gradual (< 1 mmHg sec-1) downward drifts in basal LOSP to < 2 mmHg. After lipid infusion two acidification episodes occurred, both during transient LOS relaxation. Mean duration of transient LOS relaxation was 8.0 +/- 0.4 sec. All infusions increased occurrence of transient LOS relaxation to a similar extent, each of which ended with primary peristalsis. We conclude that gastric infusion of glucose, lipid and gas are all effective in provoking gastro-oesophageal reflux in ferrets. Reflux occurs through similar mechanisms to those seen in humans, i.e. increased triggering of transient LOS relaxation. The conscious ferret is therefore an appropriate model for future studies of manipulation of mechanisms giving rise to gastro-oesophageal reflux.

Vagal and sympathetic influences on the ferret lower oesophageal sphincter

This study has investigated the relative involvement of cholinergic, adrenergic, nitric oxide and tachykininergic transmission in extrinsic neural influences on the lower oesophageal sphincter (LOS) in urethane anaesthetized ferrets. A micromanometric assembly (OD 1.75 mm) incorporating a sleeve sensor was used for high-fidelity oesophageal, LOS and gastric pressure measurement at low perfusion rates (< 0.1 ml/min). The LOS response to vagal and splanchnic nerve stimulation (0.5 ms pulse width, 10 s duration) was frequency- and voltage-dependent. LOS responses to stimulation at 20 V, 10 Hz were investigated in separate groups of animals with either L-NAME (100 mg/kg), hexamethonium (15 mg/kg), guanethidine (5 mg/kg), CP96,345 (NK-1 antagonist, 4 mg/kg), atropine (0.4 mg/kg) or propranolol (1 mg/kg). Propranolol treatment was followed by yohimbine (1 mg/kg) and prazosin (0.25 mg/kg). Vagal stimulation caused an immediate decrease in LOS pressure, followed by increase on cessation of stimulation, followed by a prolonged decrease (77 +/- 2%) for up to 5 min. L-NAME did not affect inhibition, but increased excitation 4-fold (p < 0.001). Guanethidine and CP96,345 had no major effect. Hexamethonium decreased the inhibitory (p < 0.05) and excitatory (p < 0.01) responses. Atropine reduced the excitatory response (p < 0.05). Some inhibition still remained if all treatments were combined. Splanchnic stimulation reduced LOS pressure by 70 +/- 6% for 101 +/- 17 s. L-NAME, guanethidine, hexamethonium and CP96,345 all independently significantly reduced inhibition. The combination of guanethidine and CP96,345 usually abolished splanchnic-induced inhibition. Atropine was without effect. Propranolol (1 mg/kg) changed the splanchnic-induced response from mainly inhibition to excitation (100 +/- 44% increase). LOS responses to noradrenaline (1-10 micrograms close IA) showed similar features to responses to splanchnic stimulation. We conclude that vagal stimulation evokes LOS relaxation via activation of established cholinergic and NANC mechanisms and other, unidentified mechanisms. Splanchnic stimulation activates adrenergic neurones probably via nicotinic and non-nicotinic ganglionic mechanisms, which in turn elicit beta adrenergic inhibitory effects on the LOS. Splanchnic stimulation also antidromically activates spinal afferent fibres. These may release substance P from peripheral myenteric plexus and prevertebral ganglionic endings causing activation of myenteric NANC inhibitory neurones and sympathetic neurones, respectively.

Contribution of acetylcholine, vasoactive intestinal polypeptide and nitric oxide to CNS-evoked vagal gastric relaxation in the rat

Several in vitro models of gastric relaxation have elucidated a role of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) in non-adrenergic, non-cholinergic (NANC) vagally mediated gastric relaxation. However, these models do not necessarily mimic the events leading to gastric relaxation in the whole animal. We have recently described a vagally mediated gastric relaxation evoked by micro-injection of substance P (SP) into the nucleus raphe obscurus (NRO). The present study was performed to elucidate whether this CNS-stimulated in vivo gastric relaxation involved acetylcholine, NO and VIP. Atropine (1 mg kg-1 i.v.), reduces both the rapid nadir and sustained gastric relaxation evoked by SP in the NRO, and the residual responses are abolished by NG-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 10 mg kg-1 i.v.), an NO synthase inhibitor. Blockade of NO synthase alone is not sufficient to abolish the effect of SP into the NRO on intragastric pressure. A VIP antagonist, [p-chloro-D-Phe6, Leu17]VIP (32 micrograms i.v.) alone, or with the addition of L-NAME, does not affect the nadir of the gastric relaxation in response to SP microinjected into the NRO; however, both antagonists reduce the CNS-evoked sustained intragastric pressure relaxation. We conclude that, in CNS-evoked gastric relaxation, inhibition of cholinergic pathways is potentially important for both the rapid nadir and sustained gastric relaxation, and both NO and VIP contribute to sustained gastric relaxation.